Breeze, Page 2
 

This concept is even more powerfully conveyed graphically. In this chart, bars below the line represent wind-caused Heat Index lowering, bars above the line represent wind-caused Heat Index elevation.  Each bar is a different relative humidity, with relative humidity rising from left to right for each temperature cluster.

Wind effect on heat index vs. temp and rel humidity-2.gif (5930 bytes)


For example, at an air temperature of 95F, with a relative humidity of 40%, a 33.6 mph wind makes the Heat Index 0.7F warmer than it would be with a gentle breeze of 5.6mph.

It must be remembered that this chart refers to the effects of a high wind, 33.6mph, which according to the Beaufort scale of wind is properly termed a "moderate gale". Lesser winds will have proportionately smaller effects.

Here is what can be said of the effect of wind on Heat Index, the way that the air "feels":

  • The effect of wind cannot be accurately determined unless the air temperature and relative humidity are also known.
  • At high temperatures (95F and above) with low relative humidities (less than 40%), wind tends to produce worsening of heat stress on the athlete. (The vast majority of temperatures 95F are associated with relative humidity 40%.)
  • Even a "moderate gale" usually produces fairly limited effects, a 2 to 7F change, and lesser winds have proportionately less effect on heat stress.

An anecdote to illustrate this concept: Every year in late August, Wichita Falls, Texas, plays host to the Hotter 'n Hell Hundred Bicycle Ride. Since the inception of this race in 1982, more than 120,000 riders have participated in what has become the largest sanctioned 100-mile bicycle race in the US. Wet Bulb Globe Temperature usually averages above 85F, placing a great deal of heat stress on the riders. In 1994, 1800 riders were unable to finish the race and were assisted back to the starting line area in "sag wagons". Even riders who are well-conditioned and have recently finished a hundred mile race may succumb to the heat. Race Medical Director Dr. Larry Magruder says of the heat casualties, "The most consistent predictor of heat injury is wind, not heat. The years with the wind speed greater than 20 mph see the most heat-related injuries, not the years with the highest WBGT."

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