15.3–15.5: Interacting physical forces create weather.

Storm clouds gather over a Utah cornfield.

Temperature and rainfall, that’s all. The type of terrestrial biome depends on little else besides these two aspects of the weather. But what determines the temperature and rainfall in a particular part of the world? Differences in both ultimately result from one simple fact: the earth is round. As we’ll see, the earth’s curvature influences temperature, and rainfall patterns are an inevitable consequence of the variation in temperature across the globe.

Wherever you are, begin walking toward the equator. As you get closer, it gets hotter. Nearly everyone is aware of this universal trend (and the reverse as well: it gets colder as you move away from the equator and approach the North or South Pole). What is responsible for the increased warmth at the equator?

The sun shines most directly on the equator (FIGURE 15-5). At the equator, solar energy hits the earth and spreads out over a relatively small area. Away from the equator, the earth curves toward the North and South Poles. Because of this curvature, the same amount of solar energy hitting the earth at the Poles is spread out over a much larger area and also travels a greater distance through the atmosphere, which absorbs or reflects much of the heat. With the energy dispersed over a large area, there is less warmth at any one point on the earth’s surface. It’s similar to the fact that at noon, the sun’s rays hit the earth at a more direct angle than they do later in the day. That is why the sun provides less warmth late or early in the day. It is also why the risk of sunburn and skin cancer is greatest around noon and the nearer you are to the equator.

Figure 15.5: Why is it warmer at the equator than the poles?

Global patterns of rainfall can be predicted just as easily, by taking into account that warm air holds more moisture than cold air. We’ll start at the equator again, where the greatest warming power of the sun hits the earth, and some of that energy radiates back, warming the air. This starts a three-step process: (1) hot air rises; (2) as it rises high into the atmosphere, it cools; and (3) because cool air holds less moisture, as the air rises, clouds form, and the moisture that can no longer be held in the air falls as rain (FIGURE 15-6). The equator is hot, but it is also very wet.

Figure 15.6: Rainmaking.

The high, cold air in the atmosphere over the equator expands outward, to about 30° north and south. Here, the cold air, which is heavier than warm air, begins to fall downward toward the earth and is warmed by heat radiated off the earth’s surface. As the air gets warmer, it can hold more and more moisture, rather than releasing it as rain. In fact, the falling and rapidly heating air can hold so much moisture that it sometimes sucks up moisture from the land itself. For this reason, at about 30° north and south, around the world, there is very little rainfall, the ground is very dry, and deserts form (FIGURE 15-7). The Atacama Desert of South America, which lies at approximately 30° south of the equator, is an extreme example. In some parts of this large desert, no rainfall has ever been recorded. Other great deserts of the world are also at 30° latitude, including the Sahara, Kalahari, Mojave, and Australian deserts (although some deserts do occur at other latitudes). These areas stand in stark contrast to the equator, where it is not uncommon for an area to receive more than 3 meters (almost 120 inches) of rain in a year.

Figure 15.7: Desert formation. Circulating masses of air, caused by solar energy hitting different parts of the earth at different angles, determine rainfall patterns throughout the world.

As the air falls near 30° north and south and hits the earth, it spreads equally toward the equator and toward the Poles, gradually getting warmer, until, at around 60° latitude, it begins to rise because of its accumulated heat. Again, the rising air loses its moisture as rainfall. So at 60° latitude, two-thirds of the way toward the Poles from the equator, it’s not particularly warm but there is a great deal of rain. Not surprisingly, at these latitudes lie huge temperate forests with extensive plant growth. And finally, around the Poles, air masses again descend. As they do, because they can hold more moisture, very little rain falls. The Poles are cold, but with little precipitation—they resemble frozen desert.