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.
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.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.
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
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.winds. When the upper winds reach the North Pole, this cold air sinks and then flows southward back toward the equator.
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.
The Coriolis Effect
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 southwesterly 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.
