Abstract
Liquid water skins on spongy deposits of hailstones that grow while gyrating in a wind tunnel environment, have been routinely observed to be supercooled at the water skin–air interface to as low as −5°C and more. The average water skin thickness (up to 1 mm) in the main growth region is calculated on the basis of the molecular conduction of the latent heat of freezing from the spongy substrate at the base of the water skin to its surface. The heat transfer is gradient-driven and relates directly to the speed of ice accretion on the hailstone. An extrapolation of an equation for the ice growth speed in supercooled bulk water suggests a supercooling of the ice–water interface of the order of −0.3°C.
The physical picture emerging is that of an ice sponge from which a fragile dendrite mesh grows into the water skin with a very homogeneous front and an advance speed that is controlled by the diffusion of heat (heat conduction) away from the ice front. Combining all results, it can be categorically stated that all surface points of growing hailstones have temeperatures below the freezing point of water.
