The atmosphere is a layer of gas surrounding the Earth which extends to a height of several hundred kilometers. On the basis of composition, it can be divided into three main regions, the homosphere, the heterosphere, and the exosphere. Figure 1 shows these regions and the temperature profile through the atmosphere. The vertical structure of the atmosphere is discussed in more detail by Wallace and Hobbs (1977).
The homosphere is the lowest layer and accounts for virtually all (i.e., 99.999%) the mass of the atmosphere. This layer comprises approximately 78% Nitrogen, 21% Oxygen, and 1% Argon, with a variable amount of water vapor. Many other gases are found in small quantities, the most important of which is carbon dioxide (~0.03%). In the heterosphere, the photodecomposition leads to a high concentration of atomic oxygen, which is the dominant constituent over much of this layer. The exosphere is composed mainly of the light elements, helium and hydrogen.
The homosphere can be subdivided into three strata, the troposphere , the stratosphere, and the mesosphere. The troposphere is separated from the stratosphere by the tropopause, which is a transition from an unstable to a stably stratified layer. Very little mixing between the two strata takes place. Most of the atmospheric ozone is found in the stratosphere; this allotrope is important in that it shields the Earth against UV radiation. However, the ozone is currently being depleted through the effects of freons and other pollutants which have been released into the atmosphere.
The synoptic-scale (i.e., ~1000 km) circulation of the atmosphere is governed largely by the Earth’s rotation (Coriolis Effect) and this leads to cyclonic and anticyclonic weather systems where the fluid velocity is perpendicular to the pressure gradient. Thermal circulations and centrifugal forces are important in the small-scale features of the atmospheric motions. The dynamics of the atmosphere are discussed at length by Holton (1979). It is worth mentioning that the popular belief that the direction of rotation of water flowing down a plug hole is determined by the rotation of the Earth is a myth.
A discussion of the heat balance of the atmosphere can be found in Wallace and Hobbs (1979). The Albedo of the earth is heavily dependent on the amount of water vapor and droplets in the atmosphere, but on a global average, 30% of the incident solar radiation is reflected (Figure 2). The outgoing solar radiation is mainly in the infrared part of the spectrum, and some of this is absorbed by carbon dioxide and other gases. Over the last few decades, there has been great concern that increases in atmospheric carbon dioxide, methane and other gases which absorb infrared could upset the radiation balance and so bring about an overall warming.
Further information about the atmosphere is given in the article Atmosphere (Physical Properties of).
Holton, J. R. (1979) An Introduction to Dynamic Meteorology. Academic Press, New York.
Wallace, J. M. and Hobbs, P. V. (1977) Atmospheric Science ѕ An Introductory Survey, Academic Press Inc, Orlando, Florida.
- Holton, J. R. (1979) An Introduction to Dynamic Meteorology. Academic Press, New York.
- Wallace, J. M. and Hobbs, P. V. (1977) Atmospheric Science Ñ• An Introductory Survey, Academic Press Inc, Orlando, Florida.
Heat & Mass Transfer, and Fluids Engineering