Hail in Southwestern France. II: Results of a 30-Year Hail Prevention Project with Silver Iodide Seeding from the Ground

Abstract

A nonrandomized weather modification project, hail prevention by seeding from the ground, has been run since 1952 in a large area of southwestern France. From the beginning of the experiment, the parameter proposed to measure the seeding efficiency in the area covered by AgI ground generators was the loss-to-risk ratio derived from insurance data. The analysis of the trend of this parameter in three parts of France which constitute a target, a buffer and a control area, brings to light a relative decrease of the damage during the last years in the protected area. A new statistical test for detecting a shift in precipitation series, applied after a log-transformation to the loss-to-risk ratio series, indicates a decrease significant at the 0.01 level in the damage due to hail during the period 1965?1982 in the protected area, while no significant change has been observed in the buffer area. Since there has been a large increase in the number of generators, and, above all, the setting up of better equipment since 1965, seeding is a reasonable explanation for the hail decrease. A double-ratio calculation with the target and control data gives a value of 41% for the decrease of the damage in the seeded area. Within this area, the global result is strengthened by the positive departmental correlation between the number of seeding stations per unit area and the hail decrease. The benefit-to-cost ratio of the project appears to be about 24. The hypothesis of a seeding effect leads to the following main physical implications: 1) The seeding effect is only perceptible in the area where the generators are distributed and not downwind of this area; this is in keeping with the observation that the ice-forming nucleus concentration is only locally increased over the seeded area. 2) At least 65% of the hail situations in southwestern France are related to cold fronts; the decrease of the hail damage corroborates the results of the Argentinian hail suppression project where a beneficial influence of AgI ground seeding was found to be significant under cold front situations.