The Atmosphere of Earth
The atmosphere that surrounds Earth is essential to life. The lower portion of the atmosphere contains the oxygen required by all land animals and many land plants. The atmosphere acts as an insulator that prevents extreme temperature variations between day and night, such as those that occur on the moon. The atmosphere also absorbs some radiation, such as most ultraviolet rays, that would be harmful to living organisms if it reached Earth's surface. Earth's atmosphere is made up of a series of layers, each of which has a different set of characteristics.
The Troposphere ("sphere of change") is the atmospheric layer closest to Earth. It extends from Earth's surface to an average altitude of about seven miles (11 km). The upper limit of the troposphere is lower at the poles and higher near the Equator. About 75 per cent of the total mass of the atmosphere is within the troposphere. Most weather changes occur within this layer. In the upper troposphere are high-speed currents of wind called jet streams. The temperature of the air in the troposphere decreases as the altitude increases. The tropopause is the upper boundary of the troposphere.
The Stratosphere extends from the tropopause to an altitude of about 30 miles (48 km) above Earth's surface. The stratosphere contains the ozone layer, which extends from about 9 to 19 miles (15 to 30 km). Ozone is a form of oxygen in which each molecule contains three atoms of oxygen instead of the two atoms found in ordinary oxygen. The ozone layer absorbs most of the ultraviolet radiation from the sun and prevents it from reaching Earth's surface. In the stratosphere the temperature increases slowly with increasing altitude until the bottom of the ozone layer is reached; the temperature then increases rapidly through the upper stratosphere.
The air within the stratosphere is dry. Clouds occur only rarely. High-speed aircraft frequently fly in the lower stratosphere because of the lack of clouds and storms and because air resistance is low. Planes that fly in the stratosphere must have pressurized cabins and a supply of oxygen because the air is too thin for breathing. The upper boundary of the stratosphere is the stratopause.
The Mesosphere extends from the stratopause to an altitude of about 50 miles (80 km). Temperatures in this layer decrease with altitude up to the mesopause, which is the upper boundary of the mesosphere.
The Thermosphere extends from the mesopause to about 300 miles (480 km) above Earth. Temperatures in the thermosphere increase as the altitude increases. The thermosphere contains a high concentration of electrically charged particles consisting of atoms and molecules that have lost one or more electrons. These particles, called ions, are produced by ultraviolet radiation, X rays, and cosmic rays. The thermosphere, together with the upper part of the mesosphere, which also contains a high concentration of ions, is often called the ionosphere. Various levels of the ionosphere reflect radio waves of certain frequencies. The luminous phenomena called auroras also occur within the ionosphere.
The Magnetosphere, or Exosphere, begins at an altitude of about 300 miles (480 km). Cosmic rays and other high-energy particles from space are trapped within the magnetosphere by magnetic forces resulting from Earth's magnetism. These trapped particles occur in greatest concentration as the Van Allen Radiation Belts. The magnetosphere has the general shape of an elongated teardrop, extending far into space in the direction away from the sun.