What is a Snow Survey?
Snow is difficult to measure because it is subject to drifting, melting and natural compaction. Any point measurement may be quite different from a reading taken only a few metres away. Thus, to measure the quantity of snow in an area, a systematic method for measuring snow has been developed and this is generally referred to as a "snow survey". Snow survey measurements have been made in British Columbia since 1935.
The basic principle guiding the measurement scheme is that measurements should be made consistently at the same locations, usually at monthly intervals during the winter, so that previous and subsequent measurements can be compared, month to month and year to year. The collection of measurements from snow surveys in a given region are used as indices that reflect the quantity of snow in that region. The monthly snow survey information is used for water supply and flood forecasting.
An ideal snow survey site is located in a relatively sheltered area with as little tree canopy overhead as possible. A forest clearing where the area is at least as wide as the height of the trees is ideal. Sites are chosen at elevations representative of the area and that will have snow for a substantial portion of the winter. The majority of snow courses are located between 1000 and 2000 metres above sea level. Five or ten points within the area are chosen and referenced to trees or local static features which will be visible when there is snow on the ground. At each sampling period, measurements are made at all of the points at the snow survey site and the "reading" for that site is the average snow depth and water equivalent of the points measured.
Measurements are made with the "Standard Federal Snow Sampler" which consists of graduated aluminum tubes with a cutter bit fixed to the first section of the tubing. The tubes are driven through the snow to the ground and then carefully withdrawn, extracting a core of snow in the tube. The tubes and core are then weighed using a scale specially calibrated in centimetres of water. The difference between the empty weight of the tubes deducted from the weight of the tubes and core is the snow water equivalent.
The number of snow courses measured at each sampling period varies through the year. Because the maximum reading at most snow courses occurs near the beginning of April, the greatest number of measurements is made at this date. Also, later in the season, lower elevation snow courses seldom have snow cover, so are not scheduled for sampling.