Ask an Explainer

Air is made up of molecules, that bump into each other occasionally. When they do, there is an exchange of energy. When in a big open space, they don't hit each other very often. Up in the higher atmosphere, where the air is thin, they barely do, so it's very cold. But when you compress air, the molecules are forced into closer quarters.  They colide and interact with each other more and more, increasing the heat!