Using SNOTEL Data to Estimate Snow Load Amounts
Snow survey data can be used to help determine the weight the snowpack
exerts on the ground at the site in that area or elevation zone. To determine
the snow load, one needs to know how much the snowpack weighs. The weight
of the snow varies with water content of the snowpack. The snow water equivalent
(SWE) or water content of the snowpack, is the amount of water in the snowpack
measured in inches if you were to melt the snowpack.
One could isolate a column of snow, melt it, and weigh it to determine
the weight over that area. However, this is very difficult, especially
if the snowpack is 5 feet deep and much more difficult if the snowpack
is 10-15 feet deep. An easier method to determine snow loads (if you have
a set of snow measuring tubes) is by measuring the snow water content and
using the following formula. If you are concerned about the snow load in
your area, many of the Natural Resources Conservation Service Field Offices,
located in most counties, have snow tubes and can assist you in
determining the current snow load information.
(62.418 lbs) x
(1 ft) x SWE
(inches) = Snow Load (lbs/ft2)
(1 ft3 of water) (12 inches)
Or just remember the conversion factor of "5.2" (or rounded to
5) to multiply the SWE value to estimate the snow load.
SWE (inches) X 5.2 = Snow Load (pounds/square-foot)
The average weight of water at 32 degrees Fahrenheit is 62.418 pounds per
And 1 cubic foot of water is also 7.48 gallons.
Snow water equivalent amounts can be quickly obtained the Natural Resources
Conservation Service's SNOTEL (SNOw TELemetry) stations. There are 75 automated
snow measuring sites in Idaho. These SNOTEL stations transmit daily snow
water equivalent, precipitation, and temperature data on a daily or more
frequent basis. Some sites also transmit depth of snow at the site. Click
here to view the Idaho SNOTEL Daily Snowpack / Precipitation
The following table illustrates the ground snow loads for selected snow
measuring stations in the Idaho Panhandle Region in 1997. The average SWE
for the selected dates is shown for reference.
||Snow Load (pounds/foot2)
The snow load calculated above is for ground snow loads and will provide
a indication of the roof snow load. Roof snow loads can vary depending
upon melting and re-freezing of snow and ice, roof slope, type of roof,
aspect, drifting, etc. Building codes vary depending on the elevation zone
and amount of snow and precipitation that falls. Building codes may also
vary depending upon the codes in effect when the structure was built.
Individuals can contact their local city or county agencies for specific
building codes in their area and the codes in effect when the structure
was built. Additional rain on snow can quickly increase the snow load because
a snowpack can absorb rain until its density is about 45%. Typically, the
snowpack is about 25-30% dense in January and 40-45% dense in April. The
snowpack will start melting when the density is about 45%. New snowfall
has an average density of 10%, i.e., 1 inch of snow water or rainfall =
10 inches of new snowfall.
Additional information can be obtained by contacting your local NRCS
office, or in 1986, the University of Idaho, Department of Civil Engineering,
published Ground and Roof Snow Loads for Idaho, by Ronald
L. Sack and Azim Sheikh-Taheri. To obtain a copy of this report, email the Department of Civil
Engineering at the University of Idaho or by phone 208-885-6783.