Unit 2 – Tropical Revolving Systems(TRS)

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Tropical Revolving Storms: Local names, seasons andareas affected; Origin, structure, movement andlifespan of TRS; Weather associated with TRS;Definitions and nomenclature; Warning signs of anapproaching TRS; Characteristics of TRS; Forecastingtechniques Past and present; Action to be taken whenthe presence of TRS is confirmed; Cyclone trackingand warning bulletins under international conventions;Practical Rules for navigation for maneuvering in thevicinity of TRS; Avoidance of storm centers and thedangerous quadrants; Ideal conditions for theformation of TRS; Comparison between a TRS and atemperate latitude depression; Avoiding TRS – 1-2-3theory and sector theory. 

TROPICAL REVOLVING STORMS(TRS)

TROPICAL CYCLONESTROPICAL CYCLONES

Tropical Revolving Storm(TRS)•

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A tropical revolving storm is a rapidly-rotating stormcharacterized by low pressure centre, strong windsand a spiral arrangement of thunderstorms.

TRS forms over large bodies of warm water and theydevelop the energy from the ocean surface.

These tropical cyclones take birth in the warmoceanic region and move towards the continents.

TRS tends to move in counter-clockwise in Northernhemisphere and clockwise in Southern hemisphere

Great Danger to Shipping

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TRSs in the Atlantic and Pacific are generallymore violent than in the Arabian sea and the Bayof Bengal(Wind sp reach 100 Kts).Nevertheless, TRS are a great danger, toshipping regardless of where they areencountered and require a special study.Whenever a vessel is in an area where TRSs arelikely to be encountered, careful watch should bekept for the warning signs of an approachingTRS and take early evasive action.

TROPICAL CYCLONEDEFINED!

A warm core, non-frontal,synoptic scale systemwith cyclonically rotatingwinds

A warm core, non-frontal,synoptic scale systemwith cyclonically rotatingwinds Anti Clockwise inNH & Clockwise in SH,characterized by a rapiddecrease in pressure andincrease in winds towardthe center of the storm.Cyclones develop overtropical or subtropicalwaters and have adefinite organizedcirculation

A warm core, non-frontal,synoptic scale systemwith cyclonically rotatingwinds

A warm core, non-frontal,synoptic scale systemwith cyclonically rotatingwinds Anti Clockwise inNH & Clockwise in SH,characterized by a rapiddecrease in pressure andincrease in winds towardthe center of the storm.Cyclones develop overtropical or subtropicalwaters and have adefinite organizedcirculation.

Different Names – Different Areas•

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Tropical Revolving Storms: Tropical revolvingstorms are severe low pressure systems of tropicalareas. They can be viewed as an intense vortex orwhirl in the atmosphere. They are called by differentnames in different areas.Hurricane in Atlantic OceanTyphoons in Pacific OceanWilly Willy in Australian regionCyclonic storms in Indian regionBaguio in Philippines andTaifu in JapanThese tropical cyclones take birth in the warmoceanic region and move towards the continents.Very high wind speeds are seen in association withthese low pressure systems. There are differentstages of these TRS which are given below based onthe wind speed.

Different Names(Local Names) in Different Areas

• Tropical revolving storms are severe low pressuresystems of tropical areas. They can be viewed as anintense vortex or whirl in the atmosphere.

(i) Hurricane in North Atlantic (Western Side)- Jun to Nov

(ii)Typhoons or Baguios in North Pacific(Western Side)- Allthe year round. Worst period From June to November

(iii)Hurricane or Cordonazo in North Pacific(Eastern Side)-June to November

(iv)South Pacific(Western Side) Hurricane - Dec to Apr

Different Names in Different Areas

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(v) South Indian Ocean (Eastern Side) Willy Willy in (Australianregion also) – December to April(vi) South Indian Ocean(Western side) Cyclone - Dec - April(vii) Cyclone in Indian region – Arabian sea – During change ofmonsoon : Mid Apr

Mid June, October & November . Worst months are May,October & November. Bay of Bengal – May to Dec. Worst months are May, Oct,Nov and December.

(viii) Baguio in Philippines and (ix) Taifu in Japan These tropical cyclones take birth in warm oceanic region & moveto-wards the continents. Very high wind speeds seen in low pressureaarea

Tropical revolving storms occur all aroundthe world, but are called different names.

The map below shows where the seven basins noted for TCdevelopment are located and typical tracks for each. It also hasthe average number of tropical storms, and hurricanes, created ineach basin.

TROPICAL CYCLONEFORMATION,

LOCATION

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Hurricanes Around the GlobeAtlantic and eastern Pacific tropical cyclones are known ashurricanes, while over the western Pacific they are referred toas typhoonsOver the Indian Ocean and Australia they are known as cyclonesThe eastern North Pacific has the highest frequency of tropicalcyclones for the globe with an annual average of 16The South Atlantic produces none as the basin is too small toinitiate cyclogenesis

Tropical cyclonegenesis areas andrelated storm tracks

Different Stages of TRS Based on wind Sp.

How are they measured?Although developed in the USA, the Saffir-Simpson scale isused to grade tropical storm wind strength in many parts ofthe world.

Categorykm/hrkm/hr

Wind SpeedWind SpeedWind Speedkm/hr

Wind SpeedMph Surge mSurge m

StormStormStormSurge m

Tropical storm 0-62 0

depressiondepressionTropicalTropicalTropicaldepression

63-117 0-0.9

1 119-153 74-95 1.2-1.52 154-177 96-110 1.8-2.43 178-209 111-130 2.7-3.74 210-249 131-155 4.0-5.55 >250 >155 <5.5

HURRICANE CATEGORIES

Category 1 (Minimal) - Winds 64 to 82 knots, storm surge 4 to 5 ftabove normal. No real damage to building Structures. Low lyingcoastal areas flooded, minor damage to piers. Examples Irene1999 and Allison 1995Category 2 (Moderate) - Winds 83 to 95 knots, storm surge 6 to 8 ftabove normal. Minor damage to structures, poorly constructedbuildings major damage. Coastal and low lying escape routesflooded over, considerable pier damage. Examples: Bonnie 1998,Georges (FL & LA) 1998 and Gloria 1985

Category 3 (Extensive) - Winds 96 to 112 knots, storm surge 9 to 12ft above normal. Major damage to structures, poorly constructedbuilding destroyed. Serious flooding along the coast, extensiveflooding may extend inland 8 miles. Examples: Keith 2000, Fran1996, Opal 1995, Alicia 1983 and Betsy 1965

Category 4 (Extreme) Winds 113 to 136 knots, stormsurge 13 to 18ft above normal Extensive roofing and windowdamage,complete destruction of mobile homes. Areas above 10ftflooded inland up to 6 m, major erosion of beaches,massiveevacuation of coastal areas.

Category 5 (Catastrophic)Winds above 137 knots, storm surge

greater than 18ft above normal. Complete failure of roofstructures and very severe window and door damage, somecompletebuildings fail. Major damage tostructures lower than15 ft abovesea level, massive evacuations of residential unitswithin 10 miles ofthe coast.

What drives a Tropical Storm?

Cold polar airmoves towards

the warmequator.

Hot tropical airmoves towardsthe cold poles.

The planet is not evenly heated. AtA the heating is more direct =intense.Whereas at B it is spread over alarger surface area = less intense.

What causes wind?• This uneven heating causes air to move

around the planet, trying to even out thetemperature.

around theplanet causesour wind, whichgenerallyfollows a knownpattern.

around theplanet causesour wind, whichgenerallyfollows a knownpattern.

Air moving

planet causesour wind, whichgenerallyfollows a knownpattern.

Air moving

planet causesour wind, whichgenerallyfollows a knownpattern.

Air movingaround the

our wind, whichgenerallyfollows a knownpattern.

Air movingaround the

our wind, whichgenerallyfollows a knownpattern.

Air movingaround theplanet causes

generallyfollows a knownpattern.

Air movingaround theplanet causes

generallyfollows a knownpattern.

Air movingaround theplanet causesour wind, which

follows a knownpattern.

Air movingaround theplanet causesour wind, which

follows a knownpattern.

Air movingaround theplanet causesour wind, whichgenerally

pattern.

Air movingaround theplanet causesour wind, whichgenerally

pattern.

Air movingaround theplanet causesour wind, whichgenerallyfollows a known

Air movingaround theplanet causesour wind, whichgenerallyfollows a known

Air movingaround theplanet causesour wind, whichgenerallyfollows a knownpattern.

Tropical stormsform between5ºand 20º North& South of theequator.They need warmwater ~ above27ºc – hencetheir location.As the Earth rotates, this provides the ‘spin’needed to start the tropical storm on its journeyacross the Atlantic towards America.

Favourable/Ideal conditions for the formation of TRS

1. Sea surface temperature must be 27oC or more. Such ahigh temperature is needed to produce steep lapse rate introposphere to maintain vertical circulation.2. Fair amount of Coriolis force (Latitude more than 5Deg North/South): The circulation has to be sustainedand hence the formation can take place where Coriolisforce is available. Hence no Cyclogenesis occurs close tothe equator. That is, within a belt of 5 deg latitude of bothsides of equator no cyclone forms. The maximumcyclonic development takes place around lat 150. About65% of TRS of the world develop between lat 10 to 200.3. Low pressure area surrounded by areas of HighPressure: Cyclones develop from a pre-existing trough ordiffused low-pressure areas associated with shear lines.This makes the cyclones to have preferred period andregions of formations in the oceans. The periods arelinked with the oscillations of Inter tropical ConvergenceZone (ITCZ) and they form on Doldrums on the sea area.

Conditions Contd……4.Convection current(daytime over large islands):Ensure that air rises continuously so that adiabaticcooling results in condensation that liberates latentheat, which provides energy for the TRS.)5. Weak prevailing winds(During change of season)-If the prevailing winds are strong, the air would notrise vertically. It would be carried off horizontally,thereby not allowing a TRS to form. It exists duringday time over large tropical islands, in mid ocean,between lat 5 & 20 Deg, during change of monsoonin Indian waters, mid Apr – mid June & from Oct toDec.6. High humidity is required which is available inmaritime atmosphere. TRS form with warm moistair below and steep lapse rate aloft. They form onthe western side of the ocean in summer or autumn.

Here is a check listof what is neededfor a tropicalrevolving stormto grow.

A storm can travel anywhere from 15 to 40mph

Once the stormhas developedit can grow 400miles wide.

How do they develop?

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Favorable environmental conditions that must bein place before a tropical cyclone can form:

Warm ocean waters (at least 80°F / 27°C).

An atmosphere which cools fast with height(potentially unstable).

Moist air near the mid-level of the

troposphere (16,000 ft / 4,900 m). Generally a minimum distance of at least300

miles (480 km) from the equator. A pre-existing near-surface disturbance. Little vertical wind shear between thesurface and

the upper troposphere. (Vertical wind shear isthe change in wind speed with height.)

Outflow aloft/exhaust

Common Areas for TRS

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1. North Atlantic It extends upto lat 30oN in the AtlanticOcean. About 8 TRS occur in a year. Their subdivisions area.Gulf of Mexico- from June to October (coastal areas ofMexico and USA).b.West Indies- Its TRS are very violent. Florida coast isworst affected during the period June to October.c.Carribean - The northern part is affected by TRS duringJune to October while the southern part affected duringMay to November.

Island of Cape de verde - TRS affect the island duringAugust to September (close to Africa).

2.North Pacific : This is situated close to Mexico andCentral America. 7-8 TRS affect every year during June toNovember.

3. China Sea [North West Pacific Ocean] : This includesthe coasts of Philippines, Japan, Vietnam and South China.About 22 Typhoons occur every year during May toDecember. This is where the frequency of TRS is maximumin the world.4.Australian Sea : It extends from east coast of Australiato Society island, Christian island Hawaiian island. DuringApril to December, TRS affect this area.5.North Indian Ocean : These are cyclonic storm affectingIndia from Bay of Bengal (5 to 6) or Arabian sea (1 or 2)during April to December.6.South Indian Ocean : It Includes Madagascar, Reunionand Mauritius. The frequency of TRS is 1 to 2 in a year fromNov - April. In South Atlantic Ocean, the TRS do not formbecause the area of their origin, which is doldrums, isalways north of the Equator (Over Northern Africa) withrespect to South Atlantic Ocean.

Origin, Movement of TRS•

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TRS originate in latitudes between 5 & 20 Deg and travelbetween W and WNW in NH and between W and WSW inSH, at a sp. of about 12 knots. Somewhere along theirtrack, they curve away from the Equator–curve to N andthen recurve to NE in NH; curve to S and then recurve to SEin SH.The recurving is such that the storm travels around theoceanic high (which is situated at about 30 deg N.& 30deg S in the middle of large oceans). After recurving, thespeed Of travel increases to about 15-20 Kts. Sometimes,a TRS does not curve or recurve at all, but continues on itsoriginal path, crosses the coast and dissipates quicklythereafter due to friction and lack of moisture.All TRSs do not follow such definite paths and speeds. Intheir initial stages, occasional storms have remainedpracti- cally stationary or made small loops for as long as4 days. The life span of TRS anything from 1-19 daysAverage 6 days.

Life Cycle of TRS• Life period of a TRS is 4 to 7 days. The four

stages are formation stage, Immature stage,matured stage and the decaying stage. Thewinds around a low pressure may persist forseveral days and then strengthen. Cloudsbegin to show a formation. Afterwards thegrowth stage starts. Pressure beings to fallrapidly and near circular pattern of winds andIsobars at the center take place. Wind speedstarts increasing enormously and a centraleye region forms. In the matured stage thepressure fall at center and increase of

Life Cycle of TRS Contd….wind speed is arrested but the system startsexpanding horizontally. A TRS can be 200 to500 kms in horizontal extent and vertically, itcan be upto 5 to 10 kms tall with strongexpanding horizontally. A TRS can be 200 to500 kms in horizontal extent and vertically, itcan upto 5 to 10 kms tall with strongconvective clouds. After crossing coast thewind speed decreases and the cloudorganization gradually dissipates. The lifecycle consists of 4 stages.

Life Cycle of TRS

TROPICAL DEPRESSION(Formative Stage)

Winds < 34 kts

Tropical wave develops aweak cyclonic circulation Identified by thickeningclusters of thunderstormson satellite Central pressure fallsrapidly below 1002mb ifsystem intensifies

Life Cycle of TRS•

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3. Matured stage : At this stage, the pressure fall and increasein wind speed are stopped. The circulation expands outwardsand an asymmetry sets in with high wind speed extendingmore to the right side of the system, which constitutes anarea of Dangerous semicircle.

4.Dissipating stage : When a TRS enters the land it startdecaying due to the:

i) Moisture source being cut off the energy of the systembegins to weaken.

ii)Due to the roughness of the land, the frictional drag retardsthe wind speed. The system also decays when it moves over acold ocean surface where the sea surface temperature is lessthan 26oC. This happens when TRS moves northwards tohigher latitudes

TROPICAL STORM(Immature to Mature Stage)

Winds 34 - 63 kts Closed formation expands withspiralbands becoming better organized Increasing sea state makesnavigation near the centerincreasinglydifficult and dangerous

HURRICANE / TYPHOON / WILLY-WILLY(Mature Stage)

Winds > 63 kts DANGEROUSLY HIGHSEAS navigation severely impaired Radius of strong winds may exceed350 NM Gale Force Winds extend out furtherin right front quadrant (typically 120NM)

Structure of TRS• Both horizontal and vertical wise, the inner most portionof the matured cyclone is called ‘eye’. It has a diameter of10 to 30 km. This area is calm with little cloud and low rain,as the centrifugal force keeps the cloud mass away. The temp. inside the eye may be about 5oC higher than the surrounding. If outside the storm temperature is 26oC, at the eye it can become about 30oC. Next region is the ‘eye wall’ or ‘inner ring’ area. This is 30-50 km wide. This areahas maximum wind speed. Here the wind speed can reachup to 100kts or even more. Terrible thunder, continuouslightning, heavy continuous pouring rain from thick darkNimbostratus and Cumulonimbus clouds, violent windsand squally weather with gales, tidal waves, storm surgesare common

Structure of TRS Contd…..

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Next is the ‘outer ring’ area. This is from50-200 km. In this area, the strength ofwind speed decreases rapidly andpressure rises fast and the intensity ofrainfall keeps on decreasing.

The eye and eye wall regions togetherconstitute the core of the cyclone. That isupto 50 KM

Vertical Structure of 3 Layers1. Inflow layer : It is from the ground to 3kmwhere there will be mass convergence of the air.The air motion is in a spiral band and at thecenter maximum spin or vorticity is seen. In theeye region some sinking motion of air can beseen.2. Mid tropospheric layer : From 3 to 8 km, largescale vertical motion of the atmosphere will beexperienced. There is no radial motion of thewind into the system in mid tropospheric layer.3. Outer flow layer : It is encountered above12km, where there is mass divergence of theatmospheric air. Beyond 300km, the diverging airwill start sinking groundwards.

Vertical Structure

Vertical Structure

Tropical Storm Feature•

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Circular feature of average 400/500 nmdiameterCentre eye diameter 15/30 milesSteep pressure gradient with high wind speedsThe eye wall which is approx.15 nm wide has anarea of dense cloud associated with heavyrainfall and high waves.Generate between Latitudes 5 to 10 deg North orSouth of the Equator.

Principle Danger of TRS

Gales and strong windsDamage installations, dwellings, communicationsystems, trees., etc. Resulting in loss of life andproperty.

Torrential rainMay cause river floods

Storm surges or high tidal waves A storm surge is an abnormal rise of sea level nearthe coast caused by a severe tropical cycloneAs a result, sea water inundates low lying areas ofcoastal regions drowning human beings and live-stock, eroding beaches and embankments,destroying vegetation and reducing soil fertility

Here is a 3Dimage of aTropicalrevolvingstorm. Noticethe lower levelwinds beingdrawn in andspirallingcounterclockwise.

The lines ‘isobars’ get closer together,indicating faster flowing air.

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Sinking air, warming adiabatically, causes air in the eye to bewarmer than elsewhereRelative humidities are lower in this region due to highertemperatures

Tropicalcyclonestructure

Tracks of TRSThe TRS travels at the speed of 10-30 km/hr.In a day, it can cover a distance of about 300KM. It moves from east to west, steered byhigh level easterly wind. Sometimes themovement is erratic, with loops and wobbling.The TRS gets pulled polewards and thereforegeneral motion is northwest in the northernhemisphere. There is a tendency to move inareas of warm water. They weaken as theyenter in cold water or strike a coast.

Tracks of TRS

Track of TRS in Bay of Bengal/Arabian Sea

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In the Bay of Bengal, 2 to 3 TRS are commonduring Oct- Nov. Rarely, 1 to 2 TRS are seen inApril and May. The track of most of the TRS isnorth westerly and strike the eastern coast ofIndia. Sometimes, off Orissa coast, TRS recurveand cross near Burma (Myanmar) coast.

For Arabian Sea, the formation of TRS are rare.However, the storms that form over Bay ofBengal move across Peninsular India andemerge out in Arabian Sea. The TRS getrejuvenation over the Arabian Sea, move in aNortherly course and strike the Saurashtra-kutchcoasts. The Frequency of storms in Arabian Seais very much less compared to Bay of Bengal

Evidence for Determination of TRS•

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Geographic conditions of storms experienced Lat 5 – 35Deg.Location and Season NH - Jun-Nov & SH – Dec – MayA heavy swell develops, usually from direction of stormand may be experienced upto 1000 nm from the stormscentre.An unsteady barometer or a cessation in the diurnal range.Increased wind velocity or a change in the trade wind,becoming violent.Open ocean, high sea temperature over 27 deg.A growth of cumulus and/or cumulonimbus will developwith bands of showersThunderstorms may occur within 100 nm radius of storm.Oppressive atmosphere, with squally and heavy rainfall inthe vicinity of the storm.Changing appearance of the sky cirriform cloud.

Recurvature of TRSWhen the TRS reach higher latitudes, they changethe directions. They move north and then north eastin northern hemisphere. In southern hemisphere,they move south and then south east. This processis called ‘recurvature’. Before the recurvature, theyremain stationary for a day or more. Afterrecurvature, they move very fast. The recurvature isdecided by upper level winds, which is calledsteering level. When two cyclones exist near eachother on the ocean, they interact and moveanticlockwise with respect to each other. In India theTRS after recurvature get broken up over Himalayasand wakens. Elsewhere over the world, as in Atlanticocean after recurvature, the TRS acquires theproperties of an Extra tropical depression

Cyclone Map of India

Definitions of TRS••

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VORTEX: The central calm of the storm.Right Hand Semicircle(RHSC): That half of the storm centrethat lies to the right of the observer who faces along the pathof the storm.Left Hand Semicircle(LHSC): That half of the storm centrethat lies to the left of the observer who faces along the path ofthe storm.DANGEROUS SEMI-CIRCLE: The half of the storm, which lies tothe right of the path in the Northern Hemisphere and to the leftof the path in the Southern Hemisphere.DANGEROUS QUADRANT: The advance quadrant of the RHSCin NH & LHSC in SH. When the existence of a TRS om thevicinity has been established, evasive action has to be taken tokeep the vessel out of this quadrant.NAVIGABLE SEMI-CIRCLE: The half of the storm, which lies tothe left of the path in the Northern Hemisphere and to the rightof the path in the Southern Hemisphere.TROUGH LINE: A line through the centre of the storm at rightangles to the path.  The dividing line between falling and risingpressure

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PATH: The direction in which the storm is moving.TRACK: The area, which the storm centre hastraversed.STORM FIELD: The horizontal area covered by thecyclone conditions of the storm.SOURCE REGION: The region where the storm firstforms.VERTEX or Cod: The furthest westerly point reachedby the storm centre.EYE OF THE STORM: The storm centre.BAR OF THE STORM: The advancing edge of thestorm field.ANGLE OF INDRAUGHT: The angle, which the windmakes with the isobars

Weather in TRSPressure : In the eye region the lowest pressure occurs.Here, 15 to 20 HPA fall of pressure is common. In extremecase, the central pressure was as low as 920 HPA. In theouter storm area, the fall of pressure ahead of the trough,and the rise of pressure behind it, is slow. The semi-diurnalvariation of pressure may still be visible on the trace of abarograph. In the eye-wall, the fall of pressure ahead of thetrough, and the rise of pressure behind it, is very sharp.The trace of the barograph is very steep, nearly vertical.Semi-diurnal variation is not visible on it. The pressuregradient in this region can be as high as 11 mb in 15 miles.The pressure keeps increasing rapidly behind a TRS acrosseye wall and outer storm area.Wind : Wind speed will be very high in eye wall region of theorder of greater than 100 kmph normally. If the wind veerscontinuously, the ship is in the right hand semi circle and ifthe wind backs continuously, the ship is in the left handsemi circle, which is valid in both hemispheres.

Wind•

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Angle of indraft: The angle of indraft, in the outer fringesof the storm, is about 45 Deg and gradually decreasesuntil it is 0 Deg in eye wall.Wind Force: The wind force will increase as theatmospheric pressure falls and after the trough, or eye asthe case may be, has passed, wind force will graduallydecrease as the atmospheric pressure increases.Wind direction & Force in the Eye: As soon as a vesselpasses from the eye-wall into the eye, the wind dies downinto light airs but the swell is mountainous and confused.It must not be presumed that a vessel in the eye of a TRSis in a comfortable and safe position. On the contrary, sheis in a most dangerous situation. After a short while, asthe vessel passes into the eye-wall behind the trough, thesudden hurricane force wind from the opposite directionas before, strikes the vessel and may cause it to heel overby as much as 80 deg. Or more and would hold it like that,leaving practically no margin for rolling further.

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Clouds : All types of clouds can be seen from eye wallcircle onwards. Nimbostratus clouds will give very heavycontinuous rain. In the outer fringers of the storm, cirrusin the form of strands or filaments generally so aligned,that may be said to point towards the storm centre.Then cirrostratus followed by altostratus. Around theeye-wall, thick nimbostratus(giving continuous rain) andsmall patches of cumulus, may be seen. At the eye wall,towering anvil shaped cumulonimbus gives torrentialrain. Directly above the eye, a small circular patch ofblue sky may be seen, indicating an absence of cloudtherein and consequent cessation of precipitation.Atmospheric Temperature. Since a TRS exists in one air-mass only, no drastic changes of atmospheric temp. areexperienced on its passage. However, atmospheric temp.would decrease during rain. In the eye, a slight increasemay be registered due to adiabatic heating of thesubsiding air.

Visibility & Storm Surge

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Visibility : Visibility will be good except during rainwhich occurs in the eye wall and outer storm circles.Around the eye-wall, under the nimbostratus clouds, itbecomes poor due to rain. In the eye-wall, it is poordue to driving rain and spray. In the eye, it is poor dueto mist or fog.Storm Surge : When pressure becomes low, water willrise which is known as inverted barometeric effect.The strong winds also increase the wave. The wavesreach as high as 20 metres or so. This is called stormsurge or tidal wave or storm wave.

The storm surge will get amplified when the stormcrosses the land in a bay with shallow water or in ariver estuary especially during spring tide.In a TRS the maximum damage is due to stormsurge. About 90% of the death toll is due to stormsurge which affects about 25 km inland.

Services provided for StormWarnings

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Storm warnings are transmitted over variousmeans of transmission.They include:Navtex – Continuous however available only incoastal areas – not for long ocean passagesVHF – port informationWeather FAX – all over the worldInmarsat C – Same broadcast as Navtex butfor coverage area of InmarsatHigh Frequency (HF) broadcasts – Oceanpassages

Strom Warnings/Cyclone Forecast

Weather reports based on satellite pictures andobservations from other vessels may contain stormwarnings. A satellite picture cannot indicate theatmospheric pressure at the storm centre. A vesselwhich notices the warning signs of a TRS, is the firstand only one to do so and must warn others about it.She should first send out a safety message containingthe storm warning and thence increase the frequencyof its weather reports. In a Cyclone forecast 4important aspects are:1. Position of the cyclone2. Its strength or intensity, whether it is a depressionor cyclone.3. Whether the system is intensifying or weakening.4. Future path or course of the system

Major basins and warningcenters.

Basins Warning Centre(RSMC/TCWC

North Atlantic National Hurricane Center (US)

North-East Pacific National Hurricane Center (US)

North-Central Pacific Central Pacific Hurricane

Center (US)

North-West Pacific Japan Meteorological Agency

North Indian Ocean India Meteorological

Dept(India)

South-West Indian Ocean India Meteorological

Dept(India)

Four Aspects(hints) of Deciding Cyclone Forecast

i)Weather charts which consists ofobservations on the surface as well asupper atmospheric data observed at fixedtime like 00,03,06,09,12,18,21, UTC.ii) Cloud observations from satelliteimageries and also from Radar observations.iii) Ship and aircraft observationsiv) Weather buoys.

Cyclone Movement Prediction

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Cyclones move in a path where there is maximum.1. Pressure fall, 2.Pressure departure from normal,3.Clouds & rainfall4. Path depends on the wind directions at about 10kmin the upper atmosphere which is called steering level.5. When the isobaric analysis is carried out the cyclonefield has elliptical shaped isobars. The cyclone has thetendency to move along the major axis of the ellipse.6. Future path depends on the past track also, which iscalled aspersistent climatology.7. A Cyclone track atlas is maintained by each cycloneprone country which covers hundreds of tracks ofcyclones in the past numbers of years. The presentcyclone is compared with the past tracks dependingupon its positions and its future path is decided.

The cyclone prediction is not based on one factor orother but it is based on the combination of the factorscontributing to its path and movement

Warning Signs of an approaching TRS

a) SWELL: Swells are the waves generated by a distantTRS. They have a long wave length and appear in aquick succession. From the eye-wall region, theyemerge radially in all directions and reach the coasthundreds of miles away.b) Pressure: Falls steadily. After filtering diurnalvariation of surface pressure if the pressure is lessthan 5 HPA or more, it means there is a TRS is in thevicinity.c) Lot of clouding, heavy continuous rain with squallygale force wind is a clear warning signal.d) Storm warning bulletins issued by MeteorologicalDepartment is a good guidance for mariners

Weather•

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Cirrus clouds in bands or filaments alignedtowards the direction of the storm centre.Unusually clear visibility may occur.Sometimes peculiar dark red/copper colour ofsky is seen at sunset before a TRS.Increase of wind force as the pressure falls.Threatening appearance of dense, heavy cloudson the horizon.Frequent lightning may be seenSuccession of Squallls, with or without rain.

Obtain the bearing of the storm centre.(a)

(b)

‘Face the wind, and according to Buys law the storm

centre will lie 8 – 12 points on your right in the NH, Leftin the SH. If the pressure has fallen 5 mb below normal,allow 12 points as it means that either the vessel is inthe outer fringes of a well developed TRS, or that a newTRS is forming in the vicinity.If the pressure has fallen 20 mb or more below normal,

allow 8 points as it means that the vessel is near theeye of a well developed TRS.

(c) If the wind is veering continuously, the ship is in righthand semi circle. If the wind is backing continuously, theship is in the left hand semi circle.(d) If the ship is at the port at the time of crossing of a TRSit is advisable to sail off the shore, so that the ship can beprevented from collision with the port structure or runningaground. While out in the high seas, efforts should betaken to keep the ship away from the storm centre

Dangerous Semi Circle: Right Hand Semicircle in NH& Left Hand Semicircle in SH

Navigable Semi Circle: Left Hand Semicircle inNH & Right Hand Semicircle in SH

To ascertain in which semicircle vessel lies•

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For a stationary observer, if the wind veers, vessel is in theRHSC and if it backs, LHSC. This holds good for bothNH & SH. While determining the semicircle, thefollowing points should be noted.Wind Observations, though logged every hour duringbad weather, should be compared with that 2 hoursearlier. This is to give time for significant veering orbacking and hence weed out errors that may becaused by irregular gusts of wind.Veering or backing, once detected, should becontinuous while the observer remains stationary, ie. aveering wind should continue to veer and a backingwind should continue to back. If the wind veers at firstand then backs, or if it backs at first and then veers,the vessel must have passed from one semicircle intoanother, due to change of path of the storm.

Semicircle vessel lies contd……•

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During the two-hour interval betweenobservations, while veering or backing of wind isbeing decided, the observer must bestationary(ie. Vessel should remain hove to). Ifnot, the conclusion arrived at regarding RHSC orLHSC may be wrong and disastrousconsequences may result.If a vessel was overtaking a TRS or it if wasapproaching a stationary TRS from its rear, thewind would veer in the LHSC and back in theRHSC. An unwary navigator would then arrive ata wrong conclusion regarding semi-circle andtake wrong action, which action, instead oftaking him away from the storm centre, would

Track

Equator

20˚N

Vertex

20˚S

Vertex

Characteristic Track

Alternative Path

Eye

Eye

DangerousSemicircle

DangerousSemicircle

NavigableSemicircle

NavigableSemicircle

Alternative Path

Characteristic Track

Action when approach of a TRS isconfirmed

1. Obtain the bearing of the storm centre.

2. Ascertain in which semi-circle the vessellies.

3. Take avoiding action.

Avoiding Action•

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Any avoiding action should aim to keep the vessel wellout of eye-/eye-wall. If a vessel is in the Dangerous Quadrant: Proceed asfast as practicable with the wind 1-4 points on the stbdbow(port bow in SH)-1 point for slow vessels (lessthan 12 kts) altering course as the wind veers(backs inSH). This action should be kept up until the pressurerises back to normal ie. Until vessel is outside the outerstorm area. If there is insufficient sea room, the vesselshould heave to with the wind on the stbd bow(portbow in SH) until the storm passes over.If vessel is in the path of the storm or if in theNavigable semi-circle: Proceed as fast as practicablewith the wind about 4 points on the stbd quarter (portquarter in SH), altering course as the wind backs(veersin SH). This action should be kept up until thepressure rises back to normal ie. Until vessel is outsidethe outer storm area.

1-2-3 Rule

In NH, while in Right Hand Semi Circle, that is dangerous semicircle, proceed starboard side by about 1 – 4 points(for a fast

moving vessel) till the pressure becomes normal and windbecomes light (outside the storm)

If the ship is in the circle in SH, it should proceed along withthe wind by 4 points on the Stbd side and get out of the storm

area.

Tropical Storm / Hurricane Key FactsHurricanes are located in the low-pressure belt near the equator as the sunheats the oceans to a critical temperature of 27C. The oceans heat up all through the summer making their warmesttemperatures in the Autumn (specific heat capacity); this is Hurricaneseason in the Atlantic. The hurricane starts as a tropical depression, gaining more and moreenergy from the warm ocean as it crosses the Atlantic. The circulation ofthe depression gets tighter (isobars are closer together) and the windspeed increases into a tropical storm. The wind speeds continue to increase, becoming a category 1 up tocategory 5 for the most violent storm. As the hurricane makes landfalldamage is done by the winds and rain and an advancing storm surge,which raise sea level by 5m+ in some cases. The hurricane quickly looses its energy as it crosses the cooler land, as theenergy supply has been cut off. The hurricane dies. Sometimes remnants of hurricanes are brought across back across theAtlantic at the mid latitudes giving us very strong depressions in the UK.

Warm OceansThe ‘food’ of a tropical stormis the warm moist waterfound near the equator. Theair here is under LOWpressure, which means itcan lift easily. This liftingencourages the air to cooland condense, as it doeslatent heat is released. Thisis the name given to energyproduced when a substancechanges state ie vapour to aliquid.

Cyclone Movement Prediction

1.

Cyclones move in a path where there is maximum.

Pressure fall. 2. Pressure departure from normal

3. Clouds and rainfall

4. Path depends on the wind directions at about 10km in the upper atmospherewhich is called steering level.

5.When the isobaric analysis is carried out the cyclone field has elliptical shapedisobars. The cyclone has the tendency to move along the major axis of theellipse.

6.Future path depends on the past track also, which is called as persistentclimatology.

7.A Cyclone track atlas is maintained by each cyclone prone country whichcovers hundreds of tracks of cyclones in the past numbers of years. Thepresent cyclone is compared with the past tracks depending upon its positionsand its future path is decided. The cyclone prediction is not based on one factoror other but it is based on the combination of the factors contributing to its path

Ideal conditions for the formation ofTRS

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(a) High Relative Humidity(open sea) & (b) HighTemperature(Tropical areas)-Ensure that Large quantity ofwater vapour is present in the air.(c) LP Area surrounded by areas of HP & (d) Convectioncurrent(daytime over large islands) - Ensure that air risescontinuously so that adiabatic cooling results in condensationthat liberates latent heat, which provides energy for the TRS.(e) Fair amount of Coriolis force(latitude more than 5 Deg N/S)- Ensures that when the winds blow, from surrounding areasof HP to LP area inside, they get deflected sufficiently to blowspirally inwards(Cyclonic)(f) Weak prevailing winds(During change of season)- If theprevailing winds are strong, the air would not rise vertically. Itwould be carried off horizontally, thereby not allowing a TRS toform.It exists during day time over large tropical islands, in midocean, between lat 5 & 20 Deg, during change of monsoon inIndian waters, mid Apr – mid June & from Oct to Dec.