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.”
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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
Nick
The reason people believe that kind of nonsense is because that is what they have been told.
http://en.wikipedia.org/wiki/Carl_Sagan
Sagan established that the atmosphere of Venus is extremely hot and dense with pressures increasing steadily all the way down to the surface. He also perceived global warming as a growing, man-made danger and likened it to the natural development of Venus into a hot, life-hostile planet through a kind of runaway greenhouse effect.
Here is one from the American Institute of Physics :
http://www.aip.org/history/climate/Venus.htm
George Turner: May 10, 2010 at 3:13 pm
re dr.bill: May 10, 2010 at 5:22 pm
I made a typo in my response to you.
Here’s what I said:
By the time you get to the top of the stratosphere, however, all but a tiny fraction of the mass of the atmosphere is below you, there is precious little buoyancy left anyway, and thus there is simply a cooling gradient (negative lapse rate, if you wish) downward from the tropopause. In any case, the stratosphere is hotter at the top because that is where the heating occurs, and the troposphere is hotter at the bottom because THAT is where the heating occurs.
I should have said: “downward from the stratopause“, which is roughly at a height of 50km. That negative lapse rate persists downward for about 30km from there, and then there is an isothermal region about 10km high that has the same termperature as the tropopause.
Sorry for any confusion,
/dr.bill
Yes, very prominent in the WNW sky in Canberra, very little light pollution where I live. I was checking it out two nights ago in my 10 power binoculars when a meteor passed just under Venus, within my field of view. Just amazing !
My ‘Astronomy Australia 2010’ tells me that Venus will be 2deg above the thin crescent moon (1deg in WA) on the 16th May and an occultation will be observed across North Africa, the Middle East, China and Indonesia.
By the 15th of May, Venus will have a diameter of 12.o” and a magnitude of -3.9.
Steven, I had hoped to get a response from dr.bill in my post at wayne says: May 10, 2010 at 6:58 pm. I have given much thought to you two articles on Venus. Very intriguing. See if this aligns with your thoughts on the lapse rate at a more molecular level.
Every planet with an atmosphere also has a lapse rate. Earth has one, Venus & Mars too, and even though I have never read it, I assume the major gas planets have one also. I had to stop here and think. Why? Much of what I am going to state below you will just say, I know that, but I have to state it so we are parallel and you know exactly what I am aware of.
As you have been saying of Venus, it is the pressure. The atmospheres of planets create a very special case in physics. You cannot create experiments in the lab that would ever exactly mimic the physics driving the atmosphere in this one aspect, gravity holds the gases in place with increasing pressure as the altitude descends, no work being performed on the gases. That property is unique to atmospheres, because of the gravity. But why would temperature naturally increase with the pressure?
It took me a while thinking on that point before the light bulb went off… all velocities of the molecules must equalize naturally throughout the entire atmosphere. Of course they are never the same but that is the tendency. That is how photonic heat absorbed in the upper atmosphere is transferred to lower (and hotter) levels.
This is very anti-logic and anti-normal-thermodynamics! Thermodynamics states that heat can only move from hotter to colder, never the opposite direction, but in the special case of atmospheres held by gravity, that is exactly what happens and it is because the molecular velocities are striving to equalize.
Isolate this in your mind to one single molecule for simplicity. If that molecule accepts a photon and is accelerated to a higher velocity, it’s increased velocity will be distributed to the neighboring molecules in ALL directions, even though lower molecules happen to be, because of gravity, at a slightly greater pressure. If all molecules tend to have equal velocities throughout the entire atmosphere, the surface WILL be hotter than higher in the atmosphere.
Steven, on a molecular level is that not what you are saying, that is why lower levels are always hotter and that is how radiation in the upper atmosphere makes it way to the surface at a higher temperature? I never had thought so deep on the definition of temperature itself, never needed to before. If all of the above is true, most definitions for temperature you find on the internet are somewhat mis-stated.
For dr.bill:
I am assuming you would have written back to my question above that the temperature of the cubic cm from the surface would read much higher than the cubic cm taken high in the atmosphere even though the molecular velocities in each case were equal. That makes sense, it contains more kinetic energy per volume than the one up high at lower pressure.
dr.bill says:
May 10, 2010 at 7:58 pm
Just noticed you did respond. So you are saying temperature has nothing to do with the total energy within a given volume which matters directly with pressure (density)? In case one, taken at the surface, with 90 times the molecules at ½mv² it has 90 times the energy as case two taken at a high altitude, all molecular velocities being the same. Are you sure of that?
I would have said that for the two samples, to have the same temperature (OK, too small for the thermometers, make them liters), the one at high pressure would have to have √90 of the velocity so the energy per volume would be the same. Like just grabbing some numbers: ½·10·500² = ½·(10*90)·(500/√90)².
I thought temperature measured the average kinetic energy, not simply average molecular velocity, one ignores mass. Are you sure of your statement? After all of these years following physics I find the EXACT definiotion of temperature still somewhat eludes me.
Dr. Bill,
Regarding one of your recent above posts (I’m working backwards and a little tipsy from watching “Hancock”), I think one of the problems with Stefen-Boltzmann’s law is that we use it to reduce infinitely variable and complex spectral emissions into a simple number, thus destroying information in return for a handy-dandy yardstick more useful for marketing fluorescent bulbs than doing physics.
As a side note on “Hancock”, I’m upset that Charlize Theron went out with Keanu Reeves last week when we all know she deserves a sceptical climatologist.
Bill,
Regarding your comment at 5:22?, (ie. no work is done on descending air parcels.)
Yep, that’s external work done on the gas as it trades off potential energy (from PE=mgh).
To compress a gas is to do work on it, as it requires a force applied through a distance, which is the definition of work (W=F*d).
It may seem like no work is being done on a descending air column, but believe me, the work is enormous (though largely undocumented until we figure out how to tax it).
re: wayne: May 10, 2010 at 9:19 pm
and wayne: May 10, 2010 at 9:55 pm
Hi Again Wayne,
You said a couple of contradictory things there, perhaps accidentally, but your belief that temperature is equivalent to kinetic energy, and not just velocity, is correct. The exact definition of temperature is the one I gave in my previous note to you. Here it is again: (1/2)mv² = (3/2)kT, where k is the Boltzmann constant. This is just equal to the Gas Constant divided by Avogadro’s Number, so it is like a “gas constant per molecule”. You can replace m by M (the molar mass), and k by R (the usual gas constant), and the equation still holds, as long as v still refers to the average speed of a molecule.
You are also not wrong about your belief that heating only occurs from hotter to colder. There are three mechanisms by which one object can heat another. They are Conduction, Convection, and Radiation. If you turn on the burner of your stove, for example, there are three ways to tell that it is hot. You can touch it (conduction), or hold your hand above it (convection), or just look at it (radiation).
For the first two, only a trivial amount of energy will generally be transferred from a cold object to a hot one, but radiation is “exempt” from that, in a certain sense, but not entirely. A cold object can emit lots of photons that can be absorbed by a hotter object, thus increasing its internal energy. The hot object will, at the same time be emitting even more photons toward the colder object, more than making up for what it absorbed. In the end, the hotter object heats the colder one, but there can still be a back-and-forth during the process.
The hotter to colder rule is with respect to the NET transfer between the objects, and the hot one always “wins”. On the other hand, you need to keep in mind that all Thermodynamic processes are statistical averages over time, and at the microscopic level there are fluctuations in both directions.
In addition to these three mechanisms, there is also Mechanical Work, which is what is being done on a gas when you compress it. This, however, requires an external agent of some kind that exerts a force over a distance interval. You might find my note to George Turner (dr.bill: May 10, 2010 at 5:22 pm) useful in that respect.
Hope that helps,
/dr.bill
Gail Combs says:
May 10, 2010 at 3:38 pm
I remember PV=nRT as the ideal gas law.
Are you stating that since Venus is an “open” system the gas law does not apply? And if so what about the effects of gravity?
P=ρRT/M where ρ is the density (n/V)
hydrostatic equation dP=-ρgdh where h is altitude
So dP/P=-Mρgdh/ρRT = -Mgdh/RT
Integrating from the surface to the altitude h: P(h)=P(0)exp(-Mgh/RT)
I.e. the barometric formula.
re: George Turner: May 10, 2010 at 10:24 pm
and George Turner: May 10, 2010 at 10:31 pm
Hi George,
Sorry to hear about your troubles with that unappreciative Charlize.
Perhaps in time she’ll see the light. 🙂
Regarding your other comment about work being done by gravity, I didn’t actually say that, nor did I intend it. You are perfectly correct in saying that gravity does work on the descending gas molecules.
There are, however, a couple of caveats. For a variety of reasons, and even in the absence of atmospheric friction or drag, the descending molecules will have less kinetic energy when they arrive back at the ground than they had when they headed upward. If this weren’t the case, we would have unlimited energy due to a perpetual motion machine.
And then, of course, THERE IS the atmospheric friction, which is just as real for air molecules moving through each other as it is for your hand moving through the air if you stick it out the window of your car while driving at highway speeds (and molecules move around much faster than that). This drag slows the molecules down, and transfers much of the energy they picked up from gravity to the other molecules through which they are moving. By the time they get to the bottom, they’re pretty well tuckered out and ready for a recharge from the ground.
As it always seems in atmospheric phenomena, there are partial exceptions to this as well. The case of Chinooks, Foehn winds, and other descending adiabatic processes that involve movement of entire air masses will avoid this energy loss to some extent my “moving as a whole”, so that the drag is limited to the surface of the air mass, and not to its interior.
Lots and lots of book-keeping! No wonder those GCM’s don’t work!
/dr.bill
RE:dr.bill says (May 10, 2010 at 5:26 pm) “… You are saying that the troposphere is hotter at the bottom because of adiabatic heating, whereas I am saying that the troposphere is cooler at the top because of adiabatic cooling.”
Just a thought: Is it possible that the reason that temperatures at the tropopause are so cold is because the heat [energy] there is being radiated by a distributed volume of trace gases such that any given surface is only emitting a fraction of the total thermal energy required to balance the thermal input from the sun?
I believe in the case of the Earth the ‘nominal’ non-reflected earthshine is supposed to be 235 W/m2 which requires a black-body radiation temperature of about 253.7 deg K. I understand that the temperature at the tropopause is may be as low as 217 deg K which can only force a black-body energy flux of about 126 W/m2. While one might say the other 109 W/m2 comes directly from the surface, I wonder if we really do know what is actually driving the temperature and altitude of the tropopause.
Perhaps the narrow band nature of the radiation spectra of trace gases distributed in a transparent volume makes this region a more effective energy radiator than we now expect. [Pure Speculation.]
Dr Bill
Quote:
If you heated some layer higher up in the atmosphere, it will NOT start heating the air layers BELOW it, because its density will decrease as it heats up, and it will thus tend to RISE because of the buoyancy effect of our gravitational field.
Rubbish.
As long as you have differential heating, then you will get convection, which will eventually encompass the entire air mass.
A hotter column of air, heated half way up, will expand and find itself at a higher pressure at altitude, and thus spread outwards. This will lower the pressure AT THE SURFACE (not half way up). This will induce convection at the surface, as well as half way up.
(This is a standard atmospheric convection explanation)
.
dr.bill,
and
I know that the molecules will have less potential energy when they arrive back at the ground and I guess that that energy has been spent in downward movement due to friction under gravity. But can this solid textbook information be applied to Venus?. Can you say then that the Venusian surface is heating the air? That is the only outcome from this reasoning.
Hi again dr.bill –
Hate to press a point but the figures do jive. You speak to me as if I were a young student and that’s OK but you do need to realize I have loved and followed physics to great depths for most of my life so even though we are by no means equals, I don’t need to be led. My questions are much, much deeper.
Figured the best way to show you is by figures which, since you teach thermodynamics, you will understand. This is using the equations you stated above of which I am familiar. Some assumptions apply: 100% co2 atmosphere, in this example all of the atmosphere is of the same constant temperature, all output figures are SI (internally converted).
Precision(4); = {4} k = ‹R/NA›; = {1.381E-23} // This is at 1 atm molPerL = (1.562‹g/mL›/44.010‹g/mol›)→‹mol/L›; = {35.49} /* CASE ONE AT SURFACE */ P = 90‹atm›; = {9.119E+06} V = 1‹L›; = {0.001} n = 90‹atm›•molPerL; = {3.237E+08} T = P•V/(n•‹R›); = {3.389E-06} m = 90•1.562‹kg/L›; = {1.406E+05} v1 = √(3•k•T/m); = {3.16E-17} /* CASE TWO AT ~52 km */ P = 1‹atm›; = {1.013E+05} n = 1‹atm›•molPerL; = {3.596E+06} T = P•V/(n•‹R›); = {3.389E-06} m = 1.562‹kg/L›; = {1562} v2 = √(3•k•T/m); = {2.998E-16} v2/v1; = {9.487} √90; = {9.487}Notice especially the bottom two figures. That is the √90 reduction in the molecular velocity I spoke of above. You see, this is why I think Steven is onto something. This is the key physics question excluding other physical effects on purpose: can an atmosphere as a whole exist with a huge difference in the velocity of molecules across pressure gradients? That is one good question!
I think basically no. Once again, ignoring convection, conduction rules over the radiative transfer, partially due too the short path length. Of course the hot gas near the surface is going to be radiating a great amount of LW but at such pressures the free path length of the photons will be very short, radiatively skipping from molecule to molecule, but even this I want to make that a separate issue. Home in on the velocity of the molecules. Therefore the velocities will always tend to equalize and therefore lower layers will always be hotter per the lapse rate, radiation and convection attempting to counter this to a certain point in a actual atmosphere.
Steven, I think you’ve got something there! Great hypothesis. One of the best new thoughts I have come across in a long time.
Oh, and by the way, the work done on the gases by gravity occured when Venus came to be, except for packets of rising convection and such, there is no work done on the atmosphere, in gravity case anyway. Right?
Mods—
Oh, brother! Why is “PRE” not using a font with UNICODE such as Lucida Console?
It sure didn’t like the tiny brackets!
Ralph: (May 10, 2010 at 4:42 pm) “Nonsense… The temperature of the air at 35,000ft does not suddenly warm to 20oc, just because the atmosphere is not convective that day.”
Not that day, but perhaps 100,000 years later if that condition (no convection anywhere in the atmosphere) persists from then on.
Further to my last, you would also need an unstable lapse rate to continue the convection. I am presuming that the composition of this atmosphere can lead to an unstable lapse rate – perhaps Steve could enlighten us on that.
.
Steven Goddard says, in reply to me:
“If you push a piston down into a cylinder, you are exerting a force which directly increases the pressure. That is why the temperature increases. Your claim that “everything else is held equal” is incorrect.”
First, on a point of accuracy which anyone can check, I did not use the words “everything else is held equal”. I used the phrase “other things being equal” as part of a hypothetical fallacious argument which I then showed to be fallacious precisely because other things were not equal!
Second, and more important, I said that the increase of temperature when a gas is compressed is due to the transfer of kinetic energy from the moving piston to the molecules of the gas. Is Steven Goddard disputing this? Surely it is basic physics. The temperature of a gas is the average kinetic energy of its molecules. If the kinetic energy of the gas molecules is increased, that energy must come from somewhere. The only source of energy directly impinging on the gas from outside is the movement (i.e. kinetic energy) of the piston. Does Goddard disagree with this? If so, where else does he think the energy is coming from?
Goddard also says that the temperature increases because the pressure increases. I think that is a misleading way of putting things. The pressure of a gas on the walls of a container is due to the impact of gas molecules. The amount of pressure depends on (a) the number of impacts in a given time and (b) the kinetic energy of each impact. The first of these factors depends partly on the number of particles per unit volume and partly on the velocity of the particles, since fast-moving particles will hit the walls more frequently in a given time than slow-moving ones. An increase in temperature increases both the velocity of the particles and the average kinetic energy of each impact. Other things being equal, this will result in an increase in pressure. To say that the temperature increases because the pressure increases is inverting the direction of causation. It is also possible to increase pressure without an increase in temperature, by putting more gas into the container, or by reducing the volume of the container while maintaining it at a constant temperature by cooling.
In any case, whether we describe the increase in temperature under compression as due to a transfer of kinetic energy or to an increase in pressure, in practice the increase of temperature is always only temporary, as the extra heat is eventually conducted, convected, or radiated away.
Spector says:
RE: Ralph: (May 9, 2010 at 11:59 pm) “You don’t need convection for an adiabatic lapse rate . . .”
Apples and Oranges . . . If there are no rising or falling parcels of air then there can be no adiabatic cooling or warming.
Ralph is correct. You get the lapse rate with or without. Deviations from the lapse rate occur, but that is what forecasters used to look for 50 years ago when they plotted temps on skew-T diagrams to determine atmospheric stability and the likelihood of getting your picnic ruined.
stevengoddard says:
The ideal gas law works just fine, and gravity has nothing, nada, zippo to do with the accuracy of the equation.
stevengoddard says:. . .
Gas pressure P = nRT/V . There is no term for either mass or gravity
stevengoddard says:
Mike M [different Mike]
As I have stated about 14 times now, the atmospheric pressure (in temperature ranges where it is a gas) is set by the weight of the atmosphere above it. P is fixed by the weight of the column of air above.
Weight requires gravity.
Nullius:
“During the Hadean period, an atmospheric pressure of 200 atm has been suggested.”
Sure, I could have suggested it myself. Do you have a source? I’d love to read it. From a scientific paper or book? Not from the blogosphere.
Unfortunately even if there were a proxy for atmospheric pressure in rocks, and I haven’t read that there is, no rocks exist from earlier than about 3 billion years ago, so there’s no way to find data to confirm it.
Steve G: “On earth water vapor saturates between 0 and 100mbar”
So what’s your point?
The maximum vapor pressure of water in an atmosphere depends on the temperature. If the surface of a planet were sustained at 99C by radiation or GH effect, then the vapor pressure of water at the surface would be 1 bar. I wasn’t talking about the surface of OUR Earth.
Steve: Your May 10 7:50PM comment about misinformation from the scientific community.
This post is by one person. I’m sure the climate science community consensus does not support the idea of AGW turning the Earth into Venus. Individuals can make mistakes. That’s why we need conferences, peer review, journals, etc. to separate the wheat from the chaff.
There is not enough fossil fuel in the ground to raise atmospheric CO2 more than maybe a couple thousand ppm, and this give nowhere near enough GHE to create a Venus-style runaway GH.
Because they aren’t rocks, it is actually an ancient Venusian road. They kept cutting down trees, building structures and roadway turning the whole planet into one big city.
So eventually the entire planet became one big urban disaster, (imagine you keep ending up in Detroit no matter which direction you go). The Venusians starved to death long ago waiting for their welfare checks and all that remains to remind us of them is how they turned UHI into UHP, (Urban Heat Planet).
During the building spree, (financed mostly by carbon credit speculation), a Venusian prophet allegedly said, “This whole damn planet’s going to Hell!”
I have an idea though … let’s send a few gigaplethora pounds of finely ground titanium oxide to Venus and explode it in its upper atmosphere to float down and coat the whole planet. Then more heat heat will be reflected and maybe the planet will cool off enough for us to visit in person some day?
Dr bill and George,
I think the energy exchange goes like this. At the exact dry adiabat, rising or falling air is always at the same temperature as ambient, so there is no buoyant force. But if the gradient is less than the adiabat (closer to isothermal) then rising air becomes cooler than ambient. Work is done to make it rise. Falling air becomes warmer than ambient. It too needs a push. Both motions transport heat downward. It’s a heat pump, and the energy comes from the random motions of the air. That’s why air below the dry adiabat is stable to convection. It tends to damp these motions.
The random motions of the air arise from turbulence, driven by the sun’s energy creating temperature differences and hence wind. Pumping heat downward causes the gradient to move towards the neutral dry adiabat.
If the gradient exceeds the adiabat, the air is convectively unstable. The relations are reversed. Air that rises becomes warmer than ambient, and acquires a buoyant upthrust. It now carries heat upward. Motion is created, and heat is moved from warm to cooler. It’s a heat engine.
But again the effect is to move the gradient back towards the dry adiabat.
I’ve been putting these arguments on my blog.