Atmospheric Vertical Structure

THE ATMOSPHERE COMPRISES the thin envelope of gases held by the Earth's gravitational force. Extending for several hundred miles above the Earths 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 eu. 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, mésosphère, and thermosphère.

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 [Page 95]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.

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