SALT LAKE CITY (Feb. 5, 2025) — Cloud seeding in Utah is effective for many reasons. First, clouds here are greatly impacted by both natural and unnatural sources of aerosols (airborne particles). Second, the terrain enhances how clouds grow and how efficient they can become. Third, and actually foremost: 95% of the state’s water resources come from its snowpack. Before we talk about why all of these factors matter, it’s important to understand how precipitation happens.

Precipitation — it’s not just water

Precipitation cannot occur unless a cloud has condensation nuclei inside of it. These nuclei can be aerosols like ice crystals, salt or dust, to name a few examples. Nuclei give airborne water molecules a place to collide and coalesce — they stick together. When enough water coalesces around a nucleus, it becomes a large collector droplet, causing more and more of the airborne water to come together. When it’s sufficiently heavy, it falls as precipitation.

For a cloud to be a healthy and efficient precipitator, its cloud condensation nuclei must have proper size and distributions. If a cloud’s nuclei are too small, the surface tension of droplets will be reduced, resulting in droplets hitting and bouncing off one another as opposed to colliding and coalescing. This means plenty of very small droplets that, when exposed to freezing temperatures, will not freeze. (This is due to changes in curvature of the droplet and a lack of surface area.) For distribution, it’s more complicated. If a cloud does not have enough nuclei, droplets lacking an ice nucleus will stay in a liquid form. But if too many nuclei are present, moisture then becomes evenly spread out amongst the nuclei — which can mean fewer large collector droplets.

How Utah’s clouds are impacted by aerosols

Utah’s clouds are already influenced by existing nuclei sources. As you’re probably aware, dust, sulfates and aerosols from human activities can all have great impacts on the health of our clouds — especially those near Great Salt Lake and the Wasatch Front. However, it is to be noted that any airborne salts from Great Salt Lake can have a positive natural seeding effect from time to time, as salts are great seeding agents and are hygroscopic (water attracting) in nature. What human-driven cloud seeding does is intentionally manage this process. Cloud seeding nuclei are in the 3-10 micrometer range, the perfect size for large collector droplets to form around. This also means moisture distribution is more likely to be optimal for precipitation.

How Utah’s clouds are impacted by terrain

Meanwhile, the aggressive terrain influences how clouds move and cool in Utah. Often, our clouds are pushed up against our mountain ranges. This rapid lift can expose smaller droplets to freezing temperatures before any collision and coalescence can take place, so that instead of becoming ice, they become supercooled water. At this point, droplet growth can be difficult. This makes cloud seeding efforts in Utah more vital than they might otherwise be — introducing nuclei to supercooled water gives it the opportunity to freeze and therefore, fall.

How seeding Utah’s clouds can give us more snow

Now, let’s talk more about supercooled water. Again, for a droplet to become supercooled, it is a liquid but with temperatures at or below the freezing level. Droplet growth can’t occur unless these droplets collide with an ice particle or snowflake, causing them to freeze. To expedite this process, cloud seeding introduces cloud condensation nuclei with the appropriate size and structure, closely resembling that of an ice crystal.

The material used for cloud seeding condensation nuclei — silver iodide — was first recognized as an ice nucleating particle in 1946 by American scientists Bernard Vonnegut and Vincent Schaefer. Silver iodide, which has a strong covalent bond, is insoluble in water, making it a  great cloud seeding agent since the material is distributed in supersaturated environments. This allows it to act like an ice crystal, helping the supercooled droplets to freeze. Once the freezing process takes over, droplet growth becomes much more efficient and supercooled droplets, turning to ice and snow, fall as increased precipitation. In other words, once the cloud has been seeded and some ice is present, natural processes sustain the increased precipitation.

How phase change extends a cloud’s lifetime

Digging even deeper into the science behind cloud seeding and cloud physics, the phase change from water vapor to liquid to solid provides an additional benefit that extends beyond the target area of our cloud seeding programs. First, when the supercooled liquid water converts into an ice crystal, latent heat is released into the cloud. Much like a hot air balloon, when this heat is released, it allows a cloud to expand horizontally and grow vertically. This allows the cloud to tap into even more available moisture allowing it to become even more efficient but also live longer as it moves across the region. This isn’t the only phase change taking place, however. Droplet growth through the process of diffusion is also taking place, which pulls water vapor from the gas form and deposits onto liquid cloud droplets in an attempt to balance the droplet size distribution. The atmosphere is always fighting for balance, and it’ll use any resource available to do so. This process also adds heat to the cloud allowing for continued expansion which adds to the duration of the cloud’s lifetime. 

Because Utah’s primary source of precipitation is snowfall, seeding winter clouds so that they can begin to form ice crystals gives us the best possible chance to enhance our water resources long-term.

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