On the Calibration of Hailpads

Abstract

A model of the vertical collision of a sphere with a hailpad predicts that the dent volume is proportional to the impact kinetic energy, and gives a relationship between dent diameter and sphere diameter for ice spheres. Laboratory calibration experiments confirm the essence of the theory but cast some doubt on the validity of the assumption of a constant resistance pressure. Further experiments simulating windblown ice spheres show that for the conditions we considered, the horizontal partition of energy has a small effect on the minor axis diamter of the dent. Consequently, if the wind speed is unknown, no more than a 10% error may occur if the sphere diameter is determined using the no-wind relation. Finally, field calibration of the hailpads with hailstones falling in a natural hail shaft tend to support both the laboratory calibrations and the model predictions.