Wind
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Wind is moving air. The faster the air is moving, the more wind there is. It is caused by the movement of air from an area of high barometric pressure to an area of low barometric pressure. It is the atmosphere's way of equalizing the pressure.
If there is a high pressure system (that rotates clockwise in the northern hemisphere) near a low pressure system (that rotates counter-clockwise), the air will move from the high pressure to the low pressure to try and even out the pressures. If there is a small difference in pressure, there will be breezes and light winds. If there is a big difference in pressure, the winds will be high, and in some storms, such as hurricanes, typhoons, cyclones, or tornadoes, the pressure differences can cause winds faster than 200mph (320kph). This can cause damage to houses and other buildings, and can also lead to death.
Wind can also be caused by the rising of hot air, or the falling of cold air. When hot air rises, it creates a low pressure underneath it, and air moves in to equalize the pressure. When cold air drops (because it is denser or heavier than warm air), it creates a high pressure, and flows out to even out the pressure with the low pressure around it.
The wind is usually invisible, but rain, dust, or snow can let you see how it is blowing. A weathervane can also show you where the wind is coming from. The Beaufort scale is a way to tell how strong the wind is. It is used at sea, when no land can be seen.
[change] Planetary wind belts
There are a few large systems of winds that span the whole planet. These are usually called planetary wind belts, or simply wind belts. The most important wind belts are the Doldrums, Trade winds, Westerlies, the Horse latitudes and the Polar easterlies.
The doldrums are in the equatorial region. It is a wind belt where air is warm, with little pressure variations. Winds are light there. Another name for doldrums is Intertropical Convergence Zone (ITCZ).
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