How Everything Works
How Everything Works How Everything Works

Question 756

If heat rises, how come snow accumulates on mountains? Why is it colder up there instead of down here? — HG, Grand Prairie, TX
On a local scale, hot air does rise through cold air. That's because when hot air and cold air are at the same temperatures, the hot air has fewer air molecules per liter than the cold air and so each liter of hot air is lighter than each liter of cold air. In short, hot air is less dense than cold air and it floats upward in cold air. But when hot air rises a long way through the atmosphere, something begins to happen to the hot air. It cools off! That's because the air pressure decreases with altitude. The air pressure that's around us on the ground is only present because the air down here must support the air overhead. The air down here must push upward on the air overhead and it does this by developing a high pressure. But as you move upward in the atmosphere, there's less air overhead and therefore less air pressure around you.

So as the hot air rises upward, the air pressure around it gradually diminishes and the hot air expands. It has to expand because whenever its pressure is higher than the surrounding pressure, its molecules experience outward forces that cause them to spread out. But this expansion process uses some of the hot air's thermal energy—the hot air must push the surrounding air out of the way as it expands. With less thermal energy in it, the hot air becomes cooler. Dry air loses about 10° C for every kilometer it rises, while moist air loses about 6° or 7° C per kilometer. This cooling effect explains why air at higher altitudes, such as the air on mountains, is colder than the air at lower altitudes, such as the air in valleys.

Furthermore, whenever cold air descends through the atmosphere, it is compressed and its temperature rises! This warming process also increases the air's water-carrying ability so that it becomes relatively dry. That effect explains the special "Katabatic" winds that blow warm and dry out of the mountains—including the Santa Ana winds near Los Angeles, the Chinook in the Rocky Mountains, the Foehn in the Alps, and the Zonda in Argentina.


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