Wikipedia:Reference desk/Archives/Science/2014 December 2

= December 2 =

why is it cold at the north and south poles of the earth?
why is it cold at the north and south poles of the earth?Whereismylunch (talk) 00:36, 2 December 2014 (UTC)


 * Because the sun doesn't shine as strongly there as it does at the equator. See our article on seasons for some detail. If the north pole faced the sun, it would be much hotter than the equator, and the south pole would be even colder because it would never see sunshine.   D b f i r s   00:41, 2 December 2014 (UTC)


 * I think you mean rather to say that if the Earth's rotational axis was directly parallel to the plane upon which its orbit lays, such that if the North Pole pointed directly towards the sun, then it would result in the situation you describe. Point in fact, the North Pole does face the sun sometimes and gets just as much average sunlight, adjusting for cloud cover and other climatological factors, as any other point.  In fact, during half the year it gets more sunlight than the equator.  The differences in temperature have more to do with the timing and consistency of this radiation and the way temperature is dispersed along the Earth's surface, and with the fact that snow reflects a good deal of that radiation, than it does with the amount of light that actually falls on a particular patch over the course of the year.  S n o w  talk 01:20, 2 December 2014 (UTC)
 * It actually never gets more sunlight (in terms of total energy) than the equator. It gets more hours of sunlight for a good portion of the year, but the light it gets is dimmer because the light always strikes at an oblique angle.  Sit in a dark room and aim a flashlight directly at the table.  You see a bright circle.  At the same distance, if you tip the light at an angle, you find that the brightness of the circle dims considerably.  The light from the sun strikes the poles at an angle, even at the solstice the angle is pretty pronounced, so even when the north pole is leaning towards the sun, it is still receiving less total light than the equator is on any given day.  It's the angle the light strikes at, more than anything else, which determines the average temperatures of the various latitudes.  -- Jayron  32  01:37, 2 December 2014 (UTC)


 * You're part right. Averaged over a year, the poles receive about half as much total energy as the equator.  That difference is the main reason the poles are much colder than the equator.  However, there are days (such as the solstices) when the poles actually receive more total energy per day than the equator.    Dragons flight (talk) 01:57, 2 December 2014 (UTC)
 * Thanks for that link. I stand corrected.  -- Jayron  32  02:03, 2 December 2014 (UTC)


 * Yes, good caveat -- absolutely, obliquity influences the overall amount of photons that strike a given surface point on the Earth, or any body in relation to a light source. For the starting purposes of the OP I was most interested that he first receive a correct and proper visualization as to the relative alignment of the astral bodies in question, which is what I meant to be correcting there.  But I suppose I should have said that any given point gets an even given surface point of the Earth gets a roughly equal amount of exposure to the sun, as opposed to an identical amount of sunlight.  Of course, even that isn't exactly perfectly accurate, owing to variations in surface topography.   S n o w  talk 02:22, 2 December 2014 (UTC)


 * I should have said averaged over the year, the poles get less than half the total amount of sunshine (insolation) that the equator gets. The sun is never as strong (rate of transfer of heat) at the poles as it is at the equator.    D b f i r s   08:14, 2 December 2014 (UTC)


 * I once read somewhere or other (I have heaps of sources just as reliable!) that if the axial tilt were more than 54° then the poles would be warmer than the equator. I never got around to trying the math on that one. —Tamfang (talk) 09:28, 2 December 2014 (UTC)


 * There are some good illustrations and relevant info at insolation. SemanticMantis (talk) 17:34, 2 December 2014 (UTC)


 * Yes, there's a formula that should prove Tamfang's claim, but the problem with applying just the formula is that a change in axial tilt would dramatically and unpredictably change the airflow and cloud cover, so a much more complex model would be needed to make accurate predictions about polar temperatures.   D b f i r s   19:05, 2 December 2014 (UTC)

Would a square, if I hold it perpendicular to the sun's rays, receive almost the same amount of sunlight everywhere on Earth?--Senteni (talk) 20:17, 2 December 2014 (UTC)
 * No. If you held the square at the poles, it would always receive less light per second than at the equator.  The exact amount of light it receives per second will depend on exactly where you are and what day of the year it is, but the amount of light falling on your square varies; the reason it varies is not because any part of the earth is any greater distance from the sun, but because the angle the light strikes the surface of the earth is markedly different at different points on it.  The Earth is, after all, a sphere (or very close to it). -- Jayron  32  20:23, 2 December 2014 (UTC)


 * Jayron, I think Senteni meant to hold the square perpendicular to the sun's rays.   D b f i r s   20:28, 2 December 2014 (UTC)
 * Just because that's what Senteni said? You're so trusting! —Tamfang (talk) 10:29, 6 December 2014 (UTC)


 * No, it would receive quite a bit less when the sun is near the horizon because then the sunlight has much more air to travel through before it hits your square, and it loses light and some heat as it passes through the air (more loss towards the blue end of the spectrum).   D b f i r s   20:25, 2 December 2014 (UTC)

The previous comments above answer the question perfectly. The sun doesn't hit the north and south poles, I just wanted to add another source that I found climatekids.nasa.gov/polar-temperatures/ that may seem helpful. In this post I would have liked to see more references, even though the hyperlinked to wikipedia pages were helpful to your answer, I just would like to see more references. Dcmack8 (talk) 01:06, 6 April 2015 (UTC)Dcmack8

Phenylketonuria and Obesity
Because PEA is made via Decarboxilation of L-Phenyalaline, I guess that PKU Patients have somewhat lower levels of Phenylethylamine... Now, because this Neuromodulator is associated with some forms of Appetite regulation (especially "Emotional appetite"), do you find it logical to assume that PKU subjects will have a slightly bigger chance for being overweight? thanks. Ben-Natan (talk) 02:56, 2 December 2014 (UTC)
 * Note, I ask the question after encountering some articles that have seem to contradict one eacherother. Ben-Natan (talk) 03:02, 2 December 2014 (UTC)
 * Well, what you're asking for is more in the realm of speculation on our part than sources (which you already have), which puts the request a bit outside the realm of what we are meant to be supplying here; but even putting the Ref Desks' specific guidelines out of the picture for the moment, this strikes me as the type of question that just about any neurophysiologist would decline to speculate on under most contexts, unless they had foreknowledge of, or access to, clinical findings exploring that very question. Neuromodulation of any sort is a fantastically complex process and just because one modulator has been associated with a given metabolic, behavioural, or cognitive/perceptual process does not necessarily mean one could predict an observable effect to the pathway in question as a result of a specific threshold of availability, especially not a prediction that runs cleanly and consistently in one-direction.   Would you mind providing the conflicting sources in question?  I for one feel more on solid ground assessing sources on the matter, and possibly supplying new clarifying sources, than I do speculating on a matter that would involve a massive chain of assumptions, any of which could be erroneous.   S n o w  talk 04:19, 2 December 2014 (UTC)
 * For example: 1, 2, 3, 4. Ben-Natan (talk) 15:01, 2 December 2014 (UTC)
 * , sorry for the delay in response.  I only have access to the abstracts of those papers just at the moment, but I believe that alone may be sufficient to explain the discrepancies you've been seeing.  These papers do indeed look into the issue of whether those suffering from PKU are predisposed to a higher BMI and rates of obesity, but they do not speculate as to the mechanism you were suggesting (that a reduction of phenylethylamine leads to poor modulation of appetite).  Rather the concern stems from the fact that one common aspect of treatment for PKU is a diet low in phenylalanine, which often means that said diet is high in carbohydrates, which in itself tends to predispose one to being overweight or obese.  The studies do seem to have vaguely divergent findings as to whether there is a significant degree of predisposition to higher BMI in PKU patients relative to the general population, but it's worth noting that there is a lot of variety in both the populations utilized (for both control and PKU patients), in terms of nationality, age, and sex -- so a certain degree of variety in the findings is to be expected.  My overall (but mind you very vague) impression of the overall research is that there are at least slightly higher rates of occurrence of overweight and obese states for those who have PKU, but that's not altogether shocking to me, given the typical prescribed diet.  As to the role of phenylethylamine, none of this work specifically touches upon it, nor was I immediately able to find any other research which examines the intersection between the amine, PKU, and obesity.  Hope this helps!  S n o w  talk 01:28, 4 December 2014 (UTC)

What is the reason that the covalent bonds are least specific?
I read the following sentence and I would like to get some help to understand it: "Drugs fit receptors using the lock and key model. Covalent bonds are the strongest and the least specific.". What is the reason that the covalent bonds are least specific? 149.78.231.106 (talk) 06:14, 2 December 2014 (UTC)
 * This wouldn't be a general statement, but just within the world of pharmaceuticals. Most drugs don't form covalent bonds with their targets, instead relying on things like van der Waals interactions. In order for a drug to form a covalent bond with the target, it needs to be more chemically reactive than most drugs. I think that is what is meant here, as they are more chemically reactive, they will be more likely to bind to non-target proteins. Fgf10 (talk) 07:51, 2 December 2014 (UTC)
 * It looks like this is a quiz generality.  The thing about generalities is that biology doesn't know theory, so they will always be wrong sometimes.  Something like puromycin or carbon monoxide can be pretty specific to a particular process; you can argue that these reactions only proceed specifically because of the compounds' initial noncovalent interactions but that's a sort of No true Scotsman argument because any compound that interacts covalently with another will always have a certain degree of "noncovalent" interaction with the other reactants as part of the model for whatever reaction takes place.  I suppose Fgf10's explanation is likely the reason, but I don't like the generalization. Wnt (talk) 09:34, 2 December 2014 (UTC)

Why is keratin not as conductive as the skin on your finger?
My iPhone does not react at all when I use the tip of my nail. But when I use the fleshy part of my finger, the iPhone reacts. 166.137.12.31 (talk) 19:16, 2 December 2014 (UTC)


 * Keratin contains very little free water or mobile ions - unlike the generous supply of both in skin - thus it's electrical conductivity is much lower. Roger (Dodger67) (talk) 19:20, 2 December 2014 (UTC)


 * There are several types of touch-sensitive screen: on some, a nail works well, but on others that depend on a flow of electrons, conductivity is important. Occasionally, I have to wet my finger slightly to get a screen to respond.    D b f i r s   20:38, 2 December 2014 (UTC)
 * Skin doesn't conduct. Sweat conducts. Touch screens that don't need a conductor are little produced anymore. Jim.henderson (talk) 00:14, 3 December 2014 (UTC)
 * You are correct, of course, but I still have two that work perfectly with a nail.   D b f i r s   21:14, 5 December 2014 (UTC)

desert geography
How big would an island need to be to have both a desert and farmland as part of its geography?184.147.124.158 (talk) 23:21, 2 December 2014 (UTC)
 * Molokai is not very large, and has quite a variable climate. ←Baseball Bugs What's up, Doc? carrots→ 00:59, 3 December 2014 (UTC)
 * See the right-hand image in the "See also" section of Center-pivot irrigation. It's quite possible to have productive farmland in the desert if you water it enough, so if you had a way of getting the water there, the only possible size restriction would be on the farming question: is the island big enough that farming is practical?  You can't easily farm an island that's one are in size, i.e. 0.01 hectares.  If you want more information, look at aerial or satellite views of the Nile; it's in a desert, but there are numerous small islands in the river, and many are farmed.  Nyttend (talk) 03:42, 3 December 2014 (UTC)


 * When you have high mountains, you can have moist land and desert just a few miles apart, because if the winds blow consistently in the same direction, the mountains can create a rain shadow. Looie496 (talk) 17:49, 4 December 2014 (UTC)