Over its history of 50 years, modern cloud seeding has involved projects of various types in nearly 50 countries around the world.  Some individual projects have been in operation nearly continuously for decades, with a few operating for nearly fifty years.  As water needs increase world-wide, the demand for weather modification services will also increase.  Similar increase in demand will occur in hail-prone regions.

            Reference: 5, 8.

Cloud seeding materials are released via ground-based and/or airborne systems (see pictures).  Determination of the best suited method or combination of methods for a given project is based upon an assessment of a variety of factors.  The seeding materials are applied to the clouds (sometimes targeted very carefully into very specific portions of clouds) so that the material has adequate time to affect the precipitation process, so the effect will be focused over the intended geographic area.

            Reference: 1, 4, 5.

The materials used in cloud seeding include two primary categories, tied to the type of precipitation process involved.  One category includes those which act as glaciogenic (ice-forming) agents, such as silver iodide, dry ice and compressed liquid propane or carbon dioxide, which are appropriate in cloud systems where the precipitation process is primarily cold (colder than freezing).  Of the ice-forming materials, the most commonly used is silver iodide.  The second major category is focused on cloud systems where the warm (coalescence) process predominates.  In those environments, hygroscopic (water attracting) materials such as salt, urea and ammonium nitrate can be utilized.  Of the hygroscopic materials, the most commonly used are salts.

            Reference: 5.

In cold cloud seeding. the introduction of an ice-forming nucleating agent, e.g., silver iodide, into the appropriate cloud regions causes supercooled liquid water droplets to freeze.  Once these droplets freeze, the initial ice embryos grow at the expense of the water droplets around them (sublimation) and through contact with these neighboring water droplets (riming).  These embryos, if they remain in favorable cloud conditions, will grow into snowflakes, falling to the surface as snow if surface temperatures are below or near freezing, or as raindrops at warmer surface temperatures.  This process mimics nature where certain airborne substances, e.g., soil particles have the ability to act as ice-forming nuclei and initiate the freezing process.  A secondary effect of this process can occur, wherein the freezing of water droplets releases latent heat of fusion into the cloud.  This addition of heat, under the right circumstances, can cause the treated clouds to grow larger and last longer than would have naturally occurred. 

The first freezing process is often referred to as a static seeding effect, increasing the efficiency of the precipitation process within the seeded cloud volume.  The second freezing process, resulting from release of additional heat into the cloud, is often called the dynamic effect, whereby the treated clouds are invigorated, thus processing more moisture.

Although one might initially develop the impression that cold cloud seeding is strictly appropriate to winter clouds, it can work equally well during the summer when the precipitation forming process is active in the upper (colder) portions of cumulus clouds and adequate amounts of supercooled water droplets are available.

            Reference: 1, 2, 5.

Nature can produce rainfall from clouds that are warmer than freezing.  Tiny water droplets that form during condensation and define the cloud can grow as they collide with one another within the cloud.  This process is known as collision/coalescence.  Cloud seeding of this type of cloud involves introduction of additional condensation nuclei (e.g., salt particles) which can cause additional water droplets to condense within the cloud.  Various modeling and research studies have indicated that this type of seeding is effective in continental clouds, but ineffective in maritime clouds.

            Reference: 1, 2, 5.

Many detailed studies have been conducted to address these questions.  These efforts have ranged from chemistry-focused work to broad ranging environmental investigations.  The bottom line is that no significant environmental effects have been observed.  Seeding materials are applied in very small amounts relative to the size of the geographic areas being affected, so the concentrations of the seeding materials in rainwater or snow are very low.  Using silver iodide (the most common seeding material) as an example, the typical concentration of silver in rainwater or snow from seeded cloud systems is less than 0.1 micrograms per liter.  This is much below the U.S. Public Health Service=s stated acceptable concentration of 50 micrograms per liter.  As another example, the concentration of iodine in rainwater from seeded clouds is far below the concentration found in common iodized table salt. 

            Reference: 1, 2, 5, 9.

The answer to this commonly-posed question is no.  Of the total atmospheric moisture passing over any point, the proportion falling as natural precipitation is quite small, typically less than 10-15%.  Cloud seeding-induced increases in precipitation of the order of 5-30% still results in a small overall proportion (<20%) of the total available moisture reaching the ground.  Further, especially when cumuliform clouds are present, and over mountainous terrain where air is forced to rise, the cloud-bearing layer of the atmosphere undergoes nearly continuous moisture replenishment.  Analyses of precipitation data from areas downwind of several cloud seeding projects have indicated small percentage precipitation increases extending as far as 100 miles downwind of the intended areas of effect on projects that had indications of increases in the intended target area.

            Reference: 1, 10, 11, 12, 13.

In many jurisdictions, government agencies are responsible for regulation of cloud seeding in the public interest.  These agencies commonly require licenses and/or permits for cloud seeding, to help assure that the projects are properly designed and that those conducting such operations are properly qualified.  In the U.S., for example, nearly two-thirds of the fifty states have developed rules and regulations specific to cloud seeding activities.  These regulatory groups generally also maintain records of cloud seeding activities.

            Reference: 5.

The cost of cloud seeding varies greatly, depending on a large number of factors, such as which seeding methods and materials are appropriate to a specific application, the frequency of seedable conditions, the size of the intended area of effect and the duration of the project.  Most cloud seeding projects carry favorable benefit/cost ratios, ranging over 20:1 in some cases.  Cost questions are best addressed via direct discussion with a well qualified cloud seeding company/consultant.

As water needs steadily increase worldwide, the demand for weather modification services will also increase.  Focused research efforts will continue to yield incremental refinements to the technology.  Sponsors will increasingly enjoy the benefits of cloud seeding at very attractive benefit/cost ratios, and a growing number of those sponsors will incorporate cloud seeding as an integral part of their ongoing water resource management strategies.