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AWRP Snow Gauge

"How hard can it be to accurately measure snow? After all, doesn't snow pile up until you can stick a ruler into it?"

Well, to accurately measure snow you must also measure its water content. Six inches of snow from one snowstorm can have more water in it than a foot of snow from another storm. Meteorologists have also been measuring something that's called "liquid equivalent precipitation rate"; meaning the amount of water precipitating at any given time, regardless of temperature.

Hotplate Gauge
The Hotplate Gauge with rime ice covering the stand. Scott Landolt Photo

The current method of determining the snowfall rate using the National Weather Service real-time surface snowfall intensity measurements based on visibility, however, can often be misleading due to the occurrence of high liquid equivalent snowfall rates.

The Federal Aviation Administration (FAA) is particularly interested in "liquid equivalent precipitation rate" because knowing how much water is falling in the snow at an airport will greatly improve the decision making process and lower the costs of de-icing airplanes prior to takeoff.

A key safety issue with de-icing is to prevent the buildup of ice or snow on critical aircraft surfaces. This type of buildup has been a cause or a factor in numerous aircraft takeoff accidents due to its large effect on aircraft performance, with as little as 1 millimeter of rough ice reducing lift and increasing drag by 25% during the takeoff roll.

Geonor precipitation gauge
The Geonor precipitation gauge with a heater tape wrapped around it. Scott Landolt Photo

With funding from the FAA; The National Center for Atmospheric Research- Research Applications Program (NCAR-RAP) and The Center for Wind, Ice and Fog Research (CWIFR) tested two devices that measure liquid equivalent precipitation rate. One of the instruments is a Norwegian gauge, manufactured by Geonor, which uses a collection bucket, the weight of which alters a high-pitched frequency that can be converted into a measurement of liquid-equivalent precipitation amount. The bucket is filled with antifreeze to melt the snow, and a thin coat of oil keeps it from evaporating before a reading is taken.

There are no mechanical moving parts. The sensor, an electrically excited vibrating wire, is an integral part of the weighing system. The change in frequency of the vibrating wire is a function of the precipitation. The vibrating wire measurement technique is a well proven system extensively used in dam instrumentation and for offshore installations. Results show a very accurate measurement of precipitation amount. The frequency signals can be easily transmitted and interfaced to most data acquisition systems in common use.

The other instrument is called a Hotplate. The hotplate gauge consists of two plates, one on top of the other. The plates are heated to 200F. As snow falls and hits the top plate, the plate begins to cool. The amount of energy required to raise the plate back to what it was is measured and compared to the energy of the lower plate. From that, a relationship can be determined and a snowfall amount can be calculated.

Although, snow can fall on the summit during any month of the year, testing of the snow gauge took place between the months of October to May.

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Eric Kelsey, Director of Research
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ekelsey@mountwashington.org

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