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at the same altitudes. For example, data from all stations measures atmospheric pressure at 500 millibars. "Normal" weather would show this pressure at about 18,000 feet, so the critical datum is at what altitude and at what geographic node this pressure exists when the measurement is taken. If it is higher at 18,000 feet, this represents a high-pressure zone. If lower, it obviously means a low-pressure zone or front. The pressure gradients are shown on the surface maps by a series of curved lines called isobars. Where the lines are close together, the winds are very strong. Where they loosen up, the winds are light. On upper level charts, where the lines are closest together, are the jet streams. By the same consensus, weather maps show the wind direction as parallel to the isobars, with lower pressure to the left, looking downwind. Wind speed is calculated, in knots, by counting the number of isobars, analyzed at every 4 millibars, that fall within a spacing of 50 latitude, and multiplying by ten. For example, if 3 X isobars lie across 50, the wind is 3.5 X 10 = 35 knots.11 Consecutive sets of maps show the track of the different weather systems.

Numerical-calculator enthusiasts were given a new lease on life with the invention of the first computers in the 1940s. Late in the decade, mathematician John von Neumann at Princeton put together a team of colleagues and a few meteorologists to have another go at the problem. The team's director, Jule Charney, figured he could overcome Richardson's data swamp by using the computer and at the same time filtering out whole sets of data, such as sound and gravity waves. Indeed, in April 1950, Charney's group made a series of successful twenty-four-hour forecasts over North America, and by the mid-1950s, numerical forecasts were being made on a regular basis.

A decade later, on April 1, i960, the first polar-orbiting data-collecting satellite, TIROS 1, was launched. It worked for less than four months, but it gave the world's weather people the first ever pictures of Earth and its cloud cover12

Weather forecasting remained an art. But it now had real numbers, and real-time pictures, to back up its intuitions.

III

Part of the long struggle to understand and then to forecast weather was to accurately measure wind—and then to find some way of depicting it that others would readily understand. The new meteorological offices springing up across the industrialized world needed some way of describing wind force to their clients, at first merchant mariners and sailors, but then all kinds of industrial and shore-based users. As data collection became more widespread, standardization became more and more necessary. It was no use for a telegraph operator on the Prairies to refer to a gale, without some notion of what that gale meant, or how strong it really was. A gale may be a storm to some, just a fresh breeze to others. Notoriously, what fishermen from Gloucester, Massachusetts, considered a bracing wind would send yachtsmen from New York scurrying back to port—or that's what the Gloucestermen said, anyway. In any case, whoever they were, sailors needed something more precise than a "brisk breeze" (or, as some say of gales in New England waters, "a breeze o' wint") to describe what they were likely to experience.

By the late twentieth century scales had been devised for winds of all kinds—regular winds, hurricanes, and tornadoes. We even had scales for windchill and other esoteric matters. But the first of all these was the Beaufort scale.

This is one of these simple measures that seems to have been around forever. Most people know what it is meant to show—a simple numerical relationship to wind speeds, based on real observations of wind's effects. But its history is a little more complicated.

The eponymous Beaufort was Rear Admiral Sir Francis Beaufort, Knight Commander of the Bath, who was born in Ireland in 1774, son of a country parson, an emigre from France with a doctorate in law and a mischievous penchant for acquiring massive debts and then dodging the debt collectors. Young