|MadSci Network: Environment & Ecology|
The National Weather Service records precipitation from snow in two ways, inches of snowfall and water equivalent (how much rain would have fallen if the snow had been rain instead of snow). To measure the snowfall...human observers use a snowboard and a ruler. This may seem very low tech...but it works. Each time a measurement is taken...the snowboard is moved to the top of the snow. National Weather Service observations include the rate of current snowfall along with the amount of snow falling in the last hour, last six hours, and the total amount of snow on the ground.
Snow measurements are taken from a variety of locations around the observing site in order to obtain an average snowfall for the site. This avoids measurements that are too high in drifts, or too low in spots where the snow level may be scoured by the wind or physical obstacles. Automatic observing systems can measure snowfall...but with a great deal of difficulty and a large margin of error.
Automatic observing systems measure snowfall in a variety of ways. The most common are weighing the snow and converting the weight to a precipitation amount, and collecting it for measurement (whether frozen or liquid) by a visiting human weekly or even monthly.
The water equivalent of snow varies a great deal from place to place and from season to season. The accepted average is 10 inches of snow to one inch of water. In the western mountains...dry powder snow may have a water equivalent of 15 inches of snowfall to one inch of water. Very wet snow can yield one inch of water from 5 inches of snow. Observers in some locations take a core sample of the snow on the ground and melt it to find the actual water equivalent. In addition to meteorologists and climatologists who record precipitation, this information is useful to hydrologists interested in melting runoff, water supplies, and flooding potential. In the mountains, this information is useful to agencies that monitor avalanche potential.
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