The mesosphere is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature increases with height, but the average temperature decreases as altitude increases. This characteristic is used to define limits,: it begins at the top of the stratosphere, and ends at the mesopause, which is the coldest part of Earth's atmosphere, with temperatures below −143 °C. The exact upper and lower boundaries of the mesosphere vary with latitude and with season, but the lower boundary is usually located at altitudes from 47 to 51 km above sea level, and the upper boundary is usually from 85 to 100 km.