Vapor Pressure Measurement of Supercooled Water

Abstract

A new dewpoint hygrometer was developed for subfreezing temperature application. Vapor pressure of supercooled water was determined by measuring temperatures at the dew-forming surface and the vapor source ice under the flux density balance, and by application of measured vapor pressure over ice from the Smithsonian Meteorological Table. The measured vapor pressure of supercooled water agreed well with the tables above approximately ?20°C, but below that temperature, a significant lowering of the pressure was discovered. An empirical equation to best fit the measured data was obtained. At ?30°C, the estimated specific latent heat of condensation became slightly higher than the table value by 3.4%, that of fusion considerably lower by as much as 66%, and the specific heat of supercooled water amounted to as much as 3.7 cal g?1 °C?1. Possible implications of the new results are pointed out. For example, the latent heat associated with cloud glaciation at temperatures colder than ?20°C, and especially colder than ?30°C, is found to be less than previously thought.