Snow Water Equivalent (many times referred to as "SWE") is a common measurement of the amount of water contained in snowpack. It can be considered as the depth of water that would theoretically result if the whole snow pack instantaneously melts. The snow water equivalent is the product of snow depth and snow density. New snow commonly has a density of around 8 to 10% of water. Once the snow is on the ground, it will settle under its own weight (largely due to differential evaporation) until its density is approximately 30% of water.
For example, let's assume that 25 inches (depth) of new powdery snow fell last night in the mountains at a 10% snow water density. If the powdery snow instantly melted, you would see a layer of water 2.5 inches deep. In this case, the SWE of the new snowpack equaled 25" x 10% = 2.5 inches. However, once that powdery snow is exposed to the sun and freezes and melts throughout a 24-hour cycle, the density increases as the snow pack settles.
At the manual snow course sites in Wyoming, snow surveyors measure the snowpack depth and weigh the snow core sample to help determine the density. Both measurements are then computed to determine the SWE of an individual snow course site.