By Kevin O’Connell
Date: October 18th, 2024
While experiments conducted in recent decades have found correlations between seeded clouds and the onset of precipitation further downwind of said clouds, the experiments have been too far and few between to imply causation, and further research into different cloud seeding methods, as well as more sophisticated atmospheric measurement instrumentation is needed to ensure its future reliability.
In order to accurately determine the current reliability of cloud seeding methods, we must look into the history of weather seeding experimentation and its evolution up until the present day. Vincent Schaefer, a research scientist for General Electric, is widely recognized for conducting the first cloud seeding experiments in the 1940’s. During and after World War 2, the primary focus of Schaefer’s work was to prevent aircraft from icing while in flight, as he dumped about 6 pounds of crushed up dry ice into a cloud above the Adirondack mountains of northern New York in 1946 (Harvey, 2021). One of the primary limitations with Schaefer’s research is the lack of proof that the dry ice was the catalyst for the onset of snowfall, and that it did not occur due to the natural state of the atmosphere during the experiment. Cloud physics and precipitation forecasting were still very much in their infancy during Schaefer’s time, and while his findings were certainly striking for the scientific community, the lack of modern technology greatly inhibited the refinement of his methods.
The lack of adequate instrumentation continued to be a massive barrier to early cloud seeding research throughout the 20th century. After Bernard Vonnegut, a colleague of Schaefer’s at General Electric, discovered the effectiveness of silver iodide in aiding cloud drop formation in the late 1940’s, governments and militaries across the world immediately harnessed his strategy for their own cloud seeding endeavors (Chen, 2018). These experiments were fraught with uncertainties as they did not have instruments that could measure cloud drop size in real time, so much like in Schaefer’s experiment, they had no way of proving that silver iodide was the catalyst for precipitation onset, and not natural processes (Chen 2018). While silver iodide certainly appeared to be an effective method at cloud seeding, hence its widespread use throughout the 20th century, lack of sophisticated instrumentation to measure its effects haunted the early development of cloud seeding technology. Nevertheless, the persistent demand for methods to control precipitation in clouds has paved the way for ongoing cloud seeding research that continues to the present day.
Clad with the latest and greatest cloud measurement technology, as well as statistical methods of proving the reliability of cloud seeding, atmospheric scientists have progressed leaps and bounds since the early experiments of Schaefer and Vonnegut. However, the natural variability of atmospheric parameters continues to be a limitation of modern cloud seeding methods. Statistical methods of determining success of cloud seeding strategies often involve conducting multiple experiments within the same geographical environment, half with and the other half without using seeding material. One such experiment was conducted between 1994-1998 in the Bhumibol catchment area in northwestern Thailand, where scientists intended to determine the effectiveness of silver iodide seeding in enhancing an area of rainfall just over 2,000 km² (Silverman, 2001). The results based on 8 seeded experiments and 7 unseeded experiments 300 minutes after each trial indicated about 2.73 times greater rainfall amounts in the seeded areas versus the unseeded areas (Silverman, 2001). These findings brought about many new perspectives within the cloud seeding research community due to its statistical cloud seeding evaluation in a tropical climate. It also proved crucial for developing new experimentation methods that have ushered in the modern era of weather modification research.
Equipped with modern cloud drop size measuring equipment, a group of atmospheric scientists took to the skies in southwestern Idaho in 2018 in an effort to track the complete process of precipitation enhancement, using silver iodide for seeding material. With 2 ground based radars, a seeding aircraft, and a cloud physics measurement aircraft, these atmospheric physicists tracked 8 millimeter snowflakes falling to the ground within 30 minutes of the seeding (French, 2018). Having such a wide variety of cloud physics instrumentation, radars, and aircraft at their disposal allowed the scientists to depict a clear evolution of cold orographic precipitation enhancement in real time. This was an important milestone in the evolution of cloud seeding capabilities since many studies conducted in the past on the process of seeded cloud precipitation enhancement had yielded ambiguous results. Despite their success, scientists still remain uncertain on the potential long term effects of weather seeding operations.
Since the atmosphere contains a wide variety of constantly changing parameters, tracking the effects of cloud modification experiments can be increasingly difficult with time after the operation. A study published in The Journal of Weather in 2001 discusses how persistence effects of cloud seeding can impact the development of precipitation in a seeded cloud for up to several days after the seeding occurred, leading to the results of these experiments underestimating the actual impact of the seeding on precipitation development (Long, 2001). These persistence effects will need to be taken into account in future research operations in order to ensure the safety and reliability of cloud seeding.
Various research projects conducted over recent years have brought about useful insight to new potential cloud seeding methods, as well as the tools that will be needed to refine said methods. A project conducted in the southern Korean peninsula in April 2019 aimed to test the effects of calcium chloride seeding using crewed atmospheric research aircraft as well as multiple UAVs. The validity of the experiment’s successful seeding was inhibited by the lack of statistical verification, yet the physicists were determined to refine the method with continued dual aircraft experiments, indicating that new UAV sensors, seeding methods, numerical modeling, and ground experiments will be vital to future research (Jung et. al, 2022). The primary issue with getting statistical verification is that most locations throughout the world have not had enough seeding operations done to conduct any meaningful statistical analysis. Moreover, the scientists’ analysis highlights how the state of weather enhancement experimentation is still very much in its infancy, and that there is a large amount of methodology research and technological innovation needed before it is verifiably consistent enough for widespread usage.
Technological limitations aside, many atmospheric scientists around the world are actively testing and developing new, more efficient methods of cloud seeding. In an interview for MIT Technology Review, Dr. Linda Zou of Khalifa University of Science and Technology discusses her ground breaking research using nanotechnology to maximize the efficiency of cloud drop production. She also reveals how conventional seeding material requires humidities of at least 75% to be activated, while the material she is developing can nucleate cloud drops at humidities as low as 65% (Zou, 2022). Dr. Zou’s research presents a massive step forward for weather modification technology as it is one of the first projects to harness this experimental technology for use in cloud seeding. Her material’s ability to condense water vapor at a humidity 10% lower than conventional materials creates promise for the reliability and efficiency of future weather seeding operations.
In spite of the considerable evolution weather modification capabilities have undergone ever since their original implementation in the 1940s, scientists have still been unable to produce methods that are consistent and reliable enough for widespread use. However, numerous breakthroughs are currently being made to circumvent current efficiency setbacks and create new technology to further evolve cloud seeding capabilities.
Chen, A. (2018, January 22). Does cloud seeding really work? An experiment above Idaho suggests humans can turbocharge snowfall [Review of Does cloud seeding really work? An experiment above Idaho suggests humans can turbocharge snowfall]. Science. https://www.science.org/content/article/does-cloud-seeding-really-work-experiment-above-idaho-suggests-humans-can-turbocharge
French, J. R., Friedrich, K., Tessendorf, S. A., & Blestrud, D. R. (2018). Precipitation formation from orographic cloud seeding [Review of Precipitation formation from orographic cloud seeding]. Proceedings of the National Academy of Sciences, 115(6), 1168–1173. https://www.pnas.org/doi/10.1073/pnas.1716995115#sec-1
Harvey, C. (2021, March 16). Eight States Are Seeding Clouds to Overcome Megadrought [Review of Eight States Are Seeding Clouds to Overcome Megadrought]. Scientific American. https://www.scientificamerican.com/article/eight-states-are-seeding-clouds-to-overcome-megadrought/
Jung, W., Cha, J. W., Ko, A.-R., Chae, S., Ro, Y., Hwang, H. J., Kim, B.-Y., Ku, J. M., Chang, K.-H., & Lee, C. (2022). Progressive and Prospective Technology for Cloud Seeding Experiment by Unmanned Aerial Vehicle and Atmospheric Research Aircraft in Korea [Review of Progressive and Prospective Technology for Cloud Seeding Experiment by Unmanned Aerial Vehicle and Atmospheric Research Aircraft in Korea]. Advances in Meteorology, 2022(1), 1–14. Wiley Online Library. https://onlinelibrary.wiley.com/doi/10.1155/2022/3128657
Long, A. B. (2001). Review of Persistence Effects of Silver Iodide Cloud Seeding [Review of Review of Persistence Effects of Silver Iodide Cloud Seeding]. The Journal of Weather Modification, 33(1), 9–23. https://www.journalofweathermodification.org/index.php/JWM/article/view/236/277
Silverman, B. A. (2001). A Critical Assessment of Glaciogenic Seeding of Convective Clouds for Rainfall Enhancement. Bulletin of the American Meteorological Society, 82(5), 903-924. https://doi.org/10.1175/1520-0477(2001)082<0903:ACAOGS>2.3.CO;2
Wu, X., Yan, N., Yu, H., Niu, S., Meng, F., Liu, W., & Sun, H. (2018). Advances in the evaluation of cloud seeding: Statistical evidence for the enhancement of precipitation. Earth and Space Science, 5, 425–439. https://doi.org/10.1029/2018EA000424
Zou, L. (2022, March 28). Scientists advance cloud-seeding capabilities with nanotechnology (L. Ruma, Interviewer) [Review of Scientists advance cloud-seeding capabilities with nanotechnology]. In MIT Technology Review. https://www.technologyreview.com/2022/03/28/1048275/scientists-advance-cloud-seeding-capabilities-with-nanotechnology/
Questions? Suggestions? Inquiries? Please email Kevin O'Connell at [email protected]