The density and structure of hailstones

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Abstract
Macklin's (1962) experimental study of ice accretions, together with estimates of the typical conditions in the cumulonimbus of cold air masses and of summer continental air masses, is used to deduce the density of ice in the hailstones produced by these clouds. It is shown that generally small hail has a low density; it may reach the ground in the cold air masses but melts into rain in summer conditions. Large hail may have a low density core if grown upon a small frozen cloud droplet, but otherwise always has a high density. The presence of alternate layers of cloudy and clear ice is attributed to transitions between dry and wet growth.It is shown that large hailstones can be grown during a slow second ascent in a steady updraught whose speed increases with height. Under certain simplifying assumptions it is also established that their surface temperatures are then always close to 0$C, and therefore that minor fluctuations in cloud liquid‐water content may cause growth transitions. When the simplifications are removed this result cannot be demonstrated, but it seems reasonable that alternate layers of clear and opaque ice could be produced in a similar way, and need not indicate successive rises and falls in an intermittent updraught, as formerly thought.

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