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March 2006

Let it snow

RAL is taking part in a five-year project to determine whether cloud seeding will bolster mountain snowpacks.

Dan Breed and Tara Jensen
Dan Breed and Tara Jensen. (Photo by Carlye Calvin.)

Things haven't changed too much since Mark Twain commented that in the arid Western states, "Whiskey is for drinkin', water is for fightin' over." More than a century after his famous observation, scientists in RAL are planning a field experiment in Wyoming to evaluate cloud seeding as a means of coaxing more snow from winter storms.

The experiment builds on past research to address the broad question of whether cloud seeding is effective as a water management technique. If it is, Westerners could find more water flowing down their strained river basins.

Launched by the state of Wyoming, the Wyoming Weather Modification Pilot Project aims to bolster the state's mountain snowpack and thereby increase spring runoff. RAL's role in the five-year project is to design, monitor, and evaluate the experiment to determine if and when seeding winter storms with silver iodide increases precipitation, especially given the scale and complexity of mountainous terrain.

"Hopefully, we'll be able to collect enough of the right data to provide a definitive conclusion as to whether cloud seeding is a viable tool for water resource management in Wyoming," says Dan Breed, project lead.

Does cloud seeding really work?

The concept of seeding cold clouds with silver iodide has been around for about 60 years. The way it works is that the amount of precipitation a cloud can produce depends on a balance between the number of ice nuclei inside it and the amount of water available to grow around those nuclei. Clouds often have a dearth of naturally occurring ice nuclei, so injecting them with silver iodide particles (which are very similar in structure to ice) increases the number of nuclei and makes the clouds more efficient at producing precipitation.

Several physical studies and a few seeding experiments over limited areas have demonstrated the concept of seeding. But whether the process in fact works during large storms, over extensive areas, and in complex terrain has been a subject of debate among scientists. Many studies have produced indications that cloud seeding increases snowpack, but have fallen short of meeting scientific standards by demonstrating statistical significance. In 2003, the National Academy of Sciences described the science behind cloud seeding as "too weak" and called for more research.

The Wyoming project is unique in that it not only includes an independent evaluation—something that many other cloud seeding experiments have lacked—but it also runs for five years, long enough for the researchers to take natural snowfall variability into account. "I think we're going to take some good steps forward in being able to provide a definitive answer," RAL's Tara Jensen says. "Five years is the minimum we need to really evaluate this. It allows us as researchers to look at the climatology of the storms and the year-to-year variations in the snow."

The project will likely have implications for water management in Wyoming and other Western states. "If the results of this pilot program are sufficiently robust, I think Wyoming will be able to look at cloud seeding as a long-term water management tool and it will draw the attention of many of the other states," Tara says.

Operations in the field

This winter, Dan, Tara, and RAL colleagues Roelof Bruintjes, Tressa Fowler, and others are collecting data and gearing up for the field experiment. Full-scale operations will begin next November in Wyoming's Wind River Range and Medicine Bow/Sierra Madre mountains (see map).

"Amidst all this complexity,
the challenge is to determine whether snowfall levels would have occurred anyway, or if they clearly
resulted from the seeding."

Roelof Bruintjes

The team's tasks include deciding where to place cloud-seeding generators, running real-time weather forecasts, carrying out modeling studies, and creating the experimental design and evaluation plan. Instruments placed at selected sites are already tracking precipitation rates, meteorological variables, background air quality, and ecosystem characteristics. For example, a microwave radiometer on loan from the South Dakota School of Mines and Technology is supplementing ground-based observations of cloud variations at a Forest Service research site in Wyoming's Snowy Range.

The researchers are also strengthening collaborations, having recently coordinated with the University of Wyoming on airborne studies undertaken for a winter storms project in the Snowy Range. In addition, they are working with researchers at the Desert Research Institute in Reno to collect baseline snow samples for ultra-trace analysis of silver. Weather Modification, Inc. (WMI), a private company based in Fargo, North Dakota, is providing the bulk of the equipment and will conduct the project's day-to-day field operations and logistics.

Pending permits, seeding will commence next winter. Based on climatology, optimal seeding conditions are expected about 6 to 10 times per month during the operational period of November 15 to March 31.

To accomplish the ground-based seeding component, 24 silver iodide generators will be placed approximately 12 to 20 kilometers (about 7 to 12 miles) upwind of the center of four different target areas. Generator sites consist of small platforms, 10- to 20-foot (3- to 6-meter) towers, and electronics boxes run by solar-charged batteries for remote control and monitoring. The sites will be located away from environmentally sensitive areas, such as major winter ranges for elk and deer. When weather conditions are right, a propane flame will burn silver iodide solution, producing the ice nuclei that function as seeds for ice crystals. The nuclei are then dispersed and transported into the clouds by orographic (upslope) winds.

The researchers expect to seed from the sky as well, using WMI's Piper Cheyenne II, an aircraft that is equipped for both cloud physics research and cloud seeding.

The Wyoming project is notable for the new technology it applies to cloud seeding. In addition to better instrumentation, the researchers will use improved fine-scale models that show how weather systems develop and interact with terrain and how this influences where snow falls. The silver iodide solution used to seed the clouds has also been improved with the addition of a calcium component that activates it at slightly warmer temperatures.

Water and the growing West

The potential to increase the West's mountain snowpack is especially significant because the snowpack acts like a reservoir, holding moisture until spring when water from melting snow flows down to cities, farms, and ranches. Nearly three-quarters of the West's water comes from this cycle, according to the Rocky Mountain Climate Organization.

Any efforts at bolstering the snowpack could help alleviate water shortages caused by changes in the region. Scientists project that warmer temperatures associated with global climate change will cause more winter precipitation to fall as rain rather than as snow, resulting in decreased snowpack and changes in the timing of spring runoff. This is especially worrisome because of the rapid population growth in the West. In addition, studies have shown that increased particulates and air pollution can change the character of clouds and in some cases might make them less likely to precipitate.

"Amidst all this complexity, the challenge is to determine whether snowfall levels would have occurred anyway, or if they clearly resulted from the seeding," says Roelof, pointing out that even a 10% increase would fall within the range of natural variability of a single storm or a whole season. "To attribute cause, we need this kind of careful, objective analysis independent of the operations."

Wyoming map
Area of operations. The Wyoming cloud-seeding project will center on Wyoming's Wind River Range and Medicine Bow/Sierra Madre mountains.

• by Nicole Gordon


Also in this issue...

The atmospheres of cities
Sidebar: Urban heat islands

Remembering Andrew Crook and Diana Josephson

Let it snow

AGU, AMS presidencies

Delphi Questions: Toilet paper; professional memberships

Just One Look


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