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<br />171 <br /> <br />to be around 25 ~m at an airspeed of 50 m/sec. <br /> <br />A variable speed movie camera is mounted beside the <br /> <br />pilot's head in the cockpit, facing either forward or back- <br /> <br />ward. <br /> <br />Humidity sensing is a problem at low temperatures. We <br />o <br />hoped to be working at temperatures well below -40 C and <br /> <br />needed to have reliable methods for finding ice-supersaturated <br /> <br />air. In 1977 the pilot dropped small chunks of dry ice <br /> <br />through a'smal1 port in the cockpit. <br /> <br />It was hoped that <br /> <br />the dry ice would nucleate a trail of ice particles if the <br /> <br />air through which it fell was ice-supersaturated. This <br /> <br />proved to be unsatisfactory as the pilot was never able to <br /> <br />observe trails of tiny ice crystals from these chunks, even <br /> <br />in air that appeared to be ice-supersaturated from other <br /> <br />indications. Besides that, the hole produced enough of a <br /> <br />draft to give the pilot frostbitten extremities. <br /> <br />In 1978 two small CO2 gas cylinders were strapped to <br /> <br /> <br />the wheel struts. Liquid CO2 was allowed to trickle through <br /> <br /> <br />a small hole in a plate sealed to the outlet. In ice- <br /> <br />supersaturated air under favorable viewing conditions, the <br /> <br />pilot was aole to observe two pencil thin trails persisting <br /> <br />behind the aircraft which were nucleated by the CO2 <br />trickling from these bottles. <br /> <br />However, the most reliable indication of ice-supersatura- <br /> <br />tion proved to be the contrail produced by the Schweizer. <br /> <br />The threshold temperature below which the Schweizer would <br />