Dry ice was frequently used in early cloud seeding programs in the United States in the 1950s and early 1960s. Dry ice has received increased attention in recent years due to its low cost and high effectiveness. Silver iodide has been the preferred seeding agent in the majority of cloud seeding programs in the United States. Particles of silver iodide are usually produced through a combustion process followed by rapid quenching which forms trillions of effective freezing nuclei per gram of silver iodide consumed. Cloud seeding by silver iodide can be carried out using ground-based or aerial generators.
Liquid propane is a freezing agent much like dry ice.
0 C. Dispensing is limited to ground-based systems because it is a flammable substance. As temperatures approach -40 o C, water vapor in the air rapidly condenses into trillions of cloud droplets which immediately freeze and grow into tiny ice crystals. Propane is used operationally in clearing supercooled fog from airports in Alaska and the northern portion of the continental U.S. Pseudomonas syringae , a bacterium thought to reduce frost damage in plants, has been shown to be an effective nucleating agent. Use of this bacterium as a seeding agent has been limited to producing snow in ski resorts, although there have been some experiments with aerial applications. Cloud Seeding Delivery Systems.