Tempest News | June 2023

Scientists have been studying and experimenting with cloud seeding since the 1940s, but what was initially heralded as the solution to drought has faced skepticism over the difficulty of proving its success rate outside of a lab. That's what Katja Friedrich set out to do with the SNOWIE Project. Friedrich is a professor of atmospheric and oceanic sciences at the University of Colorado, Boulder, and was a principal investigator on the SNOWIE Project (Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment). In 2017, Friedrich’s research group had a breakthrough while conducting a cloud-seeing experiment.

Q: CAN YOU EXPLAIN WHAT CLOUD SEEDING IS AN HOW IT CREATES PRECIPITATION?

A: Cloud seeding can be applied to a variety of clouds. For instance, you could seed thunderstorm clouds to prevent hail. We can seed cumulus clouds in the summer to extract more precipitation. With the SNOWIE Project, we are seeding orographic winter clouds. These are clouds that occur in the winter over mountainous terrain. They’re pretty homogeneous, so there's not a lot of turbulence (and I will explain why turbulence actually messes things up shortly). We want to increase the snowpack in the Rocky Mountains because the snowpack is a very cheap and easy way of storing water that we can then distribute throughout the river basin during the year. So that's why our focus is on orographic wintertime clouds. 

We are trying to extract water out of these clouds that would otherwise not fall down as any kind of precipitation. So these are tiny supercooled liquid cloud droplets. ‘Supercooled’ means they’re below freezing, and they're really, really tiny. And because they're so small, they don't fall onto the ground. So what we are doing with cloud seeding is we are putting silver iodide into the cloud, which is similar to ice. And what that substance does is basically act as ice-forming nuclei, attracting the supercooled liquid droplets until they grow large and heavy enough to fall to the ground as precipitation. So overall, it's primarily always trying to extract more water in a cloud that otherwise would not fall down as any kind of precipitation.

Q: HOW IS THE SILVER IODIDE RELEASED INTO THE CLOUD?

A: There are two ways you can release the iodide. The most efficient way is to take an aircraft and fly on top of those clouds or through those clouds, disbursing silver iodide particles. This enables us to gauge how much moisture is in the cloud because these aircraft usually have instruments to measure the amount of liquid in the clouds. And you can put the seeding material directly into the cloud, where it has the greatest chance of success. 

The second method is seeding from the ground. It basically uses the same method, taking flares of silver and generating silver iodide by burning these flares. And then you hope that the silver iodide will be picked up by an updraft and reach the cloud that way. This method is relatively cheap because you just need to have the stations, and you just need to burn the silver iodide. The problem is you don't really know whether the material will be transported to where you need it the most.

Q: CAN YOU EXPLAIN THE SNOWIE EXPERIMENT AND WHAT YOUR ROLE WAS?

A: Cloud seeding has been very controversial since the beginning. There's no question whether it works, as we have seen that it does in the lab, and the physics is pretty clear about this. The questions around this are, “Can I efficiently get the seeding material into the area where I need it?” And the second biggest question is, “How much precipitation can be produced?”

When cloud seeding started in the 1940s and 1950s, people were overly enthusiastic and basically conveyed the message that it was going to solve any kind of drought problem. So people basically seeded the heck out of any kind of clouds and then realized that we don't really have scientific proof that it was really working. There were new experiments in the 1960s and 1970s. But this was a lot of controversy because even scientists couldn't really quantify just how much precipitation could really be generated. The problem with that is that clouds are pretty efficient in seeding themselves. So for us, it's really difficult to distinguish the natural precipitation from the seeded precipitation. 

In the 1990s  into the 2000s, we really improved on American models, where we can simulate these kinds of scenarios. We can say I'm taking this weather system and my simulators with seeding and without seeding, and the difference basically generates what the added contribution from seeding is. The caveat with models is that they have a certain error bar. And we don't really know what they should be. So that's why we designed the SNOWIE Project because we just wanted to go out and collect a lot of data so we can validate our numerical models and conduct these kinds of experiments with more confidence. 

We were piggybacking on an operational seeding operation that the Idaho Power Company runs every winter. As they release the silver iodide, it moves through the cloud and generates precipitation. And then, we fit the data into our model to validate the model. So as we started this experiment, we had clouds, but they were not yet precipitating. So when we started to seed, we could actually see that as we were seeding, we were generating snowfall, and the snowfall was falling on the ground. As we flew, the precipitation fell in zigzag lines, which followed the exact path of the seeding aircraft. That was something that really doesn't occur naturally, so it was clearly being generated. So that's how we knew that this was precipitation we were generating. And that basically revolutionized this entire cloud seeding business. Because for the first time, we could actually show the entire process, from when we put the silver into the cloud to when the seed precipitation fell to the ground. But we were also able to quantify it, saying, “We can generate X amount of snowfall,” which people before could not really do. 

Q: IS THERE ANY DANGER OR RISK IN USING SILVER IODIDE?

A: All these companies who are doing the cloud seeding have to follow Environmental Protection Agency (EPA) guidelines. So they have to take water samples, they have to take snow samples, and they do have to measure the silver that is in the groundwater. The levels cannot exceed the limitations that the Environmental Protection Agency sets. Yes, you’re putting silver iodide or silver into the cloud. But you don't you don't really put that much silver iodide into one location, because you are distributing it over a large area over a relatively short flight, usually about 2 hours long.

Q: HAVE YOU ENCOUNTERED ANY COMMON MISCONCEPTIONS ABOUT CLOUD SEEDING?

A: Yeah, I think everyone has an opinion. You hear people that say, “Oh, cloud seeding generates drought because you're taking moisture out of the cloud,” or you have others that say, “Cloud seeding causes flooding.” But cloud seeding is pretty well regulated. For instance, if the snowpack reaches a certain depth, the Idaho Power Company has to stop seeding. So when we did this in 2017, that was also a very snowy winter, so we had to stop midway through the project, and we were not permitted to seed anymore because the snowpack was just so deep. When people make the argument that you're reducing the moisture in the cloud, I always say it's difficult really to pinpoint because we don't know whether these cloud droplets would fall as precipitation. They could evaporate and not produce any precipitation. Or they could fall as precipitation, but we don't know where and how. So, yes, we are taking moisture out of these clouds. There's no question about it. But I also think we also showed that the amount is relatively small that we're taking out.

Q: YOU MENTIONED THAT  YOU TEACH ANYTHING TO DO WITH CLOUDS. IS THIS A TOPIC THAT HAS ALWAYS INTERESTED YOU? HOW DID YOU GET INTO THIS?

A: It was more coincidence. In my research, I’m primarily using weather radars, whether ground-based or airborne, to understand processes that occur in clouds. So I'm looking at how hail forms, I'm looking at how heavy precipitation events forms, how snow forms. So basically, my colleagues that have been doing that asked me if I could oversee the entire Weather Radar Operations of the SNOWIE Experiment. Cloud seeding is really interesting from a teaching perspective because once you know basic concepts, you can start learning how to manipulate the clouds to do what you want them to do.

Q: AS TEMPERATURES INCREASE, DO YOU THINK THAT CLOUD SEEDING WILL BECOME MORE OF A PRIORITY FOR OTHER UNIVERSITIES AND COMMUNITIES? 

A: That is a really good question. The problem with clouds is how we seed requires sub-freezing conditions, and you need a certain temperature range. So what we see now with warmer winters is that the cloud seeding time gets shorter over the year. So we can’t efficiently seed it like people did just five or six years ago. So that has definitely changed. But on the other hand, there are a lot of engineering companies that look into ways to do it differently. Again, it goes back to the question of how to store the water because we don't necessarily want to have it as rain, since it's easier to store as snow. So there's a lot of discussion about maybe just seeding mountain ranges that are higher up and are not as affected as lower-elevation ranges. And we need to have a moist system coming through for seeding to work. So for example, in the Rocky Mountains, we need to have Pacific systems coming through that have a lot of moisture; otherwise, we can’t seed it. So if we had a very stable high-pressure system over the entire winter, we could not cloud seed it. So there are a lot of bits and pieces that people need to think about when it comes to cloud seeding in the future with a changing climate.