The atmosphere produces winds and storms which try to restore the balance. The atmosphere is the most dense near the surface because of gravity. As one goes higher, the air gets thinner and thinner. At an altitude of about ten kilometers, the atmosphere is only half as thick as at the surface. The last important property of the atmosphere is humidity.
Humidity is a measure of how much water is in the atmosphere.
When water evaporates from lakes and oceans, it turns into a gas and becomes part of the atmosphere. It is this water that sometimes condenses into clouds and becomes rain.