From Glossary of Meteorology
A twofold process in which a localized region on the surface of a piece of ice melts when pressure is applied to that region (pressure melting) and then refreezes when pressure is reduced.
Regelation was discovered by Faraday, who found that two pieces of ice at 0°C would freeze together if pressed against each other and then released. Regelation occurs only for substances, such as ice, that have the property of expanding upon freezing, for the melting points of those substances decrease with increasing external pressure. The melting point of pure ice decreases with pressure at the rate of 0.0072°C per atmosphere. Since this rate is very small, regelation only occurs at ice temperatures of 0°C or very slightly less. The fact that snowballs can be packed well at near 0°C but not at much colder temperatures is a consequence of regelation.