THE ATMOSPHERE COMPRISES the thin envelope of gases held by the Earth's gravitational force. Extending for several hundred miles above the Earth's surface with no clear boundary, the atmosphere is often sub divided into vertical layers according to a distinctive physical properties. These properties include thermal characteristics, chemical composition, electrical attributes, or density. The vertical structure of the atmosphere is defined by changes in these physical properties, with each change determining a new layer.
The density of the atmosphere refers to the mass of atoms and molecules per unit volume of air. As with all gases, air molecules are not in a fixed, rigid arrangement, and can move randomly through space. No upper threshold to the mass contained per unit volume exists, and, as such, air density can fluctuate. The average density of air at sea level at a temperature of 68 degrees F (or 20 degrees C) is 1.2 kg. per cu. m. (or approximately 1.2 oz. per cu. ft.), decreasing rapidly at lower heights and more gradually with increasing altitude.
This nonlinear relationship between air density and altitude results from another common property of gases, compressibility: the atmosphere expands (and contracts) with decreasing (or increasing) pressure. Atmospheric pressure results from the force exerted by the mass of air molecules subjected to gravitational acceleration over a surface; in other words, the weight of the air above.
The average sea-level pressure of the Earth is 1011 hectoPascals (hPa), but can have considerable local variability. Because the atmosphere is compressible, air molecules are more compact closer to the surface, thereby increasing the density and pressure of the air at lower altitudes. As with air density, pressure decreases at a decreasing rate with increasing height. Approximately one-half of the atmosphere lies below a height of 3.5 mi. (5.5 km.), corresponding to a pressure of 500 hPa. Nearly 90 percent of the atmosphere lies below 10 mi. (16 km.) and a pressure of 100 hPa.
In comparison to density and pressure, the structure of the atmosphere according to thermal characteristics is more complex. Interactions among various atmospheric gases and radiant energy from the Sun and Earth result in distinct variations in the vertical temperature profile. Based on these thermal changes, the atmosphere is commonly subdivided into four major layers known as the troposphere, stratosphere, mesosphere, and thermosphere.
The troposphere encompasses the lowest portion of the atmosphere, from the surface to an average height of 7.5 mi. (12 km.). The temperature of the troposphere is primarily influenced by the radiant energy exchanges from the underlying surface. The further the air molecules are away from the surface, the less the atmosphere will warm. Tropospheric temperature decreases with height at an average rate of 3.5 degrees F per 1,000 ft. (6.5 degrees C per km.), a measurement termed the normal lapse rate. The top of the troposphere, known as the tropopause, has an isothermal layer, a zero lapse-rate region, where temperature does not change with altitude. The tropopause occurs at heights ranging from 5.6-9.9 mi. (9-16 km.), depending on latitude and season.
Beyond the tropopause lies the stratosphere. In this region, the isothermal layer gives way to a temperature increase with height or temperature inversion. This temperature increase occurs because of a localized concentration of ozone molecules (O3) found 9-19 mi. (15 to 30 km.) above the Earth's surface. These molecules absorb ultraviolet solar radiation that warms the stratosphere.
Stratospheric warming terminates around 30 mi. (50 km.) at the stratopause, which marks the transition zone between the stratosphere and the much colder mesosphere. In the mesosphere, temperatures decrease rapidly with height. This layer contains the lowest temperatures of the atmosphere, averaging minus 130 degrees F (minus 90 degrees C) at 50 mi. (80 km.) above the surface. These lows are reached at the mesopause, the boundary between the mesosphere and thermosphere.
The thermosphere represents the outer layer of the Earth's atmosphere. Temperatures in this layer increase substantially with height from the absorption of solar energy by relatively few oxygen molecules (O2).
While temperatures can register to values more than 1,800 degrees F (1,000 degrees C), the thermosphere holds an insignificant amount of heat due to the extremely low density of atmospheric gases. The thermosphere (and the upper mesosphere) also contains a large concentration of ions and free electrons in a sub-layer known as the ionosphere. Between 50-250 mi. (80-400 km.), these ionized molecules interact with charged particles from the sun to produce brilliant light displays at high latitudes known as the aurora borealis (northern lights) or aurora aus-tralis (southern lights), in the Northern and Southern Hemispheres, respectively.
SEE ALSO: Atmospheric Composition; Aurora; Mesosphere; Stratopause; Stratosphere; Thermosphere; Tropopause; Troposphere.
BIBLIOGRAPHY. Edward Aguado and J.E. Burt, Understanding Weather and Climate (2006, Prentice Hall); C.D. Ahrens, Meteorology Today (Thomson Brooks/Cole, 2007).
Jill S.M. Coleman Ball State University
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