Buys-Ballot's law

In meteorology, Buys Ballot's law (Dutch pronunciation: [ˌbœy̯s bɑˈlɔt]) may be expressed as follows: In the Northern Hemisphere, if a person stands with his back to the wind, the atmospheric pressure is low to the left, high to the right. This is because wind travels counterclockwise around low pressure zones in the Northern Hemisphere. It is approximately true in the higher latitudes of the Northern Hemisphere, and is reversed in the Southern Hemisphere, but the angle between the pressure gradient force and wind is not a right angle in low latitudes.

As early as the 16th century extensive weather observations were included as part of a ship's log. These observations as well as other log information, were turned over to national hydrographic institutes in various nations, most notably Germany and England and later the US. The information from many ships about individual voyages was compiled ashore and later became what today is still published by England, a 3 volume set complete with charts titled "Sailing Directions for the World". Additionally the US Defense Mapping Agency publishes a 47 volume set Sailing Directions which serves much the same purpose. The information is the distillate of empirical observations of thousands of ships masters over thousands of voyages spanning several hundred years.

Buys Ballot's law, which was first deduced by the American meteorologists J.H. Coffin and William Ferrel, is a direct consequence of Ferrel's law. The law takes its name from C. H. D. Buys Ballot, a Dutch meteorologist, who published it in the Comptes Rendus, November 1857. While William Ferrel theorized this first in 1856, Buys Ballot was the first to provide an empirical validation.

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