Deformed Snow Slab on Roof

January 17, 2011

Photographer: Jim Bromley; Heather Renyck
Summary Author:  Bryce Hand; Heather Renyck

The photo above showing an unusual slab of snow draped over a carport rooftop was snapped in Meaford, Ontario on December 11, 2010. In some ways, it's acting like a wet-base (temperate climate) glacier where basal slippage is facilitated by water at its freezing/melting point. Evidently, the water film was just thin enough so that roughness elements of the shingles kept the entire layer from quickly sliding off. Deformation of the slab involves a number of processes that probably wouldn't have been nearly as effective in keeping the layer intact if it was bent suddenly. Note that the shrubs appear to have stalled or at least slowed the slab's progress. Of course, the snow that forms glacial ice (firn) is very dense, perhaps four or five times more dense than the snow on this roof.

Photo details: Camera Maker: Panasonic; Camera Model: DMC-TZ4; Focal Length: 17.5mm (35mm equivalent: 103mm); Aperture: f/4.7; Exposure Time: 0.025 s (1/40); ISO equiv: 160; Exposure Bias: none; Metering Mode: Matrix; Exposure: program (Auto); White Balance: Auto; Light Source: Flash; Flash Fired: Yes (Auto, return light detected); Orientation: Normal; Color Space: sRGB.