The mesosphere (from the Greek words mesos = middle and sphaira = ball) is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. The mesosphere is located from about 50 km to 80-90 km altitude above Earth's surface. Within this layer, temperature decreases with increasing altitude due to decreasing solar heating and increasing cooling by CO₂ radiative emission. The minimum in temperature at the top of the mesosphere is called the mesopause, and is the coldest place in the atmosphere. ^[1] The main dynamical features in this region are atmospheric tides, internal atmospheric gravity waves (usually just called "gravity waves") and planetary waves. Most of these waves and tides are excited in the troposphere and lower stratosphere and propagate upward to the mesosphere. In the mesosphere, gravity-wave amplitudes can become so large that the waves become unstable and dissipate. This dissipation deposits momentum into the mesosphere and largely drives its global circulation.

Atmosphere diagram showing the mesosphere and other layers. The layers are not to scale.

Because it lies between the maximum altitude for aircraft and the minimum altitude for orbital spacecraft, this region of the atmosphere has only been accessed through the use of sounding rockets. As a result, it is the most poorly understood part of the atmosphere. This has led the mesosphere and the lowest thermosphere to be disparagingly referred to by scientists as the ignorosphere [1] [2].

Temperatures in the upper mesosphere fall as low as -100°C (-146°F or 173 K) [3], varying according to latitude and season. Millions of meteors burn up daily in the mesosphere as a result of collisions with the gas particles contained there; this creates enough heat to vaporize almost all of the falling objects long before they reach the ground, resulting in a high concentration of iron and other metal atoms there.

The stratosphere and mesosphere are referred to as the middle atmosphere. The mesopause, at an altitude of 80-90 km, separates the mesosphere from the thermosphere—the second-outermost layer of the Earth's atmosphere. This is also around the same altitude as the turbopause, below which different chemical species are well mixed due to turbulent eddies. Above this level the atmosphere becomes non-uniform; the scale heights of different chemical species differ by their molecular weights.

Noctilucent clouds are located in the mesosphere.

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. The ionization is so weak that when night falls, and the source of ionization is removed, the free electron and ion form back into a nuetral molecule.