Transvaal hailstones

Characteristics of a large number of hailstones which have fallen in the Pretoria‐Johannesburg area in the past five years have been summarized under the following headings: size distributions, shapes, growth centres, layers and internal structures. Most hailstorms did not produce large hailstones. Only 4 per cent of over 5,000 hail reports were of stones exceeding 3 cm diameter. With increase in size of the largest hailstones, broader size spectra tended to occur and the modal size of hailstones reported as being most common increased. Spheroidal shapes were encountered more often than others in practically all size groups but their relative frequency diminished with size. A high proportion of the hailstones had opaque growth centres. An investigation of layering revealed that many hailstones have more changes of opacity than of crystal structure. Two, three and four crystal layers were often found in medium, large and extra‐large stones. A simple up‐and‐down trajectory can account for the structure of many stones. Slushy ice may form on large hailstones but it is not an essential growth stage.