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Water freezes into very different forms of ice. Here we look at three that affect winter climbers very much. Rime is what normally makes mixed climbs white enough to go and climb. Hoar Frost grows on horizontal surfaces in a cold, clear night and can create a very weak layer in the snowpack. Verglas is an invisible sheen of ice covering rocks, making them incredibly difficult to move over.  Rime is a type of ice crystal that grows on any surface exposed to humid air being blown onto it in a sub-zero temperature. It is often seen on fence posts and, perhaps confusingly, grows into the wind. So, you need a wind blowing cloud onto the crag and the temperature to be below zero. No snowfall is required at all. After a westerly gale, choose a crag that faces west and has been in the cloud. The best conditions in which I’ve climbed snowed-up rock have included well-frozen rocks and a light rime of a couple of centimetres that is easily brushed away to reveal (hopefully) cracks and ledges underneath. The crags have been totally white at the start of the day, with the climbs brushed free of rime by climbers on various routes. Sufficient rime to make crags and climbs white enough to climb can form overnight. Twelve hours of strong winds blowing freezing clouds onto the crag is enough to form a couple of centimetres of rime over every surface of the crag.
Delicate dry rime can fall off the crag in a strong wind and is likely to fall off in very dry, cold air. This means that the crag can be white one day and black the next despite the temperature staying below zero. Once the crag is out of the cloud the rime will start to deteriorate. Sunshine will also strip rime from the rocks faster than you can climb them. Over time, rime can grow to be a metre deep, and it turns very icy after thaw-freeze cycles. In March, the summit observatory ruins on Ben Nevis are often covered by incredibly thick rime ice that has built up over the previous three or four months and survived many thaw-freeze cycles. This type of rime makes life difficult if you want to climb the rock underneath it. Thick, icy rime it can be a monumental struggle to clear off the rocks for a whole pitch.  Hoar frost is very different to rime, apart from being a type of ice crystal. Hoar frost forms during a cold, still, starlit night and is what glistens on the grass and in the trees after a hard frost. Crystals of ice grow upwards as moisture in the air falls onto horizontal surfaces. Since hoar frost only forms on horizontal surfaces, it doesn’t turn steep crags white. So, by itself, it is not sufficient to create the winter conditions required. Hoar frost grows on the surface of the snow as well as anywhere else. When snow builds up on top of a layer of hoar frost, this buried hoar frost will stay intact and remain as a weak layer within the snowpack. The hoar frost crystals are strong enough to hold a substantial layer of snow on top, but they are prone to collapsing in a layer across a wide area. They act a bit like playing cards standing upright. In this position they can hold a lot of weight but if they are pushed in the right direction they will all fall over at the same time. Hoar frost crystals can grow very fast and a very thin layer of them is required to create a weak layer if they are later buried. So, a night of snow showers and clear spells can create a snowpack with weak layers. Remember that it does not need to be snowing for accumulations of snow to build up. The wind can transport snow that is already lying on the ground to other places where there is shelter from the wind. Clear, cold, windy nights can a create areas of windslab witting on a weak layer of hoar frost crystals.  Verglas covering the rocks on the Inaccessible Pinnacle Verglas is a thin covering of ice on all the rocks. When you look at the ground and you see rocks that look like they might be wet, but actually turn out to be icy, you've got verglas. Not surprisingly, verglas is really tough to deal with. Walking on a path can be tortuous, scrambling over verglas covered rocks is incredibly tenuous and climbing anything can be made impossible. Not only are the handholds and footholds very slippery, but cracks can be made useless for protection and slings slide off spikes very easily. Crampons can help a little but it feels like walking on bare rocks with crampons on which is never much fun. If the ground is very cold after a week of freezing weather, verglas can form instantly when the weather warms up a little. A moist cloud blowing over super cold rocks will freeze water droplets onto the rocks, creating a cover of very thin ice. If the temperature continues to rise the verglas will not survive very long, but there is a period when the rocks stay icy because they are much slower to warm up than the air. Alternatively, if the rocks are all wet when the temperature drops suddenly below freezing, the water on the rocks will freeze to create the same thin layer of ice everywhere. This verglas can last a lot longer if the temperature stays below freezing. I found conditions like this once when I walked in to climb Curved Ridge on Buachaille Etive Mor. We decided that wearing crampons for the path was the best way forward even though it looked like bare, wet rocks. On the way down we took off the crampons for the loose snow in Coire na Tullach before putting them back on for the path back to the van at the end of the day. 
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 When planning a walk it is very easy to fall into the trap of selecting a particular mountain days or weeks in advance and then just going with it, even though the conditions on the day are terrible. In this blog post we will take you through the steps to follow when planning your next walk. What is the weather forecast? You probably already have a rough area in mind, or a limit on how far you are willing to drive. So what is the weather going to be doing in that area? Firstly, you need to find the correct weather forecast - the weather on the mountain summits is very different to down in the towns. There are a lot of options so have a look at as many as you can. The forecasts will vary slightly from one another but by looking at all of them you will get a good idea of what is likely to happen. Here are a few that we use regularly:
 The weather can vary greatly in different parts of the country so, if you are fortunate enough to live in the middle of Scotland, then you could go east or west to find better weather. The prevailing weather comes from the south west so when it is raining and snowing on the west coast you may well find sunshine in the Cairngorms (and vice versa if the weather is coming from the east). If you don't have that option and the weather is not ideal then you can pick a mountain that offers you some shelter for much of the day meaning you only have to be out in the wind and snow while you quickly tag the summit. Or do you need to go to a summit at all? It is possible to have a nice day out in the mountains without going to a summit. If you're desperate for a summit remember there are a lot of lovely smaller hills and mountains around. It's not all about bagging the Munros! You should also think about the terrain that you will be on during your walk. If high winds are forecast or a particularly gusty wind then an exposed ridge is not going to be a good place to be. Sticking to broader, open slopes is a safer option for windy days. It is also worth thinking about which direction you do your walk. Will you be walking into the wind or with the wind when you are heading back to the car with tired legs? Is the weather going to improve or deteriorate through the day? If you are going onto a narrow ridge will it be better earlier or later in the day? Should you get an early start or would you be better off waiting for the weather to clear up a little before setting off? There are lot of things to consider with the weather but with a bit of practice you can select routes that are appropriate for the weather, and still have a pleasant walk on some fairly wild days.  What is the avalanche forecast? Here in Scotland we are incredibly fortunate to have daily avalanche forecasts for six different areas of the country. These are provided by the Scottish Avalanche Information Service (SAIS) and the areas covered are Glencoe, Lochaber, Creag Meagaidh, Northern Cairngorms, Southern Cairngorms and Torridon. If you know you are going out at the weekend start reading the avalanche forecasts during the week so by the time it gets to the day of your walk you have a good idea about what is going on within the snow pack. So what are the key bits of information you need when you are planning your walk? There's a lot more about How to Read an Avalanche Forecast in our blog here. Firstly, READ THE TEXT! This will give you a lot more information than just looking at the diagram. So what is the avalanche hazard on different slope aspects (the direction the slope faces)? What is the altitude of the avalanche hazard and where does it change? What are the hazards? How will the hazard level change through the day?  Once you have all the information you will know which slopes you don't want to be on or under (even if the ground you are on is not going to avalanche it might be at risk of avalanche from above) and you can think about your safe options. With the prevailing wind coming from the south-west, quite often (but not always!) south, south-west and west facing slopes will be wind scoured and have a lower avalanche hazard. In a south-westerly wind the snow will be picked up and transported to north, north-east and east facing slopes, increasing the hazard level here. But this doesn't rule out every single slope that faces north, north-east and east. Look at the altitude of the avalanche hazard and perhaps you can stay below it. And remember that avalanches are most likely to occur on a slope angle between 30 and 45 degrees. Before you send your brain into meltdown trying to count contour lines and work out the gradient, check out FATMAP. This is a fantastic and free tool that, with the use of the gradient layer, allows you to quickly see how steep the terrain is and which slopes you should be avoiding. By staying on slopes that are less than 30 degrees you will greatly reduce your chance of being caught in an avalanche. Look closely at the terrain you will be on or under. Coires, or bowls, tend to collect a lot of snow so not only will it be really hard work to plough through a coire filled with fresh snow, but the avalanche hazard is likely to be higher. On the other hand, ridges stick up into the wind and get a lot of the snow blown off them, usually making them a safer option. As with the weather there is a lot to know about snow and avalanches. If in doubt stick to wind scoured slopes, keep the slope angle down and remember that "ridges are bridges". If you want to learn more you can delve into the vast amount of information on the SAIS website or book yourself onto an Avalanche Awareness or Winter Skills course. You'll be amazed at what you'll learn about snow!  What about you? You have read the weather and avalanche forecasts, but what about you and anyone who is joining you on your walk? It would be silly to plan a long day with lots of steep, technical terrain if you are just getting over a cold and your friend has never used crampons and an ice axe before. Think about the group's ability and experience level and make sure you plan something that is suitable for every member. Are you confident and comfortable using crampons and an ice axe on steep ground? Can you navigate in a whiteout using a map and compass? Do you have enough warm and waterproof clothing for the conditions? Are you feeling physically and mentally up to the route that you have planned? What do you want to get out of the day? It is really important to be flexible in winter and maybe have plans B, C, D etc to fall back on. The weather might not be as forecast, you may have woken up feeling unwell or perhaps the snow has been transported to unexpected places. Remember that there is no shame in turning around and the summits will always be there on another day.  |
