Response of Natural Ice Nuclei to Deposited Silver Iodide

Aim: To demonstrate that deposition of silver iodide particles on the ground creates secondary ice nuclei and that these interfere with analyses of experiments designed to increase rainfall.

Methods: Measurements of ice nucleus concentrations following the cessation of deposition of silver iodide during cloud seeding show that persistent secondary nuclei had been generated. The information is used to predict the effects on analyses of cloud seeding experiments. Effects on rainfall in large areas surrounding cloud seeding operations are then examined.

Results: The predictions have explained effects such as a decrease in seeding effects with increasing application of silver iodide or with the duration of its application. Widespread downwind increases in rainfall appear to have accompanied all cloud seeding experiments. The explanation offered is that silver iodide increases the airborne concentration of ice nucleating bacteria or their properties. The secondary nuclei appear to be more efficient in stimulating rainfall than the silver iodide nuclei.

Conclusions: Attempts to increase precipitation from clouds by seeding with silver iodide ice nuclei has been motivated by water shortages in many parts of the world. This paper has indicated that the frequent apparent slight or negligible success of such operations may have been due to the influence of the silver on ice nuclei of microbiological origin. An understanding of that influence is urgently required. Much larger and more widespread gains in rainfall than have been found in the past might then be obtained.