Whirling psychrometer consists of a set of ’dry’ and ’wet’ bulb thermometers. It is used to calculate the Relative Humidity of the atmosphere.
In order to get the accurate readings of the dry and wet bulb thermometers, the whirling psychrometer is rotated. This is because the maximum depression of the wet bulb will depend upon the rate of evaporation.And, it is determined that the maximum rate of evaporation will happen when the speed of wind flowing through it is greater than 7 knots. This can be achieved by rotating the whirling psychrometer. Rotation of psychrometer will result in generation of air speed in excess of 7 knots passing by the wet bulb resulting in maximum depression of the wet bulb and hence better readings.