Air is a mixture of gases enveloping Earth. It is mostly composed of nitrogen and oxygen—approximately 78 percent and 21 percent, respectively—along with several other gases found in much smaller quantities. They include argon, neon, helium, methane, krypton, hydrogen, nitrous oxide, and xenon. Air also contains several gases that are found in small, variable quantities. Among them are sulfur dioxide, ozone, carbon dioxide, nitrogen dioxide, and water vapor. The gases that are vital for life on Earth are oxygen, nitrogen, water vapor, and carbon dioxide, but the other gases are also important. Together, they form the atmosphere, which screens Earth from lethal radiation.
Although the atmosphere extends outward some 620 miles (1,000 km), it is really only the first few miles (normally measured in thousands of feet) that are significant, since three-quarters of the atmosphere’s weight lies within 60,000 ft. (18,000 m). It is well within this layer, usually from around 15,000 ft. (4,500 m) down to the surface, that weather patterns occur, because at these levels, there are varying amounts of water vapor, which condense into cloud formations. This layer is called the troposphere, and meteorological conditions within it are affected by changes in temperature, because the air gets progressively colder and thinner the higher it is. These changes in temperature also lead to changes in pressure, which cause large bodies of air to move from one location to another to cause wind. The point at which temperature decline ceases is the tropopause, and the layer above it is the stratosphere. Ozone found in the upper part of this region, which extends to about 35 miles (56 km), protects Earth from harmful ultraviolet radiation from the Sun.
The mesosphere separates the stratosphere from the ionosphere, which begins at about 34 miles (55 km). In the ionosphere, there is no protective ozone layer to block solar radiation so that the atmospheric molecules are bombarded with ultraviolet rays, becoming electrically charged, or ionized. It is this ionized layer that reflects radio waves down to Earth and enables long distance signals and communications to be bounced around Earth instead of escaping into space.
Origins
When Earth formed, around 4.5 billion years ago, it was an extremely hot volcanic planet devoid of life. As it began to cool, the gases and steam that escaped from the cooling rock were prevented from escaping into space by Earth’s gravity. Steam condensed to form the primordial oceans, and an atmosphere of nitrogen and carbon dioxide formed, providing the basis for life to develop.
This combination of gases enabled the development of the first simple forms of plant life around 3.5 billion years ago. Gradually, these plants caused the mixture of gases in the atmosphere to change as a result of photosynthesis. Photosynthesis is the manufacture of organic compounds (such as sugar and starch) from inorganic substances (like water and carbon dioxide) by living plant cells using the energy of sunlight absorbed by the plant pigment chlorophyll. One of the by-products of photosynthesis is oxygen, which plants then release into the atmosphere. With oxygen in the atmosphere, animal life, which uses oxygen as fuel, was able to develop and in turn produce carbon dioxide and nitrogen as waste for the plants to feed on, resulting in a state of equilibrium between these gases.
Today, however, the concentration of gases found in air is not only influenced by plants and animals. Automobile and airplane engines, industrial processes, and the large-scale clearance of oxygen-producing, nitrogen-consuming forests all have an effect on the ecological balance, putting an ever greater strain on green plants to keep the relative quantities of oxygen and carbon dioxide as well as nitrogen in the air constant.
