An attempt to describe the mechanism of surface hoar growth from valley clouds

Abstract

In the Columbia Mountains of western Canada, avalanches release from buried surface hoar layers which form when stratus clouds extend across valleys on consecutive nights. Water vapor diffusion over distances of even a meter is much too weak to account for the growth, so non-diffusive movement in some form is necessary. One likely mechanism for the formation of surface hoar is rapid deposition of vapor as the cloud thickens and moves up and/or along the snow slope. Field evidence shows cloud movement both by rocking and lateral flow of the entire cloud. Since the time over which deposition could take place on any given night is short, successive nights are needed to produce a substantial deposit of hoar. More direct field observations of surface-hoar growth are needed to be certain that we have identified the dominant mechanisms.