WBGT in one sentence
WBGT estimates how much heat stress a body is under while working in direct sun. Like the heat index it uses temperature and humidity, but it also accounts for wind speed, sun angle, and cloud cover.
WBGT and the heat index
The heat index assumes you are in the shade. WBGT is measured in direct sunshine, and the National Weather Service notes that direct sunlight can add as much as 15 degrees to the heat index. A breeze works the other way, helping the body cool. That is why two days with the same heat index can call for very different practices.
How it is calculated
Outdoors, WBGT combines three readings:
- Natural wet bulb (70 percent): a wetted sensor open to the air, which shows how well sweat can evaporate.
- Black globe (20 percent): a dark globe that heats up in the sun, which captures radiant heat.
- Dry bulb (10 percent): ordinary air temperature.
The wet bulb reading, driven by humidity and how well sweat can evaporate, carries the most weight.
Who uses it
The military has long used WBGT to judge heat stress, and OSHA, many other countries, and college and high school athletic programs use it to guide how hard people work in direct sun. Many state athletic associations set heat rules by WBGT: as the number rises, the chart calls for more rest breaks, shorter practices, less equipment, and eventually no outdoor practice at all.
Measured or estimated
A WBGT meter measures the spot where it sits, at the moment someone takes the reading. Where a state association requires readings from an approved instrument, that meter reading is the reading of record.
Sports Safe estimates WBGT for each field you monitor from current temperature, humidity, wind, and sun, checks it against your state's chart where Sports Safe has one, and alerts coaches when a field moves into a stricter band. It is an estimate, not a meter, and a field's calibration only ever makes the reading more cautious.
Last reviewed October 14, 2026. This guide is general safety information, not a substitute for your organization's emergency action plan or the instructions of local authorities.