A Brief Guide to Mountain Weather


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A Brief Guide to Mountain Weather

On the 11 February the BBC reported that four tourists were rescued from the top of Ben Nevis after they got in danger in a blizzard.

The weather is different on mountains, and here we will give you some explanation why and ultimately why it makes sense to be aware of the local weather forecast before you go up one!

Prepare for the cold at the top

The first thing you need to be aware of is that it will often be a lot colder at the top of a mountain than at the bottom where you left to assault the summit.

Saturated (wet) air drops in temperature 0.5 degrees for every 100 metres you go up.

Dry air gets cooler around 1 degree per hundred metres you go up.

A climb to the top of Ben Nevis from Glen Nevis Visitor Centre is around 1,311 metres.

Ben Nevis Summit

In wetter weather, 1,311 divided by 100 metres divided by 2 = 6.555 degrees C.

That means it is 6.555 degrees C colder at the top on a wet day than it is at the Visitor Centre.

On a typical cold winter’s day of around 5 degrees C at the bottom it could be perfect snow conditions at the top of the mountain.

Dry air on a sunny day could make the air at the top of Britain’s highest mountain a whole 13 degree C lower than it is on the ground.

If it is a balmy 20 degrees C at the Visitor Centre when you get to the top it will be an extremely chilly 7 degrees!

It can also be warmer at the top than in the valley!

The land cools in the evening when there are no clouds above and this cools the air.

In valleys this can be quite pronounced and you can find that it is bitingly cold down there.

However, the sun will have warmed the air above.

This is called a ‘temperature inversion’ and this can lead to fog in the valley as the warm air above cools and falls down the hillside in ‘katabatic winds’ and dumps its moisture.

temperature inversion

It could be wet on the way up

The next thing to consider is that as air gets higher it gets colder and is less able to store water vapour up there.

Air masses will be heading in a general westerly direction and as the air on the ground meets a mountain it will climb up, get colder and dump its moisture on the mountain.

The ‘orographic effect’ as it is called is why you should always have a set of waterproofs with you when you go hill or mountain walking.

Orographic Effect

Particularly on mountains that sit in the way of westerly airflows such as Snowdon and those on the west coast of Scotland will be facing warm wet air from the Atlantic.

The air comes off the sea, ascends the mountain and drops the water as it goes.

Until recently the Cairngorms were almost guaranteed to have snow on them every year.

Ignoring the fact this has changed due to the climate, the Cairngorms are a large plateau of mountains that effectively catch the warm moist air and due to their height takes the moisture as snow in winter.

The west coast of Scotland gets 250mm of rain a year while the east coast can get just 75mm as a result of the mountains!

Snowdonia is also known as the wettest place in the UK for similar reasons. But did you realise that in catching that moisture, it is much drier on the other side?

One extreme example of this is in the Himalayas where the Mustang range is arid (you can even tour it in the Monsoon) while the Dhaulagiri range to the south is snowcapped thanks to the Dhaulagiris capturing all the precipitation from the airmass.

Fierce winds too?

Another impact of mountains on the weather is that the winds can be a lot stronger as you go up.

The air is plodding along over the sea when it finds land and has to go upward.

As it does it accelerates.

Where it comes to valleys and mountain passes that are facing the west, the air is funnelled through them so it can be a lot windier on a high mountain pass than it is on the ground.

Mountain pass

The Cairngorms can have the highest wind speeds in the UK when there is a high pressure system overhead.

This is because the pressure in the atmosphere gives the air below even less places to go (it can’t go upwards) so by the passes and summits of the plateau it absolutely screams along.

For you the mountain walker, that means you should be aware before you go looking east over a cliff in a good blow as a good gust could take you off your feet, and that would be far from pleasant with a decent drop below you.

On the side of the wind direction you will find that as the wind climbs it forms clouds and dumps the rain.

This is why visibility can be reduced and you ‘walk through the clouds’, sometimes coming to the top of the mountain with the clouds beneath you – a view that you may well have grown to appreciate or certainly seen on social media!

On the side that the winds are heading to (the less side), so the air warms up and is dry again.

This is why sometimes you can experience warm dry winds blasting down the mountainside as you head up an east-facing mountainside.

In the Rockies this is known as the ‘chinook’ wind and in the Alps, the Föhn.

Local weather forecasts

Now we have explained just what the mountains can do to the weather it is down to you to work out just what it will be like when you plan an expedition or a mountain walk.

The first thing you need to do is look for a local weather forecast site for the mountains you plan to climb.

Snowdon Weather Forecast

A brief search on the internet shows a Met Office mountain weather forecast for Snowdonia here and another for the Lake District here.

The above forecasts give projected windspeed and direction at 300 metre intervals above sea level as well as temperatures as well as forecast precipitation at 800 metres above sea level.

The next thing to consider is how far ahead you can safely watch the weather.

On the Monday of the week this article was written there was no mention of Storm Jorge that is due to hit the UK this weekend.

It is now Thursday and there are major weather warnings going out.

Essentially the closer the weather forecast is to the day you plan to hit the hills the more accurate it will be.

Weather forecasting is an educated guess with supercomputers modelling several different outcomes and assessing their probabilities.

From 24 hours out they are generally extremely accurate these days.

The probabilities of the forecasters and supercomputers being wrong increase every 12 hours you add to that.

That’s why the weather forecast for next Friday given on the BBC’s Countryfile on the Sunday before can be totally different to what will actually happen.

In short, check the weather forecast the night before you plan to climb the mountain.

Ben Nevis Weather Forecasting Stone

Is it going to be within your limits?

If so, make the decision to go, preparing for the different conditions higher up.

If not?

It’s better to be warming your toes by the B&B fire moaning about the weather than being out there fighting for survival!

So there we have it…

This has been a light and general explanation of mountain weather.

We didn’t go into why the Canadian Rockies effectively make the UK’s weather so miserable or how the Gulf Stream keeps the snow off lower ground in winter – that would be for a much longer piece!

What we have done is explain what you are likely to find higher up to give you an idea of why it makes sense to bring waterproofs and warm clothing up the mountain with you even when it’s quite a nice day at lower levels.

We also hope you have taken it aboard that you should check the weather forecast carefully the evening before you make your ascent as that could make all the difference between a challenging hike and being in such distress the local mountain rescue team have to get out of their warm living rooms to drag you out of harm’s way!

Ultimately being rescued puts those rescuing you in danger too, so a bit of good sense is safer for everyone!

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