Fractal shape of hail clouds

We present a fractal analysis of severe convective storms using data collected during a recent hail suppression experiment ‘‘Grossversuch IV’’ in Switzerland. For larger clouds, a fractal shape is observed and from an analysis of the perimeter-to-area relation the fractal (i.e., Hausdorff) dimension is found to be ${\mathrm{D}}_{\mathrm{s}}$=1.36±0.1 in agreement with the value 1.35 from the theory of relative turbulent diffusion and an earlier analysis of cloud shapes. Moreover, a particle drift, present in strong lateral winds as well as in the center of hail clouds, is presumably responsible for a rather sharp crossover to a smooth behavior (${\mathrm{D}}_{\mathrm{s}}$=1) showing the existence of an apparent characteristic length.