This post is a little more rough and ready than I’d like, and I haven’t been able to put the sort of time into refining the explanations I would like. Nonetheless, the ideas are pretty straightforward and, I hope, of some use. If there is a message it is a simple one: Learn to use the weather forecasts in relation to windspeed, and learn to understand the impact of windspeed in context.
Wind, or to use a more relative term, windspeed, is one of the significant weather features in the mountains. The impact of strong wind on high ground can range from uncomfortable to deadly. Judging the impact of windspeed relative to the nature of the ground is important for your own safety and that of others. For example, a windspeed of 25mph on low ground may have little effect but it may make it too dangerous to cross a rocky ridge. And a windspeed of 25mph on low ground could translate to a windspeed of double or triple that, maybe more, on the high tops.
A windspeed of 40mph on high ground is often used as a benchmark for safety and is sometimes considered too strong to take a group on high ground, especially in bad weather conditions. This needs to set in context of nature of the ground and the experience of the group. If the high ground does not have steep and rocky ground, and/or if the group is sticking to well-marked paths, and it is clear weather, they may well manage with winds of that speed, possibly higher. However, the risk increases with steep and rocky the ground and if the group lacks relevant experience and capability.
Strong winds also gust, more so in the mountains, and gusts can be many times stronger than the wind. Good quality weather forecasts usually give an indication of the speed of gusts and these need to be taken into account.
One of the difficulties, of course, is that windspeed, wind direction, and turbulence can be heavily affected by local features. Wind can change direction in a mater of a few metres as it is affected by different local features, funnel up valleys, become turbulent over ridges and around rock formations, and so on.
For example, we’ve been out in winds of around 60mph but along a small section of a ridge (with a well-marked wide path) the gusts were reaching 80mph. We climbed down off the ridge and after we’d lost around 10m of height there was virtually no wind.
It’s important to be able to judge the impact of windspeed on both yourself and your group. This is not the same as being able to accurately measure windspeed, not least because it will change markedly with local features. And it is the impact of windspeed on you and your group that matters, not the actual windspeed. For example, you may be able to manage the effects of a 50mph wind on high ground, members of the group may struggle with windspeeds far less than that.
The Met Office mountain weather forecasts provide summit specific forecasts that include windspeed and gust speeds on specific summits. While these may not be available for the summits you are visiting you can usually get a good idea of overall windspeed from the nearby summits.
Mountain Weather forecasts may not be available for where you are going, and you need to be able to assess localised windspeed and its impact. Having a rule of thumb to assess windspeed can help your decision making.
A good rule of thumb is the Pint Rule. Most people find this useful as a general indicator not only of windspeed but of the impact of windspeed.
Simply put, the Pint Rule says that for the average person walking down Oxford High Street every 10mph of wind has the effect of one pint of beer. One Pint = 10mph = little effect. Two Pints = some effect, mild difficulty keeping to a straight line. Three Pints = 30mph = some effect = slight loss of footing. Four Pints = some loss of balance, unsure footing. Five Pints = loss of balance, disorientating, can’t walk in a straight line. Six Pints = 60mph = stumbling, fall over easily, hard to walk in a straight line, loss of orientation, hard work making progress. At Ten Pints = 100mph = flat on the ground and unable to move (and there are Mountain Rescue reports of people being blown completely off their feet and through the air by wind reaching this sort of speed).
The Pint Rule can be applied not only to estimate windspeed but also to estimate the impact of windspeed on the group and individuals. It makes it easy to understand why a 40mph wind can be dangerous. Indeed, it makes it easy to understand why a 20mph wind could be dangerous on some ground.
The Pint Rule also works in reverse. If, for example, the forecast is for 30mph winds but a member of the group is losing their balance and unsure of their footing in the wind then they are likely experiencing the wind as though it were, say, ‘40mph’. The point being that it is not necessarily the actual windspeed that matters but being able to judge the impact of the windspeed on yourself and group members.
This can all be summarised into some general rules:
- Check a good weather forecast, preferably a mountain weather forecast if one is available, and note not only the windspeed but also the speed of gusts.
- Check summit forecasts, if available, and look at the windspeed on several different summits all close to each other.
- If no mountain weather forecast is available then be aware that windspeed tends to increase markedly with height and you need to take account of this. Windspeeds can be two or three times greater at height (c. 600m+) than they are at ground level.
- As a general rule somewhere around 40mph is a threshold. Winds of 40mph or more pose a substantial risk on high ground. Indeed, 30mph may be difficult for some people. And on steep and rocky ground the threshold may be lower (e.g. a windspeed of 25mph may be too risky to cross a rocky ridge).
- Put windspeed into context. The impact of windspeed varies with height, the type of ground, the experience and capability of yourself and the group, and the weather. Rain, mist, fog, and so on add to the risk posed by high winds, as does steep and rocky ground.
- Put windspeed into personal context. While one person may cope with winds of 40mph another person may find them challenging.
- Remember that windspeed is heavily affected by local features and can vary hugely over a small distance – a few hundred metres, or less – depending on the features.
- Monitor the effects of windspeed on yourself and others as walking on high ground in strong wind increases both the mental and physical effort required.
- Make use of the Pint Rule, or any other method that works for you, to monitor windspeed and the impact of windspeed. It helps understand the impact of windspeed on yourself and others.
When it comes down to it you are going to have judge the impact of windspeed on safety and that means putting windspeed into the appropriate context of height, type of ground, weather, and the experience and capability of yourself and your group. The core skill to develop is the ability to assess this impact from both weather forecasts and how it is affecting you and others when you are out on the mountain.