Cool air is more denser and forces up the warm air which is less denser. Cold air is more denser than warm air because molecules are closer since the bonds are absorbing less energy.
The concept of cold air and warm air is important in the formation of breezes. For example in sea breeze which occurs during the day, when the land heats up more rapidly that the sea.
Warm air over the land rises as it is less denser creating a region of low pressure at the surface and a higher pressure at the height it rises to,
Cool air which is more denser from over the sea moves to the lower pressure area over the land.
