Windswept_ The Story of Wind and Weather - Marq de Villiers [38]
The prevailing winds, showing the three major bands of steady planetary winds: the trade winds, the midlatitude westerlies, and the subpolar easterlies.
North Atlantic weather is complicated further by more or less permanent zones of low pressure around the 60 degree mark, the latitude of Greenland, and zones of high pressure in the horse latitudes, around the arid zones of the Middle East and the American southwestern deserts. In the Atlantic, weather forecasters call this the Bermuda high, and its presence, strength or weakness, is crucial for forecasting Atlantic weather; it can help "steer" hurricanes and other low-pressure systems. This high migrates east and west with varying central pressure. In summer and fall it is located near Bermuda; in the winter and early spring it is primarily centered near the Azores Islands, and then—surprise!—it is called the Azores high. The actual flow depends on a number of incalculables, including seasonality and jet-stream positioning. The circulating air tends to skirt the highs and get funneled into the lows, a tendency that is further complicated by topography (air flows more smoothly over water than over land, and less smoothly over mountains than plains), and even further complicated by the smoothly flowing stratospheric winds, that sometimes have the benign effect of shearing the tops off massive cyclones before they can coalesce into hurricanes.
Tropical cyclones such as hurricanes or typhoons usually decay as they reach cooler higher latitudes. But sometimes they decay in dangerous ways, interacting with the unstable baroclinic environment in higher latitudes and turning into what are called extratropical cyclones, a process known as extratropical transition, or ET for short. ET events, as they are called, can reenergize waning hurricanes and carry massive amounts of moisture, with the risk of flooding on land and storm surges along the shores. By some measures, almost half of the tropical cyclones that form in the Atlantic undergo some form of ET. New Zealand gets a few of these in the southern hemisphere, but the global champion in ET events, the one place where they are more likely to occur than any other, is northern New England and Atlantic Canada. One of the tasks of the Canadian Hurricane Centre has been to better understand ET; in the year 2000, Chris Fogarty, a meteorologist who operates out of the center, and others flew into Hurricane Michael while it was undergoing transition, and reported on the results in the journal of the American Meteorological Society. At the very least, they concluded, there is an urgent need to develop numerical and conceptual models that will enable weather forecasters to better anticipate changes and to improve warnings.10
A "baroclinic" storm enveloped us in the winter of 2004. On the morning of February 17, 2004, the forecasters told us to expect snow. By midday, the forecast had changed to "expect heavy snow," and by evening a blizzard warning was in effect. They had spotted a low-pressure system that had organized southeast of Cape Hat-teras, in North Carolina; it had taken on the classic cyclonic shape as cold air from eastern North America clashed with the warm waters of the Gulf Stream, and was heading northeast at almost 24 miles an hour. By the morning of the eighteenth, there was a broad cloud shield and a high cirrus deck spreading out over the region, the storm already beginning to stretch out to the northeast in a comma shape. A strikingly sharp cold front extended southward as far as the Bahamas. The barometer was dropping fast, and in just over twenty-four hours had gone from 1,000 millibars to 959.
The forecast did its job. Everyone in the storm's path knew it was coming. Snowplow crews gassed up, power company workers checked their equipment and cranked up the AT Vs. We stocked up on drinking water in case the power went off, and inventoried