Experimental Determination of Ice Nucleation by Falling Dry Ice Pellets

Abstract The ability of dry ice to generate ice crystals in supercooled clouds has been studied under conditions which closely simulate the “free-falling” dry ice pellets of a cloud seeding operation. In conjunction with separate measurements of the sublimation rate of dry ice, and the fall velocity of the dry ice pellets, the number of ice crystals generated per gram of dry ice sublimed was determined. This number, about 8 × 1011, was independent of both the pellet size (0.3 to 2 cm in diameter) and the air temperature (−2 to −12C). The surface temperature of dry ice pellets falling at their terminal velocities in the size range was estimated from the sublimation rates to be about −100C. Empirical equations for terminal velocity, surface temperature and sublimation rate, applicable to other atmospheric conditions, were obtained.