Atmospheric Structure and the Earth's Boundary Layer

Abstract

The earth's atmosphere is divided into four layers: the troposphere, stratosphere, mesosphere, and thermosphere (figure 4.1). These layers are defined by alternating decreases and increases in air temperature with height. The boundaries between the layers are called the tropopause, stratopause, and mesopause. The troposphere, the lowest layer of the atmosphere, supports life on the planet and is the layer in which “weather” occurs. It extends about 7 mi (11 km) above sea level and is characterized by a mean temperature decrease with height (—ΔT/ Δz) of about 3.5°F per 1000 ft, or 6.5°C per km. This decrease explains the lower temperatures encountered at higher elevations in the mountains. Although the mean temperature decreases with height in the troposphere, the atmospheric structure, particularly at the base of the troposphere, varies significantly over time as the earth warms during the day and cools at night, as the seasons change, and as weather systems move through the atmosphere. The vertical structure of the atmosphere is characterized by an exponential decrease in air density and pressure with height. Air density is the mass per unit volume of the atmosphere as expressed, for example, in kilograms per cubic meter or pounds per cubic foot. Air pressure is the force exerted on a unit area by the weight of the air molecules above the measurement point as expressed, for example, in millibars or pounds per square inch. Air pressure at any given level is thus a measurement of the weight of a column of air above that level. Although there is no "edge" to the earth's atmosphere, approximately 99.9% of the air molecules (and therefore the weight of the atmosphere) are found below 31 mi (50 km). Temperature, density, and pressure are interrelated, so that a change in one will result in changes in the other two. The mathematical description of this relationship is called the gas law (appendix A). The gas law allows any one of these variables to be calculated if values for the other two variables are known.