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Meteorological instruments -- Humidity

Measurement of Humidity

Humidity is generally measured by a psychrometer:

Mercury-in-glass type Psychrometer
Mercury-in-glass type Psychrometer
weather family member -- Sponge Platinum Resistance type Psychrometer
Platinum Resistance type Psychrometer
Definition of relative humidity


Relative humidity is defined as the ratio, in per cent, of the observed vapour pressure to the saturation vapour pressure with respect to  water at the same temperature and pressure.

A hygrometer is an instrument for measuring the humidity of the atmosphere.  A psychrometer is a type of hygrometer that is most widely used.

Meteorological requirements


Humidity measurements are required for weather analysis and prediction, hydrological and agricultural application, and for climate studies.  They are  particularly important for their relevance to the changes of state of water in the atmosphere.

A hygrometer should be placed in a screen or shed to protect it from direct solar radiation, atmospheric contaminants, rain and wind.

Unit of measurement

per cent (%)