The atmosphere of Earth is the envelope of gases and suspended particles surrounding the planet, retained by gravity. Its lower portion, commonly called air, supplies gases essential to life and provides the setting for weather. The atmosphere also influences climate, absorbs much incoming high-energy solar radiation, and exchanges heat and moisture with the surface. It becomes progressively thinner with altitude rather than ending at a sharply defined boundary. (noaa.gov)
Composition
By volume, dry air near the surface contains approximately 78.08 percent nitrogen, 20.95 percent oxygen, and 0.93 percent argon. The remainder consists of trace gases, including carbon dioxide, neon, helium, methane, and nitrous oxide. “Dry air” excludes water vapor: atmospheric water in its gaseous state varies from almost none in very dry conditions to approximately 4 percent by volume in warm, humid tropical air. Consequently, the percentages of other gases in moist air are lower than their dry-air values. (noaa.gov)
Abundance alone does not determine a constituent’s importance. Nitrogen and oxygen dominate the mixture, but comparatively scarce gases strongly influence radiation and atmospheric chemistry. Water vapor, carbon dioxide, and methane are examples of greenhouse gases. Suspended solid particles and liquid droplets, collectively called aerosols, also alter the atmosphere’s interaction with radiation. Their sources include volcanic eruptions, wildfires, and human activities such as fuel combustion. (science.nasa.gov)
Pressure and vertical structure
Atmospheric pressure reflects the weight of the air above a location. Both pressure and air density generally decrease with altitude because progressively less air remains overhead. Standard sea-level pressure is 101,325 pascals, equivalent to 1,013.25 hectopascals. This is a reference value, not a constant observed everywhere: actual pressure varies with elevation and meteorological conditions. About half the atmosphere’s mass lies below an altitude of roughly 5.5 kilometers. (noaa.gov)
The atmosphere is conventionally divided into layers according to how temperature changes with height. Layer boundaries vary geographically and over time; the following altitudes are approximate. (noaa.gov)
- Troposphere: The lowest layer extends from the surface to roughly 6–20 kilometers, with a lower upper boundary near the poles and a higher one in the tropics. Temperature generally decreases upward. It contains most atmospheric mass and nearly all ordinary weather.
- Stratosphere: Extending to about 50 kilometers, this layer becomes warmer with height because ozone absorbs ultraviolet radiation. Its relatively stable stratification limits vertical mixing.
- Mesosphere: Between approximately 50 and 85 kilometers, temperature again decreases upward. Its upper boundary, the mesopause, is among the coldest parts of the atmosphere.
- Thermosphere: Above about 85 kilometers, absorption of energetic solar radiation produces increasing temperatures. High temperature here describes particle motion, not the heating capacity of dense air: the gas is extremely sparse.
- Exosphere: Beginning around 600 kilometers, this tenuous outer region gradually merges into space, and some particles escape. (noaa.gov)
The ionosphere is a different kind of division, defined by the presence of free electrons rather than by a temperature trend. Its electrical properties influence radio-wave propagation, and its structure changes with solar radiation and geomagnetic conditions. (ngdc.noaa.gov)
Circulation and moisture
Unequal heating by the Sun drives atmospheric motion. Tropical regions receive more solar heating than polar regions, and the atmosphere and oceans redistribute this energy. Rising warm air, sinking cooler air, and horizontal winds participate in this transfer. Earth’s rotation modifies moving air through the Coriolis effect, helping organize broad circulation patterns rather than allowing a simple equator-to-pole flow. (science.nasa.gov)
A simplified description recognizes Hadley, Ferrel, and polar circulation cells in each hemisphere. These patterns help explain the trade winds, midlatitude westerlies, and broad belts of rising or descending air. Narrow bands of strong upper-level winds, known as jet streams, are associated with this circulation and influence the movement of weather systems. Actual circulation is more variable than this idealized pattern. (prod-01-alb-www-noaa.woc.noaa.gov)
Atmospheric transport is also integral to the water cycle. Evaporation transfers water and energy from the surface into the air; condensation and precipitation return water to the surface. These processes connect atmospheric circulation with rainfall and the redistribution of heat. (science.nasa.gov)
Radiation and environmental change
The atmosphere helps determine Earth’s energy budget by absorbing, reflecting, and transmitting electromagnetic radiation. Clouds and particles reflect some incoming sunlight, while the surface and atmosphere absorb the rest. Earth releases energy as thermal infrared radiation. Greenhouse gases absorb and emit infrared radiation, including emission toward the surface, producing the natural greenhouse effect. (science.nasa.gov)
Different atmospheric constituents interact with different wavelengths. Stratospheric ozone absorbs ultraviolet radiation, whereas greenhouse gases strongly affect outgoing infrared radiation. Changes in gas concentrations and aerosol abundance can therefore alter the planet’s radiative balance. Human activities, particularly burning fossil fuels, increase greenhouse-gas concentrations and contribute to climate change. (noaa.gov)
Geological development
Earth’s atmosphere has changed substantially over geological time. Early conditions involved intense impacts and volcanic gases; the later atmosphere developed alongside the cooling surface and oceans. Its composition was subsequently transformed by biological processes rather than remaining a fixed product of planetary formation. (science.nasa.gov)
Oxygen-producing photosynthesis, particularly by cyanobacteria, was central to this transformation. Geological evidence places a major rise in atmospheric oxygen—the Great Oxidation Event—around 2.4 billion years ago, although oxygen production and localized oxygenated environments preceded widespread accumulation. The atmosphere remains connected to the carbon cycle, exchanging carbon with organisms, oceans, and rocks through biological activity and geological processes. (astrobiology.nasa.gov)