Moderator: Moderators
Phil Winkler wrote:Allen,
I can't imagine any cave not being affected by barometric pressure. When one considers the entire volume of a cave and that it may only have one entrance, then a change in air pressure is always going to make it "breathe". Rapid changes would have more effect then gradual ones, too.
WVCaver2011 wrote:Is there anyway to determine weither it's a large cave by the amount of airflow? Assuming the cave only has one entrance (which is possible) how can I determine weither the cave is large (5 miles or more) only using airflow?
WVCaver2011 wrote:If there was a second entrance wouldn't it be possible to determine how far away the other entrance is by measuring the windspeed and refering to isobars on a weather map of that day?
WVCaver2011 wrote:I have been surveying a cave that is located in Pendleton County, West Virginia and most of my survey trips to the cave were during low pressure events however, just this past week I revisited the cave and there was a high pressure system setting right overtop of WV. The cave's air reversed from it's usual outward flow to an inward frow of approximately the same magnitude. I entered the cave at around 7pm and left the cave at around midnight on friday it was low 80's when we went in and mid to upper 50's when we left...The temperature had no effect on the airflow because it stayed the same while we were in the cave (it continuously preceded to blow inward). This is why i'm asking this question.
WVCaver2011 wrote:Is it possible that the cave is, or could be barometrically driven?
driggs wrote:WVCaver2011 wrote:I have been surveying a cave that is located in Pendleton County, West Virginia and most of my survey trips to the cave were during low pressure events however, just this past week I revisited the cave and there was a high pressure system setting right overtop of WV. The cave's air reversed from it's usual outward flow to an inward frow of approximately the same magnitude. I entered the cave at around 7pm and left the cave at around midnight on friday it was low 80's when we went in and mid to upper 50's when we left...The temperature had no effect on the airflow because it stayed the same while we were in the cave (it continuously preceded to blow inward). This is why i'm asking this question.
Allen, I'm going to phrase this answer in the form of "thinking points" that you can use to refine your ideas on what processes are at work. I encourage you to check out books from the Grotto library if you would like to research this further; Art Palmer's Cave Geology has a well-written section on cave air movement that you can refer to, and it even covers barometric air movement (which generally only affects very, very large caves).
This weekend, I believe, is the first trip you've taken into that cave when the outside temperature was greater than the in-cave temperature (~54-degrees). Think of the cave as a giant sealed gas bottle with only one valve. Think about how temperature affects the density of a gas. Now, given two situations - outside warmer than inside, outside colder than inside - what happens in each situation at the "valve" as the "gas bottle" tries to equalize its pressure? Consider a third situation, where the daily temperature swings from 80-degrees at noon down to 40-degrees at midnight, what would you observe if you sat in the entrance all day long?
Once you have considered these situations with a single-entrance cave, you can move on to the more complicated chimney-effect situation; imagine now that your cave has two entrances with significant elevation between them. How do the above scenarios affect airflow at each entrance, given that the temperature outside (and assumed density) is the same at both outside entrances? (hint: think about how dense or cold air "falls" to the bottom of a room)
jharman2 wrote:WVCaver2011 wrote:Is it possible that the cave is, or could be barometrically driven?
The sad truth about the "real world" is that questions like this often do not yield to straightforward theoretical analysis. Having been to the cave in question I can say with certainty that there are several variables at work which are not easily decoupled; barometric and chimney effects are likely both present. I am certainly not in a position to say which effect is dominant. To determine that would require a carefully crafted experiment and observations during different weather conditions over a long period of time. As an academic exercise it would be interesting, however, you won't learn as much from the airflow analysis as you will from systematically exploring and mapping the cave.
WVCaver2011 wrote:The cave dropped 66.7 feet within this distance and is still dropping.
driggs wrote:WVCaver2011 wrote:The cave dropped 66.7 feet within this distance and is still dropping.
What is the total surveyed vertical extent thus far? If your cave is dropping down the flank of the anticline, it could easily have very significant elevation difference between two (or more) entrances, giving you a gale-force wind at constrictions inside the cave due to the above-mentioned chimney effect. Unfortunately there is no way to tell if the other entrances are accessible to humans other than simply pushing it.
Think about the scenario with an upper and lower entrance (pretend that you found another "mystery" hole on the hillside moving air), and you should be able to come up with a surefire way to determine if a chimney effect is responsible for the air movement at both entrances and between them.
jaa45993 wrote:Allen,
I think everyone that has replied has given you good information, but they are perhaps being a little too pessimistic. There are lots of interesting things you can learn about a cave based on its airflow. I was involved with a multi-year study of the cave airflow of the southern Black Hills, including Wind and Jewel Caves. The things we learned about Jewel Cave were in some cases quite astonishing. It is true that cave volume and cave length are hard to correlate, but you can still compare surveyed volume to measured volume and make some predictions about potential.
Based on what I understand of your cave, it could have barometric or chimney effect winds. It is really hard to say. However, your explanation of the pressure and temperature at the time of your visit sounds a bit barometric to me. I apologize if you already said it, but is your entrance near the top or bottom of the limestone? If the outside temperature is warmer, you would expect the air to rise through the cave in a chimney effect. If colder, it would sink through.
In a barometric situation, the cave would breathe in high pressure and breathe out when the pressure is low. This gets tricky when you don't know how many entrances there are. The trick is to have instrumentation at every known entrance and blowhole, coupled with pressure and temperature data. That way, you can catch it when all entrances breathe in the same direction simultaneously. In a multi-entrance cave, this is almost a sure sign of barometric airflow.
In a one entrance cave it is easier. If you set up an anemometer or temperature logger inside the entrance of your cave, and assume that it is the only entrance, and monitor the local surface pressure and temperature, you could start to make some inferences about the nature of the airflow. These would of course have to be re-evaluated as soon as you discover another entrance!
Users browsing this forum: No registered users