8th February 2015
Air, snow surface and deeper snow temperatures are all over the place at the moment. We had a sharp frost in the glen overnight (the Met station at Tulloch Bridge recorded -3.8 degrees C. at 6am) and barely below freezing summit temperatures. An increase in the wind speed later pretty effectively mixed the air, including the temperatures, across all altitudes later. The upshot of all this was some scratchy skinning on crunchy snow this morning at Aberarder, which relented above about 600m to 850m, then firmer again nearer summits. Had the whole gamut of surface snow conditions to ski on from hard snow ice to firm slab as well as some proto-corn snow quite often at unexpected altitudes and times of day.
The anti-cyclonic conditions we’re experiencing at the moment (including freshening winds) are certainly producing some upside-down weather and snow conditions.
Glide cracks can appear quite alarming if you unexpectedly come across them as one hillwalker did in the last day or so (footprints just visible in the photo deviating around these gaping chasms). What the photo doesn’t show well is the buckling of the deep snow below the glide cracks where the slowly descending slab piles up into less mobile, better anchored snow downslope.
The glide cracks can persist for ages or the slab downhill of them fail suddenly releasing a full-depth avalanche. It’s a bit of a crapshoot, really. The SAIS doesn’t pretend to predict full depth avalanches because, like cornices, it’s really difficult to measure the forces and understand the mechanics involved. What we do know is that they often release when there’s sufficient free-water around to lubricate the bed-surface, often when it’s milder then….but they surprise us all the time, hence the gambling analogy.
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