Atmospheric temperature always decreases as altitude increases.

Near Earth’s surface, going higher usually means getting colder. Mountains are colder than nearby lowlands, and nineteenth-century balloon measurements found the same pattern through the lower atmosphere.

It was natural to assume that the cooling simply continued upward.

Around 1902, Léon Teisserenc de Bort in France and Richard Assmann in Germany independently sent instrumented balloons high enough to test that assumption. At roughly 10–15 kilometres above the surface, the steady cooling stopped.

That boundary separates the troposphere, the lowest atmospheric layer where most weather occurs, from the stratosphere above it. Through much of the stratosphere, temperature rises with altitude instead of falling. Ozone absorbs ultraviolet radiation from the Sun and converts some of that energy into heat.

Higher still, the direction changes again: temperature falls through the mesosphere and rises through the thermosphere.

The atmosphere is therefore divided into layers partly by these temperature changes. Cooling with height remains a useful rule in the troposphere, but it is not a rule for the atmosphere as a whole.