The Oblateness of Large Hailstones

Abstract

Ninety large oblate hailstones have been photographed to display thin sections containing their minor and major axes. The ratios of their minor to major axes have been determined throughout their growth from the shapes of the layered growth transitions. It has been found that the majority of hailstones became increasingly oblate throughout growth, but that a further increase in oblateness accompanied the melting of the hailstones during their descent to the ground. Although even oblate hailstones are believed to tumble to some extent, they fall for most of the time with their minor axis vertical. Evidence is presented which suggests that during growth the proportion of unfrozen water is often greater at either end of the minor axis than at the sides of the hailstone. It is proposed that the increase in oblateness during growth is due to spongy ice on the upstream face being driven toward the sides of the hailstone, where it subsequently freezes, and that the further increase in oblateness during melting is a consequence of the more rapid melting of the relatively spongy ice remaining at the two ends of the minor axis. In extreme cases this process leads to the development of apple-shaped hailstones with pronounced indentations at one or both ends of the minor axis.