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What Causes Global Wind Patterns?

The sun is the ultimate source of energy for Earth. As you have previously learned, incoming solar energy (100%) balances the energy reflected in Earth's atmosphere (30%) and the energy which is absorbed at Earth's surface and reradiated as heat or reradiated from clouds and greenhouse gases (70%). The total: 100% incoming = 100% outgoing. This is a general way to view the solar energy balance. Think about the albedo effect on Earth's materials and atmosphere. In high latitudes such as the North and South Poles, the angle of incoming solar radiation is greater than that received at the equator. Remember when determining angles on the surface of the Earth, you must determine the angle by visually constructing a line coming from the center point of the earth to the surface and connecting it with the incoming rays from the sun. Thus, rays at the equator vary from 0 degrees to 23 degrees at the tropics of Cancer and Capricorn, and at the poles, both north and south, the degree of angle is 90. Because of this the intensity of solar radiation is not as great at the poles. In addition to this, snow and ice blanket much of the north and south poles, where much of the received solar radiation is reflected back into space. On the other hand, incoming solar radiation at or near the equator, the tropics, is more intense, and much of it is absorbed by the dense vegetation existing there. See Figure 1. The intensity of solar radiation also warms the oceans within the tropical areas.
In order to help you understand this more clearly, refer to the photo. In this photo a grid of equal dimensions has been placed on a projector screen, superimposed upon a globe. Because of the curvature of the globe which represents Earth, the grids are not uniform. Notice at the equator the grids are proportionally smaller than they are at the poles. This represents the solar radiation being scattered over a greater distance at the poles. At the equator the grids are not distorted, meaning solar radiation is not scattered near as much. Because of this, the air, ocean, and Earth's surface gets heated at the equator more than at the poles.

If we were to compare these two regions we would find there is a net loss of solar radiation in the higher latitudes and a net gain in the tropics. This difference in solar radiation creates a movement of air, or wind, from the warmer tropical regions toward the cooler northern and southern regions, the poles. See Figure 2.

This moving air creates oceanic surface currents that move the water in the same direction as the wind, that is, currents from the equator and tropics move warm water northward toward higher latitudes, and currents from the poles move cooler water southward toward lower latitudes. Do winds in your area blow from the north toward the south or south toward the north? If you analyze wind patterns where you live, you probably will discern that the winds generally have a westerly or an easterly component to the north-south directions, such as northwest, southeast, southwest or west southwest. What is the reason for this? Why don't winds blow just north and south, moving the ocean currents north and south according to Figure 2? Great question. Research will help provide the answer.
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Updated October 24, 2008 by: Glen Westbroek

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