Nuclei in Snow and Ice Crystals on the Greenland Ice Cap under Natural and Artificially Stimulated Conditions

Abstract

Natural snow crystals and artificially stimulated snow and ice crystals were collected on a mesh prepared for electron microscopy at Site 2, a research facility on the ice cap about 320 km cast of Thule, Greenland. The nuclei were observed at about 10,000?magnification with an electron microscope. The procedures and results were: 1) Natural snow crystals which developed a cloud at the temperatures between ?5 and ?20C were collected on the microscope grids. Almost all of the nuclei of 356 snow crystals were found to be in the center. Only 3.7 per cent had no observable nuclei at the center. 2) Ice crystals were made by dry-ice seeding of a supercooled fog in a cold chamber. Of the 104 crystals so produced and collected, 10 per cent had observable nuclei of 0.1? diameter order and 90 per cent had no observable nuclei. 3) A low-level supercooled stratus over the ice cap was seeded with dry ice. Of the resultant 11 snow crystals collected, 5 crystals had observable nuclei while 6 crystals had no observable nuclei under the electron microscope. The electron microscope study indicates that the natural snow crystals occurring during the summer on the Greenland Ice Cap are formed mainly on clay mineral particles by heterogeneous nucleation. The ice and snow crystals with no observable nuclei which were produced by dry ice seeding in the aerosol-limited air may have been initiated by homogeneous nucleation.