When we think of Edmond Halley, if we think of him at all, we remember him for his confident and accurate prediction in 1705 that a bright comet seen in 1531, 1607 and 1682, and now named after him, would appear again in 1758. Less well known, however, is the fact that Halley made important contributions to the development of meteorology.
Halley was born 345 years ago, on November 8th, 1656, the son of a prosperous London soap manufacturer. He travelled extensively in his younger days, and his careful observations of the wind patterns he encountered allowed him to draw up the first complete map of the prevailing winds of the world, showing the trade winds, the monsoons, and other well known seasonal features. And then he proceeded to explain them.
Many of these wind patterns had been known for centuries. Most notable were the trade winds, which blow steadily from a north-easterly 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 Masefield's 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." Half a century before Halley's time, Galileo had attempted an explanation of these predominantly easterly trade winds in the lower latitudes. 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 the air. Among other things, however, this theory did not explain the semi-permanent zone of equatorial calm where the trade winds meet, a region well known to sailors as the doldrums.
But Halley assigned the global wind system to differential heating of the Earth's surface by the sun. 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.
Halley developed the essentials of the theory of the general circulation of the atmosphere that we know today, and his map describing it, presented to the Royal Society in London in 1686, was the very first of its kind to be devised.