"Every year without knowing it, I have passed the day," wrote the poet William Stanley Merwin. He was referring to the most important and inevitable, and yet totally unknowable, date in everybody's calendar, the date on which they must ultimately die.No doubt throughout his life Christopher Buys Ballot saw no significance in February 3rd, and yet it was on this day in 1890 that the meteorologically famous Dutchman passed away. But he left a lasting legacy, a principle almost as important as the law of gravity. Buys Ballot's Law provides us with that fundamental relationship between atmospheric pressure and the wind; essentially that in the northern hemisphere the wind blows anticlockwise along the isobars that delineate depressions.Anaximander of Ionia, a Greek philosopher who lived in the sixth century BC, was allegedly the first to wonder whither came the wind, and he defined it, not inaccurately, as "a flowing movement of the air".Then Aristotle, 200 years later, fudged the issue somewhat by trying to be too specific. "The sun heats up and dries the Earth," he said, "and this produces exhalations. The exhalation containing the greater amount of moisture is the origin of rain, and the dry exhalation is the origin and natural substance of the winds."After that rather unconvincing explanation, no one thought about the matter much for a full 2,000 years. But with the advent of the age of exploration, mariners began to notice that there was a pattern to the global winds.Finally Edmond Halley, he of comet fame, having travelled extensively in his younger days, used his carefully collected observations to draw the first complete map of the world's prevailing winds. It was published in 1686 and showed the trades, the monsoons and all the other wellknown seasonal features quite familiar to us now. Halley also provided a scientific explanation. Before him it was suspected that the easterly trade winds in the vicinity of the equator occurred because the atmosphere was unable to keep up with the rapidly rotating Earth below; it lagged behind, producing an apparent westward movement of the air.Halley, however, assigned the global wind system to differential heating of the Earth's surface by the sun and gave the essentials of the theory of the general circulation that we know today.But it was Buys Ballot who unlocked the finer detail. As director of the Royal Netherlands Meteorological Institute, he had daily weather maps compiled for large areas of Europe and in 1857 unveiled his famous law. It has been a cornerstone of meteorology ever since.