Snow is vital to the environmental health of the western United States. Unlike rainfall, it doesn't come as one burst that's here and then quickly gone. Instead, it tends to accrue over the winter and slowly melt as the temperatures rise in the spring. The melt typically continues into the early summer, providing a steady, consistent level of water in mountain watersheds. This is particularly important for a state like California, which receives almost no summer rainfall.

Snow comes in a variety of forms. It can be light and airy, almost dry, or it can heavy and dense. For example, light, fluffy snow that measures four feet deep might actually have the same volume of water as heavier, denser snow that is two feet deep.To account for this variation a measure called snow water equivalence is used to normalize the amount of liquid water in a given level of snow.

Compare the average snow/water levels by year and elevation. The general trend is towards drier conditions, but wet years are interspersed with drier years. Notice that 2011 was a very wet year across all elevations, while 2015 was extremely dry.