Persistent Effects of Cloud Seeding with Silver Iodide

A statistical examination of precipitation records in and near areas where cloud seeding experiments have taken place in Australia strongly suggests the delayed effects of seeding. The most conspicuous effect is an increase in precipitation 1 to 3 weeks after a seeded day, which would have caused the conventional statistical estimates of success to have been misleading. Through comparison with earlier findings and some observations of prolonged increases in ice nucleus concentrations following the application of silver iodide to the ground, it is argued that secondary ice nuclei are involved in the persistent effects of seeding. If this is so, then they must have been more effective in enhancing precipitation than the silver iodide, itself; this leads to the possibility that there may be better ways of stimulating precipitation than those used so far.