Earth's atmosphere is about 78 percent nitrogen, 21 percent oxygen, with trace amounts of water, argon, carbon dioxide and other gases. The exact composition, as well as the temperature and pressure of the atmosphere, varies with its height. The atmosphere is divided into layers: The troposphere is the lowest layer and extends from Earth's surface up to a height of about 30 miles (48 kilometers); it is where most of Earth's weather occurs. Above the troposphere is the stratosphere, where the ultraviolet-blocking ozone layer is found. Above that is the mesosphere, the thermosphere and the ionosphere. Earth's atmosphere is home to different types of clouds, the auroras, different types of lightning.