Snow Climates

Snow cover is variable on both the broad geographic scale (i.e., Antarctic snow is quite different from snow found in the Cascade Mountains of North America, which is quite different from the snow in the Southern Rocky Mountains of the United States) and the microscale (i.e., snow conditions may vary greatly from one side of a rock or tree to the other). All snow crystals are made of the same substance (i.e., water molecules), but local and regional environmental conditions control the type and character of the snow that is found at a given location.

To better understand the geographic differences that affect avalanche initiation, it is helpful to consider some basic climate conditions. The character of the snow and the avalanche propensity in different mountain ranges around the world can be described as one of three types—maritime, continental, or transitional—on the basis of the average snow conditions of that particular region.

Heavy snowfall and relatively mild temperatures characterize the maritime snow climate. The snow cover is deep and the new snow is dense as a result of moist ocean storms coming ashore at lower altitudes. Rain is common, and it is a significant cause of avalanches when it falls on deep fresh snow. Arctic air intrusions are uncommon but can occur each winter. In general, the snowpack gains strength quickly with time in this snow climate. In North America, examples of maritime ranges include the Sierras, Cascade Mountains, and Coast Range of British Columbia.

Far removed from coastal areas, the continental snow climate is characterized by low snowfall, cold temperatures, and higher altitudes. Snowfall is light and of low density, and wind is a key instigator of avalanches. Avalanche cycles are often the result of buried structural weaknesses that occur in shallow snowpacks and that may cause avalanche danger to persist for days, weeks, or even months. Avalanches that are released from these old, persistent, weak layers are a distinguishing trait of a continental climate. In this climate, especially during the early months of the winter, the shallow snow cover loses strength with time. Continental ranges include the Canadian Rocky Mountains, Southern Rocky Mountains, and Brooks Range.

In between the maritime and continental regions is a transitional snow climate that, in North America, is often referred to as the “intermountain” snow climate. Many mountain regions in this class tend to exhibit intermediate features that reflect both of the other types of climates (Table 2-1). Examples of these mountain ranges include the Wasatch Range in Utah, Teton Range in Wyoming, and Columbia Mountains of British Columbia.

TABLE 2-1 Characteristics of Maritime, Transitional (Intermountain), and Continental Snow Climates on the Basis of 15-Year Means

Physical Properties

Although snow cover appears to be nothing more than a thick homogeneous blanket that covers the ground, it is in fact one of the most complex materials found in nature. It often exists concurrently in solid, liquid, and gaseous phases. Snow is highly variable; it may go through significant changes during relatively short periods as a result of environmental factors.

At any single site, the seasonal snow cover varies from top to bottom and results in a complex layered structure, the study of which is referred to as stratigraphy. Individual layers may be quite thick or very thin, and they may vary greatly in strength and in their ability to adhere to one another. In general, thicker layers represent consistent conditions during one storm, when new snow crystals that fall are of the same type, when wind speed and direction vary little, and when temperature and precipitation are fairly constant. Thinner layers that are perhaps only millimeters in thickness often reflect conditions between storms. Examples include a melt-freeze crust or sun crust that is formed during fair weather or a hard wind crust that is formed during a period of strong winds. A brittle buried surface hoar layer represents what were once delicate feather-shaped crystals of surface hoar on the surface of the snow that were deposited from the moist atmosphere onto the cold snow surface during a clear night. These crystals are very fragile and weak; after they are buried by subsequent snowfalls, they may be a persistent and major contributor to avalanche formation.

One property of snow is strength or hardness, which is of great importance in terms of avalanche formation. Snow can vary from light and fluffy, easy to shovel, and especially delightful to ski through to heavy and dense, impossible to penetrate with a shovel, and firm enough to make it very difficult for a skier to carve a turn. The arrangement of the ice skeleton (i.e., the lattice of ice crystals within the snowpack) and the changing density (i.e., the mass per unit volume, which is typically represented as kg/m³) produce this wide range of conditions. In the case of snow, density is determined by the volume mixture of ice crystals and air. In general, the denser the snow layer, the harder and stronger it becomes, as long as it is not melting.

The density of new snow can have a wide range of values. This depends on how closely the new snow crystals pack together, which is controlled by the shapes of the crystals. The initial crystals that fall from the atmosphere have a variety of shapes, and some pack more closely together than do others (Figure 2-1). For example, needles pack more closely than stellars; thus the density of snow made of compressed needles may be three to four times greater than that of snow composed of stellars.

FIGURE 2-1 International classification of solid precipitation.

(From the International Association of Scientific Hydrology, with permission.)

Wind can alter the shapes of new snow crystals and break them into much smaller pieces that pack very closely together to form wind slabs. Such slabs may possess a density that is five to ten times that of new stellars falling in the absence of wind. Because these processes occur at different times and locations at the surface of the snow cover and are buried by subsequent snowfalls, a varied heterogeneous layered structure results. Thus minor variations in atmospheric conditions can have an important influence on the properties of snow on the ground.

After snow has been deposited on the surface, snow density increases as the snow layer compacts vertically as a result of the effects of gravity, weather, and crystal metamorphosis (i.e., settlement). Because an increase in density often equals an increase in strength, the rate at which this change occurs is important with respect to avalanche potential. Snow is composed predominantly of air pockets within an ice skeleton of crystals, and it is therefore highly compressible. In a typical layer of new snow, 85% to 95% of the volume is comprised of air pockets, and it can settle under its own weight. Individual ice crystals can move and slide past each other, and, because the force of gravity causes them to move slowly downward, the layer shrinks. A heavier snow layer or a warmer temperature speeds settlement.

At the same time, the complex and intricate shapes that characterize the new snow crystals begin to change. They become more rounded and suitable for closer packing. Intricate crystals (e.g., stellars) possess a shape that is naturally unstable and that changes quickly. New snow crystals have a large surface area–to–volume ratio and are composed of a crystalline solid that is close to its melting point. In this regard, snow crystals are unique among the materials that are found in nature. Surface energy physics dictate that this unstable condition will lead to a change in the crystalline shape toward an energy equilibrium; in other words, the warmer the temperature, the faster the change. Under very cold conditions, the original shapes of the snow crystals are sometimes still recognizable after they have been in the snow cover for several days or even after a week or two. However, as temperatures warm and approach the melting point, such shapes disappear within a few hours to a day. Changes in the shape or texture of snow crystals are examples of metamorphism, which in geologic terms defines changes that result from the effects of temperature and pressure. As the crystal shapes simplify, they can pack more closely together, thereby enhancing further settlement and strength (Figure 2-2, online).

FIGURE 2-2 Settlement. As the crystal shapes become more rounded, the crystals can pack more closely together, and the layer settles or shrinks in thickness.

New snow metamorphic changes generally occur within hours to a few days. However, the structure of a seasonal snow cover changes over a period of weeks to months via other processes. Settlement, which may initially have been rapid, continues at a much slower rate. Other factors begin to exert dominant influences on metamorphism throughout the snowpack. These factors include the difference in temperature measured upward or downward in the snow layer, which is called the temperature gradient.

Averaged diurnally, snow temperatures are generally coldest near the surface and warmest near the ground at the base of the snow cover, which creates a temperature gradient across a snow layer that is sandwiched between cold winter air and the relatively warm ground (Figure 2-3, online). The effect of temperature gradients is an ongoing dynamic process that can cross ice, large empty spaces filled with air, and dense snow.

FIGURE 2-3 When an insulating layer of snow separates the warm ground from the cold air, a temperature gradient develops across the snow layer.

Warm air contains more water vapor than does cold air; this holds true for air that is trapped within the snow cover. The greater the amount of water vapor, the greater the pressure; therefore both a pressure gradient and temperature gradient exist through the snow cover. When a pressure difference exists, the difference naturally tends to equalize, just as adjacent high and low atmospheric pressure centers cause the movement of air masses. Pressure differences within snow cause vapor to move upward through the snow layers. The air within the layers of the snow cover (i.e., in the pore spaces between grains) is saturated with water vapor, with a relative humidity of 100%. When air moves from a warmer to a colder layer, the amount of water vapor that can be supported in the pore spaces diminishes. Some vapor changes to ice and is deposited on the surrounding ice grains. We witness a similar process when warm, moist air in a heated room comes in contact with a cold windowpane. The invisible water vapor is cooled to its freezing point, and some of the vapor changes state and is deposited as frost on the window.

Figure 2-4 shows how the texture of the snow layer changes during this temperature-gradient process. Water molecules sublimate from the upper surfaces of a grain. The vapor moves upward along the temperature (and vapor) gradient and is deposited as a solid ice molecule on the underside of a colder grain above. This process will continue as long as a strong temperature gradient exists. If the gradient continues long enough, all grains in the snow layer are transformed from solid to vapor and back to solid again; in other words, they recrystallize. The new crystals are completely different in texture and shape from their initial form. They become loose, coarse crystals with faceting, straight sides, and sharp angles (also known as faceted crystals or sugar snow), and they may eventually evolve into a large, striated, hollow-cup form. Examples of these crystals are shown in Figure 2-5. This process is called temperature-gradient metamorphism or kinetic metamorphism. Well-developed crystals that typically form at the basal layer of the snow cover are commonly known as depth hoar. Depth hoar and faceted crystals are of particular importance to avalanche formation, because these crystals are very weak, with little or no cohesion (bonding) at the grain contacts. They can form the weak layer that fails under a slab and causes an avalanche.

FIGURE 2-4 In the temperature-gradient process, ice sublimates from the top of one grain, moves upward as water vapor, and then is deposited on the bottom surface of the grain above. If conditions allow this process to continue long enough, all of the original grains are lost as the recrystallization produces a layer of new crystals.

FIGURE 2-5 A, Mature depth-hoar grains. Facets and angles are visible. Grain size, 3 to 5 mm (0.12 to 0.20 inch). B, Advanced temperature-gradient grains attain a hollow cup-shaped form. Size, 4 mm (0.16 inch).

(Courtesy Doug Driskell.)

Kinetic Metamorphism

In the presence of a strong temperature gradient (e.g., ≥1° C [1.8° F] per 10 cm [3.9 inches]), kinetic metamorphism can occur anywhere within a snowpack. Typically, temperature gradients are described as extending upward from the warm ground toward the colder snow surface; however, very strong temperature gradients on the order of 2° C (1.8° F) per centimeter or more (20° C [18° F] per 10 cm [3.9 inches]) can extend for a couple of centimeters from a warm melt-freeze crust into colder snow. Strong temperature gradients can exist in the upper 10 to 30 cm (3.9 to 11.8 inches) of the snowpack, which is often related to the effects of alternating incoming solar radiation (short-wave radiation) and outgoing radiational cooling (long-wave radiation) resulting in near-surface facets. These coarse and poorly bonded loose crystals, which are typically 1 to 3 mm (0.04 to 0.12 inch) in size, have straight sides and sharp angles. Backcountry skiers often describe this snow as “recycled powder,” because it often forms on colder and northerly facing slopes with prolonged periods of cold and clear conditions. A temperature gradient forms because radiational cooling (which occurs often—but not exclusively—on clear nights) causes the surface snow to become very cold relative to the snow 10 to 30 cm (3.9 to 11.8 inches) below the surface, which changes more slowly and which may still be under the effects of daytime warming.

In the colder temperatures of continental snow climates, strong temperature gradients often exist just above the warm ground and can persist for months when the snow cover remains shallow (typically <1 m [3.3 feet] deep). The persistent gradient and warm basal snow layer temperature can cause very large and complex faceted grains to form as well-developed depth hoar. These large, cup-shaped crystals, which are typically 4 to 10 mm [0.16 to 0.39 inch] in size, are characterized by straight sides, sharp angles, and multiple layers of faceted faces. The grains can be described as looking like etched crystal glass, and their organized striations can make them sparkle.

The strong temperature gradients that drive kinetic metamorphism are aided by low-density new snow, because the larger pore spaces allow for the easier migration of water vapor. Typical faceted crystals or grains, which were previously referred to as temperature-gradient snow, are commonly called squares. Depth hoar is reserved for well-developed, cup-shaped, and large faceted crystals. Any faceted layer in the snow cover can lead to a persistent weak layer, especially layers of larger grains, which are slower and more resistant to change or to gaining strength.

Equilibrium Metamorphism

In the absence of a strong temperature gradient, a totally different type of snow texture develops. When the gradient is less than about 1° C (1.8° F) per 10 cm (3.9 inches), there are still vapor pressure differences, but upward movement of vapor through the snow layers occurs at a much slower rate. As a result, water vapor that is deposited on a colder grain tends to cover the total grain in a more homogeneous manner rather than showing the preferential deposition that is characteristic of faceted crystals. This process produces a grain with a smooth surface of a more rounded or oblong shape. Over time, vapor is deposited at the grain contacts (concavities) as well as over the remaining surface of the grain (convexities). Connecting bonds that are formed at the grain contacts give the snow layer strength over time (Figure 2-6, online). Bond growth, which is called sintering, yields a cohesive texture, which is in contrast to the cohesionless texture of depth hoar and other forms of faceted crystals. This type of grain has been referred to by various terms: equilibrium snow, equitemperature snow, equilibrium metamorphism, or simply rounds. These grains can generally be described as fine and well-sintered (bonded) snow. Such bonded and interconnected grains are shown in Figure 2-7. Weak temperature gradients and high-density new snow force water vapor molecules to form bonds and thus drives equilibrium metamorphism.

FIGURE 2-7 Bonded or sintered grains that result from equitemperature metamorphism. Grain size, 0.5 to 1 mm (0.02 to 0.04 inch).

(Courtesy Doug Driskell.)

FIGURE 2-6 Equitemperature grain growth. In the presence of weak temperature gradients, bonds grow at the grain contacts.

The preceding paragraphs described the big picture in terms of what happens to snow layers after they have been buried by subsequent snowfalls. If the temperature layer is below freezing and no melting is taking place, one of the two processes described previously is occurring, or perhaps a transition exists between the two. Within the total snow cover, these metamorphic processes may occur simultaneously, but only one can take place within a given layer at a given time, depending upon the strength or weakness of the temperature gradient. Both processes accelerate with warmer snow temperature, because more water vapor is involved.

Avalanche Types

There are two basic types of avalanche release. The first is a point-release avalanche or a loose snow avalanche (Figure 2-8). A loose snow avalanche involves cohesionless snow; it is initiated at a point, and it spreads out laterally as it moves down the slope to form a characteristic inverted “V” shape. A single grain or a clump of grains slips out of place and dislodges those below on the slope, which in turn dislodge others. The avalanche continues as long as the snow is cohesionless and the slope is steep enough. In dry snow, this type of avalanche usually involves only small amounts of near-surface snow. However, in wet snow, which is caused by warm air temperatures or rain, these avalanches can be very large and destructive.

FIGURE 2-8 Loose snow or point-release avalanche.

(Courtesy U.S. Department of Agriculture Forest Service.)

The second type of avalanche, the slab avalanche, requires a cohesive layer of stronger snow over a layer of weaker snow. The cohesive blanket of snow breaks away simultaneously over a broad area (Figure 2-9). A slab release can involve a range of snow thicknesses, from the near-surface layers to the entire snow cover down to the ground. Slab avalanches can occur in dry or wet snow. In contrast with a loose snow avalanche, a slab avalanche has the potential to involve very large amounts of snow.

FIGURE 2-9 Slab avalanche.

(From the U.S. Department of Agriculture Forest Service; Williams K, Armstrong B: The snowy torrents, Jackson, Wyo, 1984, Teton Bookshop, with permission. Top courtesy Alexis Kelner.)

Because dry slab avalanches are responsible for 95% of U.S. fatal accidents, these avalanches receive the interest of researchers.⁴ The majority of the information in this chapter deals with dry slab avalanches or dry loose snow avalanches. Because wet snow avalanches have received little research, relatively little is known about the processes that cause these avalanches. Recent research reaffirms the challenges to “observe, measure, and quantify the characteristics leading to wet snow avalanches.”³²

Slab Avalanche Formation

To understand the conditions of snow cover that contribute to dry slab avalanche formation, it is essential to reemphasize that snow cover develops layer by layer. The layered structure is directly tied to the two ingredients that are essential to the formation of slab avalanches: the cohesive layer of snow and the weak layer beneath. If the snow cover is homogeneous from the ground to the surface, slab avalanche danger is low, regardless of the snow type. If the entire snow cover is sintered, dense, and strong, stability is very high. Even if the entire snow cover is composed of depth hoar, there is still no hazard from slab avalanches, because the cohesionless character precludes formation of a slab, which is one of the essential ingredients. Loose snow avalanches may still occur in this situation on steep slopes. The combination of a basal layer of depth hoar with a cohesive layer above provides the ingredients for slab avalanche danger. For successful evaluation of slab avalanche potential, information is needed about the entire snowpack and not just its surface. A hard wind slab at the surface may intuitively appear strong and safe. However, when it rests on a weaker layer that may be well below the surface, it may fail under the weight of a skier and be released as a slab avalanche. Many snow structure combinations can contribute to slab avalanche formation, but the prerequisite conditions are a cohesive layer over a weak layer sitting on a bed surface. Figure 2-10 describes other combinations that result in brittle or cohesive layers of snow on a weak layer.

FIGURE 2-10 Snow-layer combinations that often contribute to avalanche formation.

Mechanical Properties: How Snow Deforms On a Slope

Almost all physical properties of snow can be easily seen or measured in the field. A snow pit provides a wealth of information about these properties, layer by layer, throughout the thickness of the snow cover; however even detailed knowledge of these properties does not provide all of the information that is necessary to evaluate avalanche potential. The current mechanical state of the snow cover must be considered. Unfortunately, for the average person, these properties are virtually impossible to directly measure.

Mechanical deformation occurs within the snow cover just before its failure and the start of a slab avalanche. Snow cover has a tendency to settle simply from its own weight. When this occurs on level ground, the settlement is perpendicular to the ground, and the snow layer increases in density and gains in strength. The situation is not so simple when snow rests on a slope. The force of gravity is divided into two components: one that tends to cause the snow layer to shrink in thickness and a new component that acts parallel to the slope, which tends to pull the snow down the slope. Down-slope movement within the snow cover occurs at all times, even on gentle slopes. The speed of movement is slow, generally on the order of a few millimeters per day, but it can be up to millimeters per hour within new snow on steep slopes or with warming temperatures. The evidence of these forces is often clearly visible in the bending of trees and damage to structures built on snow-covered slopes. Although the movement is slow, when deep snow pushes against a rigid structure, the forces are significant, and even large buildings can be pushed off of their foundations.

Snow deforms in a highly variable fashion, and is described as a viscoelastic material. Sometimes it deforms as if it were a liquid (viscous), and at other times it responds more like a solid (elastic). Viscous deformation implies continuous and irreversible flow. Elastic deformation implies that, after the force that is causing the deformation is removed, some part of the initial deformation is recovered. The elasticity of snow is not so obvious, primarily because the amount of rebound is very small as compared with those of more familiar materials.

With regard to avalanche formation, it is important to know when snow acts primarily as an elastic material and when it responds more like a viscous substance. These conditions are shown in Figure 2-11. Laboratory experiments have shown that conditions of warm temperatures and the slow application of force favor viscous deformation. One sees examples of this as snow slowly deforms and bends over the edge of a roof or sags from a tree branch. In such cases, the snow deforms but does not crack or break. By contrast, when temperatures are very cold or when force is applied rapidly, snow reacts like an elastic material. If enough force is applied, it fractures. We think of such a substance as brittle; release of stored elastic energy causes fractures to move through the material. In the case of snow cover on a steep slope, forces associated with accumulating snow or the weight of a skier may increase until the snow fails. At that point, stored elastic energy is released and is made available to drive brittle fractures over great distances through the snow slab.

FIGURE 2-11 Depending on prevailing conditions, snow may deform and stretch in a viscous or flowing manner, or it may respond more like a solid and thus fracture.

The slab avalanche provides the best example of elastic deformation in snow cover. Although the deformation cannot actually be seen, evidence of the resultant brittle failure is clearly present in the form of the sharp, linear fracture line and crown face of the slab release (Figure 2-12). The crown face is almost always perpendicular to the bed surface, which is evidence that snow has failed in a brittle manner.

FIGURE 2-12 The consistent 90-degree angle between the crown face and the bed surface of the avalanche shows that slab avalanches result from an elastic fracture.

(Courtesy A. Judson.)

To fully understand the slab avalanche condition or the stability of the snow cover, its mechanical state must be considered. Snow is always deforming in a down-slope fashion, but, throughout most of the winter, the strength of the snow is sufficient to prevent an avalanche. The snow cover is layered, and some layers are weaker than others. During periods of snowfall, blowing snow, or both, an additional load or weight is being applied to the snow in the starting zone, the snow is creeping faster, and these new stresses are beginning to approach the strength of the weakest layers. The weakest layer has a weakest point somewhere within its continuous structure. If the stresses caused by the load of the new snow or the weight of a skier reach the level at which they equal the strength of the weakest point, the snow fails completely at that point (Figure 2-13); this means that the strength at that point immediately goes to zero. This is analogous to what would happen if someone on a tug-of-war team were to let go of the rope. If the remainder of the team was strong enough to make up for the lost member, not much would change immediately. The same situation exists with snow cover. If the surrounding snow has sufficient strength to make up for the fact that the strength at the weakest point has now gone to zero, nothing happens beyond perhaps a localized movement or collapse in the snow, which is often heard as a “whumpf” sound. If the surrounding snow is not capable of compensating, the area of snow next to the initial weak point fails, then at the next, and so forth, until a propagating chain reaction begins.

FIGURE 2-13 Slab avalanche released by a skier. A skier is descending a slope (A) and causes the release of a slab of avalanche (B), but is able to ski off to the side to escape (C).

(Courtesy R. Ludwig.)

As the initial crack forms in the now unstable snow, the elastic energy is released, which in turn drives the crack further and releases more elastic energy. The ability of snow to store elastic energy is essentially what allows large slab avalanches to occur. As long as the snow properties are similar across the avalanche starting zone, the crack will continue to propagate, thereby allowing entire basins that are many acres in area to be set in motion within a few seconds.

Slope Angle

Avalanches occur with greatest frequency on slopes of 30 to 45 degrees. These are the angles at which the balance between strength (i.e., the bonding of the snow trying to hold it in place) and stress (i.e., the force of gravity trying to pull it loose) is most critical. The easiest way to create high stress is to increase the slope angle; gravity works that much harder to stretch the snow out and to rip it from its underpinnings. A slope of 45 degrees produces many more avalanches than a slope of 30 degrees. On even steeper slopes (i.e., >45 degrees), the force of gravity wins; snow generally rolls or sloughs off, thus preventing the buildup of deep snowpacks. Exceptions exist, especially in maritime snow climates or when strong winds plaster damp snow onto steep slopes. Dry snow avalanches have occurred on slopes of 22 to 25 degrees, which is the angle of repose for granular round substances like sand; however these are rare, because snow grains are seldom round and seldom touch without forming bonds. Although an avalanche release requires a steep slope, it is possible to trigger an avalanche from shallow and even flat slopes at the bottom of steep slopes. A collapse in a persistent weak layer in these areas could send fractures upslope, thus releasing the avalanche. This is analogous to pulling a log out from the bottom of a wood pile.

When snow is thoroughly saturated with water, a slush mixture is formed, and an avalanche can release on low-angle terrain. For example, a wet snow avalanche in Japan occurred on a beginner slope at a ski area. The slope was only 10 degrees, but the avalanche was large enough to kill seven skiers. This extreme situation applies only to a water-saturated snowpack, which behaves more like a liquid than a solid.

Orientation

Avalanches occur on slopes facing every point of the compass. Steep slopes are equally likely to face east, west, north, or south. In the northern hemisphere, certain factors cause more avalanches to occur on slopes that are facing north, northeast, and east than on those facing south through west; these relate to slope orientation with respect to sun and wind. In the southern hemisphere, more avalanches occur on east, southeast, and south-facing slopes. The sun angle in northern hemisphere winters causes south slopes to get much more sunshine and heating than do north slopes, which frequently leads to radically different snow covers. North slopes have deeper and colder snow covers, often with a substantial layer of depth hoar near the ground. South slopes usually carry a shallower and warmer snow cover that is laced with multiple ice layers that are formed on warm days between storms. Most ski areas are built on predominantly north-facing slopes to take advantage of deeper and longer-lasting snow cover. At high latitudes (e.g., in Alaska), the winter sun is so low on the horizon and the heat input to south slopes is so small that there are few differences in the snow covers of north and south slopes.

The effect of the prevailing west wind at mid latitudes is important. Storms most often move west to east, and storm winds are most frequently from the western quadrant (i.e., southwest, west, or northwest). Storm winds pick up fallen snow and redeposit it on slopes that are facing away from the wind (i.e., northeast, east, and southeast slopes). These are the slopes that are most often overburdened with wind-drifted snow. The net effect of sun and wind is to cause more avalanches on north-facing through east-facing slopes.

Avalanche Terrain Paths

The frequency with which a path produces avalanches depends on a number of factors, with slope steepness being a major one. The easiest way to create high stress is to increase the slope angle; gravity works that much harder to stretch out the snow and to rip it from its underpinnings. A slope of 45 degrees produces many more avalanches than does one of 30 degrees, however, specific terrain features are also important.

Broad slopes that are curved into a bowl shape and narrow slopes that are confined to a gully efficiently collect snow. Those that have a curved horizontal profile, such as a bowl or gully, trap blowing snow that is coming from several directions; the snow drifts over the top and settles as a deep pillow. Alternatively, the plane-surfaced slope collects snow efficiently only if it is being blown directly from behind. A side wind scours the slope more than it loads the slope.

The surface conditions of a starting zone often dictate the size and type of avalanche. A particularly rough ground surface (e.g., a boulder field) does not usually produce avalanches early in the winter, because it takes considerable snowfall to cover the ground anchors. After most of the rocks are covered, avalanches pull out in sections, with the area between two exposed rocks running one time and the area between another two other rocks running another. A smooth rock face or a grassy slope provides a surface that is usually too slick for snow to grip; therefore full-depth avalanches are distinctly possible. If the avalanche does not run during the winter, it is likely to run to ground in the spring after melt water percolates through the snow and lubricates the ground surface.

Vegetation

Vegetation has a mixed effect on avalanche releases. Bushes provide anchoring support until they become totally covered; at that point, they may provide weak points in the snow cover, because air circulates around the bush and provides an ideal habitat for the growth of depth hoar. It is common to see that the fracture line of an avalanche has run from a rock to a tree to a bush, because these are all places of healthy depth hoar growth.

A dense stand of trees can easily provide enough anchors to prevent avalanches. The reforestation of slopes that are devoid of trees as a result of logging, fire, or avalanche is an effective means of avalanche control. Scattered trees on a gladed slope offer little if any support to hold snow in place. Isolated trees may do more harm than good by providing concentrated weak points on the slope.

Snowfall

New snowfall is the event that leads to most avalanches. More than 80% of all avalanches run during or just after a storm. Fresh snowfall adds weight to existing snow cover. If the snow cover is not strong enough to absorb this extra weight, avalanche releases occur. The size of the avalanche is usually related to the amount of new snow. Snowfalls of less than 15 cm (6 inches) seldom produce avalanches. Snows of 15 to 30 cm (6 to 12 inches) usually produce a few small slides, and some of these harm skiers who release them. Snows of 30 to 60 cm (1 to 2 feet) produce avalanches of larger size that present considerable threats to skiers and that pose closure problems for highways and railways. Snows of 60 to 120 cm (2 to 4 feet) are much more dangerous, and snowfalls of more than 120 cm (4 feet) produce major avalanches that are capable of large-scale destruction. These predictions are guidelines that are based on data and experience and must be considered with other factors to arrive at the true hazard. For example, a snowfall of 10 inches (25 cm) whipped by strong winds may be serious; a fall of 60 cm (2 feet) of feather-light snow in the absence of wind may produce no avalanches.

Snowfall Intensity

The rate at which snowfall accumulates is almost as important as the amount of snow. A snowfall of 90 cm (3 feet) in one day is far more hazardous than the same amount of snow falling over the course of 3 days. As a viscoelastic material, snow can absorb slow loading by deforming or compressing. Under a rapid load, the snow cannot deform quickly enough and is more likely to crack, which is how slab avalanches begin. A snowfall rate of 2.5 cm (1 inch) or more per hour that is sustained for 10 hours or more is generally a red flag with regard to avalanche danger; the danger worsens if the snowfall is accompanied by wind.

Rain

The age of the surface snow is an important factor with regard to changes in stability when rain falls. Within hours of light rain falling on fresh or recent snow (i.e., from within several days), avalanching can occur for reasons that are not well understood. The rain somehow causes a loss of strength in the new snow. Significant rain (usually about 2.5 cm [1 inch]) falling on an old snow surface (i.e., several days old or older) produces few or no avalanches. Despite the apparent addition of weight caused by rain (2.5 cm [1 inch] of rain is the equivalent in weight to 25 to 30 cm [10 to 12 inches] of snow), the rain tends to drain down through the older snow grains and invariably freezes into an ice crust, which adds strength to the snow cover. At a later time, the smooth crust could become a sliding layer beneath new snowfall. Heavy rain (usually >2.5 cm [1 inch]) greatly weakens the snow cover. It adds weight to the snowpack, but it adds no internal strength of its own (in the form of a skeleton of ice, as new snow would create); it dissolves bonds between snow grains as it percolates through the top snow layers, thus reducing strength even further.

New Snow Density and Crystal Type

A layer of fresh snow contains only a small amount of solid material (ice); the majority of the volume is occupied by air. It is convenient to refer to snow density as a percentage of the volume occupied by ice. New snow densities usually range from 7% to 12%, depending on the snow climate. In the high elevations of Colorado, 7% is an average value; in the more maritime climates of the Sierras and the Cascades, 12% is a typical value. Density becomes an important factor in avalanche formation when it varies from average values. Avalanche danger increases when heavier, more-dense snow falls on lighter, less-dense snow, which can occur from storm to storm or even within a single storm (sometimes called an upside-down storm).

Wet snowfalls or falls of heavily rimed crystals (e.g., graupel) may have densities of 20% or more. Graupel is a type of snowflake that has been transformed into a pellet of soft ice because of riming inside a cloud. A layer of heavier-than-normal snow presents a danger because of excess weight. Snowfall that is much lighter than normal (e.g., 2% to 4%) can also present a dangerous situation. If the low-density layer quickly becomes buried by snowfall of normal or high density, a weak layer has been introduced into the snowpack. By virtue of low density, the weak layer has a marginal ability to withstand the weight of layers above, thereby making it susceptible to collapse. Storms that begin with low temperatures but then warm up produce an unstable layer of weak, light snow beneath a stronger, heavier layer, thereby acting as a slab and thus as one of the necessary ingredients for an avalanche.

Density is closely linked to crystal type. Snowfalls that consist of graupel, fine needles, and columns can accumulate at high densities. Snowfalls of plates, stellars, and dendritic forms account for most of the lower densities.

Wind Speed and Direction

Wind can transport snow into avalanche starting zones at rates that are much greater than can snow falling from clouds. Wind drives fallen snow into drifts and cornices from which avalanches begin. Winds pick up snow from exposed windward slopes and drive it onto adjacent leeward slopes, where it is deposited into sheltered hollows and gullies; this can quickly turn a 1-foot snowfall into a 3-foot drift in a starting zone. The rate at which blowing snow collects in bowls and gullies can be impressive. In one test at Berthoud Pass, Colorado, the wind deposited snow in a gully at a rate of 45 cm (18 inches) per hour.

A speed of 7 m/s (15 mph) is sufficient to pick up freshly fallen snow. Higher speeds are required to dislodge older snow. Speeds of 9 to 22 m/s (20 to 50 mph) are the most efficient for transporting snow into avalanche starting zones. Speeds of more than 22 m/s (50 mph) can create spectacular banners of snow streaming from high peaks, but much of this snow is lost to sublimation in the air or is deposited far down the slope away from the avalanche starting zone.

Winds also increase the avalanche potential, because blowing snow is denser after deposition than before transport. This is because snow grains are subjected to harsh treatment in their travels; each collision with another grain knocks off arms and rounds sharp angles, thereby reducing the grain’s size and allowing the pieces to settle and pack together into a denser layer. The net result of wind is to fill avalanche starting zones with additional, heavier, and more cohesive snow than if the wind had not blown.

Temperature

The role of temperature in snow metamorphism occurs over a period of days, weeks, and even months. The influence of temperature on the mechanical state of the snow cover is more acute, with changes occurring in minutes to hours. The actual effect of temperature is not always easy to interpret; whereas an increase in temperature may contribute to the stabilization of the snow cover in one situation, it might at another time lead to avalanche activity.

In several situations, an increase in temperature clearly produces an increase in avalanche potential. In general, these conditions include a rise in temperature during a storm or immediately after a storm or a prolonged period of warm, fair weather such as occurs with spring conditions. In the first example, the temperature at the beginning of a snowfall may be well below freezing, but, as the storm progresses, the temperature increases. As a result, the initial layers of new snow are light, fluffy, low density, and relatively low in strength, whereas the later layers are warmer, denser, and stiffer. Thus, the essential ingredient for a slab avalanche is provided within the new snow layers of the storm: a cohesive slab resting on a weak layer. If the temperature continues to rise, the falling snow turns to rain; this situation is not uncommon in lower-elevation coastal mountain ranges. When this happens, avalanches are almost certain; as the rain falls, additional weight is added to the avalanche slope, but no additional strength is provided as it is whenever a new layer of snow accumulates.

The second example may occur after an overnight snowstorm that does not produce an avalanche on the slope of interest. By morning, the precipitation stops, and clear skies allow the morning sun to shine directly on the slopes. The sun rapidly warms the cold, low-density, new snow, which begins to deform and creep down slope. The new snow layer settles, becomes denser, and gains strength. At the same time, it is stretched downhill, and some of the bonds between the grains are pulled apart; thus the snow layer becomes weaker. If more bonds are broken by stretching than are formed by settlement, there is not enough strength to hold the snow on the slope, and an avalanche occurs.

In these first two examples, the complete snow cover generally remains at temperatures that are below freezing. A third example occurs when a substantial amount of the winter’s snow cover is warmed to the melting point. During winter, sun angles are low, days are short, and air temperatures are cold enough that the small amount of heat gained by the snow cover during the day is lost during the long cold night. As spring approaches, this pattern changes, and eventually enough heat is available at the snow surface during the day to cause some melt. This melt layer refreezes again that night, but the next day more heat may be available, so eventually a substantial amount of melting occurs and melt water begins to move down through the snow cover. As melt water percolates slowly downward, it melts the bonds that attach the snow grains, and the strength of the layers decreases. At first the near-surface layers are affected, with the midday melt reaching only as far as the uppermost few inches and with little or no increase in avalanche hazard. If warm weather continues, the melt layer becomes thicker, and the potential for wet snow avalanches increases. The conditions that are most favorable for wet slab avalanches occur when the snow structure provides the necessary layering. When melt water encounters an ice layer or an impermeable crust—or, in some cases, a layer of weak depth hoar—wet slab avalanches are likely to occur.

Depth of Snow Cover

Of the snowpack factors that contribute to avalanche formation, the depth of the snow cover is the most basic. When the early winter snowpack covers natural anchors (e.g., rocks, bushes), the start of the avalanche season is at hand. However a word of caution is necessary for backcountry travelers in the weak, depth-hoar–prone, continental snow climate. The early winter avalanche danger starts not when the natural anchors are covered but when the snow fills in the spaces between the obvious anchors. North-facing slopes are usually covered before other slopes. A scan of the terrain usually suffices to discern this clue, but another method can be used to determine the time of the first significant avalanches. Long-term studies show a relationship between snow depth at a study site and avalanche activity. For example, along Red Mountain Pass, Colorado, it is unlikely that an avalanche large enough to reach the highway will run until close to 90 cm (3 feet) of snow covers the ground at the University of Colorado’s snow study site. At Alta, Utah, when 130 cm (52 inches) of snowpack has built up, the first avalanche to cover the road leading from Salt Lake City can be expected.

Weak Layers

Any layer that is susceptible to failure and fracture as a result of the overburden of additional weight is a weak link. Of the snowpack contributory factors, this is the most important, because a weak layer is essential to every avalanche. Fracture in the weak layer propagates along what is called the failure plane, sliding surface, or bed surface.

The most troubling layers are called persistent weak layers, which are usually made of snow crystals and include larger faceted snow, depth hoar, and surface hoar. These crystals are slow to change shape and gain strength, so they may persist for days, weeks, months, or even all season. One common weak layer is an old snow surface that offers a poor bond for new snow. Another weak layer that forms on the snow surface is hoar frost or surface hoar (see Physical Properties, earlier). On clear and calm nights, this hoar forms a layer of feathery, sparkling flakes that grow on the snow surface. The layer can be a major contributor to avalanche formation when it is buried by snowfall as a result of its frequent persistence in the snowpack. Many avalanches have been known to release from a buried layer of surface hoar, and sometimes this layer is more than 1 month old and 180 cm (6 feet) or more below the surface.

A weak layer that is almost always found deep within the snowpacks that blanket the Colorado Rocky Mountains (continental snowpack) is large temperature-gradient snow (facets) or depth hoar. One way to decide whether a temperature-gradient layer is near its collapse point is to test the strength of the overlying layers and the support provided by specific stability testing performed near the edge of the slope. This is no easy task, and results may be unreliable as a result of the spatial variability of weaknesses on the slope. In addition, most field-stability tests do not test for deep instabilities in the snowpack. Another method is to try jumping on your skis while standing on a shallow test slope of similar aspect. Collapse is a good indication that comparable snow cover on a steeper slope will produce an avalanche. Often skiers and climbers cause inadvertent collapses while skiing or walking on a depth-hoar–riddled snowpack. The resulting “whumpf” sound is a warning of weak snow below.

In recent years, attention to the frequency of faceted snow layers near the surface of the snowpack and their relation to avalanches—particularly in the deeper snow cover of the transitional (intermountain) snow climate—have been well studied. Several mechanisms involved in the evolution of these layers may account for the majority of avalanches in this region, because significant depth hoar-related avalanches are less common.

Graupel, which are pellet-like heavily rimed crystals, can act as ball bearings after they are buried in the snowpack; they can be responsible for the layer that is involved in avalanche initiation.

Finally, a weak layer can be created within the snow cover when surface melting or rain causes water to percolate into the snow and then fan out on an impermeable layer, thereby lubricating that layer and destroying its shear strength. This phenomenon can be seen during the winter months in the maritime snow climate of the West Coast mountains.

Combining the constellation of contributory factors on a day-by-day basis is the avalanche forecaster’s art. Every avalanche must have a weak layer on which to release, so knowledge of snow stratigraphy or layering and what level of applied load will cause a layer to fail form the essence of forecasting.

Identifying Avalanche Terrain

Because most avalanches release on slopes of 30 to 45 degrees, judging angle is a prime skill for the recognition of potential avalanche areas. An inclinometer can be used to measure slope angles. Some compasses are also equipped for this purpose; a second needle and a graduated scale in degrees can be used to measure slope angles. A ski pole may be used to judge approximate slope angle. When dangled by its strap, the pole becomes a plumb line from which the slope angle can be “eyeballed.”

Evidence of fresh avalanche activity (i.e., the presence of fracture lines and the rubble of avalanche snow on the slope or at the bottom) identifies avalanche slopes. Other clues are swaths of missing trees or trees that are bent downhill or damaged, especially with the uphill branches removed. Above the tree line, steep bowls and gullies are almost always capable of producing avalanches.