Heat Index vs Wet Bulb Globe Temperature — What Is the Difference?

The heat index uses only air temperature and humidity and is calculated for shady conditions. Wet bulb globe temperature (WBGT) is an indicator of heat stress on the body at work or play in direct sunlight, and it also accounts for wind speed, sun angle and cloud cover. That is the National Weather Service's own distinction. In practice: the heat index is a good everyday check from weather data, and WBGT is the better tool when people will be working hard or exercising outdoors in the sun.

What goes into each measure

The WBGT formula

The National Weather Service gives the outdoor formula as WBGT = 0.7 Tw + 0.2 Tg + 0.1 Td, where Tw is the natural wet-bulb temperature, Tg is the temperature of a black globe thermometer, and Td is the ordinary dry-bulb air temperature. The weights matter. The wet-bulb term, which reflects humidity and evaporative cooling, carries 70% of the result. The globe term, which captures sun and radiant heat, carries 20%. Plain air temperature carries only 10%.

Worked example: the same air temperature, different sun and humidity

The readings below are assumed for illustration, not measured on a real site, and all are in degrees Fahrenheit. Each WBGT is computed with the formula above and rounded to one decimal place.

Moving from the overcast row to the full-sun row changes only the globe reading, from 90 to 125, and adds 7.0 degrees of WBGT. Moving from humid to drier air in full sun lowers the wet-bulb reading by 8 degrees and takes 5.6 degrees off. A simple sensitivity check using the weights: 5 degrees more wet-bulb adds 3.5, 20 degrees more globe adds 4.0, and 10 degrees more air temperature adds only 1.0. The same formula works in Celsius: a wet bulb of 25°C, globe of 45°C and air of 33°C give a WBGT of 29.8°C, which is 85.6°F.

What the heat index misses in the same weather

Take a 92°F afternoon at 65% relative humidity. The heat index calculator's formula gives 108.3°F. The National Weather Service says heat index values assume shade and that full sunshine can add up to 15°F, which would put the felt value near 123.3°F. The heat index cannot tell you whether a particular job site, with a particular sun angle and cloud cover, sits at the low or high end of that range. A globe thermometer can.

Which to use when

WBGT is not a single universal cutoff. OSHA's heat pages refer to WBGT measurement and to limits that depend on the work being done, and the National Weather Service pages on WBGT point to activity-association thresholds that tie rising WBGT to shorter periods between breaks. That is why a team or employer usually adopts one specific table and one specific way of measuring, rather than borrowing numbers from a weather app.

Common mistakes and limits

• Applying heat index category cutoffs to a WBGT reading. The two are on different scales and are not interchangeable; WBGT limits depend on the guideline in use, the workload and acclimatization.
• Converting between them with a fixed offset. There is no single offset, because the gap depends on sun, wind and humidity, as the table above shows.
• Using a WBGT estimate without knowing its source. Instruments and weather-data estimates can disagree, so follow the method your employer or sport's governing body specifies.
• Treating either number as a personal safety guarantee. Age, medications, health and fitness all change individual risk.