Earth's Atmosphere

Air temperature does not just drop steadily the higher you go. It falls, then rises, then falls, then rises again, and each switch marks a new layer. Near the ground, in the troposphere, the sun heats Earth's surface and that heat radiates upward, so air gets colder as you climb away from the source, by about 6.5 degrees Celsius per kilometer. That ends at the tropopause, around 12 kilometers at mid-latitudes. Above it, in the stratosphere, ozone molecules absorb incoming ultraviolet light directly, so temperature rises with altitude even though you are farther from the ground. Higher still, the mesosphere has little ozone or surface heat to draw on, so it cools again. Finally the thermosphere, exposed to intense solar radiation with almost no air to spread the energy through, heats up sharply.

Each reversal is a clue to what is doing the heating in that layer: the ground below, ozone absorbing UV, or the thin outer air absorbing direct solar energy. Reading a temperature-versus-altitude graph for the atmosphere means tracking these slope changes, not just the numbers. A jet cruising at 11 kilometers, below a 12 kilometer tropopause, sits in the cooling troposphere; one cruising at 20 kilometers sits in the warming stratosphere.

This layering connects to other earth-space topics like the greenhouse effect and solar radiation, because both involve absorption of light by gases, just at different altitudes and wavelengths. Keeping the layers and their mechanisms straight prevents mixing up two separate atmospheric stories that happen to both involve absorbing radiation.