Arctic Engineering Module 4
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This is Jon Zufelt of the US Army Cold Regions Research and Engineering Laboratory and UAA Adjunct Faculty,
welcoming you to Module Number 4 Snow Properties. The module deals specifically with Snow Properties with an
emphasis on how they relate to Snow Hydrology. Snowmelt and runoff are the main causal factors for ice jams and ice
runs and hence the topic is important in understanding Ice Engineering. Special thanks are given to Don Cline of the
National Operational Hydrologic Remote Sensing Center of the National Weather Service for providing many of the
images in this presentation.
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So why is it important for us to understand snow? Whether it is an appreciation of the variability of the snow properties
aerial extent and ways to measure it, or how it affects us in terms of recreation, transportation and water supply, snow is
an interesting topic. We will cover properties and measurements in this presentation with focus on Snow Hydrology. A
new on-line course titled Snow Engineering is soon to be available in the Spring Semester 2004, which will cover snow in
much more detail.
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While the snow hydrologic cycle covers snow fall to melting and ground water recharge. The topic that receives the most
attention is flooding induced by snowmelt. The Red River of the North Floods in 1997 from Wahpeton, North Dakota to
Winnipeg, Manitoba caused billions of dollars in damages and loss of lives. The North East Floods in January 1996 were
equally as bad. Some parts of the Kennebec river basin in Maine lost over three feet of snowpack in the course of one
weekend, resulting in massive ice jams along the Kennebec and its tributaries.
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In order to avoid damages like these in the future we must be aware of the potential danger of the snowmelt flooding. To
do this we must understand the processes of snow accumulation, ablation or melting and melt water run off. Several
numerical models attempt to predict flow in stages in rivers based on the understanding of these processes.
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Arctic Engineering Module 4
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Understanding the Snow Hydrology involves the quantification of several processes which are often estimated due to
gaps in knowledge or due to complexity of these processes. The quantity of water held in the Snowpack can be measured
or estimated by empirical formulas based on long term averages. Water lost to sublimation and interception as well as the
timing and magnitude of the melt are functions of many physical variables that are often not measured. The fate of the
melt water and how it makes its way to the rivers and streams is often based on engineering hydrology models of runoff.
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Arctic Engineering Module 4
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If snow fell on completely flat terrain with no variations in vegetation, incident sun angle, accumulation rate or other
factors, we might have an easier time to predict how it would melt. Elevation, aspect angle, shading from and interception
by trees, wind, drifting and variations in the density and intensity of falling snow all give rise to variations in the snow cover
distribution.
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Variations in Snow Cover Distribution can be considered in terms of spatial scales. With Macro Scale representing
variations that would be expected to result from changes in major weather systems, such as differences in snow fall from
the North Central Planes in the US vs. a major coastal storm heading up the east coast. Meso Scale variations would be
due to the terrain features wind and vegetation zones such as differences between the snow fall at the base and the top of
the mountain at Alyeska Ski Resort. Micro Scale differences are those that are due to factors such as shade behind a
boulder the interception by a tree canap or the difference between the roadway and the ditch, due to drifting.
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In general terms, the depth of seasonal snow cover increases with elevation, if all other factors remain constant. This is
due to the increased number of the snow fall events lower temperatures and decrease in evaporation and melt that occur
with increased elevation. The increase with elevation is not constant from year to year just as the maximum pack of snow
depth in any location varies from year to year. Other factors, such as: wind weather systems aspect etc. can also offset
any elevation vs. depth relation.
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Vegetation can also affect the snow distribution on meso- and micro-scales. Air flow above and within the canap can
redistribute falling snow in the vicinity of the canap and surrounding open areas. The canap may intercept falling snow
holding it and allowing it to sublimate or later fall to the ground, increasing the density of the snow when it hits the ground.
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The difference between the interceptions of various coniferous species is low compared to the difference between
conifers and deciduous trees, with conifers being much higher. Cohesion between the snow particles allows the snow to
remain in the tree canap longer; allowing for the sublimation losses.
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Studies also show that there is a greater accumulation of snow in clearings than within the forested areas. The main
reason for these differences is interception and sublimation losses, rather than snow blowing from the trees into the
clearings.
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In open and exposed terrains, meso- and micro-scale terrain and vegetation differences give rise to large variations in
snow distribution. This slide shows examples of wind causing variations on the mountain on the lee side of a hill but also
on the lee side of a single rock. Aspect or shading can also have a profound effect.
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Even among open environments snow distribution can vary based on the local ground snow cover and topography. The
table here shows the relative accumulation values normalized to the accumulation on level plains under fallow conditions
(or ungrazed, unfarmed pasture).
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Snow fall and snow accumulation are distributed irregularly across the lower 48 and Alaska as well The top figure shows
the mean annual snow fall, while the bottom figure shows the average number of days per year when there is at least one
inch of snow on the ground. Both show the effects of latitude and elevation, as evidenced by the mountains, but the snow
fall is also much greater around the Great Lakes region because the lake affects the snow storms.
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Winter precipitation may fall as snow sleet or freezing rain. Sleet and freezing rain are very close to the density of freezing
rain for being mostly ice, while snow can vary significantly. Snow is usually measured in terms of its water equivalent or
the water depth equivalent that would result if you melted the snow fall. As a general rule, the water equivalent of the
newly fallen snow is about 10% of the snow depth. The density of the fresh snow can actually vary from 40 to 320kg/m3
or the water equivalent of 4 to 32% of snow depth.
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Reported values of snow include snow fall or the amount of the new snow that has fallen since the new report, typically a
24 hour period measured at the same time each day. Also snow depth or snowpack is typically reported and consists of
the total measured depth of snow at the reporting location, which includes the new snow as well as the old snow that has
not melted.
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The parameter most often studied is the peak snow water equivalent or (SWE). The peak SWE is the maximum value of
accumulated water equivalent contained in a snowpack over the season. Values of SWE are periodically measured during
the winter and this parameter is often modeled in snow accumulation and ablation models.
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Snow Measurements generally are made to determine the snow water equivalent, but also for determination of other
snow properties. Chlorine, depth of measurements, snow courses and snow pits are all used to characterize the
snowpack.
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Ground observations to measure snow water equivalent include snow pillows at the SNOTEL stations in the western US
or snow courses. Repeatable transects where snow depth and density is measured periodically over the winter. Snow
tubes or cutters are used to measure volume and mass of the cores, to determine density and SWE. Snow pits are dug to
make series of detailed measurements such as vertical profiles of SWE or density within a snowpack.
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Some of the measurement devices used in determining characteristics of snow includes the snow board which is placed
on the ground or at a specific snow depth to make snow depth measurements easier and more consistent. Snow stakes
are semi-permanent stakes marked in inches for measuring snow depth at stations where repeated measurements are
made.
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Snow gages come in a variety of sizes and types depending on the frequency of needed measurements and the total
amount of snow expected. One type is an 8-inch vertical cylinder that accumulates snow. The contents are then melted
and measured in a smaller diameter graduated cylinder.
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For stations that receive only infrequent visits over the winter, storage gauges are used. These large cylinders can be 1.2
to several meters tall and are charged with environmentally acceptable antifreeze to prevent freeze-thaw damage. The
precipitation is then weighed to calculate the water equivalent.
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Recording gauges periodically weigh the precipitation. This can be done on a storage-type gauge by fitting it with a
pressure transducer that monitors the total depth of precipitate or by a tipping bucket type gauge. These gauges
accumulate precipitation in a small reservoir until full (usually on the order of 0.1 inch of water equivalent or less) and then
dump it out and record the increment. Tipping bucket gages must be kept heated to avoid freeze damage and to measure
snow water equivalent.
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The snow pillow is a flat, fluid reservoir usually made out of stainless steel. A transducer measures the pressure inside the
pillow which can then be related to the water depth equivalent of the snow resting on top of it.
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This slide shows a typical snow measuring site such as those used at the SNOTEL sites. A snow pillow sits in the center
and surrounded by the snow steaks. A storage gage is in the background with a wind screen protecting the opening. The
shed houses recording and t telemetry equipment.
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The SNOTEL (or Snow Telemetry) system is a cooperative Federal-State-private snow survey program administered by
the Natural Resources Conservation Service (NRCS) which was formerly the Soil Conservation Service (SCS) and this
was in the western United States. The web address for the SNOTEL system is provided on this slide. Some data
stretches back to 1935. The data is updated continuously to the website.
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Snow courses are locations where measurements are made several times each winter and consist of snow depths and
cores for determining density and snow water equivalent. Data on snow course measurements can also be found on the
SNOTEL website.
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Snow pits are usually dug to provide detailed information about the snowpack. The properties of the layers from different
storms can be assessed as well as the danger of avalanche.
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NOA also conducts an airborne Snow Survey Program, which attempts to measure the snow water equivalent by
correlating it to the attenuation of naturally occurring terrestrial gamma radiation. Specific flight lines are flown several
times over the winter, as well as before and after the snow is present, to provide background radiation values.
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Snow measurement by optical remote sensing whether an airborne or a satellite some perform cloud cover problems. In
fact that water and ice properties are very similar in the optical wave lengths, observations of the snow covered area (or
SCA) can be easily made, however, and can be correlated to average snow depth measurements to provide SWE
estimates.
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What is a snowpack? It is porous medium composed of ice, air and sometimes of liquid water. It is generally composed of
distinct layers that correspond to individual storm or snow fall events. The ice can be in the form of delicate crystals of
newly fallen snow, all the way to coarse grains near the base of the snowpack.
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This slide shows the interrelationship of the many physical characteristics of a snowpack.
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As stated before, the snow water equivalent (or SWE), is the most commonly measured characteristic of the snowpack. It
is related to the snow density and depth by the relation; SWE equals the snow depth, times the snow density, divided by
the density of water.
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The density of the various layers in the snowpack can vary quite a bit from the snow surface down to its base; the chart
here shows both the density in kg/m3 and the amount of snow depth necessary to obtain one inch of water equivalent for
various types of snow; for types ranging from wild snow (or the snow falling in the air) all the way to the older fern.
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Grain shape can tell the most about the evolution that the snow crystals have undergone. Grain shape of the individual
layers can also give an idea of the potential for avalanches.
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Grain shape is further described as to its appearance, surface conditions and its inter-connection with the adjacent grains.
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This slide shows some electron micrographs of several types of grain shapes, note the differences between single
crystals with early rounding, such as fresh snow near the surface; wind blown grains are all broken apart, melt freeze
grains with and without liquid water present in the hollow faceted grain of older fern near the base of the snowpack.
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Grain size or the maximum dimension of a single discernable particle, whether a single crystal or an agglomeration of
many crystals, is termed very fine to extreme based on the sizes in the chart shown.
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Liquid water content or wetness of the pack is measured in % volume. Dry snow exists at temperatures below 0 degrees
Celsius and exhibits very little tendency for the snow grains to stick together. On the other hand, good Snowman-making
snow is somewhere between (Moist) and (Wet).
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The temperature of a snowpack is either isothermal or there is a temperature gradient ranging from near 0 degrees
Celsius at the base of the pack to a value less than 0 degrees Celsius at the snow surface. The temperature gradient will
depend on snowpack thickness, density, porosity, and air temperature above the snow.
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The temperature gradient also exhibits a diurnal variation as depicted in this slide. The variations in air temperature above
the snow surface dissipate with depth into the snowpack. On this slide you see variations in temperature of day to evening
are gone to about 20 centimeters in depth.
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Snow crystals will change in size and shape with time, temperature, and pressure. This is referred to as snow
metamorphism.
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Characteristics of the snowpack that are changed by metamorphism are density, porosity, strength, thermal conductivity,
and albedo.
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Snow undergoes metamorphism because it is close to its melting temperature and it is thermodynamically unstable. New
snow crystals (think of a new snow flake) have a high surface area to volume ratio and thus a large surface free energy.
The minimum surface free energy to volume ratio is for a sphere. Thus the natural progression from high surface energy
to low would result in the flakes gradually losing their structure and turning into snow blobs. Snow layers are also
compacted due to pressure by the weight of newly fallen snow above them.
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There are two types of snow metamorphism. Dry metamorphism occurs at temperatures less than 0 C and when no
liquid water is present. In this case, the solid crystals are in equilibrium with the water vapor surrounding them. Wet
metamorphism occurs at 0 C when liquid water is present.
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Dry metamorphism is driven by water vapor movement within the pores of the snowpack. The water vapor movement is
driven by the vapor pressure gradient and is controlled by temperature, grain size, and radius of curvature of the snow
crystals.
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Equitemperature dry metamorphism is destructive and destroys the crystal structure. Temperature gradient dry
metamorphism, on the other hand, is constructive and builds larger grains.
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Under equitemperature dry metamorphism, the surface free energy is reduced to its stable state. Think of the shape of a
snowflake. There is a higher vapor density over the points and the vapor will travel from the points to the hollows. The
surface area to volume ratio is reduced and therefore density increases which in turn increases the strength of the
snowpack. The smaller, rounder individual crystals come into contact with an increased number of other crystals,
increasing the bonds and snowpack strength.
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For temperature gradient dry metamorphism, the rate of vapor transport is very fast and there must be a temperature
gradient of at least 10 C/m of snow depth. Since the vapor pressure is higher where the temperature is greater, the most
vapor and therefore changes occur near the base of the snowpack. Angular, faceted grains are formed which have very
poor bonding with one another. The strength decreases and the density can also decrease. The snow density must
remain below 350 kg/cubic meters to maintain adequate vapor flow.
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Wet snow metamorphism occurs when there is liquid water content in the snowpack. Rates are accelerated over the dry
equitemperature metamorphism and smaller grains are destroyed preferentially. Larger grains become rounded with the
end result of large, bonded, round grains. This is the ice pellet-like snow left in your yard at the end of winter.
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This presentation was meant to provide an understanding of some of the characteristics and properties of snow, the
measurement techniques used for determining snow water equivalent, and how snow changes with time, temperature,
and pressure.
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