A psychrometer consists essentially of two thermometers exposed side by side, the surface of the sensing element of one being covered by a thin film of water or ice and termed the wet or ice bulb, as appropriate. The sensing element of the second thermometer is simply exposed to the air and is termed the dry bulb.
The temperature measured by the wet-bulb thermometer is generally lower, due to evaporation of water from the wet-bulb, than that measured by the dry-bulb. The difference in the temperatures measured by the pair of thermometers is a measure of the humidity of the air; the lower the ambient humidity, the greater the rate of evaporation and, consequently, the greater the depression of the wet-bulb temperature below the dry-bulb temperature. The magnitude of the wet-bulb depression is related to the ambient humidity by a psychrometer formula.
This psychrometric method is in widespread use for observational purposes. Instruments applying this method are also commonly used as working standards.