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By Root 342 0

winds are organized into a sequence of high-pressure ridges and low-pressure troughs, with a wavelike motion. The largest of these wave patterns, the so-called standing waves, have three or four ridges and a corresponding number of troughs in a broad band. Shorter waves, several hundred miles in length, are called traveling waves. These form the upper parts of near-surface cyclones and anticyclones, and guide their movements and development. These high altitude winds are global in scope and are called geostrophic winds; they are driven by temperature and consequently pressure differentials, and are not influenced by the surface of the earth. These winds were unknown before weather balloons and are now generally measured by aircraft.

At low altitudes winds are essentially circular, organized into cyclones and anticyclones (low-pressure and high-pressure areas). As we have seen, cyclones rotate counterclockwise around lows in the northern hemisphere and clockwise in the southern hemisphere. Anticyclones revolve in the opposite direction. Extratropical cyclones are generally benign, little more than eddies in the overall system, but they are nonetheless significant: They are an essential part of the transference of excess heat received in tropical latitudes from the sun to polar regions;18 without them the poles would be much colder and the tropics much hotter. In tropical latitudes the cyclones are smaller, usually not much more than 300 miles or so across, but they can carry winds of terrifying violence. When the sustained winds at their centers reach 74 miles an hour, they are called hurricanes or typhoons.

All in all, it seems like an orderly system, and it is. But on closer analysis it gets more complicated—much more complicated, for in other ways winds make up the most intricately beautiful and complex of the great engines that sustain life on the planet. Winds are steered and diverted and distorted by continents, mountains, forests, deserts, oceans, and large lakes, even cities, that muddy their flow and retard their passage. They also change winds' intensity. Land heats faster than water, and so localized pressure differentials are caused at every coast. Deserts, for their part, radiate heat faster than grasslands, and grasslands faster than forests, and each retains different degrees of moisture. All these factors complicate wind patterns.

Out to sea the patterns are simpler, and much more direct. If the world were perfectly flat—all ocean, perhaps—and didn't rotate, air would flow smoothly in perfectly predictable directions. Aerodynamicists call smoothly flowing air "laminar flow," as opposed to "turbulent flow"—we'd call it "streamlined." It is possible only where little interferes with the movement of air, and in nature this condition is very uncommon, so in practice pretty well any movement of air greater than 2 or 3 miles an hour is predominantly turbulent. It is one of the main tasks of car and aircraft designers to change turbulent flow into laminar flow to reduce resistance.

Large air masses don't mix easily. If they come together at speed, which is common enough, they set in motion complicated, swirling eddies of turbulent air. These eddies are what we call "weather."19

IV

Winds and weather are complicated in three other ways that make their understanding, and their prediction, much harder.

The first of these is the large-scale and relatively long-term cyclical fluctuations in the movement of air masses.

The second is the almost universal tendency of air to coalesce into vortexes. Hurricanes and tornadoes are the most obvious of these, perhaps because of their destructive potential, but there are many others, both useful and apparently useless.

The third is the purely local or regional winds affected by mi-croscale topographic and geographic features that are not plottable on larger weather maps, but which nevertheless have significant consequences. Examples are the notorious harmattan of the Sahara, the mistral of the Mediterranean, and, closer to my home, Ies Suites of Cape Breton Island, whose