Cold air is denser than warm air.
Density is a measurement that compares the amount of matter (mass) an object has to its volume. An object with more mass in a certain volume has high density. The density of warm air is less in comparison to cold air. Warm air rises up because when heat is provided, air expands. When the air expands, it becomes less dense than the air around it. The less dense warm air then floats above the denser cold air much like oil floats on water.