The problem with active hail prevention mainly comes from the assessment of results : it is extremely difficult to make out the effect of a treatment from the strong natural variations of the phenomenon in time and space.
The ANELFA first noted that damages recorded by the hail insurers were weaker in the departments equipped with generators than in the surrounding regions. The decrease is about 40 %.
Around 1988, so as to check this observation with physical measurements, networks of hailpads were added. Those hailpads are devices enabling the record of the impacts of hailstones that fall on a pad of Roofmate-type extruded polystyrene, laid horizontally at 1.20 m above the ground. A sufficient number of hailfall observations are now available to enable comparisons between the operation of the generators and the hail intensity.
For the 24 major hail days in the Midi-Pyrénées region, the correlations between the generator emission data and the hailpad measurements show that, in a statistically significant way, the kinetic energy of a hailfall is inversely proportional to the quantity of silver iodide scattered by the generators in the area where the storm was developing about 66 minutes before the hailfall.
- Thus, to an emission density of 11 generators distributed on 1000 km2 (which represents about one generator each 10 km), corresponds an average decrease of 48%.
Compared with other hail prevention methods –by rockets or by aircraft– the ground generators have the advantage of an easy operation and of low operating costs. The only difficulty in the field comes from the need to set generators beyond the protected area so as to treat sufficiently early the rapidly moving storms. The hope lies in the fact that, in the future, the interest for that method will entail its expansion to wider territories, thus enabling the penalizing effect of the unseeded surrounding areas to relatively lose its significance.