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Cloud Seeding

January 16, 2023

One experiment that I remember from high school chemistry was crystallization of copper(II) sulfate, CuSO4 from an aqueous solution. In this simple experiment, each student created a saturated solution of CuSO4, poured some into a watch glass, and then placed these on a radiator in an adjacent room. My high school was heated by the same type of cast iron radiator used as a plot point in some comedy films and animated cartoons of the early 20th century. Beautiful blue crystals were found in place of the blue solution the next day.

copper(II) sulfate pentahydrate crystals

Copper(II) sulfate pentahydrate, CuSO4·5H2O, crystals.

Copper(II) sulfate forms hydrated crystals of the form, CuSO4·nH2O, where n can range from 1 to 7. However, the pentahydrate (n = 5) is the most common, and its crystals have the bright blue color shown. One ancient name for this crystal is blue vitriol, derived from oil of vitriol, the ancient name for sulfuric acid.

(Wikimedia Commons image by Crystal Titan. Click for larger image.)

As I wrote in an earlier article (Supercooled Water, September 24, 2018), some things are learned, not from textbooks, but from an oral tradition passed from professor to student. That's how I learned the old chemist's tale that bearded chemists had more luck crystallizing products from solutions. The supposed reason was that crystals need to nucleate, and the dust from a chemist's beard provides such nuclei when it falls into solution.

Clouds at low altitudes are composed of water droplets that are so small that they are buoyant in air. Higher altitude cirrus clouds are composed of small ice crystals that are also buoyant in air. In some cases, a cloud will contain supercooled water, and this water can be nucleated into ice particles that are heavy enough to fall and become rain as they warm in the lower atmosphere. Supercooled water is liquid water below its freezing point, and it stays liquid since the ice hasn't nucleated. Supercooling was first identified by Daniel Fahrenheit (1686-1736), for whom the Fahrenheit temperature scale is named, in 1724.[1]

Daniel Fahrenheit (1686-1736)

Daniel Fahrenheit (1686-1736). At the right is a commemorative plaque at the birthplace of Daniel Fahrenheit in Gdansk, Poland. Although born in Poland, Fahrenheit spent most of his life in the Dutch Republic. (Left image of Fahrenheit via Wikimedia Commons. Right image, an enhanced portion of a Wikimedia Commons image by Starscream. Click for larger image.)

Magazines of the 1950s published many articles about cloud seeding, the idea that injection of material into clouds can make rain. Unlike other dubious methods in the history of rainmaking, cloud seeding has some science behind it, the same nucleation process exploited by our bearded chemists. The unlikely pioneer of cloud seeding was General Electric, which found that dry ice and crystals of silver iodide would efficiently nucleate ice in a supercooled water atmosphere.[2] In the United Kingdom, Project Cumulus was a series of cloud seeding experiments between 1949 and 1952.

A decade later, Project Skywater was funded as a weather modification program in the US Bureau of Reclamation with the goal of demonstrating that cloud seeding was environmentally and economically practical.[3] The initial project funding was $100,000, which is about a million dollars in today's money.[3] A few years after the inception of Project Skywater, the US National Academy of Sciences released a report that was critical of the idea that weather could be changed so easily.[3] Project Skywater produced no data that confirmed the feasibility of beneficial rainmaking.[3]

The 1970 Colorado River Basin Pilot Project was a large winter cloud-seeding program in the Colorado River basin.[3] This program seeded about a third of the winter storms during its operation and concluded that more than half of those seedings resulted in higher precipitation.[3] In 2015, the Wyoming Weather Modification Pilot Project completed a six year study on the effectiveness of cloud seeding for increased snowfall. The study found that seeding increased snowfall by 5-15% in the mountains of Wyoming. This is a small effect, but it appeared that all of the more than a hundred trials increased snowfall by a measurable amount.

Other countries appear to have had success in weather modification. The China Meteorological Administration uses anti-aircraft cannons to inject silver iodide into the atmosphere with the claim that this action created 250 billion metric tons of rain over a seven-year period.[3] One problem in using silver iodide as the seeding material is its small toxicity, and there's the possibility that long term cloud seeding will cause silver to be accumulated in the atmosphere, groundwater, and plants.[3]

A recent open access paper in Geophysical Research Letters by scientists at the University of Reading (Berkshire, UK) and the University of Bath (Bath, UK) reports on a method of cloud seeding that doesn't rely on a chemical agent.[4] This research was specifically directed to modification of fog clouds, and the agent for modification was electric charge delivered by an unmanned aerial vehicle (UAV).[4-6] The study authors were perhaps inspired by the notorious English fogs (see image).

London Fog, 1847

This engraving of a London fog by Henry Linton (1815–1899), published in The Illustrated London News of January 2, 1847, shows people carrying flaming torches to lead the way for others.

The burning of soft coal containing sulfur within the city during the 19th century created a thick, poisonous fog. This fog, called a pea soup fog because of its yellow-green color, was likely responsible for many deaths in the city.

(Portion of A Wikimedia Commons image, Wellcome Trust photo no. L0032640. Click for larger image.)

Prior research established that electrically charged water droplets are more likely to adhere to each other when they collide, so the droplet size increases.[4] Too much charging will result in electrical forces that exceed the droplet surface tension, at which point the large droplets will fragment.[4] Such droplet charging in clouds has been realized by injecting corona discharge ions upwards from the ground.[4] Fog droplets are typically about 1-10 micrometer in diameter, smaller than cloud droplets, so electrical effects are proportionally greater, and the UAV propulsion batteries are available to provide the charging.[4] An unpopulated valley with frequent fog in Somerset, England, at 51.09788°N, 2.486905°W, near Castle Cary, was the area used for this study.[4]

Giles Harrison, lead author of the study and a professor of atmospheric physics in the Department of Meteorology at the University of Reading, is quoted in The Guardian as saying,
“Electric charge can slow evaporation, or even - and this is always amazing to me - cause drops to explode because the electric force on them exceeds the surface tension holding them together."[5]

In the experiments, the UAV flew in a circular, horizontal pattern above the measurement site, and the electric field and droplet characteristics were recorded.[5-6] The positive and negative charge emitters of the UAV were alternately switched on, and an onboard downwards-looking optical sensor monitored ground visibility.[6] The optical sensor showed that the injection of ions of both polarities caused brief changes in the fog reflectance, and this was indicative of changes in the droplet size distribution.[4,6] The maximum change in fog reflectivity from charging was about 2%, and this occurred with about half a minute lag.[4]

Time series of surface Potential Gradient

Time series of surface potential gradient (PG). Switching of the positive (red) and negative (blue) charge emitters are marked. Figure 2(b) of ref. 4, licensed under the terms of the Creative Commons Attribution License. (Click for larger image.)

These experiments demonstrated the feasibility of using charge-injecting UAVs to modify fog; and, potentially, seeding clouds without chemicals.[5-6] Despite its limited efficacy, cloud seeding is still a big thing. The US National Oceanic and Atmospheric Administration has identified more than 50 cloud seeding programs started since 2000.[6] The US state of Utah spends about $700,000 annually on cloud-seeding projects, and cloud seeding has been proposed a a way to save its Great Salt Lake.[6]


  1. Experimental explanation of supercooling : why water does not freeze in the clouds, European Synchrotron Radiation Facility Website, April 21, 2010.
  2. Tomas Kellner, "Cool Science: How Kurt Vonnegut's Brother Tried To Break Up Hurricanes," General Electric Website, July 8, 2020.
  3. Jedediah S. Rogers, "Project Skywater," Historic Reclamation Projects, Bureau of Reclamation, 2009 (Reformatted, reedited, and reprinted by Andrew H. Gahan, July, 2013. PDF file.)
  4. R. Giles Harrison, Keri A. Nicoll, Graeme J. Marlton, Douglas J. Tilley, and Pejman Iravani, "Ionic Charge Emission Into Fog From a Remotely Piloted Aircraft," Geophysical Research Letters, vol. 49, no. 19 (October 16, 2022), Article no. e2022GL099827, https://doi.org/10.1029/2022GL099827. This is an open access publication with a PDF file available at the same URL.
  5. Kate Ravilious, "Scientists zap clouds with electricity to make them rain," The Guardian, November 3, 2022.
  6. Baba Tamim, "Scientists electrically charge ‘constipated’ clouds to make them rain," Interesting Engineering, November 4, 2022.

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