(4) The stratosphere is the second major layer of Earth’s atmosphere, just above the troposphere, and below the mesosphere. About 20% of the atmosphere’s mass is contained in the stratosphere. The stratosphere is stratified in temperature, with warmer layers higher and cooler layers closer to the Earth. The increase of temperature with altitude is a result of the absorption of the Sun’s ultraviolet radiation by ozone. Commercial airliners typically cruise at altitudes of 9–12 km(30,000–39,000 ft) which is in the lower reaches of the stratosphere in temperate latitudes. it allows the airliner to fly faster for the same amount of drag. It also allows them to stay above the turbulent weather of the troposphere. Because the temperature in the tropopause and lower stratosphere is largely constant with increasing altitude, very little convection and its resultant turbulence occurs there.