Unit Eight. The Living Environment
The world contains a great diversity of ecosystems because its climate varies a great deal from place to place. On a given day, Miami and Boston often have very different types of weather. There is no mystery about this. The tropics are warmer than the temperate regions because the sun’s rays arrive almost perpendicular (that is, dead on) at regions near the equator. As you move from the equator into temperate latitudes, sunlight strikes the earth at more oblique angles, which spreads it out over a much greater area, thus providing less energy per unit of area (figure 36.11). This simple fact—that because the earth is a sphere some parts of it receive more energy from the sun than others—is responsible for much of the earth’s different climates and thus, indirectly, for much of the diversity of its ecosystems.
The earth’s annual orbit around the sun and its daily rotation on its own axis are also both important in determining world climate. Because of the daily cycle, the climate at a given latitude is relatively constant. Because of the annual cycle and the inclination of the earth’s axis, all parts away from the equator experience a progression of seasons. In summer in the Southern Hemisphere, the earth is tilted toward the sun as shown in figure 36.11, and rays hit more directly, leading to higher temperatures; as the earth reaches the opposite position in its annual orbit, the Northern Hemisphere receives more direct rays from the sun and experiences summer.
Figure 36.11. Latitude affects climate.
The relationship between the earth and sun is critical in determining the nature and distribution of life on earth. The tropics are warmer than the temperate regions because the sun's rays strike at a direct angle, providing more energy per unit of area.
The major atmospheric circulation patterns result from the interactions of six large air masses. These great air masses (shown as circulating arrows below) occur in pairs, with one air mass of the pair occurring in the northern latitudes and the other occurring in the southern latitudes. These air masses affect climate because the rising and falling of an air mass influence its temperature, which, in turn, influences its moisture-holding capacity.
Near the equator, warm air rises and flows toward the poles (indicated by arrows at the equator that rise and circle toward the poles). As it rises and cools, this air loses most of its moisture because cool air holds less water vapor than warm air. (This explains why it rains so much in the tropics where the air is warm.) When this air has traveled to about 30 degrees north and south latitudes, the cool, dry air sinks and becomes reheated, soaking up water like a sponge as it warms, producing a broad zone of low rainfall. It is no accident that all of the great deserts of the world lie near 30 degrees north or 30 degrees south latitude. Air at these latitudes is still warmer than it is in the polar regions, and thus it continues to flow toward the poles. At about 60 degrees north and south latitudes, air rises and cools and sheds its moisture, and such are the locations of the great temperate forests of the world. Finally, this rising air descends near the poles, producing zones of very low precipitation.
Key Learning Outcome 36.6. The sun drives circulation of the atmosphere, causing rain in the tropics and a band of deserts at 30 degrees latitude.