Surface Hoar

If you are out and about in the backcountry this weekend, you may encounter surface hoar, beautiful but weak ice crystals that form on the snow surface when it cools below the frost point.  We saw plenty of it yesterday at lower elevations during our ski tour.  

Surface hoar is sometimes described as recrystallized powder, but as I understand it, that's incorrect.  Surface hoar is created through the growth of new crystals.  The formation process is similar to frost.  The snow surface cools below the frost point and ice crystals grow from the deposition of atmospheric water vapor onto the snow surface.  This process becomes highly effective under clear skies, when the snow surface can become quite cold, and when there is a strong temperature inversion.  It is also very effective near open water where there is a source of water vapor.  

In contrast, recrystallized powder, or near-surface facets, form through the metamorphism of existing snow crystals.  Near-surface facets are typically smaller than surface hoar crystals.  They form in the upper portion of the snowpack when strong temperature gradients exist near the snow surface (see Recycled Powder and Other Types of Near-Surface Faceting by Karl Birkeland).  There are several pathways for near-surface facets to form, including situations not favoring surface-hoar formation.  

Regardless, either surface hoar or near-surface facets can be your next weak layer.  The surface hoar we skied through yesterday was thin and fragile, shattering like shards of glass as we skied through it.  Not exactly bedrock to serve as the foundation for a stable snowpack.  Near-surface facets are also weak.  Beautiful today, but potentially scary tomorrow.