This paper describes some experiments on the deterioration of silver iodide freezing nuclei produced with a kerosene burner. It is found that the number of particles effective as freezing nuclei decreases with time as does the rate of decay itself. The decay rate is critically dependent on the ambient air temperature and to a lesser extent on the air pressure. The results suggest that for artificial rainmaking, silver iodide smoke released from the ground in warm climates is of little use but it may be very effective when released from high mountains or aircraft.

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* Division of Radiophysics, Commonwealth Scientific and Industrial Research Organization/University Grounds, Sydney, NSW, Australia.

Pakistan Meteorological Department (at present at the Radiophysics Laboratory).