Measurements of the effectiveness of natural and artificial freezing-nuclei were carried out in a room-size cold chamber. The concentration of natural nuclei varied from approximately one per liter at about −13C to 1000 per liter at −25C. Since a concentration of 1 to 10 ice crystals per liter is sufficient to produce continuous precipitation, clouds which reach the temperature level of about −15C should precipitate. In general, this is verified by the observations of several investigators. Investigations of the effectiveness of silver iodide, cadmium iodide and cobalt iodide indicate that cobalt iodide is almost as effective a freezing nucleus as is silver iodide. However, due to its high hygroscopicity, cobalt iodide probably can not be used to seed clouds from the ground. The threshold temperature at which silver iodide and cobalt iodide are effective is approximately 10C higher than that at which natural aerosols of well industrialized and populated regions are effective. Consequently, whenev... Abstract Measurements of the effectiveness of natural and artificial freezing-nuclei were carried out in a room-size cold chamber. The concentration of natural nuclei varied from approximately one per liter at about −13C to 1000 per liter at −25C. Since a concentration of 1 to 10 ice crystals per liter is sufficient to produce continuous precipitation, clouds which reach the temperature level of about −15C should precipitate. In general, this is verified by the observations of several investigators. Investigations of the effectiveness of silver iodide, cadmium iodide and cobalt iodide indicate that cobalt iodide is almost as effective a freezing nucleus as is silver iodide. However, due to its high hygroscopicity, cobalt iodide probably can not be used to seed clouds from the ground. The threshold temperature at which silver iodide and cobalt iodide are effective is approximately 10C higher than that at which natural aerosols of well industrialized and populated regions are effective. Consequently, whenev...