Physical Properties of the Snowpack
Richard Brandt – University of Washington
Paul Smith’s College
21 Feb 2006
OVERVIEW
Microphysical properties:
Snow grain types
Mass density
Metamorphism
Thermal properties
Thermal Conduction
Thermal Convection
Optical properties
Spectral absorption in pure ice
Scattering vs absorption
Spectral albedo
Solar (allwave) albedo
Internal spectral fluxes and heating rates
Other properties we don’t have time to discuss:
Permeability
Gas transfer
Chemistry
Ice surface chemistry
Internal ice chemistry
Mechanics
Grain scale (micro)
Continuum scale (macro)
Microphysical properties of snow
Crystalline structure of ice:
Microphysical properties of snow
Primary snow grain types:
New snow crystal type depends on temperature and water vapor
supply.
Plate (new snow)
Column (new snow)
Dendrite (new snow)
Images from:
The Snowflake: Winter’s Secret Beauty
-- by Kenneth Libbrecht and Patricia Rasmussen
Microphysical properties of snow
Old (metamorphosed) snow crystal type depends on temperature
and temperature gradients in the snowpack.
Scroll or cup (TG metamorphism)
Rounded ice grains (ET metamorphism)
(Melt- Freeze Metamorphism)
(Firnification)
Microphysical properties of snow
Snow and ice density:
Snow/ice type
metric tons/m3 or grams/cm3
Dry new snow
0.05-0.07
Damp new snow
0.1-0.2
Settled snow
0.2-0.3
Depth hoar
0.1-0.3
Wind packed
0.35-0.4
Firn
0.55-0.83
(0.83 = air bubble closure)
Glacier ice
0.83-0.91
Bubble free ice
0.917
Microphysical properties of snow
Metamorphism:
As snow ages, the crystalline structure changes depending on the
temperature and temperature gradient, the presence of liquid water, and
the weight of the overlying snow:
“Destructive” Metamorphism increases snow density and grain size
ET (equitemperature) rounds the grains by vapor diffusion:
FT (freeze-thaw) makes grains cluster and round
Firnification compacts and rounds grains
“Constructive” Metamorphism decreases snow density and increases
grain size (also weakens snowpack and leads to avalanches)
TG (temperature gradient) vapor migration makes depth hoar
Thermal properties of the snowpack
Thermal conductivity k is a measure of a material’s ability to transfer
heat. A high value of k is a good conductor while a low value of k is a
good insulator. k has units of Wm-1K-1
Fresh Snow
Old Snow
Ice
0.03 (better than fiberglass insulation!)
0.4
2.1
Snow conductivity increases with density:
k 0.138-1.01r+3.233r2
Thermal properties of the snowpack
Snow conductivity also increases with ET metamorphism.
Snow will dampen temperature swings at the surface. For example, a
1 degree temperature change in 15 min at surface will only change
the temperature at 20cm depth by 0.1 degree, and at 1m depth by
0.01 degree.
Thermal Convection:
In rare instances of fresh snow with a large temperature gradient,
Thermal convection has been observed.
Optical properties of the snowpack
Sunlight can penetrate the snowpack, although the depth of solar
penetration is extremely dependent on the color of the light. While
ice does not absorb much in the ultraviolet, blue, and green parts of
the spectrum, it is highly absorbing in the red and near infrared.
Absorption coefficient s is the fraction of light absorbed in 1m.
1/s is the ‘e-folding’ of light penetration.
In snow, light is both scattered by grains and absorbed:
Optical properties of the snowpack
Albedo is a measure of light that exits upward through the
snowpack surface.
Albedo = Upward Flux / Downward Flux
Spectral Albedo depends on:
Wavelength (highest in the visible)
Grain size (inversely)
Solar elevation (inversely)
Snow depth (directly)
Water Content (inversely)
Optical properties of the snowpack
Solar or Allwave Albedo is the average albedo at all wavelengths.
Allwave Albedo depend on the variables above for Spectral
Albedo plus the cloud cover.
Clouds absorb red and infra red, so the light that is transmitted
through the cloud produces a higher average snow albedo than on
a clear day.
Allwave snow albedo
Clear sky
Cloudy Sky
0.45 to 0.81
0.55 to 0.87
Optical properties of the snowpack
Internal Spectral Fluxes
Spectral transmission is the percent of a particular color of light at a
certain depth in the snowpack, compared to the light at the surface.
The deeper light travels in the snowpack, the more the red and infrared
light is absorbed. At 1-2 m depth, all that is left is the blue light.
Snow Heating rates:
In dry snow over 90% of the solar heating is in top 1 cm. Light
deeper in the pack has a high albedo and is scattered out.
Dark objects, water, and ice lenses in the snow or at it’s base can
absorb the light deep in the snowpack which can strongly increase
heating rates.
Download this talk from the link at the bottom of:
http://www.atmos.washington.edu/~brandt/