Usuário:WOtP/Testes

Origem: Wikipédia, a enciclopédia livre.
A typical thunderstorm over a field

Uma trovoada, também conhecida como tempestade elétrica, é um tipo de tempestade caracterizada pela presença de raios e seus efeitos acústicos, os trovões.[1] Trovoadas ocorrem associadas a um tipo de nuvem, o cúmulo-minbo. São normalmente acompanhadas por fortes ventos, chuvas intensas e eventualmente neve, grãos de gelo, granizo ou mesmo não apresentar precipitação. Podem vir em série ou formando uma faixa de chuva conhecida como linha de instabilidade. Trovoadas fortes e severas podem rotacionar, criando supercélulas. Enquanto a maior parte destas tempestades têm seu movimento determinado pelo vento médio predominante nas camadas médias da troposfera, ventos cisalhantes verticais causam um desvio em seu curso em ângulo perpendicular à direção do vento cisalhante.

Warm, moist updraft from a thunderstorm associated with a southward-moving frontal boundary. Taken from Texarkana, Texas looking north.

Trovoadas resultam do rápido movimento ascendente de ar quente e úmido. Conforme a altitude aumenta, o ar se resfria e a umidade condensa, formando cúmulos-nimbos que podem atingir alturas superiores a vinte quilômetros. Conforme gotículas de água e gelo se formam, caem e absorvem outras gotículas, tonando-se maiores. Esta queda produz uma corrente de ar frio descendente, que se espalha ao atingir a superfície, causando as rajadas de vento comumente associadas às trovoadas e eventualmente neblina.

Estas tempestades podem se formar e desenvolver em qualquer local do globo, mas estão concentradas em áreas localizadas em latitudes médias, onde ar quente e úmido colide com massas de ar mais frio.[2] Trovoadas são responsáveis pela formação e desenvolvimento de vários fenômenos de tempo severo, que representam perigo para a população. Os prejuízos provém da ocorrência de fortes rajadas de vento, grandes pedras de granizo e enchentes rápidas causadas pela intensa precipitação. Trovoadas ainda mais fortes são capazes de dar origem à tornados e trombas de água.

Há quatro tipos de tempestades: de célula única, multicélulas em aglomerado ou em linha e supercélulas. Esta última é a mais forte, associada aos eventos de tempo severo. sistemas convectivos de mesoescala formados por ventos cisalhantes favoráveis nas regiões tropicais e subtropicais são responsáveis pelo desenvolvimento de furacões. Trovoadas sem precipitação podem originar incêndios florestais, a partir da queda de descargas elétricas. Muitos métodos são utilizados para o estudo de trovoadas, como radares meteorológicos, estações meteorológicas e videofotografia. Povos antigos criaram vários mitos e lendas sobre a natureza das trovoadas e seu desenvolvimento. Trovoadas também já foram observadas fora da Terra em Júpiter e Vênus.

Ciclo de vida[editar | editar código-fonte]

Estágios do ciclo de vida de uma tempestade convectiva.
Ver também: Nuvem

Ar quente tem uma densidade menor que o ar frio, fazendo com que o primeiro suba, no processo de convecção atmosférica. [3][4] Nuvens se formam quando o ar relativamente mais quente, carregando umidade, atinge grandes alturas onde a temperatura menor faz com que se resfrie e o vapor de água se condense.[5] Quando isso ocorre, a água libera energia chamada de calor latente de vaporização, o que permite com que o ar continue mais quente em relação aos seus arredores e o processo de convecção continue.[6] Se a instabilidade atmosférica que deu origem ao processo persistir, ocorrerá a formação de nuvens de grande desenvolvimento vertical, os cúmulos-nimbos, que produzem muitos raios.[7]

Todas as tempestades, independente de seu tipo, passam por três estágios: o de desenvolvimento, o estágio maduro e a dissipação.[8] Uma trovada de tamanho médio possui um diâmetro de 24 km. Dependendo das condições atmosféricas, estes três estágios ocorrem em um intervalo de trinta minutos.[9]

Desenvolvimento[editar | editar código-fonte]

O primeiro estágio de uma tempestade é o estado de desenvolvimento, no qual massas de ar úmido são elevadas para o alto na atmosfera. Podem ser iniciados pela insolação que produz correntes de ar ascendentes chamadas térmicas, pela convergência de ventos superficiais, forçando o movimento vertical ascendente ou ventos que incidem sobre áreas de relevo ascendente. Em grandes altitudes, o ar se resfria e o vapor se condensa em gotículas de água liquida, formando nuvens cúmulos. Conforme se condensa, a água libera o seu calor latente de vaporização, mantendo o ar ao seu redor mais quente e favorecendo a continuidade da convecção. A corrente de ar ascendente cria uma zona de baixa pressão sob a nuvem. Em uma trovada típica, aproximadamente 5×108 kg de vapor de água são elevados da superfície para o alto da troposfera.[10]

Estágio maduro[editar | editar código-fonte]

Neste estágio do ciclo de vida de uma trovoada, o ar quente continua a subir até chegar a uma camada de ar igualmente quente, interrompendo seu trajeto. Normalmente isto ocorre quando a corrente ascendente chega à tropopausa. O ar é então forçado a se espalhar horizontalmente, dando ao topo do cúmulo-nimbo a aparência característica de uma bigorna, formando o cumulonimbus incus. As gotículas de água e partículas de gelo então coalescem e se fundem para formar a chuva. Se a corrente de ar ascendente é forte o bastante, as gotas são mantidas no interior da nuvem para se tornarem grandes o bastante para não conseguirem se fundir completamente, caindo sob a forma de gelo, o granizo. A chuva quando cai cria uma corrente de ar frio descendente. A presença de correntes de ar ascendentes e descendentes é característica desta fase, na qual, no interior da nuvem, ocorre considerável turbulência, que se manifesta pela ocorrência de fortes ventos e muitos raios.[11]

Normalmente, se há pouco vento cisalhante, a tempestade vai rapidamente entrar no estágio de dissipação, uma vez que os ventos ascendentes e descendentes se cancelam, e desaparecer,[8] mas se houver substancial mudança na velocidade do vento e da direção da corrente de ar descendente, as duas correntes opostas podem se separar, estendendo seu ciclo de vida por várias horas.[12]

Dissipação[editar | editar código-fonte]

Nesta fase, a trovoada é dominada pelo vento descendente. Se as condições atmosféricas não propiciarem o desenvolvimento de uma supercélula, este estágio ocorre rapidamente, após 20 a 30 minutos desde o início da tempestade. O vento descendente vai atingir a superfície e se espalhar. O ar frio carregado para a superfície corta o processo convectivo que alimentava a tempestade, a corrente ascendente cessa e a tempestade se dissipa. Trovadas que ocorrem onde não hajam ventos cisalhantes se enfraquecem assim que a corrente descendente se inicia.[13] O vento descendente quando atinge a terra cria uma frente de rajada. Esta pode causar rajadas de vento, que representam perigo para aeronaves que voam em seu interior, já que ocorre uma súbita mudança na velocidade e direção do vento.[14]

Classificação[editar | editar código-fonte]

Condições favoráveis para a formação de alguns tipos de tempestade.

Há quatro tipos principais de trovoadas: de célula única, multicélulas, linhas de instabilidade e supercélula. Qual tipo vai se formar depende da instabilidade e das diferentes condições do vento em diferentes camadas da atmosfera. Tempestades de célula única se formam em ambientes com pouco vento cisalhante e duram de vinte a trinta minutos. Tempestades mais organizadas e dispostas em linha ou aglomerados podem ter um ciclo de vida maior, uma vez que formam-se em ambientes com vento cisalhante vertical significativo, que favorece o surgimento de correntes ascendentes e vários tipos de tempo severo. A supercélula é a mais forte das trovoadas, comumente associada com grandes pedras de granizo, ventos fortes e formação de tornados.

Uma tempestade de célula única sobre Wagga-Wagga, na Austrália.

Célula única[editar | editar código-fonte]

Ver artigo principal: Tempestade de massa de ar

Este termo tecnicamente se aplica a uma única tempestade com uma corrente de ar ascendente principal. Também conhecida como tempestade de massa de ar, são as tormentas típicas de fim de tarde de verão em regiões mais quentes. Também ocorrem em massas de ar frio instáveis que sucedem a passagem de uma frente fria oceânica durante o inverno. Em um aglomerado de tempestades, o termo "célula" refere-se a cada corrente de ar ascendente principal. Células de trovoadas ocasionalmente se formam isoladas, enquanto que a ocorrência de uma tempestade pode criar uma frente de rajada que favorece o desenvolvimento de uma nova trovoada. Estas tempestades raramente se tornam severas e resultam de instabilidade atmosférica local (daí o termo "tempestade de massa de ar"). Quando estas tempestades têm um breve período de tempo severo associada a elas, são conhecidas como tempestades severas em pulso. Nestas condições, são fracamente organizadas e ocorrem aleatoriamente no tempo e espaço, sendo difícil sua previsão. Tempestades de célula única geralmente duram de vinte a trinta minutos.[9]

Tempestades multicelulares[editar | editar código-fonte]

Um grupo de tempestades sobre o Brasil, fotografadas a partir do ônibus espacial Challenger.

Este é o tipo mais comum de desenvolvimento de trovoadas. Células em seu estágio maduro são encontradas próximo ao centro do conjunto, enquanto que células em dissipação estão na direção a favor do vento. Tempestade de células múltiplas se formam como a aglomeração de várias tempestades indivuais, mas podem evoluir e formar uma ou mais linhas de instabilidade. Enquanto cada célula do grupo pode durar somente vinte minutos, o conjunto pode durar por horas. Frequentemente surgem de correntes convectivas ascendentes próximo a montanhas ou limites de frentes meteorológicas, como em frentes frias ou locais de baixa pressão atmosférica. Este tipo de tormenta são mais fortes que as tempestades de massa de ar, mas ainda bem mais fracas que supercélula. Os perigos advindos destas trovoadas incluem granizo de tamanho moderado, enchentes relâmpago e tornados fracos.[9]

Linhas de intabilidade multicelular[editar | editar código-fonte]

Ver artigo principal: Linha de instabilidade

Uma linha de instabilidade é uma linha alongada de tempestades que se formam ao longo ou à frente de uma frente fria.[15][16] No início do século XX, o termo era usado como um sinônimo de frente fria.[17] A linha de instabilidade provoca precipitações intensas, granizo, muitos raios, fortes rajadas de vento em linha reta e eventualmente tornados e trombas de água.[18] Desta forma, ventos fortes em linha reta podem ser esperados em áreas onde a linha de instabilidade adquire um formato de eco em arco, dentro da porção da linha que se curva mais.[19] Tornados podem ocorrer em uma linha de eco ondulada, onde áreas de baixa pressão em mesoescala estão presentes.[20] Alguns ecos de arco que ocorrem durante o verão são chamados derechos, e se movem mais rapidamente.[21] Na parte posterior do escudo de precipitação associado a uma linha de instabilidade madura, uma mesobaixa, que é uma área de baixa pressão em mesoescala que se forma atrás de uma área de mesoescala em alta pressão, pode se formar, eventualmente associados a surtos de calor.[22] Este tipo de tempestade é também conhecida como "vento do lago rochoso" (do chinês tradicional 石湖風 – shi2 hu2 feng1, chinês simplificado: 石湖风) no sul da China.[23]

Supercélula[editar | editar código-fonte]

Ver artigo principal: Supercélula
Uma supercélula sobre Chaparral, Novo Mexico.
The setting sun illuminates the top of a classic anvil-shaped thunderstorm cloud in eastern Nebraska, United States.

Supercélulas são extensas, usualmente severas

Supercell storms are large, usually severe, quasi-steady-state storms that form in an environment where wind speed or wind direction varies with height (an area of "wind shear"), and they have separate downdrafts and updrafts (i.e., where its associated precipitation is not falling through the updraft) with a strong, rotating updraft (a "mesocyclone"). These storms normally have such powerful updrafts that the top of the supercell storm cloud (or anvil) can break through the troposphere and reach into the lower levels of the stratosphere, and supercell storms can be 15 milhas (24 km) wide. Research has shown that at least 90 percent of supercells cause severe weather.[12] These storms can produce destructive tornadoes, sometimes F3 or higher, extremely large hailstones (4 polegadas ou 10 centímetros diameter), straight-line winds in excess of 80 mph (130 km/h), and flash floods. In fact, research has also shown that most tornadoes occur from this type of thunderstorm.[24] Supercells are the most powerful type of thunderstorm.[9]

Severe thunderstorms[editar | editar código-fonte]

A storm is considered severe if winds reach at least 93 quilômetros por hora (58 mph), hail is 1 polegada (25 mm) in diameter or larger, or if funnel clouds or tornadoes are reported.[25][26][27] Although a funnel cloud or tornado indicates a severe thunderstorm, a tornado warning is issued in place of a severe thunderstorm warning. In Canada, a rainfall rate greater than 50 milímetros (2 in) in one hour, or 75 milímetros (3 in) in three hours, is also used to indicate severe thunderstorms.[28] Severe thunderstorms can occur from any type of storm cell. However, multicell, supercell, and squall lines represent the most common forms of thunderstorms that produce severe weather.[12]

Mesoscale convective systems[editar | editar código-fonte]

Ver também: Mesoscale convective system
MCC moving through New England: August 2, 2006 0600 UTC

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more.[29] A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and Mesoscale Convective Complexes (MCCs), and generally form near weather fronts. Most mesoscale convective systems develop overnight and continue their lifespan through the next day.[8] The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.[30][31]

Forms of MCS that develop within the tropics use either the Intertropical Convergence Zone or monsoon troughs as a focus for their development, generally within the warm season between spring and fall. More intense systems form over land than over water.[32][33] One exception is that of lake-effect snow bands, which form due to cold air moving across relatively warm bodies of water, and occurs from fall through spring.[34] Polar lows are a second special class of MCS. They form at high latitudes during the cold season.[35] Once the parent MCS dies, later thunderstorm development can occur in connection with its remnant mesoscale convective vortex (MCV).[36] Mesoscale convective systems are important to the United States rainfall climatology over the Great Plains since they bring the region about half of their annual warm season rainfall.[37]

Motion[editar | editar código-fonte]

Thunderstorm line viewed in reflectivity (dBZ) on a PPI (NOAA)

The two major ways thunderstorms move are via advection of the wind and propagation along outflow boundaries towards sources of greater heat and moisture. Many thunderstorms move with the mean wind speed through the Earth's troposphere, or the lowest 8 quilômetros (5,0 mi) of the Earth's atmosphere. Younger thunderstorms are steered by winds closer to the Earth's surface than more mature thunderstorms, as they are less tall. Organized, long-lived thunderstorm cells and complexes move at a right angle to the direction of the vertical wind shear vector. If the gust front, or leading edge of the outflow boundary, races ahead of the thunderstorm, its motion will accelerate in tandem. This is more of a factor with thunderstorms with heavy precipitation (HP) than with thunderstorms with low precipitation (LP). When thunderstorms merge, which is most likely when numerous thunderstorms exist in proximity to each other, the motion of the stronger thunderstorm normally dictates future motion of the merged cell. The stronger the mean wind, the less likely other processes will be involved in storm motion. On weather radar, storms are tracked by using a prominent feature and tracking it from scan to scan.[12]

Back-building thunderstorm[editar | editar código-fonte]

A back building thunderstorm, commonly referred to as a training thunderstorm, is a thunderstorm in which new development takes place on the upwind side (usually the west or southwest side in the Northern Hemisphere), such that the storm seems to remain stationary or propagate in a backward direction. Though the storm often appears stationary on radar, or even moving upwind, this is an illusion. The storm is really a multi-cell storm with new, more vigorous cells that form on the upwind side, replacing older cells that continue to drift downwind.[38] When this happens, catastrophic flooding is possible. In Rapid City, South Dakota, in 1972, an unusual alignment of winds at various levels of the atmosphere combined to produce a continuously training set of cells that dropped an enormous quantity of rain upon the same area, resulting in devastating flash flooding.[39] A similar event occurred in Boscastle, England, on 16 August 2004.[40]

Hazards[editar | editar código-fonte]

Each year, many people are killed or seriously injured by severe thunderstorms despite the advance warning. While severe thunderstorms are most common in the spring and summer, they can occur at just about any time of the year.

Cloud-to-ground lightning[editar | editar código-fonte]

Ver também: Acid rain, Lightning e Wildfire
A return stroke, cloud-to-ground lightning strike during a thunderstorm.

Cloud-to-ground lightning frequently occurs within the phenomena of thunderstorms and have numerous hazards towards landscapes and populations. One of the more significant hazards lightning can pose is the wildfires they are capable of igniting.[41] Under a regime of low precipitation (LP) thunderstorms, where little precipitation is present, rainfall cannot prevent fires from starting when vegetation is dry as lightning produces a concentrated amount of extreme heat.[42] Direct damage caused by lightning strikes occurs on occasion.[43] In areas with a high frequency for cloud-to-ground lightning, like Florida, lightning causes several fatalities per year, most commonly to people working outside.[44]

Precipitation with low potential of hydrogen levels (pH), otherwise known as acid rain, is also a frequent risk produced by lightning. Distilled water, which contains no carbon dioxide, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are bases. “Clean” or unpolluted rain has a slightly acidic pH of about 5.2, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid (pH 5.6 in distilled water), but unpolluted rain also contains other chemicals.[45] Nitric oxide present during thunderstorm phenomena,[46] caused by the splitting of nitrogen molecules, can result in the production of acid rain, if nitric oxide forms compounds with the water molecules in precipitation, thus creating acid rain. Acid rain can damage infrastructures containing calcite or other solid chemical compounds containing carbon. In ecosystems, acid rain can dissolve plant tissues of vegetations and increase acidification process in bodies of water and in soil, resulting in deaths of marine and terrestrial organisms.[47]

Hail[editar | editar código-fonte]

Ver artigo principal: Hail
Hailstorm in Bogotá, Colombia.

Any thunderstorm that produces hail that reaches the ground is known as a hailstorm.[48] Thunderclouds that are capable of producing hailstones are often seen obtaining green coloration. Hail is more common along mountain ranges because mountains force horizontal winds upwards (known as orographic lifting), thereby intensifying the updrafts within thunderstorms and making hail more likely.[49] One of the more common regions for large hail is across mountainous northern India, which reported one of the highest hail-related death tolls on record in 1888.[50] China also experiences significant hailstorms.[51] Across Europe, Croatia experiences frequent occurrences of hail.[52]

In North America, hail is most common in the area where Colorado, Nebraska, and Wyoming meet, known as "Hail Alley."[53] Hail in this region occurs between the months of March and October during the afternoon and evening hours, with the bulk of the occurrences from May through September. Cheyenne, Wyoming is North America's most hail-prone city with an average of nine to ten hailstorms per season.[54] In South America, areas prone to hail are cities like Bogotá, Colombia.

Hail can cause serious damage, notably to automobiles, aircraft, skylights, glass-roofed structures, livestock, and most commonly, farmers' crops.[54] Hail is one of the most significant thunderstorm hazards to aircraft. When hail stones exceed Predefinição:Convert/LoffAoffDbS1 in diameter, planes can be seriously damaged within seconds.[55] The hailstones accumulating on the ground can also be hazardous to landing aircraft. Wheat, corn, soybeans, and tobacco are the most sensitive crops to hail damage.[50] Hail is one of Canada's most costly hazards.[56] Hailstorms have been the cause of costly and deadly events throughout history. One of the earliest recorded incidents occurred around the 9th century in Roopkund, Uttarakhand, India.[57] The largest hailstone in terms of maximum circumference and length ever recorded in the United States fell in 2003 in Aurora, Nebraska, USA.[58]

Tornadoes and waterspouts[editar | editar código-fonte]

The F5 tornado that struck Elie, Manitoba in 2007.
Ver artigos principais: Tornado e Waterspout

A tornado is a violent, rotating column of air in contact with both the surface of the earth and a cumulonimbus cloud (otherwise known as a thundercloud) or, in rare cases, the base of a cumulus cloud. Tornadoes come in many sizes but are typically in the form of a visible condensation funnel, whose narrow end touches the earth and is often encircled by a cloud of debris and dust.[59] Most tornadoes have wind speeds between 40 and 110 mph (64 and 180 km/h), are approximately 250 pés (76 m) across, and travel a few miles (several kilometers) before dissipating. Some attain wind speeds of more than 300 mph (480 km/h), stretch more than 1 milha (1,6 km) across, and stay on the ground for dozens of miles (more than 100 km).[60][61][62]

The Fujita scale and the Enhanced Fujita Scale rate tornadoes by damage caused. An EF0 tornado, the weakest category, damages trees but not substantial structures. An EF5 tornado, the strongest category, rips buildings off their foundations and can deform large skyscrapers. The similar TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes.[63] Doppler radar data, photogrammetry, and ground swirl patterns (cycloidal marks) may also be analyzed to determine intensity and award a rating.[64]

Formation of numerous waterspouts in the Great Lakes region. (North America)
A flash flood caused by a severe thunderstorm

Waterspouts have similar characteristics as tornadoes, characterized by a spiraling funnel-shaped wind current that form over bodies of water, connecting to large Cumulonimbus clouds. Waterspouts are generally classified as forms of tornadoes, or more specifically, non-supercelled tornadoes that develop over large bodies of water.[65] These spiralling columns of air are frequently developed within tropical areas close to the equator, but are less common within areas of high latitude.[66]

Flash flood[editar | editar código-fonte]

Ver artigo principal: Flash flood

Flash flooding is the process where a landscape, most notably an urban environment, is subjected to rapid floods.[67] These rapid floods occur more quickly and are more localized than seasonal river flooding or areal flooding[68] and are frequently (though not always) associated with intense rainfall.[69] Flash flooding can frequently occur in slow-moving thunderstorms and is usually caused by the heavy liquid precipitation that accompanies it. Flash floods are most common in densely populated urban environments, where few plants and bodies of water are present to absorb and contain the extra water. Flash flooding can be hazardous to small infrastructure, such as bridges, and weakly constructed buildings. Plants and crops in agricultural areas can be destroyed and devastated by the force of raging water. Automobiles parked within affected areas can also be displaced. Soil erosion can occur as well, exposing risks of landslide phenomena.

Downburst[editar | editar código-fonte]

Ver artigo principal: Downburst
Trees uprooted or displaced by the force of a downburst wind in northwest Monroe County, Wisconsin.

Downburst winds can produce numerous hazards to landscapes experiencing thunderstorms. Downburst winds are generally very powerful, and are often mistaken for wind speeds produced by tornadoes,[70] due to the concentrated amount of force exerted by their straight-horizontal characteristic. Downburst winds can be hazardous to unstable, incomplete, or weakly constructed infrastructures and buildings. Agricultural crops, and other plants in nearby environments can be uprooted and damaged. Aircraft engaged in takeoff or landing can crash.[8][70] Automobiles can be displaced by the force exerted by downburst winds. Downburst winds are usually formed in areas when high pressure air systems of downdrafts begin to sink and displace the air masses below it, due to their higher density. When these downdrafts reach the surface, they spread out and turn into the destructive straight-horizontal winds.[8]

Safety precautions[editar | editar código-fonte]

Ver também: Emergency management e Tornado preparedness

Most thunderstorms come and go fairly uneventfully; however, any thunderstorm can become severe, and all thunderstorms, by definition, present the danger of lightning.[71] Thunderstorm preparedness and safety refers to taking steps before, during, and after a thunderstorm to minimize injury and damage.

Preparedness[editar | editar código-fonte]

Preparedness refers to precautions that should be taken before a thunderstorm. Some preparedness takes the form of general readiness (as a thunderstorm can occur at any time of the day or year).[72] Preparing a family emergency plan, for example, can save valuable time if a storm arises quickly and unexpectedly.[73] Preparing the home by removing dead or rotting limbs and trees, which can be blown over in high winds, can also significantly reduce the risk of property damage and personal injury.[74]

The National Weather Service (NWS) in the United States recommends several precautions that people should take if thunderstorms are likely to occur:[72]

  • Know the names of local counties, cities, and towns, as these are how warnings are described.[72]
  • Monitor forecasts & weather conditions and know whether thunderstorms are likely in the area.[75]
  • Be alert for natural signs of an approaching storm.
  • Cancel or reschedule outdoor events (to avoid being caught outdoors when a storm hits).[75]
  • Take action early so you have time to get to a safe place.[75]
  • Get inside a substantial building or hard-topped metal vehicle before threatening weather arrives.[75]
  • If you hear thunder, get to the safe place immediately.[75]
  • Avoid open areas like hilltops, fields, and beaches, and don't be or be near the tallest objects in an area when thunderstorms are occurring.[72][75]
  • Don't shelter under tall or isolated trees during thunderstorms.[75]
  • If in the woods, put as much distance between you and any trees during thunderstorms.[75]
  • If in a group, spread out so that you increase the chances for survivors who could come to the aid of any victims from a lightning strike.[75]

Safety[editar | editar código-fonte]

While safety and preparedness often overlap, “thunderstorm safety” generally refers to what people should do during and after a storm. The American Red Cross recommends that people follow these precautions if a storm is imminent or in progress:[71]

  • Take action immediately upon hearing thunder. Anyone close enough to the storm to hear thunder can be struck by lightning.[74]
  • Avoid electrical appliances, including corded telephones.[71] Cordless and wireless telephones are safe to use during a thunderstorm.[74]
  • Close and stay away from windows and doors, as glass can become a serious hazard in high wind.[71]
  • Do not bathe or shower, as plumbing conducts electricity.
  • If driving, safely exit the roadway, turn on hazard lights, and park. Remain in the vehicle and avoid touching metal.[71]

The NWS stopped recommending the "lightning crouch" in 2008 as it doesn't provide a significant level of protection and will not significantly lower the risk of being killed or injured from a nearby lightning strike.[75][76][77]

Frequent occurrences[editar | editar código-fonte]

Ver também: United States rainfall climatology
A mild thunderstorm over Niagara Falls, Ontario.

Thunderstorms occur throughout the world, even in the polar regions, with the greatest frequency in tropical rainforest areas, where they may occur nearly daily. Kampala and Tororo in Uganda have each been mentioned as the most thunderous places on Earth,[78] a claim also made for Bogor on Java, Indonesia and Singapore. Other cities known for frequent storm activity include Darwin, Caracas, Manila and Mumbai. Thunderstorms are associated with the various monsoon seasons around the globe, and they populate the rainbands of tropical cyclones.[79] In temperate regions, they are most frequent in spring and summer, although they can occur along or ahead of cold fronts at any time of year.[80] They may also occur within a cooler air mass following the passage of a cold front over a relatively warmer body of water. Thunderstorms are rare in polar regions because of cold surface temperatures.

Some of the most powerful thunderstorms over the United States occur in the Midwest and the Southern states. These storms can produce large hail and powerful tornadoes. Thunderstorms are relatively uncommon along much of the West Coast of the United States,[81] but they occur with greater frequency in the inland areas, particularly the Sacramento and San Joaquin Valleys of California. In spring and summer, they occur nearly daily in certain areas of the Rocky Mountains as part of the North American Monsoon regime. In the Northeast, storms take on similar characteristics and patterns as the Midwest, but with less frequency and severity. During the summer, air-mass thunderstorms are an almost daily occurrence over central and southern parts of Florida.

Types of lightning[editar | editar código-fonte]

3-second video of a lightning strike within a thunderstorm over Island in the Sky, Canyonlands National Park, Utah
Cloud to ground lightning over Pentagon City in Arlington, Virginia
Lightning storm over Sydney
Ver artigo principal: Lightning

Lightning is an electrical discharge that occurs in a thunderstorm. It can be seen in the form of a bright streak (or bolt) from the sky. Lightning occurs when an electrical charge is built up within a cloud, due to static electricity generated by supercooled water droplets colliding with ice crystals near the freezing level. When a large enough charge is built up, a large discharge will occur and can be seen as lightning.

The temperature of a lightning bolt can be five times hotter than the surface of the sun.[82] Although the lightning is extremely hot, the duration is short and 90% of strike victims survive. Contrary to the popular idea that lightning does not strike twice in the same spot, some people have been struck by lightning over three times, and skyscrapers like the Empire State Building have been struck numerous times in the same storm.[83] The loud bang that is heard is the super heated air around the lightning bolt expanding at the speed of sound. Because sound travels much more slowly than light the flash is seen before the bang, although both occur at the same moment.

There are several types of lightning:

  • In-cloud lightning is the most common. It is lightning within a cloud and is sometimes called intra-cloud or sheet lightning.
  • Cloud to ground lightning is when a bolt of lightning from a cloud strikes the ground. This form poses the greatest threat to life and property.
  • Ground to cloud lightning is when a lightning bolt is induced from the ground to the cloud.
  • Cloud to cloud lightning is rarely seen and is when a bolt of lightning arcs from one cloud to another.
  • Ball lightning is extremely rare and has several hypothesized explanations. It is seen in the form of a 15 to 50 centimeter radius ball.[84]
  • Cloud to air lightning is when lightning from a cloud hits air of a different charge.[85]
  • Dry lightning is a misnomer that refers to a thunderstorm whose precipitation does not reach the ground.
  • Heat Lightning refers to a lightning flash that is seen from the horizon that does not have accompanying thunder.[86]
  • Upper-atmospheric lightning occurs above the thunderhead.
  • Clear-air lightning is used widely to describe lightning that occurs with no apparent cloud close enough to have produced it. In the U.S. and Canadian Rockies, a thunderstorm can be in an adjacent valley and not be observable, (either visually or audibly), from the valley where the lightning bolt strikes. European and Asian mountainous areas experience similar events. Also in clear areas where the storm cell is on the near horizon (within 26 km (16 mi) a strike can occur, and as the storm is so far away, the strike is referred to as clear-air.{{carece de fontes}}

Energy[editar | editar código-fonte]

Ver também: Sprite (lightning), Upper-atmospheric lightning e St. Elmo's fire
How thunderstorms launch particle beams into space

If the quantity of water that is condensed in and subsequently precipitated from a cloud is known, then the total energy of a thunderstorm can be calculated. In a typical thunderstorm, approximately 5×108 kg of water vapor are lifted, and the amount of energy released when this condenses is 1015 joules. This is on the same order of magnitude of energy released within a tropical cyclone, and more energy than that released during the atomic bomb blast at Hiroshima, Japan in 1945.[10]

The Fermi Gamma-ray Burst Monitor results show that gamma rays and antimatter particles (positrons) can be generated in powerful thunderstorms.[87] It is suggested that the antimatter positrons are formed in terrestrial gamma-ray flashes (TGF). TGFs are brief bursts occurring inside thunderstorms and associated with lightning. The streams of positrons and electrons collide higher in the atmosphere to generate more gamma rays.[88] About 500 TGFs may occur every day worldwide, but mostly go undetected.

Studies[editar | editar código-fonte]

In more contemporary times, thunderstorms have taken on the role of a scientific curiosity. Every spring, storm chasers head to the Great Plains of the United States and the Canadian Prairies to explore the scientific aspects of storms and tornadoes through use of videotaping.[89] Radio pulses produced by cosmic rays are being used to study how electric charges develop within thunderstorms.[90] More organized meteorological projects such as VORTEX2 use an array of sensors, such as the Doppler on Wheels, vehicles with mounted automated weather stations, weather balloons, and unmanned aircraft to investigate thunderstorms expected to produce severe weather.[91] Lightning is detected remotely using sensors that detect cloud-to-ground lightning strokes with 95 percent accuracy in detection and within 250 metros (820 pé) of their point of origin.[92]

Mythology[editar | editar código-fonte]

Thunderstorms strongly influenced many early civilizations. Greeks believed that they were battles waged by Zeus, who hurled lightning bolts forged by Hephaestus. Some American Indian tribes associated thunderstorms with the Thunderbird, who they believed was a servant of the Great Spirit.[93] The Norse considered thunderstorms to occur when Thor went to fight Jötnar, with the thunder and lightning being the effect of his strikes with the hammer Mjölnir. Christian doctrine accepted the ideas of Aristotle's original work, called Meteorologica, that winds were caused by exhalations from the Earth and that fierce storms were the work of God. These ideas were still within the mainstream as late as the 18th century.[94]

Outside of Earth[editar | editar código-fonte]

The clouds of Venus are capable of producing lightning much like the clouds on Earth. The lightning rate is at least half of that on Earth.[95] A thin layer of water clouds appears to underlie the ammonia layer within Jupiter's atmosphere, where thunderstorms evidenced by flashes of lightning have been detected. (Water is a polar molecule that can carry a charge, so it is capable of creating the charge separation needed to produce lightning.)[96] These electrical discharges can be up to a thousand times as powerful as lightning on the Earth.[97] The water clouds can form thunderstorms driven by the heat rising from the interior.[98]

See also[editar | editar código-fonte]

References[editar | editar código-fonte]

  1. ↑ National Weather Service (21 de abril de 2005). «Weather Glossary – T» (em inglês). National Oceanic and Atmospheric Administration. Consultado em 23 de agosto de 2006 
  2. ↑ National Severe Storms Laboratory (Setembro de 1992). «tornadoes...Nature's Most Violent Storms». A PREPAREDNESS GUIDE. National Oceanic and Atmospheric Administration. Consultado em 3 de agosto de 2008 
  3. ↑ Albert Irvin Frye (1913). Civil engineers' pocket book: a reference-book for engineers, contractors (em inglês). [S.l.]: D. Van Nostrand Company. p. 462. Consultado em 31 de agosto de 2009 
  4. ↑ Yikne Deng (2005). Ancient Chinese Inventions (em inglês). [S.l.]: Chinese International Press. p. 112–13. ISBN 978-7-5085-0837-5. Consultado em 18 de junho de 2009 
  5. ↑ FMI (2007). «Fog And Stratus – Meteorological Physical Background». Zentralanstalt für Meteorologie und Geodynamik. Consultado em 7 de fevereiro de 2009 
  6. ↑ Predefinição:Citar book
  7. ↑ David O. Blanchard (Setembro de 1998). «Assessing the Vertical Distribution of Convective Available Potential Energy». American Meteorological Society. Weather and Forecasting. 13 (3): 870–7. Bibcode:1998WtFor..13..870B. doi:10.1175/1520-0434(1998)013<0870:ATVDOC>2.0.CO;2 
  8. ↑ a b c d e Michael H. Mogil (2007). Extreme Weather (em inglês). New York: Black Dog & Leventhal Publisher. p. 210–211. ISBN 978-1-57912-743-5 
  9. ↑ a b c d National Severe Storms Laboratory (15 de outubro de 2006). «A Severe Weather Primer: Questions and Answers about Thunderstorms». National Oceanic and Atmospheric Administration. Consultado em 1 de setembro de 2009 
  10. ↑ a b Gianfranco Vidali (2009). «Rough Values of Various Processes» (em inglês). University of Syracuse. Consultado em 31 de agosto de 2009 
  11. ↑ Pilot's Web The Aviator's Journal (13 de julho de 2009). «Structural Icing in VMC». Consultado em 2 de setembro de 2009 
  12. ↑ a b c d Jon W. Zeitler and Matthew J. Bunkers (March 2005). «Operational Forecasting of Supercell Motion: Review and Case Studies Using Multiple Datasets» (PDF). National Weather Service Forecast Office, Riverton, Wyoming. Consultado em 30 de agosto de 2009  Verifique data em: |data= (ajuda)
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  16. ↑ Glossary of Meteorology (2009). «Prefrontal squall line». American Meteorological Society. Consultado em 14 de junho de 2009 
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  18. ↑ Office of the Federal Coordinator for Meteorology (2008). «Chapter 2: Definitions» (PDF) (em inglês). NOAA. p. 2–1. Consultado em 3 de maio de 2009 
  19. ↑ Glossary of Meteorology (2009). «Bow echo». American Meteorological Society. Consultado em 14 de junho de 2009 
  20. ↑ Glossary of Meteorology (2009). Line echo wave pattern. [S.l.]: American Meteorological Society. ISBN 1-878220-34-9. Consultado em 3 de maio de 2009 
  21. ↑ Robert H. Johns; Jeffry S. Evans (12 de abril de 2006). «About Derechos». Storm Prediction Center, NCEP, NWS, NOAA Web Site. Consultado em 21 de junho de 2007 
  22. ↑ {{cite ar livro|autor= Glossary of Meteorology|título=Heat burst |editora= American Meteorological Society |ano= 2009 |url= http://amsglossary.allenpress.com/glossary/search?id=heat-burst1 |isbn= 1-878220-34-9| acessodata=14 de junho de 2014}}
  23. ↑ «Squall lines and "Shi Hu Feng" – what you want to know about the violent squalls hitting Hong Kong on 9 May 2005». Hong Kong Observatory. 17 de junho de 2005. Consultado em 23 de agosto de 2006 
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  25. ↑ National Weather Service (21 de abril de 2005). «Weather Glossary – S». National Oceanic and Atmospheric Administration. Consultado em 17 de junho de 2007 
  26. ↑ Kim Runk (2009). 1" Hail (.wmv). Silver Spring, Maryland: NOAA 
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  28. ↑ Environment Canada Ontario Region (24 de maio de 2005). «Fact Sheet – Summer Severe Weather Warnings». Consultado em 3 de setembro de 2009 
  29. ↑ Glossary of Meteorology (2009). «Mesoscale convective system». American Meteorological Society. Consultado em 27 de junho de 2009 
  30. ↑ William R. Cotton, Susan van den Heever, and Israel Jirak (2003). «Conceptual Models of Mesoscale Convective Systems: Part 9» (PDF). Colorado State University. Consultado em 23 de março de 2008 
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  32. ↑ Semyon A. Grodsky and James A. Carton (15 de fevereiro de 2003). «The Intertropical Convergence Zone in the South Atlantic and the Equatorial Cold Tongue» (PDF). University of Maryland, College Park. Consultado em 5 de junho de 2009 
  33. ↑ Michael Garstang, David Roy Fitzjarrald (1999). Observations of surface to atmosphere interactions in the tropics. [S.l.]: Oxford University Press US. pp. 40–41. ISBN 978-0-19-511270-2 
  34. ↑ B. Geerts (1998). «Lake Effect Snow». University of Wyoming. Consultado em 24 de dezembro de 2008 
  35. ↑ E. A. Rasmussen and J. Turner (2003). Polar Lows: Mesoscale Weather Systems in the Polar Regions. [S.l.]: Cambridge University Press. p. 612. ISBN 978-0-521-62430-5 
  36. ↑ Lance F. Bosart and Thomas J. Galarneau, Jr. (2005). «3.5 The Influence of the Great Lakes on Warm Season Weather Systems During BAMEX» (PDF). 6th American Meteorological Society Coastal Meteorology Conference. Consultado em 15 de junho de 2009 
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  38. ↑ National Weather Service (1 de setembro de 2009). «Types of Thunderstorms». National Weather Service Southern Region Headquarters. Consultado em 3 de setembro de 2009 
  39. ↑ National Weather Service Forecast Office, Rapid City, South Dakota (15 de maio de 2007). «The Rapid City Flood of 1972». National Weather Service Central Region Headquarters. Consultado em 3 de setembro de 2009 
  40. ↑ David Flower (9 de fevereiro de 2008). «Boscastle Flood 2004». Tintagel – King Arthur Country. Consultado em 3 de setembro de 2009 
  41. ↑ Scott, A (2000). «The Pre-Quaternary history of fire». Palaeogeography Palaeoclimatology Palaeoecology. 164: 281. doi:10.1016/S0031-0182(00)00192-9 
  42. ↑ Vladimir A. Rakov (1999). «Lightning Makes Glass». University of Florida, Gainesville. Consultado em November 7, 2007  Verifique data em: |acessodata= (ajuda)
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  44. ↑ Charles H. Paxton, J. Colson and N. Carlisle (2008). «P2.13 Florida lightning deaths and injuries 2004–2007». American Meteorological Society. Consultado em 5 de setembro de 2009 
  45. ↑ G. E. Likens, W. C. Keene, J. M. Miller and J. N. Galloway (1987). «Chemistry of precipitation from a remote, terrestrial site in Australia». Journal of Geophysical Research. 92 (13): 299–314. Bibcode:1987JGR....92..299R. doi:10.1029/JA092iA01p00299 
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  49. ↑ Geoscience Australia (4 de setembro de 2007). «Where does severe weather occur?». Commonwealth of Australia. Consultado em 28 de agosto de 2009. Cópia arquivada em 21 de junho de 2009 
  50. ↑ a b John E. Oliver (2005). Encyclopedia of World Climatology. [S.l.]: Springer. p. 401. ISBN 978-1-4020-3264-6. Consultado em 28 de agosto de 2009 
  51. ↑ Dongxia Liu, Guili Feng, and Shujun Wu (February 2009). «The characteristics of cloud-to-ground lightning activity in hailstorms over northern China». Atmospheric Research. 91 (2–4): 459–465. doi:10.1016/j.atmosres.2008.06.016  Verifique data em: |data= (ajuda)
  52. ↑ Damir Počakal, Željko Večenaj, and Janez Štalec (2009). «Hail characteristics of different regions in continental part of Croatia based on influence of orography». Atmospheric Research. 93 (1–3): 516. doi:10.1016/j.atmosres.2008.10.017 
  53. ↑ Rene Munoz (2 de junho de 2000). «Fact Sheet on Hail». University Corporation for Atmospheric Research. Consultado em 18 de julho de 2009 
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  56. ↑ Damon P. Coppola (2007). Introduction to international disaster management. [S.l.]: Butterworth-Heinemann. p. 62. ISBN 978-0-7506-7982-4 
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  61. ↑ «Doppler On Wheels». Center for Severe Weather Research. 2006. Consultado em 29 de dezembro de 2006 
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Further reading[editar | editar código-fonte]

  • Burgess, D. W., R. J. Donaldson Jr., and P. R. Desrochers, 1993: Tornado detection and warning by radar. The Tornado: Its Structure, Dynamics, Prediction, and Hazards, Geophys. Monogr., No. 79, American Geophysical Union, 203–221.
  • Corfidi, S. F., 1998: Forecasting MCS mode and motion. Preprints 19th Conf. on Severe Local Storms, American Meteorological Society, Minneapolis, Minnesota, pp. 626–629.
  • Davies, J. M., 2004: Estimations of CIN and LFC associated with tornadic and nontornadic supercells. Wea. Forecasting, 19, 714–726.
  • Davies, J. M., and R. H. Johns, 1993: Some wind and instability parameters associated with strong and violent tornadoes. Part I: Helicity and mean shear magnitudes. The Tornado: Its Structure, Dynamics, Prediction, and Hazards (C. Church et al., Eds.), Geophysical Monograph 79, American Geophysical Union, 573–582.
  • David, C. L. 1973: An objective of estimating the probability of severe thunderstorms. Preprint Eight conference of Severe Local Storms. Denver, Colorado, American Meteorological Society, 223–225.
  • Doswell, C.A., III, D. V. Baker, and C. A. Liles, 2002: Recognition of negative factors for severe weather potential: A case study. Wea. Forecasting, 17, 937–954.
  • Doswell, C.A., III, S.J. Weiss and R.H. Johns (1993): Tornado forecasting: A review. The Tornado: Its Structure, Dynamics, Prediction, and Hazards (C. Church et al., Eds), Geophys. Monogr. No. 79, American Geophysical Union, 557–571.
  • Johns, R. H., J. M. Davies, and P. W. Leftwich, 1993: Some wind and instability parameters associated with strong and violent tornadoes. Part II: Variations in the combinations of wind and instability parameters. The Tornado: Its Structure, Dynamics, Prediction and Hazards, Geophys. Mongr., No. 79, American Geophysical Union, 583–590.
  • Evans, Jeffry S.,: Examination of Derecho Environments Using Proximity Soundings. NOAA.gov
  • J. V. Iribarne and W.L. Godson, Atmospheric Thermodynamics, published by D. Reidel Publishing Company, Dordrecht, the Netherlands, 1973
  • M. K. Yau and R. R. Rogers, Short Course in Cloud Physics, Third Edition, published by Butterworth-Heinemann, January 1, 1989, EAN 9780750632157 ISBN 0-7506-3215-1

External links[editar | editar código-fonte]

Commons
Commons
O Commons possui imagens e outros ficheiros sobre WOtP/Testes

Ver também[editar | editar código-fonte]

Referências

Ligações externas[editar | editar código-fonte]

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