WBGT (Wet-Bulb Globe Temperature)
A composite heat metric used by athletic governing bodies and the US military, factoring in temperature, humidity, sun, and wind.
WBGT (Wet-Bulb Globe Temperature) is a composite index designed for assessing outdoor heat stress on workers and athletes. It uses three measurements: dry-bulb temperature (regular thermometer), wet-bulb temperature (humidity-adjusted), and globe temperature (a black globe sphere that absorbs solar radiation and represents radiant heat load).
The formula: WBGT = 0.7 × wet-bulb + 0.2 × globe + 0.1 × dry-bulb. The wet-bulb dominates (70% weight) because evaporative cooling is the body's primary heat dissipation mechanism.
WBGT is the preferred metric over heat index for outdoor sports and labor because it directly accounts for sun load, which the heat index does not. A WBGT meter (a digital device with three sensors) measures it in the field.
Activity thresholds vary by population:
- WBGT 82 F: caution for trained athletes, modify intensity for non-acclimatized.
- WBGT 88 F: cancel practice for non-acclimatized, reduce for trained.
- WBGT 90 F: cancel practice entirely.
US Army uses similar bands: Yellow flag (WBGT 80-85), Red flag (85-88), Black flag (above 88, no strenuous outdoor activity). Most college and high school athletic associations have adopted similar tiered systems.