Windswept_ The Story of Wind and Weather - Marq de Villiers [66]
It is a curious fact of wind that even in the very worst storms, and even in an open field with no obstructions, the velocity at ground level is effectively zero. When hurricane forecasters refer to surface wind speeds they really mean velocities at a standard height of 33 feet above the surface; from 33 feet wind is assumed to increase in speed with height, and generally does (see Appendix 7). The Saffir-Simpson scale, then, refers to sustained wind speed, measured over a full minute, at 33 feet. Gusts can be considerably higher.16 Ratings assigned to hurricanes by the weather service are used to give an estimate of the potential property damage and flooding expected along the coast from a hurricane landfall, but wind speed is always the determining factor, as storm surges depend to a considerable degree on the slope of the continental shelf in the landfall region, on the elevation of the nearby land, and on topographical peculiarities— is there, for instance, the possibility of a funnel effect, which would push a surge higher than normal?
Anything above a Category 2 is considered a major hurricane, likely to do considerable damage to buildings and the landscape. (For the complete scale, and a representative sampling of major storms', see Appendices 3, 4, 5, and 6).
Only three Category 5 storms have ever hit continental America. The first was an unnamed storm that struck the Florida Keys in 1935, when the barometer fell to an unbelievable 892 millibars (26.35 inches). This Labor Day storm killed more than four hundred people; some of the victims were quite literally sandblasted, reduced to bones, leather belts, and shoes.17 The second was Hurricane Camille of 1969, which struck the Mississippi coast with sustained winds of over 190 miles an hour and a storm surge of 25 feet above the mean tide levels—a three-story wave rolled through Pass Christian, knocking over apartment buildings, and one appalled survivor who had retreated to his attic was forced to break the window and swim to a nearby transmission tower, from which he saw the water submerge the peak of his roof. He lived two miles from the ocean. Camille is still the most intense storm ever known to have made landfall in America; the winds were so strong—probably 200 miles an hour from Long Beach to the ironically named Waveland— that entire sections of the Mississippi coast vanished. Although there had been plenty of warning, and evacuations had gone on apace, hundreds of people were killed and more than 14,000 homes completely destroyed. It probably didn't much encourage the survivors when President Nixon ordered the dropping of a hundred thousand pounds of the pesticide Mirex on the ravaged communities in an effort to destroy the plague of rats that followed.
Hurricane Andrew of 1992 was the third. Originally classified as a Category 4, Andrew blew away the antennae and radar disks of the National Hurricane Center, which subsequently, prudently, moved inland. The storm's rating was upgraded to a 5 more than a decade later, in late 2003.
Katrina, the storm that mauled New Orleans in 2005, was briefly a Category 5, but was Category 4 at landfall. Ivan, the storm I had been tracking, reached Category 5 not once, not twice, but three times. No other storm on record has done that.
A wind-intensity guide for tornadoes exists too. Because of their explosive but transitory nature, they are really only categorized after the fact, by the damage they have caused.
The scale was first written in 1971 by Theodore Fujita of the University of Chicago, together with Allen Pearson, then director of the National Severe Storm Forecast Center. It is called the Fujita scale, and for those of us with weather anxiety, it makes ominous reading, even if Fujita was, in the upper reaches, somewhat stretching the limits of adjectival vocabulary. Fujita 1, or "moderate" tornadoes, range from 74 to 112 miles an hour, and will