9th February 2023
Another burly day was experienced in the Cairngorms today, with Storm Force winds at higher elevations. The maximum gust recorded on Cairn Gorm summit was 106 mph, while the mean windspeed was 62 mph. This does explain why it was a bit of a battle over to my snow profile location.
With winds from the West the Northern Cairngorms presents an atypical picture with regard to avalanche hazard. When the mountains are viewed from Glenmore little snow is seen currently apart from the lee side of the broad ridges, snow patches and ribbons in the burn lines. However, in the coires and further into the interior there is a slightly different picture.
Following light snow showers last night, unstable windslab was deposited throughout the day on North-East to South-East aspects above 950 metres. This is generally found in steep wind sheltered locations where it overlies older firm and consolidated snow.
The distribution of windslab is variable due to the storm force winds. Where powerful down droughts exist, snow has been carried to lower elevations or has ablated into the atmosphere e.g. in Coire Cas and Coire an t-Sneachda. The deep sided valleys that run north-south through the Cairngorms (more on them in a bit) provide broad easterly facing slopes ideal for windslab accumulation.
This is the situation that prevails at the moment, with the greatest accumulations noted in Strath Nethy, and the Lairig Ghru (seen fleetingly today). Avalanche activity was noted on a North-East facing slope of around 1100 metres, this most likely triggered by cornice collapse overnight.
Windslab will continue to accumulate overnight, but there will be a slowly stabilising trend tomorrow as the freezing level rises above the summits by dawn.
The big valleys of Strath Nethy and the Lairig Ghru have always fascinated me, partially given the fact that they bisect the Cairngorm Granite pluton. The pluton is such a large chunk of granite that I have often wondered as to the structural controls on their development.
A bit of research tells me that here in the Cairngorms that the valleys align closely to quartz veins and linear alteration zones, which are some of weakness in the granite pluton. Quite unusual as the dominant control of these things would normally be joint sets in other rock types. (To be continued…)
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