Wet bulb, dry bulb
An explanation of the terms "wet bulb" and "dry bulb" in reference to temperature measurement, and their particular usefulness. March 6, 2000
Q.
Does anybody have an easy explanation of the meaning of "wet bulb" and "dry bulb" as they pertain to temperature measurement and other readings?
A.
The dry-bulb is a thermometer that is dry. The D-B temperature is the temperature that you hear on the weather report or that you read on a thermostat in your house.
The wet-bulb is a regular thermometer with a wet (distilled water) muslin wick covering it, and brisk air flow across the wick (600 feet per minute). As a result of evaporative cooling, the W-B temperature will be cooler than the D-B temperature.
If you know the D-B and W-B temperatures, you can look up relative humidity (RH), equilibrium moisture content, dew point, etc. This relationship was established in about 1909 by the U.S. Weather Bureau -- I think the scientist's name was Marvin.
Gene Wengert, forum moderator
About the wet-bulb temperature:
As Gene stated, the wet-bulb temperature will be lower than the dry-bulb temperature because of evaporative cooling (assuming less than 100% RH). The difference between the dry-bulb temp and the wet-bulb temp is known as the "depression." The larger the depression (larger difference between wet- and dry-bulb), the lower the RH.
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