While we generally tell people it stands for Wet Bulb Globe Temperature…it seems that explanation falls short.  A more complete answer involves understanding how this all started with the US military utilizing several methods to evaluate the “real” heat conditions they were training/working/fighting in.  It involved factoring in the conditions that make us feel hot…like, direct sunlight, or no direct sunlight, air movement/wind, and humidity.   With some crude tools some evaluation of each of these factors separately was taken into account.  By applying some mathematical relationships between the data they were able to build a consolidated reading that is known today as a wet bulb temperature.  I found this definition in wikipedia and thought it was pretty good at explaining the mathematical relationships I mentioned above.

[From Wikipedia, the free encyclopedia]

The Wet Bulb Globe Temperature (WBGT) is a composite temperature used to estimate the effect of temperature, humidity, 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:

WBGT = 0.7Tw + 0.2Tg + 0.1Td

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:

WBGT = 0.7Tw + 0.3Tg

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 Flag color

1 <=79.9 No flag

2 80-84.9 Green

3 85-87.9 Yellow

4 88-89.9 Red

5 =>90 Black

The WBGT index was developed by the United States Marine Corps 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.

References

* Air Force Pamphlet 48-151

* U.S. Army Technical Bulletin Medical 507/Air Force Pamphlet 48-152

* Zunis Foundation background article