Accretion in mixed clouds

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Abstract
An object on which ice is deposited when it moves through a supercooled cloud is warmer than its environment owing to the release of heat of fusion. If the rate of accretion is sufficiently great, the temperature of the object is raised to 0°C and its surface becomes wet. It has often been assumed that water in excess of that which can be frozen is shed into the wake of the object. The present experiments show that, at temperatures below – 2°C, the excess water is not shed but is incorporated into the ice structure, giving a ‘spongy’ deposit. Even in the temperature range – 2 to 0°C, the excess water can be retained if a sufficient concentration of ice particles is introduced into the droplet cloud. It has also been found that over a wide range of conditions, considerable quantities of ice particles may be accreted in addition to the supercooled droplets, and the crystalline structure of the deposit is modified accordingly. It is considered that at temperatures near 0°C, and possibly at lower temperatures the presence of ice particles in the supercooled cloud is necessary in order that none of the unfrozen water is shed.The restriction formerly thought to be imposed on the growth rate of hailstones in the wet condition may now be removed.

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