Methods available for the measurement of relative humidities lying between 0.5 and 10% are discussed. A dew-point apparatus for measuring dew points down to[long dash]40[degree]C. is described, also a hygrometer using wet and dry thermocouples. Calibration curves are given showing the relation between the depression of the wet-junction temp. and the relative humidity over the above range for air-temps. between 10[degree]C. and 90[degree]C.