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Show Meteorological Classification of Natural Snow Crystals 329 fore the second type was classified as a " sheath" as seen in Nlc, Nld and N2b in Table 1. The snow crystals of sheath type are formed in the temperature range between - 6 and - 8° C and the snow crystals of the original needle type are formed in the temperature range between - 4 and - 6° C, as seen in Fig. 2. Shimizu14> found snow crystals of solid thin column type, namely the third type in the Antarctica. The snow crystals of this type have an outward appearance similar to the second type, but the temperature condition of the third type is quite different from that of the sheath. According to Shimizu's observation they were formed at a temperature range colder than - 30° C. Recently Kobayashi15* obtained such solid columns experimentally around - 50° C. The meteorological meaning for the difference between hollow and solid columns is st, ated below. 3.2 Distinction between solid columns and hollow columns Most of columnar crystals which are observed, are hollow ones, in other words, are of skelton form. However Kobayashi9*. 10' found that solid columns were easily obtained artificially in nearly supersaturated conditions over a wide temperature range. In addition to that, minute solid columnar crystals are frequently observed in the early stage of mature columnar crystals. From the theory of crystal growth, it may be considered that snow crystals develop into solid columnar form under a nearly equihbrium state, and into hollow columnar forms under a considerably supersaturated condition. Therefore the distinction between solid and hollow columns is important meteorologically. In the present classification, all columnar crystals ( needles, columns, bullets and thick plates) were further divided into solid and hollow types, as seen in Table 1. 3.3 Distinction between columns and thick plates In Nakaya's classification, the term " thick plates" are used for two types, namely short columns and densely rimed plane crystals. In the present classification, the latter is named " densely rimed crystals", in order to avoid confusion. In the nomenclature, it would be better to have a clear criterion to distinguish the thick plates from the columns. After Zamorsky16) and' Higuchi17), the ratio of the length to the diameter of a column ranges from 0.5 to 0.8 in the nearly equihbrium state. However in the present classification, columns are called a thick plate when its length is shorter than its |