The wet-bulb temperature is the lowest temperature to which unsaturated air can be cooled by evaporating liquid water into it.
This may sound horribly obscure, but it serves as a very useful measure of human comfort. It combines atmospheric humidity and temperature in a way that helps determine our comfort level while in the shade, especially during hot and humid conditions.
Wet-bulb temperature is related to other humidity indicators such as dew point temperature. For example, at 100% relative humidity, the wet-bulb temperature is equal to air temperature. At lower humidity, the wet-bulb temperature is lower than air temperature due to evaporative cooling.
We determine wet-bulb temperature by using a thermometer that has a water-moistened cloth, or wet wick, wrapped around its bulb. The wet wick is connected to a water reservoir and then ventilated with ambient air. As air flows over the wet wick, water evaporates. Evaporation requires energy. As water evaporates from the wick, it absorbs heat from the surrounding air and the thermometer itself, lowering the temperature reading. After a few minutes, the temperature of the wet bulb stabilizes, and that temperature is the wetbulb temperature.
The world is getting hotter and there is a limit to how much heat our bodies can endure. Our body’s ability to cool itself relies on sweating and evaporative cooling. High wet-bulb temperatures indicate dangerous heat stress levels, as the body’s ability to cool itself becomes compromised when evaporation is limited. Exposure to a wet-bulb temperature of 95 degrees Fahrenheit (35 degrees Celsius) for longer than six hours is dangerous, as the human body cannot cool itself. Prolonged exposure to these conditions can become life-threatening.
The wet-bulb globe temperature is another heat stress index: It extends the wet bulb temperature by combining air temperature, humidity, radiant heat and wind to assess the risk of human heat exposure.
Steve Ackerman and Jonathan Martin, professors in the UWMadison department of atmospheric and oceanic sciences, are guests on Wisconsin Public radio at noon the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
