Summer Ice Crystal Precipitation at the South Pole

Abstract

Ice crystal precipitation was observed and the crystals replicated, at the South Pole during January and February 1974. The crystals were of columnar form. These columns were hollow or prismatic, and sometimes were in the form of combinations of bullets. These combinations were very fragile, disintegrating into individual bullets upon impaction. Smaller ?diamond dust? crystals were observed on two occasions. NOAA-NWS radiosonde data showed that the air was supersaturated with respect to ice at 650 to 600 mb (i.e., just above the surface) throughout the period. Ice crystal precipitation was only observed at the surface when cirrus bands were present at higher altitudes. It is likely that ice crystals, descending from the cirrus only a short distance above, grew to the larger columnar crystals while falling through the moist layer. As these layers were able to remain saturated, without precipitating or forming ice fogs or clouds at temperatures of ?35°C, heterogeneous freezing nuclei were probably absent at these levels throughout the period.