The mesosphere (/'m?so?sf??r/; from Greek mesos "middle" and sphaira "sphere") is the layer of the Earths atmosphere that is directly above the stratosphere and directly below the mesopause. In the mesosphere, temperature decreases as the altitude increases. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring place on Earth with temperatures below -143 °C (-225 °F; 130 K). The exact upper and lower boundaries of the mesosphere vary with latitude and with season, but the lower boundary of the mesosphere is usually located at heights of about 50 kilometres (160,000 ft; 31 mi) above the Earths surface and the mesopause is usually at heights near 100 kilometres (62 mi), except at middle and high latitudes in summer where it descends to heights of about 85 kilometres (53 mi; 279,000 ft). The stratosphere, mesosphere and lowest part of the thermosphere are collectively referred to as the "middle atmosphere", which spans heights from approximately 10 kilometres (33,000 ft; 6.2 mi) to 100 kilometres (62 mi; 330,000 ft). The mesopause, at an altitude of 80–90 km (50–56 mi), separates the mesosphere from the thermosphere—the second-outermost layer of the Earths 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 masses. Within the mesosphere, temperature decreases with increasing height, due to decreasing solar heating and increasing cooling by CO2 radiative emission. The top of the mesosphere, called the mesopause, is the coldest part of Earths atmosphere. Temperatures in the upper mesosphere fall as low as -101 °C (172 K; -150 °F), varying according to latitude and season.