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I I I I I I I I I I I I I I I I. I I I - l03 - Our aircraft observations confirm the deduction of Magono and Lee, and Ono, that sheaths grow between -6 to -8oC. It' appeared from the aircraft observations that the sheaths are hexagonal in shape. This agrees with the findings of Ono. Ono states that sheaths reached a limiting a-axis dimension of 80 ~m regardless of length, however, due to the limiting capabilities of his sampling apparatus, sheaths longer than about 500 ~m may not have been recorded. In our airborne studies sheaths generally occurred as elementary sheaths (Nlc), although bundles (Nld) and a few combinations of sheaths (N2b) were also observed. The average length of the sheaths was about 800 ~m with lengths varying from about 300 ~m to 2000 j.lm. Limited ground observations indicated a similar trend in range of lengths of sheaths, and the average len~th was somewhat less than 1500 ~m. Sheaths with an a-axis dimension as large as 140 ~m were observed. Both air and ground observations indicated that sheaths can undergo a limited amount of break-up in the air. 4.2.3 Solid and hollow Columns (Cle, Clf, N2c, C2b) Magono and Lee state that columnar growth occurs between -5 to -lOoC (warm region columns) and below -20oC (cold region columns). Ono (1970) states that columnar growth occurs between about -3.5 to -9.5oC and below -220C. Ono's observations indicate that columns reach a limiting a-axis dimension of 100 ~m, regardless of their length. However, Auer and Veal (l970) reported columns increasing in diameter with increasing length with no apparent limit to the diameter of a column. Our aircraft observations confirm that warm region columns originate and grow at temperatures between -5 to -lOoC and that the temperatures at which