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Temperatures at the equator are actually a bit cooler than we might predict based on how directly solar radiation strikes Earth at that latitude; and the poles are somewhat warmer than a diagram of solar radiation like Fig. 48.1 would predict. To reconcile predicted and measured temperatures, we need to consider another set of processes, those that transport heat from low to high latitudes. This heat transport is carried out by wind and ocean currents.

The gas molecules that make up air are constantly in motion, and when they are heated they move faster, and (unless confined) the volume of the air expands. For this reason, warm air is less dense than colder air and so it rises through the atmosphere. Not surprisingly, the rise of warm air is particularly strong at the equator (Fig. 48.3). The air cools as it rises, and, once it reaches an altitude of 10–15 km above Earth’s surface, the air no longer continues to rise but spreads toward the poles, continuing to cool as it goes. Eventually, at about 23 to 30 degrees north and south latitude, this cooling air becomes dense enough to sink back to the surface. This air continues to move along Earth’s surface, some of it moving toward a pole, while the rest moves back toward the equator. In either case, the air will warm again as it moves across Earth’s surface, to rise once more at the equator or at about 60 degrees north and south latitude. As shown in Fig. 48.3, Earth’s lower atmosphere is organized into cells of rising and falling air masses, and these transport heat from the equator toward the poles.

As warm air expands upward, cool air moves in to replace it. We experience this movement as wind, and as sailors have known for thousands of years, the prevailing direction of the wind differs from one latitude to another (Fig. 48.3). Prevailing winds reflect those cells of rising and falling air, but to understand them more fully, we have to consider one further aspect of our planet: its rotation about an axis.

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Earth rotates in a counterclockwise direction, moving from west to east. In the course of one daily rotation, a spot at the equator will move through a distance equivalent to our planet’s circumference—nearly 25,000 miles, or 40,000 km. Points at higher or lower latitude travel a shorter distance in a single rotation (Fig. 48.4a). So, in a period of 24 hours, a point at the equator will have traveled farther than a point at higher or lower latitude, which is to say that the point at the equator moves more quickly than the point closer to the poles. As the wind moves air north and south from the equator, the land beneath it rotates to the east, but at a slower speed than the land at the equator (Fig. 48.4b). The wind, then, appears to have deflected to the right in the Northern Hemisphere, and winds in the Southern Hemisphere deflect to the left. This phenomenon is called the Coriolis effect, and, in conjunction with the cells of rising and descending air masses, it explains the pattern of prevailing winds shown in Fig. 48.3.

Winds, in turn, push on water in the oceans, directing surface currents as shown in Fig. 48.5. Water can carry much more heat than air, and so ocean currents transport a great deal of heat to higher latitudes. For example, the ocean current known as the Gulf Stream (and its northeasterly extension, the North Atlantic Drift) transports warm water north along the eastern margin of North America and then east toward Europe. The heat it carries helps to keep northwestern Europe much warmer than land at equivalent latitudes in North America. Subtropical plants ornament gardens in coastal Scotland but would be unthinkable in Newfoundland, Canada, which is at approximately the same latitude.

Just as colder air sinks below warmer and less dense air, cold waters sink beneath less dense water masses. At high latitudes, the sinking cold waters begin to move slowly along the seafloor toward the equator. The result is a complex three-dimensional circulation of water through Earth’s oceans that plays an important role in the determination of regional climates.

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Wind and water, then, transfer heat from the equator toward the poles, modifying the direct effects of solar radiation. Together, these processes result in the temperature distribution observed across the planet, explaining why palm trees that cannot tolerate freezing are confined to low latitudes, while deciduous trees that lose their leaves every fall occur mostly in the seasonal climates found at mid-latitudes. At the same time, however, if you drive due west from New York to northern California, you will pass sequentially through deciduous forest, grass-dominated prairies, and montane stands of conifers. There is more to the distribution of biomes than temperature alone.