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A thunderstorm is an intense local storm associated with a large, dense cumulonimbus cloud in which there is very strong updrafts of air. Thunder and lightning normally accompany the storm, and rainfall is heavy, often of cloudburst intensity, for a short period. Violent surface winds may occur at the onset of the storm.
Recent studies of cumulonimbus clouds show that a single thunderstorm consists of individual parts, called cells. Air rises within each cell as a succession of bubble-like air bodies, rather than as a continuous updraft from bottom to top. As each bubble rises, air in its wake is brought in from the surrounding region, a process called entrainment. Rising air in the thunderstorm cell can reach vertical speeds up to 3000 ft (900 m) per minute. Rapid condensation will be in the form of rain in the lower levels, mixed water and snow at intermediate levels, and snow at high levels.
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Upon reaching high levels, which may be on the order of 20,000 to 40,000 ft (6 to 12 km) or even higher, the rising rate diminishes and the cloud top is dragged downwind to form an anvil top. Ice particles falling from the cloud top act as nuclei for condensation at lower levels, a process called seeding. The rapid fall of condensation adjacent to the rising air bubbles exerts a frictional drag upon the air and sets in motion a downdraft. Striking the ground where precipitation is heaviest, this downdraft forms a local squall wind, which is sometimes strong enough to fell trees and do severe structural damage to buildings.
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Thunderstorms may be classified into several types, based on the mechanism or cause of initial lift of the air column that set off the spontaneous growth of the storm. One common type is the thermal, or air mass thunderstorm, which is .set off by thermal convection caused by solar heating of the ground and lower air layer. Storms of this type are often widely scattered over a large region covered by warm, moist air. The time of occurrence is typically in the late afternoon when air temperatures near the ground reach their highest levels.
In the orographic thunderstorm, air is forced to rise over a mountain barrier, if the air is warm and moist, with unstable properties, it readily breaks into heavy showers and thunderstorms. The torrential monsoon rains of the Asiatic and East Indian mountain ranges are largely of this type. For example, Cherrapunji, a hill station facing the summer monsoon air drift in northeast India, averages 426 in (1082 cm) of rainfall annually. In the arid southwestern United States, isolated mountain ranges and high plateaus receive abundant summer rain in the form of thunderstorms orographically induced. Here rich forests grow while the surrounding lower areas are barren or sparsely vegetated deserts.
