The development, testing and use of an airborne pyrotechnic cloud seeding system is described. Pyrotechnic flares producing 50 gm of silver iodide smoke each were developed by two industrial corporations and laboratory tested for nucleation effectiveness in the Colorado State University cloud chamber. A delivery rack and firing system were developed, under ESSA supervision, and installed on its B-57 jet aircraft. Night flight tests were made of reliability, burn time and flare trajectory.
The flare system was used in a Florida cumulus seeding experiment in May 1968 conducted jointly by ESSA and the Naval Research Laboratory, with the participation of the U.S. Air Force, the University of Miami Radar Laboratory, and Meteorology Research, Inc. A randomized seeding scheme was used on 19 supercooled cumuli, of which 14 were seeded and 5 were studied identically as controls. Of the 14 seeded clouds, 13 grew explosively. Seeded clouds grew 11,400 ft higher than the controls, with the difference significant at better than the 0.5% level. Rainfall from seeded and control clouds was compared by means of calibrated ground radars. Large increases in rainfall were found from seeded clouds, but at a significance level ranging from 5–20% depending on the statistical test used. A single successful repeat of the experiment could result in rainfall differences significant at the 3% level with the most stringent test.