Water droplets in clouds resist freezing because the shape of soil and dust particles in the air are not compatible with ice crystals and neither is dry ice (CO2), so droplets will not adhere to them until very low temperatures are reached (Breuer 1976). Therefore, since silver iodide's structure is so similar, ice crystals can readily adhere to it (Breuer 1976). If sufficient amounts of dry ice or silver iodide are injected into the cloud all the water will fall to the ground and the cloud will dissipate completely (Breuer 1976).
There are three basic kinds of cloud seeding: hail suppression, rain enhancement and fog dispersion- and two basic methods: cloud top and cloud base seeding (Weather Modification Inc). Hail suppression deals with a concept called beneficial competition (Dick 2001). Large hail forms when there are insufficient amounts of ice nuclei to absorb the super cooled water, usually because temperatures are hovering right around freezing (Breuer 1976). If this is the case, water droplets will continue to adhere to the few nuclei available creating larger and larger hail (Dick 2001). By dispersing silver iodide (most commonly used in this procedure) more ice nuclei are available for the water to adhere to therefore creating smaller hail, or none at all (Dick 2001). .
Rain enhancement follows the same procedure and only differs in the results that are sought (WMI). It is done in certain types of clouds under certain conditions, usually in warmer clouds, so as to create rain but not hail (WMI).
Fog dispersion is commonly used on airport runways and similar areas to dissipate fog (WMI). Fog is called a "warm cloud" because it lacks ice nuclei, so it is dispersed by using condensation nuclei (which are the same as ice nuclei at higher temperatures), usually kinds of salts, which have a high affinity to water, to attract droplets (Dick 2001). This same procedure is used in tropical dry climates for rain enhancement because clouds at these latitudes are usually "warm clouds" (Dick 2001).
