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The Wet Globe Kit pictured (NSN 6665-01-381-3023) utilizes the WBGT index to measure the potential heat stress risk. Temperature and humidity are generally monitored to evaluate risk, however the WBGT Index also accounts for the impact that direct sunlight has on the subject.

The Wet Globe Kits manufactured by Sigma Products include three thermometers to measure three different environmental factors:

Dry Bulb (DB) - Shielded from direct sunlight - reads actual air temperature.
Wet Bulb (WB) - With moistened wick over the bulb - will read varying temperatures depending on the rate of evaporation. Rate of evaporation varies with the humidity level.
Black Globe (BG) - With black copper sheath over bulb - A temperature measurement that will vary with the intensity of the sun.

The calculator included with the Wet Globe Kit will convert these three measurements into the WBGT reading. You may actually calculate the WBGT as follows:

WBGT Index = 0.7 WB + 0.2 BG + 0.1 DB

Wet Globe Instruction Manual