The mass of snow per unit volume, but often expressed as a percent water content. New fallen powder has a low density (3-10%), while heavy or wet snow is more dense (10-20%).
The stability of the snowpack is influenced by many factors, but two of the most important is the strength of the weak layer and the load it has to support. The weight of the snow resting on a weak layer is a factor of the depth of the slab and its density. Snow density can be thought of in technical terms and numbers (% density, kg/m3) but most people have an intuitive feel for snow density, even if you don’t realize it. Snow that is light and billows up in your face while you’re riding is very low density, while high density snow feels thick, heavy, or even wet.
New snowfall has an initial density, usually in the 3-20% range. Once it accumulates on the old snow surface, metamorphism takes over causing the snow to gradually become more dense. The rate at which new snow becomes densifies depends on temperature, among other things. We all know warm weather can quickly ruin light powder snow, while cold temperatures can slow down the densification process and preserve powder for quite some time.
Another important factor to consider regarding snow density is trends during a storm. If the temperature is warm when the snow starts falling, and then becomes colder, we have what we call a “right side up” storm. The snow is light and fluffy on top and becomes more dense with depth. A far less desirable scenario is called an “upside down” storm and is the result of increasing temperatures during snowfall. The result is heavy, denser snow on top of lighter snow — you can see what we’re getting at can’t you? An upside down storm can result in a slab (dense snow) over a weak layer (less dense snow), providing the necessary ingredients for slab avalanches.