A thin envelope of gases surrounds Earth and makes life possible. That envelope is the atmosphere. Roughly 78 percent nitrogen and 21 percent oxygen, it also holds water vapor, carbon dioxide, and trace gases that control temperature and weather.

The atmosphere thins rapidly with height. Most weather occurs in the lowest layer, the troposphere. Above it the stratosphere holds the ozone layer that shields the surface from ultraviolet radiation. Higher still lie the mesosphere and thermosphere, where temperatures and densities change dramatically.

Without the atmosphere, daytime temperatures would soar and nights would plunge far below freezing. The gases trap outgoing heat and distribute energy around the globe through winds and storms.

• Mixture of nitrogen, oxygen, and trace gases
• Layered structure from troposphere upward
• Enables weather and protects from radiation
• Supports the planet’s energy balance