Thermal means related to heat or temperature. As a noun, 'thermals' can mean rising currents of warm air or warm clothing designed to keep body heat in.
From Greek *thermos* 'hot, warm'. The same root appears in 'thermometer' and 'thermodynamics'.
When birds or gliders ride 'thermals', they’re surfing invisible rivers of warm air rising from the ground. The 'thermal' in your underwear and in 'thermal energy' is the same idea: managing heat.
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