Relative Humidity is determined by means of wet bulb and dry bulb thermometers. The dry bulb temperature is the temperature of air as determined by a standard thermometer. The wet bulb temperature is determined by tying a wet wick over the bulb dipped in a reservoir containing distilled water. Airflow around the wick causes the evaporation of moisture thus lowering the temperature and producing a reading lower than that on the dry bulb thermometer.
The comparison between the two readings gives us the measurement of water vapour in the air.
1. The lesser the difference – the wetter the air.
2. The greater the difference – The dryer the air.
The readings can be plotted on a chart known as the psychrometric chart from where the properties of air vapour mixture like relative humidity, absolute humidity, dewpoint can be directly determined.