Abstract
A summer (June and July) cumulus cloud seeding experiment was conducted in Canada near Yellowknife in 1975 and 1976, and Thunder Bay in 1977 and 1978. Microphysical and dynamical measurements were made with three instrumented aircraft, flying in the vicinity of and within 58 clouds near Yellowknife and 66 clouds near Thunder Bay. Using wing-mounted AgI pyrotechnic flares, a total of 25 of these clouds were seeded as an aircraft penetrated each cloud at the −5 to −10°C temperature level ∼ 300 m below cloud top. The microphysical properties of each cloud before and after seeding were compared with a statistical summary of the microphysical characteristics of natural or non-seeded cumuli at the same temperature level. Higher concentrations of ice particles were observed after seeding in ∼50% of the seeded clouds, with the magnitude of the increase and the post-seeding concentration being abnormally high in most cases. Approximately 40% of the Yellowknife cumuli produced rain after seeding and examples are given of how this rain could have been produced by an artificially stimulated Bergeron-Findeisen process. Near Thunder Bay, no rain was generated by the AgI. Even though Thunder Bay cumuli contained more cloud water and fewer large particles than Yellowknife cumuli, seeding was less successful because cloud lifetimes were too short for artificial precipitation to form.
