In deciding when it is too hot to play sports depends on the heat index. In modifying or cancelling sports practices, keep in mind the following:
Hot, dry weather can be extremely dangerous. Because sweat evaporates very quickly in such conditions, your child won't feel sweaty, and neither you nor your child may recognize how much water he or she has lost.
As the relative humidity increases, the effectiveness of sweating in cooling the body decreases.
When the relative humidity is high, sweat drips off the skin so that the cooling benefit of evaporation is lost even at cooler temperatures, resulting in a build-up of body heat.