Heat Index vs Wet-Bulb Globe Temperature (WBGT)

Heat index uses two inputs: air temperature and relative humidity. WBGT uses four: dry-bulb (regular thermometer), wet-bulb (humidity), globe temperature (black sphere absorbing solar radiation), and wind. The extra inputs are why WBGT is preferred for outdoor activity decisions.

What each captures

Heat index assumes shade and light wind. The output represents the apparent temperature for a generic adult under those assumptions. It misses two things that matter outdoors: direct solar radiation and air movement.

WBGT formula: 0.7 × wet-bulb + 0.2 × globe + 0.1 × dry-bulb. The wet-bulb dominates (70% weight) because evaporative cooling is the body's main heat dissipation mechanism. The globe captures sun load. The dry-bulb is the smallest term because air temperature alone is the least relevant input.

The decision threshold differences

Heat index of 100 F (NWS "Extreme Caution") would suggest limiting strenuous activity. WBGT of 88 F would suggest canceling practice entirely for non-acclimatized athletes. The same outdoor conditions can produce both readings simultaneously.

Why the difference: WBGT accounts for the fact that an athlete or worker is generating substantial internal heat. The environment that is "Extreme Caution" for a casual outdoor walk is "cancel practice" for a runner.

Who uses which

NWS issues heat advisories in heat index because it is the public-facing metric.

US Army uses WBGT in their flag system (yellow / red / black).

NCAA, NFHS (high school athletics), and USA Track and Field use WBGT for practice and competition decisions.

OSHA's worker rules use heat index because the general workforce is more aligned with the casual-effort assumption.

Endurance event directors (marathons, triathlons) use WBGT.

In practice

For a backyard barbecue: heat index is enough. For a soccer practice: WBGT. For a school district deciding whether to cancel outdoor PE: WBGT. For a roofer scheduling a job site: heat index suffices but should add a sun correction (HI + 15 F when in direct sun for hours). For a marathon: WBGT, and the threshold to start considering modifications is much lower than the public would expect (WBGT 82 F).