Weather: Where the wind blows

Aristotle was perfectly clear about wind. Since everything in the world was, in his philosophy, made of earth, air, fire and water, which in turn originate from each other, there had to be something filling the space between the earth and the nearest stars. And the only likely candidate was air. Now the air, much of which was formed by the evaporation of water, was dragged around by the rotation of the heavens. And that's what wind was.

Modern science takes a more complicated view. We live on a globe that is generally hot around the middle (where the sun's rays hit it directly) and cold at the top and bottom (where heat from the sun reaches it obliquely). The warm air around the equator expands and spreads out, producing an area of lower pressure than the polar zones.

Air is pushed from areas of high pressure to areas of low pressure, thus producing a basic pattern of cold air flowing from the poles to the equator, and warm air from equator to poles. But while this is going on, the earth is rotating, which makes it all much more complicated because of the Coriolis effect (also known as the Coriolis force, though it isn't in fact a force at all).

Here's what the French mathematician Gaspard Coriolis (1792-1843) discovered. The spinning of the earth means that some places are moving faster than others. In fact, points on the equator are moving at a rate of about 1600kph, while the poles are standing still. So if a man at the North Pole throws a series of tennis balls towards the equator, they will bounce in a line one behind the other stretching from east to west, because the earth is rotating from west to east and their intended destination point has always moved on by the time the ball arrives.

A ball thrown northwards from the equator starts with an eastward component of velocity of 1600kph, so will land east of its intended target. So - and this is the important bit - winds blowing towards the equator seem to curve towards the west, and winds blowing away from the equator seem to curve east.

We can now describe the general circulation of the atmosphere, known as the prevailing winds.

Warm air at the equator rises and is displaced by cold air from the poles. Those are the trade winds which, because they flow towards the equator, appear to blow from the east. (Trade winds from the north and south meet in a calm but rainy area around the equator called the doldrums.)

Further away from the equator, the dominant winds are caused by air that began at the equator flowing towards the poles. Since they flow away from the equator, their apparent direction (according to Coriolis) is eastwards. These are knows as the prevailing westerlies.

Finally, we have air chilled by the polar regions becoming heavier, sinking and spreading out towards the equator. Coriolis again makes these seem to blow from the east, hence they are known as the polar easterlies.

Polar easterlies meet the prevailing westerlies at the polar front, which is where most of our chilly weather comes from. The trade winds and the prevailing westerlies blow away from each other, leaving a calm area in between known as the horse latitudes, where Spanish ships bringing horses to America all too frequently became becalmed and ran out of water.

You could tell when you were there by the dead horses floating past in the ocean.

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