Atmospheric Structure and Global Circulation

Earth's atmosphere is organized into distinct layers defined by how temperature changes with altitude. The troposphere, extending from the surface to roughly 8-9 km at the poles and up to about 16-18 km at the equator, contains nearly 75-80% of atmospheric mass and is where almost all weather occurs, with temperature generally decreasing with height at an average lapse rate near 6.5°C per kilometer. Above it lies the stratosphere, reaching to about 50 km, where temperature rises with altitude because ozone (O3) absorbs incoming ultraviolet radiation, forming the protective ozone layer concentrated between roughly 15 and 35 km. Higher still, the mesosphere cools again to the coldest temperatures in the atmosphere before the thermosphere, where sparse molecules absorb high-energy solar radiation and temperatures climb steeply, and finally the exosphere fades gradually into space. By composition, dry air is about 78% nitrogen and 21% oxygen, with argon, carbon dioxide, and other trace gases making up the remainder; water vapor, concentrated almost entirely in the lower troposphere, varies widely and is the most important gas for weather processes.