thunderstorm n : a storm resulting from strong rising air currents; heavy rain or hail along with thunder and lightning [syn: electrical storm, electric storm]

English

Etymology

English, from thunder, and storm.

Pronunciation

• AHD: thŭn'dər-stôrm'
• Hyphenation

thun·der·storm

Noun

1. A storm consisting of thunder and lightning produced by a cumulonimbus, usually accompanied with rain or hail. A more severe thunderstorm can cause mesocyclones.

Translations

A thunderstorm, also called an electrical storm or lightning storm, is a form of weather characterized by the presence of lightning and its attendant thunder.

Life cycle

<div class="thumb tright" style="background-color: #f9f9f9; border: 1px solid #CCCCCC; margin:0.5em;"> Airflow diagrams showing three stages of a thunderstorm life cycle.Warm air is less concentrated than cool air, so warm air rises within cooler air, similar to hot air balloons. Clouds form as warm air carrying moisture rises within cooler air. As the warm air rises, it cools. The moist water vapour begins to condense. When the moisture condenses, this releases energy that keeps the air warmer than its surroundings, so that it continues to rise. If enough instability is present in the atmosphere, this process will continue long enough for cumulonimbus clouds to form, which support lightning and thunder.

All thunderstorms, regardless of type, go through three stages: the cumulus stage, the mature stage, and the dissipation stage. Depending on the conditions present in the atmosphere, these three stages can take anywhere from 20 minutes to several hours to occur.

Cumulus stage

The first stage of a thunderstorm is the cumulus stage, or developing stage. In this stage, masses of moisture are lifted upwards into the atmosphere. The trigger for this lift can be insolation heating the ground producing thermals, areas where two winds converge forcing air upwards, or where winds blow over terrain of increasing elevation. The moisture rapidly cools into liquid drops of water, which appears as cumulus clouds. As the water vapor condenses into liquid, latent heat is released which warms the air, causing it to become less dense than the surrounding dry air. The air tends to rise in an updraft through 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 during a month.

Mature stage

Supercell storms are large, severe quasi-steady-state storms which feature wind speed and direction that vary with height ("wind shear"), separate downdrafts and updrafts (i.e., precipitation is not falling through the updraft) and a strong, rotating updraft (a "mesocyclone"). These storms normally have such powerful updrafts that the top of the cloud (or anvil) can break into through the troposphere and reach into the lower levels of the stratosphere and can be wide. These storms produce destructive tornadoes, sometimes F3 or higher, extremely large hailstones (4 inch or 10 cm diameter), straight-line winds in excess of 80 mph (130 km/h), and flash floods. In fact, most tornadoes occur from this kind of thunderstorm.

Severe thunderstorm

A severe thunderstorm is a term designated to a thunderstorm that has reached a predetermined level of severity. Often this level is determined by the storm being strong enough to inflict wind or hail damage. In the United States, a storm is considered severe if winds reach over 50 knots (58 mph or 93 km/h), hail is ¾ inch (2 cm) diameter or larger, or if a funnel cloud or tornadoes are spotted. Though a funnel cloud or tornado indicates the presence of a severe thunderstorm, a tornado warning would then be issued in place of a severe thunderstorm warning.

In Canada, a severe thunderstorm is defined as either having tornadoes, wind gusts of 90 km/h or greater, hail of 2 centimetres in diameter or greater, a rainfall rate greater than 50 millimetres in 1 hour or 75 millimetres in 3 hours.

Severe thunderstorms can occur from any type of thunderstorm, however multicell and squall lines represent the most common forms. Supercells are often the most powerful type of severe thunderstorm.

Back-building thunderstorm

A back-building thunderstorm is a thunderstorm in which new development takes place on the upwind side (usually the west or southwest side in North America), such that the storm seems to remain stationary or propagate in a backward direction. Although the storm often appears to be stationary or even moving upwind on radar, this is actually an illusion. The storm in reality is a multi-cell storm with new, more vigorous, cells being formed on the upwind side replacing older cells which continue to drift downstream.

Mesoscale Convective System

Multicell or squall line systems may form within a meteorologically important feature known as Mesoscale Convective System (MCS) stretching for hundreds of kilometres. The Mesoscale Convective Complex is a closely related phenomenon. 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 Midwestern United States and the Canadian Prairies during the summer months and produce much of the region's important agricultural rainfall. 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 meteorologist can predict with high accuracy the percentage of the MCS that will be affected by thunderstorms. However, the meteorologist still cannot predict exactly where each thunderstorm will occur within the MCS.

Energy

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 (3.6 joule), which is equivalent to a 20-kiloton nuclear warhead. A large, severe thunderstorm might be 10 to 100 times more energetic.

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. Kampala and Tororo in Uganda have each been mentioned as the most thunderous places on Earth, an accolade which has also been bestowed upon Bogor on Java, Indonesia or Singapore. Thunderstorms are associated with the various monsoon seasons around the globe, and they populate the rainbands of all tropical cyclones. 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. 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 and dangerous thunderstorms occur over the United States, particularly 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, but they occur with greater frequency in the inland areas, particularly the Sacramento and San Joaquin Valleys of California. Furthermore, in spring and summer, they occur nearly daily in certain areas of the Rocky Mountains. In the Northeast, storms take on similar characteristics and patterns as the Midwest, only less frequently and severely. Probably the most thunderous 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. In more contemporary times, thunderstorms 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 to explore the visual and scientific aspects of storms and tornadoes.

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. 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 hotter than the surface of the sun. Although the lightning is extremely hot, the short duration makes it not necessarily fatal. 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. 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 arches from one cloud to another.
• Ball lightning is extremely rare and has several hypothesized explanations. It is seen in the form of a 20 to 200 centimeter ball.
• Cloud to air lightning is when lightning from a cloud hits air of a different charge.
• Dry lightning is a misnomer which refers to a thunderstorm whose precipitation does not reach the ground.

Mythology

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.

See also

• Cumulonimbus
• Severe thunderstorm warning
• Severe thunderstorm watch
• Supercell
• Thunder
• Tornado warning
• Tornado watch

References

• 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.
• ______, 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.
• _____, 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.

Further reading

• Evans, Jeffry S.: Examination of Derecho Environments Using Proximity Soundings. http://www.spc.noaa.gov/publications/evans/bowpaper/bowpaper.htm
• J.V. Iribarne and W.L. Godson, Atmospheric Thermodynamics, published by D. Reidel Publishing Company, Dordrecht, the Netherlands, 1973, 222 pages
• M K Yau and R.R. Rogers, Short Course in Cloud Physics, Third Edition, published by Butterworth-Heinemann, January 1, 1989, 304 pages. EAN 9780750632157 ISBN 0-7506-3215-1

External links

• Anatomy of a thunderstorm
• Thunderstorm photography in Germany
• Electronic Journal of Severe Storms Meteorology
• Social & Economic Costs of Thunderstorms & High Winds from "NOAA Socioeconomics" website initiative

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