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Motoi Kumai

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Motoi Kumai

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A total of 380 electron micrographs and electron diffraction patterns of 93 snow crystal nuclei were analyzed in this observation. The nuclei were identified as mainly clay minerals and sodium chloride particles. The clay mineral nuclei were illite 20%, kaoline 8%, halloysite 4%, verniiculite 3%, and related minerals 24%. For the other nuclei, sodium chloride accounted for 20%, and unidentified nuclei accounted for 5%. Fifteen percent of the snow crystals did not appear to have nuclei. Therefore, all nuclei found in snow crystals were terrestrial substances from oceans and continents. The mass of nuclei was from 10−16 to 10−12 for both clay minerals and sodium chloride.

The shapes of snow crystals were single bullets, combinations of bullets, and hexagonal hollow columns. The snow crystals formed at temperatures from −30 to −35°C. The snow crystal diameters were from 0.1 to 1.0 mm, and the crystal mass was from 10−8 to 10 −5 g The mean mass concentration of sodium chloride in snow crystals was 40.6 ppb and that of clay minerals was 15.4 ppb. The sodium chloride nucleus concentration coincided within the experimental error with data taken from the chemical analysis of the South Pole snow cover made by several workers. It was concluded that most of the sodium chloride contained in the South Pole snow cover was due to the sodium chloride nuclei of snow crystals.

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Motoi Kumai

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Arctic fog droplets were sampled on narrow glass plates precoated with chloride-sensitive gelatin film at Point Barrow, Alaska, in the summer of 1971. The relation between the radius of the log droplet and the radius of the imprint on the film was determined experimentally. The collection efficiency of the fog droplet was determined. About 20,000 fog droplet radii were measured.

The results of the analysis of the concentration and the size distribution of fog droplets are presented in the form of tables and figures. It is shown that the concentration and the size distribution changed rapidly with time and space; the droplet radii ranged widely between 3.3 and 65 µm; the maximum concentration was 24 droplets cm−3 and the liquid water content was 0.09 gm m−3 at a visibility of 250 m. Calculations were made of the attenuation by fog at wavelengths of 0.55 and 1.06 µm for the observed size distributions and concentrations of fog droplets. The values of the visual range calculated at a threshold constant of 5% were closer to the observed values than those at a threshold constant of 2%.

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Motoi Kumai

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Experimental work was conducted chiefly in an igloo, half way up Mt. Taisetsu in Hokkaido, at an altitude of 1050 m. Snow crystals were received on the collodion film of the holder of an electron microscope, and left in a desiccator kept between −4 and −8C. The crystals sublimed, and the supposed nuclei remained on the collodion film. These specimens were brought under electron-microscope investigation.

One solid nucleus (center nucleus) was always observed in the central portion of a snow crystal. The center nuclei were between 0.5 and 8µ in largest extent. In the other parts of snow crystals, numerous smaller nuclei (condensation nuclei) were observed. These were nearly the same in size as condensation nuclei in the free atmosphere. 43 successful photographs of center nuclei were obtained. Most of them are considered to be soil particles, some being a lump of carbon particles, a micro-organism, or a hygroscopic particle of some chemical compound. 60 photographs of condensation nuclei were taken, and a frequency curve of size was made from 1200 data. Condensation nuclei are found to be of two kinds, the larger ones most frequently having a diameter of about 0.15µ, the smaller ones about 0.05µ.

The mechanism of snow-crystal growth is discussed on the basis of these data.

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Motoi Kumai

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The early stages of ice crystal formation in supercooled fogs were studied in detail by electron microscopy, and ice nucleation experiments using liquid propane seeding were conducted in a thermostatically controlled coldroom. Ice crystals, formed by rapid cooling created by the evaporation of liquid propane from a fine nozzle at temperatures from −0.1 to −40°C, were collected and replicated on filmed grids for electron microscope examinations. Most of the ice crystals formed immediately after the liquid propane seedings were spherical (although ∼20% were hexagonal) with diameters ranging from 0.3 to 3 μm and with a mean diameter of 1.5 μm. Electron microscopy revealed a grain boundary in some of the ice crystals.

The production rates of ice crystals per gram of liquid propane seeding were measured at temperatures from −0.1 to −20°C. The production rate increased exponentially at temperatures from −0.1 to −4°C, and remained at about 1011 ice crystals per gram of liquid propane seeding at temperatures below −5°C.

Experiments of supercooled fog dissipation by liquid propane seeding were performed in the coldroom. The results showed that supercooled fog dissipation becomes effective at temperatures colder than −0.5°C.

The habit of early stage ice crystals formed at temperatures from −0.1 to −40°C and −90 to −160°C was studied. Two basic types of hexagonal plates and columns were observed at temperatures from −22 to −40°C. A cold stage was used with the electron microscope to investigate the structure of the crystals formed at temperatures of −90 to −160°C. Plates were observed at −100°C, and were analyzed as having a hexagonal form by their electron diffraction patterns. Cubic forms of ice crystals were observed below −100°C.

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Motoi Kumai

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Snow crystals are born at high altitudes and grow into various forms while falling through the atmosphere. In this research work, the relations observed among snow crystals were crystal form, nucleus, and temperature and humidity of the mother cloud based on radiosonde sounding data. Almost all crystal forms which are shown in “Snow crystals” by Nakaya (1954) were observed in a winter season at Houghton (Keweenaw Field Station), Michigan which is situated on a small peninsula along the southern shore of Lake Superior. The factors that influence snow-crystal form are mainly the air temperature and the humidity at which the crystal grows. Previously there have been no data for the formation of pyramid-shaped snow crystals in natural conditions. However, in the observations of Houghton, it was found that the pyramid shaped crystals were formed in the clouds at temperatures between −6C and −1OC. This coincides with the condition of growth of artificial snow crystals of the cup and scroll type. Needle crystals are made in the temperatures between −4C and −6.5C; pyramids, bullets, and columns, between −6C and −1OC; hexagonal plates between −8C and −12.6C; and dendritic forms between −14C and −16C. These observations agree well with the Nakaya-Hanajima diagram obtained from measurements made in convection snow-making apparatus and with the Mason-Hallet diagram obtained with a diffusion chamber.

Three hundred snow crystals were collected ; successful electron micrographs were obtained of the center nucleus of 271 of these. The nucleus of snow crystals can be classified as clay-mineral particles, hygroscopic particles, combustion products, microorganism and unknown (unidentified) materials. Clay-mineral nuclei accounted for 87 per cent, hygroscopic nuclei 1 per cent, combustion products 2 per cent, unknown material 9 per cent, and no nuclei 1 per cent of the sample.

A relation was found between the sizes of the snow-crystal nuclei and the snow-crystal forms. The size of the maximum frequency of needle-crystal nuclei is 3.5 μ, and that of the hexagonal-crystal nuclei is 1 μ. In other words, the sizes of maximum frequency of the nuclei of snow crystals which are formed at warmer temperatures are larger than those at colder temperatures. In this investigation, no relationship between crystal forms and the substances of the nuclei was found.

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Motoi Kumai and Karl E. Francis

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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 0.1μ 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.

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