I know a hot air balloon goes up because the density of the hot air inside is lower. But what about free air, not trapped in a balloon? You often see exactly the same explanation, hot air is less dense so it goes up, but this does not make any sense if you consider the fact that air is just a bunch of molecules flying around freely and bouncing into each other a lot, and temperature is a measure of their average momentum. There's no such thing as a "pocket" of hot air that somehow pushes other air away while going up. All you can say is that in a certain area the average speed of the air molecules is higher. So why would this cause the air to rise, to such an extent that it even draws surface winds that fill the gap? I mean, the effect is real, gliders use it all the time, but what is really going on on a molecular level? The faster molecules should surely be pushing in all directions, not just up? I would expect them to transfer their excess energy to nearby molecules through collisions until an equilibrium is reached, but can't imagine why a whole "pocket" of air would tend to rise and leave a low pressure underneath, even drawing in surrounding air instead of pushing it away.