If hot air rises, why are mountaintops so cold and snowy?

—Thomas Thompson, Barnegat, New Jersey

First things first: Under normal conditions, matter exists as either a solid, a liquid or a gas. Air is a gas. When you put pressure on a gas, it gets warmer. And when you release the pressure, the gas gets cooler. You may have noticed this phenomenon when inflating your bicycle tires with a bike pump. (By the way, refrigerators work by strategically employing air pressure changes.)

Anyway, air pressure decreases as air rises for two reasons: First is gravity, which hugs the air to the ground as much as possible. (The lower the air, the higher the pressure.) Meanwhile, the number of gas molecules in a given volume of air decreases as it rises. This results in what skiers and vacationers call “thin air.” And as the heat content of a system is directly related to the amount of matter it contains, higher altitudes correspond to lower air temperatures.

As colder air is drier, less evaporation is occurring in the system. More moisture in the air usually means more rain, but at higher altitudes, it means more snow. In addition, due to the prevalence of decreased temperatures, that snow lingers and lingers.