Orographic clouds containing supercooled liquid water (SLW) (liquid water at temperatures below 0°C) offer a unique opportunity for increasing precipitation. They may persist for hours at a time over a mountain range producing little or no precipitation, apparently because they do not contain enough natural ice crystals to form snow efficiently. The introduction of AgI crystals or other artificial ice nuclei into such clouds results in the formation of ice crystals, which grow into snowflakes at the expense of the SLW. The lifting of the air on the upwind side of the mountain barrier suggests that orographic clouds can be seeded cheaply and efficiently by AgI generators on the ground.
A typical winter orographic cloud seeding project involves the deployment of 10 to 20 manually operated AgI generators on the ground upwind to seed a target area of several thousand square kilometers. Some operators use radio-controlled generators to improve coverage of remote areas. Aircraft seeding is conducted on some projects to supplement the effects of the ground-based generators or to reach otherwise inaccessible areas.
The initial acceptance of weather modification technology was due to a perception that large economic benefits would be obtained from it. Subsequent analyses have confirmed that the economic impact of a successful weather modification program could be very large. A Bureau of Reclamation analysis of a hypothetical 10 percent increase in Colorado River streamflow showed average annual benefits of $48 million from increased water supplies for irrigation and for municipal and industrial use; $34 million from increased hydropower generation; and $62 million from improved water quality (chiefly from a reduction in salinity). As a program to seed all the major runoff-producing subbasins of the Colorado could be mounted for some $10 million per year, the potential benefit-cost ratio is large.
One point of concern in considering economic impacts of weather modification is that a weather modification program may harm people receiving few or none of the perceived benefits. The proposed program to increase snowpack in the Colorado River basin is one such case. The program, if implemented, would involve inconvenience and additional costs for snow removal for residents of the high-elevation regions within the basin, notably