A clear layer of ice formed when rain falls on the snow surface then freezes.
Rain crusts tend to be much smoother than sun crusts. Unlike sun crusts, rain crusts form uniformly on all aspects but rain crusts--like rain--is highly elevation dependent. Typically rain falls at lower elevations and as you ascend through the freezing level the rain progressively turns to snow.
Prolonged rain on new snow forms drainage channels down the fall-line and make a corrugated pattern in the surface snow. These drainage channels (also called "rill marks," are a strong sign of stability).