Wikipedia:Reference desk/Archives/Science/2017 November 14

= November 14 =

USGS measurements
How the United States Geological Survey is able to measure earthquake magnitudes around the world? That said, do they have their own stations across the world or do they measure indirectly at home, deducing the magnitude from available data? Thanks.--212.180.235.46 (talk) 09:00, 14 November 2017 (UTC)
 * Instruments in the US, others around the world, and international agreements to share data - see National Earthquake Information Center Wymspen (talk) 09:51, 14 November 2017 (UTC)

Earth's energy budget
I looked at this article and the talk, but didn't get the answer i want. Quite basic calculation (*) shows that, if greenhouse effect were absolutely perfect, atmosphere absorbing each and every parcel of energy from the surface (it doesn't matter whether it is absorbed through conduction, convection, phase transition, radiation or whatever), then back-radiation (let's call it B) peaks at a maximum A + C where A: absorbed by atmosphere (77.1 according to the picture in the article) C: absorbed by surface (163.3, same source) A+C: 240.4 BUT B is supposed to be 340.3 (same source), 100 higher that the calculated maximum.

Well, I don't expect NASA to be that wrong, and I think any error would have been long corrected, so i have to suppose that somehow Back-radiation is currently HIGHER than in a perfect greenhouse effect world. My question is: how?

(*) we are looking for a steady state, at equilibrium, stable (things get back there if some noise disturbs the system) solution. I leave you the easy calculation to get there, just gives you the only solution -- nothing else works.
 * surface receive directly C and A+C from back-radiation, for a total of A+2C, which are then all send up so surface is at equilibrium.
 * atmosphere gets A directly, plus those A+2C from the surface, for a total of 2A+2C; half of it (A+C) goes down (the same as back-radiation used just above, sanity check OK), half of it (A+C) goes up (which is just as much as absorbed, sanity check OK)

185.24.186.192 (talk) 11:42, 14 November 2017 (UTC)


 * Does the simplified schematic from greenhouse effect help? The greenhouse effect is based on a circular flow of energy trapped in the system (i.e. heat).  If you look at the schematic, the total energy entering each level is equal to the total energy leaving each level, which corresponds to an equilibrium.  (There is actually a slight imbalance these days due to global warming.)  However, it is not the case that the back-radiation must equal the total radiation from the sun.  The amount of back-radiation depends on the temperature of the atmosphere.  Similarly, the amount of energy transfer from the surface depends on the temperature of the surface.  The surface and atmosphere will warm up until they reach a temperature where the energy flows out equal those coming in.  The warm temperatures at the surface are maintained, in part, by a circular flow of energy which we know as the greenhouse effect.  The energy flows from surface to atmosphere and back again happen to be larger than those from the sun, but that isn't a problem as long as we are talking about a closed loop.  Dragons flight (talk) 11:58, 14 November 2017 (UTC)
 * Thanks, but no, it doesn't help at all : figures are only slightly different (67 + 168 = 235, Vs 324 BR instead of 77 + 163 = 240, Vs 340), but share the same issue
 * There is equilibrium in each level indeed, and you would have the same equilibrium at each level by adding just any value, positive or negative, to both back radiation and upward radiation. Subtract 324 to back radiation (putting it at zero), and also 324 to upward radiation (down from 452 to 128), ant it still works. Add another 324 to back radiation (putting it at 648) and also 324 to upward radiation (up from 452 to 776), and it also works. Well, no, it doesn't. The system is then, in both case, out of equilibrium (even though each level is at equilibrium). A zero back radiation would also mean a zero up radiation from the atmosphere, so it would warm up and emits more and more back radiation, until reaching equilibrium value. Similarly a 648 back radiation is way too much, meaning huge loss to space, cooling down atmosphere, lowering back-radiation, until the equilibrium is reached
 * The point is, basic (too basic ?) calculation put the said equilibrium at a maximum of 240 (or 235, depending on schematic) in the perfect GHE case. While each schematic says that in a NON perfect GHE case, back-radiation is much higher, when it should be lower (nothing can beat perfect GHE scenario).
 * 185.24.186.192 (talk) 13:39, 14 November 2017 (UTC)
 * Its just a very simplified model representation and you added elements which are not in that simple model. One result of that is of course that the numbers in the model no longer add up because you changed the "formula" that model is using (to result in equilibrium). Find another model that contain your elements or "manufacture" a model yourself (which you already kinda tried (wrong) with your question). --Kharon (talk) 14:01, 14 November 2017 (UTC)
 * I added elements which ARE not in that simple model, taken from wikipedia article or schematic provided by talk
 * I may be wrong, indeed i asked "how", so your answser "you are wrong" is just not an answser...
 * 185.24.186.192 (talk) 21:40, 14 November 2017 (UTC)
 * Perhaps it is unclear, but the radiation from the surface and the atmosphere is determined by the temperature of each component not the flux. So, you can't just put in random values without also changing those temperatures (flux emitted is roughly proportional to T4).  Why do you believe 240 is the maximum?  It's not.  Let's consider a different analogy.  Consider an oven.  It consists of a heating element, some food you want to cook, and an insulated box.  If you want to maintain a constant temperature, then the heat being put into the heating element must equal the heat leaking out of the insulated box.  If the insulation is pretty good hopefully, then not much energy is leaking, so that necessary flux to maintain a constant temperature is low.  However, the flux of energy being radiated between the food and the box and back will be much higher.  That's because the inside of the box can get much hotter than the outside.  If the insulation were nearly perfect, you could imagine the oven being able to getting ridiculously hot and the internal energy fluxes between the food and the box getting arbitrarily large.  This is true even if the heating element is only providing a relative trickle of new energy, since the heat can build inside until an equilibrium is achieved.  It's the same with the greenhouse effect in planetary atmospheres.  The sun provides new energy, which at equilibrium counters the losses, but the internal transfers of energy can become much larger than the source flux depending on the characteristics of the atmosphere.  For a thin atmosphere (like Mars) nearly all surface radiation escape directly to space, the back-radiation is very low, and the temperature enhancement is negligible.  For a thick atmosphere (like Venus), essentially all surface radiation is captured by the atmosphere, the back-radiation is enormous, and the temperature enhancement is huge.  Earth happens to lie in between these extremes.  Dragons flight (talk) 16:27, 14 November 2017 (UTC)
 * more food for the though here, thanks.
 * the radiation from the surface and the atmosphere is determined by the temperature of each component not the flux, but the flux determines the temperature:higher flux in or out respectivly warms or cool the element until flux in and out balance again.
 * Your oven analogy is perfect. Even a perfect insulation box radiates energy out because of its own temperature, and this temperature will increase until radiation out perfectly match radiation received by the insulation box from inseide. And you can even calculate it, and that is just what i did:
 * the heating element brings C, heating the insulating box until its temperature rise at appropriete level to radiating out C, no more, no less; A is zero (no direct heating to the insulating box, neither from the outside nor from the heating element inside); the insulating box also radiates C back into the oven (Back-radiation B = C), because othewise it would either cool or warm (if it were more or less), so the food actually gets B+C=2C heating (C from the heating element+ B=C backradiation), which it also send back to insulating box (so it receive 2C, send C out and C back in: balance respected), and everything balance perfectly, and stay so because this is a stable equilibrium. So it doesn't gets ridiculously hot inside the oven, the maximum heating being A+2C, as calculated above, with A=0 in your oven case.
 * And that's why I believe 240 is the maximum backradiation: because calculation shows it to be. It is not a "random value". It is the absolute maximum in the most perfect insulation case (unless something is wrong here, but what?)
 * Now, I understand your point that surface emperature being more or less known, the surface upward radiation cannot be very different from 452. and so the back-radiation must be whatever needed to balance things out, and that's 324 from your schematic. Higher than 235
 * Well, the only sensible conclusion is that atmosphere is better than a simple insulation layer. A heat pump. Heat pump exist, we build some, so why not nature, but I don't see how this works nor where it would pump heat from, and it is not explained in wikipedia, if it were so. Back to the start: how is this possible?
 * 185.24.186.192 (talk) 21:58, 14 November 2017 (UTC)
 * The insulating box doesn't radiate at the same rate inwards and outwards. 93.136.80.194 (talk) 08:20, 15 November 2017 (UTC)
 * I think you are right, but this doesn't explain why, and this actually is just another way to put my initial question: why would the insulating box (a perfectly absorbing, choked full of GHG, atmosphere) radiate at different rate inwards and outwards?
 * 185.24.186.192 (talk) 11:58, 15 November 2017 (UTC)
 * Imagine a box made of two thin shells. Each shell is perfectly absorbing and radiates at the same rate inwards and outwards. When the inner shell receives 1 unit of energy, 0.5 is backradiated and 0.5 is sent to the outer shell. Of the latter 0.5, 0.25 is radiated out and 0.25 is backradiated onto the inner shell. Of that 0.25, 0.125 is radiated inside (total for inside is 0.625 now), and 0.125 is backradiated onto the outer shell, and so on. In the end, 2/3 of the energy is backradiated and 1/3 is let through outside. If you add more shells, you can make the fraction radiated out as small as you want.
 * If this box has reached equilibrium, the amount of heat radiated to the outside is equal to the amount being received by the system. But to get to that point, the box contents might have received far more energy than it could radiate for a long time, and this would have caused an arbitrarily large buildup of energy. The system may receive 1 W and radiate 1 W, but that doesn't preclude that there's 200 W bouncing off the box's inner walls (and that doesn't necessarily imply that the box has been heated to its capacity as an insulator and will start to disintegrate and radiate out much more than its usual fraction). 93.136.80.194 (talk) 19:13, 15 November 2017 (UTC)
 * (indent out)
 * I see, but, as you points out, this require 2 (or more) PERFECT boxes, not a single perfect one.
 * If the too boxes are not perfect, but rather 2 imperfect, each of it absorbing half of incoming energy from innerward, so that the multilayers system is still perfect, what happens? is there any multiplicative effect?
 * for "ground": initial heating: C ; backradiation from inner to bottom: C ; total emission : 2C, from which C to inner layer and C to outer layer
 * for outer layer: directly from ground:C ; send downward: C ; radiated outward: C ; received from inner layer: C
 * for inner layer: directly from ground:C ; radiated downward: C ; send to outer layer : C ; received from outer layer: C
 * No multiplicative effect. A perfect box is a perfect box, whether it is single layered or multilayered to achieve perfection. You can change the number of layer to infinite, change the ratio received by each layer, no matter what, you cannot beat perfection.
 * Well, you can, but you need some sort of heat pump, pumping energy from the outer layer(s) to the inner layer(s)
 * However, you made me think of a real engine, able to power such a heat pump, and it is gravity, powering lapse rate. Lapse rate allow the top of atmosphere to be lower temperature that bottom, so it allows higher emission downward that upward. It is it starting to make better sense.
 * It is already stated in relevant article that GHE was a misnomer, I now know it is a double misnomer: Lapse rate is involved, despite not being mentioned (methink it should, but i guess fixing the article is not that easy)
 * thanks, consider the question answered
 * 185.24.186.192 (talk) 11:21, 16 November 2017 (UTC)
 * The "perfect" multilayered box you describe does not exist because radiation cannot "skip" layers. At each layer is absorbed and dissipated in all directions including back, so naturally less energy reaches the outer layers. Besides, what you're talking about doesn't describe the Earth's atmosphere because it simply wouldn't be an insulator; Earth's atmosphere's lapse rate proves that it does insulate. 93.136.10.152 (talk) 20:35, 16 November 2017 (UTC)