Size‐distribution of ice crystals in cumulonimbus clouds

Ice‐crystal size‐distributions have been measured in clouds associated with the intertropical convergence zone and more isolated cumulonimbus clouds in temperate zones. Mean size‐distributions fit curves N D = 1,000/D 3 and N D = 1,000/D 2·3 in tropical and temperate clouds respectively where N D δD is the number per m 3 with diameters between D and D + δD mm. The tropical curve is a close fit throughout the measured range of D (0·05 to 4 mm) but the temperate curve is applicable in the range up to D = 2 mm only, there‐after a curve N D = 10,000/ D 5·4 fits the curve for D = 2 to 5 mm. In temperate clouds there is little variability with temperature except for a decrease in size of the largest particles observed as temperature decreases. There is some evidence that the particles caught are graupel of mean density 0·6 g cm −3 and, on this assumption, variation with total ice‐crystal concentration suggests an empirical distribution law for particles with diameters 0·25 mm or greater of \documentclass{article}\pagestyle{empty}\begin{document}$ N_D = 10^{4.1} \exp [ - \frac{{2.67}}{{W^{\frac{1}{3}} }}(D - 0.25)] $\end{document} where W is the ice‐crystal content in g m −3 .