What is a Psychrometer (hygrometer)?

A psychrometer is a type of hygrometer (meteorological instrument) used to measure the amount of humidity in the atmosphere.

Principle, Working & Construction

A psychrometer consists of two thermometers i.e. wet-and-dry-bulb thermometer, one that is dry and one that is kept moist with distilled water on a sock or wick. The two thermometers are thus called the dry-bulb and the wet-bulb. At different temperatures , evaporation of water from the wick 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 by comparing the readings using a calculation table that compares the ambient temperature (the temperature given by the dry bulb) to the difference in temperatures between the two thermometers.