GLG150 Earth’s Dynamic Ocean and Atmosphere WEEK 4

As we will see in more detail in Chapter 17, the tilt of Earth's axis also helps determine which points on the surface receive the most heat at different times of the year. For now, it is important to understand that at the spring and fall equinoxes the maximum solar energy (about 1366 watts per square meter) falls on the Equator. In December, because of the 23.5° tilt of Earth's axis of rotation to the plane of its passage around the Sun, solar input is most intense at 23.5° S, the Tropic of Capricorn. In June, the incoming solar energy is most intense at 23.5° N, the Tropic of Cancer (see Figure 17.4).

Winds and ocean currents are the natural processes by which the Earth system redistributes the heat more evenly. Currents move heat from the Equator—where the input of solar heat is greatest—toward the poles, where it is least. Unequal heating of Earth's surface causes convection loops in the atmosphere, as we saw in Figure 15.5. Heated air near the Equator expands, becomes lighter, and rises. Near the top of the troposphere it spreads outward toward the poles. As the upper air travels northward and southward toward the poles, it gradually cools, becomes heavier, and sinks. Upon reaching the surface, this cool air flows back toward the Equator, warms up, and rises again, thereby completing a convective cycle. In reality, it is not quite so simple: Global atmospheric circulation actually organizes itself into three convection cells that interlock like gears (Figure 15.12). We met these same convection cells in Chapter 6 (see Figure 6.1), because they play a major role in the locations of deserts.

Figure 15.12    Global atmospheric circulation Huge convection cells transfer heat from equatorial regions, where the input of solar energy is greatest, toward the poles, where the solar input is least. Because Earth is rotating, the flow of air toward the poles and the return flow toward the Equator are constantly deflected sideways, creating the circulating air masses you may have seen on weather maps. The convection cells are permanent features of Earth's atmosphere and therefore have a great influence both on day-to-day weather and on long-term climate.