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Wyoming Weather Modification Program: FAQ

A: Cloud seeding in Wyoming is focused on increasing snow pack in mountainous areas as part of a larger strategy for flow augmentation. In Wyoming, cloud seeding coupled with adequate storage is considered a long-term water management tool used to mitigate the effects of drought conditions.

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 was used and evaluated in the pilot project 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 snow pack 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.

A:There is no substantial evidence showing that a decrease in precipitation occurs downwind from the target location.

The speculation that cloud seeding might impact the amount of downwind precipitation has been voiced many times during the modern era of cloud seeding. Some downstream water users are therefore concerned that upwind seeding projects may reduce the quantity of water that would be otherwise available to them. This impression is inaccurate, and understanding the hydrologic cycle and the potential effects of cloud seeding will help alleviate concerns.

Many dynamic forces interact in the atmosphere to produce precipitation. The hydrologic cycle is much more complex than a simple water pipeline transporting a finite amount of moisture, and is not confined to discrete barriers. The production of the liquid water condensate that becomes clouds is governed by humidity and vertical motions in the atmosphere, constrained by the temperature profile, winds, and topography. Sustained upward motions result in cooling, which in turn produces cloud droplets when 100% relative humidity is achieved. Downward motions rapidly evaporate the tiny cloud droplets, as the relative humidity quickly falls below 100%.

Large scale atmospheric motions related to cold fronts and low pressure centers are primarily responsible for moisture transport and cloud formation and dissipation within winter storms. The vertical motion associated with airflow over a mountain barrier occurs on a localized scale, and is another process that results in cloud production and decay. However, without the large scale weather attributes prepping the atmosphere over a mountain barrier, the effects of orographic uplift, and leeward descent, will have minimal effect. The upward motion must occur in an already moist atmosphere for clouds to form, and snow to fall.

To gain an increased appreciation for the moisture budget in a typical Wyoming winter storm, representative temperature and moisture profiles were examined in the context of orographic flow. A typical wintertime moist air flow approaching the Wind River Range in storm conditions, at a height of 3.0 km above mean sea level, at a pressure of about 700 hectopascals (hPa, equivalent to millibars), might have a temperature of about -10℃, and a relative humidity of about 90%, creating a dew point temperature of about -11℃, as measured by balloon-borne weather instruments released twice daily by the National Weather Service in Riverton. In a typical wintertime storm systemmoist flow is forced upward as it encounters the mountains. As the air rises and cools, clouds and precipitation form over the mountains. Nature typically will condense just over 20% of the total water vapor in the air as the moist air rises up and over the mountains. The other 80% of the moisture (in the form of water vapor) remains uncondensed because the air never gets cold enough to condense all the moisture. Winter storms are typically about 30% efficient at converting condensed cloud water into precipitation, so 30% of the 20% translates to about 6% of the total atmospheric moisture that ends up falling out naturally as precipitation. If cloud seeding is successful in increasing the natural precipitation by 15%, that amounts to 15% of the 6%, or about 0.9%, more of the total atmospheric water that might be precipitated when seeding is conducted. These thermodynamic calculations do not consider that within the hydrologic cycle, this additional water, now on the ground instead of in the air, is not removed from the water cycle, but is now available to sublimate, evaporate, or be transpired by plants back into the air. Moisture remaining on the ground may contribute to eventual runoff (and re-evaporation from surface water bodies), or may be tapped by plant life if it infiltrates the soil. Moreover, not all of the atmosphere is ever seeded, nor are seeding operations ever continuous. Thus, the net effect of cloud seeding upon the atmospheric water budget is only a fraction of the estimated 0.9%.

The tendencies of mountains to dry the air forced to pass over them is well known, and responsible for a rain shadow effect. Ice crystals created by seeding would be subjected to the same descent and subsequent sublimation as natural ice crystals as they pass over the crest line downwind of the seeding generators and aircraft, and would likely not survive. However, there is a possibility that the ice nuclei themselves might survive in the atmosphere, if not scavenged by other precipitation, and could thus be in a position to initiate new ice growth during renewed ascent upwind and over a downwind mountain range thereby potentially increasing precipitation in the area surrounding and downwind of the target location.

A: Silver iodide is utilized in cloud seeding activities for clouds which contain supercooled liquid water (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, 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.

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