A Physical Evaluation of a Hail Suppression Project with Silver Iodide Ground Burners in Southwestern France

Abstract

The large-scale hail prevention program operated by the Association Nationale d?Etude et de Lutte contre les Fléaux Atmosphériques in southwestern France combines the seeding of hail clouds by a network of silver iodide ground generators with a survey of hailfalls by a network of hailpads. Using the joint data from the two networks, a physical method has been developed to measure the change in hailfall severity in the seeded hailstorms. The method associates the number of hailstones larger than 0.7 cm in a point hailfall at the ground (the ?hailstone number?) to the amount of seeding material released in the area where the storm was developing just before hailstone growth. The fundamental but not proven hypothesis employed is that a simple negative correlation should exist between the two parameters. During the years 1988?95, 630 point hailfalls were recorded on 53 hail days with seeding. The method indicates that the hailstone number is basically responsive to the amount of seeding material released 80 min before the time of the point hailfall in a 13-km radius area centered on the place where the storm was developing. The decrease in the hailstone number is in linear relation with the seeding amount?the more heavily seeded hailfalls decrease by 42%. The results are based on significant but weak correlations that have to be strengthened with a larger sample of hailfalls. However, they agree with former evaluations by insurance data of the same hail prevention program. The method gives a model that can be used to arrange the generator networks according to the movement of the storms.