MARITIME
INSTITUTE
These bad weather systems originate on the Equatorial side of the oceanic sub-tropical anticyclones, on the periphery of the Trade Wind flow.
These bad weather systems originate on the Equatorial side of the oceanic sub-tropical anticyclones, on the periphery of the Trade Wind flow.
‘Trrade Wind ‘T Wind inversion’ inversion’ plays a significant part in their formation. ubsiding air in the anticyclone heats adiabatically at the dry lapse rate and a mar!ed inversion forms at around "##m on the Eastern side of the ocean. $ir below inversion is moist and tends towards instability.
T%E TRA&E IN&S The trade winds are a pattern of wind that are found in bands around the Earth's equatorial region. The trade winds are the prevailing winds in the tropis! blowing fro" the high#pressure area in the horse latitudes towards the low#pressure area around the equator. The trade winds blow predo"inantl$ fro" the northeast in the northern he"isphere and fro" the southeast in the southern he"isphere
&ownstrea"! the inversion wea(ens as the degree of subsidene dereases! the inversion level rises! and )u"ulus towers* penetrate through the inversion spreading "oisture into the dr$ air above.
Tropial waves! or easterl$ waves! also (nown as Afrian easterl$ waves in the Atlanti region! are a t$pe of at"ospheri trough! an elongated area of relativel$ low air pressure! oriented north to south! whih "ove fro" east to west aross the tropis ausing areas of
loudiness and thunderstor"s.
West-moving waves can also form from the tail end of frontal %ones in the subtropics and tropics and may be referred to as easterly waves, but these waves are not properly called tropical waves& they are a form of inverted trough sharing many characteristics with fully tropical waves.
$ll tropical waves form in the easterly flow along the southern side of the subtropical ridge or belt of high pressure which lies north and south of the 'ntertropical (onvergence )one *'T()+.
Tropial waves are generall$ arried westward b$ the prevailing easterl$ winds along the tropis and subtropis near the equator.
The$ an lead to the for"ation of tropial $lones in the north Atlanti and northeast +aifi basins.
eather Sequene of an Easterl$ ave In the ridge )ahead of the trough*, fine weather! sattered u"ulus! so"e ha-e. )lose to the trough* , ell developed u"ulus! oasional showers! i"proving visibilit$. )/ehind the trough* , 0eering of wind! heav$ u1b! "oderate1heav$ showers! good visibilit$ e2ept in rain.
aves in the trade winds in the Atlanti 3ean
4ew easterl$ wave depressions develop into losed isobari strutures with $loni voties.
5. Tropial &epressions +rinipal onditions for the for"ation of $loni "otion6 a. 7atitude north1south of 8#9 deg N1S b. Surfae water te"p. at least 5:deg . . +resene of groups of s"all islands inreases onvetive lift due to higher insolation. d. IT; should be well re"oved fro" the Equator.
onditions b and are found on the estern sides of the N Atlanti! N and S +aifi and S Indian oean
Tropial depressions are al"ost un(nown in the S Atlanti and in the eastern side of the S +aifi for the following reasons , a=Sea surfae te"ps! are low due to the ool urrents brought about b$ upwelling of old water fro" below. b= There are no island arhipelagoes. = The IT; never e2tends south of the equator in these regions.
:8> of all tropial depressions our in the Northern %e"isphere.
?. Tropial %urrianes , 6on the western side of the Trade ind where the inversion is higher and less ative! the surfae te"p. is highest! Tropial &epressions "a$ develop into hurrianes
In "eteorolog$! a tropial $lone
&E4INITI3N of a TRS , A tropial revolving stor" "a$ be defined as a roughl$ irular at"ospheri vorte2! originating in the tropis or subtropis! wherein the winds whih blow in onverging spiral tra(s < antil(ws in N% and lo(ws in S%= Reah or e2eed gale fore @.
Intensit$ of winds in tropial depressions , .)Tropial depression* less than fore :. 5.)Moderate tropial stor"* fore @ to B. ?.)Severe tropial stor"* fore C to . D.)%urriane* reah fore 5 and above.
+re#requisite for develop"ent of a tropial stor", .Sea surfae te"p. in e2ess of 59.8deg 5.7atitude "ust not be less than 8deg N or S. ?.S"all vertial wind shear. D.A pre#e2isting depression.
Model vertical profile of a mature typhoon. The central portion is the eye and eye wall.
Model vertical profile of a mature typhoon The central portion is the eye and eye wa
6A distintive feature of the hurriane is the war" vorte2! whih developes through the ation of large nu"ber of b towers releasing latent heat of ondensation into the at"osphere
6the war" ore is vital to hurriane growth beause it intensifies the upper anti$lone! and sti"ulates the low level influ2 of heat and "oisture! whih further intensifies onvetive ativit$ and therefore the upper level high pressure s$ste".
6In the e$e! whih has a dia"eter of ?C#DC ("! adiabati war"ing of desending air aentuates the high te"peratures and results in al" onditions and so"eti"es lear s(ies. The te"p. of the desending air in the e$e "a$ be up to @deg war"er than the te"p in the sa"e plane in the asending air
6The suppl$ of heat and "oisture o"bined with low fritional drag at the sea surfae! the release of latent heat through ondensation! ondensatio n! and the re"oval of air aloft! are essential onditions for the "aintenane of hurriane intensit$. As soon as one of these ingredients is lost! the stor" dea$s
6If the vorte2 passes overland inreased frition hastens the proess of filling!while the utting off of the "oisture suppl$ re"oves one of the "aor soures of heat. If the vorte2 passes over a ool sea! ooler air drawn into the irulation results in a rapid dea$ of the s$ste".
6Radar and satellite photograph$ reveal that loud and preipitation within a hurriane are arranged in spiral bands whose a2es are parallel to the surfae wind strea"lines.
Saffir-Simpson Hurricane Scale Category
Wind speed
Storm surge
mph (km/h)
ft (m)
5
≥156 (≥250)
>18 (>5.5)
4
131–155 (210–249)
13–18 (4.0–5.5)
3
111–130 (178–209)
9–12 (2.7–3.7)
2
96–110 (154–177)
6–8 (1.8–2.4)
1
74–95 (119–153)
4–5 (1.2–1.5)
Additional classifications Tropical storm Tropical depression
39–73 (63–117)
0–3 (0–0.9)
0–38 (0–62)
0
Wind speed record of Typhoon or! Sept 1"""
Indiations of a Tropial %urriane in the 0iinit$ a= Swell b= hange in Trades = Irregularit$ in se"i#diurnal range d= 3ppressive at"osphere
a=A heavy swell approahing fro" the stor" field6 7arge waves generated b$ the stor" pass be$ond the li"its of the disturbane! and travel great distanes. The swell travels at a "uh greater velocity than the stor" entre! an observer "a$ be forewarned of the e2istene of the stor" b$ 5 or ? da$s! and thus giving first warning and the stor" bearing.
b= A change in the intensity or direction of the Trades.
= Irregularity in the se"i#diurnal range! or the baro"etri pressure more than 3 mbs! below seasonal nor"al for the partiular region and ti"e of da$.
d= An oppressive atmosphere with increasing humidity ! and an ugly, lurid sky at sunset 1sunrise.
Sunset view of %urriane rain#bands
Stor" tra(s worldwide
Tra(s followed b$ tropial revolving stor"s
a= The$ tend to "ove parallel to isobars whih lie outside the stor" field at about C to 5 (ts. b= The$ re#urve pole#ward around the western "argin of the sub#tropial high Fpressure ells! entering the irulation of the westerlies.
Tra( of a t$phoon
<=6.Move"ent of Tropial %urrianes , e= irri#for" louds! up to CCC("s fro" the entre! in thi( bands onverging on the entre. f= +RESSURE6 4ro" 8CC" to 5C" fro" entre, slow fall of pressure fro" 5C" to 9C" fro" entre, distint fall of pressure fro" 9C" to entre , a rapid fall. g= Rain showers give wa$ to torrential rain! ith visibilit seriousl redued.
T$pial barogra" of a hurriane
. To find the bearing of the stor" entre, /$ /u$s /allot*s 7aw and faing the wind6
+lotting the probable path of the entre
+lotting the probable path of the entre6 Radio reports of the presene of a tropial hurriane will indiate the present position ourse and speed of the entre. As the infor"ation is orret up to the last observation! the "aster an onl$ "a(e an appro2i"ate esti"ation of the future path. In an$ one 5D hour period! the entre will nor"all$ be e2peted to "ove into an area overed b$ a setor e2tending DC deg either side of its present path! so the "aster should re#plot the entre on reeipt of ever$ "essage! and endeavor to avoid that area.
Use of safet$ setor for (eeping a ship lear of a tropial stor" in the Northern %e"isphere.
Avoiding tropial hurrianes
+ratial Rules for Avoiding Tropial %urrianes6 . In the dangerous se"i#irle! "a(e speed with the wind on the starboard /3 and haul round to starboard as the wind veers! to lear the entre.
+ratial Rules for Avoiding Tropial %urrianes6 5. In the diret path of the stor" "a(e speed with the wind abaft the starboard bea"! and run into the navigable se"i#irle! hauling to port as the wind ba(s.
+ratial Rules for Avoiding Tropial %urrianes6 ?. In the navigable se"i#irle run with the wind on the starboard quarter! hauling to port as the wind ba(s.
Note6 If the sea and swell are too heav$ to arr$ out these "anoeuvres! %eave#to with the wind on the starboard bow.
Note6 The shift of the wind is ver$ i"portant! and are "ust be e2erised to avoid being ta(en unawares b$ the stor" entre re#urving.
Note6 are "ust also be e2erised to avoid running before inreasingl$ heav$ seas! as the dangers of broahing in s"all ships partiularl$! annot be over# stressed. The results of failing to ta(e ti"el$ avoiding ation are evidened b$ "arine disasters < e2a"ple6 US fleet &ee"ber BDD=
Move"ent of a ship in a tropial stor" near the oint of re#urvature N%
Avoiding the %urriane entre , before ta(ing avoiding ation6 .4ind the bearing of the entre 5.Establish the se"i#irle in whih the ship lies ?. +lot the probable path of the entre.
Dangerous semi-circle… is that in whih! if a vessel runs with the wind aft! she will beo"e further involved with the entre
Navigable semi-circle… Is that whih! if the vessel runs with the wind aft! she will tend to run lear of the entre.
Se"i#irle6 a= If the wind shifts to the right
Se"i#irle6<ontinued= = If the wind diretion re"ains stead$! vessel is either , .In the diret path of the stor" being overta(en b$ it
Se"i#irle6<ontinued= If neessar$! stop the vessel and establish the real shift of the wind. The rule of the se"i#irle applies to both he"ispheres6 N%### R% se"i#irle is dangerous S%### 7% se"i#irle is dangerous
