From Glossary of Meteorology
The precedence of a thaw in a valley, sometimes by many hours, by a thaw or marked rise in temperature at mountain level in the same vicinity.
The phenomenon is usually caused by the arrival at higher levels of the warm air in advance of a surface warm front. It may also be caused by the subsidence and dynamical heating of air at the higher level. The associated inversion of the normal temperature lapse rate is a contributory cause of glaze.