i have been flying my planes for many years, but it has been in warm weather...until now.
in those years, i got into thermals regularly, and grew under the impression that it had to be warm weather in order to climb with the help of hot air (the so called 'thermals'). but recently i have been flying near freezing, and have witnessed my planes climbing and staying up there for a long time. some say that is the differential temperature that makes air go up. that is, if there is air at, let's say, -20c, and surrounding air is -10c, that mass of air will go up. so if in freezing air, and then we encounter not-so-freezing air, that makes planes go (or stay-up) there, why call them 'thermals'? shouldn't be more correct to call them something like 'rising air'? or 'risers'?
the original word comes from greek: therme=heat. see:
i found this in etymologies:
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.

and the last 1 brings the key word (relative), as i mentioned when i said that it is the differential temperature. 'relatively warm air' means that what matters is that some mass of air is warmer than the surroundings.
there is some interesting data about their shape here: https://www....Continue Reading

at the beginning, sailplanes flew from slopes, like at elmira, new york, where an upward draft kept them airborne. no need to go anywhere, just stay into the upward wind. like the planes at slopes do, so a shorter wing was better for smaller diameter turns.
then they began to get into thermals, and as they gained altitude could go somewhere else and get into another thermal. so distance was the goal, and then wings began to get longer and thinner-the trend that we see today. but models that copy the shape of full size narrow wings find themselves into trouble, as they tip stall easily. the problem is due to the air density, that makes the smaller wings less efficient the smaller they get (that Reynolds number thing).
and if we look at birds, sure those that stay around, like buzzards, haws, and eagles, have short wings, and those who go places, like albatross, have long, narrow wings. and the chord of all is almost the same all through the wing. no taper nothing. just the tips. so they may know better, after millions of years of evolution. and they use turbulators too. just think about it.