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General Wind Circulation |
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By Bill Giles O.B.E. Wind is the horizontal movement of air across the Earth's surface and is formed by unequal heating of the atmosphere by the sun. |
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The sun is higher in the sky near the equator than at the poles,therefore more heat is received over a given area. As a result there is an imbalance that creates the wind. This tries to redress the balance by moving the excess heat from the equator to the poles. The very warm moist air in the equatorial regions rises, cools and produces the huge cumulonimbus clouds that give rise to the tropical rain forests, and then starts to flow north and south at high levels of around 50,000 feet. The descending air warms up and dries out the atmosphere...
Some of the air moving towards the poles starts to descend at about 30 degrees north and south. The descending air warms up and dries out the atmosphere clearing the cloud and creating high pressure at the surface of the Earth. These semi-permanent high pressure systems help to form deserts, such as the Sahara, and are responsible for the Azores anticyclone. The winds at ground level split with some flowing back towards the equator and others towards the poles.
Meanwhile at the Poles, cold air is sinking through the atmosphere forming high pressure with the out flowing cold surface winds moving towards the equator. In the northern hemisphere this cold air meets the wind coming from the south close to our latitude, and it is the meeting of these two wind systems that helps to form the well known weather fronts that brings us so much of our rain. Winds would blow directly from high pressure to low pressure in much the same way as water flows from high ground to low ground, and if our Earth was not rotating this would be the case. The resulting wind patterns in the northern hemisphere would be with northerly winds moving out from the poles down to about 60 degrees, a southerly wind from 60 to 30 degrees and a northerly from there to the equator. The reverse would be true in the southern hemisphere. ...this causes the winds to be deflected to the right in the northern hemisphere... However, our Earth does rotate once every twenty-four hours and this causes the winds to be deflected to the right in the northern hemisphere and to the left south of the equator.
This is known as the Coriolis Effect and is why the winds blow clockwise around high pressure and anticlockwise around low pressure in the northern hemisphere and not directly from high to low. (Actually they are deflected a small amount at the surface by friction and the angle of deflection depends on the stability of the air and the ground or sea surface over which it is travelling) Predominant southwesterly winds across much of our latitudes bring us the rain...
So the resulting general circulation wind pattern ends up, in the northern hemisphere, with cold northeasterly winds blowing out from the North Pole. Predominant southwesterly winds across much of our latitudes bring us the rain bearing fronts from the Atlantic but give us regular rainfall for growing our crops.
The northeast trade winds further south, so called because they blow in this direction for much of the year, was where the sailing clippers used to head for guaranteed wind. Near the equator we find the Doldrums, which is an area of light and often very variable wind regimes. You can see these illustrated in the Weatherwise feature on global circulation. Winds, of course, do not always stick to these patterns especially in our area of the globe since they are greatly modified by the weather systems themselves but that is another story in itself. | |||||
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