This is an aspirated type portable hygrometer for the accurate measurement of atmospheric humidity. It consists of two thermometers exposed side by side in a steady airflow produced by an attached aspirator. One of the thermometers serves as a dry bulb while the other, with its bulb kept moist with wet sleeve acts as a wet bulb. As water evaporates, the temperature of wet bulb is lowered to an equilibrium point. This depression of W.B. temperature along with the ambient D.B. temperature is a measure of moistness or humidity of the ambient air which can be readily obtained from a computed table or slide rule specially made to express in units of relative humidity, dew point temperature or vapour pressure.