Hygrometer

Hygrometers are instruments used for measuring humidity. The simplest form of a hygrometer consists of two thermometers, one of which has its bulb constantly kept wet. Evaporation from the bulb lowers the temperature so that this thermometer shows a lower temperature. Humidity is computed from the ambient temperature as shown by the "dry bulb thermometer" and the difference in temperature shown by the "wet bulb" and dry bulb thermometers. There are several other types of thermometers that are commonly used. Some of them can record the value of humidity on a piece of graduated paper so that the values can be read off the chart. Modern instruments use electronic means of recording the information.

Some hygrometers use the stretching of a human or animal hair under tension to determine the ambient humidity.

Besides green houses, and industrial spaces, hygrometers are also used in some saunas, humidors and museums.