Wikipedia:Reference desk/Archives/Science/2014 September 26

= September 26 =

Is 0° dew point a special number?
If the airport says it's 1°C and 0° dew point or less near sunrise (no pre-existing ice crystal nuclei, snow patches etc.), what H2O can freeze/sublime? (at the same distance from downtown, in a level region (cause even tiny low spots are colder and mountains complicate weather)). The blackbody temperature of the sky must be very low, metal is cold and has microscopic nucleation sites, evaporative cooling cools things, there might be a lot of solid water, right? Sagittarian Milky Way (talk) 01:47, 26 September 2014 (UTC)


 * Both zero and one degree are well below freezing. What are you on about? μηδείς (talk) 04:55, 26 September 2014 (UTC)
 * Read it again. Hint: "°C". --65.94.51.64 (talk) 05:33, 26 September 2014 (UTC)


 * Please ignore User:Medeis'S unhelpful comment. 1°C is above freezing, 0° dew point is right on the edge of freezing and melting. There is very little difference between the two, so any clouds are likely to be very low, and there could be fog in the area. Frost may form as the ground temperature could drop below the air temperature and dew point. Because there is little difference between dew point and air temperature, the humidity is high, and there won't be much cooling by evaporation, there won't be much evaporation.  Note that the temperature could vary at different places. Graeme Bartlett (talk) 05:43, 26 September 2014 (UTC)
 * I'm sure I've seen the "opposite" kind of question asked and answered here, where the temp can be BELOW 0 degrees, yet there be liquid puddles around. The answer is usually because the ground is a very good insulator and even if the air is below 0, the ground can remain at above freezing for a long time. Not sure about the other way around as the OP is asking here, but i don't see any difficulty imagining local conditions, wind chill, radiation etc that could cause frost or even ice when the air remains just above freezing. Vespine (talk) 06:09, 26 September 2014 (UTC)
 * Can a wet rag freeze in the desert when you don't feel cold? How warm can it feel and still have a wet rubbing alcohol rag freeze (70 or 91% isopropyl)? What do they use when an ordinary wet bulb thermometer would freeze? Sagittarian Milky Way (talk) 00:24, 27 September 2014 (UTC)
 * to answer the the third question there. We use either a muslin sleeve (sock) or a coating of ice depending on how far below freezing it is. From 0 C to -10 C the sleeve is used and after that the ice coating. Section 5.4.6.3 Removal of muslin sleeves is the part of MANOBS with the information. You can see a muslin sleeve on the top thermometer in the picture. A dewcell works slightly different. By the way the desert I live was not cold this afternoon, about -5 C and a wet rag would have easily frozen. CBWeather, Talk, Seal meat for supper? 03:23, 28 September 2014 (UTC)
 * I meant with a T-shirt and jeans of course. I'd call 10°C the limit of doing that, and only with brisk walking and low wind. In fact, I now recall Medieval Muslims making 40 or 32F with drying alcohol at room temperature. Though maybe the best weather to try to freeze water in short sleeves and feel mild in is ~11°C wind chill and I doubt the scale goes that high so I didn't specify a temperature.
 * Also see freezing point depression. Water on the ground frequently isn't pure water, and solutions freeze at a colder temperature than pure solvent.  -- Jayron  32  10:44, 26 September 2014 (UTC)
 * Well if seawater (3.5% saline) starts to freeze at -2°C then fresh dead vertebrates at least should do so at -1/2°C, and the internet says solutes that don't dissociate have half the effect of salt so the FPD is small. People care about this mostly in the context of first and last frosts (plants), which tend to occur toward the middle of cold air masses so low wind. Dew would make the humidity % high, the ground might not have time to cool to freezing. All put together, maybe that might make the gardening convention of light frosts being air at freezing not that bad? Sagittarian Milky Way (talk) 00:24, 27 September 2014 (UTC)

I just realised what the OPs very first question was. Did you read dew point or psychrometrics? You could have a temperature of 10 C and a wet-bulb temperature of 5 C. This would give a dew point of -1.5 C and a relative humidity of 45%. Even though the DP is below freezing nothing will freeze because the temperature is well above freezing. Also there is not much chance of cloud as the air is too dry. Twice a day we release a radiosonde and one of the things it measures is the RH. From the RH we can make a more informed guess as to the height of clouds. So we pass on RH peaks that exceed 80% and ignore lower numbers. CBWeather, Talk, Seal meat for supper? 03:47, 28 September 2014 (UTC)

Wankel engine
How does the rotor (A) in a Wankel engine drive the output shaft (B)? The drive shaft rotates 3:1 with the rotor so it cannot be directly connected. According to our article it is via an eccentric wheel and eccentric (crank-shaped) shaft. This arrangement is not shown in our animation. The eccentric wheel is shown in blue in this animation and the wheel and eccentric shaft are shown in a three-dimensional animation on this page. But neither of these, nor any other site I've looked at, satisfactorily explain how the rotor is connected to the eccentric. SpinningSpark 01:41, 26 September 2014 (UTC)


 * The 'crankshaft' has a bearing surface that the rotor is free to spin on. The internal gear is recessed into one side of the rotor. The matching external gear is fixed to the engine case. Here are some pictures that may help:

Wankel crankshaft. Rotor, case and crank. Several diagrams here, most helpful one is under the heading "Eccentric Output Shaft." 50.53.245.175 (talk) 03:11, 26 September 2014 (UTC)


 * I don't think the gears have anything to do with how the eccentric wheel gets turned, but never mind, I think I understand it now. SpinningSpark 14:51, 26 September 2014 (UTC)


 * I had the same difficulty you did understanding how motion was imparted to the central gear, until I saw this: Wankel engine rotor, gears and output shaft in oblique view. Help any?  The central gear, as it's turned by the rotor's inward facing ring gear, precesses along an eccentric path, exerting leverage on and turning the output shaft, transferring power outside the engine.
 * The animation in our article doesn't show that, and one can be forgiven for thinking the central gear doesn't have to rotate to let the rotor move around it. It's the idea I got from that animation, too - it doesn't show that the central gear is actually located on an eccentric output shaft, or that its motion is ALWAYS off-center to the rotor, exerting great leverage on and power to the shaft.   loupgarous (talk) 15:54, 26 September 2014 (UTC)
 * The central gear is fixed relative to the engine casing and cannot be responsible for turning the output shaft. It is only there to give the rotor something to turn around.  The eccentric shaft moves entirely independently of it, it goes through the centre of the fixed gear only because the two need to be on the same axis.  You are right that the eccentricty of the rotor is responsible for the torque on the eccentric shaft, but it cannot be directly coupled because their is a 3:1 ratio in rotational speeds.  There is another component, not shown on the diagram, the eccentric wheel, which is held to the rotor on the same eccentric path, but is free to rotate relative to the rotor centre.  It is this wheel that pushes the eccentric shaft around.  It is 3:1 because the rotor cycles through its eccentricity three times for every rotor revolution. I did not understand this when I first posted, but have now answered my own question. SpinningSpark 18:09, 26 September 2014 (UTC)
 * As long as we're all happy... thanks for the catch re: the central gear vs. the eccentric gear. loupgarous (talk) 00:14, 27 September 2014 (UTC)
 * http://www.youtube.com/watch?v=gcrVxI0Rk-w --Kharon (talk) 07:07, 27 September 2014 (UTC)

Canada Goose collar
Why do Canadian Geese have a white patch on the side of their heads? IkarosMyAngel (talk) 13:19, 26 September 2014 (UTC)
 * Evolution. -- Jayron  32  13:35, 26 September 2014 (UTC)
 * I suppose the OP is asking (or could be asking) what evolutionary advantage there is to the white collar. I didn't see anything about it in the article, so the OP may need to do a Google search on the subject. ←Baseball Bugs What's up, Doc? carrots→ 14:10, 26 September 2014 (UTC)
 * with below: Camouflage explains some of the broad trends; explaining the specific advantages provided by animal coloration is something that goes back in the literature all to Darwin himself. Reading the articles I linked here, and sources mentioned in those articles, may lead the OP in the direction they are looking for.  -- Jayron  32  14:17, 26 September 2014 (UTC)
 * While Jayron is correct, the terse answer can be unpacked a bit to give more explanation of what role the patch might play, and why it might be advantageous to have white patches. A key concept to remember: not all traits of an organism are adaptive traits. While specific studies are occasionally done to determine to what extent a certain trait is adaptive, in general we can't know all the conditions that led to such a trait. For example, the white patch could be a result of sexual selection, or it could be an example of background selection, or even just genetic drift.
 * As for the adaptiveness and function of this specific trait: this scientific article reasons that the white patch helps to aid in visibility of head movements, which are used to communicate things like intent to fly, social status, and other things that help a flock of geese survive . Though science can never prove anything in the strong sense of the word, the claims of the article are supported by observations and reasoning of an expert behavioral ecologist.
 * If we assume that this claim is basically correct, that the white patch helps the survival of the flock as a whole, then things like group selection or kin selection may also be at play. I'll also note that many other social geese use head motions to communicate (e.g. Bar-headed_goose, Nene_(bird), Greater_white-fronted_goose, perhaps most of the Anserini) and they also have contrasting patches on the head/neck. So the idea that white patches are adaptive traits that function to aid in communication is at least parsimonious. If you use google scholar, you can see what papers have cited the reference I gave. Perhaps more recent work has elaborated the function, or cast the claim into doubt... SemanticMantis (talk) 14:14, 26 September 2014 (UTC)
 * I suspect it just helps to distinguish it from other species. It's important to be able to identify others of the same species at a distance, for mating purposes, etc.  Having a nice white patch may also be an indicator of health, used to find good mates.  Birds are widely known for "decorative markings" which have no apparent purpose other than what I've mentioned (and perhaps communication, as noted above). StuRat (talk) 14:17, 26 September 2014 (UTC)


 * While I am tempted to say they reason they have white patches on their heads is because they don't have pockets to put them in, the reason is actually because they belong to the white-cheeked geese family. See Evolution and taxonomy of White-cheeked Geese. μηδείς (talk) 17:14, 26 September 2014 (UTC)
 * Interesting link, thanks. From that review of the book: "Anderson did not undertake any genetic work" - meaning that this new-ish taxonomy isn't supported by any systematics, though of course it might turn out that this guys taxonomy turns out to later also be supported by genetic work.
 * Also, of the 15 so-called 'species' of geese presented, "9 species ... correspond to Greater Canada ... and 6 species ... Lesser Canada/Cackling Goose." So in effect, this article says that they have white cheeks because they are what is commonly known as Canada Geese, either of the greater or lesser variety. The whole book is about attempting to resolve (sub)species within the greater/lesser Canada goose complex, not about how the Canada goose is related to other (i.e. non-Canada) geese. SemanticMantis (talk) 19:42, 26 September 2014 (UTC)
 * Yes, I was reticent to use the word family, which is why there's no link. I could have said clade.  A google for white cheeked goose gives a ton of info. μηδείς (talk) 01:18, 27 September 2014 (UTC)

Help with a calculation
Given a thousand identical semiconductor atoms in a 10 x 10 x 10 cube, with 100 nanodiodes connecting in from one side and 100 connecting out from the opposite side (each connected to one atom), and insulation around the four other sides, what's the formula for the pattern of electrons emerging for each input pattern? Hcobb (talk) 15:57, 26 September 2014 (UTC)
 * Individual electrons cannot be tracked. See uncertainty principle.  -- Jayron  32  16:00, 26 September 2014 (UTC)
 * The "nanodiodes" are the chief determinants of the pattern. All the semiconductor atoms do is impose a "forbidden zone" of voltage and current into which the electrons cannot move within this lattice you've described.  But the "nanodiodes" already do that.  Your experimental scheme is low on information about the semiconductor lattice you're describing, but I'd expect current to flow through it in the direction dictated by the "nanodiodes."  loupgarous (talk) 16:16, 26 September 2014 (UTC)
 * Diodes cannot be made from identical semiconductor atoms: they require impurities called dopants.--Phil Holmes (talk) 17:09, 29 September 2014 (UTC)

nut allergy
This is not a Qs, merely possible information update in regards to peanut allergies. Initial results of a recent Oxford study shows an increased allergic reaction to roasted / heat treated peanuts, which is contrary to information on nut allergy page. — Preceding unsigned comment added by 166.179.68.31 (talk) 19:45, 26 September 2014 (UTC)


 * Here is a link to the study (make sure to click on full text), if anyone wants to update the page: http://www.jacionline.org/article/S0091-6749%2814%2901031-8/abstract Justin15w (talk) 21:42, 26 September 2014 (UTC)

It only grows to fit the size of its enclosure
Is this actually true for any animal? I've heard it said for fish, snakes, turtles and crocodiles for sure - and it seems to be one of those pieces of 'common wisdom' that is usually proven wrong, eventually leaving an animal keeper with a large problem on his/her hands. --Kurt Shaped Box (talk) 20:03, 26 September 2014 (UTC)


 * This paper from 1960 concludes that "Growth rates and size at metamorphosis of Rana pipiens tadpoles fed maximally increase with container size over the range studied." -- so it seems to be supported by at least one animal, but density of conspecifics and food availability will also play a role. SemanticMantis (talk) 20:32, 26 September 2014 (UTC)


 * Well, it's certainly true of plants. After all, how could they grow larger than their enclosure, unless they break through it ?  As for animals, some may just grow too big, have their circulation cut off, and die. StuRat (talk) 22:38, 26 September 2014 (UTC)


 * Most fish and reptiles have indeterminate size; as long as they are alive they continue to grow, but at a slowing rate. Mammals have a determinate growth size based on the growth plates on their bones, combined with nutrition and so forth.  News of snakes too big for their containers eating their masters and the police being called to remove 6' alligators from 5' bathtubs is not all that rare.  Mammals kept in cages that small end up horribly maimed, as often happens with apes and great cats reared as pets. Animals like tadpoles are a strange case since they need to survive to metamorphose before their pool dries up or they get eaten by predators or relatives--a sensitivity to crowding makes sense here. μηδείς (talk) 00:57, 27 September 2014 (UTC)


 * Thanks for the answers. I've certainly heard the stories of alligators/crocs that have been bought as tiny youngsters and stubbornly refused to adhere to the 'they'll only grow big enough to fit the tank you keep them in' advice provided by the seller. I personally know someone who was told this about a snake - which he still has, despite it being huge now, and has ended up spending a large amount of money on housing it comfortably and safely. Snapping turtles are another one. Hmmmm. It would seem absolutely ludicrous for someone to say the same sort of thing about a mammal, wouldn't it? --Kurt Shaped Box (talk) 06:49, 27 September 2014 (UTC)


 * Well, mammals growth can be stunted by poor nutrition and a few other things, but for a tight space to limit growth, we're talking about something like foot binding, which causes deformity. StuRat (talk) 19:32, 27 September 2014 (UTC)


 * Yes, that was my point. There was a recent news report of a lioness held in too small a cage.  When she was rescued her paws were permanently crippled. Quite disgusting, and makes you want to punch people.  This is not a limitation of growth to the space allowable, it is deformation due to abuse. μηδείς (talk) 01:29, 28 September 2014 (UTC)

Paddle shifter on antique car
Two weeks ago I saw an old car parked round the corner from my work. A big one with a soft top and a leather strap holding the bonnet down. I think it was a Daimler. The top was down and I could see inside, and I noticed a tiny gearstick on a stalk off the steering column. It was clearly marked with the gear directions (4 forward and 1 reverse) but it was far too small to be a normal mechanical gearstick. You could have moved it with your thumb. How might it have worked - some sort of hydraulic effect? Surely there were no fly-by-wire electronic controls in the 1930s? 129.67.118.198 (talk) 22:01, 26 September 2014 (UTC)


 * Perhaps a preselector gearbox? DuncanHill (talk) 22:03, 26 September 2014 (UTC)


 * I just looked at that article actually. Maybe I misunderstood, but would a preselector allow for controls that weren't mechanically connected to what they were controlling? I wish I'd had my phone with me when I saw this car... that picture doesn't clearly show anything but the steering wheel. 129.67.118.198 (talk) 22:19, 26 September 2014 (UTC)


 * If you click on the picture it gets bigger, and you can clearly see the preselector in the upper right hand quadrant of the steering wheel. I think it would have had a cable connexion. DuncanHill (talk) 22:25, 26 September 2014 (UTC)


 * There is more information, and are more pictures, at this link. DuncanHill (talk) 22:28, 26 September 2014 (UTC)
 * Thanks, I didn't know pictures here worked like that. In the full version I can see what you mean. A bit different from what I saw but I'm sure it could have done the same job. Were they electronic though, like today's paddleshifts? 129.67.118.198 (talk) 22:37, 26 September 2014 (UTC)
 * Certainly not electronic. Remember, they didn't even have transistors, and the tubes then commonly available were nowhere near robust enough for use in a vital part of car. --65.94.51.64 (talk) 07:30, 27 September 2014 (UTC)
 * Apart from the risk of the car jumping to life if you start it with a gear engaged, why did this system fall out of use? It seems like a much easier way to drive. Was it much more fragile than today's manual transmissions? 129.67.118.198 (talk) 22:40, 26 September 2014 (UTC)


 * Don't confuse the 1930's preselector with the modern paddle shifters. With a paddle shifter, there is just one operation - flip the paddle.  With the 1930's style gizmo, you preselected the gear you would want next with the hand lever, and then pushed a foot pedal in and out to make it happen.  Since engines of the time were kinda crappy, you'd also have to use the gas pedal to get the revs of the engine to roughly match the new gear or very bad things would happen.


 * The preselector has some definite advantages, and in the 1930's, before the invention of synchromesh (early 1950's), it was a MASSIVE advantage. Having to learn to use a clutch is bad enough on a modern stick-shift car - but having to learn to double-declutch is a nightmare!


 * The likely reason it fell from common use was that it required two distinctly consecutive steps - one to move the hand lever and another to depress and release the foot pedal while also adjusting the accelerator pedal to match the new gear. With a modern synchromesh shifter and a moderately expert driver, the clutch-in/shift/adjust-gas/clutch-out is a smooth overlapping motion.  I'm pretty sure you can shift more rapidly with synchromesh than with a preselector.


 * SteveBaker (talk) 14:53, 27 September 2014 (UTC)
 * Minor factual correction - the first car to be fitted with synchromesh was the Cadillac 341 (No article! Even though Al Capone had one!) in 1928 (see Hydramatic, which is not the first place I would have looked). The Porsche 356 (1952) was the first car with the modern cone-and-ring synchromesh system, but the basic principle is rather older. Tevildo (talk) 15:58, 27 September 2014 (UTC)
 * Harumpf... The German WP has an article: — And Italian:   (What's "harumpf" in Italian?) —71.20.250.51 (talk) 17:25, 27 September 2014 (UTC)