Heat is a big hurdle to overcome in getting players acclimatized, especially in this age of air conditioning.  Our players go from air-conditioned houses/schools/cars to a field and much physical activity and exertion.  All of the guidelines suggested by U.S. Soccer — the WBGT chart, the work-to-rest ratios, and suggestions for parameters for cancellation of training and match play — can be found in the Recognize to Recover program at ussoccer.com.