Atmospheric ice crystals over the Antarctic Plateau in winter

Falling ice crystals were collected daily on a gridded glass slide at South Pole Station during the antarctic winter of 1992, and photographed through a microscope. Nine types of ice crystals are identified, which fall into three main categories: "diamond dust", blowing snow, and snow grains. The dimensions of about 20,000 crystals were measured on scanned images of the photomicrographs. The predominant crystal types are hexagonal columns and plates ("diamond dust") and rounded particles of blowing snow. Diamond-dust crystals have a large range of lengths (2 to 1000 micrometers) and aspect ratios (0.1 to 100). Diamond-dust crystals can usually be classified as either columns or plates; nearly-equidimensional crystals are rare. "Long prism" crystals with aspect ratios greater than 5 were collected often, and very long prisms ("Shimizu" crystals), 1000 micrometers long but only 10 micrometers thick, were collected occasionally. The extreme Shimizu crystals were predominant on only one winter day, but the meteorological conditions on that day were not unusual. Some precipitation was observed on every day; even when the dominant crystal type was blowing snow, there were always in addition some snow grains or diamond dust. Blowing-snow particles dominate by number and contribute nearly half of the total surface area. Bullet clusters and blowing snow each contribute about one third of the total volume of atmospheric ice. Size distributions of the equivalent spherical radius are obtained for each of nine crystal types, as well as for the three main categories of crystals, using the volume-to-area ratio to specify the equivalent spheres. In addition, the effective radius for each day when crystals were sampled is computed. Many of the distributions are approximately log-normal. The effective radius (area-weighted mean radius) of the entire size distribution of diamond dust is 12 micrometers in winter, somewhat smaller than in summer (15 micrometers). The small size of wintertime blowing snow allows it to reach heights of tens of meters in winter, compared to only a few meters in summer. The average effective radius was 11 micrometers for blowing snow and 24 micrometers for snow grains. The most probable effective radius for any given day in winter is about 11 micrometers.