Clouds – those enigmatic formations of condensed water vapor which drift above our heads, forming rivers in the sky – can take many forms. They can be wispy and non-threatening, dark and menacing, towering or diaphanous. We tend to think of them as beyond human reach, as harbingers of forces outside of our control, but as scientists learn more about them, it’s increasingly clear that humanity isn’t merely subjected to whatever weather a cloud portends – we also create and influence it through our everyday actions. And scientists now regularly harness their moisture and pull it to Earth, bringing water to parched communities and landscapes around the world.  

This rain-inducing technique, called cloud-seeding, has been around for more than 60 years. The process involves “seeding” existing clouds with a harmless substance called silver-iodide to give water droplets a particle to converge around, allowing them to form an ice crystal. Every snowflake you’ve ever seen has initially formed this way – a small speck of dust or pollen floating around the atmosphere collects freezing drops of water, forming the intricate designs that we’re familiar with. The only difference between cloud-seeding and natural precipitation is that instead of dust or pollen, the nucleus of the ice crystal is a tiny particle of silver iodide that scientists released into the cloud. Although not a panacea for drought-stricken regions, cloud-seeding can increase seasonal precipitation by about 10%. In the Reno area alone, winter cloud-seeding efforts are estimated to add enough water to supply about 40,000 households for a year. 

DRI researchers first joined pioneering efforts to draw more precipitation from otherwise reluctant clouds in the early 1960s, using it to increase the mountain snowpacks that supply much of the West’s water. Today, our scientists continue cloud-seeding efforts around Nevada, including in the Sierra Nevada mountains near Reno, the Spring Mountains near Las Vegas, and the Ruby Mountains near Elko. To bolster the snowpack feeding the Colorado River, DRI runs additional cloud-seeding efforts in Colorado.  

“I feel really passionate that we can improve water resources across the Western U.S. with these cloud-seeding programs,” said Frank McDonough, DRI’s cloud-seeding program director. “And we can run them relatively inexpensively. It’s really the only way to add precipitation to a watershed.” 

Above: A map of DRI’s cloud-seeding locations around Nevada. DRI also has operations in Colorado. 

Cloud-seeding in a time of increasing drought 

Nevada is the driest state in the nation, with the statewide precipitation average a mere 10.3 inches — only about a third of the nationwide average of 30 inches. The rest of the Western U.S. isn’t faring much better, as the 21st century has been a time with the most severe drought in 1,200 years.  

Scientists examined tree rings to decipher the historical record for soil moisture across the region, finding that the years 2002 and 2021 were among the driest. Although historical records tell us the Western U.S. is prone to fluctuations in climate, the growing greenhouse effect of burning fossil fuels is exacerbating drying conditions. In 2022, DRI researchers published a study showing that as temperatures rise, the atmosphere pulls more moisture from streams, soils, and vegetation – what’s known as “atmospheric thirst.” Under these conditions, even consistent levels of precipitation will result in less available water for humans and ecosystems.  

We see the effects of this playing out before us as the country’s largest reservoirs, Lakes Mead and Powell, are at their lowest levels on record. As of February 2023, Lake Mead stood at about 70% empty, while Lake Powell was little more than 20% of capacity. Under these arid conditions, and with so much uncertainty surrounding water availability from season to season and year to year, cloud-seeding is one tool to help alleviate some of the human and ecological impacts of persistent drought.  

“Now, it’s not going to solve all our problems, because we need storms in the area to do cloud seeding,” McDonough says. “But I think that cloud seeding and making clouds more efficient at producing precipitation is a huge tool in water managers’ tool belt.” 

As the impacts of climate change reduce mountain snowpacks around the arid West and the world, the Intergovernmental Panel on Climate Change (IPCC) has recommended cloud-seeding as one way to help communities adapt to drier climates.  

DRI researcher Patrick Melarkey performs maintenance on weather monitoring equipment in the Sierra Nevada Mountains above Lake Tahoe. 

The complex science of cloud formation 

Cloud formation is a complicated field of research, and scientists are still learning about the way that water droplets and ice crystals interact with atmospheric particles to produce the many different types of clouds we observe. However, one thing is clear: without microscopic particles for water vapor to latch onto — like dust or salt from the sea — clouds cannot form.  

Small particles of water scatter in the air and require another speck of something microscopic in order to come together into larger, more visible water droplets. The effect is demonstrated well in a simple video taken in 2008 by scientists in the Arctic, who show that their breath fails to form visible water vapor due to the low atmospheric particle count (at sea, in areas with little to no wind, atmospheric aerosol levels are very low). They then proceed to steep a cup of tea, which also fails to form much of a visible cloud as the tea evaporates into the cold air. But when they spark a lighter over the top, small particles produced during fuel combustion grab onto the surrounding water vapor and a small cloud forms instantly.  

Due to this relationship between water vapor and atmospheric aerosols, human activities impact clouds in a number of ways. Atmospheric aerosols now include a wide range of pollutants produced by industrial emissions, tiny bits of plastics and rubber that wear from car tires and brakes, and vehicle tailpipe emissions.  

Not all atmospheric aerosols have the same impact on clouds. Research has shown that air pollution can prevent rainfall, because the water droplets in polluted clouds are too small – they float around in the atmosphere without merging to form large enough droplets to fall to the ground. A single drop of precipitation requires more than one million of these small droplets to converge. These pollutants can also prevent ice formation in subfreezing clouds. This means that our everyday activities in urban and industrial areas are already altering global rainfall patterns.  

“There’s increasing research showing that air pollution is actually having a negative impact on a cloud’s ability to produce precipitation,” McDonough says. “So, in some ways, we’re trying to restore the cloud’s ability to produce precipitation to what it would have been prior to all the air pollution coming in upstream.” 

Above: An old Beechcraft C-45 plane that DRI researchers used to seed clouds in 1966. Credit: DRI

A snowstorm in a freezer inspired decades of research 

The history of cloud-seeding begins in the 1940’s with scientists who wanted to understand why ice sometimes accumulated on planes, creating dangerous flying conditions. Realizing they needed to know more about clouds that contain supercooled water (which is below freezing temperature but still in liquid form), researchers at General Electric simulated these conditions with a repurposed home freezer, as shown in this video. When they dropped dry ice into the freezer to mix with the water vapor from their own breath, millions of ice crystals formed, simulating a miniature snowstorm. In 1946, one of these scientists was the first to drop dry ice from a plane, watching as streams of snow fell from the cloud.  

Following these initial experiments, the research team (including Bernard Vonnegut, brother to famed author, Kurt Vonnegut) turned to silver iodide for its structural similarity to ice crystals. Silver iodide continues to be used today by DRI researchers, as a harmless substance that effectively creates a central point – or nucleus – for water droplets to converge around.  

Although many groups around the world continue to use airplanes for cloud-seeding, DRI scientists turned to a ground-based program after a fatal accident on March 2, 1980. The plane crash killed DRI researchers Peter Wagner, William Gaskell, John Latham, and Gordon Wicks. Following this tragic event, one of DRI’s pioneering experts on cloud-seeding, John Hallett, dedicated his expertise on ice formation in clouds to improving airplane safety.  

Hallett was recruited in 1966 to be one of the institute’s founding scientists: with his expertise in atmospheric physics, he helped establish DRI as a worldwide leader in the field. His focus on the behavior of ice in the atmosphere led to the discovery of a key mechanism for the transition of water molecules into ice, which is now known as the “Hallett-Mossop ice multiplication mechanism.” DRI continues to be a leader in cloud-seeding efforts and research around the Western U.S.  

Above: A DRI weather monitoring station perched on top of Slide Mountain overlooking Lake Tahoe. Credit: DRI

Measuring success in the chaos of a storm 

Taking cloud-seeding from a freezer to the skies meant finding evidence of the technique’s impact in the real world. Measuring the impact of cloud-seeding attempts isn’t simple, as it requires comparing precipitation from seeded and unseeded clouds under identical conditions – hardly an easy task due to the complex nature of the atmosphere and changing conditions over time. However, scientists have found several ways to assess their impact. By sampling levels of silver iodide in a mountain snowpack following cloud-seeding activities, researchers found it incorporated into ice crystals and deposited as snow – evidence that the compound works as an ice nucleating agent. A comprehensive review of available research published in 2019 concluded “clear physical evidence has been obtained that orographic clouds containing supercooled water, when seeded with silver iodide, produce plumes of ice particles that originate downwind of the seeding location and reach the ground through precipitation growth and fallout.”  

In 2020, a ground-breaking study known as the SNOWIE project used advanced radar and cloud-measuring technology to show that cloud-seeding coaxed moisture out of supercooled clouds, producing enough snow to fill 282 Olympic-sized swimming pools over approximately two hours. Studies like this allow scientists to build computer simulations that can facilitate more research, overcoming the difficulty of conducting field experiments under challenging conditions.  

Clouds with supercooled liquid water often form around mountain ranges as the air rising over them quickly cools with the elevation change. This is why DRI’s cloud-seeding efforts focus on mountain regions, including the Sierra Nevada and Spring Mountains. A small team of researchers, led by McDonough, plants generators in strategic locations for intercepting incoming storm clouds. These generators vaporize silver iodide particles with acetone, allowing them to rise into the air and enter supercooled clouds. The silver iodide causes the tiny drops of water in the cloud to freeze and go on to grow ice crystals large enough for gravity to pull them to the ground. Ground-based cloud-seeding allows the research team to safely and cost-effectively conduct their work, with each acre-foot of water produced only costing a few dollars.  

“There’s been a lot of good research done over the last decade or so that has really nailed down how well this works,” McDonough says. “We’re feeling more and more comfortable about our understanding of when cloud-seeding techniques work, and the scope of the impact.” 

Above: A DRI cloud-seeding generator and maintenance truck in a wintery, mountain-top setting. Credit: Jesse Juchtzer/DRI

Unfortunately, the internet contains many misleading ideas about cloud-seeding. Below is a series of misconceptions and questions about the common scientific practice.  

 FAQ:  

1. Is cloud-seeding producing so-called “chem-trails”? 

No. Those fluffy white lines zig-zagging across the sky are jet contrails, and they are the aviation equivalent of visible plumes of steamy breath on a cold morning. Warm water vapor produced during jet fuel combustion interacts with the cold atmospheric air to create strings of ice crystals that behave like high-altitude cirrus clouds. When a plane passes through an area of high pressure, which leads to low winds and clear skies, the trails will linger. Jet contrails have no connection with cloud-seeding activities.  

2. Is cloud-seeding “geoengineering”? 

Cloud-seeding is a well-researched and monitored form of small-scale weather modification. Other examples of ways that humans change the weather and the global climate include: driving a car, deforestation, and air pollution from industry.  

3. Who is funding cloud-seeding programs? 

Cloud-seeding programs occur worldwide. In the Western U.S., state and agency-supported efforts occur across California, Nevada, Colorado, New Mexico, Wyoming, Kansas, Oklahoma, Texas, North Dakota, Utah, and Idaho.  

4. Is silver iodide toxic? 

No. The silver used in cloud-seeding is silver iodide (AgI, or silver bonded to iodine), which can be confused with other molecular forms of silver. When silver is isolated as an ion (Ag+) it is biologically active, meaning it interacts with bacterial or fungal cell walls — which is why it’s often used for medicinal purposes and for sterilizing drinking water. Silver ion (Ag+) can be hazardous in aquatic environments because it can also interact with proteins and other parts of cell membranes, but silver iodide (AgI), not silver ion (Ag+), is used for seeding clouds. Silver iodide retains its form in water and does not break down into the potentially toxic silver ion. When the silver iodide particle falls to the ground with rain or snow, it separates from the water molecules that formed an ice crystal around it, essentially becoming a speck of dust no different from the silver naturally occurring in the soil.  

Although the chemistry can be a bit complicated, you can think of it as the difference between water (H2O) – the life-giving force that forms much of your own body – and hydrogen peroxide (H2O2), which is used as a sterilizer and bleaching agent and is hazardous at high concentrations .  

5. Is cloud-seeding used for military purposes? 

Following the (now declassified) use of cloud-seeding by the U.S. military during the Vietnam War, a 1977 international treaty banned the use of weather modification in warfare.  

6. Does DRI continue cloud-seeding during intense winters like the winter of 2022-2023?

DRI pauses all cloud-seeding activities when the snowpack reaches 150% of the historical average. In the Lake Tahoe region, this means that cloud-seeding activities halted in mid-December, 2022, due to the remarkable amount of natural snowfall occurring. 

More information:  

The Cloud Seeders
A short video about DRI’s cloud-seeding team

Where to find more water: eight unconventional resources to tap
The Conversation
By Manzoor Qadir and Vladimir Smakhtin, Deputy Director and Director of the United Nations Institute for Water, Environment, and Health 

Can cloud seeding help quench the thirst of the U.S. West?
Yale e360 

Wintertime Orographic Cloud Seeding—A Review 
Journal of Applied Meteorology and Climatology, Vol. 58, No. 10 (October 2019), pp. 2117-2140 

Quantifying snowfall from orographic cloud-seeding
PNAS, Vol. 117, No. 10 (February 2020), pp. 5190-5195 

Does cloud seeding really work? An experiment above Idaho suggests humans can turbocharge snowfall
Science Magazine 

More Information

To learn more about DRI’s cloud-seeding program, go to https://www.dri.edu/cloud-seeding-program/. 

DRI Interns Join the Search for Elusive Desert Tortoises in Tule Springs Fossil Beds National Monument

DRI’s Behind the Science Blog continues with the second installment of our fall 2022 Research Immersion Internship Series

This fall, DRI brought eleven students from Nevada’s community and state colleges to the Las Vegas and Reno campuses for a paid, immersive research experience. Over the course of the 16-week program, students worked under the mentorship of DRI faculty members to learn about the process of using scientific research to solve real-world problems.

Our Behind the Science Blog is highlighting each research team’s accomplishments over a series of five stories. Click here to read the first installment in our internship series.

In this story, we follow Tiffany Pereira’s student interns as they track elusive and threatened desert tortoises in the Las Vegas desert.

Student Researchers: Amelia Porter and Akosua Fosu

Faculty Mentor: Tiffany Pereira, M.S., Ecologist and Assistant Research Scientist

Despite their enormous size, desert tortoises are elusive desert dwellers, often spending most of their lives in underground burrows – giving them their scientific name, Gopherus agassizii.  They occur across the Mojave and Sonoran deserts of California, Nevada, Utah, and Arizona. Listed as threatened under the Federal Endangered Species Act since 1990, their numbers are declining due to a number of threats. Understanding the size and health of their populations is a priority for both government agencies and researchers. 

Desert tortoises occur across the Mojave and Sonoran deserts of California, Nevada, Utah, and Arizona.

Credit: Map courtesy of the U.S. National Park Service. 

“Desert tortoises face predation by ravens and other large birds, canids including coyotes and foxes, as well as insects such as fire ants,” said intern Amelia Porter. “They’re also victims of urbanization, military activity, mining, and alternative energy projects, which destroy their natural habitat as well as their food and water sources and deposit a multitude of pollutants.”

For their internship, Amelia Porter and Akosua Fosu worked with DRI ecologist Tiffany Pereira to survey parts of the Tule Springs Fossil Beds National Monument near Las Vegas for desert tortoises. Tule Springs was established in 2014 to protect the delicate desert habitat as well as rare, preserved fossils of Ice Age life – including mammoths, ground sloths, dire wolves, and American lions. The park borders northern Las Vegas and Highway 95.

“When it comes to urban-wildlife interface, Tule Springs acts as a barrier between active human development and pristine desert tortoise habitat,” said intern Akosua Fosu. “The thing is, Tule Springs National Monument is literally in people’s backyards, and it borders a major highway. The goal is to continue to use this park and enjoy all it has to offer, but to do so in a way that doesn’t disturb the desert tortoises within the monument.”

Interns Akosua Fosu and Amelia Porter locate a desert tortoise while conducting surveys. 

Credit: DRI.

Searching the Desert Landscape for Clues 

To help Tule Springs resource managers better understand how many desert tortoises occur across the park, as well as how they use the landscape, the research team used a method called occupancy sampling. This method combines field surveys with computer modeling to help researchers determine the proportion of habitat within an area containing evidence of a targeted species. Occupancy sampling allows scientists to determine the abundance of a species that is otherwise elusive or difficult to track.

“One way to understand where tortoises are in the park is to walk transects across the entire monument – but that’s just not feasible,” said faculty mentor Tiffany Pereira. “So, we did a different type of population sampling that could provide information on where the tortoises are. Going to the site with the interns every week has been really fun.”

The research team conducted field surveys across 20 plots, each of which they visited four times. As they walked focused lines called transects, they recorded signs of tortoise occupancy including scat, tracks, and of course, observations of live tortoises.

Intern Akosua Fosu surveys the desert landscape for signs of desert tortoise, including scat, tracks, and evidence of burrows.

Credit: DRI.

“Tortoise scat is cylindrical in shape and has lightened edges,” said Fosu. “It’s almost like a cigar – and when you break it apart, it contains plant material.”

When the researchers found a possible tortoise burrow, they looked for evidence of recent activity, like an “apron” in the dirt indicating digging, or visible tracks.

“A tortoise burrow is a half-circle shape with the top of it rounded and smooth, due to the shell eroding it over time,” Porter said. “They’re usually located in rocky areas or under vegetation.”

Desert tortoises spend much of their lives in burrows that protect them from the harsh desert sun.

Credit: Tiffany Pereira/DRI.

The students recorded any sign of desert tortoises in a survey app, including the measurements and characteristics of burrows and the presence of live tortoises or carcasses. Using the survey data, the researchers marked each plot as active or inactive for desert tortoises using a binary system. The results are analyzed using the software program PRESENCE, which provides an estimated occupancy probability of desert tortoises within an area of interest.

“Tule Springs is a newer park,” said Porter, “so they will use the data that we’ve collected over the course of the season to help determine where to place signage, or hiking trails, without disturbing desert tortoise habitat.”

One of the most important findings from their study is that many tortoises are using parts of the park that are near human activity. “That’s a big deal for management of the park,” Pereira said. “In one case, we found a tortoise less than 200 meters from a paved road. When they think about their management plans, they need to account for that.”

Joshua trees at Tule Springs Fossil Beds National Monument.

Credit: Matthew Dillon.

Embracing the Research Experience

For student interns Porter and Fosu, joining Pereira’s research team and spending time in the field was a truly immersive experience into the world of science.

“Being able to see some of our native flora and fauna up close was a highlight,” said Fosu. “There were days where we came across tortoises, snakes, and even jack rabbits. I also got to learn about some of our native plant species.”

Fosu, a student at Nevada State College studying biology and chemistry, entered the school year with plans to pursue a veterinary career. Her time working with Pereira reinforced her interest in working with animals, she says. “I may even consider conducting veterinary research in the future.”

Student intern Akosua Fosu finds a desert tortoise while conducting surveys.  

Credit: DRI.

“Overall, I think this was a very eye-opening experience,” Fosu continued. “My goal was to gain some research experience before I graduate and I’m glad I was able to gain that through this internship. I would definitely recommend this to anyone considering a career in a STEM field.” 

Intern Amelia Porter conducting a survey for desert tortoises in Tule Springs Fossil Beds National Monument.

Credit: DRI.

Porter, a student at the College of Southern Nevada studying environmental conservation biology, agreed that the DRI internship helped her feel more confident in her career choice. “It has not only confirmed my passion for a career in ecology and wildlife studies,” she said, “but sparked an interest in park service and in field surveying as well.”

The highlight of the semester, Porter said, was “feeling immersed in the methods of an established ecologist, and the opportunity to feel like I was a part of a project that benefitted the surrounding area.”

“I think the entire immersion program has been a fantastic opportunity,” she said, “and I hope that the program continues so it can be as inspiring to others as it has been to me.”

More Information

To learn more about the DRI Research Immersion Internship, go to https://www.dri.edu/immersion/.