Ice-forming nuclei in transvaal, Republic of South Africa

Abstract

Concentrations of ice-forming nuclei (IFN) active by sorption and by condensation-followed-by-freezing were assessed during February and March, 1986 in Transvaal, Republic of South Africa. Concentrations were found to be very low; at S w just below 0% and at 2%, IFN concentrations below 10 m −3 at a temperature of −15°C were present on 44 and 28% of the days, respectively. Number concentrations and the rates of activation of IFN active through condensation-followed-by-freezing were found to be larger than those active by sorption. IFN active by condensation-followed-by-freezing usually nucleated ice at higher temperatures than the IFN active by sorption. The number concentration of IFN active through condensation-followed-by-freezing was found to be independent of water vapor supersaturation over liquid water in the range between 0 and 6% in the temperature range from −12° to −22°C. Evaporation and condensation of water vapor on IFN active through condensation-followed-by-freezing did not deactivate them.