Weather Modification, a.k.a Cloud Seeding, a Technology Whose Time Has Come
Little did Dr. Vincent Schafer realize, while working in a General Electric laboratory in 1946, that he would stumble upon the first scientific indication that man might beneficially modify clouds. Dr. Schaefer was doing research on a hot summer day and cold temperatures were required. He was using a chest-type deep freezer, and he decided to lower the temperature further by placing a chunk of dry ice into the deep freeze. He noticed an unexpected reaction. While working over the open freezer, his breath had created a small cloud of “supercooled” (colder than freezing) water droplets. These droplets appeared as a sort of haze in the freezer when light was shone through them. Introducing dry ice caused the water droplets to freeze due to the very cold temperature of the dry ice. They froze forming tiny ice crystals that scintillated in the light. Dr. Schaefer’s serendipitous discovery demonstrated that “supercooled” cloud water droplets (common in clouds) could be artificially induced to freeze. This classic experiment is easily replicated.
There are some famous photos taken in the 1940s and 50s when Dr. Schaefer flew in an airplane equipped to drop dry ice particles into “supercooled clouds”. Ice crystals formed via the “seeding” grew into snowflakes which fell to the ground, leaving a hole in the seeded cloud deck. Further research conducted on different types of particles that might also cause “supercooled” water droplets to freeze on them identified silver iodide as an excellent particle to cause such freezing. It remains the most widely used cloud seeding agent for seeding cold (below freezing) clouds.
These developments were greeted with enthusiasm in the 1950s. Research programs in the United States and other countries were conducted to determine if precipitation could be increased through “cloud seeding.” These programs and others following in the 1960s though the 1990s showed mixed results. Difficulties were due to a number of factors including the complex cloud interactions involved, seeding coverage variability, short experimental period length, and large precipitation variability that can mask the seeding effects. Some disillusionment developed within the scientific community. Research in the field declined to near zero in the latter 1990s.
The acid test adopted to determine if a seeding experiment increased precipitation was whether the indicated results were “statistically significant.” This was the model of randomized trials used in pharmaceutical testing exported to the atmosphere to “prove” that cloud seeding worked in research experiments. A 5 percent statistical significance level was written into the design of weather modification research programs. Attaining a 5 percent significance level would indicate that there was only a 5 percent chance that the experimental results would randomly occur without the cloud treatment or stated differently, 95 percent confidence that observed differences were due to seeding. Some research programs that demonstrated positive seeding results were rejected by purists because the 5 percent significance level was not obtained. These pioneering and positive experiments were unfortunately and unjustly labeled as failures.
Coincident with the beginning of research programs in the 1950s,”operational” cloud seeding programs began in the United States and a number of other countries. These programs were designed to create positive benefits, for example, increased spring and summer streamflow from the melt of augmented snowpacks or augmented rainfall on croplands. Continued to the present time, these operational programs are truly an international phenomenon involving Argentina, Canada, Chile, China, Cuba, France, Greece, India, Indonesia, Italy, Israel, Jordan, Morocco, Philippines, Spain, Thailand, United States, United Arab Emirates, Venezuela, and Zimbabwe. Some programs in the Sierra Nevada of California date back to the 1950s and early 1960s. Estimates of the results achieved from precipitation augmentation programs typically range from 5 15 percent seasonal increases.
The obvious $64,000 question (not adjusted for inflation) might be: Why are large numbers of operational programs conducted around the world despite some skepticism within the scientific community? Several factors may be at play but I believe the primary reasons are: 1) the potential for “new” water from precipitation augmentation programs; 2) a perceived substantial return on investment, and 3) a lower expectation of “proof” that cloud seeding “works.”
Various studies of U.S. programs indicate additional streamflow derived from winter snow augmentation costs a few dollars per acre foot to produce, often resulting in estimated benefit to cost ratios of 10/1 or higher. Managers of water districts, municipalities, hydroelectric companies, irrigated agricultural districts, etc. do not often have the luxury of demanding a 95 percent confidence level when making workday decisions. Why then should they demand this level of confidence to fund a cloud seeding program?
Cloud seeding offers the potential to tap an “atmospheric ocean” to provide additional precipitation. Contrary to popular belief, studies have indicated that precipitation is actually increased, not decreased, downwind of cloud seeding programs. Few other technologies offer the potential for producing “new” water. One example is desalinization. It is quite expensive, costing over about $1000 per acre foot compared to an estimated cost of a few dollars per acre-foot for water produced with cloud seeding.
An ever increasing worldwide population and growing per capita demand for water will increase demands on existing fresh water supplies. Varied approaches are needed to satisfy these increased demands, with weather modification one such approach. A technology whose time has come, it will become increasingly important in the future.