A snow layer melted by radiation from the sun and subsequently refrozen.

Sun Crusts:

A frozen sun crust sometimes forms a hard bed surface for future avalanches to run upon, but just as often does not. When new snow falls on a sun crust, it may produce loose snow avalanches and but this avalanche activity is short-lived. Over time through complex processes of heat and vapor transfer, small facets can form near a crust. These facets can become the weak layer for future slab avalanches.

Sun crusts, of course, form only on sunny slopes and not at all on the shady ones. So we find them mostly on southeast, south, southwest and west facing slopes at mid latitudes in the Northern Hemisphere (and conversely forms more uniformly on all aspects in tropical and arctic latitudes). On these aspects many sun crusts can form during a season. Many do not become an avalanche concern while some do.

Hot Tip:
When new snow falls on a sun crust, it's important to check out whether the sun crust is wet or frozen when the snow starts. If it's wet, the new snow will stick to it and you most likely won't have any immediate avalanche problem, but if the crust is frozen, then the new snow does not tend to bond very well.

Sun Crust Summary:

By strong sun on the snow surface.

Looks like:
Shiny with slightly rough surface.

Distribution pattern:
Forms only on sunny aspects, none on shady aspects - moderately elevation dependent.

Additional Terms:
Anchors Hard Slab Avalanche Slide
Aspect High Danger Sluff
Avalanche High Marking Snowpit
Avalanche Path Isothermal Soft Slab Avalanche
Avalanche Transceiver Layer, Snow Stability
Bed Surface Leeward Stability Test
Collapse Loading Starting Zone
Concave Slope Loose Snow Avalanche Stepping Down
Considerable Danger Low Avalanche Hazard Sun Crust
Convex Slope Melt-Freeze Snow Surface Hoar
Cornice Metamorphism, Snow Sympathetic Trigger
Corn Snow Moderate Danger Temperature Gradient
Couloir Persistent Weak Layers Terrain Trap
Cross Loading Point-Release Track
Crown Face Probe Trigger
Danger Ratings Propagation Trigger Point
Deep Slab Avalanche Rain Crust Upside-Down Storm
Density, Snow Remote Trigger Weak Layer
Depth Hoar Rime Weak Interface
Dry Snow Avalanche Runout Zone Wet Snow Avalanche
Extreme Danger Sastrugi Windward
Faceted Snow Settlement Wind Loading
Fracture Ski or Slope Cut Wind Slab
Glide Skinning, Skin Track Whumpf
Graupel Slab