QotW: Why is it cold up mountains?
Wayne asked "We've always learned that heat rises but it's normally cooler in the mountains. Shouldn't their higher elevation make it warmer there?"
Sally Le Page reached out to atmospheric physicist Simon Clark for the answer...
Simon - Hot air rises because it's warmer than surrounding air yet under the same atmospheric pressure. Air pressure and temperature are connected via some rules that describe how all gases behave. Specifically, a pocket of air at the Earth's surface that's warmer than its surroundings must be less dense than its surroundings, and so, like a bubble of less dense air in a swimming pool of very dense water, it rises above the surrounding air.
Sally - I see, heat rises at ground level because it is less dense than the air around it. But Wayne is right, it is much colder up in the mountaintops. What is different at high altitudes?
Simon - To make things more complicated, however, as air rises in the atmosphere, it also cools. This is because as it rises it is subject to less and less atmospheric pressure. Atmospheric pressure is just the weight of air above pushing down on you, and so the higher you go in the atmosphere, the less pressure you experience.
Sally - And what effect does there being less pressure pushing down on me (pushing down on you) have on hot air?
Simon - As a bundle of warm air rises through the atmosphere, there is less and less air above pushing down on it. This release of pressure allows the air parcel to expand and, in the process, cool down. The higher a pocket of air rises, the cooler it becomes. Generally a parcel of warm air will only rise so far before stopping, having cooled to the point where it is the same temperature as its new surroundings. This means it's the same density as its surroundings, and so has no motivation to rise any further. This process, known as adiabatic cooling, combined with the fact that the atmosphere is heated from below by the Earth, means that the atmosphere gets colder as you get higher and higher, and hence why mountaintops are cold.
Sally - So that solves it. Hot air rises because it is less dense than cold air, but the higher up it goes, the more it cools down leading to snow-capped mountains. Next week’s question might be a little more hairy, as Beth asked:
Beth - “My dog is always licking her fur but never gets hairballs. Why don’t dogs get hairballs?”