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Snowpack & Its Assesment Table of Contents

SECTION 1 - INTRODUCTION

1.1 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.

2 photographs, one of the Alps, the other of a skilift

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.

Two photographs, one of snow removal, the other of an avalanche rescue

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.

Two photographs, one of a mountain stream fed by melting snowpack, the other of farmland in a drought.

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.

1.2 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.

"Hi, my name is Ethan Greene and I'm the director of the Colorado Avalanche Information Center. We're responsible for assessing the avalanche conditions around the state of Colorado and we do this through meteorology, forecasting, and avalanche forecasting. Part of avalanche forecasting is collecting information from the field so today we're up here in Caribou above the town of Nederland in the Front Range mountains of Colorado to take a look at the snowpack, see how it's formed, and how the stability is progressing."

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

“Information on snowpack is very important for hydrologic concerns about water supply and the potential for flooding or drought. The difference between replenishing groundwater in one case and flood-producing runoff in another can be simply a matter of the timing, speed, and efficiency of snowmelt.

Snowpack characteristics like water content, density, structure, and temperature profile all provide important information about when, how quickly, and how efficiently snow may melt. Combined with information about the weather conditions above and the soil conditions below, hydrologic forecasters can better predict the impacts of snowmelt.”

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

“For forecasters, snowpack assessment information is vital to carrying out their mission of enhancing winter mountain safety, predicting floods, and informing emergency managers and transportation representatives of dangerous snowpack conditions. Forecasters in complex terrain predict snowfall densities operationally. This information is used in two primary ways: first, to predict the depth of new snowfall and the impact of the storm; and second to update the snowpack status in terms of new layers and stability conditions.

The real-time precipitation-type, distribution of winds and temperature, and the potential for dust deposition are all primary components of atmospheric forecasts that contribute directly to snowpack evolution. Through these daily prognoses, forecasters work in collaboration with avalanche specialists, hydrologists, and public safety officials to reduce risks to human recreation and travel as well as property loss.”

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

“Hello, my name is Jay Irwin and I’m fortunate to be an avalanche survivor. Last year, I was in the backcountry of CO, skiing a well-known backcountry slope called Cupcakes, right off of Ptarmigan Peak. I had checked the Colorado Avalanche Information Center website and seen that the danger rose had been reduced from red to orange—from high to considerable. One of my rules is to never ski in the backcountry when it’s red or black—high or extreme. It’s just not worth it. However, orange is a gray area for me. On this particular day, I knew there were three feet of fresh powder. But when I saw that the snow had consolidated, I felt a little safer and decided that I would be able to do the run. As I went down, I got mesmerized and continued to ski down through the powder and the trees. It was phenomenal. It felt safe. And my ego got the better of me. When I reached Ptarmigan Knob, a well-known avalanche chute, I stood there for a while and looked at the snow. I knew that it had reduced and packed down, and I decided not to dig a hasty pit.

I’d been skiing for a few days and had had phenomenal snow and phenomenal success. I decided not to go down the chute and started hiking back up. But with three feet of snow, it was slow going. So I decided to try and see if I could cut the slope off at the top and see if it would go. As I made that slice, nothing went. So I thought that I could make five turns and be in the trees and be safe ... or go into an opening and potentially outrun anything that came after me.

I made three turns, three of the most incredible turns of my life. Snow over the head, coming over the shoulder. But as I made the third turn, I watched the entire face of the mountain break loose. It spidered. It whumpfed. And I saw it go fifteen to twenty feet over to my right. I took it all the way back to where I had made the cut and then across the back. And the entire slab of snow took me down. It took me down Two-birds Chutes, hitting trees along the way at high speed.

I broke my pelvis and leg, but was fortunate enough to swim and stay above the slab. I was only buried to my chest. I was able to dig out and get out of there eight hours later.

When I look back on it, I know I didn’t do things I should have done. I didn’t dig a hasty pit. I didn’t make sure I knew where I was going. And I went down a 38-degree slope that’s perfect for an avalanche to go. Those mistakes almost cost me my life.”

1.3 Module structure and 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.

Photographs of both snow-covered flat terrain and mountains 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.

SECTION 2 - SNOWPACK ENVIRONMENT

2.1.0 Global environment

2.1.1 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.

View of Earth with insets for the three levels of climate drivers (mesoscale and synoptic- and global-scale)

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.

2.1.2 Climate zones

Various schemes are used to categorize the world's climate zones, notably the Köppen-Geiger climate classification scheme. Roll your mouse over the names on the graphic to see the regions that they encompass.

Which zones would you expect to have snowpack during part or all of a given year? Select the correct choices below, then click Done.

The correct answers are a, b, c, d, e, f, and h.

Regions of semi-permanent snowpack (Polar tundra and Polar ice cap) are found at high latitudes across the globe. Seasonal snowpack is found at high elevations in all latitudes (Highland, Moist with severe winters, and Semi-arid).

Terrain is a major factor in determining the environment for snowpack. Most mountain ranges of the world receive enough snowfall to create annual or permanent snowpack and are in the Highland climate zone.

2.1.3 Snowpack types

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

Definitions of the types of global snowpack based on the Sturm et al. classification scheme.

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.

Maps of the northern hemisphere with the type of global snowpack indicated; based on the Sturm et al. classification scheme.

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?

2.1.4 Snow classes

Table that defines the various snowpack zones.

The table has been expanded to include the depth ranges of the various types of snowpack. Notice that on a global scale, maritime areas receive the deepest snowfalls. Why is this so? Select the correct answers, then click Done.

The correct answers are b and c.

The ocean supplies atmospheric moisture and relatively warm air temperatures that create large snow depths and heavy, wet, high-density snow.

2.1.5 Major mountain ranges

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

Image locating five of the major mountain rainges in the world

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

2.1.6 Elevation, latitude, and terrain

Map of the worldwide climate zones using the Köppen-Geiger climate classification scheme, with several of the zones highlighted for an exercise

Although maritime areas receive the largest snow events, high-elevation areas at middle and high latitudes (30 to 35 degrees and higher) have the largest average annual snowpacks. Why do they accumulate such deep snowpacks? Select the correct answers, then click Done.

All of the options are correct.

  • As latitude increases, winter temperatures get colder, creating conditions that are conducive to significant snowfall events. The cold temperatures also minimize cold season melting. Note that while some regions are cold enough for snow, they do not receive enough cold season precipitation to establish an extensive snowpack.
  • The polar jet stream defines the path of the strongest midlatitude storm systems, which produce heavy precipitation.
  • On average, higher elevations have colder temperatures.
  • Wind blowing upslope cools. If there's a large elevation gain and the air is sufficiently humid, clouds will form and precipitation will fall.

2.1.7 Annual precipitation in complex terrain

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.

CONUS annual number of days with snow cover

Most mountain ranges in the United States are generally oriented north-south. Which side of these mountains would you expect to receive the greatest annual number of days of snowfall? Select the correct answer, then click Done.

The correct answer is b.

Due to the large-scale prevailing west-to-east flow aloft that drives midlatitude storm systems, the upwind (or west in the CONUS) sides of mountains typically receive several times the snowpack depth of downwind sides at the same elevation.

2.1.8 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.

Glacier calving, Johns Hopkins glacier, Glacier Bay, AK, 1991

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.

Global glacier locations of sizes 1-5 km

2.1.9 Solar variation

Length of day progressing throughout year at latitude 45 ° NAt latitudes where snowpack is typically found, the amount of solar energy varies significantly throughout the year.

Although the smallest amount of solar energy reaches the Earth's surface in the Northern Hemisphere in late December, snowpack depths peak during late winter/early spring. This is due to two main factors.

  • Snowpack is cumulative, typically deepening throughout the cold season
  • Snowpack depth does not decrease significantly until melting offsets replenishment by new snowfall. In areas north of the Equator, daytime lengths increase most rapidly during the late March period—the point at which days become longer than nights. On average, this is when daytime melting exceeds new snow accumulation.

You might think that the coldest temperatures of the year would correlate with the shortest days. But that's not typically the case. The coldest ground and atmospheric temperatures of the year typically lag behind the winter solstice (December 21 in the Northern Hemisphere) when the minimum solar energy reaches the Earth's surface. That's because the thermal energy stored in the ground from the previous summer and fall delays the coldest temperatures until after the winter solstice.

2.1.10 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.

February 2009 remotely-sensed snowcover for the northern hemisphere

2.2.0 Regional and smaller-scale environment

2.2.1 Introduction

This plot shows how seasonal snowfall amounts vary across sites in the western United States, with striking differences occurring during some ENSO events.

Annual snowfall at 3 selected US western mountain sites

The peak times for annual snowfall maxima vary from early to late winter based on factors such as the typical progression of the jet stream, surface storm systems, and moisture sources. For example, the Cascade Mountains receive most of their snowfall in December and January, while the Denver area east of the Rocky Mountains peaks in March.

What causes these variations? What creates the different types of snowpack found around the world? In the previous section, we examined the global-scale factors that play a role in snowpack development. Now we'll explore the regional- and local-scale factors that have an impact, such as geography, precipitation type, and wind.

2.2.2 Influence of geography and 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.

Snow covered mountains in Alaska

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.

Map of Utah showing the east/west orientation of the Uinta Mountains

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.

2.2.3 Mountainous terrain, 1

Graphic for interaction that ask users to select the location on a mountain that's most likely to experience various conditions (most snowfall from an upslope event, fastest snowmelt, lots of drifted snow, and little snow accumulation due to slope steepness)

2.2.4 Mountainous terrain, 2

Below the graphics are detailed descriptions of each feature.

Area A, fastest snowmelt:

photograph of a mountain with a bare south-facing slope and a snowy north-facing slope

While the sun has a generally uniform effect over snowpack surfaces on flat terrain, it's different in complex terrain. The interaction of aspect (the direction that a slope faces), time of day, temperature, and cloud cover determine how snowpack depth evolves on mountain slopes. In general, in the Northern Hemisphere, south-facing slopes get greater solar exposure, which causes the most frequent and rapid snowmelt, and thus the lowest snow depths.

Area B, the most snowfall from an upslope event

Diagram of an upslope precipitation event

The prevailing wind direction during a precipitation event is critical for the resulting snow accumulation, with the upwind side of a mountain (the upslope side) typically receiving the most snow. Downslope winds tend to dry out the atmosphere and quickly reduce snowfall.

 

 

 

 

Area C, lots of drifted snow

Electron microscopy photo of a single stellar snow crystal with sectorlike extensions with an arm broken off during descent through the atmosphere

Wind has three primary effects on snowfall and snowpack.

  1. It fractures snowflakes in the air, breaking them into small pieces of ice that pack together and form dense layers of wind-deposited snow where they land.
  2. Wind picks up the top portion of the snowpack, fractures the crystals, and deposits them downwind, increasing snow depth and density where they fall.
  3. Wind enhances sublimation, the evaporation of snowpack. When relatively dry air is present, sublimation is always occurring (the top of the pack is always evaporating). Wind enhances the process by quickly removing the water molecules near the ice surface. Note that snow in tree canopies can sublimate particularly quickly due to the enhanced exposure to dry air and wind. In some areas, nearly half of the annual snowfall can sublimate in this way.

Spencer Logan of CAIC discusses wind redistribution of snow.

Here's Spencer Logan of CAIC dicsussing wind and its impact on snowpack: "We're standing in the Front Range of Colorado, where it's very windy. Behind me, you can see the effects of it. Above tree line, we see areas that have been blown bare during the winter, with the snow transported somewhere. Some gets picked up and moved higher into the atmosphere, but a lot of it gets deposited downwind. The big round roll is a very thick deposit of snow, which will probably last through most of the summer."

Remember to select the last option, D.

Area D, little snow accumulation due to slope steepness

The window of terrain steepness for avalanche occurrence.

In general, snow does not accumulate much on slopes steeper than 45 degrees. It slides off in avalanches during or immediately after snowstorms. (An avalanche is a large mass of snow or other material that moves swiftly down a mountainside or over a precipice.)

Note that the steepness angle can be as high as 50- or even 60-degrees in maritime climates, such as the coastal mountain ranges of North America.

Most avalanches tend to occur on slopes between 30 and 45 degrees. These avalanches are often large and can remove much of the snow from the slope. Avalanches rarely occur on slopes less steep than 30 degrees.

 

 

 

 

2.2.5 Variations in snow accumulation

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

Table of relative accumulation amounts of snow on various types of landscapes

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).

2.2.6 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.

NWS snow water equivalent analysis over U.S. for 22 March 2006.

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)

2.2.7 Density question

Assume that you're a hydrologist and want to know how much runoff a particular snowpack will produce. Let's assume that the snowpack has an SLR of 5:1 (meaning that if it were melted, the depth of water would be 20% of the original snow depth). If the snow depth is 20 in (51 cm), how much water is that equivalent to? Select the correct answer, then click Done.

The correct answer is b.

To get the liquid equivalent or SWE, you divide the snow depth (20 in) by the SLR (5:1). In this case, the SWE is four inches.

2.2.8 Snowpack depth and 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.

SWE from snowpacks with different snow-to-liquid ratios.

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.

Snow-liquid Ratio info for the CONUS

2.2.9 Density and wind

Two fresh snowfall scenarios

Consider these scenarios. Both take place in flat terrain, where it's snowing moderately. In the first scenario, the surface temperature is 30ºF (-1ºC) and it's windy. In the second scenario, the surface temperature is 20ºF (-7ºC) and the wind is calm. Which situation would you expect to have higher-density snowpack, all other factors being equal? Select the correct answer, then click Done.

The correct answer is a.

Scenario one is likely to have much denser snow accumulation due to the warmer temperature, which enables the air to hold more moisture. When the surface temperature is at or near freezing, the density of new snowfall is usually high. In addition, the stronger wind in the first scenario will fracture the snowflakes prior to depositing them, resulting in packed, denser snowpack.

2.2.10 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.

2.2.11 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)

Graph relating temperature, excess vapor pressure, and common crystal habits.

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.

depiction of dendrite, column, and needle crystal habits (types)

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.

Depiction of a plate crystal habit (type)

For more information on atmospheric microphysics, see the COMET module “Topics in Precipitation Type Forecasting” at http://www.meted.ucar.edu/norlat/snow/preciptype/.

SECTION 3 - SNOWPACK EVOLUTION

3.1 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:

“A lot of people think of snowpack as a bunch of snowflakes that fell and just, kind of, lie against each other, like cornflakes in a bag. Really what's happening in a snowpack is you're getting physical bonds forming between these ice crystals, so we actually form, maybe, more like an ice sponge that's got this complicated ice network and complicated air network within it, but it's really a solid material that's layered. The particles are not laying against each other, they're sintered and actually physically attached to each other.”

3.2 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.

Basic metamorphosis processes for level snowpack

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.

3.3 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.

Basic depiction of incoming shortwave and outgoing longwave radiation for snowpack

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.

Schematic of 2 scenarios at night: one with snowpack, one without.

3.4 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.

Melting is a phase change from solid ice to liquid water. As ice melts, it absorbs a great deal of energy from the surrounding environment, which significantly slows the warming of the snowpack. As a result, the temperature rise in a warming snowpack stalls at 0°C (the melting point) as ice changes to water. Illustration of the energy associated with melting snow
Freezing is a phase change from liquid water to solid ice. As water freezes, it actually releases heat, which significantly slows the cooling of the snowpack. Illustration of the energy associated with melting snow
Sublimation is a phase change from ice directly to water vapor. It occurs most commonly and quickly at the surface of a snowpack on dry, sunny days. Sublimation absorbs much more energy from the surrounding environment than melting, creating a shallow, stable layer just above the snowpack. This occurs despite the sunny conditions and inhibits the melting of the pack. Effect of sublimation on a snowpack

3.5 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).

3.6 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.

Illustration of convection in snowpack

SECTION 4 - SCENARIO 1: SNOWPACK ON FLAT LAND

4.1 Initial Conditions

4.1.1 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.

Scenario 1: snowpack on flat land

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).

Given the relatively dry snow, what crystal habits and sizes would likely comprise most of the snowpack? Select the correct answer, then click Done.

The correct answer is a.

The snow-to-liquid ratio is approximately 14:1 (40 divided by 2.9), which is generally considered an average to slightly-below-average SLR for this elevation. Due to the low density, we can infer that the majority of the snowfall consists of dendrites and aggregated dendrites. Small and rimed crystals have a lower SLR and result in denser snow.

4.1.2 Day 1, 6AM: 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.

Illustration of the general effects of trees on the snowfall distribution

How much more snow would you expect to accumulate in clearings than in treed areas? Select the correct answer, then click Done.

The correct answer is b.

The ground in treed areas receives 20 to 45% less snow than in adjoining clear areas primarily because of the interception by tree branches. But the size of the clearing is important too. If it's larger than the height of the trees, the area can become windswept, with the windblown snow accumulating in the forest. Note that when the snow on forest floors is untouched by the sun and wind, it's less prone to subsequent sublimation.

Ethan Greene on the impact of vegetation on snowpack:

Vegetation can have a large impact on snow. During precipitation events in winter, snow collects on the ground as well as on pine boughs. Because of the wind, the snow on the boughs densifies, temperatures change, and snow sheds and forms a moat around the trees. There's much denser snow next to the trees than in open areas.

If the snow on the pine boughs melts onto the pack, it changes the properties of the snow surface. The pine needles also add contaminants to the snow, changing its albedo. In addition, the trees can be a source of energy for the snowpack. Since they're much warmer than the surrounding environment, we can get lateral temperature gradients around the trees. The trees also act like terrain features in regard to the wind, causing drifts around vegetation stands and small tree clumps.

4.1.3 Day 1, 2PM: 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.

Afternoon of the first day, and melting of the snowpack has begun.

What is the albedo of the snow likely to be in the early afternoon? Select the correct answer, then click Done.

The correct answer is c.

The clear skies and fresh snow result in relatively high albedos, probably in the range of 0.7 to 0.8. This means that 70 to 80% of the solar radiation is reflected back to space.

4.1.4 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.

Small-scale effect of the sun's aspect on a large snow-covered boulder.

4.1.5 Day 1, 2PM: Temperature profile

On the afternoon of the first day, solar warming begins to melt the top layer of the snowpack. This graphic forms the basis for an exercise on thermal gradients; 4 arrows are drawn, each indicating a different direction and intensity, and users must select the correct one.

To what approximate depth does the sun heat the snowpack at 2PM? In which direction is a thermal gradient created? Select the vector corresponding to the correct strength and direction of the gradient, then click Done. Note that the vectors point from cold to warm, with their widths representing the strength of the gradient (strong vs. weak).

The correct answer is b.

Since we're in late autumn near the winter solstice, the upper snowpack surface will melt relatively slowly. The melting will last a few hours each day if atmospheric conditions remain calm and clear. This means that the thermal gradient is small and is directed upward. Recall that with new snow, solar radiation will warm 15 to 30 cm (6 to 12 in) of the top of the pack.

4.1.6 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.

The lowest portion of the snowpack, next to the ground, is warming slowly from conduction.

4.1.7 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.

Schematic of radiational cooling at night for the snowpack.

What happens to the very top portion of the snowpack, which melted earlier in the day? Select the correct answer, then click Done.

The correct answer is b.

Melting will stop because the surface temperature of the snowpack falls well below freezing after sunset. Crusting (the freezing of the snowpack surface) occurs quickly around sunset.

4.1.8 Day 1, 6PM: Temperature gradient

graphic for interaction asking users to select the right direction and strength of the thermal gradient in a nighttime snowpack scenario with radiational cooling

What would you expect to happen to the temperature gradient in the upper portion of the snowpack during the night if atmospheric conditions are clear and calm. Select the vector that corresponds to the correct strength and direction of the thermal gradient, then click Done. Note that the vectors point from cold to warm, with their widths representing the strength of the gradient (strong vs. weak).

The correct answer is b.

Radiative cooling will maximize as the evening progresses. The top few millimeters of the snowpack will cool very rapidly, resulting in a strong temperature gradient directed downwards.

4.1.9 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.  

Depiction of the top layer of a snowpack during early evening when strong radiational cooling occurs, causing crusting

4.1.10 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

Early morning scenario during frost and surface hoar formation.

4.1.11 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.

Snowpack for scenario 1 showing gravity always at work

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.

4.1.12 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.

Electron microscopy images of depth hoar crystals, one showing very small crystals, the other showing them at large, mature size

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.

4.2 New weather events

4.2.1 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.

piction of conditions the morning after the initial snowfall (day 2)

4.2.2 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.

Photograph of people being blown by strong winds during a snowstorm

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.

4.2.3 Impact of wind

2 snowpack scenes: on the left, drifted snow slab (day 6 of Scenario 1). On the right, fresh, undrifted snowfall (day 1 of Scenario 1).

Which snowpack would be impacted more by winds greater than the 5 m/s (~10 kt) threshold? Select the correct answer, then click Done.

The correct answer is a.

Drifted snowpack is harder to lift and move because the snow grains are typically fractured, smaller crystals. As they accumulate, they pack together more tightly, making them more resistant to wind. For a given wind speed, the denser the snowpack, the less the snow will be blown and redistributed.

4.2.4 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 deposition evident on the snowpack near Fremont Pass, Colorado, May 2009

As you can see, dust discolors the top of snowpack. What impact would you expect this to have on the albedo of the pack? Select the correct answer, then click Done.

The correct answer is c.

If the dust is not covered by additional clean snow, it will significantly reduce the albedo of the snowpack, causing the top of the pack to melt significantly during daytime. In fact, general springtime snowmelt speeds up significantly when “dirty” snow is present.

4.2.5 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.

4.2.6 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

Scenario for day 6 after a new snowfall.

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)

4.2.7 Impact of new snow

Scenario for day 6 after a new snowfall.

What are the most important impacts of the new snowfall on the snowpack? Select the correct answer(s), then click Done.

The correct is d.

Option A is incorrect because the wind speed is below the 5 m/s (10-kt) threshold for drifting. Option B is incorrect because depth hoar originates within the snowpack, not on top. Option C is incorrect because the new snow is relatively dry and lighter than the old snow (the SLR is 15:1 compared to ~10:1). Option D is correct because although the new snow is relatively light, it's deep enough that the additional weight will likely compact the old snow even further.

4.2.8 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.

Scenario for an upside-down snowfall.

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.

4.2.9 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.

Scenario for day 6 after a new snowfall.

What immediate impact would each type of precipitation have on the snowpack? Select the correct answer in the box beside each statement. When you are finished, click Done. Note that each precipitation type only matches one statement so it should only be selected once.

4.2.10 More about the precipitation types

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

New freezing rain event.

Freezing rain accumulates as a very dense, hard layer of ice on top of the snowpack.

Graupel falling on the layer of new snow.

Graupel acts as a very dense, heavy layer of particles, which can compact the top of the old snow. When graupel is buried, it is a weakly bonded layer.

Freezing drizzle event after the new snowfall.

Freezing drizzle accumulates as an ultra-thin, very dense, hard layer of ice on top of the snowpack.

Sleet falling on top of the new snow layer.

Sleet accumulates as a dense layer on the top of the snowpack and compacts the top of the old snow layer.

Rain falling on the new snow layer

Raindrops percolate into the top few cm of the snowpack, creating a wet snow layer. The mixture will eventually freeze when temperatures fall. When crusting occurs, the associated heat release caused by the freezing produces a temperature gradient in the snowpack and facets grow on the snow crystals just below the crust, forming flat edges. If snow falls after that point, we'll have a buried layer of faceted crystals next to a hard ice layer, which could lead to cracking and collapsing in the new snow layer.

4.3 Spring melting

4.3.1 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.

Dye test showing the transport of water through a snowpack

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.

4.3.2 Rain

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

Rain falling on snowpack

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.

SECTION 5 - SCENARIO 2: SNOWPACK IN MOUNTAINOUS AREAS

5.1 Initial conditions

5.1.1 Introduction

A depiction of the mountainous terrain in scenario 2, with an inset of the terrain in Scenario 1

This scenario takes place in mountainous terrain rather than the flat terrain of Scenario 1. How would you expect snowpack evolution to differ in the two scenarios based solely on the difference in terrain? For each process or event, select the correct answer, then click Done.

5.1.2 Scenario description

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).

A depiction of the mountainous terrain in scenario 2, with an inset of the terrain in Scenario 1

5.1.3 Gravity

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

Snowpack for scenario 1 showing gravity always at work

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.

5.1.4 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.

The window of terrain steepness for avalanche occurrence.

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:

5.1.5 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.

Orientation of the friction force, F, on the snowpack.

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.

The role of friction in 2 scenarios: a coffee mug on a plane of glass, and a coffee mug on a sandstone surface.

5.1.6 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.

A layer of snow has slid off a greenhouse window and deformed

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

5.1.7 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.

Which of the following grain types commonly form persistent weak layers within a snowpack? Select the correct answers, then click Done.

The correct answers are a and d.

Depth hoar and other faceted grains have edged, flat surfaces. These grains inherently form weak layers that can reduce stability over long periods of time. The other types of grains, such as pristine snow crystals, do not last very long or form strong, well-bonded layers. Note that if a strong layer overlies a weak one, the situation can be unstable.

5.2 Additional events

5.2.1 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.

A depiction of scenario 2 with its 40 cm of new snow on a sloped surface

5.2.2 Wind

New event: 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.

Snow distribution at the conclusion of the wind event.

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.

Wind scouring snow off of the windward side of the peak and depositing it on the leeward side. (Photograph courtesy of Richard Armstrong.)

5.2.3 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.

If the dust layer is covered by new snowfall, it will become internal to the snowpack. What impact might this layer have on the pack's stability? Select the correct answer, then click Done.

The correct answer is b.

If the dirty layer is close enough to the surface to absorb solar radiation (15 to 30 cm or 6 to 12 in), two things may occur:

  • The dust-laden surface will lower the albedo, creating a local temperature gradient; this will cause edge growth on the snow grains and create a weak layer
  • The submerged layer may melt, possibly leading to an avalanche of wet, slushy snowpack

5.2.4 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.

Snowpack scenario in mountainous terrain showing addition of new snow

What impact might the new layer of wet, heavy snow have on the pack's stability? Select the correct answer, then click Done.

The correct answer is b.

Since the lower portion of the snowpack is six days old, it has had time to evolve and could well contain depth hoar (a weak layer). The added weight of the new, heavy snow poses a risk that the weak layer may fracture or collapse.

5.2.5 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.

Snowpack scenario in mountainous terrain showing addition of new snow, with a higher SWE

5.2.6 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.

Depiction of a rain event followed by a wet, heavy snowfall

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.

5.3 Spring melting

5.3.1 Introduction

Dye test showing the transport of water through a snowpackIt's now March and springtime melting has begun. Melting produces liquid in or on the top of the snowpack, which is pulled along the slope or down through the pack by gravity. The rest of the process is similar to that described in the flat land scenario.

(To summarize, the routing of the water depends on the structure of the snowpack. If the liquid runs into obstacles, it will travel along the path of least resistance, which a layer may provide. If there's too much water, it will either pond or travel vertically. If the water encounters a capillary barrier, it will move parallel to the snow layers rather than through them.)

5.3.2 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.

Melting increases the potential for flooding.

The correct answer is a, true.

Snowpack runoff can cause flooding depending on the amount and capacity of the drainages.

Melting decreases stability at layer interfaces that have refrozen during cold periods.

The correct answer is a, true.

Layer interfaces can become liquid paths and subsequently freeze at night or during cold periods. These crusted layers may lead to significant hoar formation over long periods of time due to local temperature gradients.

Melting warms the atmosphere, which can subsequently accelerate the rate of snowmelt.

The correct answer is b, false.

Melting actually cools the atmosphere near the snowpack. If this latent cooling didn't occur, the snow would melt even more quickly.

5.3.3 Springtime snowpack stability

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

“In midwinter, snowpack stability changes as snowstorms come in and quickly load the pack, causing it to be unstable. As the snow adjusts to the new load, stability increases over the course of several days or even a week or more.

Right now, so much melt water is being produced in the snowpack that in the morning, after it's been frozen solid all night, it's relatively stable and very strong. As things heat up during the day, the bonds between the crystals start eroding and we lose stability. The pack will probably be the most unstable in the afternoon.”

5.3.4 Walking on springtime snowpack

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

“In springtime, the ease with which you can travel across snowpack changes dramatically, especially with elevation and daytime temperatures. During the middle of the day, you'll sink into the snow pretty deeply. Earlier in the day, the pack is frozen and hard, making it easy to cross.”

5.3.5 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.

A mountain stream fed by melting snowpack.

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:

“The water running down this road is melted snow that's flowing across the top of the ground. In thin areas of snow, the snow might actually be melting from all sides. The sun comes down into the top, melts the surface, heats up rocks on the ground, and melts the snow from the sides and bottom. Where snowpack is deeper, melt water is produced near or on the snow surface.

When the sun heats the snow and melts it, the water percolates down through the pack, hits a hard layer of snow, ice or perhaps the ground surface, and runs out onto the road, where the water flows downhill. As we go through several freeze/melt cycles, vertical columns of ice form within the snowpack. The melt water finds the path of least resistance and forms columns of preferential flow through the pack. When the water freezes at night, it forms an ice column.”

5.3.6 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? Select the best answer for each scenario. (Note that each should only be chosen once.) When you are finished, click Done.

SECTION 6: SNOWPACK ASSESSMENT

6.1 Onsite measurements

6.1.1 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.

Montage of insitu and remotely sensed snow measurements

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.

6.1.2 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.

Montage showing 2 onsite snow measurement techniques: SNOTEL and manual

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.

6.1.3 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.

Ethan Greene discusses the initial stage:

“This snow has been melting as we move into spring. The upper surface is pretty consolidated. It's actually pretty thick. It feels like ice layers are forming in the lower part of the snowpack.”

Ethan continues his discussion:

“Now we're looking at a cross section of the snowpack. You can see some of the layers that have formed due to the weather (heating, cooling, snow events, and wind events). At the top, we have just a little bit of snow that's undergoing melt today. You can actually see some of the suspended crust that's forming on the surface. A thicker crust is forming underneath, the result of multiple melt/freeze cycles over the last week or so. We see a few other little crusts forming from water percolation.”

6.1.4 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.

A hypothetical full snow profile, showing the temperature and hardness data

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.

6.1.5 Degree of wetness and 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.

Classification table showing the various degrees of wetness of snowpack samples.

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.

6.1.6 Degree of wetness and density, 2

Click the links to hear Ethan discuss the following topics.

“A federal sampler is a device that lets you easily measure the snow water equivalent through the entire depth of the snowpack. It's commonly used by federal agencies that are trying to figure out how much water is in certain basins for runoff and agricultural forecasting.

When you're in the field, you just need to write down a few things. Most of the math has already been done so you can read the numbers directly. First, you measure the weight of the empty tubes. I'll put this in the cradle here. We get a “tear” weight, which we'll write down. Then we'll take the tube and slowly push it through the snowpack. It's got some sharp teeth on the bottom. You really want to capture the snowpack in its natural setting and not press it too much. We read the depth of the snowpack off the height scale (99 cm in our case). We'll pull out the core and put it back on the scale. It's a bit tricky to balance it because it's a little off-kilter now. The scale reads our snow water equivalent directly. In our case, it looks like it's 65 cm.”

“One of the parameters that is useful to measure is the density of individual layers in a snowpack. To do this, we take a sample, which is a known volume, and weigh it. By knowing the weight of the sample and its volume, we get the density. This particular cutter is a 1000-cm volume. We gently push it into the snowpack, trying not to compress the snow at all. Then we slide the cutter down on top of the wedge, pull it out, and see if we have a good wedge. We've got a few gaps here but it's not too bad. Then we brush the snow off the side that's not part of our volume and weigh the sample. For smaller layers, say less than 10-cm tall, we use a 250-cc cutter. We gently slide the cutter in and cut the wedge. In this case, our sample is not very good, so we'll discard it and try again. There, we've got a good sample, so we can brush off the excess snow and record its weight.”

“A pit wall is a cross-section of a snowpack. It shows the evolution of the pack, from the winter's worth of snow that accumulated on the ground and metamorphosized through the different winter weather events and to its condensation and melting in springtime.

To examine the layers, I'll run my hand across the pit wall and feel for hardness changes. Since it's such a warm day, there's subsurface melt just below the snow surface, and a suspended crust is starting to form that might eventually turn into fermspegial. Underneath, we've have melt forms (polycrystals) that are undergoing melt right now. Below that, there's a harder crust that's the result of multiple melt-freeze cycles over the last few weeks.

The upper part of the pack is melting and starting to get some free water during the day. At night, it refreezes into a very hard layer.

This is what's left of the hard layer from last night. It will refreeze into a very strong layer tonight.

As we move down, different stages of melt metamorphism are occurring. A few, little, thin ice layers are forming as the water percolates down through the snowpack, pools in certain places, and refreezes. That's really what's happening in the upper part of the snowpack. We have a couple of harder layers.

This might have been the snow surface several few weeks ago when we were in a warm period (in March).

Here we see a fairly thick ice layer formed from melt water pooling down to a capillary barrier and refreezing.”

6.1.7 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.

“This may not be the type of snowboard you're used to seeing, but it's the snowboard we use to measure new snow events. You need to find a site where you can place the board right on the snow surface so it's as even to it as possible. That will allow snow from precipitation events to accumulate on the board without the board really affecting the accumulation very much.

Unfortunately we have a bright spring day with no new snow to measure, so I'll simulate a new snow event. … Snow is falling onto the board in a relatively even fashion. To get both the height and snow water equivalent of the new snow, you take this tube and press it down through the snow until it hits the board. That lets us measure its depth. Then we take the whole sample and weigh it with this calibrated scale. We can actually read the SWE directly. This one simple measurement provides both the depth of the new snow and its water content. ”

“One of the biggest problems that we face in regional avalanche forecasting is dealing with the spatial variability of snowpack properties. Snow changes as we move across elevation bands, slope aspects, and slope angles. The wind and sun cause some of these variations, for example, where there's very deep or shallow snow or melted layers vs. areas that stay dry. We need to take a lot of measurements in calculated places to get a full picture of what's happening over a large range. But sometimes, we get less measurements than we'd like.”

6.1.8 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.

Chart depicting the hand hardness test, which identifies a snowpack layer's degree of hardness, which has impacts for avalanche potential.

6.1.9 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.

Illustration of a ram penetrometer, a formal test for measuring the hardness of a layer of snowpack

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

Sample hardness profile measured by the ram penetrometer

What do you think the thin layer near the top of the snowpack consists of? Select the correct answer(s), then click Done.

The correct answers are b and c.

Penetrometer resistance is very high for the thin layer, indicating that the grains are tightly bonded and likely to be crust. It could also be a buried dirt layer, since these often form hard ice layers. It cannot be depth hoar or low-density grains; these have a low degree of bonding, which makes them easy to penetrate.

6.1.10 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.

Plate IX of "Studies among the Snow Crystals ... " by Wilson Bentley knows as the "The Snowflake Man." From the "Annual Summary of the "Monthly Weather Review" for 1902. Bentley's hobby was photographing snow flakes.

6.1.11 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.

A typical snowpack temperature profile. The solid line is the nighttime profile, while the dotted line is the daytime profile.

6.1.12 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.

Graphic depicting the manual Rutschblock test used to assess the shear quality of snowpack

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.

6.1.13 Snow stability and 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:

Diagram showing preparation for the Extended Column Test used to test snowpack stability

  • Isolate a vertical column 90 cm (35 in) across a slope by 30 cm (12 in) downslope
  • Apply pressure to one end of the column (the “loading” area) and see how much pressure it takes to transmit stress across the column to fracture it. Tap the end of the column up to thirty times: ten times from the wrist, then ten times from the elbow, and finally ten times from the shoulder
  • Note the the number of taps required to initiate a fracture; the number of additional taps required for the fracture to spread across the full column; and the depth of the fracture from the surface
  • Stop the test when the fracture has spread across the entire column or you have given it 30 taps

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 http://www.avalanche.org/~nac/NAC/techPages/articles/06_ISSW_Simenhois.pdf.

6.1.14 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.

A hypothetical full snow profile, showing the temperature and hardness data

Which of the following statements are true of the temperature profile (the line in blue)? Select the correct answer(s), then click Done.

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.

A hypothetical full snow profile, showing the temperature and hardness data; the 20-cm to 70-cm layer is marked for the feedback to the question

6.1.15 Snow profiles, 2

A hypothetical full snow profile, showing the temperature and hardness data

What can you infer from the hardness profile in red? Select the correct answer(s), then click Done.

The correct answers are a and c.

The extremely high hardness values at 15 and 70 cm (6 to 28 in) are indicative of ice. The deep snowpack above these shallow layers may be less stable and prone to slide if the slope is steep enough. The top 35 cm (14 in) or so exhibits very low hardness and is probably low-density snow. Given the layers with highly variable degrees of hardness, it's clear that there have been periods of melting and snowfalls of varying densities during the winter; it has not not been monotonous.

6.1.16 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. 

hotograph of a sample SNOTEL site that takes routine, standardized in-site snow measurements

The table shows the physical properties measured and tools involved. 

A list of the physical properties measured and tools involved in SNOTEL measurements

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.  

Snow Water Equivalent (SWE, in inches) for the Truckee #2 site in California, for 2008 and 2009. The average SWE progression at that site is also shown.

6.2 Remotely sensed data

6.2.1 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.

Aqua AMSR-E 89GHz three-day average of snow and ice cover over North America on 26 April, 2006.

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.

6.2.2 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.

NWS snow water equivalent analysis over U.S. for 22 March 2006.

6.2.3 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.

NOAA / NESDIS Snow Cover product for 03 Feb 2005

6.2.4 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.

NWS snow depth analysis over U.S. for 21 January 2006.

6.2.5 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.

NWS snowpack temperature analysis over U.S. for 24-hour period ending 6 UTC, 21 January 2006.

6.2.6 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.

NWS snow melt analysis over U.S. for 24-hour period ending 5 UTC, 22 March 2006.

6.2.7 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.

Screen grab of NOHRSC's interactive snow information tool

SECTION 7: 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.