A storm of heavy rain, thunder, and lightning which occurs when cumulonimbus clouds develop in unstable, humid conditions. As air rises, condensation releases latent heat, and this increases the available energy, reinforcing the rising tendency of the air. Above the level at which condensation occurs, supercooled water droplets coalesce to form precipitation-sized droplets. As rain falls, instability decreases, the cloud ceases to grow, and precipitation soon stops. However, a series of cumulonimbus clouds may allow storms to continue. During a thunderstorm, an electric charge builds up at the base of the cloud, and when large enough, is discharged in the form of lightning. Thunderstorms are often associated with the passage of a cold front during a depression, and with intense heating and moisture availability at low latitudes.
A thunderstorm, also called an electrical storm, is a form of weather characterized by the presence of lightning and its attendant thunder produced from a cumulonimbus cloud. Thunderstorms are usually accompanied by heavy rainfall and they can also be accompanied by strong winds, hail and tornadoes. In the winter months, snowfall can occasionally take place in a thunderstorm.
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.
Thunderstorms have had a lasting and powerful influence on early civilizations. Thunderstorms were associated with the Thunderbirds, held by Native Americans to be a servant of the Great Spirit.
According to Encyclopedia Britannica, 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 an average thunderstorm, the energy released amounts to about 10,000,000 kilowatt-hours, which is equivalent to a 20-kiloton nuclear warhead. A large, severe thunderstorm might be 10 to 100 times more energetic.
Classification
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. 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. Squall line or multicell line storms are formed as an organized line or lines of multicell storms frequently with a gust front. This kind of storm is also known as "Wind of the Stony Lake" (Traditional Chinese:石湖風, Simplified Chinese: 石湖风) in southern China. These lines can move swiftly and in some parts of the line, bow echoes can form, bringing with it high winds, dangerous lightning, and possibly tornadoes. Supercell storms are large, severe quasi-steady-state storms which form when the wind speed and direction vary with height ("wind shear") separates downdrafts from updrafts (i.e., precipitation is not falling through the updraft) and contain a strong, rotating updraft (a "mesocyclone"). These storms normally have such powerful updrafts that the top of the cloud (or anvil) can reach miles into the air and can be 15 miles wide. These storms produce destructive tornadoes, sometimes F3 or higher, extremely large hailstones (4 inch—10 cm—diameter), straight-line winds in excess of 80 mph (130 km/h), and flash flood.Multicell or squall line systems may form within a meteorologically important feature known as mesoscale convective system (MCS) stretching for hundreds of kilometres. Prior to the discovery of the MCS phenomenon, the individual thunderstorms were thought of as independent entities, each being effectively impossible to predict. The MCS is amenable to forecasting, and a meteorlogist can now predict with high accuracy the percentage of the MCS that will be affected by thunderstorms. However, the meteorlologist still cannot predict exactly where each thunderstorm will occur within the MCS.
Severe thunderstorm
A severe thunderstorm is a thunderstorm with winds 92.5 kilometers/hour (57.5 mph) or greater, 1.9 centimeter (¾ in) or larger hail, funnel clouds or tornadoes. These storms may contain frequent cloud-to-ground lightning and heavy downpours which can lead to localized flooding. 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.
Where thunderstorms occur
Thunderstorms occur throughout the world, even in the polar regions, with the greatest frequency in tropical rainforest areas, where they may occur nearly daily. Thunderstorms are rare in polar regions due to the cold climate and stable air masses that are generally in place, but they do occur from time to time, mainly in the summer months.
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
summer to explore the visual and scientific aspects of storms and tornadoes.
Life cycle
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. 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 during 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. 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 not sufficient moisture in the lower air to sustain the cycle and the thunderstorm dissipates.
Anvil shaped thundercloudLightning
Lightning is an electrical discharge that occurs in a thunderstorm. Lightning occurs when a charge is built up within a cloud. Contrary to the popular idea that lightning doesn’t 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. Cloud to Ground Lightning is when a bolt of lightning from a cloud strikes the ground. 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 arches from one cloud to another. Cloud to Air Lightning is when lightning from a cloud hits air of a different charge. Johns, 1993: Some wind and instability parameters associated with strong and violent tornadoes. Leftwich, 1993: Some wind and instability parameters associated with strong and violent tornadoes. EAN 9780750632157 ISBN 0-7506-3215-1
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