When moving several abreast they may form a squall line. Once precipitation begins the updraft which initiated the cloud's growth weakens and is joined by a downdraft generated by the precipitation. On the ground the updrafts and downdrafts of air are felt as rapid gusts of wind . Most storms are composed of several cells that form, survive for about half an hour, and then die. New cells may replace old ones, and it is possible for some storms to continue for several hours.
Lightning always accompanies the thunderstorm.
Lightning arises from a discharge of electrical energy which has built up within the cumulonimbus cloud as a result of repeated separation and splitting of water and ice particles in the turbulent conditions that prevail. Although air is a fairly good insulator, eventually the separation of electric charge becomes so great that the insulation breaks down and a lightning strike results. Lightning discharge may occur entirely within the cumulonimbus cloud or between the cloud and the ground. The lightning strike causes a rapid heating of the surrounding air, resulting in a sudden expansion and contraction of air that is heard as thunder. Close to the lightning strike the thunder may be heard as a short loud crack.