How is the dew point measured?

Weather Blog

Dear Tom,

How is the dew point measured?

Thanks, Abby Rosen

Dear Abby,

Today the city’s official dew point is measured electronically by sensors in automated weather stations.  The sensor measures humidity through capacitance.  It derives the dew point through a calculation based on measured relative humidity and the measured temperature of the heated capacitive element.  Before automation, the dew point was calculated from the difference between the air temperature and the “wet bulb” temperature, a reading obtained by placing a wet wick on a thermometer bulb and noting the temperature drop as the water evaporated. This was done with a sling psychrometer, an instrument that consisted of two thermometers (a wet bulb and a dry bulb) mounted atop a handle that was vigorously twirled, allowing the water on the wet wick to evaporate.

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