Science desk
< December 6	<< Nov \| December \| Jan >>	December 8 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

December 7

Mountain gaps

According to its article, Virginia's Rockfish Gap in the southwest-northeast Blue Ridge is a wind gap. According to its article, a wind gap is essentially a dry water gap, a kind of mountain pass in which a river flows all the way through the mountain uninterrupted, thus producing a pass with no elevation change. All very nice.

If you drive U.S. Route 250 over the Rockfish Gap, as I do every week, you'll gain significant elevation in a short distance; less than a mile southeast of the 1903-foot col is a location below 1000 feet, the terrain drops rather fast to the northwest, and all in all the col is far higher than anything else around, except of course the ridge to the northeast and southwest. How can it be a wind gap when it's so far above everything else? Dry up a water gap, e.g. the James River Gorge near Glasgow, Virginia, and you get a canyon whose "summit" is essentially the same elevation as the ground at the base of the mountain (otherwise the river couldn't flow through; you'd get a pair of box canyons), not something where the nearest low ground is 900+ feet below the summit of the pass. Nyttend (talk) 04:03, 7 December 2016 (UTC)[reply]

One possibility is that geologic uplift, such as tectonic uplift, occurred after the gap was formed. StuRat (talk) 04:35, 7 December 2016 (UTC)[reply]

Yes see 3D Fold Evolution for some example of elevated wind gaps forming. Graeme Bartlett (talk) 06:37, 7 December 2016 (UTC)[reply]

This book has a description and some nice diagrams that seem to answer the OPs questions very nicely. --Jayron 32 10:08, 7 December 2016 (UTC)[reply]

The whole point of a wind gap is that the water used to flow, creating a gap at the level of a valley, but when it stopped, the mountain began to be recreated by differences in erosion again. Note that the Appalachian Mountains aren't really the remnants of mountains in the naive sense that some of the height hasn't been worn down yet - rather, there are layers of rock embedded in anticlines and synclines that form a few weird little loops and a lot of long lines, and the differential erosion keeps them always present at a certain height, barring something like a river crossing them. Look at a satellite map and it should become clearer. Wnt (talk) 12:01, 7 December 2016 (UTC)[reply]

Alleghanian orogeny explains a bit more details about Wnt's statements. If one wanted to know more, they should research that topic. --Jayron 32 13:02, 7 December 2016 (UTC)[reply]

Air pollution and what kind is the worst for your health

All things being equal, which is worst?

PM2.5
O3
NO2
SO2
CO

Many thanks.

Anna Frodesiak (talk) 05:16, 7 December 2016 (UTC)[reply]

PEL=permitted, REL=recommended, IDLH = immediate danger

substance	PEL ppm	REL ppm	IDLH
O3	0.1	0.1	5
NO2	5	1	20
SO2	5	2	100
CO	50	35	1200

Looks like carbon monoxide is the "friendliest" poison gas. But as it has no odour, you may not notice. Graeme Bartlett (talk) 06:49, 7 December 2016 (UTC)[reply]

PM2.5 standards are about 100 to 1000 times lower in mass. So it suggests that is the worst, but I suspect it will be ozone that kills the quickest (50ppm in 0.5 hour). Graeme Bartlett (talk) 07:00, 7 December 2016 (UTC)[reply]

Thank you so, so much for taking the time on this one, Graeme. You have been really helpful. We have been getting a lot of PM2.5 over here -- in fact around the whole country. My very best wishes, and several people here reading this thank you also!!!! You're aces in my book. :) Anna Frodesiak (talk) 07:42, 7 December 2016 (UTC)[reply]

Yeah, China's air pollution sucks. [1] Dragons flight (talk) 08:04, 7 December 2016 (UTC)[reply]

For most of the world, particulate matter (e.g. PM2.5) is the most significant pollutant, accounting for 75-95% of air pollution related mortality. Most of the balance is associated with O3. For PM2.5 it is the effect of chronic exposure that is most important (leading to increased rates for stroke, heart disease, lung cancer, and other illnesses), while for O3 we tend to worry more about the effects of acute exposure (e.g. asthma and other breathing difficulties). NO2 and SO2 are almost never high enough to pose a significant direct mortality hazard, though people can become aware of them (e.g. reporting unpleasant / uncomfortable odors) at concentrations that are still well below the level that poses a direct mortality risk. By contrast, PM2.5 can significantly impact mortality risk even at levels that ordinary people don't perceive as unpleasant. As a result the health impacts of PM2.5 are often higher in many countries than local residents might expect. One should note that in addition to health impacts, there are also significant environmental impacts from many forms of air pollution (e.g. acid rain from SO2). Dragons flight (talk) 08:00, 7 December 2016 (UTC)[reply]

Emigrate to Finland before it is too late Count Iblis (talk) 08:22, 7 December 2016 (UTC)[reply]

I live in the Smoky Mountains. The air is nice and clean (except for the recent rash of forest fires). However, there is a huge problem with indoor air pollution from radon. My understanding (I haven't researched it) is that radon is a very nasty pollutant. 209.149.113.4 (talk) 15:54, 7 December 2016 (UTC)[reply]

Maybe. As noted at Radon, it can be a serious risk factor for lung cancer; it is the second greatest linked cause of lung cancer after cigarette smoking. That being said, the only known group for whom Radon exposure is significant enough for risking lung cancer is Miners, who spend a long time underground in stale air that isn't often recycled at the surface. Back in the 1980s, there was a major Radon scare, as people began to worry about radon leaking into homes through basements and getting trapped in the house. As noted here however, few studies have found any significant risks from radon exposure in the home; it's possible if you a) lived in a basement with b) very poor ventilation, you COULD be at a higher risk of radon exposure; but most people live in homes that are "leaky" enough to avoid building up radon to dangerous levels. Which is not to say that there is no chance, but for most people, it shouldn't be a concern; except for people who run companies willing to charge you thousands of dollars to "seal" your foundation against radon getting in. They want you to be very scared. The science, (see source) is less sure. --Jayron 32 18:26, 7 December 2016 (UTC)[reply]

Jayron's radon link is to a book on sexually transmitted diseases (see next section). Useful information can be found at [2] and [3]. 86.176.120.111 (talk) 19:59, 7 December 2016 (UTC)[reply]

So corrected. --Jayron 32 21:24, 7 December 2016 (UTC)[reply]

Thank you all very, very much!!! Anna Frodesiak (talk) 03:33, 9 December 2016 (UTC)[reply]

Sex with a Hepatitis C positive woman during her period

There is some controversy as to the degree to which Hepatitis C is sexually transmittable in bodily fluids other than blood. My question is, has anyone considered the issue of the risk involved to a man who has penile-vaginal sex with a Hepatitis C positive woman during her period, at a time when she is experiencing significant blood flow, such that her blood will inevitably get on his penis? I have no plans of having sex with such a woman, and to avoid "medical advice" problems, probably best if we can stick to linking to sourced answers which have looked at this question. (It would likely be of significant concern to the sex industry, where a significant portion of the prostitutes are IV drug users, and thus not infrequently get infected with Hepatitis C. On the other hand, many prostitutes either take a break during their period, are on the pill or implant and thus don't have a period, and/or universally insist on condom use. No, I am not involved whatsoever in the sex industry). Eliyohub (talk) 14:23, 7 December 2016 (UTC)[reply]

Some reading material: [4], [5], [6], [7]. --Jayron 32 18:19, 7 December 2016 (UTC)[reply]

All other things equal is building fire a bigger or smaller risk at high altitude?

(moved to Science Desk)

Especially as high as La Paz/Lhasa a fire would more easily use up oxygen till you black out right? Woe to those who are lowlanders and unacclimated. Would low O₂ balance things out though by making fires grow slower? Sagittarian Milky Way (talk) 19:40, 5 December 2016 (UTC)[reply]

This sounds like a science question to me, not a humanities question. I don't know the answer, but perhaps you should switch desks? Eliyohub (talk) 04:46, 6 December 2016 (UTC)[reply]

Note that breathing-wise, the lack of oxygen isn't the usual problem in a fire, it's the presence of toxic fumes. So, with that in mind, I suspect that the fire-retardant nature of thin air would make fires somewhat less dangerous at altitude. It would be interesting to find stats that support or counter this. StuRat (talk) 16:51, 6 December 2016 (UTC)[reply]

Think, the basics should be always kept in mind in answering this. The percentage of oxygen remains the same regardless of which floor one is on. The lowering of ambient pressure however, lowers the thermodynamic efficiency of combustion (think of it the other way around where the higher the compression ratio in an infernal combustion engine, the higher the efficiency). Also, the lowering of pressure, lowers the partial pressure between O2 and CO2 exchange in the lungs. So one may start panting to lower the blood CO2 only to inhale more toxic fumes. Modern furniture also tends to emit cyanide (which inhibits haemoglobin efficiency) therefore the nature of the fuel (in this hypothetical towering inferno) must be considered. There is nothing like an in vivo experiment to settle the matter – any volunteers?--Aspro (talk) 21:56, 7 December 2016 (UTC)[reply]

NFPA has a statistics on structure fires here [8]. Here [9] are all their pages that mention 'elevation'. You can contact their Fire Protection Research Foundation representative or statistics and data representative directly here [10]. SemanticMantis (talk) 22:04, 7 December 2016 (UTC)[reply]

Those reference appear to be concerned only with the hydrostatic pressure head of water not air. This form of liquid monoxide tends not to support combustion (although I am willing to be corrected, e.g. aluminium swarf fires).--Aspro (talk) 22:38, 7 December 2016 (UTC)[reply]

I think you are probably right, though I haven't checked every instance. While not related to O2 concentrations, water pressure, and the change due to elevation, is still an important factor in the eyes of the NFPA, and thus I thought it broadly relevant to OP's question of structure fire danger and elevation. P.S. OP: if you're interested in altitude/elevation effects on wildfire, let me know; there is tons of classic and ongoing research in that area, and I'd be happy to supply scholarly refs. SemanticMantis (talk) 23:32, 7 December 2016 (UTC)[reply]

I would be curious what elevation does to wildfires. What have people found? Sagittarian Milky Way (talk) 15:18, 8 December 2016 (UTC)[reply]

It would be difficult to judge, since vegetation and moisture levels are also different at altitude. So, you're comparing apples to oranges. StuRat (talk) 15:22, 8 December 2016 (UTC)[reply]

There is no one simple answer, and this is not something that we can wave our hands on and come up with something useful from first principles. I'd recommend starting here [11], where a mathematical modal of probabilistic fire risk is presented by some folks from the USFS. You'll have to read the whole paper to fully understand it, but Eq. 4 shows how elevation enters in to the model as spatially explicit variable.

Here [12] is another model specifically focused on risk assessment. It is free to read online if you register.

Here [13] is some work about using GIS data to calculate specific risks for specific areas.

Here [14] is a fairly readable case study for CO.

Mulivariate regressions and other fancy analyses show that elevation, slope, and aspect are all very important, as are the associated changes in plant cover. Fig 2. of the CO study lays out a schema for important variables, and Fig 1 of the first link shows how in OR the fire occurrence is related to elevation.

Fire will generally burn upslope, and generally risks will be higher in rain shadows of the region. I am not an expert in wildfire, but it is something I read a lot about. To my understanding, the most general effect of elevation on fire risk that is supported by data is the joint effect of Aspect_(geography) with elevation, with slope being a close third. With those three, within a certain region, you can make a pretty OK first pass fire risk map, assuming you also know how those affect the type of dominant plant cover and precipitation. Hope that helps, SemanticMantis (talk) 16:10, 8 December 2016 (UTC)[reply]

External temperature sensors on cars

I would like to know how these work and how they interface to the electronics systems on cars especcially Vauxhall Astra. When I look up temperature sensor on Wikipedia I get directed to thermometer. This is not a sensor but a combined sensor and display. Can someone create a new page for temperature sensor that is not a thermometer but is just a sensor like a thermistor??--86.187.164.158 (talk) 23:23, 7 December 2016 (UTC)[reply]

Are you looking for something like List of temperature sensors? Looie496 (talk) 23:30, 7 December 2016 (UTC)[reply]

Sort of. But I think there should be a page on temp sensors not just a list. A sensor senses but does not 'measure'. A thermoMETER senses and displays as stated in its name (..METER). With sensors, other mechanisms/devices convert the physical sensor quantity to a reading of temperature. --86.187.164.158 (talk) 00:00, 8 December 2016 (UTC)[reply]

Different sorts of temperature sensor and how they work and their applications would be the basis of the page. The list of sensors can be incorporated into that page. Searching temperature sensor should DEFINITELY NOT send users to the Themomoeter page! A theromoeter is NOT a sensor.--86.187.164.158 (talk) 00:05, 8 December 2016 (UTC)[reply]

A thermistor seems the most probable. Email them and just ask,.. request a circuit diagram: >vauxhall.customerassistance@vauxhall.co.uk< If you think there should be a page on this subject you can create one yourself Wikipedia:Your first article. You can then assist us to improve Wikipedia.--Aspro (talk) 00:08, 8 December 2016 (UTC)[reply]

Yeah but as for WP, I think the article currently named List of temperature sensors should be renamed Temperature sensors and a lot more stuff added in to the page. How do I do that?--86.187.163.86 (talk) 01:01, 8 December 2016 (UTC)[reply]

This would be considered a "move" I think. You could create a new article at temperature sensor (after deleting the redirect to "thermometer"), them paste in the "list of..." content, then delete "list of...". But while you can be WP:BOLD, for a move/rename like this, I think you should seek WP:CONSENSUS, at the talk page of "list of temperature sensors". SemanticMantis (talk) 17:14, 8 December 2016 (UTC)[reply]