GLG150 Earth’s Dynamic Ocean and Atmosphere WEEK 4

Convection in the atmosphere (see Chapter 15) creates huge belts or cells of rising and falling air masses (see Figure 6.1). This results in three global belts of high rainfall and four belts of low rainfall. The high-rainfall belts are regions of convergence where huge, moist air masses meet and rise upward. These belts lie in the equatorial region and along the two polar fronts, at approximately 50°N and S latitudes, resulting in the warm-humid (tropical) and cold-humid (polar front) climate zones. The four global belts of low rainfall are regions of divergence, where cool, dry air masses move downward and apart. These belts lie in the two polar regions and in the subtropical regions along the 30°N and S latitudes. The dry air masses and resulting low annual precipitation in these regions create two belts of dry climate in the subtropical regions, as well as the dry, cold climates of the polar regions.

Figure 6.1    The world's deserts A.   This map shows the distribution of arid and semiarid climates and the major deserts associated with them. Many of the world's great deserts are located where belts of dry air descend along the 30°N and 30°S latitudes. Notice also that regions of cold, descending air also surround both poles.