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<br />164 <br /> <br />6.0 INTRODUCTION <br /> <br />Persistent contrails are formed haphazardly every day <br /> <br />by jet aircraft over various parts of the globe. Ships, <br /> <br />industrial plants and electric power-generating stations <br /> <br />. <br /> <br />also can initiate the growth of water/ice clouds with their <br /> <br />exhaust, which typically contains large amounts of water. <br /> <br />Intentional clear-air seeding experiments have been <br /> <br />performed by Fraser (1949), Vonnegut and Maynard (1952), <br /> <br />Schaefer (1953), Bigg and Meade (1971a), and Jayaweera et <br /> <br />al. (1972, 1975). The most extensive and well-documented <br /> <br />effort was that of Bigg and Meade, using dry ice seeding <br /> <br />techniques over the Tasmanian Sea. Their program involved <br /> <br />30 flights in both summer and winter conditions, generally <br /> <br />between 5 and 8 km above sea level. They noted that they <br /> <br />stimulated spectacular convective growth of clouds in pre- <br /> <br />viously clear air in five of these cases. Ice crystals <br /> <br />falling out of these long-lived clouds modified lower super- <br /> <br />cooled clouds. In one or two cases, it appeared that con- <br /> <br />vection in the shadow beneath the artifically produced <br /> <br />clouds was damped. <br /> <br />Jayaweera and Ohtake (1972) report seeding clear air <br /> <br />with dry ice between 300 and 600 m above the ground near <br /> <br />Fairbanks. The cloud grew to be about 1 km in diameter and <br /> <br />more than 150 m thick befOre it drifted out of sight after <br /> <br />one and one-half hours. Further work is described in <br /> <br />Jayaweera et al., 1975. <br />