The mesosphere extends from the stratopause (about 50 km) to the mesopause (approximately 85 km), constituting the third layer of the atmosphere. It is the coldest layer, with temperatures dropping from 0 °C at its base to approximately -90 °C at the mesopause—the lowest temperatures anywhere in Earth's atmosphere. This cooling occurs due to the absence of ozone to absorb ultraviolet radiation and the low air density that prevents efficient heat retention. Notable phenomena occur in the mesosphere: meteors (shooting stars) disintegrate upon entering this layer due to friction with air molecules, generating the luminous trails visible from the surface. Noctilucent clouds (polar mesospheric clouds) also form here—the highest clouds on the planet at 76–85 km altitude, composed of tiny ice crystals visible during twilight at high latitudes. Their increasing frequency is considered a possible indicator of climate change. The mesosphere is the least directly studied layer because it is too high for weather balloons and radiosondes and too low for orbital satellites. It is primarily investigated using sounding rockets and scatter radars. Although it does not directly influence surface weather, mesospheric dynamics affect the propagation of atmospheric waves connecting the stratosphere with upper layers.