(note: Excerpts are from: Weather Modification Association (WMA) Capability Statement on Weather Modification- Adopted 2005)
It has been established that certain aspects of the weather, specifically cloud microphysical and precipitation processes, can be intentionally modified under various circumstances.
It has also been established that unintentional anthropogenic effects (those caused by human activity) on weather do occur, and are commonly referred to as inadvertent weather modification
Concerns about water supplies are producing increasing interest in the application of cloud seeding for precipitation augmentation.
Hail damage to crops and property and fog-induced problems continue to produce interest in their mitigation. These factors, combined with the typically attractive benefit/cost ratios associated with operational seeding programs, have fostered ongoing and growing interest in intentional weather modification.
The potential environmental impacts of cloud seeding have been addressed in many studies. No significant adverse environmental impacts have been found due to use of silver iodide, the most commonly used seeding material, even in project areas where seeding has been conducted for fifty years or more.
The capability to increase precipitation from wintertime orographic cloud systems has now been demonstrated successfully in numerous "links in the chain" research experiments. The evolution, growth and fallout of seeding-induced (and enhanced) ice particles have been documented in several mountainous regions of the western U. S.
Technological advances have aided winter precipitation augmentation programs. Fast-acting silver iodide ice nuclei, with higher activity at warmer temperatures, have increased the capability to augment precipitation in shallow orographic cloud systems.
Wintertime snowfall augmentation programs can use a combination of aircraft and ground-based dispersing systems.
Although silver iodide compounds are still the most commonly used glaciogenic (causing the formation of ice) seeding agents, dry ice is used in some warmer (but still supercooled) cloud situations.
Liquid propane also shows some promise as a seeding agent when dispensers can be positioned above the freezing level on the upwind slopes of mountains at locations adequately far upwind to allow growth and fallout of precipitation within the intended target areas.
The capability to augment summer precipitation from convective clouds has been reasonably well demonstrated. Assessments of some operational and research programs that have seeded selected individual clouds or clusters of clouds with either glaciogenic or hygroscopic nuclei have found that seeded clouds tend to last longer, expand or travel farther to cover larger areas, and are more likely to merge with nearby clouds and produce more precipitation.
Technological advances have aided summer precipitation augmentation programs. These include fast-acting silver iodide ice nuclei, new hygroscopic seeding formulations, sophisticated radar and satellite data processing and analysis capabilities, airborne cloud physics instrumentation and continued improvements in numerical modeling.
While there are a number of concepts regarding the formation and mitigation of hail, the most common treatment method for hail suppression involves the addition of high concentrations of ice nuclei (usually silver iodide smoke particles) into the new growth regions of storms from aircraft or ground-based sources to manipulate the hail embryo formation process and thus limit the growth of hailstones.