The specific surface area of fresh dendritic snow crystals

Abstract

The surface area to mass ratio or specific surface area (SSA) is an often neglected characteristic of the snowpack that varies substantially with time, and with the shape of the individual snow crystal for fresh snow. The SSA for the dendritic shape of snow crystals was computed using a series of images photographed by W. A. Bentley. The specific images were dendritic crystals (P1d, P1e, P1f) and crystals that take a partial dendritic form and have ends or extensions (P2a, P2b, P2d, P2e, P2f, P2g) according to the Magono and Lee snow crystal classification. Image analysis, using known geometric relationships between length and width, and particle size distributions, examined the spatial properties of 50 sample snow crystals. Probability distribution functions were derived for SSA and these compared well with measured and other computed estimates of fresh snow SSA. For the non-rimed condition, the average SSA was 0.182 m 2 /g with a range from 0.09 to 0.33 m 2 /g. The presence of rime is discussed, and depending on the shape of the rime particles and the degree of surface coverage, the SSA can be doubled (20% coverage for needle or plate shaped rime). Fractal analysis was performed to determine various geometric relationships that characterize the dendritic form of snow crystal.