Go up through the atmosphere and the air thins out. The temperature does something stranger: it falls, rises, falls and rises again. Those turns mark four layers.
| Layer | Height (about) | Temperature going up | What happens there |
|---|---|---|---|
| Troposphere | 0 to 12 km | Falls, about 6.5 °C per km | All weather; nearly all water vapor; most of the air's mass |
| Stratosphere | 12 to 50 km | Rises | The ozone layer absorbs ultraviolet sunlight |
| Mesosphere | 50 to 85 km | Falls to the coldest air on Earth, near −90 °C | Meteors burn up |
| Thermosphere | above 85 km | Rises, to hundreds of degrees or more | Auroras; the International Space Station orbits here |
Key idea
Temperature changes direction with height, marking four layers: troposphere (weather), stratosphere (ozone), mesosphere (coldest) and thermosphere (very thin and hot).
Why it works
The troposphere is warmed from below by the sunlit ground, so it gets colder as you go up. In the stratosphere, ozone absorbs the Sun's ultraviolet light, which heats that layer from within, so it gets warmer with height. Above the ozone the heating stops and the air cools again. At the top, the few molecules of the thermosphere absorb the most energetic sunlight and move very fast, which is what a high temperature means.
Worked example: A mountain hike
At the base of a mountain it is 20 °C. In the troposphere, air cools about 6.5 °C for each kilometer up. About how warm is it at the top, 2 km higher?
- Cooling: $2 \times 6.5 = 13$ °C.
- $20 - 13 = 7$ °C.
Answer: About 7 °C
Worked example: Which layer?
A meteor glows and burns up about 70 km above the ground. Which layer is it in?
- The mesosphere runs from about 50 to 85 km.
- 70 km is inside it.
Answer: The mesosphere
Common mistake
Expecting the air to get colder all the way up. It does in the troposphere, but ozone warms the stratosphere, and the thin thermosphere is very hot. Check: a temperature graph of the atmosphere zigzags; it is not a straight line.
"Hot" in the thermosphere does not mean you would feel warm there: the air is so thin that very few fast molecules would hit you, so you would lose heat and feel cold.