AWH 4-3

Chapter 4, The Earth’s Atmosphere 4-3 4.2.1 Air Parcel An air parcel is an imaginary volume of air to which any or all of the basic properties of atmospheric air may be assigned. A parcel is large enough to contain a very large number of molecules, but small enough so that the properties assigned to it are approximately uniform. It is not given precise numerical definition, but a cubic centimeter of air might fit well into most contexts where air parcels are discussed. In meteorology, an air parcel is used as a tool to describe certain atmospheric processes, and air parcels will be referred to in this handbook. 4.3 Vertical Structure The Earth’s atmosphere is subdivided into five concentric layers (see Figure 4-1) based on the vertical profile of average air temperature changes, chemical composition, movement, and density. Each of the five layers is topped by a pause, where the maximum changes in thermal characteristics, chemical composition, movement, and density occur. 4.3.1 Troposphere The troposphere begins at the Earth’s surface and extends up to about 11 kilometers (km) (36,000 ft) high. This is where people live. As the gases in this layer decrease with height, the air becomes thinner. Therefore, the temperature in the troposphere also decreases with height. Climbing higher, the temperature drops from about 15 degrees Celsius (°C) [59 degrees Fahrenheit (°F)] to -56.5 °C (-70 °F). Almost all weather occurs in this region. The vertical depth of the troposphere varies due to temperature variations that are closely associated with latitude and season. It decreases from the Equator to the poles and is higher during summer than in winter. At the Equator, it is around 18–20 km [11–12 miles (mi)] high; at 50° N and 50° S latitude, 9 km (5.6 mi) high; and at the poles, 6 km (3.7 mi) high. The lowest portion of the troposphere is known as the planetary boundary layer. The height of the boundary layer varies depending on terrain and time of day and is directly affected by surface heating and cooling. It has an important role in transporting heat and moisture into the atmosphere. The transition boundary between the troposphere and the layer above is called the tropopause. Both the tropopause and the troposphere are known as the lower atmosphere. 4.3.2 Stratosphere The stratosphere extends from the tropopause up to 50 km (31 mi) above the Earth’s surface. This layer holds 19 percent of the atmosphere’s gases but very little water vapor. Temperature increases with height as radiation is increasingly absorbed by oxygen molecules, leading to the formation of ozone. The temperature rises from an average -56.6 °C (-70 °F) at the tropopause to a maximum of about -3 °C (27 °F) at the stratopause due to this absorption of ultraviolet radiation. The increasing temperature also makes it a calm layer, with movements of the gases being slow. Commercial aircraft often cruise in the lower stratosphere to avoid atmospheric turbulence and convection in the troposphere. Severe turbulence during the cruise phase of flight can be caused by the convective overshoot of thunderstorms from the troposphere below. The disadvantages of flying in the stratosphere can include increased fuel consumption due to warmer temperatures, increased levels of radiation, and increased concentration of ozone.