The stratosphere is a layer of the earth's atmosphere located above the troposphere and below the mesosphere. As with the other atmospheric layers, it does not have a fixed height because the height of the tropopause directly below it is not constant. Its bottom can drop to as low as seven kilometers near the poles during winters, but may be as high as 20 kilometers near the equator, and its top extends to about 50 kilometers above the the earth's surface. This layer contains the ozone layer, which absorbs most of the deadly ultraviolet radiation from the sun, transforming it into heat and thereby protecting life below.