Snow-thaw-freeze.

I wasn't out climbing today, which was a shame because it turned out to be a nice day for ice climbing. Friends of mine were out climbing The Cascade and Green Gully on Ben Nevis. Both were thin but gave good fun ice climbing.

​After two and a half weeks of soggy, relatively warm weather, we are finally starting to get more normal weather and temperatures for the time of year. The big high pressure is finally moving away, taking with it the stream of air from the Canary Islands that has slowly melted away some of our snow and ice. In its place, low pressure systems will sweep across Scotland bringing strong winds and fluctuating temperatures, and plenty of precipitation. This is good news (honestly).

Our most common and most famous style of winter climbing is snow-ice climbing in gullies. Point Five Gully is possibly the most famous example and is well known the world over. Green Gully and Comb Gully on Ben Nevis, Crowberry Gully on Buachaille Etive Mor and SC Gully on Stob Coire nan Lochan are also snow-ice climbs.

​Snow-ice does not form just with cold weather. Instead we need snow fall and the right direction of wind to fill the gully with snow; just enough thaw to make the snow wet without melting too much away; then a good freeze to make it solid. This snow-thaw-freeze cycle needs to be repeated a few times to form sufficient snow-ice in the gully to climb. Too little thaw before the freeze will result in firm snow rather than solid snow-ice, or a surface layer of snow-ice on top of softer snow. Then we need a calm day with no snowfall and a freezing level below the bottom of the route for it to be in condition.

The West Coast of Scotland typically benefits from lots of snowfall accompanied by strong winds and temperatures that can change by 10℃ in just a few hours (and sometimes back again in the next few hours). Soft snow is transported by the wind to collect in sheltered slopes and is funnelled by gullies. It is warmed up in the following thaw and becomes wet from partial melt or through rainfall. It then freezes into a more solid version of snow if there is a subsequent freeze. This is how snow-ice is formed and the quality of the snow-ice depends on the precise balance of volume of snow, depth and duration of thaw, rain, and how well frozen it is afterwards. With very little change in any one of these variables, the quality of the resulting snow-ice changes greatly.

​At it’s best, snow ice can offer the perfect combination of plastic, dependable ice that does not shatter when you place a pick but is sufficiently solid to place good ice screws into. The “honeycomb” structure provided by millions of tiny air pockets in the snow creates a texture that absorbs the strike of a pick instead of shattering like cascade ice. At its worst, a snow ice climb can be vertical snow that is too soft to hold an ice axe, let alone an ice screw.

How do you know if the balance is just right and the snow-ice will be perfect? Well, that comes with practice, and a lot of checking forecasts and actual weather data. It looks to me like the weather forecasts for the next week or so, combined with the snow and ice cover we already have, the background temperature of the rocks and the amount of water in the rocks from recent rainfall, all add up to a good ice building period coming our way. Fingers crossed!

Here are some useful resources to track progress.

Mountain Weather Information Service - https://www.mwis.org.uk/forecasts/scottish/west-highlands
Met Office - https://www.metoffice.gov.uk/weather/specialist-forecasts/mountain/southwest-highlands

Scottish Avalanche Information Service - https://www.sais.gov.uk/lochaber/

Weather data from Aonach Mor at 1200m - https://holfuy.com/en/weather/1365
Weather data from Aonach Mor at 900m - https://holfuy.com/en/weather/195
Weather data from Aonach Mor at 650m - https://holfuy.com/en/weather/296

Weather data from Glencoe Mountain (Meall a’Bhuiridh) at 360m, 750m and 1100m - https://www.glencoemountain.co.uk/weather/

Webcam images and weather data from CIC Hut, Ben Nevis - https://www.smc.org.uk/cicwebcam/cic_weather.php