On a still, sunny afternoon in the tropics, the air temperature can read a comfortable 88°F while a person doing physical work outdoors is quietly heading toward heat exhaustion. The thermometer misses it because a thermometer only measures one thing. WBGT was built to measure everything that matters.
The formula, and what each part measures
For outdoor conditions in direct sun, WBGT is a weighted average of three separate temperatures:
WBGT = 0.7 · Tnwb + 0.2 · Tg + 0.1 · Ta
- Tnwb, natural wet-bulb temperature (70%). The temperature of a wetted wick exposed to natural airflow and sun. It captures how effectively sweat can evaporate, which depends on both humidity and wind. In humid air, evaporation stalls and this term climbs. It is the single most important input, hence the 70% weight.
- Tg, globe temperature (20%). The temperature inside a matte-black copper sphere sitting in the sun. It absorbs solar radiation exactly the way a human body or a dark uniform does, capturing the radiant load that a shaded thermometer never sees.
- Ta, dry-bulb air temperature (10%). The ordinary air temperature. Necessary, but on its own it badly under-warns outdoors, which is exactly why it carries the smallest weight.
Indoors or fully in the shade, where there is no solar load, the weighting shifts to 0.7 · Tnwb + 0.3 · Tg. ClimaSafe uses the full outdoor form because that is the situation the U.S. Virgin Islands live in.
Why WBGT beats air temperature
The human body sheds heat two ways: by radiating it and by evaporating sweat. Both fail in the tropics. High humidity means sweat cannot evaporate, so the body's main cooling system stops working, and full sun pours radiant heat back in faster than the body can lose it. WBGT accounts for all four of the factors that govern this balance:
- Air temperature, the baseline heat load.
- Humidity, whether sweat can evaporate at all.
- Wind, moving air that strips heat away and aids evaporation.
- Solar radiation, direct and diffuse sun, and its angle in the sky.
The familiar heat index only uses the first two, and it assumes you are standing in the shade. That makes it a reasonable estimate for how hot it feels on a porch, and a poor one for a construction site, a ball field, or a hiking trail. See our full comparison of WBGT vs. the heat index.
The threat categories
ClimaSafe colors every reading against the National Weather Service WBGT threat scale. The bands are guidance, not guarantees, a fit, acclimatized worker and an unacclimatized visitor face very different risk at the same number.
| WBGT (°F) | Category | Typical guidance |
|---|---|---|
| Below 80.0 | Low threat | Normal activity; stay hydrated. |
| 80.0 to 84.6 | Elevated threat | Add fluids; watch unacclimatized people. |
| 84.7 to 87.7 | Moderate threat | Rest breaks and shade; adjust intensity. |
| 87.8 to 89.7 | High threat | Curtail strenuous work and sport. |
| 89.8 and above | Extreme threat | Many bodies cancel outdoor activity. |
The honest limitations
WBGT is a measure of environmental heat stress, not a verdict on any one person. It does not know your workload, your clothing, your medications, or how many days you have been acclimatizing. Reef-black asphalt can run far hotter than the regional reading, and a shaded, breezy porch far cooler. Treat the number as the baseline the environment sets, then adjust for the specific human and the specific spot, which is exactly what the analysis panel in thelive map helps you do.
Sources
- U.S. National Weather Service. WBGT, Wet Bulb Globe Temperature. weather.gov/ict/WBGT
- OSHA. Heat, Hazard Recognition. osha.gov/heat-exposure/hazards
- Liljegren, J.C., et al. (2008). Modeling the Wet Bulb Globe Temperature Using Standard Meteorological Measurements. J. Occup. Environ. Hyg. doi:10.1080/15459620802310770