Earth's Atmosphere

Climb straight up from the ground and temperature does not just fall forever. In the troposphere, the bottom layer up to about 12 km, air is warmed from below by the sun-heated ground, so it cools as you rise away from that heat source. Above that, in the stratosphere up to about 50 km, temperature reverses and climbs with altitude. That is because the stratosphere holds the ozone layer, and ozone molecules absorb incoming ultraviolet radiation from the sun. Absorbing that UV energy heats the surrounding air, and since more UV gets absorbed higher up where it first hits concentrated ozone, the top of the stratosphere is warmer than its base.

Above the stratosphere sits the mesosphere, up to about 85 km, where there is no ozone left to absorb UV and little ground heat reaches that high, so temperature drops again, making it the coldest layer. Then comes the thermosphere, stretching to roughly 600 km, where temperature rises sharply because its thin gas directly absorbs high-energy solar radiation, including X-rays and extreme UV. Each reversal in the pattern traces back to a different energy source switching on or off.

Temperature in the thermosphere can read over 1000 degrees Celsius, but that number describes molecular speed, not something that could burn skin. The air there is so thin that molecules rarely collide with anything, so almost no heat actually transfers to an object floating in it. This layering connects to pressure and density, which both just decrease steadily with altitude as less air sits above you, no reversals, unlike the temperature zigzag.