Atmosphere, spheroidal envelope of gas and vapor surrounding a planet, retained by gravity. The composition of the earth's atmosphere and most of its physical properties vary with altitude. About 75% of the total mass of the atmosphere and 90% of its water vapor are contained in the troposphere, the lowest zone, which extends from the earth's surface to an altitude of about 5 mi (8 km) at the poles and 10 mi (16 km) at the equator. The stratosphere, where the ozone layer filters out the sun's ultraviolet radiation, extends from the troposphere to about 30 mi (50 km); the mesosphere ranges from there to about 50 mi (80 km); the ionosphere, containing electrically charged particles that reflect radio signals, goes to about 400 mi (640 km); finally, the exosphere merges into the interplanetary medium. Overall, the atmosphere is about 78% nitrogen by volume. Other major components include oxygen (21%), argon (0.93%), and carbon dioxide (0.03%).