One way to measure the dew point is based on the observation that a bathroom mirror will “fog up” when the temperature equals the dew point during a bath or shower.
A dew point hygrometer consists of a small mirror, a narrow light beam and a light detector. The light beam is reflected off the mirror into the detector, which measures the brightness of the reflected light. When the mirror surface cools to the dew point temperature, dew forms on the mirror.
The water droplets on the chilled mirror block light from reaching the detector, causing the reflected light beam to look less bright. The reduction in the measured reflected light indicates the mirror is at the dew point temperature.
A psychrometer is an alternative instrument for determining the dew point. A psychrometer consists of two thermometers, one of which has a wet wick around its bulb and is called the wet-bulb thermometer.
Evaporation of water off the wick removes heat from the thermometer. The temperature of the wet-bulb thermometer drops according to the rate of evaporation, which depends on the dew point.
The thermometers have to be ventilated by either whirling the instrument around or using a fan.
After a few minutes, the temperature of the wet-bulb stabilizes at a particular temperature.
The temperature difference between the two thermometers is converted into a dew point temperature using a chart.