LONG, long ago sailors discovered that there was a prevailing pattern to the wind in various parts of the world. Most notable of all were the trade winds, blowing steadily from a northeasterly direction in the zone from about 10 degrees to 30 degrees north of the equator, and the corresponding south easterly trades, their mirror image in the southern hemisphere. Insofar as people wondered why, their attitude might have been summed up by John Mascfield's little ditty:
A very queer thing is the wind,
I don't know how it beginned,
And nobody knows where it goes;
It is wind, it beginned, and it blows.
For along time, the accepted explanation for the more or less easterly trades was that offered by Galileo in the first half of the 17th century. His theory was that in the vicinity of the equator the atmosphere was unable to keep up with the rapidly rotating earth: it lagged behind, producing an apparent westward movement of air. Among other things, however this theory did not explain the semi permanent zone of equatorial calm where the trade winds meet, a region known to sailors as the doldrums, and to meteorologists as the Intertropical Convergence Zone.
Edmund Halley, better known for the comet that fears his name, provided a more convincing explanation. He assigned the global wind system to differential heating of the Earth's surface. Within the tropics the sun's rays are nearly vertical throughout the year, while on the polar ice caps they are almost horizontal during the half of the year that the sun is above the horizon at all solar heating is therefore strong in equatorial latitudes and very weak near the poles, a differential which leads to semi permanent areas of high and low pressure in various regions of the world.
The pattern is revealed by a global map of the average barometric pressure. Semi permanent areas of high pressure lie about 30 to 35 degrees north and south of the equator, with an area of low pressure in between, more or less along the Line itself, and these dictate the winds in low latitudes.
If the earth were not revolving on its axis, air would flow directly towards the equator from the two regions of high pressure. But the Earth's rotation complicates the issue by deflecting the air to the west as it moves towards lower latitudes. The result is the northeast trade winds in the northern hemisphere, the south east trades south of the equator, and an equatorial zone of calm where these winds meet.