This paper discusses the occurrence and characteristics of small ice crystals that are encountered in the lower troposphere of the Arctic during the cold half of the year, when large reductions in visibility are present. Because of the difficulties in observing these ice crystals, both from surface and satellite observations, and owing to the ambiguities of cloud classification in the polar regions, these ice crystals are not included in current cloud climatologies. A summary is presented of what is known about the Arctic ice crystal climatology, formation mechanisms, and physical properties. Data obtained from the University of Washington Convair C-131A research aircraft during April 1983 and 1986 are presented, including the frequency,of occurrence of ice crystals as a function of height, temperature, and relative humidity. Radiative transfer calculations were made using the observed ice crystal size distributions. The radiative transfer through the ice crystal layers is inferred to have a substantial impact on visibility in the Arctic, the vertical temperature structure of the Arctic troposphere, and on the surface energy balance.

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