If warm air rises, why is Everest so cold?
You can't blame anyone for being confused. It's a fact of weather life that warm air rises and cold air sinks. Yet as anyone who's climbed a mountain knows, the air temperature usually goes down as you scale a peak. This seeming paradox has a subtle explanation. Warm air expands and cools as it rises, and cool air is compressed and warms up as it sinks. When conditions are dry, the rising and felling air are essentially trading identities, because they warm and cool at a fixed rate – roughly 10°C per km (5.5°F per 1000ft).
Thus, warm air rising toward the top of Mount Everest becomes cold by the time it gets there, and a downslope wind that starts out cold at mountaintop level can be very toasty when it reaches sea level. However, things get more complicated when the air is saturated with moisture. If you lift air that's at 100 percent relative humidity, some of the water vapour gets condensed. This produces clouds and releases enough heat to cancel out over half of the ascent-forced cooling. But when the air subsides, it normally warms at the rate noted above, which means that if it descends all the way to its starting point, it ends up warmer than it was originally. Because of this asymmetry, the atmosphere is constantly shifting to adjust to temperature variations, both horizontally and vertically.