The hydrological cycle plays a critical role in continuously delivering freshwater precipitation to the land while maintaining water levels, both on the Earth’s surface and in underground aquifers. Consistency in the cycle is necessary for maintaining life. Of course, each of the Earth’s regions receives a different amount of rainfall. The coastal deserts of Chile and Namibia, for instance, receive less than 5 mm of precipitation a year, while tropical rainforests experience around 2,000 mm of precipitation annually. Figure 8 shows mean annual rainfall distributed across the Earth.
Notice the patterns that emerge from this map. Regions closest to the equator receive significant amounts of rainfall, while some of the driest regions of the world are located 30 degrees North and South of the equator.
There are many factors that determine how much precipitation a region will receive. One important factor is the global climate pattern driven by the temperature difference between the equator and the poles. At the equator the Earth receives intense solar radiation, causing increased evaporation. This moist, warm air rises, eventually cooling and condensing to form clouds that produce abundant rainfall in equatorial regions (thus, equatorial regions are hot and wet). Once the rising air mass loses its water, causing it to cool even more, it becomes denser and thus has a higher air pressure. This dense, high pressure air circulates north or south and sinks at 30 degrees north or south of the equator. The falling dry air heats as it descends, forming the world’s major desert regions such as the Sahara, the Atacama, and the Namib at 30 degrees north and south latitude. These air currents are called the Hadley cells (see Figure 9), named after George Hadley who hypothesized their existence in the 1700s.
Like fluids, air flows into regions of lower pressure and flows out of regions of higher pressure. As air rises at the equator, the rising air leaves a slight negative pressure (vacuum effect) at the surface of the earth. For that reason air in subtropical regions of 30°N and 30°S (high pressure areas) tends to move toward the equator and the poles (low pressure areas; see red arrows depicting NE and SE trade winds in Figure 9). This process also influences rainfall patterns. For instance, air traveling north from the subtropical high (30°N) meets cold, dense polar air moving south. The place where they collide is called the polar front. The colliding air masses rise and form clouds, resulting in much of the rainfall over North America and Europe.