A crust that forms when a wet snow layer (from melt or rain) refreezes.

A melt-freeze crust that formed from the combination of rain and warm temperatures followed by colder weather. Credit: Flathead Avalanche Center

Melt-freeze crusts are caused by the snow surface melting or wetting and subsequently refreezing. Wetting the snow surface may be caused by rain, strong sunshine, warm air temperatures, and/or a ‘greenhousing’ solar radiation effect.  The thickness and hardness of these crusts depends on the amount of water available to re-freeze and the intensity of the freeze.  Melt-freeze crusts can vary from thin (several mm to 1 cm) to thick (>5 cm).  

A stack of melt-freeze crusts caused by solar radiation and warm temperatures during the heat of the day, and subsequent refreezing overnight. Credit: Crested Butte Avalanche Center

A melt-freeze crust on a crystal card. Credit: snowcrystals.it

Melt-freeze snow grains under magnification. Credit: The International Classification for Seasonal Snow on the Ground / Fierz