Читать книгу Wind Power Basics - Dan Chiras - Страница 28

To understand why prevailing winds deviate from the expected patterns based solely on convection, let’s start with the trade winds. As shown in Figure 2.4c, the trade winds in the Northern Hemisphere flow not from north to south, as you might expect, but from the northeast to southwest. Why?

Because they are “deflected” by the Earth’s rotation.

In reality, the Earth’s rotation doesn’t deflect winds. It makes it appear as if the winds have been deflected. The apparent deflection in wind direction in the tropics is a planetary sleight of hand, an illusion produced by the rotation of the Earth on its axis. To understand this phenomenon, consider a simple example. Imagine that you board a plane leaving the North Pole. The pilot plots a course that will take you due south toward an airfield on Sri Lanka just south of India. If the pilot flies the plane due south the entire trip, however, the plane will end up somewhere over the Arabian Sea because of the Earth’s rotation.

As shown in Figure 2.5, as the plane travels south, the Earth rotates beneath it. The Earth rotates eastward. If you plot the flight path of the plane it appears to have been deflected. In reality, it only looks that way.

The apparent deflection of the plane’s path is the Coriolis effect. In the Northern Hemisphere, the deflection is to the right of the direction of travel. In the Southern Hemisphere, the deflection is to the left.

Winds flowing north or south also appear to be deflected thanks to the Coriolis effect. The south-flowing trade winds, for instance, appear to flow from northeast to southwest.

In the temperate zone, as shown in Figure 2.4c, the low-level north-flowing winds that sweep across the surface of the Earth flow across the North American continent not from south to north but from the southwest to northeast. These are the prevailing south-westerly winds that blow across the Great Plains of North America. Many a wind farm and many a small wind operation depend on them.

Fig. 2.5: The Coriolis Effect. The rotation of the Earth causes an apparent deflection in the path of winds. This can be understood by observing the flight of a plane that begins at the North Pole and heads south toward Sri Lanka. The plane appears to veer off course. It hasn’t. The Earth’s rotation makes it look that way.