• Wet bulb: a thermometer wrapped in a wet wick. Evaporation cools it, the way sweat cools skin, so it reads humidity and wind. Humid, still air means less cooling.
• Black globe: a thermometer inside a black sphere, which heats up in the sun to show the radiant heat from sun and hot surfaces.
• Dry bulb: the ordinary air temperature.

Outdoors they're weighted 70% wet bulb, 20% globe and 10% dry bulb. Humidity counts the most because sweat that can't evaporate doesn't cool.

Temperature alone misses humidity, sun and wind. The heat index adds humidity but is calculated for shade and light wind, so it can understate the risk on a sunny field. WBGT measures the conditions athletes are actually in, which is why it's what the National Athletic Trainers' Association, the Korey Stringer Institute, the American College of Sports Medicine and the military base their heat guidelines on, and why a growing number of state high school associations require it.

Exertional heat stroke is one of the leading causes of death in high school athletes, and with monitoring and the right response it is almost always preventable. A WBGT policy turns a reading into a clear decision: practice as normal, add rest breaks, shorten practice, remove equipment, or stop.

• Measure on site with a WBGT meter, in the sun, where the athletes are.
• Take a reading before practice starts and every 30 minutes during it.
• Follow the level for that reading, below, and record what was done.
• Some National Weather Service offices publish WBGT forecasts, which help plan ahead.