Adequate snow cover is crucial to the ovewintering success of many small mammals. Although air temperature can be frigid, it increases rapidly under depths of snow to keep warm, grouse dig snow roosts; taiga voles' communal nests can be 50 degrees warmer than the air above the snow. At the snowpack's bottom lies the depth hoar, a layer of sugarlike snow through which small animals can move easily. Various snow types can comprise a snowpack: A crustlike lens forms when snow melts, then refreezes. Hoarfrost, winter's equivalent of dw, resembles freezer ice. Strongwinds produce densely packed snow and in cold, dry conditions, snow falls as a light powder. A small fraction of sunlight can prenetrate up to six feet of snow, allowing some plant acivitiy. And some insects produce glycerol, which inhibits ice formation in their tissues, enabling survival in subzero temperatures.