Air is a mixture of gases that forms the Earth's atmosphere. It surrounds the planet and is found everywhere, though invisible. The atmosphere has different layers - the troposphere nearest the surface, then the stratosphere containing the ozone layer, followed by the mesosphere and thermosphere. The outermost layer is the exosphere. Air is comprised primarily of nitrogen (78%) and oxygen (21%), but also contains smaller amounts of water vapor, carbon dioxide, and other trace gases.