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Measuring snowfall a complex procedure
Tuesday, 13 January 2004
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You hear that your favorite ski area just received 16 inches of new snow and you're raring to try out that new snowboard. But was there really 16 inches? How is it measured, and by whom?

Unlike many weather elements, measuring snowfall is more than just reading a number off a dial. It is probably the most difficult weather parameter to measure accurately and consistently.

We usually hear only about the depth of snow, but this is just one of the parameters measured to characterize snowfall. As any powder rider will tell you, the snow in the Sierra is often a heavy, wet accumulation of snow, not so fondly referred to as ``Sierra cement'' when compared to fluffy powder in the Rockies or else where.

This is a matter of the water content of the snow, an important factor for water managers and dam operators when it begins to melt.

The recommended method of measuring snow depth is to use ``snowboards.'' Unlike the object you strap to your feet before careening down a snowy slope, this type of snowboard is simply a flat wooden surface upon which snow can accumulate. Snowboards are preferable to uneven surfaces or an object with heat that might affect the snow's characteristics.

The National Weather Service standard for measuring snow is to take a measurement every six hours, starting with a snow-free board, with four six-hour amounts added together for a 24-hour total. If measurements are taken less frequently, the weight of the snow causes it to compact, and the depth that is measured will be smaller.

The location of snow measurements is also a crucial factor. Ideally, readings should be taken away from structures or terrain features that cause drifts or valleys of snow. Multiple readings should also be taken in the same area, with an average value being representative of the overall depth.

New technologies to measure snow depth include ultrasonic sounders. These devices are mounted on poles above the snow and bounce a sound wave off of the top of the snow to determine its depth.

The water content of snow is also measured using a variety of methods. One of the simplest ways involves a heated rain gauge that melts snow as it hits the gauge and then is measured as liquid. On another device, large platforms called snow pillows respond to the weight of snow upon them. The density of the snow is measured as an antifreeze liquid is displaced in proportion to the weight of the snow.

Cosmic rays are sometimes used. Sensors that measure cosmic radiation are placed above and below the snow. The water content of snow attenuates these rays, and the difference in the amount of radiation reaching the top and the bottom sensors is calibrated to the density of the snow water.



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