Within the mesosphere, temperature decreases with increasing altitude. The top of the mesosphere, called the mesopause, is the coldest place on Earth.
This is due to decreasing solar heating and increasing cooling by CO2 radiative emission.
Temperatures in the upper mesosphere fall as low as -100 °C
The mesosphere is also the region of the ionosphere known as the D layer. The D layer is only present during the day, when some ionization occurs with nitric oxide being ionized by Lyman series-alpha hydrogen radiation.