The Angry Corrie 58: Jul-Sep 2003


Vortex shedding, firnification and fairy motorbikes

Dougie Lockhart, on page 2 of TAC57, described a curious incident on the Bealach an Lapain in Kintail when he was buzzed by an object which "shot past us emitting a steady whooshing or swishing sound like a large Guy Fawkes rocket". He went on to mention that various other incidents are detailed on his website at http://www.graupius.pwp.blueyonder.co.uk/, and asked for further reports and theories from TAC readers...

THE WHOOSHES that Dougie Lockhart describes are caused by a well-known (to engineers) phenomenon called vortex shedding. It's the same thing that happens when you "blow" a blade of grass to make a whistling sound. The vortices are shed from the blade of grass and make it vibrate, producing the sound. Air flow, even at modest speeds, causes free vortices (pockets of spinning air) to form downstream of some objects in the airstream. In certain conditions these vortices break free of the object and are carried in the direction of the airstream. On the hill there are a lot of objects that are quite capable of causing vortex shedding - hillwalkers included. The (spin) speed of a shed vortex can be quite high and can make quite a loud noise. I have been buzzed by shed vortices more than once, not always on the hill.

The reason why a lot of tall chimneys have helical spoilers at the top is to prevent vortex shedding that would make the chimney vibrate and eventually collapse. The collapse of the Tacoma Narrows bridge in the 1950s was caused by vortex shedding. A steady wind created vortices that were shed off the downwind side of the bridge and this created a series of regular pulses pushing the bridge downwind. The frequency of the pulses hit the natural frequency of the bridge and it began to oscillate by as much as 10 metres and, inevitably, it collapsed. The collapse was filmed and has been shown to generations of engineering students worldwide as an example of what not to do! Fortunately no one was killed, but a car left on the bridge was lost.

Anything, from boulders to mobile phone masts, can cause vortex shedding in the right conditions.

Ron Anderson

I WAS OVER in the Fforest Fawr for a couple of days in April and experienced something which hopefully helps throw further light on some of the "mysterious noises" reported by Dougie Lockhart.

Although not especially warm, the day was cloud-free and there was just a slight easterly breeze. I was descending westwards off Cefn Perfedd when I became aware of a strange sound, a little way upslope of me, that initially seemed to be a gurgling stream - impossible as there were no streams nearby, and in any case the ground was extremely dry. As it grew louder I thought it more closely resembled the extreme rustling of dry vegetation. Yet it was virtually calm. Then for about five seconds the wind increased and the sound was all around me, before both died away.

My impression was that I'd experienced a whirlwind travelling down the slope - although there was no swirling vegetation in the air to give it form - and that the sound was caused by the way it agitated the dry grass. As it was otherwise calm and quiet, this sound seemed louder and more distinct than it would on a normal windy day when such noise, being continuous, would hardly be noticed. Significantly, had I been a few yards further away and not actually felt the wind increase, I would have been aware only of a sound moving down the slope, and seemingly coming from the ground.

In the reports of similar experiences (Meall Horn, Beinn Dronaig, Saileag), I notice that it was also fairly calm and sunny. Surely no coincidence.

I'm quite certain therefore that at least some of these events are caused by whirlwinds - of varying size and intensity - which do indeed form in such weather conditions. While similar "dust devils" are commonly seen on lower ground, in the hills there may not always be sufficient loose vegetation or soil to be lifted into the air to give them form - hence they are heard but not seen.

One other thought - the experience also reminded me of folk tales of the Sluaigh - in earlier times it would be quite easy to attribute the sound to a large number of fairy folk moving invisibly down the hillside...

Andy Mayhew

I HEARD A Strange Sound just a few months ago. It was on the north top of Na Gruagaichean in the Mamores, on 17 February 2003, at about 1pm. The weather was clear, with high pressure, and there was a stiff breeze of perhaps 20-30mph. We were above the snowline: the snow was firm névé, requiring crampons.

The sound resembled two sharp flaps of a large sheet of polythene. It appeared to come from just down the western flank. Two of us heard it, and were surprised, and couldn't work out what it was. I tried to peer over the edge, but it was slightly corniced and that slope is very steep; if anything had been there I would not have been able to see it.

My hypotheses at the time, starting with the least plausible, were:

Later that afternoon a rescue helicopter was searching all the western Mamores. In view of Hypothesis 2, I reported the Strange Sound to Glencoe MRT when I met them on the descent. They were seeking a missing walker whose route-plan had been an all-encompassing "Mamores or somewhere". The Strange Sound seemed a conceivable clue, and at about 4:30pm their helicopter investigated the slope.

There seem to be three different sorts of Strange Sound being described. There is the editor's Very Small Whirlwind; there is the Flapping Plastic of Charlie Stephen and myself; and there is the original fast-moving swish. I think I can explain the fast-moving swish as a process called firnification.

Firn is the German word for the French névé, meaning firm snow suitable for crampons. It is part of the process of recrystallisation by which freshly fallen snow is transformed into, eventually, glacier ice. On hot afternoons with the snowpack at around freezing/melting point, the recrystallisation can take place quite suddenly, stimulated by a passing footfall. There is nothing to see, but the sound is a mystifying swish. It's like an invisible horseman galloping rapidly away in all directions on skis. (Illustrate that if you can, Chris Tyler!)

Firnification occurs as a literary device in Cat's Cradle by Kurt Vonnegut, the only work of fiction to feature as villain a rogue crystalline state of water, known as Ice Nine. The ocean firnifies into Ice Nine, ending life on Earth, with the sound "Ah-whoom".

Re the pint-size tornado, this is called the "willy-willy" in The Songlines by Bruce Chatwin, p107 in the Picador edition: "The willy-willy roared and crackled as it approached; sucked up leaves, branches, plastic, paper and scraps of metal sheet, spiralling them into the sky and then sweeping across the camp-ground and on towards the road. A moment or two of panic - and everything was back to normal." This is somewhere in the Australian desert, general area of Ayers Rock.

I suggest that when investigating Strange Sounds, you enquire about snow conditions (if any). And rather than an astronomer, consult a few meteorologists and perhaps the snowpack experts of the Scottish Avalanche Information Service at Glenmore or SLF (the Swiss Federal Institute for Snow and Avalanche Research) at Davos in Switzerland. We know that UFOs are caused by various different phenomena - lenticular clouds, Venus by daylight, ball lightning, meteors - and the same is probably true of Scotland's Strange Sounds. The sound I heard was not firnification but I think Dougie Lockhart's one may well have been.

We need a snappy name for unidentified sounds on Scottish hills: Invisible snowballs? Hill whispers? Ah - yes - fairy motorbikes. That is it exactly. Now to just get two other people to refer to them as such, then I can put together a piece about them for a Sunday newspaper.

Ronald Turnbull

RE DOUGIE LOCKHART's report in TAC57, my not-long-after-the-spot account reads:

"16 June 1989. Ben Aden east ridge about 1730ft. Gained a rocky platform and as we paused we heard the sharp sound of blowing plastic sheeting (or a flapping tent!) from just above. 'It' came over the lip, marked its slow passage with a flattening of the grass a couple of yards from us, went down the inside of the shelf, and hit a low parapet at the end with a loud crack. A wholly novel experience, aptly described by [my friend] Richard as a 'windflap'. I wondered what sticking an arm in its path would have felt like. Richard encountered one two days later which sent a butterfly spiralling aloft. Odd in fairly light general winds - must be the terrain."

It was quite warm I recall, as my taste in T-shirts came under criticism. A newspaper Weatherfile piece soon after described this as a "calm-weather whirlwind typical of hilly country" in discussing crop circles (!)

Since then I have met several lesser versions, and am pretty sure that Dougie Lockhart's Kintail missile can be interpreted as a multi-sensory illusion. Although the vortex travels quite slowly, the apparent speed is very high (given reaction time to the unexpected, and post-rationalisation time), as with any sound source going 180 degrees past you very close by in a second or so.

I remain curious as to why an upward-moving heat column (as with the butterfly) appears to flatten the grass. Presumably this is due to air being sucked in at ground level, so the grass is being laid flat from all sides in a spiral sequence, not squashed from above.

It is notable that all your reports and my case are in the heart of the Western Highlands where the terrain is most extreme and where meteorological gradients are also steepest (mountains closest to the sea and high inland terrain). It is no coincidence that this is also the area where I have observed almost all evidence of lightning strikes - hence the OS abandonment of the Sgurr na Ciche trig point (see TAC57, p16). Kintail is also the current seismic hotspot of Britain, but this appears to be a complete red herring.

Finally, it troubles me that the editor's choice of title for the TAC57 piece, "Rocket science", is the name of the business which the Richard in my story set up.

David Jarman

I DON'T KNOW what the cricket ball-sized object was that Dougie Lockhart encountered in Kintail, but he's right to doubt the pronouncement of the folk at Astronomy Now: it certainly wasn't a meteorite. Meteorites of that size arrive dropping vertically at their terminal velocity, and for reasons that don't take rocket science to figure out:

Atmospheric pressure is around 15 pounds per square inch, so there are 15 pounds of air above every square inch of the Earth's surface (more than that, actually, since the stuff higher up weighs less, but we needn't quibble).

A cricket ball is nine inches around, giving it a cross-sectional area of 6.4 square inches.

So on its way to the ground from outer space, a cricket ball-sized object must shoulder aside at least 6.4 x 15 = 96 pounds of air.

But to arrive horizontally, it has to come in from the horizon - and atmospheric optics tell us that there's about 38 times more air between you and space in a horizontal direction than in a vertical direction.

So that's 96 x 38 = 3648 pounds of air, or rather more than a ton and a half.

But a cricket ball-sized lump of rock weighs just a pound or so. No matter how briskly it's moving initially, by the time it has slammed into a ton and a half of air it has effectively been reduced to a dead halt, and so ends up just dropping freely under its own weight.

On a similar I-know-what-it-ain't note, I was intrigued by the huge paw-print, featuring claw-marks, that Badger Bill encountered on Ben Wyvis (TAC57, p18). This seems unlikely to have been left by an Alien Big Cat, since almost all the big cats sheath their claws when walking - the exception is the cheetah, which uses its claws for running traction rather than prey-grabbing. So cheetah tracks do show the marks of four claws on the fore-foot, but they're rather smaller than the one Bill described - palm-sized rather than hand-sized. It's also difficult to imagine a super-specialist hunter like a cheetah lasting for very long out of its natural environment.

Giant dog, anyone?

Grant Hutchison

Ed. - Ashley Tuck has likewise been in touch to say that Badger Bill's "cat" footprint was not feline. "Cats have retractable claws," he writes, "and retract them when they walk. They therefore leave no claw marks in their prints."


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