Size 2 or Size 3?

20th February 2021

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This blog is intended to provide hazard and mountain condition information to help plan safer mountain trips.
After a mild and spring like morning, it was a damp afternoon. On Ben Nevis debris from a number of avalanches which came down either yesterday afternoon or last night were noted. The largest of these was in Coire na Ciste and is shown in the photos below.

Ben Nevis: Looking into Coire Leis with the profile of North East Buttress on the right.

This block of ice pushed out of the Lochan in Coire na Ciste by a size two or three avalanche at some point a few weeks ago. The dirty snow to the immediate left of the forecaster is likely debris from that avalanche. 

An wet snow avalanche in Coire na Ciste. This likely came down in the mild conditions yesterday afternoon or last night. The debris fan on the left was measured as being 58m across. 

Crossing the debris fan to measure it’s width. I have tweaked the colours of the photo to make the debris stand out better. 

The European Avalanche Warning Service size scale.What size was the avalanche in the photos above. 

Judging the size of an avalanche can be quite a difficult task. Lets take the avalanche shown above as an example,  what size would you estimate it as. Here is my working and result:

Length; in terms of vertical fall distance it ran from an altitude of very roughly 1200 metres down to around 950 metres, giving a fall hieght around 250 metre mark. It’s actual length is probably going to be around 50% or so more, so lets say very roughly 400 metres. This implies somewhere between a size 2 and size 3.

Volume; The debris fan was 58m across where measured, but lets say its average width is around 50 metres. I would say the debris fan is roughly 4 times longer than it is wide, so lets go with a value of 200 metres long. For depth I would say generally between 30cm and 1m, so to make the numbers easier lets say an average value of 50cm. This gives a debris volume of the order of 5000 metres cubed. Again this implies somewhere between a size 2 and size 3.

Destructive power; This avalanche could definitely injure and bury people, but I am not convinced it would bury and destroy cars, destroy a building or damage a truck. Again this implies somewhere between a size 2 and size 3.

These values consistently indicate that this avalanche is between a size 2 and size 3.  It might be best described (in my estimation) as a large size 2 avalanche. That is my answer at least, feel free to put your answers, comments or views in the comments section, it is always interesting to get feedback on what we write in the blogs.

Comments on this post

  • Brian
    20th February 2021 5:07 pm

    Size 2.5

  • Ben F
    20th February 2021 8:05 pm

    Really interesting post, does a size 2.5 exist?
    Out of interest what is the biggest size slide you have seen in Scotland? And where was it?
    Thanks for your time.

    • lochaberadmin
      22nd February 2021 6:17 pm

      Yes, the scale is sometimes split into half sizes, and so size 2.5 does exist. I think the largest avalanche I have seen in Scotland was the one down to the halfway lochan on Ben Nevis in 2010. At some point if the weather is poor and there is not much to report on the blog I might try and post a photo of is.

  • Lee
    21st February 2021 12:21 am

    I think you’re right, large size 2.
    Thanks for the explanation.

  • Tom C
    21st February 2021 9:48 am

    I’m no expert on avalanche forecasting, so I’m not going to venture an opinion on the size of this specimen, but I am going to say that I enjoy these glimpses of process. I remember a blog a couple of years ago, estimating the mass of snow somewhere in Lochaber. That was fascinating. It would be interesting context to gauge the mass of snow involved in this – but I can’t remember the mass of 1m3 of snow. Is it the same as water?

  • C Cuthbertson
    21st February 2021 10:03 am

    Enthusiasm for a subject is magic to see – especially if it is backed up by clear practical mountain experience – and where time has been taken to commit this to “paper” and shared. Look forward to seeing these blogs with a morning cup of coffee!

  • Ross MacRae
    21st February 2021 4:17 pm

    Thank you for this explanation.

    I frequently read all the SAIS blogs as they are a great source of advice about mountain conditions and winter awareness.

    Not sure why there are few comments but I know the reports are very helpful to many people.

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