Copyright © 2007 Dorling Kindersley
Wind is the movement of the atmosphere. The atmosphere moves because the Sun heats the Earth’s surface, causing air to increase in temperature, expand, and rise upwards. Cool air moves in to replace the warm air, and we feel this movement of air as wind. The air flows from areas of high pressure to areas of low pressure around the globe. The strongest winds occur during HURRICANES and TORNADOES.
At the Earth’s poles, the air is at high pressure and low temperature; at the Equator, it is at low pressure but higher temperature. This, together with the spin of the Earth on its axis, creates a pattern of warm and cool winds around the globe. Continents and high mountains also produce wind patterns, such as the monsoon winds over southern Asia. Areas on the Equator where the winds are very light are called the doldrums. Sailing ships used to get stuck there for long periods.
Winds often blow from one direction most of the time. If these prevailing winds are strong, trees grow lopsided. Wind direction is always given by the direction the wind is blowing from, rather than where it is blowing to. A southerly wind, for example, blows from the south towards the north. Wind speed is measured in kph or mph. At sea, it is measured on the Beaufort Scale, from force zero (no wind) to force 12 (hurricane).
A hurricane is a huge, spinning storm with very high-speed winds. A hurricane starts life as a group of thunderstorms near the Equator. If the storms begin to spin together, they form a tropical storm. If the storm’s winds reach more than 119 kph (74 mph), it is called a hurricane. In the Pacific Ocean, hurricanes are called typhoons; in the Indian Ocean, they are called cyclones.
A hurricane contains spiral bands of thunderstorms spinning around a still centre called the eye. Warm air within the bands flows around the eye and upwards. The air pressure in the eye is so low that, over sea, water bulges upwards. If the hurricane hits land, the bulge turns to a mass of water that floods the coast in a storm surge.
The central eye and swirling storm clouds of a hurricane can be seen in a satellite image taken high above the Earth. The hurricane’s strongest winds are in the eye wall (the towering wall of cloud around the eye). These often reach speeds of 300 kph (190 mph). Inside the eye, however, there is almost no wind. The eye is usually between 8 and 25 km (5 and 15 miles) in diameter. The hurricane itself can be up to 800 km (500 miles) across.
A tornado is a spinning, funnel-shaped column of air. Inside, winds can blow at speeds of more than 480 kph (300 mph) – the fastest winds on Earth. These violent winds destroy buildings in their path. Tornadoes form underneath giant thunderstorms, and they can be anything from a few metres to 800 m (half a mile) across. Most tornadoes happen in the USA, especially in Tornado Alley in central USA.