Here are some basic details for interpreting snow pillow data.
- Name, Date, Time and Temp are what they seem.
- RGuage measures the depth in millimeters ( mm) of antifreeze liquid in a container. In the winter snow is instantly melted as it falls into the container. When it precipitates (rain or snowfall) the level of the liquid in the container goes up (as new water is added to the antifreeze). When it is warm, moisture evaporates from the container and the level goes down.
- SW measures the "Snow Water Equivalent" in millimeters (mm) of the snow above a pressure plate. This reading is essentially a measure of the weight of the snowpack above the pressure plate which is then translated into the estimated depth of water the melted snow woudl turn into. To make use of this number you must convert the Snow Water Equivalent into depth of snow. For this you must estimate the density of the snowpack above the pressure plate. Some rough guidelines are (these are guesses):
- Fresh snow - 100 kilograms per cubic metre
- Mean Coastal Snowpack in March - 350 to 400 kilograms per cubic metre
- Spring Snowpack in Mt Hood - 400 kilograms per cubic metre
- June snowpack in the Coast Moutains - 500 to 580 kilograms per cubic metre
- To calculate snow depth use the following equation:
- depth (metres) = SWE (millimetres) / density (kilogram per cubic metre).
- For example the Chilliwack River Snow Pillow for June 16th, 2006:
- Depth in metres = SWE 690 ÷ 550 (spring density) = 1.25 m of snow
More about Snow Density
The density of snow (usually expressed in units of kilogram per cubic metre) is a measure of the mass per unit volume of snow, and is an indicator of the compactness of a snowpack. New snowfall typically has a density of around 100 kilograms per cubic metre, but this increases rapidly once snow is on the ground; winter snowpacks typically have mean densities in the 200 to 300 kilogram per cubic metre range. The density of a snowpack reflects the characteristics of the various snowfall events, as well as various processes, such as snow compaction and snow melt and refreeze cycles. Information on mean snow density is essential for determining the snow water equivalent of a snowpack, and knowledge of the vertical density structure is critical for avalanche-risk forecasting. Information on the density of the snow surface layer is important for assessing snow trafficability and potential for blowing snow.
Figure 2 shows mean snow density for March from available snow observations. The spatial pattern is characterized by higher snowpack densities in warmer coastal regions and lowest densities over the boreal forest zone.
More about Snow Water Equivalent
- Snow water equivalent (SWE) is defined as the depth of water (in millimetres) of snow cover on a horizontal surface area if that snow cover is completely melted. SWE is related to snow depth and density by
- SWE (millimetres) = depth (metres) x density (kilogram per cubic metre).
- The conversion of SWE (millimetres) from a mass of snow per surface area to a depth of water is based on the fact that 1 millimetre of water spread over an area of 1 square metre weighs 1 kilogram.
- A variety of surface-based and satellite methods can be used to measure SWE. The most commonly used approach for determining SWE is the gravimetric method, which involves taking a vertical core through the snowpack and weighing or melting the core to obtain the SWE.