18.104.22.168. Measurement of Humidity
The amount of water vapour in the air can be described in at least 5 ways, in terms of:
1) the water-vapour pressure;
2) the relative humidity;
3) the absolute humidity
4) the mixing ratio
5) the dewpoint.
A full account of these definitions may be found in Linacre (1992). The standard instrument for measuring humidity is a psychrometer. This is a pair of identical vertical thermometers, one of which has the bulb kept wet by means of a muslin moistened by a wick dipped in water. Evaporation from the wetted bulb lowers its temperature below the air temperature (measured by the dry bulb thermometer). The difference between the two measured values is used to calculate the air's water-vapour pressure, from which the other indices of humidity can be determined.