Because hot air has more energy, the molecules of air are moving more and pushing each other away more, so its less dense. because of this hot air rises above colder air masses somewhat like a bubble rises in water, only much slower.

as you go higher up in the atmosphere, thanks to gravity the air naturally gets less dense. As the air gets less dense the air is allowed to expand out and cool, a process called Adiabatic Cooling. This same principle is why any liquid or gas sprayed out of a compressed canister, like compressed air or a fire extinguisher is cold.