- Published: 29 July 2009
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The prevailing westerly winds blow across most of North America, carrying most of our changing weather from the west to the east. Our westerlies occupy a belt between 30 and 60 degrees north of the equator. North of the westerly wind belt, the polar easterlies circle the top of the world in the opposite direction. The southern hemisphere has similar belts of westerly and easterly prevailing winds.
The trade winds are the prevailing winds which blow towards the equator from the northern and southern hemispheres. They blow to meet each other from the north and the south, but they are twisted a little by the rotating earth, The trades of the northern hemisphere blow from the northeast and the trades of the southern blow from the southeast.
Near the equator, the trade winds meet a belt of warm, rising air called the doldrums. Instead of clashing head on, the moving air changes direction and rises perhaps a mile or two above the surface of the earth. Some of the air aloft changes direction again and starts blowing away from the equator, These currents upstairs, blowing in the opposite direction from the surface trade winds, are called the antitrades.
They blow towards the poles, cooling as they go. At latitudes 30 degrees north and south of the equator, the antitrades come downstairs in currents of cool, descending air. At these latitudes they meet the westerly wind belts. Some of the descending air mingles with the westerlies and some turns around to become the surface trade winds blowing towards the equator. In the main, the trade winds blow at a steady 10 to 15 miles an hour, day and night. Over land, they tend to be gusty because they are interrupted by solid objects.
But most of their journey is over the sea and here they blow steadily around and around the globe.
There are times when the trade winds are not so neatly divided at the equator. In our summer season, the southeast trades often cross over and tangle with the northeast trades. In our winter season, the northern trade winds often cross the equator and tangle with their neighbors in the southern hemisphere.