By Steve Goddard
ESA’s Venus Express mission has been studying the planet and a basic atmospheric model is emerging.
Venus has long been the CO2 bogeyman of climate science. In my last piece about Venus I laid out arguments against the claim that it is a runaway greenhouse which makes Venus hot. This generated a lot of discussion. I’m not going to review that discussion, but instead will pose a few ideas which should make the concepts clear to almost everybody.
If there were no Sun (or other external energy source) atmospheric temperature would approach absolute zero. As a result there would be almost no atmospheric pressure on any planet -> PV = nRT.
Because we have a sun providing energy to the periphery of the atmospheric system, the atmosphere circulates vertically and horizontally to maintain equilibrium. Falling air moves to regions of higher pressure, compresses and warms. The greater the pressure, the greater the warming. Rising air moves to regions of lower pressure, expands, and cools. The amount of warming (or cooling) per unit distance is described as the “lapse rate.” On Earth the dry lapse rate is 9.760 K/km. On Venus, the dry lapse rate is similar at 10.468 K/km. This means that with each km of elevation you gain on either Earth or Venus, the temperature drops by about 10C.
It is very important to note that despite radically different compositions, both atmospheres have approximately the same dry lapse rate. This tells us that the primary factor affecting the temperature is the thickness of the atmosphere, not the composition. Because Venus has a much thicker atmosphere than Earth, the temperature is much higher.
dT = -10 * dh where T is temperature and h is height.
With a constant lapse rate, an atmosphere twice as thick would be twice as warm. Three times as thick would be three times as warm. etc. Now let’s do some experiments using this information.
Experiment # 1 – Atmospheric pressure on Venus’ surface is 92 times larger than earth, because the atmosphere is much thicker and thus weighs more. Now suppose that we could instantly change the molecular composition of Venus atmosphere to match that of Earth. Because the lapse rate of Earth’s atmosphere is very similar to that of Venus, we would see little change in Venus temperature.
Experiment #2 – Now, lets keep the atmospheric composition of Venus constant, but instead remove almost 91/92 of it – to make the mass and thickness of Venus atmosphere similar to earth. Because lapse rates are similar between the two planets, temperatures would become similar to those on earth.
Experiment #3 – Let’s take Earth’s atmosphere and replace the composition with that of Venus. Because the lapse rates are similar, the temperature on Earth would not change very much.
Experiment #4 – Let’s keep the composition of Earth’s atmosphere fixed, but increase the amount of gas in the atmosphere by 92X. Because the lapse rates are similar, the temperature on Earth would become very hot, like Venus.
Now let’s look at measured data :
http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-t.gif
http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-p.gif
Note that at one Earth atmospheric pressure on Venus (altitude 50km) temperatures are only about 50 degrees warmer than earth temperatures. This is another indication that atmospheric composition is less important than thickness.
Conclusions : It isn’t the large amount of CO2 which makes Venus hot, rather it is the thick atmosphere being continuously heated by external sources. It isn’t the lack of CO2 on Earth which keeps Earth relatively cool, rather it is the thin atmosphere. Mars is even colder than earth despite having a 95% CO2 atmosphere, because it’s atmosphere is very thin. If greenhouse gases were responsible for the high temperatures on Venus (rather than atmospheric thickness) we would mathematically have to see a much higher lapse rate than on Earth – but we don’t.
WUWT commentor Julian Braggins provided a very useful link which adds a lot of important information.
“The much ballyhooed greenhouse effect of Venus’s carbon dioxide atmosphere can account for only part of the heating and evidence for other heating mechanisms is now in a turmoil,” confirmed Richard Kerr in Science magazine in 1980.
The greenhouse theory does not explain the even surface temperatures from the equator to the poles: “atmospheric temperature and pressure in most of the atmosphere (99 percent of it) are almost identical everywhere on Venus – at the equator, at high latitudes, and in both the planet’s day and night hemispheres. This, in turn, means the Venus weather machine is very efficient in distributing heat evenly,” suggested NASA News in April 1979. Firsoff pointed out the fallacy of the last statement: “To say that the vigorous circulation (of the atmosphere) smooths out the temperature differences will not do, for, firstly, if these differences were smoothed out the flow would stop and, secondly, an effect cannot be its own cause. We are thus left with an unresolved contradiction.”
======================================================
An update for those interested in what Venus looks like at the surface.
On March 1, 1982, the Soviet Venera 13 lander survived for 127 minutes (the planned design life was 32 minutes) in an environment with a temperature of 457 °C (855 °F) and a pressure of 89 Earth atmospheres (9.0 MPa). The photo composite above shows the soil and rocks near the lander.
Here’s another Venera image that shows a hint of yellow atmosphere. – Anthony
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wayne: May 12, 2010 at 6:43 pm
Hi Wayne,
The conduction effect is there, in our atmosphere and anywhere else that things are in contact, but it isn’t the dominant effect on Earth; the convection and radiation processes are, as I’m sure you know.
Regarding temperature gradients and your thoughts about molecular speeds, think again about that iron bar. One end is hot. One end is cold. Once it stabilizes, every little slice along the bar is at a different, but stable, temperature, and energy is being transported along the bar from the hot end to the cold end. Each little slice stays at a fixed temperature as long as the hot end stays hot and the energy is “dumped” at the cold end. That means that the velocity of the molecules in each adjacent slice will be different, and the faster ones transfer energy to the colder ones, but they don’t actually go very far. They just sit there, move back and forth, and give their extra energy to the next ones in line, and so on until the end of the bar. Each slice gets continually “re-heated” and they keep on doing that, which is why it is possible to have adjacent slices with different speeds (and thus temperatures) for the molecules in those slices. In the atmosphere, when energy gets transported upward in this manner, it is also dumped at the cold end (the top) in the form of outward radiation. In the bar, you could dump it into a big ice-bucket or something.
It’s a lot like an electrical circuit with a battery connected to a resistor in the form of a long wire. The energy stored in the battery creates an electric field (a voltage gradient) that drives the current along the wire. As long as the battery stays topped up, and maintains that voltage gradient, the process will continue moving energy along the wire. The voltage gradient in that case serves the same purpose as the temperature gradient for thermal processes.
Regarding the 5 and 3 business, it’s only the 3 that matters for temperature (and thus speed). The 5 is what’s needed for heat capacity because such molecules can have 5x(1/2)kT units of energy at ordinary temperatures. Two of those units go into making them spin, but not actually making them go anywhere. At much higher temperatures, there are extra degrees of freedom due to bending (or vibrational) modes, that can also suck up energy (but again, don’t increase molecular speed).
Here’s a short slideshow you might find useful about all that:
Introduction to Molecular Physical Chemistry
Keep thinking (as will I)! 🙂
/dr.bill
RE: Nick Stokes (May 12, 2010 at 12:11 am) “The arithmetic goes like this. About 40 W/m2 outward IR comes from ground through the atmospheric window (unabsorbed frequencies), at about 288K. Another 60W/m2 approx goes out from cloud tops, also fairly warm. The rest comes from varying altitudes, with about half from the tropopause at about 225 K. That all adds up to an average 255K.”
Thanks. I am still not sure that I have ever heard a good explanation of why the tropopause regions of the Earth and especially Venus have such strong ice-locker (negative greenhouse) effects.
It seems to me that there might be some special factor perhaps related to thermal radiation from a distributed volume that might be making tropopause regions of these planets so effective in expelling convected heat that they cool far below the nominal thermal radiation temperature. I would expect that in the case of Venus, most of the non-reflected heat returned to space is from the tropopause on up. I note that the tropopause on Venus is nominally at the end of the normal adiabatic lapse-rate at 60 km.
It seems that many educational sites I see on the web are describing Venus as the result of a run-away CO2 greenhouse effect, yet I also find a statement saying that if the Earth were ever moved to the orbit of Venus, there would be a run-away water-vapor driven greenhouse effect that would eventually boil away the oceans. This would allow UV to disassociate water molecules in the upper atmosphere, where the hydrogen would escape to space. Then the freed oxygen would be able to combine with all the carbon on the Venus-like surface and anything else it took a liking to.
Spector,
There is much confusion over the term “runaway greenhouse effect.” As oroiginally postulated it was a hypothetical scenario involving water that may have occured in Venus’ distant past, but not in effect today. From there it morphed into whatever maintains the current temperature on Venus. From there it morphed into a passion play about discarded deodorant cans, or something.
RE: George Turner (May 13, 2010 at 12:07 am) “…it was a hypothetical scenario involving water that may have occured in Venus’ distant past, but not in effect today. “
I am not sure that CO2 alone is responsible for the greenhouse effect on Venus as there are many gaps in its absorption spectrum, but with an atmosphere that dense, perhaps almost all gases are ‘greenhouse gases.’
The scenario I was referring to implied that moving the Earth to orbit of Venus would cause the Earth to eventually acquire a Venus-like atmosphere. I do not want to try that experiment.
George Turner, Steve, and all –
There is a website called SpectralCalc.com with a free public tool (pay for unlimited usage, all outputs) for calculating transmittance spectra of a gas or gas mixture. With this I calculated the CO2 transmittance in Venus’s atmosphere and found that the CO2 spectral lines broaden DRAMATICALLY when you go from 380ppm on Earth to “88,000,000 ppm” equivalent on Venus. Most of the IR spectrum from 1.4 – 28 microns is blocked, and water vapor blocks everything longer. There are narrow IR “windows” at
1.70 – 1.73 microns (max 58% transmittance)
1.82 – 1.84 microns (max 34% transmittance)
2.21– 2.42 microns (>90% fr0m 2.27 to 2.41)
3.28 – 3.56 microns (>90% from 3.36 – 3.47)
5.85 – 6.68 microns, but blocked by water vapor
>0% at 27 microns and higher, but blocked by water vapor
So SpectralCalc gives you George your VIRTIS window at 2.3 microns, but leaves most of the IR-B and C bands with 0% transmittance.
The IR-A band is pretty clear except for some water vapor peaks (and maybe other gases), but I’m guessing even a 733K surface doesn’t radiate much IR-A.(? Don’t have tool.)
But how can the absorption lines broaden so much when as Steve says “the greenhouse effect is logarithmic. After the first few percent, additional CO2 makes much less difference to the temperature”?!
The reason – you don’t stop at 100% CO2 in the atmosphere. When you go from Earth to Venus, it’s like having 88,000% CO2 in the atmosphere!!
It is because the transmittance of radiation is multiplicative based on the mass of CO2 through which the radiation must pass.
If 0.6 grams CO2/cm2 (the amount in Earth’s atmosphere) transmits 99.99% of radiation, when you add another 0.6 grams below that, it transmits 99.99% of what’s left (at constant T & P), and so on. So if you have 138,000 grams of CO2/cm2 (Venus), the transmittance is 99.99% raised to the (138,000/0.6)th power.
So, take the CO2 transmittance spectrum on Earth, with 380ppm of gas.
On Venus, you have 88 MILLION ppm of the gas, relative to Earth
Or 231,579 times the mass/m2.
If the data point on your CO2 transmittance graph for Earth is 99.99% at some wavelength “x”, you won’t even see it on your Wikipedia graph for Earth, it’s basically “100%”, but raise 99.99% to the 231,578th power, and your transmittance goes from virtually transparent on Earth to virtually zero transmittance on Venus. Let’s do the numbers:
380 ppm –> 99.99% transmittance at “x” microns
3800 ppm –> 99.90% – So far so good.
38,000 ppm –> 99.00% – Hangin’ in there.
380,000 ppm –>90.48%
3.8 atmos. –> 36.79% transmittance Uh oh!
38 atmos. –> 0.005% transmittance
88 atmos. –> 9 E-11 transmittance. Complete blackout!
Yes, on Earth the logarithmic effect makes little difference, but when you increase the CO2 by a very large number, you see that the effect is quite dramatic. The absorption lines can broaden a great deal when you are changing by many orders of magnitude.
These numbers also illustrate why wildly extrapolating the “1-3 deg C per doubling of CO2” rule of thumb does not apply. This rule was estimated for Earth for up to several thousand ppm of CO2. You can infer from above that CO2 behavior is not going to change dramatically over that narrow range. The rule is based on calculations and geological data including the ice ages and going back to the Paleocene/ Eocene period when Earth had several thousand ppm of CO2 and northern Canada was warm enough for crocodiles to survive. However, there is no data for dozens of atmospheres of CO2. Also, the rule ONLY applies for 1 atmosphere surface pressure, AND it includes all the feedbacks inherent in Earth’s climate system – clouds, ice sheets, humidity changes, etc. Thus, not only should this “rule” NOT be extrapolated by orders of magnitude, it applies only to Earth and has nothing whatsoever to do with Venus, Mars, Jupiter, or any other planet!
Thus two arguments are demolished: the “it can’t be CO2 because the greenhouse is logarithmic” one and the “it can’t be CO2 because of the 1 – 3 deg C per doubling rule” argument. Both fail because they don’t apply for 5 orders of magnitude of change. All we need now is a calculation of Venus’s prodigious radiative energy balance kilometer by kilometer in the lower atmosphere to prove conclusively that it is CO2 that is responsible for Venus’s temperature profile and that the lapse rate is merely a symptom of this layer-by-layer absorption and re-radiation of a large flux of energy.
Jbar:
“If the data point on your CO2 transmittance graph for Earth is 99.99% at some wavelength “x”, you won’t even see it on your Wikipedia graph for Earth, it’s basically “100%”, but raise 99.99% to the 231,578th power, and your transmittance goes from virtually transparent on Earth to virtually zero transmittance on Venus.”
99.99% to the 231,578th power is for all practical purposes completely opaque. And if the spectral lines “broaden DRAMATICALLY,” all wavelengths would essentially be captured by CO2.
So why is there plenty of observed daylight on Venus?
When there is a discrepancy between models and observation, the models are wrong.
A simple experiment can put all this to rest quite easily. Take a long insulated cylinder of some gaseous mix at some pressure and temperature with temperature and pressure sensors at each end and … put it in a centrifuge cylinder long axis parallel to the radius of of the centrifuge. I doubt there is a way to generate enough G’s to get the inside cylinder end pressure to drop to zero but we really only need to generate a measurable pressure difference between the two ends not simulate an entire column out to space.
paul in UK says:
May 12, 2010 at 4:24 pm
I’m imagining trying to model a hypothetical simple atmosphere where all we need for the model is PV=nRT. So, if air is rising, the pressure (P) drops, but I don’t understand why in this simple model temperature (T) drops. Would not P1xV1 = P2xV2? i.e. the gas is free to expand as it rises and the pressure drops; i.e. the volume (V) increases? Isn’t PV a constant; as P drops, V increases, therefore P1V2 = P2V2 therefore no change in T? If V did not change, then I presume T would drop, but presumably V is increasing, that’s why the air gets thinner with altitude?
That’s why it’s more convenient to use this form of the Gas Law: P=ρRT where ρ is density and =n/V. Then as Bill showed above all you need is the Hydrostatic equation, dP/dy = -ρg.
George Turner says:
May 12, 2010 at 4:15 pm
re Phil: May 11, 8:25
I ran CO2 spectra at ~90 atm and there’s a transmission window from 3840^cm-1 to 4600^cm-1, so yes there is a narrow window.
I calculated that 799 W/m^2 could be emitted into that window (blackbody), and since parts of the window are used to image the surface from orbit, some of the radiation is making it directly into space. Other parts of the window are being used to look at constituents of the lower atmosphere, so CO2 emissions (and thus absorption) aren’t occuring there.
Even is the surface emissivity is 0.25, that would still be 200 W/m^2 radiating up from Venus surface, which is more radiation than the surface receives from the sun. If the surface has this negative radiative flux then its temperature is being maintained by atmospheric convection/conduction (or it requires that as supplemental energy).
Or the clouds and other GHGs (H2O, COS, SO2, CO etc), are reducing the losses (refer to Table1 in the VIRTIS site you linked).
Well, like that little choo-choo train in the children’s book my wife read to our kids when they were very little, I’ve been watching this discussion, telling myself “I think I can! I think I can!” gain an understanding of Venus versus Earth environmental lapse rates and surface temperatures.
The first stumbling block I had to overcome was to figure out why adiabatic lapse rate would be the base factor for environmental lapse rate. The first is a calculable quantity based in physics and unrelated to specific environments, only the chemistry of the gases in question. Environmental lapse rate, however, is an observed value. The two are similar but not exactly the same for the atmospheric gases encountered. Furthermore, environmental lapse rate is specified for still air conditions, not moving masses.
Justifying the similarity of the two lapse rates was not an obvious step for me. I guess that I’ve always dealt with fluids or gasses in situations where pressure gradients due gravity are not measurable with available instrumentation. I had to sit back and think a bit. What I came up with is looking at what might happen in a theoretical situation where adiabatic lapse rate might observable and guess what is happening as the distance the gas moves is reduced in steps until it approaches that due simply to thermal energy. I can come up with no reason, thinking of ideal gases at least, why adiabatic lapse rate should not be consistent down to that level. Sensible up and down drafts may not be necessary for gas temperatures to follow an adiabatic lapse rate. OK, maybe this was obvious to everyone else but I don’t remember seeing it mentioned above.
Adiabatic lapse rate is a dynamic value. A delta for delta kind of calculation with no absolute value specified. Environmental lapse rate though is dealing with real temperatures at specified elevations allowing us to say: At elevation X we will experience temperature Y. What establishes an “anchor” pressure/temperature point from which we may then use environmental lapse rate to calculate that Y from X values? So far, no specific single factor has stood out for me.
I suppose what I’d like to see is that there is some convenient black body temperature were incoming solar radiation plus internal planetary heat source energies are balanced by radiation to space. That then would be the base from which all other temperatures are derived. Of course, a straight line dry adiabatic lapse rate is not a good model for the full depth of real Venus and Earth atmospheres. Energy movement through real atmospheres encompasses radiation, convection, and conduction. Clouds can impede radiative energy transfer, molecular thermal energy levels effect both absorption and radiation wavelength bandwidths. Phase and chemical changes introduce interesting and often non-linear variations in lapse rate. There is probably a very large number of things that effect environmental lapse rate.
Now, as to the question of “runaway greenhouse effect.” Assuming my previous statements are rational, I think it is necessary to separate out the specific impact of a CO2 rich atmosphere on environmental lapse rate to make a definitive statement. However, after working my way through this long run of comments, I have come to the conclusion that “runaway” not a scientific description. For it to have been a runaway in the past, the atmospheric temperature would have to have been much cooler than today. I see no descriptions of what we have divined about Venus’ history to believe that there was a time when Venus was as cool as Earth is today.
As for the greenhouse part. I’m not going to worry at this point about a 90% CO2 atmosphere having nothing in common with a Earthly greenhouse. I’ll just assume greenhouse means warming specifically due to CO2’s presence versus other possible atmospheric gasses. Since the CO2 that is there is at both high pressure and high temperature and is nearly opaque to infrared, it does seem like it should have some effect, and probably warming at that. Of course, that poor transmission of infrared is both upward and downward. I does not appear that using the effect of CO2 in Earth’s atmosphere to describe that gas’s effect within the Venus atmosphere is rational. “Runaway Greenhouse Effect” may make a fun sound bite but is far from accurate from an engineering perspective.
Anyway, that is how it plays out for me, looking at the subject as an engineer.
Jbar says:
May 13, 2010 at 3:43 am
The reason – you don’t stop at 100% CO2 in the atmosphere. When you go from Earth to Venus, it’s like having 88,000% CO2 in the atmosphere!!
This is the critical issue which was the basis of my Question #3 to Steven in my post of May 11, 2010 at 11:47 am.
Smokey says:
May 13, 2010 at 4:11 am
So why is there plenty of observed daylight on Venus?
Because major components of “observed daylight” are not infrared?
Frustrated.
Venus’s atmosphere most likely started out like ours. However, on Earth much of the secondary atmosphere became part of the planet surface, (carbon dioxide and sulfur dioxide dissolved in the oceans or were absorbed with surface rock). If all of the dissolved or chemically combined carbon dioxide on the planet were released back into our atmosphere, its new comp would be 98% carbon dioxide and 2% Nitrogen and the pressure would be about 70 times its current value. A lot like Venus, huh? The difference is that Venus’s greenhouse gases never left the atmosphere the way ours did.
Venus’s runaway greenhouse effect, (and that is actually a scientific term), results from it’s composition. Period. It’s atmosphere consists of 96.5% carbon dioxide. Radiation enters the atmosphere, strikes the surface, and is reflected back towards space where the thick blanket of atmosphere (remember its comp – 96.5% of those nasty co2 molecules) absorbs 99% of all the infrared radiation. This, in turn, rewarms the planet with its own radiated heat (hence the term greenhouse effect). This simple explanation should also explain the tag “runaway”.
And, yes, there is temperature equilibrium. The reason we can enjoy our balmy 280K surface temp is a direct result of this. Our planet needs greenhouse effect in order to maintain a comfy temp, but it also needs to stay in balance. Without the greenhouse effect, average surface temp of the planet would be 40K cooler. So, maintaining a balance of incoming radiation and re-radiated absorbed energy is extremely important. Long wavelength infrared radiation is partially blocked by Earth’s atmosphere, mainly because of carbon dioxide and water vapor, both of which absorb very efficiently in the infrared portion of the spectrum. Even though these two gases account for only a tiny fraction of the atmosphere, they manage to absorb a large fraction of all the infrared radiation emitted from the surface. Consequently, only some of that radiation escapes back into space; the rest is radiated back to the surface, causing the surface temp to rise. With this info, you can quickly realize that even a small difference in the number of co2 emissions we contribute to the atmosphere can have large consequences.
My question is this: Why take the chance? Why not make major changes in our ways that make our planet a healthier place to live? What’s the harm? If I’m wrong (I’m not, trust me) in the end, we still have a healthy planet to give our children. If the conservatives are wrong and nothing is done to turn global warming around, we’re screwed. Period. I understand that the conservatives think that there are more important things to spend money on, but the funny thing about it is that they won’t give a shit about those things when they’re dealing with a major global climate revolution.
Correy Dukes: May 13, 2010 at 1:11 pm
Give it a rest Correy. You’re just wasting bandwidth with that “think of the children” crap, and your “book of revelations” needs a rewrite.
/dr.bill
My question is this: Why take the chance? Why not make major changes in our ways that make our planet a healthier place to live? What’s the harm? If I’m wrong (I’m not, trust me) in the end, we still have a healthy planet to give our children.
Economics is about trade-offs. It’s about recognizing opportunity costs, and all of the knock-on effects of a proposed policy.
If I could recommend only one thing to you, it would be to read “Economics in One Lesson“.
The “harm” in such an approach could, in fact, be staggeringly large. If you’re “wrong” (and trust me, you most definitely could be), there could be fewer children to leave the “healthier world” to.
You might think that trade is “worth it”. Somehow, I doubt that they would.
Returning to the baffling topic of why is the average temperature on Venus reportedly uniform all over, when there is such a long nighttime to lose heat:
I mentioned that the cooling period was ~117x longer than on Earth, but have since realized that because the year length is of similar order to the day length for Venus, its presentation pointing away from the sun is ~122 earth days, not ~59 Earth days.
It is of course impossible for there to be no loss of heat over such a long period. For a start, there are some small windows for shorter wavelength infrared to escape, and there is no such thing as a perfect insulator.
Ref: Sidereal rotation period (hrs) -5832.5 (Length of day (hrs) 2802.0)
Bob_FJ: May 13, 2010 at 5:18 pm
Jbar had a comment higher up on this thread (May 9, 2010 at 5:26 pm) which might have some relevance. It’s of “sketchy provenance”, but that wouldn’t necessarily make it un-factual. I’m not sure how truly uniform the surface temperature of Venus is, but with a very large mass and not much happening at ground level, it might be possible to get a fairly “sluggish” lower layer.
I don’t know if there is a data set giving the pressure and temperature profile of Venus right down to the surface, or if what Steve showed on his graphs is all that exists. If the whole range is available, the density profile can be found from that, and then maybe some viscosity calculations could shed some light. Anyone know about a complete datat set?
/dr.bill
Correy Dukes ,
I won’t claim to understand the vertical structure of atmospheric temperature , but one thing I know is that your 40k number for earth’s “greenhouse effect” is a BS number based on a misleading , mathematically intractable assumption confounding albedo with spectrum positing a body which has a non-zero reflectance yet perfect emissivity . Far more relevant is that we are about 9k warmer than a gray ( flat spectrum ) ball in our orbit , how dark or light does not matter .
As Barry Kearns points out , you similarly appear to assign zero cost to trying to suppress the amount of available carbon in the biosphere , ignoring the significant greening of the planet promised by increasing the substance which combined with H2O by sunlight forms over 90% of every bite of food , and therefore every warmist and realist on the planet .
Even with all the discussion both here and on counter-blogs , I still see no alternative to internal heating being the cause of Venus’s steady state surface temperature , more than twice that of a ball totally absorbing facing the sun and reflective facing away , on both its , as Bob_FJ just reiterated , extremely long day and night sides .
Dr.bill, Reur May 13, 2010 at 8:45 pm
[1] I would describe some of Jbar’s comments as shaky rather than sketchy. If he would respond to my earlier comment here, I might debate his May 9, of 5:26PM that you cite.
[2] Let me put it this way; in my past life, when signing-off any draft report from my engineers, I would apply to their data or whatnot what I call: A test of reasonableness. If by analogy they had submitted analyses like that reported from the ESA here, I would have “politely” told them it had failed the “reasonableness test” and to go away and do it again.
The point is, that in the applied sciences, such as in engineering, we cannot make ideological assumptions, because like for example; people might get killed, or there could be bad infrastructure failures. (although mistakes do still happen). However it is a much more flippant approach in academia, where securing future research funding may well be a primary consideration.
Here is an interesting extract from the link immediately above:
The [temperature] map comprises over a thousand individual images, recorded between May 2006 and December 2007. Because Venus is covered in clouds, normal cameras cannot see the surface, but Venus Express used a particular infrared wavelength that can see through them.
Well that is interesting; over such a long time! How were the individual 1,000+ images cross calibrated? Were they all taken at the same time of day? ……. No ‘limb darkening’ then? (are some things springing to mind)
[3] I would think that the only T versus altitude and atmospheric pressure (also surface wind speed) data were derived from short survival time Soviet probes that obviously only had single non vertical descent paths and landing points.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
BTW Bill, I look forward to your response to my comment here.
Bob Armstrong: May 13, 2010 at 10:49 pm
Hi Bob,
I just had a browse around your website, and found your plot of planetary temperatures vs distance from the Sun very interesting. Pictures can make you think, and that graph is a good example. Venus sure does stick up there like a sore thumb. Thanks.
/dr.bill
Can you show us any other instance where nature magically violates the 2nd law of thermodynamics?
Bob_FJ: May 14, 2010 at 12:57 am
Hi Bob,
I missed that earlier message from you, and am off to work in a bit, so I’ll get back to you later about that. Regarding your other comments, I certainly have to agree with your “rubber hits the road” attitude. I beat that into my students until they’re numb from hearing about it. Too bad it isn’t a more prevalent requirement in ‘climate science’.
/dr.bill
Bob_FJ: May 14, 2010 at 12:57 am
Actually, I do have a few more minutes till I’m out the door.
For the opaque paint on the concrete, what I was getting at was that ‘things going on inside’ will surely affect what happens at the paint layer, but the paint layer is where the energy emerges, and the Planck distribution and S-B results are expressed in terms of what happens at surfaces. I think of it like the nozzle of a fire-hose. Any number of things may be happening ‘back the line’, but the nozzle is where the water finally gets out. The pressure behind it will affect what happens at the nozzle, but the characteristics of the nozzle also shape the effect of the pressure.
That analogy is a bit weak, however, in the case one object (say your ice cube) emitting ‘from inside’ of another object (the water layer outside of it), since water in a fire-hose, being ‘materially substantial’, can’t pass through other water molecules the way that radiation from one place can pass through radiation from another, and then combine to give the composite emission pattern. I’m glad you brought up this example. I’ve never given much thought to the idea of a ‘two-layer’ or ‘multi-layer’ Planck emitter before, and it’s a very interesting question that I’ll explore further.
/dr.bill
Mike M says:
May 14, 2010 at 4:24 am
Correy Dukes: :absorb a large fraction of all the infrared radiation emitted from the surface”…. “only some of that radiation escapes back into space; the rest is radiated back to the surface”
Can you show us any other instance where nature magically violates the 2nd law of thermodynamics?
No it doesn’t in this case either! You appear to misunderstand the 2nd Law, your version appears to hold that a photon knows what the temperature of its ultimate target is before it is emitted and only leaves in that direction if the target is cooler. The 2nd law as far as radiation is concerned says that net transfer is from hotter to cooler.
MikeM,
“Can you show us any other instance where nature magically violates the 2nd law of thermodynamics?”
There’s no violation, for two reasons – firstly, the 2nd law only talks about the net flow and that’s still directed upwards, and secondly, because heat can indeed go from colder to hotter if there is an input of work. A refrigerator transfers heat from the cold interior to the warm exterior, without breaking the second law. In the case of the atmosphere, convection supplies this work.
The model of heat being radiated back is a description appropriate to a non-convective atmosphere, and if it wasn’t for convection, the atmosphere would indeed operate this way. (See here for more detail.) However, because the atmosphere does have convection, the pure radiative model is incorrect, and grossly misleading. It’s also contradicted by experiment.
Correy is wrong, but not because there’s a problem with radiation shining from a cold sky to a warm surface. He’s wrong because convection prevents any such surface heat build-up before it can occur.