Windswept_ The Story of Wind and Weather - Marq de Villiers [32]
Contemporaneous with Maury was the work of William Ferrel, whose Essay on the Winds and Currents of the Ocean rediscovered the forgotten work of Gustave-Gaspard Coriolis. It was in this essay that Ferrel, a self-taught farm boy from what is now West Virginia, proposed his famous model for the midlatitude circulation of the earth's atmosphere that was late to be called the Ferrel cell. His theory was that air flows toward the pole and eastward near the earth's surface, and toward the equator and westward at higher altitudes. His theory doesn't match precisely what actually happens, but it was still the first real explanation for the westerly winds in the middle latitudes of both hemispheres.
Only after manned flight in the twentieth century were the overall patterns of air circulation finally plotted. The work was given some urgency in the First World War, because the commanders of the new air forces desperately needed data they could use to protect their lethal but nonetheless fragile little bombers. By the late 1920s it was understood that winds were the continuing collision of huge air masses in waves, fronts, ridges, and troughs, all caused by solar radiation and the rotation of the planet. The final piece of the puzzle—the discovery of the high-altitude stratospheric winds and the jet streams—had to wait until aircraft could fly higher still. By the Second World War a real understanding of winds was, finally, in place.
CHAPTER FOUR
Wind's Intricate Patterns
Ivan's story: By the afternoon of September 3 the system had moved almost 300 miles farther into the tropical Atlantic, and was located 863 miles south southwest of the Cape Verde Islands. Around it and above it the air was still, but the system itself was still spinning slowly, moving just south of west at about 30 miles an hour. Strong winds stretched outward about 60 miles from the center. Within a few hours the winds picked up and passed the 39-mile-an-hour threshold that turned the system officially from a tropical depression into a tropical storm. A bulletin from the National Hurricane Center in Miami assigned the system the name Ivan, according to custom and its place in the annual cycle, and found it was strengthening steadily. By five P.M. on the 3rd, Bulletin 3 found that the storm's central pressure had dropped to 1,000 millibars, and maximum sustained winds were in the region of 30 miles an hour, with higher gusts. This was still well shy of the 74 miles an hour of a hurricane, but the bulletin was cautious: Strengthening was expected, and it could turn westward onto a new track.
In the small print of the bulletin, issued under the signature of Forecaster Beven, there were hints of what Miami thought would happen. "Thought," because they didn't know; winds are predictable in their larger patterns and behaviors, but horribly intricate in their local behavior. Aircraft data had found flight level winds of g3 knots near the center.1 Beven expected the system to turn slightly north and reach hurricane status within two days. Further ahead than 36 hours, he wrote, the forecast was of low confidence.
He expected Ivan would head for the Gulf of Mexico, but the system could still go anywhere, do anything.
That bulletin was the first time Ivan impinged on my consciousness. I logged on to the National Hurricane Center Web site and squinted along the predicted track. It looked likely that Ivan could go to Puerto Rico and then Cuba, and after that southern Florida and the Gulf. It didn't appear dangerous for the northeast corner of America. But that "low confidence" niggled. The northern edge of the track forecast would take the storm north of the Bahamas, where it could curve northward, as most storms do under the influence of the Coriolis force, and head for Bermuda. That could be Trouble.
At least for me. I confess I was rather less concerned at this point about Florida than I was about the storm showing up in the northern