Snow measurement determines both the physical depth of the snowpack and the amount of water it contains. Accurate measurements help meteorologists forecast weather events and allow water resource managers to predict spring runoff, which affects water supply and flood control. The techniques employed vary significantly depending on whether the goal is to find the total accumulation on the ground or the liquid content of the snow.
Measuring Total Snow Depth
Determining the total snow depth involves measuring the entire accumulated snowpack currently resting on the ground, regardless of when it fell. The standard method requires a simple measuring stick or yardstick, which is inserted vertically until it makes contact with the underlying ground surface. It is important to push the stick straight down without compressing the snow layers, which would result in an artificially lower reading.
To ensure the reading is representative of the entire area, multiple measurements should be taken in an open space, away from obstructions like buildings, large trees, or artificial snow piles. These objects can cause localized drifting or sheltering that distorts the true depth. By taking several measurements across a flat, undisturbed area and calculating the average, a more reliable total depth is obtained, typically rounded to the nearest whole inch for reporting.
Isolating and Measuring New Snowfall
Measuring only the snow that has fallen since the last observation requires a technique that isolates the fresh accumulation from the existing snowpack. The most effective way to do this is by using a specialized surface called a snow board, which is a flat piece of white-painted plywood or similar material. The board is placed directly on the ground or on top of the existing snow surface before the new snowfall begins.
The white color is specifically used to prevent heat absorption from the sun, which could prematurely melt the snow and skew the measurement.
When the new snow has finished falling, a ruler is plunged perpendicular to the board until it reaches the surface. The depth is read to the nearest tenth of an inch to capture precise detail of the new event.
After the measurement is recorded, the surface must be completely cleared of snow. The board is then placed back on top of the newly fallen snowpack to capture the next layer of accumulation. This process is essential for observers who track daily snowfall amounts.
Calculating Snow Water Equivalent
Snow Water Equivalent (SWE) is the measurement of the liquid water content within a snowpack, expressed as the depth of water that would result if all the snow were melted. This value is scientifically significant because it directly relates to the potential for spring runoff, making it a foundation for hydrological forecasting and flood risk assessment.
The density of snow can vary widely; for example, light, fluffy snow has a much lower SWE than heavy, wet snow, even if their depths are the same.
A practical method for calculating SWE involves taking a core sample of the snow using a straight-sided container of a known volume, such as a can or a cylindrical tube. The container is carefully pushed down through the snow layer to the ground to capture a complete sample. The snow sample is then brought indoors and allowed to melt completely into its liquid form.
Once melted, the resulting water depth is measured with a ruler; this value is the Snow Water Equivalent. Comparing the original snow depth to the resulting water depth provides the snow-to-water ratio. This ratio can vary widely from the traditional 10:1 ratio, sometimes being as low as 4:1 for very wet snow. Professionals often use specialized tools like the Federal snow sampler, but the melt technique provides accurate data.