The air always has some water mixed with it. This is called humidity. When it cools, the water can change from a gas to a liquid through condensation. Then the water can fall from the sky as rain or snow. After the air rises, it gets colder and goes back towards the ground. Because the air lost its water before, it is dry when it comes back to the ground. When two air masses of different temperatures meet, this is called a warm front or cold front. The ways that the air moves around the Earth are called atmospheric circulation.