Читать книгу Wind Power Basics - Dan Chiras - Страница 27

Local winds can be a valuable source of energy. The winds on which most people rely, however, are those produced by much larger air masses that result from regional and global air circulation. They create dominant wind-flow patterns, known as prevailing winds.

Prevailing winds, like local winds, are created by the differential heating of the Earth’s surface, but on a much larger scale. Here’s how they are formed: As shown in Figure 2.3, the Earth is divided into three climatic zones: the tropics, temperate zones and poles. Because the tropics are more directly aligned with the Sun throughout the year, they receive more sunlight and are, therefore, the warmest regions on Earth.

Fig. 2.3: Climate Zones. The Earth is divided into three climate zones in each hemisphere. In the Northern Hemisphere, warm air from the tropics flows northward by convection, creating the global circulation pattern.

The temperate zones lie outside the tropics, in both the Northern and Southern Hemispheres.They receive less sunlight than the tropics and so are cooler. The North and South poles receive the least amount of sunlight and are the coolest regions of our planet.

As shown in Figure 2.4a, global air circulation is created by hot air produced in the tropics. This air expands and rises (as in local air circulation patterns). Cool air from the northern regions — as far north as the poles — moves in to fill the void. The result is huge air currents that flow from the poles to the equator.

Although air generally flows from the North and South poles toward the equator, circulation patterns are a bit more complicated. In the Northern Hemisphere, some of the warm air moving northward cools and sinks back to the Earth’s surface, as shown in Figure 2.4b. It then flows back toward the equator creating the trade winds. Because the trade winds blow quite consistently, they are a potentially huge and reliable source of energy for residents and nations fortunate enough to lie in the winds’ path.

As shown in Figure 2.4b, a substantial amount of the northward-moving air continues on toward the North Pole, traveling over the temperate zone.

The figure also shows air masses flowing northward across the temperate zone split into two, creating higher- and lower-level winds. When the upper winds reach the North Pole, this cold air sinks and then flows southward back toward the equator.

Fig. 2.4: Global Air Circulation. (a) As shown here, warm tropical air rises and flows toward the poles. Cold polar air flows toward the equator. (b) Warm tropical air loses some of its heat and sinks toward the Earth’s surface, then flows back toward the equator, creating the trade winds. Air masses moving over the temperate zone split into upper and lower winds. (c) Wind patterns caused by the Coriolis effect, resulting from the rotation of the Earth on its axis.

If no other forces were at work, winds flowing back to the equator would flow from north to south. As shown in Figure 2.4c, they don’t. Other factors influence the movement of air masses across the surface of the planet. One of the most significant is the Earth’s rotation, which results in a phenomenon known as the Coriolis effect.