船舶实用导航文集(英文)Tropical Cyclones

CAUSES AND DESCRIPTION OF TROPICAL CYCLONES

3600. Introduction

Atropicalcycloneisacycloneoriginatinginthetrop-icsorsubtropics.Althoughitgenerallyresemblestheextratropicalcycloneofhigherlatitudes,thereareimpor-tantdifferences,theprincipalonebeingtheconcentrationofalargeamountofenergyintoarelativelysmallarea.Tropicalcyclonesareinfrequentincomparisonwithmiddleandhighlatitudestorms,buttheyhavearecordofdestruc-tionfarexceedingthatofanyothertypeofstorm.Becauseoftheirfury,andbecausetheyarepredominantlyoceanic,they merit special attention by mariners.

Atropicalstormhasadeceptivelysmallsize,andbeautifulweathermaybeexperiencedonlyafewhundred

milesfromthecenter.Therapiditywithwhichtheweathercandeterioratewithapproachofthestorm,andtheviolenceofthefullydevelopedtropicalcyclone,aredifficulttoimagine if they have not been experienced.

OnhissecondvoyagetotheNewWorld,Columbusen-counteredatropicalstorm.Althoughhisvesselssufferednodamage,thisexperienceprovedvaluableduringhisfourthvoyagewhenhisshipswerethreatenedbyafullydevelopedhurricane.Columbusreadthesignsofanapproachingstormfromtheappearanceofasoutheasterlyswell,thedirectionofthehighcirrusclouds,andthehazyappearanceoftheat-mosphere.Hedirectedhisvesselstoshelter.Thecommanderofanothergroup,whodidnotheedthesigns,lost most of his ships and more than 500 men perished.

Figure 3602. Areas in which tropical cyclones occur. The average number of tropical cyclones per 5˚ square has beenanalyzed for this figure. The main season for intense tropical storm activity is also shown for each major basin.

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3601. Definitions

TROPICAL CYCLONES

3602. Areas Of Occurrence

Tropicalcyclonesoccuralmostentirelyinsixdistinctareas,fourintheNorthernHemisphereandtwointheSouthernHemisphereasshowninFigure3602.Thenamebywhichthetropicalcycloneiscommonlyknownvariessomewhat with the locality.

1.NorthAtlantic.Atropicalcyclonewithwindsof64knots or greater is called ahurricane.

2.EasternNorthPacific.Thenamehurricaneisusedas in the North Atlantic.

3.WesternNorthPacific.Afullydevelopedstormwithwindsof64knotsorgreateriscalledaty-phoon or, locally in the Philippines, abaguio.4.NorthIndianOcean.Atropicalcyclonewithwindsof 34 knots or greater is called acyclonic storm.5.SouthIndianOcean.Atropicalcyclonewithwindsof 34 knots or greater is called acyclone.

6.SouthwestPacificandAustralianArea.Thenamecy-cloneisusedasintheSouthIndianOcean.AseveretropicalcycloneoriginatingintheTimorSeaandmovingsouthwestandthensoutheastacrosstheinte-rior of northwestern Australia is called awilly-willy.

“Tropicalcyclone”isthetermforcyclonesoriginatinginthetropicsorsubtropics.Thesecyclonesareclassifiedbyform and intensity as they increase in size.

Atropicaldisturbanceisadiscretesystemofappar-entlyorganizedconvection,generally100to300milesindiameter,havinganonfrontalmigratorycharacter,andhav-ingmaintaineditsidentityfor24hoursormore.Itmayormaynotbeassociatedwithadetectabledisturbanceofthewindfield.Ithasnostrongwindsandnoclosedisobarsi.e.,isobars that completely enclose the low.

Atitsnextstageofdevelopmentitbecomesatropicaldepression.Atropicaldepressionhasoneormoreclosediso-barsandsomerotarycirculationatthesurface.Thehighestsustained (1-minute mean) surface wind speed is 33 knots.

Thenextstageistropicalstorm.Atropicalstormhasclosedisobarsandadistinctrotarycirculation.Thehighestsustained(1-minutemean)surfacewindspeedis34to63knots.

Whenfullydeveloped,ahurricaneortyphoonhasclosedisobars,astrongandverypronouncedrotarycircu-lation,andasustained(1-minutemean)surfacewindspeedof 64 knots or higher.

Figure 3603a. Storm tracks.The width of the arrow indicates the approximate frequency of storms; the wider the arrowthe higher the frequency. Isolines on the base map show the resultant direction toward which storms moved. Data for the

entire year has been summarized for this figure.

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TropicalcycloneshavenotbeenobservedintheSouthAtlantic or in the South Pacific east of 140°W.3603. Origin, Season And FrequencySeeFigures3603aand3603b.Origin,season,andfre-quencyofoccurrenceofthetropicalcyclonesinthesixareas are as follows:NorthAtlantic:Tropicalcyclonescanaffecttheen-tireNorthAtlanticOceaninanymonth.However,theyaremostlyathreatsouthofabout35°NfromJunethroughNovember;August,September,andOctoberarethemonthsofhighestincidence.SeeFigure3603b.About9or10tropicalcyclones(tropicalstormsandhurricanes)formeachseason;5or6reachhurricaneintensity(windsof64knotsandhigher).Afewhurricaneshavegeneratedwindsestimatedashighas200knots.Earlyandlatesea-sonstormsusuallydevelopwestof50°W;duringAugustandSeptember,thisspawninggroundextendstotheCapeVerdeIslands.Thesestormsusuallymovewestwardorwestnorthwestwardatspeedsoflessthan15knotsinthelowerlatitudes.AftermovingintothenorthernCaribbeanorGreaterAntillesregions,theyusuallyeithermoveto-507

wardtheGulfofMexicoorrecurveandaccelerateintheNorthAtlantic.SomewillrecurveafterreachingtheGulfofMexico,whileotherswillcontinuewestwardtoaland-fall in Texas or Mexico.EasternNorthPacific:TheseasonisfromJunethroughOctober,althoughastormcanforminanymonth.Anaverageof15tropicalcyclonesformeachyearwithabout6reachinghurricanestrength.Themostintensestormsareoftentheearly-andlate-seasonones;theseformclosetothecoastandfarsouth.MidseasonstormsformanywhereinawidebandfromtheMexican-CentralAmer-icancoasttotheHawaiianIslands.AugustandSeptemberarethemonthsofhighestincidence.ThesestormsdifferfromtheirNorthAtlanticcounterpartsinthattheyareusu-ally smaller in size. However, they can be just as intense.WesternNorthPacific:MoretropicalcyclonesforminthetropicalwesternNorthPacificthananywhereelseintheworld.Morethan25tropicalstormsdevelopeachyear,andabout18becometyphoons.Thesetyphoonsarethelargestandmostintensetropicalcyclonesintheworld.Eachyearanaverageoffivegeneratemaximumwindsover130knots;circulationscoveringmorethan600milesindi-ameterarenotuncommon.MostofthesestormsformeastAREA AND STAGENORTH ATLANTICTROPICAL STORMSHURRICANESTROPICAL STORMS AND HURRICANESJAN***JANFEB***FEB***FEB0.30.20.4FEB2.81.14.1FEB2.21.13.3FEB***MAR***MAR***MAR0.30.20.5MAR2.41.33.7MAR1.70.82.5MAR**0.1APR***APR***APR0.20.70.9APR1.30.31.7APR0.60.41.1APR0.10.10.3MAY0.1*0.2MAY*0.30.3MAY0.40.91.3MAY0.3*0.3MAY0.2*0.2MAY0.30.50.7JUN0.40.30.7JUN1.50.62.0JUN0.51.21.8JUN0.2*0.2JUN***JUN0.50.20.7JUL0.30.40.8JUL2.80.93.6JUL1.22.73.9JUL*0.10.1JUL***JUL0.50.10.6AUG1.01.52.5AUG2.32.04.5AUG1.84.05.8AUG*0.10.1AUG***AUG0.4*0.4SEP1.52.74.3SEP2.31.84.1SEP1.54.15.6SEP***SEP***SEP0.40.10.5OCT1.21.32.5OCT1.21.02.2OCT1.03.34.3OCT0.1*0.1OCT0.3*0.3OCT0.60.41.0NOV0.40.30.7NOV0.3*0.3NOV0.82.12.9NOV0.40.30.7NOV0.3*0.4NOV0.50.61.1DEC**0.1DEC***DEC0.60.71.3DEC1.50.52.0DEC0.80.51.4DEC0.30.20.5ANNUAL4.25.29.4ANNUAL9.35.815.2ANNUAL7.517.825.3ANNUAL10.93.814.8ANNUAL7.43.811.2ANNUAL3.52.25.7EASTERN NORTH PACIFICTROPICAL STORMSHURRICANESTROPICAL STORMS AND HURRICANES***JANWESTERN NORTH PACIFICTROPICAL STORMSTYPHOONSTROPICAL STORMS AND TYPHOONS0.20.30.4JANSOUTHWEST PACIFIC AND AUSTRALIAN AREATROPICAL STORMSHURRICANESTROPICAL STORMS AND HURRICANES2.70.73.4JANSOUTHWEST INDIAN OCEANTROPICAL STORMSHURRICANESTROPICAL STORMS AND HURRICANES2.01.33.2JANNORTH INDIAN OCEANTROPICAL STORMSCYCLONES10.1*0.1TROPICAL STORMS AND CYCLONES11Winds≥ 48 Kts.* Less than .05Monthly values cannot be combined because single storms overlapping two months were counted once in each month and once in the annual.Figure 3603b. Monthly and annual average number of storms per year for each area.508TROPICAL CYCLONES

trueduringOctober,themonthofhighestincidenceduringthetropicalcycloneseason.Itisalsoduringthisperiodthattorrentialrainsfromthesestorms,dumpedoveralreadyrain-soaked areas, cause disastrous floods.

SouthIndianOcean—Overthewaterswestof100°E,totheeastAfricancoast,anaverageof11tropicalcyclones(tropicalstormsandhurricanes)formeachseason,andabout4reachhurricaneintensity.TheseasonisfromDe-cemberthroughMarch,althoughitispossibleforastormtoforminanymonth.Tropicalcyclonesinthisregionusu-allyformsouthof10°S.Thelatitudeofrecurvatureusuallymigratesfromabout20°SinJanuarytoaround15°SinApril.Aftercrossing30°S,thesestormssometimesbecomeintense extratropical lows.

SouthwestPacificandAustralianArea—Thesetropicalwatersspawnanannualaverageof15tropicalcy-clones4ofwhichreachhurricaneintensity.TheseasonextendsfromaboutDecemberthroughApril,althoughstormscanforminanymonth.ActivityiswidespreadinJanuaryandFebruary,anditisinthesemonthsthattropi-calcyclonesaremostlikelytoaffectFiji,Samoa,andtheothereasternislands.Tropicalcyclonesusuallyforminthewatersfrom105°Eto160°W,between5°and20°S.StormsaffectingnorthernandwesternAustraliaoftende-velopintheTimororArafuraSea,whilethosethataffecttheeastcoastformintheCoralSea.Thesestormsareof-tensmall,butcandevelopwindsinexcessof130knots.NewZealandissometimesreachedbydecayingCoralSeastorms,andoccasionallybyanintensehurricane.Ingen-eral,tropicalcyclonesinthisregionmovesouthwestwardand then recurve southeastward.

ofthePhilippines,andmoveacrossthePacifictowardthePhilippines,Japan,andChina;afewstormsformintheSouthChinaSea.TheseasonextendsfromAprilthroughDecember.However,tropicalcyclonesaremorecommonintheoff-seasonmonthsinthisareathananywhereelse.ThepeakoftheseasonisJulythroughOctober,whennear-ly70percentofalltyphoonsdevelop.Thereisanoticeableseasonalshiftinstormtracksinthisregion.FromJulythroughSeptember,stormsmovenorthofthePhilippinesandrecurve,whileearly-andlate-seasontyphoonsmoveonamorewesterlytrackthroughthePhilippinesbeforerecurving.

NorthIndianOcean—TropicalcyclonesdevelopintheBayofBengalandArabianSeaduringthespringandfall.Tropicalcyclonesinthisareaformbetweenlatitudes8°Nand15°N,exceptfromJunethroughSeptember,whenthelittleactivitythatdoesoccurisconfinednorthofabout15°N.Thesestormsareusuallyshort-livedandweak;how-ever,windsof130knotshavebeenencountered.TheyoftendevelopasdisturbancesalongtheIntertropicalCon-vergenceZone(ITCZ);thisinhibitssummertimedevelopment,sincetheITCZisusuallyoverlandduringthismonsoonseason.However,itissometimesdisplacedsouthward,andwhenthisoccurs,stormswillformoverthemonsoon-floodedplainsofBengal.Ontheaverage,sixcy-clonicstormsformeachyear.Theseincludetwostormsthatgeneratewindsof48knotsorgreater.Another10trop-icalcyclonesneverdevelopbeyondtropicaldepressions.TheBayofBengalistheareaofhighestincidence.Howev-er,itisnotunusualforastormtomoveacrosssouthernIndiaandreintensifyintheArabianSea.Thisisparticularly

ANATOMY OF TROPICAL CYCLONES

3604. Formation

Hurricaneformationwasoncebelievedtoresultfromanintensificationofconvectiveforceswhichproducethetoweringcumulonimbuscloudsofthedoldrums.Thisviewofhurricanegenerationheldthatsurfaceheatingcausedwarmmoistairtoascendconvectivelytolevelswherecon-densationproducedcumulonimbusclouds,which,afteraninexplicabledropinatmosphericpressure,coalescedandwere spun into a cyclonic motion by Coriolis force.

Thishypothesisleftmuchunexplained.Althoughsomehurricanesdevelopfromdisturbancesbeginninginthedol-drums,veryfewreachmaturityinthatregion.Also,thehighincidenceofseeminglyidealconvectivesituationsdoesnotmatchthelowincidenceofAtlantichurricanes.Finally,thehypothesisdidnotexplainthedropinatmosphericpressure,so essential to development of hurricane-force winds.

Thereisstillnoexactunderstandingofthetriggeringmechanisminvolvedinhurricanegeneration,thebalanceof

conditionsneededtogeneratehurricanecirculation,andtherelationshipsbetweenlarge-andsmall-scaleatmosphericprocesses.Butscientiststoday,treatingthehurricanesys-temasanatmosphericheatengine,presentamorecomprehensive and convincing view.

Theybeginwithastartermechanisminwhicheitherinternalorexternalforcesintensifytheinitialdisturbance.Theinitialdisturbancebecomesaregionintowhichlow-levelairfromthesurroundingareabeginstoflow,acceler-atingtheconvectionalreadyoccurringinsidethedisturbance.Theverticalcirculationbecomesincreasinglywellorganizedaswatervaporintheascendingmoistlayeriscondensed(releasinglargeamountsofheatenergytodrivethewindsystem),andasthesystemissweptintoacounterclockwisecyclonicspiral.Butthisincipienthurri-canewouldsoonfillupbecauseofinflowatlowerlevels,unlessthechimneyinwhichconvergingairsurgesupwardis provided the exhaust mechanism of high-altitude winds.

Thesehigh-altitudewindspumpascendingairoutof

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Figure 3604. Pumping action of high-altitude winds.

thecyclonicsystem,intoahigh-altitudeanticyclone,whichtransportstheairwellawayfromthedisturbance,beforesinkingoccurs.Thus,alargescaleverticalcirculationissetup,inwhichlow-levelairisspiraledupthecyclonictwist-ingofthedisturbance,and,afteratrajectoryoverthesea,returnedtoloweraltitudessomedistancefromthestorm.Thispumpingaction-andtheheatreleasedbytheascendingairmayaccountforthesuddendropofatmosphericpres-sureatthesurface,whichproducesthesteeppressuregradient along which winds reach hurricane proportions.

Itisbelievedthattheinteractionoflow-levelandhigh-al-titudewindsystemsdeterminestheintensitythehurricanewillattain.Iflessairispumpedoutthanconvergesatlowlev-els,thesystemwillfillanddieout.Ifmoreispumpedoutthanflows in, the circulation will be sustained and will intensify.

Scientistshavefoundthatanyprocesswhichincreasestherateoflow-levelinflowisfavorableforhurricanedevel-opment,providedtheinflowingaircarriessufficientheatandmoisturetofuelthehurricane’spowersystem.Ithasalsobeenshownthatairabovethedevelopingdisturbance,atal-titudesbetween20,000and40,000feet,increases1°to3°intemperatureabout24hoursbeforethedisturbancedevelopsintoahurricane.Butitisnotknownwhetherlow-levelin-flowandhigh-levelwarmingcausehurricanes.Theycouldverywellbemeasurablesymptomsofanothereffectwhichactually triggers the storm’s increase to hurricane intensity.

Theviewofhurricanesasatmosphericenginesisnec-essarilyageneralone.Theexactroleofeachcontributorisnotcompletelyunderstood.Theengineseemstobebothin-efficientandunreliable;amyriadofdelicateconditionsmustbesatisfiedfortheatmospheretoproduceahurricane.Theirrelativeinfrequencyindicatesthatmanypotentialhurricanes dissipate before developing into storms.3605. Portrait Of A Hurricane

Intheearlylifeofthehurricane,thespiralcoversan

areaaveraging100milesindiameterwithwindsof64knotsandgreater,andspreadsgale-forcewindsovera400-milediameter.Thecyclonicspiralismarkedbyheavycloudbandsfromwhichtorrentialrainsfall,separatedbyareasoflightrainornorainatall.Thesespiralbandsas-cendindecksofcumulusandcumulonimbuscloudstotheconvectivelimitofcloudformation,wherecondensingwa-tervaporissweptoffasice-crystalwispsofcirrusclouds.Thunderstormelectricalactivityisobservedinthesebands,both as lightning and as tiny electrostatic discharges.

Inthelowerfewthousandfeet,airflowsinthroughthecy-clone,andisdrawnupwardthroughascendingcolumnsofairnearthecenter.Thesizeandintensitydecreasewithaltitude,thecycloniccirculationbeinggraduallyreplacedabove40,000feetbyananticycloniccirculationcenteredhundredsofmilesaway,whichistheexhaustsystemofthehurricaneheatengine.

Atlowerlevels,wherethehurricaneismoreintense,windsontherimofthestormfollowawidepattern,liketheslowercurrentsaroundtheedgeofawhirlpool;and,likethosecurrents,thesewindsaccelerateastheyapproachthecenterofthevortex.Theouterbandhaslightwindsattherimofthestorm,perhapsnomorethan25knots;within30milesofthecenter,windsmayhavevelocitiesexceeding130knots.Theinnerbandistheregionofmaximumwindvelocity,wherethestorm’sworstwindsarefelt,andwhereascendingairischimneyedupward,releasingheattodrivethestorm.Inmosthurricanes,thesewindsreach85knots,and more than 170 knots in severe storms.

Inthehurricane,windsflowtowardthelowpressureinthewarm,comparativelycalmcore.There,convergingairiswhirledupwardbyconvection,themechanicalthrustingofotherconvergingair,andthepumpingactionofhigh-al-titudecirculations.Thisspiralismarkedbythethickcloudwallscurlinginwardtowardthestormcenter,releasingheavyprecipitationandenormousquantitiesofheatenergy.Atthecenter,surroundedbyabandinwhichthisstrongvertical circulation is greatest, is theeye of the hurricane.

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Figure 3605. Cutaway view of a hurricane greatly exaggerated in vertical dimension. Actual hurricanes are less than

50,000 feet high and may have a diameter of several hundred miles.Ontheaverage,eyediameterisabout14miles,al-thoughdiametersof25milesarenotunusual.Fromtheheatedtowerofmaximumwindsandcumulonimbusclouds,windsdiminishrapidlytosomethinglessthan15milesperhourintheeye;attheoppositewall,windsin-creaseagain,butcomefromtheoppositedirectionbecauseofthecycloniccirculationofthestorm.Thissuddentrans-formationofstormintocomparativecalm,andfromcalmintoviolencefromanotherquarterisspectacular.Theeye’sabruptexistenceinthemidstofopaquerainsquallsandhur-ricanewinds,theintermittentburstsofblueskyandsunlightthroughlightcloudsinthecoreofthecyclone,andthegalleriedwallsofcumulusandcumulonimbuscloudsare unforgettable.

Everyhurricaneisindividual,andthemoreorlessor-derlycirculationdescribedhereomitstheextremevariabilityandinstabilitywithinthestormsystem.Pressureandtemperaturegradientsfluctuatewildlyacrossthestormasthehurricanemaintainsitserraticlife.IfitisanAuguststorm,itsaveragelifeexpectancyis12days;ifaJulyorNovember storm, it lives an average of 8 days.3606. Life Of A Tropical Cyclone

Reportsfromshipsinthevicinityofaneasterlywave(awestward-movingtroughoflowpressureembeddedindeepeasterlies)mayindicatethattheatmosphericpres-sureintheregionhasfallenmorethan5millibarsinthepast24hours.Thisiscauseforalarm,becauseintheTrop-icspressurevarieslittle;thenormaldiurnalpressurechangeisonlyabout3millibars.Satellitepicturesmayin-dicatethickeningmiddleandhighclouds.Squallsare

reportedaheadoftheeasterlywave,andwindreportsin-dicateacycloniccirculationisforming.Theformereasterlywave,nowclassifiedatropicaldisturbance,ismovingwestwardat10knotsunderthecanopyofalargehigh-pressuresystemaloft.Seasurfacetemperaturesinthe vicinity are in the 28°-30°C range.

Within48hourswindsincreaseto25knotsnearthecenterofdefinitecirculation,andcentralpressurehasdroppedbelow1000millibars.Thedisturbanceisnowclas-sifiedasatropicaldepression.Soonthecirculationextendsoutto100milesandupwardto20,000feet.Windsnearthecenterincreasetogaleforce,centralpressurefallsbelow990millibars,andtoweringcumulonimbuscloudsshieldadeveloping eye; a tropical storm has developed.

Satellitephotographsnowrevealatightlyorganizedtropicalcyclone,andreconnaissancereportsindicatemax-imumwindsof80knotsaroundacentralpressureof980millibars;ahurricanehasdeveloped.Ashiptotheright(leftintheSouthernHemisphere)ofthehurricane’scenter(lookingtowardthedirectionofstormmovement)reports30-footseas.Thehurricaneisrapidlymaturingasitcontin-ues westward.

Afewdayslaterthehurricanereachesitspeak.Thesatellitephotographsshowatextbookpicture,as120-knotwindsroararounda940-millibarpressurecenter;hurri-cane-forcewindsextend50milesinalldirections,andseasarereportedupto40feet.Thereisnofurtherdeepeningnow,butthehurricanebeginstoexpand.In2days,galesextendoutto200miles,andhurricanewindsoutto75miles.Thenthehurricaneslowsandbeginstorecurve;thisturning marks the beginning of its final phase.

Thehurricaneaccelerates,and,uponreachingtemper-

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atelatitudes,itbeginstoloseitstropicalcharacteristics.Thecirculationcontinuestoexpand,butnowcoldairisin-truding(coldair,coldwater,dryairaloft,andland,aidinthedecayofatropicalcyclone).Thewindsgraduallyabateastheconcentratedstormdisintegrates.Thewarmcoresur-

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vivesforafewmoredaysbeforethetransformationtoalarge extratropical low-pressure system is complete.

Notalltropicalcyclonesfollowthisaveragepattern.Mostfalterintheearlystages,somedissipateoverland,andothers remain potent for several weeks.

Figure 3606. Satellite photograph of a hurricane.

FORECASTING AND PREDICTING TROPICAL CYCLONES

3607. Weather Broadcasts And RadiofacsimileThemarineweatherbroadcastandradiofacsimileweathermapsarethemostimportanttoolsforavoidingtrop-icalcyclones.Thesebroadcasts,coveringalltropicalareas,

provideinformationaboutthetropicalcyclone’slocation,maximum winds and seas, and future conditions expected.

TheUS.Navy,theNationalOceanicandAtmosphericAdministration,andtheU.S.AirForcehavedevelopedahighlyeffectivesurveillancesystemforthetropicalcy-

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andwarningprogram,andradioaids,refertoSelectedWorldwideMarineWeatherBroadcasts,publishedjointlybytheNavalMeteorologyandOceanographyCommandandthe National Weather Service.

AlthoughtheareasofforecastingresponsibilityarefairlywelldefinedfortheDepartmentofDefense,theinter-nationalanddomesticciviliansystemprovidesmanyoverlapsandisdependentuponqualitativefactors.Forex-ample,whenatropicalstormorhurricaneistravelingwestwardandcrosses35°Wlongitude,thecontinuedissu-anceofforecastsandwarningstothegeneralpublic,shippinginterests,etc.,becomestheresponsibilityoftheNationalHurricaneCenteroftheNationalWeatherServiceatMiami,Florida.Whenatropicalstormorhurricanecrosses35°Wlongitudetravelingfromwesttoeast,theNa-tionalHurricaneCenterceasestoissueformalpublicadvisories,butwillissuemarinebulletinsonanydangeroustropicalcycloneintheNorthAtlantic,ifitisofimportanceorconstitutesathreattoshippingandotherinterests.TheseadvisoriesareincludedinNationalWeatherServiceMarineBulletinsbroadcasttoshipsoverradiostationNAMNor-folk,Virginia.Specialadvisoriesmaybeissuedatanytime.IntheAtlanticOcean,DepartmentofDefenseresponsibili-tyrestswiththeNavalAtlanticMeteorologyandOceanography Center in Norfolk, Virginia.

IntheeasternPacificeastoflongitude140°W,respon-

clone-proneareasoftheworld.Routineandspecialweatherreports(fromlandstations,shipsatsea,aircraft;weathersatelliteimagery;radarreportsfromlandstations;specialreportsfromshipsatsea;andthespeciallyinstrumentedweatherreconnaissanceaircraftofNationalOceanicandAtmosphericAdministrationandtheU.S.AirForce)enableaccuratedetection,location,andtrackingoftropicalcy-clones.Internationalcooperationiseffective.Databuoys,bothmooredanddrifting,provideanothersourceofinformation.

Thetropicalwarningserviceshavethreeprincipalfunctions:

1.Thecollectionandanalysisofthenecessaryobser-vational data.

2.Thepreparationoftimelyandaccurateforecastsand warnings.

3.Therapidandefficientdistributionofadvisories,warnings, and all other pertinent information.Toprovidetimelyandaccurateinformationandwarningsregardingtropicalcyclones,theoceanshavebeendividedintooverlappinggeographicalareasofresponsibility.

Fordetailedinformationontheareasofresponsibilityofthecountriesparticipatingintheinternationalforecasting

NOAA/NATIONAL HURRICANE CENTER MARINE ADVISORY NUM-BER 13 HURRICANE LADY 0400Z SEPTEMBER 21 19--.

HURRICANE WARNINGS ARE DISPLAYED FROM KEY LARGO TOCAPE KENNEDY. GALE WARNINGS ARE DISPLAYED FROM KEY

WEST TO JACKSONVILLE AND FROM FLORIDA BAY TO CEDAR KEY.HURRICANE CENTER LOCATED NEAR LATITUDE 25.5 NORTHLONGITUDE 78.5 WEST AT 21/0400Z. POSITION EXCELLENT AC-CURATE WITHIN 10 MILES BASED ON AIR FORCE RECONNAISSANCEAND SYNOPTIC REPORTS.

PRESENT MOVEMENT TOWARD THE WEST NORTHWEST OR 285DEGREES AT 10 KT. MAX SUSTAINED WINDS OF 100 KT NEARCENTER WITH GUSTS TO 160 KT.

MAX WINDS OVER INLAND AREAS 35 KT.

RAD OF 65 KT WINDS 90 NE 60 SE 80 SW 90 NW QUAD.RAD OF 50 KT WINDS 120 NE 70 SE 90 SW 120 NW QUAD.RAD OF 30 KT WINDS 210 NE 210 SE 210 SW 210 NW QUAD.REPEAT CENTER LOCATED 25.5N 78.3W AT 21/0400Z.

12 HOUR FORECAST VALID 21/1600Z LATITUDE 26.0N LONGI-TUDE 80.5W.

MAX WINDS OF 100 KT NEAR CENTER WITH GUSTS TO 160 KT.MAX WINDS OVER INLAND AREAS 65 KT.

RADIUS OF 50 KT WINDS 120 NE 70 SE 90 SW 120 NW QUAD.24 HOUR FORECAST VALID 22/0400Z LATITUDE 26.0NLONGITUDE 83.0W.

MAX WINDS OF 75 KT NEAR CENTER WITH GUSTS TO 120 KT.MAX WINDS OVER INLAND AREAS 45 KT.

RADIUS OF 50 KT WINDS 120 NE 120 SE 120 SW 120 NW QUAD.STORM TIDE OF 9 TO 12 FT SOUTHEAST FLA COAST GREATERMIAMI AREA TO THE PALM BEACHES.NEXT ADVISORY AT 21/1000Z.

Figure 3607. Example of marine advisory issued by National Hurricane Center.

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sibilityfortheissuanceoftropicalstormandhurricaneadvisoriesandwarningsforthegeneralpublic,merchantshipping,andotherinterestsrestswiththeNationalWeath-erServiceEasternPacificHurricaneCenter,SanFrancisco,California.TheDepartmentofDefenseresponsibilityrestswiththeNavalPacificMeteorologyandOceanographyCenter,PearlHarbor,Hawaii.Formaladvisoriesandwarn-ingsareissueddailyandareincludedinthemarinebulletinsbroadcastbyradiostationsKFS,NMC,andNMQ.

InthecentralPacific(betweenthemeridianandlongi-tude140°W),thecivilianresponsibilityrestswiththeNationalWeatherServiceCentralPacificHurricaneCen-ter,Honolulu,Hawaii.DepartmentofDefenseresponsibilityrestswiththeNavalPacificMeteorologyandOceanographyCenterinPearlHarbor.Formaltropicalstormandhurricaneadvisoriesandwarningsareissueddai-lyandareincludedinthemarinebulletinsbroadcastbyradio station NMO and NRV.

Tropicalcycloneinformationmessagesgenerallycon-tainpositionofthestorm,intensity,directionandspeedofmovement,andadescriptionoftheareaofstrongwinds.Alsoincludedisaforecastoffuturemovementandintensi-ty.Whenthestormislikelytoaffectanylandarea,detailsonwhenandwhereitwillbefelt,anddataontides,rain,floods,andmaximumwindsarealsoincluded.Figure3607providesanexampleofamarineadvisoryissuedbytheNa-tional Hurricane Center.

TheNavalPacificMeteorologyandOceanography

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CenterCenter-West/JointTyphoonWarningCenter(NP-MOC-W/JTWC)inGuamisresponsibleforallU.S.tropicalstormandtyphoonadvisoriesandwarningsfromthe180thmeridianwestwardtothemainlandofAsia.Asecondaryareaofresponsibilityextendswestwardtolon-gitude90°E.WheneveratropicalcycloneisobservedinthewesternNorthPacificarea,seriallynumberedwarn-ings,bearingan“immediate”precedencearebroadcastfromtheNPMOC-W/JTWCat0000,0600,1200,and1800 GMT.

TheresponsibilityforissuinggaleandstormwarningsfortheIndianOcean,ArabianSea,BayofBengal,WesternPacific,andSouthPacificrestswithmanycountries.Ingeneral,warningsofapproachingtropicalcycloneswhichmaybehazardouswillincludethefollowinginformation:stormtype,centralpressuregiveninmillibars,windspeedobservedwithinthestorm,stormlocation,speedanddirec-tionofmovement,theextentoftheaffectedarea,visibility,andthestateofthesea,aswellasanyotherpertinentinfor-mationreceived.Allstormwarningmessagescommencewith the international call sign “TTT.”

Thesewarningsarebroadcastonspecifiedradiofrequencybandsimmediatelyuponreceiptoftheinformationandatspe-cificintervalsthereafter.Generally,thebroadcastintervalisevery6to8hours,dependinguponreceiptofnewinformation.

Bulletinsandforecastsareexcellentguidestothepresentandfuturebehaviorofthetropicalcyclone,andaplot should be kept of all positions.

AVOIDING TROPICAL CYCLONES

3608. Approach And Passage Of A Tropical CycloneAnearlyindicationoftheapproachofatropicalcy-cloneisthepresenceofalongswell.Intheabsenceofatropicalcyclone,thecrestsofswellinthedeepwatersoftheAtlanticpassattherateofperhapseightperminute.Swellgeneratedbyahurricaneisabouttwiceaslong,thecrestspassingattherateofperhapsfourperminute.Swellmaybeobserved several days before arrival of the storm.

Whenthestormcenteris500to1,000milesaway,thebarometerusuallyrisesalittle,andtheskiesarerelativelyclear.Cumulusclouds,ifpresentatall,arefewinnumberandtheirverticaldevelopmentappearssuppressed.Theba-rometerusuallyappearsrestless,pumpingupanddownafew hundredths of an inch.

Asthetropicalcyclonecomesnearer,acloudsequencebeginswhichresemblesthatassociatedwiththeapproachofawarmfrontinmiddlelatitudes.Snow-white,fibrous“mare’stails”(cirrus)appearwhenthestormisabout300to600milesaway.Usuallytheseseemtoconverge,moreorless,inthedirectionfromwhichthestormisapproach-ing.Thisconvergenceisparticularlyapparentataboutthetime of sunrise and sunset.

Shortlyafterthecirrusappears,butsometimesbefore,

thebarometerstartsalong,slowfall.Atfirstthefallissogradualthatitonlyappearstoaltersomewhatthenormaldailycycle(twomaximaandtwominimaintheTropics).Astherateoffallincreases,thedailypatterniscompletelylost in the more or less steady fall.

Thecirrusbecomesmoreconfusedandtangled,andthengraduallygiveswaytoacontinuousveilofcirrostratus.Belowthisveil,altostratusforms,andthenstratocumulus.Thesecloudsgraduallybecomemoredense,andastheydoso,theweatherbecomesunsettled.Afine,mist-likerainbe-ginstofall,interruptedfromtimetotimebyrainshowers.The barometer has fallen perhaps a tenth of an inch.

Asthefallbecomesmorerapid,thewindincreasesingustiness,anditsspeedbecomesgreater,reachingperhaps22to40knots(Beaufort6-8).Onthehorizonappearsadarkwallofheavycumulonimbus,calledthebarofthestorm.Thisistheheavybankofcloudscomprisingthemainmassofthecyclone.Portionsofthisheavycloudbecomede-tachedfromtimetotime,anddriftacrossthesky,accompaniedbyrainsquallsandwindofincreasingspeed.Betweensqualls,thecirrostratuscanbeseenthroughbreaks in the stratocumulus.

Asthebarapproaches,thebarometerfallsmorerapidlyandwindspeedincreases.Theseas,whichhavebeengradu-

514TROPICAL CYCLONES

Figure 3608. Typical hurricane cloud formations.

allymounting,becometempestuous.Squalllines,oneaftertheother,sweeppastineverincreasingnumberandintensity.

Withthearrivalofthebar,thedaybecomesverydark,squallsbecomevirtuallycontinuous,andthebarometerfallsprecipitously,witharapidincreaseinwindspeed.Thecentermaystillbe100to200milesawayinafullydevel-opedtropicalcyclone.Asthecenterofthestormcomescloser,theever-strongerwindshrieksthroughtherigging,andaboutthesuperstructureofthevessel.Asthecenterap-proaches,rainfallsintorrents.Thewindfuryincreases.Theseasbecomemountainous.Thetopsofhugewavesareblownofftominglewiththerainandfilltheairwithwater.Visibilityisvirtuallyzeroinblindingrainandspray.Eventhelargestandmostseaworthyvesselsbecomevirtuallyunmanageable,andmaysustainheavydamage.Lesssturdyvesselsmaynotsurvive.Navigationvirtuallystopsassafe-tyofthevesselbecomestheonlyconsideration.Theawesomefuryofthisconditioncanonlybeexperienced.Words are inadequate to describe it.

Iftheeyeofthestormpassesoverthevessel,thewindssuddenlydroptoabreezeasthewalloftheeyepasses.Therainstops,andtheskiesclearsufficientlytopermitthesunorstarstoshinethroughholesinthecomparativelythincloudcover.Visibilityimproves.Mountainousseasap-proachfromallsidesincompleteconfusion.Thebarometerreachesitslowestpoint,whichmaybe11/2or2inchesbe-lownormalinfullydevelopedtropicalcyclones.Asthewallontheoppositesideoftheeyearrives,thefullfuryofthewindstrikesassuddenlyasitceased,butfromtheop-positedirection.Thesequenceofconditionsthatoccurredduringapproachofthestormisreversed,andpassesmorequickly,asthevariouspartsofthestormarenotaswideinthe rear of a storm as on its forward side.

Typicalcloudformationsassociatedwithahurricaneare shown in Figure 3608.

3609. Locating The Center Of A Tropical CycloneIfintelligentactionistobetakentoavoidthefullfuryofatropicalcyclone,earlydeterminationofitslocationanddirectionoftravelrelativetothevesselisessential.Thebul-letinsandforecastsareanexcellentgeneralguide,buttheyarenotinfallible,andmaybesufficientlyinerrortoinduceamarinerinacriticalpositiontoaltercoursesoastounwit-tinglyincreasethedangertohisvessel.Oftenitispossible,usingonlythoseobservationsmadeaboardship,toobtainasufficientlycloseapproximationtoenablethevesseltoma-neuver to the best advantage.

Thepresenceofanexceptionallylongswellisusuallythefirstvisibleindicationoftheexistenceofatropicalcy-clone.Indeepwateritapproachesfromthegeneraldirectionoforigin(thepositionofthestormcenterwhentheswellwasgenerated).However,inshoalingwaterthisisalessreliableindicationbecausethedirectionischangedbyrefraction,thecrestsbeingmorenearlyparalleltothebottom contours.

Whenthecirruscloudsappear,theirpointofconver-genceprovidesanindicationofthedirectionofthestormcenter.Ifthestormistopasswelltoonesideoftheobserv-er,thepointofconvergenceshiftsslowlyinthedirectionofstormmovement.Ifthestormcenterwillpassneartheob-server,thispointremainssteady.Whenthebarbecomesvisible,itappearstorestuponthehorizonforseveralhours.Thedarkestpartofthiscloudisinthedirectionofthestormcenter.Ifthestormistopasstooneside,thebarappearstodriftslowlyalongthehorizon.Ifthestormisheadingdi-

TROPICAL CYCLONES

rectlytowardtheobserver,thepositionofthebarremainsfixed.Oncewithintheareaofthedense,lowclouds,oneshouldobservetheirdirectionofmovement,whichisal-mostexactlyalongtheisobars,withthecenterofthestormbeing90°fromthedirectionofcloudmovement(leftofdi-rectionofmovementintheNorthernHemisphere,andrightin the Southern Hemisphere).

Thewindsareprobablythebestguidetothedirectionofthecenterofatropicalcyclone.Thecirculationiscy-clonic,butbecauseofthesteeppressuregradientnearthecenter,thewindsthereblowwithgreaterviolenceandaremore nearly circular than in extratropical cyclones.

AccordingtoBuysBallot’slaw,anobserverwhosebackistothewindhasthethelowpressureonhisleftintheNorthernHemisphere,andonhisrightintheSouthernHemi-sphere.Ifthewindfollowedcircularisobarsexactly,thecenterwouldbeexactly90°frombehindwhenfacingawayfromthewind.However,thetrackofthewindisusuallyin-clinedsomewhattowardthecenter,sothattheanglefromdeadasternvariesbetweenperhaps90°to135°.Theinclina-tionvariesindifferentpartsofthesamestorm.Itisleastin

515

frontofthestorm,andgreatestintherear,sincetheactualwindisthevectorsumofthepressuregradientandthemo-tionofthestormalongthetrack.Agoodaverageisperhaps110°infront,and120-135°intherear.Thesevaluesapplywhenthestormcenterisstillseveralhundredmilesaway.Closertothecenter,thewindblowsmorenearlyalongtheisobars,theinclinationbeingreducedbyoneortwopointsatthewalloftheeye.Sincewinddirectionusuallyshiftstem-porarilyduringasquall,itsdirectionatthistimeshouldnotbeusedfordeterminingthepositionofthecenter.Theap-proximaterelationshipofwindtoisobarsandstormcenterinthe Northern Hemisphere is shown in Figure 3609a.

Whenthecenteriswithinradarrange,itwillprobablybevisibleonthescope.However,sincetheradarreturnispredominantlyfromtherain,resultscanbedeceptive,andotherindicationsshouldnotbeneglected.Figure3609bshowsaradarPPIpresentationofatropicalcyclone.Iftheeyeisoutofrange,thespiralbands(Figure3609b)mayin-dicateitsdirectionfromthevessel.Trackingtheeyeorupwindportionofthespiralbandsenablesdeterminingthedirectionandspeedofmovement;thisshouldbedoneforat

Figure 3609a. Approximate relationship of wind to isobars and storm center in the Northern Hemisphere.

516TROPICAL CYCLONES

Figure 3609b. Radar PPI presentation of a tropical

cyclone.least1hourbecausetheeyetendstooscillate.Thetrackingofindividualcells,whichtendtomovetangentiallyaroundtheeye,for15minutesormore,eitherattheendofthebandorbetweenbands,willprovideanindicationofthewindspeed in that area of the storm.

Distancefromthestormcenterismoredifficulttode-terminethandirection.Radarisperhapsthebestguide.However,therateoffallofthebarometerissomeindication.

3610. Statistical Analysis Of Barometric PressureThelowest-sea-levelpressureeverrecordedwas877millibarsintyphoonIda,onSeptember24,1958.Theob-servationwastakenbyareconnaissanceaircraftdropsonde,some750mileseastofLuzon,Philippines.Thisobserva-tionwasobtainedagainintyphoonNoraonOctober6,1973.ThelowestbarometricreadingofrecordfortheUnit-edStatesis892.3millibars,obtainedduringahurricaneatLowerMatecumbeKey,Florida,inSeptember1935.InhurricaneCamillein1969,a905millibarpressurewasmeasuredbyreconnaissanceaircraft.Duringa1927ty-phoon,theS.S.Sapoeroearecordedapressureof886.6millibars,thelowestsea-levelpressurereportedfromaship.Pressurehasbeenobservedtodropmorethan33mil-libarsperhour,withapressuregradientamountingtoachange of 3.7 millibars per mile.

Amethodforalertingthemarinertopossibletropicalcycloneformationinvolvesastatisticalcomparisonofob-servedweatherparameterswiththeclimatology(30yearaveragedconditions)forthoseparameters.Significantfluc-tuationsawayfromtheseaverageconditionscouldmeantheonsetofsevereweather.Onesuchstatisticalmethodin-volvesacomparisonofmeansurfacepressureinthetropicswiththestandarddeviation(s.d.)ofsurfacepressure.Anysignificantdeviationfromthenormcouldindicateproxim-

itytoatropicalcyclone.Analysisshowsthatsurfacepressurecanbeexpectedtobelowerthanthemeanminus1s.d.lessthan16%ofthetime,lowerthanthemeanminus1.5s.d.lessthan7%ofthetime,andlowerthanthemeanminus2s.d.lessthan3%ofthetime.Comparisonoftheob-servedpressurewiththemeanwillindicatehow“unusual”the present conditions are.

Asanexample,assumethemeansurfacepressureintheSouthChinaSeatobeabout1005mbduringAugustwithas.d.ofabout2mb.Therefore,surfacepressurecanbeexpectedtofallbelow1003mbabout16%ofthetimeandbelow1000mbabout7%ofthetime.Ambientpressureanylowerthanthatwouldalertthemarinertothepossibleonsetofheavyweather.Chartsshowingthemeansurfacepressureandthes.d.ofsurfacepressureforvariousglobalregionscanbefoundintheU.S.NavyMarineClimaticAt-las of the World.

3611. Maneuvering To Avoid The Storm Center

Thesafestprocedurewithrespecttotropicalcyclonesistoavoidthem.Ifactionistakensufficientlyearly,thisissimplyamatterofsettingacoursethatwilltakethevesselwelltoonesideoftheprobabletrackofthestorm,andthencontinuingtoplotthepositionsofthestormcenterasgivenin the weather bulletins, revising the course as needed.

However,thisisnotalwayspossible.Iftheshipisfoundtobewithinthestormarea,theproperactiontotakedependsinpartuponitspositionrelativetothestormcenteranditsdirectionoftravel.Itiscustomarytodividethecir-cular area of the storm into two parts.

IntheNorthernHemisphere,thatparttotherightofthestormtrack(facinginthedirectiontowardwhichthestormismoving)iscalledthedangeroussemicircle.Itisconsid-ereddangerousbecause(1)theactualwindspeedisgreaterthanthatduetothepressuregradientalone,sinceitisaug-mentedbytheforwardmotionofthestorm,and(2)thedirectionofthewindandseaissuchastocarryavesselintothepathofthestorm(intheforwardpartofthesemicircle).

Theparttotheleftofthestormtrackiscalledthelessdangeroussemicircle,ornavigablesemicircle.Inthispart,thewindisdecreasedbytheforwardmotionofthestorm,andthewindblowsvesselsawayfromthestormtrack(intheforwardpart).Becauseofthegreaterwindspeedinthedangeroussemicircle,theseasarehigherthaninthelessdangeroussemicircle.IntheSouthernHemi-sphere,thedangeroussemicircleistotheleftofthestormtrack,andthelessdangeroussemicircleistotherightofthestorm track.

Aplotofsuccessivepositionsofthestormcentershouldindicatethesemicircleinwhichavesselislocated.However,ifthisisbaseduponweatherbulletins,itmaynotbeareliableguidebecauseofthelagbetweentheobservationsuponwhichthebulletinisbasedandthetimeofreceptionofthebulletin,withtheever-presentpossibilityofachangeinthedirectionofthestorm.Theuseofradareliminatesthislagatshortrange,butthereturnmaynotbeatrueindicationofthecenter.Perhapsthemostreliableguideisthewind.Within

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517

thecycloniccirculation,awindshiftingtotherightinthenorthernhemisphereandtotheleftinthesouthernhemi-sphereindicatesthevesselisprobablyinthedangeroussemicircle.Asteadywindshiftoppositetothisindicatesthevessel is probably in the less dangerous semicircle.

However,ifavesselisunderway,itsownmotionshouldbeconsidered.Ifitisoutrunningthestormorpullingrapidlytowardoneside(whichisnotdifficultduringtheearlystagesofastorm,whenitsspeedislow),theoppositeeffectoccurs.Thisshouldusuallybeaccompaniedbyariseinatmosphericpressure,butifmotionofthevesselisnearlyalonganisobar,thismaynotbeareliableindication.Ifindoubt,thesafestactionisusuallytostoplongenoughtode-finethepropersemicircle.Thelossintimemaybemorethanoffsetbytheminimizingofthepossibilityoftakingthewrongaction,increasingthedangertothevessel.Ifthewinddirectionremainssteady(foravesselwhichisstopped),withincreasingspeedandfallingbarometer,thevesselisinornearthepathofthestorm.Ifitremainssteadywithdecreasingspeedandrisingbarometer,thevesselisnear the storm track, behind the center.

Thefirstactiontotakeiftheshipiswithinthecycloniccirculationistodeterminethepositionofhisvesselwithre-specttothestormcenter.Whilethevesselcanstillmakeconsiderablewaythroughthewater,acourseshouldbese-lectedtotakeitasfaraspossiblefromthecenter.Ifthevesselcanmovefasterthanthestorm,itisarelativelysim-plemattertooutrunthestormifsearoompermits.Butwhenthestormisfaster,thesolutionisnotassimple.Inthiscase,thevessel,ifaheadofthestorm,willapproachnearertothecenter.Theproblemistoselectacoursethatwillpro-ducethegreatestpossibleminimumdistance.Thisisbestdeterminedbymeansofarelativemovementplot,asshownin the following example solved on a maneuvering board.Example:Atropicalcycloneisestimatedtobemovingindirection320°at19knots.Itscenterbears170°,atanes-timateddistanceof200milesfromavesselwhichhasamaximum speed of 12 knots.

Required:

(1)The course to steer at 12 knots to produce the

greatest possible minimum distance between thevessel and the storm center.

(2)The distance to the center at nearest approach.(3)Elapsed time until nearest approach.

Solution:(Figure3611)Considerthevesselremaining

atthecenteroftheplotthroughoutthesolution,ason a radar PPI.

(1)Tolocatethepositionofthestormcenterrelativetothevessel,plotpointCatadistanceof200miles(scale20:1)indirection170°fromthecenterofthediagram.Fromthecenterofthediagram,drawRA,thespeedvectorofthestormcenter,indirection320°,speed19knots(scale2:1).FromAdrawalinetangenttothe12-knotspeedcircle(labeled6at

scale2:1)onthesideoppositethestormcenter.Fromthecen-terofthediagram,drawaperpendiculartothistangentline,locatingpointB.ThelineRBistherequiredspeedvectorforthe vessel. Its direction, 011°, is the required course.

(2)ThepathofthestormcenterrelativetothevesselwillbealongalinefromCinthedirectionBA,ifbothstormandvesselmaintaincourseandspeed.Thepointofnearestap-proachwillbeatD,thefootofaperpendicularfromthecenterof the diagram. This distance, at scale 20:1, is 187 miles.

(3)ThelengthofthevectorBA(14.8knots)isthespeedofthestormwithrespecttothevessel.Markthisonthelow-estscaleofthenomogramatthebottomofthediagram.TherelativedistanceCDis72miles,bymeasurement.Markthis(scale10:1)onthemiddlescaleatthebottomofthediagram.Drawalinebetweenthetwopointsandextendittointersectthetopscaleat29.2(292at10:1scale).Theelapsedtimeistherefore 292 minutes, or 4 hours 52 minutes.

Answers:(1) C 011°, (2) D 187 mi., (3) 4h 52m.

Thestormcenterwillbedeadasternatitsnearestapproach.

Asageneralrule,foravesselintheNorthernHemi-sphere,safetyliesinplacingthewindonthestarboardbowinthedangeroussemicircleandonthestarboardquarterinthelessdangeroussemicircle.Ifonthestormtrackaheadofthestorm,thewindshouldbeputabout160°onthestar-boardquarteruntilthevesseliswellwithinthelessdangeroussemicircle,andtheruleforthatsemicirclethenfollowed.IntheSouthernHemispherethesameruleshold,butwithrespecttotheportside.Withafasterthanaveragevessel,thewindcanbebroughtalittlefartheraftineachcase.However,asthespeedofthestormincreasesalongitstrack,thewindshouldbebroughtfartherforward.Iflandinterfereswithwhatwouldotherwisebethebestmaneuver,the solution should be altered to fit the circumstances.

Ifthevesselisfasterthanthestorm,itispossibletoovertakeit.Inthiscase,theonlyactionusuallyneededistoslow enough to let the storm pull ahead.

Inallcases,oneshouldbealerttochangesinthedirec-tionofmovementofthestormcenter,particularlyintheareawherethetracknormallycurvestowardthepole.Ifthestormmaintainsitsdirectionandspeed,theship’scourseshould be maintained as the wind shifts.

Ifitbecomesnecessaryforavesseltoheaveto,thecharacteristicsofthevesselshouldbeconsidered.Apowervesselisconcernedprimarilywithdamagebydirectactionofthesea.Agoodgeneralruleistoheavetowithheadtotheseainthedangeroussemicircle,orsterntotheseainthelessdangeroussemicircle.Thiswillresultingreatestamountofheadwayawayfromthestormcenter,andleastamountofleewaytowardit.Ifavesselhandlesbetterwiththeseaasternoronthequarter,itmaybeplacedinthispo-sitioninthelessdangeroussemicircleorintherearhalfofthedangeroussemicircle,butneverintheforwardhalfofthedangeroussemicircle.Ithasbeenreportedthatwhenthe

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windreacheshurricanespeedandtheseasbecomecon-fused,someshipsrideoutthestormbestiftheenginesarestopped,andthevesselislefttoseekitsownposition,orlieahull.Inthisway,itissaid,theshiprideswiththestormin-stead of fighting against it.

Inasailingvesselattemptingtoavoidastormcen-ter,oneshouldsteercoursesasnearaspossibletothoseprescribedaboveforpowervessels.However,ifitbe-comesnecessaryforsuchavesseltoheaveto,thewindisofgreaterconcernthanthesea.Agoodgeneralruleal-waysistoheavetoonwhichevertackpermitstheshiftingwindtodrawaft.IntheNorthernHemisphere,thisisthestarboardtackinthedangeroussemicircle,andtheporttackinthelessdangeroussemicircle.IntheSouthern Hemisphere these are reversed.

Whileeachstormrequiresitsownanalysis,andfre-quentorcontinualresurveyofthesituation,thegeneralrules for a steamer may be summarized as follows:

Northern Hemisphere

Rightordangeroussemicircle:Bringthewindonthe

starboardbow(045˚relative),holdcourseandmakeasmuchwayaspossible.Ifnecessary,heaveto with head to the sea.

Leftorlessdangeroussemicircle:Bringthewindon

thestarboardquarter(135°relative),holdcourseandmakeasmuchwayaspossible.Ifnecessary,heave to with stern to the sea.

Onstormtrack,aheadofcenter:Bringthewind2

pointsonthestarboardquarter(about160°rela-tive),holdcourseandmakeasmuchwayaspossible.Whenwellwithinthelessdangeroussemicircle, maneuver as indicated above.

Onstormtrack,behindcenter:Avoidthecenterby

thebestpracticablecourse,keepinginmindthetendencyoftropicalcyclonestocurvenorthwardand eastward.

Figure 3611. Determining the course to avoid the storm center.

TROPICAL CYCLONES519

Southern Hemisphere

Leftordangeroussemicircle:Bringthewindonthe

portbow(315°relative),holdcourseandmakeasmuchwayaspossible.Ifnecessary,heavetowithhead to the sea.

Rightorlessdangeroussemicircle:Bringthewind

ontheportquarter(225°relative),holdcourseandmakeasmuchwayaspossible.Ifnecessary,heaveto with stern to the sea.

Onstormtrack,aheadofcenter:Bringthewind

about200°relative,holdcourseandmakeasmuchwayaspossible.Whenwellwithinthelessdan-gerous semicircle, maneuver as indicated above.Onstormtrack,behindcenter:Avoidthecenterbythe

bestpracticablecourse,keepinginmindthetendency

oftropicalcyclonestocurvesouthwardandeastward.

Itispossible,particularlyintemperatelatitudesafterthestormhasrecurved,thatthedangeroussemicircleistheleftoneintheNorthernHemisphere(rightoneintheSouth-ernHemisphere).Thiscanoccurifalargehighliesnorthofthestormandcausesatighteningofthepressuregradientinthe region.

TheTyphoonHavensHandbookfortheWesternPa-cificandIndianOceansispublishedbytheNavalOceanographicandAtmosphericResearchLab(NOARL)Monterey,California,asanaidtocaptainsandcommandingofficersofshipsinevaluatingaty-phoonsituation,andtoassistthemindecidingwhethertosortie,toevade,toremaininport,ortoheadfortheshelter of a specific harbor.

CONSEQUENCES OF TROPICAL CYCLONES

3612. High Winds And Flooding

Thehighwindsofatropicalcycloneinflictwidespreaddamagewhensuchastormleavestheoceanandcrossesland.Aidstonavigationmaybeblownoutofpositionorde-stroyed.Craftinharbors,oftenliftedbythestormsurge,breakmooringsordraganchorandareblownashoreandagainstobstructions.Ashore,treesareblownover,housesaredamaged,powerlinesareblowndown,etc.Thegreatestdamageusuallyoccursinthedangeroussemicircleashortdistancefromthecenter,wherethestrongestwindsoccur.Asthestormcontinuesonacrossland,itsfurysubsidesfaster than it would if it had remained over water.

Windinstrumentsareusuallyincapableofmeasuringthe175to200knotwindsofthemoreintensehurricanes;iftheinstrumentholdsup,oftenthesupportingstructuregivesway.Dopplerradarmaybeeffectiveindeterminingwind speeds, but may also be blown away.

Windgusts,whichareusually30to50percenthigherthansustainedwinds,addsignificantlytothedestructive-nessofthetropicalcyclone.Manytropicalcyclonesthatreachhurricaneintensitydevelopwindsofmorethan90knotssometimeduringtheirlives,butfewdevelopwindsofmore than 130 knots.

Tropicalcycloneshaveproducedsomeoftheworld’sheaviestrainfalls.Whileaverageamountsrangefrom6to10inches,totalsnear100inchesovera4-dayperiodhavebeenobserved.A24-hourworld’srecordof73.62inchesfellatReunionIslandduringatropicalcyclonein1952.Forwardmovementofthestormandlandtopographyhaveaconsiderableinfluenceonrainfalltotals.Torrentialrainscanoccurwhenastormmovesagainstamountainrange;thisiscommoninthePhilippinesandJapan,whereevenweaktropicaldepressionsproduceconsiderablerainfall.A24-hourtotalof46incheswasrecordedinthePhilippinesduringatyphoonin1911.AshurricaneCamillecrossed

southernVirginia’sBlueRidgeMountainsinAugustof1969,therewasnearly30inchesofraininabout8hours.Thiscausedsomeofthemostdisastrousfloodsinthestate’shistory.

Floodingisanextremelydestructiveby-productofthetropicalcyclone’storrentialrains.Whetheranareawillbefloodeddependsonthephysicalcharacteristicsofthedrainagebasin,rateandaccumulationofprecipitation,andriverstagesatthetimetherainsbegin.Whenheavyrainsfalloverflatterrain,thecountrysidemaylieunderwaterforamonthorso,andwhilebuildings,furnishings,andunder-groundpowerlinesmaybedamaged,thereareusuallyfewfatalities.Inmountainousorhillcountry,disastrousfloodsdevelop rapidly and can cause a great loss of life.

Therehavebeenoccasionalreportsintropicalcyclonesofwavesgreaterthan40feetinheight,andnumerousreportsinthe30-to40-footcategory.However,intropicalcyclones,strongwindsrarelypersistforasufficientlylongtimeoroveralargeenoughareatopermitenormouswaveheightstodevel-op.Thedirectionandspeedofthewindchangesmorerapidlyintropicalcyclonesthaninextratropicalstorms.Thus,themaximumdurationandfetchforanywindconditionisoftenlessintropicalcyclonesthaninextratropicalstorms,andthewavesaccompanyinganygivenlocalwindconditionsaregen-erallynotsohighasthoseexpected,withsimilarlocalwindconditions,inthehigh-latitudestorms.InhurricaneCamille,significantwavesof43feetwererecorded;anextremewaveheight reached 72 feet.

Exceptionalconditionsmayarisewhenwavesofcer-taindimensionstravelwithinthestormataspeedequaltothestorm’sspeed,thus,ineffect,extendingthedurationandfetchofthewaveandsignificantlyincreasingitsheight.ThisoccursmostoftentotherightofthetrackintheNorthernHemisphere(leftofthetrackintheSouthernHemisphere).Anotherconditionthatmaygiverisetoex-ceptionalwaveheightsistheintersectionofwavesfrom

520TROPICAL CYCLONES

twoormoredistinctdirections.Thismayleadtoazoneofconfusedseasinwhichtheheightsofsomewaveswillequalthesumsofeachindividualwavetrain.Thisprocesscanoccurinanyquadrantofthestorm,soitshouldnotbeassumedthatthehighestwaveswillalwaysbeencounteredtotherightofthestormtrackintheNorthernHemisphere(left of the track in the Southern Hemisphere).

Whenthesewavesmovebeyondtheinfluenceofthegeneratingwinds,theybecomeknownasswell.Theyarerecognizedbytheirsmooth,undulatingform,incontrasttothesteep,raggedcrestsofwindwaves.Thisswell,particu-larlythatgeneratedbytherightsideofthestorm,cantravelathousandmilesormoreandmayproducetides3or4feetabovenormalalongseveralhundredmilesofcoastline.Itmayalsoproducetremendoussurfoveroffshorereefswhich normally are calm.

Whenatropicalcyclonemovesclosetoacoast,windoftencausesarapidriseinwaterlevel,andalongwiththefallingpressuremayproduceastormsurge.ThissurgeisusuallyconfinedtotherightofthetrackintheNorthernHemisphere(leftofthetrackintheSouthernHemisphere)andtoarelativelysmallsectionofthecoastline.Itmostof-tenoccurswiththeapproachofthestorm,butinsomecases,whereasurgemovesintoalongchannel,theeffectmaybedelayed.Occasionally,thegreatestriseinwaterisobservedontheoppositesideofthetrack,whennortherlywindsfunnelintoapartiallylandlockedharbor.Thesurgecouldbe3feetorless,oritcouldbe20feetormore,de-pending on the combination of factors involved.

Therehavebeenreportsofa“hurricanewave,”de-scribedasa“wallofwater,”whichmovesrapidlytowardthecoastline.Authenticatedcasesarerare,butsomeoftheworld’sgreatestnaturaldisastershaveoccurredasaresultofthiswave,whichmaybearapidlyrisingandabnormallyhighstormsurge.InIndia,suchadisasteroccurredin1876,betweenCalcuttaandChittagong,anddrownedmorethan100,000 persons.

Alongthecoast,greaterdamagemaybeinflictedbywaterthanbythewind.Thereareatleastfoursourcesofwaterdamage.First,theunusuallyhighseasgeneratedbythestormwindspoundagainstshoreinstallationsandcraftintheirway.Second,thecontinuedblowingofthewindto-

wardlandcausesthewaterleveltoincreaseperhaps3to10feetaboveitsnormallevel.Thisstormtide,whichmaybe-ginwhenthestormcenteris500milesorevenfartherfromtheshore,graduallyincreasesuntilthestormpasses.Thehigheststormtidesarecausedbyaslow-movingtropicalcycloneoflargediameter,becausebothoftheseeffectsre-sultingreaterdurationofwindinthesamedirection.Theeffectisgreatestinapartlyenclosedbodyofwater,suchastheGulfofMexico,wheretheconcavecoastlinedoesnotreadilypermittheescapeofwater.Itisleastonsmallis-lands,whichpresentlittleobstructiontotheflowofwater.Third,thefuriouswindswhichblowaroundthewalloftheeyecreatearidgeofwatercalledastormwave,whichstrikesthecoastandofteninflictsheavydamage.Theeffectissimilartothatofaseismicseawave,causedbyanearth-quakeintheoceanfloor.Bothofthesewavesarepopularlycalledtidalwaves.Stormwavesof20feetormorehaveoc-curred.About3or4feetofthiswaveisduetothedecreaseofatmosphericpressure,andtheresttowinds.Likethedamagecausedbywind,damageduetohighseas,thestormsurgeandtide,andthestormwaveisgreatestinthedanger-oussemicircle,nearthecenter.Thefourthsourceofwaterdamageistheheavyrainthataccompaniesatropicalcy-clone.Thiscausesfloodsthataddtothedamagecausedinother ways.

Therehavebeenmanyinstancesoftornadoesoccur-ringwithinthecirculationoftropicalcyclones.MostofthesehavebeenassociatedwithtropicalcyclonesoftheNorthAtlanticOceanandhaveoccurredintheWestIndiesandalongthegulfandAtlanticcoastsoftheUnitedStates.Theyareusuallyobservedintheforwardsemicircleoralongtheadvancingperipheryofthestorm.Thesetorna-doesareusuallyshort-livedandlessintensethanthosethatoccur in the midwestern United States.

Whenproceedingalongashorerecentlyvisitedbyatropicalcyclone,anavigatorshouldrememberthattimeisrequiredtorestoreaidstonavigationwhichhavebeenblownoutofpositionordestroyed.Insomeinstancestheaidmayremainbutitslight,soundapparatus,orradiobea-conmaybeinoperative.Landmarksmayhavebeendamagedordestroyed,andinsomeinstancesthecoastlineand hydrography may be changed.