Thursday, 21 April 2016

Snow cover in Lochaber 3

It is again roughly the time of year when snow depth reaches it's maximum in the high North and East facing gullies and hollows. The amount of snow in these areas is the biggest factor in how long it survives, and how likely various patches are to make it through to next winter. For this reason, and because of the popularity (in relative terms) of my previous blog posts on this subject, I thought it would be a good time to write about the current snow situation.

Below are a series of pictures of the South side of Coire an Lochan of Aonach Mor taken in April this year, and previous Aprils.
 April 2016
April 2015
April 2014
April 2013
April 2012
April 2011
April 2010

April 2009
March 2008 (I could not find an April picture for this year)
By studying these pictures, and in particular how buried some of the rock are, I would say that in terms of the amount of snow  that in this location this year there is;
The point Five patch on Ben Nevis last
August. A fair bit of this now very dense
snow must be buried under this winter's
Less than 2015.
Less than 2014.
More than 2013.
A lot more than 2012.
More than 2011.
A lot more than 2010.
More than 2009.
Less than 2008.

I would say that this year is the 4th snowiest in the 9 years that I have pictures for.

The strongest single factor in which patches survive the summer is the depth of snow in the spring.  The three years snowiest winters, as ranked using the photos above, led to the survival of the snow in this coire through the following summer. This did not occur in any of the summers following the years winters rated less snowy than this winter. This suggests it will be touch an go as to whether the coire holds snow through to next winter.

Another source of data on April snow volume is the cumulative snow fall totals as recorded by the SAIS forecasters through the winter. This tells a similar, perhaps slightly more positive tale as the pictures. This year's total being put this year as being the joint second (shared with 2015) winter in the last 8 winters (note that the data only went back to 2009, I suspect we would have been beaten by 2008 if that had been included in the analysis).

However, there are of course other factors than just April snow depth which affects how much snow survives through to the next winter. One of these factors is the length of the melt season. It it now mid to late April, and the forecast is for a north winds, and consequently a low freezing level, until at least the end of the month. Therefore there is no sign of the melt season beginning properly until into May. Although cold weather at this time of year is not unusual, every day that the snow is not thawing helps. 
A picture taken August 2015. You can see the first year snow in the foreground peeling away from the much denser multi-year snow in the background. There is presently plenty of multi-year snow  in some locations.
Another factor is the amount of multi-year snow which exists in some of the snow patch locations, particularly the Point Five and Observatory Gully patches on Ben Nevis.  What I mean by multi-year snow is snow which fell in previous winters which did not melt through the summer.  This has compressed into a high density snow-ice.  This will give these patches large icy hearts which will be it much more resistant to thaw than first year snow.

In summary due to the quite reasonable amount of snow that fell over the winter, the so far cold spring, and the multi year snow in some locations I am currently quite optimistic about prospects of snow patch survivals this season. I think it is unlikely to be an exceptional year, but I suspect (and hope) it is better than average.