Precipitation

The irregular and frequently unpredictable nature of precipitation has led to a number of direct attempts to either stimulate or hinder the precipitation process for the benefit of humans. In warm clouds, large hygroscopic particles have been deliberately introduced into clouds in order to increase droplet size and the likelihood of collision and coalescence to form raindrops. In cold clouds, ice nuclei have been introduced in small quantities in order to stimulate precipitation by encouraging the growth of large ice crystals; conversely, large concentrations of ice nuclei have been used to try to reduce numbers of supercooled droplets and thereby inhibit precipitation formation. Silver iodide, which has a crystalline structure similar to that of ice, is frequently used as an ice nucleus in these "cloud seeding" experiments. Although certain of these experiments have shown promising results, the exact conditions and extent over which cloud seeding works and whether apparent successes are statistically significant is still a matter of debate.