atmosphère

An atmosphere is the envelope of gases that surrounds a planetary body and is retained by the body's gravity. On Earth, it extends from the surface into space and is structured into layers known as the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The atmosphere’s primary roles are to provide the air for living beings to breathe, regulate surface temperatures, protect the surface from meteoroids and ultraviolet radiation, and drive weather and climate through the movement of air and moisture.

Earth’s atmosphere is composed mainly of nitrogen (about 78%) and oxygen (about 21%), with argon, carbon dioxide, neon, helium, methane, and other gases making up the remaining 1%. The exact composition and the thickness of each layer vary with altitude, location, and season. Atmospheric pressure at sea level averages about 1013 hPa and decreases with height.

Temperature changes with altitude are a key feature of the atmospheric structure: the troposphere generally cools with height, the stratosphere warms due to ozone absorption of ultraviolet radiation, the mesosphere cools again, and the thermosphere shows rising temperatures with altitude. The atmosphere is an active system influenced by solar energy, cloud formation, and human emissions.

The greenhouse effect, driven by water vapor and other greenhouse gases, traps heat and helps keep Earth's surface warmer than it would be otherwise. Human activities are altering the atmosphere’s composition, impacting climate, air quality, and the planet’s radiative balance.

Ever since observations and missions began, atmospheres around other planets and moons have been studied to understand their compositions and pressures, revealing a wide diversity beyond Earth.