The Stratosphere

Above the troposphere, as shown in Figure 5, is the stratosphere. The stratosphere (STRAT uh sfeer) extends from an altitude of about 12 kilometers to about 50 kilometers. The temperature of the stratosphere remains nearly the same from the boundary with the troposphere to an altitude of about 20 kilometers. Above that height, temperature increases as altitude increases. The temperature in the lower stratosphere remains about −57°C. Above 20 kilometers, the temperature rises to about 0°C.

The upper stratosphere is warmer than the lower stratosphere because of the presence of the ozone layer, a region of high ozone concentration. Recall that ozone is a highly reactive gas whose molecules are composed of three oxygen atoms (O₃). Most of the oxygen molecules you breathe are composed of two oxygen atoms (O₂). In the stratosphere, the energy of sunlight is great enough to split O2 molecules into single oxygen atoms (O). When an oxygen atom (O) collides with a molecule of oxygen (O₂), ozone (O₃) is formed.

Ozone absorbs ultraviolet (UV) radiation in sunlight and filters much of it out before such radiation can reach Earth's surface. The energy absorbed from UV radiation is converted into thermal energy, warming the upper stratosphere.

Because UV radiation can be harmful to living things, the presence of the ozone layer is extremely important to life on Earth. In humans, UV radiation can cause a deadly form of skin cancer. Certain chemical pollutants have been depleting ozone in the stratosphere, permitting more UV radiation to reach Earth's surface. However, the release of such chemicals was limited by international agreements and national policies adopted in the 1990s. As a result, ozone levels in the stratosphere appear to have stabilized.

The Mesosphere

The layer above the stratosphere is the mesosphere. The mesosphere (MEZ oh sfeer) begins at an altitude of about 50 kilometers and extends to about 80 kilometers. In the mesosphere, temperature decreases as altitude increases. At the top of the mesosphere, the temperature approaches −90°C.

The air is very thin in the mesosphere. Studies in the 1990s discovered that air in this layer may move at speeds of more than 320 kilometers per hour. Most meteoroids that enter the atmosphere burn up in the mesosphere.

Figure 5 The atmosphere has four principal layers—the troposphere, the stratosphere, the mesosphere, and the thermosphere. The ionosphere overlaps the lower thermosphere. Interpreting Diagrams In which layer do auroras appear?

d

In which layer of the atmosphere do most meteroids burn up?

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