A method utilizing ice crystals to circumvent nucleation processes in cloud seeding activities is discussed in the framework of nuclei activation concepts. Ice, in the form of small crystals, would be a highly efficient cloud seeding materia up to 0 degrees Celsius. The lower limit on humidity would necessitate dispensing the crystals into air that is saturated with respect to ice but no requirement for 100 percent RH(relative humidity) was found. In fact, the lower limit varies with temperature linearly from about 95 percent RH at -5 degrees Celsius to 75 percent RH at -30 degrees Celsius. Preparation of small crystals was found possible at liquid nitrogen temperatures and the volume which could potentially be seeded seems to be large enough to seriously consider the ice crystals as a possible triggering material.
Hubbard, Kenneth G., "Weather Modification Studies: The Potential for Creating and Utilizing Ice Crystals in Weather Modification Activities" (1977). Reports. Paper 404.