Dear Tom,
How is relative humidity measured?
— Robert Kufka
Dear Robert,
It’s done in three ways. Automated weather systems measure the reflection of a beam of light from a heated mirror. The mirror is chilled until moisture in the air condenses on the mirror and distorts the beam, giving the dew point temperature, which is easily converted into the relative humidity value.
Before automation, weather observers used a sling psychrometer, a thermometer whose bulb is covered with a wet sleeve. Evaporation lowered the psychrometer to the wet-bulb temperature, from which relative humidity was calculated.
Finally, there is the human hair hygrometer. Swiss geologist Horace Benedict de Saussure discovered that human hair expands and contracts as humidity increases and decreases, and that led to his invention of the hygrometer in 1783. Hair hygrometers are reliable, inexpensive and excellent instruments for measuring indoor relative humidity.
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Weather Report is prepared by the WGN-TV Weather Center, where Tom Skilling is chief meteorologist. His forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.
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