Thunderstorms, generated by temperature imbalances in the atmosphere, are a violent example of convection. Warming of the air near the earth's surface and/or cooling of the air above puts warmer, lighter air layers below colder, denser layers. The resulting instability causes convective overturning of the layers, with heavier, denser layers sinking to the bottom and the lighter warmer air rising rapidly.

Mechanical processes are also at work. Warm, buoyant air may be forced upward by the wedge-like undercutting of a cold air mass or by flowing up a mountain slope. Winds blowing into the center of a low-pressure area may force warm air near that center upward.

In the first stage of thuderstorm development, an updraft carries warm air to a level where the air becomes saturated with moisture and visible droplets appear as a cloud begins to form. Continued upward movement produces large clouds resembling rising mounds, domes, or towers, known as cumulus clouds. Strong winds above the developing clouds may further enhance the updraft or convection.

As the cloud forms, water vapor changes to liquid and/or frozen cloud particles. This results in a release of latent heat that takes over as the principal source of energy for the developing cloud. Once the cloud starts to form by other forces, this release of heat helps keep it growing.

The cloud particles grow by colliding and combining with each other, forming rain, snow, and hail. When the droplets become heavy enough to fall against the updraft, precipitation begins.

Having reached its final stage of growth, the cumulonimbus cloud, called a thunderstrom, may be several miles across its base and often towers to altitudes of 40,000 feet or more. High level winds shred the cloud top into the familiar anvil form. These cloud towers are sometimes visible as lonely giants and at other times while moving several abreast are known as a squall line.

This final stage is also marked by a change in wind flow within the storm cells. The updraft which initiated the cloud's growth no longer prevails and is joined by a downdraft generated by the precipitation. This updraft-downdraft couplet constitutes a single storm "cell". Most storms are composed of several cells that form, survive for perhaps 20 minutes, and then die. New cells may replace old ones, and it's possible for some storms to last for several hours.

Strong gusts of cold wind from the downdraft or heavy precipitation--rain or hail--often occur at the ground beneath and outward from the mature storm. Lightning always accompanies the thunderstorm. These are nature's warnings that the thunderstorm is in its most violent stage. Tornadoes may also be associated with the thunderstorm.

Even so, the thunderstorm cell has already begun to die. The violent downdraft, having shared the circulation with the sustaining updraft, now strangles it. Precipitation weakens and the cold downdraft ceases. The thunderstorm cell, a short-lived creation spreads and dies.

Source: Pamphlet by U.S. DEPARTMENT OF COMMERCE. This pamphlet is available at:

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