Original artwork by Windows to the Universe staff (Randy Russell).
The top of the mesosphere is the coldest part of the atmosphere. It can get down to -90° C (-130° F) there! As you go higher in the mesosphere, the air gets colder.
The air is much thinner (less dense) in the mesosphere than in the stratosphere below. There are fewer air molecules to absorb incoming electromagnetic radiation from the Sun. That includes molecules of ozone, which absorb ultraviolet radiation and heat the stratosphere. In the mesosphere, the thin air and small amounts of ozone prevent the air from warming much.
Carbon dioxide in the mesosphere also helps make this layer cold. CO2 molecules absorb heat energy when they bounce off of other molecules. The CO2 releases some of that energy as photons in a process called radiative emission. Some of those photons travel upward into space. This carries heat away from the mesosphere.