Frequently Asked Questions

General Cloud Seeding Questions

Frequently Asked Questions

A:  To increase snowfall in mountainous areas and increase runoff for water supplies at lower, semi–arid elevations and other uses (farming and ranching, fisheries, industry, etc.)

Cloud seeding (weather modification) technology has developed to the point where it can be an effective and economic tool for water managers. Cloud seeding technology will be used and evaluated in the proposed pilot projects as a long–term water management tool rather than as a tool to mitigate the effects of drought conditions. Cloud seeding is most effective under normal or near–normal weather conditions. However, the benefits during dry years cannot be ignored.

Winter cloud seeding to increase snowfall in mountainous areas is designed primarily to increase runoff for water supplies at lower, semi–arid elevations. Increases due to cloud seeding can improve soil moisture, stream flows, and reservoir levels. These effects may reduce the need for groundwater mining and improve water supplies for municipalities, industry, and irrigation. Irrigated crops can be successfully cultivated without mining (pumping) as much groundwater, dry–land farming can be more successful, and increased water supplies within reservoirs will mean that more water is available for hydropower generation, irrigation, and municipal and industrial use. Recreational boating and fishing opportunities may also be enhanced. Increased stream and river flows may aid recovery of special status species and may improve overall water quality in some locations by diluting previously turbid waters.

Present technologies to increase precipitation through cloud seeding can supply additional water, but only when clouds amenable to seeding are naturally present. Cloud seeding works best in normal or near–normal weather conditions. In drought situations, few clouds suitable for cloud seeding operations develop, and the opportunity to increase precipitation in a meaningful way will be very limited. In seasons with precipitation well above normal, seeding operations cease because when plentiful water supplies exist, seeding is not needed or desired.

The Sierra Madre/Medicine Bow Mountains and the Wind River Range receive 25 to 60 inches of precipitation annually. Data on storm frequencies suggest that seeding opportunities could occur on at least 60 to 80 days during the winter months. About 40 to 70 percent of annual precipitation in these mountain ranges falls in the winter, mostly in the form of snow, with totals of more than 250 inches of snow during the winter months. This pattern is especially evident in the Wind River Range, where 60 to 70 percent of the annual precipitation on the highest peaks falls from October through March. The Wind River Range also contains 63 glaciers covering 17 square miles, an area larger than that covered by all other glaciers in the American Rockies. Snowpack augmentation may not only increase streamflow in the Wind River Range but may also help protect these glaciers from further recession.

Under a moderate growth scenario, future water demands in the Green River Basin show an estimated increase in surface water use from 73 percent to 82 percent of the allocation given in the Colorado Compact. In the Wind River/Bighorn Basin, projected needs would increase to an estimated 88 percent of the available flow under moderate population growth. All of the water in the Platte River Basin is presently allocated. A long–term strategy of snowpack augmentation would help with storage and future water needs in these basins.

A:  Seeding does not measurably impact the availability of moisture downwind, and extra–area effects appear to increase, not decrease, precipitation in the area surrounding and downwind of the target location

The development of precipitation can be quite variable and also largely inefficient. Storm efficiency, the percentage of the cloud mass (water and ice) that falls as precipitation, is about 30 percent for average winter storms. Less intense storms tend to be less efficient, while intense storms with heavy snowfall are likely to be more efficient. The situations in which nature is not efficient can be recognized in real time through targeted monitoring and direct observations. Under some conditions, human intervention can improve cloud efficiencies and repeated interventions can increase snowpack on an area–wide basis.

If cloud seeding is successful in increasing the natural precipitation by a nominal amount, say 15 percent, the additional percentage of total atmospheric water that might be precipitated would still be quite small. Typically, about 20 percent of the total water vapor in the air condenses to form clouds as it rises over mountains. The remaining 80 percent of the moisture remains uncondensed because the temperature of the air typically does not get cold enough.

As mentioned earlier, winter storms are typically about 30 percent efficient, so only a portion of the water vapor that condenses naturally when rising over mountains (30 percent of the 20 percent that was condensed), or 6 percent of the total moisture, ends up falling out naturally as precipitation during an average winter storm. An increase in precipitation of 15 percent translates into only an additional 0.9 percent of the total atmospheric moisture available. Therefore, about 7 percent of the total atmospheric water might be precipitated when seeding is conducted. Instrumentation presently used by the National Weather Service would have a difficult time detecting a change on the order of 1 percent, along with the confounding influences of natural variability. These calculations do not consider that this additional water, now on the ground instead of in the air, remains in the hydrologic cycle. For example, a portion of this water would return to the atmosphere on relatively short time frames through evapotranspiration.

There are two mechanisms that may cause extra-area (or so-called "downwind") effects on precipitation:

  1. Downwind transport of ice nuclei and ice crystals from the seeding source
  2. Invigoration of clouds by release of latent heating of freezing and their subsequent propagation out of the target area. Long (2001) provides an excellent summary of previous findings.

These findings show:

  • Extra–area effects appear to increase precipitation in the area surrounding and downwind of the target location
  • There is no substantial evidence that a decrease in precipitation occurs downwind from the target location

Long, A.B, 2001: Review of downwind extra–area effects of precipitation enhancement. J. Wea. Mod., 33, 24–45.

The WWMPP will also be investigating potential extra-area effects, primarily through numerical modeling studies.

A:  Silver iodide is utilized in cloud seeding activities for clouds which contain supercooled water (existing as a liquid at temperatures below the freezing point), because it’s molecular structure is similar to that of water in its frozen state (ice).  This structure promotes the freezing (by contact) of the supercooled cloud droplets into ice crystals and leads to subsequent growth of the ice crystals by the preferential attraction of water vapor to the crystals at subfreezing temperatures.  While other suspended fine liquid and solid particles (known as aerosols) from both natural and man-made sources serve this function in clouds, the addition of a modest amount of silver iodide to such clouds improves the efficiency of these cloud processes, especially at warmer temperatures (-5 to -15 degrees Celsius), producing more precipitation.

A:  For cloud seeding operations, silver iodide is either dissolved in a flammable solution or combined with flammable solids in a flare or similar device.   The mixtures are burned to produce submicron-sized silver iodide-based molecules which are transported into the cloud by the air circulation surrounding the cloud (most often the rising air ---the updraft--- that supports the cloud’s existence).

A:   The amounts of silver iodide used in cloud seeding are quite small, typically no more than 25 grams (9/10 of an ounce) per hour from ground generators. 

A:   Measurements of silver iodide concentrations resulting from cloud seeding in snowpack, water bodies and soils have been made in many regions of the globe, partly due to concerns about environmental impacts.  Fortunately, this is possible because silver iodide is insoluble (doesn’t dissolve) in water and therefore can be traced.  Measured concentrations in snowpack, lakes and soils have been in the low parts per trillion (ppt) which is three orders of magnitude less than the lowest measured levels of silver in what are considered "clean" environments.  Several dozen studies, some commissioned by the U.S. Environmental Protection Agency (EPA), have repeatedly demonstrated that cloud seeding contributes levels of AgI far below those from all other sources, and far below the levels considered safe by the EPA and environmental regulatory agencies in other countries.

A:    The measurements of silver iodide discussed in the preceding answer have been taken at some sites over periods as long as 30 years; concentrations of silver remain well in the ppt range at these long-term study sites and represents many thousands of samples tested. This suggests that long-term cumulative deposition of silver iodide does not pose a significant health risk.  

Summary of trace chemistry sampling in the Wyoming project [pdf file]