9-6 Like Earth’s interior, our atmosphere has a layered structure

While life has shaped our atmosphere’s chemical composition, the structure of the atmosphere is controlled by the influence of sunlight. Scientists describe the structure of any atmosphere in terms of two properties: temperature and atmospheric pressure, which vary with altitude.

Pressure, Temperature, and Convection in the Atmosphere

Atmospheric pressure at any height in the atmosphere is caused by the weight of all the air above that height. For a useful analogy, if 10 students form a vertical stack—one lying on top of another—it is the bottom student who feels all the weight from those above. It is similar for air, with the greatest air pressure at Earth’s surface. The average atmospheric pressure at sea level is defined to be 1 atmosphere (1 atm), equal to 1.01 × 10⁵ N/m² or 14.7 pounds per square inch. As you go higher in the atmosphere, there is less air above to weigh down on you. Hence, atmospheric pressure decreases smoothly with increasing altitude.

Figure 9-24: Temperature Profile of Earth’s Atmosphere This graph shows how the temperature in Earth’s atmosphere varies with altitude. In the troposphere and mesosphere, temperature decreases with increasing altitude; in the stratosphere and thermosphere, temperature actually increases with increasing altitude.

Unlike atmospheric pressure, temperature varies with altitude in a complex way. Figure 9-24 shows that temperature decreases with increasing altitude in some layers of the atmosphere, but in other layers, it actually increases with increasing altitude. These differences result from the individual ways in which each layer is heated.

The lowest layer, called the troposphere, extends from the surface to an average altitude of 12 km (roughly 7.5 miles, or 39,000 ft). It is heated only indirectly by the Sun. Sunlight warms Earth’s surface, which heats the lower part of the troposphere. By contrast, the upper part of the troposphere remains at cooler temperatures. This vertical temperature variation causes convection currents that move up and down through the troposphere (see Section 8-1). Much of Earth’s weather is a consequence of this convection.

Convection on a grand scale is caused by the temperature difference between Earth’s equator and its poles. If Earth did not rotate, heated air near the equator would rise upward and flow at high altitude toward the poles. There it would cool and sink to lower altitudes, at which it would flow back to the equator. However, Earth’s rotation breaks up this simple convection pattern into a series of convection cells. In these cells, air flows east and west as well as vertically and in a north-south direction. The structure of these cells explains why the prevailing winds blow in different directions at different latitudes (Figure 9-25).

Figure 9-25: Circulation Patterns in Earth’s Atmosphere The dominant circulation in our atmosphere consists of six convection cells, three in the northern hemisphere and three in the southern hemisphere. In the northern temperate region (including the continental United States), the prevailing winds at the surface are from the southwest toward the northeast. Farther south, within the northern tropical region (for example, Hawaii), the prevailing surface winds are from northeast to southwest.

Upper Layers of the Atmosphere

Almost all the oxygen in the troposphere is in the form of O₂, a molecule made of two oxygen atoms. But in the stratosphere, which extends from about 12 to 50 km (about 7.5 to 31 mi) above the surface, an appreciable amount of oxygen is in the form of ozone, a molecule made of three oxygen atoms (O₃). Unlike O₂, ozone is very efficient at absorbing ultraviolet radiation from the Sun, which means that the stratosphere can directly absorb solar energy. The result is that the temperature actually increases as you move upward in the stratosphere. Convection requires that the temperature must decrease, not increase, with increasing altitude, so there are essentially no convection currents in the stratosphere.

Above the stratosphere lies the mesosphere. Very little ozone is found there, so solar ultraviolet radiation is not absorbed within the mesosphere, and atmospheric temperature again declines with increasing altitude.

The temperature of the mesosphere reaches a minimum of about -75°C (= -103°F = 198 K) at an altitude of about 80 km (50 mi). This minimum marks the bottom of the atmosphere’s thinnest and uppermost layer, the thermosphere, in which temperature once again rises with increasing altitude. This temperature increase is not due to the presence of ozone, because in this very low-density region oxygen and nitrogen are found as individual atoms rather than in molecules. Instead, the thermosphere is heated because these isolated atoms absorb very-short-wavelength solar ultraviolet radiation (which oxygen and nitrogen molecules cannot absorb).

The Cosmic Connections figure compares the layered structure of our atmosphere with that of Earth’s interior.

COSMIC CONNECTIONS

Comparing Earth’s Atmosphere and Interior

Earth’s atmosphere (which is a gas) and Earth’s interior (which is partly solid, partly liquid) both decrease in density and pressure as you go farther away from Earth’s center.