The stratosphere is the second layer of Earth's atmosphere, extending from the tropopause (approximately 12 km at mid-latitudes) to the stratopause at about 50 km altitude. Its most distinctive feature is the temperature inversion: while in the troposphere temperature decreases with altitude, in the stratosphere it increases from about -56 °C at its base to near 0 °C at its upper boundary, due to ultraviolet radiation absorption by the ozone layer. This temperature inversion makes the stratosphere extremely stable: warm air above cold air prevents vertical convection, so there are virtually no clouds or conventional weather phenomena. The only clouds that form here are nacreous clouds (polar stratospheric clouds), visible at 15–25 km altitude in polar regions during winter, and which are involved in catalytic ozone destruction. The stratosphere plays a crucial role in protecting terrestrial life by filtering 97–99% of harmful ultraviolet radiation. Long-haul commercial aircraft fly in the lower stratosphere (10–13 km) to take advantage of the absence of turbulence and the strong winds of the jet stream at the tropopause. Major volcanic eruptions can inject sulphate aerosols into the stratosphere, affecting global climate for months or years by reflecting solar radiation.