Thunderstorms

A THUNDERSTORM IS a localized storm that is produced by a cumulonimbus cloud and always contains thunder and lightning. Thunderstorms form in conditionally unstable environments, meaning there is cold, dry air aloft over warm, moist surface air. This causes the air to become buoyant and allows for rising air motion. A lifting mechanism is also needed to start the air moving. Such lifting mechanisms include surface heating, surface convergence, lifting caused by mountains, or lifting along frontal boundaries.

Worldwide, it is estimated that 50,000 thunderstorms occur every day, and over 18 million occur per year.

The heat and the humidity of the summertime can often produce what are called ordinary thunderstorms or air mass thunderstorms. These are the type of thunderstorms that seem to suddenly pop up, last less than an hour, and are rarely severe. A severe thunderstorm is defined by the National Weather Service as having three-quarter-inch diameter hail or surface winds exceeding 58 mi. (93 km.) per hour or producing a tornado. Ordinary thunderstorms also do not usually have excessive vertical wind shear, meaning that the wind speed or direction does not change greatly with height. They usually go through a series of stages from birth to decay. The first stage is known as the cumulus stage and is dominated by updrafts. The [Page 968]updrafts bring in warm, moist air, which then cools and condenses as it rises. When the clouds further develop and precipitation starts to fall, a downdraft is produced. This marks the beginning of the mature stage, which is the most intense stage. During this stage, the strong updraft is still present, supplying the warm, moist air, but the strong downdraft is also evident. The gust front is located at the boundary of the updraft and the downdraft. This is an area where the wind velocity changes rapidly. Eventually, the down-draft will cut off the supply of warm, moist air in the updraft. When this occurs, typically 15 to 30 minutes after the mature stage, the thunderstorm will start to weaken and enter the dissipating stage as a result of the deprivation of energy from the updraft.

If the vertical wind shear increases, this allows for the thunderstorm to tilt. Therefore, the down-draft is less likely to cut off the updraft, which allows the thunderstorm to persist longer. Sometimes the downdraft can slide underneath an updraft, which can produce multiple cell thunderstorms or simply multiceli storms. If the vertical wind shear becomes extremely strong, the shear can produce a large rotational thunderstorm known as a supercell. Supercell thunderstorms are large storms that last longer than an hour and are often severe and can produce tornadoes. The strong wind shear creates horizontal spin, which can then rotate vertically when the updraft encounters the vortex.

Thunderstorms can occur as a line of multiple-cell thunderstorms known as squall-line thunderstorms. These usually form along or slightly ahead of a cold front. The line of thunderstorms can extend over 500 mi. (805 km.) and often exhibit severe characteristics. When thunderstorms occur in a large circular pattern, they are known as a mesoscale convective complex, or MCC. An MCC is a large, convectively driven system that usually lasts more than 12 hours and covers more than 38,610 sq. mi. (100,000 sq. km.) Many thunderstorms are embedded within the MCC and often form during the summer in the Great Plains. As warm, moist air is brought in from the Gulf of Mexico, the tops of the very high clouds cool rapidly by emitting radiation into space. This makes the atmosphere very unstable and allows for the MCCs to generate and persist. Because MCCs are usually located underneath weak upper-level winds, they tend to travel very slowly, which can cause locally heavy rains and flooding events.

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