Used as a noun, the term refers to air that rises in a column over some localized heat source; it rises through the atmospheric environment by convection, as a result of its reduced density that causes cooler, denser air to sink beneath it. Intense heating over land (and particularly over some types of surface) results in adiabatic expansion and uplift. When thermals rise to condensation level, short-lived clouds (e.g. fair-weather cumulus) may form. Continued strong thermals in moist air can result in the growth of the cloud into large cumulus or cumulonimbus. When the upward growth reaches a stable layer, the cloud top may spread laterally.