By Steve Goddard
The classic cure for hyperventilation is to put a paper bag over your head, which increases your CO2 levels and reduces the amount of Oxygen in your bloodstream. Global warmers have been hyperventilating over CO2 on Venus, ever since Carl Sagan made popular the idea of a runaway greenhouse effect. That was when he wasn’t warning about nuclear winter.
Sagan said that marijuana helped him write some of his books.
I bought off on the “runaway greenhouse” idea on Venus for several decades (without smoking pot) and only very recently have come to understand that the theory is beyond absurd. I explain below.
The first problem is that the surface of Venus receives no direct sunshine. The Venusian atmosphere is full of dense, high clouds “30–40 km thick with bases at 30–35 km altitude.” The way a greenhouse effect works is by shortwave radiation warming the ground, and greenhouse gases impeding the return of long wave radiation to space. Since there is very little sunshine reaching below 30km on Venus, it does not warm the surface much. This is further evidenced by the fact that there is almost no difference in temperature on Venus between day and night. It is just as hot during their very long (1400 hours) nights, so the 485C temperatures can not be due to solar heating and a resultant greenhouse effect. The days on Venus are dim and the nights are pitch black.
The next problem is that the albedo of Venus is very high, due to the 100% cloud cover. At least 65% of the sunshine received by Venus is immediately reflected back into space. Even the upper atmosphere doesn’t receive a lot of sunshine. The top of Venus’ atmosphere receives 1.9 times as much solar radiation as earth, but the albedo is more than double earth’s – so the net effect is that Venus’ upper atmosphere receives a lower TSI than earth.
The third problem is that Venus has almost no water vapor in the atmosphere. The concentration of water vapor is about one thousand times greater on earth.
Composition of Venus Atmosphere
0.965 CO2
0.035 N2
0.00015 SO2
0.00007 AR
0.00002 H2O
Water vapor is a much more important greenhouse gas than CO2, because it absorbs a wider spectrum of infrared light – as can be seen in the image below.
http://www.globalwarmingart.com/images/7/7c/Atmospheric_Transmission.png
The effects of increasing CO2 decay logarithmically. Each doubling of CO2 increases temperatures by 2-3C. So if earth went from .04% CO2 to 100% CO2, it would raise temperatures by less than 25-36C.
Even worse, if earth’s atmosphere had almost no water (like Venus) temperatures would be much colder – like the Arctic. The excess CO2 does not begin to compensate for the lack of H2O. Water vapour accounts for 70-95% of the greenhouse effect on earth. The whole basis of the CAGW argument is that H2O feedback will overwhelm the system, yet Venus has essentially no H2O to feed back. CAGW proponents are talking out of both sides of their mouth.
So why is Venus hot? Because it has an extremely high atmospheric pressure. The atmospheric pressure on Venus is 92X greater than earth. Temperatures in Earth’s atmosphere warm over 80C going from 20 kPa (altitude 15km) to 100 kPa (sea level.) That is why mountains are much colder than the deserts which lie at their base.
The atmospheric pressure on Venus is greater than 9,000 kPa. At those pressures, we would expect Venus to be very hot. Much, much hotter than Death Valley.
http://en.wikipedia.org/wiki/File:Emagram.GIF
Wikipedia typifies the illogical “runaway greenhouse” argument with this statement.
Without the greenhouse effect caused by the carbon dioxide in the atmosphere, the temperature at the surface of Venus would be quite similar to that on Earth.
No it wouldn’t. 9000 kPa atmospheric pressure would occur on earth at an altitude many miles below sea level. No such place exists, but if it did – it would be extremely hot, like Venus. A back of the envelope estimate – temperatures on earth increase by about 80C going from 20 to 100 kPa, so at 9,000 kPa we would expect temperatures to be in the ballpark of :
20C + ln(9000/(100-20)) *80C = 400C
This is very close to what we see on Venus. The high temperatures there can be almost completely explained by atmospheric pressure – not composition. If 90% of the CO2 in Venus atmosphere was replaced by Nitrogen, it would change temperatures there by only a few tens of degrees.
How did such bad science become “common knowledge?” The greenhouse effect can not be the cause of the high temperatures on Venus. “Group Think” at it’s worst, and I am embarrassed to admit that I blindly accepted it for decades.
Blame CO2 first – ask questions later.
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UPDATE: Lubos Motl has written an essay and analysis that broadly agrees with this post. See it here
Harmlessly? Then what happens? As more and more molecules ‘fall’ to the surface on this supposedly ‘dead’ planet isn’t it’s mass going to increase? As it’s mass increases doesn’t this mass exert a (gravitational) force on all those molecules? When you impart a force onto molecules, what happens?
@joel Shore
‘In the earth’s thermosphere the temperature becomes even higher than at the earth’s surface ( http://envam1.env.uea.ac.uk/images/tprofile.jpg ) and yet the pressure is down by a factor of more than a million!
No matter how you slice it, you can’t have a surface temperature as high as Venus’s is in the absence of an IR-absorbing atmosphere. It would radiate way more than it absorbs and rapidly cool down. It really is as simple as that.’
Actually you can. It doesn’t matter the atmosphere if you don’t add pressure, with one pressure you’re left to the planet core, and the planets behavior in its orbit, and the amount of sun light/radiation the planet receives. Our moon has virtually zero atmosphere, and zero atmospheric pressure, yet reaches about 190 f in the sun, which is about the mean for Mercury which has trace atmosphere at best, virtually no co2 at all or nitrogen or water vapor or pressure, but it is a whole lot closer to the sun though. Venus, for all its weird ass behavior in its sneaky orbit, has everything, yet co2 only comes at third place when it comes to temperature but then only as part of the collective thing that is its planetary greenhouse effect, first comes pressure, second solar radiation.
The thermosphere can reach like 1500 degrees C, yet a person would freeze to death at 0C if he was waiting to get boiled to death, assuming of course he had oxygen to last. :p
Well for starters Venus’s atmosphere is made out of much heavier gases, it has a Mean molecular weight: 43.45 g/mole where Earth’s atmosphere has a Mean molecular weight: 28.97 g/mole so it weighs 1.49 times as much under the same pressure and temperatures conditions.
Note that I expect my previous post to get shot down from where I start proposing how the Venusian heating could be working. After all, “of course” such possibilities must have been considered before.
However, it’s hard to find anything else but the CO2 and greenhouse “noise” regarding Venus. I should learn some good information in the debunking of my proposed system. I will also learn something if it is not soundly debunked. 😉
******
stevengoddard says:
May 7, 2010 at 10:05 am
beng
What you are implying is that the temperature is the same in Death Valley and on Mt. Everest.
*******
No, not at all. The atmosphere is mostly heated at the bottom by the warm, irradiated surface — water or land. That’s why it’s warmer down there, not because of pressure. If you took earth into deep, cold intergalactic space, the solid surface and air at all elevations/pressures would chill down to the background ambient temp — and there’d be no lapse rate. This is assuming the air didn’t liquefy and there was no internal heat, of course.
stevengoddard says:
May 7, 2010 at 10:53 am
Lon Hocker
“So what you are saying is the high temperatures on Venus are due to high atmospheric pressure. I could have sworn that was the point of this article.”
Pressure alone wouldn’t accomplish anything. It needs CO2 or some other IR active molecule in a high concentration in combination with the high pressure to reach Venus’ extreme state. Swap in Ar, N2 or O2, and the atmosphere would be transparent, and Venus would cool off quickly. It even would have convection cells, and could support Willis’ thermostat.
I see some confusion here.
The pressure at a planetary surface is a function of gravity and atmospheric density.
The atmosphere of Venus is extremely dense compared to those of Earth or Mars. It is that density combined with the gravity of Venus that dictates both pressure and temperature.
The temperature will be higher for the same quantity of CO2 than for say Nitrogen because of their different molecular weights and thus their contributions to total atmospheric density.
Steve Goddard has just used the term pressure as meaning the composite effect of gravity and atmospheric density at the surface. It is true that the higher the pressure at a planetary surface then the higher also will be the temperature given a constant solar input.
Hence the differences between Venus, Earth and Mars with the greenhouse effect idea being a mere unnecessary distraction.
Extra GHGs will only affect temperaure if they also affect total density enough to make the difference noticeable. The amount of CO2 in Earth’s air is so small that it could never have a measurable effect on total atmospheric density from anything humans could ever achieve.
Stephen Wilde says:
May 7, 2010 at 1:40 pm
I see some confusion here.
The pressure at a planetary surface is a function of gravity and atmospheric density.
The atmosphere of Venus is extremely dense compared to those of Earth or Mars. It is that density combined with the gravity of Venus that dictates both pressure and temperature.
The temperature will be higher for the same quantity of CO2 than for say Nitrogen because of their different molecular weights and thus their contributions to total atmospheric density.
Steve Goddard has just used the term pressure as meaning the composite effect of gravity and atmospheric density at the surface. It is true that the higher the pressure at a planetary surface then the higher also will be the temperature given a constant solar input.
Only if the atmosphere has IR opacity that depends on pressure, i.e. a greenhouse atmosphere. For an atmosphere of nitrogen the pressure will have no effect.
Hence the differences between Venus, Earth and Mars with the greenhouse effect idea being a mere unnecessary distraction.
No it is the essential feature!
Extra GHGs will only affect temperaure if they also affect total density enough to make the difference noticeable. The amount of CO2 in Earth’s air is so small that it could never have a measurable effect on total atmospheric density from anything humans could ever achieve.
No, it has nothing to do with density.
Scientists must always try the simplest explanations first. This principal is called Occams Razor. Physics comes before chemistry.
The gas law PV = nRT is difficult to visualise. Many find it hard to accept that an increase in pressure leads to increases in temperature.
The chemistry of CO2 absorption depends on its being made up of two elements. It is easy to visualise.
Mrs Thatcher was a chemistry graduate and popularized the CO2 theory of surface temperatures in her 1988 speech to the Royal Society.
beng
The top of the Grand Canyon receives just as much solar radiation as the bottom of the Grand Canyon, yet it tends to be 20-30 degrees F cooler.
How does that fit into your theory “The atmosphere is mostly heated at the bottom by the warm, irradiated surface…That’s why it’s warmer down there, not because of pressure. “
Nice to see I am not the only one with questions about Venus. Does anybody mind if I claim precedence on this idea?
This is the first of four Omniclimate blogs on the topic, from 2008. And this is the original blog from Aug 2007.
stevengoddard says:
May 7, 2010 at 2:07 pm
beng
The top of the Grand Canyon receives just as much solar radiation as the bottom of the Grand Canyon, yet it tends to be 20-30 degrees F cooler.
How does that fit into your theory “The atmosphere is mostly heated at the bottom by the warm, irradiated surface…That’s why it’s warmer down there, not because of pressure. “
There are breezes at the top of the canyon due to convection cells pulling away hot air and replacing it with cool air. Not much in the way of breezes at the bottom of the canyon as I remember. Also fewer steradians of cool sky to radiate into.
Please Steve, this article is awash with problems, give up on the CPR.
Oops…this is the link from Aug 2007.
OceanTwo says:
May 7, 2010 at 1:08 pm
======================
Richard M says:
May 7, 2010 at 9:58 am
A thought experiment. Let’s create a fictional planet with no external source of energy. Drop in an atmosphere like that of Venus. Gravity would start bouncing all those little molecules around. Heat would be generated. However, eventually the heat would be dissipated to space by radiation. The molecules would slowly but surely fall to the surface where the gravitation energy would be transferred to the underlying surface. No more energy to heat the atmosphere. Eventually all the molecules would lie harmlessly on the surface.
=========================
Harmlessly? Then what happens? As more and more molecules ‘fall’ to the surface on this supposedly ‘dead’ planet isn’t it’s mass going to increase? As it’s mass increases doesn’t this mass exert a (gravitational) force on all those molecules? When you impart a force onto molecules, what happens?
—————————————–
What happens is the force of gravity from the extra molecules is mostly directed at the internal structure of the planet. It would necessarily be warmed slightly. Assuming this heat reaches the surface you might see occasional motion of the molecules. Would this be enough to keep the entire atmosphere in motion?
I venture it takes a lot more energy to keep the atmosphere in motion. Now, there are other possible sources. However, I just don’t see these as sufficient as the energy provided would radiate away quickly. It seems to me you would have a dead planet.
My point still stands, it seems that the heat due to the motion of the atmosphere is not enough to prevent the atmosphere from eventually going dormant. Keep in mind this only a starting point for what happens next.
” beng says:
May 7, 2010 at 1:30 pm”
Beng, I think you and Steven each have half the story there. Some people are saying that somehow pressure causes high temperature. That’s wrong – a cylinder of compressed oxygen will generally be at ambient temperature.
What is true is that a layer of air in motion tends to a temperature gradient – the dry adiabat. The motion causes compression and rarefaction, which pumps heat downwards. The theoretical gradient is g/c, where g=9.8 m/s2, and c is specific heat of air at constant pressure.
That makes it work like a battery, which has a voltage difference, say 1.5V, from end to end. That doesn’t mean that the pointy end is at high or low voltage, only that there is a difference. The actual voltage depends on what it is connected to.
So 1 km of dry air in motion will have about a 10C difference, bottom to top. Then, as you say, if the temperature is fixed by heat absorbed and radiated at the bottom, it will be warm there, and cooler as you go up. In winter, there is less sunlight, so it’s cooler at the bottom, and again, drops off at the same rate from there as you go up.
The engineers and sicentists from the European Space Agency that designed the Venus Express spacecraft have my utmost admiration. Think about their task. They had escape a delicate payload from Earth’s gravity well, aim it at Venus and trasmit data successfully from such a hostile environment. It is an astonishing achievement realised in 2009.
Does it seem a little odd to anyone that their scientists have not considered atmostpheric pressure and its relation to temperature??
To quote the European Space Agency’s web site (and note Venus has a very high albedo (reflectivity)):
“On the global scale, Venus’s climate is strongly driven by the most powerful greenhouse effect found in the Solar System. The greenhouse agents sustaining it are water vapour, carbon dioxide and sulphuric acid aerosols.
About 80% of the incoming solar radiation is reflected back to space by the cloud layer, about 10% is absorbed by the atmosphere and only 10% manages to get through it and heat the surface. However, the thermal radiation emitted by the surface gets trapped by the same atmosphere. The result is an amazing 500 °C difference between the surface and cloud-top temperatures.”
Michael R says:
May 6, 2010 at 8:44 pm
“Unfortunately, what you have declared so adamantly as fact is, by it’s very nature, what is under dispute currently. In fact, what you have described is, in effect, what the whole argument of global warming is and consequently, people who do not agree that we understand fully the processes by which this plant achieves its temperature balance, are even less than willing to accept that you (or anyone else) can then ascribe said theory – with another planet altogether – and expect to be 100% correct.
I note you were particularly blunt with your critisms and so shall I – I can only assume that the complete lack of uncertainty relating to process we know only partially how they work and arrogance in assuming you do was a result of your Planetary Geology degree being …. honorary?”
Michael,
I was blunt because of the pointlessness and cyclical nature of arguments about global warming, and particularly comparative planetology, on the blogosphere. To your point, no, the process I pointed out is not in dispute. The fact that you and others unfamiliar with the field continue to dispute it does not reflect a corresponding dispute amongst those who have actually spent significant time studying planets. And with the honorary degree swipe to your credit, I’ll say up front that this is why scientists are usually not involved in the discussion. It usually winds up being a waste of time because those who are ignorant of the facts and details end up believing what they want to anyway. If it offends you to discover that someone knows more than you do, my friend, then no one can help you.
To the point:
1) Upper-atmospheric disassociation of H2O is the only viable model anyone has thought of that can replicate Venus’s current conditions, unless you believe that there was a mysterious asymmetry in the composition of the solar nebula during condensation. If you’ve got a better model, then fire away. If not, keep your honorary degrees to yourself.
2) To point out how silly this entire conversation is, let’s also talk about the 900-lb gorilla in the room: Hydrogen Sulfide, which composes a sizeable ~2 ppm of the atmosphere and is also an incredibly effective (and perhaps dominant) “greenhouse gas.” Sulfuric acid cloud layers between 45 and 75 km altitude (because there the temperature is cool enough for H2S to condense into droplets) act as terribly effective re-radiating layers, such that – yes – Venus can be hotter on the surface with only a fraction of the insolation because, due to the cloud cover (as mentioned above), Venus does receive less solar energy than Earth does. This is just one example of why the conversation here is crippled before we start – most of the participants aren’t familiar (no offense) with the basics.
3) You also seem to misunderstand something critical here – I was talking about Venus at its distance from the sun, not Earth. Nothing about this “theory” is being asribed from Earth to Venus, as you say. This is basic chemistry worked out from scratch under specific planetary conditions. So, for you to extrapolate that I believe the Earth’s “greenhouse effect” is the same is erroneous, and secondly, to say that I’ve just described the whole “global warming” argument show show little you understand about it. Earth’s “global warming” argument involves planetary feedback mechanisms not even in play on Venus, including the formation of limestones and dolostones (the Earth’s natural CO2-removal mechanism,) oceanic density current temperature-regulation mechanisms, and micrometeorology that people are only starting to describe. I do believe that Earth’s atmosphere at present is much more dynamic than Venus’s (though Venus’s is more impressive to us). If you were to ask me about the threat of “global warming” on Earth, I suspect you would be surprised by my answer. In fact, I did a blog interview on climate change a couple of months back, which I think you would find enlightening. Link: http://jordanspeak.wordpress.com/2010/02/21/an-interview-on-climate-change/
If we’re going to talk about climate change, that’s one thing. If we’re going to drift into comparative planetology, that’s something entirely different, and come prepared.
Lon Hocker
The reason why the rim of the canyon is much cooler than the bottom has nothing to do with “breezes.” You can travel anywhere along the 200 mile long 7,000 foot elevation Mogollon Rim in Arizona, and experience approximately the same temperature, 20-30 degrees cooler than the bottom of the Grand Canyon or the Sonoran Desert. Wind conditions have nothing to do with it.
I’m sorry that you don’t understand. Don’t hike up Mt. Everest in shorts though to display your lack of understanding.
jeff brown
Have a look at this graph.
http://en.wikipedia.org/wiki/File:Emagram.GIF
It shows very clearly the relationship between temperature and atmospheric pressure on earth. The fact that you don’t understand something does not make it untrue.
If you don’t accept the relationship between atmospheric pressure and temperature, then book your next ski vacation in Yuma, Az instead of Aspen. They have nice mountains there too.
Willis Eschenbach says:
May 6, 2010 at 8:31 pm
Willis, thanks for your post, and I’ll do my best with each of your questions:
“1. You say that in the early days, the atmosphere of Venus was similar to that of the early earth, mostly nitrogen. What evidence is there for that?”
-As far as direct evidence is concerned, not much, yet. We don’t have a time machine, our landers melt before long, and direct observations are related to re-worked areas of Venus’s crust and what minerals we can infer (their exact crystal structure is a function of their formation temperature). However, as far as circumstantial evidence goes, there’s TONS. First, the average distribution of chemical elements we see in condensing stellar nebula across the galaxy is fairly uniform, so it would be quite unlikely that Venus simply started out very differently (i.e., we’ve NEVER seen globular asymmetries). Second, what differences we do see between the planets is a predictable function of which minerals would cool and crystallize out first as the stellar nebula began to cool around the young Sun, (which is how we can tell that a given meteorite on Earth came from Mars and not Venus, for example.) – For the geologists amongst us, this tends to follow a loose version of Bowen’s Reaction Series, (and also explains why all of the “rocky” planets are close to the sun, and all of the “gas giant” planets are past 3 AUs from the sun.) It’s geochemistry. And, since Venus does follow this predictable geochemical pattern, it’s even less likely that Venus would be deficient in a single molecule, like H2O.
“2. At present, the atmosphere of Venus contains very little nitrogen (~ 3.5%). What happened to all the early nitrogen?”
Two things – One, I was generalizing when I described the atmosphere in my earlier post, so I wasn’t implying that there was 80% nitrogen or anything. Just a significant amount. Two, the nitrogen is all still there, it’s just been overwhelmed by CO2. Imagine if we took all water on Earth and spontaneously turned it into CO2 – well, our atmosphere would be far, far denser than it is now. We haven’t contributed any new nitrogen, so the percentage of nitrogen in the atmosphere would have plummeted.
“3. You say that the cause for what you call the “runaway greenhouse effect” is the stronger ionizing radiation at the the top of the Venusian atmosphere. If that is the case, wouldn’t a much more accurate term be the “runaway ionization effect”, and as such, be something that could never happen on earth?”
Good question. For starters, the “greenhouse effect” was never absolutely literal to begin with – greenhouses don’t work like atmospheres – it’s a general term. But having said that, yes, emphasizing the ionization would make it more accurate. However, this ionization effect IS already happening on Earth. A very tiny fraction of the water vapor that makes it to the upper atmosphere is separated and is lost to space. However, water tends to cool, condense, and return to the lower atmosphere before it gets there, so we’ve “trapped” the water here in a way that Venus couldn’t. The very concern is that slight increases in the atmospheric temperature with a “greenhouse gas” might send more water to the upper atmosphere – and any we lose, we lose forever. Hydrogen by itself is too speedy for Earth’s gravity to hang on to. However, ask me if I think we’re anywhere close to that here on Earth? Nope. Human beings’ effects are on the scale of a large volcano, which the Earth has plenty of mechanisms to mitigate.
“4. You say that the ionizing radiation at the very top of the atmosphere dissociated the water vapor and that the “free oxygen quickly bound to plentiful carbon to make CO2″. Why would carbon be plentiful at the very top of the atmosphere? And why would the oxygen not reform as O2?”
This is where the devil is in the details. Carbon is plentiful as CO2 and other compounds in the atmosphere, which is twice as effective at holding oxygen, but oxygen could also easily form up as diatomic oxygen. Think on a bigger scale, though. For starters, free oxygen could have been lost to space during disassociation. Also, there are other chemicals in play in the atmosphere: hydrogen sulfide can snap up free oxygen to make sulfuric acid (H2SO4), which we see. However, imagine also the O2 concentration going up slightly in a general sense, which would eventually be cycled through the atmopshere and brought into contact with the planet surface, where chemical erosion occurs. There’s lots of evidence of oxidation on the surface – lots of O2 ended up there, and plenty of carbon to bond with.
“5. Why is there still water vapor in the Venusian atmosphere (20 ppmv)? What has prevented the ionization of the last of the water?”
Essentially, the atmosphere itself has prevented all water from being lost. That would require all water vapor getting to the upper atmosphere, and there’s plenty of atmosphere to shield it from UV these days. However, if Venus ever had a shallow ocean, (which the isotopic composition of the hydrogen we see in the Venusian atmosphere supports,) it’s been long evaporated, fried, and lost to space.
Hope this helped to answer some of your questions, but as I said earlier, this barely scratches the surface about the discussions/research people are having/performing about Venus’s geologic history – it’s an awesome field of study!
kadaka
Nice explanation!
Time to chime in for a moment…
Here on Earth temperatures are warmer than they would be w/out a greenhouse effect. In the past (and in most college books), the warming was estimated to be 33C, as it’s obtained as the difference between the globally observed surface air temperature (Ts) of 15C and the radiative equilibrium temperature of Earth (Te) of -18C, and represents the longwave radiation effect of the atmosphere. Te is computed from the Earth’s radiation balance with the observed planetary albedo of 0.3 [Trenberth et al., 2009].
The warming effect of our atmosphere trace gases (i.e. Co2, H2O, O3, NO2) on the Earth’s surface temperature is quantitatively computed as the temperature difference with and without these gases in the atmosphere.
In reality Te should be recomputed by considering clouds so that the planetary albedo of 0.3 is replaced by the combined reflectance of the atmosphere and the surface, which would be 0.14. Doing so would show there is a 20C difference between Ts and Te. Note Ts is caused by the atmospheric radiation effect (primarily from GHGs and clouds) and surface sensible and latent heat fluxes due to fluid motions in the atmosphere. W/out these heat fluxes the differences between Te and Ts would be larger.
Seems to me we should frame the discussion about Venus in a similar framework
What would happen to atmospheric pressure if the sun turned off?
PV = nRT
If T dropped to zero, then P would also drop to zero. It is the sun which provides the energy that keeps the molecules moving, and keeps the pressure up.
stevengoddard says:
May 7, 2010 at 2:07 pm
beng
The top of the Grand Canyon receives just as much solar radiation as the bottom of the Grand Canyon, yet it tends to be 20-30 degrees F cooler.
How does that fit into your theory “The atmosphere is mostly heated at the bottom by the warm, irradiated surface…That’s why it’s warmer down there, not because of pressure. “
The lapse rate just sets the stable temperature gradient, it has nothing to do with setting the temperature of the planet. All you have done (and Motl) is transfer the problem to the tropopause. The temperature is set by the radiation balance, the gradient by the adiabatic lapse rate. If there are condensible species in the atmosphere then it’s the ‘wet’ lapse rate that applies (H2O on earth, H2SO4 probably on Venus).
Ryan says:
May 7, 2010 at 8:20 am
On your last sentence, that applies to a container of gas. In a planetary atmosphere, the pressure is mostly due to the weight of the column of air above the barometer. Move up, the pressure goes down. Light a fire, a little air moves up (convection) and the barometer reads the same. Get a large whirlpool of air driven by temperature differences and the planet’s rotation (i.e. a storm) and the pressure goes down a little bit. On Earth, sea level pressure only varies between about 960 and 1060 millibars except for small scale heat engines called hurricanes, but note the pressure changes there have little to do with surface temperature.
As for “In other words, you say that the gas pressure above Venus is what causes the higher temperature. What I am telling you is that you are 180 degrees facing the wrong direction,” I’m not sure how that would read -180° facing the right direction. Hike to the top of your favorite mountain, take a bagful of air and stick it in a bag made of a perfect insulator, hike with it down to the valley. The air will be warmer, all in accordance with the (nearly) ideal gas law.
Do the same sort of thing on Venus – start from a point where the atmosphere is radiating out to space, refill your bag, and take it down to the surface. Being an ideal bag, it won’t melt, but the air/CO2 inside will be mighty hot. If the heating at the planet’s surface is greater than the outgoing radiation, then convection will mix the atmosphere and you’ll get a nice adiabatic lapse from ground to tropopause.