Tests for Persistent Effects of Cloud Seeding in a Recent Australian Experiment

Abstract

An analysis of cloud seeding experiments in Australia prior to 1984 that used silver iodide dispensed from aircraft as the seeding agent suggested that there were systematic aftereffects of seeding at least one month after a given seeded day, as revealed by mean target?control precipitation ratios a given number of days after a seeded day. Confirmation has been obtained from a new experiment in southeastern Australia in which both precipitation and ice nuclei were measured. The timing of the aftereffects on precipitation agreed closely with those of the earlier experiments, were confined to the regions that could be expected to have silver iodide deposited on them in rainwater, and were dependent on the amount of silver iodide used in the individual events. Correlations with the persistence time series of earlier work were significant at levels better than 0.1%. However, when unseeded days that were suitable for seeding formed the basis of the persistence time series for the new experiment, there was no significant correlation, showing that the aftereffects were certainly seeding related. Although substantially independent of the precipitation data, ice nucleus concentrations showed similar mean variations following a seeded day, which correlated with those of the previous work at significance levels better than 1%. The cause of the persistent effects of seeding is therefore likely to be due to stimulation of natural ice nucleus production by silver iodide deposited in precipitation. It is speculated that a microbiological pathway is involved. The importance of these observations is that an analysis of the effects of seeding will be incomplete if it is confined to seeded days and may be highly misleading if it relies on comparisons with unseeded days within the seeding season.