|MLA Citation:||Bloomfield, Louis A. "Question 756"|
How Everything Works 18 Jun 2018. 18 Jun 2018 <http://howeverythingworks.org/print1.php?QNum=756>.
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.