I get that hot air is less dense, because the molecules are moving faster. But given random Brownian motion of the molecules, why do they move up? What causes the molecules with higher kinetic energy to sort themselves to the top of an air column?
In: Physics
The lower density alone is responsible for it.
You have two important forces acting on anything thats floating in our atmosphere – bouyancy and weight.
Bouyancy pushes things up. It exists because the air pushes against an object on all sides, but since the air pressure is slightly higher below any object than above (due to it being deeper inside the atmosphere) a net upward force is generated.
Weight pushes things down according to its mass. I think i dont need to explain how that works to you 🙂
If anything has exactly the same density as the surrounding air, Bouyancy and Weight cancel each other out.
But what happens if you make something less dense? Bouyancy stays the same (as its only related to the surface of an object, not to whats inside that surface). But the weight decreases. So a lighter object has the same bouyancy pushing it up, and less weight to pull it down. And so it rises up.
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