Of all environments, air is the simplest one and it occurs in a single phase gas. The relative quantities of various gases in air, by volume percentage are nitrogen 78%, oxygen 21 %, argon 0.9%, carbon dioxide 0.03%, hydrogen 0.01 % and other gases in trace amounts. In addition to various gases, dust and condensed vapour may also be found in air.
Various layers can be recognized in the atmosphere upto a height of about 1000km. The layer nearest to the earth is called as troposphere. In temperate regions, troposphere extends upto about 11 km whereas in tropics up to about 16km. This troposphere is characterized by a heavy load of microorganisms.The temperature of the atmosphere varies near the earth's surface. However, there is a steady decrease of about 1 DC per 150m until the top of the troposphere. Above the troposphere, the temperature starts to increase.
The atmosphere as a habitat is characterised by high light intensities, extreme temperature variations, low amount of organic matter and a scarcity of available water making it a non hospitable environment for microorganisms and generally unsuitable habitat for their growth. Nevertheless, substantial number of microbes are found in the lower regions of the atmosphere.