A thunderstorm is a form of severe weather characterized by the presence of lightning and its attendant thunder. It is often accompanied by copious rainfall, or, on occasion, snowfall.

Thunderstorms form when significant condensation, resulting in the production of a wide range of water droplets and ice crystals, occurs in an atmosphere that is unstable and supports deep, rapid upward motion. This often occurs in the presence of three conditions: sufficient moisture accumulated in the lower atmosphere, reflected by high dewpoint temperatures; a significant fall in air temperature with increasing height, known as a steep lapse rate; and a force such as mechanical convergence along a cold front that will focus the lift.

Thunderstorms have had a lasting and powerful influence on mankind. Romans thought them to be battles waged by Jupiter, who hurled lightning bolts forged by Vulcan. Thunderstorms were associated with the Thunderbird, held by Native Americans to be a servant of the Great Spirit . In more contemporary times, thunderstorms now have taken on the role of a curiosity. Every spring, storm chasers head to the Great Plains to explore the visual and scientific aspects of storms and tornadoes.

Life cycle

A given cell of a thunderstorm goes through three stages: the cumulus stage, the mature stage, and the dissipation stage. This life cycle was identified in 1949 as the result of the U.S. Weather Bureau's landmark Thunderstorm Project.

In the cumulus stage of a thunderstorm cell, masses of moisture are pushed upwards; the moisture rapidly cools into liquid drops of water vapor, which appears as cumulus clouds. Not only are the masses of water vapor warmer than the surrounding air, but water vapor is less dense than dry air, and for both of these reasons the warm humid air will tend to rise in an updraft due to the process of convection. This creates a low-pressure zone beneath the forming thunderstorm. In a typical thunderstorm, some 5×10⁸ kg of water vapor are lifted and the amount of energy released when this condenses is about equal to the energy used by a city (US-2002) of 100,000 over a month.

In the mature stage, the warm air rises and while it's rising atmospheric pressure is pushing it down forcing the top to spread out thus giving it its anvil shape. The resulting cloud is called cumulonimbus. The water vapor will coalesce into heavy droplets and ice particles, which will fall onto the area below as rain. If temperatures in the upper atmosphere are cold enough, some of these droplets may actually form into masses of ice and fall as hail. While updrafts are still present, the falling rain creates downdrafts as well. The presence of both updrafts and downdrafts during this stage can cause considerable internal turbulence in the storm system, which sometimes manifests as strong winds, severe lightning, and even tornadoes.

Finally, in the dissipation stage, updraft conditions no longer exist, and the storm is characterized largely by weak downdrafts. Because most of the moisture has precipitated out as rain or ice (precipitation) there is no longer sufficient moisture in the lower air to sustain the cycle.

Classification

Thunderstorms are often classified into a spectrum based on their cell structure: single cells, multicellular storms, and supercells. The type of storm depends on the instability and relative wind conditions at different layers of the atmosphere (shear).

The single-cell (unicell) thunderstorm is the singular three-stage situation as described above, usually lasting about 30 minutes from the start of significant precipitation. A severe unicell storm is often referred to as a pulse thunderstorm. The life cycle of the storm usually takes place within a one-hour span.

In a multicell thunderstorm, several thunderstorm cells merge into a larger system. The cloud becomes divided into updraft and downdraft regions separated by a gust front. The gust front may extend for several miles ahead of the storm, bringing with it increases in wind speed and atmospheric pressure, decreases in temperature, and shifts in wind direction. The storm itself will have different portions sequentially going through the various thunderstorm stages. In many cases the immature cells develop along a line known as a flanking line, resulting in what is known as a line multicell.

The supercell is the most dangerous type of thunderstorm, as it typically contains violent gusts of wind, large hail, and damaging tornadoes. It is caused by strong instability or strong helicity in the flow feeding the storm. The supercell possesses a mesocyclone, the results of which are strong vertical shear, differences in wind speed at different layers and separate updraft and downdraft regions, with the effect being that the storm will both last longer and continue to grow larger and more dangerous.

In a thunderstorm, the Earth's Coriolis effect can twist the huge vertical motions of air and moisture into horizontal rotation - a vortex - which can result in tornadoes in larger thunderstorms.

Geographic features (such as mountain ranges) or linear boundaries (such as warm or cold fronts) may create lines of thunderstorms which move across the landscape. A special case of this is the squall line, which usually occurs in the warm sector of a cyclone. The squall line is propelled by its own outflow, which reinforces continuous development of updrafts along the leading edge.

Multicell or squall line systems may form a meteorologically-important feature known as mesoscale convective system (MCS) stretching for hundreds of miles. They are large enough to have a pronounced effect on the upper-level and surface weather pattern, and may influence forecasts over half of a continent. MCS systems are common in the Midwest region of the United States during the summer months and produce much of the region's important agricultural rainfall.

See also

hurricane, weather, lightning