ATMOSPHERIC STRUCTURE. The vertical distribution of temperature, pressure, density, and composition of the atmosphere constitutes atmospheric structure. These quantities also vary with season and location in latitude and longitude, as well as from night to day; however under the topic of atmospheric structure, the focus is on the average variations with height above sea level. Although it is impossible to define an absolute depth of the atmosphere, most of the atmosphere is confined to a narrow shell around the planet, with the pressure and density of air decreasing rapidly with altitude and gradually merging into the emptiness of space. Fifty percent of the mass of the atmosphere is within 5.5 kilometers (3.4 miles) of sea level; 90 percent is within about 16 kilometers (10 miles) of sea level, and 99.9 percent is below 49 kilometers (about 30 miles). Since the mean radius of the Earth is 6,370 kilometers (3,960 miles), the atmosphere is a very thin coating around our planet. At altitudes of 500 to 600 kilometers (about 350 miles) it is still possible to detect air, although the density of gases there is less than 10-12 (one trillionth) of that at sea level. Figure 1 displays the vertical temperature structure and the pressure distribution of the atmosphere. The names given to the various layers, defined based on the temperature change with height, and the boundaries between these layers are also shown. The heights, pressures, and temperatures in the diagram are based on the U.S. Standard Atmosphere, which represents average conditions above the middle latitudes. One might expect temperature to decrease steadily with height as the pressure decreases, since air cools as it expands into lower pressure regions, but this is not everywhere the case. Temperature cools with height throughout the troposphere, but itthen warms through the stratosphere, only to cool again through the mesosphere; finally it heats up in the thermosphere and exosphere. This distribution comes about through changing interactions among shortwave radiation from the Sun, longwave radiation from the Earth, and various gases in the air. Scientists made rapid progress in understanding the structure of the atmosphere starting at the beginning of the 20th century. In 1902, Léon Teisserenc de Bort discovered the constant temperatures with height in the lower stratosphere with the use of rapidly-ascending hydrogen balloons. The inference of high temperatures around 50 kilometers was first made in 1923 by Frederick A. Lindemann and Gordon M. B. Dobson using measurements of meteor trails; study of long-distance sound propagation from cannonades during World War I and controlled experiments afterward provided further confirmation of the high temperatures near the stratopause. Meteorological rockets later allowed examination of the temperature and composition of upper layers of the atmosphere otherwise unreachable by weather balloons. Remote sensing of the atmosphere with satellites began in the early 1960s and remains extremely important for measuring properties of the different layers of the atmosphere.Composition of the Atmosphere. Air is a mixture of a number of gases, but the most abundant are molecular nitrogen (N2) and molecular oxygen (O2), with a tiny amount of the inert gas argon (Ar). These gases make up more than 99.9 percent of the mass of dry air; the ratio of the number of molecules of each is nearly constant up to a height of about 80 or 90 kilometers (about 60 miles). Other gases, whose relative concentrations vary, exist only in small quantities. The most important of these are water vapor (H2O) and carbon dioxide (CO2), which absorb and emit longwave radiation, and ozone (O3), which Dargan M. W. Frierson! 4/13/09 2:27 PMComment: There is are some typos in this figure: “Mesospause” should read “Mesopause” in both the upper panel and the lower panel. Also, “29.2 inches” should read “29.92 inches”absorbs ultraviolet radiation from the Sun as well as some longwave radiation from the Earth. The distribution of these “trace” gases therefore affects the vertical temperature distribution. [See Longwave Radiation; Trace Gases.] In addition to the layers of the atmosphere shown in Figure 1, which are defined based on temperature, there are also atmospheric layers defined based on the composition of air. The region below 80 to 90 kilometers is called the homosphere because the main constituents of air are homogeneously distributed regardless of weight. Above this level, molecules and atoms tend to separate, with the heavier gases beneath the lighter ones, in a layer called the heterosphere. Above about 64 kilometers (40 miles), in the upper homosphere and through the heterosphere, gases can be readily ionized by very shortwave radiation; that is, they can lose an electron from their atoms. Thus free electrons and ions are plentiful, giving this region the name ionosphere. The ionosphere is very important in long-range radio transmission. [See Ionosphere; Isotopes; Radiation; Shortwave Radiation.] Ultraviolet radiation from the Sun interacts with oxygen molecules in the stratosphere to form ozone (O3). The peak ozone concentration is found at about 25 kilometers (15.5 miles). Although the total amount is very small—only about 10 molecules of O3 per million molecules of air at highest concentrations—ozone blocks much ultraviolet radiation that would otherwise damage living things. [See Ozone.] The amount of water vapor in the air is quite variable and drops off rapidly with height. At peak concentrations near the surface, water vapor can make up around 4 percent of the mass of air, but in total water vapor makes up only around 0.33 percent of the mass of the atmosphere. At altitudes of about 10 kilometers the concentration ofwater vapor is only about 1 percent of that at the ground, and higher levels have much less. This occurs primarily because very little water vapor is needed to saturate cold air, and excess water will precipitate as rain or snow. Unlike water vapor and ozone, carbon dioxide is fairly well mixed in the air. Despite the low concentration of CO2 (of 1 million molecules of air near the surface, only about 400 are CO2) it is vital for photosynthesis as well as to maintaining the radiation balance. Water vapor and carbon dioxide both absorb little of the Sun’s radiation, but they do absorb some of the Earth’s radiation and reradiate it both upward and downward. This