If hot air rises, why is all the snow on top of mountains?
In: Physics
Why is there snow on top of mountains? To put it another way, why is it colder at higher altitudes? Because the air around us is not being heated directly by the sun above us, but actually by the ground below us.
A lot of the energy from the sun is visible light. This light comes from the sun and goes through our atmosphere hardly affecting it at all. The light is not blocked by the atmosphere, it doesn’t heat up the atmosphere, nothing: straight through. Then the light hits the ground, and is absorbed, and heats up the ground and everything on the ground. The reason things are warmer at lower altitudes is that they are really being heated from below.
It’s like if you had a heated bathroom floor: sure the heat will rise, but it will still be warmer right next to the floor.
So that explains why it is colder at higher altitudes, but it doesn’t explain the problem with “hot air rises”. Is “hot air rises” wrong then? How can “hot air rises” and “it’s colder higher up” both be true? Well, “hot air rises” is one of the most misunderstood and misapplied statements in science in my experience (this is not a criticism of OP; it’s a good question).
I really don’t like “hot air rises”. I prefer “hot air doesn’t sink very fast”. Less pithy, but less misleading.
Here’s a crucial point: if you leave things alone long enough, and don’t heat them up from the outside or anything, then the temperature will end up the same everywhere. It doesn’t matter if it started hotter on top, hotter on the bottom, whether it is in air, water, or the vacuum of space, eventually if you can isolate something, the temperature in it will end up the same everywhere. So if something is hotter at the bottom, the heat will rise. If something is hotter at the top, the heat will sink (yes, it’s true that “heat sinks” too!).
But here’s the thing: in air and water, heat will rise to even things out much more quickly that heat will sink to even things out. “Hot air rises” is about how quickly temperatures will change, not about what their final temperature will be.
This is why, if you are going to heat up some soup, you should do it from underneath. The hot air (and hot water) will rise fairly quickly to even out the temperature. But you will never get it to be hotter on top by heating it at the bottom. On the other hand, if you want a juicy grilled steak, heat it from the top. The air will not sink very fast, and you’ll get the top really hot (nice and browned) without the heat sinking into the steak and over-cooking it.
So anyway, heat rises, but only sometimes, and heat sinks too, just much slower.
(Usual disclaimer: this is of course simplified, especially the blanket statement that temperatures will end up the same everywhere. I could have thrown around phrases like equilibrium state and the second law of thermodynamics, but I’m trying to keep it ELI5).
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