Though we live on the surface of the Earth, we actually live at the bottom of an ocean of air. Dynamic layers of air interact with the Earth's surface and the Sun's energy to produce the phenomenon of weather. The atmosphere is classified into layers based upon the characteristics each layer exhibits. Most weather occurs in the troposphere and most flying occurs in the troposphere and stratosphere. The atmosphere is comprised of air. Air is a mixture of gases. It is about 80 percent nitrogen and 20 percent oxygen. Water vapor and many other gases constitute the remainder of the gas mixture.