Why does hot air rise?



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

Imagine when you are cold, you are cowering into a ball. Cold air does the same, so it sinks to the ground, actually pushing warmer air up.

Or like when you fall into a ball pit, you’re the cold air and all the balls, loosely spread, are warm air rising, while you sink throug them to the ground.

Basically think of air as a box sponge balls. When the box gets wet (warm/hot). The sponge balls get bigger and bounce around much more. When they dry out (cold) the balls shrink and slow down.

Have you ever seen men in black? That little ball that J touches in MIB head quarters. That’s 1 of countless hot air molecules. When k catches it with the glove that when air is cold.

Since hot air molecules expand and move around faster. The same amount of air takes up more space. So colder air that takes up less space is heavier. So cold air sinks and hot air rises. This process is called convection. It’s responsible for a lot of interesting things.

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.

Hot air molecules move around a lot. Think of them moving around in beach balls (less dense). Cold air molecules move around less. Think of them moving in tennis balls (more dense). What happens when you put beach balls and tennis balls in a pit and mix or stir the balls?

The smaller balls go to the bottom and bigger balls go to the top. That’s how it is!

Some answers here talk about buoyancy or pressure, but I think that’s really the end result of what’s happening to these individual molecules. Here’s my understanding of what’s happening at that molecular level.

In that column of air, from the point of view of individual molecules, there’s really two important things: gravity and the energy of each molecule. Those molecules with low energy will act mainly under the influence of gravity and so slowly settle to the bottom (although lots of jostling will still be happening). Those molecules with higher energy will be racing around in random directions, and so will be more likely to find and occupy the larger spaces that emerge as the slower particles drop down the column. The faster the molecule, the more likely it will be to move into areas with more gaps, and so the warmer air will rise.

This is all me trying to interpret my high-school physics lessons, so corrections would be very welcome.