Skip Content

Meteorological instruments -- Measurement of Humidity

Psychrometer

A psychrometer consists 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.

Mercury-in-glass type psychrometer and platinum resistance type psychrometer are commonly used for meteorological purpose.

Mercury-in-glass type Psychrometer
Mercury-in-glass type Psychrometer
Platinum Resistance type Psychrometer
Platinum Resistance type Psychrometer