What is "premelting"

n. (context physics English) The formation of a quasi-liquid film on the surface of crystal below its melting point

Premelting (also: Surface melting) describes the fact that, even below its melting point (T), quasi-liquid films can be observed on crystalline surfaces. The thickness of the film is temperature (T) dependent. This effect is common for all crystalline materials. Premelting shows its effects in e.g. frost heave, the growth of snowflakes and, taking grain boundary interfaces into account, maybe even in the movement of glaciers. Considering a solid-vapour interface, complete and incomplete premelting can be distinguished. During a temperature rise from below to above T, in the case of complete premelting, the solid melts homogeneously from the outside to the inside; in the case of incomplete premelting, the liquid film stays very thin during the beginning of the melting process, but droplets start to form on the interface. In either case, the solid always melts from the outside inwards, never from the inside.