Some Dry January Statistics.
4th February 2025
It was another damp day in Lochaber. The freezing level was above/around the summits during the morning before dropping during the afternoon. The snowpack is looking a bit sorry for itself compared to a few days ago, but on the other hand there is a lot more than there was around the middle of January. Hopefully, drier, colder, clearer conditions tomorrow will allow a decent view of what has survived the mild damp conditions of the past few days.

With all the rain over the last 24 hours there is plenty of water about. Hopefully, there is enough snow still left in the higher gullies to form some good snow-ice when it does turn cold.
I thought it might be interesting to have a look at some weather data for the season so far, and in particularly January. January is easier to compare to other Januarys, as there is a decent record of summit weather each day of that month going back to 2008. This contrasts to December where the SAIS weather data record is not complete for the month due to the varying start date during December in previous seasons. It turns out that, in terms of temperature, this January has been about average (for the period 2008-2024) even if it has felt far from average, in not a good way, in terms of overall wintery feel.

The Aonach Mor midday summit temperature so far this season. Also shown are the minimum, average and maximum values recorded for the winter 2007/08 to 2023/24. Data is missing for the 21st of December.

The midday summit temperature anomaly up the end of January this season as compared to the 2007/08-2023/24 average. During January the cold spells at the start and late in the month cancelled out the mild spell in the middle, and right at the end of the month, meaning that January 2025 was very close to the 2008-2024 mean midday summit temperature of -2.1°C. Much of the second half of December was mild.

The average January Summit temperature (2008-2024) is about -2.1°C. This graph shows the overall temperature anomaly for all Januarys since 2008. This January, happened to be -2.1°C, which gives an anomaly of 0°C, which is why the entry for this year superficially appears to be missing. From 2008 to 2015, Januarys were predominantly below the 2008-2024 mean. Since 2016 Januarys have been predominantly above the mean, although with the notable exception of 2021. However, there might not be as much of a trend as this data suggests, as a number of Januarys prior to this dataset, specifically 2002, 2005 and 2007 were also very mild.

The Standard Deviation of the daily January temperatures through the years. The standard deviation is a measure of the amount of variation of the value (in this case the temperature) around its mean. It can be seen that this season is the one with the largest Standard Deviation and thus spread of daily temperatures around that months mean. This is unsurprising if you look at some of the previous graphs, January went from being cold for the first 12 days of so to unusually mild for a period mid month with only a few days with close to average around the 20th of the month.

Standard Deviation measures the spread around the mean. It does not however, take into account the order in which those values, in this case midday temperatures, occurred. So a month in which the weather was cold for the first half and then mild for the second half would give the same standard deviation as one in which a cold day was followed by a mild day, then a cold day and so on throughout the month. However, those two scenarios would give a very different feeling of the variability of the temperature even though the standard deviations were the same. Therefore I thought that looking at the average absolute temperature change from day to day though January might be a better measure of how changeable it has felt. By this measure this month was only slightly more changeable than the average January. As an aside the largest change from midday one day to the next was a drop of 13.7°C from +8.3°C to -5.4°C on the 27th to 28th of January 2024.

There were six days when the Aonach Mor midday summit temperature was above freezing, this is pretty typical and is only 2 days more than, for example, the memorably cold January of 2010. However, the last few days of December were mild and temperatures were above freezing on the tops, as has been the case for a number of days in early February. Therefore if we looked at for example the number of days above freezing since Christmas for example, this season would rank much higher.
So given that January was average temperature-wise, why has it felt so poor winter-wise? Well one reason is the mild conditions at the end of December and so far in February have made the period from around the 20th of December until now milder (both relatively and absolutely) than January alone. Almost all the cold days we have had this season have been in January, which has pulled that month down temperature wise during a generally warm season so far. However, probably the greatest effect has been the lack of snow. I don’t have precipitation data for Lochaber specifically but looking at the Met Office stats (which can be found at https://www.metoffice.gov.uk/research/climate/maps-and-data/uk-temperature-rainfall-and-sunshine-time-series ) it has been one of the driest Januaries of the 21st century. The two other similar Januarys in terms of precipitation are 2001 and 2010, both of which felt very wintry.
Little snow fell in the final 10 days of December due to the mild conditions. There was a dusting of snow early on in January which made the hills look white, but disappeared rapidly when milder weather arrived mid month. I don’t think I have ever seen the hills holding as little snow at the middle of January. Much of the snow we have had since Christmas was associated with Storm Eowyn, and the deep low pressures which followed over the next couple of days. Things definitely improved snow cover wise, but the mild wet days of early February have stripped all that back.
So essentially the problem is that is has generally been mild, but with some cold but dry spells. Cool (or at least not mild) temperature and precipitation are required to build up a snow pack, and we have not had that combination so far this season. The pattern we have seen so far looks to be continuing with the mild damp conditions of the past few days transitioning to a colder but drier period. Looks like the skis will have to stay at home for a while yet, but on the other hand I might be able to leave my waterproof jacket in the rucsac for the next few days.
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John
4th February 2025 4:32 pm
Insightful analysis. Many thanks for sharing this.
Jason
4th February 2025 6:01 pm
Can i ask: are the warm / cold cycles getting longer? For example the two weeks of very cold weather at the start of the year followed by prolonged warm weather? Is this part of the reason why snow can’t consolidate?
lochaberadmin
5th February 2025 7:26 pm
Good question, one I will try and look into at some point. We have certainly had long warm spell March 2012 the freezing level was above the tops most of the month. What is noticeable this winter is that we are very much having blocks of one type of weather then another.
Kenny
4th February 2025 8:38 pm
Very thoughtful analysis, really enjoyed not only the stats (complete with standard deviations instead of ‘lies, damned lies…’) but also the insight and reflections. Thanks!
And agree on looking for the positives, I had five days in a row of mostly cold/dry weather you mention from 19 Jan, with almost no snow around. Whilst it was a shame the axe and crampons let alone snow shoes didn’t get used… but likewise, the waterproofs stayed in rucksack and the frozen ground made for some brisk walking and some bike/hikes I’d not normally think of doing in winter.
And all that said, have fingers crossed for snow again sometime soon!
Allan Crawford
5th February 2025 11:28 am
I fish for salmon on the river Oich and this year is the warmest water for January I can remember at around 4.5C usually expect to see it between 3 and 4C. .