The north face is also one of Scotland's foremost venues for winter mountaineering and ice climbing, and holds snow until quite late in the year; in a good year, routes may remain in winter condition until mid-spring. Most of the possible rock routes are also suitable as winter climbs, including the four main ridges; Tower Ridge, for example, is grade IV on the Scottish winter grade, having been upgraded in 2009 by the Scottish Mountaineering Club after requests by the local Mountain Rescue Team, there being numerous benightments and incidents every winter season. Probably the most popular ice climb on Ben Nevis is The Curtain (IV,5) on the left side of the Càrn Dearg Buttress. At the top end of the scale, Centurion in winter is a grade VIII,8 face climb.