This essay examines the transformation of a local rule of thumb into a widely acknowledged meteorological law, generally known as Buys Ballot's law. This law relates wind direction to atmospheric pressure. From 1857 to 1867, Christophorus Buys Ballot (1817'1890) actively lobbied in the international arena for his wind rule, which he regarded as a promising basis for a system of storm warnings. At the same time he was reluctant to generalize his rule beyond the Dutch boundaries or to make strong claims about its predictive nature. Initially he failed to interest foreign meteorologists in his work, partly because of a widespread scepticism with regard to meteorological predictions, and partly because some of his foreign colleagues favored competing theories. One of his main rivals in this respect was Robert Fitzroy, director of the British Meteorological Office, who had set up his own warning system. This practice provoked the wrath of the Royal Society, as its members regarded Fitzroy's theories and the resulting predictions as unscientific. After his death the Society took the British Meteorological Office under its control and abolished the practice of storm warnings. The resulting wave of protests from people who felt they had benefitted from the warnings landed the Society in an awkward predicament. The warnings could only be reintroduced without losing face if they had a "scientific" basis, and therefore finding a sound basis for storm predictions became a matter of urgency. At last Buys Ballot found a willing ear for his campaign. A rapid verification of his wind rule in Britain sufficed for the introduction of the unprecedented expression "Buys Ballot's law" in the Royal Society reports. From these authoritative reports the designation rapidly spread all over the world, thus becoming a current expression.