Low Temperature Cloud Chamber Studies on Water Vapour

Abstract

The critical supersaturation for the precipitation of water droplets or ice crystals from the vapour was measured between 0°C and -75°C by an expansion-chamber technique. Nitrogen was used as the carrier gas, and measurements were made both in the presence and in the absence of an electric field. The temperature dependence of critical supersaturation was found to be the same in both situations. Thus existing critical supersaturation data for nucleation of water droplets from the vapour in the presence of an electric field probably represent heterogeneous nucleation on ions. Comparison of the data taken in the presence of a field with the Becker-Doring equation for homogeneous nucleation shows fair agreement at the higher temperatures but marked negative deviation at lower temperatures. The precipitate particles appeared to be spherical above -65°C, while at lower temperatures precipitation was in the form of ideomorphic ice crystals.