- the gas envelope surrounding Earth, held by gravity
- composition: 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% CO2, trace gases, water vapor
- layers: troposphere (weather), stratosphere (ozone), mesosphere, thermosphere, exosphere
- troposphere contains 75% of atmospheric mass and nearly all water vapor and weather
- the ozone layer in the stratosphere absorbs UV radiation, shielding life
- greenhouse gases (CO2, CH4, N2O, H2O) trap infrared radiation, maintaining habitable temperature
- driven by solar energy: differential heating creates wind, pressure systems, and climate zones
- medium for the water cycle (evaporation, cloud formation, precipitation)
- reservoir in the carbon cycle and nitrogen cycle
- the magnetic field protects the atmosphere from solar wind stripping
- thermodynamics of atmospheric convection drives all weather systems