Introduction

Importance of Snowpack

Snowpack is the lifeblood of the U.S. Rocky Mountain west and other mountainous areas of the world. Although mountain ranges cover a very small fraction of the Earth's land regions, the water stored in their snowpack feeds lakes and rivers, which irrigate crops and supply water for many of the world's heavily populated areas.

Ski areas bring in tourist dollars that bolster or even form the backbone of local economies.

Snowpack is not always beneficial though. Large amounts of money are spent on public transportation and safety during snow events, and dozens of people are killed annually in avalanches.

As winter transitions to spring, rapid snowmelt can cause flooding and flash flooding, even when atmospheric conditions are dry. In addition, dry winters can worsen drought conditions.

Snowpack assessment is the measurement and interpretation of snowpack. It is needed by hydrologists and those involved with commerce, public safety, and recreation to deal with the concerns outlined above. The data are also critical to climate researchers who use them to monitor climate trends. Scientists in other disciplines use snowpack data as well. For example, biologists use them to study the impact of snow on animal migration and the daily life cycles of burrowing animals, and chemists use them to examine how chemical components become part of the clouds, precipitate into snowpack, and are contained in runoff.

Perspectives on Snowpack

Dr. Ethan Greene is the director of the Colorado Avalanche Information Center (CAIC), which issues backcountry avalanche forecasts and educates the public about avalanche conditions and dangers. At various points throughout the module, Ethan will discuss topics related to snowpack development and evolution. He will also demonstrate manual snowpack assessment techniques. Here is his introduction.

Matt Kelsch is a meteorologist at the University Corporation for Atmospheric Research. Hear him discuss hydrologic concerns regarding snowpack.

Dr. Doug Wesley is a meteorologist at the University Corporation for Atmospheric Research. Here he talks about forecasting concerns and snowpack assessment.

Jay Irwin is a backcountry skier who survived an avalanche in 2009. Here's his story.

Module structure & goals

This module introduces you to the science of snowpack and its assessment. We begin by exploring the factors involved in snowpack development and evolution. The evolution section is structured around two scenarios differentiated by terrain: one takes place in a relatively flat area, the other in a mountainous region. The final section examines both in-situ (onsite) and satellite-based snowpack assessment techniques.

By the end of the module, you should be able to:

• Describe the key factors involved in snowpack development at the global, regional, local, and micro scales
• Describe the factors involved in snowpack evolution
• Describe the primary in-situ and remote sensing techniques used to assess and monitor snowpack

The module is intended for those interested in snowpack and its assessment, including weather forecasters, emergency managers, hydrologists, recreationalists, and climate researchers. To get the most out of the module, it is useful to have a basic understanding of Earth's surface and atmosphere, including geography, precipitation, wind, and temperature.

Snowpack Environment

Global

Introduction

Why do certain parts of the world receive a lot of snow while other places at similar latitudes are arid? The difference is due to climate, climate being the set of meteorological conditions that prevail in a particular place over a long period of time.

Climate is determined by a wide range of factors at the global, regional, and local scales. Examples are displayed on the graphic.

In this section, we will describe the types of snowpack found throughout the world and examine the global-scale factors that create and impact snowpack.

Climate Zones

Various schemes are used to categorize the world's climate zones, notably the Köppen-Geiger climate classification scheme.

Mouse over each classification name, in the text above the photo, to see its areas on the map.

Moist Tropical
(A)

Moist with Mild Winters
(C)

Moist with Severe Winters
(D)

Polar
(E)

Af/Am Tropical Rain Forest

Cfa Humid Suptropical

Dfa/b Humid Continental

ET Polar Tundra

Aw Tropical Wet/Dry

Cfb/c Marine

Dfc/d Subpolar

EF Polar Ice Cap

Cs Mediterranean

Dw Dry Winter

Dry
(B)

Cw Dry Winter

Highland
(H)

BW Arid Desert

H Highland

BS Semi-Arid

Snowpack Types

The factors that create climate also produce various types of snowpack.

This graphic shows the type of snowpack received by each region of the Northern Hemisphere. As you can see, tundra snowpack covers the largest portion, followed by taiga snowpack.

If you live in an area that receives snow, what type of snow does it get? How well does the description match your experience of the area?

Snow Classes

Major Mountain Ranges

Here are the major mountain ranges of the world, all of which accumulate deep, annual snowpacks.

We've finished looking at the types and locations of snowpack. Next, we'll examine the global-scale factors that create and impact snowpack.

Elevation, Latitude, Terrain

Annual Precipitation

The impact of terrain is clearly evident in this plot of the annual number of days of snow cover for the continental U.S. (CONUS). The areas of greatest annual snowfall coverage correlate with mountain ranges as well as latitude. Notice that areas within the Rocky Mountain region of the western U.S. have the most days with snow cover.

Glaciers

Glaciers are semi-permanent, large masses of ice that form when snow accumulates over time, turns to ice, and starts flowing downhill under the pressure of its own weight. In polar and high-altitude alpine regions, glaciers accumulate more snow in the winter than they lose in the summer from melting and sublimation. As more snow accumulates, air spaces collapse, snow grains recrystallize, and the buried layers slowly grow together to form a thickened mass of ice. This dense ice usually looks somewhat blue.

Glaciers are common at high latitudes and in high-elevation areas of the lower-latitude mountain ranges of the Andes, Rocky Mountains, Alps, and Himalayas.

Solar Variation

Remote Sensing and Monthly Snowpack Observations

How do we know how much of the world is covered in snow or where the deepest snowpacks are located? While snowpack measurements taken at particular sites provide point-specific data, satellite observations provide global information about snowpack and have revolutionized snowpack assessment on a global scale. For example, they enable us to observe and quantify the percentage of land covered with snow as well as snowpack depths and temperatures around the world.

This satellite product shows snow coverage for the Northern Hemisphere in February 2009. For latitudes higher than 40° to 50°, we see that much of the land surface is covered with snow, which is typical for midwinter continental conditions.

Regional and Smaller Scale

Introduction

Influence of Geography & Wind on Snow Distribution

While large-scale terrain features impact snowpack on the global scale, terrain plays an even more important role in snowpack development and evolution at regional and local scales. Regional topographic features, such as mountain ranges on the scale of tens of kilometers, produce large annual snowpacks due to their influence on the prevailing atmospheric flow. Mountains force the prevailing flow upward, producing moisture-laden clouds that enhance snowfall.

Mountain ranges oriented north-south, such as the Rocky Mountains, enhance snowfall when winds are perpendicular to the mountains (from the west or east). In contrast, ranges oriented east-west, such as the Uintas, have the heaviest terrain-enhanced snowfall with southerly or northerly flow.

Terrain is even more complex at the local level, with individual peaks, valleys, moisture sources, and other features creating their own local impacts on snowpack. This results in large horizontal variations in snowpack.

In the next few pages, we will examine other factors that impact snowpack development.

Mountainous Terrain, 1

Mountainous Terrain, 2

Click the highlighted areas in the graphic for more information.

Variations in Snow Accumulation

The table describes snowpack accumulation in various settings, with the difference in amounts due largely to wind.

Assume that a 10-cm (4-in) snowfall with significant wind occurs over level plains that are fallow; that is, they haven't been planted, plowed, or harvested in a long time.

A tree cluster would slow down the wind, with snow accumulations as much as 2.4 times higher than in open areas (up to 24 cm or approximately 10 in).

Grazed plains, however, would probably get just over half as much accumulation as the fallow plains.

Around twice as much snow would accumulate in ditches and drainages.

In contrast, windswept ridges and hilltops would have much lower snowpacks.

Wind speeds typically diminish on steep hillsides, with accumulation totals ranging from 28.5 to 42 cm (~11 to 16.5 in).

Density

Although geography sets the stage for snowpack development, each type of precipitation event has its own impact on snowpack depth and density.

Before examining the precipitation types, we'll take a minute to discuss density. Density is the most important aspect of snowpack since it determines the amount of runoff and the stability of a snowpack. Stability is critical for avalanche considerations.

When discussing density, we tend to think in terms of weight per volume, with a typical snow density being 0.1 gram per cubic cm (g/cm3). That makes snow one tenth as dense as water because it is composed of both water and air.

When talking about snowpack, we express density as the depth of the pack vs. the depth of the water that would be produced if the snow were melted. This is known as the snow water equivalent or SWE. For example, a snowpack that is 100 cm (39.4 in) deep will melt to produce a volume of water 10 cm (3.9 in) deep.

Hydrologists like the term SWE because it tells them how much water will run off when the snowpack melts. Meteorologists typically use another term to discuss the density of snow: snow-to-liquid ratio or SLR. SLR is a unitless ratio of snow depth to liquid depth (SWE).

The SLR is the inverse of density, meaning that the higher the SLR, the lower the density of a snowpack. Using our previous example, snowpack with a density of 0.1 has an SLR of 10:1.

• SLR is high for light, powdery snowpack (up to about 40:1)
• SLR is low for snowpack with older or drifted snow (as low as 6:1 or 7:1)
• SLR is even lower for very wet snowpack (as low as 2:1)

Density Question

Snowpack Depth & SWE

Does a deeper snowpack always produce more water? The snow depth on the left side of the graphic is 10 in (25 cm), a dense snowpack with an SLR of 2:1 that would melt down to a SWE of 5 in (13 cm). In contrast, the snow depth on the right is 20 in (51 cm), the SLR for that low-density, fluffy snowpack is 20:1, and the SWE is only 1 in (2.5 cm). The snowpack on the left will produce five times as much snowmelt as the one on the right, even though it's only half as deep.

This map shows how average snowpack density values vary across CONUS. The lower SLR values (the wetter, denser snow) correspond to more maritime climates, while the higher values are characteristic of colder and/or higher elevation snowfalls in the interior of the country.

Density & Wind

Precipitation Types

Various types of precipitation impact snowpack: dry snow, wet snow, graupel, sleet (also called ice pellets), freezing rain, rain, freezing drizzle, and drizzle.

What impact does each precipitation type have on the density and depth of snowpack? Assume that the snowpack is fresh and relatively dense.

Select the correct answers in the listboxes for each precipitation type, then click Done for the feedback.

Snow Microphysics

All snow, be it wet or dry, originates as ice crystals in the atmosphere. These crystals form different shapes (habits) depending on the temperature and moisture content of the atmosphere. The primary habits are dendrites, plates, columns, and needles. In general, each is produced at the following atmospheric temperatures.

• Dendrites and plates: -22°C to -10°C (-8°F to 14°F)
• Needles: -10°C to -3°C (14°F to 26°F)
• Columns: -10°C to -3°C (14°F to 26°F) and colder than -22°C (-8°F)

Dendrites are the fastest-growing crystals and tend to aggregate into larger snowflakes that result in low-density snowpack at cold temperatures. In contrast, smaller crystals, such as columns, needles, and plates, tend to accumulate into a higher-density snow layer.

Other factors that also increase snowpack density include:

• Riming, which occurs when any type of ice crystal passes through a super-cooled cloud (one whose liquid water droplets are below 0°C or 32°F). Rimed crystals are partially or completely coated in tiny frozen water droplets and are associated with higher-density snow accumulation.
• The presence of broken crystals. Crystals can fracture when they hit each other as they descend through the atmosphere or when they strike the ground. Higher winds lead to more fractured crystals, which increases snowpack density.

For more information on atmospheric microphysics, see the COMET module “Topics in Precipitation Type Forecasting” at https://www.meted.ucar.edu/training_module.php?id=179.

Evolution

Introduction

Snowpack evolution is often referred to as snowpack metamorphism. The characteristics of a snowpack, such as its depth, density, layering, degree of bonding, and temperature, change over time regardless of whether more precipitation falls. Metamorphic processes become more important as dry periods persist.

Our study of snowpack evolution spans three sections.

• In this first section, we examine the basic processes that affect snowpack evolution, such as conduction and radiation
• In the next two sections (Scenarios 1 and 2), we use scenarios to explore other factors that affect snowpack evolution, such as the different types of weather events; both scenarios occur over the same period of time (from fall to spring) but take place in different types of terrain (a relatively flat, open area vs. a mountainous region)

Before getting started, we need to define some terms.

Snow grains refer to the ice crystals within a snowpack rather than those in the atmosphere.

Bonding refers to the degree to which grains are aggregated (clustered together) or not. For new snowfall, the degree of bonding is generally greater if the crystals are of similar sizes.

Ethan Greene on bonding:

Gravity & Conduction

The basic metamorphism processes that affect snowpack evolution are gravity, conduction, radiation, vapor diffusion, and, to a lesser extent, convection. We’ll describe each process on this and the following pages of this section.

Gravity

In general, gravity acts to pull snowpack straight downward toward the ground on flat slopes. It increases the density of the snow after the snowpack becomes established in a process called settling.

Over sloped surfaces, a portion of the gravitational force is directed parallel to (along) the slope rather than vertically. This portion increases with slope steepness and is responsible for moving snow downhill.

Conduction

Conduction is the direct transfer of thermal energy from warmer to cooler substances that are in contact with each other. Conduction is present in snowpack when there are changes in temperature within the pack or at the top or bottom. Conduction often occurs in fall when ground temperatures are warmer than the snowpack. A temperature gradient forms, causing the ground to heat the lower portion of the snowpack.

Note that ground temperatures just below the snowpack are typically near 0°C except in permafrost regions of the high latitudes, where they can be significantly colder. In these areas, snowpack temperatures more than a few centimeters away from the ground can be as low as -60°C depending on the ambient air temperature.

Radiation

Radiation is primarily responsible for inducing the melt/freeze process, which results in crusting and other types of crystal evolution. Two types of radiation are important for snowpack evolution.

Incoming shortwave (solar) radiation

Solar energy reaching the Earth's surface is reflected, absorbed, or scattered, depending on the type of surface. Snowpack is particularly reflective, especially when the top of the pack contains newly fallen snow. The degree of a surface's reflectivity is referred to as its albedo or ratio of reflected solar energy to incoming solar energy. The albedo of snow is relatively large—typically in the range of 0.3 to 0.9. This means that 30% to 90% of the energy is reflected back to the atmosphere.

The albedo depends largely upon the age of the snow at the surface, with old snow having lower albedos than new snow: 0.3 to 0.5 as compared to 0.6 to 0.9. That's largely due to the presence of foreign matter such as dirt and dust, which are less reflective and have lower albedos. Albedo is also dependent on the size of the crystals in the top of the snowpack, with smaller crystals having larger albedos.

Outgoing infrared radiation

All surfaces on Earth, be they bare ground or snowpack, constantly emit infrared radiation. The amount is primarily controlled by factors such as the temperature of the surface and the presence of nearby or overhanging vegetation. For snowpack, the warmer and more vegetation-free the surface, the greater the rate of radiative loss. This cools the snowpack at the very top few mm of the surface. Contrast this with the warming effect of incoming solar radiation, which heats up the top 15 to 30 cm (6 to 12 in) of snowpack.

On a calm, clear night, a snow-covered surface will cool much more quickly than a bare one given the same surface temperatures. That's due to several factors. Even though both surfaces have the same temperature at sunset, the snow radiates heat very efficiently, increasing the rate of heat loss at the surface. Snow is also a very good insulator. This prevents heat from rising through the snowpack, which allows the surface to cool quickly. In contrast, bare soil conducts much more heat upward from below, which helps slow down cooling at the surface.

Phase Changes, Including Vapor Diffusion

Radiation and conduction are processes that transfer thermal energy. Each induces changes in snowpack between solid ice, liquid water, and water vapor.

Vapor Diffusion

Now we'll discuss the important process of microphysical vapor diffusion, which is critical for snowpack evolution.

This simple schematic shows how ice molecules move within a snowpack when the ground is warmer than the snowpack. The process involves millions of ice and water vapor molecules.

Snow crystals (snow grains) are interspersed with microscopic air pockets. These air pockets have a given temperature and vapor pressure (the part of air pressure that's due to water vapor). The air at the bottom of the pockets is warmer than that above since it's closer to the (warmer) ground. This results in temperature and vapor pressure gradients.

As the animation below shows, the water molecules move from high to low vapor pressure by sublimating off the snow grain at the bottom of the pocket and moving upward and attaching onto the snow grains at the top. This involves changing from vapor back to ice, a process called vapor deposition. The result is that the upper snow grains grow at the expense of those below, resulting in a net transport of ice mass upwards.

If the temperature gradient is large, the upper crystals will grow quickly. The vapor molecules attach to the bottom of the crystals in flat layers rather than simply enlarging the size of the grains. These flat edges are called crystal facets.

In sum, two phase changes occur with warm temperatures below and cool temperatures above in snowpack:

• Sublimation of the lower crystals (a phase change from ice to vapor)
• Vapor depositional growth of the upper crystals (a phase change from vapor to ice)

The process is reversed when the ground is colder than the snowpack (an unusual situation).

Convection

In the lower portion of the snowpack, convective processes, albeit weak ones, can be important if the snowpack is relatively porous. We see this with Arctic snowpack that has evolved over days and weeks into very porous layers of snow grains. The convection is caused by warm air at the bottom of the snowpack rising into the porous layers above. The rising motion can extend over a meter upward depending on the depth of the pack and the porosity of the layers. The primary effect of convection is to transport small amounts of heat upwards.

Scenario 1: Flat Land

Initial Conditions

Introduction

This section uses a scenario to explore snowpack evolution in flat terrain. The scenario begins on November 15 in a region of flat, open terrain with some areas of vegetation. The area sits at an elevation of 2.5 km (8,202 ft) and latitude of 45°N. (You could find this kind of place in Wyoming.) The area was free of snow when it experienced a significant snowstorm last night.

It's now sunrise, which we'll say is 6:00 am local time. The snow has stopped, skies are clear, and there are 40 cm (15.7 in) of fresh snow on the ground. The temperature at the top of the snowpack is -3°C (27°), and the ground temperature is 0°C (32°F). The water content of the snowpack (its SWE) is 2.9 cm (1.1 in).

Effect of Trees on Snow Distribution

Vegetation has a strong influence on the initial snowfall distribution and its subsequent redistribution, with more snow accumulating in clearings than in adjoining forests.

Melting

Since we're in late autumn and approaching the winter solstice, the daytime solar energy is near its annual minimum. The solar energy heats the upper snowpack surface relatively slowly throughout the middle part of the day and begins melting the snow around noon.

Day 1, 2pm: Aspect

Aspect is the direction that an object or tilted ground surface faces. We're looking at a large tree stump covered in snow. Notice the dramatic melting on the south-facing side and the undisturbed snowpack on the north-facing side. The snow on the dark, cold side remains as it was when it fell, whereas a melted layer has formed (and subsequently crusted) on the south-facing side.

Day 1, 2pm: Temperature Profile

Day 1, Conduction Starting at Sunrise

Conduction began when the snow started accumulating. At sunrise, the temperature was 0°C (32°F) near the base of the pack and -3°C (27°F) at the top (the same as the air temperature above). This caused a small temperature gradient, with the snow next to the ground warming slightly from conduction. This process will continue as long as the adjacent ground surface stays warmer than the snowpack.

Day 1, 6pm: IR Cooling

In the early evening, radiative cooling quickly takes over and the top of the snowpack cools very quickly. In contrast to solar absorption, the vast majority of radiative loss happens within a few millimeters of the top of the snowpack. In our case, since the outgoing radiative loss is much larger than the daily solar radiative gain, nighttime temperatures at the top of the snowpack are much lower now.

Day 1, 6pm: Temperature Gradient

Day 2, 6am: Radiative Recrystallization

The intense vertical thermal gradient present the next morning (day two) has transported moisture from lower down in the snowpack to the top layer, causing the ice grains in about the top 5 cm (1.9 in) of the snowpack to grow. This process, known as radiative recrystallization, involves two phase changes: one from ice to water vapor in the lower snowpack, the other from water vapor back to ice in the upper snowpack.

Day 2, 6am: Surface Hoar

Some of the water vapor particles moving up through the snowpack escape into the atmosphere right above the pack and freeze upon contact with the colder air temperatures. This results in the formation of surface hoar at the top of the pack. Surface hoar is large, rounded, feathery crystals (snow grains) with flat edges that grow rapidly.

To summarize, surface hoar forms when:

• Atmospheric conditions are calm and clear and the snow at the top of the pack has cooled rapidly overnight due to longwave radiative cooling
• A strong temperature gradient in the snowpack is accompanied by a vapor pressure gradient that drives water vapor out of the snow and into the atmosphere very close to the snow surface
• The water vapor freezes, forming hoar crystals

Day 2, 6am: Gravity

Gravity is always at work, causing snow depth to decrease over time. As the snowpack ages or settles, its density gradually increases, which generally makes the snowpack more stable.

Gravity never works in isolation. Other processes, such as wind events and precipitation, often have a more dramatic effect on snowpack density and stability.

Note that when we mention stability in the context of snowpack, we're really talking about the likelihood of avalanche formation. A stable snowpack is less likely to fail and form an avalanche.

Day 4, 6am: Depth Hoar Formation

Long, undisturbed periods of cold atmospheric temperatures and radiative loss at the top of the snowpack have a cumulative effect, with most of the snowpack steadily cooling. Since the ground remains relatively warm because it's insolated by the snow, the temperature gradient increases. If it gets large enough, depth hoar can form. Depth hoar is highly faceted, large, feathery crystals that grow on the edges of existing snow grains.

These electron microscopy images show depth hoar crystals, first at a very small size (a few tens of micrometers), then at a large, mature size. Notice that the depth hoar grows at the expense of the pre-existing snow grains.

Depth hoar typically forms over several days. Although the crystals are bonded, together they form a weak, brittle structure. If you try to pick up or move a layer dominated by depth hoar, it will disintegrate. Note that weak layers of depth hoar are a significant concern from an avalanche perspective, since they can be the source of fractures and slides.

New Events

Introduction

It's been six days since the initial snowstorm. How would a wind storm, a dust storm, another snowfall, and other types of precipitation events impact the snowpack? We'll see what happens, returning to day six at the outset of each event so we can examine its impact on the same set of conditions.

Snow Redistribution by Wind

New Event: Wind

A windstorm sweeps through the area six days after the initial snowfall, with speeds up to 18 m/s (35 kt). Before we examine its impact on the snowpack, we'll take a general look at wind and snowpack.

The redistribution of snow by wind is a critical aspect of snowpack evolution. Even in relatively flat terrain, strong winds can cause widely varying snow depths in adjoining areas. While a flat, open, wind-blown area might be snow-free, deep snowdrifts can surround obstacles or form in small terrain depressions. (That's why it's important to choose a representative location when measuring snow depth, avoiding scoured areas or snowdrifts.)

Wind can transport snow when wind speeds are above ~5 m/s (~10 kt). This threshold depends on the characteristics of the snow surface though. Weaker winds can move low-density snow whereas older, hardened snow surfaces may only begin to move with much stronger winds.

Impact of Wind

Dust

New Event: Dust

Six days after the initial snowfall, a dust storm sweeps across the region. Dust storms are created by very strong low-level winds moving across arid regions. The dust moves downstream in the atmosphere and resettles on snowpack when the wind speed decreases or the dust particles are scoured out by precipitation. Scouring is a process in which precipitating ice or water particles collide with other airborne particles, such as dust. These particles are carried along with the precipitation down to the ground.

Dust Layers

Dust deposits on snowpack become a layer—first at the top of the snowpack and then submerged if additional snow falls. When the dust is in the top layer, the decrease in albedo accelerates melting. As melting proceeds, the dirt remains on top and merges with any previously established dust layers to form one strong, thick dirty layer.

Furthermore, the dust embedded within the snowpack can induce internal melting and freezing. The resulting internal layers may exhibit a reduced degree of bonding that can affect snowpack stability in mountainous regions.

More Snow

New Event: Snow

Let's say that in the six days since the initial snowstorm, the following has happened:

• The old snow has settled to a depth of 14 in (35 cm)
• There's a layer of surface hoar
• The SWE has decreased slightly from 1.30 to 1.26 in (3.3 to 3.2 cm) due to sublimation (evaporation of snowpack)
• The snow-to-liquid ratio (SLR) is ~10:1

We've just received approximately 12 in (30 cm) of new snow.

• The air temperature is -5°C (23°F)
• The SLR is ~15:1
• The SWE is 0.8 in (2.0 cm)
• The wind was 3 to 4 m/s (6 to 8 kt) when the new snow was falling but has calmed down and is now 2 m/s (~5 kt)

Impact of New Snow

High-Density Snow Over Low-Density Snow

Assume that the SWE of the new snow is 1.6 in (4.0 cm) rather than the 0.8 in (2.0 cm) that we just discussed, making it a very dense, wet snow layer. This type of snow tends to develop with warmer atmospheric and surface temperatures.

Higher-density snow overlying lower-density snow can lead to an unstable snowpack, one prone to collapse, since the bonds in the lower layer may not be strong enough to withstand the additional weight. Unstable snow situations occur in flat areas all the time but do not lead to avalanches due to insufficient slope steepness.

When the density of the snow increases during a single snow event, we get an upside-down snow situation. Generally speaking, the higher-density layer is stronger than the lower-density layer. In flat areas, the denser, upper layer speeds the compaction of the lower layer, but on a steep slope, the situation can lead to an avalanche.

Other Precipitation Types

New Events: Various Precipitation Types

Consider what would happen if the following precipitation events occurred after the second snowfall: rain, freezing rain, sleet, freezing drizzle, and graupel.

More About the Precipitation Types

Click on each type of precipitation to learn more about it.

Springtime Melting

March and Beyond

We've skipped forward to March. An upper ridge (a high-pressure system associated with clear, dry weather) has developed in the atmosphere. Daytime temperatures are in the +3°C to +8°C range (37° to 46°F). Since that's above freezing, significant melting is occurring on the top of the snowpack.

The warmer the atmospheric temperature, the greater the rate of melting. In general, melting adds liquid to the top of the snowpack, which increases the density of the upper layer.

The vast majority of liquid from melting or rain moves down through vertical channels in the snowpack unless it encounters an ice layer, in which case the water will pool above it, freezing if temperatures fall. If atmospheric temperatures stay warm, the water will keep moving horizontally until it finds another vertical channel.

If the water encounters a capillary barrier, it will move parallel to the snow layers rather than through them. A capillary barrier forms when a layer with small pore spaces rests above one with large pore spaces. The gradient in pore sizes creates a barrier that the water runs along.

At night, when the air temperature falls back below freezing, the top layer freezes into a crust whose density is near that of water (many times higher than snowpack). The latent heat release from the crusting process enhances temperature gradients within the snowpack. This can lead to edge growth (faceting) on the snow grains and create layers of reduced stability.

The cycling of melting and freezing can occur diurnally or with the passage of storm systems that are common during the spring season.

In the spring (and early summer at higher elevations), snowpack coverage typically becomes spotty. Melting accelerates at the bottom fringes of the snowpack due to conduction from the adjacent warmer, bare ground.

Rain

Rain falling on the snowpack is inherently warmer than the pack and provides additional energy for melting, which accelerates the melt process.

No land mass is perfectly flat. Areas with just a slight slope are prone to flooding from excessive snowmelt.

If the land under the snow is frozen, it cannot absorb much water from the snowmelt. The water will travel downhill even if the slope is minimal and can lead to flooding.

Scenario 2: Mountains

Initial Conditions

Introduction

Scenario

Assume that a similar sequence of meteorological events occurs in this scenario as in the first one.

• Snow falls on bare ground on November 15, followed by several clear, calm nights. Like Scenario 1, this initial snowfall brings 16 in (40 cm) of new snow, with the surface temperature at -3°C (27°F) at the end of the storm. The SWE is 1.3 in (3.3 cm equivalent) and the SLR is 12:1. This scenario is also at 45°N latitude.
• Then four more weather events occur, followed by springtime melting. As in scenario 1, we will treat the four events as if they occur independently of each other, not in consecutive order.

We are going to focus on a ridge line whose slopes are 30 and 40 degrees respectively. Because of the slopes, we have to consider snowpack movement, which involves several important factors. These include gravity, friction, and deformation, which we'll explore on the following pages (before the additional weather events occur).

Gravity

Gravity causes avalanches to occur on sloped but not flat terrain. We'll use these graphics to see why.

Like all forces, gravity can be described in terms of vectors, with the gravity vector drawn to indicate the direction of its pull. The total gravity vector remains constant.

In flat terrain (A), gravity pulls the snowpack straight downwards towards the ground.

When a slope is introduced (B, C, D), the total force of gravity remains the same but the gravity vector can be represented by two components:

• One that's parallel to the slope (that runs along it), g1
• One's that's perpendicular to the slope, g2

There are several important relationships between the two components.

• The component that's parallel to the slope increases as the slope steepens. Conversely, the component that's perpendicular to the slope decreases as the slope steepens.
• When the slope is vertical (E), the slope-perpendicular vector is reduced to zero so all of the gravitational force is directed parallel to the slope.

Avalanches

Avalanches form on slopes mild enough for snow to accumulate but steep enough for it to slide. Generally, this includes slopes between 30 and 45 degrees, although the slope threshold can be as steep as 60 degrees in maritime climates.

Every slope has a limit as to how deep the snowpack can be without sliding. That limit decreases as steepness increases, meaning that the steeper the slope, the less snowpack it can hold. If a slope is too steep, snow will not accumulate significantly so the slope will remain bare.

Different types of avalanches form depending on the characteristics of the snowpack. These include slab, point-release, slough, wet, and dry avalanches.

For more information on avalanches, please refer to other training materials, such as:

• COMET's Avalanche Weather Forecasting module (https://www.meted.ucar.edu/training_module.php?id=764)
• The avalanche.org website (http://www.avalanche.org/)

Friction

Friction inhibits the movement of the snowpack at its interface with the ground and is always directed parallel to the slope. However, its orientation is directly opposite that of the slope-parallel gravitational component. Frictional resistance increases as surface roughness increases.

Friction is the primary factor that lets snowpack build up on sloped surfaces, rather than just sliding downslope. To illustrate this, imagine a plane of glass with a coffee mug on it. You don't have to tilt the glass much to get the mug to slide off. The glass is very smooth and produces little frictional resistance. If you change the glass to a sandstone surface, you'll have to tilt it much higher to get the same effect. This is due to the rough surface of the sandstone, which inhibits objects resting on it from moving due to its high frictional force.

Snow Deformation

Many people think that snowpack behaves as a solid mass, unable to stretch, compress, or bend. In fact, snow is a viscoelastic material, meaning that an entire snowpack or particular layer can shear and stretch. Understanding this is important when assessing the stability of a snowpack.

Snow layers can move in several ways:

• By gliding, where the entire snowpack detaches at the bed (underlying surface) and moves slowly down the slope; the same process occurs with avalanches but at a much faster pace
• By creep, the slow, differential movement of a slab down the slope, with the upper portion traveling faster than the lower portion; this process occurs slowly, but can produce tension in the snowpack that can eventually produce a slide

Grain Types and Stability of Layered Snowpack

Layers form in a snowpack from the various snowpack processes that we've discussed, including thermal gradients, melting/freezing, radiative processes, and precipitation events. Layers can represent weaknesses in the snowpack because fractures tend to occur along their interfaces.

New Events

Overview

We've examined the factors that characterize snowpack in complex terrain. Now we'll see what happens when additional atmospheric events occur: a wind storm, a dust storm, another snow event, and a rainstorm, all of which lead to a layered snowpack. Remember that layers are a primary determinant of the stability of a snowpack in regard to avalanche formation. Multiple events can create snowpack layers of varying densities, which can lead to unstable conditions.

As in Scenario 1, we'll treat the four new precipitation events as if each one occurred several days after the initial snowfall rather than in consecutive order. This will let us examine the effect of each event on the same set of conditions. Then we'll skip ahead to March, when springtime melting begins.

Wind

Six days after the initial snowfall, a 15-m/s (30-kt) wind event occurs, which lasts twelve hours.

In general, wind can significantly redistribute snowpack in the mountains where high wind speeds are common. With wind speeds of 15 m/s, snow depths can be at least 50% higher in redistributed areas. The depth is typically greatest on the lee or downwind side of a ridgeline. The location of the maximum is typically dependent on the wind speed, with higher speeds usually corresponding to distances further downwind from the crest.

When wind redistribution occurs relatively quickly (on the order of several hours), it produces stress that a formerly stable snowpack may not be able to resist. The quickly added weight may lead to an avalanche.

Redistributed snow is inherently of higher density than undisturbed snow. When it accumulates on low-density snowpack, it forms a slab. The slab increases the weight of the snowpack, to the point where it may exceed the counteracting force of friction. When this occurs, the slab can fail, causing an avalanche.

If subsequent snowfall covers redistributed snow, high-density layers will be created within the pack.

Dust

New Event: Dust

Let's see what happens if the initial snowfall is followed by a high wind event that deposits dust on top of the snowpack.

Snow

New Event: Snow

The initial 16-in (40-cm) snowfall had an SLR of 12:1, which has probably decreased to about 11:1 in the six days following the event. Then we get 8 in (20 cm) of wet, heavy snow, with an SLR of 8:1.

A Variation

New Event: Snow Variation

What would happen if the original snow layer was much drier—if the SLR was at least 15:1? If the slope was steep enough, inconsequential, point-release avalanches would probably occur after the initial snowfall. Then, with the addition of the second, heavier snow, the top slab would be even more likely to slide, creating a larger, heavier, and longer avalanche or slide.

Rain

New Event: Rain

Let's say that in the six days following the initial snowfall, the original 16-in snowpack became slightly denser while its depth decreased slightly. The SLR was about 11:1. Then the surface temperatures warm, a storm approaches, and significant rainfall occurs. Most of the rain (about 0.2 in or 0.5 cm) occurs at 36°F (2°C). The top of the snowpack is transformed into a high-density layer, with some crust forming after the rain and the passage of a cold front. As is typical in mountainous locations, snow falls after the cold front passes and is wet and heavy, with a liquid equivalent of 2.4 cm (1 in) and an SLR of approximately 8:1 to 9:1.

Now we have a relatively thick layer of crust and high-density snow that's 20 cm (8 in) down in the pack. Snow grains in the crusted layer have a low degree of bonding, so the presence of this weak layer can decrease friction, thereby reducing the stability of the snowpack.

Spring Melt

Introduction

Impacts of Snowmelt

Let's look at the impacts of snowmelt. For each statement below, decide if it is right or wrong, then click Done.

Springtime Snowpack Stability

Here are comments by Ethan Greene about snowpack stability in springtime.

Walking on Springtime Snowpack

What's it like to walk across snowpack in springtime when snowmelt is occurring?

Snowmelt in Mountainous Areas

Snowmelt is discharged into drainages in mountainous terrain and into ponds, lakes, and relatively flat rivers in level terrain. Each drainage has a limit as to the amount of runoff it can handle, beyond which flooding will occur.

The rate of melting depends on several factors:

• The low-level temperatures of the atmosphere, with warmer temperatures leading to more melting.
• The presence and amount of liquid precipitation. In general, rain melts snow when atmospheric temperatures are relatively warm. But when they are very close to freezing, the rain usually freezes as it percolates into the snowpack. The heat release associated with this freezing warms the pack to near the melting point so that any additional heating will melt it much more quickly.
• The type of ground cover. If the surface is frozen soil or rock slab, little snowmelt will be absorbed; most of it will move horizontally, pool, or move downhill, increasing runoff. If the surface is unfrozen soil, the ground can absorb water more easily, decreasing runoff.

Of course, snowmelt runs off faster in sloped regions than flat areas.

Here Ethan Greene discusses springtime snowmelt:

Ideal Snowpack

The notion of an “ideal” snowpack depends on one's perspective and interests. What's good for farmers may be bad for road crews. Consider the following scenarios. Whose interests does each one best match?

Snowpack Assessment

Onsite Measurements

Introduction

A hydrologist needs to monitor water supplies to determine flooding potential. An avalanche forecaster needs to decide whether to issue any warnings. A climate researcher needs lots of ground and atmospheric observations to assess and monitor climate trends. These are the types of situations that require a steady stream of snowpack data gathered regularly throughout the cold season.

Snowpack is monitored extensively over many areas of the world, usually by government agencies. There are two primary methods: remotely-sensed (primarily satellite-based) and onsite (in-situ). Both methods measure a standard set of characteristics throughout the full snowpack depth to provide a complete profile analysis.

This section describes the various techniques used to assess snowpack, primarily in the context of avalanche stability. The techniques range from simple, hand-based tests to those that use complex tools to obtain precise measurements.

Snow Courses

Onsite measurements, known as snow courses, are an essential part of determining avalanche conditions—both current and past as well as those that may occur in the future. Onsite measurements are taken at fixed sites on both flat and sloped surfaces at regular intervals throughout the cold season. They are also taken at particular sites to address specific needs, such as the likelihood of avalanche formation. In these cases, a partial or full profile of the snowpack is assessed.

In the United States, there are two types of onsite measurements: manual techniques and routine, automated, in-situ measurements called SNOTELs. SNOTEL stands for snowpack telemetry. As of 2009, there were more than 1,200 manually-measured snow courses and over 750 SNOTEL sites in the western U.S. states, including Alaska.

Before starting the discussion of onsite measurements, it is strongly recommended that you review two chapters of the American Avalanche Association's "Snow, Weather, and Avalanches: Observational Guidelines for Avalanche Programs in the United States" guide.

• "Snowpack Observations” at https://www.americanavalancheassociation.org/swag. Please refer to it as you read through the module's descriptions of snowpack assessment techniques since it provides more detailed information than can be included here. The chapter can also be downloaded and used as a guide in the field.
• "Manual Snow and Weather Observations,” provides background information on snow and weather observations. https://www.americanavalancheassociation.org/swag

Manual Measurements

Manual, onsite observations are typically taken in snow pits dug out with shovels. Visual observations are particularly good at revealing crystalline and other characteristics that are important to snowpack stability and melting.

Ideally, snow should be observed in its native state, undisturbed by tracks, vegetation, rocks, or collapsed layers. The walls of the pit should be vertical and clean, and straight, vertical columns should be sampled.

Snow Profiles

The primary goal of measuring a snowpack in the context of avalanches is to assess the stability of the layers. The measurement of all of the layer qualities is called a snow profile.

The snow profile is created by recording the following characteristics for each layer:

• Degree of wetness
• Density and snow water equivalent
• Depth
• Hardness
• Snow grain size and type (including whether bonding exists)
• Temperature gradient
• Shear quality

We'll examine these characteristics in more detail on the following pages.

Degree of Wetness & Density, 1

You can get a quick sense of a snowpack's degree of wetness (its liquid water content) by doing a simple hand-based test that lets you characterize the snowpack as dry, moist, wet, very wet, or slush. Simply squeeze a handful of snow in your gloved hand and observe the amount of water present. Use this chart to determine the snow class.

A more accurate and formal way of assessing the water content of a layer is to first measure its density (mass per unit volume, for example, grams per cubic centimeter). This is done by cutting out a predetermined volume (chunk) of snow and weighing it.

The density is then converted to water content (snow-to-liquid ratio) by dividing the measured density by the density of water.

Degree of Wetness & Density, 2

Click the links to hear Ethan discuss the following topics.

Depth

Depth measurements are important to the avalanche community, hydrologists, and ski resorts since they indicate the amount of snow on the ground and provide an estimate of the amount of snow that would slide were an avalanche to occur.

A simple way to measure the thickness of a layer or the entire snowpack is to use a ruler. Since snow depths can vary widely in complex terrain, it is important to take multiple measurements.

Click the links to hear Ethan Greene describe another technique for measuring snow depth and the impact of spatial variability on depth measurements.

Hardness, 1

To help determine the stability of a snowpack, we measure the relative hardness of the layers. This refers to the number of bonds per volume, which indicates the degree to which the grains hold together under external pressure.

A very hard layer over a very soft layer decreases stability, while a very soft layer over a very hard layer increases stability.

In the field, a layer’s degree of hardness is defined by the largest object that can penetrate the snow. You can get a quick sense of it by gently pressing objects of varying sizes into the snow and determining the largest object that can penetrate it. In order of increasing hardness, these are a fist, four fingers, one finger, a pencil, and a knife.

Hardness, 2

The more formal way of determining hardness is to use a ram penetrometer, which measures resistance as it is thrust into the snowpack layer. The greater the resistance, the greater the layer’s degree of hardness.

Here’s a hardness profile obtained with a ram penetrometer.

Snow Grain and Type

Snow grain type and size are important to measure because of their relation to snowpack stability. Hoar and other facetized grains are typically large in size (over ~2mm in diameter) and reduce snowpack stability.

Simple magnifiers are used to assess the type of snow grain within a layer. The graphic shows different grain types.

Temperature

The temperature of a snowpack layer should be measured with a thermometer in the shade and not be contaminated by heat from the fingers. You can estimate temperature gradients by taking measurements at several depths in the pack.

Notice the difference between day and night temperatures in the top of this snowpack. As you’ll recall, solar radiation warms the top 15 to 30 cm (6 to 12 in), whereas outgoing infrared radiation cools the very top few mm.

Shear Quality

Shear quality is the resistance of the layers in a snowpack when an amount of pressure is applied to one layer, such as the top, and the rest are left alone.

Shear quality is measured by exerting increasing amounts of vertical pressure on the top of the snowpack until the top layer begins to move down the slope. There are several types of shear quality tests.

The Rutschblock test is a rather crude, time-consuming technique done on-site by a snowpack expert on skis. First, the skier digs a u-shaped trench around an undisturbed block of snow, leaving the block attached on the uphill side. Then he or she performs a series of increasingly stressful maneuvers to see if the block will fail at any weak layers and begin to move. These range from gently stepping on the block to jumping up and down on it. The point at which the block moves is recorded.

The Stuffblock test is a variation on the Rutschblock test. It is more quantifiable in that pre-set amounts of weight are exerted on a column to see when it will fail.

To do the test, you fill a stuff sack with ten pounds of snow and drop it from increasingly greater heights onto a shovel blade on top of a column of snow, noting the point at which the column fails. The test can be done by skiers and non-skiers alike, which is an advantage in some situations.

Snow Stability & the Extended Column Test

The Extended Column Test (ECT) is a relatively new technique for evaluating snowpack stability. While the Stuffblock and other tests identify layers in a snowpack that are likely to initiate a fracture, the ECT identifies layers that are likely to both initiate and propagate a fracture. This helps focus attention on unstable areas, where an avalanche really might occur. To do the test:

It’s pretty easy to interpret the results. Fractures typically propagate across the entire column within one or two additional loading steps on unstable slopes. If the fracture propagates across one or more layers or breaks, it’s unlikely that fracture propagation will occur.

There are some limitations to be aware of. The test can overestimate snowpack instability, for example, when a thick hard layer overlies a weak one, or the upper layers of a pack are soft.

Like other tests, the ECT should be done in an area that is representative of the entire slope.

For more information on the ECT, access the paper “The Extended Column Test: A Field Test for Fracture Initiation and Propagation” at https://avalanche.org/wp-content/uploads/2018/08/06_ISSW_Simenhois.pdf.

Snow Profiles, 1

Here’s a snow profile—the culmination of the in-situ, snowpit-based, snowpack measurement process. The results are depicted vertically on the plot, making easy to interpret the stability of the snowpack layers.

Which of the following statements are true of the temperature profile (the line in blue)?

Select the correct answer(s), then click Done.

a) Conditions are well below freezing near the frozen ground in the lower portion of the pack

b) Conditions are warmer lower in the snowpack

c) The snowpack is melting rapidly

d) There is a significant vertical temperature gradient in the snowpack

The correct answers are b and d.

Conditions are warmer in the lower portion of the snowpack: near 0°C (32°F), which is just barely freezing. Melting is not significant because all of the temperatures are at or below freezing. The vertical temperature gradient in the 20-cm to 70-cm (8-in to 28-in) layer is significant since it’s conducive to both the transport of moisture upward and the growth of the upper grains at the expense of the lower grains. This leads to the formation of depth hoar, which decreases snowpack stability.

Please make a selection.

Snow Profiles, 2

SNOTELs

The United States’ Natural Resources Conservation Service operates an extensive, automated system for collecting snowpack and related climatic data in the Western United States and Alaska. This system is called SNOwpack TELemetry or SNOTEL for short. The SNOTEL network consists of over 750 measuring sites over complex terrain.

The table shows the physical properties measured and tools involved.

This example shows SNOTEL SWE measurements for a site in California in 2008 and 2009 (up to mid-March). The plot is useful because it shows the data in a historical context, since 1971. Note that the maximum values of SWE typically occur in late February/early March.

Satellite Data

Microwave Wavelengths

Remote sensing of snow cover is done by low-orbiting satellites that have the spatial and temporal resolution required for accurate snow-related measurements.

Satellite observations have advantages over traditional ground measurements. For example, they cover large areas with near-uniform resolution and retrieve data from remote regions of the world where onsite measurements can be time consuming or nearly impossible to take.

Microwave wavelengths are best for assessing snow properties from a remote sensing perspective—far more so than visible or infrared wavelengths. That’s because microwave energy can penetrate snowpack and is reflected and emitted both from the surface and deeper within the pack. This makes it sensitive to parameters such as snow depth, snow water equivalent, snowpack temperature, snow crystal type, wet-dry state, as well as soil conditions below the snowpack.

In addition, microwave instruments also penetrate cloud cover and operate during both day and nighttime. This makes them useful for detecting snowpack conditions on a 24-by-7 basis.

The rest of this section presents products made from satellite-derived snow observations.

Snow Water Equivalent (SWE) Product

Snow water equivalent (SWE) estimates have widespread applications in hydrology, agriculture, and emergency management.

This example, created from satellite and surface observations of SWE, shows snowpack conditions during late winter in the CONUS. Notice the high SWE values over the western mountain ranges and the extreme northern parts of New England, and the lower values (10 to 15 cm or 4 to 6 in) over some of the flatter areas of the northern fringes of the U.S.

Snow Cover Product

Snow cover estimates made primarily from satellite data are important to hydrology and provide critical input to numerical weather prediction models.

In this example, notice the extensive snow coverage over the higher latitudes of the Northern Hemisphere in February 2005.

Snowpack Depth Product

The snowpack depth product is made from microwave satellite data and is used to determine post-snowstorm depth. This information is of interest to, for example, transportation agencies and those involved with snow recreation.

Snow Top Temperature Product

Microwave satellites assist in determining the detailed distribution of temperatures at the top of snowpack, which is useful to hydrologists and others interested in assessing and predicting snowmelt.

Temperatures at the top of the snowpack are also used to assess snowpack stability, including the potential for crusting at the top of the snowpack and surface hoar formation.

Snowmelt Product

Real-time estimates of snowmelt are possible using microwave satellite data. The product helps identify areas under threat from flooding and is used to analyze soil moisture through the late winter and springtime months.

Product Access

NOAA’s National Operational Hydrologic Remote Sensing Center (NOHRSC, http://www.nohrsc.nws.gov/) and the National Snow and Ice Data Center (http://nsidc.org/) display these types of products for operational users.

The NOHRSC site has an “Interactive Snow Map” feature that lets you display all of the remotely sensed and in-situ global snowpack data for any site of interest in the United States.

Summary

Factors impacting snowpack development

• At the global scale: Climate, elevation, latitude, terrain, solar variation
• At regional and smaller scales: Geography, precipitation type, wind

Types of snowpack

• Tundra: Thin, cold, windblown snow usually found above or north of tree line
• Taiga: Thin to moderately deep, low-density snowpack found in forests in cold climates
• Alpine: Intermediate to cold, deep snow cover, typically low density
• Maritime: Warm, deep snow cover with coarse-grained snow due to wetting
• Prairie: Typically thin, moderately cold snow cover with substantial wind drifting
• Ephemeral: Thin, extremely warm snow cover that melts soon after being deposited

Snowpack density measurements

• Snow water equivalent (SWE): The depth of water produced if the snowpack melted
• Snow-to-liquid ratio (SLR): A unitless ratio of snow depth to SWE; high for light, powdery snowpack (up to about 40:1), low for snowpacks with older or drifted snow (as low as 6 or 7:1), even lower for very wet snow

Crystal types (habits)

• Dendrites: Fastest-growing crystals; are produced at atmospheric temperatures from -11°C to -17°C (12°F to 1°F); tend to aggregate into larger snowflakes that result in low-density snowpack at cold temperatures
• Columns, needles, and plates: Smaller crystals that tend to accumulate into a higher-density snow layer; needles form with atmospheric temperatures of -10°C to -3°C (14°F and 26°F); columns from -10°C to -3°C (14°F to 26°F) or colder than -22°C (-8°F)
• Riming occurs when any type of ice crystal passes through super-cooled cloud (liquid water droplets below 0°C); rimed crystals are partially or completely coated in tiny frozen water droplets, and are associated with higher-density snow accumulation
• Broken crystals: Crystals can fracture when they hit each other in the atmosphere or strike the ground; higher winds lead to more fractured crystals

Basic processes that affect snowpack development

• Gravity: Pulls snowpack straight downwards on flat land, increasing its density; over sloped surfaces, a portion of the gravitational force is directed parallel to the slope; this increases with steeper slopes and is responsible for moving snow downhill
• Conduction: The direct transfer of thermal energy from warmer to cooler substances that are in contact with each other; occurs at temperature gradients within a snowpack or at its top or bottom
• Radiation:
  • Primarily responsible for inducing the melt/freeze process, which causes crusting and other types of crystal evolution
  • Two types are important for snowpack evolution:
    • Incoming solar radiation: Snow is highly reflective (has a high albedo or ratio of reflected solar energy to incoming solar energy), with albedos typically 0.3 to 0.9; incoming solar radiation heats up the top 15 to 30 cm (6 to 12 in) of snowpack
    • Outgoing infrared radiation: Cools the very top few mm of the snowpack; the warmer and more vegetation-free the snowpack surface, the greater the rate of radiative loss; snow radiates heat very efficiently and is a very good insulator (prevents heat from rising through the pack)
• Phase changes: Radiation and conduction transfer thermal energy and induce changes in snowpack between solid ice, liquid water, and water vapor. All phase changes induce either cooling or warming of the surrounding air depending the type. Evaporation and sublimation cool the immediate atmosphere while condensation warms it.
  • Melting: Phase change from solid ice to liquid water
  • Freezing: Phase change from liquid water to solid ice
  • Sublimation: Phase change from ice directly to water vapor
  • Microphysical vapor diffusion: Water molecules move from warm to cool in microscopic air pockets, attaching onto other snow grains in flat layers; this process usually occurs from the warmer ground upward toward the snow surface
• Convection: Occurs in relatively porous snowpack when warm air at the bottom rises into the porous layers above, transporting small amounts of heat upwards

Factors involved in snowpack evolution

• Vegetation: Impacts snowfall distribution/redistribution, with more snow (20 to 45%) accumulating in clearings than adjoining forests
• Daytime solar energy: Heats the upper surface of the snowpack
• Aspect (the direction a tilted surface faces): Melting is strongest on sun-facing sides (south in the Northern Hemisphere)
• Radiative cooling after sunset: Quickly cools a few millimeters of the top of the pack
• Crusting: Freezing of a snowpack surface that has previously melted due to either solar radiation or warm temperatures; often occurs just after sunset
• Radiative recrystallization: Occurs when an intense vertical thermal gradient transports moisture from the lower part of the snowpack to the top layer, causing ice grains in the ~top 5 cm (1.9 in) to grow
• Hoar: Large, rounded, feathery crystals with flat edges that grow rapidly
  • Surface hoar: Forms when snow at the top of the pack cools rapidly overnight; a strong, upward temperature gradient develops along with a vapor pressure gradient that drives water vapor out of the snow and into the atmosphere; the water vapor freezes and forms hoar
  • Depth hoar: Takes several days of strong temperature gradients in the snowpack to form; causes highly faceted hoar crystals to grow on the edges of existing snow grains; although the crystals are bonded, they form a weak, brittle structure that can cause a fracture or slide
• Wind: Can transport snow when speeds are over 5 m/s (~10 kt); weaker winds can move low-density snow whereas older, hardened snow surfaces may only start moving with much stronger winds
• Dust storms: Created by very strong low-level winds moving across arid regions; the dust moves downstream in the atmosphere and resettles on snowpack, causing the top to melt significantly during daytime; when the pack melts, the top dust layer merges with other dust layers, forming a strong, thick, ‘dirty’ layer
• High-density snow over low-density snow: Can lead to an unstable snowpack, one prone to collapse
• Precipitation types
  • Dry snow: Typically decreases snowpack density and increases snowpack depth
  • Wet snow: Generally increases snowpack density and depth, but if a thin layer of wet snow falls on a thin layer of dry snow, the depth can actually decrease due to compaction
  • Sleet (ice pellets): Adds a high-density layer of ice to the very top of the snowpack; increases the density of a relatively dense snowpack while decreasing its depth
  • Freezing rain: Freezes upon contact with the snowpack, forming a thin layer of dense, hard ice on top; typically increases snowpack density and decreases snowpack depth
  • Freezing drizzle: Adds an ultra-thin, high-density layer to the top of the snowpack
  • Graupel: High-density frozen precipitation that typically increases snowpack density and depth
  • Rain: Creates a wet snow layer in the top few cm of the snowpack; can lead to melting and refreezing in the top of the pack; increases snowpack density but decreases snowpack depth
• Springtime snowmelt
  • The warmer the atmospheric temperature, the greater the rate of melting (adds liquid to the top of the pack, increasing the density of the upper layer)
  • Most liquid from melting and rain moves down through vertical channels in the snowpack unless it encounters an ice layer, in which case it will pool above it (freezing if temperatures fall) and move horizontally
  • Water moves parallel to snow layers when there’s a capillary barrier (a gradient of pore sizes, from smaller to larger) that causes water to run through the upper layer rather than draining into the lower one
  • At night, when the air temperature is below freezing, the top layer freezes into a crust whose density is near that of water (many times higher than snowpack)
  • The cycling of melting and freezing can occur diurnally or with the passage of storm systems

Aspects of snowpack evolution specific to mountainous terrain

• Snowpack is often unstable in both flat and sloped terrain but the impacts are far more severe in mountainous areas
• Gravity has a greater impact in sloped terrain since it can cause layers to detach and slide
• Friction is the primary factor that lets snowpack build up on sloped surfaces rather than just sliding downslope
• Terrain affects the type and amount of snowfall, with upslope areas typically receiving more precipitation than surrounding locations
• Wind can significantly redistribute snowpack in the mountains where high wind speeds are common; snow depths can be at least 50% higher in redistributed areas; the weight of the additional snow can destabilize formerly stable snowpacks
• Snow layers move by:
  • Gliding, where the entire snowpack detaches at the bed and moves slowly down the slope; the same process occurs with avalanches but at a much faster pace
  • By creep, the slow, differential movement of a slab down the slope, with the upper portion traveling faster than the lower portion; although creep occurs slowly, it can eventually produce a slide
• Avalanches form on slopes mild enough for snow to accumulate but steep enough for it to slide; the steepness threshold generally ranges from 30 and 45 degrees (up to 60 in maritime areas)

Snowpack assessment

• Two primary methods: Onsite (in-situ) and remotely-sensed (satellite-based); both measure snow depth, SWE, density, temperature, and the nature of the layers throughout the snowpack to provide a complete profile analysis
• Onsite measurements (snow courses)
  • Taken at fixed sites at regular intervals throughout the cold season; taken at particular sites as needed
  • Manual observations
    • Taken in vertical snow pits dug out with shovels, with straight, vertical columns sampled
    • Snowpack wetness: Characterizes snowpack as dry, moist, wet, very wet, or slush; to measure it, squeeze a handful of snow and observe the amount of water; or cut out a volume of snow and weigh its density, then divide the measured density by the density of water
    • Snowpack depth: Use a ruler to measure the thickness of a layer or the snowpack; take multiple measurements to get representative samples
    • Snowpack hardness: Gently press a fist, four fingers, one finger, pencil, knife into the snow and determine the largest object that can penetrate it; or use a ram penetrometer to determine resistance as it’s thrust into a layer
    • Snow grain type and size: Use a simple magnifier to assess the type of snow grain within a layer; hoar and other facetized grains are typically large and reduce snowpack stability
    • Snowpack temperature: Use a thermometer in the shade; take measurements at several depths to estimate temperature gradients
    • Shear quality (the resistance of layers when an amount of pressure is applied to one layer): Exert increasing amounts of vertical pressure on the top of the snowpack until the top layer begins to move down the slope; the Rutschblock test is done by a skier who cuts out a u-shaped trench and applies pressure on it (from gently stepping on it to jumping up and down) to see when it fails; the Stuffblock test is more quantifiable since it uses pre-set amounts of weight; the Extended Column Test identifies layers that are likely to both initiate and propagate a fracture, helping to focus attention on unstable areas where an avalanche might really occur
• SNOTELs: An extensive, automated system for collecting snowpack and related climatic data in the Western U.S., including Alaska; measures solar radiation, RH, snow depth, SWE and snow weight, air temperature, wind direction/speed, soil moisture, soil temperature, precipitation, barometric pressure
• Remote sensing
  • Covers large areas with near-uniform resolution, retrieves data from remote regions
  • Microwave wavelengths are best for assessing snow properties because their energy penetrates snowpack and is reflected/emitted from the surface and deeper within the pack; are sensitive to snow depth, SWE, snowpack temperature, snow crystal type, wet-dry state, soil conditions; penetrate cloud cover; operate day and night
  • Products: SWE, snow cover, snow depth, snow top temperature, snowmelt

You have reached the end of the Snowpack & Its Assessment module. Please consider taking the Quiz and sending a User Survey.

Contributors