Earth's air reaches 1500 °C high up, yet would not feel hot
In the thermosphere the thin gas can hit about 1500 °C, but a person there would not feel warm, because molecules are so scattered that each drifts about a kilometre before meeting another. Our planet also leaks roughly 3 kg of hydrogen into space every second from its ragged outer fringe.
Dry air is about 78.08% nitrogen, 20.95% oxygen and 0.93% argon, with carbon dioxide near 0.04% and a scattering of trace gases. Water vapour is the wild card, ranging from around 10 parts per million in the coldest air to as much as 5% in hot, humid masses. One puzzle persists: xenon is far scarcer than the other noble gases, and ideas for where it went include hiding in the lower mantle or reacting with iron and nickel in the core.
The whole envelope weighs about 5.15×10^18 kg, and three quarters of that sits within roughly 11 km of the surface; below 30 km lies 99% of it. There is no sharp edge to space, so the Kármán line at 100 km serves as a convention. Up to about that height turbulence keeps the gases well mixed, and higher up lighter gases increasingly dominate.
Temperature, not pressure, marks the layers, because it swings up and down with height. The stratosphere, from roughly 12 km to about 50 km, holds the ozone layer. Above it the mesosphere is the coldest region on the planet, averaging around −85 °C; ice crystals there form noctilucent clouds, the highest of all, visible an hour or two after sunset. Most meteors burn up in this layer, which is too high for jets and balloons and too low for orbiting craft, so it is reached mainly by sounding rockets.
In the thermosphere, the green aurora glows from atomic oxygen between 120 and 400 km, and the International Space Station circles at 370 to 460 km. Beyond lies the exosphere, so tenuous that some scientists count it as interplanetary space; by some definitions it stretches about halfway to the Moon.
Source: Atmosphere of Earth
