1. Pertaining to temperature or heat. 2. A discrete buoyant element in which the buoyancy is confined to a limited volume of fluid. See plume. 3. A relatively small-scale, rising current of air produced when the atmosphere is heated enough locally by the earth's surface to produce absolute instability in its lowest layers. The use of this term is usually reserved to denote those currents either too small and/or too dry to produce convective clouds; thus, thermals are a common source of low-level clear-air turbulence. It is generally believed that the term originated in glider flying, and it is still very commonly used in this reference.