Extremely hot or humid weather may require that sports practices or games be modified or even cancelled because of the risk of heat illness. In deciding whether to do so, you should keep in mind that 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 also 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.