‘Warm car’ day.
10th March 2016
Spent ten minutes scraping frost off the car windscreen this morning then set the heating controls on the dashboard to ‘hot’ and ‘defrost’.
Nothing exceptional about that, other than the contrast with when I got off the hill this afternoon. The vehicle cabin was warm having been parked in the open with the sun on it. Warm enough to warrant setting the heating controls to ‘cold’ for the return journey.
So, the first ‘warm car’ day of the year. For me this is a very particular and personal benchmark which marks the arrival of Spring! Somewhere in a dusty corner of the SAIS Co-ordinator’s office will be a collection of handwritten diaries and inside of some of them will be scribbled references to ‘warm car day’ from when I used to work in the Northern Cairngorms many, many moons ago.
Haven’t seen any frogs cavorting in puddles yet, nor any daffodils (though some snowdrops are just beginning to poke through in the garden) but spring is definitely upon us at Creag Meagaidh….since it’s an official ‘warm car day’.
These diurnal (night to day to night etc) changes in temperature are affecting the existing snow cover in quite contrasting ways.
Note how the heavily indented exit ramps of the Post Face (an East aspect) have retreated into shade. The warming/cooling cycles will help strengthen slab layers which overlie buried weaknesses. In simple terms, this effect is called ‘bridging’. If the buried weak layers are ’rounding out’ (steadily diminishing) in these places then this is considered to be part of a consolidating or stabilising trend. However, although this diurnal ‘cycling’ is helping, we’re not quite there yet at ‘Meggie! There’s a lot of deep snow on East aspects on our patch which is maintaining cold temperatures deep down. Some East aspects at lower altitudes in Coire Ardair receive limited direct sunlight due to the shading effect of the high coire walls, so remain cold all day.
The problem with ‘bridging’ is that the higher tensile strength of the slab could, in the event of a high loading (collapsing large cornice etc), result in a big avalanche. It’s complicated, often delicately balanced, and always quite a ‘squeaky-bum’ judgement call when deciding avalanche hazard. That’s why you need to pay special attention to the formal hazard scale and travel advice guide on our website when considering the published avalanche hazard category. Read the stuff about loading really carefully.
NE aspects have just stayed cold right through the night and day. Our last storm cycle loaded up NE aspects with a lot of snow, with buried weaknesses developing due to the incessant refrigerated embrace of the shade.
(Above) Proper corn snow when skinning up a South aspect of Creag na Calliche. This will have been quite hard and/or crusty after the overnight frost but some warmth from the sun softened the surface enough for easy skinning.
(Above) Wide variety of snow surfaces. Soft looser snow overlying a hard crust. Couldn’t get an edge when skinning across it and ended up just slip-sliding away. Fall line slope angle is in the high 30s here. (Note to self: harscheissen would have been useful).
(Above) Looking up to the top end of the Moy Corrie from the formal snow pit today. Good cover! The forthcoming rain and mild temperatures in the next few days will bring big changes to the superficial snow cover.
(Above) Very large cornices on Beinn a Chaorainn and above many other very steep lee slopes. The largest are above NE through E to SE aspects. High loading on the slopes below in the event of collapse when the milder temperatures and rain arrive in the next day or so.
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