Cave Air by Bruce A. Zerr

Article submitted by Bruce A. Zerr 11/09/2001

There are very few barometric cave wind studies that have been done in the States. Here is what I remember. Nameless Cave, a commercial cave located about 4 miles West of Rapid City, on the North side of Dark Canyon, was the first confirmed barometric breather I know of. I believe that the owners determined that the cave has a cave volume of about 35,000,000-ft3. Yet the length of the cave is only about 250 ft. The volume of the explored cave is
maybe 10,000-ft3.  As you can see by this small explored volume, the cave has no big rooms.  (the air slips out thru the cave walls in about a half-dozen places.)  The cave air leaves the cave thru a portal about the size of a cinder block - at the top of the stairs leading down into the cave.  Wind velocities I saw range up to about 10-mph.
Now I did a study on Brooks Cave in 1971.  This cave is located about 5 miles Southwest of Rapid City, on the South side of Dark Canyon. This cave too has a volume of about 35-55 million Ft3. Its explored length is about 1/2 mile. Volume of the explored cave itself is only about 200,000-ft3.  The entrance to the cave is just a little bigger than a caver is crawling - about 2-ft2. Most of the time the cave wind is light - 5-10 mph; but I have seen cave winds blowing out of the cave strong enough to blow out my carbide lamp facing into the wind from 15 feet away. So I would say that the max wind I have seen from this cave is about 30-mph, but I didn't have an anemometer back then.The interesting thing about this cave is that the cave air volume is x100 times larger than the caver explored volume of the cave.
At Lechuguilla, I measured max cave winds of +49-mph, and -43-mph. The wind is blowing out of the end of a pipe culvert that measures 2' in diameter.  Now Lechuguilla has an explored length of 106 miles with an average passage size of 20'w x 20'h.  That gives you an caver-explored volume of 223,872,000-ft3. I measured a cave wind that blew out of the cave at an average velocity of 35-mph, for 84 hours. The cave air volume of Lechuguilla was calculated at 3,000,000,000-ft3.
Big Manhole Cave, a mile to the East of Lechuguilla was tested too, at the same time.  This cave has a plugged entrance. Air flow up from broken rock and breakdown into the entrance room in a couple of places. This cave has a cave volume of at least 200,000,000-ft3.  Caver explored volume is 100'x100'x40'.
Because Lechuguilla and Big Manhole are so close together, I consider them to be coming from the same cave and cave airofir.
Carlsbad Caverns is probably barometeric.  They have cave winds going thru the wall at one place that reach 20-30 mph - headed right toward Lechuguilla.
Wind Cave has had cave winds of 100-mph. It just went over 100 miles in length this last August.  Its average cave winds, blowing out a standard sized door, average 10-15 mph. Herb Conn determined that the cave volume, or airofir had a volume of 1,500,000,000-ft3.
Jewel Cave has been explored to 126 miles. It has an average hallway size of 10'w x 20'h. or 133,000,000-ft3 of caver explored volume. The airofir there has a volume of 3,000,000,000-ft3. Cave winds at the entrance to
Jewel are lighter than they are at Wind because they come from an airofer that has the shape of a long tunnel, whereas the airofir at Wind Cave is shaped like a round balloon.
To find the shape of the airofirs of a cave you are interested in you must find out a couple things. Look for geologic and hydrologic barriers to the cave. The hydrologic barrier is usually easy - just find the depth to the watertable.  Then assume this hydrologic barrier is impurvious to air - and that the watertable is flat over the area of interest. Next barrier is usually the geologic roof of the cave.  The cave is going to be in a limestone belt, but barometric breathers seem to prefer to hide underneath a "sandstone caprock". This is your second barrier. At Brooks Cave you had
an unconformity contact at this junction.  The limestone was exposed above the ocean surface, eroded, had sinkholes form, had karst develop - but most important, had several feet of red residium clay (made up of the insoluable residue of the dissolved limestone) spread over its top layer when the land sank underneat the ocean again and the Minnelusa Sandstone unit was deposited on top of it.(It makes a nice barrier, or seal.) What you are interested in here is the height above your cave entrance to the top of the limestone/sandstone contact.  Also find out the local dip and
strike of the rock units of the limestone and sandstone. From the dip, and the height from the watertable to the underside contact of the sandstone you can use geometry of a right triange to calculate what I call the triple point - that point where the flat top of the watertable touches the limestone/sandstone contact plane.  This triple point is your third barrier to the cave - for the cave air can not extend beyond it, because the air would have to be able to go underwater!  No way! At Brooks Cave the dip of the rock was to the East 4,000-ft away from the entrance.Now you must check for other barriers to your cave. At Brooks Cave the northern boundary was Dark Canyon, half a mile to the North.  This deep canyon was a cave airofir barrier because of its depth.  The water table was only 15' below the bottom of the canyon.  Rapid Creek flowed right across the limestone belt; but it is the only surface stream to do so in the Black Hills. (Rapid Creek does lose about 30% of its base flow to the limestone, though.)  To the West, the cave airofir barrier was caused by the updip of the rock.  The limestone was completely eroded away, giving way to an older,deeper, non-limestone formation - thus a barrier.  The last barrier was determined to be close by - because the cave air volume was so small
(35,000,000-ft3) as compared to Wind (1,500,000,000-ft3) or Jewel(3,000,000,000-ft3).  See what I mean.  This boundary was found to lie about a mile south of the cave.  It was a fault that cut perpendicularly across the limestone belt.
So what good is this extra work, you ask?  What it does is let you determine the porosity of the limestone formation the cave is in.  At Brooks Cave you had 35,000,000-ft3 volume in an area of 1-1/2 sq mi. x 350' H.; or 1.5%  Since this is secondary and tertiary volume you are calculating, it gives you the volume of water per unit volume this limestone unit will store.
So taking this a step further, Wind Cave had an airofer of 45-sq miles, and Jewel Cave has an airofer of 90-sq miles!  Actually, when you start drafting the barriers locations - one finds out that Wind and Jewel share the same
airofir. They are one and the same cave - with two entrances. This get real interesting since this airofir was found to have an area of 144 sq miles; and since Jewel cave has a cave passage density of 50 miles of cave passage/sq mile - you have the potential of finding a cave having over 7,000 miles of cave passage here!
The airofir of Lechuguilla appears to involve 45-sq miles, and has a cave passage potential of 2,400 miles; but this does not include Carlsbad Cavers, or Wind Cave there. (Wind Cave is located about 15 miles away - it can suspend a beach ball up in the air all day in the wind.  Sure sounds barometric to me.)
Breathing Cave, Virginia is barometric.  Volume is about 4-8,000,000-ft3.  Length is a couple of miles - it should be listed in the long of the World if you are interested in looking up its length.
Bruce Zerr can be contacted via email at:

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