I asked, watching buzzards, white-backed vultures, and tawny eagles corkscrew skyward on thermals.
1756, "having to do with hot springs," from French thermal (Buffon), from Greek therme "heat," from PIE *ghwerm-/*ghworm- "warm" (cf. Latin fornax "an oven, kiln," formus "warm," Old English wearm; see warm). Sense of "having to do with heat" is first recorded 1837. The noun meaning "rising current of relatively warm air" is recorded from 1933.
thermal ther·mal (thûr'məl)
Of, relating to, using, producing, or caused by heat.
Intended or designed in such a way as to help retain body heat.
Adjective Relating to heat.
Noun A usually columnar mass of warm air that rises in the lower atmosphere because it is less dense than the air around it. Thermals form because the ground surface is heated unevenly by the Sun. The air usually rises until it is in equilibrium with the air surrounding it.