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<br />172 <br /> <br />generate a contrail was around _400C. <br /> <br />(The exact threshold <br /> <br />varies a couple of degrees centigrade, of course, with alti- <br /> <br />tudes and throttle settings.) The Schweizer contrail rarely <br /> <br />persisted longer than one-half minute or so, but persisted <br /> <br />longest as humidity increased and temperature decreased. <br /> <br />After several flights, the pilot proved able to gauge the <br /> <br />proper conditions for seeding by the length of his contrail. <br /> <br />To use this approach, however, seeding had to be done at <br />o <br />temperatures lower than -40 C. <br /> <br />6.3 OPERATIONS <br /> <br />The base of operations was Peterson Air Force Base in <br /> <br />Colorado Springs, Colorado. The airstrip lies on the rolling <br /> <br />plain to the east of Pike's Peak and the Front Range of <br /> <br />the Colorado Rockies. <br /> <br />Eight seeding flights per season were carried out in <br /> <br />February and March of both 1977 and 1978. Seeding was <br /> <br />carried out either (a) between Taos and Cimarron in northern <br /> <br />New Mexico, or (b) in eastern El Paso County, Colorado, to <br /> <br />the east of Peterson Field. Seeding was performed between <br /> <br />28,000 and 32,000 feet MSL (roughtly between 8 1/2 and 10 km). <br /> <br />6.4 RESULTS <br /> <br />With respect to the original aims of the program, we <br /> <br />reached the following conclusions. <br />