thermal(n = noun.phenomenon) - rising current of warm air;
is a kind of air current, current of air, wind
Derived form adjective thermal₁

1.  thermal(a = adj.pert) caloric, thermic - relating to or associated with heat; "thermal movements of molecules"; "thermal capacity"; "thermic energy"; "the caloric effect of sunlight"
Antonym: nonthermal
Derived form noun thermal₁

2.  thermal(a = adj.pert) - of or relating to a hot spring; "thermal water"

3.  thermal(s = adj.all) - caused by or designed to retain heat; "a thermal burn"; "thermal underwear"

1.  Of or pertaining to heat; warm; hot; as, the thermal unit; thermal waters. [1913 Webster]
"The thermal condition of the earth." [1913 Webster]

2.  caused by or affected by heat; as, thermal springs. [PJC]

3.  designed to retain heat; as, thermal underwear. [PJC]

thermal, adj. & n.
--adj.
1 of, for, or producing heat.
2 promoting the retention of heat (thermal underwear).
--n. a rising current of heated air (used by gliders, balloons, and birds to gain height).

Idiom:
British thermal unit the amount of heat needed to raise
1 lb. of water at maximum density through one degree Fahrenheit, equivalent to 1.055 x 10(3) joules. thermal capacity the number of heat units needed to raise the temperature of a body by one degree. thermal neutron a neutron in thermal equilibrium with its surroundings. thermal reactor a nuclear reactor using thermal neutrons. thermal springs springs of naturally hot water. thermal unit a unit for measuring heat.

Derivative:
thermalize v.tr. & intr. (also -ise). thermalization n. thermally adv.

Etymology:
F (as THERM)

N  levity, lightness, imponderability, buoyancy, volatility, feather, dust, mote, down, thistle, down, flue, cobweb, gossamer, straw, cork, bubble, balloon, float, buoy, ether, air, leaven, ferment, barm, yeast, lighter-than-air balloon, helium balloon, hydrogen balloon, hot air balloon, convection, thermal draft, thermal, light, subtile, airy, imponderous, imponderable, astatic, weightless, ethereal, sublimated, gossamery, suberose, suberous, uncompressed, volatile, buoyant, floating, portable, light as a feather, light as a thistle, light as air, lighter than air, rise like a balloon, float like a balloon.

N  heat, caloric, temperature, warmth, fervor, calidity, incalescence, incandescence, glow, flush, fever, hectic, phlogiston, fire, spark, scintillation, flash, flame, blaze, bonfire, firework, pyrotechnics, pyrotechny, wildfire, sheet of fire, lambent flame, devouring element, adiathermancy, recalescence, summer, dog days, canicular days, baking, heat, white heat, tropical heat, Afric heat, Bengal heat, summer heat, blood heat, sirocco, simoom, broiling sun, insolation, warming, sun, pyrology, thermology, thermotics, thermodynamics, thermometer, calorie, gram-calorie, small calorie, kilocalorie, kilogram calorie, large calorie, British Thermal Unit, B, T, U, therm, quad, degrees Kelvin, kelvins, degrees centigrade, degrees Celsius, degrees Fahrenheit, hot, warm, mild, genial, tepid, lukewarm, unfrozen, thermal, thermic, calorific, fervent, fervid, ardent, aglow, sunny, torrid, tropical, estival, canicular, steamy, close, sultry, stifling, stuffy, suffocating, oppressive, reeking, baking, red hot, white hot, smoking hot, burning, hot, piping hot, like a furnace, like an oven, burning, hot as fire, hot as pepper, hot enough to roast an ox, hot enough to boil an egg, fiery, incandescent, incalescent, candent, ebullient, glowing, smoking, live, on fire, dazzling, in flames, blazing, in a blaze, alight, afire, ablaze, unquenched, unextinguished, smoldering, in a heat, in a glow, in a fever, in a perspiration, in a sweat, sudorific, sweltering, sweltered, blood hot, blood warm, warm as a toast, warm as wool, volcanic, plutonic, igneous, isothermal, isothermic, isotheral, not a breath of air, whirlwinds of tempestuous fire.

british thermal unit, solar thermal system, thermal barrier, thermal emission, thermal equilibrium, thermal pollution, thermal printer, thermal reactor, thermal resistor, thermal spring