A hygrometer is any instrument utilised for measuring the humidity, or water vapour content, of a gas (usually air).

The simplest form of a hygrometer is known as a "psychrometer". A psychrometer consists of two thermometers, one uses a dry bulb to measure the dry temperature and the other is kept wet to measure wet-bulb temperature.  Evaporation from the wet bulb lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer.

Relative humidity is calculated from the ambient temperature as shown by the dry-bulb thermometer. Alternatively it can be determined by locating the intersection of the wet- and dry-bulb temperatures on a psychrometric chart. 

The temperature at which dew or frost forms is a measure of the absolute humidity—the weight of water vapour per unit volume of air or other gas at the temperature before cooling.