UKC

The  met office forecast for Ben Nevis on Friday evening is quite spectacular; minus 6, heavy snow, mean wind speed 120mph, gusts 130.

Doesn't sound very survivable to me. If that mean wind speed isn't an error then the gust speed would surely be even higher. Hang on to your anemometers!

In reply to pasbury:

Yes, it’s showing sustained wind speeds of over 110mph for 4 hours tomorrow evening- that is category 3 hurricane territory 😳

In reply to pasbury:

There are probably other hills which will be worse, it's the angle and orientation of the slope into the wind which will ramp it up. Peak wind is from SW, so you want a hill straight off the sea, upto 3/4000' with a good ramp like SW aspect. Northern lakes has a similar forecast to Glasgow, going to be fun out there. I'll bet there'll still be trucks going north up the 74. 

Edit. I'm thinking places like Goat Fell, or hills near Oban. 

In reply to ExiledScot:

The upper slopes of Skiddaw don't half ramp up the wind speed, due to the convex slopes. I've been reduced to crawling, before retreating from just below the summit ridge. Lower down it was blowy, but very walkable.

In reply to The Groak:

I can testify to that place!  Many years ago (1974) I crawled the length of that summit ridge having been lifted up bodily into the air and dropped back on the ground such was the wind.

In reply to pasbury:

I'm surprised at the forecasted temperature, I'd expect it to be lower. Although I'm sure a 120mph wind chill factor will be significant! 

In reply to pasbury:

It is also giving 101% humidity. How does that work?

In reply to wercat:

I'm glad someone else had that experience!

In reply to aln:

MWIS Lakes for Friday is forecasting a wind chill of minus 19 degrees C at 750m, approximately 2,500 feet.

In reply to The Groak:

Funnily enough I was on Ben Chonzie again this past Tuesday (lovely day) with a pal and we were discussing the similarities between that hill and Skiddaw. Both have a lot of smooth slopes that seem to ramp up the wind speed on top, and they're both known to be tricky hills in those terms as there's not much shelter on them, Chonzie in particular. They're both 931m too, oddly. I've crawled on both of them including a day on Chonzie when there was a 120 mph gust in Glen Ogle and I was pinned down for a while at 900m with crawling not even an option. Happy not to be trying that again this Friday.

It's calm here in Stirling just now, btw - I've just done ten minutes along the road at the edge of town and it felt like a pleasant late evening.

In reply to Dave Hewitt:

That's a nice similarity Dave. It's maybe something that the mountain weather forecast sites (MWIS) could take into account. I know LDNPA Weatherline sometimes do.

In reply to ExiledScot:

In the mid 80's when my friend Robin and I were starting out in hillwalking we had a trip to Arran in Spring. A few sunny days were great as we arsed about doing this and that. We headed up Goat Fell. Halfway it started getting cloudy, by the time we got on top it was blowing a hoolie and there was frost and ice forming. Time for a sharp exit. The wind became so strong we were being blown off our feet and falling over. Gusts were literally lifting us off our feet and throwing us 6ft or so. At one point I heard a gust coming, it sounded like a roar, I sat down to avoid being lifted. The wind blew me onto a big flat slab covered in ice and I became a human curling stone sliding uncontrollably towards a nasty looking drop. Stopped in time. I think the only reason we survived was getting angry, it felt like the wind was bullying us, pushing us around, and we took offence to that!🤣

When we got down we found out the wind had been 90mph at sea level. God knows what it was at the top, and what the gusts went up to.

In reply to Dave Hewitt:

There might be something in that. The strongest wind I've ever encountered was on the red cuillin tops!

In reply to The Groak:

I've been reduced to crawling there as well.

How does the physics work? What exactly is happening with respect to the shape of the hill?

In reply to abcdefg:

If I remember right, in Martin Moran's book there's an explanation for the "Cairngorm boomer" when a fairly static air mass in the upper atmosphere 'presses' the lower atmosphere meaning any wind is confined to the lower couple of kilometres, half of which is occupied by the hill, resulting in stupid wind speeds.

In reply to Andy Hardy:

Sounds a bit like what happens with the Helm wind.

In reply to pasbury:

I once had problems getting through one of the Red Cuillin cols - knocked over etc. I think the problem with smooth dome-like hills of the Red Cuillin/Skiddaw//Chonzie sort is that there's very little on the flanks to break up the airflow - plus there's the quite common thing of it being at its strongest just where a slope levels off. If I'm on the central Ochils on a day with a strong SWerly blowing I'm very used to the idea that the couple of minutes getting off the top of the west side of the Ben Cleuch plateau down towards the Ben Ever col is likely to be the roughest part of the day.

You also sometimes get what feels like the opposite on more conical hills - eg I've more than once gone up Ben More from the Crianlarich side and had it really windy at that band of crag around the 1000m level, then calm enough on top for lunch to be eaten. Similar too on the standard northern approach to the eastern Vorlich - it's like the wind whips round the upper slopes as a sort of short cut rather than going over the top.

Has been steadily blowy here in Stirling since maybe 6am-ish. Nothing too drastic as yet - there was a gale last winter here that was noisier than this and a bit scary. The windspeed numbers forecast for Glasgow are five or six mph higher than for here, so it could be that we are just far north enough to miss the worst, assuming it stays on the forecast path. I've not stepped outside yet, mind you!

I remember once failing to get to the top of Marsco due to the wind - I was possibly over-cautious but I was on my own and the last bit is quite narrow. Later I learned that someone I knew had done a Cuillin traverse that day! 

I also had a very strange experience that day - as I was getting higher up the hill, I came to a place where there was some weird eddy effect so that I could sit and eat my lunch in a dead calm while listening to the wind whistling past.

I still remember strugling to get back across the col at the head of Coire Odhar (Ben Lawers) after skiing a long way northwards towards Glen Lyon. Skinning back up to the col en route to the car at the NTS visitors centre we bacame aware that the wind was getting stronger and after a while we added ski crampons to stop us being blown backwards. Closer to the col & we were on foot (wearing crampons) & wondering if we'd have to drop down to Glen Lyon & try & hitch round to get to the car. The final 100m we were crawling & using our ice axes to help make progress. Thankfully we did get to the col & a welcome rest behind some rocks before we poled our way downhill into the wind back to the car

Must have been 30 years ago but can still remember the couple of hours very clearly.

In reply to Robert Durran:

Bust model working well outside the expected input parameters, I'd suggest

In reply to Robert Durran:

The app is calculating relative humidity from other fields which are not represented to sufficient precision. 

In reply to pasbury:

It's approaching the Highlands. Central Belt is showing nr 90mph steady winds with 124mph gusts at 450m

https://holfuy.com/en/weather/1362

In reply to aln:

As a bit of hyperbole that fair lifted the spirits on a miserable day 

In reply to abcdefg:

Well... the hill's in the way. The air that wanted to go where the hill is can't divert sideways because there's already air there. It can't divert upwards because there's already air there. The air following along behind it wants to keep coming at the speed it's coming at so it can't dawdle - it has to hurry. 

That's simplistic but it's the basic point. Why can't the air go up or sideways ? (you might ask) as in daily life you don't find a problem with air getting in your way. Air's heavy - a cubic metre weighs over a kilogramme. OK, higher up, it weighs less, but once you start picturing a 100 x 100 x 100m cube of air weighing 1000 tonnes, or a column of atmosphere reaching ~100km up, it helps your intuition with why it's slow to get out of the way. 

In practice, of course some of the air diverts sideways and some upwards, but point is, it's constrained and has to accelerate. It can then decelerate afterwards - tucked down a steep lee slope just below a cliff or cornice you may find it very slow indeed as the big stuff races over the top and only a few little eddies peel off into the space that you're huddling in. (Saying among aerodynamicists - for subsonic flow, it's the rearward-facing surfaces that make the difference...) 

Surface friction makes a difference to the first few metres, there's an equation (easy found on Wikipedia) which shows how quickly the windspeed rises (from zero at ladybird height to the full "free-stream velocity") and it's quite a steep curve. Worth tucking down behind cover if any. 

Y

In reply to DaveHK:

Yes, that's right. If there is a inversion a short way above the summits, the air being forced up over the hill cannot rise due to its negative buoyancy with respect to the overlying layer, and so gets squeezed and accelerated over the hill top. You can also get a hydraulic jump downstream of the hill as the flow readjusts to the extra vertical space available. This results in an area of strongly rising air downstream of the hill, which can form a cloud if the humidity is right (e.g. the Helm Bar). 

I have experienced this on Skiddaw once in an easterly - the summit winds were at least double what was forecast and the hydraulic jump cloud was present to the west of the hill. 

In reply to Yanchik:

Thanks. I get that, I think - but I was really musing on the 'convex slope' comment upthread.

Your arguments apply irrespective of the hill shape, I think.

In reply to pasbury:

I failed to get up Catbells when a huge squall came off Skiddaw towards us. We were pinned down for what felt like ages before we could start crawling down. The two E5 climbers with me swore us all to silence as they didn't want anyone to know they had failed on Catbells.

In reply to timparkin:

Conditions report from the NE edge of Stirling - only about 10m above sea level but a long way inland. Have just done 20 minutes along the road - quite an exposed rural stretch. Wasn't very pleasant (partly because it was raining), but was manageable. Was getting shoved along on the outward/northbound stretch, then it was a bit of a push coming back. The red zone time period started while I was out - if it only gets a little worse here than at present (the forecast reckons another 5mph by lunchtime, then levelling off) it'll be OK, I think. I've been out in worse, several times, along the same stretch of road.

We're at the very northern edge of the red zone here insofar as one can tell from squinting at the rather vague map. The BBC hourly windspeed thing has Glasgow at 80mph around 11am - our highest is given as 74mph at 1pm, quite a difference. We have friend a who lives in Dunure on the South Ayrshire coast: the BBC is showing 90mph there at 11am - I wouldn't fancy that.

There was talk of snow down to 300m but from what little I can see of the Ochils through the rain there's nothing below 500m.

That's me for the next few hours, anyway - will happily stay indoors until it starts to calm down mid-afternoon. The cat's gone back to his bed while I've been writing this, very sensible.

In reply to pasbury:

87.6mph recorded at Tower 17 on Aonach Mor at apx 650m. 

Otherwise it doesn't feel like much is happening in Fort William. 

In reply to Dave Hewitt: Pressure reading from my house at 0900 this morning, the needle on my old barometer fell off the scale. My house sits around the 170m contour on the south side of Falkirk.  Hopefully I'll be heading to Kinlochleven for the weekend after the winds peak mid afternoon.... finger's X'd, it's ma mates turn to drive. 

In reply to Joak:

Impressive. Don't envy you being on the Heights of Falkirk. Steadily wild and wet here now - think we're about halfway through the worst four-hour slot, should be starting to ease by half-two-ish.

Good luck! There's talk of Sunday being quite nice...

In reply to The Groak:

Yes!  I remember going up there one February.  No issues with wind at all anywhere else in the valley or going up the slopes but that rounded ridge between the two high points was almost impassable.

I wonder if the fact that Skiddaw is fairly isolated had an added effect - nothing else to interfere with the wind before it hits the mountain.

In reply to Dave Hewitt:

Not much snow up here... Just the remains of the last days dump at 850m+ (I can see Beinn Beithir/Sgorr Bhan from the house). Wind has been strong here in Ballachulish but nothing major, probably 40mph gust.

In reply to pasbury:

In Glasgow just now and went out for a short period, otherwise indoors and looking at the trees bending! Power still on though brief flashes in the lights. Family at Loch Lomond have had a power cut.

Doesn't feel as though there has been much rain volume here, I think we just escaped it but there's been some mental gusts and a few of them have been truly violent, even from inside. Impressive!

In reply to kwoods:

Still pretty hectic here in Stirling, has been wet for several hours now - not a complete deluge, looks worse than it is because of the gale but not much fun being out in it. A couple of fence panels of the elderly (89 but very spry) neighbour had broken off, so I've just been out with another neighbour laying them flat and dragging them to a safer place. She also has a bizarre number of old wheelie bins so we tucked them into a sort of alcove bit before they disappeared off down the road. Other than that nothing seems to have broken hereabouts that I've heard of or seen, but it'll be a case of seeing what's what once it starts to calm down in an hour or two. I'm glad my weekly online chess tournament was yesterday evening rather than today - would have been worried about the connection going kaput at some critical stage!

Interesting to hear the not-much-snow reports. I was never convinced by the forecasts of a snow dump ahead of the main storm - it never seemed chilly enough here yesterday although I'd imagine there's been some above 600m and it'll have been scouring and drifting like mad.

In reply to Root1:

A friend and I went up Catbells just before xmas a few years ago, not long after the big Keswick flood. We had to get down on our hands and knees a couple of times and had to crawl to the summit. My friend's glasses blew off, a couple on their way up found them about 20m or so further down the path. 

I've done the Forcan Ridge in winds that made us suddenly crouch now and again, not to be repeated. I have been lifted off my feet in Coire Mhic Fhearchair, over 35 years ago. It was my first visit to it and it made quite an impression.

In reply to Dave Hewitt:

The CIC hut shows no more snow on the DB than yesterday... 

In reply to Robert Durran:

There are more complete and technical explanations out there if you wish to Google for them, this is the simple version. Call it a dummies version, because it was written by a dummy 😁

100% is based on the maximum amount of water vapour the water is "happy" to hold. It's a balancing point, it doesn't mean 100% water like a bucket of water.

If the humidity is >100% it has more water than it's "happy" to hold, so it will start precipitating water droplets out, it'll start forming mist or thickening any existing mist (or creating snow, hail, dew, rain etc). But if it's <100% it is either not misty or the mist is getting thinner.

If the humidity is exactly 100%  then the mist (or lack of) will stay the same.

So if the summit is -6 and >100% humidity then rime ice is going to be deposited.

Sounds lovely for a breezy walk, it'll be very bracing.  

In reply to CantClimbTom:

Thanks! That makes sense. So not actually uncommon then?

In reply to Robert Durran:

No, certainly not uncommon walking in the hills and cloud comes down and you feel everything getting damp. BTW.. stupid typing error above, hopefully obvious? but I'd meant to say

100% is based on the maximum amount of water vapour the air is "happy" to hold.

In reply to Pids:

There was no hyperbole. It was a terrifying experience and we were lucky to survive it. But obviously not an experience you've had, therefore you don't believe it and feel the need to cast doubt. Fill yer boots. 

In reply to timparkin:

There has been fresh snow in the end - looks to have been on the trailing edge of the storm. Is white down to around 300m on the south side of the Ochils this morning, and the Ben More webcam shows similar at Crianlarich. Winds back to normal, I think - haven't been outside as yet as it's started raining!

In reply to Dave Hewitt:

Same here in Ballachulish - probably 200m here.. 

In reply to timparkin:

Brightened up here now - am just about to go out for a look although it'll be tomorrow before I can get on the hill. Was sleeting a few minutes ago - is quite chilly. The sun's out now (sort of).

In reply to Dave Hewitt:

Me and two pals had a very enjoyable winters hill day in the Mamores today.... yesterday's drive up to Kinlochleven during storm Eowyn was far from enjoyable. 

In reply to Joak:

Sheildhill?

To the side of our house there is a small garden, mostly grass, that faces west and is bounded by a solid wooden fence. After the storm the grass has been flattened pointing away from the house, which is interesting in terms of what the wind was doing.

In reply to fimm:

It's probably because the wind drops in speed toward the ground and the wall is blocking it as well. However, when the wind hit's the side of the house, it splits, some goes up and some goes down. The wind that goes down will create a circulating pattern that will head award from the house and then up the wall, causing the grass to flatten

In reply to fimm:

A very simplified 2D fluid dynamics sim, if you want to draw your house and the wall

https://physics.weber.edu/schroeder/fluids/

As fimm noted, you likely got counter‑rotating vortices whose lower half rotated away from the 2nd obstacle (house), thus flattening the grass such way.