The Earth's atmosphere (or air) is a layer of gases surrounding the planet Earth that is retained by the Earth's gravity. Dry air contains roughly (by volume) 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, and trace amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night.

There is no definite boundary between the atmosphere and outer space. It slowly becomes thinner and fades into space. An altitude of marks the boundary where atmospheric effects become noticeable during reentry. The Kármán line, at , is also frequently regarded as the boundary between atmosphere and outer space. Three quarters of the atmosphere's mass is within of the surface.

Temperature and layers

The temperature of the Earth's atmosphere varies with altitude; the mathematical relationship between temperature and altitude varies among five different atmospheric layers (ordered highest to lowest, the ionosphere is part of the thermosphere):

Exosphere: From up to , contain free-moving particles that may migrate into and out of the magnetosphere or the solar wind.

Exobase: Also known as the 'critical level', it is the lower boundary of the exosphere.

Ionosphere: The part of the atmosphere that is ionized by solar radiation stretches from and typically overlaps both the exosphere and the thermosphere. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. Because of its charged particles, it has practical importance because it influences, for example, radio propagation on the Earth. It is responsible for auroras.

Thermopause: The boundary above the thermosphere, it varies in height from .

Thermosphere: From to over , temperature increasing with height. The temperature of this layer can rise to . The International Space Station orbits in this layer, between .

Mesopause: The temperature minimum at the boundary between the thermosphere and the mesosphere. It is the coldest place on Earth, with a temperature of .

Mesosphere: From the Greek word "μέσος" meaning middle. The mesosphere extends from about to the range of . Temperature decreases with height, reaching in the upper mesosphere. This is also where most meteors burn up when entering the atmosphere.

Stratopause: The boundary between the mesosphere and the stratosphere, typically . The pressure here is 1/1000th sea level.

Stratosphere: From the Latin word "stratus" meaning spreading out. The stratosphere extends from the troposphere's range to about . Temperature increases with height. The stratosphere contains the ozone layer, the part of the Earth's atmosphere which contains relatively high concentrations of ozone. "Relatively high" means a few parts per million—much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from approximately above Earth's surface, though the thickness varies seasonally and geographically.

Ozone Layer: Though part of the Stratosphere, the ozone layer is considered as a layer of the Earth's atmosphere in itself because its physical and chemical composition is far different from the Stratosphere. Ozone (O₃) in the Earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O₂), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O₂ to create O₃. O₃ is unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O₂ and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle. This occurs in the ozone layer, the region from about above Earth's surface. About 90% of the ozone in our atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about , where they range from about 2 to 8 parts per million.

Tropopause: The boundary between the stratosphere and troposphere.

Troposphere: From the Greek word "τρέπω" meaning to turn or change. The troposphere is the lowest layer of the atmosphere; it begins at the surface and extends to between at the poles and at the equator, with some variation due to weather factors. The troposphere has a great deal of vertical mixing because of solar heating at the area. This heating makes air masses less dense so they rise. When an air mass rises, the pressure upon it decreases so it expands, doing work against the opposing pressure of the surrounding air. To do work is to expend energy, so the temperature of the air mass decreases. As the temperature decreases, water vapor in the air mass may condense or solidify, releasing latent heat that further uplifts the air mass. This process determines the maximum rate of decline of temperature with height, called the adiabatic lapse rate. The troposphere contains roughly 80% of the total mass of the atmosphere. Fifty percent of the total mass of the atmosphere is located in the lower of the troposphere.