ther·mal / ˈ[unvoicedth]ərməl/ • adj. of or relating to heat. ∎ another term for geothermal. ∎  (of a garment) made of a fabric that provides exceptional insulation to keep the body warm: thermal underwear.• n. 1. an upward current of warm air, used by gliders, balloons, and birds to gain height.2. (usu. thermals) a thermal garment, esp. underwear.DERIVATIVES: ther·mal·ly adv.

thermal 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 as a result of its reduced density. 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 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.

thermal (noun) Air that rises in a column over some localized heat source, as a result of its reduced density. When thermals rise to the condensation level, short-lived clouds (e.g. fair-weather cumulus) may form. In continued strong thermals, the cloud may develop into large cumulus or cumulonimbus. When the upward growth reaches a stable layer, the cloud top may spread laterally.

thermal Small-scale, rising current of air produced by local heating of the Earth's surface. Thermals are often used by gliding birds and human-built gliders.

thermal pert. to hot springs XVIII; pert. to heat XIX. -F. thermal, f. Gr. thérmē heat; see -AL1.