Heating of the earth's land surface often causes thermals, or parcels of warm air, to rise to the level of condensation. If the thermals contain sufficient moisture and the atmosphere is conditionally unstable, towering cumulus clouds may form. Under suitable conditions the cumulus towers may merge to form cumulonimbus clouds, which have an anvil-shaped top. Cumulonimbus clouds produce thunderstorms. These are local storms accompanied always by lightning and thunder; often by strong gusts of wind, heavy rainfall, and sometimes hail; and occasionally by tornadoes.

Sometimes upper-atmospheric optical phenomena known as "red sprites" and "blue jets" also occur. Although long reported, these phenomena were not actually photographed until the late 1980s and early 1990s. They are best seen from aircraft or outer space. Sprites are weak but often vast flashes observed above clouds at the same time as lightning strokes. Blue jets are stronger upward ejections to the active core regions of thunderstorms, often taking the form of narrow cones. They appear to be integral parts of moderate to large storms, but they are not yet fully understood.

Thunderstorms play an important role in the earth's hydrologic cycle and general circulation by vertically transporting a large part of the heat and water vapor that enters the atmosphere from the earth's surface in response to solar heating. Thunderstorms produce a large fraction of the annual rainfall in many areas, especially semiarid and tropical equatorial regions. In most areas of the world, especially those regions in the United States, Canada, and Ukraine where corn and wheat are the major crops, rainfall during the peak growing season is supplied by thunderstorms. Thunderstorms are also the engines of tropical disturbances that bring a large fraction of the annual rainfall to tropical coastal regions.

Formation and Circulation
During the cumulus stage, the growing cloud is characterized by the initial formation of small precipitation elements and the presence of generally active updrafts throughout its vertical extent. Updraft velocities may exceed 10 m/sec (33 ft/sec) in the interior of the cloud. In the mature stage the cloud has expanded laterally and grown to greater heights in the atmosphere. A strong updraft over a broad area exists in the cloud's upper levels. In a less intense thunderstorm a downdraft originating at heights of 3 to 5 km (2 to 3 miles) above the cloud base often pinches off the updraft air at low levels, leading to the eventual demise of the cloud. New clouds frequently form along an advancing air mass that is chilled by the evaporation of rain and that emanates from the downdraft spreading laterally beneath the cloud. Thunderstorms often contain precipitation in the form of raindrops, graupel, and small hail in the interior of the cloud and snowflakes in the outflow levels and the cirrus anvil. Moderate to heavy rainfall occurs over broad regions below the cloud base. Eventually the updraft weakens and a downdraft spreads throughout the cloud, weakening the rainfall and dissipating the cloud.

Isolated thunderstorms occasionally form over small hot spots above flat terrain or by the lifting of moist air over ridges or mountaintops. Generally, however, thunderstorms consume so much moisture that they require a well-organized field of low-level moisture convergence for their sustenance. Thunderstorms thus typically form in conditionally unstable air masses in conjunction with a larger-scale circulation such as that produced by a monsoon, a sea breeze, or a mid-latitude or tropical cyclonic disturbance.

Destructive Potential
A small fraction of the total number of thunderstorms develops into exceptionally large and intense severe local storms that produce violent windstorms, tornadoes, large hailstones, heavy rainfall, and intense lightning. Severe local storms typically form in air masses that exhibit high conditional instability, have a well-organized source of low-level moisture convergence, and exhibit substantial shear, or change in speed and direction of the horizontal wind per unit of height, through a deep layer of the atmosphere.

Thunderstorms are notorious for their destructive potential. Thunderstorm-associated lightning causes 100 to 200 deaths and several hundred million dollars worth of property damage and ignites approximately 10,000 forest fires each year in the United States alone. Hail annually causes several hundred million dollars worth of crop loss in the United States. Downbursts, the outflow from intense downdrafts, can produce maximum surface wind speeds in excess of 80 m/sec (180 mph), causing considerable structural damage and creating a potential hazard for aircraft landing or departing from airports. Flash floods, generally caused by relatively stationary and persistent thunderstorms forming over watersheds feeding deep valleys and canyons, are rapidly becoming one of the highest-ranking forms of storm-related killers because of the continued expansion of urban areas into flood-prone areas.

by William R. Cotton

Bibliography: Cotton, W., and Anthes, R., Storm and Cloud Dynamics (1992; repr. 1997); Kessler, E., ed., Thunderstorm Morphology and Dynamics, 2d rev. ed. (1992); Houze, R., Cloud Dynamics (1994); MacGorman, D., The Electrical Nature of Storms (1998); Zimmer, Carl, "Heaven's New Fires," Discover, July 1997.