1. The Atmosphere The atmosphere is the layer of gases that surrounds the Earth. Earth’s atmosphere is a mixture of nitrogen, oxygen, water vapor, and many other gases.

2. The Atmosphere Air pressure is the force exerted by a column of air on a surface. As altitude increases, air pressure and density decrease. Scientists measure air pressure with an instrument called a barometer.

3. Layers of the Atmosphere The troposphere is the lowest layer of Earth’s atmosphere. The height of the troposphere ranges from 9 km above the poles to 16 km above the tropics. (average = 12 km) In the troposphere, temperature decreases as altitude increases.

4. Layers of the Atmosphere The stratosphere extends from an altitude of about 12 km to about 50 km. The temperature is about -57°C up to 20 km, then 0°C above 20 km. The warmer temperatures are caused by the presence of the ozone layer, a region of high ozone concentration.

5. Layers of the Atmosphere Ozone, O 3, absorbs ultraviolet (UV) radiation in sunlight and filters much of it out before the radiation can reach Earth’s surface. The radiation is converted into thermal energy, which warms the upper stratosphere.

6. Layers of the Atmosphere The mesosphere begins at an altitude of about 50 km and extends to about 80 km. In the mesosphere, temperature decreases as altitude increases. At the top of the mesosphere, the temperature approaches -90°C. Most meteoroids that enter the atmosphere burn up in the mesosphere.

7. Layers of the Atmosphere The outermost layer of the atmosphere, the thermosphere, begins at an altitude of about 80 kilometers and extends outward into space. In the thermosphere, temperature increases rapidly with altitude, from about -90°C to more than 1000°C.

8. Layers of the Atmosphere The ionosphere is not a distinct layer of the atmosphere. It is a region of charged particles, or ions, that overlaps the lower thermosphere. In the ionosphere, gas molecules become positively charged ions as they absorb solar radiation. An aurora is a colorful display of light in the sky, produced when charged particles from the sun are attracted to Earth’s magnetic poles.