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A roll cloud associated with a heavy or severe thunderstorm over Enschede, The Netherlands.
A roll cloud associated with a heavy or severe thunderstorm over Enschede, The Netherlands.
A thunderstorm, or an electrical storm, is a form of weather characterized by the presence of lightning and its attendant thunder. It is usually accompanied by copious rainfall, hail, or rarely, snowfall in the winter months. (which can be called thundersnow)Thunderstorms can happen nearly anywhere in the world, 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 thundery place on earth, an accolade which has also been bestowed upon Bogor in Java, Indonesia. In temperate regions, they are most frequent in spring and summer, though can occur in cold fronts at any time of year. Probably the most thundery region outside of the Tropics is Florida. During the summer, violent thunderstorms are an almost daily occurrence over central and southern parts of the state. The most powerful and dangerous severe thunderstorms also occur over the USA , particularly in the Midwest and over the southern States. These storms can produce very large hail and powerful tornadoes.Thunderstorms form when significant condensation, resulting in the production of a wide range of water droplets and ice crystals, occurs in an atmosphere that is unstable and supports deep, rapid upward motion. This often occurs in the presence of three conditions: sufficient moisture accumulated in the lower atmosphere, reflected by high dewpoint temperatures; a significant fall in air temperature with increasing height, known as a steep lapse rate; and a force such as mechanical convergence along a cold front that will focus the lift.Thunderstorms have had a lasting and powerful influence on early civilizations. Romans thought them to be battles waged by Jupiter, who hurled lightning bolts forged by Vulcan. Thunderstorms were associated with the Thunderbirds, held by Native Americans to be a servant of the Great Spirit. In more contemporary times, thunderstorms now have taken on the role of a curiosity. Every spring, storm chasers head to the Great Plains of the United States and the Canadian Prairies in Canada to explore the visual and scientific aspects of storms and tornadoes.

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Life cycle

Classification - Contents

There are four main types of thunderstorms: single cell, multicell, squall line (also called multicell line) and supercell. Which type forms depends on the instability and relative wind conditions at different layers of the atmosphere (" wind shear"):
  • Single cell storms form when the atmosphere is unstable, but there is little or no wind shear, meaning precipitation falls back down through the updraft that led to it, cooling it and eventually killing it. These storms are short lived, and last for less than an hour after becoming strong enough to produce lightning. Days with suitable weather conditions often see the repeated forming and dissipation of such storms, leading them to be known as "pulse" storms.
  • Multicell storms are groups of cells in different stages of development which have merged into a larger system. The cloud becomes divided into updraft and downdraft regions separated by a gust front. The gust front may extend for several miles ahead of the storm, bringing with it increases in wind speed and atmospheric pressure, decreases in temperature, and shifts in wind direction. The storm itself will have different portions sequentially going through the various thunderstorm stages. In many cases the immature cells develop along a line known as a flanking line, resulting in what is known as a line multicell.
  • Squall line storms are formed as an organized line or lines of multicell storms often with a gust front. They often form from convective updrafts in or near mountain ranges and linear weather boundaries, usually strong cold fronts. The squall line is propelled by its own outflow, which reinforces continuous development of updrafts along the leading edge. Heavy rain and hail may occur in squall lines. Damaging winds can also occur. In the United States, a particularly damaging form of wind called a derecho can occur, although they can also occur in other parts of the world.
  • Supercell storms are large, severe steady-state storms which form when the wind changes speed or direction with height ("wind shear") producing a separate downdraft from the updraft (i.e., precipitation is not falling down through the updraft) and contain a strong, rotating updraft (a " mesocyclone"). These are the most damaging type of thunderstorm, and may produce heavy rain, hail, damaging outflow winds and tornadoes.
Multicell or squall line systems may form a meteorologically important feature known as mesoscale convective system (MCS) stretching for hundreds of kilometres. Mesoscale convective complex is another term for this phenomenom. They are large enough to have a pronounced effect on the upper-level and surface weather pattern, and may influence forecasts over a large area. MCS systems are common in the Midwest region of the United States and the Canadian Prairies during the summer months and produce much of the region's important agricultural rainfall.

Severe thunderstorm
A severe thunderstorm is a thunderstorm with winds 90 km/h or greater, 2 cm or larger hail, funnel clouds or tornadoes. These storms also can contain frequent cloud-to-ground lightning and heavy downpours which can lead to localized flooding. This is a general definition which varies by country and is somewhat contentious. An otherwise weak thunderstorm which produces a wind gust of the required strength would be defined as 'severe' whereas a very violent thunderstorm with continuous lightning and very heavy rain ( but without the required wind gusts, hail or tornado/funnel cloud ) would not. Many of the violent local thunderstorms which affect Florida so frequently during the summer months would not be defined as severe.Severe thunderstorms may occur as supercell thunderstorms, although multicell and squall lines are the most common forms.

Life cycle - Contents

An anvil thundercloud over Bristol, England.
An anvil thundercloud over Bristol, England.
A given cell of a thunderstorm goes through three stages: the cumulus stage, the mature stage, and the dissipation stage.In the cumulus stage of a thunderstorm cell, masses of moisture are pushed upwards. The trigger for this can be solar insolation heating the ground producing thermals, areas where two winds converge forcing air upwards, or where winds blow over areas of high ground. The moisture rapidly cools into liquid drops of water, which appears as cumulus clouds. As the water vapour condenses into liquid, latent heat is released which warms the air, causing it to become less dense than dry air, and so the air will tend to rise in an updraft due to the process of convection (hence the term convective precipitation). This creates a low-pressure zone beneath the forming thunderstorm. In a typical thunderstorm, some 5×108 kg of water vapour are lifted and the amount of energy released when this condenses is about equal to the energy used by a city ( US- 2002) of 100,000 over a month.In the mature stage, the warmed air continues to rise until it reaches existing air which is itself warmer, and the air can rise no further. Often this 'cap' is the tropopause. The air is instead forced to spread out, giving the storm a characteristic anvil shape. The resulting cloud is called cumulonimbus incus. The water droplets will coalesce into heavy droplets and freeze to become ice particles. As these fall they melt, to become rain. If the updraft is strong enough, the droplets are held aloft long enough to be so large that they do not melt completely as they fall and fall as hail. While updrafts are still present, the falling rain creates downdrafts as well. The presence of both updrafts and downdrafts during this stage can cause considerable internal turbulence in the storm system, which sometimes manifests as strong winds, severe lightning, and even tornadoes. If there is little wind shear, the storm will rapidly 'rain itself out', but if there is sufficient change in wind speed and/or direction the downdraft will be separated from the updraft, and the storm may become a supercell.Finally, in the dissipation stage, updraft conditions no longer exist, and the storm is characterized largely by weak downdrafts. Because most of the moisture has precipitated out there is no longer sufficient moisture in the lower air to sustain the cycle and the thunderstorm dissipates.
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