Hygrometer is a weather instrument used for the Measurement of Relative Humidity of Air. Hydrometer with fewer technicalities in a much simpler form is known as psychrometer. A psychrometer has two thermometers with two bulbs, a dry and a wet one. The wet bulb is used to measure wet-bulb temperature.

The wet-bulb temperature is measured in a hygrometer when the temperature lowers in the wet bulb after evaporation. The dry bulb temperature gradually becomes high. The wet bulb gets coated with a thin layer of ice if the air temperature is below freezing. However it may still be warmer than the dry bulb.

The Relative Humidity is calculated firstly from the temperature shown by the dry-bulb thermometer and then the difference of temperatures between the wet-bulb and dry-bulb thermometers. Besides, the relative humidity can also be calculated from the point of intersection of the wet-bulb temperature and the dry-bulb temperature on the psychrometric chart.

Another hygrometer that uses the wet and dry bulb method to measure the relative humidity is the sling psychrometer. In this device there are thermometers attached to a handle. They are then spun around for some time in the air. These hygrometers are generally used in outdoor.

Another type of hygrometer is a Dew point Hygrometer. The word dew point means the temperature at which moist air reaches the water vapor saturation point under constant pressure. The cooling of the air, at this saturation temperature, results in condensation of water. A chilled mirror and optoelectronic mechanism are used by the Dew point Hygrometer for detecting the condensation temperature.