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Measuring snowfall accurately has always been a problem for meteorologists -- all too often, the snow refuses to cooperate. Unlike rain, which tends to behave similarly under almost all conditions, wet snow may stick to the sides of a snow gauge that employs a funnel-type receptacle. This can cause the snowfall to be unreported while it's happening, and overreported when the frozen precipitation later melts.
Another difficulty is that wind usually flows around the top of a snow gauge, carrying snowflakes with it. So instead of attracting snow at the catch point, a snow gauge can actually repel it. Meanwhile, beyond the gauge, wind-driven snow piles up in some places and leaves almost bare ground in others.
There are several types of snow gauges, the simplest of which is simply a measuring stick secured inside a metal container and anchored to the ground. This works well for casual observers of winter weather, but generally is not accurate enough for meteorologists bent on determining snowfall to a fraction of an inch. Sticking and wind fluctuations are particular problems with this type of snow gauge.
Meteorologists and hydrologists are more likely to use a copper container attached to a funnel shaped gauge. The catch basin is just over 50 inches (51-plus centimeters) in depth, and the snow within it is melted when the basin is full or the snowfall has ended. The snowfall is then arrived at by multiplying the water equivalent times 10.
Some subjectivity still exists with this method, however, especially when precipitation is mixed snow and rain. Similarly, a snow gauge device known as a snow pillow is prone to variations caused by wind. A sensor inside the "pillow" determines how much snow is sitting on top.
Yet another type of snow gauge was developed within the last few years by the National Weather Service, which tested it on Mount Washington in New Hampshire, the Denver International airport and sites in Ames, IA and Worthington, MN. It consists of two plates, one facing upward and the one below facing down. They are insulated from each other, and the difference in power required to heat the top plate compared to the bottom plate provides the raw data for determining snowfall.
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