Wikipedia:Reference desk/Archives/Science/2009 September 15

= September 15 =

Integrins and signal transduction
It seems to me to be overkill for there to be tensile/contractile sensation in the form of integrins on the surface of cells that subsequently induce signal transduction. Why do proteins need to be made to order with immediate initiation of RNA transcription just because the chondrocytes in my nose get pushed a little when I scratch my nose. I suppose my question may derive from an oversimplification of this process as it was explained to me.  DRosenbach  ( Talk 00:00, 15 September 2009 (UTC)

Grapes undissolvable?
I was holding a grape in my mouth for what seemed like ages, wondering when it would dissolve entirely by action of the saliva. Nothing seemed to change, except it was a very warm grape when I gave up and bit into it. Are grapes undissolvable by human saliva alone? Is it the skin perhaps?-- The Ninth Bright Shiner 01:11, 15 September 2009 (UTC)


 * I can't see it dissolving at all. Chewing is an extremely important part of the process of breaking it down, it then gets broken down further by acids and enzymes in the mouth, stomach and intestines. Food doesn't really dissolve until it reaches the intestines and gets from there into the blood - until it is in the blood, it isn't dissolved (I'm not sure how dissolved it is then, it might just be suspended). --Tango (talk) 01:18, 15 September 2009 (UTC)


 * I'm pretty sure that the enzymes in your mouth are only there to break down starches into sugar. Other than that, it's really only things that'll dissolve in water.  Since the skin of the grape is waterproof (Grapes on the vine don't dissolve when it rains!) - and they evidently aren't mostly made of starch - they will indeed just sit there until you bite into them. SteveBaker (talk) 01:32, 15 September 2009 (UTC)


 * Ah, that's nice. Occasionally when I'm eating a lot of something (Milk Duds at the movies, Cheez-It whenever), I'll see how long it takes to become...well, its technically not dissolved, but more like liquefied. You know what I mean. I have been defeated by grapes! But through defeat, comes wisdom...of saliva. Meh. Thanks y'all!-- The Ninth Bright Shiner 02:04, 15 September 2009 (UTC)


 * (ec) In addition to the fact that amylase in your saliva can only go so far, a large portion of any plant matter is made up of cellulose which can't be digested by humans at all. You need to chew in order to expose the parts that you can break down.  So even without the skin a grape wouldn't dissolve in your mouth. Rckrone (talk) 02:08, 15 September 2009 (UTC)
 * There are enzymes other than amylase (the starch one) in saliva (see Saliva), but I think amylase is the only one there is significant quantities that is active in the mouth. --Tango (talk) 02:43, 15 September 2009 (UTC)
 * Fletcherizing might be the best you can do with a grape, where you chew it until it basically liquefies and slithers down your throat. An important advance in the field of nutritional psychoceramics!  --Sean 08:59, 15 September 2009 (UTC)

You'll get cavities going too far into this research! Sagittarian Milky Way (talk) 16:10, 15 September 2009 (UTC)

stem cell
my sister has a cervical cancer. she has already undergone many chemotheraphy. How can stem cell help a patient with cervical cancer? —Preceding unsigned comment added by 125.212.67.220 (talk) 03:08, 15 September 2009 (UTC)


 * I am sorry for your sister's health trouble. Unfortunately, we at Wikipedia are not going to give any medical advice, nor should you take anything you read here, or anywhere on the Internet, as a substitute for qualified medical advice from a real physician.  If you want to read a basic overview of stem cell therapy, we have an article titled Stem cell treatments, but even after reading that, you should have your sister consult her oncologist for any treatments she may receive.  The doctor may be aware of some alternative/experimental treatments availible, inlcuding, perhaps, some stem cell-based treatments. -- Jayron  32  04:49, 15 September 2009 (UTC)


 * Jayron is correct; we can't possibly know all of the circumstances related to a person's medical condition or provide any advice. With regard to the question "How can stem cell help a patient with cervical cancer?" -- sometimes cancer patients are given a bone marrow transplantation or hematopoietic stem cell transplantation after receiving aggressive chemotherapy treatment.  Whether this applies at all to your sister's situation is a question for her treatment team. --- Medical geneticist (talk) 13:14, 15 September 2009 (UTC)

Why are commercial culinary funnels made with pipes?
I bought a set of funnels from a shop the other day. You know, the sort of things you can put over an empty jar and pour sugar into, so that it doesn't go all over the bench. They are basically a cup with a pipe at the bottom. Then I noticed how often granules of sugar or rice or whatever get stuck in that dispensing pipe after you pour your mixture in the funnel cup. And then I thought, WHY OH WHY do you even NEED a pipe there at all, which only seems to serve the purpose of getting things stuck in it. Why not just the funnel cup with a hole at the bottom? Or if the cup needs some kind of stem to keep it stable, why not a little pole instead of a pipe? But I've never seen a funnel like this. Does anyone agree with my design critique? Myles325a (talk) 08:46, 15 September 2009 (UTC)
 * If you wanted to fill a bottle like a soda or ketchup bottle, your proposed device would be far inferior: you'd need one hand on the funnel while you pour, it would probably still leak some, and if it's thick stuff like ketchup you'd have to stand around holding it while it bloops down on its own schedule. --Sean 09:05, 15 September 2009 (UTC)
 * They do exist. I've seen them 2 feet in diameter and made out of metal for filling beer barrels, or 2 inches in diameter and made out of plastic for filling jam jars. --TammyMoet (talk) 10:22, 15 September 2009 (UTC)
 * The elongated end may helpful in directing the pour of liquids, as was already suggested above. Bus stop (talk) 11:25, 15 September 2009 (UTC)
 * In addition to stability and directing the flow, I'd also suggest that using a section of pipe would promote relatively laminar flow at the exit (the material exits as a stream} versus turbulent flow (the material spews out in all directions). A simple opening at the bottom of the receiving cup would only work if the vessel below was wide enough to contain spattering material. Franamax (talk) 22:05, 15 September 2009 (UTC)

Op myles325 back. I had thought of Sean's point to some extent. That is why I suggested that instead of a pipe you could have a short pole sticking down from the cone so that the whole contraption would be weighted down and you would NOT need to stand around holding it. The other main drawback to the current designs (and I've seen dozens of them, all sizes and shapes and they all have that damn pipe) is that as your jar fills up you have to keep raising the funnel so that the pipe is clear of the stuff you have already poured in there. That means at the end, you DO have to hold the funnel up ABOVE the jar and this means you can spill stuff. I did that the other day funneling sugar from a sugar bag into my sugar jar. The sugar was lumpy and of course it kept getting stuck in the pipe. I had to keep pressing it down and shaking the funnel. That was ok when the pipe was deep down in the jar, but as the level went up, I had to raise the funnel, and of course in no time at all, I had sugar all over my bench. That's when I had this brain wave. With my device you could easily perch the funnel on top of the jar and it would be weighed down by the rod. Also, I do take the point about the pipe encouraging laminar flows, but I think a very short pipe (only say a quarter inch) would do the job quite well. Myles325a (talk) 03:15, 16 September 2009 (UTC)


 * Not ALL funnels have long pipes. Jam funnels dont (see http://www.cookability.biz/hutzler-jam-funnel/b_1263.htm?SRC=GB1 for example) - you may want to try one of these for your sugar task. --Phil Holmes (talk) 13:50, 16 September 2009 (UTC)

The cause of gravity.
So, I'd like to know what the cause of gravity is. Einstein viewed gravity geometrically; as a distortion of space-time itself. But I'd like to know: Why does this happen? What causes mass to exert gravity? Or is this just a fundamental property that came with the universe?Unique and proud of it (talk) 12:39, 15 September 2009 (UTC) -Unique and proud of it. —Preceding unsigned comment added by Unique and proud of it (talk • contribs) 12:38, 15 September 2009 (UTC)
 * At present, "fundamental property" is as good an explanation as any. Gravity remains the only un-unified fundamental interaction not quantized under the Standard Model, and a theory of quantum gravity is one of the most highly sought-after prizes of the physics world.  The linked articles can take you much further down the rabbit hole, depending on how much reading you'd like to do.  &mdash; Lomn 13:01, 15 September 2009 (UTC)
 * The Strong interaction has yet to be unified acceptably with the Electroweak interaction; it has been quantized, though, which is presumably what you meant. Algebraist 13:31, 15 September 2009 (UTC)
 * Thanks, I've tried to correct/clarify the statement. &mdash; Lomn 13:54, 15 September 2009 (UTC)
 * I thought it had been shown that they unify at extremely high energy levels (fraction of a second after the big bang type stuff). --Tango (talk) 17:00, 15 September 2009 (UTC)
 * That's the electroweak interaction, per the article lead. &mdash; Lomn 18:07, 15 September 2009 (UTC)
 * No, I'm definitely thinking of the strong force unifying with electroweak. Looking it up, I see I'm talking about Grand unification theory, which is still a work-in-progress. I was jumping the gun a bit! --Tango (talk) 21:28, 15 September 2009 (UTC)
 * Because nature seems to like laws that can be described by a short formula that have complicated implications. Best I can come up with. See The Unreasonable Effectiveness of Mathematics in the Natural Sciences. Dmcq (talk) 14:30, 15 September 2009 (UTC)
 * Gravity and all the rest of the forces and particles in the Standard Model pop out of String theory automatically. Gravitons appear as the vibrations in closed loops in many dimensions, but string theory has a lot of holes in it. Fences  &amp;  Windows  21:56, 15 September 2009 (UTC)
 * especially if you take all the strings and make little loops out of them...-- Jayron  32  02:30, 16 September 2009 (UTC)
 * If I could answer that question, you could just ask me why that's true. If I were able to answer that, you could ask me why that's true. At some point, there is no reason why. It's just the way the universe is. Perhaps this is that point. Perhaps string theory is that point. Nobody knows. — DanielLC 03:39, 16 September 2009 (UTC)

Thermite
How much thermite is needed to melt an inch or so hole through 1/16" thick steel? I'm using powdered rust and aluminum foil cut into small pieces. How small should the pieces be for maximum safety? What will contain that amount of thermite without cracking or hot substance leaving? I have two concrete blocks, 4" x 8" x 12", about 3 cm wall thickness, will stacking them: weight - concrete block "lid" - block #2 "reaction cavity" - 3 or 4 1/16" steel plates seperated by air spaces, or maybe just another block - kitchen sink be ok? If there are any problems with that, other possibilities I can think of are filling the blocks with dry sand, bucket of sand on the bottom (plus you get a fulgurite!), bricks. I've heard that match tip material burns hot enough to start the reaction, would that really work? Sagittarian Milky Way (talk) 16:05, 15 September 2009 (UTC)


 * Aluminum foil and flake rust will do nothing. If you want to make your own Thermite (and hey, they're your fingers... If you don't need them all, who am I to say anything), then you need very finely powdered aluminum and very finely powdered iron (iii) oxide.  Foil has far two problems; first it has WAY too small a surface area to be effective, and secondly it tends to have an impervious coating of aluminum oxide on the surface, making it all but useless for a solid-state reaction like thermite.  Secondly, rust isn't really pure enough iron (iii) oxide.  Rust is a combination of multiple iron compounds, including iron oxides, iron hydroxides, and iron carbonates, in both iron (ii) and iron (iii) oxidation states.  For an effective thermite reaction, you really need finely powdered and scrupulously pure iron (iii) oxide (Fe2O3).  Thirdly, a match tip MAY possibly start the reaction, but not reliably.  What you need is something WAY hotter, like a blowtorch, or even better, a chemical starter like sodium chlorate.  All of this is MUCHO dangerous, and I probably wouldn't be messing around in any room you don't want burned to the ground.  If you want to do real thermite, you need to order finely powdered aluminum and finely powdered iron (iii) oxide from a reliable chemical supply house; just be aware that they are likely required to report such orders to authorities, so don't be surprised if you get a call from the po-po before they even show up at your door, wondering what you are doing ordering ingredients for a dangerous explosive... -- Jayron  32  17:55, 15 September 2009 (UTC)


 * Welcome to the Wikipedia Reference Desk. Your question appears to be a controlled demolition question. I apologize if this is a misevaluation, but it is our policy here to not to tell people how to blow up tall buildings as part of a massive government conspiracy. Please attempt to solve the problem yourself. Fences  &amp;  Windows  21:50, 15 September 2009 (UTC) Lol. Sagittarian Milky Way (talk) 17:09, 16 September 2009 (UTC)
 * I recall no such policy. We've given advice on how to hide the body before, which can't be much better. Vimescarrot (talk) 09:01, 16 September 2009 (UTC)
 * I believe Wiktionary has the definition of facetious, which may prove informative in understanding the subtext in Fences and windows comments. -- Jayron  32  12:05, 16 September 2009 (UTC)

I wouldn't be holding the match while it went off, lol. I'd make a rudimentary fuse. If the oxidation film is so bad, very finely powdering it will make it worse, and since powder lasts long enough to work, you could polish your foil before cutting. Does wood stop burning if it has low surface area to mass? It just burns slower. Sagittarian Milky Way (talk) 17:09, 16 September 2009 (UTC)


 * But if the thermite burns slow enough, there will never be enough activation energy present to maintain the reaction. That's the point, the thermodynamics of the reaction are invariant whether you use aluminum foil or powdered aluminum. However, the chemical kinetics are the controlling factor here. Aluminum foil's low surface area slows down the reaction SO MUCH that it will happen, but on the order of magnitude of days rather than nanoseconds. You can recreate the endproducts of thermite if you have ever left a rusty steel pan in contact with an aluminum pan. After a week or so, the aluminum pan will develop a white powder (aluminum oxide) all over it. Its exactly the same reaction as Thermite. Absolutely no difference, except it is happening VERY SLOWLY. The difference is the surface area. To use your wood analogy, its the difference between a slowly smoldering piece of paper and a sawdust explosion which has taken out many a wood shop... Same material, but what in one case is a slow, controlable reaction becomes a violent reaction only because of the surface area of the particles. In thermite, the relationship is magnified even more. It is ALL about using the powdered chemicals here. -- Jayron  32  17:21, 16 September 2009 (UTC)

If problems are not insurmounatble, the beach in winter would be a better place. No one's there, the shielding/target material is plentiful, and you can use as much as you want. Run! Sagittarian Milky Way (talk) 17:15, 16 September 2009 (UTC)
 * Careful with the beach...one of the dangers of the standard thermite demo with sand to catch the results is that the sand needs to be dry. Otherwise the molten iron lands on damp sand, the heat vaporizes some of the water, and you get some ejection of sand and hot metal. This exact expermental setup has been used as a model of some processes related to volcanic activity (for example, 10.1016/S0377-0273(01)00280-3). DMacks (talk) 20:27, 16 September 2009 (UTC)
 * As a point of further clarification, I have personally done the thermite reaction in front of a classroom of students. If one is properly trained, it is safe.  But you must be aware of the dangers, and take proper precautions, and know exactly how, and why, and exactly what happens in the reaction.  I didn't just learn how to do it as an educational demonstration from a book either, or tried to figure it out myself.  I worked personally with an ex-professor of mine who showed me exactly how to do it right.  Like anything else of this magnitude, it is OK to do in the right hands, but if you have never worked with Thermite before in a supervised situation, it is NOT the sort of stuff you want to play around with.  -- Jayron  32  17:26, 16 September 2009 (UTC)

rice growing
The instructions I have for growing rice seeds says to soak them for 36 hours and then to let them dry out for 24 hours and then to plant then in potting soil covered with several inches of water. However, after soaking some seeds have already sprouted roots. Should these seeds be dried for 24 hours too? -- Taxa  (talk) 20:36, 15 September 2009 (UTC)
 * Without even researching this, I'd say divide the sprouted seeds into two groups. Plant one group right away, dry the other group for 24 hours before planting. Let us know the results! The main rationale I can see for the drying step is to prevent rotting. Totally WP:OR on my part! ;) Franamax (talk) 22:21, 15 September 2009 (UTC)
 * About half do not have roots so I'll have 2 test and 2 control groups. -- Taxa  (talk) 23:39, 15 September 2009 (UTC)
 * Looks like the ones put in yesterday with only an hour or so of drying have shoots (or upsidedown root tips) that are doubling in length every 12 hours. I've setup a crude timelapse camera. -- Taxa  (talk) 03:05, 17 September 2009 (UTC)
 * I guess this has to do something with dormancy breaking for which I can't find an article. But there is an article on seed dormancy. 95.112.171.137 (talk) 18:10, 16 September 2009 (UTC)

matter, gravity, space and time
How can matter influence space and time? I mean sure if you have a tree lying in the road and have to take a longer route then matter (the tree) can increase distance and time but that is not what I mean. -- Taxa  (talk) 20:39, 15 September 2009 (UTC)


 * Matter has mass - mass distorts space and warps time. Einsteins discovery of special relativity was only widely accepted after an expedition measured the amount by which the light of a distant star was bent by the curvature of space due to the sun's mass during a solar eclipse.  In everyday experience, the effect is far too small to measure - but for things like stars - it's very important. SteveBaker (talk) 20:57, 15 September 2009 (UTC)
 * That's general relativity, Steve. GR is SR+gravity. --Tango (talk) 21:20, 15 September 2009 (UTC)
 * The traditional analogy is one of masses placed on a trampoline, they all indent the surface, and all to some extent lie in each others indentations, and thus roll together; ergo gravity. Einsteins revelation was that time and space are a single fabric which is distorted by mass, acceleration of the observor and appears differently from different reference frames (an important implication being that there is not absolute frame, hence relativity). The best places to start are introduction to special relativity and introduction to general relativity, the former not relating to distortions by mass, but is bust understood prior to the latter. Elocute (talk) 23:17, 15 September 2009 (UTC)
 * That analogy is more the "what" than the "how". The "how" is more difficult, I don't think we really have an explanation for that, it's just the way it is. --Tango (talk) 23:22, 15 September 2009 (UTC)
 * Just as an aside...how were things like this explained prior to the advent of the items we generally use in analogies to explain them. I mean, before trampolines existed, before thin sheets of rubber and foam existed, and all they had was, I don't know...sticks, ,bricks and papyrus, how did people speculate how this would appear and work?  DRosenbach  ( Talk 02:13, 16 September 2009 (UTC)
 * Well, the flat-earth nut-jobs believe that gravity doesn't exist at all - it's just that the earth itself is accelerating at 1g straight upwards. SteveBaker (talk) 19:26, 16 September 2009 (UTC)
 * One thought that came to mind which your question give me the opportunity to say is that any solid object in air at sea level sort of forms a dent or hole or something in the air. The space part is easy to see and so is the time distortion if the object is moved. In fact you could probably apply the analogy of the need to go around a solid sphere located in air as an increase in distance and time. Mass occupies space and two masses can't occupy the same space at the same time. Think of a solid sphere and air. -- Taxa  (talk) 05:21, 16 September 2009 (UTC)
 * General relativity was first presented in 1915. Predecessors to the trampoline date back at least as far (Trampoline). --Tango (talk) 02:17, 16 September 2009 (UTC)
 * Tango is right to point out that the trampoline by far predates the concept of spacetime. In fact the most abstract of physical theories are all fairly recent, electromagnetism, GR and SR all within the last 150 years, all being somewhat hard to get your head round, and of course QM which has no conventional interpretation, was a product of the 20th century. Before this, I think it is fair to say, that theories explained things in terms of Newtonian hardball particles, which essentially act like billiard balls, and waves, like the ones in the sea, so theories were largely tangible in terms of common natural phenomena. Elocute (talk) 09:18, 16 September 2009 (UTC)

Bugs, Sun, Moon
Bugs seem attracted to light. At least, it seems so at night while I'm inside, the lights are on, and a window is open. Questions: (a) Is there a Wikipedia article on this phenomenon? (b) Do adventurous bugs therefore try to reach the Sun during day, and the Moon during night? Comet Tuttle (talk) 22:06, 15 September 2009 (UTC)
 * The phenomenon is called phototaxis and its existence in insects is very well known but not very well understood. Our article doesn't even mention it, but you can find stuff on the web, for example this page. (b) No, but one hypothetical explanation of the phenomenon is that they use the sun and moon as navigation cues. Looie496 (talk) 23:29, 15 September 2009 (UTC)


 * I wonder if this is what moths that fly several miles above the earth in very large groups at night are doing? -- Taxa  (talk) 23:49, 15 September 2009 (UTC)
 * Several miles? What makes you think there are moths several miles up? --Tango (talk) 02:18, 16 September 2009 (UTC)
 * Maybe not several but probably higher than a mosquito would fly. -- Taxa  (talk) 05:13, 16 September 2009 (UTC)
 * QI says moths orientate themselves by moonlight. When Man comes along and creates a million tiny artificial moons, moths get confused. Vimescarrot (talk) 08:57, 16 September 2009 (UTC)
 * Not that I'd want to argue with Mr Fry's researchers but here is a bit more information on this behaviour in moths. There still seems to be some debate. Martlet1215 (talk) 13:33, 16 September 2009 (UTC)

Design specifications for industrial pipeline flame arrester
I am a manufacturer's representative and have been approched about selling a line of flame arresters,tank vent and vacuum release valves, and tank seal valves. I'm attempting to evaluate how acceptable these products will be in the U.S. market by detemining if they meet the most commonly required U.S. design and operating specs. I have not been able to find a source that calls out those specs.

Can anyone tell me what they are, or suggest where I might find them? 69.152.195.244 (talk) 22:21, 15 September 2009 (UTC)
 * Right off the bat, I'd suggest contacting Underwriters Laboratories directly. They are one of the US NRTL's who would have good information. Possibly our ASME article or links therefrom would have some contacts also. Franamax (talk) 22:27, 15 September 2009 (UTC)

Why is it colder in the winter?
1. Angle of incidence of sun's rays. 2. Depth of atmosphere through which sun's rays must travel. 3. Length of day.

Does anyone know the relative importance of each of these factors? I would like to be able to say that x% of the difference is due to factor 1, y% to factor 2, etc. -- even if the x and y are just ballpark figures. —Preceding unsigned comment added by 86.134.9.51 (talk) 23:44, 15 September 2009 (UTC)


 * Start with the Winter article and see where it takes you. Baseball Bugs What's up, Doc? carrots 23:49, 15 September 2009 (UTC)


 * Already been there I'm afraid. The linked Effect of sun angle on climate makes the rather vague statement that "the angle of sunlight" is the "basic mechanism" but does not give the specific comparative information that I'm after. It also isn't clear in context whether "the angle of sunlight" includes the depth-of-atmosphere effect. 86.134.9.51 (talk) 00:01, 16 September 2009 (UTC).


 * For all intents and purposes, the first item is the only one that matters. From the first sentence of that linked article, "The amount of heat energy received at any location on the globe is a direct effect of sun angle..."  Moreover, the change in the length of the days is technically also due to the tilt of Earth's axis, and you can relate the depth of the atmosphere to that as well.  If you want numbers, you can say that the angle of incidence is responsible for about 100% of the difference, although location in orbit does have a small influence.  This of course only applies macroscopically, as local patterns and locations (currents, water, mountains, etc.) can change things radically. ~ Amory ( user  •  talk  •  contribs ) 00:41, 16 September 2009 (UTC)


 * However, for a factor that is unrelated to axial tilt, consider the effect of the eccentricity of Earth's orbit about the sun. &mdash; Lomn 00:48, 16 September 2009 (UTC)


 * For half of the planet -- the Northern Hemisphere -- that operates against the other effects. The Sun is closest in January. --Anonymous, 04:30 UTC, September 16, 2009.


 * Earth%27s_orbit also discusses this. Mitch Ames (talk) 11:52, 16 September 2009 (UTC)


 * The length of the day certainly matters - but the angle of incidence is the critical factor. The amount of sunlight hitting one square meter of the earths' surface is proportional to the cosine of the angle between the direction of the sunlight and a line at right angles to the surface.  Since the earth's axis is tilted by 23 degrees - when the bit of land you happen to live on is tilted away from the sun - in winter it intercepts less sunlight.  Suppose you happen to live in Havana, Cuba - which (by an amazing coincidence) is at 23 degrees latitude.  In the middle of summer, the sun's rays are absolutely vertically above you at midday.  The angle between "up" and the sun is zero degrees and cosine(0)=1.  So 100% of the sun's energy is beating down on you.  In mid-winter in Cuba, the axis has tilted and Rio de Janeiro is now right under the sun and in the middle of it's (southern hemisphere) summer.  Havana is now pointing 46 degrees away from the sun at midday.  cosine of 46 degrees is about 0.7 - so you're only getting 70% as much sunlight in midwinter as in midsummer at noon.  However, that ratio varies depending on where you are on the planet.  At the equator, the sun's angle changes by the same number of degrees as it does in Havana - but when you look at the shape of a cosine curve, you see there isn't any difference between -23 degrees and +23 degrees.  On the other hand, if you are within 23 degrees of the North pole - the angle between "up" and the sun becomes 90 degrees and you don't get any sunlight falling on flat ground at all!
 * The day length varies too - check out File:Day length.jpeg - and you'll see that that Havana's day length changes from around 10.5 hours in the winter to about 13.5 hours in the summer. That's a +/- 15% variation - so there is about a total 30% variation between summer and winter - which is about the same as the amount of variation due to the angle of the sun's rays.
 * The amount of atmosphere that the sun's rays have to pass through is much less important - if the sunlight is absorbed by the air - then the air must heat up. The total amount of heat that hits the ground is less when the path-length is long - but there is more heat being dumped into the air.  What's more difficult to understand is that when sunlight hits the ground - if the ground is (for example) bright white snow - then it's going to be reflected away again without warming it up much.  But on (say) dark soil, most of it will be absorbed and turned to heat.  So heating up the air on the way in (and out again) actually saves some heat.


 * But it's not really that simple - there are vast numbers of other variables to consider. The amount of cloud cover for example.  Bright white high altitude clouds will reflect the sun away before it gets any chance to be absorbed at all.  The amount of humidity in the air will alter the ability of the air to hold the heat.  If the prevailing winds shift from on-shore to off-shore or from blowing across a major desert or not.


 * It would be rash to come up with a single answer here. The result certainly depends dramatically on your latitude. SteveBaker (talk) 00:44, 16 September 2009 (UTC)
 * Steve, good explanation; but I got a little lost reading your explanation about atmospheric absorption. Categorically, if the insolation is being absorbed by a lot of atmosphere, then it's warming air somewhere else (e.g. at a lower latitude).  Complicated, as you say.  Primarily, I think the incident energy per square meter (which is dictated by the incoming solar illumination angle), is the chief driving factor in seasonal temperature variation.  Large-scale convection in the atmosphere (e.g. "prevailing winds") would be the next most prominent effect (noting, of course, that these prevailing winds are coupled to the seasonal solar heating in a complex way).  Nimur (talk) 14:31, 16 September 2009 (UTC)
 * Thanks for the replies guys... 86.134.9.51 (talk) 01:15, 16 September 2009 (UTC).


 * Heh. Now I'm trying to imagine an environment which is warmer in winter due to vertical elements. Something like a sparse forest full of tall trees, with snow on the ground all year round. 81.131.45.124 (talk) 09:50, 16 September 2009 (UTC)


 * There's another factor to consider, and that's the "delayed effect" of heat buildup and dissipation. I don't know what the technical terms are for it, I just know its effect. Based on angle of sun and shortness of day, you'd think December 21 would be the coldest day of the year and June 21 the hottest. But the "height of summer" and the "depth of winter", in terms of heat (or lack thereof) really occur roughly halfway into the season, i.e. early August and early February, and some days or weeks either side of those points. Baseball Bugs What's up, Doc? carrots 01:40, 16 September 2009 (UTC)


 * That delay effect is based on the heat capacity (more specifically the Thermal mass) of the atmosphere and ground. If you were standing on an atmosphereless and perfectly reflective sphere, then the hottest day WOULD be the summer solstice.  However, since you are not, the material of the atmosphere and the ground takes time to absorb energy and change temperature accordingly, and that time is a direct consequence of the heat capacity of the stuff the earth is made of.  -- Jayron  32  02:27, 16 September 2009 (UTC)
 * Actually, if the earth had no atmosphere and it was perfectly reflective, would there be any heat at all, other than geothermic heat? If you were standing anywhere on that surface, absorbing the sun's rays with no protection other than your space suit, wouldn't you pretty much be equally hot no matter where you were? Baseball Bugs What's up, Doc? carrots 04:35, 16 September 2009 (UTC)
 * My space suit is height h and w x w when viewed from above (it's the new cubist model). Standing at a pole at equinox I get h/w more warmth than standing at the equator. Cuddlyable3 (talk) 08:51, 16 September 2009 (UTC)


 * The intensity of the sun's radiation hitting the surface of the Earth is proportional to the sine of the angle of the sun above the horizon. We can find this value based on the time of day, latitude, and time of year.  In particular if A is the strength of the sun directly overhead, then the strength of the sun during daytime in general is
 * $$f(t) = A[\cos(\frac{2\pi}{day}t)\cos\psi\cos\theta - \sin\psi\sin\theta]$$
 * where t is the time of day in relationship to noon, θ is the latitude, and ψ is the angle that the Earth's axis is tilting away from the sun at the given time of year (at the north end). Let α be the overall tilt of the axis, which is about 23.5°, then ψ = α in the (northern hemisphere) winter, ψ = -α in the summer and ψ = 0 at the equinoxes.  More generally
 * $$\psi = \sin^{-1}(\sin\alpha\cos(\frac{2\pi}{year}T)) \approx \alpha\cos(\frac{2\pi}{year}T)$$
 * where T is the time of year in relation to the northern hemisphere winter solstice. From this information, we can find sunset based on latitude and ψ, which is the time t0 after noon where f(t0) = 0.  That gives
 * $$t_0 = \frac{day}{2\pi}\cos^{-1}(\tan\psi\tan\theta)$$
 * Similarly sunrise is at t0 before noon, and so the length of the day is 2t0. Note that if |ψ|+|θ| > π/2, there's no solution for t0, since this corresponds to places that are experiencing 24 hours of light or darkness so there is no sunset or sunrise.  To find the total incident radiation per area over the course of the day, we can integrate f(t) which yields
 * $$R = \int_{-t_0}^{t_0}f(t)dt = 2A[\frac{day}{2\pi}\sin(\frac{2\pi}{day}t_0)\cos\psi\cos\theta - t_0\sin\psi\sin\theta]$$
 * for places with day and night and
 * $$R = \int_{-\frac{day}{2}}^{\frac{day}{2}}f(t)dt = -A(1day)\sin\psi\sin\theta$$
 * for places with 24 hours of daylight. With these tools you can get some actual values for how the length of day and amount of sun you get where you are varies over the course of the year.  This ignores any atmospheric effects and also the fact that the Earth's orbit isn't totally circular, which messes things up a little, but I'm definitely not going to attempt to correct for that.  Rckrone (talk) 06:45, 16 September 2009 (UTC)


 * The example given of a planet without an atmosphere, thus removing air absorption from the discussion, leaves us with the question of why the sun angle matters. If you position yourself such that you're facing the sun directly, you're going to get the same number of photons hitting you directly from the sun, no matter where you are on the half of that sphere that's facing the sun. The difference, I suppose, would be the number photos reflected and hitting you, which would tend to be greater or lesser depending on the angle at which those photons are hitting the planet at the point you happen to be standing. Baseball Bugs What's up, Doc? carrots 02:33, 17 September 2009 (UTC)


 * That answer doesn't make any sense. Being without an atmosphere would kinda make the whole thing meaningless.  You can't talk about 'weather' without considering the atmosphere!  However, despite that, the temperature of the ground would vary depending on the angle of the sun to the surface - and assuming you're not trying to measure the temperature of the vacuum - or that of your own body - neither of which have anything to do with this question - then what are you measuring if not the temperature of the ground - which is most certainly affected by the angle of the incident sunlight?!? SteveBaker (talk) 12:23, 17 September 2009 (UTC)