Thunderstorms form when three conditions are present: sufficient moisture accumulated in the lower atmosphere, a significant fall in air temperature with increasing height, and a force (such as the pressure differential of a cold front) that will push the moisture into the low-temperature upper regions. A given cell of a thunderstorm typically goes through three stages: the cumulus stage, the mature stage, and the dissipation stage.
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. The masses of water vapor are warmer than the surrounding air, and therefore 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 5E8kg 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 accumulated water vapor has become large, with the top layer often spreading out into an anvil formation. 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.
Thunderstorms can be generally classed into three categories, largely in order of increasing severity: single cells, multicellular storms, and supercells. Largely the type of storm depends on the relative wind conditions at different layers of the atmosphere (shear). The single cell thunderstorm is the typical three-stage situation as described above, usually lasting about 30 minutes from the start of significant precipitation.
In a multicellular storm, several of these 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 sequentually going through the various thunderstorm stages.
The supercell is the most dangerous form of storm system, as it may produce violent gusts of wind, large hail, and more damaging tornadoes. It is caused when updrafts through the forming cumulonimbus cloud are twisted to proceed along the anvil. It 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.
Geographic features (such as mountain ranges) or atmospheric conditions (such as extended warm or cold fronts) may create lines of thunderstorms which move across the landscape. A special case of this is the squall line, which forms along the warm sector of a cyclone. When conditions are right, several multicell systems may merge into large "thunderstorm convective complexes" stretching for hundreds of miles; in the presence of cyclones or upper-level troughs, even larger clusters of thunderstorms may cover thousands of miles.