Weather

If the Sun provides the energy by which weather patterns can develop, certain features of the earth itself determine the precise forms in which those patterns may be exhibited. One example has already been provided above. Earth's surface is highly variable, ranging from oceans to deserts to cultivated land to urbanized areas. The way solar energy is absorbed and reflected from each of these regions is different, accounting for variations in local weather patterns.

Other characteristics of the planet account for more significant variations in weather patterns. These characteristics include such features as the tilt of Earth on its axis in relation to its plane of revolution, and the variations in Earth's distance from the Sun.

The fact that Earth's axis is tilted at an angle of 23 1/2° to the plane of its orbit means that the planet is heated unevenly by the Sun. During the summer, sunlight reaching the Northern Hemisphere strikes more nearly at right angles than it does in the Southern Hemisphere. In the winter, the situation is reversed.

The elliptical shape of the earth's orbit around the Sun also affects weather conditions. At certain times of the year the planet is closer to the Sun than at others. This variation means that the amount of solar energy reaching the outer atmosphere will vary from month to month depending on Earth's location in its path around the Sun.

Even Earth's rotation on its own axis influences weather patterns. If Earth did not rotate, air movements on the planet would probably be relatively simple: air heated along the equator would rise into the upper atmosphere, travel northward toward the poles, be cooled, and then return to the earth's surface at the poles.

Earth's rotation causes the deflection of these theoretically simple air movements. Instead of a single overall equator-to-poles air movement, global winds are broken up into smaller cells. In one cell warm air rises above the equator, moves northward in upper altitudes, is cooled, and returns to the earth in the regions around 30° north and south latitude. A second cell consists of air that moves upward in the regions around 60° north and south latitude, across the upper atmosphere, and then downward at about 30° north and south latitude. The final cell contains winds traveling upward at 60° north and south latitude and then downward again at the poles.