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hurricanes en tyfoons Info

National Hurricane Center

 
 
Logo van National Hurricane Center.

Het National Hurricane Center te Miami is een onderdeel van het Amerikaanse Tropical Prediction Center, dat op zijn beurt ressorteert onder de National Weather Service. Het National Hurricane Center legt zich toe op het analyseren van, voorspellen van en waarschuwen voor tropische cyclonen in het bassin van de Atlantische Oceaan en het oostelijk deel van de noordelijke Grote Oceaan (oostelijk van de 140e breedtegraad westerlengte). Het National Hurricane Center geeft dan ook tropische-stormwaarschuwingen, -observaties, orkaanwaarschuwingen en orkaanobservaties uit, als daartoe aanleiding is.

Geschiedenis

In 1898 verordonneerde president William McKinley het Amerikaanse Weather Bureau, het huidige National Weather Service tot het opzetten van een waarschuwingsdienst voor orkanen. Dit netwerk werd later gecentraliseerd en ondergebracht bij de afdeling van het Weather Bureau in Miami. In 1967 werd het National Hurricane Center opgericht, dat zich destijds alleen op het Atlantisch bassin toelegde. In 1984 werd het National Hurricane Center zelfstandig van het Weather Bureau en in 1988 kreeg het ook de verantwoordelijkheid voor het oostelijk deel van de Grote Oceaan. De Wereld Meteorologische Organisatie heeft het National Hurricane Center benoemd tot regionaal gespecialiseerd meteorologisch centrum en daarmee tot zenuwcentrum voor alle tropische cyclonen in de Atlantische en de oostelijke Grote Oceaan. Daarmee vaardigt het waarschuwingen uit voor de bovengenoemde bassins, onafhankelijk of de betreffende tropische cyclonen een bedreiging vormen voor de Verenigde Staten of andere landen. In 1992 werd het National Hurricane Center zelf slachtoffer van een tropische cycloon, toen orkaan Andrew Miami trof en een weerradar van het gebouw stuk blies. Één van de gewezen directeuren (1967-1973) van het centrum is Dr. Robert (Bob) Simpson, die samen met Herbert Saffir de Schaal van Saffir en Simpson construeerde. De huidige voorzitter is Britt Max Mayfield.

Werkzame gespecialiseerde meteorologen

Gedurende het seizoen staan specialisten continu paraat in diensten van 8 uur. De dienstdoende meteoroloog ondertekent alle uitgaande waarschuwingen, beschouwingen en observaties. Buiten het seizoen werken zij aan onderwijs en voorlichting.

Leidende orkaanspecialisten

  • Dr. Lixion Avila, specialist sinds 1987
  • Dr. Jack Beven, specialist sinds 1999
  • Dr. James Franklin, specialist sinds 1999
  • Dr. Richard Knabb, specialist sinds 2005
  • Dr. Richard Pasch, specialist sinds 1989
  • Stacy Stewart, specialist sinds 1999 en meteoroloog, die voor de coördinatie van waarschuwingen verantwoordelijk is

Andere orkaanspecialisten

  • Eric Blake, specialist sinds 2006
  • Dan Brown, specialist sinds 2006
  • Michelle Mainelli, specialist sinds 2006
  • Jamie Rhome , specialist sinds 2006
  • Chris Landsea, Science and Operations Officer (SOO) sinds 2005

 

Tropische cycloon

 
 
Orkaan Elena, Golf van Mexico. 1 september 1985. Foto: NASA

Een tropische cycloon, ook wel orkaan, cycloon, tyfoon of taifoen, is een tropische storm, waarvan de windsnelheden de orkaandrempel, windkracht 12, overschrijden. Dat houdt in dat er windsnelheden voorkomen van meer van 63 knopen, ongeveer 117 kilometer per uur.

Tropische cyclonen kunnen ontstaan in de tropen en subtropen (tot ongeveer de 35ste breedtegraad), maar niet te dicht aan de evenaar, doordat daar het corioliseffect te zwak is. Dit wordt ook wel de intertropische convergentiezone genoemd. Een tropische cycloon is per definitie een tropische depressie, die al naargelang haar kracht een tropische storm of een tropische cycloon kan worden. Naar gelang zijn locatie wordt deze orkaan, taifoen of cycloon genoemd. Een storm zonder tropische kenmerken, die de orkaandrempel overschrijdt, heet een (extra-tropische) storm met orkaankracht. Alle tropische depressies, dus ook tropische cyclonen, voldoen aan drie kenmerken: I Voldoende atmosferische convectie, II een gesloten circulatie, dat wil zeggen volledig rond en niet onderbroken door een front en III een warme kern: de warmste lucht bevindt zich in het oog van de depressie.

De term cycloon heeft betrekking op de cyclonale circulatie, de luchtstroming in dezelfde richting als de draairichting van de aarde. Dit is het geval bij lagedrukgebieden die dan ook wel cyclonen worden genoemd.

 

Verschillende namen

Welke titel een tropische depressie met windsnelheden van 12 Beaufort krijgt, is afhankelijk van de plaats waar zij de orkaandrempel overschrijdt. In het Nederlands spreekt men van orkaan of tropische cycloon. In de Atlantische Oceaan en het oostelijke deel van de noordelijke Grote Oceaan worden ze hurricanes genoemd. In het westelijke deel van de noordelijke Grote Oceaan heten ze tyfoon, terwijl daar ook geregeld tropische cyclonen voorkomen met windsnelheden boven de 130 knopen(241 kilometer per uur), supertyfoons genaamd. In Australië werd aan de westkust wel van willy-willies gesproken en aan de oostkust van queenies, maar tegenwoordig wordt hier veelal de generieke naam gebruikt. In de Indische Oceaan en in de Golf van Bengalen hebben ze de algemene naam van tropische cycloon.

In de Tweede Wereldoorlog kregen tropische cyclonen een letter als herkenningsnaam, maar bij telegrafische berichten bestond de kans op misverstanden. Vandaar is men vanaf het orkaanseizoen 1950 overgeschakeld op het gebruik van meisjesnamen. Tegenwoordig is dit om de beurt een meisjes- en een jongensnaam volgens van tevoren per gebied in alfabetische volgorde opgestelde lijsten.

Een overzicht:

Amerika

In de Atlantische Oceaan en het oostelijk en centraal deel van de Grote Oceaan spreekt men van een orkaan. Het woord is afkomstig van Huracan, de naam van de god van de wind en vernietiging bij de Maya's in Mexico en Centraal-Amerika. Vergelijk hurricane in het Engels, Ouragan in het Frans en Huracán in het Spaans.

In het Engels wordt tropical cyclone ook gebruikt als verzamelnaam voor tropische depressies, tropische stormen en orkanen (hurricanes).

Azië

In Azië spreekt men bij hetzelfde verschijnsel van een taifoen of cycloon. Het woord taifoen komt uit het Japans en betekent: Grote wind. Het wordt ook wel geschreven als tyfoon. Dit is een gedeeltelijke vernederlandsing van de Engelse spelling typhoon, die bovendien aanleiding geeft tot een verkeerde uitspraak. De verkeerde uitspraak is te verklaren uit het Griekse woordbeeld, dat door de spelling met ph nog versterkt wordt.

Australië

Er wordt vaak gedacht dat men in Australië een tropische cycloon willy-willy noemt, maar dat is onjuist. Een willy-willy is de Aboriginalnaam voor wind- of stofhoos (dust devil). In Australië heet een orkaan gewoon een tropical cyclone.

Kenmerken

Het opvallendste kenmerk van een tropische cycloon is het oog met de laagste luchtdruk en de grote windsnelheden rond dit centrum, terwijl de windsnelheid en bewolking in het oog gering zijn of niet aanwezig. Daarnaast is er een zware regenval en kunnen stormvloeden in kustgebieden zware overstromingen veroorzaken. In tegenstelling tot stormen op gematigde breedte hebben tropische cyclonen geen front. De depressie heeft een gesloten, concentrische circulatie. Deze geeft op satellietfoto's een spiraal- of schijfvormig beeld. Deze concentrische circulatie is in alle lagen van de aardatmosfeer aanwezig. Bij lagedrukgebieden van de gematigde zone kan de continuïteit van de circulatie worden onderbroken door fronten en is dan niet meer echt concentrisch. Hieruit volgt ook dat zolang een systeem gekenmerkt wordt door fronten, er geen sprake kan zijn van een tropische cycloon.

Een tropische cycloon heeft rond het centrum een hoge en brede wolkenmuur van cumulonimbi met aan de bovenzijde naar buiten uitwaaierende wolkenbanden en hoge cirrusschermen. Deze muur ontstaat door convectie. Dit is een zichzelf stimulerend fenomeen van het verdampen en condenseren van relatief warm zeewater. Zeewater verdampt en stijgt op. Deze damp condenseert (het gaat regenen) en daarbij komt energie vrij. Deze energie doet de lucht opstijgen, waardoor de luchtdruk daalt, de nog aanwezige waterdamp afkoelt en condenseert. Door de dalende luchtdruk wordt vochtige lucht uit de omgeving aangezogen. Deze condenseert (het gaat harder regenen) en er komt energie vrij, waardoor de luchtdruk verder daalt (het gaat harder waaien).

Schaal van Saffir en Simpson en nomenclatuur

 
Orkaan Celia vanuit de ruimte gezien

De meteorologie hanteert de schaal van Saffir en Simpson om orkanen naar hun kracht in te delen. Alle tropische cyclonen zijn gevaarlijk, maar sommige zijn gevaarlijker dan andere. Daarom is er een classificatie ontwikkeld om onderscheid te kunnen maken tussen bijvoorbeeld krachtige en verwoestende orkanen en om zich beter op de te verwachten schade te kunnen voorbereiden. De schaal werd opgesteld in 1968 door consultant Herbert Saffir, gespecialiseerd in stormschade aan gebouwen, en Bob Simpson, directeur van het National Hurricane Centre.

Echter niet overal wordt de schaal van Saffir en Simpson gebruikt; in Australië is een vijfpuntige schaal in gebruik, die cyclonen classificeert op basis van windstoten in plaats van de doorstaande wind en kracht.

Hier een overzicht van de schaal van Saffir en Simpson:

Klasse 1: stormvloed die 1,2 tot 1,6 meter boven normaal is. Schade: licht

Klasse 2: stormvloed die 1,7 tot 2,5 meter boven normaal is. Schade: dak- en vensterschade en belangrijke schade aan bomen en gewassen

Klasse 3: stormvloed die 2,6 tot 3,7 meter boven normaal is. Schade: grote vernielingen aan gebouwen

Klasse 4: stormvloed die 3,8 tot 5,4 meter boven normaal is. Schade: daken weggeblazen, veel waterschade op de begane grond van gebouwen aan de kust

Klasse 5: stormvloed die de 5,4 meter boven normaal overstijgt. Schade: catastrofaal: vrijwel alle daken weggeblazen, evenals kleinere lichtere bouwsels en grote schade aan gebouwen

Orkaanseizoen

Op het noordelijk halfrond treden de meeste tropische cyclonen op in de maanden augustus, september en oktober. Het officiële Atlantisch orkaanseizoen loopt van 1 juni tot 30 november. Het seizoen wordt ten eerste ingeperkt door het voorhanden zijn van warm zeewater, een zeewatertemperatuur van ten minste 26 graden Celsius is een goede voedingsbodem voor tropische cyclonen. Maar deze eis bleek in 2005 niet zo hard te zijn als wel werd gedacht; Vince, Delta, Epsilon en Zeta ontwikkelden zich bij zeewater van 21 tot 23 graden. Vince en Epsilon groeiden zelfs uit tot een orkaan en ook Delta en Zeta waren geen minimale tropische stormen. Ten tweede wordt het seizoen beperkt door de aanwezigheid van windstilte ten noorden van de evenaar. Waar het wateroppervlak het warmst is, ontstaat een doldrum. Aan het eind van de zomer vormen zich vaak doldrums in de Caribische Zee, ten westen van Mexico, en in de buurt van de Filipijnen. De passaatwinden ontmoeten elkaar bij een doldrum, doordat het een gordel is van lage druk, vanwaar de aangezogen lucht opstijgt. Als er een doldrum ten noorden van de evenaar ontstaat, ontmoet de zuidoost passaat de noordoostpassaat niet op de evenaar. De zuidoostpassaat klapt dan als hij de evenaar overschrijdt om in een zuidwestpassaat. Als zij elkaar ontmoeten, kan dit het corioliseffect doen beginnen, doordat beide passaatwinden een hoek van 180 graden maken in plaats van 90 graden, wanneer zij elkaar op de evenaar zouden ontmoeten. Een derde oorzaak waardoor tropische cyclonen in de winter veel zeldzamer zijn is de straalstroom en andere stromingen in de atmosfeer, deze ruïneren de delicate structuur van een tropische cycloon. Ook voor delen van de Stille Oceaan en voor het noorden van de Indische Oceaan geldt dit orkaanseizoen, maar er zijn ook gebieden waar cyclonen het hele jaar door kunnen voorkomen en ontstaan.

Schade door orkanen

De meeste schade wordt veroorzaakt op het moment dat de subtropische cyclonen de kust bereiken. De slachtoffers vallen vooral door vloedgolven, die tot zes meter hoog kunnen worden. Inhammen kunnen het vloedgolfeffect nog versterken: de Bathurst Bay Hurricane veroorzaakte in 1899 in de gelijknamige baai in Australië een opzet van 13 m.

Als record voor het aantal slachtoffers gold lange tijd de cycloon van 7 oktober 1737 in de Golf van Bengalen, waarbij meer dan een kwart miljoen slachtoffers te betreuren waren. Inmiddels is men van mening dat de Cycloon Bhola in Bangladesh uit 1970 daar niet voor onderdeed; sommige bronnen schatten het aantal doden bij die catastrofe zelfs op 300.000.

Ook als de schade in het kustgebied gering is, kan overvloedige neerslag meer landinwaarts plotseling opkomende overstromingen teweegbrengen. Soms komen neerslaghoeveelheden voor van 750 mm, dat is evenveel als er in Nederland in een heel jaar valt. De neerslag vormt bij alle tropische stormen die het land optrekken, een even grote bedreiging. Boven zee treden extreem hoge golven op, bijvoorbeeld 30 m bij 'hurricane Luis' uit 1995. Behalve dat er door de wind hoge golven ontstaan, wordt het zeewater door de zeer lage atmosferische drukken in het centrum van een orkaan ook nog eens wat omhoog gezogen ten opzichte van het normale evenwichtsniveau.

In de laatste decennia (tot 2003) eisen de tropische cyclonen vooral een hoge tol in bijvoorbeeld Bangladesh, waar vloedgolven makkelijk ver het land kunnen binnendringen en de infrastructuur ontbreekt om de bevolking te beschermen, te evacueren of tijdig te waarschuwen. In de Verenigde Staten viel het aantal slachtoffers de laatste decennia mee. Deels komt dit door tijdiger signalering dankzij weersatellieten, nauwkeuriger verwachtingen, effectievere berichtgeving, tijdiger evacuaties, een beter voorbereid publiek en een professioneler 'calamiteitenmanagement' door de Amerikaanse overheid. Daarnaast is er sprake van toeval of, zo men wil, geluk: naar verhouding weinig belangrijke orkanen trokken in dichtbevolkte streken het land op. Door de sterke groei van de bevolking langs de Amerikaanse oostkust en de complexiteit van een evacuatie, is ook in de Verenigde Staten een ramp in de toekomst toch niet uit te sluiten.

Op 20 oktober 2004 werd Japan geraakt door één van de zwaarste taifoens uit de laatste decennia.

Op 29 augustus 2005 richtte de orkaan Katrina zeer veel schade aan in de Amerikaanse staten Louisiana, Alabama en Mississippi. Vooral zwaar getroffen waren de steden Biloxi, New Orleans en Mobile. Meer dan een miljoen mensen zaten zonder stroom en in Biloxi alleen maakten de eerste berichten gewag van meer dan 80 doden. Een week na de ramp was de toestand in het gebied nog steeds totaal ontregeld met een groot stuk van de stad New Orleans dat nog steeds onder water staat. Het totaal aantal doden door deze orkaan beliep enkele honderden tot ca 1000.

Verloop

Ontstaan

Er is bij het ontstaan van een orkaan of tropische storm een lagedrukgebied aanwezig; convectie alleen zal nooit "spontaan" gaan draaien.

Deze convectie is de belangrijkste energiebron van een tropische depressie. Eenmaal boven land wordt de kracht snel minder. Stijgt de windsnelheid waarmee een tropische depressie gepaard gaat boven windkracht 8, dan spreekt men van een tropische storm en krijgt deze een naam. Als een tropische storm 64 knopen (windkracht 12) bereikt, dan verandert de naam van deze entiteit, afhankelijk van de locatie op aarde.

Gerekend over de gehele aardbol zijn er elk jaar 80-90 tropische stormen; ongeveer 2 op de 3 stormen ontwikkelen zich tot een orkaan. Op de Noord-Atlantische Oceaan loopt het aantal tropische stormen per jaar uiteen van 4 (1982) tot 21 (1936); gemiddeld zijn het er ongeveer 10.

Sommige, naar het noorden afgebogen, tropische cyclonen komen terecht in de westelijke stroming van de gematigde breedten. Ze gaan dan over in een 'normale' depressie en worden in de richting van Europa gevoerd. Voor zover bekend maakte een tropische cycloon slechts eenmaal de oceaanoversteek af en kwam terecht aan de Ierse westkust, waar hij veel schade aanrichtte; dat was Debbie in 1961. In onze omgeving wordt gemiddeld eens per jaar een ex-cycloon gesignaleerd; vaak gaat het dan om actieve depressies die veel regen en wind meebrengen.

Ontwikkeling

 
Routes van alle tropische cyclonen van 1985 - 2005.

Tropische orkanen zijn kleinschaliger dan de hoge- en lagedrukgebieden van de gematigde breedten; ze hebben een doorsnede van 500–1500 km. De orkaanwinden doen zich uitsluitend voor in de kern, die slechts 1-4 procent beslaat van de totale omvang. De levensduur bedraagt gewoonlijk 5-10 dagen, maar is soms veel langer. In 2002 had Kyle een levensduur van 22 dagen; Ginger hield het in 1971 28 dagen vol.

De meeste tropische orkanen komen tot ontwikkeling in een gebied tussen 5 en 20 graden noorder- of zuiderbreedte. Verder van de evenaar weg is het zeewater te koud; hierboven is al gemeld dat dit bij voorkeur 26 graden moet zijn of warmer. De orkaan moet echter ook voldoende nieuwe warme lucht van boven zee kunnen aanzuigen: daardoor ontstaan er in de Middellandse Zee, Perzische Golf en Rode Zee, die 's zomers tot ver boven de 26 graden opwarmen, geen cyclonen. Verdampend zeewater is namelijk een belangrijke bron van energie voor een orkaan; daarnaast levert de achtergrondstroming een bijdrage, evenals de condensatiewarmte die vrij komt in de zware buien in de buurt van het centrum van de orkaan. Overigens wordt slechts 2,5 % van die energie gebruikt voor het aandrijven van de orkaanwinden. Het gaat natuurlijk wel om grote hoeveelheden energie: een 'gemiddelde' orkaan bevat een hoeveelheid energie die gelijk is aan vijf maal het totale energiegebruik van de hele mensheid in 1990.

Het corioliseffect in de atmosfeer dat optreedt als gevolg van het roteren van de aarde speelt een belangrijke rol bij het ontstaan van cyclonen en daardoor komen cyclonen niet dicht bij de evenaar; hoe dichter bij de evenaar hoe zwakker het corioliseffect in de atmosfeer, en op de evenaar zelf is het nul. Om tropische cyclonen tot ontwikkeling te laten komen moet verder de atmosfeer boven de tropische oceaan voldoende onstabiel zijn, zonder dat de wind er weer te veel verandert met de hoogte. Ze ontstaan niet 'uit het niets'; er moeten in de buurt van het aardoppervlak al storingen aanwezig zijn (zogeheten 'easterly waves'), die onder de hierboven genoemde voorwaarden uit kunnen (niet noodzakelijkerwijs moeten) groeien tot een tropische storm. Voor de hurricanes in het Caraïbische gebied geldt dat die storingen meestal afkomstig zijn uit Afrika; ze drijven al ontwikkelend met de noordoostpassaat - ten zuiden van het subtropisch hogedrukgebied bij de Azoren langs - de Atlantische Oceaan over. Deze stroming drijft ze naar het Caraïbische gebied, waar ze over de eilanden kunnen razen, aan land kunnen gaan of naar het noorden afbuigen. Boven land neemt de windsnelheid door wrijving enigszins af, maar de vlagerigheid neemt toe. Doordat juist de windstoten de meeste schade veroorzaken, raakt men zo van de regen in de drup, pas na enkele uren boven land begint de orkaan in kracht af te nemen; hij raakt dan namelijk afgesneden van zijn belangrijkste energiebron: het warme zeewater.

Restanten

Als een tropische cycloon (onafhankelijk van welke sterkte) zijn tropische kenmerken verliest (convectie en gesloten, concentrische circulatie), dan valt de depressie meteorologisch niet meer onder tropisch weer en de meteorologen van bijvoorbeeld het National Hurricane Center staken het uitgeven hun waarschuwingen over dat systeem. Zo'n ex-tropische cycloon wordt dan aangeduid met de term extra-tropische cycloon. Eigenlijk wil dit niets meer zeggen dat het niet om een tropische cycloon gaat, dus is iedere depressie van de gematigde zone een extra-tropische cycloon. Maar de term wordt uitsluitend gebruikt voor ex-tropische depressies die hun tropische kenmerken hebben verloren. De status extra-tropische cycloon hoeft niets te zeggen over het potentieel gevaar van zo'n depressie; een extra-tropische cycloon kan nog steeds respectabele orkaanwinden hebben en heel veel regenval, zoals Orkaan Karl in 2004. Extra-tropische cyclonen worden in hun eindfase vaak meegenomen door de westenwinden, de straalstroom. Als zij door de straalstroom worden gegrepen, zwakken zij snel af, doordat de structuur van het systeem aan flarden wordt gescheurd. De convectie is een verticaal bewegen van lucht, die snel moet worden afgevoerd als door een schoorsteen. De straalstroom (of een andere sterke luchtstroming in de atmosfeer) verstopt deze schoorsteen. Dit fenomeen wordt in het Engels windshear genoemd. Een voormalige orkaan neemt veel energie en warmte mee en kan daarmee een andere depressie voeden. Dat kan leiden tot veel neerslag en stormachtige winden, maar het weer kan ook heel anders worden. Blijven de restanten ten westen van Europa dan kunnen ze hier zorgen voor een sterke warme zuidelijke luchtstroming en wordt het warm (na)zomerweer, zoals in de herfst van 2005 gebeurde. In de herfst levert dat vaak een mooie periode op, eind september de oudewijvenzomer genaamd. Pas zodra de depressie verder trekt en onze kant op komt verslechtert het weer. De rekenmodellen waarop de weersverwachtingen zijn gebaseerd, hebben vaak moeite met orkaanrestanten, zodat de prognoses van dag tot dag kunnen veranderen.

Oog

 
Orkaan Catarina in 2005, met een duidelijk zichtbaar oog. De richting waarin de cycloon draait, verraadt dat Catarina op het zuidelijk halfrond raasde.

Het opvallendste kenmerk van een tropische cycloon is het wolkenvrije oog waar dalende luchtbewegingen optreden. Het oog heeft een diameter van 30–50 km en is op satellietbeelden gewoonlijk goed te zien. De luchtdruk is in het oog het laagst. Rondom het oog bevindt zich een 'muur' van actieve bewolking; daar gaat de lucht met snelheden van 100–150 km per uur omhoog. Aan het aardoppervlak direct onder de 'muur' treden de hoogste windsnelheden op. Aan de bovenzijde op zo'n 18 km hoogte, stroomt de lucht met bewolking weer spiraalsgewijs naar buiten. Daardoor vormt er zich aan de bovenkant van de cycloon een kap van ijswolken, die eveneens op satellietbeelden markant zichtbaar is.

Direct buiten de 'muur' bevinden zich regenbanden die evenwijdig aan de wind naar het centrum toe lijken te spiraliseren. Deze banden zijn 5–50 km breed en 100–300 km lang. Ze veroorzaken neerslagintensiteiten van 25 mm per uur of meer over een klein oppervlak, ongeveer 10% van het totale gebied waar de cycloon het laat regenen.

Wie zich in een cycloon bevindt merkt dat het oog overtrekt aan de plotseling afnemende wind, en de zon die ineens door de wolken breekt. Uiteraard duurt dit hooguit een uurtje, waarna de orkaan met volle kracht uit de andere richting zal blazen. Op zee is een oog zeer gevaarlijk vanwege de hoge golven, die uit alle richtingen komen.

Namen van stormen

Tropische stormen en cyclonen worden officieel benoemd sinds 1945 en de namen worden zodanig gekozen dat bij prognoses en waarschuwingen ze communicatie vergemakkelijken tussen meteorologen en het publiek. Namen verminderen ook verwarring ten aanzien van welke storm wordt beschreven, aangezien meer dan één storm in dezelfde regio op hetzelfde moment kan voorkomen. Het bekendst zijn de jongens- en meisjesnamen, maar in landen als Korea, Thailand, Vietnam, China en Japan worden historische namen van goden gebruikt, zoals Prapiroon (Regengod), Wukong (Apenkoning) of Dianmu (Moeder van de Bliksem).

Orkanen in het Caraïbische gebied werden een aantal eeuwen lang aangeduid met de naam van de heilige van de dag waarop de orkaan optrad. Zo werd Puerto Rico op 26 juli 1825 aan het begin van een vroeg seizoen getroffen door de verwoestende orkaan Santa Ana (Sint Anna), op 13 september 1876 door de San Felipe-orkaan (Sint Phillip) en op 8 augustus 1899 door de Sint Cyriacus-orkaan. Toen er in 1928 op dezelfde dag opnieuw een orkaan toesloeg, werd dat San Felipe 2. Overigens paste men in een ver verleden in Nederland een zelfde principe toe. Zo herinnert de Biesbosch thans nog steeds aan de Sint Elisabethsvloed van 18 november 1421 en de vloed op 'St Felix Quade Saterdach', 5 november 1530, gaf de aanzet tot de ondergang van Reimerswaal op Zuid-Beveland.

Na de heiligennamen kwamen de geografische coördinaten in gebruik, maar deze praktijk vertraagde de communicatie en gaf aanleiding tot veel fouten in de berichtgeving.

De Australische meteoroloog Clement Wragge gebruikte aan het eind van de 19e eeuw als eerste meisjesnamen voor tropische stormen. In de Tweede Wereldoorlog werd dat gebruik van meisjesnamen in alfabetische volgorde de normale praktijk in het Caraïbische gebied. Elke tropische depressie met ten minste windkracht 8 krijgt een eigen naam. In 1978 deden, na protest uit het feministische kamp, voor het eerst jongensnamen hun intrede, aanvankelijk alleen in het gebied van de Stille Oceaan voor de Amerikaanse westkust, een jaar later ook op de Atlantische Oceaan en in de Golf van Mexico.

Voor de Atlantische stormen heeft de Wereld Meteorologische Organisatie zes namenlijsten van telkens 21 namen opgesteld; om de zes jaar komen dezelfde namen dus weer terug. Het is een alfabetische lijst met alternerend een vrouwelijke en een mannelijke naam. De letters Q, U, X, Y en Z worden niet gebruikt omdat er te weinig namen zijn.

De naam van verwoestende orkanen die weergeschiedenis hebben geschreven en die men zich nog generaties lang kan heugen wordt van de lijst afgevoerd en vervangen door een nieuwe. Sinds 1954 is dat 67 keer gebeurd. De recentste voorbeelden zijn Dennis, Katrina, Rita, Stan en Wilma uit 2005, Charley, Frances, Ivan en Jeanne uit 2004, Fabian, Isabel en Juan uit 2003 en Isodore en Lili uit 2002. Om politieke redenen is ook de naam Adolph geschrapt, net als de naam Israël.

Subtropische cyclonen

Soms gebeurt het, dat een gewone depressie, een depressie van de gematigde zone, terecht komt in bijzondere omstandigheden en tropische kenmerken gaat vertonen (convectie en gesloten concentrische circulatie). Zij vertoont dan zowel kenmerken van een gewone, als van een tropische depressie. In dat geval spreekt men van een subtropische cycloon. Een subtropische cycloon kan een subtropische storm worden. In de nomenclatuur wordt een subtropische storm net zo behandeld als een tropische storm. Als een subtropische cycloon zijn niet-tropische kenmerken verliest, wordt het een gewone tropische storm.

Voorbeelden van orkanen

  • Sint Cyriacus-orkaan (1899)
  • Galveston (1900)
  • Camille (1969)
  • Gilbert (1988)
  • Andrew (1992)
  • Mitch (1998)
  • Galifo (2004)
  • Ivan (2004)
  • Katrina (2005)
  • Rita (2005)
  • Stan (2005)
  • Wilma (2005)
  • Ioke (tyfoon) (2006)
  • Nargis (cycloon) (2008)
  • Gustav (2008)
  • Ike (2008)
  • Omar (2008)
  • Irene (2011)
  • Sandy (2012)
  • Phailin (2013)
  • Haiyan (2013)
  • Hagupit (2014)

 

Tropical cyclone

From Wikipedia, the free encyclopedia
"Hurricane" redirects here. For other uses, see Hurricane (disambiguation).
 
Hurricane Isabel (2003) as seen from orbit during Expedition 7 of the International Space Station. The eye, eyewall, and surrounding rainbands, characteristics of tropical cyclones, are clearly visible in this view from space.

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by names such as hurricane, typhoon /tˈfn/, tropical storm, cyclonic storm, tropical depression, and simply cyclone.

Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which ultimately recondenses into clouds and rain when moist air rises and cools to saturation. This energy source differs from that of mid-latitude cyclonic storms, such as nor'easters and European windstorms, which are fueled primarily by horizontal temperature contrasts. The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, they rarely form within 5° of the equator. Tropical cyclones are typically between 100 and 4,000 km (62 and 2,485 mi) in diameter.

Tropical refers to the geographical origin of these systems, which form almost exclusively over tropical seas. Cyclone refers to their cyclonic nature, with wind blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis effect.

In addition to strong winds and rain, tropical cyclones are capable of generating high waves, damaging storm surge, and tornadoes. They typically weaken rapidly over land where they are cut off from their primary energy source. For this reason, coastal regions are particularly vulnerable to damage from a tropical cyclone as compared to inland regions. Heavy rains, however, can cause significant flooding inland, and storm surges can produce extensive coastal flooding up to 40 kilometres (25 mi) from the coastline. Though their effects on human populations are often devastating, tropical cyclones can relieve drought conditions. They also carry heat energy away from the tropics and transport it toward temperate latitudes, which may play an important role in modulating regional and global climate.

 

Physical structure

 
Typhoon Nabi as seen from the International Space Station, on September 3, 2005.

 

Tropical cyclones are areas of relatively low pressure in the troposphere, with the largest pressure perturbations occurring at low altitudes near the surface. On Earth, the pressures recorded at the centres of tropical cyclones are among the lowest ever observed at sea level. The environment near the center of tropical cyclones is warmer than the surroundings at all altitudes, thus they are characterized as "warm core" systems.

 

Wind field

 

The near-surface wind field of a tropical cyclone is characterised by air rotating rapidly around a centre of circulation while also flowing radially inwards. At the outer edge of the storm, air may be nearly calm; however, due to the Earth's rotation, the air has non-zero absolute angular momentum. As air flows radially inward, it begins to rotate cyclonically (counter-clockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere) in order to conserve angular momentum. At an inner radius, air begins to ascend to the top of the troposphere. This radius is typically coincident with the inner radius of the eyewall, and has the strongest near-surface winds of the storm; consequently, it is known as the radius of maximum winds. Once aloft, air flows away from the storm's center, producing a shield of cirrus clouds.

 

The previously mentioned processes result in a wind field that is nearly axisymmetric: Wind speeds are low at the centre, increase rapidly moving outwards to the radius of maximum winds, and then decay more gradually with radius to large radii. However, the wind field often exhibits additional spatial and temporal variability due to the effects of localized processes, such as thunderstorm activity and horizontal flow instabilities. In the vertical direction, winds are strongest near the surface and decay with height within the troposphere.

 

Eye and center

 

 

 

At the center of a mature tropical cyclone, air sinks rather than rises. For a sufficiently strong storm, air may sink over a layer deep enough to suppress cloud formation, thereby creating a clear "eye". Weather in the eye is normally calm and free of clouds, although the sea may be extremely violent. The eye is normally circular in shape, and is typically 30–65 km (19–40 mi) in diameter, though eyes as small as 3 km (1.9 mi) and as large as 370 km (230 mi) have been observed.

 

The cloudy outer edge of the eye is called the "eyewall". The eyewall typically expands outward with height, resembling an arena football stadium; this phenomenon is sometimes referred to as the stadium effect. The eyewall is where the greatest wind speeds are found, air rises most rapidly, clouds reach to their highest altitude, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclone's eyewall passes over land.

 

In a weaker storm, the eye may be obscured by the central dense overcast, which is the upper-level cirrus shield that is associated with a concentrated area of strong thunderstorm activity near the center of a tropical cyclone.

 

The eyewall may vary over time in the form of eyewall replacement cycles, particularly in intense tropical cyclones. Outer rainbands can organize into an outer ring of thunderstorms that slowly moves inward, which is believed to rob the primary eyewall of moisture and angular momentum. When the primary eyewall weakens, the tropical cyclone weakens temporarily. The outer eyewall eventually replaces the primary one at the end of the cycle, at which time the storm may return to its original intensity.

 

Intensity

 

Storm "intensity" is defined as the maximum wind speed in the storm. This speed is taken as either a 1-minute or a 10-minute average at the standard reference height of 10 meters. The choice of averaging period, as well as the naming convention for classifying storms, differs across forecast centers and ocean basins.

 

Size

ze descriptions of tropical cyclones
ROCIType
Less than 2 degrees latitude Very small/midget
2 to 3 degrees of latitude Small
3 to 6 degrees of latitude Medium/Average
6 to 8 degrees of latitude Large
Over 8 degrees of latitude Very large[17]

 

There are a variety of metrics commonly used to measure storm size. The most common metrics include the radius of maximum wind, the radius of 34-knot wind (i.e. gale force), the radius of outermost closed isobar (ROCI), and the radius of vanishing wind. An additional metric is the radius at which the cyclone's relative vorticity field decreases to 1×10−5 s−1.

 

On Earth, tropical cyclones span a large range of sizes, from 100–2000 km as measured by the radius of vanishing wind. They are largest on average in the northwest Pacific Ocean basin and smallest in the eastern Pacific Ocean basin. If the radius of outermost closed isobar is less than two degrees of latitude (222 km (138 mi)), then the cyclone is "very small" or a "midget". A radius of 3–6 latitude degrees (333–670 km (207–416 mi)) is considered "average sized". "Very large" tropical cyclones have a radius of greater than 8 degrees (888 km (552 mi)).Observations indicate that size is only weakly correlated to variables such as storm intensity (i.e. maximum wind speed), radius of maximum wind, latitude, and maximum potential intensity.

 

Size plays an important role in modulating damage caused by a storm. All else equal, a larger storm will impact a larger area for a longer period of time. Additionally, a larger near-surface wind field can generate higher storm surge due to the combination of longer wind fetch, longer duration, and enhanced wave setup.For example, Hurricane Sandy, which struck the eastern U.S. in 2012, barely attained hurricane intensity prior to landfall yet was one of the costliest landfalling hurricanes in U.S. history because of its extremely large size.

 

The upper circulation of strong hurricanes extends into the tropopause of the atmosphere, which at low latitudes is 15,000–18,000 metres (50,000–60,000 ft).

 

Physics and energetics

 

 
Tropical cyclones exhibit an overturning circulation where air inflows at low levels near the surface, rises in thunderstorm clouds, and outflows at high levels near the tropopause.

 

The three-dimensional wind field in a tropical cyclone can be separated into two components: a "primary circulation" and a "secondary circulation". The primary circulation is the rotational part of the flow; it is purely circular. The secondary circulation is the overturning (in-up-out-down) part of the flow; it is in the radial and vertical directions. The primary circulation has the strongest winds and is responsible for the majority of the damage a storm causes, while the secondary circulation is slower but governs the energetics of the storm.

 

Secondary circulation: a Carnot heat engine

 

A tropical cyclone's primary energy source is the evaporation of water from the ocean surface, which ultimately recondenses into clouds and rain when the warm moist air rises and cools to saturation. The energetics of the system may be idealized as an atmospheric Carnot heat engine.First, inflowing air near the surface acquires heat primarily via evaporation of water (i.e. latent heat) at the temperature of the warm ocean surface (during evaporation, the ocean cools and the air warms). Second, the warmed air rises and cools within the eyewall while conserving total heat content (latent heat is simply converted to sensible heat during condensation). Third, air outflows and loses heat via infrared radiation to space at the temperature of the cold tropopause. Finally, air subsides and warms at the outer edge of the storm while conserving total heat content. The first and third legs are nearly isothermal, while the second and fourth legs are nearly isentropic. This in-up-out-down overturning flow is known as the secondary circulation. The Carnot perspective provides an upper bound on the maximum wind speed that a storm can attain.

 

Scientists estimate that a tropical cyclone releases heat energy at the rate of 50 to 200 exajoules (1018 J) per day, equivalent to about 1 PW (1015 watt). This rate of energy release is equivalent to 70 times the world energy consumption of humans and 200 times the worldwide electrical generating capacity, or to exploding a 10-megaton nuclear bomb every 20 minutes.

 

Primary circulation: rotating winds

 

The primary rotating flow in a tropical cyclone results from the conservation of angular momentum by the secondary circulation. Absolute angular momentum on a rotating planet M is given by

 

where f is the Coriolis parameter, v is the azimuthal (i.e. rotating) wind speed, and r is the radius to the axis of rotation. The first term on the right hand side is the component of planetary angular momentum that projects onto the local vertical (i.e. the axis of rotation). The second term on the right hand side is the relative angular momentum of the circulation itself with respect to the axis of rotation. Because the planetary angular momentum term vanishes at the equator (where f=0 ), tropical cyclones rarely form within 5° of the equator.

 

As air flows radially inward at low levels, it begins to rotate cyclonically in order to conserve angular momentum. Similarly, as rapidly rotating air flows radially outward near the tropopause, its cyclonic rotation decreases and ultimately changes sign at large enough radius, resulting in an upper-level anti-cyclone. The result is a vertical structure characterized by a strong cyclone at low levels and a strong anti-cyclone near the tropopause; from thermal wind balance, this corresponds to a system that is warmer at its center than in the surrounding environment at all altitudes (i.e. "warm-core"). From hydrostatic balance, the warm core translates to lower pressure at the center at all altitudes, with the maximum pressure drop located at the surface.[10]

 

Maximum potential intensity

 

Due to surface friction, the inflow only partially conserves angular momentum. Thus, the sea surface lower boundary acts as both a source (evaporation) and sink (friction) of energy for the system. This fact leads to the existence of a theoretical upper bound on the strongest wind speed that a tropical cyclone can attain. Because evaporation increases linearly with wind speed (just as climbing out of a pool feels much colder on a windy day), there is a positive feedback on energy input into the system known as the Wind-Induced Surface Heat Exchange (WISHE) feedback.[25] This feedback is offset when frictional dissipation, which increases with the cube of the wind speed, becomes sufficiently large. This upper bound is called the "maximum potential intensity", v_p, and is given by

 

v_p^2 = \frac{C_k}{C_d}\frac{T_s - T_o}{T_o}\Delta k

 

where T_s is the temperature of the sea surface, T_o is the temperature of the outflow ([K]), \Delta k is the enthalpy difference between the surface and the overlying air ([J/kg]), and C_k and C_d are the surface exchange coefficients (dimensionless) of enthalpy and momentum, respectively.[29] The surface-air enthalpy difference is taken as \Delta k = k^*_s-k, where k^*_s is the saturation enthalpy of air at sea surface temperature and sea-level pressure and k is the enthalpy of boundary layer air overlying the surface.

 

The maximum potential intensity is predominantly a function of the background environment alone (i.e. without a tropical cyclone), and thus this quantity can be used to determine which regions on Earth can support tropical cyclones of a given intensity, and how these regions may evolve in time. Specifically, the maximum potential intensity has three components, but its variability in space and time is due predominantly to the variability in the surface-air enthalpy difference component \Delta k.

 

Derivation

 

A tropical cyclone may be viewed as a heat engine that converts input heat energy from the surface into mechanical energy that can be used to do mechanical work against surface friction. At equilibrium, the rate of net energy production in the system must equal the rate of energy loss due to frictional dissipation at the surface, i.e.

 

W_{in} = W_{out}

 

The rate of energy loss per unit surface area from surface friction, W_{out}, is given by

 

W_{out} = C_d \rho |\mathbf{u}|^3

 

where \rho is the density of near-surface air ([kg/m3]) and |\mathbf{u}| is the near surface wind speed ([m/s]).

 

The rate of energy production per unit surface area, W_{in} is given by

 

W_{in} = \epsilon Q_{in}

 

where \epsilon is the heat engine efficiency and Q_{in} is the total rate of heat input into the system per unit surface area. Given that a tropical cyclone may be idealized as a Carnot heat engine, the Carnot heat engine efficiency is given by

 

\epsilon = \frac{T_s-T_o}{T_s}

 

Heat (enthalpy) per unit mass is given by

 

k = C_pT + L_vq

 

where C_p is the heat capacity of air, T is air temperature, L_v is the latent heat of vaporization, and q is the concentration of water vapor. The first component corresponds to sensible heat and the second to latent heat.

 

There are two sources of heat input. The dominant source is the input of heat at the surface, primarily due to evaporation. The bulk aerodynamic formula for the rate of heat input per unit area at the surface, Q_{in:k}, is given by

 

Q_{in:k} = C_k \rho |\mathbf{u}|\Delta k

 

where \Delta k = k^*_s-k represents the enthalpy difference between the ocean surface and the overlying air. The second source is the internal sensible heat generated from frictional dissipation (equal to W_{out}), which occurs near the surface within the tropical cyclone and is recycled to the system.

 

Q_{in:friction} = C_d \rho |\mathbf{u}|^3

 

Thus, the total rate of net energy production per unit surface area is given by

 

W_{in} = \frac{T_s-T_o}{T_s}\left(C_k \rho |\mathbf{u}|\Delta k + C_d \rho |\mathbf{u}|^3\right)

 

Setting W_{in} = W_{out} and taking |\mathbf{u}| \approx v (i.e. the rotational wind speed is dominant) leads to the solution for v_p given above. This derivation assumes that total energy input and loss within the system can be approximated by their values at the radius of maximum wind. The inclusion of Q_{in:friction} acts to multiply the total heat input rate by the factor \frac{T_s}{T_o}. Mathematically, this has the effect of replacing T_s with T_o in the denominator of the Carnot efficiency.

 

An alternative definition for the maximum potential intensity, which is mathematically equivalent to the above formulation, is

 

v_p = \sqrt{\frac{T_s}{T_o}\frac{C_k}{C_d}(CAPE^*_s-CAPE_b)|_m}

 

where CAPE stands for the Convective Available Potential Energy, CAPE^*_s is the CAPE of an air parcel lifted from saturation at sea level in reference to the environmental sounding, CAPE_b is the CAPE of the boundary layer air, and both quantities are calculated at the radius of maximum wind.[32]

 

Characteristic values and variability on Earth

 

On Earth, a characteristic temperature for T_s is 300 K and for T_o is 200 K, corresponding to a Carnot efficiency of \epsilon = 1/3. The ratio of the surface exchange coefficients, C_k/C_d, is typically taken to be 1. However, observations suggest that the drag coefficient C_d varies with wind speed and may decrease at high wind speeds within the boundary layer of a mature hurricane.[33] Additionally, C_k may vary at high wind speeds due to the effect of sea spray on evaporation within the boundary layer.[34]

 

A characteristic value of the maximum potential intensity, v_p, is 80 m/s. However, this quantity varies significantly across space and time, particularly within the seasonal cycle, spanning a range of 0–100 m/s.[32] This variability is primarily due to variabliity in the surface enthalpy disequilibrium ( \Delta k ) as well as in the thermodynamic structure of the troposphere, which are controlled by the large-scale dynamics of the tropical climate. These processes are modulated by factors including the sea surface temperature (and underlying ocean dynamics), background near-surface wind speed, and the vertical structure of atmospheric radiative heating. The nature of this modulation is complex, particularly on climate time-scales (decades or longer). On shorter time-scales, variability in the maximum potential intensity is commonly linked to sea surface temperature perturbations from the tropical mean, as regions with relatively warm water have thermodynamic states much more capable of sustaining a tropical cyclone than regions with relatively cold water.[36] However, this relationship is indirect via the large-scale dynamics of the tropics; the direct influence of the absolute sea surface temperature on v_p is weak in comparison.

 

Interaction with the upper ocean

 

 
Chart displaying the drop in surface temperature in the Gulf of Mexico as Hurricanes Katrina and Rita passed over

 

The passage of a tropical cyclone over the ocean causes the upper layers of the ocean to cool substantially, which can influence subsequent cyclone development. This cooling is primarily caused by wind-driven mixing of cold water from deeper in the ocean with the warm surface waters. This effect results in a negative feedback process that can inhibit further development or lead to weakening. Additional cooling may come in the form of cold water from falling raindrops (this is because the atmosphere is cooler at higher altitudes). Cloud cover may also play a role in cooling the ocean, by shielding the ocean surface from direct sunlight before and slightly after the storm passage. All these effects can combine to produce a dramatic drop in sea surface temperature over a large area in just a few days.

 

Major basins and related warning centers

 

Main articles: Tropical cyclone basins and Regional Specialized Meteorological Center

 

Basins and WMO Monitoring Institutions[38]
BasinResponsible RSMCs and TCWCs
North Atlantic National Hurricane Center (United States)
North-East Pacific National Hurricane Center (United States)
North-Central Pacific Central Pacific Hurricane Center (United States)
North-West Pacific Japan Meteorological Agency
North Indian Ocean India Meteorological Department
South-West Indian Ocean Météo-France
Australian region Bureau of Meteorology (Australia)
Indonesian Meteorological and Geophysical Agency
Papua New Guinea National Weather Service
Southern Pacific Fiji Meteorological Service
Meteorological Service of New Zealand
: Indicates a Tropical Cyclone Warning Center

 

There are six Regional Specialized Meteorological Centers (RSMCs) worldwide. These organizations are designated by the World Meteorological Organization and are responsible for tracking and issuing bulletins, warnings, and advisories about tropical cyclones in their designated areas of responsibility. In addition, there are six Tropical Cyclone Warning Centers (TCWCs) that provide information to smaller regions.[39] The RSMCs and TCWCs are not the only organizations that provide information about tropical cyclones to the public. The Joint Typhoon Warning Center (JTWC) issues advisories in all basins except the Northern Atlantic for the purposes of the United States Government.[40] The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) issues advisories and names for tropical cyclones that approach the Philippines in the Northwestern Pacific to protect the life and property of its citizens.The Canadian Hurricane Center (CHC) issues advisories on hurricanes and their remnants for Canadian citizens when they affect Canada.

 

On March 26, 2004, Cyclone Catarina became the first recorded South Atlantic cyclone and subsequently struck southern Brazil with winds equivalent to Category 2 on the Saffir-Simpson Hurricane Scale. As the cyclone formed outside the authority of another warning center, Brazilian meteorologists initially treated the system as an extratropical cyclone, but later on classified it as tropical.

 

Formation

 

Main article: Tropical cyclogenesis

 

 
Map of the cumulative tracks of all tropical cyclones during the 1985–2005 time period. The Pacific Ocean west of the International Date Line sees more tropical cyclones than any other basin, while there is almost no activity in the Atlantic Ocean south of the Equator.

 

 
Map of all tropical cyclone tracks from 1945 to 2006. Equal-area projection.

 

Worldwide, tropical cyclone activity peaks in late summer, when the difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month, while September is the most active month. November is the only month in which all the tropical cyclone basins are active.

 

Times

 

In the Northern Atlantic Ocean, a distinct cyclone season occurs from June 1 to November 30, sharply peaking from late August through September. The statistical peak of the Atlantic hurricane season is September 10. The Northeast Pacific Ocean has a broader period of activity, but in a similar time frame to the Atlantic. The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and March and a peak in early September. In the North Indian basin, storms are most common from April to December, with peaks in May and November. In the Southern Hemisphere, the tropical cyclone year begins on July 1 and runs all year-round encompassing the tropical cyclone seasons, which run from November 1 until the end of April, with peaks in mid-February to early March.

 

Season lengths and averages
BasinSeason
start
Season
end
Tropical
Storms
HurricanesRefs
North Atlantic June 1 November 30 12.1 6.4 [47]
Eastern Pacific May 15 November 30 16.6 8.9 [47]
Western Pacific January 1 December 31 27.0 17.0 [47]
North Indian January 1 December 31 4.8 1.5 [47]
South-West Indian July 1 June 30 9.3 5.0 [47][48]
Australian region November 1 April 30 11.0   [49]
Southern Pacific November 1 April 30 7.4 4 [50]
Global January 1 December 31 86.0 46.9 [47]

 

 

 

Factors

 

 
Waves in the trade winds in the Atlantic Ocean—areas of converging winds that move along the same track as the prevailing wind—create instabilities in the atmosphere that may lead to the formation of hurricanes.

 

The formation of tropical cyclones is the topic of extensive ongoing research and is still not fully understood. While six factors appear to be generally necessary, tropical cyclones may occasionally form without meeting all of the following conditions. In most situations, water temperatures of at least 26.5 °C (79.7 °F) are needed down to a depth of at least 50 m (160 ft); waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms. Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone. High humidity is needed, especially in the lower-to-mid troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop. Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation.Tropical cyclones generally need to form more than 555 km (345 mi) or five degrees of latitude away from the equator, allowing the Coriolis effect to deflect winds blowing towards the low pressure center and creating a circulation. Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather. Tropical cyclones will not form spontaneously. Low-latitude and low-level westerly wind bursts associated with the Madden-Julian oscillation can create favorable conditions for tropical cyclogenesis by initiating tropical disturbances.

 

Locations

 

Most tropical cyclones form in a worldwide band of thunderstorm activity near the equator, referred to as the Intertropical Front (ITF), the Intertropical Convergence Zone (ITCZ), or the monsoon trough.Another important source of atmospheric instability is found in tropical waves, which contribute to the development of about 85% of intense tropical cyclones in the Atlantic ocean and become most of the tropical cyclones in the Eastern Pacific.The majority forms between 10 and 30 degrees of latitude away of the equator, and 87% forms no farther away than 20 degrees north or south. Because the Coriolis effect initiates and maintains their rotation, tropical cyclones rarely form or move within 5 degrees of the equator, where the effect is weakest.However, it is still possible for tropical systems to form within this boundary as Tropical Storm Vamei and Cyclone Agni did in 2001 and 2004, respectively.

 

Movement

 

The movement of a tropical cyclone (i.e. its "track") is typically approximated as the sum of two terms: "steering" by the background environmental wind and "beta drift".

 

Environmental steering

 

Environmental steering is the dominant term. Conceptually, it represents the movement of the storm with the background environment, akin to "leaves carried along by a stream". Physically, the flow field in the vicinity of a tropical cyclone may be decomposed into two parts: the flow associated with the storm itself, and the large-scale background flow of the environment in which the storm is embedded. In this way, tropical cyclone motion may be represented to first-order simply as the advection of the storm by the local environmental flow. This environmental flow is termed the "steering flow".

 

Climatologically, tropical cyclones are steered primarily westward by the east-to-west trade winds on the equatorward side of the subtropical ridge—a persistent high-pressure area over the world's subtropical oceans. In the tropical North Atlantic and Northeast Pacific oceans, the trade winds steer tropical easterly waves westward from the African coast toward the Caribbean Sea, North America, and ultimately into the central Pacific ocean before the waves dampen out. These waves are the precursors to many tropical cyclones within this region. In contrast, in the Indian Ocean and Western Pacific in both hemispheres, tropical cyclogenesis is influenced less by tropical easterly waves and more by the seasonal movement of the Intertropical Convergence Zone and the monsoon trough. Additionally, tropical cyclone motion can be influenced by transient weather systems, such as extratropical cyclones.

 

Beta drift

 

In addition to environmental steering, a tropical cyclone will tend to drift slowly poleward and westward, a motion known as "beta drift". This motion is due to the superposition of a vortex, such as a tropical cyclone, onto an environment in which the Coriolis force varies with latitude, such as on a sphere or beta plane. It is induced indirectly by the storm itself, the result of a feedback between the cyclonic flow of the storm and its environment.

 

Physically, the cyclonic circulation of the storm advects environmental air poleward east of center and equatorward west of center. Because air must conserve its angular momentum, this flow configuration induces a cyclonic gyre equatorward and westward of the storm center and an anticyclonic gyre poleward and eastward of the storm center. The combined flow of these gyres acts to advect the storm slowly poleward and westward. This effect occurs even if there is zero environmental flow.

 

Multiple storm interaction

 

Main article: Fujiwhara effect

 

A third component of motion that occurs relatively infrequently involves the interaction of multiple tropical cyclones. When two cyclones approach one another, their centers will begin orbiting cyclonically about a point between the two systems. Depending on their separation distance and strength, the two vortices may simply orbit around one another or else may spiral into the center point and merge. When the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will orbit around it. This phenomenon is called the Fujiwhara effect, after Sakuhei Fujiwhara.

 

Interaction with the mid-latitude westerlies

 

See also: Westerlies

 

 
Storm track of Typhoon Ioke, showing recurvature off the Japanese coast in 2006

 

Though a tropical cyclone typically moves from east to west in the tropics, its track may shift poleward and eastward either as it moves west of the subtropical ridge axis or else if it interacts with the mid-latitude flow, such as the jet stream or an extratropical cyclone. This motion, termed "recurvature", commonly occurs near the western edge of the major ocean basins, where the jet stream typically has a poleward component and extratropical cyclones are common.[70] An example of tropical cyclone recurvature was Typhoon Ioke in 2006.

 

Landfall

 

See also: List of notable tropical cyclones and Unusual areas of tropical cyclone formation

 

Officially, landfall is when a storm's center (the center of its circulation, not its edge) crosses the coastline. Storm conditions may be experienced on the coast and inland hours before landfall; in fact, a tropical cyclone can launch its strongest winds over land, yet not make landfall; if this occurs, then it is said that the storm made a direct hit on the coast. As a result of the narrowness of this definition, the landfall area experiences half of a land-bound storm by the time the actual landfall occurs. For emergency preparedness, actions should be timed from when a certain wind speed or intensity of rainfall will reach land, not from when landfall will occur.

 

Dissipation

 

Factors

 

 
Tropical Storm Franklin, an example of a strongly sheared tropical cyclone in the North Atlantic hurricane basin during 2005

 

A tropical cyclone can cease to have tropical characteristics in several different ways. One such way is if it moves over land, thus depriving it of the warm water it needs to power itself, quickly losing strength.Most strong storms lose their strength very rapidly after landfall and become disorganized areas of low pressure within a day or two, or evolve into extratropical cyclones. There is a chance a tropical cyclone could regenerate if it managed to get back over open warm water, such as with Hurricane Ivan. If it remains over mountains for even a short time, weakening will accelerate.Many storm fatalities occur in mountainous terrain, when diminishing cyclones unleash their moisture as torrential rainfall. This may lead to deadly floods and mudslides, as was the case with Hurricane Mitch in 1998. Without warm surface water, the storm cannot survive.

 

A tropical cyclone can dissipate when it moves over waters significantly below 26.5 °C (79.7 °F). This will cause the storm to lose its tropical characteristics, such as a warm core with thunderstorms near the center, and become a remnant low-pressure area. These remnant systems may persist for up to several days before losing their identity. This dissipation mechanism is most common in the eastern North Pacific.Weakening or dissipation can occur if it experiences vertical wind shear, causing the convection and heat engine to move away from the center; this normally ceases development of a tropical cyclone. In addition, its interaction with the main belt of the Westerlies, by means of merging with a nearby frontal zone, can cause tropical cyclones to evolve into extratropical cyclones. This transition can take 1–3 days.Even after a tropical cyclone is said to be extratropical or dissipated, it can still have tropical storm force (or occasionally hurricane/typhoon force) winds and drop several inches of rainfall. In the Pacific Ocean and Atlantic Ocean, such tropical-derived cyclones of higher latitudes can be violent and may occasionally remain at hurricane or typhoon-force wind speeds when they reach the west coast of North America. These phenomena can also affect Europe, where they are known as European windstorms; Hurricane Iris's extratropical remnants are an example of such a windstorm from 1995. A cyclone can also merge with another area of low pressure, becoming a larger area of low pressure. This can strengthen the resultant system, although it may no longer be a tropical cyclone. Studies in the 2000s have given rise to the hypothesis that large amounts of dust reduce the strength of tropical cyclones.

 

Artificial dissipation

 

In the 1960s and 1970s, the United States government attempted to weaken hurricanes through Project Stormfury by seeding selected storms with silver iodide. It was thought that the seeding would cause supercooled water in the outer rainbands to freeze, causing the inner eyewall to collapse and thus reducing the winds. The winds of Hurricane Debbie—a hurricane seeded in Project Stormfury—dropped as much as 31%, but Debbie regained its strength after each of two seeding forays.[83] In an earlier episode in 1947, disaster struck when a hurricane east of Jacksonville, Florida promptly changed its course after being seeded, and smashed into Savannah, Georgia.Because there was so much uncertainty about the behavior of these storms, the federal government would not approve seeding operations unless the hurricane had a less than 10% chance of making landfall within 48 hours, greatly reducing the number of possible test storms. The project was dropped after it was discovered that eyewall replacement cycles occur naturally in strong hurricanes, casting doubt on the result of the earlier attempts. Today, it is known that silver iodide seeding is not likely to have an effect because the amount of supercooled water in the rainbands of a tropical cyclone is too low.

 

Other approaches have been suggested over time, including cooling the water under a tropical cyclone by towing icebergs into the tropical oceans.Other ideas range from covering the ocean in a substance that inhibits evaporation, dropping large quantities of ice into the eye at very early stages of development (so that the latent heat is absorbed by the ice, instead of being converted to kinetic energy that would feed the positive feedback loop), or blasting the cyclone apart with nuclear weapons. Project Cirrus even involved throwing dry ice on a cyclone. These approaches all suffer from one flaw above many others: tropical cyclones are simply too large and short-lived for any of the weakening techniques to be practical.

 

Effects

 

 
The aftermath of Hurricane Katrina in Gulfport, Mississippi.

 

Main article: Effects of tropical cyclones

 

Tropical cyclones out at sea cause large waves, heavy rain, flood and high winds, disrupting international shipping and, at times, causing shipwrecks. Tropical cyclones stir up water, leaving a cool wake behind them, which causes the region to be less favorable for subsequent tropical cyclones.On land, strong winds can damage or destroy vehicles, buildings, bridges, and other outside objects, turning loose debris into deadly flying projectiles. The storm surge, or the increase in sea level due to the cyclone, is typically the worst effect from landfalling tropical cyclones, historically resulting in 90% of tropical cyclone deaths.[92] The broad rotation of a landfalling tropical cyclone, and vertical wind shear at its periphery, spawns tornadoes. Tornadoes can also be spawned as a result of eyewall mesovortices, which persist until landfall.

 

Over the past two centuries, tropical cyclones have been responsible for the deaths of about 1.9 million people worldwide. Large areas of standing water caused by flooding lead to infection, as well as contributing to mosquito-borne illnesses. Crowded evacuees in shelters increase the risk of disease propagation. Tropical cyclones significantly interrupt infrastructure, leading to power outages, bridge destruction, and the hampering of reconstruction efforts. On average, the Gulf and east coasts of the United States suffer approximately US $5 billion (1995 US $) in cyclone damage every year. Majority (83%) of tropical cyclone damage is caused by severe hurricanes, category 3 or greater. However, category 3 or greater hurricanes only account for about one-fifth of cyclones that make land every year.

 

Although cyclones take an enormous toll in lives and personal property, they may be important factors in the precipitation regimes of places they impact, as they may bring much-needed precipitation to otherwise dry regions. Tropical cyclones also help maintain the global heat balance by moving warm, moist tropical air to the middle latitudes and polar regions, and by regulating the thermohaline circulation through upwelling.[98] The storm surge and winds of hurricanes may be destructive to human-made structures, but they also stir up the waters of coastal estuaries, which are typically important fish breeding locales. Tropical cyclone destruction spurs redevelopment, greatly increasing local property values.

 

When hurricanes surge upon shore from the ocean, salt is introduced to many freshwater areas and raises the salinity levels too high for some habitats to withstand. Some are able to cope with the salt and recycle it back into the ocean, but others can not release the extra surface water quickly enough or do not have a large enough freshwater source to replace it. Because of this, some species of plants and vegetation die due to the excess salt. In addition, hurricanes can carry toxins and acids onto shore when they make landfall. The flood water can pick up the toxins from different spills and contaminate the land that it passes over. The toxins are very harmful to the people and animals in the area, as well as the environment around them. The flooding water can also spark many dangerous oil spills.

 

Observation and forecasting

 

Observation

 

Main article: Tropical cyclone observation

 

 
Sunset view of Hurricane Isidore's rainbands photographed at 7,000 feet (2,100 m)

 

Intense tropical cyclones pose a particular observation challenge, as they are a dangerous oceanic phenomenon, and weather stations, being relatively sparse, are rarely available on the site of the storm itself. In general, surface observations are available only if the storm is passing over an island or a coastal area, or if there is a nearby ship. Real-time measurements are usually taken in the periphery of the cyclone, where conditions are less catastrophic and its true strength cannot be evaluated. For this reason, there are teams of meteorologists that move into the path of tropical cyclones to help evaluate their strength at the point of landfall.

 

Tropical cyclones far from land are tracked by weather satellites capturing visible and infrared images from space, usually at half-hour to quarter-hour intervals. As a storm approaches land, it can be observed by land-based Doppler weather radar. Radar plays a crucial role around landfall by showing a storm's location and intensity every several minutes.

 

In situ measurements, in real-time, can be taken by sending specially equipped reconnaissance flights into the cyclone. In the Atlantic basin, these flights are regularly flown by United States government hurricane hunters. The aircraft used are WC-130 Hercules and WP-3D Orions, both four-engine turboprop cargo aircraft. These aircraft fly directly into the cyclone and take direct and remote-sensing measurements. The aircraft also launch GPS dropsondes inside the cyclone. These sondes measure temperature, humidity, pressure, and especially winds between flight level and the ocean's surface. A new era in hurricane observation began when a remotely piloted Aerosonde, a small drone aircraft, was flown through Tropical Storm Ophelia as it passed Virginia's Eastern Shore during the 2005 hurricane season. A similar mission was also completed successfully in the western Pacific ocean. This demonstrated a new way to probe the storms at low altitudes that human pilots seldom dare.

 

 
A general decrease in error trends in tropical cyclone path prediction is evident since the 1970s

 

Forecasting

 

See also: Tropical cyclone track forecasting, Tropical cyclone prediction model and Tropical cyclone rainfall forecasting

 

Because of the forces that affect tropical cyclone tracks, accurate track predictions depend on determining the position and strength of high- and low-pressure areas, and predicting how those areas will change during the life of a tropical system. The deep layer mean flow, or average wind through the depth of the troposphere, is considered the best tool in determining track direction and speed. If storms are significantly sheared, use of wind speed measurements at a lower altitude, such as at the 70 kPa pressure surface (3,000 metres or 9,800 feet above sea level) will produce better predictions. Tropical forecasters also consider smoothing out short-term wobbles of the storm as it allows them to determine a more accurate long-term trajectory. High-speed computers and sophisticated simulation software allow forecasters to produce computer models that predict tropical cyclone tracks based on the future position and strength of high- and low-pressure systems. Combining forecast models with increased understanding of the forces that act on tropical cyclones, as well as with a wealth of data from Earth-orbiting satellites and other sensors, scientists have increased the accuracy of track forecasts over recent decades. However, scientists are not as skillful at predicting the intensity of tropical cyclones. The lack of improvement in intensity forecasting is attributed to the complexity of tropical systems and an incomplete understanding of factors that affect their development.

 

Classifications, terminology, and naming

 

Intensity classifications

 

Main article: Tropical cyclone scales

 

 
Three tropical cyclones at different stages of development. The weakest (left) demonstrates only the most basic circular shape. A stronger storm (top right) demonstrates spiral banding and increased centralization, while the strongest (lower right) has developed an eye.

 

Tropical cyclones are classified into three main groups, based on intensity: tropical depressions, tropical storms, and a third group of more intense storms, whose name depends on the region. For example, if a tropical storm in the Northwestern Pacific reaches hurricane-strength winds on the Beaufort scale, it is referred to as a typhoon; if a tropical storm passes the same benchmark in the Northeast Pacific Basin, or in the North Atlantic, it is called a hurricane.   Neither "hurricane" nor "typhoon" is used in either the Southern Hemisphere or the Indian Ocean. In these basins, storms of a tropical nature are referred to as either tropical cyclones, severe tropical cyclones or very intense tropical cyclones.

 

As indicated in the table below, each basin uses a separate system of terminology, which can make comparisons between different basins difficult. In the Pacific Ocean, hurricanes from the Central North Pacific sometimes cross the 180th meridian into the Northwest Pacific, becoming typhoons (such as Hurricane/Typhoon Ioke in 2006); on rare occasions, the reverse will occur. It should also be noted that typhoons with 1-minute sustained winds greater than 67 metres per second (m/s), over 150 miles per hour (240 km/h), are called Super Typhoons by the Joint Typhoon Warning Center.

 

Tropical depression

 

"Tropical Depression" redirects here. For the Filipino reggae band, see Tropical Depression (band). For the EP by Elephant Micah, see Tropical Depression (EP).

 

A tropical depression is an organized system of clouds and thunderstorms with a defined, closed surface circulation and maximum sustained winds of less than 34 knots (63 km/h). It has no eye and does not typically have the organization or the spiral shape of more powerful storms. However, it is already a low-pressure system, hence the name "depression". In the Philippines, the practice is to name tropical depressions from their own naming convention when the depressions are within the country's area of responsibility.

 

Tropical storm

 

A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 34 knots (63 km/h) and 64 knots (119 km/h). At this point, the distinctive cyclonic shape starts to develop, although an eye is not usually present. Government weather services first assign names to systems that reach this intensity (thus the term named storm).Although tropical storms are less intense than a hurricane they can produce significant damage. The shear force of winds can blow off shingles and air borne object can cause damage to power lines, roofing and siding. More dangerous is the heavy rain fall causing inland flooding. It is also at this point the nation hurricane center issues Watches and warnings for coastal areas.

 

Hurricane or typhoon

 

Main articles: Atlantic hurricane, Pacific hurricane and typhoon

 

A hurricane or typhoon (sometimes simply referred to as a tropical cyclone, as opposed to a depression or storm) is a system with sustained winds of at least 34 metres per second (66 kn) or 74 miles per hour (119 km/h).A cyclone of this intensity tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the center of circulation. The eye is often visible in satellite images as a small, circular, cloud-free spot. Surrounding the eye is the eyewall, an area about 16 kilometres (9.9 mi) to 80 kilometres (50 mi) wide in which the strongest thunderstorms and winds circulate around the storm's center. Maximum sustained winds in the strongest tropical cyclones have been estimated at about 85 metres per second (165 kn) or 314 kilometres per hour (195 mph).

 

[hide]Tropical cyclone classifications [115][116]
The
Beaufort
scale
1-minute sustained winds10-minute sustained windsNE Pacific &
N Atlantic
NHC/CPHC
NW Pacific
JTWC
NW Pacific
JMA
N Indian Ocean
IMD
SW Indian Ocean
MF
Australia & S Pacific
BOM/FMS[117]
0–7 <32 knots (37 mph; 59 km/h) <28 knots (32 mph; 52 km/h) Tropical Depression Tropical Depression Tropical Depression Depression Zone of Disturbed Weather Tropical Disturbance
Tropical Depression
Tropical Low
7 33 knots (38 mph; 61 km/h) 28–29 knots (32–33 mph; 52–54 km/h) Deep Depression Tropical Disturbance
8 34–37 knots (39–43 mph; 63–69 km/h) 30–33 knots (35–38 mph; 56–61 km/h) Tropical Storm Tropical Storm Tropical Depression
9-10 38–54 knots (44–62 mph; 70–100 km/h) 34–47 knots (39–54 mph; 63–87 km/h) Tropical Storm Cyclonic Storm Moderate Tropical Storm Category 1
tropical cyclone
11 55–63 knots (63–72 mph; 102–117 km/h) 48–55 knots (55–63 mph; 89–102 km/h) Severe Tropical Storm Severe Cyclonic Storm Severe Tropical Storm Category 2
tropical cyclone
12+ 64–71 knots (74–82 mph; 119–131 km/h) 56–63 knots (64–72 mph; 104–117 km/h) Category 1 hurricane Typhoon
72–82 knots (83–94 mph; 133–152 km/h) 64–72 knots (74–83 mph; 119–133 km/h) Typhoon Very Severe
Cyclonic Storm
Tropical Cyclone Category 3 severe
tropical cyclone
83–95 knots (96–109 mph; 154–176 km/h) 73–83 knots (84–96 mph; 135–154 km/h) Category 2 hurricane
96–97 knots (110–112 mph; 178–180 km/h) 84–85 knots (97–98 mph; 156–157 km/h) Category 3 hurricane
98–112 knots (113–129 mph; 181–207 km/h) 86–98 knots (99–113 mph; 159–181 km/h) Intense Tropical Cyclone Category 4 severe
tropical cyclone
113–122 knots (130–140 mph; 209–226 km/h) 99–107 knots (114–123 mph; 183–198 km/h) Category 4 hurricane
123–129 knots (142–148 mph; 228–239 km/h) 108–113 knots (124–130 mph; 200–209 km/h) Category 5 severe
tropical cyclone
130–136 knots (150–157 mph; 241–252 km/h) 114–119 knots (131–137 mph; 211–220 km/h) Super Typhoon Super Cyclonic Storm Very Intense Tropical Cyclone
>137 knots (158 mph; 254 km/h) >120 knots (140 mph; 220 km/h) Category 5 hurricane

 

Origin of storm terms

 

 
Taipei 101 endures a typhoon in 2005

 

The word typhoon, which is used today in the Northwest Pacific, may be derived from Arabic ţūfān (طوفان) (similar in Hindi/Urdu and Persian), which in turn originates from Greek Typhon (Τυφών), a monster from Greek mythology associated with storms. The related Portuguese word tufão, used in Portuguese for typhoons, is also derived from Typhon. The word is also similar to Chinese "táifēng" (Simplified Chinese: 台风) (fēng = wind), "toifung" in Cantonese (Traditional Chinese: 颱風), "taifū" (台風) in Japanese, and "taepung" (태풍) in Korean.

 

The word hurricane, used in the North Atlantic and Northeast Pacific, is derived from huracán, the Spanish word for the Carib/Taino storm god, Juracán. This god is believed by scholars to have been at least partially derived from the Mayan creator god, Huracan. Huracan was believed by the Maya to have created dry land out of the turbulent waters. The god was also credited with later destroying the "wooden people", the precursors to the "maize people", with an immense storm and flood. Huracan is also the source of the word orcan, another word for a particularly strong European windstorm.

 

Naming

 

Main articles: Tropical cyclone naming and Lists of tropical cyclone names

 

Storms reaching tropical storm strength were initially given names to eliminate confusion when there are multiple systems in any individual basin at the same time, which assists in warning people of the coming storm. In 1953, the United States abandoned a plan to name hurricanes using a phonetic alphabet, and began using women's names. The practice of using women's names exclusively came to an end in 1978, when both men's and women's names were added to the Eastern North Pacific storm lists. The lists for the Atlantic and Gulf of Mexico followed in 1979. In most cases, a tropical cyclone retains its name throughout its life; however, under special circumstances, tropical cyclones may be renamed while active. These names are taken from lists that vary from region to region and are usually drafted a few years ahead of time. The lists are decided on, depending on the regions, either by committees of the World Meteorological Organization (called primarily to discuss many other issues) or by national weather offices involved in the forecasting of the storms. Within all basins except for the Southwestern or North Indian Ocean, the names of particularly destructive or deadly storms may be retired and replaced with a new name.

 

According to a study done by researchers at the University of Illinois hurricanes with feminine names cause up to three times more fatalities because people don't see them as threatening compared to hurricanes with masculine names. Every hurricane in the US between 1950 and 2012 except for Hurricane Katrina (2005) and Hurricane Audrey (1957) were studied and results show that hurricanes with masculine names cause 15.15 deaths and hurricanes with feminine names caused 41.84 deaths.

 

Notable tropical cyclones

 

Main articles: List of notable tropical cyclones, List of Atlantic hurricanes and List of Pacific hurricanes

 

Tropical cyclones that cause extreme destruction are rare, although when they occur, they can cause great amounts of damage or thousands of fatalities. The 1970 Bhola cyclone is the deadliest tropical cyclone on record, killing more than 300,000 people and potentially as many as 1 million after striking the densely populated Ganges Delta region of Bangladesh on November 13, 1970. Its powerful storm surge was responsible for the high death toll. The North Indian cyclone basin has historically been the deadliest basin. Elsewhere, Typhoon Nina killed nearly 100,000 in China in 1975 due to a 100-year flood that caused 62 dams including the Banqiao Dam to fail. The Great Hurricane of 1780 is the deadliest North Atlantic hurricane on record, killing about 22,000 people in the Lesser Antilles. A tropical cyclone does need not be particularly strong to cause memorable damage, primarily if the deaths are from rainfall or mudslides. Tropical Storm Thelma in November 1991 killed thousands in the Philippines, while in 1982, the unnamed tropical depression that eventually became Hurricane Paul killed around 1,000 people in Central America.

 

Hurricane Katrina is estimated as the costliest tropical cyclone worldwide,causing $81.2 billion in property damage (2008 USD) with overall damage estimates exceeding $100 billion (2005 USD). Katrina killed at least 1,836 people after striking Louisiana and Mississippi as a major hurricane in August 2005. Hurricane Sandy is the second most destructive tropical cyclone in U.S history, with damages totaling $68 billion (2012 USD), and with damage costs at $37.5 billion (2012 USD), Hurricane Ike is the third most destructive tropical cyclone in U.S history. The Galveston Hurricane of 1900 is the deadliest natural disaster in the United States, killing an estimated 6,000 to 12,000 people in Galveston, Texas. Hurricane Mitch caused more than 10,000 fatalities in Latin America. Hurricane Iniki in 1992 was the most powerful storm to strike Hawaii in recorded history, hitting Kauai as a Category 4 hurricane, killing six people, and causing U.S. $3 billion in damage.Kauai was also struck by Hurricanes Dot (1959) and Iwa (1982) (see List of Hawaii hurricanes). Other destructive Eastern Pacific hurricanes include Pauline and Kenna, both causing severe damage after striking Mexico as major hurricanes.In March 2004, Cyclone Gafilo struck northeastern Madagascar as a powerful cyclone, killing 74, affecting more than 200,000, and becoming the worst cyclone to affect the nation for more than 20 years.

 

Hurricane Sandy which hit the United States east coast in late October 2012, caused unprecedented damage, flooded subways, closed down all major airports, resulted in cancellation of over 20,000 flights, costing the aviation industry nearly 200 million dollars. It claimed more than one-hundred lives. Eleven states and the District of Columbia were declared disaster states. Estimates for the damage cost vary from 50–70 billion dollars.

 

The relative sizes of Typhoon Tip, Cyclone Tracy, and the Contiguous United States

 

The most intense storm on record was Typhoon Tip in the northwestern Pacific Ocean in 1979, which reached a minimum pressure of 870 hectopascals (26 inHg) and maximum sustained wind speeds of 165 knots (85 m/s) or 190 miles per hour (310 km/h). Tip, however, does not solely hold the record for fastest sustained winds in a cyclone. Typhoon Keith in the Pacific and Hurricane Allen in the North Atlantic currently share this record with Tip.Typhoon Nancy in 1961 had recorded wind speeds of 185 knots (95 m/s) or 215 miles per hour (346 km/h), but recent research indicates that wind speeds from the 1940s to the 1960s were gauged too high, and this is no longer considered the storm with the highest wind speeds on record. Likewise, a surface-level gust caused by Typhoon Paka on Guam was recorded at 205 knots (105 m/s) or 235 miles per hour (378 km/h). Had it been confirmed, it would be the strongest non-tornadic wind ever recorded on the Earth's surface, but the reading had to be discarded since the anemometer was damaged by the storm.

 

In addition to being the most intense tropical cyclone on record based on pressure, Tip was the largest cyclone on record, with tropical storm-force winds 2,170 kilometres (1,350 mi) in diameter. The smallest storm on record, Tropical Storm Marco, formed during October 2008, and made landfall in Veracruz. Marco generated tropical storm-force winds only 37 kilometres (23 mi) in diameter.

 

Hurricane John is the longest-lasting tropical cyclone on record, lasting 31 days in 1994. Before the advent of satellite imagery in 1961, however, many tropical cyclones were underestimated in their durations.John is also the longest-tracked tropical cyclone in the Northern Hemisphere on record, which had a path of 8,250 mi (13,280 km). Cyclone Rewa of the 1993-94 South Pacific and Australian region cyclone seasons had one of the longest tracks observed within the Southern Hemisphere, travelling a distance of over 5,545 mi (8,920 km) during December 1993 and January 1994.

 

Changes caused by El Niño-Southern Oscillation

 

See also: El Niño-Southern Oscillation

 

Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies. When the subtropical ridge position shifts due to El Niño, so will the preferred tropical cyclone tracks. Areas west of Japan and Korea tend to experience much fewer September–November tropical cyclone impacts during El Niño and neutral years. During El Niño years, the break in the subtropical ridge tends to lie near 130°E which would favor the Japanese archipelago. During El Niño years, Guam's chance of a tropical cyclone impact is one-third more likely than of the long-term average.The tropical Atlantic ocean experiences depressed activity due to increased vertical wind shear across the region during El Niño years. During La Niña years, the formation of tropical cyclones, along with the subtropical ridge position, shifts westward across the western Pacific ocean, which increases the landfall threat to China and much greater intensity in the Philippines.

 

Long-term activity trends

 

See also: Atlantic hurricane reanalysis

 

 
Atlantic Multidecadal Cycle since 1950, using accumulated cyclone energy (ACE)

 

 
Atlantic Multidecadal Oscillation Timeseries, 1856–2013

 

While the number of storms in the Atlantic has increased since 1995, there is no obvious global trend; the annual number of tropical cyclones worldwide remains about 87 ± 10 (Between 77 and 97 tropical cyclones annually). However, the ability of climatologists to make long-term data analysis in certain basins is limited by the lack of reliable historical data in some basins, primarily in the Southern Hemisphere, while noting that a significant downward trend in tropical cyclone numbers has been identified for the region near Australia (based on high quality data and accounting for the influence of the El Niño-Southern Oscillation).In spite of that, there is some evidence that the intensity of hurricanes is increasing. Kerry Emanuel stated, "Records of hurricane activity worldwide show an upswing of both the maximum wind speed in and the duration of hurricanes. The energy released by the average hurricane (again considering all hurricanes worldwide) seems to have increased by around 70% in the past 30 years or so, corresponding to about a 15% increase in the maximum wind speed and a 60% increase in storm lifetime."

 

Atlantic storms are becoming more destructive financially, as evidenced by the fact that five of the ten most expensive storms in United States history have occurred since 1990. According to the World Meteorological Organization, "recent increase in societal impact from tropical cyclones has been caused largely by rising concentrations of population and infrastructure in coastal regions." Pielke et al. (2008) normalized mainland U.S. hurricane damage from 1900–2005 to 2005 values and found no remaining trend of increasing absolute damage. The 1970s and 1980s were notable because of the extremely low amounts of damage compared to other decades. The decade 1996–2005 was the second most damaging among the past 11 decades, with only the decade 1926–1935 surpassing its costs. The most damaging single storm is the 1926 Miami hurricane, with $157 billion of normalized damage.

 

Often in part because of the threat of hurricanes, many coastal regions had sparse population between major ports until the advent of automobile tourism; therefore, the most severe portions of hurricanes striking the coast may have gone unmeasured in some instances. The combined effects of ship destruction and remote landfall severely limit the number of intense hurricanes in the official record before the era of hurricane reconnaissance aircraft and satellite meteorology. Although the record shows a distinct increase in the number and strength of intense hurricanes, therefore, experts regard the early data as suspect.

 

The number and strength of Atlantic hurricanes may undergo a 50–70 year cycle, also known as the Atlantic Multidecadal Oscillation. Nyberg et al. reconstructed Atlantic major hurricane activity back to the early 18th century and found five periods averaging 3–5 major hurricanes per year and lasting 40–60 years, and six other averaging 1.5–2.5 major hurricanes per year and lasting 10–20 years. These periods are associated with the Atlantic multidecadal oscillation. Throughout, a decadal oscillation related to solar irradiance was responsible for enhancing/dampening the number of major hurricanes by 1–2 per year.

 

Although more common since 1995, few above-normal hurricane seasons occurred during 1970–94.[164] Destructive hurricanes struck frequently from 1926 to 1960, including many major New England hurricanes. Twenty-one Atlantic tropical storms formed in 1933, a record only recently exceeded in 2005, which saw 28 storms. Tropical hurricanes occurred infrequently during the seasons of 1900–25; however, many intense storms formed during 1870–99. During the 1887 season, 19 tropical storms formed, of which a record 4 occurred after November 1 and 11 strengthened into hurricanes. Few hurricanes occurred in the 1840s to 1860s; however, many struck in the early 19th century, including an 1821 storm that made a direct hit on New York City. Some historical weather experts say these storms may have been as high as Category 4 in strength.

 

These active hurricane seasons predated satellite coverage of the Atlantic basin. Before the satellite era began in 1960, tropical storms or hurricanes went undetected unless a reconnaissance aircraft encountered one, a ship reported a voyage through the storm, or a storm hit land in a populated area.

 

Proxy records based on paleotempestological research have revealed that major hurricane activity along the Gulf of Mexico coast varies on timescales of centuries to millennia. Few major hurricanes struck the Gulf coast during 3000–1400 BC and again during the most recent millennium. These quiescent intervals were separated by a hyperactive period during 1400 BC and 1000 AD, when the Gulf coast was struck frequently by catastrophic hurricanes and their landfall probabilities increased by 3–5 times. This millennial-scale variability has been attributed to long-term shifts in the position of the Azores High, which may also be linked to changes in the strength of the North Atlantic Oscillation.

 

According to the Azores High hypothesis, an anti-phase pattern is expected to exist between the Gulf of Mexico coast and the Atlantic coast. During the quiescent periods, a more northeasterly position of the Azores High would result in more hurricanes being steered towards the Atlantic coast. During the hyperactive period, more hurricanes were steered towards the Gulf coast as the Azores High was shifted to a more southwesterly position near the Caribbean. Such a displacement of the Azores High is consistent with paleoclimatic evidence that shows an abrupt onset of a drier climate in Haiti around 3200 14C years BP, and a change towards more humid conditions in the Great Plains during the late-Holocene as more moisture was pumped up the Mississippi Valley through the Gulf coast. Preliminary data from the northern Atlantic coast seem to support the Azores High hypothesis. A 3000-year proxy record from a coastal lake in Cape Cod suggests that hurricane activity increased significantly during the past 500–1000 years, just as the Gulf coast was amid a quiescent period of the last millennium.

 

Global warming

 

See also: Effects of global warming, Hurricane Alley and Hurricane Katrina and global warming

 

According to IPCC SREX 2012, "attribution of single extreme events to anthropogenic climate change is challenging".On one hand, the report said that there is "medium evidence" that long-term trends in normalized losses have not been attributed to tropical and extratropical [winter] storms. On the other hand, the report also noted that much more research is needed in part due to "confounding factors" that might have increased losses, such as increased population and development in at-risk areas, and those that might have decreased losses, such as better forecasting, emergency alert systems, emergency management, building codes, and near-instantaneous media coverage of weather emergencies.

 

Some experts who agree that we can not yet detect any increase in frequency or intensity of tropical cyclones include Thomas Knutson, and Roger Pielke Jr..

 

Others say that there is evidence for a causal connection. The U.S. National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory performed a simulation to determine if there is a statistical trend in the frequency or strength of tropical cyclones over time. The simulation concluded "the strongest hurricanes in the present climate may be upstaged by even more intense hurricanes over the next century as the earth's climate is warmed by increasing levels of greenhouse gases in the atmosphere".

 

In another simulation done by Kerry Emanuel, application of a tropical cyclone downscaling technique to six CMIP5-generation global climate models run under historical conditions and under the RCP8.5 emissions projection indicates an increase in global tropical cyclone activity, most evident in the North Pacific region but also noticeable in the North Atlantic and South Indian Oceans. In these regions, both the frequency and intensity of tropical cyclones are projected to increase. This result contrasts with the result of applying the same downscaling technique to CMIP3-generation models, which generally predict a small decrease of global tropical cyclone frequency.

 

 

 

Costliest U.S. Atlantic hurricanes 1900–2010
Total estimated property damage, adjusted for wealth normalization[161][176]
RankHurricaneSeasonCost (2010 USD)
1 "Miami" 1926 $164.8 billion
2 Katrina 2005 $113.4 billion
3 "Galveston" 1900 $104.3 billion
4 "Galveston" 1915 $ 71.3 billion
5 Andrew 1992 $ 58.5 billion
6 "New England" 1938 $ 41.1 billion
7 "Cuba–Florida" 1944 $ 40.6 billion
8 "Okeechobee" 1928 $ 35.2 billion
9 Ike 2008 $ 29.5 billion
10 Donna 1960 $ 28.1 billion
Main article: List of costliest Atlantic hurricanes

 

 
Costliest US Atlantic Hurricanes 1900–2010

 

In an article in Nature, meteorology professor Kerry Emanuel stated that potential hurricane destructiveness, a measure combining hurricane strength, duration, and frequency, "is highly correlated with tropical sea surface temperature, reflecting well-documented climate signals, including multidecadal oscillations in the North Atlantic and North Pacific, and global warming". Emanuel predicted "a substantial increase in hurricane-related losses in the twenty-first century". In more recent work, Emanuel states that new climate modeling data indicates "global warming should reduce the global frequency of hurricanes." According to the Houston Chronicle, the new work suggests that, even in a dramatically warming world, hurricane frequency and intensity may not substantially rise during the next two centuries.

 

P.J. Webster and others published an article in Science examining the "changes in tropical cyclone number, duration, and intensity" over the past 35 years, the period when satellite data has been available. Their main finding was although the number of cyclones decreased throughout the planet excluding the north Atlantic Ocean, there was a great increase in the number and proportion of very strong cyclones.[180]

 

The strength of the reported effect is surprising in light of modeling studies that predict only a one-half category increase in storm intensity as a result of a ~2 °C (3.6 °F) global warming. Such a response would have predicted only a ~10% increase in Emanuel's potential destructiveness index during the 20th century rather than the ~75–120% increase he reported. Second, after adjusting for changes in population and inflation, and despite a more than 100% increase in Emanuel's potential destructiveness index, no statistically significant increase in the monetary damages resulting from Atlantic hurricanes has been found.

 

Sufficiently warm sea surface temperatures are considered vital to the development of tropical cyclones. Although neither study can directly link hurricanes with global warming, the increase in sea surface temperatures is believed to be due to both global warming and natural variability, e.g. the hypothesized Atlantic Multidecadal Oscillation (AMO), although an exact attribution has not been defined. However, recent temperatures are the warmest ever observed for many ocean basins.

 

In February 2007, the United Nations Intergovernmental Panel on Climate Change released its fourth assessment report on climate change. The report noted many observed changes in the climate, including atmospheric composition, global average temperatures, ocean conditions, and others. The report concluded the observed increase in tropical cyclone intensity is larger than climate models predict. In addition, the report considered that it is likely that storm intensity will continue to increase through the 21st century, and declared it more likely than not that there has been some human contribution to the increases in tropical cyclone intensity.[185] However, there is no universal agreement about the magnitude of the effects anthropogenic global warming has on tropical cyclone formation, track, and intensity. For example, critics such as Chris Landsea assert that man-made effects would be "quite tiny compared to the observed large natural hurricane variability". A statement by the American Meteorological Society on February 1, 2007 stated that trends in tropical cyclone records offer "evidence both for and against the existence of a detectable anthropogenic signal" in tropical cyclogenesis. Although many aspects of a link between tropical cyclones and global warming are still being "hotly debated", a point of agreement is that the strength of destructiveness no individual tropical cyclone or season can be attributed entirely to global warming. Research reported in the September 3, 2008 issue of Nature found that the strongest tropical cyclones are getting stronger, in particular over the North Atlantic and Indian oceans. Wind speeds for the strongest tropical storms increased from an average of 230 kilometres per hour (140 mph) in 1981 to 251 kilometres per hour (156 mph) in 2006, while the ocean temperature, averaged globally over all the regions where tropical cyclones form, increased from 28.2 °C (82.8 °F) to 28.5 °C (83.3 °F) during this period.

 

Related cyclone types

 

 
Gustav on September 9, the first system to be given a name as a subtropical cyclone

 

See also: Cyclone, Extratropical cyclone and Subtropical cyclone

 

In addition to tropical cyclones, there are two other classes of cyclones within the spectrum of cyclone types. These kinds of cyclones, known as extratropical cyclones and subtropical cyclones, can be stages a tropical cyclone passes through during its formation or dissipation.An extratropical cyclone is a storm that derives energy from horizontal temperature differences, which are typical in higher latitudes. A tropical cyclone can become extratropical as it moves toward higher latitudes if its energy source changes from heat released by condensation to differences in temperature between air masses; although not as frequently, an extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone.[192] From space, extratropical storms have a characteristic "comma-shaped" cloud pattern. Extratropical cyclones can also be dangerous when their low-pressure centers cause powerful winds and high seas.

 

A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. They can form in a wide band of latitudes, from the equator to 50°. Although subtropical storms rarely have hurricane-force winds, they may become tropical in nature as their cores warm. From an operational standpoint, a tropical cyclone is usually not considered to become subtropical during its extratropical transition.

 

 

Hagel (neerslag)

 
 
 
Samengeklonterde hagelsteen van 6 centimeter doorsnee, de kleinere stenen zijn zichtbaar. NOAA Photo Library

Hagel is een vorm van neerslag die uit kleine gelaagde ijsklompen bestaat. Hagel kan in verband met de ontstaanswijze alleen in buien voorkomen en niet in frontale neerslaggebieden.

 

Ontstaan

 
Kleine hagelstenen van minder dan 1 mm diameter zijn doorgesneden en 246 x vergroot. De opname werd gemaakt met een Low Temperature Scanning Electron Microscope (LT-SEM).
 
Zware hagelbui tijdens hevig onweer. De grootste hagelstenen lijken een doorsnee te hebben van 5 tot 8 centimeter.

Hagel ontstaat wanneer kleine ijs- en sneeuwkristallen terechtkomen in luchtlagen met grote onderkoelde waterdruppels. Het bovenste deel van een buienwolk, waar het meer dan 20 °C vriest, bevat ijskristallen, terwijl het onderste deel, met temperaturen tussen -10 en -20 °C, onderkoelde druppels bevat. Dalende en stijgende luchtbewegingen in de wolk jagen ijsdeeltjes door niveaus met veel onderkoeld water. Zo komen ze in botsing met andere onderkoelde druppels en ijs. De onderkoelde druppels zetten zich af op de ijskristallen, die groeien en ten slotte als hagel uit de wolk vallen.

Hagelstenen bestaan vaak uit laagjes die afwisselend mat en helder zijn. In het matte deel zijn op grote, koude hoogten de botsende deeltjes of druppels onmiddellijk vastgevroren. In het heldere deel is vloeibaar water op lagere warmere hoogte ingevangen dat pas later op koudere hoogte bevroren is.

Kenmerken

In Nederland komt zomerhagel met een diameter van 2 centimeter of meer gemiddeld vijf keer per jaar ergens in het land voor. Vrijwel elke zomer komen plaatselijk ook grotere hagelstenen omlaag: op 23 juli 1996 vielen er in Apeldoorn en omgeving hagelstenen van circa 6 centimeter. Op 6 juni 1998 zijn extreem grote hagelstenen gevallen tot bijna 10 centimeter doorsnee. Meldingen kwamen uit de Alblasserwaard, Elburg en Zeewolde. In deze gevallen ging het om een supercel met een roterende stijgstroom.

Grote stenen zijn door hun gewicht gevaarlijker dan kleine. Bovendien slaan ze met grotere snelheid in. Bolvormige stenen van 3 centimeter halen zo'n 50 km/h, stenen als tennisballen van 6 centimeter wegen ongeveer 100 gram en vallen met 120 km/h. Nog grotere stenen zouden snelheden kunnen bereiken van 300 km/h. Hagelstenen hebben vaak langwerpige uitsteeksels en daarom zegt het gewicht meer over het gevaar dan de afmeting. Grote hagelstenen ontstaan in gigantische onweerswolken die soms tot meer dan 10 kilometer hoogte uitgroeien en waarin krachtige omhoog en omlaag gerichte luchtstromingen voorkomen. Zulke buien gaan vergezeld van valwinden, windstoten of windhozen. Hagel kan grote schade aanbrengen, in het bijzonder in de landbouw. Voor de (glas)tuinbouw bestaan daarom speciale hagelschadeverzekeringen. De plaats waar het vaakst hagel valt is Kericho (Kenia) met 132 dagen hagel per jaar. De grootste hagelsteen (voor zover bekend) ooit viel in 23 juli 2010 in Vivian in de Amerikaanse staat South Dakota met een diameter van 20 centimeter en een gewicht van 0,76 kilogram.

 
Grootste hagelsteen uit Vivian, circa 8 inches (20 centimeter).

Symbolen

De volgende symbolen worden gebruikt op weerkaarten:

SymboolNr.Beschrijving
Symbol Shower3.png
27 Hagelbui of korrelsneeuwbui in het afgelopen uur
Symbol Shower9.png
89 Lichte hagelbui, met of zonder regen en/of regen en sneeuw, en nog zonder donder
Symbol Shower8.png
90 Matige of zware hagelbui, met of zonder regen en/of regen en sneeuw, en nog zonder donder
Symbol Thunder3.png
93 Onweer in het afgelopen uur en lichte sneeuw, regen en sneeuw, of hagel op het moment van waarneming
Symbol Thunder10.png
94 Onweer in het afgelopen uur en matige of zware sneeuw, regen en sneeuw, of hagel op het moment van waarneming
Symbol Thunder5.png
96 Licht of matig onweer met hagel of korrelsneeuw
Symbol Thunder11.png
99 Zwaar onweer met hagel of korrelsneeuw

 

Shelf cloud

 
 
Shelf cloud boven Enschede op 17 juli 2004.

Een shelf cloud (letterlijk plankwolk) is een wolk die soms voorafgaat aan zware onweersbuien. In de meteorologie worden zulke bijzondere wolkenvormen, samen met de rolwolk als een type gezien van een arcus boogwolk.

De onheilspellende shelf cloud ontstaat wanneer koudere lucht die met de onweersbui meekomt vanaf enige hoogte, in aanraking komt met veel warmere lucht aan het aardoppervlak. De koude lucht drukt dan de warme vochtige lucht omhoog, waardoor de vochtige lucht condenseert. In de lucht kan dan een vaak wat afgeplatte wolkenrol ontstaan die er zeer onheilspellend uitziet. Aan de onderzijde zijn soms gerafelde randen waarneembaar waarin roterende en 'kolkende' delen te zien zijn.

 
Shelf cloud trekt over Zwolle op 26 juli 2008

Shelf clouds ontstaan bij snel verplaatsende en zware onweersbuien waarbij de warme lucht aan de voorkant van het torenhoge wolkcomplex sterk stijgt terwijl op meer dan 10 kilometer hoogte in de buienwolk die er achter zit, koude luchtmassa's omlaag storten. Een shelf cloud wordt vergezeld door enorme en plotselinge windstoten van soms 100 tot 150 kilometer per uur. Het is een voorkeursplaats voor windhozen, maar vaak blijft het bij een begin van hoosvorming in de lucht dat tuba genoemd wordt. In dat geval zit er meestal in de buienlijn een HP-supercel verscholen, op de buienradar is dat te zien aan inkepingen in het neerslagpatroon aan de voorkant van de buienlijn.

Een shelf cloud verschilt van een rolwolk omdat een rolwolk helemaal los staat van de basis van de onweersbui of van andere wolken. Rolwolken zijn relatief zeldzaam.

Een zeer duidelijke shelf cloud was in Nederland te zien op 17 juli 2004. De angstaanjagende shelf cloud ging vergezeld van zware tot zeer zware windstoten. Hoek van Holland registreerde 101 km/uur en in het binnenland schoten de windmeters uit tot 80 à 95 km/uur. De passage van de buienlijn ging vergezeld van een plotseling luchtdrukverandering van enkele hPa, zichtbaar als onweersneus in het barogram.

Een shelf cloud komt niet uitsluitend voor bij zware onweersbuien. Iedere convectieve wolk kan in de juiste situatie een shelfcloud veroorzaken, en niet elke zware onweersbui laat een shelfcloud zien.

Shelfcloudsquall.jpg