Weather Modification
Weather modification, or cloud seeding, has long been recognized as a means to enhance existing water supplies. Cloud seeding had its beginnings in 1946 at the General Electric Research Laboratories in Schenectady, New York. Cloud seeding can assist nature in the formation of precipitation, with appropriate types and numbers of nuclei at the proper times and places. Cloud seeding projects have been carried out in over 20 countries. Projects are generally conducted either during the winter or summer months. While wintertime projects target the enhancement of mountain snow-pack within a watershed, summertime projects are aimed at enhancing precipitation and/or reducing damage from hail.

"Seeding" winter storm clouds over mountains is well established and understood. Clouds form as moist air is lifted and cooled during its passage across mountain ranges. Left to nature, many clouds are highly inefficient precipitators, retaining more than 90 percent of their moisture. By cloud seeding, the precipitation efficiency can be greatly improved. Generally, silver iodide is used in ground generators to produce artificial ice nuclei that form ice crystals. Spreading the nuclei via aircraft is also common. These crystals attract moisture from the surrounding air forming droplets that grow large enough to fall to the ground as snow. Some projects using ground-based silver iodide generators to seed winter storms over mountain areas in the western United States have operated continuously since 1950. The process for seeding mountain clouds is shown in Figure 1.

Precipitation data from a number of cloud seeding projects have been examined in detail for evidence of downwind effects. Results from these analyses show a slight increase in precipitation in areas up to 90 miles downwind from the project area. No decrease in precipitation has been detectable farther downwind from any long-term cloud seeding project.

The first cloud seeding project in Utah began in the early 1950's in the central portion of the state. Cloud seeding started again in 1973 and has continued to the present. In 1973 the Utah Legislature passed the Utah Cloud Seeding Act. This law provided for licensing cloud seeding operators and permitting cloud seeding projects by the Utah Division of Water Resources.

 The act states that for water right purposes all water derived from cloud seeding will be treated as though it fell naturally. The act also allowed for the Division to sponsor and/or cost share in cloud seeding projects. Since 1976, the state through the Division and Board of Water Resources has cost shared with local entities for cloud seeding projects. Recent cost sharing by the board has been 37-50 percent.

Utah wintertime project areas that have been seeded in the past are shown in the figure to the right. Some ski resorts have conducted cloud seeding projects.  The resorts do not cost-share with the Board of Water Resources; their cloud seeding projects are completely self-funded.