Wet Bulb Globe Temperature
The Wet Bulb Globe Temperature (WBGT) is a composite temperature used to estimate the effect of temperature, humidity, wind speed (wind chill) and solar radiation on humans. It is used by industrial hygienists, athletes, and the military to determine appropriate exposure levels to high temperatures. It is derived from the following formula:
<math> WBGT = 0.7T_w + 0.2T_g + 0.1T_d </math>
Where
- Tw = Natural wet-bulb temperature (humidity indicator)
- Tg = Globe thermometer temperature (measured with a globe thermometer, also known as a black globe thermometer, to measure solar radiation)
- Td = Dry-bulb temperature (normal air temperature)
- Temperatures may be in either Celsius or Fahrenheit
Indoors, or when solar radiation is negligible, the following formula is used:
<math> WBGT=0.7T_w + 0.3T_d </math>
The American Conference of Governmental Industrial Hygienists publishes threshold limit values (TLVs) that have been adopted by many governments for use in the workplace. The process for determining the WBGT is also described in ISO 7243, Hot Environments - Estimation of the Heat Stress on Working Man, Based on the WBGT Index.
In hot areas, some US military installations display a flag to indicate the heat category based on the WBGT. The military publishes guidelines for water intake and physical activity level for acclimated and unacclimated individuals in different uniforms based on the heat category.
| Category | WBGT °F | WBGT °C | Flag color |
|---|---|---|---|
| 1 | <=79.9 | <=26.6 | No flag |
| 2 | 80-84.9 | 26.7-29.3 | Green |
| 3 | 85-87.9 | 29.4-31.0 | Yellow |
| 4 | 88-89.9 | 31.1-32.1 | Red |
| 5 | =>90 | =>32.2 | Black |
The WBGT index was developed by the United States Marine Corps (Drs. David Minard and Constantine Yaglou) at Parris Island in 1956 to reduce heat stress injuries in recruits and has been revised several times.
The heat index used by the National Weather Service and the humidex used by the Meteorological Service of Canada are also measures of perceived heat, but they do not account for solar radiation.
See also
References
- Air Force Pamphlet 48-151
- U.S. Army Technical Bulletin Medical 507/Air Force Pamphlet 48-152
- Zunis Foundation background article
External links
- Excellent description of WGBT and apparent temperature. Formulae in metric units
- http://www.wetglobe.com
- Fort Drum implementation
- Canadian Centre for Occupational Health and Safety guidelines
- Occupational Safety and Health Administration guidelines
- "About the WBGT and Apparent Temperature Indices" from Australian Bureau of Meteorology (includes a formula to approximate WBGT from dry bulb temperature and relative humidity)fr:Température au thermomètre-globe mouillé
