Snow day

Video shows how cloud seeding's biggest question could be answered

Snow created during these trials could fill over 200 Olympic swimming pools.

Cloud seeding, a process in which clouds are chemically manipulated into producing precipitation in the form of snow, might sound like something straight out of science fiction. But for places like the American West or the Middle East where water can be scarce, perfecting this process is vital. But while cloud seeding is becoming increasingly common, experimentally confirming just how significance its results are can be extremely challenging.

To improve this process, scientists have designed an approach that moves beyond numerical models to use radar and snow gauges in the field to measure and quantify snow created through seeding and conclusively tell it apart from naturally occurring snow. The results of this study were published Monday in the journal Proceedings of the National Academy of Sciences.

The process of cloud seeding itself is activated when the chemical compound silver iodine, which behaves very similarly to ice, is introduced to the cloud system. But, as these fluffy monstrosities are thousands of feet in the air, actually reaching clouds to introduce the chemical can pose a challenge unto itself. In this study, which took place in Idaho, the University of Colorado Boulder researchers were able to introduce the silver iodine to the cloud using planes to drop flares containing the compound.

From there, lead researcher and co-author on the study, Katja Friedrich, tells Inverse the lightweight water droplets in the cloud begin to form and condense around tiny aerosol particles created by the iodine and form snow.

"What we're doing is putting something very similar to ice, silver iodide, [whose] crystal structure is very similar to ice, into the cloud and then the cloud forms snow that is big enough or heavy enough to fall to the ground," said Friedrich.

Without the introduction of the aerosol, says Friedrich, the water droplets in these clouds would be too light to ever turn into precipitation that could fall to the ground. And when it comes to toxicity caused by injecting these compounds into clouds, Friedrich says the concentrations are well within EPA guidelines and do not pose a significant threat to health or the environment.

Image of one of the project's radar dishes used to anticipate expected natural snowfall.Joshua Aikins

Using this injection method, the researchers performed cloud seeding on three different days in January 2017 and observed the process from initial injection all the way to snowfall. The researchers observed that different environmental variables, like wind or mountains, effected how well the cloud seeding was able to produce precipitation and that snowfall was improved as it went over high terrain. Overall, the researchers observed enough snowfall to produce 282 Olympic-sized swimming pools worth of water. This snow can then be stored and used throughout the year to provide water to dry areas of the West.

An important experimental component of this study that allowed researchers to make such measurements of the cloud-seeded snow was that they used both radar images to track expected natural snowfall as well as snow gauges to keep track of how much snow actually fell to the ground in areas underneath the seeded clouds. With this dual approach, the researchers were able to subtract expected natural snowfall from the actual snow measurements and make much more accurate calculations of how much snow could be attributed to the seeded clouds.

With the success of this approach, says Friedrich, future research will now be able to apply this knowledge back to numerical models and start to answer cloud-seeding's biggest question: how much precipitation can be created in a year and turned into usable water?

Friedrich says that numerical models will be able to run these kinds of scenarios ad nauseam and predict just how useful extended cloud seeding could really be.

"With numerical models, we can turn on and off cloud seeding and see how much water we could produce with cloud seeding," said Friedrich. "The next thing would be to run a snow model, meaning at the end of the water year how much snow is still on the ground, how much snow has been sublimated into the atmosphere? And then once the snow starts to melt, how much water really ends up in the reservoir? Because again this is the ultimate goal -- [we] want to know how much more snow we get in our reservoir."

Abstract: Climate change and population growth have increased demand for water in arid regions. For over half a century, cloud seeding has been evaluated as a technology to increase water supply; statistical approaches have compared seeded to nonseeded events through precipitation gauge analyses. Here, a physically based approach to quantify snowfall from cloud seeding in mountain cloud systems is presented. Areas of precipitation unambiguously attributed to cloud seeding are isolated from natural precipitation (<1 mm h−1). Spatial and temporal evolution of precipitation generated by cloud seeding is then quantified using radar observations and snow gauge measurements.This study uses the approach of combining radar technology and precipitation gauge measurements to quantify the spatial and temporal evolution of snowfall generated from glaciogenic cloud seeding of winter mountain cloud systems and its spatial and temporal evolution. The results represent a critical step toward quantifying cloud seeding impact. For the cases presented, precipitation gauges measured increases between 0.05 and 0.3 mm as precipitation generated by cloud seeding passed over the instruments. The total amount of water generated by cloud seeding ranged from 1.2 × 105 m3 (100 ac ft) for 20 min of cloud seeding, 2.4 × 105 m3 (196 ac ft) for 86 min of seeding to 3.4 x 105 m3 (275 ac ft) for 24 min of cloud seeding.
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