What Causes Thunderstorms?

A thunderstorm is a storm with lightning and thunder.

Thunderstorms are produced by towering cumulonimbus clouds associated with a fall in atmospheric pressure, strong gusts of wind, heavy rain and sometimes hail.

Individual clouds are often known as thunderstorm cells because of the basically convective cellular motion which ultimately causes the cloud formation. Cells are usually 5 to 10 km in diameter and may extend up to the tropopause 10 to 15 km depending on the latitude. The lifetime of a simple thunderstorm cell is only one to two hours and in this time the cloud goes through three distinct stages: youth (updraft only), maturity (updraft and downdraft existing together) and old age (downdraft occupying most of the cloud). Although simple single cells occur, a thunderstorm usually comprises several cells, each at different stages in their lives.

The movement of thunderstorms depends largely on their size and their internal circulation. Small storms (perhaps single cells) tend to move with the mean wind of the layer in which they form. As storms get larger, with a diameter greater than 15 km (9 mi) their internal circulation becomes much more complicated than the simple upward and downward motion outlined above, and they tend to propagate new storm-clouds rather than simply moving with the wind.

The internal circulation of a mature, severe thunderstorm features a system of coexisting updraft and downdraft which helps to prevent the atmosphere stabilizing itself. Warm, moist air from a low level is swept up into the storm from the front, becoming an updraft that follows a tilted path toward the rear of the cloud. The downdraft originates from cold, dry air at middle levels which curls around the updraft before sinking and diverging as it hits the ground. In a storm of this kind, the cold downdraft air helps to trigger the upward motion of warm, moist air in the front right quadrant. In this way the storm propagates further storms to the right of the mean wind of the layer of the atmosphere in which the storm occurs.

Thunderstorms occur as a result of atmospheric instability and broadly represent the violent rearrangement of air layers having different densities, in order to achieve a more stable arrangement. The instability usually results either from direct heating of the air near the ground, which leads to fairly small, often single-cell storms, or from large-scale cooling and drying in the middle and upper atmosphere, giving rise to massive outbreaks of families of storms, usually along or ahead of cold fronts.

Large-scale outbreaks ahead of fronts (known as squall-lines) occur frequently in the central USA and some other mid-latitude areas . In the tropics, thunderstorms often break out where warm, moist air rises over steep mountains .

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