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. NIGHT VERSUS DAY CLOUD SEEDING IN LANGMUIR'S PERIODIC EXPERIMENT ~ Wallace E. Howell Division of Atmospheric Water Resources Management Bureau of Reclamation U.S. Department of the Interior On October 12, 1950, a delegation from the National Academy of Scil~nce visited Irving Langmuir at his Schenectady laboratory to hear a presen- tation of preliminary results from the periodic cloud-seeding experiment then just completing its twelfth 28-day cycle. They went away uncon- v;nced of a connection between the weekly release of silver iodide smoke from Socorro (later, Alamogordo), New Mexico, and the unusually strong weekly periodicity in precipitation over a large part of th,e central United States during February, March, and April of that year (Langmuir, 1962). To furnish, if possible, more convincing proof, Langmuir changed the seeding schedule beginning 3 days later. Instead of one silver iodide smoke generator operated between 8 a.m. and 4 p.m. on Tuesdays, Wednesdays, and Thursdays, for the next ten 28-day cycles three generators were operated 1 day a week at Alamogordo on the following schedule (op. cit., pp. 545-547): Cycle number 13, 17, 21 14, 18, 22 15, 19 16, 20 Seeding Schedule Mondays, 4 p.m. to midnight Mondays, 8 a.m. to 4 p.m. Fridays, 4 p.m. to midnight Fridays, 8 a.m. to 4 p.m. The entire experiment ended with cycle 22. The purpose of the 8-week alternations between Mondays and Fridays was to see if the phase of the weekly periodicity in widespread precipita- tion and associated weather elements would show a corresponding shift. The purpose of the 4-week alternations between nighttime and daytime seeding was to see if evidence of photodeactivation of the silver iodide by sunlight, then under study (Inn, 1951), could be found in the behavior of widespread weather. Subsequently, both Langmuir (op. cit.) and the U.S. Weather Bureau (Brier, 1955) made analyses of the Monday- Friday phase shifts. Curiously, Langmuir1s analysis does not mention night-day differences, nor did the Weather Bureau investigation extend to these differences (Brier, personal communication). While reviewing the periodic-seeding experiment in preparation for a workshop on wide-area effects of cloud seeding, held at Fort Collins, Colorado, August 8-12, 1977, the author was struck by this omission.