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The wet-bulb globe temperature (WBGT) is a measure of environmental heat as it affects humans. Unlike a simple temperature measurement, WBGT accounts for all four major environmental heat factors: air temperature, humidity, radiant heat (from sunlight or sources such as furnaces), and air movement (wind or ventilation).[1] It is used by industrial hygienists, athletes, sporting events and the military to determine appropriate exposure levels to high temperatures.
A WBGT meter combines three sensors, a dry-bulb thermometer, a natural (static) wet-bulb thermometer, and a black globe thermometer.[2]
For outdoor environments, the meter uses all sensor data inputs, calculating WBGT as:
where
- Tw = Natural wet-bulb temperature (combined with dry-bulb temperature indicates humidity)
- Tg = Globe thermometer temperature (measured with a globe thermometer, also known as a black globe thermometer)
- Td = Dry-bulb temperature (actual air temperature)
- Temperatures may be in either Celsius or Fahrenheit
Indoors the following formula is used:
If a meter is not available, the WBGT can be calculated from current or historic weather data.[2] A clothing adjustment may be added to the WBGT to determine the "effective WBGT", WBGTeff.
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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. The American College of Sports Medicine bases its guidelines on the intensity of sport practices based on WBGT.[3]
In hot areas, some[4] 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 University of Georgia adapted these categories for use in college sports as a guideline for how strenuous practices can be.[5]
| Category | WBGT (°F) | WBGT (°C) | Flag color |
|---|---|---|---|
| 1 | ≤ 78–81.9 | ≤ 25.6–27.7 | White |
| 2 | 82–84.9 | 27.8–29.4 | Green |
| 3 | 85–87.9 | 29.5–31.0 | Yellow |
| 4 | 88–89.9 | 31.1–32.1 | Red |
| 5 | ≥ 90 | ≥ 32.2 | Black |
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The heat index used by the U.S. National Weather Service and the humidex used by the Meteorological Service of Canada, along with the wind chill used in both countries, are also measures of perceived heat or cold, but they do not account for the effects of radiation.
The NWS office in Tulsa, Oklahoma, in conjunction with Oral Roberts University's mathematics department, published an approximation formula to the WBGT that takes into account cloud cover and wind speed; in limited experimentation (four samples), the office claimed the estimate was regularly accurate to within 0.5 °F (0.28 °C), even with a simplification that reduces the equation from a four-degree polynomial to a linear relationship (the authors noted that the linear approximation was not tested for air temperatures under 68 °F (20 °C) since the WBGT is designed to measure heat stress, which seldom occurs below that threshold).[6]
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