Wet Bulb Globe Temperature
Encyclopedia
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:
Where
Indoors, or when solar radiation is negligible, the following formula is used:
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.
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.
Wind chill
Wind chill is the felt air temperature on exposed skin due to wind. The wind chill temperature is always lower than the air temperature, and the windchill is undefined at the higher temps...
) 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:
Where
- Tw = Natural wet-bulb temperatureWet-bulb temperatureThe wet-bulb temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor. Wet bulb temperature is the lowest temperature that can be reached by the evaporation of water only. It is the...
(humidity indicator) - Tg = Globe thermometer temperatureMean radiant temperatureThe Mean Radiant Temperature is a concept arising from the fact that the net exchange of radiant energy between two objects is approximately proportional to their temperature difference multiplied by their ability to emit and absorb heat . Mean radiant temperature is simply the area weighted mean...
(measured with a globe thermometer, also known as a black globe thermometer, to measure solar radiation) - Td = Dry-bulb temperatureDry-bulb temperatureThe dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. Dry bulb temperature is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It is the...
(normal air temperature) - Temperatures may be in either CelsiusCelsiusCelsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
or FahrenheitFahrenheitFahrenheit is the temperature scale proposed in 1724 by, and named after, the German physicist Daniel Gabriel Fahrenheit . Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees...
Indoors, or when solar radiation is negligible, the following formula is used:
The American Conference of Governmental Industrial Hygienists
American Conference of Governmental Industrial Hygienists
The American Conference of Governmental Industrial Hygienists is a professional association of industrial hygienists and practitioners of related professions, with headquarters in Cincinnati, Ohio...
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
United States Marine Corps
The United States Marine Corps is a branch of the United States Armed Forces responsible for providing power projection from the sea, using the mobility of the United States Navy to deliver combined-arms task forces rapidly. It is one of seven uniformed services of the United States...
(Drs. David Minard and Constantine Yaglou) at Parris Island
Marine Corps Recruit Depot Parris Island
Marine Corps Recruit Depot Parris Island is an military installation located within Port Royal, South Carolina, approximately south of Beaufort, the community that is typically associated with the installation. MCRD Parris Island is used for the training of enlisted Marines...
in 1956 to reduce heat stress injuries in recruits and has been revised several times.
The heat index
Heat index
The heat index is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature — how hot it feels, termed the felt air temperature. The human body normally cools itself by perspiration, or sweating, which evaporates and carries...
used by the National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
and the humidex
Humidex
The humidex is an index number used by Canadian meteorologists to describe how hot the weather feels to the average person, by combining the effect of heat and humidity. The humidex is a unit-less number based on the dew point, but it is equivalent to dry temperature in degrees Celsius...
used by the Meteorological Service of Canada
Meteorological Service of Canada
The Meteorological Service of Canada , also known as "The Canadian Weather Service", is a division of Environment Canada, which primarily provides public meteorological information and weather forecasts and warnings of severe weather and other environmental hazards...
are also measures of perceived heat, but they do not account for solar radiation.
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)