It can be OK to practice in 100-degree weather, or it could be dangerous. It depends on the humidity factor.
So impactful is humidity that, for example, 86-degree weather can be lethal if humidity is extremely high.
“Humidity is probably the key factor that could be the most difficult on the body with regards to exercise, because with the humidity, your body's ability to dissipate heat is stunted,” says Dr. George Chiampas, U.S. Soccer’s Chief Medical Officer. “You can’t sweat it off because the environment outside of you is hotter than yourself.”
Therefore, coaches must figure out the Wet Bulb Globe Temperature (WBGT) -- when deciding whether to cancel or delay training, or games.
“WBGT is becoming a more and more a common term in youth sports and in sports in general,” he says.
The R2R “Heat Guidelines” provide a graph to estimate WBGT (for which there are also apps, such as WeatherFX, and WBGT monitors). Also included in the Guidelines is a list of indicators of heat-illness and heat stroke (which can be life-threatening), management and recovery information, acclimatization procedure, and work-to-rest ratio formulas.
Emphasized also is access to cooling and hydration resources.
“We’re understanding more and more in regards to heat and acclimatization,” Chiampas said. “We also see that there have been extremes in temperatures over the last 10 years.”
The Guidelines break the nation into three geographic parameters based on how acclimated players are to heat, because, "Players who are playing in the South are more acclimated to warmer weather compared to players who are playing the North," Chiampas says.
The main emphasis of the Guidelines is player safety, but they also serve to optimize performance.
“If as coach, you start learning more about exercising in the heat, this is gives you some insight into the importance of it and that it is a factor in the ability of your players to play at a high level,” Chiampas says. “It also gives you some action items of things that you can do, such as adjust your work-rest ratio. But I think most importantly it's providing coaches, parents and players, as well as referees, a user-friendly process to implement a very important safety component.”
• The U.S. Soccer Federation R2R Guidelines are mandated for Development Academy clubs, serve as guidelines for USSF members, and meant as a resource for American soccer outside its membership.
• For Heat Guidelines, go HERE.
• The R2R home page is HERE.