Discussion of “Supercooling and crystallization”

Abstract

The writer would like to present some data obtained during the spring of 1948 on observations of the spontaneous freezing temperatures of supercooled water drops.The apparatus used consisted of a thermally controlled chamber at atmospheric pressure. The drops of distilled water were suspended from paraffin‐coated Wollaston wire or glass fiber after the technique of Houghton [1933]. The temperature of the drops was measured using a thermocouple located one centimeter below the drop. Theory [Houghton, 1933] and test showed that when the ambient relative humidity (the relative humidity of the surrounding air at a great enough distance from the drop so as to be uneffected by the evaporation from the drop) is at saturation, the ambient and drop temperature are the same. In experiments with supercooled water drops under isothermal ambient conditions, some evaporation of the drop occurs. This is explained by the fact that the drop has a vapor pressure saturated in respect to its own water surface while the chamber is saturated in respect to ice particles on its walls. This results in the chamber being slightly unsaturated in respect to the vapor pressure of the drop. The question immediately arises as to the validity of the temperature measurements of the drop using the ambient temperature when the ambient temperature is changing. Experimental evidence indicates that the surface temperature of a drop in the range of radii used (150 to 1700 microns) follows the changes in the ambient temperature and relative humidity so closely that the total error in measuring the surface temperature of the drop by means of the ambient temperature is believed to be not in excess on one degree C. The direction of the error is such that the drop is colder than indicated by the ambient temperature.