Which is the coldest layer of the atmosphere?

Earth's atmosphere is divided into layers based on how temperature changes with height: the Troposphere (temperature falls with height), the Stratosphere (temperature rises because ozone absorbs solar UV radiation), the Mesosphere (temperature falls sharply and continuously with height), the Thermosphere (temperature rises sharply due to direct absorption of high-energy solar radiation), and the Exosphere. Each layer's temperature profile is governed by how it interacts with incoming solar and outgoing terrestrial radiation.

Among the layers listed in this question, temperature falls with height in both the Troposphere and the Mesosphere, but the fall continues much further in the Mesosphere, reaching the lowest value found anywhere in the atmosphere near the mesopause (roughly -85°C to -100°C) — far colder than the roughly -60°C reached at the top of the Troposphere. That is why the Mesosphere, not the Troposphere, is identified as the coldest layer of the atmosphere.

• Thermosphere: here temperature rises sharply — it can exceed 1500°C — because this layer directly absorbs high-energy X-ray and ultraviolet solar radiation, making it the hottest part of the atmosphere rather than the coldest.

• Troposphere: temperature does fall with height here, but only down to about -60°C at its upper boundary, the tropopause — a much smaller drop than what is seen higher up in the atmosphere.

• Ionosphere: this region is defined by its high concentration of ions and free electrons (useful for reflecting radio waves), not by a temperature boundary, and it overlaps parts of the Mesosphere and Thermosphere rather than forming a separate temperature-ranked layer.

Because it records the lowest temperatures found anywhere in the atmosphere, the Mesosphere is the coldest layer.