For you runners (and we dancers) out there - from the New York Times [requires subscription]:
http://www.nytimes.com/2008/07/03/health/nutrition/03Best.html?em&ex=1215230400&en=56c7d9eb4e2e67ef&ei=5087%0A

To Beat the Heat, Learn to Sweat It Out
(Personal Best, New York Times, July 3, 2008)
by Gina Kolata

...An elite runner capable of finishing in less than two and a half hours on a cool day (41 to 50 degrees) would be 2.5 percent slower in warmer climes (68 to 77 degrees.) A three-hour marathoner on a cool day would be slowed by 12 percent in the heat, the researchers reported.
It may seem like a brilliant idea, then, to pour water over your head to cool down. That is what Floyd Landis did during a grueling ride on a hot day in the Alps during the 2006 Tour de France.
And last month, on that balmy Saturday, amateur runners used the same trick, dousing their heads, in an 8-kilometer race in Moorestown, N.J. Town residents also squirted runners with their garden hoses.
It is a useless ploy, said Samuel N. Cheuvront, another researcher at the Army institute. “Sweat must evaporate to provide cooling,” he said. “Dripping does not help.”
In fact, he added, if you get too wet you risk hidromeiosis, when sweat pores become blocked, which makes you even hotter.
At least most races are held in the morning, when it is usually cooler and more humid, than later in the day, when it is hotter and drier.
Cold and humidity stresses the body less; you heat up less when it is cooler. Relative humidity may be greater on cool mornings, but what really matters for sweat evaporation is water vapor pressure. And water vapor pressure is lower when the air is cooler, meaning sweat evaporates faster.
Dr. Cheuvront said that if you have to choose between exercising in the morning when it is 60 degrees and 80 percent humidity, or in the evening when it is 90 degrees and 50 percent humidity, choose the morning.
Yet as challenging as heat and humidity are, people can acclimate. Blood volume expands, which reduces the strain on the heart from the increased demand for blood flow to the skin and muscles. And sweating increases — people who are heat adapted sweat sooner and more profusely, allowing their bodies to cool more efficiently.

For example, if you are not acclimated and run for an hour in 98-degree heat, your core temperature may go up to 103 degrees, bordering on the danger zone, said Craig Crandall, who studies heat acclimation at the University of Texas Southwestern Medical Center in Dallas. But if you are acclimated, your temperature might be 101 degrees after an hourlong run, which is well within the safety zone. Acclimation takes at least five days, Dr. Cheuvront found.