layer of the Earth's atmosphere lying between the troposphere and the mesosphere. The lower portion of the stratosphere is nearly isothermal (a layer of constant temperature), whereas temperatures in its upper levels increase with altitude. The stratosphere extends from the tropopause at about 6 to 17 km (4 to 11 miles) altitude to its upper boundary (the stratopause) at about 50 km (30 miles), and it also contains the ozone layer. See atmosphere.