Why do snow crystals become rounder after they fall into a snowpack?

Once snow crystals fall into a snowpack, their intricate, high-curvature shapes—such as hexagonal plates or stars—become unstable. To minimize this instability, molecules evaporate from the sharp points of the crystals and condense into the hollows of the ice grains. This process leads to a general rounding of the grains, allowing them to fit more closely together and increasing the density of the snowpack.1

Additionally, wind can break off the delicate points of these crystals, further contributing to tighter packing. Over time, touching grains may develop connecting "necks" of ice through a process called sintering, or individual grains may rotate to improve their fit. If liquid water is present, these changes occur much more rapidly due to the melting of ice from grain extremities and subsequent refreezing elsewhere, as well as the compacting force of surface tension.1