Copyright © 2007 Dorling Kindersley
Winds are moving currents of air, which can blow over a small local area or over a much larger region. Global winds help to moderate temperatures worldwide by carrying warm air away from the tropics and cold air from the poles. Winds bring changing weather conditions, such as clear, sunny skies or torrential MONSOON rain. Strong winds can bring storms and hurricanes.
Winds are caused when the Sun heats air masses in different parts of the world unevenly. Air warmed by the Sun becomes less dense, or lighter, and rises. This creates an area of low pressure where there is less air pressing down on Earth. Because air always flows from a region of high pressure to one of low pressure, cooler air flows in to fill the space left by the rising air. This is a wind.
Regular wind patterns across Earth’s surface are affected by the planet’s spin. This is called the Coriolis effect. In the northern hemisphere, the Coriolis effect causes winds to swirl clockwise. In the southern hemisphere, it causes winds to spin counterclockwise. The speed and direction of the wind are also affected by air currents blowing over natural features, such as mountains.
Three main belts, or bands, of prevailing winds blow on either side of the equator. In the Tropics, trade winds blow from the northeast or southeast toward the equator. In temperate zones, the prevailing winds are westerlies (blowing from the west). Cold easterly winds (from the east) blow in the polar regions.
Monsoon winds are massive winds that bring heavy, seasonal rain to subtropical regions, such as southeast Asia and India, in summer. In winter, they bring dry, cooler weather. Monsoon winds are the strongest in Asia, but they also blow in West Africa, northern Australia, and parts of North and South America.
Monsoon winds change course because of seasonal temperature differences between the land and the sea. Water absorbs heat more slowly than dry land but holds the heat for longer. This makes the sea cooler than the land during the summer, and warmer during the winter. The difference in temperature causes monsoon winds to blow onshore (from sea to land) in summer, and offshore (from land to sea) in winter.