The Earth’s Atmosphere

The earth’s atmosphere is composed of distinct layers. The troposphere extends upward from the earth to a height of about 5 miles (8.1 kilometers) at the poles, to about 7 miles (11.3 kilometers) in mid-latitudes, and to about 10 miles (16.1 kilometers) at the equator. The air in the troposphere is in constant motion, with both horizontal and vertical air currents. Throughout the troposphere, temperature decreases with altitude. Above the troposphere is an atmospheric ozone layer, which is also the lower layer of the stratosphere. Temperature changes little with altitude in the stratosphere, which extends upward to about 30 miles (50 kilometers). Above this layer is the mesosphere, which extends to about 50 miles (80 kilometers) above the earth; the temperature sharply decreases at the base of the mesosphere before it begins to rise at the top of the mesosphere. The next layer is the thermosphere, which extends upward from the mesosphere to about 400 miles (640 kilometers); its temperature increases rapidly with altitude because of the absorption of shortwave radiation by ionization processes, although, because of the thinness of the air, little heat energy is available. The final layer is the exosphere, which gradually gets thinner as it reaches into the vacuum of space at around 435 miles (700 kilometers); the atmosphere is so attenuated at this altitude that the average distance air molecules travel without colliding is equal to the radius of the earth.