Since the discovery of glaciogenic materials more than 40 years ago, silver iodide has been the most widely used cloud seeding material. Silver iodide enhances the ice crystal concentration in clouds by either nucleating new crystals or freezing cloud droplets.

The static mode of cloud seeding is based on the concept that clouds are deficient in ice nuclei and therefore additions of silver iodide crystals that mimic the structure of ice should result in a more efficient precipitation producing cloud system. Cotton and Pielke (1995) suggest that seeding using this hypothesis is limited to:

• Clouds which are relatively cold-based and continental;
• Clouds having top temperatures in the range -10 to -25 ºC;
• A time scale limited by the availability of significant supercooled water before depletion by entrainment and natural precipitation processes.

This hypothesis has been tested and scrutinized during the last decade in experiments with mixed results. Although there are constant indications that seeding can increase precipitation, a number of recent studies have questioned many of the positive results, weakening the scientific credibility of some of these experiments. As a result, there is some uncertainty as to the methodology of such a hypothesis.
 

The concept of dynamic seeding is a physically plausible approach that offers an opportunity to increase rainfall by much larger amounts than the static concept. This concept is to seed supercooled clouds with large enough quantities of ice nuclei to cause glaciation of the cloud. Due to seeding, supercooled liquid water is converted into ice particles, releasing latent heat, increasing buoyancy, and thereby invigorating cloud updrafts. In favorable conditions, this will cause the cloud to grow larger, process more water vapor, and yield more precipitation (Bruintjes, 1999). Enhanced downdrafts from the seeded clouds may also promote regions of convergence and the initiation of convection in the surroundings.
Rainfall increases of seeded clouds versus unseeded clouds are documented regularly in Texas. Although most of these evaluations show increases in rainfall mass estimated by radar, evidence on what the effect on area rainfall would be has not been documented.

References for this section:
Bruintjes, R. T.,1999: A review of cloud seeding experiments to enhance precipitation and some new prospects. Bulletin of the American Meteorological Society: Vol. 80, No. 5, pp. 805-820.

Cotton, W. R., and R. A. Pielke, 1995: Human Impacts on Weather and Climate. Cambridge University Press, 288 pp.