Temperature, Humidity and Heat Index Definitions

Dry Bulb Temperature.  This is the ordinary air temperature that can be obtained from any ordinary thermometer that is hung in the air, in the shade.

Wet Bulb Temperature. This temperature is measured by making a minor modification to the same thermometer used for Dry Bulb Temperature. The modification is to place a cotton wick (like a little cotton sleeve) around the bulb of the thermometer, then soak that wick with water. What this does is to create an evaporative halo around the thermometer bulb, and by means of that evaporation the Wet Bulb Temperature will always be lower than the Dry Bulb Temperature, UNLESS relative humidity is 100%. At that point, the two temperatures will be the same. The Wet Bulb Temperature measures the ease with which sweat will evaporate.

Relative Humidity. This common measure of atmospheric water content describes the amount of water vapor in the air compared with the amount of water vapor that the air can hold at a given temperature. The warmer the air, the more water it can hold.

WBGT. This stands for Wet Bulb Globe Temperature. It is the heat stress standard developed by the US Marine Corps to guide training at their Parris Island center. This standard has been very successful, and it has been adopted by a wide variety of sports organizations, medical advisory groups and governmental agencies. WBGT is a weighted average of three temperatures: (0.7 x Wet Bulb)+(0.1 x Dry Bulb)+(0.2 x Black Globe). The Wet Bulb and Dry Bulb Temperatures are those outlined above. The Black Globe Temperature is measured by placing a standard thermometer into a black copper globe. This measures the radiant heat (mostly sunshine) component of heat stress.

Dew Point. This is a measure of humidity, the temperature at which the atmosphere can hold no more water and dew forms. It is a superb measure of human comfort, much more useful than Relative Humidity.