Some snow, regardless of how much we get, just won’t pack and stick together. There is a simple reason for this… the snow literally isn’t “sticky”. That non-packing snow forms when we see a snowfall during very cold temps that are well below freezing. We are talking about temperatures in the low to mid teens. There is less moisture in the atmosphere at these temperatures so the snow flakes tend to be small and almost “dry”. On the other hand, wet packing snow is seen when it snow with a temperature closer to freezing (32°). At these temperatures, the snow flakes have more moisture in the atmoshpere to latch on to. The extra moisture acts like a sort of “glue” that bind the flakes together when they are packed.
from http://www.addins.wrex.com/blogs/weather/2012/12/what-makes-packing-snow