The atmosphere supports life on Earth by protecting it from dangerous electromagnetic radiation, by creating and controlling weather and climate and by providing the gases that plants and animals need to breathe. The atmosphere is composed of the troposphere, the tropopause, the stratosphere, the mesosphere and the ionosphere.
The ozone layer, which lies in the upper stratosphere, is the planet's primary protector from electromagnetic radiation in the ultraviolet band of the spectrum. UV rays are invisible to the human eye and are harmful to life. By absorbing the UV radiation, the ozone layer keeps the bulk of rays from reaching Earth's surface. The ionosphere also helps to support life by absorbing energetic protons from the sun.
Weather and climate take place in the troposphere. The troposphere is also where the water cycle takes place. Water evaporates from the oceans and forms clouds. The clouds then release the water over land as rain, snow and other precipitation. Some of this precipitation is absorbed into the ground, where it provides water for all living things. The rest forms streams and rivers that flow into the ocean, and the cycle begins again.
Chemical reactions in the atmosphere yield the gases that make life possible. Oxygen in the troposphere allows human life, while carbon dioxide allows plant life to thrive.