Snow

Snowflake Formation

The hexagonal (6-sided) structure of snowflakes reflects the molecular structure of water. Solid water forms when the molecules bond in a hexagonal lattice. Small and simple snow crystals (grains) grow slowly, with low temperatures and low humidity. Large and more complex snow crystals (flakes) form quickly with higher temperatures and higher humidity.

QUESTION: What general snowflake shape forms at (a) higher humidity; (b) lower humidity?

Snow definitions

More images of snow crystals

Electron microscope images of snowflakes

For a complete description of snow cover properties click here.

Click here for more information about snow formation.

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Metamorphism

The snowpack consists of ice crystals, water vapour and, with temperatures near 0°C, liquid water. Unlike ice, snow is permeable allowing air and water to circulate through the snow pack.

Snow metamorphism is the change in the structure and texture of snow grains which result from variations in temperature, migration of liquid water and water vapor, and pressure within the snow cover.

The flow chart shows the general pattern of snow crystal growth and decay. Equi-temperature (ET) metamorphism is destructive while temperature gradient (TG) metamorphism is constructive. Melt-freeze (MF) metamorphsim occurs when the air temperatures fluctuate above and below freezing.

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Equi-temperature Metamorphism (ET)

ET metamorphosis occurs:

• in snowpacks that are nearly isothermal or
• when the snowpack is deep

In weak temperature gradients (

QUESTION: Why is equi-temperature metamorphism referred to as "destructive"?

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Complete description of equi-temperature metamorphism snow grain types.

MORPHOLOGICAL CLASSIFICATION			PROCESS-ORIENTED CLASSIFICATION		ADDITIONAL INFORMATION ON PHYSICAL PROCESSES AND STRENGTH
Basic Classification	Subclass	Shape	Place of Formation	Classification	Physical Processes	Dependence on most important parameters	Common effect on strength
Decomposing and fragmented precipitation particles	Partly decomposed precipitation particles	Partly rounded particles, characteristic shapes of precip. particles still recognizable	Recently deposited snow	Initially rounding and separation	Decrease of surface area to reduce surface free energy at low temperature gradients	Speed of decomposition decreases with decreasing snow temperature gradient	Strength decreases with time; felt like arrangement of dendrites has modest initial strength
	Highly broken particles	Packed, shards or rounded fragments of precipitation particles	Saltation layer	Wind-broken particles; initially fractured then rapid rounding due to small size	Fragmentation particles are closely packed by wind; fragmentation followed by rounding and growth	Fragmentation and packing increase with wind speed	Quick sintering results in rapid strength increase
Rounded grains (monocrystals)	Small rounded particles	Well-rounded; particles of size	Dry Snow	Small equilibrium form	Decrease of specific surface area by slow decrease in number of mean grain diameter; equilibrium form may be partly faceted at lower temperatures	Growth rate increases with increasing temperature gradient; growth slower in high density snow with smaller pores	Strength increases with time, density and decreasing grain size
	Large rounded particles	Well rounded particles of size >0.5mm	Dry Snow	Large equilibrium form	Grain-to-grain vapor diffusion due to low to medium temperautre gradient; mean excess vapor density remains below critical value for kinetic growth Same as above	Strength increases with time and density and decreasing grain size
	Mixed forms	Rounded particles with few facets which are developing	Dry Snow	Transitional form as temperature gradient increases	Growth regime changes if temperature gradient increases above critical value of about 10°C.m	Grains are changing in response to an increasing temperature gradient	Desintering and decrease in strength

Click here for more snowflake images.

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Temperature Gradient Metamorphism (TG)

QUESTION: Why is temperature-gradient metamorphism called "constructive"?

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Complete desciption of temperature gradient metamorphism snow grain types.

MORPHOLOGICAL CLASSIFICATION			PROCESS-ORIENTED CLASSIFICATION		ADDITIONAL INFORMATION ON PHYSICAL PROCESSES AND STRENGTH
Basic Classification	Subclass	Shape	Place of Formation	Classification	Physical Processes	Dependence on most important parameters	Common effect on strength
Faceted crystals	Solid faceted particles	Solid faceted crystals; usually hexagonal prisms	Dry snow	Solid kinetic growth form	Strong grain-to-grain vapor diffusion driven by large temperature gradient; excess vapor density above critical value for kinetic growth	Growth rate increases with temperature gradient and decreasing density; may not occuring in high-density snow because of small pores	Strength decreases with increasing growth rate and grain size
	Small faceted particles	Small faceted crystals in surface layer;	Near surface	Kinetic growth form at early stage of development	May develop directly from precipitation particles or partly decomposed precip. Particles due to large, near-surface temperature gradients	Temperature gradient may periodically change sign but remains at a high absolute value	Low strength snow
	Mixed forms	Faceted particles with recent rounding of facets	Near surface	Transitional form as temperature gradient decreases	Faceted grains are rounding due to decrease in temp. gradient
Cup-shaped crystals; Depth hoar	Cup crystals	Cup-shaped striated crystal; usually hollow	Dry Snow	Hollow or partly solid cup-shaped kinetic growth crystals	Very fast growth at large temperature gradient	Formation increases with increasing vapor flux	Usually fragile but strength increases with density
	Columns of depth hoar	Large, cup-shaped striated hollow crystals arranged in columns (	Dry Snow	Large cup-shaped kinetic growth forms arranged in columns	Intergranular arrangement in columns; most of the lateral bonds between columns have disappeared during crystal growth	Snow has almost recrystalized; high recrystalization rate for long period at low snow density and high exterrnal temperature gradient facilitates formation	Very fragile snow

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Other Forms of Metamorphism

Gravitational metamorphism

Snow settles as it accumulates so that the depth of snow on the ground is always less than the initial amount of snowfall, especially where snow falls as large flakes and settles under the usually milder conditions near the ground. The rate of settling is directly related to snow density and depth.

Melt-freeze (MF) metamorphism

Melt-freeze metamorphism occurs when the sun melts the upper layers of the snowpack during the day, but freezing still takes place at night. With MF metamorphism, larger grains grow at the expense of smaller ones, helping to strengthen the snowpack in most cases. With MF metamorphosis, the density of the snow pack can increase to 0.6 g cm^-3.

All photographs by S. Colbeck

MORPHOLOGICAL CLASSIFICATION			PROCESS-ORIENTED CLASSIFICATION		ADDITIONAL INFORMATION ON PHYSICAL PROCESSES AND STRENGTH
Basic Classification	Subclass	Shape	Place of Formation	Classification	Physical Processes	Dependence on most important parameters	Common effect on strength
Wet Grains	Clustered rounded grains	Clustered rounded crystals held together by large ice-to-ice bonds; water in internal veins among three crystals or two-grain boundaries Wet Snow	Grain clusters without melt-freeze cycle	Wet snow at low water content, pendular regime; clusters form to minimize surface free energy	Meltwater can drain; too much water leads to slush; freezing leads to melt-freeze particles	Ice-to-ice bonds give strength
	Rounded poly-crystals	Individual crystals are frozen into solid polycrystaline grain; may be seen either wet or frozen	Wet Snow	Poorly bonded; rounded single crystals	High liquid content; equilibrium form of ice in water	Water drainage blocked by impermeable layer or ground; high energy input to snow cover by solar radiation, high air temperature or water input	Little strength due to decaying bonds

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Conductive Heat Flow

The conductive heat flux though snow on lake ice can be calculated from a few simple parameters: snow temperature gradient, snow density and snow thermal conductivity.

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Conductive Heat Flux through Snow on Lake Ice

What is conductive heat flow?

1. When water freezes, latent heat is produced.

2. In the case of water freezing on the bottom of a lake ice (or sea ice) cover, the latent heat is conducted through the ice and snow to the atmosphere along the negative temperature gradients (red).

3. The magnitude of the conductive heat flow is determined by the thermal conductivity of and the temperature gradients in the snow and ice. The latter are a function of the depth of the snow and ice, and the temperature at the top and bottom of the snow surface and at the bottom of the ice.

4. To calculate the conductive heat flow from the bottom of the ice to the top of the snow, we only need to know the snow variables (depth, top and bottom temperature, density [from which we derive thermal conductivity]).

Snow temperature gradient (Tgrad)

The change in the temperature of the snow with depth expressed by the equation:

Tgrad = (Ts - Tb) / Zs

where Ts: snow surface temperature (°C),
Tb: snow bottom temperature (°C), and
Zs: snow depth (m).

Tgrad is expressed as °C m-1 (°C/m).

The snow temperature gradient increases as air temperatures (snow surface temperatures) decrease.

Snow Density (ρ)

Snow density is the mass per volume and is expressed by the equation:

ρ = Mass (g)/Volume (cm3) with units g cm-3 (g/cm3)
ρ = Mass (kg)/Volume (m3) with units kg m-3 (kg/m3).

Snow densities vary between as little as 50 kg/m3 (new snow) and 500 kg/m3 (wet snow). Higher density (heavier) snow typically results from higher temperatures and/or winds while lower density (lighter snow) usually results from colder air with less wind. The density will increase over time due to snow settlement.

Snow Thermal Conductivity
(keff, i.e. effective thermal conductivity)

The thermal conductivity of the snow cover can be calculated as a function of its density.

If snow density is ρ If 0.156 > ρ

keff is expressed as W m-1 K-1 (W/m/K, Watts per metre per Kelvin)

Conductive Heat Flow (Fa)
The conductive heat flow is determined by the temperature gradient in the snow and its thermal conductivity.

Fa = (Tgrad)( keff )

Fa is expressed as W m-2 (W/m2).

QUESTIONS:

1. What is the conductive heat flux through the top pair of snow covers?
2. What difference does the snow density make?
3. What is the conductive heat flux though the bottom pair of snow covers?
4. What difference does the temperature gradient make?

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Other Properties

Snow Albedo

Snow albedo changes as the state of the snow cover changes. New snow albedo is 0.90 and it decreases as the snow goes through the densification process or begins to melt.

The albedo effect of the snow-temperature feedback can be described as follows. If a snow covered area warms and the snow melts, the albedo decreases, more sunlight is absorbed, and the temperature tends to increase. The converse is also true: if snow forms, a cooling cycle happens. The intensity of the albedo effect depends on the magnitude of the change in albedo and the amount of insolation.

QUESTION: When will snow albedo be the highest? lowest?

Snow Water Equivalent (SWE)

Snow Water Equivalent (SWE) is the amount of water contained within the snowpack. It can be thought of as the depth of water that would theoretically result if you melted the entire snowpack instantaneously. Changes to the water equivalent snow depth result from snow accumulation and snow melt.

Snow water equivalent can be presented in units of kg/m3 or meters of depth of liquid water that would result from melting the snow. SWE is the product of depth and density:

SWE = depth (m) x density (kg/m3) (units: kg/m 2)
SWE = depth (m) x density (kg/m 3) / density of water (kg/m 3) (units: m)

(Remember: 1 cm3 (1 millilitre) has a mass of 1 gm or 1000 cm3 (1 litre) has a mass of 1000 grams (1 kilogram).)

QUESTION: What is the SWE of 15.7 cm of snow with a density of 0.175 gm/cm3?

See mean monthly snow depth and snow water equivalent for sites in Alaska.

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Snow Module

After completing this module, you will be knowledgeable about snow, snowflakes, and ice. You will have all the information and tools you need to create your own activities and lesson plans.

You will be able to show your students how to:

• Use weather data to predict the shapes of snowflakes (grades 3-8);
• Design experiments to study snow (all grades);
• Investigate ice freeze-up and break-up of a stream, river, lake or pond (grades 5-8) (Globe protocol);
• Measure snow temperature, mass and depth (various grades);
• Calculate snow volume and density (grades 5-12);
• Calculate the rate of heat flow from the ground through the snow to the atmosphere or from the water through the river/lake ice and snow into the atmosphere (grades 5-12);
• Use mathematics within science (all grades); and
• Create and use mathematical calculations in an excel spreadsheet (grades 5-12).

All teaches are encouraged to become members of Teachers' Domain. It’s easy and it’s free. Becoming a member gives you access to an extensive library of free digital media resources and lesson plans produced by public television, designed for classroom use and professional development.

Delena Norris-Tull, a science educator, and Kim Morris and Martin Jeffries, two snow and ice scientists, designed these web pages for teachers. A number of science teachers have provided inspiration and developed activities. Martin first started working with teachers through the Teachers Experiencing the Arctic and Antarctica (TEA) project, taking a teacher with him to conduct research in Antarctica. Martin, Delena, and Kim, and middle school science teacher Ron Reihl started collaborating with teachers in Alaska in 2000. The Alaska Lake Ice and Snow Observatory Network (ALISON) project was the outgrowth of that collaboration. Quickly, teachers from other states became interested in research on ice and snow. To date, teachers from Alaska, Connecticut, Montana and Oregon have been involved in the ALISON project.

So now, let’s get started........