Wikipedia:Reference desk/Archives/Science/2011 July 23

= July 23 =

About thiruchendur nazhi kinaru
what is the reason for the pure water in thiruchendur nazhi kinaru? since it has backish water in the some well and sea water near by — Preceding unsigned comment added by Panneerzeus (talk • contribs) 06:53, 23 July 2011 (UTC)


 * I don't know anything about the area, or where the wells are. But often there is a halocline in underground caverns, or in groundwater, or even in coastal ocean waters.  Here is a diagram for the area of fresh water beneath a small island, for example.  A well in the right place might tap the fresh water that runs from higher ground, even if there is salt water in a deeper well.  But I don't know that is true for thiruchendur. Wnt (talk) 22:01, 23 July 2011 (UTC)

Regarding contour
integral((1/p)exp(-(p+img.a)^2)   where img.-imaginary iota, a-constant, integeral limit -infinity to +infinity  — Preceding unsigned comment added by Md qutubuddin (talk • contribs) 07:19, 23 July 2011 (UTC)
 * If you type that expression into Wolfram Alpha, with img changed to i, you get an answer. --Heron (talk) 09:11, 23 July 2011 (UTC)
 * Using this http://www.wolframalpha.com/input/?i=+integral%28+%281%2Fp%29exp%28-%28p%2Bi.a%29^2%29%29+dp+p%3D-infinity..%2Binfinity I get integral does not converge, with a substituted with some example numbers. Graeme Bartlett (talk) 13:50, 23 July 2011 (UTC)

Scale factor with distance
By what factor does an object shrink as you move away from it? More specifically, if you are looking at a circle of radius 1 at a distance of 1 unit and you move back to 2 units, what will the radius be? I've looked around for an answer, but haven't been able to find one, I'm sure there is a wealth of information out there on such subjects, I just can't figure out the right search terms; everything seems to bring up discussions on 20/20 vision and zoom lens, neither of which are what I'm looking for. Thank you:-)

Edit: I just realized that the circle isn't the best example since if it is flat, then not every point is equidistant to you. Basically, how does size change with distance from an object? 209.252.235.206 (talk) 07:32, 23 July 2011 (UTC)


 * The article called Perspective (visual) may help. ←Baseball Bugs What's up, Doc? carrots→ 08:44, 23 July 2011 (UTC)


 * Basically, if you double the distance, the radius appears halved, the area appears one quarter of the size, and the volume appears one eighth. Similarly, if you triple the distance, radius will be 1/3, area 1/9 and volume 1/27.  The scientific principle is that apparent linear size is inversely proportional to distance; the area factor is the square of the linear factor, and the volume factor is the cube.  If you are interested in artistic representation then Vanishing point also needs to be considered.    D b f i r s   11:23, 23 July 2011 (UTC)


 * Inverse square law may be useful. --Cookatoo.ergo.ZooM (talk) 12:20, 23 July 2011 (UTC)   [[Image:Inverse square law.svg|thumb|200px|The lines represent the [[flux]] emanating from the source.]]


 * It sounds like you want a simple equation to calculate magnification. Magnification is trivially defined as "apparent length / true length."  But, this will not work for simple viewing of a distant object.  There is no "apparent length" to put into the equation!  When you view a distant object, you are seeing an angular size, not a length.  So, you can not trivially obtain a magnification relative to the object's true size.
 * The angle subtended is θ ; the distance to the subject is r; and the object's length can be denoted s; so you can use either the standard circle formula, s = r θ (which is a slight approximation); or the triangular formula, s = 2 r tan (θ/2) ≈ r θ for small angles. The formulas I presented above work with the angle subtended, not the magnification factor; you need to have a reference size or reference angle in order to compute optical magnification as a ratio.  Typically, in photography, you would compare the object's true size to the size of an image projected onto 35 mm film.  You could also compare to the image projected inside your eyeball: you would use the geometry/physiology of your eyeball's retina imaging plane and your eye lens.  In cameras, the "magnification" factor used by the marketing department is the ratio of subtended angles at the smallest and largest focal-lengths that a lens can focus at.
 * If you so desired, you could pick an arbitrary projection frustum to represent the approximate behavior of a "human eye." We use this geometric tool in computer graphics all the time to help us solve the equations that define how large an object should appear on a computer-screen.  Essentially, we "estimate" that your eye is the point of a frustum, and your computer-screen is the top "flat", and then we select an arbitrary bottom-"flat" to generate any geometry we choose.  (You may consider that to define the "zoom" or "optical magnification factor").
 * The common rule of thumb in photography is that the human eye "zoom geometry" is sort of like a 50 mm focal-length lens projecting onto 35 mm film. You'll find heated debates among photographers about whether this is anywhere close to the ballpark of the actual physiology of the human eye.  Nimur (talk) 16:18, 23 July 2011 (UTC)
 * Isnt this similar triangles? Something twice as far away has to be twice as tall to subtend the same angle. Therefore, moving something away twice the distance will half its apparant height, at least approximately. The OP appears not to have learnt trigonometry at school. This trigonometry calculator may be useful: http://www.carbidedepot.com/formulas-trigright.asp 92.28.245.233 (talk) 19:38, 23 July 2011 (UTC)
 * Notice that because the area is inversely proportional to the square of distance, and the amount of light received is also, the apparent brightness of the object doesn't depend on its distance. (The size of the object from your perspective increases in the same way as the amount of its light that you soak up as seen from its perspective)  So if you walk toward a yellow wall, it doesn't seem to get any brighter or dimmer as you approach.  (Though if you look at a partial eclipse of the sun, the sliver remaining is just as bright as the normal sun, but your iris opens up because overall the sky seems dimmer, making it much more hazardous to the retina.) Wnt (talk) 03:12, 24 July 2011 (UTC)

Nice in Israel, baking in NYC
So it's a nice 86 °F (30 °C) here in this part of Israel (a subtropical part) and 84 °F (28.8888 °C) in Be'er Sheba in the Negev (most decidedly a desert). I look at the weather back home and see that in my beloved NYC, the temperature is 104 °F (40 °C) and in Central Jersey it is 120 °F (49 °C). So, here is my question, why is it that the subtropical region and the desert are so much cooler than the temperate area at about the same time of day? What's cooking here? Sir William Matthew Flinders Petrie &#124; Say Shalom! 12:01, 23 July 2011 (UTC)
 * Because NYC (and much of the US) is presently under a record-setting heat wave exacerbated by high humidity. Note that NYC is characterized as having a humid subtropical climate, similar to the Mediterranean climate of coastal Israel.  Beersheba is in a desert region, and so is more arid, but "desert" and "hot" don't have a causal relationship -- its climate is still semi-Mediterranean.  So, based on your numbers, 86F is average for Tel Aviv and 84 is mildly below average for Beersheba.  NYC, meanwhile, is setting record highs.  This sort of discrepancy is actually quite common. &mdash; Lomn 12:26, 23 July 2011 (UTC)


 * The subtropical area I am in, which is called the Western Galilee (guess I should have said the numbers for Nahariyya) is actually quite humid in some places, especially in our digsite, Tel Kabri, and we've never had anything nearly that bad. Why are we having this heatwave in NYC and the other less important places though? =p Sir William Matthew Flinders Petrie &#124; Say Shalom! 12:50, 23 July 2011 (UTC)


 * Lots of news articles on the heat wave and its proximate causes are readily available. Here's one article on the heat wave itself and another on the terminology and scope of the system.  Note that even outside of the heat wave we're seeing significant weather effects -- temperatures at my home are seasonally hot, but weather systems are presently moving east-to-west instead of the near-universal west-to-east pattern that prevails here. &mdash; Lomn 13:01, 23 July 2011 (UTC)


 * Also, it sounds like the OP is making the common mistake of confusing "climate" with "weather". Merely because one region has a warmer climate than another doesn't mean that every minute of every day the cooler climate has to have temperatures which are lower than the warmer climate.  There are going to be local variations on any given day that can make the weather hotter in some parts of the world than their average is, and colder in other parts of the world than their average is.  While averaged over many decades, New York City may have a cooler, wetter climate than most of Israel that doesn't mean that for a few random days one summer New York City won't be much hotter than Israel.  -- Jayron  32  16:42, 23 July 2011 (UTC)

NYC's climate is moderated by its position on the coast. Its current weather is caused by a strong stationary high over the eastern US. Note that even now, it is better to be in NYC than Brumfiss, the most miserable place in the world to spend a summer.μηδείς (talk) 16:57, 24 July 2011 (UTC)
 * It didn't hit 120 in NJ as the OP mentioned. The max seems to have been 108 in Newark and the heat index might have reached 120 but the state record high temperature of 110 was not broken as far as I can tell. Rmhermen (talk) 18:48, 24 July 2011 (UTC)


 * A large, non-mountainous continental land mass nearby can exacerbate a heat wave, allowing air to warm rapidly over the dry areas and causing the heat wave to spread toward nearby coastal regions as well. The same happens for cold waves. ~ AH1 (discuss!) 16:24, 25 July 2011 (UTC)

Dang it, how do I always miss the meteorology questions?? The answer is a bit complicated, but the simple answer is hinted at by AstroHurricane. The greatest solar heating occurs over large continental landmasses such as North America. Given the right weather conditions, New York can be downwind of a large stretch of solar heating, since it is firmly within the band of mid-latitude prevailing westerly winds. The same can happen in Israel. Both places are also next to an ocean: New York is by the Atlantic, Israel is next to the Mediterranean Sea, which is a warm sea, but water temperatures are still rarely much warmer than 90 F. This often serves as a moderating influence on temperatures there. So the simple answer is that the general weather pattern was a west-southwest wind in New York, which brought a large fetch of warm air in, while Israel was under a pattern of northwesterly wind, which brought in cooler air from the Mediterranean. However, given the right conditions, parts of Israel has the potential to get much warmer than New York City. You may also be interested in the urban heat island effect, which can exacerbate heat waves in large cities.

You should also note that just because a place is a desert does not make it any hotter. Sure, deserts are often hotter during the day than comparable areas which are less dry, but these areas are also often colder at night! This is due to dry air having a lower specific heat capacity than moist air]], which allows the same amount of energy input to increase the temperature by more, and vice versa.- Running On Brains (talk) 10:20, 27 July 2011 (UTC)

Watershed between rivers
I often find myself bicycling over a river bridge, and then many miles later going over the bridge of another river. In between the two rivers are streams, valleys, and hills, and contour lines all over the place. Is there any simple practical method of finding where I cross the watershed: ie the dividing line at which a drop of water will drain either into one river or the other? Might it be for example the highest elevation on my route? 92.28.254.185 (talk) 14:19, 23 July 2011 (UTC)


 * It sounds like by "watershed" you mean drainage divide specifically (in the US "watershed" means the whole basin, not just its edge). Really every local maximum, however small, that you cross is a tiny watershed; these all shed water into one system or another, which eventually drain down either to the world ocean or to an endorheic system. Beyond that, it's kinda a matter of words about what constitutes a drainage system (usually a river) that you care about, and even with a single mountain, your scheme isn't sufficient. Imagine a plain with a single lonely mountain. Its north flank is drained by the river N and its south side by the river S. If N and S flow all the way to the sea without flowing together into a great river NS then we call the N and S systems basins in their own right; if N and S meet somewhere downstream of the mountain then the whole system is part of the NS basin (made up by the N and S basins, and by the area where water flows into the joined NS river).  So, with you standing there on one part of the peak of the lonely mountain, you can tell a little bit, just from looking, where water falling on one side or the other will go, but unless you know where all the streams and river in the area flow, you can't know just from what you can see how a given droplet will finally get to the sea. So no, you need a topographic survey and (because water often goes underground) a hydrological survey too. -- Finlay McWalter ☻ Talk 14:46, 23 July 2011 (UTC)

I meant watershed, although you apparantly call it a "drainage divide" where you are. The difficulty you described was the point of my question. I fogot to add that I'm mainly interested in identifying it on a topographic map. 92.28.254.185 (talk) 15:33, 23 July 2011 (UTC)


 * You basically want to identify the ridges. Visit this site and click on "Watershed/drainage basin delineation" for step by step instructions.--Shantavira|feed me 16:13, 23 July 2011 (UTC)


 * Yes, the exact watershed (British usage) is not always easy to identify on the cycle route because the highest point is not necessarily the exact watershed. Sometimes the local contours around the highest point on the route mean that water from both sides of the highest point on the route can all flow to the same river.  Near where I live is a meeting of three watersheds, where, in theory, a single raindrop could split into three, with some flowing into the Solway Firth (Scottish border), some into Morecambe Bay, and some out into the North Sea at the Humber Estuary.  I have tried but failed to identify the exact point on the ground.    D b f i r s   17:04, 23 July 2011 (UTC)


 * With a very flat area, like a mesa, something as subtle as wind direction might change the actual point. Also, drifting sand dunes might change it in relatively short order, too. StuRat (talk) 17:38, 23 July 2011 (UTC)


 * I would expect that the angle (syncline/anticline) and permeability of the underlying sediments should have some effect also. Wnt (talk) 22:19, 23 July 2011 (UTC)


 * Can do if the folds are recent (Zagros fold and thrust belt for example) but older mountains (e.g. Snowdon as I recall} are often old synclines. Mikenorton (talk) 09:08, 24 July 2011 (UTC)


 * In theory, at least, you could perform the watershed transform on the image that describes the terrain. --Martynas Patasius (talk) 17:43, 24 July 2011 (UTC)

Wholemeal starchy food
"If Americans could eliminate .... potatoes, white bread, pasta, white rice ... we would wipe out almost all the problems we have with weight and diabetes and other metabolic diseases" says http://www.latimes.com/health/la-he-carbs-20101220,0,5464425.story

Apart from wholemeal bread pasta and rice, and oats, are there any other wholemeal uncooked or semi-cooked starchy foods that are easily bought in supermarkets? 92.28.254.185 (talk) 15:45, 23 July 2011 (UTC)


 * What else can you make using wholemeal? And type 2 diabetes is caused by people not having enough exercise and not eating properly. You can ignore small details like eating ordinary pasta/wholemeal pasta, if you are stuffing yourself with Big Macs and watch t.v. all day long. Count Iblis (talk) 16:08, 23 July 2011 (UTC)


 * You can get sweet potatoes instead of white potatoes, and corn is easy to find in the "whole grain" form. Bread can also be found with all varieties of whole grains in it, beyond the common grains.  However, to lose weight, I'd eliminate all white flour, yes, but also limit carbs in total, getting most calories from protein and vegetable fats (and a little alcohol). StuRat (talk) 17:42, 23 July 2011 (UTC)


 * Its rather dangerous to give your own pet theory as gospel, particularly when no research backs it up. 2.97.210.203 (talk) 21:48, 24 July 2011 (UTC)


 * Hardly "my pet theory". It's been extensively studied, see our article: Medical research related to low-carbohydrate diets.  While we're only starting to get long-term results, they certainly aren't deadly diets, at least in the mild form I recommended: "limit carbs in total, getting most calories from protein and vegetable fats (and a little alcohol)".  Perhaps eliminating all carbs might be dangerous, but that's not what I advocated. StuRat (talk) 03:56, 25 July 2011 (UTC)


 * I struck the portion of the medical advice to which I thought people would most likely object. It's a really bad idea to make general statements like that. 99.2.148.119 (talk) 22:48, 23 July 2011 (UTC)


 * And I unstruck it. Dietary advice is not medical advice.  If it was, anyone writing a diet book would be required to have a medical degree.  Thus, the rule about not editing the posts of others applies here. StuRat (talk) 06:10, 24 July 2011 (UTC)


 * I'm a follower of the GL Diet, which limits the carbs you eat. Some of the foods I eat include bulgar wheat, couscous, quinoa (which I don't like so don't eat that much). I can eat potatoes as long as they are small and in their skins (such as Jersey Royal potato). Bananas are a no-no, as is pineapple, but I am encouraged to eat lots of berries. I could if I wished eat yams, cassava and sweet potatoes, but I'm not that adventurous (for an Englishwoman)! They are, however, all available in my local supermarket.--TammyMoet (talk) 17:55, 23 July 2011 (UTC)


 * Sweet potatoes adventurous ? I didn't realize that they were so novel to Brits.  I tend to look at calories versus nutrition.  In that context, sweet potatoes are much better than white potatoes, and bananas and pineapple are also good choices. StuRat (talk) 18:20, 23 July 2011 (UTC)


 * She is referring to yams and cassava also, not just sweet potatoes. 92.24.180.158 (talk) 23:24, 23 July 2011 (UTC)


 * Yams, sweet potatoes and their ilk are widely avaialable in British urban supermarkets but are (I suspect) mostly bought by members of immigrant communities. I don't consider myself to be conservative food-wise, but I've never eaten (or been served) any, and have no idea how to prepare or cook them. Alansplodge (talk) 16:54, 24 July 2011 (UTC)


 * Sweet potatoes are usually available at supermarkets in the UK - I eat them sometimes, but they are also too sweet for me. I've never eaten a yam or cassava, and are not available in mainstream supermarkets. 2.97.210.203 (talk) 21:39, 24 July 2011 (UTC)


 * My younger compatriots think I'm a fuddy-duddy. I just find them too sweet and I'd rather have a nice King Edward potato, thank you. --TammyMoet (talk) 19:11, 23 July 2011 (UTC)


 * I can see why you might find a sweet potato too sweet once covered in brown sugar, melted marshmallows, etc., but it's hard to see how a plain one would be too sweet for a Brit, considering how sweet you like your chocolate bars. StuRat (talk) 20:07, 23 July 2011 (UTC)


 * Much as I like Kit-Kats I don't put them on the plate next to my roast beef and Yorkshire puddings! Context is everything! --TammyMoet (talk) 20:11, 23 July 2011 (UTC)


 * So, to clarify, you consider sweet potatoes with nothing more than butter and cinnamon on them, to be too sweet ? (I'm talking about ones you buy in the produce section, not "candied yams" in a can.)  Note that cinnamon, being rather bitter without the added sugar, tends to counter excessive sweetness. StuRat (talk) 20:24, 23 July 2011 (UTC)


 * As far as I am aware American food has routinely much more sugar in it than British food. So what might not seem too sweet to an American can to a Briton. 92.28.245.233 (talk) 20:37, 23 July 2011 (UTC)


 * Candied yams, if they exist in the UK and not sold in American food isles or something similar are unlikely to be sweet potatoes. And who the heck puts marshmallows or brown sugar on sweet potatoes? Nil Einne (talk) 21:02, 23 July 2011 (UTC)


 * Lots of Americans do: . While I'm aware that yams are not sweet potatoes, I wasn't sure if a Brit would know, hence the question. StuRat (talk) 21:08, 23 July 2011 (UTC)
 * Yam in American English often means sweet potato, as is explained in the articles. So-called true yams, the ones from Africa, are not widely available in the States, but are more likely to be found in Britain, so actually they're more likely to make the distinction than we are. --Trovatore (talk) 21:13, 23 July 2011 (UTC)
 * At least they are available in the US I presume. Unlike here in NZ where I've never seen any sort of true yam (only Oxalis tuberosa). Although looking now I find which suggests they probably exist (unless the person is using some sort of powder or something) and various sources    suggest they were once cultivated by the Māori Nil Einne (talk) 21:31, 23 July 2011 (UTC)


 * (ec) Well TammyMoet was discussing sweet potatoes (they did mention yams but as distinct from sweet potatoes) so it seems rather unlikely they were thinking of candied sweet potatoes sold as candied yams when they said sweet potatoes were too sweet. Nil Einne (talk) 21:16, 23 July 2011 (UTC)


 * I'm just so baffled by how anyone can think sweet potatoes without added sugar are too sweet, that I'm grasping at straws trying to figure it out. I wonder if carrots, beets, and corn are also "too sweet" for Tammy. StuRat (talk) 21:44, 23 July 2011 (UTC)
 * Other countries don't stuff so much sugar in their food as the US does, so thats probably the reason. 2.97.210.203 (talk) 21:46, 24 July 2011 (UTC)
 * I can see how the sweetness could be distracting paired with certain meats, probably ones I don't eat anyway. Italians for sure wouldn't like that (try ordering lemon chicken in a Chinese restaurant in Rome, to see how the local culture influences the interpretation).  It does seem a little incongruous coming from the country that invented boiled lamb with mint jelly, though.  --Trovatore (talk) 21:50, 23 July 2011 (UTC)
 * I add salt to my vegetables and not sugar. This is common in the UK. Carrots and sweetcorn are OK but beetroot are too sweet without vinegar. I'm used to eating lamb with mint sauce, which is made with mint, water and vinegar - not sugar! Never heard of candied yams or candied sweet potatoes! As I said earlier, some of my younger compatriots think I'm a fuddy-duddy. But I'm not alone. Maybe the American propensity to put sugar with everything is what is leading to their obesity epidemic (and the OP's comment, to bring this back on topic). --TammyMoet (talk) 07:37, 24 July 2011 (UTC)
 * Yams (sweet potatoes) cooked with brown sugar and/or honey, and sometimes marshmallows, is a very common American dish at Thanksgiving and Christmas dinner. Personally I like everything except the marshmallows.  No idea how the marshmallows got to be popular; to me they ruin the dish. --Trovatore (talk) 21:07, 23 July 2011 (UTC)

I suppose the quote above is another way of saying that you should eat low Glycemic index foods and not high GI foods. See also Insulin index. 92.28.245.233 (talk) 20:02, 23 July 2011 (UTC)


 * Type 2 diabetes is likely caused by eating a high fat, low carb diet. Since low carb diets have become popular in the US more than 20 years ago, the obesity and diabetes rates have skyrocketed. People in the US seem to think that potatoes are not healthy while having no problems eating a huge steak for dinner. The last time I was in the US, I had difficulties getting normal foods in restaurants. One day I ordered a potatoes and steak dish, but what I got was a huge steak and just a few potatoes. When I asked for a lot more potatoes (a full plate), the (very obese) waiter said: "What? so many potatoes! That's not healthy!". Then what he brought was about half of what I normally eat, so I had to order again.


 * Trying get a proper meal with bread for lunch is also quite difficult. A ham sandwich is not a piece of bread with some ham in it. What you get is a huge piece of ham with some sauce, covered by a paper thin piece of bread. The only way you can eat properly, is to go to some supermarket and buy bread yourself and then make your own lunch. I normally eat about 500 grams of bread per day and about 1 kg of potatoes. This is not a big amount at all if you exercise a lot and need to get more than 3000 Kcal per day.


 * And then just walking on the streets and seeing so many obese people everywhere is very strange. Clearly, US citizens have indoctrinated themselves that the type of unhealthy diet they eat is healthy, and that healthy foods like potatoes, bread etc. are unhealthy. The higher the obesity rate gets, the more they stick to their idea of what is healthy, making the problem even worse. Count Iblis (talk) 16:19, 24 July 2011 (UTC)


 * Your assertion at the start is doubtful. 2.97.210.203 (talk) 21:46, 24 July 2011 (UTC)


 * I suspect you visited either NYC or Los Angeles, and ate at some "chic" restaurants, because the "bread and butter" restaurants here in the Midwest push bread like it's going out of style. Many restaurants bring unlimited bread with the meal, and fast food restaurants like Subway are happy to put your sandwich on an entire loaf of bread.


 * There's also an obvious problem with the logic of saying that anything Americans do must cause (or at least must not prevent) obesity. For example, many Americans drink bottled water, but I see no reason why that would cause obesity.  On the contrary, drinking lots of water, bottled or not, should actually help. StuRat (talk) 03:41, 25 July 2011 (UTC)


 * Yes, New York and Toronto. It isn't much better in Canada. The US doctrine about diet does seem relevant to me. Another isses may be that people don't get enough exercise. What's also striking is the large numbers of obese children. And they eat what their parents give them to eat. Count Iblis (talk) 16:39, 25 July 2011 (UTC)


 * Actually, one of the worst problems seems to be food provided by schools. Many of the schools are paid off by junk food makers and soft drink companies to sell unhealthy food in vending machines, etc.  The food actually provided by school cafeterias tends to be unhealthy, too, such as chocolate milk instead of plain milk (the cacao isn't the issue, it's all the sugar added with it) and lots of fried foods, with veggies few and far between.


 * There is no one "American diet". There are many different diets, based on geographic region, socio-economic class, ethnicity, age, etc. StuRat (talk) 17:16, 25 July 2011 (UTC)