Jean Dessens

Day-of-the-week variability of hail in southwestern France

A study of the day-of-the-week variability of hailfall has been made using hailpad data, collected for 11 years in a large area of southwestern France. In the Atlantic region of this area, point hailfall frequency and intensity are not different on weekends and weekdays. In the inland region, the frequency is the same during both week periods, but the mean kinetic energy per hailfall is about twice as important on weekends as on weekdays. An analysis of the corresponding mean hailstone size distributions for the two periods shows that both intercept and slope parameters of exponential size distributions, which fit with the observed ones are significantly different, and that the reduction in hail severity on weekdays is due to a shift from large to small hailstones. The day-of-the-week variability of air pollution measured in the inland region surrounding Toulouse suggests that anthropogenic emissions of ice-forming particles may be responsible for the observed hail change.

Return periods of severe hailfalls computed from hailpad data

Abstract Hail is an important economic problem in several countries, and there would be a need for climatological studies not limited to general data such as space and time frequencies of ordinary hailfalls. A large network of hailpad stations, which operated continuously in southwestern France since 1988, gives the first opportunity to compute return periods of very severe point hailfalls characterized either by the total kinetic energy of hailstones or by the diameter of the largest hailstones. The Gumbel distribution has been used to represent the probability density function of the maximum annual value of these two parameters. Preliminary results indicate that the area located just north of the central Pyrenees is three times more exposed to damaging hailfalls than the Atlantic border. The results show that the computation of an areal return period depends upon the hailpad network density, which makes data normalization necessary for inter-network comparisons. This study also offers the possibility of determining return periods of severe hailfalls at a point. As an example, a provisional estimation indicates that a hailfall with hailstones of 3–4 cm diameter occurs every 22 years at any point in the hail core region north of the Pyrenees. The hope is that this study may be progressively expanded to other European hailed regions in which similar hailpad networks are in operation.