The troposphere is the lowest layer of Earth's atmosphere, extending from the surface to the tropopause at approximately 8–15 km altitude (lower at the poles, higher at the equator due to intense convection). It contains about 80 % of the atmosphere's mass and virtually all its water vapour, making it the layer where essentially all weather occurs — from gentle breezes to the most violent hurricanes and tornadoes.
The defining characteristic of the troposphere is that temperature decreases with altitude at an average rate of about 6.5 °C per km (the environmental lapse rate). This temperature structure makes the troposphere inherently convectively unstable — warm air at the surface can rise, creating the clouds and precipitation that define our weather. The tropopause — where temperature stops decreasing — acts as a ceiling: cumulonimbus clouds spread into anvils when they hit this boundary, and most weather systems are confined below it.
The troposphere is divided into several sub-layers of meteorological interest: the boundary layer (lowest 1–2 km, directly influenced by the surface — turbulence, friction, diurnal cycle, fog, urban heat islands), the free troposphere (above the boundary layer — where weather systems develop and the jet stream flows), and the tropopause transition zone. Within the troposphere, the composition is remarkably uniform (78 % N₂, 21 % O₂, 1 % Ar, 0.04 % CO₂) except for water vapour, which varies from near 0 % (cold, dry air) to 4 % (hot, humid tropical air) — this variability in water vapour is the engine of weather.