The behaviour of a single-hair hygrometer under varying conditions of temperature and humidity

A sensitive single-hair hygrometer is described which enables observations to be carried out, over an extended range, on the effects due to variations in relative humidity and in temperature. The temperature coefficient of a stretched hair in an atmosphere kept at a relative humidity of 100 per cent is negative for temperatures above about 1.5° C. and positive in the range below that temperature. If the hygrometer is kept at a constant temperature and the relative humidity of the air is decreased from 100 per cent there is an immediate increase in length, corresponding to an apparent increase in relative humidity of about 3 per cent, followed by a contraction to the equilibrium length characteristic of the lower humidity. It seems probable that this effect may account for the apparent supersaturation sometimes seen in balloon meteorograph records. With this hygrometer the scale value for relative humidity was dependent on temperature. The lag in response of the single hair varied with experimental conditions, and was often considerable. It will have its maximum effect when humidity is varying rapidly as in a rising balloon, but will be of much less importance with hair hygrometers used on the ground, since there conditions alter relatively slowly, and the hair will always be in approximate equilibrium with the air surrounding it.