Temperature gradient and initial snow density as controlling factors in the formation and structure of hard depth hoar

We investigate how temperature gradient and initial density influence depth-hoar growth in snow and seek to better define the range of conditions under which cohesive, or hard, depth hoar forms. I Samples of 400 kg m(-3) sieved snow were exposed to temperature gradients of 20-80degreesC m(-1), and samples of four different densities were exposed to a 40degreesC m(-1) temperature gradient. Following exposure to temperature gradients, penetrometer tests were made on samples to determine the presence of solid and/or hard depth hoar. Grain bond orientation was analyzed in section planes by two-dimensional stereological techniques where hard depth hoar developed. Results indicate that hard cohesive depth hoar forms from rounded-grain snow having a density of 400 kg m(-3) or greater, following exposure to a temperature gradient of 20degreesC m(-1) or greater. Hard depth hoar appears to consist of solid-type depth-hoar grains connected by necks, with vertically preferred directions of grain elongation and organization of grain-to-grain chains. This work corroborates Atikaya's (1974) results, but extends his observation of formation of hard depth hoar to weaker temperature gradients for high-density snow. Our results also indicate that hard depth hoar is composed of faceted solid-type (anhedral) grains.