Secondary Ice Nucleus Generation by Silver Iodide Applied to the Ground

Abstract

Ice nuclei were monitored, using the membrane filter technique at eight sites around the 17-km perimeter of a rough circle and at one site in a 40 ha wheat field within the circle. The wheat field was then sprayed in fine weather with 2000 1 of an aqueous colloid, made by rapidly diluting a stock cloud seeding solution of 1.7 kg of silver iodide in 3 1 of acetone containing 340 gm of sodium iodide. From 13 to 21 h later there was a large increase in ice nucleus concentrations at sites downwind from the sprayed area during that period. It took several days for concentrations to subside to the background level established over the preceding 5 months. At intervals of 10 days, high ratios of concentrations in the sprayed area relative to those in the control were repeated out to the 40th day, soon after which the experiment was terminated. This behavior suggests that secondary ice nuclei released into the air following the addition of cloud seeding chemicals to the ground could have been responsible for apparent persistent effects in earlier cloud seeding experiments.