Cornices…
6th December 2025
Following last night’s wild weather it seemed prudent to visit the coire rim of Coire an t-Sneachda once again. The area around “Windy Col” readily collects snow on South-Easterly winds and there was no sign of my snow profile site from the previous day.
Snow overnight had been subject to heavy drifting, despite the storm force winds. With a maximum gust of 104 mph recorded on Cairn Gorm overnight, it would be an obvious assumption that any snow had been blasted clear of the mountains. Paradoxically, plenty of snow had accumulated around the coire rim and some large and fragile looking cornices have appeared.
For those of a meteorological persuasion these sticky accumulations of humid or moist windslab and cornices represent a visual cue as to the difference between the Dry and Wet Bulb freezing levels.
These cornices will present the greatest hazard as we go into tomorrow. Fresh unsupported cornices can be seen around ‘Jacob’s Ladder’ in particular, and will no doubt also be around ‘The Vent’ in Coire an Lochain. These will present a persistent hazard, with an ongoing threat of collapse exacerbated by the onset of rain.
An altogether moist looking picture from the summit of Cairn Gorm this morning.
A climber at the top of ‘The Mess of Pottage’, Coire an t-Sneachda. Localised accumulations of snow are present in the foreground while cover can be seen to be patchy at lower elevations.
Fresh and unstable cornices are present, pictured here at the top of ‘Jacob’s Ladder’. A group from the University of Stirling Mountaineering Club are wisely walking the other side of the rocks. As the SAIS Senior Forecaster from Creag Meagaidh area always says – never trust a cornice!
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Sputan Dearg
9th December 2025 11:20 pm
I don’t understand your comment regarding “a visual cue as to the difference between the Dry and Wet Bulb freezing levels”. The relative humidity at the summit of Cairn Gorm has been essentially 100% since the start of December, i.e. dry bulb and wet bulb temperatures (and freezing levels) have been the same.
ncairngormsadmin
13th December 2025 3:55 pm
Good question. I wrote that based on the observations made during the day (notably a fair bit of windslab and cornice development around 1000 metres). Overnight the temperatures were below freezing on the summit of Cairn Gorm, and there were quite hight winds. I was anticipating that any snow falling would be blasted away. But the observed conditions were quite different i.e. a lot of snow had “stuck”. My assumption (and I could be wrong) was that the Wet Bulb Freezing Level was much lower resulting in snow sticking around the coire rims, which are approx 200 metres lower. Hence, what I was seeing was a visual representation of this. You are quite correct that we have had 100% humidity and therefore the dry and wet bulb temperature would be the same. You have piqued my interest though and I will have a look at the historical data…
As an aside the other thing that happens quite frequently with a southerly flow in that milder air over-rides the colder air in the Northern Coires on Cairn Gorm. This results in slight inversion conditions, with cooler air in the coires and in isolated areas of the plateau. There was a slight radiation freeze after a clear night on the plateau and the snow surface was very firm. All of this results in a confusing picture, although it does support the value in getting the ground truth and actually putting forecasters on the ground. Hopefully we won’t get overtaken by AI just yet?