why does hot air rise? why does cold air sink?

Simply put, warm air molecules are more active and thus spaced farther apart--warm air is less dense (lighter) than cooler air and therefore rises.

Hot air doesn't rise because it is hot, nor does cold air sink because it is cold. Denser air sinks, less dense air rises.

If you go up in the troposphere you will see that colder air is overlying warmer air with the warmest air on the ground and the coldest at the tropopause. If the rise and fall of air was due to temperature, you would not find that situation.

Although the air higher in the troposphere is colder than the air below it, it is less dense because it has fewer molecules per unit volume. The densest air is at the ground.

If you warm the air close to the ground so that it becomes less dense than the air above it, it will rise but it does so because of density not temperature.

Ok here goes. Warm air is less dense than cold. That means it takes up more space for the same about of actual stuff. just like a blown up balloon takes up more space than a flat one, but there's no more rubber in the blown up one, it just takes up more space. Well, things that are less dense float in things that are more dense. LIke, a ship floats because it takes up more space with less "stuff" (metal and wood) than the same amount of water would. So hotter gas floats up onto the top of colder gas for the same reason. It is called convection, but it is really an extension of Archimedes Principle, which you can google that.

By the way, that's what makes wind outside too. It's hotter 30 miles away than it is here, so the air over there rises, and the cooler air from here flows over there to fill the hole made by the rising hot air.

Let us think about why some things sink and some things float in water.

A thing will float if it ways less than the water it displaces. Let us

imagine you have a slice of white bread. If you put in water, it

will float. Now take another slice of white bread and SQUEEZE it into

a tiny little dough ball. Put that ball in water and it sinks. It is

the same amount of bread - it ways the same - but it is denser.

Now back to your question. When you heat air, it expands. When you

cool air, it "shrinks" (like the bread being SQUEEZED). The hot

air is less dense than the cold air, so it floats - or rises above the co

air.

Hot air rises because it has received a high boost of energy from, example, heating. The air particles begin to move at a very high speed in any environment, e.g. hot-air balloons.

On the other hand, cold air does not have that same amount of energy, thus its particles move at a relatively slow speed. Not good for hot-air balloons or other objects in this case.

In hot air, the molecules move in and out among each other very fast. This means that the air takes up more space. Taking up more space makes it lighter than cold air, where the molecules move more slowly.

when somethings cold it's more dense, hence heavier, when somethings hot (like water) it expands and gets lighter. so hot air raises because it's lighter then the cold air under it, the cold air sinks right through the heat

Think of the molecules as people. When they are cold the huddle together, thus more of them can fit into a space making the space heavier. When it is hot they spread out and away from each other, meaning less can fit into the same sized space, making it lighter. Good luck with your project.

I think because hot air is mre dense than cold air.

hot air rise becoz it contains only steam and rise upto atmospheric towards up

cold air sink becoz it contains only fog ie water and sink to atmospheric