Air

Air is the mixture of gases that envelopes Earth, forming the atmosphere. The major constituents of dry air are nitrogen (about 78%) and oxygen (about 21%), with smaller amounts of argon, carbon dioxide, neon, helium, methane, krypton, hydrogen, and others. Water vapor is variable and can reach several percent of the volume depending on location and conditions. Trace gases and aerosols also influence air quality and climate.

Air is colorless, odorless, and tasteless at standard conditions. It behaves as a compressible gas: its density decreases with height and increases with pressure and decreasing temperature. At sea level, dry air has a density around 1.225 kilograms per cubic meter at 15°C; humidity lowers density slightly because water vapor is lighter than nitrogen and oxygen.

Air supports life and most combustion processes. Animals obtain oxygen by respiration, while plants exchange carbon dioxide and oxygen during photosynthesis. Air also transports sound and serves as the medium for weather, climate, and many industrial processes. The atmosphere is layered: the troposphere contains weather and decreases in temperature with height; above lie the stratosphere, mesosphere, thermosphere, and exosphere, with pressure and density thinning rapidly.

Wind results from horizontal differences in air pressure driven by solar heating and Earth’s rotation. Human activities alter the composition of air, increasing pollutants and greenhouse gases such as CO2, methane, and nitrous oxide, which influence climate and air quality.