Snow and Precipitation Data
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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.
(1 US gallon) x (1 ft3) x (62.418 lbs) x (1 ft) x SWE (inches) = Snow Load (lbs/ft2)
(0.1337 ft3) (7.48 gallons) (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 cubic foot.
Snow water equivalent amounts can be quickly obtained from the Natural Resources Conservation Service's SNOTEL (SNOw TELemetry) stations. There are 48 automated snow measuring sites in Alaska. Most of 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.
The snow load calculations above are for ground snow loads and will only provide an 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.
