The Origin of Cave Draughts - Question

The Old Ruminator

Well-known member
Well the question in this particular case. Aisholt Cave is well known for its temperature controlled draught. Strongly outward in warm weather . Cold at ten degrees below outside. Winter its cold air going in. Right up to the terminal boulder choke. B, fumes just disappear here and are not seen outside nor is there any other entrance. So what drives this and where does the cold air go if not coming out again. Same in Summer . Loads of cold air coming out with no let up.

 

pwhole

Well-known member
We have a similar situation at one of our digs in Derbyshire. A TinyTag temperature monitor left well inside over July/August showed temps of average 6-7° inside, compared to average of 21° under a large boulder outside. We had to move the monitor to a safer spot halfway through, and the temp jumped by one degree immediately (it was further away from the main draught), and then stayed flat again for the rest of the time. Digging last weekend showed the draught was now sucking in - it was only 1-2° outside, so warmer inside. Out conjecture obviously is that there's a larger system somewhere beyond, possibly involving water, but we're nowhere near an obvious way in yet other than removing more boulders.

This is another hole we found 10m away from the main entrance - in May:

 

Pitlamp

Well-known member
Seems like the answer to OR's question is convection, to / from a higher entrance.

In winter, exactly the same as what makes a fire grate "draw" as soon as the air in the flue has begun to warm up (and become less dense, so it rises) when the fire is lit. In summer, a "reverse chimney effect" (as air in the cave system is cooler than surface air temperatures, so more dense, so it sinks and your cave draughts outwards).
 

mrodoc

Well-known member
The chimney effect is the obvious explanation but the site is a lens of Devonian limestone in a quarry that has exposed the cave at the base of a 10+ metre cliff - presumably original ground level. My current gloomy explanation is that where the dig is at the moment the back wall consists of collapsed beds of shale and I suppose that if it goes all the way to the surface it could allow enough air in for a temperature differential to develop.
 

The Old Ruminator

Well-known member
The chimney effect is the obvious explanation but the site is a lens of Devonian limestone in a quarry that has exposed the cave at the base of a 10+ metre cliff - presumably original ground level. My current gloomy explanation is that where the dig is at the moment the back wall consists of collapsed beds of shale and I suppose that if it goes all the way to the surface it could allow enough air in for a temperature differential to develop.
Its not shale now but large boulders. The end dig is 2m away from a steep bank going upwards so soon will be 20m plus below any surface. The surface is impervious mud so I cant see how air comes up through it.
 

Pitlamp

Well-known member
20 m height difference is certainly enough to drive a convectional draught. (Again, think about a house chimey which is a fraction of that height but will draught well as soon as the flue even starts to get warmer than surrounding air.)

If you're struggling to envisage where the air is emerging topside, if and when we get the next dollop of snow, grab the opportunity for a stroll around looking for a blowhole.

Small favour OR, could you post an image (fairly close up) of this Devonian limestone? I'm interested to compare it with a small outcrop near Cowan Bridge (in the Dales) which them as know better than me have suggested may also be Devonian.
 

The Old Ruminator

Well-known member
20 m height difference is certainly enough to drive a convectional draught. (Again, think about a house chimey which is a fraction of that height but will draught well as soon as the flue even starts to get warmer than surrounding air.)

If you're struggling to envisage where the air is emerging topside, if and when we get the next dollop of snow, grab the opportunity for a stroll around looking for a blowhole.

Small favour OR, could you post an image (fairly close up) of this Devonian limestone? I'm interested to compare it with a small outcrop near Cowan Bridge (in the Dales) which them as know better than me have suggested may also be Devonian.
All in the video or here -





 

The Old Ruminator

Well-known member
Better to buy cold canned smoke from Maplins .No smelly smoke and no premature death for the smoker !
We have been using canned smoke for many years. See it in the second image above. It dissapates too quickly for positive tests and plumbers smoke bombs give too much smoke though the matches work OK.
 

Pitlamp

Well-known member
Thanks OR, those photos are helpful.

So is this Devonian limestone stratigraphically above the Old Red Sandstone which forms the core of the Mendip Hills? (In other words, younger than it but older than the Carboniferous limestone?) Or is it interbedded with the O.R.S.?

Sorry, my knowledge of the geology of your neck of the woods isn't very detailed.
 

pwhole

Well-known member
Maybe ensure the digging team has at least one smoker of some variety so that any draughts or airflows underground are illustrated

I'm usually the one doing the capping, so that's fairly easy to achieve. Though Alex had to endure the last smoke-test as he was doing some capping with me behind, but it proved the point :cool:
 

Leclused

Active member
Well the question in this particular case. Aisholt Cave is well known for its temperature controlled draught. Strongly outward in warm weather . Cold at ten degrees below outside. Winter its cold air going in. Right up to the terminal boulder choke. B, fumes just disappear here and are not seen outside nor is there any other entrance. So what drives this and where does the cold air go if not coming out again. Same in Summer . Loads of cold air coming out with no let up.


In winter the cold will cool down the rocks and in summer the rock will start to warm up. Check out the temp of the rock in early spring and you will see that this will be far below the average temp of the air in the cave.

But there will somewhere an upper entrance, or multiple small ones

There is a very good write up about airflows in caves called “Vent de ténèbres” by Lismonde

Part1 : http://cds38.org/wp/wp-content/uploads/2013/04/climatologie_monde_souterrain-tome1.pdf
 

The Old Ruminator

Well-known member
Thanks OR, those photos are helpful.

So is this Devonian limestone stratigraphically above the Old Red Sandstone which forms the core of the Mendip Hills? (In other words, younger than it but older than the Carboniferous limestone?) Or is it interbedded with the O.R.S.?

Sorry, my knowledge of the geology of your neck of the woods isn't very detailed.
We are working in the Quantock Hills not on Mendip though still in Somerset. Here we believe that caves exist but are all hypogenic offering maze like caves or large caverns. All would be found by quarrying as no natural entrances exist. Jackdaw cavern and Pen Park Hole are very large hypogenic caverns. Holwell cavern is a maze. The Quantocks are a truncated anticline rather like those periclines on the Mendips. Morte Slates underlie parts of the Quantocks to a depth of c 1,000m. There is a slate pit and mine to the SE. Devonian limestones and associated slates form ribbons from Dodington to Cothelstone where they terminate at the Cothelstone fault. The strips run deep and mostly dip to the east. Many were quarried for lime burning. Lenses of sandstone bury the limestones in places where at the surface sub aerial erosion has reduced the rock to sand.
 

2xw

Active member
Our dig is quite deep in the cave and has cold air rising into it from a 45 degree slope (we can't quite get into yet). Does this mean we are at the top of the chimney? I tried reading the pdf up above but my french is rusty and the document wasn't a breeze
 

Graigwen

Well-known member
Better to buy cold canned smoke from Maplins .No smelly smoke and no premature death for the smoker !
I used to buy canned smoke from Maplins until they folded, but I am not sure the new Maplins sells it.

I have in recent years been buying from Stage Electrics, they used to offer bulk discounts. I see it is now up to £13+ a can, but used carefully a can lasts a long time. Usually a couple of seconds burst is sufficient.

Magican Fog Spray
.
 

Ali M

Active member
We find Arctic Hayes Strikes Smoke Matches excellent. They have a burn time of 20 seconds which makes them ideal for testing draughts.
 

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