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
Temperatures in the upper mesosphere fall as low as -100°C (-146°F or 173 K) , 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.