At Least There's A Breeze: Wind/Heat

"Evenings in that part of the country must have been a kind of sad relief.
But today, with the sun bearing down, making the whole landscape shimmer with heat,
it was inhuman and oppressive."  Albert Camus in The Stranger

The sirocco is a hot, dry south wind coming out of northern Africa, Camus country. This parching wind can blow continuously for a day or two, but occasionally it lasts much longer. There have even  been cases in which a lawyer has cited the sirocco as part of the defense in a crime of passion, claiming that the defendant had been driven mad   by unrelenting, swirling heat. The wind is hardly a constant ally.

Still, faced with severe heat, most athletes would hope for a breeze. Humans have extraordinarily moist skin, and wind blowing across the skin is almost always lower in water vapor pressure than the skin surface, resulting in some degree of skin surface evaporation and cooling. Of course, the degree of evaporation is vitally dependent on the relative humidity of the air. As the air becomes more humid, less evaporation occurs, and there is less cooling effect from wind.

Wind also provides transfer of heat by convection. A cool wind blowing across warm skin will result in cooling of the skin by heat transfer independent of evaporation. However, when air temperature reaches 95F, the temperature gradient is reversed, and heat begins to flow from the air into the athlete. (See Sweat, Perspiration and Glow for a detailed account of this process.) The friendly breeze becomes oppressive.

In 1979, R.G. Steadman published "The Assessment of Sultriness, Parts I and II." This seminal work was adopted by the US National Weather Service as the basis for the Heat Index which the NWS uses in its Heat Advisories. Steadman examined every aspect of the interaction between humans and the environment,  including the effect of wind. Steadman observed that, as wind speed increases, the rate of convective heat exchange increases more or less proportionately. This results in progressive athlete cooling if the air temperature is less than 95F, but progressive heating if air temperature is 95or higher. Steadman performed calculations relating air temperature and relative humidity to change in Heat Index at wind speed of 33.6 mph, as compared to the "built-in" breeze of 5.6 mph which is always present in the Heat Index equation. Here is the result of those calculations:

          Heating or Cooling Effect on Heat Index  (in F) of a 33.6 mph Wind
                            Ambient Relative Humidity
Air Temp in F 0% 10% 20% 40% 60% 80% 100%
68 -7.74 -7.74 -7.74 -7.56 -7.2 -7.02 -7.38
77 -8.1 -7.56 -6.84 -5.94 -5.94 -6.84 -7.74
86 -2.34 -2.7 -3.24 -3.96 -5.58 -8.1 -12.6
95 +2.34 +2.16 +1.8 +0.72 -2.88
104 +5.76 +5.4 +4.5 0
113 +7.38 +6.3 +4.32
122 +8.28 +6.12

For example, at an air temperature of 86F and a relative humidity of 60%, a 33.6 mph wind would make the Heat Index (the way that the air feels) 5.58F cooler than it would be with a gentle breeze of 5.6 mph.

As this table shows, the wind tends to provide more cooling as temperature drops and relative humidity rises.  Due to  Dalton's Law of Partial Pressures, high temperatures are usually associated with low relative humidity.  For example, in a survey of temperatures in Tulsa, OK,  85% of temperatures of 95F and above were associated with a relative humidity of 40% or less. For Phoenix, the ratio is 99.8%. In the presence of such high temperatures with low relative humidity, wind has the adverse effect of heating the athlete.

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Copyright 1998 The Zunis Foundation. Last Modified: April 2, 2009.