Cairngorms snow cover in May, how usual is it?

Welcome to my first post on Scottish snow. Here we are at the end of what feels like the second winter this year in Scotland. Many of us thought that snow had gone for good, including the Cairngorm ski resort during the record breaking warm weather in March. Since then there has been considerable snowfall with avalanche observers out in force. Only now in late May is the high level snow beginning to melt in Lochaber.

But how often has there been snow cover in May in the past? Fortunately the Met Office collated observations of snowline from 1953 until 2007 via a network of voluntary observers. Before this, data was collected by the British Glaciological Society from 1945 until 1953. This data was the basis of the Snow Surveys of Great Britain. Whilst these records are subjective as they are based on the observers judgement of the snowline visible from low land, they do provide a unique national dataset chronicling snowline across more than half a century.

The SAIS observer on the 19th May indicated that snow cover in the northern Cairngorms is good from 800 m ASL (above sea level). What does 800 m snow cover look like on a map? How big an area is this? Using a GIS and a digital terrain model called Panorama available from the Ordnance Survey I have produced Figure 1 that shows the 800 m contour for the Cairngorms. The land above 800 m falls into two separate areas, the westerly one being 135 km2 and the easterly one being 61 km2.

Figure 1: Area of the Cairngorms above 800 m ASL.

If we assume this May’s snow cover was approximately 800 m ASL, how does this compare to the observations of the Snow Survey of Great Britain? There are two long record stations that look towards the western Cairngorms, Achnagoichan and Aviemore. Achnagoichan ran from 1954 until 1983 and Aviemore ran from 1982 until 2002. These records are not complete with some missing years and months (11 out of 48 May months missing). This aside I have used the data to estimate the mean elevation of snow, when present, in May as 810 m ASL. This figure is quite crude and doesn’t take into account days with no snow lying so how does May 2012 compare with the number of days during May with snow lying at 800 m ASL? Well, the Snow Survey observers recorded data in bands and the nearest band is 750 m ASL, which is close enough for the purpose of this exercise. Again, to make a simple assessment there are 9.4 days on average (mean) in May with snow lying at 750 m ASL and above, compared to a mean of 4.8 days with snow lying below this. You can take a more detailed look at this using the histograms below (Figures 2 and 3).

Figure 2: Histogram showing the number of days in May with snow lying below 750 m ASL.

Figure 3: Histogram showing number of days with snow lying above 750 m ASL in May.

So to conclude, based on the assumptions above May 2012 has been a bit snowier than usual with at least 19 days with a snowline around 800 m ASL, compared to an average over 37 years of of 9.4 days during May at 750 m ASL.

Other than the obvious mountain sports like skiing, why would we be interested in snowline? One major reason is to help estimate flood risk. SEPA is the authority that oversees, monitors and forecasts flooding and is requires an understanding of snowline to estimate potential river flow from snowmelt. Ecologists and biologists are also interested in long term snow cover trends as these influence the types of plants that can grow in our upland areas.

If you’ve further information on snowline in the Cairngorms for May 2012 please get in touch via the comments below and I’ll try to provide an update to this post.