Today I climbed up to Col Ratti in the Haute Savoie. Due to laziness we didn’t get on the hill til about 1330, got to the top at about 1600 and hung about there for a good hour taking in the epic views in today’s great vis.

When we started up at 1400 the snow seemed pretty good with fairly loose powder and some equally powdery hoar frost areas, it seemed like it’d be a good run down. During the course of the ascent I noticed a bit of a crust developing and by the time we made our way down the snow was somewhat caramelly - not too bad on the untracked bits on a snowboard but (as I was emphatically told...) very hard work on skis. The route has a S/SW aspect.

Is this the usual progression of the surface conditions during a sunny day? How - given yesterdays weather was essentially the same - do things return to the much better state we saw earlier in the day, in other words, presuming it was caramelly yesterday afternoon, how did it return to powderiness?

sounds like the same conditions we have here, timing is everything

I think your answer is diurnal recrystallization. Were the crystals about .5 mm to 2mm or so?  The snow has radiated heat overnight creating a high temperature gradient in the top few cm of the snow. Enough to google?

The conditions further south where I am (south of Chambery) are probably similar. We had fresh snow last week, I think fresh was on Friday. Then a very sunny weekend.

I was indoors all day yesterday but today was touring in the N.Belledonne. We started out at 780m at 10am (quite late but it was a NW route). Snow was crusty between 780 to 1000m, probably a thaw/freeze cycle.  Between 1000-1600m it was powder with a good layer of surface hoar, if it stays cold and clear this will eventually transform a good portion of the snowpack, turning it into what the American’s refer to as “loud powder”. It is a very unstable layer when fresh snow falls. I think this is what Ise is talking about.

At 1600m we turned onto S to SW slopes, this had a light crust on the surface. At 1800m we turned more onto west slopes and these were powder again. Finally we skied a steep NW couloir which was about 10-20cm of surface powder on a firm but not frozen base.

On the ski down the crust at 1600m and lower down below 1000m had melted and the conditions were heavy powder. Much better than a crust.

A day not to be out too early although we would have been better off starting before 9am or climbing faster.

Loud or recycled powder are terms I’ve heard. Not a great layer for new snow to fall onto but conditions look nice for a few days now.

In the Haute Savoie the today is the first day of the season where the Avalanche Bulletin has said that a particular aspect is more dangerous. (atleast I think it is the first time, SLF have had aspects most of the winter) And unsurprisingly with the strong sun the south based aspects are more dangerous.  Just driving around between Geneva/Morzine, and Geneva/Chamonix you can see many self released purges to full depth on south based aspects.  Mole has had a big brown patch for many many days, but I wonder if this will stop the Bonneville locals who seem to ‘tick it off’ every new snowfall.

Both Meteo.fr and SLF are now talking about weak layers near the base.  Now its question time for you experts.  Does that form from the cold weather and big temperature gradient in the snow pack?  Or the heating during the day, and then the cool temperatures at night?  Or something else?

I understand what Davidof is saying about surface hoar forming in the very cold surface layers and then becoming and extremely weak layer if snowed upon and also horrible to ski on as doesn’t support you at all!

These are good questions, it’s just made me re-read the bulletin and the language is odd. I’m just on the way out but I looked at the local bulletin and a couple from the Valais. There’s certainly mention of a buried weak layers but it is qualified by saying it’s shallow snow packs. In fact, here’s what one says

Avalanches can in very isolated cases be released in deep layers, especially in areas where the snow cover is rather shallow as well as in areas close to the tree line

I’d say that could leave people scratching their head a bit, how can there be a deep layer in a shallow snowpack? I think they’re using deep to mean lower which explains that.

This is a good example of how shallow snow packs are often more dangerous than deep snow packs. It’s certainly due to a big temperature gradient which is often the nature of shallow packs, given a ground temperature around zero, an air temperature of -10 then the gradient must always be higher where the snow is shallower. Basically the cold air is pulling water vapour up through the snow pack forming faceting in lower layers. This isn’t a fixed situation, if the temperature gradient improves the snow will restructure again.

Worst case is sustained cold weather, day after day, where nothing is halting or reversing the faceting process. Around here, at this height, that’s not occurring of course, we’re getting warm days which slows or halts that. At the obvious expense of some moisture dropping down into the snowpack to lubricate sliding layers. Which, like you say is causing some releases on sunny slopes. By and large that’s fairly safe, mostly people don’t get hurt by slides on south faces like that because they’re fairly predictable.

There’s some cloud here today which is going to have an warming effect, now heat radiating from the snowpack is being reflected back at it which is increasing the overall snowpack temperature and restructuring some snow. In fact, it’s going to be quite a marked effect today, the cloud is thin enough to let solar radiation in but thick enough to bounce it back. How do you know when that’s the case? My rule of thumb is if it’s cloudy and I still need dark sunglasses then we’re in that situation.

Quite good really, I’ve a had a couple of days off, I’m only doing a half day today but from tomorrow it’s everyday for a while and I like the look of the snow conditions because they’re not going to restrict my route choices much. And, fairly normal this time of year, sunny days, overnight temperatures down to -10 is all pretty standard for mid-Feb.

probably forgot something crucial there but I’ve got to go.

Whats the verdict for next week?  I am off to La Grave on Saturday and have been watching the forecasts for the past month or so.. its been pretty similar - apart from temps and this week the sun for a week or so and new snow forecast later on.

Anyone wiling to give an opinion about next week?

The weather service are still warning of residual slabs above 2500 meters on shaded north sector slopes in the area. A weather front is moving in now. There is supposed to be some snow in the Hautes-Alpes tomorrow but it won’t get over the col du Lauteret then another weather front starting Sunday. In between cold which seems to be creating surface hoar on slopes that see the sunshine, we saw a lot on Tuesday but the snowpack is pretty deep on North sector slopes which appears to be slowing down the formation of depth hoar. Plenty of hard snow/ice on ridges plus some big coriches to watch out for.

So keep an eye on that weather front on Sunday/Monday to see how much snow it brings otherwise things are pretty stable. That said soldier killed at nearby Valloire yesterday on a risk 1-2 day.

@ise: was the snowpack partly wet when it slid? Here in Switserland we have the same problem but the full depth slides occur on all aspects (even NE-NW!) and consist of dry snow.

The bulletins have been mentioning slides on north faces often with shallow snow packs and/or steep slopes, the cold weather has left that untouched. Here’s quite a big fracture I noticed yesterday on a north slope, you can see not only the fracture but there’s been a slide onto the set of tracks :

click on the photo for a full size version

If anything that’s actually steeper than than it looks on the photo. It’s a ridge line, the south side is 30’ as well so it wouldn’t be good to fall off either side.

We saw some surface purges today but everything seems pretty solid, for the moment. Lots of surface facets in the usual 1500-1800m range in forest clearings.

As for snow, crust from SW -> SE with dense powder (5-10cm on solid base) to deep powder (20-40cm) elsewhere. We had a lot of snow over the weekend, there was cloud on the summits which deposited a very light powder but nothing much at all in the valleys despite the intense cold.

Sorry no pics, still no Internet access here apart from my telephone.

I did look a little today, at 1850m 240cm deep, about 30cm of fresh snow at the top with a little surface hoar. A hard rain layer at around a meter and some signs of a depth hoar forming near the ground. But fairly stable currently, I got a failure on a isolated column on the rain layer but it took a lot of force. That was on a northwest aspect. Actually, a super day to be out.