since it includes the humidity factor as well as the air temperature. For example, work
that is relatively easy at a dry bulb temperature of 95F with a relative humidity of 50
percent (80F wet bulb) becomes impossible at the same dry bulb temperature when
the relative humidity approaches 100 percent (96F wet bulb). At a given wet bulb
temperature, the dry bulb temperature can vary upward over a wide range without much
physiological effect on an individual. However, the wet bulb temperature is not a good
index of the heat stress imposed upon individuals exposed to the sun. Air movement
also is not taken into account in these measurements. Many attempts have been made
to devise a single index combining all four of these factors which is simple and can be
used without elaborate equipment or training of personnel. Investigations suggest that
the WBGT index provides such a standard, as does the more portable Botspill.
4-22. WET BULB GLOBE TEMPERATURE INDEX
a. The WBGT index is computed from readings of (1) a stationary wet bulb
thermometer exposed to the sun and to the prevailing wind, (2) a black globe
thermometer similarly exposed, and (3) a dry bulb thermometer shielded from the direct
rays of the sun. All readings are taken at a location representative of the conditions to
which men are exposed. The wet bulb and globe thermometers are suspended in the
sun at a height of 4 feet above ground (figure 4-1) for at least 20 minutes before
readings are taken.
b. The wet bulb thermometer is a standard laboratory glass thermometer with its
bulb covered with a wick (heavy white corset or shoe-string). The wick is dipped into a
flask of clean, preferably distilled, water. The mouth of the flask should be about three-
fourths of an inch below the tip of the thermometer bulb. The water level in the flask
should be high enough to ensure thorough wetting of the wick. The water should be
changed daily after rinsing out the flask and washing the wick with soap and water. To
avoid erroneous readings, the water and wick must be free of all salts and soap.
c. The globe-thermometer apparatus consists of a 6-inch hollow copper sphere
painted flat black on the outside and containing a thermometer with its bulb at the center
of the sphere. The thermometer stem protrudes to the outside through a rubber stopper
tightly fitting into a brass tube soldered to the sphere (figure 4-1). The sphere has two
small holes near the top used for suspending the sphere with wire or strong cords. The
globe must be kept dull black at all times and free of dust or rain streaks, by dusting,
washing, or repainting if necessary.
d. The WBGT Index is computed as follows:
WBGT = 0.7 x wet bulb temperature
+ 0.2 x black globe temperature
+ 0.1 x dry bulb temperature (shade)
Example:
wet bulb temperature
80F x 0.7
= 56F
black bulb temperature
105F x 0.2
= 21F
dry bulb temperature
90F x 0.1
= 9F
WBGT Index
= 86F
MD0008
4-19