Thunderstorms (cumulonimbi) are violent convective clouds that fill the depth of the troposphere. Thunderstorms look like mushrooms or anvils. The most violent thunderstorms are called supercells, in which the whole thunderstorm rotates as a mesocyclone. Other storm organizations include airmass thunderstorms, multicell storms, orographic storms, mesoscale convective systems, squall lines, and bow echoes.
Four conditions are needed to form thunderstorms: high humidity; instability; wind shear, and a trigger to cause lifting. Thermodynamic diagrams and hodographs are used to determine the moisture availability, static stability and shear. A variety of stability and shear indices have been devised to aid thunderstorm forecasting. Trigger mechanisms are often mountains or airmass boundaries such as synoptic fronts, dry lines, sea-breeze fronts, and gust fronts, such as determined from weather radar, satellite, and surface weather analyses.
Thunderstorms are like gigantic engines that convert fuel (moist air) into motion and precipitation via the process of condensation and latent heat release. Once triggered, thunderstorms can often sustain themselves within a favorable environment for 15 minutes to several hours. They are most frequent in the late afternoon and evening over land. The explosive growth of thunderstorms, their relatively small diameters, and their sensitivity to initial conditions make it difficult to forecast thunderstorms.