The manually operated frost-point hygrometer and a frost-point hygrometer employing photo-electric indication of the dew or hoar-frost deposit, have both shown their value as meteorological instruments; both can measure absolute humidities as low as 1 mg/m3, corresponding to a frost point of −80°C. To reduce the observational skill required, a fully automatic hygrometer was developed; the conditions which the automatic instrument must fulfil are outlined and some of the special difficulties are discussed. A brief description is given of an instrument which has fulfilled laboratory tests and will measure any frost point from room temperature down to −80°C. Operation below about −80°C is impossible, owing to the slow rate of crystal growth at such temperatures.