SALT LAKE CITY (Feb. 5, 2025) — Cloud seeding can be a complex science for many, which opens the door to many myths and misconceptions. First and foremost is the idea that silver iodide is toxic. We’ll cover that, as well as other myths and misconceptions.

Silver iodide: safe in the environment

The misconception arises from the silver in silver iodide. Free silver ions — meaning silver ions that are not attached to other ions, atoms or molecules — can be toxic to living things, especially when dissolved in water. However, silver iodide does not cause problems like free silver.

This is because in silver iodide, the silver and iodide ions have a strong covalent bond, due to their differences in the electronegativity. In fact, the electronegativity of silver iodide is three times the threshold needed to be considered strongly bonded. What exactly does this mean? It means that silver iodide stays in a solid state when introduced to supersaturated environments and/or water. It does not dissolve. This makes it both a great seeding agent but also environmentally safe as the free silver ion stays attached to the iodine. 

If any silver ions do get released (dissolved), research has shown that the maximum concentration of the soluble silver peaks at 0.984 micrograms per liter. This is far below the safe standards set for drinking water (100 micrograms per liter), impacts on aquatic life (4-6 micrograms per liter) and impacts to terrestrial plants (10-10,000 micrograms per liter). Additionally, naturally-occurring background levels of free silver across parts of southern Utah, Nevada, and California are in the 4-6 micrograms per liter range, which is 400-600% more than what could be released via silver iodide. Previous research has occurred in Australia, Nevada, California and Idaho. For Utah, it’s important to confirm that environmental  impacts mirror those of those previous studies. Therefore a comprehensive environmental study will be taking place in the next 18 months.

Cloud seeding, not cloud stealing

Next is the idea of robbing Peter to pay Paul (i.e. stealing water that would’ve fallen somewhere else). Clouds are an open system, so while moisture is precipitated out of the cloud, more moisture can move in to replace it. The life of the cloud is more dependent upon the energy available, whether through the heating of the atmosphere, or interaction with a frontal boundary like a cold front, warm front or a dryline. 

Research has shown that your typical summertime thunderstorm is only 19% efficient: meaning only 19% of the moisture in the cloud is precipitated out. Here in Utah, our work is surrounding larger scale cloud systems that are only 1% efficient. If only 1% of the moisture is precipitated out, and cloud seeding impacts that cloud by 10%, that means 1.1% of the cloud is precipitated out. This has no downwind impact. In fact, our downwind neighbors to the east, Colorado, support the Utah cloud seeding program as much as they support their own program. In this series’ second article, we talked about the release of latent heat in a cloud. This allows the clouds to expand, resulting in the clouds being able to last longer and tap into additional moisture resources. Studies in Utah have shown up to a 5-10% increase of precipitation up to 75 miles downwind. Therefore, we are paying Paul, yes, but we are always paying Peter!

White lines in the sky? Contrails, not cloud seeding

The last thing I’d like to cover is when cloud seeding can occur. Cloud seeding is a weather modification technique that requires clouds to be present. Cloud seeding work is never conducted in clear skies, at high altitudes or, in Utah’s case, away from the mountains. Cloud seeding introduces cloud condensation nuclei directly into an already precipitating cloud. Thus, white lines in the sky never mean cloud seeding. 

So what are those white streaks you sometimes see in the sky? Most likely they are jet contrails. Contrails are like the exhaust you see coming from a car on a cold winter morning, but exaggerated! Conditions at 30,000 feet are much colder, and the exhaust from jets is much hotter. Why do some contrails stay and spread out instead of evaporating away? Remember, the atmosphere is a fluid, and very complex. Moisture can change vertically and horizontally based on boundaries, mixing and changes in wind direction and speed. On days where moisture is more prevalent, evaporation doesn’t take place as fast. This allows the streak to spread out as mixing and winds separate the material. Sometimes, a plane can fly through a moist part of the atmosphere and cross a boundary to a drier portion of the atmosphere, causing the streak to start evaporating quicker. So, it’s completely normal to see variation in how long contrails stay in the sky, and how their shapes change as they evaporate away.

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