Sir, Unusual hailstones

On 20 May 1938 in Leningrad near the Finnish railway station, hail was falling. It lasted only three minutes, but the hailstones were very unusual. They were about 7 × 10 mm in size, and each hailstone was in the form of a hexagonal pyramid which consisted of six pyramids (one inside the other). Three pyramids were of transparent ice, the other three of milk-white ice (Fig. 1). Each milk-white pyramid consisted of sub-individuals (minute hillocks of growth) with air bubbles amongst them.

Fig. 1.

Shape of hailstones observed in Leningrad, 20 May 1958. Each hailstone consisted of a series of pyramids alternately milk-white and transparent. Size 7 × 10 mm.

If an alum crystal is placed in a highly supersaturated solution of alum (about 1 kg l⁻¹), the crystal becomes covered by sub-individuals and becomes milk-white. If it is put into a slightly supersaturated solution (3 g l⁻¹), the sub-individuals disappear and a transparent layer is formed on the surface.

It is obvious that the pyramidal hailstones described above grew in a slightly and highly supersaturated water vapour environment. When they grew in the highly supersaturated water vapour, sub-individuals formed and the hailstones became milk-white; when they grew in the slightly supersaturated environment the sub-individuals disappeared and the hailstones became transparent at the surface.

On 17 July 1959 in Moscow during a hailstorm lasting several minutes, the hailstones were disc-shaped. Their diameter was 12 mm, and their thickness 2 mm. They consisted of concentric white and transparent rings

8 November 1968