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- 104 - cold temperature columns occur is below -20oC. Crystals were assumed to have formed within t lOC of the temperature at which they were collected if their length or diameter did not exceed 100 ~m. However, warm region columns were also observed to form at temperatures above -4oC. Mossop ~~. (l967) also observed columns forming and growing at temperatures above -40C but Ono (1970) found only plates above -3.50C. Columns were also found at temperatures above -20oC, in fact at temperatures near -l8oC. This suggests that the transition from plate-like to columnar-like growth at -200C is not sharp. The aircraft observations indicated that columns were generally present as individual crystals (Cle, Clf) although combinations of columns (N2c and C2b) were observed. At the ground, columns often occurred in aggregates. The average lengths of the warm and cold region columns were about 500 ~m and 350 ~m respectively. Cold region columns seldom exceeded 600 ~m in length whereas some warm region columns had len~ths in excess of 2000 ~m~ Ground observations indicate that the average length of all columns was about 500 ~m. There appears to be a fundamental difference in shape between many warm region and cold region columns. Warm region columns were generally slender and had length to width ratios well in excess of three. By contrast, cold region solid and hollow columns were invariably stubbier, having length to width ratios of about three. C-axis growth appears to dominate more in the warmer region (-5 to -lOOC) than in the cold region (below -200C). Thus, only long, slender c-axis growth crystals are usually found between :.5 to -lOoC (e.g., sheath and needles) but other crystals (e.g., cold region plates) exhibiting a-axis growth are observed at temperatures below -20oC. At the ground both ,I I. I I I I I 1 I- I- I- I I- I- I 1- I~. I I