Those Window-Rattling Booms on Powder Days? The Howitzers Behind Them Are Nearly Gone

For almost as long as humans have been riding chairlifts in the big mountains of Western North America, avalanche artillery mitigation has been at the center of a steadily evolving program to understand, predict, and control snowpack hazards. Many a skier has woken up to the thunderous, window-rattling booms of artillery fire from a nearby highway or ski area and known that sound is almost universally a sign of a good ski day. The combination of new avalanche mitigation strategies, dwindling supplies of the surplus ammunition used for mitigation, and retirements of people who know how to service an aging arsenal of howitzers has culminated in a more than decade-long push to eliminate reliance on artillery for avalanche mitigation. Despite the continual development of new technologies for avalanche mitigation and a deepening understanding of where and why avalanches form, military artillery offers distinct advantages over other methods. Howitzers and recoilless rifles, the centerpiece of many avalanche mitigation programs for 75 years, are certainly on their way out, but the holes they leave behind are not so easily filled.

How Avalanche Artillery Mitigation Began: Monty Atwater and the 75 mm Howitzer

Monty Atwater, who first developed the use of explosives to artificially trigger avalanches during his time as a Snow Ranger in Little Cottonwood Canyon, also pioneered the use of artillery to trigger avalanches in 1949. Initially, Atwater would call the Utah National Guard, who would tow a World War I-era 75 mm howitzer up the road and shoot at whatever he told them to, in an effort to protect the town of Alta, Utah, and the budding ski area. Eventually, Atwater began firing the howitzer himself, and after surviving a bureaucratic firestorm, he helped start the partnership between the Army and the U.S. Forest Service that continues to this day. The 75 mm howitzer was the initial weapon of choice, or rather, the weapon that was available. From the 1950s to the 1990s, nearly all ski areas and highway programs using artillery for avalanche mitigation employed either 75 mm or 105 mm recoilless rifles, a slightly different artillery design from the howitzer, lighter in weight. In December 2002, two separate in-bore explosions occurred 13 days apart at Mammoth Mountain, California. These accidents were likely caused by manufacturing defects in the ammunition, which caused the charges to go off within the gun. Luckily, the crews operating the guns were behind a protective barrier and were unharmed, a safety measure implemented after a fatality related to an in-bore explosion at Alpine Meadows, California, in 1995. Following these accidents, the Forest Service switched all of its artillery users over to the 105 mm howitzer.



Why Artillery Works: The Physics Behind the Blast

Avalanches are caused by mechanical failures, or fractures, within a snowpack. These fractures occur at weak points between the layers of the snowpack, sometimes in a layer of an especially weak type of snow crystal, and sometimes where new snow has not yet bonded with the old snow it is falling on. A skier, in the wrong place at the wrong time, can initiate a fracture and eventually an avalanche, but that is not the only way of initiating an avalanche. The blast from an explosion can provide more than enough mechanical energy to initiate a fracture, and if that fracture occurs in a starting zone, or in a part of the mountain with a slope angle steeper than roughly 30 degrees, an artificially triggered avalanche is likely. The rest of the development of an avalanche mitigation strategy essentially comes down to an engineering problem, though certainly not a simple one. How can explosives effectively be delivered to an avalanche start zone when avalanches can be expected to occur?



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Many ski areas rely on a combination of chairlifts, snowmobiles, snowcats, and hiking to get ski patrollers into position to throw relatively small bombs or hand charges into start zones. In contrast, an artillery piece uses a controlled explosion within the barrel to launch a shell with a bomb inside it to a starting zone, often from very far away. Because of humanity’s unfortunate fascination with developing better ways to kill each other, modern artillery pieces can deliver a shell to within a few meters of a target from several kilometers away. One of the most obvious advantages is that humans do not need to be near starting zones to start avalanches with artillery, a huge safety advantage. Further, multiple slide paths or starting zones can be addressed from a single location, rather than moving between them or using multiple teams to quickly mitigate avalanche hazards.



Of course, using artillery, or tools designed to kill and injure people, is not risk-free. Hand charges deployed by ski patrollers produce dangerous, but relatively small explosions. It is easy to throw a hand charge a safe distance away. Artillery shells, even when they carry the same amount of explosive material as a hand charge, are far more dangerous because they produce shrapnel. Bits of metal from the artillery shell can cause serious injury to people a far distance from the blast. Out of an abundance of caution, entire highways or sides of a ski area are typically closed when using artillery. In comparison, multiple teams can safely work in relatively close proximity to each other with hand charges, which do not produce shrapnel.

The U.S. Army has not used 105 mm howitzers in combat for some time, and ammunition for these weapons is running out. All avalanche artillery programs are run by the U.S. Forest Service, which leases the weapons from the Army. The reliability and accuracy of howitzers for avalanche mitigation work come in part from the Army’s diligent servicing of the guns. But the people with the expertise to service the aging arsenal of guns number fewer each year. The Army asked all of the members of the Avalanche Artillery Users of North America Committee to plan for an artillery-free future.

Eliminating Avalanche Artillery at Snoqualmie Pass, Washington

The Washington State Department of Transportation’s (WSDOT) avalanche program is responsible for forecasting and mitigation along several highways, most notably Interstate 90 over Snoqualmie Pass. Mitigation of the 40 slide paths that can impact the highway has traditionally involved a combination of hand charges, ski cutting, bomb trams, howitzers, and, for a brief time, an M60 tank, which fired a projectile similar to that of a recoilless rifle. Since the early 1980s, WSDOT has used bomb trams to deliver explosives to starting zones, inspired by pioneering work at Bridger Bowl, Montana. Over the years, trams have ranged in design, often constructed with spare materials lying around the shop. From the start, bomb trams offered several advantages: they eliminated hand-charging or ski-cutting locations that were dangerous or difficult to access, reduced the number of artillery shots required, and enabled the delivery of air blasts.



It turns out that because of the mechanical properties of snow, explosions above the surface of the snowpack are far more effective at triggering avalanches than explosions on the surface or within the snowpack. Bomb trams offer a simple way to deliver an air blast. Dangle a bomb off a simple cart on a cable, use a bicycle or a hand crank to position the bomb over the starting zone, then blow it up while it dangles above the surface of the snow. As WSDOT’s fleet of trams grew, so did the maintenance required to keep them running. Ice buildup on tram lines, carriers breaking, tangling in the wind, trees falling on the lines, and occasionally, anchor trees falling over all pose challenges to keeping trams operational. By 2010, WSDOT had 16 trams in place to mitigate most of the avalanche hazard. Only three hand charge locations remained, and artillery took care of the rest.



Many of the avalanche paths mitigated with artillery were in terrain that made on-foot access extremely difficult. John Stimberis, WSDOT’s avalanche forecasting supervisor, described this challenge to SnowBrains during a phone interview. “We have all scratched our heads numerous times, like, how can we get to that terrain and eliminate, or at least reduce the artillery footprint? We did in some places; initially, some artillery shots were replaced. But we couldn’t do the whole thing,” he said. Several of the last remaining artillery shots on avalanche paths that could affect the highway are at lower elevations and do not always develop snowpacks deep enough to trigger an avalanche, yet they are in steep terrain that prevents the WSDOT avalanche team from installing trams or deploying hand charges. If conditions were right for these paths to trigger avalanches, additional artillery fire could be used to mitigate the hazard. But with an artillery-free future ahead, another solution to mitigate these difficult-to-access starting zones had to be found. Stimberis was convinced that these paths could be controlled with a remote avalanche control system, or RAC.



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There are several different kinds of RAC systems; one of the earliest and most widely adopted in North America is the Gazex tube. Gas-based systems like the Gazex ignite a propane-oxygen mixture inside a metal tube to create a blast in the start zone of an avalanche. The blast can be ignited remotely with radio, and storage tanks can hold enough gas for dozens of shots throughout the winter. Stimberis had his eye on a different system, the Wyssen Tower, which is rapidly growing in popularity. Stimberis said he had been thinking about RAC systems for I-90 for roughly 10 years, including a self-funded trip to Europe to examine various RAC designs.



The Wyssen tower has a box full of pre-built explosives, and when triggered by radio, one charge drops from the tower, dangling by a cord, where it delivers an air blast. Wyssen towers can be reloaded throughout the winter by helicopter or stored and serviced off-site over the summer, just leaving a tower behind. RAC systems of any kind can cost several hundred thousand dollars and require construction in high-angle alpine environments, usually by helicopter. After studying several types of RAC systems and talking with other users, Stimberis had decided that the Wyssen tower would most effectively address avalanche problems in Washington’s dense snowpack. He worked with the Forest Service to complete environmental reviews for potential RAC system installations before he had the budget to purchase them, ready to move the project forward if funding became available. “Last spring, some money opened up, and we were told that if you can use this in the next two months, you can get your system. And so it really paid to have all that decision-making and the permitting process completed. We basically had everything ready to go,” Stimberis told SnowBrains.

With six Wyssen towers installed along the interstate, the final artillery shots were eliminated permanently. Operationally, the Wyssen towers have dramatically changed what avalanche mitigation looks like on I-90, cutting hours from road closures and firefighting missions. In Washington’s maritime snowpack, addressing avalanche problems more quickly leads to more effective mitigation, since the window of time when the snowpack is most unstable can close quickly compared to other climates, such as Utah or Colorado. “It is a big deal, shutting down an interstate highway,” Stimberis said. “It takes hours and hours to even get to the point of being able to shoot. Then, you’re shooting a full 20 or 22 targets, and it might take us an hour and a half to do. Then there’s clean up. Sometimes, by the time you’re shooting your artillery, the avalanche cycle has already happened. And you’re kind of just cleaning up what nature has done.” Now, with the Wyssen towers, only the westbound lanes of the interstate need to be closed, which could happen in an emergency in about 30 minutes. Shooting all six Wyssen towers also happens in as little as ten minutes. Washington’s push to eliminate artillery usage along I-90 required years of planning, but resulted in a system that allows avalanche problems to be addressed much faster than previous artillery missions.



Little Cottonwood Canyon, Utah, the birthplace of artillery-based avalanche mitigation, is also working on phasing out its artillery use. In 2023, Alta, Utah, used its howitzer for the last time. Similar to Snoqualmie Pass, Wyssen tower installations throughout the ski area have eliminated the last few artillery shots needed to protect the ski area. These remote systems are used alongside traditional hand charging by the Alta Ski Patrol, but the Wyssen towers, like the artillery shots before them, provide greater worker safety by mitigating avalanche risk in terrain not easily accessible on foot.  Down the road, the Utah Department of Transportation (UDOT) has been actively working for decades to cut down on its artillery use. State Route 210 is threatened by 64 avalanche paths that previously have been mitigated with upwards of one hundred different artillery targets. UDOT has been working to install RAC systems to reduce its artillery usage since 2007. Wyssen towers and Gazex tubes can cost upwards of $100,000 per unit, and require tricky construction projects in high-alpine terrain. UDOT was the first installer of Wyssen towers in North America, and currently has more than 60 RAC systems in Little Cottonwood Canyon. A few remaining artillery targets in the lower canyon are in wilderness areas, precluding the use of RAC systems to mitigate those avalanche paths. UDOT has considered asking for an exemption to allow for the construction of RAC systems in the wilderness, but is currently pursuing the construction of passive defenses, snowsheds, along the road.

Beyond Washington and Utah, Wyoming and Colorado are also moving away from artillery use in their avalanche mitigation programs, largely driven by RAC systems. The Wyoming Department of Transportation (WYDOT) still occasionally uses its howitzer, the last time was in January 2022, but a combination of RAC systems and helicopter bombing has nearly made the program artillery-free. Colorado’s Department of Transportation (CDOT) ended its use of howitzers to mitigate avalanche paths along Interstate 70 last year, thanks to the installation of three Wyssen towers. RAC systems are replacing artillery shots all over the mountain west, but the high cost of these systems means that complete replacement of an artillery program can take many years. New drone-based approaches to avalanche mitigation may offer a cheaper route to the elimination of some of the remaining artillery shots.

Strapping Bombs to Drones: The Future of Avalanche Control?

Drones may offer a unique combination of advantages, combining the benefits of remote systems with the mobility and flexibility of artillery. The concept is relatively simple: strap a bomb to a drone, fly over an avalanche path, and drop the bomb. However, convincing the government and avalanche specialists that drones can safely and reliably fly around, carry explosives, and drop them exactly where needed is no easy feat. Drone Amplified, a Nebraska-based drone payload company, has been developing the MONTIS System for avalanche mitigation for more than four years. Dan Justa, Drone Amplified’s vice president for business development, told SnowBrains in an interview that some of the ideas for their avalanche mitigation system started with their IGNIS system, which drops pellets of combustible material to start controlled burns, and is now used for fuel reduction and wildfire fighting in 36 states.



The results of the years of development and several rounds of testing with Alaska’s Department of Transportation are impressive. The MONTIS system can scan the area for humans, mountain goats, eagle’s nests, and other wildlife with a thermal camera, then drop an explosive that reliably produces an airblast a meter or two off the surface of the snow, within a meter or two of the intended target, and observe the resulting avalanche with high-resolution video. If the drone has a problem and needs to return or land somewhere in the field, the explosives are disarmed, and the drone is safe to approach. The secret to both the safety and effectiveness of the precision airblast has been changing the ignition system used for the explosives.



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The explosive charges used for hand-charging, bomb trams, helicopter bombing, and several types of RAC systems are typically ignited by a combination of a blasting cap, a safety fuse, and a pull-wire igniter. The blaster “pulls smoke” using a pull-wire igniter that starts a timing fuse. When the timing fuse is used up, it ignites a blasting cap, which sets off the rest of the charge. The problem with putting this system on a drone is that once the timing fuse is ignited, there is no way to safely disarm the explosive. If a drone flying around with a lit fuse encounters a problem, there may be no safe location to drop the charge. Further, there is no reliable way to time the fuse so that the charge detonates directly above the snowpack surface. Once dropped, charges may roll out of the intended target zone before they detonate, diminishing their effectiveness. Drone Amplified began using electronic triggers, widely used in the mining industry, to enable safer, more precise blasting. Electronic triggers will only ignite when a coded electronic signal is sent. They are designed to remain in a safe state for potentially months on end, meaning that if a drone has to drop its charge early or land due to a problem, the charge will not go off. Upon receiving the detonation signal, the trigger can ignite the charge within milliseconds. The precision blasting side of Drone Amplified’s system comes from a combination of the drone’s GPS and laser rangefinders being able to sensitively measure the height of the drone relative to the snow surface, drop the charge, then send the detonation signal at the precise moment that the charge is a meter or two off the surface of the snow.



Justa said that Drone Amplified’s system has a range of around 2 miles, and the combination of cameras, laser rangefinders, and GPS gives the operator extremely fine control over the placement of the charge. With long range and control over shot placement, the MONTIS system offers many of the same capabilities as a howitzer or a recoilless rifle. However, the Federal Aviation Administration (FAA) currently restricts drone flights to line-of-sight operation, meaning that, regardless of flight capability, drone-based avalanche mitigation systems could not be used in the middle of a storm or in heavy fog. Justa said that they will not fly in conditions where a helicopter would not, and that, overall, drone-based avalanche mitigation is very similar to helicopter bombing, albeit with greatly reduced costs and precision blasting. Drone Amplified is still working out the pricing for the MONTIS system, but expects that it will be well under the cost of a typical RAC system.



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Aerial Vehicle Safety Solutions, or AVSS, is a Canadian-based company that is pursuing a slightly different strategy for drone-based avalanche mitigation. AVSS got its start manufacturing parachutes for drones, both as a safety feature for the drone and as a way of dropping payloads from the drone without landing.  Josh Ogden, the CEO and co-founder of AVSS, told SnowBrains that he was approached about the possibility of using parachutes to deliver explosives from drones for avalanche mitigation. “We quickly realized parachutes and high wind conditions in mountains don’t make sense,” Ogden said. Leaning on their expertise in drone safety systems and guided delivery systems, AVSS began developing its own approach for drone-based avalanche mitigation, centered around a specially designed explosive charge called the SnowDart.

Ogden said that between tests with Transport Canada and the Department of National Defense, the SnowDart system has been almost continuously improved since its inception. “Every time we test, we get new ideas,” Ogden said. Last year, AVSS received nationwide approval from Transport Canada to use its Precision Avalanche Management System for active avalanche mitigation. Testing in Jasper National Park held promising results, and according to Ogden, AVSS is planning on working with five to ten locations next winter to continue to explore approaches to drone-based avalanche mitigation. AVSS is not seeking to directly replace howitzers with drones, but rather add a tool alongside other strategies for remote avalanche mitigation, like Wyssen towers, bomb trams, and hand charging. With regulatory approvals in hand, AVSS will be able to expand to more mountainous test locations and continue to build experience using the Precision Avalanche Management System to trigger avalanches in a variety of weather conditions.

Avalanche Artillery Mitigation at Alpine Meadows, California

Few places in the world deal with an avalanche problem on the scale of Alpine Meadows and Palisades Tahoe, California. Jeff Goldstone, mountain manager at Alpine Meadows, said on a typical storm day, Alpine Meadows Ski Patrol will use 360 hand charges and put 30 rounds through their howitzer. “Large storm cycles come in off the Pacific and deliver massive amounts of snow with very high water densities. Those are accompanied by very, very tough conditions to be able to operate avalanche control programs in,” Goldstone said. The howitzer is essential to operations at Alpine Meadows because of the intensity of the storms the Sierra can experience. No matter how hard the wind blows or how much snow accumulates every hour, artillery can deliver explosives to starting zones to keep the avalanche hazard from growing too large. The conditions during Sierra mega-storms can make deploying hand charges from a ridge line a formidable task. The placement of the lodges, chairlifts, and other infrastructure at Alpine Meadows means that avalanche danger can reach devastating levels if not diligently mitigated, as history has shown. “Unless you’re doing continual mitigation work, the hazard is going to build up to a point where it’s going to get out of control, and you’re going to have to take other measures to keep people safe,” Goldstone said.

The howitzer in Alpine Meadows enables safe, effective mitigation work to protect both the ski area and the roadway during the increasingly frequent, severe storms in the Sierra. The scale of the avalanche problem can be managed by artillery fire without exposing ski patrollers to the most dangerous weather and snowpack conditions in a storm cycle, and when they can go out on their mitigation routes, they do so with greater margins of safety because many of the largest avalanche paths have already been at least partially addressed. RAC systems appear to offer the same advantages as artillery for mid-storm mitigation, but the Sierra has proven challenging to operate any system in. Alpine Meadows has more than a dozen Gazex tubes, which have replaced a few artillery shots and hand-charging routes. Goldstone said that during several storm cycles, Gazex tubes have been completely buried, the creep of the snowpack has led to gas pipeline fittings loosening, or antenna masts have been ripped off in hurricane-force winds.



Many avalanche programs, such as Washington’s Department of Transportation, are moving toward explosives-based RAC systems like the Wyssen tower. California currently only allows gas-based RAC systems, but Goldstone said that Alpine Meadows, along with Mammoth Mountain and other ski areas, is working with the state government to get approvals for explosives-based RAC systems. A recent study from Montana State University found that Wyssen towers deliver larger blasts than Gazex tubes, meaning more trigger points within a start zone may be sufficiently impacted to initiate avalanches. The wind, however, still creates some uncertainty around the potential effectiveness of Wyssen towers in some of the most exposed terrain at Alpine Meadows. “You’d think somewhere else in the world, somebody would be in our situation in California, but there’s not really that much of that. With our extreme winds and rime that will fully encompass one of these devices, we’re still unsure about what the right tool is to be able to accomplish these ridge-line shots,” Goldstone said.



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Once Alpine Meadows secures permission to use explosives-based RAC systems, initial installations in more sheltered locations on the mountain will help the ski patrol become familiar with the performance and operational differences between Wyssen towers and Gazex tubes. This experience will help Goldstone and the avalanche mitigation team plan for the right mitigation strategy to eventually replace ridge-line howitzer shots. Goldstone’s decades of experience with the weather conditions in Alpine Meadows helped inform planning around where the first Wyssen tower might go, and the initial seasons of use will help inform where the next Wyssen tower will go. While the first few RAC system installations may not replace artillery shots in the near future, the methodical development of an understanding of new systems will help ensure their success. “That’s moving us towards getting rid of our reliance upon military weapon systems,” Goldstone said. Behind nearly every Wyssen tower, or Gazex tube, or Avalanche Guard, or other RAC system is an expert like Goldstone in Alpine Meadows, or Stimberis on Snoqualmie Pass, with intimate knowledge of local weather and snowpack conditions, who knows precisely where to put an explosion for optimal results, be it from a hand charge, 105 mm howitzer shell, or charge dropped out of a Wyssen tower.

Replacing Avalanche Artillery Mitigation: The Road Ahead

The replacement of artillery in any avalanche program, whether at a ski area or along a highway, is no small task. Artillery has long enabled highway crews to mitigate multiple avalanche paths along a roadway from a single location, and its replacement has entailed the expensive installation of remote systems in alpine environments, such as Wyssen towers and Gazex exploders. The long range and high accuracy of artillery, regardless of weather conditions, are advantages not readily replicated by other mitigation strategies. Retiring an artillery program means devising and implementing a new mitigation strategy for each shot an artillery piece would have fired. A routine howitzer mission with a dozen shots must be slowly, methodically transformed into a handful of bomb trams, a few Wyssen towers, a Gazex tube or two, and a helicopter bombing mission every other year. The economic importance of an interstate highway means that highway avalanche programs can more easily fund remote avalanche control system installations than ski areas. Helicopter use for mitigation is likely too expensive to become routine at ski areas, but drone-based solutions may offer a way forward without replacing every artillery shot with an RAC system.



The advances in our understanding of snow science, atmospheric science, avalanche forecasting, and mitigation would be astounding to Monty Atwater and the other early avalanche hunters who first pulled firing chains on artillery pieces. The buildup of knowledge and experience with the artificial release of avalanches, decades of which are represented in every avalanche artillery program, is why the replacement of artillery with other mitigation strategies is possible. Aging artillery arsenals and dwindling ammunition supplies necessitate replacing these weapons for avalanche mitigation, but that replacement will be far from a simple exchange for another mitigation strategy. As the remaining avalanche-artillery users work to implement new solutions for avalanche mitigation, howitzers will continue to provide the safety margins that our ski areas and highways have relied on for more than 75 years. When the last guns fire their final shots, they will conclude an era where weapons designed for destruction and death found a peaceful place delivering safety to mountain communities across North America.

More from Zach Armstrong:



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History with SnowBrains: The Incredible Notes from a Swiss Avalanche Researcher’s Travels to Western USA in 1949

Why Ski Patrollers at Park City Initiated The Biggest Ski Patrol Strike In History

How Eldora, CO, Ski Patrollers Finally Got Their Union

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