Quirks & Quarks

If hot air rises, why is it cold at the top of mountains?

As warm air rises up a mountain side, it loses pressure, expands, and cools as it works against its surroundings

As warm air rises up a mountain side, it loses pressure, expands, and cools

A process called Adiabatic cooling explains why rising hot air becomes cold as it reaches the top of a mountain, like this one in the Canadian Rockies near Banff, Alberta (Joe Passaretti/CBC Still Photo Collection)

This week's question comes from David Hopkins in Delta BC. He asks: 

If hot air rises, why is it colder up in the mountains than it is at sea level? Because hot air rises, shouldn't it be the other way around, with it being warmer up in the mountains than at sea level?

Long Li, a Professor in the Department of Earth and Atmospheric Sciences at the University of Alberta explains that as a warm air mass rises up a mountainside, the resulting pressure drop causes the air to expand. During this expansion, the air mass spends heat as it works against its surroundings. The subsequent drop in air temperature is a process known as adiabatic cooling. 

After the air mass passes over the mountain top, it starts to descend down the leeward side of the mountain. At this point, Adiabatic heating takes place. In this process the air is warmed as its volume shrinks and air pressure increases. This process also explains the warm and dry Chinook winds experienced in southern Alberta. 




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