Windswept_ The Story of Wind and Weather - Marq de Villiers [83]
Some of the most powerful winds, curiously, come directly downward— not just in tornadoes, but in so-called downbursts associated with thunderstorms. In fact, more damage is done by downbursts than any other kind of winds, perhaps because they are more frequent—a recent study found that more than two thirds of all high-intensity winds that did damage to buildings and structures came during thunderstorms. 16 Unlike hurricanes, the worst effects on downbursts are close to the ground, and so affect low-rise buildings more than high-rises. Construction engineers and wind tunnel experts have been testing novel geometries and odd-looking protrusions on low-rises to reduce the wind-induced uplift of roofs. They include leading-edge spoilers, porous fences at building corners, similarly porous parapets, and roof-edge circular cylinders to induce downward flowing vortexes to counteract lift. In 2001, two researchers were granted a U.S.
patent for "visually subtle spoilers . . . to achieve a reduction in peak negative pressures along the roof leading edge of low-rise buildings." Other research has shown that the best building to withstand high winds is a brick home with perimeter wall, roof, and balcony alignments all designed to provide the wind with a path of least resistance. Still, an experimental home built with precisely those factors in mind blew apart during Cyclone Tracy in Darwin, Australia, in 1974, along with half of the city. The cause—inadequate component fasteners. The building was a good idea; the attention to detail not good enough; the result complete failure.17
III
We now know what hurricanes and their typhoon cousins are. We know where they start, and some of the hows and whens, and all this knowledge is useful. We don't know much about the whys, though. Nor is it likely that we will ever be able to control hurricanes, though that hasn't stopped people from trying.
The first clues to the hows and whens of hurricane formation lie in the wheres—where they start and, more interestingly, where they don't. Hurricanes never start on the equator, for example— there is no Coriolis force at the equator, and so no way to get a storm spinning. They always start in latitudes just high enough for the Coriolis force to be appreciable. But they never start at high latitudes either—the ocean is too cool there, the sea surface temperatures, or SSTs, as the hurricane hunters call them, far too low. They may have their remote origins over land, especially where high heat and cool air produce thunder cells, but hurricanes proper never form on land—evaporated moisture is their fuel. Hurricanes hardly ever form in the southern Atlantic either—before they can properly organize they are broken up by the prevailing westerlies, which in the southern hemisphere are much closer to the equator, although in 2004, for the first time, Hurricane Catarina struck Brazil, the extraordinary product of high sea-surface temperatures, low vertical wind shear, and strong mid-to-low-level blocking currents. Few tropical cyclones start in the smaller Indian Ocean. Or if they do, they don't amount to much—the "fetch" of the ocean, the amount of sea available for a storm's nourishment as it travels, is too small.
Many of the Atlantic storms, like Ivan for example, are born in the Sahara, as the superheated air of the desert meets the cooler air over the mountains,