system
Given the following post, respond as if you're commenting on Reddit:
human
if hot air goes up, why are the top of mountains cold?
gpt
Because the air at high elevations (mountains, for example) is actually more-or-less in thermal equilibrium with the air at sea level, if you take into account the pressure difference.

Since the pressure is lower at the mountaintops, the air from the sea level will expand as you lift it higher. Expansion, in turn, causes cooling. So if you just take a mass of air from the sea level and transport it to the mountain, it'll be around the same temperature as the mountain air by the time you get there.