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 due to mountains, or lifting along frontal boundaries.

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 the 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 3/4-inchdiameter hail and/or surface winds exceeding 58 mph and/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. 

Stages of a Thunderstorm

Thunderstorms usually go through a series of stages, from birth to decay. The first stage is known as the cumulus stage, which is dominated by updrafts. The 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. 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 rapidly changes. Eventually, the downdraft will cut off the supply of warm, moist air in the updraft. When this occurs, typically 15–30 minutes after the mature stage, the thunderstorm will start to weaken and enter the dissipating stage due to the deprivation of energy from the updraft.

If the vertical wind shear increases, this allows for the thunderstorm to tilt. Therefore, the downdraft is less likely to cut off the updraft, which allows the thunderstorm to persist for a longer period of time. Sometimes, the downdraft can slide underneath an updraft, which can produce multiple-cell thunderstorms, or simply multicell storms. If the vertical wind shear becomes extremely strong, the shear can produce a large rotational thunderstorm known as a supercell, thunderstorms that last longer than an hour, 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 multiplecell 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. (800 km) and often exhibit severe characteristics. When thunderstorms occur in a large circular pattern, they are known as a Mesoscale Convective Complex (MCC), a large, convectively driven system that usually lasts more than 12 hours and covers more than 386,000 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. Since MCCs are usually located underneath weak, upper-level winds, they tend to travel very slowly, which can cause locally heavy rains and flooding events.

All thunderstorms have lightning, the electrical discharge, thunder, and resulting shockwave produced by extreme heating. Lightning has a temperature of approximately 54,000 degrees F (30,000 degrees C), which is five times hotter than the surface of the sun. Lightning occurs during the mature stages of thunderstorms and can appear within a cloud, connect from one cloud to another, or travel from cloud to ground. Most lightning strikes are within a cloud.

Worldwide, it is estimated that 50,000 thunderstorms occur every day and over 18 million occur per year. Thunderstorm frequency is the most common in the tropics, especially near the Intertropical Convergence Zone (ITCZ), an area of low pressure near the equator. Lower frequencies occur in drier regions near 30 degrees north/south, which is dominated by the subtropical high pressure, and also in the polar regions. In the United States, thunderstorm activity is predominantly in the southeast, with a maximum located over Florida. Florida has over 90 days of thunderstorms per year due to the convergence of wind from the Gulf of Mexico and the Atlantic Ocean.