In avalanche operations, ski cutting involves a single avalanche practitioner attempting to trigger a snow avalanche by skiing across the upper part of a slope. There are two types of ski cutting: test skiing to determine if the snow is unstable and mitigation to remove unstable snow before the avalanches get bigger or before less skilled people (e.g. clients) get to the specific slopes. To address the wide differences in the perceived risk of injury during ski cutting, we conducted a quantitative survey that helped avalanche practitioners estimate the number of ski cuts over many winters and asked them to recall their near misses and three classes of injuries. Over 150 practitioners completed the survey with a combined career total of 1.5 million ski cuts. From the responses, we calculated various results. The median number of ski cuts per respondent is 300 per winter. The rate of triggering a size D1 to 1.5, D2 to 2.5 and D3+ avalanche was 300, 4 and 0.1 per thousand ski cuts, respectively, indicating that smaller avalanches are triggered much more often than larger ones. The rate of being caught in a size D1 to 1.5, D2 to 2.5 and D3+ per thousand triggered avalanches was 7, 25 and 80, respectively, indicating that the probability of being caught increases with the size of an avalanche triggered during ski cutting. When the survey results are scaled to a million ski cuts, about 23 resulted in light duty, 7 resulted in missed work and 3 resulted in career ending injuries. Practitioners at lift-based ski areas (ski patrollers) had lower risk per ski cut than guides for helicopter and snowcat skiing.

Most dry slab avalanches occur during or immediately following loading by snowfall or wind deposition. In the absence of further loading avalanche activity decreases over time. This suggests that avalanche release is facilitated by changes in snowpack properties during loading, and that changes following loading generally help to stabilize the snowpack. This study quantifies these longer term stabilizing changes following loading. We developed a field method to rapidly increase the load on existing weak layers, and used the technique to add 10 cm of disaggregated snow on over 30 isolated columns. We then conducted Propagation Saw Tests (PSTs) in the minutes, hours and days following loading and isolation, with tests ranging from 15 minutes to 4 days. We filmed each test at 120 fps for particle tracking velocimetry analysis, and we utilized a finite element (FE) model to simulate the experiments and better interpret our results. We found that critical crack lengths increased rapidly at first and then more slowly over time. FE simulations of the experiments suggest that changes in critical crack length over time are caused by an increase in slab elastic modulus in the first hours following loading, and then caused by both increasing slab elastic modulus and weak layer specific fracture energy in days following loading. Our results help to illustrate changes in critical crack lengths in the days following loading, and are consistent with field observations of increasing avalanche activity immediately following loading events and decreasing avalanche activity afterwards.

Avalanches not only pose a major hazard to people and infrastructure, but also act as an important ecological disturbance. In many mountainous regions in North America, including areas with existing transportation corridors, reliable and consistent avalanche records are sparse or non-existent. Thus, inferring long-term avalanche patterns and associated contributory climate and weather factors requires the use of dendrochronological methods. Through the collection of regionally distributed tree-ring data recording avalanche events, we aim to address the following questions: 1) What is the regional and path specific frequency of large magnitude avalanches in the U.S. Northern Rockies?, and 2) Are there specific seasonal weather or climate variables that contribute to large magnitude and regional avalanche events? We collected 617 cross sections and 56 cores from 12 different avalanche paths in four mountain ranges in Glacier National Park and the Flathead National Forest across northwest Montana, USA. Six of these paths affect major transportation corridors, and the other six impact heavily used winter backcountry recreation zones. We identified, cross-dated, and quality ranked damage events recorded within and between samples from each path. We then implemented a double threshold analysis to account for decreasing sample numbers through time and to ensure accurate identification and dating of avalanche events. Finally, we developed an avalanche chronology of large magnitude events for each path, mountain range, and ultimately a composite record for the region. Preliminary samples from five paths in Glacier National Park reveal 1308 growth disturbances over 27 major avalanche years from 1795 to 2017. Within the major transportation corridors of GNP, a large magnitude avalanche (size 3 or greater) is likely to occur every 5 years in at least one of the five avalanche paths. These avalanche years coincide with winters characterized by high regional snowpack anomalies. Using this developing network to understand the spatiotemporal behavior of large magnitude avalanches and the contributory climate and weather factors will ultimately improve avalanche forecasting and backcountry safety efforts within the region.

Understanding snow depth distribution and change is useful for avalanche forecasting and mitigation, runoff forecasting, and infrastructure planning. Advances in remote sensing are improving the ability to collect snow depth measurements. The development of structure from motion (SfM), a photogrammetry technique, combined with the use of uninhabited aerial systems (UASs) allows for high resolution mapping of snow depth over complex terrain. The primary objective of this study was to determine the feasibility and efficacy of SfM to examine snow depth distribution and variability in complex terrain such as avalanche path starting zones at multiple times during the season. We used a 3DR Solo quadcopter UAS equipped with a Ricoh GR II camera at 90 m above ground level to acquire images of one avalanche starting zone in northwest Montana, USA. We also placed 4 to 13 ground control points (GCPs) around the area of interest to avoid traveling in steep, avalanche terrain. Ground control measurements resulted in 5 to10 cm horizontal accuracy and 5 to 15 cm vertical accuracy for 90 to 95 % of the collected points (a minimum of 100 points collected at each GCP). In-situ measurements of snow depth difference between sampling days ranged from 20 to 60 cm. We processed the images to create point clouds and digital surface models (DSMs). The resolution of the resultant DSMs was approximately 5 cm. Preliminary DSM and point cloud differencing efforts suggest relative change detection of snow depth at 5 to 15 cm resolution. The use of these relatively low cost and easily accessible methods of snow depth data collection will enhance accuracy of snow depth change estimates in starting zones and can be used to inform avalanche forecasting and mitigation efforts.

This paper presents a method developed to capture the terrain metrics of all visible skiers on an avalanche-prone backcountry slope. A remote time-lapse camera focused on Saddle Peak, a high skier-use backcountry slope in the Bridger Mountain Range of southwest Montana, USA captured 31,960 photos of 525 skiers descending in ten-second increments on 13 unique days. Skier locations (7,499 location-points) were digitized from the photos, then transformed onto a geo-referenced digital elevation model (DEM) such that terrain metrics could be applied to each of the 7,499 skier locations. Analysis of terrain metrics for each skier point compared slope, profile curvature (downslope), and plan curvature (cross-slope) over days with three different forecasted avalanche hazards (Con, Mod, Low). Terrain metrics on Considerable avalanche hazard days differed significantly from Moderate or Low avalanche hazard days (p-value < 0.001). Skier location-points transformed from the oblique photos to a geographic coordinate system had an observed horizontal spatial accuracy of 49-m with a 95% confidence interval. By capturing all visible skiers on a slope anonymously, the data provides a large and diverse data set of the terrain preferences of backcountry skiers under varying conditions.

Time lapse photography presents a simple and inexpensive tool for effectively monitoring skiers in avalanche terrain. Skier images can be useful in determining high-use areas of skier traffic, crowding or congestion issues, documenting avalanches, and recording avalanche control operations. It has also proven to be useful with assisting avalanche emergencies by providing visual survey of the avalanche path including determining number of people (if any) involved, identifying triggers and last seen points, and assessing residual risk to responders.

Deep slab avalanches result from season-long weather beginning with the formation of snow stratigraphy conducive to deep slab avalanches. The onset of deep slab activity is commonly preceded by heavy snowfall, wind, rain-on-snow, or rapid warming events. Local slope weather (or surface weather) is mainly driven by atmospheric circulation at higher altitudes. The field of synoptic climatology relates these atmospheric processes to surface observations. Previous work has investigated the meteorology of deep slab avalanches, as well as the synoptic climatology of snowfall and avalanche occurrence in several different locations around the world. However, there is limited research that specifically relates atmospheric circulation to deep slab avalanches. This work aims to bridge a knowledge gap by investigating the synoptic climatology of deep slab avalanches across the western United States. We obtained daily 500 mb geopotential height maps from the NCEP/NCAR Reanalysis dataset for 38 consecutive winter seasons from 1979-2017, resulting in over 6,000 days of data. We have classified these daily 500 mb height maps using Self-Organizing Maps (SOM) to obtain 20 summary map patterns characterizing the observed modes of atmospheric variability over western North America and the eastern Pacific. We used Westwide Avalanche Network (WAN) data from three ski resorts across the western U.S. that represent the three dominant snow climates (maritime, continental, and intermountain) and count the number of deep slab avalanche events for each synoptic type at each location. We investigate which synoptic types tend to occur immediately prior to deep slab avalanche events. Our work improves our understanding of the relationship between atmospheric circulation and deep slab avalanches, thereby enabling practitioners to better anticipate deep slab avalanche events.

Our research expands existing knowledge of travel behavior and decision-making in avalanche terrain. We utilize GPS tracking to observe the travel behavior of lift access backcountry (LABC) skiers and in person survey data to investigate the human factors that influence their terrain choices. Our study area is the southern boundary of Bridger Bowl Ski Area, Southwest Montana, USA (N=139). We analyze travel behavior by avalanche hazard rating, and identify human factors that affect avalanche terrain choices. A subset of our results are presented here, which illustrate the demographics of our sample and highlight their travel behavior through heat maps. Our results provide a case study example of the terrain preferences and avalanche awareness of LABC skiers, and highlight specific human factors that are correlated with terrain selection. Two practical applications of this research are: 1) targeted avalanche education outreach based on our results, and 2) designing new signage to illustrate the avalanche terrain near the ski area boundary for skiers who are inexperienced in the backcountry or unfamiliar with the area.

Avalanches failing on buried weak layers tend to be easier to trigger during or immediately following snowfall, suggesting that loading favorably changes snow cover properties for avalanche release. However, previous research showed that thicker, denser slabs (with more load) tend to have longer critical crack lengths in propagation saw tests (PSTs). To address these seemingly contradictory observations, this study quantifies the effect of increasing load on weak layer fracture. In previous research weak layer strengthening may have affected the relationship between load and critical cut lengths. We therefore developed a method to rapidly increase the load on existing weak layers to ensure reasonably constant weak layer strength. On 11 sampling days we used a cardboard frame the size of a PST and added 5, 10, 15, or 20 cm of disaggregated snow on top of PST columns. We allowed the added snow to sinter for 30 to 60 minutes before performing PST experiments and filming each test at 120 fps for particle tracking velocimetry (PTV) analysis. In all cases critical cut lengths dramatically decreased with increasing load (sometimes to almost 10% of unloaded cut lengths), a finding consistent with observations of easier avalanche triggering immediately after loading. Crack propagation speeds also increased with increasing load, consistent with previous work showing higher speeds with increasing slab density. From a practical perspective, this research presents an objective, repeatable, and inexpensive method for testing the effect of loading on surface weaknesses or shallow weak layers. From a scientific perspective, these experiments offer the first controlled, field-based measurements investigating the effect of loading on weak layer fracture, providing results that may be useful for calibrating theoretical models.

The open-source Know Before You Go (KBYG) avalanche awareness program provides a 50 minute introduction to avalanche safety for motorized and non-motorized users and can be presented during a school period. Intended for a 14 year-old audience with minimal avalanche experience, the program is easily adapted for older audiences and provides a valuable tool for influencing the behavior of potential, aspiring, and beginning backcountry riders. Major revisions in 2015 include messaging based on 5 points:  Get the Gear  Get the Training  Get the Forecast  Get the Picture  Get out of Harm’s way The program begins with a video featuring prominent athletes, big avalanches, extreme riding footage, music, animation, and an introduction to backcountry social responsibility. The introductory video was posted online (kbyg.org) and has received 463,000 views since November. KBYG was presented 288 times to over 16,000 people in Utah and Colorado last winter, three-quarters of them in schools. We distributed the program content across North America, with major media sources helping to promote the program. The KBYG program content is available to all avalanche educators.

Avalanche activity in snow climates dominated by direct-action avalanches is primarily controlled by the local and synoptic scale meteorological conditions just prior to and during winter storm events. Previous work on Svalbard characterized the region’s unique, direct-action snow climate as “High Arctic maritime” and demonstrated an association between periods of snow drift and regional avalanche activity. This study uses a record of road closures due to drifting snow on a mountain road to further investigate Svalbard’s snow climate and avalanche regime by: 1) characterizing synoptic meteorological conditions leading to regional snow drift events, and 2) exploring the relationship between these periods of snow drift and regional avalanche activity using a case study approach. We couple a nine-year (2007- 2015) record of road closures with local meteorological observations and NCEP/NCAR synoptic composite maps to characterize the local and synoptic weather conditions leading to and occurring during periods of snow drift near Longyearbyen, Svalbard’s primary settlement. Then we compare this record of snow drift events with regional avalanche observations to illustrate the relationship between snow drift and avalanche activity on Svalbard. The results of this study will improve the understanding of Svalbard’s unique maritime snow climate and will help advance avalanche forecasting efforts throughout the region.

Knowing the Extended Column Test’s (ECT’s) effectiveness at different slab thicknesses is critically important for practitioners. To better understand the limitations of the ECT, we used the SnowPilot dataset to investigate the utility of ECTs for providing an index of crack initiation and propagation on varying weak layer depths. The database currently contains 5013 ECTs conducted by 386, primarily professional, users worldwide between 2007 and 2016. The broad range of observers and snowpacks in the dataset allow us to examine variations in ECT results with changing weak layer depth across seasons and locations. We found 25% of ECTP (propagating ECT) results have weak layer depths of less than 30cm, 45% have depths between 30 and 70 cm, and 30% propagate on weak layers deeper than 70 cm. We also found an increasing ratio of ECTP to ECTN (non-propagating ECT) results as depth increased. The results make intuitive sense as fracture initiation takes more force and becomes more infrequent as slab depths increase, especially once depths exceed about 1.2 m. In addition, we used these same data to look at the repeatability of ECT results in individual snow pits. In the 582 pits where two ECTs were performed on the same weak layer, 86% have similar results (either two ECTPs or two ECTNs), showing a high degree of repeatability. Our results suggest the ECT can be effectively used over a fairly wide range of weak layer depths.

The North American Avalanche Danger Scale is a tool used by backcountry avalanche forecasters to communicate the potential for avalanches to cause harm or injury to backcountry travelers. Danger ratings are the most basic component of the public forecast, providing the foundation for more nuanced descriptions of avalanche conditions. In 2010, the United States, Canada, and New Zealand adopted a consistent, five-tiered danger scale. Although widely used, we do not know how consistently the danger scale is applied both within and between avalanche forecasting operations. To address this question, we developed ten scenarios capturing a variety of avalanche conditions at the mountain range scale. We derived the scenarios from real avalanche forecasts issued by various avalanche centers throughout North America. Avalanche forecasters in the United States, Canada, and New Zealand reviewed each scenario and assigned a single danger rating for the forecast period. Results indicate that although most respondents choose ratings within one step of each other, individual forecasters can arrive at different conclusions when presented with identical information. Additionally, it appears that there are regional and/or cultural differences in how forecasters assign danger ratings.

The interaction between snowpack layers determines the snowpack stability. It follows that disrupting layers may improve stability. Ski patrols and guiding operations employ mechanical compaction methods such as boot packing, ski compaction, and explosives aiming to increase snowpack stability. High-usage backcountry areas (but still uncontrolled avalanche terrain) may become compacted, developing a snowpack with different characteristics than in lower usage adjacent terrain. A challenge in avalanche forecasting is determining how this compaction alters snowpack stability, if at all. Our results show that snow pack stability increases if compaction penetrates and impacts a weak layer, disrupts the failure plan, or affects slab cohesion. This is likely due to an increased probability of fracture arrest in a compacted snowpack from either slab or weak layer heterogeneity. While several compaction methods exist, specific research addressing different compaction techniques is lacking. This study compares the effects of different compaction methods on a snowpack. The snowpack for our case study consisted of an approx. 30 cm 1F wind slab over approx. 30 cm depth hoar. After applying mechanical compaction methods to nine slopes, we conducted ECTs and PSTs over six weeks to assess stability. We found ECTXs more common in compaction-dense areas (boot packed) than in compaction-light (skied and compaction free) areas. This research quantifies some of the effects of different compaction strategies, and provides preliminary guidance for avalanche practitioners on the most useful techniques for mitigating avalanche hazard.

Once a weak layer has fractured, slope steepness largely dictates whether or not an avalanche will release. Exceeding the critical slope angle, i.e. angle at which the slab overcomes friction, is an essential condition for dry-snow slab avalanche release. Practitioners continually take this into consideration when assessing avalanche terrain and safe travel routes in alpine environments. However, thus far, such assessments rely on rules of thumb, such as avoiding terrain steeper than 30°, rather than actual measurements. Further, research into critical slope angles has been limited and confined to dry-snow and mostly persistent weak layers and harder slabs. To address these limitations, we developed a simple smart phone app to measure the kinetic friction between the detached slab and the bed surface. We used Optical Flow method to track motion of sliding slabs. We assumed Coulomb friction to calculate the friction between the slab and bed surface and derive the critical slope angle. Using our app on existing videos allowed us to compare our measurements with prior friction research, and we found the app was able to estimate friction to within +/- 1° of previous work. Our preliminary field data consist of 16 measurements on decomposed fragments (DF) and moist faceted crystal (FC) weak layers from four pits spaced 10 meters apart on a single slope. The critical slope angle was 35 +/- 1° for the DF layer and 39 +/- 0.5° for the moist FC layer. Our data also show a constant critical slope angle within the initial 0.1 – 0.2 m. of the down-slope motion, and a decreasing critical slope angle shortly afterward. Our smartphone tool provides a method to quickly estimate critical slope angles in the field. Our goal is to link critical slope angles to specific snow cover properties, and to assess spatial and temporal variability in critical slope angle.

In 2013 the Forest Service National Avalanche Center created guidelines designed to reduce worker risk at Forest Service (FS) backcountry avalanche forecasting operations. These guidelines establish context for field operations, define worker safety philosophy and responsibility, and suggest specific risk management practices. At the conclusion of the 2015-2016 season, we conducted a survey to examine current safety practices and how operations integrated the 2013 guidelines into their safety programs. The survey results illustrate field practices and safety management at FS avalanche centers, and provide insight into how these avalanche centers might further improve workplace safety.

Particle tracking velocimetry (PTV) is a measurement technique widely used to determine displacement and velocity fields from video recordings. It is largely nonintrusive and capable of simultaneously measuring the state of deformation over an entire cross section of a sample. PTV has been used in field and laboratory experiments since the mid-1990s to study snow deformation and fracture. Initial studies focused primarily on documenting weak layer collapse and crack propagation velocities. However, recent technological and computational advances allow researchers to determine essential mechanical properties relevant to the processes involved in snow slab avalanche release. Indeed, PTV has been used to estimate the effective elastic modulus of the slab, weak layer specific fracture energy, crack propagation distance and speed, and the friction between the slab and the bed surface after fracture. In this contribution, we will give an overview of over 500 field experiments performed in Canada, USA, and Switzerland over the last 15 years, with an emphasis on relating derived snow mechanical properties to commonly observed snow cover characteristics. For instance, our results suggest that crack propagation speed, which increases with slab density, strongly correlates with crack propagation distance. Furthermore, crack face friction, which determines the critical slope angle at which an avalanche releases, is affected by the hardness differences across the weak layer. While PTV has improved our understanding of the fundamental processes involved in snow fracture, we will also highlight topics that have received little attention to date.

Predicting avalanche danger depends on knowledge of the existing snowpack structure and the current and forecasted weather conditions. In remote and data sparse areas this information can be difficult, if not impossible, to obtain, increasing the uncertainty and challenge of avalanche forecasting. In this study, we coupled the Weather Research and Forecasting (WRF) model with the snow cover model SNOWPACK to simulate the evolution of the snow structure for several mountainous locations throughout western Montana, USA during the 2014-2015 and 2015-2016 winter seasons. We then compared the model output to manual snow profiles and snow and avalanche observations to assess the quantitative and qualitative accuracy of several snowpack parameters (grain form, grain size, density, stratigraphy, etc.) during significant avalanche episodes. At our study sites, the WRF model tended to over-forecast precipitation and wind, which impacted the accuracy of the simulated snow depths and SWE throughout most of the study period. Despite this, the SNOWPACK-WRF model chain managed to approximate the snowpack stratigraphy observed throughout the two seasons including early season faceted snow, the formation of various melt-freeze crusts, the spring transition to an isothermal snowpack, and the general snowpack structure during several significant avalanche events. Interestingly, the SNOWPACK-WRF simulation was statistically comparable in accuracy to a SNOWPACK simulation driven with locally observed weather data. Overall, the model chain showed potential as a useful tool for avalanche forecasting, but advances in numerical weather and avalanche models will be necessary for widespread acceptance and use in the snow and avalanche industry.

Extended Column Tests (ECT) and Propagation Saw Tests (PST) are used to assess crack propagation; that is, the likelihood of a crack self-propagating. Yet, we present findings that show that full crack propagation to the end of the beam depends on beam length. The practical question is: are beams about 1 m long optimal for assessing stability? Finite element modeling shows that the so called “far edge attraction” becomes insignificant for beams longer than 2 m. In other words, full crack propagation be-comes independent of beam length when beams ≥ 2 m are used. To test the accuracy of 2 m tests for stability evaluation, we collected data on 135 side-by-side standard length ECTPs (full propagation) fol-lowed by 2 m ECTs. We only focused on ECTPs because we assumed 2 m ECTs would not propagate if standard length tests did not. These tests were preceded by an a priori stability assessment to reduce circularity or stability ratings based on test results. Our results show that the proportion of tests in agree-ment, i.e. ECTP and 2 m ECTP, increase with decreasing stability. We conclude that an ECTP followed by a 2 m ECTP is a clear red flag. The interpretation of an ECTP followed by a 2 m ECTN/X (no propaga-tion) is not clear. The main finding for practitioners is that a 2 m ECT can be used to give additional infor-mation about slope stability following an ECTP.

Although all avalanche types are difficult to forecast and mitigate, wet slab avalanches are particularly problematic. The dynamics of wet slabs are poorly understood and large wet slab cycles are relatively rare in comparison to dry slab cycles. We provide a case study of a historic wet slab cycle at Crystal Mountain Ski Area in Washington State in March of 2014. This cycle resulted in numerous large natural and explosive-triggered avalanches, including one particularly notable slide that destroyed a 34-year old chairlift and had an alpha angle of only 20º. The avalanches occurred after a brief period of warming followed by rain. Though the amount of precipitation (1.4 inches or 36 mm) on the preceding day was not atypical for this location, we believe the existing snow structure was a key contributing factor to the avalanches. The snowpack contained numerous crusts and layers of facets near the base due to a lack of early-season snowfall, and a deep and relatively homogenous slab formed by a 10-day storm that dropped 12′ (3.6 m) of new snow. Water penetration through the slab progressed much more rapidly than expected, perhaps due to the homogeneous nature of the slab. We discuss the snowpack evolution, contributory weather factors, operational challenges, mitigation efforts (including whether or not to use mitigation), and difficult decision-making that allowed the risks of this avalanche cycle to be managed without loss of life. A thin margin for error existed in this event, and we hope other practitioners can learn from our case study.

Though recent work on fracture has provided us with new stability tests and improved our knowledge of avalanche release, our understanding of fracture arrest is still limited. We studied fracture arrest by making modifications to weak layers and slabs in a series of propagation saw tests (PSTs). We conducted more than 100 tests at a single study plot over eight weeks during which cracks along a surface hoar weak layer consistently propagated. Slab characteristics changed dramatically over time, with increasing slab depth, density, and hardness. High speed videos of more than 70 tests allow the analysis of fracture arrest dynamics. Our results show that removing the weak layer had no effect on propagation. Fracture arrest only occurred when we replaced a 30 cm long section of the weak layer with a non-collapsible structure. Modifying the slab by introducing slope normal cracks (either from the surface down or from the weak layer up) showed that sometimes small changes to soft (F hardness) parts of a slab were sufficient to arrest fractures through slab fracture, while other times only a strong thin portion of a thick slab was capable of communicating fracture farther down the beam. Our results suggest that the tensile strength of the upper layers of the slab is a key component for crack propagation. This work demonstrates the importance of the slab in slope stability, and it also suggests avenues for developing tests capable of assessing slab characteristics conducive to fracture propagation.

Conducting stability tests in avalanche terrain is inherently dangerous since it exposes the observer to the potential of being caught in an avalanche. Recent work shows that such exposure may be unnecessary since the results of extended column tests (ECTs) and propagation saw tests (PSTs) are largely independent of slope angle, allowing for data collection in safer locations. Conversely, some past work shows that compression tests (CTs) are slope angle dependent. In this paper, we test the effect of slope angle on CTs using similar methods as the recent ECT work. We collected field data on three sep-arate days with persistent weak layers in Montana and California. Our slopes exhibited gradual changes in steepness, allowing us to sample a variety of slope angles with minimal snow structure changes. We also employed a second method to reinforce our results. Utilizing the SnowPilot dataset, we analyzed the difference between propagating ECTs and CTs on the same layer, and compared that difference with slope angle. Our fieldwork shows that the CT test results either did not change or increased slightly with increasing slope angle. Further, the SnowPilot data demonstrate that the difference between ECTs and CTs is not statistically dependent on slope angle, reinforcing conclusions from our field work. Our results have significant theoretical implications, but the practical implications are even more important since this work suggests that, in addition to ECTs and PSTs, CTs can be conducted in safer low-angle terrain.

The distribution of snow depth in avalanche starting zones exerts a strong influence on avalanche potential and character. Extreme depth changes over short distances are common, especially in wind-affected, above-treeline environments. Snow depth also affects the ease of avalanche triggering. Experience shows that avalanche reduction efforts are often more successful when targeting shallow trigger point areas near deeper slabs with explosives or ski cutting. Our paper explores the use of high resolution (cm scale) snow depth and snow depth change maps from terrestrial laser scanning (TLS) data to quantify loading patterns for use in both pre-control planning and in post-control assessment.

Dry-snow slab avalanches are generally caused by a sequence of fracture processes including (1) failure initiation in a weak snow layer underlying a cohesive slab, (2) crack propagation within the weak layer and (3) tensile fracture through the slab which leads to its detachment. During the past decades, theoretical and experimental work has gradually led to a better understanding of the fracture process in snow involving the collapse of the structure in the weak layer during fracture. This now allows us to better model failure initiation and the onset of crack propagation, i.e. to estimate the critical length required for crack propagation. On the other hand, our understanding of dynamic crack propagation is still very limited. For instance, it is not uncommon to perform field measurements with widespread crack propagation on one day, while a few days later, with very little changes to the snow-pack, crack propagation does not occur anymore. Thus far, there is no clear theoretical framework to interpret such observations, and it is not clear how and which snowpack properties affect dynamic crack propagation. To shed more light on this issue, we performed numerical propagation saw test (PST) experiments applying the discrete element (DE) method and compared the numerical results with field measurements based on particle tracking. The goal is to investigate the influence of weak layer failure and the mechanical properties of the slab on crack propagation. Crack propagation velocities and distances before fracture arrest derived from the DE simulations were in good agreement with experimental data suggesting that the simulations can reproduce crack propagation in PSTs.

Observations by practitioners suggest that fracture speeds are often much faster than re-cently reported measurements. These measurements include speeds along isolated beams ranging from 15 to 45 m/s, as well as the speed of a meadow collapse of 20 m/s. Since reported velocities appear to be slower than many observed avalanches, we analyzed a sample of 27 videos to estimate avalanche fracture speeds. Time was measured by counting video frames from the explosive trigger to visible slab fractures, and distances by on-site measurements, picture scaling, and Google Earth. Though our speeds vary widely (from 18 to 428 m/s), most of our values fall in the range of 50 to 125 m/s, which clearly exceeds previously reported values. We also investigated the relationship between fracture speed and other characteristics, such as explosive size. Interestingly, in videos with visible crowns and stauch-walls, the stauchwall opens first or at the same time 33% of the time. Our results may improve our under-standing of avalanche release, as well as providing practical guidance for explosive placements.

Though a significant amount of research examines the spatial variability of snow stability at the slope scale, most of that work focuses on tests primarily related to fracture initiation. The small number of studies examining the spatial variability of fracture propagation (utilizing tests such as the ex-tended column test (ECT)) are inconsistent. Some work reports homogenous ECT results, while later studies showed more variable and difficult to explain results. Because of these inconsistencies, we measured the spatial variability ECT results on multiple slopes in southwest Montana over the course of two winters. We sampled 23 grids, with each grid containing 28 ECTs for a total of 644 ECTs using a pre-defined semi-random 30m by 30m extent. We tested slopes with a variety of weak layers (surface hoar, depth hoar, new snow, and near surface facets), slab characteristics (slab hardness, slab depth), and snow depths. Further, we sampled during varying levels of forecasted regional avalanche danger. Our data demonstrate that considerable spatial variability in ECT potential exists on many slopes, even with-out substantial variation in snowpack structure. When the regional avalanche danger is either considerable or low, results are likely to be more consistent, but when the regional danger is moderate, results tend to be more variable. Further, the ratio of ECTPs to ECTNs can be correlated with the forecasted danger level. The practical implications of our results are that ECTs, like all other stability tests, should be inter-preted with an appropriate level of caution.

Deep slab avalanches are a particularly challenging avalanche forecasting problem. These avalanches are typically difficult to trigger, yet when they release they tend to propagate far and can result in large and destructive avalanches. For this work we define deep slab avalanches as those that fail on persistent weak layers deeper than 0.9m (3 feet), and that occur after February 1st. We utilized a 44-year record of avalanche control and meteorological data from Bridger Bowl Ski Area in southwest Montana to test the usefulness of meteorological variables for predicting seasons with deep slab avalanches. While previous studies often exclusively use data from the days preceding deep slab cycles, we include meteorological metrics over the early months of the season when persistent weak layers form. We used classification trees for our analyses. Our results showed that seasons with avalanches on deep persistent weak layers typically had drier early months, and often had maximum snow depth greater than 88cm in November, which provided ideal conditions for persistent weak layer development. This paper provides insights for ski patrollers, guides, and avalanche forecasters who seek to understand the seasonal conditions that are conducive to deep slab avalanches on persistent weak layers later in the season.

Dry slab avalanches start when a catastrophic failure along a weak snowpack layer separates the slab from the underlying snowpack over a large section of the slope. Most research and papers focus on crack propagation, but little work has been done on fracture arrest, which is critically important for understanding slab boundary locations. This study investigates some of the factors contributing to fracture arrest at boundaries of actual avalanches. We targeted these areas by conducting a series of 2m ECTs (ECT200s) along crown walls, and working toward the flanks of 15 Storm-Slab and seven Persistent-Slab avalanches with maximum crown depth between of 45 and 80 cm. At 16 of our avalanches we recorded snowpack and terrain changes in 32 locations where fractures arrested as evidenced by our tests transitioning from ECT200Ps to ECT200Ns. In four ECT200Ns (13%), we observed a sharp increase in slab density down column from fracture arrest, in 21 ECT200Ns (65%) there was a decrease in slab thickness, density or both, and in seven ECT200Ns (22%) the weak layer disappeared within the scope of the column. The slab boundaries in the other six of our 22 avalanches were dictated by the slab and weak layer friction rather than weak layer fracture arrest. In these avalanches, slab fractures appeared beyond the release areas, but slope steepness around these fractures wasn’t sufficient to overcome friction. Our work helps improve terrain management strategies and suggests terrain-related safety margins for different dry slab avalanche problems.

Workplace avalanche accidents claimed the lives of 11 avalanche workers in the United States over the last five years. Though none of the victims worked at a backcountry avalanche center, each accident reminds us of the danger of working in avalanche terrain. In response to these accidents, and in particular to an accident involving a Utah Department of Transportation avalanche forecaster, the Forest Service National Avalanche Center started a dialogue between the U.S. avalanche centers aimed at improving worker safety and increasing consistency in safety procedures. Our discussion began with the practice of solo travel by avalanche workers, but quickly transitioned into a more comprehensive project. The result was the creation of guidelines designed to reduce risk during field work. The guidelines establish context for field operations, define worker safety philosophy and responsibility, and improve risk management by requiring documentation of procedures related to check-in/check-out, required safety equipment, working alone, and emergency response. One of the key components of the guidelines is a pre-field work checklist and critical thinking exercise. U.S. avalanche centers are unique and diverse operations, adding complexity to the project. In the end, reducing or preventing accidents requires a holistic approach to safety. This approach must address the fundamental questions of who we are as a professional group, to what extent are we willing to expose our workers to potential hazards, and what methods are at our disposal to mitigate “acceptable risk”.

Understanding variations in slope scale snowpack properties influences stability assessments at the slope and regional scale. Previous studies have shown that surface hoar, a prominent weak layer type, can vary in initial crystal size and height across small, seemingly homogenous meadows and sparsely forested areas on northerly aspects. The differences in size have generally been attributed to the radiation balance, which is difficult to estimate in the field. This study aims to further investigate the effect of canopy cover and shading on the growth of surface hoar in small forested meadow openings just after initial growth and prior to burial. Two small (approx. 30m x 50m) study plots located in southwest Montana were selected. The study plots, one northern and one southern aspect, are mostly planar 10° meadows surrounded by heavy tree cover. We collected 200 samples with 2119 individual crystal observations, and estimated shading and hemispheric sky visibility to explain the difference in sizes of surface hoar in each meadow. Findings indicate that the strength of these determinants varies depending on aspect and how the surface hoar size is determined.

Surface hoar (SH) crystals, once buried, often result in a persistent weak layer within the snowpack that contributes to instability on slopes in the mountains of southwestern Montana as well as many other mountainous regions of the world. The influence of local meteorological conditions and gross-scale topographical features (e.g. slope and aspect) that influence the formation of surface hoar are relatively well understood. However, the relationships between synoptic atmospheric conditions and the formation of surface hoar are not very well known, nor have they been extensively studied. To investigate these relationships, atmospheric patterns from NCEP/NCAR synoptic composite maps were obtained for periods with varying amounts of surface hoar presence (four intervals from <20% to >80%) for our study area in SW Montana. This study used a comprehensive suite of 127 days of manual observations of surface hoar presence and absence from Pioneer Mountain within the Yellowstone Club in southwestern Montana from December 2012 – April 2014. Each of these days provides detailed observations from 16 sites distributed across several different elevations and all aspects on Pioneer Mountain. The composite maps show that higher sea level pressure, northerly winds and higher than average 500 hPa geopotential heights tend to favor more extensive surface hoar formation. This knowledge along with further research and analysis could potentially provide better insight into long-term forecasts of the spatial distribution of surface hoar presence.

At ski areas, a majority of avalanches fail in storm snow. We investigate these avalanches using stability tests and avalanche observations from California and Alaska. Collapse amplitudes during fracture, measured using particle tracking, were 1 mm for a failure layer of precipitation particles and 7 mm for a layer of unrimed sectored plates. Stability test results showed little dependence on slope angle, suggesting that both precipitation particles and older faceted crystals (persistent weak layers) fail as described by the anticrack model, with collapse providing energy. Using observations from avalanche control work at Mammoth Mountain, CA USA, a large coastal ski area where 9/10 avalanches fail in storm snow, we examined Extended Column Test (ECT) results and their relation to avalanche activity. ECT propagation was a powerful predictor; days with ECTs that propagated had significantly more and larger avalanches. Since other studies have shown that the ECT is an effective predictor of avalanches involving persistent weak layers, we suggest that the ECT is an effective test to predict both types of avalanches, those that fail in storm snow and those that fail on persistent weak layers.

Extended Column Tests (ECTs) have become increasingly popular for assessing snowpack stability. However, we still do not fully understand what happens to the block and the underlying weak layer during the test. Such work has been done for the Propagation Saw Test (PST), but not for the ECT. In order to better understand and interpret ECT results, we analyzed high-speed video (240 frames/second) of propagating ECTs using particle tracking. Our results show several things: 1) fractures initiate in an area of the weak layer directly under the shovel at the free edge of the block, 2) at the resolution of our measurements we do not see signs of progressive damage accumulation in the weak layer during tapping, but rather a single rapid collapse when fracture is initiated, 3) tapping on one side of the ECT does not affect the far side of the ECT, and 4) measured fracture speeds are similar to previously reported values, including those measured using the PST. From a practical perspective, our results suggest that the ECT is indeed measuring the propensity of a crack to propagate at the small scale of the ECT block, giving us greater confidence that we are capturing a critically important property of snow stability in our tests.

Professionals and recreationists utilize stability tests to assess snow stability. Our goal is to determine whether or not people are changing the types of tests they conduct. We utilized the SnowPilot database of over 3,600 snow pits from nine winters, with about 83% of these pits being dug by avalanche professionals. We found a dramatic shift in the tests conducted since 2004. SnowPilot users have moved away from rutschblocks and stuffblocks and moved more toward extended column tests (ECTs) and propagation saw tests (PSTs), while still conducting a large number of compression tests (CTs). ECTs are now the most popular test, being conducted in nearly 80% of all pits. Not surprisingly, this shift toward ECTs and PSTs has coincided with an increasing emphasis on the importance of propagation potential in our stability assessments. As we learn more about snow and the way it fractures, newer and more effective tests might well be advanced. Our results demonstrate that our community will quickly adopt new tests when they are useful and scientifically validated.

Understanding what controls coarse scale snowpack properties, such as surface hoar distribution, is imperative for predicting snow avalanches. Due in part to the inherent difficulties of winter travel in mountainous terrain, most spatial variability investigations of snow properties have been limited to relatively fine scales. To quantify snow surface spatial variability at the basin, region, and mountain range scales, a team of heli-skiing guides collected data throughout four major surface hoar formation periods over two heli-skiing seasons in rugged alpine terrain near Haines, Alaska across an extent of nearly 60km. Geographically weighted regression revealed a positive relationship between elevation and surface hoar crystal size with adjusted R2 values averaging near 0.40. Geostatistical analysis yielded spherical semivariogram autocorrelation ranges from approximately 3-25km, which is similar in size to many of the basins and regions within the study area. Kriging models built from the semivariograms were produced to aid geographic visualization of coarse scale snowpack processes. The results of this research suggest it may be possible to identify areas with greater surface hoar growth and persistence potentials as a consequence of synoptic onshore or offshore flow, and glacially influenced katabatic winds. These results can help in future efforts to forecast snow stability patterns over large areas.

Experience suggests that shallow, steep zones on slopes are likely spots for artificially triggering slab avalanches. However, a scientific understanding of this observation is not well quantified. We performed 108 point stability tests on a 30 x 30 m slope in central Svalbard. The slope has a rugged underlying topography and frequent wind influence both by top and cross loading. We found three persistent weak layers at different depths in the snowpack. Due to the rough nature of the study slope, snow surface does not resemble the ground topography. Weak layers forming early in the season follow the ground topography closely. As snow depth increases, the influence of ground topography diminishes. We further found a decrease in slab thickness with increasing slope and bed surface inclination. We therefore investigated the influence of slab thickness on slope stability. Our data shows that stability decreased significantly with decreasing slab thickness, which correlates to how deeply the weak layer is buried. Thus the weakest spots on the slope coincide with the shallowest and steepest spots, where the deeper buried weak layers are buried “less deep”. Such spots often occur around topographic heights such as large rocks, which are thus potential trigger zones.

We have developed a robust meteorological tower for deployment in locations with extreme conditions and for applications that require relatively maintenance-free structures. The basic design consists of a triangular base with two horizontal rails on each side, and uprights at the triangle vertices for various instrument configurations. The fabrication materials include 6061-T6 aluminum pipe (schedule 40 or 80), and cast aluminum connectors. The design is self-supporting, but may be guyed. Advantages of the design compared to conventional towers include easy assembly, readily available materials, easy adjustment for rugged location or terrain, transportable components by backpack, helicopter, boat, snowmobile, ATV or alternate method, and no need for concrete. Like all structures, the design is vulnerable to snow creep in deep-snow environs, but judicious site selection will mitigate this issue. We deployed these stations with minor variations in design in Alaska and Colorado, USA and on Baffin Island, Nunavut, Canada. The towers have survived relatively severe conditions in terms of cold temperature, wind, rime, and snow deposition. Locations have included subalpine, alpine, and arctic maritime environments. Regular data transmission has been achieved via satellite modem and satellite transmitter. We discuss design and installation.

During this past winter, southwest Montana had many large avalanches and days of high avalanche danger. In the Bridger Mountains the most prominent and active weak layer was a 30-55cm thick layer of depth hoar. This layer developed in November 2011 and persisted throughout the season. Precipitation was below average between December and early February, but each storm consistently produced artificially triggered avalanches on this layer. Higher snowfall rates and above average SWE in late February and March produced natural avalanche activity in the backcountry on the depth hoar. In late March 2012 a near isothermal snowpack, combined with a period of above freezing temperatures and heavy snowfall, produced an historic skier triggered and explosive controlled full depth wet slab cycle on the depth hoar. This event occurred on in-bounds terrain that was closed at the time, and had previously been heavily skied and controlled with explosives. We will review meteorological and snowpack factors that were associated with avalanche activity on the depth hoar within Bridger Bowl ski area and in the surrounding backcountry. Weather data from stations at Bridger Bowl and snowpack observations taken throughout the ski area will be used to discuss the factors associated with the timing of avalanche activity on the depth hoar layer. Patterns, trends, and outcomes will be highlighted which may be of wider value to the industry in managing these types of instabilities in future years.

Avalanche researchers and practitioners have long measured snowpack temperatures in snow pits with thermometers about 10 cm apart. This led to the assumptions that temperature gradients are smooth and that temperature changes are regular. For this study, we used a thermal imager in standard snow pits in the Canadian Rocky Mountains during two seasons between 2010 and 2012. We collected the first season of data in a very shallow, below treeline snowpack study plot, and the second season of data in a deeper, treeline study plot. Data included thousands of thermal images, as well as visual macro images of the snow crystals in each pit layer to monitor changes.

We observed strong temperature gradients on the scale of individual snow crystals. We found that these small scale gradients correlated with future snow crystal changes. We also found that these gradients changed quickly with the weather, even at depth. This paper focuses on our most recent findings from the 2011-12 season, and describes our overall progress in extracting data from thermal images to use for research and forecasting. We use correlations to present very general relationships between thermal data, crystal size, and layer stability tests. We also present temperature and gradient changes at depth during a period of clearing.

Field observations suggest that avalanches releasing on non-persistent weak layers tend to slide on steeper slopes than other dry slab avalanches. Recent research indicates that the initiation of fracture in a weak snowpack layer is almost unaffected by slope angle. However, this field research included only persistent weak layers. Thus, the question remains: Why are avalanches involving non-persistent weak layers slope angle sensitive? Our paper investigates the following for non-persistent weak layers: 1) the effect of slope angle on fracture initiation and 2) the friction coefficient. We performed Extended Column Tests (ECTs) on slopes where we could track the number of taps needed for weak layer fracturing on a variety of slope angles (from 8° to 46°) with minimal snow structure change. Our results show that the number of taps required to initiate a fracture that crosses the entire column (ECTP) was mostly independent of slope angle. In addition, we measured the friction coefficient of two different non-persistent weak layers types (PPsd and DFdc) within minutes after avalanches were triggered and in the following days. Our results show that the friction coefficient of non-persistent weak layers was higher than published values for persistent weak layers. From a theoretical perspective, our results are in line with the mixed-mode anticrack model for fracture in a weak snowpack layer. From a practical perspective, our results can contribute to safer pit site selection, terrain and snowpack analysis, and they give us insight into the value of observing downslope block movement after fracturing in stability tests.

Depth hoar is a persistent weak layer that is a common instability problem in the snowpack of many areas around the world. Boot packing or compaction via explosives is a technique that is widely used in an attempt to disrupt this weak layer and increase its variability across a slope, thereby increasing overall slope stability. While some data have been gathered on the results of boot packing and explosive use on slopes, no recent work has concentrated directly on the effect of boot packing on layer density, hand hardness and stability test scores. Therefore, we devised an experiment to test the changes in these metrics directly on a side-by-side boot-packed and non-boot packed slope.

A 50m x 25m, relatively uniform slope was split into two equal areas with one being extensively boot packed, while the other remained undisturbed. Observations were made in both boot packed and non-boot packed plots until both areas were unreactive with respect to results using Extended Column Tests (ECT) on this basal layer. Our results show that density and hand hardness increased in the boot packed area in comparison to the non-boot packed area. Furthermore, in the boot packed area we also observed a marked increase in ECT scores and a change in fracture character of the basal layers. In the non-boot packed area our ECTs propagated (ECTPs) for a full nine weeks longer than in the boot packed area. While this is just one case study, we are encouraged by our results, which provide quantitative data that indicate that boot packing can be an effective tool for helping to stabilize persistent basal weak layers.

Explosives are a critically important component of avalanche control programs. They are used both to initiate avalanches and to test snowpack instability by ski areas, highway departments and other avalanche programs around the world. Current understanding of the effects of explosives on snow is mainly limited to shock wave behavior exhibited through stress wave velocities, pressures and attenuation. This study aims to enhance current knowledge of how explosives physically alter snow by providing practical, field-based observations and analyses that quantify the effect of explosives on snow density and snow stability test results. Density and stability test results were evaluated both before and after the application of 0.9 kg cast pentolite boosters as air blasts. Changes in these properties were evaluated at specified distances up to 4 meters (m) from the blast center using a density gauge and Compression Tests (CTs). Statistically significant density increases occurred out to a distance of 1.5 m from the blast center and down to a depth of 60 centimeters (cm). Statistically significant density increases were also observed at the surface (down to 20 cm) out to a distance of 4 m. Results from CTs showed a decrease in the number of taps needed for column failure in the post explosive tests. The results of this study provide a better understanding of the physical changes in snow following explosives, which may lead to more effective and efficient avalanche risk mitigation.

Avalanche practitioners often assume that weak snowpack layers fracture more easily in steeper terrain. We are typically reluctant to conduct stability tests in safer, gentler terrain because we believe the results may not be reliably extrapolated to steeper areas. However, recent fracture mechanics research as well as propagation saw testing suggest that increasing slope angle has a small effect on fracture initiation. This paper investigates the effect of changes in slope angle on extended column test (ECT) results. We conducted ECTs on slopes in Montana and Alaska where gradual changes in slope angle allow us to sample a variety of angles with minimal changes in snow structure. The slope angles in the Montana datasets range from 7° to 35°, while those in the Alaska datasets range from 30° to 44°. On all slopes the weak layer consisted of buried surface hoar. The results show that the number of ECT taps required to initiate fracture that propagates across the entire column (ECTP) either did not change or increased slightly as the slope angle increased. From a theoretical perspective, this result is in line with the predictions of the mixed-mode anticrack model of fracture propagation in snow. From a practical perspective, our results suggest that, as long as the snow structure remains reasonably consistent in space, observers can conduct dependable tests in gentler, safer terrain before committing themselves to more exposed areas.

Understanding the spatial distribution of weak layers is a significant challenge for avalanche forecasters. Thus, improving our understanding of the processes that dictate the formation and persistence of surface weak layers across large areas is critically important for improving backcountry avalanche forecasting accuracy. For this work, heli-skiing guides mapped unburied surface hoar and near-surface facets across the Chilkat and Takhinsha Mountains of southeast Alaska during two major formation events of the 2010 season. After burial, we monitored weak layer persistence and avalanche activity. Our study area encompasses 900 km2 of rugged, glaciated, alpine terrain, at elevations ranging from 300 m to 2000 m. Guides collected information on crystal attributes and terrain characteristics at each location, and used handheld GPS units to reference locations. Incorporating the data into a Geographic Information System (GIS) proved to be invaluable for managing and visualizing observations. For example, the GIS allowed the creation of surface condition maps that we used operationally in guide meetings and in the helicopter for making run decisions. Data analyses quantified observed patterns. In particular, we often found that surface hoar crystal size lessened with decreasing elevation, possibly due to stronger katabatic winds in valley bottoms. Additionally, we observed areas of inhibited surface hoar formation and persistence which may have resulted from the influence of synoptic inflow and outflow drainage winds. By better understanding the distribution of surface hoar and near-surface facets, this work provides insights into improved backcountry forecasting of avalanche conditions over large areas.

On February 16th, 2010 a person accidentally kicked off a cornice that triggered a large slide on Saddle Peak immediately outside Bridger Bowl’s boundary. This slide narrowly missed killing several people, and the immense snow cloud was seen throughout the ski area. While the avalanche surprised many, avalanche professionals at the Gallatin National Forest Avalanche Center (GNFAC) and Bridger Bowl Ski Area have anticipated such an event since this sidecountry area was opened up two years ago. We have tried to educate the public about the accident potential through signage, articles, a special DVD, and education efforts targeting youth and their parents. Despite our best efforts Saddle Peak is heavily skied by all ages, with multiple people on the slope at the same time and with no regards for, and apparent knowledge of, safe backcountry travel techniques. Moreover, the slide demonstrated the fallacy of skier compaction for preventing large hard slab avalanches. In December 2009 a layer of facets was buried by a supportable hard slab impervious to the thousands of skiers who skied the slope during the season. A mid-February storm dropped over three inches of SWE, setting up the conditions leading to the slide.

In the fall of 2009 I became interested in when offices are staffed by forecasters at the various centers and what time of day products are issued. In other words are the hours inhumane at all the centers? How much do product issuance times vary between the centers? I decided to survey the various centers regarding this and a few other topics. Later I learned of a similar survey compiled by Knox Williams in 1998. So I added some questions to my survey in order to update his work. My survey turned out to have more questions than I originally imagined; but later I thought of more questions I could have asked.

So the results of this survey include various topics such as administration, pay, forecast area size, budget, hours of operation, weather stations, weather and avalanche product information, dissemination methods, product issuance days and times, avalanche education, and length of seasonal employment. I hope the results are informative and useful to any interested persons.

I would like to emphasize that I did not undertake this survey in order to show any particular results. Rather the work was undertaken out of interest in similarities and differences between the various centers.

Understanding the spatial variability of the snowpack is critical for avalanche prediction and mitigation. Previous spatial variability research focused primarily on relatively low angle slopes, many of which had fairly uniform characteristics. With snow sports progressing to steeper and more complicated terrain, a need exists to better understand the relationship between this “extreme” terrain and the snowpack variability. This research utilizes nine couloirs from Big Sky, Montana and Teton Pass, Wyoming. We used a probe to measure weak layer heights, slab thicknesses, and snow depths, we cross-verified those measurements with pits, and we georeferenced our sampling points using a sub-meter accuracy GPS. LiDAR data are used to derive terrain parameters, such as slope, aspect, elevation, and curvature in a GIS, and these data are then compared with our snow observations. Our analyses quantify the distribution of snow in the couloirs, and suggest that weak layer thickness normalized by snow depth is significantly correlated with the distance from the windward boundary when all other terrain parameters are accounted for in our sampled population of couloirs. Our results provide insights into the distribution of weak layers and snow depth in steep couloirs, which is a first step in optimizing snow pit locations and explosive placements in this terrain.

Most skiers trigger the slab avalanche in which they are caught. Preventing those accidents necessitates a better understanding of the factors contributing to the failure of the snowpack under the action of a skier. Our contribution gives a number of insights into a newly developed mathematical model of skier triggering based on the principles of mixed-mode anticracking. We give various examples of how the direction of the applied force and the penetration depth of the skis influence the chances of triggering fracture in the weak layer, and we investigate how the skier’s stance influences that risk. We also ask how the critical loads for triggering fracture depend on slope angle in general. We find that, for weak layers prone to anticracking, fracture is not easier to trigger on steep slopes, but is equally difficult or marginally more difficult to trigger the steeper the slope. We carried out extensive field experimentation to test this proposition using the extended column test method, the detailed results of which are given in a companion paper. As usual, our formulation includes simple shear cracking as a limiting case, so that the anticrack model is well-suited to emphasize the differences between the two fracture mechanisms. The results emphasize that the anticrack mechanism for fracture in snow requires scientists and practitioners to rethink previously accepted – and practically relevant – paradigms.

The number of skier triggered dry slab avalanches typically increases during or shortly after snow loading events. However, field observations and research suggest that a skier is less likely to trigger a weak layer fracture as the depth to that weak layer increases. This begs the question: Why does skier triggered avalanche activity increase when the likelihood of initiating fracture seemingly decreases? This paper presents preliminary evidence that new snow loading may decrease the chances for fracture arrest once initiated. During the winter of 08/09 in Colorado’s continental snowpack we used Extended Column Tests (ECT) and Propagation Saw Tests (PST) to track changes in the snowpack’s ability to propagate facture before and after loading events. In addition, we present two case studies from Southeast Alaska’s maritime snowpack. We used Extended Column Tests to measure the amount of additional loading required for a fracture to cross the entire column (ECTP). We compared these measurements to the natural loading at the end of the loading event and a day after the precipitation stopped. We also compared our data to avalanche activity on the same slopes. Our results suggest that in some cases the snowpack’s propensity for fracture arrest decreased with the additional loading, and that artificial loading of an extended column may be a useful tool to estimate loading thresholds for full fracture propagation.

Glide avalanches are a significant hazard that threatens people and property in many snowy climates. They are hard to control, poorly understood, and extremely challenging to forecast. This paper presents meteorological and environmental data associated with three glide avalanche cycles. It also discusses hazard reduction techniques from an operational perspective and provides possible explanations why previous attempts to artificially trigger glide avalanches rarely succeed. During Southeast Alaska’s winter of 09/10, we witnessed three glide avalanche cycles with over 35 total avalanches. During those cycles we collected data on snowpack, precipitation, temperature, relative humidity, sky coverage and streamflow, as well as slope aspect, elevation, steepness, shape and ground cover. We also recorded visual snow surface observations leading to the transition of some of the glide cracks to avalanches. Although glide avalanche activity is clearly somehow related to atmospheric events, we found no direct correlation between meteorological data and avalanche occurrences. However, we did find a rough correlation between snowpack, terrain and avalanche time distribution in two out of the three cycles. Our lack of reliable forecasting and control tools for glide avalanches implies that limiting the potential destructive size of glide avalanches throughout the entire winter may be the most effective approach to managing the hazard for some operations.

The Avalanche Danger Scale is an ordinal, five-level warning system that is a cornerstone of public avalanche information. The system was developed in Europe in 1993, and introduced to North America in 1994. Although both Canada and the United States adopted the system, different descriptors of the danger levels were developed in each country. Fifteen years of practical use revealed numerous deficiencies in this danger scale, most notably a lack of clarity during low probability/high consequence avalanche conditions. In 2005, a group of Canadian and American avalanche forecasters and researchers began to revise the system, with the goal of improving clarity and developing a single standard for North America. Initial explorations to define the problem resulted in more questions and uncovered an almost complete absence of formal underpinnings for the danger scale. The magnitude of the project subsequently changed, and in 2007 the project objectives were clarified as: 1) definitions of avalanche hazard, danger and risk; 2) methodology for assessing avalanche danger; and 3) revisions to the danger scale as a public communication tool. This paper concentrates on the third and final objective, and describes the methods and results of producing the North American Public Avalanche Danger Scale. Emphasis is placed on best practice in warning system design and the principles of risk communication, which helped reshape the avalanche danger scale into a more effective communication tool. The revised danger scale will be implemented across Canada and the United States for the 2010/11 season.

Conventional avalanche forecasting is practiced as a mix of deterministic treatment for snow and weather parameters and inductive logic to reach actual forecast decisions (LaChapelle, 1980). Avalanche forecasters subjectively integrate a complex array of data and evidence to reach their decisions, often operating under a high degree of uncertainty. Spatial scales in avalanche forecasting are widely variable, ranging from slope specific predictions to large, regional areas characterized by significant variation across space, and over time. Thus, forecasters must synthesize the available evidence, and extrapolate this across the landscape by relying on their knowledge of the terrain.

We present results on the geometry and depth distributions for transects along the crowns of small to large avalanches. Previous research suggests that power laws in aggregate distributions of crown depths result from observers recording maximum depth. We therefore studied individual crown transects to obtain information about the parent distributions, using in situ and photogrammetric measurements. The geometry shows that crowns are thinnest and decreasing toward the flanks. Depth distributions of individual crowns do not follow power laws. Instead, we find transects fit normal distributions. To reconcile this finding with observed aggregate distributions, we propose a mixture of maxima, taken from normally distributed crown depths, with power law mean parameters. We suggest normally distributed transects arise from spatially-correlated new snow depths.

The Extended Column Test (ECT) is a new stability test that aims to assess the fracture propagation potential across a 90 cm wide isolated column. Initial results with a dataset consisting of over 300 tests collected by one observer demonstrated the ECT’s effectiveness for differentiating between stable and unstable slopes. Further, we have received positive feedback on the test from a world-wide network of observers. This paper: 1) presents new recording standards for the test, 2) uses the SnowPilot dataset to further assess the effectiveness of the test by analyzing over 300 tests performed by several observers in different snow climates, 3) looks at the spatial variability of ECT results from several test grids, and 4) compares side-by-side results between the ECT and the Propagation Saw Test on stable and unstable slopes. Our results indicate that the ECT is an effective stability test, with a false stability ratio generally less than other standard snow stability tests. Results are sometimes quite spatially uniform, though occasionally slopes may exhibit variable ECT results. In comparison to the PST, our data suggest the ECT has a lower false stability rate, but a higher false instability rate. No test is perfect and all tests must be used in conjunction with additional data, but our results show the ECT is valuable additional tool for assessing snow stability.

Swimming in avalanches has recently been questioned, with detractors stating that “swimming leads to dying”. Since no direct scientific evidence exists to either refute or support the idea of swimming, we combine the practical experience of avalanche survivors with our emerging knowledge of avalanche dynamics to arrive at possible survival strategies for different parts of flowing avalanches. Practical experience and avalanche dynamics theory are largely consistent and suggest the following strategies: 1) Once an avalanche is released, every effort must be made to get off the moving slab, 2) After being caught, the victim must do everything possible to try to get toward the back, or tail, of the avalanche since this is where avalanches run out of mass and where a victim is more likely to be left behind by the slide, 3) Experience shows that in some avalanches a backstroking and log rolling motion may help the victim stay near the surface and move toward the flanks of the avalanche, and 4) If at all possible, the head of the avalanche should be avoided since the turbulent flow and large forces in this area increase the odds of injury and deep burial. Though it cannot be definitively proven, experience and avalanche dynamics theory suggest that swimming – or as some call it, “struggling” – is part of a viable strategy for surviving an avalanche once you are caught.

Understanding the spatial variability of the snowpack is a crucial step to improve accuracy in field data collection and avalanche forecasting. While there has already been a large volume of literature assessing the spatial variability of the snowpack, inconsistent sampling designs make comparing results difficult. This work uses an overlapping 10 by 10 m grid to collect Extended Column (ECT), Compression (CT) and Stuffblock (SB) test data at the slope scale across a range of environmental settings and climatic regimes in Montana and New Zealand. The overlapping grid methodology standardizes data collection between our sites, as well as allowing for repeat data collection on the same slope, thereby providing a new method for attempting to assess changes in spatial variability over time. Preliminary results suggest that the spatial variability of fracture propagation and fracture initiation may increase over time, and that the spatial variability of the fracture propagation propensity may be related to the processes causing the instability. As we collect more data, these results will provide further insight into the problem of snow pit location and representivity, both in terms of space and time.

Researchers and practitioners have long utilized a variety of penetrometers to investigate the snowpack. Identifying definitive relationships between penetrometer-derived microstructural information and stability has been challenging. The purpose of this study is two-fold: 1. We propose a simple field test that can be implemented by the scientific community to establish relationships between load and penetrometer-derived microstructural strength, 2. Utilizing the SnowMicroPen (SMP) data, we quantify changes in weak layer residual strength and structural dimensions associated with a loading event. Our dataset is from Moonlight Basin, Montana and includes three modified loaded-column tests, each paired with 5 SMP profiles. Depth hoar comprised the targeted weak layer. Results indicate that loading events cause the residual bond strength and bond frequency in large-grained weak layers to decrease significantly. Much like a compression test at a micro-scale, the force required for the SMP to rupture individual bonds as well as the micro-strength decrease significantly when the slab stress is increased by artificially adding blocks of snow. A decrease in observed bond frequency within the weak layer (or an increase in the distance between bonds) also occurs after a loading event, probably because some bonds within the weak layer have already failed or are so close to failing that the penetrometer cannot detect their rupture. Artificial removal of slab stress resulted in greater rupture forces and distances between bonds, likely due to elastic rebound. This indicates that long after a natural loading event has occurred, elastic deformation still exists within the weak layer.

Due to the time-consuming nature of traditional snowpit measurements, and the large spatial variability that often exists in alpine snowpacks, tools which can rapidly characterize snowpack properties are in great need. Microwave radar has an additional advantage in that it is non-destructive and measurements can be made remotely, providing the opportunity to make measurements over large areas rapidly from an airborne platform. Signal interpretation can be difficult, however recently several ground-based studies have shown that the technique can be used to accurately measure snow depth, snow water equivalent, and snow stratigraphy. Accurate measurements of these properties from the air is much more challenging, especially in steep terrain. We present results from two field campaigns near Valdez, Alaska, in which measurements in mountainous terrain were made from a helicopter with two different FMCW radar systems. Snow depth and stratigraphy was visible at altitudes of less than 100 feet, while steep terrain made interpretation difficult at typical flying altitudes, due to the footprint size of the radar. Refinements of helicopter-based radar measurements may eventually provide a useful tool to assist in stability evaluations for helicopter ski operators.

Understanding the spatial variability of snow at many different scales is critically important for avalanche forecasting. This research uses new techniques to reanalyze an existing spatial dataset collected in southwest Montana’s Bridger Range during the 1996-97 winter. Recent developments in statistical software have greatly increased the ease with which mixed effects and spatially correlated models can be run. Employing such advanced statistical procedures can lead to more beneficial use of available data sets and a more efficient use of limited financial funding. We reanalyzed the data using recently developed packages in SAS and the freely available software package R. The generalized linear model provides a suitable framework for categorical and / or dependent response variables. We analyzed the data using a fixed effect repeated measures model, a random effect clustered data model, and a spatially dependent fixed effects model using normally distributed continuous data. In addition, using the GLIMMIX procedure in SAS, we analyzed the data using a spatially dependent – random effects model with multinomial errors. This method allows for analysis of untransformed ordinal data, such as the data created when performing Rutschblock and stuffblock tests. Comparisons to the original analysis and suggestions for improving analytical efficiency are discussed. Our analyses help to provide a methodological context for future analyses of similar regional spatial data.

Power-laws provide a means for investigating snow avalanche frequency-magnitude relationships and their contributing factors. This research uses power laws to explore variations in avalanche size proportions through space and time, as well as investigating factors which may contribute to these variations. Data utilized for this work includes the Westwide Avalanche Network data from the western United States for regional analyses, with path-specific analyses focused on data from Utah’s Little Cottonwood Canyon. Results show power-law exponents vary through space both at the regional level and between individual avalanche paths. Avalanche size proportions, with respect to space, are the product of terrain based variables at both the mountain range and the path levels, with alpha angles significantly correlated to the proportion of small to large avalanches. This research also indicates that variation in exponents through time is indicative of changes in seasonal weather and snowpack characteristics, with mean snow height also significantly correlated to the proportion of small to large avalanches. Knowledge of power-law exponents for particular avalanche paths, and their relationship to seasonal snowpack depth, may be helpful for managing avalanches along highway corridors, in ski areas, or in backcountry forecasting operations.

Most avalanche fatalities occur due to dry slab avalanches. However, wet snow avalanches are also dangerous and can be particularly difficult to predict. The rate of change from safe snow conditions to dangerous snow conditions occurs rapidly in a wet snowpack, often in response to water production and movement. This research focuses on the relationship between snow stratigraphy and water movement in an inclined snowpack. Concentrating on the capillary barrier effect and flow finger formation within the snowpack, dye tracer was mixed with water and applied to a stratified snowpack to observe and measure the movement of water in various snow grain types, sizes, densities, and temperatures. Experiments show that even a slight textural change within dry snow grains produce a capillary barrier. The amount of water needed to produce flow fingers depends on the snow structure. Both capillary barriers and flow finger formation may play a large role in wet slab avalanche formation. Increasing global mean temperatures may increase the frequency of wet snow avalanches of all types, so a better understanding of the processes involved is important.

Many avalanches are triggered from shallower parts of a slope. Past research demonstrates that initiating fractures in such areas is easier than in deeper places where the applied stresses must penetrate through more overlying snow before affecting the weak layer. However, to our knowledge there is no work on whether fractures more effectively propagate from deeper to shallower areas, or from shallower to deeper areas. During the 2006/07 and 2007/08 winters, we looked at fracture propagation using standard Extended Column Tests, modified Extended Column Tests with column widths of 200 and 300 cm, and Propagation Saw Tests. We tested fracture propagation on slopes with highly variable weak layer depth and with reshaped slab thickness. Our results suggest that fractures are more likely to propagate further when traveling from under thin to thick slabs than in the opposite direction. We support our test results with four case studies. In these cases, slopes only partially released with big explosives applied where the weak layer was deeper, but then avalanched entirely a few days later when tested with small loads where the weak layer was shallower. Thus, shallower areas of the slab may be both the easiest place to initiate a fracture and also the best place from which to propagate a fracture. These results have broad implications for backcountry travel, avalanche avoidance, and avalanche control work.

Dry slab avalanche stability typically increases over time in the absence of active loading from new snow or wind. However, field observations suggest that occasionally slopes showing no signs of instability in the morning avalanche later in the day when the snow surface is warmed by the sun. In this paper we present evidence that dry snowpack fracture propagation propensity may increase during sunny days as the snow surface warms up and becomes wet. During four warm, sunny days in the winters of 06/07 and 07/08, we tracked changes in results for both Extended Column and Propagation Saw tests. Our data suggest that snow surface temperature affects fracture propagation propensity on inclined slopes, with fractures more likely to propagate when the snow surface is wet. We support our test results with two case studies where explosives and ski cuts produced no avalanches when the snow surface was cold and dry, but when those same slopes were re-tested after the snow surface warmed to zero degrees they avalanched. In both cases the weak layer was dry and had a temperature below zero. We hypothesize that fracture propagation propensity may increase due to increased surface creep or due to changes in the mechanical properties of the slab.

The worst nightmare for an avalanche worker is to assess an unstable slope as stable since the consequence of such an assessment is that you, your clients or the public could be caught in an avalanche. Thus, a primary goal in avalanche forecasting is to minimize such “false-stable” errors. In this paper we analyze the first season of data from the SnowPilot database. Starting with nearly 1,000 snowpits and 3,500 stability tests, we use stability test scores, shear quality, and weak layer depth to identify what we term the “critical weak layer” in each pit. We also divide the pits into “stable” and “unstable” categories based on the assessed snow stability and observations of obvious signs of instability (collapsing, cracking and recent avalanche activity). This filtering leaves us with 289 compression, rutschblock and stuffblock stability tests that fractured on the critical weak layer on unstable slopes. Of those 289 tests, 38 of them (13%) presented “false-stable” results, which we define as CT21 or greater, RB5 or greater, or SB drop heights 40 cm or greater. If we include shear quality and consider strong test results with a Q1 shear to be unstable, we decrease our false-stable cases to around 9% of the total. This implies that – if we use only stability test results – around 1 in 10 times we assess unstable slopes we will conclude that it is stable, which is unacceptably high. Recently spatial variability research has led some to argue that digging snowpits is unnecessary or futile, but we believe our data reinforce the idea that the key to analyzing snow stability lies in digging more rather than fewer pits, and using a holistic approach that considers much more than simple stability test results. Though our dataset is limited, it suggests that digging multiple pits might be an effective strategy for minimizing false-stable situations. In fact, having stability tests and associated shear quality from two different, but representative locations on the slope might decrease the chance of a false-stable error from around 10% to closer to 1%.

While doing avalanche mitigation work or traveling in the backcountry, occasionally a sizable part of a slope fractures without triggering an avalanche. An example is when a weak layer fractures with a characteristic “whumpf” sound and tensile cracks open up, but no avalanche releases. Disagreement exists among avalanche professionals about the immediate safety of these slopes. Many assume that if the slope does not slide during initial fracture propagation then it is unlikely to slide and is probably safe. Others treat the slopes with extra caution, especially immediately following the event. This paper provides a synopsis of recent research and two case studies that provide insight into this problem. Research shows that shear strength decreases immediately after a collapse, followed by differing strengthening rates. In both case studies, avalanche mitigation work with explosives resulted in the fracturing of some slab boundaries, as evidenced by tensile cracks visible on the surface. Additional explosives applied to the slopes shortly following the initial fractures resulted in sizable avalanches, casting doubt on the idea that fractured slopes are necessarily safe. Over many years and a handful of such experiences, an unofficial policy at Big Sky Ski Area has evolved whereby the snow safety group typically will not open slopes that have deep fractures until the following day. Our paper does not provide definitive answers about the safety of fractured slopes. However, it does point out uncertainties in our knowledge and, as a result, suggests taking a cautious approach toward such slopes.

Throughout the winter, avalanche forecasters issue bulletins to help the public and the managers of public facilities make avalanche safety decisions. These bulletins typically describe important  snowpack features and current weather events before rating the avalanche danger on a scale of one through five. Although the character of avalanche conditions may vary between regions, the physical processes that form avalanches are universal. In addition, the methods used to forecast avalanche activity are similar throughout North America and Europe. We use the distribution of fatal avalanche accidents with respect to forecasted avalanche danger level to examine how effectively avalanche forecast groups communicate with the public and how consistent these groups are within countries and internationally. The results show that avalanche forecast groups are effectively communicating with the public when relatively benign or very dangerous conditions exist.

For dry slab avalanches, fractures initiate and propagate in a weak layer or along an interface. Current field tests like compression or stuffblock tests are designed for assessing fracture initiation; however, these tests may not be as useful for assessing fracture propagation. Furthermore, in some cases these test may identify layers that are more likely to initiate a fracture under stress, but not necessarily those layers that initiate and propagate the fracture as well, thereby occasionally “masking” those layers of real concern in a weak snowpack. This paper describes the development of a new field test that aims to assess both fracture initiation and propagation in an isolated column. Tested during the winters of 2005-06 in Colorado and 2006 in New Zealand, this test is a variation of the compression test and can be used in the same manner with the stuffblock. By tapping on one side of an extended column of 30 cm downslope by 90 cm in the cross slope direction, the extended column allows a slab to transmit stress across the width of the column. The fracture initiation results are collected as well as the results of the fracture propagation across the extended column. Out of 68 tests of unstable snowpacks (where avalanches recently occurred, or there was whumphing or shooting cracks) the fracture propagated across the entire block in 1 or 2 loading steps every time (100%) and 63 times (93%) it fractured with a compression test load of easy to moderate. Conversely, out of 256 pits where the snowpack was stable, only 4 cases (1.6%) propagated across the entire extended column through a single layer or interface. Thus, in stable snowpacks a fracture may be initiated, but it typically does not propagate across the column. For the snowpacks we tested the extended column test effectively discriminated between stable and unstable slopes.

Wet loose snow avalanches are a significant hazard within many ski areas. Wet snow stability changes dramatically over short time periods which typically coincide with operating hours, and few quantitative tools exist for avalanche workers attempting to predict the onset of wet snow avalanching. This study documents changes in surficial shear strength during melt freeze cycles and relates these changes to observed wet loose avalanche activity. We conducted field work at two study sites in southwestern Montana over the course of four April days in 2005 and 2006.  We used a 250 cm2 shear frame to make as manay as 210 surficial shear strength measurements of melt-freeze snow per day, and adjusted our results for known shear frame size effects. We also collected SnowMicroPen penetrometer profiles in conjunction with shear strength during one melt-freeze cycle. Initial results are encouraging. Changes occurred rapidly within the melt-freeze cycle as shown by highly significant changes in shear strength within half hour intervals. SnowMicroPen data shows significant positive correlations between the microstructural hardness of snow and shear strength. Most importantly, our limited data shows an apparent association between surficial shear strength and avalanche activity. On 22 April 2006 when our shear strength measurements dropped below 250 Pa we observed, and triggered, wet loose avalanches in the immediate vicinity of study slopes. Conversely, surficial stability on our study slope improved when shear strength values exceeded 300 Pa.

Snow avalanches are a major mountain hazard that kills hundreds of people and causes millions of dollars in damage worldwide annually. Yet, the relationship between the well-documented spatial variability of the snowpack and the avalanche release process is not well understood. We utilize a cellular automata model to show that the spatial structure of shear strength may be critically important for avalanche fracture propagation. Fractures through weak layers with large-scale spatial structure are much more likely to propagate over large areas than fractures through weak layers with smaller-scale spatial structure. Our technique of integrating spatial structure into the model can improve many cellular automata models that aim to explain and predict other natural hazards, such as forest fires, landslides and earthquakes.

The U.S. Forest Service initiated avalanche control and forecasting in the United States. The Forest Service manages large tracts of publicly owned land called National Forests and they permit certain activities and businesses on those lands. The Forest Service began permitting ski areas on National Forests in 1938 when they issued a special use permit to Alta Ski Area in Little Cottonwood Canyon near Salt Lake City, Utah.

Almost immediately the Forest Service realized that avalanches threatened the public both while they traveled the Little Cottonwood Canyon Road to reach Alta Ski Area and while they actually skied. The Forest Service hired Douglass Wadsworth to help mitigate the threat and he became the first Forest Service Snow Ranger.

The Forest Service Snow Ranger Program grew to become one of the most effective and innovative avalanche control and forecasting programs in the world. Forest Service Snow Rangers pioneered the use explosives for avalanche control in the US, developed the first effective avalanche forecasting programs in the US, and initiated the use of military artillery for avalanche control in the US.

My paper will trace the development of the Military Artillery for Avalanche Control Program in the United States from its inception to today. It will explain how military artillery work, examine alternatives to military artillery, discuss three 106mm RR accidents and discuss the future of military artillery for avalanche control in the U.S.

This paper compares the spatial structure of the compressive strength of slab and weak layers from two different snow climates. Our data come from arrays of SnowMicroPen (SMP) measurements in southwestern Montana, USA, and near Davos in eastern Switzerland. In both cases, buried surface hoar comprised the critical snowpack weakness. We analyzed the SMP data by manually delineating the surface hoar layers and a number of layers within the slabs above the weak layers. Using the log-10 transformed mean of the penetration resistance for our layers of interest, we investigated the spatial structure of the data by looking for linear slope-scale trends and assessing the residuals of any trends with semivariograms. Our results demonstrate that the layers investigated had variable spatial structures, both in terms of their linear trends and their semivariograms. This suggests that each snowpack layer has a unique spatial structure possibly arising from its depositional pattern and the subsequent changes to the layer when buried. We also demonstrate that the specific layout used for the measurements strongly influences the observed spatial variability. The complicated spatial structures of individual layers, and how they interact, likely contributes to the sometimes confounding overall patterns of spatial variability of stability observed on snow slopes.

SnowPilot (www.snowpilot.org) is a free software program that allows users to collect snowpit and avalanche occurrence data onto a Personal Digital Assistant (PDA or Palm Pilot). This data is then stored on a PC where it can be viewed in a graphical snowpit format. The data is also uploaded to a database on the web where it can be viewed and accessed.

This study presents evidence that slope aspect plays a significant role in the formation, size, type, and extent of surface hoar and near-surface faceted crystals. Experimental stations were placed on the north and south-facing aspects of Pioneer Mountain in southwest Montana to measure wind speed and direction, snow temperatures from 0.1m above the snow surface to 0.35m below the snow surface, incoming and outgoing shortwave radiation, and snow surface temperature. Each time a surface hoar or near-surface faceted crystal layer formed, snowpack and meteorological variables were gathered and snow crystals were collected, measured, characterized and photographed so that crystal size and structure could be compared between aspects. Persistent weak layers formed on January 9-10, 2004, and January 12-13, 2004. Results show a statistically significant difference in the size of surface hoar and near-surface faceted crystals based on slope aspect. During these two periods, better developed near-surface facets formed on south-facing slopes, while surface hoar crystals grew larger at the north-facing study site. These aspect dependent differences are important for assessing mountain range scale spatial variability and may also play a role at smaller scales due to subtle aspect changes. The results of this research may help us better understand some of the differences in weak layer formation on different aspects which, in turn, lead to different avalanche conditions.

For over 35 years avalanche workers in the United States used observations described on “the blue and green sheets” published by the USDA Forest Service for standard snow, weather and avalanche observations. Those observation outlines served the U.S. avalanche community well and produced a valuable long-term data set. However, a new and updated set of guidelines are now needed since the blue and green sheets are out of print, and the number, types and needs of avalanche programs in the U.S. have changed.

The American Avalanche Association and the USDA Forest Service National Avalanche Center joined forces to research, write and publish a new set of observational guidelines to support avalanche programs in the United States. This effort began by licensing the Canadian Avalanche Association’s Observational Guidelines and Recording Standards for Weather, Snowpack and Avalanches.  This well established document along with the blue and green sheets formed a solid foundation for the new guidelines, which were specifically developed to fit the diverse needs of avalanche programs in the U.S. The resulting document, Snow Weather, and Avalanches: Observational Guidelines for Avalanche Programs in the United States, was published by the American Avalanche Association in the fall of 2004.

This research uses a two-dimensional cellular automaton model, with inputs taken from field data, to mimic snow slab avalanche release. The initial weak layer shear strength in the model follows a normal distribution with known mean and standard deviation. For each realization of the spatial distribution of weak layer shear strength, the model stresses all cells equally until the weakest cell fractures. The stress from the fractured cell is transferred to cells in the neighborhood of the fractured cell, possibly causing a propagation of the fracture and a model avalanche. The stochastic shear strength field makes the model avalanche size stochastic for statistically constant initial conditions. The standard deviation of shear strength strongly affects the proportion of model avalanches covering nearly all cells in the model, with low variability leading to a high proportion of large model avalanches, a result that supports previous conceptual models. Stress transfer properties in the model are also important for the proportion of large model avalanches, but no field data exist to constrain these values. Spatial auto-correlation of shear strength is likely to be important for the size of model avalanches, and will be built into a future version of the model.

Avalanche forecasting involves the prediction of spatial and temporal variability of the snowpack. To predict avalanches with more accuracy it is important to determine whether the snowpack is becoming more spatially variable or more spatially uniform. Greater variability increases uncertainty in extrapolation and prediction. Our results offer a look at the evolution of the spatial variability of shear strength and stability of a buried surface hoar layer in southwestern Montana, USA, from shortly after burial until it was no longer the weakest layer in the snowpack. We selected the study site for its 27-degree planar slope, uniform ground cover, and wind-sheltered location. This simplified the comparison of the plots by minimizing initial spatial differences so we could focus on temporal change. Within the site, we sampled four 14 m x 14 m arrays of more than 70 shear frame tests in a layout optimized for spatial analysis. Over a three-week period, the sampling of the four adjacent arrays showed temporal change. The variability of the shear strength of this layer initially decreased then became increasingly variable through time. This suggests that extrapolating test results to other locations becomes increasingly unreliable as layers age, a result that matches practical experience. The data also provide indications that shear strength has a correlation length, the distance at which test results are related, of just a few meters. This short correlation length demonstrates quantitatively why stability tests that are relatively close together can be quite different.

Few avalanche forecast models are tailored specifically for wet avalanche forecasting. Bridger Bowl (intermountain climate) is a good area to develop a wet avalanche probability model. The primary archived data consists of eight variables. The archived data for March from 1968 to 2001 (1996 data unavailable) were used to develop 68 predictor variables related to temperature, snowpack settlement, and precipitation. The original dataset was divided into days with snowfall in the past 48 hours (new snow) and days without (old snow). There were 33 significant old snow variables and 22 significant new snow variables. Six variables are common to both old and new snow. The best predictor variables for old and new snow are different. The variables were analyzed with binomial logistic regression to produce probability models for old snow and for new snow wet avalanche conditions. The old snow model uses the prediction day minimum temperature and the two-day change in total snow depth as predictor variables and has a 89% overall success rate. However, the majority of this success is due to correct prediction of days without wet avalanches (96% of all correct predictions). The new snow model uses the prediction day minimum temperature and three-day cumulative new snow water equivalent as predictor variables, but is less useful. The models are applicable only to Bridger Bowl. The numerical forecast models can be used as one of the tools in the forecasting toolbox but limited data and complexity of process require that the decisions about closure remain in the hands of the ski patrol.

A helicopter ski guide died in an avalanche at 3:00 pm on February 10, 1996 while guiding clients on a large west-facing slope in the Smoky Mountains of Central Idaho. The ski guide, his clients, and several other guides and their clients had skied over one hundred runs in the vicinity and on the same slope prior to the accident. The avalanche measured nearly one half-mile wide and involved three chutes that are separated by counter ridges. Three other guides and sixteen clients were on an unaffected counter ridge when the avalanche occurred.

The avalanche fractured on buried surface hoar and near surface facets. The avalanche varied from one foot to an estimated five feet deep and ran on 28 to 38 degrees slopes. Forty-eight hours of extreme avalanche hazard including many natural and human triggered avalanches followed the fatal accident.

Prior to the February 10 avalanche, significant WNW winds on January 27 rapidly loaded slopes throughout the area and numerous large natural and artificially triggered new snow avalanches occurred. No avalanches were reported from February 4 through February 9. An “inverted” storm dropped several centimeters of snow and three centimeters (1.2 inches) of water equivalent from February 4 – 8. Temperatures rose from –16 degrees C (2F) February 2 to –1 degree C (30F) February 9 and then spiked to 5 degrees C (41F) at 2:00 pm February 10.

Our paper will examine possible reasons why the deadly cycle occurred despite no overly dramatic weather event immediately preceding it, how the accident affected the local community including avalanche education, and possible ways to deal with similar events in the future.

The spatial variability of snow stability presents a challenge to avalanche workers and scientists attempting to assess avalanche danger. Recent research has demonstrated, and attempted to explain, snowpack and snow stability variations over individual slopes. However, these studies have provided single snapshots of an exceedingly dynamic system instead of investigating stability variations over time. This research begins to address how spatial variability at the local slope scale changes through time and the some possible mechanisms for those changes. We performed ten Quantified Loaded Column Tests (QLCTs) in each of five snowpits within a 900 m² plot on a relatively uniform slope on three different days.  We make a case that the behavior of the snow avalanche system is consistent with complex, nonlinear Earth Surface Systems (ESSs), and that research into ESS behavior in other fields might prove to be useful for examining changes in spatial variability through time. In particular, ESSs are typically characterized by sensitivity to initial conditions, which leads to increasing spatial variability through time. Our series of three QLCT trials exhibited changing stability patterns, suggesting that the spatial variability on a slope may increase through time in the absence of external forcing, but that variability may then decrease when additional load is added to the slope. This research provides some interesting initial insights into changes in spatial variability for practitioners and provides some baseline data for future scientific work in this area.

Our poster summarizes the results of Birkeland and Landry (2002), where we direct the interested reader for more details on this research. Our results show scale-invariant relationships between avalanche frequency and size in several groups of avalanche paths. Specifically, data from Gothic, Colorado, Bridger Bowl, Montana, Jackson Hole, Wyoming, and Snowbird, Utah demonstrate loglinear relationships, or power-laws, for both natural and artificially released avalanches. We also analyzed a group of snow avalanche paths at Yule Creek, Colorado for their response to 104 storms, showing that a power-law exists between the magnitude and frequency of the resultant avalanche cycles. Recently, other researchers have also documented a number of power-laws associated with snow avalanches, including acoustic emissions, crown crack heights, and crown crack lengths (Louchet et al., in press; Dendievel et al., 2002; Faillettaz et al., 2002a; Faillettaz et al., 2002b), as well as presenting evidence that slab avalanching is a chaotic process (Rosenthal and Elder, 2002). Several other natural hazards, such as earthquakes and forest fires, exhibit similar power-law relationships (Drossel and Schwabl, 1992; Olami et al., 1992; Malamud et al., 1998). Our results, as well as our avalanche forecasting experience, suggest that snow avalanches may exhibit self-organized criticality (Bak et al., 1987; 1988; Bak and Chen, 1991), an observation recently backed up by the research of others (i.e., Louchet et al., in press; Dendievel et al., 2002). Frequency-size relationships for small and medium sized avalanches, and avalanche cycles, may be useful for quantifying the risk of rarer large snow avalanches, and avalanche cycles, within a given group of avalanche paths.

The objective of this study was to investigate the importance of topography in controlling the geographic patterns of deep snow temperature gradients within a seasonal snowpack. Demonstration of the relative importance of topography in influencing spatial snowpack temperature gradients could aid future modeling of snow layer development and behavior, with benefits for avalanche and snowmelt modeling. This spatial, or geographic, analysis of the relationship of snow temperature gradient patterns to topography utilizes landscape-scale modeling in an attempt to identify responses in complex, mountainous terrain. During the snow season of 2001-2002, 30 temperature profiles were sampled on each of nine sample days. Profiles were collected through the use of a portable snow temperature profile probe (Deems, 2001). These data were used to calculate temperature gradients for each profile. Topographic attributes were derived from a digital elevation model (DEM) using a Geographic Information System (GIS). Linear regression models quantified the relationships between the topographic variables and snow temperature gradient patterns in our spatially distributed dataset, and demonstrate the relative importance of the terrain variables in determining spatial patterns of temperature gradients.. Analysis shows a complex pattern of relationships between temperature gradients and the static topographic variables. A qualitative assessment of weather variables recorded onsite suggests the utility of using more dynamic variables such as weather data in future research.

The objective of this study was to investigate the relative importance of topography in controlling the geographic patterns of snow temperature gradients within a seasonal snowpack. Regression models quantified relationships between topographic parameters and temperature gradient statistics for our spatially distributed dataset. Demonstration of the relative importance of topography in influencing spatial snowpack temperature gradients could aid future modeling of snow layer development and behavior, with benefits for avalanche and snowmelt modeling. This spatial, or geographic, analysis of the relationship of snow temperature gradient patterns to topography, utilizes landscape-scale modeling in an attempt to identify responses in complex, mountainous terrain.

During the snow season of 2001-2002, 30 temperature profiles were sampled on nine sample days. Profiles were collected through the use of a portable snow temperature profile probe (Deems, 2001). These data were used to calculate temperature gradients for each profile. Topographic attributes were derived using a Geographic Information System (GIS) and a Digital Elevation Model (DEM). Linear regression assessed the relationship between the topographic variables and snow temperature gradient patterns, and demonstrates the relative importance of the terrain variables in determining spatial patterns of temperature gradients. Analysis of the regression models shows a complex pattern of relationships between average temperature gradients and topographic variables. A qualitative assessment of weather variables suggests the utility of weather data in future modeling efforts.

This study investigates whether collecting shear quality data in conjunction with stability test results improves snowpack evaluations. Over the past six seasons we have consistently evaluated shear quality when evaluating snowpack stability. Shear quality is subjectively evaluated on a 3-tiered scale from Q1 (clean, fast shears) to Q2 (average shears) to Q3 (irregular or dirty shears). Our method is a formalization of what ski patrollers and others have been doing in the U.S. and elsewhere for at least several decades. We used a dataset of nearly 700 individual stability tests (rutschblock, stuffblock and compression tests) collected by seven observers on slopes from Alaska to Chile. In addition to stability test results, observers noted whether slopes they felt were similar to their snowpit location had avalanches, or collapsing or cracking snowpacks, on that day. Results suggest that shear quality provides important stability information, especially when stability test results appear to indicate relatively stable conditions, but the shear quality is rated Q1. This might be because stability test results are often spatially variable, while our experience indicates that shear quality is more homogeneous. Given these results, we believe formally integrating some description of shear characteristics into stability assessments may be important for avalanche workers and backcountry enthusiasts.

The majority of slab avalanche accidents occur when the victim triggers the slide. Slab hardness is an important property affecting skier-triggered avalanches because hardness partially determines whether sufficient stress reaches the weak layer to cause failure and/or fracture. This study examines how new and old snow layer hardness varies with aspect and which meteorological variables most influence those changes. Slab hardness was measured with a ram penetrometer on north and south aspects from January through March, 2000 at Jackson Hole Mountain Resort and Grand Teton National Park, Wyoming. Continuous weather data were obtained from weather stations at Jackson Hole Mountain Resort. Analyses were carried out on new and older near-surface snow layers. A temperature index was calculated for the south and north aspects to describe the delayed effect of increasing temperature on increasing hardness through sintering, settlement, and densification. The south temperature index, maximum daily temperature, and the interaction between maximum daily temperature and incoming shortwave radiation were the most significant predictors of new snow layer hardness on the south aspect. The north temperature index, maximum daily temperature, and the previous day’s wind speed were the most significant predictors of new snow layer hardness on the north aspect. The temperature index was the only significant predictor of old snow layer hardness on both the north and south aspects. The results of this research suggest that it may be possible to use meteorological factors to predict changes in snow hardness – an important component in predicting skier-triggered avalanches.

This research investigated whether single snowpits can reliably represent snowpack stability on uniform slopes. The study utilized seven carefully selected slopes, three each in the Bridger and Madison Ranges of Southwest Montana, and one in the Columbia Mountains near Rogers Pass, British Columbia. Teams performed ten Quantified Loaded Column Tests in each of five snowpits within a 900 m2 plot at a slope, measureing shear strength in a single weak layer. Collection of slab shear stress data enables the calculation of a strength/stress stability ratio. Altogether, eleven stability sampling trials were performed during 2000/01 and 2001/02, testing several weak layer types exhibiting a wide range of strengths.  Of the 54 snmowpits completed, 26 pits (48%) represented plot-wide stability and 28 pits (52%) did not one plot collapsed prior to completion of a 55th pit.  Two of the eleven plots did contain full complements of five representative snowpits.  The results of this study suggest the importance of improving our understanding of the processes affecting the variability of snowpack stability on any given day. 

Many ski areas, backcountry avalanche centers, highway departments, and helicopter ski operations record and archive daily weather parameters and the resulting avalanche activity. This paper proposes a new probabilistic method to allow avalanche forecasters to better utilize their historical weather and avalanche data by incorporating a Geographic Information System (GIS) with a modified meteorological nearest neighbors approach. This approach utilizes concepts from Geographic Visualization (GVis) and Knowledge Discovery in Databases (KDD). The resulting interactive database tool allows avalanche forecasters to visually explore regional spatial patterns of avalanche activity at multiple scales. This technique allows the correlation between weather parameters and the spatial pattern of avalanche activity. An example of this method was implemented using 23 years of historical avalanche data from the Jackson Hole Ski Area with over 10,000 avalanche events to analyze the effect of new snow, wind speed, and wind direction on the spatial patterns of avalanche activity. Patterns were found at the slide path scale and for sub-regional groups, but not for the entire region as a whole, or when slide paths were grouped by aspect categories.

To assist local avalanche forecasters, Buser (1983) developed an avalanche prediction system based on the nearest neighbor method. NXD2000 is an improved and further developed version of the program, and has been installed at several places in Switzerland, Austria, Kazakhstan, and the USA. Improvements include the introduction of explanatory variables such as settlement and a mass of wind-transported snow and functions giving certain values of a variable more load, e.g. snow surface temperature which is important in the range right below O°C, but not far below at -20°C. We explored the influence of the functions and attempted to optimize the variable loadings at Parsenn Ski Area in Davos, Switzerland and Snowbasin Ski Area in Utah, USA.

A slab avalanche occurs when a weak layer or interface below the slab fractures, causing the slab to release. A large percentage of accidents associated with slab avalanches result when the victim triggers the release. While it is recognized that avalanches are caused by the unique interaction between the weak layer and the slab, most research has focused on the composition of the weak layer and how its strength changes over time. Relatively few studies have examined how slab properties change over time and space. We expect slab properties to change over time and space because of both the inherent variations in the energy balance associated with aspect and the dependency of snow metamorphism on the energy balance. This research attempts to address the following research question: to what degree does slab hardness vary with aspect and time when elevation and slope are kept relatively constant? Slab hardness was measured with a ram penetrometer on north and south aspects from January through March, 2000 at Jackson Hole Mountain Resort and Grand. Teton National Park, WY. While hardness increased more rapidly on southern than northern aspects due to settlement and densification, solar radiation on southern aspects also contributed to greater heterogeneity in the snowpack.

Recently, a computer model has been developed by the Swiss Federal Institute for Snow and Avalanche Research that- simulates the evolution of a natural snow cover. Using common meteorological parameters as input, SNOWPACK predicts characteristics such as snowpack temperature and density, in addition to snow microstructure and layering. An investigation was conducted to evaluate the effectiveness of SNOWPACK in a Montana climate. A weather station was constructed in the Bridger Mountains near Bozeman, Montana, to provide the meteorological parameters necessary to run SNOWPACK. Throughout the 1999-2000 winter, weekly snow profiles were performed in undisturbed snow to provide a benchmark for the model output. Density, grain size. and crystallography were recorded on 10 cm intervals over the full snow depth, and the temperature profile was monitored with a thermocouple array. Finally, the meteorological parameters were input into SNOWPACK, and a statistical comparison was performed comparing the predicted snowpack to the observational data. Snowpack temperatures are predicted reasonably accurately by SNOWPACK. The modeled and observed densities correlated well, but the model typically underestimates snowpack settlement. Comparison of grain size and shape was problematic due to different definitions utilized by the model and observer, but still demonstrated some agreement.

Most historical avalanche data are stored in tabular form, and are difficult to access and visualize. This project utilizes a Geographic Information System (GIS) to spatially visualize historical avalanche data at Jackson Hole Ski Area in Wyoming, USA. Jackson Hole is an ideal location for this study because: 1) the ski patrol have been collecting avalanche and weather records for over 25 years, 2) the ski area is spatially diverse, with aspects ranging from north to east to sough, and 3) a three-meter Digital Elevation Model (OEM) and a geo-referenced orthophoto are available for the area. We have combined the OEM, orthophoto, and historical data to geographically display historical avalanche data, as well as to be used for future statistical modeling. The GIS allows greater three-dimensional visualization of historical avalanche data by displaying specific avalanche path activity and corresponding size, thereby enabling forecasters to better utilize their historical data. Additional weather information can be displayed simultaneously, and the entire picture can be viewed through time in a frame-by-frame movie format to help avalanche forecasters observe trends. The GIS can also be used to obtain data about specific avalanche paths such as slope, elevation, and aspect. We hope to use these data to train statistical models, and to eventually display historical avalanche activity that most closely resembles the current day in real-time for the ski area avalanche forecasters.

Though avalanche workers and scientists recognize that distinct patterns of snow stability exist in mountainous terrain, no field-based research has been conducted to rigorously analyze those patterns at a scale of interest to backcountry avalanche forecasters. This research investigates snow stability throughout a small mountain range. Using helicopter access, sampling teams collected data from over 70 sites on each of two sampling days. Variables generated were analyzed using a variety of statistical techniques. Results demonstrate that links exist between terrain and stability, and that those relationships change over time. The data structure is complex, providing insights into the myriad difficulties faced by avalanche forecasters.

Determining the likelihood of an avalanche occurring on a given slope is a critical decision faced by both avalanche professionals and backcountry enthusiasts. An important part of that decision-making process is using stability tests and interpreting their results. Though a variety of stability tests are available, we use the rutschblock and stuffblock tests since they both identify weak layers in the snowpack and, to a limited extent, quantify the stresses necessary for weak layer failure. However, there are several limitations to these stability tests, including site selection and interpretation of results, both of which require experience. While site selection may be the most important limitation, we focus on the interpretation of results. Such interpretations can be problematic, since those without the benefit of avalanche experience often latch onto stability test values as an absolute indicator of stability. However, our experience indicates that occasionally dangerous avalanche conditions exist when stability tests show a stable snowpack and that sometimes the snowpack is more stable than , stability tests indicate. In order to partially address the difficulties in test interpretation, we have begun to qualitatively define the quality of the shear failure into three separate categories. This paper will focus on combining the interpretation of rutschblock and stuffblock tests, and describing the shear failure quality, thereby giving useful qualitative field information about the avalanche conditions in a given location. In the end, users must realize that stability tests do not provide a numerical description of the snow stability, but are instead just a piece of the puzzle for avalanche prediction.

Avalanches may be important hydrologically if avalanche deposits alter the timing and volume of runoff. Avalanches often fail from a weak layer such as surface hoar, graupel, or faceted crystals. One important type of faceted crystal. called near-surface faceted crystals. form in the near-surface layers. During the 1997/98 winter. A study of near-surface faceted crystals was conducted along the Red Mountain Pass corridor, Colorado. Six types of near-surface faceted crystals were differentiated: small faceted crystals, radiation recrystallization grains. Faceted precipitation crystals, near-surface hoar, faceted partly-decomposed precipitation crystals and needles. A particularly well-developed near-surface facet layer that evolved in December acted as the dominant weak layer in the study area for seven weeks. Observation of 14 avalanches showed that 79% of the failures occurred on near-surface faceted layers. The majority of stability test failures occurred on near-surface faceted layers. Understanding the growth of near-surface faceted crystals and their effect on snow stability is important for avalanche forecasting

This study examines the effect of forest clearing for ski runs and artificial snowmaking on snow water accumulation at of Big Sky recreation area in southwest Montana. Extensive research shows that clearing the forest canopy for timber harvest increases water stored in the snowpack, but no studies have compared water stored in the compacted, skied, and machine groomed snowpack of ski runs with the water stored under the natural forest canopy. Thirty six sampling locations were selected, terrain variables (elevation, aspect and slope angle) were surveyed, and snow depth, density and water equivalence were measured both under the forest canopy and on the adjacent ski run. Additionally, some of the sampling locations were in areas with artificial snowmaking. Snowpack on the run has a higher density and water equivalence, and a similar depth, to snow off the ski runs. Elevation is the primary terrain variable that affects snowpack characteristics, and increases in snow water equivalence (SWE) with increasing elevation are similar to previously published research. There was no significant difference between snow water storage on ski runs with snowmaking and those without snowmaking, although the snow density on the runs with artificial snow was higher. Total increase in snowpack water storage for all ski runs at Big Sky was greater than 700,000 m3. This number is close enough to values derived from a model designed to measure water yield due to forest cutting that it appears that the clearing of ski runs causes similar increases in snowpack water storage and water yield as forest clearing for timber harvests.

In the winter of 1995-96 we investigated the formation of faceted crystals that develop in the upper levels of the snowpack. We used an array of six thermocouples connected to a datalogger to measure hourly diurnal temperature changes in the region from 0.005 m above the snow surface to 0.20 m below the snow surface. Measurements during clear sky conditions in March showed temperature gradients in excess of 200 deg C/m at night in the top 0.05 m of the snowpack, with the temperature gradient shifting direction and exceeding 100 deg C/m through this layer during the day. A significant weak layer of faceted snow formed within 36 hours with a grain size of about 1 mm in the upper snowpack. Widespread avalanche activity occurred for up to nine days after this layer was buried by 0.50 m of snow.

The stuftblock is a new snow stability test developed and used operationally by the Gallatin National Forest Avalanche Center during the 1993-94 winter. Shortcomings of other stability tests, including the inability to effectively communicate results, the complexity of the test, and the time necessary to collect a measurement make the development of a new test desirable. The stuffblock is performed on an isolated column of snow about 0.30 m (l ft) square which is cut out of the wall of a snow pit. A nylon sack (stuff sack) is filled with 4.5 kg (10 lbs) of snow, which is measured with a lightweight scale. An avalanche shovel blade is placed on top of the isolated column and the stuff sack is first placed, then dropped onto the shovel from increasing heights. The drop height is increased in increments of 0.10 m. When shear failure occurs, the drop height is noted. Initial results from the 1993-94 field season indicate that, for snowpack conditions found in southwest Montana, a positive relationship exists between stuffblock drop heights and rutschblock numbers. While the stuffblock is not perfect, it is inexpensive, quick, easy, and provides numbers that can be readily compared between observers. This latter attribute is especially useful for regional avalanche forecasters who must often compare the results of several different observers with differing avalanche skills. For the individual avalanche worker, the stuffblock provides one more useful tool for snowpack stability evaluation.

The digital resistograph (DR) is a microprocessor-based probe which is used to rapidly obtain strength index profiles similar to the profiles obtained with the ram penetrometer. The current version of the DR can collect as many as 500 strength profiles in a work day. In this study the DR is compared to the ram penetrometer for repeatability, learnability and ease of use. Additional studies were conducted to determine just how sensitive the DR readings are to penetration rate. The test results did show that the instrument has a repeatability at least as good as the ram penetrometer. The DR has a decided edge in terms of learnability. Two novices were found to give roughly the same DR profiles after a minute of instruction, whereas the ram penetrometer is widely known to show a wide range of results when used by novices. One of the the primary problems with the DR was durability, as the instrument frequently malfunctioned and failed to work properly in cold conditions. The DR shows much promise for becoming a substantial improvement over the ram penetrometer, provided a more reliable and durable version can be engineered.