SNOW CRYSTALS AND THE IDENTIFICATION OF THE NUCLEI IN THE NORTHERN UNITED STATES OF AMERICA

Abstract

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 ?, and that of the hexagonal-crystal nuclei is 1 ?. 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.