Venus Envy

By Steve Goddard

ESA’s Venus Express mission has been studying the planet and a basic atmospheric model is emerging.

Venus Express probe - Image: European Space Agency

http://astronomyonline.org/SolarSystem/Images/Venus/VenusClouds_th.jpg

Atmospheric model - Image: Astronomyonline.org - click for more

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 :

Temperatures within Venus's atmosphere

http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-t.gif

Pressures within Venus's atmosphere

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.

http://www.donaldedavis.com/BIGPUB/V13CLR2.jpg

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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kwik

What disturbs me is the following;
Personally I havent given Venus much thought before reading Steven Goddards posts here at WUWT. Why should I have. I have enough other thinks to worry about on a daily basis.
But now, reading about Venus, thinking about it, the grey cells working…over at Stephen Wildes post…it stands clear to me, at least for now….that, hey, it makes sense!
And then the disturbing thought;
Surely J. Hansen must know this too? Surely? I mean, if its your job to use the grey cells on such matters?
Surely?

John caten

Haha, love the title! Now I’ll go back and read the article.

Troels Halken

When you compress or decompress a gas, it either warms or cools. Due to the temperature differential to the surroundings, after some time it will cool or warm by exchanging heat with the surroundings until the temperature differential is approaching zero and hence the temperature of the gas approaches that of the surroundings.
Why should Venus behave any different?

Leonard Weinstein

Steve,
You are correct on all the cases you quoted, but the basic reason Venus is hot is in fact due to the presence of greenhouse gasses combined with the high pressure. It is not how much of the gas is greenhouse gasses, but the fact that there is at least some present (even <1% CO2 or water vapor and 99% N2 would do about the same). If there were no greenhouse gas at all, but the gas pressure were still high, a lapse rate would still be present (but modified due to real gas effects and greatly different temperatures), but the surface of Venus would then be the location where all of the radiation to space would balance incoming absorbed energy. This would make the surface close to Earth's temperature, and the lapse rate would result in a very cold upper atmosphere. The presence of even a small amount of greenhouse gasses moves the location of the radiation that goes to space out to the edge of the atmosphere, and the lapse rate heats the surface. The main difference is where the radiation to space occurs.

Ian l. McQueen

Experiment # 1 – Atmospheric pressure on Venus’ surface is 92 times larger than earth, because the atmosphere is much thicker and thus weighs more.
I am always puzzled by the statement that the atmospheric pressure on the surface of Venus is a large multiple of that on earth.
The MW of earth’s atmosphere is about 29, and that of Venus around 44. But gravity at the surface of Venus is only 0.904 that of earth.
Atmospheric pressure is the result of the weight of a column of gas above the surface pressing down. I won’t embarrass myself by trying to calculate the effect of higher MW and lower force due gravity on Venus, and compressibility of the gas(es), but I would expect the atmosphere of Venus to extend out a large multiple of the distance that it does on earth to produce an atmospheric pressure 92X that on earth. But I have not heard that the Venusian atmosphere extends out so far…..
So, the question remains why the atmosphere is so “thick”. It is not constrained to a fixed volume, but can expand as the mass of gas is increased.
A further niggle comes from the statement: “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.”
At 0K all gases would have condensed to liquid or solid, but the mass of the gas would still be present as a layer on the surface. I would think that the liquid/solid would be as heavy (=attraction due to gravity) as the atmosphere from which it was produced, and possibly a tiny bit higher since the molecules would be concentrated at the surface instead of extending a distance above the planet and thus minutely farther from the center of attraction of the mass of the planet.

kcom

It’s always been clear to me from the timeline of the discoveries on Venus and the genesis of the global warming panic that a strong argument can be made that the scare aspects of global warming (its original and most honest name) are an example of a massive case of medical students’ disease. James Hansen got his nose so deep into Venus he couldn’t see his way out. Just because it’s not real, doesn’t mean it doesn’t seem real. The mind is a powerful thing and can overwhelm logic when placed under enough stress. Ask any medical student if this has ever happened to them.

Leonard Weinstein
You are correct.
Earth already has plenty of greenhouse gases, so my point is that even if earth went to 100% CO2, the maximum temperature increase would be less than 36C – nothing like Venus. (In reality, it would get cooler because of the loss of water vapour.)

Ian l. McQueen
You can infer how thick the atmosphere of Venus is from this diagram:
http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-t.gif

Ian l. McQueen
Liquids are not very compressible and don’t heat up much under pressure. Consider the bottom of a swimming pool.

On Earth we have water that Venus does not have. We have a wet lapse rate that is less than a dry lapse rate because of the processes of evaporation and condensation. Condensed moisture also controls the atmospheric concentration of CO2. If we had no atmospheric moisture, CO2 concentrations could be higher and the atmosphere could be heavier. If Venus ever had water like Earth, it must of boiled off because it was too close to the Sun and not in the sweet zone that sustains life here.

John caten
Title is Anthony’s idea. He also added the ESA and Venera content.

rbateman

One of the great things that NASA did was the planetary explorer missions. Without comparison to other planets, there would be little hope of understanding Earth. It is unfortunate that some have hypochondriated that splendid knowledge into doomsday scenarios. Earth, like the other planets, has been as it is for at least the last billion years. The foundation for what separates Earth from the other planets is Carbon-based life that transformed the place. Else, who knows what it might have become, or not become. Better start thinking about taking care of Earth’s precious Carbon, as geologic forces are relentlessly conspiring to sequester it.
I’m thinking of a Carbon Appreciation Day.

RobJM

In addition to pressure, Venus also has high concentrations of other greenhouse gasses such as carbon monoxide. If one of these gasses absorbs at the frequency corresponding to earths atmospheric window then it could have a significant effect on temp. I also wonder if atmospheric pressure could explain the faint sun warm earth paradox.

Doug S

Leonard Weinstein says:
May 8, 2010 at 2:38 pm
… The presence of even a small amount of greenhouse gasses moves the location of the radiation that goes to space out to the edge of the atmosphere, and the lapse rate heats the surface. The main difference is where the radiation to space occurs.

That’s interesting Leonard. It would seem to a layman like me that a small amount of green house gas would radiate a small amount of energy back into space at the effective altitude of the gas. Are you saying that just a small amount of GHG at some altitude implies a large amount of re radiated energy is leaving the planet at that same altitude? I’m having trouble understanding that non-linear relationship between gas density and radiated energy.

Dave McK

Willis- the lapse rate of earth’s atmosphere is NOT the lapse rate of a dry atmosphere.
In fact, the profile of earth’s atmosphere is that of a refrigerant system because of the working fluid water.
Venus has no such phase change gases in its atmosphere so it has the dry lapse rate.
The lapse rate of earth’s atmosphere is definitely not that of a dry atmosphere.
The are completely different- not similar at all.

IanH

The modeling of Venus and earth atmospheres was reported (1,2): The authors agree adiabatic lapse rate is the crucial physical process. They point out three things
1) the adiabatic exponent for a hypothetical CO2 atmosphere replacing a nitrogen oxygen one of the same mass would result in a cooling of 6.4C.
2) Adding Carbon to oxygen in the atmosphere makes it slightly heavier, in principle resulting in tiny warming
3)Ultimately the CO2 dissolves in seas forming carbonate rocks, oxygen is thus lost from the atmosphere and atmospheric mass drops causing (tiny)cooling.
They conclude:
“Accumulation of large amounts of carbon dioxide in the atmosphere leads to the cooling, and not to warming of climate…. This conclusion has a simple physical
explanation: when the infrared radiation is absorbed by the molecules of greenhouse
gases, its energy is transformed into thermal expansion of air, which causes convective
fluxes of air masses restoring the adiabatic distribution of temperature in the troposphere.”

FWIW I am not a qualified climate scientist, take my reading with a pinch of salt.
1)
Environ Geol (2008) 54:1567–1572
Response to W. Aeschbach-Hertig rebuttal of ‘‘On global forces
of nature driving the Earth’s climate. Are humans involved?’’
by L. F. Khilyuk and G. V. Chilingar
2)
Energy Sources, Part A, 30:1–9, 2008
Cooling of Atmosphere Due to CO2 Emission
G. V. Chilingar a; L. F. Khilyuk a;O. G. Sorokhtin b
a Rudolf W. Gunnerman Energy and Environment Laboratory, University of Southern California, Los
Angeles, California, USA b Institute of Oceanology of Russian Academy of Sciences, Moscow, Russia

Mike McMillan

Leonard Weinstein says:
. . . but the basic reason Venus is hot is in fact due to the presence of greenhouse gasses combined with the high pressure. It is not how much of the gas is greenhouse gasses, but the fact that there is at least some present . . .

The basic reason Venus is so darn hot is that it is so darn close to the sun. The high albedo moderates the temperature. The difference in composition affects the adiabatic lapse rate, but swapping Venus’ atmosphere with the same weight of air would give you no clouds and a much hotter planet.
The difference in lapse rates boils down to the gas molecules. Air is mainly diatomic, while Venus is triatomic CO2. The specific heat ratio Cp/Cv for air is ~1.4, CO2 is ~1.3, and that affects the temperature change as you move a parcel up and down, how much energy goes to ‘work’ as opposed to what’s left for temperature. Triatomic water vapor, our main ghg, has a Cp/Cv of ~1.3 like CO2. Methane, the next ghg suspect, also has a Cp/Cv of ~1.3. Sounds like a conspiracy.
Bottom line is that things on Venus are quite in accord with what you’d expect, no “runaway greenhouse” effect. I must acknowledge, though, that with no weather, Venus has solved the problem of climate change.

Mike McMillan
There isn’t a large difference in lapse rates, and the albedo of Venus is very high, so it’s distance from the Sun has little effect on temperature.

Dr A Burns

Troels,
The atmosphere is not static and conductive heat transfer to surroundings is small. Here’s a description:
“An adiabatic temperature change occurs in a vertically displaced parcel of air due to the change in pressure and volume (refer to the gas equation in section 1.2) occurring during a short time period, with little or no heat exchange with the environment. Upward displacement and consequent expansion causes cooling; downward displacement and subsequent compression causes warming. In the troposphere, the change in temperature associated with the vertical displacement of a parcel of dry (i.e. not saturated) air is very close to 3 °C per 1000 feet, or 9.8 °C / km, of vertical motion; this is known as the dry adiabatic lapse rate [DALR]. As ascending moist air expands and cools in the adiabatic process, the excess water vapour condenses after reaching dewpoint and the latent heat of condensation is released into the parcel of air as sensible heat, thus slowing the pressure-induced cooling process.”

George Turner

Ian l. McQueen
Yep. I think the pressure going to zero argument is an oopsie. The pressure is determined by mass and gravity, and so is fixed (except for some small dynamic effects due to weather). It’s the volume and temperature of the atmosphere that vary.

Another great post. I have been trying to go through similar ideas in the Climate Sceptics group for a while myself.
May I suggest a couple of interesting links:
(1) UTexas teaching notes about why there is an adiabatic lapse rate at all (hint: there is no mention of any GH effect)
(2) From the New Mexico State University a table about lapse rates in various atmospheres in the Solar System (and showing that composition might matter, alongside gravity)
In particular, the lapse rate is roughly g/Cp, where g is the acceleration due to gravity and Cp is the atmosphere’s specific heat at constant pressure (cp) divided by the molecular weight. It would be interesting to find out why exactly Cp for Venus would only be 85% of Earth’s.

Forgot to close the hyperlink in previous comment. Apologies.

Jacob

Why is the atmosphere on Venus 92 times more dense than on Earth? That is the big question, that I don’t understand.
Now, if the atmosphere is denser – it means there are 92 times more molecules of gas in the same volume (or layer) of the atmosphere, therefore they would absorb 92 time more outgoing radiation (much more anyway). So, the heat on Venus’ surface is caused by the greenhouse effect after all, by the fact that the greenhouse effect is much stronger there, due to the density of the atmosphere, and not necessarily due to it’s composition (Co2).
If pressure alone could cause heating – why isn’t the bottom of the oceans very hot?

Savant

Steve,
You have almost got it now — here is the relevant question:
For Earth and Venus (as you note) the adiabatic lapse rate is similar; For Earth and Venus, the planetary equilibrium temperature is similar (Venus is closer to the sun but has a higher albedo). So now the question: To what altitude does the atmosphere mix vertically from the surface close to the adiabatic lapse rate (note on Earth that the true lapse rate is ~6.5o/km, e.g. the atmosphere is slightly stable)? You assume in your posts that it must be to the same pressure — but where does this assumption come from? On Earth, the temperate at 50 km is ~250K, if your 10K/km held to this altitude, the surface of Earth would be 750K — clearly something amiss. The answer is that at the tropopause OLR balances the incoming solar: outgoing energy (emission) from the atmosphere by cooler greenhouse gases and from the warmer surface (in the IR windows) balances the solar headed down.
So the important question for Venus is why is the tropopause at 60Km? You make the assumption that it is because this is (approximately) the same pressure as the tropopause on Earth. But there is no physical reason why the tropopause must be at the same pressure on other bodies. As on Earth, the tropopause is where the atmosphere must mix vertically to achieve thermal balance (e.g. read very nice discussion of radiative-convective models by Manabe). If instead of being made of IR absorbing constituents, the atmosphere was made only of N2 (even if it was as massive), the surface temperature would be much colder (and there would be a large day-night difference).
There is not much to this (at zeroth order) beyond the first law of thermodynamics in a compressible (nearly-ideal) gas.

Andrew W

“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.”
The accepted view is that present day Venus is a result of a runaway greenhouse, she got too warm, her oceans boiled, the resultant water vapour created a super GH effect, which melted the planets crust, the oxygen released from ionize H2O combined with crustal carbon to create CO2. The planets surface is hot because of its present atmosphere, its atmosphere is a product of a runaway greenhouse.
“we have a sun providing energy to the periphery of the atmospheric system”
Not sure what you mean by this, the sun’s EM radiation is absorbed by atmospheric constituents throughout the depth of the Venusian atmosphere, not at the “periphery”.
“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”
The surface pressure is almost entirely independent of the temperature, assuming that all the atmospheric constituents remain suspended.

Dave

Again, the height of the tropopause will change with the composition of the atmosphere: therefore, even if the lapse rate stays constant and the temperature at the tropopause stays constant, if reducing the amount of CO2 in the atmosphere causes the tropopause to decrease in height the surface will decrease in temperature. (And note that the troposphere in Venus contains a much larger percentage of the atmosphere than the troposphere on Earth, which would be consistent with a higher CO2 level pushing the tropopause upwards).
(also, while I agree that a runaway greenhouse on Earth is almost certainly not going to happen, I think that your back-of-the-envelope calculation of the effect of 100% CO2 atmosphere is wrong because the logarithmic forcing relationship isn’t going to hold above a couple thousand ppm CO2, and you are underestimating the potential for CO2 to lead to temperature increases at those extreme concentrations)

Stephan

OT: One gets the distinct impression from recent posts here, at CA, and RC etc, that interest in the subject (AGW), is finally waning. Would be interesting to follow the sites statistics recently, to confirm/deny this. Basically the skeptics/deniers were right.. weather, aka climate is not changing due to AG. AGW news stories have certainly dipped. We can all go back and get a life instead of looking at graphs of temps, ice, polar bears, anxiety due to AGW, etc.. Of course a few diehards will keep banging away at it until they too, just give up, a good ol hahaha and LOL!

1DandyTroll

Sometimes I figure GISS should’ve been named after you, sometimes not so much.
What defines atmospheric temperatures and what drives those temperatures? Is it just the proximity of a star? How about gravitational pul from say a black hole? Or just from being close to a gas giant like saturn and its gravitational pul?
But sure, get rid of all the stars and everything would’ve a temperature of absolute zero…. that doesn’t even compute.

Bob_FJ

For anyone still believing that CO2 GHG is the fundamental reason for high surface temperatures, please consider this, (adapted from my comment in the earlier thread):
Here is an authoritative extract from the ESA, (my bold), so let’s examine it, particularly as to why Venus has a uniform average temperature everywhere. (over 117 earth days, and see link below)
“…[1] On Venus there are no day and night variations of the surface temperature. The heat is globally ‘trapped’ under the carbon-dioxide atmosphere, with pressure 90 times higher than on Earth.
[2] Instead, the main temperature variation is due to topography. Just like on Earth, mountain tops are colder, whereas the lowlands are warmer. The ‘only’ difference is that on Venus ‘cold’ means 447º Celsius, while ‘warm’ means 477º Celsius. Such high temperatures are caused by the strongest greenhouse effect found in the Solar System…”

[1a] When facing the sun, she is said to receive at the surface about 10% of sunlight. Whether this is a midday or total facing area average, or the effects of scattering, I don’t know, but whatever, the surface receives SOME solar energy. This energy amount must be lost back to space because the planet is apparently in thermal equilibrium. The fundamental process for this should be convection and conduction. Additionally, since infrared photography etc of the surface has been accomplished there is at least one window for some infrared to directly escape to space.
[1b] At nightime she no longer receives any solar energy, but has capability to lose heat in the same way as on the daylight side, over a period 117 times longer than on Earth. What is more, because the upper atmosphere is no longer heated by solar infrared, (~40% of sunlight), the temperature gradient of the atmosphere should increase, inferring increased conductive/convective cooling. Yet, there is no change in surface temperature! Such a condition would require an impossible perfect insulation layer, and no geothermal energy, but clearly, this is not the case.
[2] This part of the extract supports Steven’s hypothesis, but see also my comment in the earlier thread concerning the strange and unexplained dynamics of the atmosphere, according to the ESA. (that may have an astonishing “mixing” effect).

Dave
You are ignoring the fact that Venus has almost 100% CO2 and it’s lapse rate is the same as Earth’s up to 60 km. The empirical evidence does not agree with your theory.

Jeff Green

(kcom says:
May 8, 2010 at 2:46 pm
The mind is a powerful thing and can overwhelm logic when placed under enough stress. Ask any medical student if this has ever happened to them.)
James Hansen came to his conclusions based on observations. If you can prove him wrong you win. Do ou have a better explanation?

Dave McK
Dry lapse rate means unsaturated (less than 100% humidity.)
Most of the troposphere is unsaturated.
http://www.physics.umt.edu/borealis/RH%2520Lab%2520Report_06.pdf

George Turner
Gas pressure P = nRT/V
There is no term for either mass or gravity.

el gordo

Stephan
Over at Deltoid its all political science these days, rarely do they discuss ‘graphs of temps, ice, polar bears, anxiety due to AGW, etc.’

wayne

Steve, I don’t see how they get the lapse rate for Venus you mention. I tried three ways and each is much lower. Assuming 740K for the surface temp per Wiki, the top graph has two possibilities:
(740K-260K)/58km = 8.3 K/km
(740K-310K)/52.5km = 8.2 K/km
The lower graph, assuming Earth’s mean surface temp at 290K, based on pressure, not temperature is:
(740K-290K)/49.5km = 9.1 K/km
All of these are between the surface and to the point where ~90% of the mass of the atmosphere is lower.
Do you think their 10.468 K/km rate is more accurate? As you were saying, if co2 has it’s hand in it then the lapse should be much higher than the lapse rate of earth and I come up lower, not higher by the equally valid graphs (they tend to mirror what you read in Wiki).

Andrew W
If that is the “accepted view” then I have even less respect for the perpetrators of this nonsense. Basalt melts at 1300 C.

MaxL

As Kwik mentions I really have not given much thought to the atmosphere of Venus either. However, this discussion has been interesting and gets one to thinking. A couple of points need more fleshing out. The statement “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.” The ideal gas law has 3 variables, T, P and V. If you decrease T, as suggested, you can do this by keeping P constant, in which case V (volume) will decrease, you can keep V constant and decrease P or a combination of both. So it is not so simple as saying there would be no pressure if there was zero temperature. The gas molecules would still be there and have the same weight. This introduces another problem, namely the hydrostatic equation which states that the pressure of a column of air is solely a result of the weight of the air above. This applies most of the time to our atmosphere on Earth and is one of the fundamental equations in numerical weather models. Offhand, however, I am not sure how well it applies to Venus.
Another problem is the discussion of lapse rates and the adiabatic process. This requires the combination of both the “First principle of Thermodynamics” which relates heat to temperature, pressure and volume, plus the idea gas equation (PV = nRT). For an adiabatic process (no heat added or subtracted), when you combine these you get Poisson’s equations. The one most familiar is the equation relating temperature and pressure: T1/T2 = (P1/P2)^k
where k is found to be 0.286 for our atmosphere.
This is the equation for the dry adiabatic lines found on Skew-T or tephigram
diagrams. You can use this to find out what the temperature here on Earth would be at be at 9000 hPa given the temperature at 1000 hPa is about 293 K.
It is difficult on a blog to go into much more detail but I hope this helps out a bit.

kwik

Stephan says:
May 8, 2010 at 4:32 pm
Yes, Stephan, but:
They are now promoting new scary stories.Acidification of the osceans seems to be the new big thing. In Norway there has been several stories in the news lately.And lo and behold, after a few months camaign a new research institute is now planned to study this.

Robin Kool

In the end there is simply the warmth given by the sun plus the warmth produced inside Venus, and the warmth radiated into space.
My opinion:
It is the heat the atmosphere gets from the planet surface that determines how far the atmosphere expands into space, not the other way around.
It is not the distance the atmosphere expands into space that determines the temperature at the surface.
The air receives heat on the planet surface, expands, rises and cools down, and falls back. As it falls back and contracts, it warms up again, but it will not gain more heat than it has lost by expanding. As it has also lost heat that is radiated out to space, it will arrive at the surface of the planet cooler than it left it.
My reasoning:
1. OK. For the moment, let’s forget the green house effect and let’s start with a very cold atmosphere, contracted very close to the surface of Venus.
2. You say: “Because we have a sun providing energy to the periphery of the atmospheric system, the atmosphere circulates vertically and horizontally to maintain equilibrium.”
The energy of the sun must go through the atmosphere and warm the surface of the planet, which then warms the lower layers of the atmosphere for circulation to start happening.
(The sun providing energy to the periphery (= outside) of the atmosphere, means the periphery warms up. The air there expands and wants to rise, not sink, thus no circulation. The word periphery is usually used for ‘outside’ and seems misplaced here.)
3. OK, when such a dense atmosphere is heated up, the resulting upward force is greater that of the thinner Earth atmosphere heated up to the same temperature. This makes the Venus atmosphere spread out further into space than Earth’s.
4. And yes, when the expanding gas has reached the outer layer, and has cooled down, it will fall and contract and heat up, but it will not gain more heat than it has lost by expanding. As it has also lost heat that is radiated out to space, it will arrive at the surface of the planet cooler than it left it. (Otherwise it would be a perpetuum mobile).
5. As Venus has no oceans and no biosphere, it has a hell of an urban heat island (UHI) effect.
The energy that the sun radiates on the surface heats up a thin layer of surface to very high temperatures. The whole planet is one big heat island.
None of it is spread over a huge body of water like our oceans or converted into plant growth.
I would expect that to be the most important reason the surface temperature of Venus is so much higher than that of the Earth.
Plus, of course, that Venus is closer to the Sun and receives more heat per square meter or foot than Earth.
6. The much denser atmosphere is much better at absorbing the heat of the planet’s surface, than Earth’s thinner atmosphere. Giving it a huge upward, expansive force, that expands it far out into space.
But the extend to which it spreads out is determined by the heat it receives on the surface, so that after the hot air has reached the cold space and – cooled down – falls back, it does warm up, but not to higher temperatures than it had when it started at the surface of the planet.
In other words, it is the heat content of the atmosphere – heat it gets from the planet surface – that determines how far the atmosphere expands into space, not the other way around.
It is not the extend to which the atmosphere expands into space, that determines the temperature at the surface.
7. The heat content of the atmosphere if Venus is so high, because of the massive UHI effect and because the dense atmosphere is so efficient at absorbing heat from the surface.
(8. I imagine that without oceans there is no thunderstorm belt that transports massive amounts of heat to the highest layers of the atmosphere, bypassing the greenhouse effect – the thermostat Earth has in the tropics.)

u.k.(us)

I’m in way over my head here, but doesn’t the “solar wind” change the depth of our atmosphere. Which would cause pressure and temperature changes?

George Turner

I’d like to suggest another experiment for Steve.
To simulate the effects of an IR blocking atmosphere on a very dim planet, build a greenhouse. The glass for the greenhouse should be selected for absolute IR opacity across the band, and it can even be triple-paned and argon-filled to limit convection losses. To best mimic the dim light on the surface of Venus, build the greenhouse deep in the forest and wait for a very dark, overcast day. Sit inside with a good thermometer and see if the temperature approaches 900 degrees F.

Jeff Green

(http://en.wikipedia.org/wiki/Atmosphere_of_Venus#Structure_and_composition
The large amount of CO2 in the atmosphere together with water vapor and sulfur dioxide create a strong greenhouse effect, trapping solar energy and raising the surface temperature to around 740 K (467°C), hotter than any other planet in the solar system, even that of Mercury despite being located further out from the Sun and receiving only 25% of the solar energy Mercury does.[11])
Based on AGW theory if venus’s atmosphere were nitrogen, then infrared would continue back out into the space without being reactive to the nitrogen. The gas in the atmosphere does make a difference as to how much heat is held in.

MaxL
Gas pressure is created by the motion of molecules. As you approach absolute zero, molecules cease to move. So there is no pressure.
Volume has to remain above zero, because the molecules have a finite size. So yes, it is that simple.

suricat

Ian l. McQueen says: May 8, 2010 at 2:40 pm
“I am always puzzled by the statement that the atmospheric pressure on the surface of Venus is a large multiple of that on earth.”
If you blow away many of the lighter elements of a planet’s atmosphere with the ‘solar wind’ and continue to warm it, the heavier elements will stay there in the atmosphere to provide a greater surface pressure.
Having said that, I’m sure you realise that the heavier gasses should form a shorter altitude distance for any change in pressure. It’s curious that the lapse rate of both Venus and Earth are similar, but is this more to do with the similar orbital distance from Sol than the atmospheric makeup of these planets?
Well I think this is possible, but perhaps it’s more to do with the natural progression of (and here’s a new term) ‘nature’!
We have the ‘nature’ of Earth, and we have the ‘nature’ of Venus.
The history of Venus has led it to its current situation, and ditto for Earth. Although Venus has a lower gravity field than Earth it has lost many of its lighter elements. Thus, Venus’s troposphere contains compounds that are much heavier than those incorporated within Earth’s troposphere and this reflects in its higher surface pressure.
Best regards, Ray Dart.

wayne

I ought to correct the percentage I noticed in my comment above, at ~50-55km altitude about 99% of the mass is below this point, not 90%. Technical, but to be as accurate as possible…

William Sears

The adiabatic lapse rate argument works best with heating from below. This might be caused by geothermal activity since the atmosphere of Venus, unlike earth, is opaque. I believe that a number of scientists have studied the strange geology of Venus’ “not quite plate tectonics” and no doubt have already made this same suggestion. Popular knowledge dies hard though, even among scientists, and the runaway greenhouse analogy lives on.

GeoFlynx

[snip – try rewording that, there’s no reason to hurl insults]

William Sears
Whether the source of heat is geothermal or absorbed SW, the lapse rate is the similar on both planets – indicating that greenhouse effect is less important on Venus than advertised.

Jeff Green

(http://en.wikipedia.org/wiki/Atmosphere_of_Venus#Structure_and_composition
The atmospheric pressure at the surface of Venus is about 92 times that of the Earth, similar to the pressure found 910 metres below the surface of the ocean. The atmosphere has a mass of 4.8 × 1020 kg, about 93 times the mass of the Earth’s total atmosphere.[1] The pressure found on Venus’s surface is high enough that the carbon dioxide is technically no longer a gas, but a supercritical fluid. The density of the air at the surface is 67 kg/m3, which is 6.5% that of liquid water on Earth.[1])
The carbon dioxide is so thick that it has more the properties of fluid rather than a gas. There are all kinds of studies and observations showing co2 blocks infrared energy.