Friday, 18 December 2020

Some More Snow Ponderings

Exploring the snow tunnels on Ben Nevis during August. 

After neglecting my blog for the last eight months or so I feel inspired to get back to it. During lockdown I didn't feel inspired to write and more recently, despite not really working and thus having a fair bit of time, the inertia required to get writing again has prevented me from getting going. However, now that I have got going, I hope to update things a bit more regularly. I thought that the easiest subject to start with would be one I have written a fair bit about before, snow. 

An impressive snow tunnel at the Point 5 Patch. 
Last spring the amount of snow on the Lochaber hills was greatest in mid to late March, just around the time that lockdown started. Although I don't have any hard data on this, I am sure this is earlier than is typical, normally snow amounts peak in April or May. The main reason for is that April was unusually dry and sunny, a fact that would not have gone unnoticed to everybody stuck in lockdown. Although there was no snow added to the snow patch locations during this period, there also would have been little melting in the high shady locations where the snow patches lie. 

As usual the majority of the melt occurred from late spring/early summer until early Autumn. Melt rate is controlled not just by temperature, but by a combination of this and rainfall, with both high temperatures and rainfall leading to increased melt. This the summer was average in terms of both temperature and rainfall, and thus likely to be pretty typical in terms of snowpatch melting. I noticed that Scott of Wanstead Meteo blog visited Ben Nevis in the summer to have a look at the snow patches, and recently wrote a nice analysis of the summer melt season (which he found to be around average) and can be found here; https://wansteadmeteo.com/2020/11/05/cathedrals-of-snow/

The depth of snow in the Observatory Gully patch in mid/late August. 

Every year there is an annual snowpatch survey occurs on or around the 20th of August, and as is often the case I headed up Ben Nevis with a few few friends to have a look around. Although it was the not snowiest I had seen the Ben at that time of year, it was not at all bad with things looking positive for a few survivals. There were certainly some impressive snow tunnels to have a look at. 

The Met Office mean temperature for the uk during the Autumn. Although not specific to the area, it is still useful to show the warm and cold periods. 

After an average to mild start to September, the first hints of Autumn arrived on the 22nd of September. The first proper snows fell about three weeks later on the 9th or 10th of October. Although there was some reasonable drifts at the tops of some of the gullies (and is seems likely some of these drifts were the lasting snow in these locations), little fresh snow made it down to the locations of the snowpatches, and any that did soon melted. The middle of the month was cool and dry which slowed melting. There was a dusting of fresh on the 23rd of the month, and then a bit more on the 29th/30th, with further falls on the 3rd and 4th of November. The middle of November was mild and wet, before more substantial fresh snow arriving on the 21st, after which is stayed cold with further snow falls. 

In Lochaber this year five snowpatches survived; two on Aonach Mor, two on Ben Nevis and one on Aonach Beag. It is about 5 years since we have had so many survivals in Lochaber. 

Coire an Lochan of Aonach Mor on the 11th of October. The two patches of last years snow are visible below the crags. Notice how with just a small dusting of snow it has built up significantly around the coire rim. 

Some years it is very clear when the lasting snow arrives at the snow patches, there is a substantial snowfall one day in October or November, and then it stays cold with additional snow until the patches are clearly buried. Other years it is a bit more subtle, with the fresh snow coming and going. It can be hard to tell whether a particular fall of fresh snow melted or not, and thus working out when the lasting snow arrived can be tricky. I think this year is a tricky one, with the date of the lasting snow (at least in my opinion) being different for the Ben Nevis and the Aonach Mor snow patches. 

The two patches that survived on Aonach Mor are both found in Coire an Lochan and are called Piranha and Pro-talus, the names referring in the first case to the climb is it situated under, and in the second, the feature that it sits in. Aonach Mor is made of granite which, due to it's chemistry, tends to have more vegetation than the rhyolite of Ben Nevis. The summit plateau of Aonach Mor is fairly flat and smooth, and ledges slopes of the the coire tend to be grassy and smooth. The smoothness of the plateau means that even during the first snow falls of the season, there is little to prevent the snow blowing along the ground and accumulating at the top of Coire an Lochan. Once there it does not take much  build up before the snow can sluff and avalanche downwards towards the snow patches which sit near the base of the crags.  

The Pro-Talus patch on Aonach Mor on the 26th of Nov. Although the old snow in visible in the middle ground, the far side of the patch (where the rucsac is) was well buried by avalanche debris. 

Avalanche debris was observed lying on around and on top of the two Coire an Lochan snowpatches on the 31st of October. A picture from the 10th of November shows additional avalanche debris surrounding the patches, and filling in the gaps at the side. It was warm and wet for the next week or so, and with no observations it is difficult to tell how much of this debris survived. My feeling is that some of it, particularity where it was packed around the snowpatches did survive. I would say therefore that the two patches on Aonach Mor the lasting snow arrived in late October, either the dusting on the 23rd or the more substantial fall during the night of the 29th. 

The Observatory Gully Patch on the 30th of October. There was a uniform layer of fresh snow while at this time the patches on Aonach Mor were buried by avalanche debris. 

The terrain on Ben Nevis is a bit different to that on Aonach Mor. The summit plateau is a boulder field, the crags themselves are rougher and the snow patches are in locations a bit more removed from the top of the crags. During any early season snowfall, a lot of the snow gets stuck between the boulders, and much more build up is required for avalanches to move the snow down to the snowpatch locations than is required in Aonach Beag. I headed up for a look on the 30th of Oct, the same days that the Coire an Lochan patches were covered in avalanche debris. Two patches that survived on Ben Nevis this year.  These are the Point Five patch and the Observatory Gully patch, both named after the gullies they lie in/at the base off. There was very little fresh snow at the former, and at the latter uniform covering of  around 15cm of fresh snow with little evidence of drifting or avalanche debris. This fresh stuff could not withstand the mild wet conditions of mid November, and when these patches were visited no fresh snow was observed on a visit to this location by a colleague on the 18th. On the 21st of November there was a dusting of fresh snow on the hill, and this continued to be added to over the few days. It soon became clear that the lasting snow had arrived here as well. 

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