The remaining question was, which variables should one map? Today, the primary variable used, at least in temperate regions, is atmospheric pressure, with isobars being drawn linking places of equal pressure; but there was no obvious reason to choose this, even though a falling barometer had long been linked with stormy weather. FitzRoy, who pioneered the construction of synoptic maps in Britain, favoured the plotting of wind speeds and directions. He thought that the direction and force of the wind determined whether the barometer rose or fell and therefore, though he sometimes drew them, he did not accept the primacy of isobars.[39] Buys Ballot, however, announced (in what came to be known as 'Buys Ballot's Law;) an empirically determined dependence of wind speed and direction on atmospheric pressure gradients.[40] The German meteorologists Ludwig Kämtz advocated isobars and seems to have persuaded the French. By the late 1860s and early 1870s, this view became widely accepted and plotting isobaric synoptic charts became standard northern hemisphere practice.[41]

Weather being no respecter of international boundaries, European meteorologists sought to extend their maps across the entire continent. Commencing in 1872, a series of pan-European conferences was held to establish more uniform recording procedures—the essential preliminary to fruitful interchanges of data. In the United States, the Army Signal Corps, which took over responsibility for the national meteorological network in 1874, began issuing synoptic weather charts three times daily.

Bureau of Meteorology