Does It Rain In The Pantheon?

March 31, 2007 4:18 PM

Pantheon Filter: I need an authoritative answer to this question. Is it true that if you get a ton of people inside the Pantheon in Greece, "air pressure" will keep rain from falling through the occulus?

Apparently rain will come through with a small crowd, but will be held back by a larger one. A tour guide told us this was true, however, an extensive Google search has revealed not a single supportive opinion of this explanation. Is it true, or is it a myth that tour guides lay on gullible American tourists?

posted by crapples to science & nature (31 comments total)

first of all, the pantheon is in rome.
and no, a crowd won't stop the rain, any more than it does at a san francisco 49ers game.
posted by bruce at 4:25 PM on March 31, 2007

I think you meant to say the Pantheon in Rome.
posted by ericb at 4:26 PM on March 31, 2007

Whoops - Of course it's in Rome. Major faux pais. Common sense tells me that this is a myth - but it came from a somewhat authoritative source (a tour guide) - so I'd like to be able to say for sure.
posted by crapples at 4:30 PM on March 31, 2007

Tour Guides are to truth as Yoda is to wisdom.
posted by Dizzy at 4:31 PM on March 31, 2007

..gullible American tourists

Surely there's no such thing?

Anyway, the answer is no and it's all a myth for the tourists. Although it does help to keep it cool.
posted by Nugget at 4:31 PM on March 31, 2007

I think you meant to say 'faux pas' :)
posted by dirtynumbangelboy at 4:37 PM on March 31, 2007

Answer: Yes, but only if Chuck Norris is one of the ones inside.

I agree that it's most likely a myth. I really don't see how people being inside would raise the air pressure an appreciable amount, or at least enough to stop rain from getting in.
posted by DMan at 4:42 PM on March 31, 2007

Yeah, that makes no sense whatsoever, more people in a room doesn't change the air pressure, and even if it did, air pressure wouldn't stop rain from coming in unless it was much higher the the outside, so that air was literally blowing out strong enough to push the rain out of the way.
posted by delmoi at 4:45 PM on March 31, 2007

Not air pressure but rising thermal currents could conceivably keep very light rain out but not likely, really:

This page points out not only is it unlikely because the Pantheon isnt heated but also there are drains in the floor - for water that comes in through the occulus.
posted by vacapinta at 4:49 PM on March 31, 2007

Wow. That's idiotic - and to hear it from a tour guide! How would a bunch of people in the room affect the air pressure, especially given that there's a big hole in the ceiling?
posted by Dasein at 4:55 PM on March 31, 2007

I think it is entirely possible if the rainfall is relatively light.

The oculus is small compared to the size of the Pantheon and is located at the top of the roof (Google images), and if the building were to be full of people, their heated breath and the air heated by their collective bodies will rise, pool and create a strong positive outflow through the oculus which would in effect, blow the rain away from the opening. Air would be sucked in to replace that air from other openings lower down, of course.
posted by jamjam at 4:59 PM on March 31, 2007

I think the idea was that enough people in the place, supplying body heat, could start convection of warmed air rising to the dome, and thus, in conjunction with outside wind creating negative pressure over the dome, create enough upward air flow to keep rain out. So, the "myth" is based not so much on "air pressure" as it is on normal convective flow, which building designers now call "stack effect" or "chimney effect." And yes, it is powerful enough to cause hazardous airflow conditions in the lobbies of modern buildings with elevator shafts, and quite probably, has been equally effective for the last several thousand years. :-)

And that's the reason it rarely rains into the Pantheon.
posted by paulsc at 5:00 PM on March 31, 2007

This is not even wrong. And I feel dumber for even having to answer this.
posted by DU at 5:34 PM on March 31, 2007

Actually, it rains pretty frequently in the Pantheon. (For this reason, the floors all slant to the center of the building where there are drains.) And though there are probably going to be more people there when it does rain, just so that they can avoid getting wet themselves, the lack of heat in the building in general and the large diameter of the oculus are really going to lessen any possible updraft effect. Maybe if there happened to be a huge gust of wind that somehow got in the door and pushed its way outward through the oculus, you would get a brief respite, but that floor's still gonna get wet.
posted by zeugitai_guy at 6:04 PM on March 31, 2007

great question, it's mathematical, thanks paulsc. hypothetically the air speed could reach very high levels, and if there is wind outside on top of that then the answer is clearly yes. did you rate du as your favourite answer?
posted by edtut at 6:09 PM on March 31, 2007

i'm surprised that nobody brought this up, so i kind of assume you already know this.

it was explained to me that the hole in the pantheon is an optical illusion, that is to say, while standing inside, it appears to be level (parallel) with the ground, when it fact it is built at a steep angle - thereby allowing light to come in normally, but making it more difficult for rain to fall in.

as far as logical conclusions go, im not sure if the fact that there are drains in the pantheon necessitates that the builders expected rain inside.
posted by phaedon at 6:20 PM on March 31, 2007

I think you meant to say 'faux pas' :)

I would have guessed that crapples meant to say 'faux pays', but was throwing in some Spanish with the French for flavor. We've already got Italy and Greece involved...
posted by aneel at 6:29 PM on March 31, 2007

...making it more difficult for rain to fall in.

I'm really loving how quickly this fact-free discussion is falling to pseudo-science.

If you can stand directly under the hole, look straight up and see a big patch of sky (which, if photos are to be believed, you definitely can), rain falling down is going to see that same cross section and fall in.
posted by DU at 6:31 PM on March 31, 2007

Yikes.
posted by Dizzy at 6:32 PM on March 31, 2007

I'm really loving how quickly this fact-free discussion is falling to pseudo-science.


I think the point I was getting at was that a window may not allow as much rain in a room as a skylight. Anyway, that appears not to be the case according to this cross-section.
posted by phaedon at 6:35 PM on March 31, 2007

DU - it's not pseudo-science that warm air rises.
posted by scarabic at 7:32 PM on March 31, 2007

"In her Memoirs of Hadrian, Marguerite Yourcenar speaks of 'this open and secret temple, conceived as a sundial. The hours were to circle the center of its carefully polished pavement where the disk of the day was supposed to rest like a golden buckler; there the rain would make a limpid pool from which prayer could transpire like smoke toward the void where we place the gods.'

Put less poetically, the thirty-foot hole in the ceiling performs like a sundial to mark the passing of the hours and the dates of equinox and solstice. The hole is the building’s only source of light. Or air. And it symbolizes a union between earth and sky that allows human prayer to ascend to the heavens unimpeded. ^*

"The Pantheon is defined by its oculus, a bright opening in the center of the dome. It renders the space an inside-out sundial: the shaft of light that shines through the oculus slowly moves across the dome over the course of the day. A well-trained eye can tell the month and the hour by the slant of light - but not, I think, the year. Time collapses inside the Pantheon. While most Roman architectural achievements are meant to communicate the civilization's mastery over nature, the Pantheon maintains a deep connection to it, to the essential elements of light and sky." ^*

posted by ericb at 7:46 PM on March 31, 2007

the hole in the pantheon is an optical illusion, that is to say, while standing inside, it appears to be level (parallel) with the ground, when it fact it is built at a steep angle

Um, no. In fact it is not. Do you people just write any random twaddle that pops to mind?
Also, I've been rained on inside the Pantheon. That's why there are drains in the floor. Sheesh.
posted by poxuppit at 9:05 PM on March 31, 2007

The oculus is indeed parallel to the ground. Yes it does rain inside the structure; any air pressure/stack effect issues are negligible considering the size of both the building and the diameter of the opening. Don't believe everything your guide tells you. I should know. I used to be one.
posted by hydatius at 11:09 PM on March 31, 2007 [1 favorite]

Paging Adam Savage.
posted by Ironmouth at 11:31 PM on March 31, 2007

yes
posted by longsleeves at 11:55 PM on March 31, 2007

I, too, have been rained upon in the Pantheon. So no, this is not true. :)
posted by antifuse at 4:29 AM on April 1, 2007

.
posted by Alt F4 at 6:22 AM on April 1, 2007

Poxuppit and Antifuse - maybe you were the only 2 in the Pantheon at the time, which is why you got wet? (I keed)

If so many people are able to create that much updraft, then they are also likely to be able create their own micro-climate and maybe even rain - from condensate accumulating on the ceiling and ultimately falling back down on them.
posted by strawberryviagra at 6:39 AM on April 1, 2007

@ antifuse. Large crowd or small? :)

How large is this "small" opening?
From here, a drop falls at 5m/s. To get air moving at that speed up, out of a 1m diameter hole, (making some rough assumptions), you would need ~8200cfm. That's not insignificant. I use a 2hp fan to drive air out at 8500cfm through an opening of equivalent size. Now, my assumption that I need 5m/s air is probably very generous, I don't know what it would take to deflect a raindrop.
Of course, the bigger the hole, the more air you would need to drive out at that speed. Driving this based on temperature generated by crowd size, I'm quite skeptical. This doesn't mean that the chimney effect can't change things though, but I don't feel like figuring out what the airspeed over the hole would need to be to drive air out of the building at a significant speed.
posted by defcom1 at 6:54 AM on April 1, 2007

This guy claims it's the rain that falls at an angle. This guy has been rained on in the Pantheon "often."

I don't buy the stack effect - I think paulsc is wrong - but I agree it'd make a good Mythbusters episode, because I think the experiment is necessary.
posted by ikkyu2 at 2:56 PM on April 1, 2007