Windswept_ The Story of Wind and Weather - Marq de Villiers [67]
A quarter of all tornadoes are marked as "significant" (F2), and only 1 percent are Fujita 3s or above, the most violent categories.18
Other useful measurement schemes devised in the twentieth century include the Mach number, the Reynolds number, and, most useful of all, at least in more northerly regions, the so-called wind chill scale. The Mach number, named after Ernst Mach (1838-1916), is used mostly by the military, by NASA, and, as a knowing aside, by former passengers of the transatlantic Concorde flights. It is, simply put, the speed of a moving object compared to the speed of sound; no winds, not even the most violent tornadoes, approach Mach 1. The Reynolds number, named after English engineer Osborne Reynolds, is on the face of it somewhat more esoteric. It's a way of expressing that how winds move through and around objects depends on their speed, density, temperature, viscosity, and compressibility. A wind's Reynolds number indicates whether the flow will be laminar (streamlined) or turbulent. It's widely used in aircraft and car design and to define wind flows around buildings.19
The wind chill, on the other hand, has immediate connections for anyone out in a winter gale. It has always been tricky to measure, and has generally been met with a certain degree of skepticism, because to some degree it measures how people "feel" in cold winds, rather than how cold they really are. But it is important, because winds do exaggerate coldness, and from about — 35° Celsius, severe frostbite sets in within ten minutes—and much faster in high winds.
On calm days, the human body is somewhat insulated from ambient temperature by warming a thin layer of air close to the skin, the so-called boundary layer. Wind interrupts the boundary layer, exposing the skin directly to the air. It takes energy to warm up a new layer, and if successive iterations are blown away, the body feels— and gets—colder. Wind has another effect: It makes you feel colder by evaporating skin-surface moisture, a process that draws away yet more heat. When the skin is wet, it loses heat much faster than when it is dry.
The original wind chill formula was derived from experiments conducted in 1939 by Antarctic explorers Paul Siple and Charles Passel. Their table, first published in 1945, was devised by hanging a small plastic cylinder filled with water on a pole and measuring how long it took to freeze in various wind and temperature conditions. Their formula was modified somewhat over the years, but it still had two basic flaws as a practical guide. Human bodies shiver, for one thing—a plastic cup just doesn't react to cold in the same way. In addition, the measurement was done at the conventional height of 33 feet, where the winds are much stronger than at an average human height. So their table was unnecessarily severe.
In the 1970s the scale was modified by Robert Steadman of Texas Technological College in Lubbock, who proposed a scale that included not only wind but the intensity of sunlight, clothing worn, and other factors.20
And there it rested until the turn of the millennium. The push for a still more accurate measure, unsurprisingly, came from north of the U.S. border—Canada is not as dismally frigid as many Americans profess to believe, but, yes, it is colder. A Canadian-sponsored international symposium on wind chill attracted participants from thirty-five