I can think of a number of examples of large boulder slopes, say for 30m up a hillside, that draught like a classic cave (i.e. in the bottom in winter, out the bottom in summer. Does anyone know of any official, or unofficial, theories supporting this phenomenon?

I imagine the boulder size distribution and depth of cover will be the predominant factors controlling the effect, but also vegetation cover and bearing.

One example is around the Strawberry Hole area in Derbyshire. Walking from the car park towards the quarry the whole slope to the left draught like this. I realise that in this case it may well be the sough below which is the source, which is kind of why i ask the question...

An interesting one.

If this is a convection effect actually within the boulder slope itself (i.e. not cave draught related) then it should be readily detectable wherever there are natural boulder slopes such as the big screes which form below crags in the Lake District. So one might expect a line of melted snow along the crest of scree slopes as a test of your hypothesis? I'll keep my eyes open to see if I can spot any.

Good question.  I guess you have read http://en.wikipedia.org/wiki/Stack_effect which covers the basic physics.  Enhancing the model described on Wiki to a cave passage is fairly simple.  But extending it further to a pile of boulders becomes really difficult.  One major concern would be the resistance to air flow through a pile of boulders which is why I guess you are asking the question.  I would have thought the movement of air through "wriggly" passages has been modelled.  Does any one know of such work? 

So thinking of experimental situations, my suggestion is that you would be better off looking at bolder piles up against quarries faces where it would seem far more likely that no cave existed behind the pile.  Pitlamp's comment about snow melt is very neat but surely for much of the time the snow cover will have sealed off the flow of air?  A smoke / smell test might just produce a result but I fear even a short passage length will scrub the smoke or smell out.  Has any one done micro pressure and temperature measurements to test cave draughts?  I vaguely recall Bryan Ellis doing air temperature measurements in Icelandic lava tubes in the early 70s but can't recall the outcome. 

Is the draught around Strawberry Hole moist or dry?

Hello Bob - on my way home from caving this afternoon I took a picture of Ingleborough (the classic view from the Kingsdale road). It was about 3 p.m. and the sun was just coming round from the south west onto the western summit slope, north of the big Falls Food landslip. The bouldery slope was still frost covered but the sun hadn't melted it, so it was possible to see that there wasn't a line of melted frost near the head of the slope. I'm not clever enough to upload the picture here but I'll email it to Rob so he can have a look at least.

Rob's is certainly an interesting question; I'd have thought if there was convectional air movement through boulder masses it'd be more pronounced if most of the surface of the boulder slope was grassed over except near the base and the top (i.e. "sealed" to leave just two "entrances" for air at top & bottom). If a good example of such a boulder mass an be identified then it'd be a good test case to study. If this boulder slope were not in a caving area at all (i.e. unlikely to be influenced by any cave draught as well) so much the better.

Rob - I'll send that picture to you this evening.

Here's John's picture:

Look's like a lovely day!

The boulder slope near Strawberry Hole consists predominantly of boulders larger than a kettle, up to the size of a car. I think this sort of boulder slope is much more likely to have open "passageways" for the air to travel through. Remember air is not like water, small apertures (i.e. high turbulence) create much more of a resistance [that's why ventilation ducting in buildings is much bigger than the water pipes]. Therefore to have any significant flow the gaps will need to be bigger than that found between pebbles and typical frost-shattered scree. I think, as Bob suggested, manmade rubble slopes around quarries are likely to be good environments for this, if any...

Also, i think it may be more of a concentrated effect than in Pitlamps example. I.e. if a good "passage" up the slope is present the main outlets for the air will be small and compact, and possibly quite dispersed, rather than whole "zones". Maybe not though!  Certainly near Strawberry Hole the bits that noticeably draft out in the summer are only under large rocks. However the key word there may be noticeably.

in cumbria? said:

Is the draught around Strawberry Hole moist or dry?

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Sorry, not sure.

EPC had a dig half way up the other side of Stoney Dale over this winter, which drafted very strongly out. After excavating ~5m it was blind, the draft coming from the rubble down slope. And i dont think this is the first time in this dale. Another dig further down the dale was against a cliff (2nd picture down on this thread: http://ukcaving.com/board/index.php?topic=9492.msg124645#msg124645 before the cliff was found!). It hasn't bottomed out fully yet, but there's been no evidence of cave so far. One theory is that the cliff acted as a barrier for the draft, so it surfaced there. 

Aye - it wasn't a bad day today. Sometimes it's tough living in the Dales - but someone's got to do it! 

In case it helps - I noticed your emphasis on the word "noticeably". Some time ago Braveduck (who often posts on here) showed me the trick of using that canned disco smoke for detecting weak draughts in caves. It's very sensitive; you just need a tiny squirt from the aerosol. On a still evening you might be able to make even very weak draughts more "noticeable" with some of that? (I think they sell it at Maplins and it's only cheap.)

Pitlamp said:

I'd have thought if there was convectional air movement through boulder masses it'd be more pronounced if most of the surface of the boulder slope was grassed over except near the base and the top (i.e. "sealed" to leave just two "entrances" for air at top & bottom).

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If cooler, descending air is hugging the hard base beneath the boulders, a top seal is unnecessary.

Here in the Rockies, our "boulder slopes" are mostly scree slopes, which often cover the transition between massive limestones above and shaley stuff below, and a draught amongst those rocks might indicate a buried cave entrance.

Been thinking about this a little more recently. Here's an example that may be applicable...

A cave has multiple entrances. The top entrance is say at an altitude ~1900m. A 100m entrance series leads to a descending streamway which traverses horizontally for ~1km and sumps at 1400m. All along the streamway other entrances link in, each with ~100m vertical development (i.e. the surface matches the trajectory of the streamway).

So this is a simplistic description of a cave in Albania. We explored it in the summer (outside temp ~20?C) and the cave temperature is roughly 4?C. One would of course expect under standard temperature driven drafts for the top entrance to draft inwards and the lowest entrance (above the sump) to draft outwards. This is the case, so nothing unexpected there.

The querry is how people would expect the other entrances to draft?!? I can think of 4 possible results:
[list type=decimal]
[*]the top half to draft inwards, the bottom outwards, and the ones nearest the middle hardly at all.
[*]the top inwards strongly, the lowest outwards strongly, no noticeable draft in the others
[*]the top inwards, all the other outwards, probably with increasing strength as you descend
[*]the bottom outwards, all the others inwards, probably with increasing strength as you ascend
[/list]

The actual theory was experienced by the direction of the draft witnessed in the streamway along it's length. In this example in Albania the draft heads all the way down to the sump, increasing in strength as it progresses, and then leaves via the lowest entrance. Therefore it is my suggestion that theory 4 was in action...!

This agrees with the boulder choke theory.

Does anyone have any other cave examples like this that agree or not with this suggestion?

I'm confused.

4. the bottom outwards, all the others inwards, probably with increasing strength as you ascend

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but then

In this example in Albania the draft heads all the way down to the sump, increasing in strength as it progresses

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Stronger as you ascend and then stronger as you descend (as it progresses). I assume you are talking about different draughts. In 4 you are talking about the draught within an entrance but later you are talking about the draught in the main streamway?

Ooops, a little mistake there. Sorry. The 4 theories should read:
[list type=decimal]
[*]the top half to draft inwards, the bottom outwards, and the ones nearest the middle hardly at all.
[*]the top inwards strongly, the lowest outwards strongly, no noticeable draft in the others
[*]the top inwards, all the other outwards, probably with decreasing strength as you descend the streamway
[*]the bottom outwards, all the others inwards, probably with increasing strength as you descend the streamway
[/list]

Did you record atmospheric pressure during your observations at this cave - and was it steady? I ask because most cave draughts are driven both by convection and barometric changes over time. If you want to try and get your head round convectional draughts you need to take observations when the air pressure is fairly stable (a bit like it was in northern England over much of last week). I keep a barograph on the mantlepiece and it's quite interesting to watch how air pressure changes on the weekly charts it produces. Last week, from Wednesday morning to Friday tea time it almost flat-lined. Those are the best conditions for studying convectional cave draughts.

Venturi effect?

Rob said:

The querry is how people would expect the other entrances to draft?!?

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Pitlamp`s observation about external conditions are right of course. They are part of the reason why it is not possible to give a firm answer to the question.
Another reason is that the air flow restrictions in the main passage and the inlets will affect things a lot.

But I am prepared to suggest what might happen in some idealised systems.

Taking a simple system as described, with equally spaced entrance series 1,2,3,4,5,6 counting from top to bottom and assuming nothing unusual is going on outside.

If entrance 1 and the main passage have large areas of X-section compared with the rest (so there is a large, constantly replenished reservoir of cold dense air inside) I would expect inwards flow at 1 and outwards flow at the others.  The internal pressure is increasing with depth, but so is the external pressure, so I wouldn`t expect that much difference in the outward flow rates.

If entrances 1 to 5 and the main passage  have small areas of X-section and 6 is big then I would expect air to go in 1 to 5 and come out of 6. This seems to be what Rob said was going on. Was the bottom entrance big?

If everything is the same size I don`t know what would happen. It`s tempting to think that 1, 2, 3 would take air in and the others would push it out but that is just a guess.
BUT if there was a restriction in the main passage between 3 & 4 then it is quite possible that air in 1 would move inwards and 3 outwards (2?), with 4, 5, 6 behaving much like 1,2,3.
In a real cave with multiple restrictions I think things become impossible.

Really useful post, chocolate fireguard. It ought to be able to design a simple laboratory experiment to test that, if someone was motivated enough.

Robert - venturi effect - yes, I'm sure that happens too.

I have a vague recollection of a certain cleric (in a certain Skipton associated club with which you're familiar) might have done some experiments along those lines himself, some years ago, when he was into ergonomics. I'm sure he'll pop up at a certain deep hole in August; we must remember to ask him.

Slightly digressing, but a month or so ago, I noticed a distinct cold draught in Notts Pot, which was dropping down the second pitch.

The day was cold (air around freezing), there was basically no wind on the surface, and the cave was virtually dry.
Given the small size of the passage just below the entrance pitch, it's not easy to imagine a cold sinking air current and a warm rising one flowing past each other without mixing (and there seemed to be a general in-draught in the crawl anyway).

If the draught was due to any kind of chimney effect between multiple entrances, given the conditions it would presumably suggest some entrance higher than the normal entrance?

Interesting, potholer.

On a cold day in February this year Kirk Pot was draughting inwards strongly - yet the pothole is all downhill. I still haven't quite got my head round that one.

Chocolate, thanks for your suggestions. Glad we can disrespect other variables for a while to concentate on simplistic situations.

Chocolate fireguard said:

...If entrances 1 to 5 and the main passage  have small areas of X-section and 6 is big then I would expect air to go in 1 to 5 and come out of 6. This seems to be what Rob said was going on. Was the bottom entrance big?

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To be perfectly honest, we did not explore the other entrances down to the streamway, they are only witnesses as avens from below, so i cannot detail the passage sizes between the aven and the surface. I can confirm that they were all relatively similar sized avens, certailny the first and last were no different. Also there is no vegetation on the surface so it is likely that all avens will connect to the surface by at least an air passable gap.

This effect was witnessed in two different caves, both with the last aven taking all the draft upwards. Means that you get really excited as you explore the streamway downstream, the draft getting bigger and bigger as you proceed, just to reach a sump with an aven above taking all the draft. Maybe that last aven bypasses the sump, but i doupt it looking at the other cave development in the area. Kind of why i'm trying to understand this draft effect....

Chocolate fireguard said:

...The internal pressure is increasing with depth, but so is the external pressure, so I wouldn`t expect that much difference in the outward flow rates...

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An extension of that idea suggests that height difference between entrances of a through trip will not impact on flow rate, which i would disagree with.

I would have thought a simple CFD simulation could be setup to test this theory, anyone here a simulator? Or are there any other examples people can think of? For example does Cwm Dwr draft in or out in winter or summer?

As for venturi, i bet it has very little impact, even where high velocities exist. It is proportional to the density of the fluid, and air is very light. A quick calc suggests that if the air velocity increases from 1m/s to 2m/s the pressure difference will be ~1.9Pa. Basically nothing compared to the temperature change forces pulling these air masses through the caves. Has anyone proven a venturi effect in a cave draft?

Could it be influenced by the stream?

Good idea, but not in this situation. Very low flow in the summer....