The mesosphere is the third layer of the atmosphere, situated directly above the stratosphere and directly below the thermosphere. Its lower boundary is usually located at altitudes from 50 to 65 km above sea level and the upper boundary is usually from 85 to 100 km, although the exact boundaries vary according to the latitude and the season (higher in the tropics and in winter and lower at the poles and in summer).
The temperature in the mesosphere decreases as the altitude increases, in contrast to the thermosphere, and its top is the coldest part of the atmosphere, at around minus 90 degrees centigrade. It is also the layer in which most meteors burn up. Because it is too high for airplanes and balloons and too low for satellites, the mesosphere remains the most poorly investigated part of the atmosphere.