Cloud Seeding – What It Is and Whether It Works


Cloud Seeding
Cloud seeding introduces particles that act as condensation nuclei and promote rain or snow.

Cloud seeding is an intriguing and controversial method of weather control that raises questions about its effectiveness and environmental impact.

What Is Cloud Seeding?

Cloud seeding aims at producing rain or snow by introducing particles into clouds that act as condensation nuclei and promote precipitation. The practice dates back to the 1940s. Goals of cloud seeding are improving water supplies and crop yields and preventing hail and storm damage to sensitive regions.

How It Works

The basic principle is that particles serve as nuclei for water or ice aggregation, eventually causing them to fall as precipitation. Common agents including silver iodide, potassium iodide, dry ice, urea, and table salt, but pollen, bacteria (especially Pseudomonas), and dust also have surfaces that promote nucleation and precipitation. Table salt, calcium chloride, and urea are hygroscopic, so they attract water to them as well as offer surfaces for nucleation. More recent methods involve pulsing lasers or delivering electrical charges that coalesce water molecules or affect latent heat within clouds and strengthen updrafts.

Cloud Seeding History

Two key figures in the development of cloud seeding were Vincent Schaefer and Irving Langmuir, both working at the General Electric Research Laboratory in Schenectady, New York in the 1940s. Credit for the discovery of cloud seeding usually goes to to Schaefer, who serendipitously discovered that dry ice transforms supercooled water droplets in a cloud into ice crystals, precipitating as snow.

The first deliberate attempt at modifying weather using cloud seeding took place later in 1946. This historic event, known as the Schaefer-Langmuir experiment, involved dropping dry ice into a supercooled cloud over Mount Greylock in Massachusetts, successfully creating snowfall.

Bernard Vonnegut contributed by discovering the effectiveness of silver iodide as an agent for cloud seeding. Silver iodide produced a similar effect to dry ice, but was more practical because it could be dispersed as a smoke. Vonnegut favored silver iodide because it offers a hexagonal crystal structure similar to that of ice. However, recent research indicates that the freezing process likely does not involve hexagonal structures.

The early experiments led to Project Cirrus (1947-1952), a collaboration between General Electric, the U.S. Army Signal Corps, the Office of Naval Research, and the U.S. Air Force. Project Cirrus conducted several experiments, including one infamous attempt to modify a hurricane. In October 1947, a hurricane off the coast of Florida, was seeded with dry ice. However, the storm changed course and made landfall near Savannah, Georgia, causing extensive damage. It’s unclear what effect, if any, cloud seeding had on the hurricane.

Research continues to the present day, particularly in the United States, China, and the United Arab Emirates (UAE).

Does Cloud Seeding Work?

Basically, cloud seeding does work, but not reliably and not particularly well.

Results from the Wyoming Weather Modification Pilot Project (WWMPP) indicate that cloud seeding raised snowfall between 5% and 15%. The remote-sensing equipment measured an increased in the number and size of snow particles in seeded clouds. Results from other countries also indicate around a 10% increase in precipitation from cloud seeding.

However, not every cloud meets the criteria for cloud seeding. Also, temperature and wind speed are important factors that influence whether or not an attempt is successful.

Pros and Cons

While cloud seeding offers an opportunity for water management, it has a low chance of success and certain disadvantages over other solutions.

Pros

  • Enhances Rainfall or Snowfall: It potentially increases precipitation. This benefits agriculture and water reservoirs and might mitigate the impact of damaging weather.
  • Cost-effective: Compared to other water augmentation strategies, cloud seeding is relatively inexpensive.

Cons

  • Uncertain Outcomes: The effectiveness of cloud seeding is variable and dependent on atmospheric conditions.
  • Environmental Concerns: There are concerns about the potential toxicity of the chemicals. For example, excessive sodium degrades soil and affects plant growth, while silver is toxic to aquatic life.
  • Weather Interference: Manipulating weather patterns in one area inadvertently affects weather elsewhere.

References

  • Breed, D., R. Rasmussen, C. Weeks, B. Boe, and T. Deshler, 2014: Evaluating winter orographic cloud seeding: Design of the Wyoming Weather Modification Pilot Project (WWMPP), J. Appl. Meteor. Climatol., 53, 282-299. doi:10.1175/JAMC-D-13-0128.1
  • Carrasco, J.; Michaelides, A.; et al. (2009). Pelley, Janet (2016). “A one-dimensional ice structure built from pentagons”. Nature Materials. 8: 427-431. doi:10.1038/nmat2403
  • Lukas, M., Schwidetzky; et al. (2022). “Toward Understanding Bacterial Ice Nucleation”. J. Phys. Chem. B. 126(9): 1861-1867. doi:10.1021/acs.jpcb.1c09342
  • Pelley, Janet (2016). “Does cloud seeding really work?“. Chemical and Engineering News. 94 (22): 18–21.
  • Vonnegut, B.; Chessin, Henry (1971). “Ice Nucleation by Coprecipitated Silver Iodide and Silver Bromide”. Science. 174 (4012): 945–946. doi:10.1126/science.174.4012.945