WBGT vs. Heat Index: Which Should You Use?

WBGT vs. Heat Index: Which Number Should You Trust?

The heat index estimates how hot it feels in the shadefrom temperature and humidity alone. WBGT measures total outdoor heat stress, adding wind, direct sun, and cloud cover. For anyone making decisions about outdoor work, sport, or exertion, WBGT is the number to use.

Last updated July 9, 2026 · Live data refreshes every 15 min

Both are "feels-like" numbers, but they were designed for different worlds. The heat index is a public-forecast tool: a quick answer to "how uncomfortable is it out there?" WBGT is a safety instrument, engineered to decide whether a specific physical activity is safe to perform. In the tropics, the gap between those two questions can be the difference between a hard practice and a medical emergency.

What each one actually measures

Factor	Heat Index	WBGT
Air temperature	Included	Included
Humidity	Included	Included
Wind speed	Ignored	Included
Direct sun & radiant heat	Ignored, assumes shade	Included
Sun angle & cloud cover	Ignored	Included
Answers	"How muggy does it feel in the shade?"	"Is it safe to exert outdoors right now?"
Used by	Weather media, public forecasts	OSHA guidance · NFHS athletics · U.S. military

Why they diverge in the tropics

Two forces pull WBGT away from the heat index, and both are strong in the U.S. Virgin Islands:

The sun pushes WBGT up. The USVI sits at 18°N; in summer the sun passes nearly overhead. On an exposed field, court, dock, or trail, radiant load can add the equivalent of up to 15°F of heat stress the heat index cannot see. A shade-only number simply does not describe an unshaded body.
The trade winds pull WBGT down. A steady easterly breeze, the USVI's near-constant companion, speeds sweat evaporation and carries heat away, holding WBGT lower than temperature and humidity alone would suggest. The heat index gives wind no credit at all.

The result: on a sunny, breezy afternoon the two numbers can point in opposite directions from the same weather. That is not a flaw in either, it is the reason you have to pick the one that matches your decision.

Rule of thumb. If the question is "should we be exerting outside right now?", use WBGT. If the question is "how sticky will the walk to dinner feel?", the heat index is fine. When the sun is out and someone is working or playing, trust WBGT.

See both, live

ClimaSafe's map reports WBGT for every point across St. Thomas, St. John, and St. Croix, computed from the same weather that drives heat-index forecasts, so you can watch the sun-and-wind difference play out in real time. Learn the basics inWhat is WBGT?, or open thelive map now.

Sources

U.S. National Weather Service. Wet Bulb Globe Temperature vs. Heat Index. weather.gov/ict/WBGT
NOAA WPC. The Heat Index Equation. wpc.ncep.noaa.gov
OSHA / NIOSH. Heat exposure and WBGT. osha.gov/heat-exposure

Frequently asked questions

Which is more accurate, WBGT or the heat index?

For outdoor activity in the sun, WBGT is more accurate because it accounts for wind and radiant heat, which the heat index ignores. For a general sense of how muggy it feels in the shade, the heat index is fine. They answer different questions.

Why can the heat index look fine when it is actually dangerous?

The heat index assumes shade and still air. In full sun, radiant heat can add the equivalent of up to 15°F of stress that the heat index never counts, so a "moderate" heat index can coincide with a high or extreme WBGT on an exposed field or job site.

Does wind lower the heat index?

No. The heat index formula has no wind term at all. WBGT does: a steady breeze measurably lowers WBGT by speeding evaporation and carrying heat away, which is why a windy 90°F beach can be safer than a calm 88°F parking lot.

Should coaches and employers use WBGT or the heat index?

WBGT. The NFHS and many state athletic associations write their activity-modification policies around WBGT, and OSHA references it for occupational heat. Both bodies chose WBGT precisely because it reflects sun and wind on real work and play.