You look at the forecast for Fort William: 20°C, sunny and a moderate breeze at a wind speed of 15mph. Brilliant! Ben Nevis, here we come. You set off with a light pack because you certainly won’t need that fleece jacket or gloves today.
Happily, you walk up The Ben…
Halfway up, you realise: It’s colder up here and quite breezy. You continue.
The wind becomes stronger and stronger the more height you gain. Cold gusts rip at your t-shirt and make you shiver. You grit your teeth and walk faster to keep warm. How you wish you had packed your fleece; and your ice blocks, formerly known as hands, would appreciate that pair of gloves now that you left in the car. This is definitely not what the weather forecast had said. Or is it?
Wind – The only force that can hold you back
Obviously, you misjudged the wind conditions. It’s a common mistake beginner hillwalkers make. I’ve also had some very chilly experiences on the hills in my early hillwalking days because I didn’t pay attention to the wind forecast.
In a very hard and cold way, I soon learned that the wind forecast is THE most important thing to look at when you prepare for your walk. You can deal with fog, with torrential rain, with snow and with hail, but there is a point when you can’t deal with wind anymore. It’s the only environmental occurrence that holds you back from reaching your destination – literally.
And even when it’s not that strong, being able to judge from the forecast what the conditions will be like on the hill is an essential mountain skill. In this post, we will explore the effect wind has on hillwalkers, how wind speed and wind chill are related, and how to know when it’s safe to go up high.
Wind speed and temperatures at high altitudes
Temperature decreases with height
Let’s look at the example from the introduction: Fort William lies at sea level – 0.0m altitude. Ben Nevis is 1,345m high. A rule of thumb says that with every 100m gained in height, the temperature drops by about 0.8°C to 1°C.
The temperature difference between Fort William and the top of Ben Nevis ranges thus between 10°C and 13°C. So if it’s 20°C in Fort William, the air temperature on Ben Nevis can easily be below 10°C.
Wind speed increases with height
Just like the temperature gets lower the higher you get, the windspeed increases. There are exceptions when the wind is stronger at the bottom of the hill than at the top, but they are rare. So for the sake of simplicity and when planning your walk, always assume that the windspeed increases with height.
Since the windspeed depends on the shape of the terrain and many other factors, it’s not as easy to derive a rule of thumb as was the case for the temperature. Hence, the following rules are heavily oversimplified. They are sufficient, though, to decide whether to go up a Munro or stay on lower ground:
- 500m – 900m height: 2x wind speed at sea level
- 900m – 1,300m height: 3x wind speed at sea level
Wind chill: The feels-like temperature
Have you ever wondered where the feels-like temperature in the weather forecast comes from and how it’s determined? The secret ingredient for that calculation is the wind speed.
The most important thing you should remember when it comes to wind chill is: The higher the wind speed, the lower the feels-like temperature.
You can easily test this yourself: Take a shower on a windy day and go outside without drying your body. Stand in a wind-sheltered spot and see how that feels. Now walk to a spot that’s exposed to the wind. Chillier, isn’t it? 😉
There is a rather complicated equation to calculate the wind chill factor from the wind speed and a certain air temperature. You don’t have to calculate it yourself, however. I’ve already done that for you and put the results into a lookup table:
Looking at this chart, it becomes obvious now that the wind speed makes a huge difference in how the air temperature truly feels on our skin.
Putting it all together
Now that we know the relationship between altitude, wind speed and wind chill, we can put all of the information together to make a rough estimate of what to expect on the hills. Let’s look at our Fort William example again: 20°C air temperature at sea level, 15mph wind speed at sea level.
We look at the best-case scenario temperature-wise, assuming a temperature drop of 0.8°C per 100m gained in height. The air temperature will be roughly 10°C on the top of Ben Nevis.
To calculate the wind speed on the summit, we triple the wind speed at sea level, which gives us 45mph.
Now we look at our wind chill chart: With an air temperature of 10°C and a wind speed of 45mph, the wind chill temperature is between 4°C and 5°C.
I would undoubtedly pack my fleece jacket and windproof gloves when expecting temperatures and wind speeds like that.
When does wind become dangerous?
We now know the relation between altitude, wind speed and wind chill temperature so that we can pack accordingly for these conditions. Still, another crucial thing to consider is how the wind speed will impact your progress on the hills. In strong winds, this can be a significant factor that will determine how long it will take to get to your destination or if you will get there at all.
Please, keep in mind that wind gusts may be much stronger than the “regular” wind speed, especially when you’re on exposed terrain. If the regular wind speed is already very strong, wind gusts can make your life on the hills much more difficult. So, always check both on the forecast: the regular wind speed and the wind gusts.
The following table might make it easier for you to decide whether you want to go hillwalking or not. Personally, my limit is 40mph. If the conditions get to this limit, I choose walks where I wouldn’t walk for hours on an exposed plateau and plan my walks in such a way that I have the wind in my back for most of the time while on higher ground.
| Wind speed (regular) | Effect and what to do |
|---|---|
| Less than 20mph | – Insignificant – Safe on all kinds of terrain |
| 20-30mph | – You will feel the wind chill – Windproof layers necessary |
| 30-40mph | – Walking against the wind becomes arduous – Wind affects your balance – Trekking poles become hard to handle – Narrow ridges are a no-go, and a safe distance must be kept from edges |
| 40-50mph | – Arduous walking that requires a lot of energy – Balance will be affected frequently – Trekking poles are more a hindrance than a support – Handling a map and compass is very challenging |
| 50-70mph | – Not fun anymore – Gusts can blow you over and lift you off the ground – You may need to crawl or go down on one knee to keep your balance – Find the safest route down and lose height as quickly as you can |
| over 70mph | – Don’t even try! – Serious danger to life as gusts can throw you for several metres |
Summing up: Don’t neglect the wind speed when looking at the forecast
You now know the dependencies of the temperature on the wind speed and the altitude. This will enable you to make better judgements when looking at the weather forecast or when different forecasts seem to contradict each other (yes, it happens).
Most mountain weather forecasts, like MWIS, give you the wind chill temperature, so it’s not necessary to check the numbers yourself in a lookup table. The chart can help you, however, to develop a better feeling and understanding of the conditions.
When it comes to wind speed, it’s important to know that it will affect not only the feels-like temperature but also your movement. Walking in wind higher than 40mph requires much more energy than at lower speeds – especially when you walk against the wind.
Over time you will get to know your limits and find out when the wind becomes too much for you to enjoy the walk. On some days, the high peaks are better admired from lower ground. 🙂
See you on the hills (or in the glens on a very windy day 😉 )
Natalie