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G. A. Isaac, R. S. Schemenauer, C. L. Crozier, A. J. Chisholm, J. I. MacPherson, N. R. Bobbitt, and L. B. MacHattie


A cloud seeding technique is proposed which has the objective of stimulating rainfall from cumulus clouds drifting over forest fires. Preliminary tests of the ice crystal production capability of the cloud seeding technique were conducted on five cumulus clouds near Yellowknife, N.W.T., Canada, during July 1975. These clouds were over forest but not near forest fires. A T-33 turbulence research aircraft performed the seeding by burning wing-mounted TB1 AgI flares while flying through the clouds at the −5 to −10°C level. The T-33 turbulence measurements enabled estimates to be made of the rate of dispersion of the AgI. Microphysical measurements were made before and after seeding by an instrumented DHC-6 Twin Otter aircraft flying at the seeding level, and these were compared with measurements in six untreated cumulus clouds. High concentrations of ice crystals appeared after seeding in four of the five seeded cumulus clouds, and on two occasions precipitation-sized particles appeared at the seeding level. The evidence indicates that the AgI aerosol produced large quantities of ice crystals.

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