(source: Weather Modification Association website)

1. When did modern cloud seeding begin?:

Modern cloud seeding dates back to the late 1940's, springing from a discovery at the General Electric (GE) labs in Schenectady, New York in 1946. The GE lab discovery led to the realization, through a series of laboratory trials, that flecks of dry ice converted supercooled water droplets (those existing as water at temperatures lower than freezing) to ice crystals . These efforts also demonstrated the ice nucleating properties of various inorganic compounds in certain cold (lower than freezing) cloud conditions. Trials in the atmosphere soon followed, and operational and research cloud seeding projects began in the late 1940’s/early 1950’s.

2. In what areas are cloud seeding projects conducted?

There are two major types of project areas; mountainous areas and crop or range lands. Projects in mountainous areas are most commonly conducted to increase winter snowpack, which subsequently increases spring and summer runoff. Other projects are conducted directly over crop or range lands, typically during summertime, to provide more precipitation for agricultural crops or range land vegetation. Projects are conducted in areas which would normally benefit from additional precipitation.

3. Who typically sponsors cloud seeding projects?

The typical types of sponsors include agricultural organizations, municipalities and hydroelectric utilities. Agricultural organizations desire additional runoff for irrigated agriculture or direct precipitation on crop or range lands during the growing season. Municipalities are interested in enhancing drinking water supplies (via reservoir storage or enhanced streamflow). Hydroelectric utilities can generate additional hydroelectric power from augmented runoff which is either flowing into reservoirs or from enhanced flows in rivers which have hydroelectric generation facilities. Some government entities ranging in size from local governments up to state governments provide cost sharing support to cloud seeding projects.

4. What seeding materials and methods are used in cloud seeding?

Two different types of seeding agents are used, depending on the temperature of the cloud volume to be treated. The agents used to seed a cloud (cloud seeding agents) drive the technology used for cloud seeding. The terms “cold rain” (developing through a process that involves ice particles) and “warm rain” (developing through a process that is entirely liquid) are often used to describe the two primary cloud microphysical processes that produce precipitation naturally.
The most common agents used in “cold rain” or snowpack augmentation cloud seeding projects are silver iodide and dry ice (frozen carbon dioxide). Other agents like liquefied gases (liquid nitrogen for example) can be used to seed clouds, The means of generating seeding agents in “cold rain” snowpack augmentation projects include pyrotechnic flares (similar to emergency highway flares), a solution of chemicals, dry ice, or compressed gas cylinders. The flares used for cloud seeding usually contain silver iodide or a salt. Flare racks can be installed on aircraft to carry the pyrotechnic flares. The solutions contain silver iodide dissolved in acetone and are dispensed via combustion from devices called silver iodide (ice nuclei or aerosol) generators. These generators can be located on the ground or attached to the wings of an aircraft. Dry Ice is usually acquired in pellet form. The pellets are placed in special insulated containers (called dry ice hoppers) for dispensing the pellets. Dry ice is most frequently dispensed from aircraft. Installations on aircraft are certified by the Federal Aviation Administration (FAA).
Similarly, “warm rain” cloud seeding agents can be produced by “hygroscopic” flares. These flares contain some type of salt (e.g., calcium chloride). When the flares are burned they produce minute particles of the salt which attract water vapor, forming cloud droplets in addition to those already present in the cloud. Hygroscopic seeding can also be done using dry common salt (NaCl) that has been processed to optimum sizes in powder form for the promotion of droplet growth in the clouds. This salt is dispersed normally from an aircraft flying immediately below or in the updraft regions of the clouds.

5. How does silver iodide or dry ice work to make rain or snow?

Silver Iodide ice nuclei are produced through the combustion of silver iodide acetone solutions or by burning pyrotechnic flares. Either method produces trillions of microscopic sized particles of silver iodide (ice nuclei). Silver iodide nuclei (aerosols) increase the probability of ice crystals forming in a cloud at temperatures of approximately -5°C or lower. The dry ice or evaporating liquefied gases cool the air down so quickly that either unfrozen cloud droplets at temperatures of -5°C or lower (termed supercooled) quickly freeze or water vapor molecules become so cold that they fuse (freeze) together forming very tiny ice crystals. The conversion of supercooled water to ice results in the release of heat (heat of fusion) to the cloudy air that can invigorate the cloud and promote its further growth and longevity. Regardless of the specific cold cloud seeding agent used, the ice crystals that form grow at the expense of the surrounding cloud moisture and they fall through the cloud, collecting unfrozen cloud drops and smaller ice particles in the process. These falling ice crystals typically become snowflakes that may melt if they fall into air that is above freezing or reach the ground if ground temperatures are below freezing.

6. How does warm cloud seeding work?

Warm rain is produced naturally from clouds that are formed in air with ambient temperatures completely above freezing, typically in the tropics and subtropics but sometimes in more temperate climates. Tiny cloud droplets form on cloud condensation nuclei (e.g. salt particles, dust, soot) when the water saturation point is reached as moist air ascends in the atmosphere. These cloud droplets may grow to a size where they collide with each other, forming some larger droplets eventually reaching rain drop size. This process is known as collision-coalescence or warm rain. Many times, especially over continental regions, the droplets in these warm clouds become similar in size and the collision/coalescence or rain process does not initiate or is inefficient. Suppression of the collision/coalescence can be especially prominent over polluted continental regions where too many cloud droplets form on the large number of smoke and other kinds of particles in the air. The cloud water is divided between the drops, so that the cloud droplets are too small to effectively collide and coalesce with each other to efficiently start the rain forming process.
Warm cloud seeding involves introducing additional nuclei upon which water vapor may condense and form cloud droplets that are of different sizes (typically larger) than those droplets forming naturally in the cloud. This is intended to promote the collision – coalescence process. The warm cloud seeding agents act to increase the probability of forming such larger drops that can collect the smaller similarly-sized droplets and grow to a size where they naturally fall as rain. Various modeling and research studies have indicated that this type of seeding may be ineffective in maritime clouds (clouds formed over or near oceans) because the natural precipitation processes are already rather efficient because such clouds may already contain many larger cloud droplets.

7. Is cloud seeding successful?

Yes, it can be very successful. But like any tool it has its limitations, and sometimes cloud seeding can be ineffective. Studies have shown certain clouds or stages of cloud development are susceptible to seeding while others are not. Cloud seeding will not “end” a drought, although it may provide some increases in precipitation even during drought periods.
The seeding agents and methodologies used in present day cloud seeding projects have been developed and refined for over 60 years. There is some consensus that cold cloud seeding technologies can increase area-wide seasonal precipitation by approximately 5-15% when the seeding is effectively applied to suitable clouds. See the Weather Modification Association’s 2011 Capability Statement on Weather Modification for details. This statement is based on the collective experiences of weather modification scientists, engineers, social & economic scientists, insurance specialists (crop and property damage), business men and women, environmentalists, policy makers, and lawyers, who have been working (or have had an interest) in the field.
Various analyses have indicated that the estimated benefit/cost ratios in a number of operational cloud seeding projects have ranged from 5/1 to 15/1 or perhaps even higher in some cases. Other analyses of long-term winter mountainous projects have indicated that additional streamflow from these projects is being produced at a cost of approximately 1 to 10 dollars per acre-foot.

8. Are there safeguards built into the conduct of cloud seeding projects?

Yes, cloud seeding projects normally have suspension criteria built into the project designs. Reasons for suspensions may include:
1) severe weather warnings;
2) forecasts of extreme rainfall amounts, especially over short time periods;
3) excess snow water contents (winter);
4) avalanche warnings (winter).
Such suspensions may be temporary or sometimes for the remainder of the scheduled seeding period.

9. Does cloud seeding have any significant negative environmental impacts?

There is no evidence that suggests cloud seeding creates any significant negative environmental impacts on the environment. Measurements made since the 1950’s indicate that the amount of silver iodide deposited in a target area after a long standing cloud seeding project falls several orders of magnitude (multiples of 10) short of the amount known to be toxic to plants, animals, trees, or humans. It is often difficult to detect any silver accumulation above the background amounts naturally present in the environment. Naturally, this kind of investigation continues. See FAQ #10, “Is Silver Iodide Harmful to the Environment?”
Warm cloud seeding is not conducted nearly as frequently as silver iodide cloud seeding, and the effect of warm cloud seeding agents on the environment is not as well known. Warm cloud seeding agents are salts. Preliminary results suggest that because the amounts of seeding agent used are so small, even these warm cloud seeding materials probably do not have any significant impacts.

10. Is silver iodide harmful to the environment?

As noted in the previous response (with respect to cloud seeding), questions sometimes arise regarding the environmental effects of silver iodide aerosols used in cloud seeding, which include silver iodide aerosol complexes such as silver iodide-silver chloride. Silver iodide is the primary component of silver iodide-based ice-nucleating complexes used in cloud seeding, and all these complexes will be referred to as silver iodide (AgI). The published scientific literature clearly shows that no environmentally harmful effects arising from cloud seeding with silver iodide aerosols have been observed; nor would they be expected to occur. Based on this work, the Weather Modification Association finds that silver iodide is environmentally safe as it is currently being dispensed during cloud seeding programs. See the WMA’s 2009 Position Statement on “The Environmental Impact of Using Silver Iodide as a Cloud Seeding Agent”.

11. Doesn’t cloud seeding rob “Peter” of rainfall to water “Paul”?

It is a common misconception that cloud seeding robs someone of rainfall, or, robs "Peter" to water "Paul". This misconception is often based on the belief that there is limited moisture in the atmosphere. Water vapor is always present in the atmosphere but because it is vapor we do not visually recognize its presence. Calculations indicate that of the amount of water (liquid and vapor) that passes over the Unites States on an annual basis only approximately 10% reaches the ground as precipitation. If cloud seeding results in approximately a 10% increase over some limited geographical area, then only approximately 1% of the water balance is impacted in this limited area. Several studies of the impact of cloud seeding downwind of the intended target areas have indicated that the precipitation is typically increased not decreased, in these downwind areas to distances of approximately 50-100 miles.

12. Is there any link between cloud seeding and chemtrails?

No. STWMA, along with the WMA, is unaware of any connection between cloud seeding as is practiced by our program and to what some refer to as “chemtrails” (chemical trails). Atmospheric scientists even dispute the existence of “chemtrails”. What some chose to call chemtrails are actually “contrails” (condensed engine exhaust trails), which are well-understood atmospheric phenomena. Contrails are defined as “streaks of condensed water vapor created by an airplane or rocket at high altitudes.” These condensation trails are the result of normal emissions of water vapor, carbon dioxide, and some carbon-containing particulates from piston engines and jet engines at high altitudes in which, given the right atmospheric conditions, the water vapor condenses into a visible cloud. Actually, due to the very cold temperatures at high altitudes, the water droplets that initially form this cloud rapidly freeze, forming an ice cloud similar to naturally occurring cirrus clouds. Under certain conditions contrails can merge or contribute to the development of a larger area of cirrus clouds. Contrails are normally observed on otherwise clear days, when cloud seeding would not be conducted. The cloud seeding nuclei from ground or airborne sources do not produce such visible clouds.