Meteorological instruments -- Measurement of Humidity
Psychrometer (mercury-in-glass type)
The psychrometer consists of two mercury-in-glass thermometers, usually being mounted vertically inside a Stevenson screen or a thermometer shed.
The surface of one of the thermometer bulbs, that is termed 'Wet Bulb', is kept wet by a cotton cover through which is threaded a cotton wick extending at least 2 cm to a reservoir of distilled water. The other thermometer is simply exposed to the air and is called the 'Dry Bulb'. The relative humidity of the atmosphere can be calculated from the temperature readings (dry and wet bulb) by making use of a humidity slide rule or a computer program developed by the staff of the Observatory.