Analysis CR2 | Effectiveness of Cloud Seeding

Roberto Rondanelli, Associate Researcher at the Center for Climate Science and Resilience CR2, and Academic from the Department of Geophysics, Faculty of Physical and Mathematical Sciences, University of Chile

“…under what conditions, if any, can seeding with artificial ice nuclei be employed to produce significant increases in precipitation on the ground in a predictable manner and over a large area? This question remains unanswered.” Wallace & Hobbs, 2006

Cloud seeding, a long-standing and debated potential short-term measure for drought mitigation, has once again come under discussion. However, despite over seventy years of research, the effectiveness of cloud seeding remains a topic of contention, with lukewarm conclusions and a lack of recommendations for its use as a palliative measure against drought.

What is cloud seeding supposed to do?

Cloud seeding is a process that aims to enhance precipitation by introducing artificial ice nuclei into clouds. The first stage in the growth of raindrops or snowflakes in a cloud is condensation, which occurs from a nucleus or particle on whose surface the water vapor present in the atmosphere begins to deposit. The idea behind cloud seeding is that these nuclei are not always present in the required quantity in the air, even when all the physical conditions to produce rain are, so to speak, present. Cloud seeding provides the final ‘push’ for rain to occur by ‘seeding’ a cloud with these particles that will serve as a substrate for water vapor or supercooled water to deposit on, thus rapidly increasing the size of these particles that will eventually give rise to rain.

Since the synthetic compound silver iodide’s structure is similar to that of ice crystals, forming a kind of hexagonal lattice (Figure 1), this molecule can be used to “deceive” the water droplets and then make them condense using this crystal as a substrate.

Since the 1940s, it has been known that under laboratory conditions, silver iodide is effective in increasing the number of nuclei that exist naturally in the atmosphere, which are essential to sustain crystal growth (Vonnegut, 1947). However, although it works well in the laboratory, this mechanism presents various complexities when put into practice. On one hand, it requires that the cloud be in a particular temperature state (below -3°C) and, on the other hand, that there is a deficit of these ice condensation nuclei. Increasing the condensation nuclei in a situation where they are already abundant could have the opposite effect, as the water vapor would be “shared” among a larger number of smaller droplets or crystals. In addition, increasing precipitation through seeding does not imply increasing the amount of water present in the cloud but simply increasing its efficiency for rain.

Despite the practical difficulties, there is evidence that the proposed mechanism for increasing precipitation using silver iodide seeding has been effective in its application and under natural conditions like those occurring in our own territory (French et al., 2018).

On the other hand, evaluating the effectiveness of this method at the basin or regional level is complex and has eluded those who have attempted it for years, due to the complexity of the statistical design of the control experiments (and perhaps also due to the small size of the signal being measured). To explain this, one must first say that precipitation has a very high spatial variability. In simple terms, precipitation, unlike, for example, temperature, exhibits large variations over short distances (in terms of Santiago, it may be raining in La Florida and, at the same time, only cloudy in San Miguel). This means that the proper evaluation of a cloud seeding plan requires many surface measurements, ideally from meteorological radars (which estimate precipitation in real-time over areas hundreds of kilometers in diameter), technology that does not exist in Chile. A second possibility for evaluating cloud seeding methods is the recent development of mesoscale numerical models that incorporate the dispersion of silver iodide and its effect on the physics of precipitation formation.

So, is cloud seeding effective in alleviating drought?

The effectiveness of cloud seeding as a short-term palliative measure against drought is difficult to justify. Its effectiveness is very low in dry years but improves in the long term and, coincidentally, in wetter years. Additionally, as the additional precipitation one aspires to in the case of cloud seeding is a fraction of what actually falls, applying cloud seeding in dry years only produces small increases. Taking the case of a normal year in La Serena (100 mm of total annual precipitation), a 5% effectiveness would produce just 5 mm of additional precipitation, while applying the measure to all precipitation systems during a very rainy year (200 mm) would deliver additional precipitation of only 10 mm, which could then be “stored” in reservoirs and glaciers.

Currently, many countries have artificial precipitation enhancement programs. It is understood that the mere fact that everyone is doing it does not constitute an argument in favor of this technique. In many countries, rain rituals are performed without being able to evaluate their effectiveness. In fact, those who have studied the anthropological origin of rain rituals and prophecies point out the positive effect generated by having these prophecies, which, even if the drought persists, carry a certain optimistic nuance that allows societies to go through droughts with hope for the future (Pennesi, 2007). Could this be the same sociological phenomenon behind the persistence of cloud seeding as a technique despite its effectiveness not being demonstrated so far?

A few other countries, including the United States and Israel, have precipitation modification research programs that guide and assist in evaluating some operational experiences using these systems. However, a recent study carried out in Israel between 2013 and 2020 shows an increase of only 1.8% in precipitation (with a confidence interval between -11% and +16%), an increase that, given the characteristics of spatial variability of precipitation, cannot be statistically attributed with certainty to cloud seeding (Benjamini et al., 2023). Following this study, Israel indefinitely suspended its cloud seeding program. The cost of these programs must then be evaluated in relation to other palliative drought programs, considering the very limited possibility of increasing the amount of rainfall even under the most favorable conditions.

Cloud seeding does not seem to be an effective action to alleviate drought, not only because of its low effectiveness but also because it diverts resources from other possibly more effective solutions.

References

Benjamini, Y., Givati, A., Khain, P., Levi, Y., Rosenfeld, D., Shamir, U., Siegel, A., Zipori, A., Ziv, B. & Steinberg, D. M. (2023). The Israel 4 Cloud Seeding Experiment: Primary Results. Journal of Applied Meteorology and Climatology, 62(3), 317–327.

French, J. R., Friedrich, K., Tessendorf, S. A., Rauber, R. M., Geerts, B., Rasmussen, R. M., Xue, L., Kunkel, M. L. & Blestrud, D. R. (2018). Precipitation formation from orographic cloud seeding. Proceedings of the National Academy of Sciences of the United States of America, 115(6), 1168–1173.

Pennesi, K. (2007). Improving forecast communication: Linguistic and Cultural Considerations. Bulletin of the American Meteorological Society, 88(7), 1033–1044.

Smith, R. L., Vickers, M., Rosillo-Lopez, M. & Salzmann, C. G. (2019). Stacking Disorder by Design: Factors Governing the Polytypism of Silver Iodide upon Precipitation and Formation from the Superionic Phase. Crystal Growth & Design, 19(4), 2131–2138.

Vonnegut, B. (1947). The nucleation of ice formation by silver iodide. Journal of Applied Physics, 18, 593–595.

Wallace, J. M. & Hobbs, P. V. (2006). Atmospheric Science: An Introductory Survey. Elsevier.