GOLDEN NATURE GUIDES •
Birds
Flowers
Trees I n sects
•
Stars
Rept i l es and A m p h i b i a n s Mammals Seashores
•
F i shes
Weather Rocks and M i nerals P h otography
( A GOLDEN HANDBOOK)
Zoo logy
•
Foss i l s
G a m e b irds Sea Shells of the World IN PREPARATION:
Moths a n d B utterflies N o n -flowering P l a nts REGIONAL GUIDES OF AMERICA
T h e South west The Southeast The Pacific Northwest Evergl a des N ati o n a l P ark The Rocky Mounta i ns
1 00100
De
WEATHER
AIR MASSES-CLOUDS-RAINFALL S TORMS-WEATHER MAPS-CLIMATE by PAUL E. LEHR Meteorologist, Scien tific Serv i ce s D i v i s i o n of U . S. A i r W e a t h e r Service
R. WILL BURNETT
Professor of Science Education, University of Illinois HERBERT S. ZIM I
l
I l l ustrated by HARRY McNAUGHT
r
).
{ A GOLDEN
NATURE GUIDE
GOLDEN PRESS
•
NEW YORK
FOREWORD Of a l l aspects of the natura l world, weather is outsta nd i n g i n its bea uty, its majesty, its terrors, a n d its conti n u a l d i rect effect o n us a l l . Because weather invo lves, f o r t h e m ost part, massive movements o f invisible a i r a n d i s con cerned with the temperature a n d pressure c h a nges of this a l most intangible substa nce, most of u s h ave o n l y a l im ited u n derstanding of what weather is a l l about. This book wi l l h e l p you to u n dersta n d it a n d a lso to u n der sta nd, i n some deg ree, how weather changes a re pre d icted. I n the difficult a ttempt to portray the weathe r simply, acc u rately, and graph ica l ly we have had inva l u a b l e assist ance from co l leag ues, experts, and many orga n izations whom we g ratefu l ly thank. The U.S. Weather B u reau (in c l u d i n g the libra ry) helped us l ibera l l y with information and photos. H e l pf u l materia l was supplie d a lso hy the Smithsonian I n stitution, the American Meteorological So c iety and its secreta ry, Ken neth F. Speng ler, the Nationa l Safety Counc il , Dr. David M. Ludlam of the F ra n k l i n I n sti tute, Lt. J o h n H. Boone ( USAF), and Lt. J o h n F. M a n n , J r . ( USAF). Bernice Burnett a -n d A d e l e F . Lehr r e a d a n d criti c ized the m a n uscript at various stages. Dr. Vincent J . Schaefer o f t h e Munita l p Foun dation exa mined both text a n d i l l ustrations a n d offered m uch h e l pfu l a dvice. P.E.L. R.W.B.
H . S.Z. Twelfth Printing, 1 963
Library of Congress Catalog Card Number: 61-8327 ®Copyright 1957 by Golden Press, Inc. All Rights Reserved. Including the Right of Reproduction in Whole or in Part in Any Form. Designed and Produced by Artists and Writers Press, Inc. Printed in the U.S.A. by Western Printing and Lithographing Company. Published by Golden Press, Inc., New York 22 N. Y. Published Simultaneously in Canada by The Musson Book Company, Ltd., Toronto
CONTENTS The effects of heat, p ressu re, wind, a n d moisture . . . Clouds
5-20
Their na tu re, types, a n d origins . . . Rainmaking . . . .
21-33
WHAT MAKES THE WEATHER?
RAIN, SNOW, DEW, AND FROST
THE ATMOSPHERE-RESTLESS OCEAN OF AIR .
.
THE
.
Its structure a n d weather function . . . . . EARTH'S
MOTIONS
AND
34-47
WEATHER
Seaso n a l cha nges a n d the ea rth's rotation a s they affect winds . . . . . . . . . . . . . . .
48-59
Pressure cells, their winds, and a ssociated weather . . . . . .
60-67
Major a i r masses of the world, their identification, a n d their role as a sou rce ·of o u r weather . . . . . . . . . . . . . .
68-76
AND FRONTAL WEATHER How fronts form, move, a n d change . . . The kinds of weather a ssociated with each type . . . .
77-95
T h e origin, developme nt, a n d effects of t h u n derstorms, to rnadoes, a n d h u rrica nes
96-111
HIGHS AND LOWS
AIR MASSES
FRONTS
STORMS
FORECASTING Weather i n struments and how they a re used i n forecasti n g 112-129
WEATHER
How data are plotted a n d cha rted • . . H o w to r e a d maps a n d make 130-146 y o u r own forecast . . • . • . . . . .
WEATHER MAPS
Average weather conditions worth knowing a n d using day by . . • . . . . 147-156 day .
WEATHER AND CLIMATE
BOOKS FOR MORE INFORMATION
and magazines to read . . . . . . .
INDEX
Books
157 158-160
EVERYBODY TALKS ABOUT T H E W EAT H E R C h a rles Dudley Warner said, " Everybody ta l ks a bout the weather, but nobody does anyt h i n g about it." Everyone, at times, feels as Warner did. The spoiled fa mily picn ic, the withered crops, a l l rem ind us how dependent we are on the weather. That is why weather is o u r m ost common topic of conversation, a factor i n much of o u r agricu ltura l , ind ustria l , a n d civic p l a n n ing, and a consta n t concern of everyone. Warner was wrong. Someth ing is being done. Today the science of weather-meteorology-is used to make our l ives safer and better. Some types of forecasts are 95 per cent accu rate. Storms are tracked and warnings a re g iven . C l o uds a re being seeded to ca use rainfa l l where i t is n eeded. A n etwork o f weather stations e n a b les p l a nes to fly safe ly. A continued program of research reveals more and more about the weather. This introduc tion to weather will help you understa n d it.
4
What M akes the Weather? Weather is the cond ition of the atmosphere in terms of h eat, pressure, wind, and moisture. These are the elements of which the weather is made. Where the atmosphere thins to noth i n g n ess, there is no weather. There is n o weather on the moon, for it has n o atmosphere. But near the sur face of the earth the atmosphere is dense and heavy. Here, in the lower atmosphere, you conti n ua l ly see the everchanging, d ra m atic, often violent weather show. But it takes more than a i r to make weather. If the earth's atmosphere were never heated, m ixed, o r moved about, there would be no weather-or, more properly, there would be n o cha nges in the weather. There would be n o w i nds, no chan ges in a i r pressure, no storms, r a i n , o r snow. Heat is the spoon that m ixes the atmosphere to make weather. All weather cha n ges are brought about by tem perature c h a n ges i n d ifferent pa rts of the atmosphere. 5
T H E S U N, sou rce of m ost of the earth's heat, is a ba l l
o f glowing gases, 9 3 m i l l ion m i les away. This giga ntic atomic fu rnace bom bards the earth with 1 26 tri l l ion horse power every seco n d . Yet this vast energy is but a half of o n e b i l l ionth of the s u n 's tota l output. Most of this solar energy is lost i n space; traces reach other p l a n ets. The s u n 's energy is transm itted as waves that a re s i m i l a r to radio waves. Some of these are visi b l e light waves; others a re invisible. Some, although not heat waves, change to heat when a bsorbed by ob jects such as soil or our bod ies. About 43 per cent of the radiation reaching our pla net h its the earth's s u rface and is changed to heat. The rest stays in the atmosphere or is reflected into space.
6
,�
W H AT H A P P E N S TO T H E SU N'S H EAT is shown in
the diagram a bove. This is for average weather-that is, 52 per cent c l o u d i n ess in the sky. A typical c l o u d refl ects back into space 75 per cent of the s u n l ig h t striking it. O n overcast days, o n ly about 25 per cent of the s u n's energy h its the g roun d. Energy that does reach the ground is a bsorbed a n d refl ected in varying deg rees. Snow reflects about 75 per cent, a bsorbs o n ly 2 5 per cent; this partly accou nts for the co l d of pol ar reg ions. Dark forests absorb about 95 per cent of solar energy a n d change it to heat. Such d ifferences i n a bsorptio- n a n d reflection acco u n t, i n pa rt, f o r regiona I d ifferences i n temperature a n d c l imate. ABSORPT I O N O F S U N L I G H T BY D I F F ERENT S U RFACES
dense forests 95%
plowed field 75 to 95%
7
'•' '• .
,
if9"' ' So l a r rays g o through g lass-
heat rays cannot
A g reenhouse "traps" solar radiation when ..short" sol a r rays cha n g e to "long" heat rays.
Earth's atmosphere is like g l a ss. It lets solar rays through but keeps most heat rays from· escaping.
8
EARTH
AS
A
G R E E N H O USE
The g lass of a g reenho use lets the s hort solar rays pass through. These a re a bsorbed by o bjects inside a n d a r e re-rad iated as l o n g heat rays. But these long heat rays c a n not get through the g lass. The heat rays a re conti n u a l l y re-absorbed a n d re rad iated inside. This h e l ps keep the g reenhouse warm on cold days. Some heat is lost by conduction through the g lass. Like a gree n h ouse, the earth's atmosphere admits most of the solar radiation. When this is a bsorbed by the earth's surface, it is re-ra di ated as heat waves, m ost of which a re trapped by water vapor in the atmosphere. Thus the e a rt h is kept warm.
T H E ATMO S P H E R E AS A T H E R MOSTAT controls the earth 's heat
a s a utomatica l ly a s i n any heati n g system . It p rotects the earth from too much so l a r radiation d u ring the day, a n d screens out d a ngerous rays. It acts as a n i nsulating blan ket which keeps m ost of the heat from esca p i n g at n ig ht. Without its thick atmosphere the earth wou l d experience tem peratu res like t h e m oo n 's. T h e m o o n 's surface tem perature reaches the poi l i n g poi nt of water (2 1 2 ° Fahren heit) d u ring the two-week lunar day. It d rops to 238 ° F below zero durin g the long lunar n ight.
Earth has thick atmosphere.
l
night 40 °F
1
Moon has a very thin atmosphere.
The earth cools faster o n bright clear nig hts than o n c l oudy nig hts, because an overcast sky refl ects a large a m o u n t of heat back to earth, where it is once again re-a bsorbed .
.
-
.
. .
perature a n d reta rds night heat loss.
o n a clear n i g h t more heat esca pes.
H EAT AND AIR MOVEM ENTS
The a i r is heated mainly by contact with the warm earth. W h e n air is Co nvection cu rrents warmed, it expa n d s a n d becomes in heated water lig hter. A layer of air, warmed by contact with the earth, rises a n d i s replaced by colder a i r which fl ows in a n d u n d e r it. This c o l d a ir, in turn, is warmed a n d rises, a n d it, too, is replaced by colder air. Such a circulating movement of w arm a n d c o l d fl uids is ca l led "convectio n . " You can see co nvection c ur rents if you drop sma l l b its of paper into a g lass conta iner in which water is being heated. The air at the eq uator receives much more heat than the air at the poles (p. 5 1 ) . So warm air at the eq uator rises and is replaced by colder a i r flowing i n from north and south . The warm, light a i r rises and moves poleward high above the earth. As it coo ls, it sin ks, rep lacing the cool s u rface air which has moved towa rd the eq uator. If the earth did not rotate, the air wou l d circ u l ate as shown . Because the earth does rotate, the circulation is different (p. 53).
Air movements over a n on-rotati n g earth
10
"" I ,
- ·,
' '
Differe n ces i n h11ating cause loca l winds.
CO NVECT I O N ca uses loca l winds and b reezes. Different
land and water surfaces absorb d iffere nt a m o u nts of heat. Da rk, plowed soi l absorbs much more tha n g rassy fields. Mounta ins a bsorb heat faster during daylight t h a n nearby va l l eys, and lose it faster at n ig ht. land warms faster tha n does water d u r i n g the day a n d cools faster at n i g ht. The air above such surfaces is warmed o r cooled accord i n g l y -and loca l w i n d s resu lt.
Mountain breezes i n daytime S e a breezes in d ayti me
Mountain breezes at n i g h t
Wate r is a lways pres ent i n the air. It eva porates from the earth, of which 70 per cent is covered with water. In the a ir, water exists in three states: solid, liquid, a n d invisib le va por. The amount of water vapor in the air is ca l l e d the " h u m idity." The "re lative humidity" is the amount of vapor the air is holding expressed as a percenta ge of the amount the a i r c o ul d h o l d at that pa rticular tem perature. Warm a i r can h o l d more water than cold. When air with a g iven amount of water vapor coo ls, its relative h u m i d ity g oes u p; when the a i r is warmed, its rel ative h u m id ity drops. As the ta b l e below shows, air at 86°F is "saturated" when it holds 30.4 grams of water vapor per cubic m eter. ( I n other words, it has a relative h u m i d ity of 1 00 per cent; it has reached its dew point.) But air at 68 ° is satu rated when it h o l ds o n ly 1 7. 3 grams per cubic m eter. That's a d ifference of 1 3. 1 grams per c u b ic m eter. So every cubic m eter of 86° saturated air that is coo led to. 6 8 ° w i l l lose 1 3 . 1 grams of water vapor as cloud d rop lets which, if cond itions are rig ht, will fa l l as rain or snow.
W AT E R I N T H E ATMOS P H E R E
R E L AT I VE H U M I D I TY 1 6%
24%
31%
45%
57%
28%
42%
54%
79%
1 00%
53%
69%
1 00%
52%
77%
1 00%
67%
1 00%
36%
I
1 00%
1 00%
4.85
7.27
9.4 1
1 3 .65
grams af water vapor per
12
1 7. 3 1
cubic meter
30A
.... ...
'
"'
�
....
...
....
� .... ...
... .....
.....
+-
.... "'
....
"'
"'
'\
'\
1 '
H EAT
AND
ATMOS P H E R I C
WAT E R
t
Heat eva porates m i l l ions of tons of wate r i nto the air d a i ly. Lakes, streams, a n d 1 1 t j oceans send up a stea dy stream of water t ' vapor. An amazing amount of water tra n +. spires from the leaves of g reen pla nts. A l s i n g l e apple tree may m ove 1 ,800 g a l l o n s j l J. of water i nto the a i r in a six-month g rowi J. i n g season. J. As mo ist warm a i r rises, it slowly cools. t t Fina l ly it cools so much that its relative l. ' 1 h u m i d ity reaches 1 00 per cent. C louds r o n i ra conditions, in certa under d, n a form .l, ; f snow comes down. This eternal process of + eva poration, condensation, a n d preci pita +. r j ion is cal led the water cyc le. J.. • +
+
)' .1'
r
When a i r is cooled be low its saturation point the water vapor in it condenses to form clouds. When water vapor at a teakettle s pout is cooled by the air around it, a s m a l l cloud forms. You r w a r m mo ist b reath forms a m i n iature c loud when it h its the cold wi nter a i r . The clouds you see nea rly every day form i n several ways but all form by the sa m e general p rocess-coo l i ng of a i r below its saturation point. HOW C L O U DS AR E FORMED
Earth radiates heat rapidly o n clear n i g h ts. Air in contact with cold earth may cool below its sat. u ratio n point a n d fa rm low clou ds, or fog {a cloud on the ground). � warm air
Warm a i r is often l ifted by a h eavier mass of cold a i r which p u shes u nder it l i ke a wedge. C louds form as warm air coo ls � below its saturation point.
..,. Warm a i r may move over a cold s u rface and be cooled below its satu ration poi nt. C l o u d s may form a s warm l a ke or ocea n a i r moves in over a cooler l a n d su rface.
col d air
Air m a y b e heated b y contact with the earth's warm su rface. I t expends, becomes l i g hter, a n d rises. Expansion lowers its tem· peratu re. The more it rises, the more it cools-at a rate of about 51f2 ° F for each 1 ,000 ft. of rise. This "adia batic coo l i n g " occurs whenever a i r rises. Most clouds form beca u se of adia batic coo l i n g .
Air movi n g up a slope loses heat adia batica lly as it rises. I f
Warm a i r often p u shes over a mass of cold a i r (a bove). C louds may form a s it cools adia batica lly because of i ts rise.
Someti mes rain or snow from high clouds may fall through warm a i r, cool it, a n d cause lower clouds to fo rm. These lower clouds w i l l genera lly be i n laye rs-often i n several levels.
it rises enough to cool below its saturation point, clouds will fo rm.
Cum ulus clouds
Stratus clouds
C louds are c lassified ac cord i n g to how they are formed. There a re two basic types: ( 1 ) C louds formed by rising air currents. These a re piled u p a n d puffy. They a re ca l led "cu m u l us," which means piled u p or acc u m u lated. (2) C louds formed when a layer of air is coo led below the saturation point with out vertica l movement. These a re i n sheets o r fog l ike layers. They a re cal led "stratus," meaning sheetlike or l ayered. C louds a re further classified by a l titude into four fa m i lies: h i g h c louds, m i d d l e clouds, low clouds, a n d towering c louds. The bases of the latter may be as low as the typi ca l low clouds, but the tops may be at or above 75,000 ft. C L O U D C LASS I F I CAT I O N
The na mes of c louds are descriptive of their type and form. The word " n i m b us," meaning rain cloud, is added to the na mes of c louds w h ic h typically produce rain o r snow. The prefix "fra cto-," meaning frag m ent, is added to na m es of wind-blown clouds that are broken into pieces. "Alto-," mea n i n g high, is used to indi cate m idd le-layer high c louds of either stratus or c u m u lus types. The pictures a n d captions on the next four pages will help you to identify m a jor cloud types and to u n d er sta n d better their relationship to the weather. C L O U D N AMES
16
H I G H C L O U DS are com posed a l m ost entirely of tiny ice crysta ls. Their bases averag e about 20,000 ft. above the earth. Three types exist: C i rrus clouds, thin, wispy, and feathery, are com posed e ntirely of ice c rysta ls. Cirrus clouds usu ally form at 25,000 ft. a n d above, where the tem perature is a lways far below freezing. These clouds are freq uently b lown about i nto feathery stra nds ca lled "mares' ta i ls." C i rroc u m u lus clouds, genera l ly form ing at 20,000 to 25,000 ft., are rarely see n . These thin, patchy clouds often form wavelike pat terns. These are the true mackerel sky, not to be confused with a lto c u m u l u s ro l l s . They a re often rip p led and a lways too thin to show shadows. C i rrostratus clouds form at the
same a ltitudes as c i rroc u m u l us. These a re t h i n sheets that look like fi ne veils or torn, wind-b lown patches of g a uze. Because they a re made of ice crysta l s, cirrostra tus clo uds form large h a l os, or l u m inous circ les, a round sun a n d moon. 17
M I D D L E C LO U DS a re basica l l y stratus or c u m u l us. Their
bases average about 1 0,000 ft. above the earth. Altostratus (above) are dense veils or sheets of g ray or b l ue. They often appear fi brous or l ig htly striped. The sun o r m oon does not form a halo, as with hig her, ice crysta l c irrostratus, b ut appears as if seen th roug h frosted g lass. Altoc u m u l u s (be low) are patches or layers of puffy or
ro l l - l i ke c l ou ds, gray or whitish. They resemble c i rrocu m u l us, but the p uffs or ro l l s a re larger a n d made of water dropl ets, not ice crysta ls. Through a ltoc u m u l u s the sun often produces a corona, o r d isk, genera l ly pa le b l u e or ye l low inside, reddish outside. The corona's color a n d s prea d d i sti n g u ish i t from t h e c irrostratus h a lo-a la rger ring, covering much more of the sky.
18
LOW C L O U DS have bases that range in height from near the earth's s u rface to 6,500 ft. There a re three main kinds: Stratus is a low, q u ite u n iform
sheet, like fog, with the base above the g ro u n d . Dul l-gray stra tus c louds ofte n make a heavy, leaden sky. O n l y fi n e drizzle can fa l l from true stratus c l o u ds, be cause there is l ittle o r no vertical move ment i n them.
N i m bostratus a re the true rain c louds. Darker than ord i n a ry stra tus, they have a wet look, a n d strea ks o f rain often exte nd to the ground. They often are ac compa nied by low sc ud clouds (fractostratus) when the wind is strong.
are i rreg u l a r masses o f c louds spread o u t in a ro l l ing or puffy layer. Gray with darker shading, stratoc u m u l us do not produce rain but sometimes c h a n g e into n i m bostratus, which do. The ro l l s or masses then fuse together a n d the lower surface becomes i n d istinct with rain. Stratocu m u l us
lii ll;ii ,� ••::JI�!t!l�r.::Eii;:;�:· lilllli '!'
C u m u lo n i m bus are the fam ilia r
thu nderheads. Bases may a l most touch the ground; violent up drafts may ca rry the tops to 75,000 ft. Winds aloft often m o l d the tops into a fl at anvil-l ike form. I n their most violent form these c louds prod uce torna does (p. 102). Cumulus a re puffy, caul iflower
l ike. Sha pes constantly change. Over land, cumulus usua l l y form by day i n risin g warm a i r, and disappear at n ight. They mean fair weather u n less they p i l e up into c u m u l o n i m b us. C u m u lus and C u m u lo n i mbus ..
"-•
..;' '
are both clouds of vertica l devel opment, u n l ike the layered clouds described on previous pages. C louds of the c u m u l u s type result from stro ng vertical currents. They form at a l m ost any a ltitude, with ba ses sometimes as h i g h as 14,000 ft.
,...
�
....
�·� ' .�
�
--
CLOUD SYMBOLS
Q \,../
B
�
20
cumulus
d
a ltostratus
stratocumulu s
\..A../
altocu m u l u s
stratus
---->
cirrus
cumulonimbus
L
cirrostratus
n i m bostratus
�
cirroc u m u l u s
Ra i n, S n ow, Dew, a n d F rost P R E C I P I TAT I O N s u c h as rain, snow, sleet, a n d h a i l can occur only if there are clouds i n the sky. But not all kinds of c l ouds can produce precipitation. Temperature, the presence of tiny foreig n partic les, or of ice c rysta ls, a l l h e l p dete r m i n e whether preci p itation w i l l occur a n d what form it will take. For exa m p le, snow will n ot fo rm u n less a i r is supersaturated (cooled below its satu ration point or dew point without its water vapor condensin g ) a n d is con sidera bly below the freezing point of water.
21
Rain fa l l s from c louds for the same reason a nyth ing fa l ls to earth. The e a rth's g ravity pulls it. But every cloud is made of water drop lets o r ice c rysta ls. Why doesn 't rain or snow fa l l con sta ntly from a l l clou ds? The droplets or ice c rysta ls in clouds are exceeding ly sma l l . The effect of gravity on them is m i n ute. A i r currents move a n d l ift droplets so that the n et downward movement is zero, even though the droplets a re i n constant motio n . Droplets a n d i c e crysta ls behave somewhat l i ke dust in the a i r made visible in a shaft of s u n l i g ht. But dust partic les are much larger than water droplets, and they fi n a l l y fa l l . The cloud droplet of average size is only 1/2500 inch i n diameter. It is so s m a l l that it would take 1 6 hours to fa l l h a lf a mile in perfectly sti l l a i r, and it does not fa l l out of moving air at a l l . Only when the d roplet g rows to a diameter of 1/125 inch o r larger can it fa l l from the cloud. The average raindrop conta i n s a m i l lion times as much water as a tiny cloud droplet. The g rowth of a c l o u d droplet to a size large enough to fa l l out is the cause of rain and other forms of precipitation. This i m por tant g rowth p rocess is cal led "coa lescence." W H AT MAKES IT RAI N ?
C l o u d drop lets (enlarged 70 times).
Smal lest raindrop (enla rged 70 times).
..
Coalescence occurs c h iefly in two ways: (1) Droplets in clouds are of d ifferent sizes. Big d ro ps move more slowly i n turbulent air and in paths d ifferent from the paths of sma l l droplets. Bigger, heavier d ro ps are not whipped around so ra pidly. So drops c o l l ide, beco m e b igger, and fi n a l ly d rop as rain. This is prob a b ly the main cause of rainfa l l from n i m bostratus and other low c louds. (2) The most i m po rta nt type of coa lescence occurs when tiny ice crystals and water d roplets occ ur to gether (as near the m i d d l e of cumu l o n i m bus c louds). Some water drop lets evaporate and then condense on the crystals. The c rysta ls grow until they drop as snow or ice pe l l ets. As these drop through warm a ir, they c h a n g e i nto raindrops. (3) lig htning d ischa rges i n a thu n derstorm form oxides of n itrogen that a re extremely hyg roscopic (water absorbing). These oxides are added to the atmosphere a n d become one of the kinds of nuclei for future con densation and eventual coa lescence and rainfa l l . But the two processes me ntioned a bove a re the m a i n a n d perhaps t h e o n l y causes o f coa les cence and hence precipitation. Re search may show other possibil ities.
" � �·
.. ., -1
., .,
�
� .,
�
�
2
3
23
Very sma l l partic les i n the a i r may a c t as n uclei u pon which water vapor will crysta l l ize to for m snow. Air m ust be su persaturated with water vapor and below the freezing point. Mic roscopic bits of soi l , c lay, sand, a n d ash a re c o m m o n n u c l e i . C l o u d tem peratu res m ust general l y be from + 1 0 ° to -4°F before snow begins to form. Va por cha nges to snow even without nuclei at high a lti tudes in supersaturated air at -38 ° F . S N OW
24
SNOW PELLETS (or granular
snow) a re w hite, a n d of various shapes. Although much like soft hail, pe l l ets a re too sma l l a n d soft to bounce. A single pel let gener a l ly forms from many su percooled cloud droplets which freeze together i nto crysta l l i n e form.
Snow pellets-magnified 3 times.
I C E P R I SMS form hexag o n a l plates, col um ns, a n d needles t h a t sometimes g l itter like d i a m o n d s as they a re b lown a b o ut. Because of their s m a l l size they fa l l very slowly. Ice need les often mdke halos a r o u n' d sun or moon. I n very cold c l i m ates ice-need le fogs form o n the ground.
Ice needles-magnified
10
times.
Halo caused by ice needles. I C E P E L L ETS (sl eet) consist of tra nsparent or tra nsl ucent
beads of ice. Sleet occurs when rain, dropping from upper warm a i r, fa l l s through a layer of freezing a i r. Rain drops fi rst be WARM AIR come freezing rain (supercoo led) and when striking the ground in this condition form g laze (p. 27). But further coo l i n g produces ice pe l lets, or true sleet, which freezing rain bounces o n h itting the g round.
ice pellets
Ice pellets-magnified 3 times.
25
H A I L forms as frozen ra i n d ro ps, formed h i g h in the cl ouds, m ove through a reas of supercooled water d roplets i n thun derc louds. Hai lston es were long thought to develop their o n io n l ike structure by being a lternately forced up ward by vertica l winds i n the thunderhead to a freezing leve l, then d ropped down to where more water was picked up. Such up-and-down trips do occur, but the growth of hai l stones resu lts mostly from ice pel lets' picking up water i n the supercooled m id d l e a n d u pper reg ions of the cloud. The layers result from d ifferences between the freezi n g rate a n d the rate at which water accumu lates on the pe l lets.
cross section • of a hailstone
26
I C E STORMS are c h a racterized by g l a ze. Glaze is a s destructive as it is beautifu l . It occurs w h e n ra i n o r d rizzle that has been supercooled (cooled below 32 ° F but not yet frozen) falls on cold su rfaces a n d immediately freezes. Glaze ice fo rmed from this freezing ra i n can snap branc hes, w ires, and poles and cause hazardous driving conditions.
27
D E W does n o t fa l l. I t i s water vapor that con denses o n so l i d s u rfaces that have cooled be low the condensation point of the a i r i n conta ct with them. This coo l i n g by radiation occurs usua l l y on c l ea r n i g hts. The "sweat" that forms on the outside o f a g l ass of cold lemonade on a hot day is a lso dew.
FROST is formed like dew but at tem peratures below freezi n g . The water vapor changes directly to sma l l, fi n e frost crysta ls without condensi n g into water drops fi rst. Frost crysta ls growing on windows devel o " p feathery pat terns as the primary frost me lts and recrysta l l izes.
28
Drainage pattern af the United States and Canada. Rain either is stored i n the ground, po n ds, o r la kes, o r runs off to b e stored i n the ocea n . It eventua l ly evaporates i nto the a i r again. S u rface runoff o f preci pitation a n d the u n dergro u n d fl o w o f wate r g ive us o u r brooks, streams, a n d rivers. Rivers a lways flow from higher to lower levels, c utting their own va l l eys. Streams and rivers merge and fi n a l ly they em pty i nto the ocea ns. When the supply of water is l a rger than the a m o unt n o rm a l ly h a n d l e d by strea m s a n d rive rs, flood ing occurs. I n areas of h a rd, baked g ro u n d o r clay soil, the runoff from a thunderstorm is a l m ost c o m p l ete, since there is l ittl e seepage i nto the soi l . Flash floods may resu lt. Water may a lso run u n derground, to em erge as springs. We l l s ta p this u n dergro u n d water, which provides many a reas with an a b u ndant supply. Large u n dergro u n d rivers exist, but these are rare. WHAT H A P P E N S TO R A I N A N D SNOW
29
Water is stored as snow and ice, as well as in lakes.
Ten inches of snow melts into one inc� of water.
Lakes a n d ponds obviously store a g reat dea l of water. Not so obvious is the i m m e nse reser voir of water stored in the po lar ice caps, in g l aciers, a n d i n s n o w o n m o u ntains a n d on t h e cold northern p l a i n s d u r i n g winter. Winte r snows in the m o u nta i n s determ i n e the water supply for irrigation and for power use. This snow me lts with the spring thaw a n d fi l ls the rivers. If spri n g is late, the melting wi l l be m o re sudden, resulting i n floods. WAT E R STO RAG E
Topsoil h o l ds a n i m mense amount of water. Some of this is tra nsferred into the a i r by g reen plants. But most of it seeps through soil a n d porous rock until it reaches non porous clay o r rock. I t forms a m assive subterra n ea n reservoir, fi l l i n g a l l the cracks a n d pores i n t h e rock a n d soi l . T h e u n derground water, l ike surface water, fl ows down h i l l and seeps out into strea m s or comes out in springs. Eventua l ly this water, too, evapo rates or reaches the sea. G RO U N D WAT E R
impervious rock
RAI N MA K I N G is an a ncient ho pe, a 1 9th-century fake, a n d a modern scientific fact. Every primi tive tribe has tried one way or an other to make it ra i n . Prim itive mag ic, r a i n dan ces, and sacrifices have a l l been used to i n d uce ra i n . B y coincidence, ra in has fo l l owed these efforts often enough to kee p a l ive the b e l ief in the efficiency of the methods. Qu ite a boom in ra i n making devel o ped in the 1 9th cen tury. Dru m s were beaten, cannons shot, a n d explosives were set off, prod ucing g reat q u a n tities of smoke. The methods were worthless and so were many of the o perators. One of the smooth est of t hese con fidence m e n used U. S. Weather B u reau c l i matolog ica l data. He never a ppea red i n a drought area until it was a l m ost certa i n that rain was but a few days away. Then he wou l d put o n his act, wait for the rain, a n d c o l lect h is fee. Modern rainmaking tec h n iq ues a re based o n known facts of coa les cence a n d g e n u i nely influence ra i n fa l l a n d snowfa l l (see pp. 32-33). A l l modern tec h n iques depend u p on the "seed ing" of a rtifi ci al n uclei i nto potential rain c l ouds. Si lver iodide crysta ls a re most common ly \Jsed.
Hopi I n d i a n s dance for rain.
1 9th-century rai nmakers.
Silver iodide generato r.
Dry ice d ropped from plane forms partly clea red strip in stratus clouds.
M O D E R N RAI NMAK I N G g rew out of studies of how a ircraft c o l lect ice on wings and other surfaces. Dr. Vincent J . Schaefer was attempting to prevent the formation of su percoo led clouds (which cause ice to form on a i rcraft). H e discovered that tiny bits of dry ice (frozen carbon di oxide) produced fa ntastic n u m bers of ice n uclei when "seeded" i nto c louds colder than 32°F.A piece of d ry ice the size of a groin of rice caused the formation of more than a tri l l ion ice crysta ls. Each g rew at the expense of supercoo led cloud droplets and formed snow (see p. 23). The technique of seeding supercoo led clouds has since been used to l essen ground fogs and, u n der certa i n con ditions, to induce snow o r rainfa l l from I o rge c u m u lus c l ouds. Dr. Irving Langmuir a n d Dr. Bernard Vonn egut pioneered further studies by seed ing c l ouds with water and s i lver iodide. The water i n itiates coalescence. Since the si l ver iodide has a molecular structure nearly identical with that of ice, it i n duces coa lescence i n m u c h the some m a n n e r as ice crysta ls wou ld.
32
C L O U D S E E D I N G experi ments continued with min iature clouds produced u n der laboratory condi tions. Va rious substa nces were sprayed into a cold chamber i n an effort to produce ice crysta l s or co a l escence. Dry ice and silver iodide ---� .- ,. both worked we l l . Finely ground ,.,.. -dry ice (with a tem perature of : - 1 08 ° F) ca used cloud drop lets to crysta l l ize a long its path. These Seeding a cumulus cloud with d ry ice. crysta ls grew rapidly at the expense of the water droplets around them, and soon be came large enough to fa l l . Si lver iodide, on the other hand, acted d i rectly as a n u c leus for ice formation. These ice c rystais, a lso, g rew until they fe l l . C louds a re n o w seeded with either dry ice o r si lver iodide. One po u n d of dry ice spread by a plane may start a shower in large c u m u l us clouds. Si lver iodide is l ess expensive to use, because it can be sent up from the ground to c l ouds from spec ia l gen Ma n-mad e snow cloud i n erators. But cloud seed ing is not a cold chamber. successful u n less conditions are nearly right for natura l precip itation. Seed ing can i n duce ra i n under t h e right conditions. It can cause more ra i n to fa l l than wou l d occur with u n d istu rbed natura l con d itions. It can n ot produce rain from fa ir-weather c u m u l u s c l o u ds. Nor is it yet possible for cloud seeding, which is sti l l i n its infancy, to in duce rain to fa l l over a widespread area .
...
Courtesy General Electric Company
...
The Atmosphere Earth's atmosphere would be more a pparent to a traveler mi les out in space than to us on the ea rth 's s u rface. The s u n 's rays, scattering i n the atmosphere, wo u l d make the l ighte d side of the earth a brig ht, fuzzy crescent, faintly b lotched with c l o uds along the twi light zone. D iffusion wou l d extend the cresce nt of l i g ht far around the earth's c u rve. O n the dark side, l ig hts of g reat cities might show as tiny, dimly s h i n ing spots through the atmospheric vei l . 34
A f u l l u n dersta nding of the weather req uires knowledge of the atmosphere. We l ive a t the bot tom of a yirtu a l ocea n of a i r. Exte n d i n g u pward perhaps 1,000 m i les, this m assive, restless ocean is fa r d ifferent, and fa r m o re tem pestuous, than the watery ocea ns that cover three-fourths of the g lobe. A na rrow band of com pacted air lying j ust above the earth is the reg ion of continuous winds. Here, the risi n g and fa l l i n g air cu rrents sometimes develop i nto violent storms. Only recently have the most ad vanced a ircraft ventured a bove this thin layer, some 5 to 11 m i l es thick. The ocea n of air differs in one major way from a n ocea n of water. Water is nearly incom pressible. A cubic foot of water on a n ocea n bottom weighs much the same as a cubic foot near the surface. But the air of the atmospheric ocea n is h i g h l y com pressible: a cubic foot of air at the surface weig hs b i l l ions of times a s much as a cubic foot a t the outer edge of the atmosphere. The atmosphere thins so rapidly as one leaves the earth that, only 3Yl miles u p, over half the atmosphere by weight would lie below you. It is c h iefly in this 3Yl- m i l e b l a n ket of heavy a i r that weather cha nges are born. The at mosphere 500 miles out is so thin that there a re o n l y a bout 22 m i l l io n m o l e c u l e s o f air p e r cubic i n c h , com pared to b i l l ions u p o n b i l l ions at the earth's sur face. Sti l l farther out, the ever-th inning atmosphere b le n ds with the stray gases a n d dust of o uter space.
Composition of air at a ltitudes above 500 miles helium 50%
oxygen 2 1 % argon 0.93 % carbon d ioxide 0.03 % a l l other gases 0.04%
36
AI R CONSISTS MAI N L Y O F G ASES
that wi l l not d irectly susta in life. Oxy gen, which a l l l iving things need, ma kes up s l ightly less than 2 1 per cent of the a i r. I nert n itrogen m a kes up 78 per cent. The remainder of the g ases, a l l tota l i n g less t h a n 1 p e r cent, are carbon dioxide, argon, neon, radon, h e l i u m , krypton , xenon, hydrogen, m ethane, n itrous ox ide, and ozone (a form of oxygen). Besides these, air conta i n s u p to 4 per cent water vapor, a l so d ust and gases such as smoke, sa lt, other chemicals from sea spray or ind ustry, carbon monoxide, a n d m icro-org a n isms. If the a i r were perfectly q u iet, the heavier pa rticles a n d g ases wou l d settle c l ose to the earth a n d the l i g htest w oul d be fou n d the fa rthest out from earth's surface. But the consta nt motion of the air near the surface m ixes the gases so that the same proportions exist from the earth's surface up to about 45 m i les. Far ther out a re fou n d c h iefly the lig hter gases. Proba bly o n ly the lig htest ga ses, h e l i u m and hyd rogen, a re fou n d at heig hts a bove 500 m i l es. In intermed iate levels are found h i g h concentrations of ozone and ionized n itrogen, together with s m a l ler q u a ntities of othe r ion ized gases. Ozo ne, by a bsorption, plays a l a rge part in the earth 's heat b a l a n ce as does the increasin g a m o u n t of m a n made carbon d ioxide.
P H E R E First a n d most im porta nt of the atmosphere's four l a . y ers is the troposphere, which lies c losest to earth. Next a bove is the strato sphere. Where the troposphere ends and the stratosphere begins is a boundary cal led the tropo pause. This averages 5 m i les above the earth n ear the poles, a n d 1 1 m i l es a bove at the equa tor. The stratosphere ·goes u p to a bout 50 m i l es. Above this is the ionosphere, extending out to about 650.m i les. Here are ion ized, or electrified, pa rticles that reflect long radio waves back to earth. F i n a l ly, a bove the ionosp here is the exosphere, about which ·l ittle is known.
TROPOSPHERE -WEAT H E R B R E E D ER In this layer are nearly all of the c l o u ds. Here is w here weather occurs. A i r, heated by co ntact with the earth, rises and is replaced by colder air. These vertica l currents create hori zonta l winds at or near the surface of the earth. Water, eva porated from the l a n d and seas, rises with the ascend ing warm a ir. As the air rises, the surrounding pressure lessens, so it steadily expa n ds. Expansion is a cooling proc ess (see below). If the air rises high enol!g h, it cools until condensation forms clo uds.
T HE
Expa nsion of any gas is a cooling process. Com pression creates h eat. The cylinder and hose of a tire p u m p get hot as air is p u m ped and com pressed. The sudden expan sion of gas rushing out of a n aeroso l bomb cools the tip. Most air conditioners work on the same principle, as shown below. A gas is compressed in the part outside the house. The heat of com pression is g iven off to the o utside air, a n d the gas condenses to a liquid. The liquid is forced through a tiny nozzle and expa nds sudden l y i n the coils inside the house. This expansion (and evaporation back to a gas) coo l s the coils and a lso the air w h ich is b l own over the coils i n to the room. H O W AN A I R C O N D I T I O N E R W O R K S
I N DOORS
house wall
OUTDOORS
exhaust air
outdoor air
7000 ft.
41.5 °F
6000 ft.
47°F
5000 ft.
52.5 °F
4000 ft.
58°F
3000 ft.
63.5 °F
2000 ft.
69°F
Adiabatic temperature cha nges as a i r travels aver a mountain.
The tro posphere has a kind of a utomatic air condition e r. The primary h eat p u m p is, of course, the s u n . It heats the earth's surface which, in turn, heats the a i r in contact with it. The air expands, becomes lig hter, a n d rises. But the higher it rises the more it expands, because the pressure aro u n d it is stead i l y lessening. A n d the more it expa nds, the more it coo l s. This is a n a utomatic cooling process which occ u rs without any loss of heat due to outside ca uses. The rising air coo ls a utomatica l ly at about 5Yz ° F f o r e a c h 1 ,000 ft. it rises. T h i s is w h a t ha ppens when a i r rises u p the side of a mountai n . When it g o e s d o w n t h e other side, it beg ins com pressin g . It w a r m s u p i n doing s o a t the same rate (5\12° F per 1 ,000 ft.) i t c o o l e d i n risin g . This a utomatic tem perature c h a n g e in risi n g or fa l l ing air is ca l led "adiabatic" warm ing or coo l i n g . Air need not be pushed u p by a mounta i n for this adia batic change to take place. Air rising over heated plains w i l l a lso cool at the same rate of a bout 5\12° F per 1 ,000 ft. rise.
39
I u ses this much heal
C O N DE NSAT I O N compl icates adia batic coo l i n g a n d
warming, a n d· ma kes the temperatures on t h e windward side of mountain ranges lower than those o n the leeward side. Ma inly because of condensation, there a re relatively cool va l l eys west of the Sierra Nevadas and hot deserts to the east. Considerab l e heat is needed to change l iquid water to water va por. Heat increases the speed of water molecu les, so that many more escape as water vapor. Changing a pan of water at the boiling point to vapor req uires six times the heat needed to ra ise the same amo unt of water from freezing to boiling. When water vapor condenses back into l iquid water, the sa m e large amount of heat is g iven off. This heat can increase the tem perature of the air considera b ly. When air is rising, two opposing influ ences operate as its moistu re condenses: adiabatic cooling tends to lower its tem perature; a n d heat of condensation tends to raise it. The net effect is a n avera ge coo l i n g of 3.2 ° F per 1,000 ft. of rise when condensation occurs. Ta ke an exa m ple: Air at 60°F moves up the west side of a mounta i n . It cools ad iabatica l l y 5 . 5 ° F for each 40
1 ,000 ft. of rise up to the clouds at 4,000 ft. As conden sation begi ns, heat is released, and the adia batic coo l i n g is t h u s partly offset. F rom 4,000 ft. to the mounta i n top, at 1 0,000 ft., the net coo l i n g is only 3.2 ° F per 1 ,000 ft. Thus the tota l coo l i n g of the wind as it sweeps u p the mounta i n is 4 x 5.5, or 22 ° plus 6 x 3.2, or 1 9. 2 ° Total 4 1 .2 °
from va l l ey floor to cloud base from cloud base to mounta in top from va l l ey to mounta i n top
The a i r that began at 60 ° F tops the mounta i n at 18 .8 ° F . A s t h e a i r pours down t h e eastern sl ope, it co mpresses and wa rms, ad ia batical ly, at the rate of 5 . 5 ° F per 1 ,000 ft. Because of the warming, no further condensation oc curs. Tota l warming of the a i r from mo untain top to val ley, 1 0,000 ft. below, is 1 0 x 5.5 ° F, or 55 ° F . Adding this to the mounta i n to p temperature of 1 8 . 8 ° F g ives 73.8 ° F as the temperature of the eastern va l l ey. The temperature at the eastern foot of the mountain is 1 3. 8 ° F higher than at the western foot. Condensation and precipitation o n the western slope ma de the d ifference. Chinook winds are a n exa mple of this down -slope warmi n g of air. They occ u r on the easte rn slope of the Rockies, often with dramatic effect. One C h i nook brought the temperature u p from -6 ° F to 3rF i n 1 5 min utes. Condensation mod ifies adia batic coo l i n g .
41
T R O P O P A U S E A N D JET
The tropopa use-the zone that ma rks the end of the troposphere and the beg i n n i n g o f t h e nearly weatherless strato sphere-was once thought to be continuous from poles to eq uator. We now know that the tropopause has breaks, giving it a n overlap ping, leaf-like structu re. These breaks are i m porta nt i n connec tion with the jet strea ms. A jet strea m is a tubular ribbon of h igh speed winds, genera l ly from the west a n d some 20,000 to 40,000 feet up. Jet streams form at the overla ps, pa rticu l a r l y of the a rctic tropopause and the extratro pica l tropopause. They are at l east par tia l ly the result of the strong tem perature contrast there. STR EAM
Typical jet stream paths.
A jet stream was d iscovered by American B-29 pilots flying to J a p a n from the Ma rianas i n World War I I . They consistently reported westerly winds with speeds far i n excess of those expected. A jet stream is usua l ly a bout 300 m i les wide a n d 4 m i les high. At its core it averages 100 m i les per hour i n winter and 50 m i les per hour in sum mer; these speeds may rise to 250 m i les per hour or more. Forming a wavy path at the top of the tropo s phere, the jet stream assists high-fl ying a irplanes travel i n g east; p l a nes g o i n g west try t o avoid these stro n g headwinds. T h e strength o f the jet winds decreases o ut ward from the core, a n d from place to place i n the stream. The n u mber of jet streams a n d their paths varies from day to day and season to seaso n . A typica l jet stream has a reas of maxi m u m winds a l o n g it t�at tend to travel eastwa rd. Two p l aces where th ese wind max imums occ u r with g reat freq uency are across Japan a n d over the N e w E n g l a n d states. I n winter, over the North American Continent, there are th ree major jet strea ms: one over northern Canada, one over the U n ited States, a n d one over the su btro pics. Structure of a jet stream
20,000 ft.
43
T H E STRATOSP H E R E is the a l most weatherless pa rt of the atmosphere. It extends from the tropopause u pward a bout 50 mi les. The name suggests its nature, for it usua l l y has very l ittle vertical air movement; it is a u niform l ayer, or stratum . Temperature drops much more slowly with height than i n the troposphere. I n fact, the tem perature begins to rise again near the top of this layer. F lying in the stratosphere is genera l ly smooth, a n d the visibil ity is a lways exce l lent. The air is thin and offers very l ittle resis tance to a pla ne; hence a gallon of fuel ca rries a plane m uc h farther through the stratosphere. A region of no weather, the stratosphere is preferred by jet pil ots, for here they can fly at top speeds with l ittle fear of turbu lence. Often too far above us to be seen, the jets c ha l k their path across the sky a s moisture from their engines · forms condensation trails-strea ks of fi n e ice crysta ls in an otherwise weatherless atmosphere.
44
T H E I O N O S P H E R E lies a bove the stratosphere, extend
ing outward some 650 mi les. Here the air is extremely thin. The scattered a i r particles are ion ized; that is, electrified by the remova l of a n electron, o r negative charge of electricity. The relentless bombardment of cosmic rays from o uter space ca uses this ion ization . Were it not for this ionized air we could not receive radio broadcasts beyond the horizon, for radio waves go in a stra ight line. Layers of ion ized air reflect the radio waves back to earth. The severa l m irrorl ike rad io-wave-refl ecti ng layers are the Ken n e l ly-Heaviside or E Layer, 50 to 80 m i les up; the F Layer, 1 50 to 200 mi les u p; and a varia b l e n u m ber of other laye rs that resu lt from spl itting of the E a n d F Layers. The air particles in the ionosphere are hot. Estim ates put them at some 1 ,000 ° F to l ,500 ° F during the day a n d 300 ° F at n i g ht. These extreme temperatures a re proba bly ca used by cosm ic-ray bomba rdment. But when space travel becomes a rea lity, we need not fea r being broiled a l ive. The a i r partic les a re so fa r apart and so tiny that we will proba b l y not notice them at a l l . Much more dan gerous wi l l be the cosmic rays.
Radio waves reflected by the ionosphere can be received beyond the horizon.
45
-
-- --�----
T H E EXOS P H E R E is the fina l a n d hig hest layer of the
atmosphere. Hotter,. even, than the stratosp here's a i r particl es, t h e ions o f g a s in t h e exosphere a r e possibly as hot as 4,500 ° F . They are bombarded so fi ercely by cosmic rays that they exist o n l y in atomic fo rm, instead of mo l ec u l es. At night, shielded from the sun's d irect rays, the temperature of the partic les drops nea r ly to absol ute zero -about -460 ° F . HIGH
ALT I T U D E
P H ENOMENA
A u roras resu l t from the action of streams of solar particles on the ionosphere, 50 to 600 m i l es up. What happens is much l ike events in a neon tube. Beca use the earth is a magnet, ion ization is stro ngest near the poles, a n d the a u roras are seen ma inly at h ig h l atitudes. Mother - of - pearl Clouds may
C u rta i n a u rora
consist of water droplets. These rare c l ouds a p pear as bands of paste l colors, 1 4 to 1 9 miles hi gh. The sky is otherwise clear at the a ltitudes where they a re seen.
Nocti l ucent C l ouds, the hig h est clouds known, a re
proba b l y formed from meteor d ust. Appearing in the western sky shortly after sunset at a height of some 50 mi les, they have a gold edge near the horizon and a re b l uish w h ite a bove. Both nocti l ucent a n d mother-of- pea r l c l o u d s travel a t terrifi c speeds-394 miles per hour was once observed . Meteors are visitors from outer space. They h it our atmosphere at tremendous speeds- perhaps 90,000 m i les per h o u r. Friction with the a i r of the upper atmosphere h eats them to incan descence, a n d most of them va porize into gases o r disinteg rate into h a r m l ess d ust before they come to within 30 m i les of the earth's surface. Thus our atmosphere protects us. Mi l l io n s of meteors, most of them sma l l e r than grains of sand, hit our atmosphere every day. Very few ever reach the ground.
The E a rth's M oti o n s a n d Weather The earth has five motions i n space. It rotates on its axis once each 24 hours, with a slow wobble ( l ike that of a top) which takes 26,000 years to com p l ete. It revolves around the sun at 1 8 Vz m i l es per second, making the circuit i n 365 1;4 days. It speeds with the rest of o u r so lar system at 12 m i les per second towa rd the sta r Veg a . F i n a l ly, o u r entire ga laxy, with its b i l lions of stars, is rotat ing in space-our pa rt of it at a speed of 1 70 m i les per second. Only two of these motions affect the weather. But their effect is profou n d. Earth 's a n n u a l trip around the sun g ives us o u r seasons and their typica l weather. Earth's d a i ly rotation not on ly resu lts in n i g ht and day; it produces the ma jor wind belts of our earth, and each has its typica l pattern of weather. 48
CAUSE OF THE SEASO NS Seaso ns result from the fact that the axis on which the earth spins is sla nted 2 3 Y2 ° to the plane of its orbit. When the North Pole is tipped to ward the sun, the northern hem isphere has sum mer. The s u n 's rays beat more directly down o n the northern hem isphere a n d the days are longer. The sun is farthest north at the summer so lstice, about J u n e 22. Then the s u n is directly over the Tropic of C a ncer, and daylight hours a re lo ngest in the n orthern hem isphere and nights are the shortest of any time in the yea r. The North Pole is i n the middle of its a n n u a l period of six months of s u n l i g ht, a n d the South Pole i n the middle of its six months of relative darkness. At the w inter solstice ( a bout Dec. 22) the North Pole is tipped farthest away from the sun, which is now di rectly over the Tropic of Capricorn. The southern hem isphere has sum mer; the n orthern hem isphere has winter. C o n ditions are just the re verse of those at the summer solstice. The Anta rctic is now the " l a n d of the midnight s u n " and the Arctic is sun less. At the fa l l and spring equinoxes (about Sept. 23 a n d March 2 1 , re spective ly) the earth's tilt is side wise with respect to the sun. light fa l ls equa l ly on northern a n d south ern hemispheres. Day and night are of equa l d uration everywhere on the earth. The equi noxes mark the be ginn ings of spring and fa l l .
SUMMER IS WARMER tha n winter for two reasons: the
days a re longer (more time for the sun to heat the earth) and the sun's rays, striking our part of the ea rt h more d i rectly, a re therefore more concentrated. Days a n d n i g hts at the eq uator a re a lways 1 2 h o u rs long. The farther north you go i n summ e r, the l o n g e r the days a n d the s horter the nig hts. T h i s is so be cause the . sun in summer shi nes across the pole. The nearer one g ets to the pole, the longer the day be comes, until it fi n a l ly becomes 24 hours long. To a n observer i n the "land of the m id n ig ht sun" the sun a ppea rs to move a l o n g the hori zon i n stead of risi n g and setting. But a s winter comes, the s u n s e e m s t o m o v e south. The farther 4-- north you go, the shorter the win ter days a n d the longer the nig hts. summer The sun 's a pparent movement relative to the e a rth in w inter and in summer is shown below. O n Dec. 22, at the latitude of Was h i n gton, D.C., the s u n a t noon is o n ly about 27° a bove the south ern horizon , The day is short because the sun's path from eastern to western horizon is short. But on J u n e 22, at the sa m e l atitude, the s u n rises farther i n the north and at noon reaches about 74° a ltitude. Its path is longer a n d so daylig ht lasts about 1 5 h o u rs.
sun at noon December 22
sun at noon June 22
'W s
s
To see the effect of the d i rectness of t h e s u n 's rays, set a flashlight 1 ft. a bove a large sheet of pa per. Mark the outline of the circle of light it ma kes. Now h o l d the flash light at a n angle to the paper-but sti l l a foot away. Mark the outline of the e l l ,i pse of light it m a kes. The sa m e a m o u n t o f l i g h t h it the pa per both times. But the fi rst time it was concentrated, the second time more spread out. When the rays were direct the s u m mer concentration for a u n it area was g reatest. sun The summer sun fo l l ows a path more nearly overhead, so its rays are more con centrated. The winter sunlight h its the earth at a g reater sla nt. The sa m e amount of sun light now spreads out over a larger area. I n addition , winter s unl ight m ust pass through more atmosphere, because it has a more sla nted path . More energy is dif fus � d by the atmosphere a n d less reaches the earth to warm it. winter sun
solar radiation under a cloudless sky
temperature average
SEASONAL LAG Aug ust is hotter than J u n e even though the sun is more n early overhead and the day is longest on June 22. I n terms of so lar radiation reaching the earth, May, J une, a n d J u ly shou l d be our warm est months. But J une, Ju ly, and August actu a l l y a re. Why? During the yea r the earth, as a whole, l oses precisely the same amount of heat it receives from the sun. But as the sun moves north in spring, our part of the earth gains heat faster than heat is l ost. On J u ne 22 it is receiving maximum solar radiation. The heat gain continues to ex ceed heat loss until maximum warmth is reached, usu a l l y i n late J u ly. Heat gain continues t o exceed h e a t l oss, at a diminishing rate, until about Aug ust 3 1. Then our part of the earth sta rts to lose heat faster tha n it receives it, a n d b e g i n s t o c o o l down. T h e process is l i k e sta rti n g a fi re in a stove: the roa ring fi re heats the room s l owly, but the room will stay warm for a while after the fi re has died down. The same heat lag accounts for the fact that the warmest time of day is usua l l y about 3 p . m .-not noon, when the sun's rays are most intense.
52
90°
. 0 mph
•
80° . 70° . 60° . 50� 40° .
1 75 340 500 640 770
30°.
865
20° .
940
1 00 .
985 . 1 000
A p p roximate speeds of the earth's rotatio n at various l atitudes.
If the earth were not rotating, h eated air wo u l d rise over the eq uator a n d move n orth a n d south h i g h a bove t h e earth's surface. I t wou l d c o o l a n d si n k down at the poles. The s i n k i n g a i r wou l d force air at t h e earth's surface toward t h e eq uator. So surface wi n ds o n a smooth non-rotating ea rth would a l l move directly towa rd the eq uator with equal speed (see p. 1 0). The earth's rotation cha nges th is. The earth at the equator is about 25,000 m i les around. The ea rth ma kes one <:omplete rotation every 24 hours. Hence every point on the eq uator moves eastward at a l ittle more than 1 ,000 mph. But every place north or south of the eq uator m oves more slowly to the east, simply because the dista nce around the earth becomes l ess a s you g o n orth or south of the eq uator. At latitude 45 ° , exactly h a lfway between the equator and the poles, the earth spins eastward at about 700 m p h . At the poles there is no movement at a l l, since these a re the ends of the earth's axis. These differences i n the earth's speed of rotation at various latitudes have a marked effect o n our winds, as we sha l l now see. EARTH'S ROTAT I O N AND W I NDS
53
WIND
DEFLECTION
IN
TH E
NORT H E R N
H EM I
S P H E R E is caused by the earth's rotation, as a n experi
ment w i l l demonstrate. Place an old phonograph record on the turnta b le. The hole a t the center represents the Noth Po le; the rim, the equator. Now spin the turnta b l e slowly by hand counter clockwise, the way the earth turns as viewed from a bove the North Po le. As the record turns, d raw a l i ne with c h a l k from the center hole straight away from you to the "equator" edge. See how the "easterly" rotating record ca uses the cha l k l ine to curve to its right ("west" as seen from a bove). In the d ia g ram, the dotted l i n e sh ows the actua l path of your chalk; the so l id line, the mark it made on the reco rd. Re peal the experi ment, but draw a stra ight line from the rim of the record to the center. Again the chalk mark shows a curve to the right. Winds o n earth shift to the rig ht exactly like the c h a l k l i ne, a n d f o r the s a m e reason . I m a g i n e a wind from the north, sta rting at C h icago ( l atitude 45 ° ) . The top drawing o n the next page shows this wind b lowin g stra ight south as viewed from a bove the North Pole. It moves i n a straight line toward sta r P, a n d point A on the eq uator. But as the w i n d moves south, the earth (and point A along with it) is movi n g eastward . So the stra ight path, a s seen from space, becomes a curved path to the right a s seen by a n observer o n the earth 's surface. Wind d eflection i l l ustrated by phonograph recor d :
54
Drawings 1 , 2, 3 show the wind sh ift as you would see it from space over the eq uator. The draw ings show meridians ( i magi nary l i n es marking the earth into sec tions, like an orange). The north wind sta rts south along Meridia n A. It moves in a stra ig h t l i n e d i rectly towa rd the equator. A s the wind m oves f r o m latitude 45 ° to 30°, earth's eastwa rd rotation moves Meri d i a n A to your rig ht, a n d Meridian B moves to a posi tion di rectly u n der you. By the time the w i n d has reached the eq uator, Merid i a n B has moved east and has been replaced by Meridian C. As seen from space, the wind sti l l moves d irectly south. But to a man o n the earth, the wind has taken a curved path a n d is now a n o rtheast w i n d . Winds m o v i n g northwa rd, a lso, c u rve to their right. This shift h e l ps explain the general circu lation of air over the earth . In the southern hemisphere th is effect is exactly reversed a n d winds sh ift t o the left. To see why, repeat the experiment on p. 54. This time, h owever, turn the record c l oc kwise, as if you were looking down on the earth from a bove the South Pole.
Path over rotati n g ea rth .
North wind at C h icago . . .
sh ifts to its right becoming N E.
3
G E N ERAL AIR MOVEMENTS IN THE NORTHERN H E M I S P H E R E beg in with air moving north h i g h a bove
the eq uator, a n d slowly sh ifti ng towa rd the east because of the earth's rotation. By the time this u pper a i r has gone a bout one-third of the way from the eq uator to the North Pole, it is moving eastward. As more a i r from the eq uator a rcs north and east i nto this region, a bout latitude 30 ° , it pi les u p, form ing a n a rea of high pressure. Some air is forced down to the surface of the eo rth. There a portion flows southwa rd, turning west as it goes. This portion forms the so-ca lled "trade winds" that b low rather stea dily from the northeast. But some of the descend ing air moves northwa rd and is deflected to the ea st. This air forms the preva i l ing wester l ies wh ic h b low over the middle l atitudes (30° to 60 °N).
56
Not a l l of the high air sinks to the surface at l atitude 30°. Some of it conti n ues north, high a bove the earth. It cools by ra diation as it goes, and fi na l ly contracts a n d becomes so heavy that it sinks down near t h e North Po le. The pressure builds u p i n that reg ion, a n d the cold, heavy air moves southward on the su rface, sh ifting towa rd the west as it goes. At about latitude 60 ° , it runs into the preva i l i ng westerlies trave l i n g . northeast. The l i n e of c o l l i s i o n is cal led the " polar front." The warm a i r from the south pushes up in a wedge over the cold, south-moving, polar air. The rising warm air is rapidly coo led, a n d un settled weather conditions resu lt. This polar front is the source of much of the changing weather in the U n ited States. The air m ass breaks through i n c o l d waves that may m ove as far south as F l orida and Mexico.
polar easte rlies
57
THE EART H'S G E N ERAL C I R C U LATI ON, its wind a n d
N . P.
weather, a re modified by m a n y things, s u c h as w i nds ca us e d by uneven l oca l heating, d ifferences i n heat a bsorption of oceans a n d land, a n d seaso n a l c h a n ges. But some general conditions are worth noting -as, for example, that the circ u l ation i n the southern hemisphere is the opposite of circu lation in G E N E RA L W I N D the northern. Winds s h ift to the C I R C U L AT I O N left for the same reason that winds north of the eq uator sh ift to the rig ht. Northern hem isphere wind patterns exist in the southern hem isph ere, but reversed . Each pat tern has its near cou nterpa rt in 30 ° N the southern he misphere.
3 0°5
60 ° 5 S.P. Note reversal of winds i n southern hemisphere
As su rface a i r moves toward the equator w i t h the north a n d south trade winds, it i s heated by the direct rays of the sun, and rises. Rising air creates the eq uatorial ca lms, o r doldrums. Day after day, week after wee k may go by without a b reeze. Sailing ships avoided this region when possible. F rom the rising m oist a i r of this region tropical typhoons, or h u rricanes, are born i n summer (p. 1 07). Here a lso oc· curs heavy tropical rainfa l l . The Equatori a l Dol d r u m s
The Trade W i n d s are well named, for these steady north east winds ma rked a pop u l a r route f o r s a i l i n g vessels. Even n o w , ai rcraft traveling west in this zone have a tail wind much of the time. Skies o re clear near loti· tude 30 ° but cloud iness and h eavy, freq uent, showery rainfall occ u r nearer the equator, where the air rises at the d o l d r u m zone.
The Horse Latitudes are another region
of ca l m . As the air sinks, and thus warms adia batically, skies tend to be cloudless. Winds ore wea k and u n dependa ble. The term " h o rse latitu d es" appa rently orig· i nated beca use soiling ships carrying horses from Spain to the N ew Wo rld often became beca lmed and ron out of food and water for the a n imals. The sea was sometimes lit· tered with bodies of sta rved horses w h ich had been th rown overboard.
The major air flow over the U n ited States is from slightly south of west to slig htly north of west. Were it not for the repeated i nvasions of cold air from the polar front, a n d compl icatio n s caused by a i r being fo rced over mountains, weather i n the U n ited Stales would be rather sta ble because of these winds-fa ir weather and clear skies a lte rnating with steady rains a s the a i r moved n o r t h a n d slowly cooled. B u t the polar f r o n t a n d o t h e r factors cause rapid and often violent weather changes. The Preva i l i ng Westerlies
Polar Easte r l i e s Bounded on the south by the polar fron t, the zone of the polar easterlies i s one of cold a i r movi ng southwest u n ti l it r u n s i n to the p reva i l i n g westerlies. The fron t itse lf moves as these two air masses p u s h back a n d forth . At the front, warm, moist air from the south is l ifted over the h eavier, cold air. Bad weather resu lts. The
H i g hs a n d Lows H I G H S A N D LOWS Uneq u a l heating of the ea rth between the equator and the poles causes north-south win ds. Rotation of the earth turns these east or westwa rd, depending on the hemisphere. Th is sh ifti ng of g reat masses of moving air creates the overa l l pattern of air circulation. But it does something else. It creates whirling mo sses of a i r cal led high-pressure cel ls, or highs, a n d low-pressure cel ls, or lows. T h e highs (a nticyc l ones) gen era l l y bring fa ir weather. The lows (cyc lo n es) bring un settled weather. In the reg ion of the preva i l i n g westerl ies (latitude 30 ° to 60 ° ) high cells fol low a general path from west to east ( moved by the preva i l i ng westerl ies) a n d a lternate with l o w cells w h i c h they generate a n d drive a l o n g . The table below summa rizes the important facts: HIGHS Weather:
Genera l ly fair
C l ockwise i n northern hemisphere C o u n te rclockwise i n sou'lhern hemisphere Lig h t Wi nds: Temperature: W a r m o r c o l d f o r rela tively long periods with out change
Circu lation:
60
LOWS Genera l ly cloudy, with rain o r snow Counterclockwise in northern h e m isphere C l ockwise i n southern hemisphere Strong Tropical lows very warm. Other lows cold, o r warm changing to cold
loca l h i g h - pressure a reas may deve l o p any p l ace where a i r cools, compresses, a n d sinks. Most importa nt in t h e northern hem isphere a re the two l a rge, we l l -defi n ed h i g h - pressure regio n s : the horse latitudes and the polar h i g h . In both p l a ces, a i r acc u m u l ates, becomes heavy, a n d settles t o earth. As a m ass of a i r sett les over a n a rea favora b l e for deve lopment of a high, it slowly deve lops i nto a c l ockwise spira l i n g a nticyc lone, or high- pressure ce l l . Air flows from the hig h - pressure a rea into the surro u n d i n g a rea of lower pressure. As it pushes out, it is twisted to its right by the earth's rota t i o n . Air moving north sh ifts to the east; a i r moving south sh ifts t o t h e west. T h e resu lt is t h e formatio n of a w h i r l i n g h i gh - pressure cel l . Such cel ls, form i n g north of th e polar front, repeatedly sweep southwa rd. H i g h s may exte n d to Mexico a n d the Gulf i n wi nter. T h e y are usua l l y severa l h u n dred m i les in d i a m eter but va ry i n size. A l a rge h i g h -pressure c e l l m a y cover the entire U n ited States east of the Rockies. HOW H I G H S A R E B O R N
HIGH
S EASO N A L
AND
G EOGRA P H I CAL
E F F ECTS
ON
If the earth were com plete ly covered with water (see a bove), there would be perma nent low- pressure bands at the equator and at a bout latitude 60° because of the rising a i r at these p laces. There wou l d be perma nent h igh-pressure belts of sinking air at the horse latitudes and the poles. But solar radiation heats ocea ns and l a n d masses differently. T h i s a n d seasona l h e a t changes m a k e the actu a l con ditions q u ite complex. As exa m p les, t h e co ldest region in winter is not over the North Pole b u t over Siberia, a bout latitude 70° N . T h e average tempera ture of San F rancisco in J u ly is lower than at Fa irba n ks, Alaska. In sum mer the land is relative ly warm and the ocea n s coo l . Because o f these factors, high- pressure zones form in a s potty fa shion instead of in wide be lts. I n the northern hemisphere, they form a l most e ntirely over the coo l ocea ns-pa rticu larly in sum mer, when the l a n d is warm. The maps show this. The two "perm a nent" h ighs that cover the eastern parts of both Pacif1c a n d Atlantic are the Pacif1 c a n d Azores anticyclones. But keep i n m i n d t h a t i n d ivid ual high s move with t h e genera l circulation o f the eart h-northeast between latitude 30 6 a n d 60°, a n d southwest between latitude 60° a n d t h e pole. H IGHS
62
Normal position of h i g h s ( H ) a n d lows ( L ) i n the northern hemisphere
63
-1o16
�
Air fl ows from h i g h pressure to low, l i ke water dow n h i l l .
The earth ' s rotation ma kes northern hemisphere air spiral clockwise.
W I N D MOVEM E NTS W IT H I N H I G H S fol low a norm a l
fl ow from h i g h pressure t o l ow pressure, j ust as water flows down a h i l l . But the flow is modified by the earth 's rotati o n . The diagram a t left shows t h e situatio n t h a t wou l d exist if the earth did not rotate. Note the " isoba rs" ( l ines con necting all p l aces of eq u a l pressure), as they a ppear on weather ma ps. The n u m bers refer to a i r pressure i n " m i l l i bars" ( u n its of pressure used by meteorolog ists) . The higher the n u m ber, the greater the pressure. Sta n dard average sea- level pressure is 1,0 13 m i l l ib a rs. On a non rotating ea rth, w i n ds from a high wou l d m ove out across the isobars in a l l directions. The diagram at right shows how the eart h 's rotation twists the o utflowi n g a i r so that winds beg i n to fl ow around the isoba rs as if water were flowing down the sides of a s p i n n i n g h i l l . Note the c h a n ges i n d i rection a s the earth's rotation twists the winds to their rig ht. 64
(side view)
H I GH -
I so b a rs rese m b l e the l ines o n co ntour m a ps. I n stead of a ltitudes, they show pressure i n tensity. Where isobars a re c l ose together, the p ressure c h a nges rapidly a n d winds a re fast. W h e re they are far a p a rt, winds are com pa rative ly slow. W I N D V E L O C I T I ES I N H I G H S
Winds o n the ground and winds a l oft At about 2,000 ft. a bove the ground, winds fo l low isoba rs closely and do not spira l outward a p preciably as they do a t the surfa ce; they are governed primarily b y the earth's rotation, and a re not affected by fricti o n with the grou n d . B u t w i n d s o n t h e ground a re slowed by surface frictio n . The earth's rotatio n a l effect i s reduced, s o a i r contin ues to spira l out u n d e r p ressure from the center. The a n g l e at which winds blow across isoba rs varies from about 10 ° ove r ocean to 45 ° or more over rough l a n d . So w i n ds overhead g e nera l ly m ove 45 ° to the right of their d i rec tion o n the ground and move about twice as fa st. Winds aloft blow about 45 ° to the right of those o n g ro u n d .
65
HOW TO LOCATE H I GHS A N D LOWS H ig h s gen era l ly bring fa i r weather; lows, poor weather. So it is useful to know whether a high or low is coming yo u r way. This can be done with fa i r accuracy by th e a p p l ication of a sim ple r u l e . Sta n d with yo u r back to the wind. Turn about 45 ° to your rig ht; now your back is to the wind as it is b l owing we l l a bove the ground. The h i g h - pressure center is to your rig ht, the low-pressure center to your left. This r u l e h o lds for the northern h e m isphere a n d is q u ite accurate for general winds. It w i l l not h o l d for loca l b reezes ca used by u neq u a l heating ( l a n d a n d sea breezes, for exa m p l e ) . In the southern hem isphere, the d irections a re reversed: the high pressure is o n you r l eft, the low o n your right. Since both highs and lows trave l genera l ly i n a n east ward direction, in the zone of preva i l i n g wester l ies, any h ig h o r low to your west w i l l l ikely move over yo u . But here is where u n certa inty enters. Hi ghs a n d lows m a y re m a i n stationary, fade o ut, or move to the north o r south. So o u r rule h a s but l i m ited usef u l n ess. Once you 've mas tered the facts about air masses a n d fronts (pp. 68-94) a n d learned to read weather maps (pp. 1 30- 1 46), you can use this general r u l e more j udiciously.
66
+
LOWS ARE FORMED by a hori
zontal wav e l ike action between two h ighs of different tem peratures (1 ). The wave g ets l a rger a n d fi n a l ly breaks like an ocea n wave. The whirling air creates a low-pressure cel l . This com p l icated process which forms major lows is d iscussed with fronts (pp. 77-94). A loca l low may form when a i r u n der a l a rg e c u m u lonimbus cloud is rapidly rising (2). This low-pressure a rea is fi l led b y surrounding a i r moving i n a n d twisting counter c l ockwise beca use of the earth's rotation . Such cyc lones a re about 20 m i les i n dia meter. A heat low deve lops over deserts a n d other intensely heated places: The hot air expan ds, rises, a n d flows outwa rd h i g her up; t h u s less air piles up i n the a rea . Pressure d ro ps, a n d surrou n d i n g a i r rushes in with a swirling motion (3). Such a low lasts most of the summer in southwester n Arizona a n d south eastern C a l ifornia. Dust devi ls a re smal l-sca l e heat l ows. Sometimes lows form on the leeward side of m o u nta in ranges, where they may cause weather d is turban ces (4). Frequently they form east of the Rock ies i n Colorado a n d i n the Texas Pa n h a n d le.
cold ai�
J l
2
4
LOW"ff� ', J , /'
�
J l'
._
wa rm air
Air mass properties at a ny level (such as A or B) are similar.
A i r M asses "Air mass," a term in constant use, means " h ig h - pressure c e l l , " but this term ca rries a d ifferent em phasis. An a i r mass i s a vast body o f a i r (often covering h u n d reds o f thousa nds o f square m i l es) in w h i c h t h e conditions o f tem perature a n d m oisture are much the same at a l l points i n a horizonta l d i rection. An air mass (anticyc lone o r h i g h ) takes on t h e temperature and moisture characteristics of the surface over which it forms. For exa m p le, an air mass for m in g over northern Canada a n d the A rctic in wi nter is very cold and dry because of the low tem perature a n d h u m i d ity there. The U n ited States is swept by air masses of g reat con trasts. The North American continent is wide a t the top. Cold, dry a i r ma sses conti n ua l ly form there and m ove southward. The southern part of the continent is narrow. The moist, hot tropic a l air masses wh ich form over the ocea ns can easily move n orth . When a hot, m o ist mass m eets a cold, dry mass, a weather "front" (pp. 77-94) usua l ly deve l o ps. 68
A I R MASSES T HAT A F F ECT O U R W EAT H E R move
ac ross the country and ca rry with them the te m perature and moisture of their orig i n . A n air m a ss is modified by the surfa c e over which it moves, but its orig i n a l c h a rac teristics te n d to persist. Air m a sses come from either of two sou rces: tropic a l reg ions or p o l a r reg ions. On weather c h a rts, tropica l a i r masses a re m a rked T , polar ones P. The t w o types of surfaces over which these a re formed a re the conti n e nta l (c) a n d ocea n ic or ma ritime ( m ) . Polar a i r formed over ocea ns is m a rked mP; po l a r air formed over l a n d is m a rked cP. F i n a l ly, a third letter is added to i n d icate w h ether a i r is colder ( k ) o r warmer ( w ) than the surface over w h i c h it is movi n g. So if polar, contin enta l a i r w e r e moving over gro u n d warmer than itself, it wou l d be m a rked c P k . A t l e a s t th ree differe nt a i r-mass c l a ssification systems are used by U.S. weather experts, but they need not concern u s here. Below a re the principa l air ma sses that affect o u r weather, and their typica l paths. N ote that the a i r- m ass paths sta rti ng i n the north are po l a r (P); those from the south are tropica l (T). Typical paths of a i r ma sses aver the U n ited States
S u n ' s e n e rgy goes a few inches into soil, but deep i nto ocea n .
Heat needed to melt ice de lays the· seasons over the ocea ns.
C O N T I N ENTAL AND MARIT I M E AIR MASSES d iffer
in tem perature and h u m id ity, so conti n e n ta l air brings weather very d ifferent from maritime a ir. L a n d a n d sea reflect solar radiation differently (p. 7). More heat is needed to raise water te m perature than to ra ise l a n d temperature. O n l y the t o p few inches o f s o l i d earth ab sorb radiation, w h i le ocea ns absorb heat to a dept h of 80 ft. or more. Fina l ly, ocean turbu lence carries heat even deeper. The res u lt of a l l this is that ocea n s a re slower to warm up a n d to cool than l a n d . Oceans l ag beh i n d continents by one or two months in their respon se to seasonal cha nges. F u rthermore, there is less than l 8 ° F d ifference between average winter and sum mer temperatu re over most open ocea n a reas! This therm a l lag and its moderat ing effects arise beca use much spring heat o n the ocea n is used in melting ice in a rctic a n d a nta rctic ocea ns. As m u c h heat is req u ired to melt a pound of ice at 32 ° to water at 32 ° F as is req uired to heat a pou n d of water from 32 ° to 1 76 ° F . Thus ocea ns warm slowly i n spring and cool slowly i n fa l l because the same great a m o u n t of heat is thrown o ff as a rctic water freezes. This heat warms the a i r and ca uses a l a g i n the com i n g of c o l d weather. Th us, m a riti me a i r t e n d s to moderate extremes of either heat or cold as it moves over the l a n d . 70
fwormf
1
Sto b i e : thermodynamically warm a i r mass
l
t
l cold+ l 1 t t
U nsta b l e : the rmodyn a mica l l y cold a i r m a s s
STA B I L ITY O F WARM A N D C O L D AIR MASSES
d etermines what weather air masses b r i n g . Vertic a l a i r c u rrents a r e c reated by mou nta ins, o r by heating o r coo l i ng of a i r by contact with t h e s u rface. Sta b l e a i r resists these vertica l c u rrents a n d q u ickly returns t o n o r m a l . U nsta b l e a i r a l l ows the vertica l c u rrents to g row. An air mass moving ove r a surface c o lder than itse lf is thermodyn a m i ca l ly wa r m . Such a n a i r mass is sta b le. An a i r mass moving over a surface warmer than itself i s thermodyna m ic a l l y cold a n d is genera l ly unsta b l e . A thermodyn a m ica l ly w a r m a i r m a s s is c o o l e d by con tact with the colder earth be low. The cooled, heavy, lower surface tends to stay at the bottom . If l ifted, it s i n ks back. Buf when a thermodynamically cold a i r m a ss is warmed by contact with the earth, the warm, l ight air rises through the c o l d a i r a bove. So a thermodynamica l l y c o l d a i r m a ss is turb u l ent, with c u m u l us-type clouds.
Air mass
Sta bil ity
T u rb u l ence
Su rface visibil ity
Clouds (if any)
Precip itatio n (if a ny)
cold
u nsta b l e
turbulent g u sty
good
cumulus forms
t h u n d e r. showers
warm
sta b l e
steady winds
poor
stratus forms
d rizzle
N O RT H AM E R I C A N WI NTER AIR MASSES
Typical paths af cP a i r.
Conti nenta l Polar (cP) air masses are
extremely cold. When trave ling over warm er su rfaces they a re conti n e ntal polar cold (cPk). These a i r masse s are u nsta ble (p. 7 1 ) a n d flying i n the lower levels i s turbulent. Their most common path i s ma rked "A" i n the map. As cPk air goes over the Great Lakes (pa rticu larly before they a re frozen), its lower pa rts a re warmed a n d moistened by the water at about 33 ° to 38 ° F . This warmed air rises through colder upper air, causing snow fl u rries or showers on the leewa rd side of the la kes. A locality o n the eastern shore of the Lakes will h ave th ree to five times as much snow in a winter as one on the western shore. Pushed h i g h e r ove r the Appalachia ns, the cPk a i r is a d i a batica lly cooled fu rther, a n d more pre cipitation resu lts. Hence skies a re generally clear east of the Appalachians as the rel atively d ry air sinks and wa rms. Sometimes cP air takes Path B o n our map. The air is turb u lent; c u m u l u s clouds may fo rm. But the lack of warm lakes and of mountains o n Path B means generally clearer skies t h a n when cP a i r is movi ng along path A. The third path, C , is rare. On this path, cP a i r hits the ocea n a n d col lects moistu re. I t brings sq u a l l s and eve n snow as fa r south as southern C a l ifornia.
cP ai r-snow in C a lifornia. Maritime P o l a r ( m P ) a i r may take a rel
atively short trip over the Pacific (Path 1 , a t left) before hitting the Coast. Rain or snow showers are common as the a i r is lift ed over coastal ran ges. It can also go over the Rockies to stag nate in the Mack enzie valley, becoming cP. If mP air is over the ocean longer (Path 2), it will warm up a n d get weller. Su ch a i r masses a re comTypical paths of m P a i r.
A I R MASSES
m o n o n t h e Pacific Coast i n wi nter. They cool o n contact with the l a n d ; rain a n d f o g resu lt. Maritime Polar a i r rising over the moun tai n s produces clouds a n d heavy precipita tion o n windward slopes. I t may stagn ate between coasta l m o u nta i n s and the Rockies in the Great Basin region. I f i t goes east over the Rockies, it worms as it descends the east slopes, a n d brings clear skies, low h u m i dity, and mild weather.
m P a i r-West Coast fog.
m P-clea r east of Rockies. �' Marit i m e Tro p i c a l ( mT) a i r is hot and
h u m id at its sou rce. Being hotter than the l a n d su rface of the U n ited States, it i s sta b l e (mTw). Stratus or stratoc u m u l u s clouds often f o r m , mostly d u ri n g night cool ing. These tend to disappear by noon be cause of warming. Little or no p recipitation occurs u n less mT air runs into a cold air mass. Then warm, light mT air rises over the cold air and precipitation results. This h a p pens ofte n (p. 86), accounting for more rain i n the U n ited States than any other cause. But mT air seldom moves over the north or northeast part of the U n ited States i n winter. When it does, the col d land sur face may cause extensive fog. It can a l so cause dense oce a n fogs as it crosses the line dividing the Gulf Stream from the cold waters of the North Atlantic. Maritime Tropical air rarely enters the U n ited States via the Pacific Coast. It does so only when extremely low p ressu re exists off C a l ifornia. Then the warm, moist mT a i r moves i n a n d rises ra p i d ly o v e r the heavier, colder air it encounters, or over the Sierra Nevad a and other weste rn ranges. I n either case, despite its sta b i l ity, the mT air pro d u ces heavy rainfall in southern C a l ifornia.
Typical paths of mT a i r .
mT a i r
,IJf cold a i r
mT over cold air-rain.
mT a i r-ra i n on m o u nta i ns.
�
SUMMER AIR MASSES Conti nenta l Polar (cP) a i r in s u m m e r
Summer cP a i r usually brings fair weather.
differs from cP a i r i n winter. The sou rce regions a re warmer and the general cir culation of the atm osphere is weaker be ca use there is less temperature difference between polar and tropical regions. Air moves slowly over the U n i ted States. Fair weather p revails; a few c u m u l u s clouds are p rod uced by local h eati n g . Rains occ u r ove r the G r e a t lakes and in the eastern mountains.
Pacific Coast mP a i r i n summer · p ro
d uces sea and coasta l fogs i n C a l ifornia. Moist maritime a i r passes over a cold ocean su rface off the coast. Cooled u nderneath, the m P air produces dense sea fog, which rolls inland a n d may hang on for weeks. Farther i n l a n d , m P air is wormed q u ickly by contaot with the land; so its relative h u midity d raps. Altho u g h the air is u n stable, its h u m i d ity is s a low t h a t s k i e s re main clear from a reas just inside the coast l i n e to the Sierras, on whose west slope showers may la l l . Summer m P air brings fog to the Cal ifornia coast.
The c o l d ocean surface that causes West
Coast fogs is a resu l t of the " u pwe l l i n g " af su bsu rface waters. F riction of the p re vailing northwest wind over the Pacific p ushes coastal su rface waters Ia its right: that is, toward the south west. T h e result: the calder subsu rface water rises o r wells up. This colder wate r coo ls the a i r a b ove it, p roducing fag and stratus which may then move in over the coastal region, p ro ducing cool weather a n d i n terfering with flying. Cold u p-we l l i n g f o r n i a coast.
Atla ntic Coast mP a i r in summer forms
over cold North Atlantic waters. I t is colder than the cP or mT air which generally sweeps the eastern states. m P air occasion a l ly flows in over land. When it does, it brings cool weather which sometimes may reach as far south as Florida. I f mP makes con tact with mT a i r from the south, frontal weather (typically low stratus clouds and d rizzle) occurs as the mT air is lifted a n d coaled. Typical path of Atl a n tic coast mP a i r. Ma ritime Tro p i c a l (mT) a i r formed aver
the Pacific is of little i m portance in summer. I t does came i n l a n d and may g o as far north as Alaska. But it is nearly always mTw, sa it is stable and generally brings l ittle disturbance. Atlantic mT a i r, o n the other hand, is more important i n summer than i n winter. Central and Easte rn stales are affected, as is sometimes the Southwest. Atla ntic mT air moves a l most continuously over the Central a n d Easte rn stales i n summer, bringing hu mid ity and o p p ressive heal. Atla ntic mT a i r brings h eal and h u m i d ity to the East.
mT air formed i n the Caribbean is extremely hot and h u m id i n summer. It is u n stable as it comes in over the hotter southern coastal stales. As land coals rapid ly at n i g h t, the air's sta b i l ity in creases and stratocu m u l u s clouds form, with bases near the gr o u nd . These clouds usually clear by m id-morning as the l a n d a g a i n warms a n d heals t h e a i r . F u rther heating, l a t e r i n t h e day, in creases i nsta b i l ity a g a i n a n d sh ow· ers a n d t h u n d e rstorms may result from rapidly rising moist air that coo ls to below its saturation poi nt.
Caribbean mT air brings showers to the South.
w a r m e d by ocea n ; water added
P o l a r a i r m a s s e s m a y u n d e r g o g re a t c h a n g e s .
POLAR AIR MASSES change rapidly a s they swee p over the U n ited States. cP air is i n itia l l y colder than l a n d over w h i c h it travels, s o bottom layers a re w a r m e d b y the e a r t h a n d rise, creating turb u l e n ce, c u m u l u s c l ouds, and prec ipitatio n . The air is furth er warmed by heat of co ndensatio n . Thus polar air te nds to mix a n d get warm. The m a p sh ows c han ges in winter a i r starting i n Siberia as a d ry c P a i r mass. Wa rmed a n d moistened by the ocea n , it becomes m P air. Coo ling by passa ge over land a n d mountains ca uses it to drop its moisture a n d it con tin ues o n as d ry, relatively warm a ir. T R O P I C A L AIR MASSES c h a n g e s l owly because they
a re warmer than l a n d over which they travel . The m a p shows t h e path o f hot, mo ist m T a ir. Bottom layers, coo l e d by l a n d do not rise-there is no m i x i n g . F u rther coo ling by Eastern mountains o r la kes i n c reases sta b i l ity further. mT a i r u s u a l l y rem a i n s the same for a l o n g time hot s u m m e r days in the Midwest a n d East go o n a n d o n . T r o p i ca l a i r masses u s u a l ly c h a n g e s l i g h tly. cooled i f cooled by g ro u n d
c o o l e d by l i fti n g o v e r m o u n ta i n s
l
Fro n ts a n d Fro ntal Weather F R O N T S form when a i r masses co l l ide, for a i r masses d o not m ix u n less they a re very sim i l a r in temperature and mo isture content. What usua l ly ha ppens is the forma tion of a boundary, or front. The colder a i r mass pushes under the warm one and l ifts it. Then, if t h e bound ary doesn 't m ove, the front becomes stationary. More commonly it does move, and one air mass pushes the other along. If the cold mass pushes the warm a i r back, we have a c o l d front; such a front is pictured a bove. If the cold a i r retreats, warm a i r pushing over it gives us a warm front. I n either case, fronta l weather is either un settled or stormy. Fronts usua l l y bring b a d weather.
77
" J Loo k i n g down on
a front (outl i n e d o rea e n l a rged b e l o w ) .
FRONT FACTS At the top of this page we a re look ing down a fro nt. A warm high is to the right, a cold h i g h at the left. A rrows show tl,e wind directions; isobar l i nes show pressure in m i l l ibars. At the bottom is a b lock v iew of the sa me front. line A-B has the same position in each diagram. Six facts, true of all fronts, a re a pparent: ( 1 ) Fronts form at margins of h i g h - pressure cel ls. (2) Fronts form o n ly between cells of d ifferent tem peratu res. (3) Warm air a lways s l o pes upwa rd over cold air. (4) A front is fou n d a l o n g a low-pressu re tro u g h (few exce ptions), so pressure drops as the front a p p roaches, rises after it passes. (5) Wind near ground a lways sh ifts c l ockwise (in the northern hemisp here) as the front passes. (6) A front a lways s l o pes upward over cold air either a head of o r to the rea r of its direction of advance. Th ree-d imensional e n l a rgement o f area outlined above.
cold high
{
�� . �o
�,
l o w-p ressure trough
�� ·'�0
78
warm high
�
B
Polar front ( d a r k b l u e line), shown at left in theoretical position, often breaks thro u g h (right). Arrows show wind flow.
POLAR A N D EQUAT O R I A L FRONTS At latitude 60°, co n d itions a re idea l for a permanent front. Preva i l i ng westerl ies f r o m the southwest ( r ed a rrows) run into polar easte r l ies ( b l u e a rrows) from the north east. The polar a i r is cold, the westerlies relative l y warm. But the position of the polar front that forms is not static. Pressure b u i l ds u p near the pole, and the polar a i r breaks through to form the masses of cP or m P a i r that move over N o rth America. Weather, especia lly in the centra l a n d eastern states, is influenced by movements of this polar front. The so-c a l led eq uatoria l front (i ntertro pica l converg ence zone) is not a true front, for temperatu res to the north and south a re about the same. But i n late summer the zone moves n orth . A rea l line of weathe r activity m a y t h e n deve lop-which accou nts in part f o r h u rricanes. T h e b l a c k l i n e s h o w s t h e positio n of t h e i ntertro p i c a l c o n v e r g e n ce z o n e i n F e b r u a ry ( l eft) a n d September ( r i g ht).
IN W I N T E R major fronts form and move d ifferently than i n sum mer. The d ifference is due to the lowering of p res sure in s u m m e r as contine nts warm up . Winter fro nts move fa r south; cold po l a r air pushes to F lorida o r farther. Stro ng wind a n d temperature differences of land and sea push fronts back and forth. The most active fronts of the northern hem isphere form ( 1 ) in northwest pa rts of the Pacific, from Aleutian low-pressure troughs, and (2) in no rthwest pa rts of the Atlantic, from the I cel andi c low. East of the Rockies, fro nts move genera l ly southeastward bringing changing weather.
W I NTER Dark b l u e lines are zones of front formatio n .
80
I N SUMMER m a jor fronts in the northern hemisphere form and m ove like those of winter. But fronts do not move so far south. They are much weaker tha n wi nter fronts beca use of the warming and expa nsion of the air by the warm conti n ents. Held back by preva i l i n g westerl ies, summer fronts do not often b u l g e down over the southern Un ited States. Mariti m e tro pica l air often swe l l s far north, bath ing most of the U n ited States i n hot, sticky weather. When a front does move south, rel ief tends to be short lived, a n d the front is soon pushed north a g a i n by mari time tropical a i r.
SUMMER At t h i s time the z o n e s o f f o r m a t i o n a re f a rt h e r n o r t h .
81
Th ree-d imensional view of a ra.pidly moving col d front_
cirrus
I
warm a i r
Actual slope of a c o l d front (bl ue wedge). left end of b l a c k l i n e i s
C O L D FRO NTS wedge their way under warm a i r a s they advance. The typica l thick wedge of a cold front develops a s friction with the ground holds back the botto m of the a dva ncing mass of cold air. So the cold air a l oft tends to p i l e i nto a rounded prow as it advances aga inst warm a i r. The long wedge diagram a bove shows the actual slope. I n the northern hem isphere, m a jor cold fronts usua l ly lie in a n o rtheast to southwest direction a n d move towa rd the east o r southeast. The reason is a pparent from the m a ps on p p . 80-8 1 . Here a l l cold, polar air is colored b l u e . As t h e general movement is east, the cold a i r masses ad v a n c e i n that d i rection. Note that cold fronts (easterly edges of cold a i r masses) are a l most a lways oriented north east to southwest. C o l d fronts usua l ly advance at speeds of about 20 m p h -faster i n wi nter than i n summer, because i n wi nter the a i r is c o l d e r a n d exerts g reater pressure. Altho u g h the sloping edge of a cold front may exten d over several h u n dred m i les horizonta l ly, the steepness o f the a dvancing edge means t h a t fronta l weather is l i m ited to an extremely narrow band (clouds i n the d rawi n g above are greatly exaggerated h orizonta l ly). The stee p sloping edge a l so prod uces a brupt l ifti ng of warm a i r, so that storms at a cold front are genera l ly brief though violent.
82
Slow-moving cold front l iftin g sta ble air. a ltostratus
warm a i r
1 m i l e a bove ground -front is on su rface (right) 1 00 miles away.
Weather at slowly moving cold fronts differs from weather accom panying rapidly moving fronts. If the warm a i r is sta b le, n i m b ostratus c l o uds will form a l m ost directly over the fron t's contact with the ground, and rain w i l l fa l l through the cold mass after the front h a s passed. If the cold front is weak, neither rain nor clouds may form. I f u nsta b l e a n d very humid a i r is pushed over a s l owly movi n g cold front, c u m u l o n i m b u s clouds wi l l form a n d th u ndershowers m a y fa l l . But t h e c h ief rainfa l l w i l l be through the cold mass after the front has passed. Typic a l ly, a steady downpour from n i m bostratus c l ouds at the l ower leve ls a lte r n ates with rain in sheets from c u m u l o n i m b u s c l o u d s towering a bove. Slow-movi n g cold front l ifti n g u nsta ble air.
cumulonimbus
col d a i r
SQUALL L I N ES may precede fast-moving c o l d fronts. They a re an u n broken line of b l ack , ominous c louds, tow ering 40,000 ft. or m o re i nto the sky, i n c l u d i n g thun der storms of a l m ost incredible violence and occasio nal tor n adoes. Such sq ua l l l i n es are extremely turbulent, some times more so than a typica l h u rrica ne. They c a n tea r a l ight a irplane apart a n d are avoided by a l l except radar e q u i p ped p l a n es. From the ground, a squa l l l i n e looks like a wa l l of ro l l i n g, boiling, b l a c k fog . Winds s h ift a n d sharpen sudden ly with t h e ap proach o f t h e sq u a l l line, a n d downwa rd-pouring ra in may carry the c l o u d clear to the ground in sharp, vertica l bands. Torrentia l rains fa l l beh i n d t h e leading edge o f t h e sq u a l l l i ne. F lash floods are common, a n d dry ravines may become raging tor rents with i n m i n utes a s r u n -off water rushes thro u g h . Sq u a l l l i n es occur w h e n w i n d s a bove a c o l d front, mov ing in the same d irection a s the front's a dva nce, prevent the l ifti ng of a warm air mass. This is why l itt l e bad weather occurs right at the s u rface front. But 1 00 to 1 50 m i l es a he a d of the fro nt the stro ng winds force u p the warm a i r with a l most explosive violence, produc i n g the sq u a l l li n e .
84
WEAT H E R S E Q U E NC ES within a
strong cold front are usua l l y the same. F i rst o n e n otices a sharpen i n g of winds from the south o r southwest a n d the a p peara nce o f a ltoc u m u l us c l ouds darke n i n g t h e horizon to the west o r northwest. The ba rometer begins to fa l l . A s t h e front a p p roaches, the c l ouds lower and c u m u l o n i m b u s c louds beg i n t o tower overhead . Rain spatters down, then inc reases i n intensity. The wind may increase a n d the barometer fa l l sti l l further. With the actual passa ge of the front over the observer, the wind s h ifts ra pidly to west or north, b low ing i n stro n g g u sts. Sq ua l l - l ike ra ins conti n u e a n d the barometer hits its l owest rea d i n g . Passage of the front usua l ly r es u l ts i n fa irly rapid clearing, but i n moist or m o u n ta i n ous reg ions, c u m u l us or stratocumu lus c l o uds m a y stay overhead for some time. The ba rometer rises rapid ly; temperature drops. Winds genera l l y become stea dy from west or n o rthwest. Afte r passage of a c o l d front we experience for a few days the ty pi c a l weather of a h i g h ( p . 60) . Then stea dily i n crea sing c l o u d i n ess usu a l ly i n d icates o n a p proaching worm front ( p p . 86-88).
� Rising � Barometer
ng f"2\Risl � Barometer
warm a i r
J!iln bostratus
cold a i r W a r m front. W a r m a i r rides ove r c o l d su rface a i r .
WARM FRO NTS are those i n which warm a i r a dva nces,
replacing colder air. In the Northern H e m isphere, warm fronts occur o n the east side of low-pressure ce l ls and a re usua l l y fol lowed by cold fronts as the preva i l i ng westerlies m ove the low ea stwa rd (p. 90). The advance of warm fronts, horizonta l ly, is usua l ly at about 1 5 mph o r slower-that is, about half the speed of cold fronts. The vertical slope between warm and cold a i r in a w a r m front is m uch less steep than i n a cold fro nt. The wa r m a i r moves gradua l l y u p the slope, without the typica l col d-front b u lge. This is beca use grou n d friction drags the bottom edge of the retreating cold air i n to a thin wedge. Wa r m -front weather extends over a n a rea h u n dreds of m i les in advance of the front l i ne at ground leve l . Typic a l c l o u d seq uences m a y be noted 1 ,000 m i les i n advance of the front, a n d often 48 hours in adva nce of its a rriva l . T h e clouds a n d preci pitation typica l o f warm fronts de velop a l o n g the contact zone of the warm and cold a i r a b o v e the ground, as shown a bove. 86
cirrus
W a r m front. Clouds of t h e stratus type form a s sta b l e warm air climbs u p a slope o f cold a i r.
Sta b l e warm a i r, lifted over cold a i r as a warm front advan ces, prod uces stratus-type clouds because the up l ift of a i r is s low and l ittle turbule nce resu lts i n the sta b l e a ir. A s the a i r l ifts (see p. 88 f o r seq uence observed from the ground), it coo ls to prod uce stratus, n i m b ostratus, a lto stratus, c irrostratus, a n d cirrus c louds i n that o rder. Pre cipitation is h eavy a t the beg i n n i n g of the l ift, but decreases g r a d u a l ly, leaving relatively d ry cirrus c l o u d s at 20,000 ft. or higher. U n sta b l e warm a i r produces more violent weather. Turbulence is h i g h, and the u n stable a i r sets up ascen d ing a i r currents creatin g c u m u lonim bus c l o u d s a n d t h u n derstorms a h e a d o f the front l i n e . The preci pitation is therefore spotty, a lternati n g between heavy dow n p o u r a n d s l ow drizzle, w i t h t h u n dersto rms i nterspersed. C u m u l o n i m b u s fo r m i n g as u n sta b l e w a r m a i r r ises u p s l o p e of c o l d a i r.
a ltocu m u l u s a n d
cold oir
cirrus
WARM-FRONT WEAT H E R SE fa i i i iiCJ bar.
..
-tl"' �"� ,. �
�
'"'
.
-
_ ·
QUENCE fi rst disp lays cirrus clouds, which have been l ifted fa rthest up the cold-air slope. These cha nge into cir rostratus, a n d be come denser as the front a dvances. (Circles represe n t ba rometer with b l ack a rrow as the needle, and red arrow showing whether pressure readings are rising o r fa l l i n g . )
If cirrocum u l us clouds ("mackerel sky") a p pear, the warm air over head is u nsta b l e and weather de scribed at the bottom of p. 87 is a l most certain to come, as old time sa i l ors and farmers k n ew. 1f the warm air overhead is sta b le, ci rrus c louds are rep laced by mid d l e-height a ltostratus ( l e a d e n sky). Unsta b l e cirroc u m u l u s a re a l so re placed by a ltostratus. Ra i n or snow begins as the a ltostratus becomes de nse, and contin ues until the front has passed. Stratoc u m u l us, n i m bostratus, stra tus, a n d-in u n sta b l e a i r-c u m u l o n i mbus clouds fi n ish the warm-front seq uence. But rain fa l l i n g through co l d a i r adds to its m oisture co ntent, prod ucing lower stratus c l o u d s that often o bsc u re h i g h e r c l o u d s . The skies c l ear as the front passes.
STAT I O N ARY FRONTS are those which move l ittle o r
not at a l l . C o n d itions a re much l i ke those acco m pa n y i n g a w a r m front, but are usua lly m o r e m i l d . Statio n a ry fronts with rain m a y h a n g on for days. WEAK FRONTS often pass u n n oticed except by weather
m e n . Occ urring when air masses are a l most identic a l i n h u m i d ity a n d temperature, they a re m a rked o n l y b y a w i n d sh ift as the weak front passes. Weather m e n watch wea k fro n ts for possible regeneration i n to stron g fronts.
L I F E H I STORY OF A FRONTAL LOW Fronts tend to move
across the U n ited States in a gen era l ly east or southeast direction . Warm fronts a r e usua l l y fol lowed c lose ly by cold fronts. To u nder sta nd why, observe the way "ex tratropica l " low- pressure cells de velop from wave like o utbreaks along fronta l l i n es. The pictu res show the deve lopment of a typi cal low as it might be seen from a bove. Black l i nes are isoba rs. Purple a reas mark precipitation. ( 1) A fronta l line exists a t a low-pressure tro u g h between cold a n d warm a i r masses. (2) Cold air begins to push under the warm air at some point, form ing a wave like pattern at the front. (3) The cold front contin ues to push back the warm a i r and go under it. Warm a i r, pushed from o n e side, b u lges out at its front into a low pressure a rea, which increases in size as cold air moves away from it. So the warm front a dvances, adding to the deve loping wave. (4) The cold fron t p l unges a head about twice as fast as the warm front. The l ow pressure area ex pands a s warm air is forced over the low, a n d t h e wave forms a distinct crest. (5) The c o l d front
fi n a l l y overta kes the warm front, l iftin g the warm air completely off the ground. {6) Only the l ow pressure ce l l of counterclockwise swir li n g air remains near the sur face. The swir l i n g low-pressure ce l l fi n a l l y disappears as air pres sure eq u a l izes. The new front l i n e is esta b l ished at the bounda ries of the cold and warm air masses. The deve lopment and d isa p pea rance of a low-pressure ce l l does n ot take p l ace i n a station ary position. Westerly winds m ove the air masses from southwest to northeast. So, as a low deve lops a n d a cold a i r mass bends a ro u n d the ton g ue of the warm air mass, a warm front m oves across the cou ntry, fo l l owed closely by a cold front. lowering air pressure and a rapidly dropping barometer precede the warm front with its stormy o r genera l l y bad weather. This is fol lowed by the higher p ressure of the warm a i r m ass, a n d a short period of clear weather may resu lt. But the adva ncing cold front is n ot fa r b e h i n d . Thus warm fronts, closely fo l l owed by cold fronts, m ove e n d l essly over the U n ited States.
Path of a, loW ceU at It develops and moVN eastward.
91
warm a i r
Cold front a p p roach ing a w a r m front p r i o r t o occlusion.
The next-to-last position shown for the low o n p. 91 marks the deve lopment of what is c a l led a n "occ luded front." This occurs when a c o l d front, c lose ly fol lowing a warm front, fi n a l ly overtakes the warm front and l ifts the warm air mass off the ground. Either the warm front (p. 93, top) o r the cold front goes a l oft. The top d rawings o n these two pages show two stages in an occl usion as seen in a side view. Study these and the diagram of the occ l u ded front as seen from a bove (lower position, next page). In it, the purple l i n e marks the occlusion. The yel low l i ne (A-B) s hows the situation when the cold front has not q u ite caught u p with the warm front. Conditions o n the ground i n this area are shown i n the picture directly a bove. Warm front weather is quickly fol lowed by co l d-front conditions. The redd ish-brown line (C-D) is lying across the occ l usion itse lf; the p icture a t the top of p. 93 s hows cond itions o n the g ro u n d. This is a low-pressure a rea; surro u n d i n g a i r flows i n, equalizing the pressure. OCC L U D E D FRO NTS
92
warm air
Occl u s i o n occu rs {above) a s t h e co l d f r o n t overtakes t h e w a r m f r o n t a n d lifts both it a n d t h e a i r m a s s off the g r o u n d .
B e l o w : d i a g r a m of t h e o c c l u s i o n a s s e e n f r o m a bove.
1012
HOW FRONTS A R E SHOWN O N WEAT H E R MAPS
Most weather changes a re brought by fronts. By c h eck ing them i n newspa per weather ma ps, you ca n judge the kind of weather that is a p p roac h i n g a n d when it will ar rive. Keep in mind that weather i n the U n ited States moves genera l l y from southwest to northeast. Cold fronts move on the average about 20 m p h, warm fronts 1 5 mph. C o l d fronts are in dicated on m a ps by b l ue lines o r l i n es with peaks (on the side of the line towa rd which the front is advancing). Warm fronts a re shown by red li nes or l i n es with rounded bu m p s (on the side of the l i n e of a dvance). Stationary fronts are shown by a lternati n g colo rs o r with peaks a n d b u m ps on o p posite sides of the line. Occ l u ded fronts a re shown by p u r p l e lines or a lternating peaks a n d b u m ps, b o t h o n the sa m e side of t h e l i n e .
.�� \
X '
It
''
i
'
-.
F R O N T S A N D FRONTAL WEAT H E R show prominently o n weather m a ps. Above is a wave ( p . 90) with both a warm a n d a cold front. Areas of stea dy precipitatio n are m a rked in s o l i d g reen; i nterm ittent preci pitation i n hatc hed g reen. N ote the sym b o l s for s h owers � a n d t h u n derstorms ; a lso the a i r m a s s c lassification i n r e d a n d b l ue.
R.
95
Sto r m s Storms feared storms, storms,
are the most dramatic, most dang erous, a n d most of a l l weather phenomena. In addition to fronta l t h ree other we l l -known types occ u r-t h u n der torn a does, and h u rricanes.
T H U N DERSTORMS represent violent vertical movements
of a i r. They occ u r as a result of strong u p l ifting, sometimes b u i l d i n g the c l ouds to heig hts i n excess of 75,000 ft. This u p l ifti n g may b e d u e to heati n g of air by the ground sur face (co m m o n i n the Midwest a n d East); the action of a c o l d front; or to tem perature d ifferences between l a n d a n d ocean (co m m o n i n F l orida a n d t h e Southeast states) . 96
The map below shows that thund erstorms are m ost fre q uent i n F lorida, the Southeast, the southern Rockies, and adjacent Great Pla ins. They occur chiefly in summer. Thunderstorms seldom occu r on the West Coast, where water-land contrasts in tem perature are l owest. C u m u l us clouds represent upd rafts; c u m u l o n imbus (thunderstorm) clouds have both upd rafts a n d downdrafts. Hu rrica nes (pp. 1 04- 1 1 1 ) a re, i n early stages, like giant t h u n der storms-with vio lent upd rafts a n d downdrafts. J U DG E T H E DISTANCE O F A T H U N D ERSTORM from
the fact that l ight trave ls at a bout 1 86,000 m i l es per sec. and so u n d at about 1 , 1 00 ft. per sec. or 1 m i l e i n a l ittle less than 5 sec. J ud g e the d ista nce of a storm by timing how l o n g it takes for thu nder to reach you after you see the l i g h t n i n g fl ash. Counting " 1 00 1 , 1 002, 1 003," etc. at a normal s peed cou nts seconds for you. Another aid is to n ote that cold a ir, ca rried down from the thun derhead by rain, may flow forwa rd about 3 m i l es i n front of the storm. So a t h u n de rsto rm genera l l y a n nounces its ap proach by a rush of co l d air that flows down a n d out over the gro u n d a head of the storm itse lf. THUN DERSTORM DISTRIBUTION
T h e n u m bers rep· rese nt the aver a g e n u m b e r of t h u n d e rsto rms per y e a r .
97
T H U N D E R STO R M
D E V E L O P
MENT takes place in th ree recog
nizable stages. The cumulus sta ge is the fi rst
•
1
=
* = s now
rain
-60 ° f
40,000 ft.
\
sta ge that each of the m a n y cells of a thun derstorm undergoes. A c u m u l u s cloud develops i nto a thunderhead when a i r c urrents in it exten d to about 2 5,000 ft.
- -'\- - - - - -,-Jt - - - -...-3 6 0 - -
"' 15 - - - - - (- - -,;; - - ---
- - -
• . ·• ------
---•
0
- - - - -- -· __ _ _
- --
"' 1
- - .-
3
0
-
\"
18°
- , _ _ _ _ _ ... - -
32 °
-
- - - - -t - - �6 °
- - - .... .....
_ ,__ _
J · i - - --- -
62
- -
.... ..... .... _ _ ... ...
°
-
82°
The mature stage begins when ascending a i r reaches such a height that precipitation occurs (p. 23). I n the earlier stage, a i r i n the cloud had been warme r tha n the sur rounding air. But now fa l l i n g ra i n a n d ice crysta ls cool the a i r a n d downdrafts a re created . N o t e how the dotted l ines of equa l tem pera ture dip with the down d raft.
- = i c e crysta l s
40,000 ft.
....,.
-60 ° F
..
- -- - - - --
- - -- -
-
-
- -
-
- -
- 36 °
- - - - - --
--
- - - -
-- - - - - -
-
- -
- 1 5° --
- - - - -
final stage occurs when downd raft areas have increased until the entire c l o u d is n oth i n g but sinking a i r being ad iabatica l ly warmed. As no a i r is ascen d i n g a n d cooling, preci pitation becomes l i g ht, then ceases. Steady u p per air w i n ds b low the c loud of ice c rysta ls at the top of t h u n derheads i nto the typica l anvil to p. The
+
L I G H T N I N G is ca used by the at
tractio n of u n l ike electrica l charges + with i n a t h u n dercloud, or between it and the e a rth. The ea rth normal ly has a negative charge (a surplus of electrons as compa red to atmos phere). Friction of rapidly moving ice particles and rain in a thunder cloud "wipes off" electrons a n d bui lds u p strong e lectrical charges. Negative c h a rges (shown here by minus sig ns) concentrate between the 32 ° F and 0 ° F leve ls. The lower part of the cloud has positive ( +) zones surrounded by negative zones. When electrica l pressure becomes h i g h enough, charges between pa rts of cloud or between c loud and earth are released by l ightning. First strokes a re with i n the cloud; 65 p e r c e n t o f a l l discharges a re t h e r e or b e twee n c l o u d s . lig htn ing t o t h e ground starts w i t h a re la tive ly thin " l eader" stroke from the c loud, fol l owed i m m e d iately by a heavy return stroke from the ground. A single l i g htning discharge strikes back a n d forth m a ny times in less than 1 / 1 0 secon d . Most leaders start from the cloud, 1 I some from the ground. A l i g htning ! ( discharge is incredibly powerful u p to 30 m i l l io n volts at 1 00,000 a m peres-but is of very sh ort d u ra Most leaders sta rt from tion; hence lightning can not be clouds; some from g r o u n d . harnessed o r used. But the tota l en ergy of a major thun derstorm far exceeds that of a n atomic bomb The sudden tremendous heat from lightn i n g causes the com pres sion or shock waves that w e ca l l thunder.
� .
;
99
lightning strikes thousa nds of times i n the U n ited States each year. Damage runs into the m i l l ions, and a bout 2 1 0 peo ple are k i l led. But if a few elem entary precautions are ta ken, the chances of being hit a re ex ceed ingly sma l l . lightn ing takes the path of least resista nce. It tends to hit the highest places. Never sta n d under a l o n e tree in a n open fi e l d , or fix y o u r T V a n te n n a , d u r i n g a thun derstorm. The mast of a sai l boat may attract l ightning. Stay away from boats and water d u r i n g sto rms. I nside your car or i n a stee l-frame building you ' l l a lways be safe. Meta l aircraft have often been hit by l ightning but seldom with harm to occ u pants, even though there is ofte n extensive d a m a g e to the radio equipm ent of the plane. The meta l skin may be pitted with ho les from l ightning str ikes. On b u i l d i ngs, l ightning rods a l low el ectrons to stream off into the air or down i nto the gro u n d a n d so te n d to prevent strikes. If struck, rods cond uct elec tricity harm lessly to the g r o u n d . TV a ntennas, properly g ro u n ded, serve the sa me pur pose. Wood structu res and trees have high e l ec trical resista nce and are heavily damaged u n less grou nded. LIGHT N I N G SAFETY
D a n g e r spafs in a sfarm
Car bady sh i e l d s a c c u p a nfs
A i r p l a n e pa sse n g e rs a r e safe
a nten n a grounded
a n te n n a n o t g ro u n d e d
fewer than 5
-
40 30 to 40 days 20 to 30 1 0 to 20 5 to 1 0
N u mber of tornado days per year over equal a reas, by states
CJ 5 to 0 TORNADOES are by far the most violent of storms. They are w h i r l pools of a i r of such violence that houses in their path may fly a pa rt like matchsticks and even ra i l road tra i n s may be l ifted from their tracks. Fortunate ly, tornadoes ( m isca l l e d cyc lones i n the Midwest) are of sma l l diameter. Their path of destruction rarely extends Ya mile to eithe r side of the dipping, twisting, f u n n e l sha ped c l o u d w h i c h m a r k s t h e vortex. An averag e of 1 24 torna does hit the U n ited States each year, a n d the g reat majority of these strike i n the l ower Mississippi Va l ley. They res u l t from a l most exp losive in sta b i l ity in the a i r, a n d they accompany heavy thun der storms and u n us u a l l y heavy rains-conditio n s typical of stron g cold fronts a n d sq u a l l l i n es (p. 84). I n a stu dy of 92 tornadoes, 72 were found to precede the cold front at a n average dista nce of 1 65 m i les. When the proper condi tions occur, tornado wa r n i n gs a re b roadcast for a reas most likely to be affected. Sometimes to rnadoes can be spotted a n d tracked by radar.
1 01
M a m m a tocu m u l u s c l o u d s fo r m .
F u n n e l f o r m s a s t o r n a d o develops.
DEVELO P M E N T O F A TORNADO is shown in fou r sta g es. Destructive effects do not c o m e from the forward speed of the sto rm (20 to 40 m ph) but from the whirling winds w h ich, near the vortex, often exceed 300 m p h . Torn a d o w i n ds, the stro ngest k n o w n , may have a force 900 times that of a 1 0-m i l e breeze and cause u n be l ieva b l e d a mage. Extreme l o w pressure i n the vortex ca uses c l osed houses o r barns l itera l l y to explode from the norma l pressure of a i r trapped inside as a i r pressure d rops d ras tica l l y in the vortex. F u n n e l sta rts its destructive cour se .
F u n n e l s u c k s u p d u st a n d d e b r i s .
To r n a d o c a n d rive straw i n to tree .
Press u re c a u s e s b a r n to e x p l o d e .
Another destructive force is a 1 00- to 2 00- m ph u p d raft at the center of the f u n n e l . This u p d raft has sucked ho uses, a n imals, and cars i nto the air and carried them h u n dreds of feet; it resu lts from the sudden setting off of the explosively u nsta b l e a ir. Tornados have l ifted frogs a n d fi s h from ponds, then d ropped them over pop u l ated a reas. Red clay when so l ifted, m ixed with rain, a n d d ropped to earth has been ca l le d " a ra i n of b l o o d . " O v e r la kes a n d s e a s the tornado f u n n e l sometimes l ifts water i nto the a i r, creating a waterspo ut. Tornado m o y "rain" frogs.
Waters p o u t : t o r n a d o over ocea n .
The eye of h u rrica ne Gloria seen from an airplane.
H U RRIC A N E S ( OR "TYPHOONS" ) are tropica l cyc lones. like a l l cyc lones they are lows. I n some ways they are like the "extratropica l " cyclones that exist as low-pressure cells a l o n g fronts (p. 90). But h u rrica nes (with winds of 75 mph or more) are not accompanied by fronts and they differ from fronta l "extratropica l " lows in i m porta nt ways. Far more i ntense than extratro pical lows, h urrica n es often have winds of ove r 1 50 mph. They are a bout one-third as large, averaging 400 mi les i n diameter_ They move s l ow ly, trave l i n g in a n a i r m a s s of u n iform tem perature rather tha n between a i r masses of d ifferent tem peratu res. Hurricanes a re amazingly sym m etrical, the centra l isobars form i n g a l most perfect circles. H urricanes deve lop o n l y over open ocean a reas covered by extremely warm m oist air masses. They break up soon after moving over land. An i nteresting d ifference between h urricanes and extratropical cyclones is the ca lm "eye" found in the center of the hurricane (p. 1 08).
1 04
,
.
.
4 H u rricanes generally form in the a reas shown in d o r k b l u e a n d travel i n the di rectio n of the red arrows.
The south western part of the North Pacific has more h urricanes (there known as "typhoons") than any other place on earth. These a re born between the Marsh a l l I s l a n ds and the P h i l i ppines, a n d move in a c lockwise fashion, fi rst westwa rd toward the C h i n a coast, then n o rthward a n d northeast o v e r t h e P h i l i pp i n es toward Korea a n d Japan. Ra n k i n g second are the h urricanes ("cyc lones") of the South I n dian Ocean. Some ("Wil ly-Wi l l ies") deve lop north of Austra l ia and c u rve down over the northwest coast. More freq uent and intense a re those that sweep westwa rd, some brush i n g Madagascar and Southeast Africa. Third in freq uency are the hurricanes that move over the West I n dies. These storms, most of w h ic h orig i nate near the b u l g i n g west coast of Africa, cause great damage to shi pping a n d to the many islands i n their path. These are the storms that h it the south and east coasts of the U n ited States o r strike Mexico or Centra l America. Of eight reg ions of occurren ce, o n ly two, the West I n dies and the Mexica n West Coast, g ive rise to h urrica nes that affect the U n ited States. The latter se ldom strike the continent as far north as C a l ifornia. The map shows the eight sources (dark b l ue) a n d paths ( red a r rows)· of h u r ricanes of the world. SOURCES A N D PATHS OF H U RRICANES
1 05
T H E B I RT H OF H U R R I C A N E S often occ urs in regions
of contrasting win ds, s i m i l a r to those at fronts. These a re n ear, but n ot at, the equator, where trade winds meet to form the intertropica l convergence zone ( ITCZ). But trop ical cyc l ones have n ever occ u rred at the equato r itse lf! This is beca use cyc lones req uire the twisti n g forces of the earth 's rotation to sta rt them spin n i n g (pp. 53-54), a n d there i s n o n e at the equator. Since t h e ITCZ moves n o rth of the eq uator i n sum mer to a reas where the w i nds are twisted to their rig ht, cyclonic swirls can be esta b l ished. A n d so begins the season of h u rricanes that affect the U n ited States. Another way h u rricanes form is by the intensification of a wave or ripple i n the easterly trade winds. These easterly waves a re watched, and when a ship repo rts stro n g w i n d speeds in its vicin ity, reco n n a issa nce a i rcraft a r e sent to look over the suspected area. I n winter, when the ITCZ in the Atlantic Ocea n is at the equator, h u rricanes never deve lop on it. However, when the zone moves south of the eq uator i n the I n d i a n Ocea n a n d in pa rt of the South Pacific in winter, the season of h u rrica nes affecting the southern h e m isphere begins. Compare the m a ps ( p . 79) showing the February and Sep tem ber positions of the ITCZ with the ta b l e of West I n d i a n h u rrica nes (be low). TOTAL WEST INDIAN HURRICANES BY MONTHS OVER A 40-YEAR PERIOD
1 06
May
June
J u ly
Aug.
Sept.
Oct.
Nov.
Dec.
0
10
13
51
66
35
6
0
A h u rricane is born in a h ot moist a i r mass over the ocea n . The cy clonic motion is often started as op posin g trade winds whirl around each other. This can occur only when the ITCZ is displaced from the eq uator so that the twistin g ef fect of earth's rotation c a n take place. The rotating low pushes air towa rd its center, forcing h ot moist air there to l ift. liftin g ca uses mo isture to condense. Heat thrown off as the m o isture conde nses (p. 1 3) furthe r warms rotating a i r, which becomes even lig hter a n d rises m o r e swiftly. A s m o r e a n d m o re m oist tropica l s e a air sweeps in to replace the rising a ir, more and more condensation takes place. So air inside the storm rises faster and faster. H u rricanes are so vio lent be cause of the tremendous e n e rgy re leased by the continuous conden sation. At fi rst the storm is like the inside of a g i a nt thundersto r m . U n l i k e a thun derstorm over l a nd, a h u rricane has a n inexha ustible source of m oistu re. The heat given off by condensation ca uses the air i n the h ur rica ne to rise faster a n d faster. Surroun d i n g a i r sweeps i n ra pidly u n ti l the h urrica n e is a giant wheel of violent winds.
Opposing w i n d s m a k e a i r whirl.
Heat of condensation l ifts mass.
300
miles
Rise accelerates like a b a l loon.
C o l u m n is fed from sides.
Sectio n thro u g h a h u rricane, showing eye a n d vertica l winds.
THE EYE OF A H U RR I CAN E is at the center of the storm -a zone of near calm or light breezes, with clear or lightly c louded skies overhead. It averages about 20 m i les in dia meter. Thousa n ds, ignorant of the h u rricane's an atomy, have gone out into the ca l m of the eye, u n aware that they wou l d soon be hit again by the fu l l m ig h t of the othe r side of the h urrica ne. The eye may be caused by centrifugal force acti n g on winds at the rim of the eye. The centrifuga l force acting on a rotating body doub les when the radius of rotation is cut in ha lf. As a i r spira ls in toward the cente r of a h u r rica n e, its centrifuga l force increases g reatly. The c loudy wa l l of the eye is where the centrifuga l force exactly bala nces the pressure forcing air i nward to the low-pres sure center. Friction with the ocea n surface slows down the w h i r l i n g air and decreases the centrifuga l force. So the eye is s m a l l at the surface. A loft, where wind speeds are great, the centrifuga l force is h i g her and the eye larger and funnel-shaped.
1 08
I
Azores
Tropic of C a n ce r C a pe Verde I s l a n ds
75 °
45 °
30°
1 5°
Path of a typical h u rrica ne from birth Ia decay.
T H E L I F E H I STORY OF A H U RR I CAN E c a n be traced
from its b i rth a s a tropica l low, through maturity and to decay as an extratropica l cyclone i n the westerlies. The m a p a bove shows a storm sta rti ng west of the C a pe Verde Islands as a tropic a l low with winds less than 32 m ph . Next w e fi n d tropica l storm "Betty" ( h urrica n e s a r e g iven wome n 's n a m es) with winds 32 to 73 m p h, a pproac h i n g t h e Leewa rd I s l a nds o f t h e West I n d ies. W h e n n ext re ported it is a mature h urricane with winds over 75 m p h , r i g h t over Guadeloupe . It contin ues westwa rd at 1 0 t o 1 2 m p h , passing south o f Puerto Rico a n d H a iti. T h e n it begins to c u rve north, sti l l at 10 to 12 mph, passing over Cuba and the Bahamas, but now with winds over 1 25 m p h . Betty then c u rves t o t h e n ortheast, movi n g at 30 m p h unti l it h its a col d front n orth of Bermuda . Hu rricane Betty in d uces a wave o n the front a n d becomes a n extratro pica l low, ending as a storm over the North Sea. H u rric a n e Betty's p a t h is typica l . It cou l d h a v e contin ued west t o h it t h e G u l f Coast, F lorida, o r t h e Atlantic seaboard. 1 09
Wind blows boats ashore.
Waves undermine b u i l d i ngs.
DESTRUCTIVEN ESS O F H U R R I C A N ES is due to winds
u p to 200 m p h , storm waves a n d tides, a n d flash floods ca used by torrenti a l rains fi l l i n g rivers t i l l they overflow as the storm m oves i n l a n d . On t h e average t h e U n ited States is hit by h u rrica n es about twice a yea r. About once every three yea rs an u n usua l l y severe h u r r i c a n e strikes, l e a v i n g a swath o f de structio n 50 to 1 00 m i les wide and sometimes h u n d reds of m i les lo n g . The v io l ent winds uproot trees, s p l i nter fra m e b u i l d i n gs, s i n k or damage s h i ps, and in combination with the h i g h tides erode beaches a n d w a s h away s h o r e i n sta l lations. The storm tides with the September 8, 1 900, h urrica n e at Ga lveston, Texas, comp letely fl ooded the c ity. More than 6,000 died and pro perty damage was w e l l over $20,000,000. Flash floods ca used the ch ief destruction and g reatest n u mber of deaths from the 1 955 h u rricanes that hit the eastern seaboard a n d New E n g l a n d .
O U R H U R R I C A N E WAR N I NG SYST E M was esta b
l ished i n 1 93 8 by joint action of the U.S. Weather Bureau, Air Force, and the Navy. P reviously, h u rric a n es h a d struck with l ittle o r n o wa rning. Now recon n a issa nce a i rc raft, f u l l y eq u i p ped with radar scopes and weather instru ments, a re sent to rec o n n o iter suspected a reas. When a h u rricane is s potted, pla nes fly into the h urrica n e once every 6 h o u rs to measure the i ntensity of the winds and exactly locate the center. B u l letins every 6 h o u rs forecast the proba b l e path of the hurricane a n d g ive wa r n i n g . People h a v e t i m e t o board u p t h e i r h o m e s o r g o to safer p l aces. Ships a t sea g e n e ra l l y c h a n g e course to avo i d t h e storm, a n d a i rcraft a r e fl own t o safe s pots away from the coast. F lying into the eye of a h u rrica ne, t h o u g h ex trem ely rou g h , is not so dangerous as o n e mi ght t h i n k . Desp ite the tho usands of fl i g hts made since the beg i n n in g of the prog ram, o n l y three recon n a issance a i rc raft have been l ost i n the period between 1 938 and 1 957. The m i croseismograph is a relatively n ew instrument
that picks up disturbances ca used by storms at sea. This u ltra-sensitive earthq uake recorder detects h u rricanes earlier so that they can be l ocated a n d tracked by recon naissa nce p l a nes. 111
We at h e r F o re c ast i n g Accurate weather forecasti ng depends o n the forecaster's knowing a s m u c h as possib l e a bout the tota l state of the atmosphere. The U.S. Weather Bureau operates a far fl u n g n etwork of stations where such information is col l ected a n d speedi l y sent to regio n a l forecasting centers. Observations a re taken hourly, day and n ig ht, at some 400 stations a long the U.S. airways. More complete obser vations are made every 6 hours at many of the stations. U pper air elements are sa mpled by rad iosonde twice daily at some 50 stations. Pilot ba l loons, measuring the u pper a i r win ds, a re sent u p four times a day at some 1 25 stations. The Coast Guard, Air Force, Navy, a n d Civil
The U n ited States is covered b y a dense network of statio ns t h a t report the weath e r for aviation n eeds. New York C ity and Los Angeles are each su rrou n ded by a dozen statio ns close at h a n d .
1 12
Aero n a utics Authority cooperate in observing a n d fore casting the weather. Over 9,000 cooperative, part-time weather stations a lso report to the U.S. Weather B urea u . Weather affecting t h e Un ited States a week o r more from now is being born today in a i r masses over other cou ntries and the oceans. A worl d-wide n etwork of weath e r stations (under the Wor l d Meteorolog ica l Org a n ization, a n agency o f the U n ited Nations) exch a n ges data with the U.S. Weather Bureau. Merchant s h i ps radio infor mation fou r to eight times a day while at sea, and a ir liners report weather cond itions encountered i n fl ig ht. Be cause of a l l this, the U.S. Weather Bu rea u is a b l e to ach ieve better than 85 per cent accu racy i n its 24-hour to 36-hour forecasts.
The stations o n this map a re those that report upper-air data. They a l l explore w i n d s a loft b y sending u p p i l o t bal loons. Those ma rked by a tria n g l e report, in addition, data obtained by radioso nde.
1 13
Colder; Snow?
24 hour forecast FIVE·-DAY FORECAST
For Jefferson Coun t y : Colder and in creash1g cloudiness tonight. \Vednesday scattered li ght rain
':Z_�.t��
or snow flurries and colder.
'--..:::.=-., night
Low
to-
around
40:
High Wednesday i n t h e 4 0 s . I.ow Wednesday night low 3 0s.
Tem pcratures will a\'crage 4 t o
7 degrees below normal. Normal ma:\imum
54 north to 63 south;
normal minimum 3 4 north to 4 0 south. Colder tonigh t , \Vcdncsday and Thursday fol lowed by warm ing trend o\·er weekend . Precipita tion
total
.50
to
I
i nches ;
rain
south and rain or snow north \iVcd ncsday and again ncar week end.
Deta iled 24-hour and general five-day forecasts.
SERVICES OF T H E U .S. WEAT H E R BU REAU i n c l ud e 24-hour deta iled forecasts, genera l 5 - d a y forecasts, 30day genera l outlook, 1 2-hour aviation forecasts (with a n accuracy of 9 5 p e r cent), reports o f weather conditions at over 300 a irports, and specia l b u l leti ns, weather maps, and reports of temperature and prec ipitation. They issue warnings on storms, frost, etc. for fa rming and industry. The 24-hour a n d 5-day forecasts go by radio o r te letype. N ewspa pers and TV broadcasters use the Weathe r Bu rea u 's weather maps to prepare their simplified m a ps. Before taking off, pilots read reports a n d forecasts received by tele typewriter in the weather b riefi ng rooms.
DEN AKO GLD G CK DDC
HL C
RSL
HU T
SLN
I CT
TOP
MK C STJ OFK
1 5�£ 3 5 �6 0® 1 0 �2 0 2 1 7/ 6 5/ 5 0�� 1 0/ 0 3 1/ E 4 0® 1 8 0$ 1 5 224/6 3 / 5 5�� 1 0/ 02 9/ B I NOV C NW 2 0�E 1 2 � 1 5 2 20/ 6 q / 5 9�7/ 026 2 0�E 7 � 1 5 1 90/ 7 0/ 6 0�2 0/ 0 1 7 9 �E 12 � 1 5R - 1 6 6 / 6 7 /6 4 �� 1 8/ 0 0 9 E 1 0 � 2 5� 1 5 2 03/68/58 � � 1 4/ 0 1 7 40�80�1 50�[ 200® 1 5 1 93 / 7 0/ 5 9v l 0 5 0�2 5 0 -®1 5 7 4 ��2 3 +28/ 0 03 £2 0® 1 2 0$ 1 5 1 8 0/ 7 4/ 6 1 �� 1 6/0 08 6 5IDE2 5 � 1 5 l q ?/ 7 7/63 �22/ 9 9 9/ TW R G CU S E -W 2 0�E 8 0®1 5 1 6 9/ 7 3/ 6 q � 1 0/ 00q M2 5® 1 2 � 1 5 1 6 4 / 7 6 / 6 5 l 2 0/ 002 2 2� E 8� 1 5 � 72/6 5 l l 2/003/�V® B I N OV C N W 2 5 0-® 1 5 + 22 7/ 6 2/ 4 6 � 1 3
A Teletype weather report for pi lots.
For a i r safety, major a i rports have stations operated by the Civil Aerona utics Authority or U.S. Weather Bureau to g ive pilots complete i nformation . Te letype reports l ike that shown at the top of the page a n d decoded at the bottom are ava i lable. While i n fl ight the pi lot "receives weather reports and forecasts every half hour, and ca n req uest information at a n y time. The g rowth of private and commercia l aviation has been paced by the g rowth of the U.S. Weather Bureau service, which has made safe flying possible. Following is a n i n terp retation of the fifth line of the Teletype at t h e t o p of t h e p a g e :
DOC Reporti n g statio n Dodge City, Ka ns. 90 (1) Scattered clouds at 9000 ft. E 1 20 E9 Cloud ceiling at a n esti mated height of 1 2,000 ft. 1 5 Visibil ity 1 5 miles R· Weather: light rain 1 66 Barometric p ressu re: 1 0 1 6.6 m i l l i bars 67 Temperatu re i n degrees F a h renheit 64 Dew point i n deg rees Fahre n h eit � · 1 8 Wind: n o rth-northeast at 18 mi les per hour 009 Altimeter setting of 30.09 inches
1 15
LATE CITY EDITION Condensat i o n of U . S . Weather B u reau F o recast
Rain and cooler today; clear ing and colder tonight. Fair, cold tomorrow. T e m perature ran ge today : 48 - 4 1 Tem perature range yesterday : 4 9 . 6 -40.2
N ot a coincidence. large stores retain p rivate forecasti ng services to know in advance which merchand ise to feature.
PRIVATE
AND
I N DUSTRIAL
M ETEORO LOG I STS
supplement the work of government weathermen and sup ply a g reat variety of weather forecasting services. If a department store is a l l set to a dvertise spring c l othes and a consulting forecaster advises that three days of ra in a n d c o o l weather are a head, the ads a re changed t o se l l rain wear a n d cool weather c lothing. If a meteoro l ogist em p loyed by a large New York bakery a dvises that tomor row's weather w i l l be rainy, the ba kery del ivers the b u l k of its products t o downtown stores, knowing t h a t h ouse wives wi l l delay their shopping in the suburbs and ca l l their h usba n ds t o "pick u p a loaf o f brea d " near their office. The da iry company ma kes more ice cream when a dvised of warm weather. The coat man ufacturer ma kes topcoats instead of overcoats on learning of a m i l d winter ahead; the rainwear man ufacturer ma kes rubbers instead of boots. The drug manufacturer makes more cold rem edies because more peop l e catch colds i n the changeable weather of a " m i l d " winter. Mil lions of d o l l a rs a re saved a n n ua l l y throug h these services. 1 16
P R I VATE METEOROLOGY is widespread in todoy's complex affairs. Most a i r l i n es employ their own meteor olog ists, who work out comp l ete details of the weather that pilots w i l l encou nter on their fl i gh ts over the U.S. or abroad. Motion picture compan ies save m i l l ions of d o l l a rs by arranging their sched u l e of outdoor shooting on the basis of c lear-day predictions of their meteorologists. Some meteoro logists hove ful l-time jobs with major uti lity com pon ies. Their predictions of cool a n d worm weather and bright a n d dull days enable the compa n ies to pump gas a n d generate electricity according to the need. Private a n d commercial m eteorolog ists get their basic data from the U.S. Weather Bureau. This agency m a kes the observations, draws the mo ps, and issues the genera l information from w h ich speci a l forecasts c o n b e mode for s pecial purposes. Basic to a l l forecasting is the c o l l ection of data throughout the U n ited States and other ports of the world, the cod i n g of these data for tra nsm ission, a n d t h e preparation o f basic weather mops. Many a irlines have their own staff meteorologists, who help pilots avoid storms by p redicting where and when they will occur.
T H E DATA basic to accu rate forecasting req uires the use of instruments. Until the inven tion of such instru ments, o u r k n owl edge of the atmosphere was l i m ited to g en era l ideas d isti l l ed from gen erations of folk wisdom. Farm ers, h u nters, and sa i l ors were a lways i n terested i n weather. T h e i r rules o f t h u m b (pp. 1 42- 1 43) genera l l y h e l d a g o o d dea l of truth. But with the invention of the ba rometer a n d thermometer and other measuring instruments, accurate ob servation was made possi b l e a n d the science of meteo rology was born. The a dvent of telegraphy and the worl d-wide sta ndardization of time zones based o n Greenwich Civil Time made possible the c o l l ection of these data at centra l points. The next step was a n a lysis and prepara tion of weather m a ps for the entire wo r l d that showed the con d itions of the atmosphere with fair accuracy. It is now possi b l e to prepa re weather maps showing simu ltaneous world wide conditions every 6 hours of the day a n d night. U pper-air cha rts can be prepared every 1 2 hours. Deta ils on modern wea ther instru ments fol low-but the teletype, ra dio, and the i nternatio n a l coopera tion of scientists are essential a lso . C O L L ECTI NG
radiosonde t r a n s m itter and re ceive r
PRESS U R E MEASUREME NTS are essentia l for the plotting of isobars ( l i nes of equal pressure) over the earth, and for dete r m i n i n g positions and movements of fronts. As pressure at mounta i n tops is less than at sea level, re g a rd less of positions of highs and lows, all pressure read ings a re converted to what they wou l d be if readings were taken at sea leve l . This a l l ows accu rate m a pping of pres sure patterns a nywhere in the world. Pressure is measured by two types of barometers. The mercuria l b a rometer is used by m ost weather stations. It is a g l ass tube c l osed at the top and fi l led with mercury. The mercury c o l u m n is supported by the pressure of the a i r, being high i n the tube when air p ressure is high and lower as air pressure decreases. The rea d i n g is conve rted into m i l l i bars. The a n eroid (no fl uid) barometer is a corrugated meta l conta i n e r from which the a i r has been removed. The cor rugations and a spring inside prevent a i r pressure from collapsing it com pletely. As a i r pressure increases, the top of the b ox bends in; as pressure decreases, the top bows out. Gears a n d levers tra nsmit these c h a n ges to a pointer on a dia l . MERCURIAL BAROMETER closed e n d 31 vacu u m 30 mercury
A i r pressure ope n p u shes against mercu ry at open end end. High pres sure fo rces mer cury up closed e n d - low pressure lets it drop.
O I
ANEROID BAROMETER
?-� lever
1
·
1
�:
1 n easing air pressure push es top af vacuu m box down. Th i s p u l l s down at tached lever a n d works gears which move pointer over dial.
1 19
reco r d i n g barometer or b a rograph
a n eroid barometer
WEAT H E R STAT I O N M E RC U R I A L BA
mercurial barometer
1 20
ROMETERS a re precision-made a n d ac curate to 1 / 1 000 inch. They respond to changes in tem perature as we l l as pres sure, beca use the mercury co l u m n itself ex pa n ds with heat. This is corrected by adj ust ing the level of a poo i of m ercury at the base of the tube. H i g h accuracy i n rea ding is made possible by a sliding vernier. The observer reads the mercury height a long etched l ines on the vernier. Aneroid ba rometers are not as accurate but have im porta nt uses. One, not related to weather, is their use as a n a ltimeter. The dial is ma rked i n feet, and as a pi lot fl ies u pward, the poi nter gives his a ltitude a bove sea leve l . An a n e roid ba rometer can be arranged a lso so that changes in pressure a re recorded on a pa per-covered rotating drum. Most weather stations h ave such a recording aneroid barometer, or barogra ph.
T E M P E R AT U R E
REA D I NGS
above -38 ° F a re taken with mer curia l thermometers ( mercury freezes at -38 ° F ) . Alcohol ther mometers are used below -38 ° F . (Alcoh o l freezes at - 1 79 ° F ) M i n i mum thermometers record l ow est temperatures. Surface tension of a lcohol keeps a sma l l sliding g lass in dex a lways at the surface level of the alcohol as it contracts with fa l l i n g tem perature. When the a l cohol rises i n the t u b e as tempera ture goes u p< the index remains at the lowest position, i ndicating the m i n i m u m tem perature reached. Maxi m u m thermo meters, l ike a doctor's thermometer, have a con striction i n the mercury tube. As temperature rises, the mercury ex pands through the constriction . As temperature fa l ls, the constriction keeps the mercury from r u n n ing back. A Thermograph, o r re cord ing thermometer, has a Bour don tube fi l led with alcohol. It flexes a s temperature changes.
mercurial thermometer
.
index
a lcohol i n d ex
Max. a n d m i n . thermometer constriction
mercury
thermograph
121
sling psychrometer
Hyg rog r a p h records data o n movi n g paper.
RELAT I V E H U M I DITY is measured with a psyc hrometer, consisting of two thermometers. One is a reg u l a r mercury thermometer. The other, a wet b u l b thermometer, has a muslin wick over its b u l b . The end is d ipped i nto water before a reading is ta ken. These thermometers a re whirled before the psych ro meter is read. Eva poration from the muslin wick lowers the temperature of the wet bulb ther mo meter. In d ry a i r, there is more evaporation and there fore more cooling than i n moist air; hence the difference i n temperatures shown by the two thermometers ind icates the rel ative h u m idity (p. 1 2 ), which is rea d from a ta ble. There are severa l types of psych rometers; they d iffer mainly i n the means for eva porating water from the wet bulb. Hygrogra phs a r e instruments f o r recording relative h u m i dity. They work like thermographs, except that the h u m id ity element consists of a sheaf of b l onde h uman h a i rs treated to remove the oil. As the relative h u m idity inc reases, the hairs increase in length a n d operate the recording mechanism. Knowledge of relative h u m i d ity is im porta nt in predicting preci pitation and icing conditions, which can force a plane down .
1 22
MEAS U R I N G P R E C I PITAT I O N is
extremely im portant. The amount of a n n ua l rainfa l l a n d its seaso n a l d istribution in a loca l ity determine the kinds of farm ing that a re prac tica ble, and may affect the whole pattern of living. The U.S. Weather Bureau keeps accurate records of precipitation throughout the coun try. Rain gauges a re of severa l types and measure not o n ly ra i n but a l l other forms o f preci pitation. Tip ping b ucket gages have a divided bucket ba l a n ced so that one side fi l ls, then tips its water i nto a con ta iner and a l l ows the other side to fi l l . The gauge is so precisely bala n.c ed that 1 / 1 00 in. of ra i n tips the bucket each time. A s t h e bucket ti ps, it sends an electric signal to a recording drum inside the weather station. later the station observer measures the water which ran i nto the conta i n e r as a check on the re cording's accuracy. Another type of record ing rain gauge conti n u o u s l y weighs the water a n d records that weight directly on a g raph as i n c hes of rainfa l l . "O ne inch of rain fa l l " is a way of sayi n g that on level ground a layer of ra i n one inch deep wou l d remain if none of the water ran off or seeped into the ground.
R E C O R D I N G RA I N G A U G E (housing removed)
1 23
Aerovan e transmitter Aerova n e indicator
W I N D S P E E D A N D W I N D D I RECT I O N I N D I C ATORS
show intensity and direction of air movements. Sudden sh ifts i n wind direction mark the passage of fronts. Pre cise data on wind direction a n d speed are determ ined by a va riety of wind vanes a n d by various types of anemometers, usu al l y recording types which a utomatica lly set down a conti n uous record. One of the most commonly used i n stru ments is the q uadruple recorder, or station meteorograph, which is connected electric a l ly to a wind vane, anemometer, ra i n ga uge, a n d a device that records sunshine d uration. The a n emometer part has three cu ps, which catch the wind a n d rotate. Gears and e lectrica l con nections tra nsmit the wind speed to a d i a l for direct reading a n d to the record ing gra p h . The Aerovane is like a sma l l airplane without wings; the nose turns into the wind, showing w i n d direc tion, a n d the whirling prope l ler g ives wind speed in m i les · per hour. Accurate information on wind di rection a n d speed helps meteoro log ists to chart the positions of highs a n d lows (pressure readings provide the basic data for such charts).
station meteorog raph or q uad ruple reco rder
1 24
Beaufort W i n d Scale I n 1 805 A d m i r a l Bea ufort of the British Navy deve loped a sca le to esti mate w i n d speeds f r o m their effect o n t h e s a i l s . H i s ta b l e is g iven be low, mod ified for use o n l a n d . Bea ufort n u m ber 3, for exa mple, with winds of 7 to 1 0 k nots (a k n ot is 1 . 1 5 m i l es per hour), is a gentle b reeze that prod uces consta nt but l ight motion of twigs and l eaves. Today, by internatio n a l agreeme nt, a l l repo rts for use i n p l otti n g weather maps use knots. Winds a re recorded o n the weather map to the nea rest 5 k n ots. ESTIMAT I N G W I NDS O N THE BEAUFORT SCALE Bea u fort n u mber
0 1 2
3 4 5
6 7 8 9 10 11 12
mph
Symb o l s o n weather maps
Description
Observa tion
calm
s m o k e rises verti c a l l y
@ calm
light air
s m o k e d rifts slowly
@
slight breeze
leaves rustle
gentle breeze
leaves and tw i g s i n m o t i o n
m o d e rate b reeze
small branches move
fresh breeze
sma l l trees sway
stro n g breeze
l a rg e b r a n c h e s sway
3
mode ra te gale
w h o l e trees i n motion
0
fresh qale
twi g s b r e a k off trees
strong aale
branches brea k
whole aale
trees s n a p a n d a r e b l own d o w n
storm
wides p rea d d a m a·ge
h u rrica n e
extreme damaae
k n ots
�1 �3 �6 � �6 �1 �7 � � �7 �5 56 63 � �1
0
ca l m
5 knots \ 1 0 k n ots 15 k n ots \� \
\\ 20 k n o t s
\\{5 k nots �\30 k n ots \\\�5 k n o ts �\�5 k n o ts ..
50 knots
� 60 k n ots
�70 k n ots
MEAS U R I N G C E I L I NG H E I G HT
Releasing ceiling balloon
Using clinometer ( left) and ceiling light projector
(the a ltitude of the base of c louds which are below 1 0,000 ft. a n d cover more t h a n h a lf o f the sky) is of partic u l a r im porta nce to a i r safe ty. Experienced observers, knowing cloud types, can estimate the ceil ing to within 1 00 ft. for low c louds, · and with i n ·1 ,000 ft. for m i d d l e a n d high c louds. Station reports often g ive "estimated" cloud heig hts a n d ceiling. C l oud a n d c e i l i n g height are a lso measured i n several ways. Air craft fly to c loud levels a n d check a ltimeter rea dings. C e i l i n g bal loons a re inflated to rise at a fi xed rate, and the time it ta kes them to dis appear into the cl oud base indi cates the height of the clouds. A c l i n o meter a n d a ceiling l ight projector a re used to measure cei ling height at nig ht. The observ er sta nds 1 ,000 ft. from the projec tor and observes the bright spot re flected from clouds thro u g h the c l i n ometer. He reads the angle on the i n dex of the c l i n om eter, then c hecks a table to get the ceiling heig ht. At la rge airports a p hoto electric cel l , w h i c h rotates verti cally through a 90° arc w h i l e a imed at a vertica l bea m of lig ht, makes these measurements a utomatica l ly.
SUNSH I N E D U RATI O N TRANS MITTERS are used to record the d u ration of sunshine each day. This, l i ke the preci pitation for a region, is im porta n t for agricultu re, ind us try, and resort areas. The tra nsm it ter has a " b l a c k b u l b " conta i n i n g mercury a n d a sea led a i r space a bove it. When the sun s h i nes, the black b u l b is q u ickly h eated a n d S U N S H I N E T R A N S M I TT E R t h e a i r expan ds, pushing mercury up the tube a n d making an electric contact. The re sulting electric signal ma kes a mark o n a rotating graph paper. When clouds cover the sun, the bulb cools rapidly a n d the mercury d ro ps, breaking the e l ectrica l 'contact. OBSERV I N G W I N DS ALOFT is im porta nt to aviation a n d i n preparing upper- l evel maps for forecasting. These winds are observed either by radar o r by use of pilot bal loo n s and a theodo l ite similar to the su rveyor:s i nstru m ent. Accurate scales measure t h e a n g l e o f the bal loon with the g round a n d its d i rection from the statio n . Speed a n d d i rection of winds at various leve ls are easily com puted from these observations. Theodol ite o n d pilot balloon help keep track of winds aloft.
Radiosonde receiver (left) and transmitter (right).
RADIOSO N D E MEASUREMENT OF U P P E R AIR is one of the more recent a n d most accurate means of fi n ding the temperature, pressure, a n d h u m idity at various heig hts i n t h e atmosphere. There a r e several types, b u t most common is the "modu lated audio-freq uency system, " involving a tiny radio transm itter that broadca sts a specia l freq uency radio wave. Attached to this tra nsm itter is a sma l l but com p l icated set of measuring instruments which a utomatica l ly convert d ata on temperature, pressure, a n d h u m i d ity into e l ectrica l impu lses, which are sent by the tra nsm itter to a receivi n g and recording set on the ground . Radiosondes a re sent up on special b a l loons twice daily by U . S. Weather Bureau stations. They sometimes reach heig hts of over 1 00,000 ft. One kind, the d ropson de, is released on a parach ute from airpla nes flying 20,000 ft. a bove ocea n a reas.
1 28
On radar, d ista nt sto rm (left) is overhead 4 h o u rs later (right).
RADAR EQ U I PMENT has come i nto pro m inent use in weather observation a n d forecasting work. Designed pri marily to track the course of storms (including h u r ricanes), some forms of radar a re carried on airplanes a n d enable pilots to go around storms or to p ick their way thro u g h t h e m by avoiding the most turbulent a reas (p. 1 1 1 ) Below is a typica l weather station radar eq uipment set-up. A rotating a nte nna constantly sca ns the skies. Radar beams shoot out at the speed of lig ht, are reflected from preci pitation cel ls i n clouds, a n d return to the receiving part of the antenna. The receiver converts the beam to electrica l impu lses which show u p on a screen much a s i n a television receiver .
Screen (left) shows information collected by rotati ng a nten na (right).
1 29
Weather M a ps W EAT H E R MAPS summa rize a l l the data sent to the
centra l stations. Two kinds of maps are made: surface charts and upper air cha rts. Surface c h a rts plot the fol lowi ng for each station: wind direction a n d speed, pressure, tem pe rature, dew point, visibil ity, current weather (rain, snow, fog, etc.), the amount a n d types of clouds a n d their heig hts, pressure changes in the past 3 hou rs, weather i n the past 6 h o u rs, and the a m o u n t and kind of precipitatio n . A l l this is shown by sym bols i n a space easily covered by a d i m e . O n c e the data a re pl otted, t h e c h a rts a re a n a lyzed b y forecasting experts, who d r a w isobars t o show l ines of eq u a l pressure. Then they mark the positions of fronts and show precipitation a reas. From these a n a lyzed cha rts, the forecasters prepa re forecasts and weather b u l letins. The methods of. making forecasts are exp l a i n e d o n pp. 1 34- 1 46. To u n dersta n d forecasting methods, learn how to rea d the synoptic or general surface weather m a ps. Officia l Weather B u reau ma ps, sent daily, can be p u rchased for 60 cents a month or $7.20 a year from Superi ntendent of Documen ts, Washington 25, D. C . Newspaper weather ma ps a re not so comp lete but g ive you data a day a head of offic i a l maps. On this U. S. Weather Bureau m a p, shaded a reas show p recipitatio n . Look for the isobars a n d for the fronts. N ote the h i g h s a n d l ows and the sym bols for a i r masses ( m P over Was h i n gton and m T on the Texas Gu lf). Sym bols for sta tion d a t a a re shown at the bottom of the page. O n the backs of the Daily Weather Maps there a p pea r, at i nter vals, a s u m m ary of weather sym bols, a n a lyses of u n us u a l weather, a n d other genera l information . 1 30
Wind speed and d i recti o n
Temperature A m o u n t of clouds
STAT I O N MODEL
�
/
{! � �
V i si b i l ity
� "'� 76
Stale o f w e a t h e r-
/68 /
�g �l�� �
Dew p o m t C l o u d type
H e i g h t of c l o u d base
8
[R ()
//.
!59
+ 2 87.::::
8 5 .35
4� ......____
Pressure (9 o r 1 0 o m i tted5 .9 ���i�alr?)l Pressure change in l a st 3 h o u rs i n tenths af m i l l ibars Press u r e l e n d e n cyn teadily
���; :
amount of c l o u d I n ches of p re c i p i t a t i o n i n last 3 h o u rs
HIGHEST AND LOWEST TEMPERATURES
! 9� 5!66o 9v66
;�
....9'1
gr'"�
58
6(
�r
DAI LY T EMPERAT U R E A N D PREC I P ITAT I O N MAPS
a lso appear as part of the Weather Bureau's dai l y weather map. Special reports appea r when needed, making this tota l service a n outsta nding h e l p in checking the weather. PRECIPITATION AREAS A N D AMOUNTS
132
N EWSPA P E R WEAT H E R MAPS va ry in deta i l . Some (as above) provide considerable data and follow the Weather Bureau pattern. Others (as below) are simplified.
Night Shew Low Tcmpero'u'•'
1 33
FORECAST I NG FROM A S I N G L E WEAT H E R MAP
is not difficu lt, a lthough accuracy is l i m ited to about 6 to 1 2 hours. Use good newspaper maps or Weather Bu rea u d a i l y ma ps. Short-range forecasts a re based on the prin ciple of persistence, which assumes, for exa m ple, that a front moving at 20 m p h wi l l continue at that rate with the same weather accom pa nying it, a n d that a stationary high will remain stationary. This principle is usua l ly followed by expert forecasters. It genera l l y works, though loca l conditions sometimes th row such predictions off. Study the weather map below. For simpl icity, o n l y front al positions are shown, with a precipitation a rea a head of the warm front line. If you l ive at Dayton, O h io, you know the cold front is 80 m i les west, a n d movin g at 20 m p h . The front s h o u l d pass Dayton in 4 hours. Your forecast? " Ra i n showers at about 2 p. m .-coo ler to night a n d to morrow." T h e m a k i n g of a sh ort-ra nge forecast. It is n o w 1 0 A M ; a col d front, still 8 0 miles from Dayton, i s moving east at 20 mph. What is the fore· cast for Dayton?
1 �4
L I M I TAT I O N S OF T H E PERSIST E N C E M E T H O D lie
i n the fact that many things can change a weather pattern. As the cold front (p . 1 34) a pproaches the Appa lachians, portio ns m a y be s l owed, a i r may be l ifted slowly here a n d rapidly there, d a y a n d n i g ht w i l l affect a i r-mass charac teristics, and so forth. An experienced and tra ined fore caster can accurately esti mate these t hi ngs. A beg i n n e r cannot-so k e e p records a n d gain experience. THE C O N T I N U ITY METHOD OF FOR ECAST I N G is the persiste nce m ethod with modifications. It can be used with fa i r accuracy for periods g reater than 12 hours. You can make contin u ity fo recasts by inte l l igent use of weather m a ps. The m a p below, and those on the n ext th ree pages, w i l l show you the method. Mon day's m a p, below, shows a wave centered i n Colorado with a cold front extending southwest out of the low, and a warm front exte n d i n g southeast. Ra i n showers lie a long the cold front a n d a stea dy rain is fa l l ing ahead of the warm front. (See pp. 77-95 o n fronts a n d their accompanying weather.)
COMPARISO N OF DAILY WEAT H E R MAPS shows
how fronts and the genera l weather pattern move a n d c h a n g e . Compare Tuesday's m a p b e l o w with Monday's on the preced i n g · page. The wave center has now moved to south centra l Nebraska, with the fronts exte n d i n g out in much the same fa shion as on Monday. But the warm sector of the wave (south of the wave center, or crest, con necting the two fronts) has become narrower. The a rea of precipitation ahead of the warm front sector has be come much wider a n d, of cou rse, the entire pattern has moved. Ca ref u l measurement, using the map sca le at the bottom, shows that the crest of the wave has moved about 300 m i les eastward. Wednesday's m a p at the top of the next page shows that the low center has moved to northwestern I l l inois. The cold a ir mass has overta ken the warm air m ass a n d formed a n occ luded front ( p p . 92-93) t h a t extends from the low center to the southern tip of I l l inois. Precipitation is much the same as before, except that stea dy ra i n now extends over the top of the occ l usion.
500 mi les Tuesday
1 36
I
Wed nesday
MAKE Y O U R FORECAST by placing a b l a n k ma p
over the Monday, Tuesday, and Wedn esday m a ps on the previous pages a n d trac ing the fronta l positions from them. Next d raw a rrows con necting the point of the wave for each day. Notice that the arrow connecti n g Tuesday and Wednesday points more northward than the fi rst a r row con necting Monday a n d Tuesday positions.
Forecast for T h u rsday
To get the Thursday position of the tip of the occ l u sion, draw a dashed arrow pointing s l ightly more to the north but about the sa m e length as the Tuesday-to-Wednesday arrow. The dashed arrow is to remind you that this is Wednesday and that you are making a forecast for Thurs day. Now m a ke arrows for the cold a n d warm fronts i n the same w a y (refer t o map at botto m o f p. 1 37). Make sure the arrows for the fronts are perpe n d i c u l a r to the fronts. Next make broken forecast arrows for the fronts and draw your weather map for Thursday-on Wedn es day. Be s u re to draw in the preci pitation belt the sa me as it was a long the previous day's fronts. The path of the point or crest of the wave on fronts (as on preceding m a ps) fol lows what is c a l l e d a storm track. This is the path of bad weather. A knowledge of storm tracks w i l l help you im prove you r forecasts. By fitting the path of the low crest from your forecast map to the storm track map below, a prediction for a period longer than 24 hours can be made. I f you systematica l ly fol low the weather m a ps and ob serve your l oca l weather a rea, you s h o u l d be able to make exce llent short-ra nge forecasts after a year o r two of experience. A tra ined meteoro log ist, knowing the prin ciples of loca l i n fl uences on weather, can make good loca l area foreca sts with i n a week or so in a new area.
Storm tracks. The a r rows show the paths which low cells gener ally follow over the U n ited States. The tracks are more wide· ly sepa rated in winter than in summer.
1 38
./l
-
upper air w i nd flow upper a i r contour s urface fronts forecast of s urface front
U P P E R A I R PATTERNS AFF ECT F O RECASTS Al though storms usua l ly fol low the storm tracks shown at the bottom of p. 1 38, they are sometimes th rown off by pat terns of a i r flow in the upper air. The picture a bove shows how this ha ppens. The green l i nes (contou rs) show the pressure pattern and the b l ack arrows show wind d i rection of the upper air. N ote the position of the u p pe r a i r "closed low" cel l-a c o m p l ete l ow cel l with the usual cou nter c l ockwise a i r rotation. It is h i g h a bove the ground and is northwest of the surface low which l ies at the crest of the wave formed at the fronta l l i nes-convention a l ly shown i n b l ue a n d r e d . Were it n ot f o r the c losed low cel l i n the a i r f a r a bove the earth, the surface low would proba b l y fol low the storm track marked A i n the map at the botto m of p. 1 38. But the low- p ressure center in the upper a i r "steers" the surface low north, resu lting i n the forecast pa1tern shown i n purple, w h ic h represents the warm sector.
1 39
L
upper air
� �
contour
s u rface fronts
forecast of s u rface low
T H E MAP ot the top of this poge shows h ow d ifferently
o surface low will move if the upper oir does not conta i n o c losed l ow-pressure ce l l n e o r it. The surface situation is identica l with thot shown o n t h e m o p on p . 1 39. B u t there is no c l osed low c e l l i n t h e upper o i r pressure pattern shown by the g reen contours ond block a rrows. The oir just a bove the s u rface low (ma rked i n red o t the tip of the fronta l crest) is sweeping stro n g l y to the northeast (wi n d i n u pper o i r fol lows con tou rs-see p. 65). So the upper o i r will steer the surface low directly ond rapidly to the northeast, resulting i n the forecast pattern shown i n purple. The low has moved a d ista nce of 900 m i les as compared to a movement of about 500 m i les when the closed cell (p. 1 39) was present. U pper air patterns are extremely i m porta nt to accurate forecasti ng by the continu ity method, but are not printed in newspapers. They o re printed daily i n the U.S. Weather Bureau mops (see p . 1 4 1 ) a n d you should study them for most occurote forecasti ng. 1 40
CO NSTANT P R ESS U R E MAPS of the u p per air have conto u r lines showing a ltitude, not p ressure. Solid l i nes show the h eight a bove sea leve l at which the a i r pres s u re is 500 m i l l ibars. The standard heig h t of the 500m i l l ibar level is 1 8,280 feet-but it can vary from 1 5,800 to 1 9,400 on a s i n g l e chart. Note o n this map that these heig hts a re l owest i n the north and highest i n the south . Two upper a ir l ows exist in the far north a n d n orthwest; a h ig h l ies over the Southwest. Dashed lines show centigrade temperature. Arrows show the wind.
1 41
MAK I N G Y O U R OWN FOR ECASTS FROM OBSER VAT I O N S Pages 1 1 2- 1 4 1 exp l a i n how professional forecasters do their work and how you can do a reason able job using good surface weather maps and upper a i r pressure charts-after a yea r or so of experience. It is possible to make fa i r-to-middling forecasts based on "weather signs," barometer, a n d weather-va ne rea d i ngs. If you use these things p l us weather m a ps a n d a carefu l reading of the preceding sections of the book, you should be a b l e to forecast your loca l weather with accuracy. However, it is the cooperative reports from h u n dreds of stations that make modern weather forecasts possib le. Most weather lore comes from farmers, h u nters, and sa i l o rs-people m ost concerned with the weather. Many weather sayings have some truth i n them-for wisdom accu m u lated a s peo p l e gained experience, even before the reasons behind the facts were d iscovered. The say ing "A ring arou n d the moon mea n s ra i n " is true 40 to 80 per cent of the time, depen ding on location, how the pressure is changing, the direction the clouds a re moving, etc. If you use this rule modify it to: "If pressure is fa l l ing rapid ly, a ring around the moon means rain i n 1 8 to 48 hours, a bout 75 per cent of the time." . . A r i n g a r o u n d t h e m o o n m e a n s rain"-sometimes!
WEAT H E R S I G N S have some prediction value if you
know the atmospheric conditions they i n d icate. Here a re the background facts for u ndersta n d i n g what some weather signs mea n . If you can expl ai n these general rules-of-t h u m b in terms of more scientifi c m eteorology, they wi l l be of use to you. It is interestin g to col lect loca l weather signs and lore. If you can ta l k with "ol d-timers" long resident in your reg ion, see what they have to say. Then exa m i n e these bel iefs i n the light of present-day meteor�logy. Weather will g e n e ra l ly r e m a i n fair when:
The wind blows gently from west o r northwest (p. 66). Barometer remains steady or rises (pp. 85-88). C u m u l us clouds dot the summer s k y i n the afternoon (p. 20). Morning fog b r e a k s or "bu rns off" by noon (evidence of clear s k y a bove) . Rainy weather or snow may come when:
Cirrus clouds thicken a n d are fol lowed by lower clouds (p. 88). (Par ticularly true if ba rometer is d ropping.) There i s a ring around the moon (pp. 17 & 88). ( P a rticularly true if barometer is d roppi ng.) Puffy cumulus clouds begin to d evelop vertica lly (p. 20). Sky is dark o n d th reatening to the west (p. 85). Southerly wind increases i n speed with clouds moving from west (p. 85). The wind-particu larly a north wind-shifts in a counterclockwise d irection-that is, from north to west to south (pp. 85-88). The barometer falls stead i ly (pp. 85-88). Weather w i l l generally clear when:
Bases of clouds show steady rise to higher types (p. 85). The wind-particu l a rly a n east wind-shifts to the west (p. 85). The ba rometer rises rapidly (pp. 85 and 88). Temperature w i l l u s u a l ly f a l l w h e n :
Wind blows from-or shifts to- north o r northwest (p. 85).
Night sky is clear a n d wind is light (pp. 9 a n d 1 4). The barometer rises steadily in winter (p. 85). Temperature will usually rise when:
Wind is from south, particula rly with cloud cover at night or clear sky d u ri n g the day (pp. 9 a n d 88).
1 43
A dependa ble, inex pensive aneroid b a r ometer for home use.
Aneroid barometer com· bined with thermometer and h umidity ind icator.
A GOOD BAROMET E R AND T H I S TABLE (p. 1 45) wil l h e l p you make reasonably good forecasts. The tab l e is adapted from a U . S . Weather Bu rea u ta ble based on average conditions over the Un ited States. Use the table in connection with good daily weather m a ps, a n d learn how con ditions in your loca l ity tend to vary (beca use of nearby mountains, la kes, ocea n, or deserts, for exa m ple). A rapid rise or fa l l in barometric pressure (see second column i n ta ble) is 0.05 to 0.09 inches or more i n 3 hours. A slow rise or fa l l is less than th is. Barometric pressu res in the ta b le are i n inches, a d justed to sea l evel . Calibrate your ba rometer to sea- l evel pressu res by checking with you r local weather station or airport. Every a neroid barometer has a device by which the pointer ca n be re-set. Once set to conform to a sea- level reading of an accurate barometer i n your loca l ity, it should req u ire no further read ju stment. Be sure to keep records of your forecasts together with records of actu a l weather changes. Only i n this way c a n y o u l e a r n h o w loca l conditions affect the a ntic ipated weather.
1 44
Barometric Wind Direction Pressure SW to N W SW to NW SW to NW SW to NW
S to SE S to SE
SE to N E SE to N E SE to N E S E to N E
30. 1 0 to 30.20ba rometer steady 30. 1 0 to 30.20risi n g rapidly 30.20 o r above ba rometer steady 30.20 or a bove fa l l i n g slowly
Fair, with l ittle temperature change for 1 to 2 days Fair, with warmer weather a n d rain within 2 days Remaining fair with l ittle tempera ture change Fair and slowly rising temperatures for about 2 days
30. 1 0 to 30.20fa l l i n g slowly 30. 1 0 to 30.20fa l l i n g rapidly
Rain with i n 24 hours
30. 1 0 to 30.20fa l l i n g slowly 30 1 0 to 30.20fal l i n g rapidly 30.00 o r below fal l i n g slowly 30.00 o r below fa l l i n g rapidly
E to N E
30. 1 0 o r above fa l l i n g slowly
E to N E
30. 1 0 o r above fal l i n g rapid ly
S to SW
30.00 or below rising slowly 29.80 or below fal l i n g rapidly
S to E
E to N
29.80 or below fa lling rapidly
Swinging to
29.80 o r below rising rapidly
w
General Forecast
Rain within 1 2 to 24 hours. Wind will rise Rain within 1 2 to 1 8 h o u rs. Wind will rise Rain within 1 2 h o u rs. Wind will rise Rai n will conti n u e 1 or more days Rain with high winds i n few h o u rs. C learing with i n 36 hou rs-colder i n wi nter S u m mer, with lig h t winds: rain in 2 to 4 days. Winter, rai n or snow within 24 h o u rs Summer: proba b l e rain in 1 2 to 24 hours. Winter: rain or snow with i n 1 2 hours C learing within a few hours. Then fair for several days Seve re storm within few hou rs. Then clearing withi n 24 hou rs-colder i n winter Severe storm (a "nor'easter" gale) i n few h o u rs. Heavy rains or snowstorm. Fol lowed by cold wave in winter E n d of storm-clearing and co lder
1 45
MAC H I N E FOR ECAST I N G as accurate as h u m a n forecasting, or m o r e s o , is n o w b e i n g d o n e by e lectron ic comp uters. The U.S. Joint N u merical Weather Prediction U n it began operation of the fi rst "electro n ic brain" for routine weather forecasting in 1 955. Mac h i n e forecasting is possible because movements of the atmosphere fo l l ow natura l laws which can be ex pressed i n mathematical equations. These eq uations are changed to coded instructions for the machine. N umerica l fi g u res, based on weather data , a re fed i nto the machine. The electronic "bra i n " works out the predictions and prints forecast maps showing hig hs, lows, and other data. Mac h i n e forecasting does not replace the m eteorolog ist. Machines m ust b e fed accurate data, and machine- pro d uced m a ps must be i nterpreted and modified i n term s of loca l conditions. But machine forecastin g w i l l save thou sands of hours of routine work. COMPUTER 36-HOUR FORECAST CHART Machine shades a reas between contou r lines and pri nts conto u r heig hts on grid points.
Weather a n d C l i m ate C l imate is the weather at a g iven place over a period of time. It involves averages, tota ls, and extremes to set a picture of the weathe r pattern. C l imate is a ffected by the same physical conditions that affect weather-latitude, preva i l i n g w i n ds, ocea n currents, mou nta i n s, nearness to the sea, a n d the l ike. C l i m ate might be c a l led the gen era l ized weather of a n area. Because c l im ate, like weather, is made u p of many fac tors, it is i m possible to c lassify c l imates s i m p l y without ignoring some of them. Two p l aces with the same ra nge of temperature may have very d ifferent a m o u nts of rain fa l l . Hence these two factors are more usefu l than either one a lone. So c l i m a tes are often c lassified by a combina tion of temperature (torrid, tem perate, frigid) a n d rain fa l l (wet, h u m id, subhum id, semi-arid, a rid). The most commonly used system stresses rainfa l l for the torrid a n d temperate zones, temperatu res f o r the c o l d e r zones. This system (see m ap below) is usef u l beca use it corresponds to the natura l vegetation of regions .
•
•
tl.
'
.• h u m id (forest) s u b h u m i d (grassla n d sem iarid (steppe) a rid (desert}
•
•
CLIMAT I C DATA are i m porta n t in agricu lture a n d in
d ustry. Farmers n eed to know the length of growing sea sons, extremes of temperatu re, average and m i n i m u m ra infa l l f o r successful growing o f crops. The sale o f a ir conditio ners, sports c l othes, a n d winter wear depends on seasona l tem perature a n d rainfa l l . Ma ny perso n a l deci sions involving pa inting, ga rden i ng, sports, trips, a n d vacations can be better made i f c l imatic d a t a are taken i nto account. The U.S. Weather Bureau furn ishes such information. The c l i m ates of the Un ited States vary considerably because of the size of the country a n d its varied terra in. U.S. Weather Bureat� figures, recorded since 1 899, g ive a ful l picture of the tem perature, h umidity, preci pitation, sunshi ne, frosts, storms, and other c l imatic factors. These data are presented o n maps for easy reference. The most i m porta nt of such data appear on the fo l l owing m a ps. AV ERAG E TEMP ERAT U R E is the most commonly used
c l i mate statistic. It is of l i m ited use and is of more va l u e w h e n used with t h e next two ma ps.
60 ° to 70 ° F a bove 70 ° F
•
0 ° to 20 ° F o bove 20°F
AVERAG E LOWEST TEMPERAT U R E is more i m porta nt
to peo ple like farmers a n d fruit g rowers than average a n n ua l tem peratures. Ma p a bove shows th is. AVERAG E TEMPERAT U R E OF HOT MONTHS h e l ps
in deciding where to l ive, in what kind of house, what pla nts to g row, or if air conditioning is needed.
AV E R A G E A N N U A L P R E C I P I TAT I O N I N I N C H E S
20-30 • 30-50 • 50-70 • 70-1 00
AVERAG E PREC I P ITATION-mainly rain and snow-is importa nt to determine types of c ro ps that can be grown. LACK OF RAIN FALL ruins fa rmers. Twenty inches is min i m u m for m ost crops. PER C E N T O F YEARS WITH LESS T H A N 2 0 I N C H E S O F P R E C I P I TAT I O N
1 0-20% 0-1 0 %
AV E R A G E N U M B E R O F H O U R S O F S U N S H I N E P E R D AY I N D E C E M B E R , J A N U A R Y, A N D F E B R U A R Y
5-6
• 6-7 • over 7
W I N T E R S U N S H I N E makes F lorida, California, a n d the
Southwest famous. It is i mportan t, too, for agricu lture. NUMBER O F C L EAR DAYS is different from average daily sunshine. Note southern F l orida and Califo r n ia.
H U M I DITY spe l l s comfo rt or discomfort. Summer humidity is high i n coasta l a reas.
R E LATIVE
NUMBER O F DAYS WITH SNOW ON G RO U N D is of interest to skiers and fa rmers. Snow means spring water. AV E R A G E A N N U A L N U M B E R OF DAYS W I T H S N O W C O V E R ( 1 INCH OR MORE)
80- 1 20 D over 1 20
OF T H E L A S T K I L L I N G F R O S T
W H E N CAN YOU SAF ELY PLANT Y O U R G A R D E N ?
Ma p shows dates of l ast killing frosts i n spring. T H E G R O W I N G SEASON determines what pla nts can
be grown a n d the n u mber of crops per season . AV E R A G E L E N G T H O F T H E F R O ST-F R E E P E R I O D I N D AY S
1 80-240 240-300 over 300
1 53
Geog ra ph ica l, biological , a n d m a n m a d e factors ofte n m a k e loca l c l i m atic conditions d if ferent from the genera l c l imate. A loca l c l i matic pattern is ca l l ed a m icroc l i m ate. large i n l a n d la kes moderate tem perature extre mes. The resu lt is c l i m atic differences be tween the w i ndward and lee sides-for exa m ple, between Milwa u kee, o n w i n dward side of lake M i chi gan, a n d G ra n d H a v e n , o n the lee side, on ly 85 m i les east (see fi rst t a b le b e l ow). Pla nts create m icroc l imatic d ifferences c h iefly by their use of water and by their effect on winds (see second ta b l e below). W i n d b reaks a r e g rown t o m a ke a favora b l e M I C R O C L I MATES
M I C R O C L I M AT I C D I F F E R E N C E S BETWEEN LAKES I D E C I T I ES
C l i matic condition
Period of year
Av. te mp.
J a n uary Aug ust
A v . temp.
Milwau kee windward shore
Grand Haven leeward shore 3.6 ° F h i g h e r
2.0 ° F h ig h e r
Precipitation
Dec.-Feb.
Precipitation
J u ne-Aug.
Snowfa l l
J a n .-Dec.
44 days more
Rei. h u midity
Dec.-Feb.
5 % more
Dec.-Feb .
1 .4 m p h more
Wind speed
2.09 in. more 0.55 i n . more
BETWEEN FORESTS A N D O P E N LAND
Fo rests
Open land
Wa rmer i n winter
Warmer in summer
Wind speeds reduced
Wind s p e e d s h i g h e r
Relative h u mid ity h i g h e r
Relative h u m idity lower
Wate r storage h i g h
Water storage low
m icroc l imate on fa rms. The p lacement of a n ew dwe l l i n g i n regard to frost drainage, loca l breezes, a n d util izati o n o f winter s u n l ig h t may make great difference i n liva b i l ity, and i n fuel economy during w inter. Heavy, c o l d air fl ows down h i l l , form i n g cold pockets in va l l eys. F rost is m u c h more common th ere; so ora n g es, g ra pes, a n d a p p les are pla nted on h i l lsides to i n s u re "frost drainage" when cold spe l l s come. The fi rst ta b l e below shows m ic roc l i matic d ifferences between va l leys and h i l ls. Cities i n d uce convection curre nts which ca use higher c louds a n d somewhat more ra i n than i n the nea rby coun try. Because of sewer drainage in cities, less wate r sta n d s a n d evaporates. Thus relative h u m idity is lower i n cities. The secon d chart below shows other d ifferen ces.
M I C R O C L I M AT I C D I F F E R E N C E S B E T W E E N VA L L E Y S A N D H I L L S
C l imatic condition M i n i m u m temp. Temp. range
Period
Val leys
daily
much lower
higher
daily & a n n u a l
l a rg e r
smaller
Hills
F rost
n ig h t
more
less
Wind speed
n ig h t
lower
higher
morning
more
less
Fog
BETWEEN CITY A N D C O U NTRYS I D E
C ity Haze and smog
C o u ntrysid e C lear
Temperature h i g h e r
Temperatu re lower
Wind speed and radiation less
Wind speed and radiation higher
Recent newspaper and magazine a rticles have claimed that our c l imate is warming up. Perhaps this is true-per haps n ot. Throug h geo logica l h istory the normal c l i m ate of the earth was so warm that subtropical weather reached to 60 ° N and S and there was a tota l absence of polar ice. It is only during less than 1 per cent of the earth's h is tory that g laciers have reached down from the polar a reas to what is now the temperate zone of the n o rthern hemisphere. The latest such advance, which started about 1 ,000,000 years ago, was ma rked by geo logica l u pheava l a n d the beginn ing of man . During this time vast ice sheets a dvan ced and retreated over the conti n ents, the last re treat occurring 30,000 to 40,000 years ago. With in recorded h istory there have been minor g lacia l advances a n d retreats. Al pine passes now covered with ice were used from A.D. 600 to 700. S h i ps sailed from Leif Ericson 's Green land throug h routes now b l ocked by ice floes. From A.D. 1 000 to 1 200 there were flourishing settlements i n Gree n l a n d . Gradual ly, coo l i n g of the c l i mate forced their abandonment about t h e yea r 1 400. I s the earth rea l ly wa rming? A 2 ° F increase i n the earth's overa l l tem perature wou l d clear the polar seas of a l l ice. Yet today i n pa rts of wester n N orth America g laciers a re perceptibly advancing w h i l e the east coast seems to be warm i n g . We h a v e routin e tem perature measurements f o r o n l y 1 00 years-and these n ot t o o re liable, because of c h a nges i n i nstrum ents a n d methods of observatio n . The o n ly concl usion t o be d rawn a bout o u r c l i mate i s t h a t we do not k n ow whether i t is c h a n g i ng d rastica l ly. Geo log ica l ly we may be at the end of the Ice Age, o r we m a y just be having a breath ing spe l l o f a few cen turies before the next advance of the g laciers. IS O U R C L I MATE C H A N G I NG?
1 56
B O O K S F O R M O R E I N F O R M AT I O N
The following books w i l l further your knowledge a n d en joyment of weather. The U.S. Weather B u reau, Washing ton 25, D.C., pu b l ishes monogra phs a n d periodica ls on a l l phases of m eteoro logy. Write for a free l ist of gov ernment weather publ ications to the Su perintendent of Documents, Washington 25, D.C. Byers, H o ra ce Robert, G E N E R A L M E T E O R O L O G Y, McG raw- H i l l B o o k Co., New York, 1 959. An introd uctory col l ege text cove ring basic prin ciples. C ritchfield, Howard J., G E N E R A L C L I M A T O L O GY, Prentice H a l l, N ew Jer sey, 1 960. A basic text coveri n g regional, physical a n d applied cli matology. Ham bridge, G rove, C L I MATE AND MAN-A S U M M A R Y . This s u m m a ry is a n overview of the a rticles i n the 1 94 1 YEAR B O O K O F AG R I C U LT U R E on meteorology a n d climatology. Haynes, B. C . , T EC H N 1 9 U E S O F OBSERVI N G T H E WEAT H E R, John Wiley & Sons, New York, 1 947. This eleme ntary book is well su ited fo r those who wish to ta ke up weather as a hobby. O n e chapter sh ows how to imp rovise a weather station . Longstret h, T . M . , U N D ERSTA N D I N G T H E W EAT H ER, T h e Macm i l l a n Co., New Yo rk, 1 953. A simple, interesti ng, non-tech nical acco u n t of the weather. Sloane, Eric, E R I C S L O A N E ' s WEAT H E R B o o K , Duell , New Yo rk, 1 95 2 . I nteresti ng weather i l l ustrati ons a n d cartoons acco m p a n y a rea d a b l e account of weather phenomena. U.
S. Weather B u reau, MA N U A L O F C L O U D F a R M S A N D C o D E S F O R STATES C l o u d photog raphs i l l u strate the Cloud Code specifica tions. Order from Su perintendent of Documents. O F THE SKY.
PERIODICALS
American Meteo rological Society, 3 Joy Street, Boston B , Mass. Non-techn ical, bi-monthly weather magazine conta i n i n g su rveys a n d many " h ow t o " a rticles. February i s s u e e a c h year i s a n almanac of previous year's weather.
WEAT H E R W I SE,
M O N T H LY WEAT H E R R E S U M E AND OUTLOOK, U . S. Weather Bureau (order from Superintendent of Documents). Graphic presen tation of preceding month's weather a n d esti mate of expected rain fal l a n d temperatures for next 30 days.
AVERAGE
1 57
I NDEX Aste r i s k s ( * ) ind icate i l l u strations. Adiabatic c h a n g e , 1 4, 1 5, 39 A i r , composition, 1 2 , 3 6 see a lso.Atmosphere Air c o n d i t i o n i n g , 3 8 , 1 49 A i r masses, 68-76 c l a s s i fi e d , 69 conti n e n t a l v s .
...
;::
0
o-
"'
m a r i t i m e , 70 North America n , *69, 72-76 s u m m e r , 74-75 w i n ter, 72 -73 A l timeter, 1 20, 1 3 1 :;; A l tocu m u l u s , 1 6, * 1 8 ! A l tostratus, 1 6, * 1 8, * 8 3 , *88 u Anemometer, * 1 1 8 , * 1 24 o Antarctic, seasons, 49 Anticyc l o n e : see H i g h -
5
� �
pressure areas . season s, 49 Arctic, Atmosphere: appearance, *34 circu l a t i o n , 1 0, 56-58 d e n s ity, 35 ! heat storage, B moderate tempera ture, 9 shield against meteors, 57 weight, 3 5 iii � see a lso A i r a * '
� ,.... � ; j i -o
�
•
; ��;�� �
t:
B a l loons, weather, 1 1 2, 1 1 8 , * 1 26, * 1 27, 1 28 Barograph, * 1 20 Barometer: aneroid, * 1 1 9, * 1 20, * 1 44 as a l t i meter, 1 20 h o m e u s e , * 1 44 merc u r i a l , * 1 1 9 , * 1 20 Beaufort w i n d s c a l e , 1 25
C a r b o n d i oxide, 3 6 C e i l i n g , 1 26 C h inook, 4 1
1 58
C i rcu l a t i o n , a t m o s pheric, 1 0, * 56 - * 58 C i rrocu m u l u s , * 1 7, 88 C i r rostratus, * 1 7 C i rrus, * 1 7 C l e a r days, * 1 5 1 C l i m a te, 1 47- 1 55 loca l , 1 54- 1 55 m a i n t y p e s , 1 47 U . S . , * 1 47 C l ou d droplets, *22 C l o u d i n e s s , average, 7 C l ou d seed ing, 3 1 -33 Clouds: c l a ssified, 1 6-20 h igh, 1 7 how formed, 1 3 - 1 5 l ow, 1 9 m id d l e , 1 8 names, 1 6 o f vertical devel opm e n t , 20 rain, 1 9 reflect s u n l i ght, 7 scud, 1 9 s y m b o l s , 20 types: clue to ce i l i n g , 1 26 w i t h fronts, *83, *86 C o a l e scence, 22-*23 i n c l o u d seed i n g , 3 1 , 32 C o l d front, 82-83 weather seq u e n ce, 82-83, 85 Cold waves, 57 Coldest region, 62 Condensation: effect on tempera lure, 1 3 , 40-41 i n h u rricanes, 1 07 Con stant pressure mops, * 1 4 1 Continental Polar air, 72, 74 Convection, 1 0 - 1 1 Corona, with a lto cumulus, 1 8 C o s m i c rays, 45 C u m u l o n i m b u s , *20, *23, *96, 98 -99
C u m u l o n i m b u s (cont.) a n v i l top, 98 form s l o w pressure a r e a , 67 C u m u l u s , * 1 6, 20, 7 1 can become t h u n derh e a d , *98 C u rrents, vertica l , 7 1 Cyclones: extratropi c a l , 90,91 i n I nd i a n Oce a n , 1 05 in Midwest: see Tornadoes tropica l , 1 06 see a lso Low pressure areas D a i l y Weath e r Map, *131 Day a n d n i g h t : length,
so Desert, 67, 1 47 Dew, *28 Dew point, 1 2 D o l d r u m s , 58-59 Drizzle, 1 9 Dropsonde, 1 28 Dust d e v i l s , 67
E layer, 45 E a rth : a x i s , 49 motion in space, 48 rotation affects w i nds, 53, 65 radiates heat, 1 4 E lectric charge, of c l o u d s , 99 Electronic computers, 1 46 Equator: rotation at, 53 heat, 1 0 Equator i a l front, 79 E q u i no x , *49 Evaporation, 30 Exosphere, 37, 46 E x p a n s i o n of a i r , 38 F layer, 45 F a l l , s e a s o n , 49 Fog, 25, 74 F o l k l ore, 1 42 - 1 43
Forecast i n g : con t i n u i ty method,
1 35- 1 38 from observ a t i o n s ,
1 42 - 1 45 from a m a p, 1 30 , 1 33 , 1 34, 1 40 persistence method, 1 34 - 1 3 5 t a b l e , 1 45 Forecasts: b y m a c h i n e , 1 46 m o n t h l y , 1 56 newspaper, 1 1 4 Forests, effect of, 7, 1 54 Fractocu m u l u s, 1 6 Fractostratus, 1 6, 1 9 F r i g i d Zone, 1 47 Fronts, 77-95 c h a racteristics, 78 c o l d : see C o l d fronts d e fi n e d , 77 m a p s y m b o l s , 94 occ l uded, *92 - *94 on charts, 1 30 P o l a r , 57, 79 stationary, 89 w a r m : see Warm fronts weak, 89 Frost, 28 in U . S . , * 1 53 i n v a l l ey s , 1 55 G l a z e , 25, 27 Great Lakes, effect, 72 Ground water, 30 Growing season, 1 53 H a i l , *26 H a los, * 1 7, *25 Heat: a b sorbed by e a r t h ,
11 c a u s e of weather, 5 of compre s s i o n , 38 of con d e n s a t i o n , 4 1 trapped b y atmosphere, 8 H i g h - pressure a r e a s : formation, 60-62 how located, 66 permanent, 62 w i n d v e l oc i t i e s , 65
H o r s e l a t i tudes, 59 H u m idity : a n d c l i mate, 1 47 water vapor, 1 2 see also Relative h u m i d ity H u rricanes, * 1 04 - 1 1 1 eye, * 1 04, * l OB forma t i o n , 1 07 sources, paths, 58,
1 05, 1 09 warn i n g system , 1 1 1 West- I nd i a n , 1 05 - 1 06 w i n d speeds, 1 04 Hygrograph, * 1 22 Ice: crysta l structure, 32 i n c l ou d s , 1 7, 22-*23 n e e d l e s , *25 p e l l ets, 23, *25 prisms, *25 I n struments: see Weath e r I n tertro p i c a l convergence zone, *39, 1 06 I onosphere, 37, *45 I rr i g a t i o n , a n d sn ow, 3 0 I sobars, 65, 1 3 0 Jet stre a m , *42- *43 Ken n e l l y - H e a v i s i d e l a y e r , 45 Knot, 1 25 l ig h t n i n g , 99- 1 00 Low-pressure areas formation, 60, 67 h o w l ocated, 66 paths over U . S . , 1 38 with fronts, *90-91 Mackere l s k y : see C i rrocu m u l u s Magnetism, of earth, 46 Mares' t a i l s , * 1 7 Maritime Polar air, 72 -75 Maritime Tropica l Air,
73, 75 Meteorogr a p h , * 1 24 Meteor o l o g i sts: private, 1 1 6 - 1 1 7 v s . mach i nes, 1 46 Meteor o l o g y : 4, 1 1 6, 118
Meteors, 47 M i c r oc l i m ates, 1 54 - 1 55 M o l e c u l e s , in a i r , 35 Moon, 5 , 9 Mot h e r - of- p e a r l c l o u d s ,
*46 M o u n t a i n s , effect of,
1 1 , 67, 72 N i g h t, l e n g t h , 50 N i m bostratus, 1 9 N i trogen, i n a i r , 3 6 Noct i l u ce n t c l o u d s , *47 North P o l e , seasons,
49, 50 N u c l e i : 2 3 , 24 a r t i fic i a l , 3 1 -33 Oce a n s , t e m p e r a t u r e of, 70 O v e r c a s t , e ff e c t of, 7, 9 O x y g e n , i n air, 36 P i lot b a l loons, 1 1 2 , 1 27 P o l a r air masses, 76 P o l a r easte r l i e s , 59 P o l a r front, 57, 79 P o l a r r e g i o n s : c l i mate, 7 Precipitation: average, U . S . , 1 50 c o n d i t i o n s for, 2 1 -23 d a i l y m a p , * 1 32 m e a surement, 1 23 m i n i m u m for f a r m i n g , * 1 50 Pressure, atmospheric: a t c o l d front, 85 a t sea leve l , 64 m e a s u re m e n t , 1 1 9-
1 20 v s . height, 1 1 9 Preva i l i n g weste r l ies,
56, 59 Proverbs, BB Psych rometer, * 1 22 R a d a r, * 1 29 detects h i g h - level w i n d s , 1 27 p i c k s up s q u a l l l i ne s ,
84 u s e t o p i lots, 1 1 1 , 1 29 Radiation, solar, 6 R a d i o s o n d e , 1 1 3 , * 1 1 8,
* 1 28
1 59
"'
Rain, 22, 23, 8 9 clouds, 1 9 gauge, * 1 1 8 , * 1 23 with cold front, 83 see also: Precipitation Rainfa l l : how measured, 1 23 statistics, * 1 50 R a i n m a k i n g , 3 1 -33 Relative h um i d ity, 1 2, 1 22 , 1 55 U . S . summer aver· age, * 1 52 Ring aroun d m o o n , 1 42
Superco o l i n g , 27 Supersaturation, 2 1 Symbo l s : c l oud, 20 front, 94 weather, 1 3 1 wind, 1 25 Synoptic maps, 1 3 0
Water: i n t h e atmosphere, 1 2 on earth, 1 2 u ndergroun d , 30 Water cycle, 1 3, 29-30 Water vapor: a m o u n t in air , 3 6 a n d h u m idity, 1 2 a n d temperature, 1 2 Teletype report, * 1 1 5 Waterspout, * 1 03 Temperate zone, 1 47 Weather, 5 Te mperature: forecasting, 1 1 2.1 1 7 c h a nges, a n d i n struments, 1 1 8- 1 29 weather, 5 lore, 1 42 - 1 43 c l i mate factor, 1 47 s i g n s, 1 43 d a i l y cycle, 52 Safety, from l i g h t n i n g , s y m b o l s , * 1 33 d a i l y map, * 1 32 1 00 Weather m aps, 1 3 0-* 1 53 maps, * 1 48 - * 1 49 Saturation, of a i r , 1 2 Daily, * 1 3 1 measurement, 1 2 1 d a i l y precipitation, z Schaefer, Vincent J . , 32 Theod o l i te, * 1 27 u Scud c louds, 1 9 * 1 32 Thermograph, * 1 2 1 !! Seasons, * 48 , * 49, 52 d a i l y temperatures, Thermometers, * 1 2 1 • Sleet: see I ce p e l lets * 1 32 Thunderhead: for p i l ots, *95 0 Snow: see C u m u l o n i m b u s M frequency of issue, 1 1 8 conditions for, 2 1 , 24 Thunderstorms, 96- 1 00 z i n newspapers, 1 30, crysta ls, *24 areas of occurrence, !! * 1 33 dete r m i n e s water 97 !! su rface, 1 30 - * 1 3 1 s u p p l y , 3 1 , 1 52 deve lopment of, 99 u pper a i r , * 1 39 - * 1 4 1 in U . S . , * 1 52 in unstable warm air, � Wil ly-Wi l l ies, 1 05 pel lets, * 2 5 87 Win ds: reflects s u n l ight, 7 � judging th e i r disa loft, 1 27 water content, 30 lance, 97 • N a n d pressute, 65 see also: Prec i p i tation Tornadoes, 1 0 1 - * 1 03 loca l, 1 1 Solstice, *49 Torrid zone, 1 47 c South Pole : seasons, 49 Wind di rect ion : Trade winds, 56, 59 Southern hemisphere, 58 and earth ' s rotati on, Tropical a i r masses, 76 Spring, 49 54-55 Tropopause, *42 floods, 30 i n h igh pressure Troposphere, 37-39 areas, 64 Squa l l l i nes, 84 Typhoons, 58, 1 05 Storm track, * 1 38 ind icators, * 1 24 U . S . Weather Bu rea u : Storms, 96- 1 1 1 sh ifts a s front passes, c l i m a te data, 1 48 Stratoc u m u lus, * 1 9 78, 85, 1 24 forecast accuracy, 1 1 3 transm itter, * 1 1 8 Stratosphere, 37, 44 m a p s ava i l a b le, 1 30 sign ificance, 1 24 Stratus, * 1 6, * 1 9 observation stations, Wind speed, 1 24 Summer: 1 1 2- 1 1 3 Beaufort sca le, 1 25 a i r masses, 74 services, 1 1 4, 1 1 7 Wind vane, * 1 24 fronts, 8 1 relative h u m i d ity, Winter: U pper air charts, 1 1 8, a i r masses, 72-73 * 1 52 * 1 39-* 1 4 1 warmth, 50- 5 1 fronts, 80 Vertica l currents, 2 0 reason for c o l d , 50-51 Sun, 6, 50 Vonnegut, Bernard, 3 2 s u n s h i n e overages, Sunsh i n e : * 1 51 Warm fronts, *86-*87, d u ration : transmitWorld Meteoro l o g i c a l 90-9 1 ter, * 1 1 8, * 1 27 Organ i zati o n , 1 1 3 weather sequence, 88 winter a v e r a g e , * 1 5 1
...
t
j
... .... .. .... ..; ;;:
1 60
. �
' •
I
J
' L
WEATHER A G O L D E N N AT U R E G U I D E
PA U L E . L E H R i s a meteoro l o g i st w i t h t h e s c i e n t i fi c S e rv i ce s D i v i s i o n of U . S . A i r W e a t h e r Se r v i c e . D u r i n g W o r l d W a r I I h e s e r v e d a s meteo r o l o g i st i n t h e southwest P a c i fi c . H e h a s t a u g h t meteo r o l og y , a n d f o r s i x y e a r s w a s a s e n i o r m e te o r o l o g ist of t h e U .
S.
A i r Force
Weather Centra l .
R . W I L L B U R N ETT, P h . D . , Professor o f Sc ience Ed ucation, U n iversity of I l l inois, has served as spec ial consultant for the War D e partment and the Civil Aero n a u tics Authority, and is author of m a n y scientific texts. H E RBERT S . ZIM, P h . D . , formerly P rofessor of Education, U n iversity of I l l i n ois, is an out sta n d i n g a uthority on science education , a n d co-a uthor of a l l t h e G o l d e n Nature G u i des. (See back cover for other titles.) HARRY McNAUGHT atte nded the P h i ladel phia Museum School of Art. H e is a fi rst- prize winner of the Society of I l l ustrators, and his work has become we l l k n own i n scientific jour n a l s a n d juvenile books, i n c l u d i n g the Golden Book of Science.
·.
T H E G O L D E N N AT U R E G U I D E S are an introduction to the world o f nature, a g u i d e to t h e m o s t common, m o s t easily seen, a n d most interest ing aspects o f the world a rou n d us. Each guide c o m b i n e s the authority o f a n eminent scientist and o f an ex p e rt i n science education-Or. Her bert S . Zim. These 160 page b ooks o verfl o w with a c c ur a te ful l c o l o r illu strati o n s
and
concise,
double
c h e c k e d information which m a k e s i dentification a n d understand i ng the subj ect easy and enjoyable. B I RDS • FLOW ERS • I NSECTS TREES • SEASHORES • STARS REPTILES AND AMP H I BIANS WEATH E R • MAMMALS F I S H ES • ROCKS AND M I N ERALS GAM E B I R DS SEA SH ELLS OF T H E WORLD FOSSI LS
Sponsored by the WILDLIFE MANAGEMENT I N STITUTE
