Another Fine Day in Lochaber
26th December 2025
As the title suggests it was another fine day on the hills in Lochaber. I was not so nice down in the freezing fog which hung around all day near various large bodies of water such as Loch Eil, Loch Lochy, Loch Oich and getting further away Loch Ness. There looked to be a band of fog running the whole length of the Great Glen.
Humidity was very low on the summits. Low relative humidity on the summits is something we see most years, and has been discussed in a couple of times previously on this blog, see the 5th of Feb 2023 and the 19th of Dec 2021 On those occasions relative humidity got down to about 20% (based on the Holfuy station on the Aonach Mor summit hut). According to the Holfy station it got down to just under 10% at around 2am last night, see graph below. However, the nearby Met Station gave a reading of around 5% at roughly the same time. Looking at the graph of humidity distributions (seen in the 19th of Dec 2021 post), such low humidity’s are recorded about once in a thousand days on Aonach Mor. They also virtually never occur in the UK at low land sites. According to a BBC news article which is also linked to, there is only one reliably documented instance of a humidity below 10% at any lowland site in the British Isles. This was at RAF Honington in Suffolk where the relative humidity fell to 8% at 1400 GMT on 30 June 1976. Further East the weather station on the summit of Cairngorm recorded a relative humidity of 2% yesterday. I don’t know how accurate this value is, and if this is a UK record. These are certainly unusual conditions.
Back in Locbaher, the air temperature on the summits has actually been above freezing for the past 24 hours. However, due to radiative cooler and evaporative cooling (both subjects that deserved a bit more explanation at some point as they both can have a significant effect on snow and it’s evolution) the ground and snow surface temperature were below freezing. At a shady location near the summit hut the ground surface temperature was just below freezing, while the snow surface was a a chilly minus seven. This meant that the snowpack which had been wet a few days ago was frozen hard. While avalanches are not currently a hazard, slipping or sliding on the firm icy snowpack is.
Temperature and Humidity from the Holfuy Station on Aonach Mor for the past 24 hours. Air temperature (blue dots in upper graph) has been consistently above freezing, the wet bulb temperature (black dots) dropped below freezing lasy night and the relative humidity (lower graph) has been consistently low, getting down to just less than 10% just before 2am this morning.
The view West from Aonach Mor this morning. The fog over Loch Eil did not clear all day.
The view to the North and North-East this morning. Although there was some diffusion of the fog, a strip remained through the Great Glen all day.
Coire nan Lochan catching the morning sunshine.
The snow at higher levels is very hard and icy. Crampons would be required to safely cross anything that was other than very gentle slopes.
North East face of Aonach Beag. Not much ice there at the moment.
Ben Nevis and Carn Mor Dearg.
Looking South over the Mamores towards the Glen Coe hills
Frost in the trees down in the glens. In shady locations the trees and ground were covered in frost all day.
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