Snow - Snowfall Measurement

Snowfall Measurement

See also: Disdrometer

Snowfall is defined by the U.S. National Weather Service as a being the maximum depth of snow on a snowboard (typically a piece of plywood painted white) observed during a six-hour period. At the end of the six-hour period, all snow is cleared from the measuring surface. For a daily total snowfall, four six-hour snowfall measurements are summed. Snowfall can be very difficult to measure due to melting, compacting, blowing and drifting.

The liquid equivalent of snowfall may be evaluated using a snow gauge or with a standard rain gauge having a diameter of 100 mm (4 in; plastic) or 200 mm (8 in; metal). Rain gauges are adjusted to winter by removing the funnel and inner cylinder and allowing the snow/freezing rain to collect inside the outer cylinder. Antifreeze liquid may be added to melt the snow or ice that falls into the gauge. In both types of gauges once the snowfall/ice is finished accumulating, or as its height in the gauge approaches 300 mm (12 in), the snow is melted and the water amount recorded.

Another type of gauge used to measure the liquid equivalent of snowfall is the weighing precipitation gauge. The wedge and tipping bucket gauges will have problems with snow measurement. Attempts to compensate for snow/ice by warming the tipping bucket meet with limited success, since snow may sublimate if the gauge is kept much above the freezing temperature. Weighing gauges with antifreeze should do fine with snow, but again, the funnel needs to be removed before the event begins. At some automatic weather stations an ultrasonic snow depth sensor may be used to augment the precipitation gauge.

Spring snow melt is a major source of water supply to areas in temperate zones near mountains that catch and hold winter snow, especially those with a prolonged dry summer. In such places, water equivalent is of great interest to water managers wishing to predict spring runoff and the water supply of cities downstream. Measurements are made manually at marked locations known as snow courses, and remotely using special scales called snow pillows.

When a snow measurement is made, various networks exist across the United States and elsewhere where rainfall measurements can be submitted through the Internet, such as CoCoRAHS or GLOBE. If a network is not available in the area where one lives, the nearest local weather office will likely be interested in the measurement.

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