Temperature inversion at every boundary
The troposphere descends from 18 km at the equator to 8 km at the poles, and within it temperature drops steadily. This is where weather lives. At the tropopause, the trend flips: the stratosphere above warms as you climb, an inversion caused by ozone absorption of ultraviolet radiation. This creates a thermal ceiling that traps rising air and defines weather's upper limit.
The mesosphere above the stratosphere cools sharply again, reaching the coldest temperatures in the entire atmosphere near 90 km altitude. Meteors burn up here. Above that, the thermosphere reverses once more, heating to over 1000 K as solar radiation ionizes atoms directly. Each boundary represents a fundamental shift in how energy moves through the atmosphere.
Density and pressure gradients
Atmospheric density falls exponentially with altitude. Half the atmosphere's mass sits below 5.5 km, where the air is thickest and weather fiercest. This concentrated mass creates the pressure we feel at sea level. Climb high enough and pressure drops so low that humans cannot survive without supplemental oxygen, yet the atmosphere continues thinly for hundreds of kilometers until it merges with the vacuum of space.
Each layer supports different phenomena: the troposphere drives weather and convection, the stratosphere protects us with ozone, the mesosphere hosts noctilucent clouds and meteor ablation, and the thermosphere supports auroras and satellite orbits. The boundaries between them are not arbitrary lines but physical transitions in how the air behaves.