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Tags atmosphere , greenhouse effect , planet formation , temperature , venus

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Old 19th June 2010, 06:20 AM   #1
wogoga
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Evolution of Venus Temperature & Climate

It is often claimed that Venus surface temperature as high as 450 - 500 degrees Celsius suggests a carbon dioxide greenhouse effect (around 96.5% of its atmosphere consists of CO2). However, if one deals with the question in an unprejudiced, honest way, then the hypothesis of such a greenhouse effect being the culprit of the high surface temperature becomes completely untenable.

What is called greenhouse effect on Venus would be quite different from both the original greenhouse effect and the global-warning greenhouse effect.

The original greenhouse effect:
The sun heats the ground of a greenhouse, the ground heats the air, and the glasses prevent the hot air from flowing outside the greenhouse
The global warming greenhouse-effect:
Radiation from the sun heats the earth, and a corresponding amount of energy is lost as thermal radiation to outer space. An increase in CO2 significantly reduces such thermal losses (outgoing radiation), whereas it does not reduce significantly the absorption of incoming radiation.
A Venus greenhouse-effect would work in this way:
Although only a small proportion of radiation from the sun reaches the ground of Venus, carbon dioxide is assumed to somehow heat up the ground.
The true reason of the high temperature on Venus is much simpler:
The very dense atmosphere of Venus has prevented the planet's surface from cooling down after its formation to a temperature similar to thermodynamic radiation-equilibrium.
So what is called greenhouse effect in the case of Venus is nothing more than atmospheric heat insulation over hundreds of millions of years. Gases are excellent insulators.

And if it were possible to cool down the whole planet Venus to zero degree Celsius, its temperature would remain near water freezing point over millions of years.

Cheers,
Wolfgang

Atmospheric carbon dioxide per square centimeter: on Venus 100 kg - on Earth 0.6 gram
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Old 19th June 2010, 06:28 AM   #2
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I understood that (theoretically at least) Venus used to be pretty much like Earth - with oceans and stuff.

How does your "always hot" theory square with that?
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Old 19th June 2010, 06:37 AM   #3
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Don't forget that Venus receives around DOUBLE the intensity of solar radiation that Earth does, since it is much closer to the sun. Comparisons with Earth are quite ridiculous for that reason alone.
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Old 19th June 2010, 06:49 AM   #4
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Although only a small proportion of radiation from the sun reaches the ground of Venus, carbon dioxide is assumed to somehow heat up the ground.

Are you going to back this statement up
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Old 19th June 2010, 06:56 AM   #5
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Originally Posted by MichaelN View Post
Don't forget that Venus receives around DOUBLE the intensity of solar radiation that Earth does, since it is much closer to the sun. Comparisons with Earth are quite ridiculous for that reason alone.
The comparrison being drawn is clearly ridiculous. However I am not aware of any planetary scientist who are making the comparrison. There is great interest in why Venus ended up the way it did and why Earth is as it is

Venus however does give us a giant chemistry kit to play with. To understand conditions in worse case senario etc.

One example of this that stands out was the destruction of the global cooling theory of the 1970's. Venus shows us what carbon can do in an atmosphere, not what it will do
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Old 19th June 2010, 07:36 AM   #6
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I doubt anyone these days makes a direct comparison between Venus and Earth. Apart from all the other mentioned differences, its rotation is so slow we have no idea what sort of extra effects that might have had on the evolution of its atmosphere, as well as its slightly lower density and the fact that the solar wind hits its atmosphere harder and the fact that it (at least currently) does not have a magnetic field to protect the atmosphere from the sun.
The comparison is sort of saying that because footballs and apples are both round, apples will never rot, because obviously that doesn't happen to footballs.
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Old 19th June 2010, 08:41 AM   #7
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Originally Posted by wogoga View Post
So what is called greenhouse effect in the case of Venus is nothing more than atmospheric heat insulation over hundreds of millions of years. Gases are excellent insulators.
Nope. First of all, gases are excellent insulators against *conduction* and only conduction. Given convection and advection, over large distances they're actually worse insulators than solids. And they're no insulation at all against radiation, except in the normal atmospheric-greenhouse-effect effect sense.
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Old 19th June 2010, 09:09 AM   #8
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Originally Posted by wogoga View Post
Although only a small proportion of radiation from the sun reaches the ground of Venus, carbon dioxide is assumed to somehow heat up the ground.
If the sunlight isn't reaching the ground because it's being absorbed by the atmosphere, then the atmosphere is being directly warmed by the sun. I don't understand the "somehow" remark - are conduction and convection not well-understood?

Quote:
The true reason of the high temperature on Venus is much simpler:
The very dense atmosphere of Venus has prevented the planet's surface from cooling down after its formation to a temperature similar to thermodynamic radiation-equilibrium.
So what is called greenhouse effect in the case of Venus is nothing more than atmospheric heat insulation over hundreds of millions of years. Gases are excellent insulators.
Do you have any supporting math?
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Old 19th June 2010, 10:43 AM   #9
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http://dipastro.pd.astro.it/planets/...ctive_Ch10.pdf

http://www.skyandtelescope.com/news/...tml?page=1&c=y

http://www.is.wayne.edu/mnissani/a&s/GREENHOU.htm

http://www.lpi.usra.edu/vexag/nov_20...ions/crisp.pdf

Not that these, in themselves, are topic ending pieces, but if you are going to attack longstanding (close to two centuries old) mainstream scientific understandings, you would think that somewhere along the way, the actual science (preferrrable with a solid reference to the science, rather than a caricatured and distorted strawman version to beat up on) itself should be presented, if for no other reason than to clearly demonstrate how and where you think it to be incomplete or wrong.
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Old 20th June 2010, 04:59 AM   #10
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Originally Posted by wogoga View Post
Although only a small proportion of radiation from the sun reaches the ground of Venus, carbon dioxide is assumed to somehow heat up the ground.
Originally Posted by dasmiller View Post
If the sunlight isn't reaching the ground because it's being absorbed by the atmosphere, then the atmosphere is being directly warmed by the sun. I don't understand the "somehow" remark - are conduction and convection not well-understood?

The question is, whether Venus surface got heated up from the sun (as suggested by the greenhouse effect ideology), or simply remained hot because of a heat flow from inside (as suggested e.g. by common sense).

It is obvious that core, mantle, and crust are hotter than the surface. The atmosphere however is colder, continuously decreasing from the surface temperature of more than 450 įC next to the surface, to around -30 įC at the upper cloud deck in 60-70 km altitude (source).

Do you actually suggest that the atmosphere (warmed by the sun) can by conduction and convection increase surface temperate beyond its own temperature?

In Greenhouse Effect and Radiative Balance on Earth and Venus, page 4, we read:
Only ~2.6% of the solar flux incident at the top of the atmosphere reaches the surface

Ė Solar flux at the surface is ~17 W/m2 (global avg.)

Surface temperature of ~730 K maintained by an efficient atmospheric greenhouse mechanism

Ė Net downward thermal flux at surface ~15,000 W/m2
It seems rather astonishing to me that as much 2.6% of solar radiation should reach the surface, despite the mass of Venus' atmosphere corresponding to a mass of more than 1 km depth of water (more than 1000 metric tons per square meter). For comparison, at 100 m underwater the light present from the sun is about 0.5% of that at the surface. (source)

In any case, how "an efficient atmospheric greenhouse mechanism" should somehow transform a "solar flux at the surface" of around 17 W/m2 into a "net downward thermal flux at surface" of around 15,000 W/m2, will probably always remain a complete mystery.

Cheers, Wolfgang

Ideology driven science is rather rule than exception
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Old 20th June 2010, 07:04 AM   #11
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Originally Posted by MichaelN View Post
Don't forget that Venus receives around DOUBLE the intensity of solar radiation that Earth does, since it is much closer to the sun. Comparisons with Earth are quite ridiculous for that reason alone.
Um, the ratio of intensity if probably 1/r2, since we are have about 90 to 67 pretty close

67 million mile vs. 93 million miles so we have that the ratio of the in intensities is the ration between the distances squared
http://www.ndt-ed.org/EducationResou...e/distance.htm

I1/ I2=(D2)2/(D1)2

So we set I two and D two as earth and we get
(X)/(1)= (93)2 ( 67)2
X=8649/4489

X=1.926

But again it doesn’t matter you can still have insulation play a much larger role that ambient radiation, you will note that the energy of Venus is NOT twice that of the earth in stored energy per unit of the atmosphere.

Somehow I doubt that 14 C (Earth) and 464 C (Venus) is twice the energy storage,.
So what is the average surface temperature for Mercury compared to Venus, I mean it is that much closer than Venus , right?

I mean we have 40 million miles vs. 67 million mile so the ration is (67)2/(40)2 or 4489/1600=2.8 or almost three times the radiation,

But what is the mean day time temperature for Mercury?
350C and 170C for night time!
And venus has a mean surface tempertature of 464C.

So Why is that? I mean really it gets three times the radiation that Venus does!

That means Mercury should be three times hotter than Venus, right?
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Old 20th June 2010, 10:26 AM   #12
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I thought it had something to do with atmospheric pressure ?
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Old 20th June 2010, 11:50 AM   #13
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Originally Posted by Dancing David View Post
Um, the ratio of intensity if probably 1/r2, since we are have about 90 to 67 pretty close

67 million mile vs. 93 million miles so we have that the ratio of the in intensities is the ration between the distances squared
http://www.ndt-ed.org/EducationResou...e/distance.htm

I1/ I2=(D2)2/(D1)2

So we set I two and D two as earth and we get
(X)/(1)= (93)2 ( 67)2
X=8649/4489

X=1.926
Keep in mind also that the power emitted by a black body is proportional to the fourth power of the temperature. If you double the irradiance on a barren rock with no atmosphere you expect the new equilibrium temperature to be ~1.2 times higher(fourth root of 2).

If Earth and Venus were identical barren rocks, with Venus 2.5 times hotter, you'd need 2.54 = ~40 times the solar irradiance on Venus.
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Old 21st June 2010, 09:11 AM   #14
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Originally Posted by wogoga View Post
The question is, whether Venus surface got heated up from the sun (as suggested by the greenhouse effect ideology), or simply remained hot because of a heat flow from inside (as suggested e.g. by common sense).
To most educated and intelligent individuals, "common sense" says that the well researched, studied and evidenced understandings created by generations of very smart people who devote their lives to investigating natural phenomena probably have much more accurate and meritorious understandings of these phenomena than individuals who lack that training, experience and foundation of knowledge.
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Old 21st June 2010, 09:54 AM   #15
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Originally Posted by MichaelN View Post
Don't forget that Venus receives around DOUBLE the intensity of solar radiation that Earth does, since it is much closer to the sun. Comparisons with Earth are quite ridiculous for that reason alone.
Venus reflects about twice as much incoming radiation as the earth does so Venus actually gets slightly less solar energy entering itís atmosphere then the Earth does.

To the OP, nope sorry there is no way for Venus to hold onto itís heat over billions of years without incoming solar energy. While relatively little sunlight actually reaches the surface of Venus the energy that does get there has no way to escape because the solar energy absorbed on the way in heats the atmosphere and heat canít escape the surface unless itís warmer then the atmosphere above.

Another error in the OP is that at equilibrium the (non reflected) solar energy entering the atmosphere is going to be in balance with the IR leaving the atmosphere. What the greenhouse gasses are doing is increasing the temperature gradient required in order to have that much outgoing IR.
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Old 21st June 2010, 09:56 AM   #16
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Originally Posted by Lukraak_Sisser View Post
I doubt anyone these days makes a direct comparison between Venus and Earth. Apart from all the other mentioned differences, its rotation is so slow we have no idea what sort of extra effects that might have had on the evolution of its atmosphere, as well as its slightly lower density and the fact that the solar wind hits its atmosphere harder and the fact that it (at least currently) does not have a magnetic field to protect the atmosphere from the sun.
Dispite it's slow rotation there is almost no difference between nighttime and daytime temperatures on Venus. This is a classic characteristic of heating caused by greehouse gasses.
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Old 21st June 2010, 11:02 AM   #17
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Originally Posted by lomiller View Post
Another error in the OP is that at equilibrium the (non reflected) solar energy entering the atmosphere is going to be in balance with the IR leaving the atmosphere. What the greenhouse gasses are doing is increasing the temperature gradient required in order to have that much outgoing IR.
I'm still sorting it out, but the OP seems to be saying that Venus is far (>450 deg C) from equilibrium. I'm a little boggled at the idea that it could be substantially out of equilibrium for billions of years, and the OP seems to have a poor understanding of the greenhouse effect. But I'll have to wade through the posts a little more (I've been busy).
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Old 21st June 2010, 12:18 PM   #18
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Originally Posted by dasmiller View Post
I'm still sorting it out, but the OP seems to be saying that Venus is far (>450 deg C) from equilibrium. I'm a little boggled at the idea that it could be substantially out of equilibrium for billions of years, and the OP seems to have a poor understanding of the greenhouse effect. But I'll have to wade through the posts a little more (I've been busy).
Equilibrium in this case means non-reflected solar energy in the top of the atmosphere vs IR out the top of the atmosphere. The OP seems to be trying to argue IR is very slightly larger then solar energy in because its initial heat has been retained by itís atmosphere for 4 billion years.

How its atmosphere is selectively letting energy from incoming sunlight escape but preventing geologic heat from escaping is a mystery to meÖ
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Old 21st June 2010, 01:57 PM   #19
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Originally Posted by Xephyr View Post
I thought it had something to do with atmospheric pressure ?
It's got a hell of a lot to do with atmospheric pressure (and thickness). Atmospheres circulate. When air rises, it expands and cools, and conversely, when air drops, it compresses and heats. Air at lower altitudes is hotter than air at higher altitudes in large part because of adiabatic expansion and compression. This works out to roughly 10 deg. C per km, both on Earth and on Venus. So the fact that Venus's atmosphere is so much thicker than Earth's atmosphere is the primary cause of its dramatically higher surface temperature. Conversely, Mars has a much thinner atmosphere, and is much colder, despite being primarily CO2.
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Old 21st June 2010, 03:54 PM   #20
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Originally Posted by Ziggurat View Post
It's got a hell of a lot to do with atmospheric pressure (and thickness). Atmospheres circulate. When air rises, it expands and cools, and conversely, when air drops, it compresses and heats. Air at lower altitudes is hotter than air at higher altitudes in large part because of adiabatic expansion and compression. This works out to roughly 10 deg. C per km, both on Earth and on Venus. So the fact that Venus's atmosphere is so much thicker than Earth's atmosphere is the primary cause of its dramatically higher surface temperature. Conversely, Mars has a much thinner atmosphere, and is much colder, despite being primarily CO2.
Surely, you aren't echoing Mr Goddard and trying to bring WUWT woo pseudoscience into a legitimate discussion of Venus,...are you?

"adiabatic" - occurring without loss or gain of heat

Highly unlikely with respect to Venus and the issue of its atmospheric and surface heat content. There are some adiabatic processes playing important roles in the atmospheric physics of Venus, but by definition, adiabatic processes are energy neutral, and don't seem particularly relevent to issues of Venus' surface/atmosphere heat, nor any proposed nullification of the CO2 greenhouse effect.

I'd recommend "The Recent Evolution of Climate on Venus" Bullock & Grinspoon as a good reference on Venus's heat.
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Old 21st June 2010, 10:43 PM   #21
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Originally Posted by TShaitanaku View Post
Surely, you aren't echoing Mr Goddard and trying to bring WUWT woo pseudoscience into a legitimate discussion of Venus,...are you?

"adiabatic" - occurring without loss or gain of heat

Highly unlikely with respect to Venus and the issue of its atmospheric and surface heat content. There are some adiabatic processes playing important roles in the atmospheric physics of Venus, but by definition, adiabatic processes are energy neutral
They are heat neutral. They are NOT energy neutral. There's a major difference.

Quote:
and don't seem particularly relevent to issues of Venus' surface/atmosphere heat
It's damned relevant. Hell, it's very relevant to earth's surface temperature too. That's why Death Valley is so damned hot: it's below sea level.

Quote:
nor any proposed nullification of the CO2 greenhouse effect.
This isn't about a nullification of greenhouse effects. It's about the relative sizes of effects. And atmospheric thickness effects will dominate over greenhouse effects when you want to discuss the difference in surface temperature between, say, Earth and Venus.

Quote:
I'd recommend "The Recent Evolution of Climate on Venus" Bullock & Grinspoon as a good reference on Venus's heat.
Composition will be VERY important to the upper atmosphere temperature. But lower down, it becomes much less important. Look at figure 2 of your paper. Below about 40 km, notice how the temp vs. altitude graph becomes pretty much a flat line, fairly close to that 10 C/km adiabatic lapse rate I cited earlier. That's not radiative heat transfer. Yes, adiabatic compression and expansion aren't the ONLY factors at play, but they are the dominant component at lower altitudes. So whatever you want to do to the upper atmosphere, you'll still have that slope at lower altitudes, and you'll still end up with a surface temperature MUCH hotter than Earth's, regardless of what you do to the composition. That paper doesn't actually claim otherwise. In fact, let's see what they say about adiabatic processes:

"Convection was treated by taking the radiative equilibrium temperature profile and adjusting the lapse rate to be adiabatic wherever the radiative
equilibrium lapse rate exceeded the adiabat (McKay et al. 1989)."

Hmm.... seems they thought it was important after all.
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Old 22nd June 2010, 09:42 AM   #22
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Thanks for the explanation Ziggurat.

It makes perfect sense... heat gets trapped in the atmosphere due to the composition of high CO2 and the atmospheric pressure forces that heat to escalate until it hits its maximum based on physical laws. At that point the temperature is maintained.

Sounds like Venus is sitting on an equilibrium to me...

But here's my next question :

What causes Venus' atmospheric pressure to be so much higher than ours ? Rotation ? Axis tilt ? Direction of revolution ? Atmospheric composition ?

Could that answer the question of what our maximum temps could possibly reach in a worst case scenario if CO2 jacks up to 95% here on earth...? Of course the differences of our hydrological cycle, etc would all have to be factored in as well in order to calculate max temp possibilities.

Or am I completely off my crock thinking of it this way ?
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Old 22nd June 2010, 10:20 AM   #23
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Originally Posted by Xephyr View Post
Thanks for the explanation Ziggurat.

It makes perfect sense... heat gets trapped in the atmosphere due to the composition of high CO2 and the atmospheric pressure forces that heat to escalate until it hits its maximum based on physical laws. At that point the temperature is maintained.

Sounds like Venus is sitting on an equilibrium to me...

But here's my next question :

What causes Venus' atmospheric pressure to be so much higher than ours ? Rotation ? Axis tilt ? Direction of revolution ? Atmospheric composition ?
The amount of gas there is. Venus has much more gas than Earth.

Quote:
Could that answer the question of what our maximum temps could possibly reach in a worst case scenario if CO2 jacks up to 95% here on earth...? Of course the differences of our hydrological cycle, etc would all have to be factored in as well in order to calculate max temp possibilities.

Or am I completely off my crock thinking of it this way ?
You might be able to ballpark it. The problem is the uncertainty. A 20 degree C uncertainty (for example - real uncertainty may be different) is plenty good enough to distinguish Earth climate from Venus climate (which is more than 400 deg. C hotter at the surface), but it's not enough to gauge impact on humans, since 20 degrees makes a huge bloody difference to our lives. So gas volume considerations are enough to tell us we'll never look like Venus. They aren't enough to tell us what we really want to know.
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Old 22nd June 2010, 12:26 PM   #24
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I agree.

As I see it, the only way we could become Venus-like is if all of our oceans suddenly vapourized within minutes (or at least most of it to permanently alter the hydrological cycle). Maybe a catastrophic event happened on Venus that vapourized all the water, or perhaps it never formed water in the first place.

So long as there is evaporation (hydrological cycle), we have a pretty steady range of barometric pressure limitations going on. Water has less mass than gas in the air. More moisture, pressure drops, less moisture, pressure rises. That plays a huge role on earth's surface (at sea level) temp min/max possibilities.

... but as for Venus, I've often wondered if rotation speed/direction plays a role in any of that as to how and why it evolved to what it is today. Or maybe it just simply boils down to the very beginning of how a planet's formation starts off that determines the path of evolution it'll see.

Or maybe I just think too damn much.



Edit to the edit.
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Old 22nd June 2010, 01:53 PM   #25
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Greenhouse gasses controls surface temperature while atmospheric composition controls how much temperature decreases as you go up and how high you go before that relationship breaks down. If Venus’s atmosphere were equally thick but opaque to neither IR nor visible light it’s surface temperature would be approximately equal to it’s blackbody temperature and if the there were any lapse rate at all temperatures would decrease from these.

IOW you can’t do this the way Ziggurat is attempting, and it does indeed seem to be based on a blog posting by Goddard that has been thoroughly debunked. Lapse rate is ultimately a function of surface temperature not a cause of it.

BTW he does seem to be echoing a blog posting from last month, and one that has received considerable debunking. Here is a good example

http://moyhu.blogspot.com/2010/05/gr....html#comments
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Old 22nd June 2010, 02:14 PM   #26
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Which political anti-science woo site was the OP copied from?
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Old 22nd June 2010, 02:19 PM   #27
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An educational resource; http://www.aip.org/history/climate/co2.html
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Old 22nd June 2010, 02:23 PM   #28
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Originally Posted by lomiller View Post
Greenhouse gasses controls surface temperature while atmospheric composition controls how much temperature decreases as you go up and how high you go before that relationship breaks down. If Venusís atmosphere were equally thick but opaque to neither IR nor visible light itís surface temperature would be approximately equal to itís blackbody temperature and if the there were any lapse rate at all temperatures would decrease from these.
In other words, if you radiatively decouple the atmosphere completely from its surroundings, so that it can only absorb and release heat through conduction, then blackbody temperature of the surface is your only constraint. Well, yeah. But that's an irrelevant scenario, with or without greenhouse gasses, because that doesn't describe the atmosphere of any planet.

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IOW you canít do this the way Ziggurat is attempting, and it does indeed seem to be based on a blog posting by Goddard that has been thoroughly debunked. Lapse rate is ultimately a function of surface temperature not a cause of it.
No. Adiabatic lapse rate is a function of gravity and constant-pressure heat capacity. Surface temperature is part of the boundary condition for the problem (as already pointed out, this lapse rate won't hold over the entire atmosphere), but you're confusing boundary conditions with the solution to the problem.

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BTW he does seem to be echoing a blog posting from last month, and one that has received considerable debunking.
And yet, your attempts to describe what's wrong with what I said are incorrect.
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Old 22nd June 2010, 03:29 PM   #29
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Where did Venus get all the CO2 gas in the first place?

Or, the earth has about the same amount of carbon as Venus, how come ours is mostly dissoved in water and in rocks?

Venus is not big enough to catch and hold that much gas during its formation early in the history of the solar system.
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Old 22nd June 2010, 03:58 PM   #30
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Originally Posted by Ziggurat View Post
No. Adiabatic lapse rate is a function of gravity and constant-pressure heat capacity.
As I said, the hypothesis you are pushing has been debunked extensively since it appeared in a blog posting a month ago but by all means go ahead and try your derivation of lapse rate from gravity and heat capacity if you want.

Originally Posted by Ziggurat View Post
Surface temperature is part of the boundary condition for the problem (as already pointed out, this lapse rate won't hold over the entire atmosphere), but you're confusing boundary conditions with the solution to the problem.
So you were not attempting to explain surface temperature in your posts above?
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Old 22nd June 2010, 04:04 PM   #31
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Originally Posted by bobdroege7 View Post
Where did Venus get all the CO2 gas in the first place?

Or, the earth has about the same amount of carbon as Venus, how come ours is mostly dissoved in water and in rocks?

Venus is not big enough to catch and hold that much gas during its formation early in the history of the solar system.
Life might have something to do with it. And I've heard that the Moon may have had an effect as well - it was a lot closer when our atmosphere was forming, so tidal forces may have liberated a significant portion of the gases present back then.
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Old 22nd June 2010, 04:58 PM   #32
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Originally Posted by bobdroege7 View Post
Where did Venus get all the CO2 gas in the first place?

Or, the earth has about the same amount of carbon as Venus, how come ours is mostly dissoved in water and in rocks?

Venus is not big enough to catch and hold that much gas during its formation early in the history of the solar system.
Liquid water plays a big role in the formation of Carbonate rock. If Venus had oceans it lost them very early
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Old 22nd June 2010, 09:12 PM   #33
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Originally Posted by lomiller View Post
As I said, the hypothesis you are pushing has been debunked extensively since it appeared in a blog posting a month ago but by all means go ahead and try your derivation of lapse rate from gravity and heat capacity if you want.
It isn't my derivation (though I have also done it myself). It's standard textbook thermodynamics. And the derivation was included in that NASA page I linked to before (I can cite thermo textbooks too if you care), but here it is again. I notice that, once again, you can only state that what I'm saying has been debunked, you can't actually offer up an argument yourself for why anything I said is wrong. In fact, given your responses, it appears that you don't even understand what I said.
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Old 23rd June 2010, 02:56 AM   #34
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Originally Posted by wogoga View Post
So what is called greenhouse effect in the case of Venus is nothing more than atmospheric heat insulation over hundreds of millions of years. Gases are excellent insulators.

Originally Posted by ben m View Post
Nope. First of all, gases are excellent insulators against *conduction* and only conduction. Given convection and advection, over large distances they're actually worse insulators than solids.

Do you know evidence of significant vertical convection on Venus?

In heights where atmospheric temperatures are (significantly) influenced by the sun, there is strong horizontal convection (zonal circulation, transporting heat between the day and night sides of the planet). At lower than 50į latitudes, such east-west winds decrease from around 100 m/s at 60-70 km height to less the 1 m/s near surface.

Heat is also transported from the equator to the poles. However, near surface, such heat exchanges are not needed, as the temperature there does not depend on sun radiation, but on the iso-thermic heat from within the crust. (see)

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And they're no insulation at all against radiation, except in the normal atmospheric-greenhouse-effect effect sense.

Also liquids and solid are "no insulation at all against radiation, except in the normal" not-being-transparent "sense".

To characterize a well-known normal physical principle by a trendy ideological concept such as greenhouse-effect seems rather problematic to me.

That Venus may seem hotter than expected, results primarily from the fact that planets with atmospheres have no clearly defined surfaces.

A black body with Venus' surface temperature (~740į Kelvin) radiates around 17,000 W/m2, whereas a black body with a temperature of Venus' upper cloud deck (~240į Kelvin) less than 200 W/m2.

If we defined the boundary between Jupiter's atmosphere and Jupiter's fluid interior as Jupiter's surface, then we also could explain its high temperature by a super greenhouse effect.

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Old 23rd June 2010, 07:13 AM   #35
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Originally Posted by Ziggurat View Post
It isn't my derivation (though I have also done it myself). It's standard textbook thermodynamics. And the derivation was included in that NASA page I linked to before (I can cite thermo textbooks too if you care), but here it is again. I notice that, once again, you can only state that what I'm saying has been debunked, you can't actually offer up an argument yourself for why anything I said is wrong. In fact, given your responses, it appears that you don't even understand what I said.
As I said weíve seen this all before. Atmospheric height is not a constant, itís a function of surface temperature and lapse rate.. The equation you cite is perfectly acceptable to us in that context. You cannot use the way you are attempting and try to pick a fixed atmospheric height and us lapse rate to derive surface temperature.
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Old 23rd June 2010, 10:16 AM   #36
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Regarding the adiabatic lapse rate: there *are* thermodynamic assumptions that go into that calculation. One of them is that there are adiabatic vertical air currents; the calculation that gives you the lapse rate is basically the calculation of how much a parcel of (high-altitude, low-pressure) air will heat up when descending and being compressed, or vice-versa. If air is moving vertically, then you *will* have this heating effect from standard textbook thermodynamics. It gives you a relationship between the derivatives of pressure and temperature---it doesn't give you the temperature itself. You apply this relationship to (a) hydrostatic equilibrium and (b) surface temperature and *all together* these constraints fix the lapse rate and the scale height. They're not independent.

But Zig is not, as far as I can tell, claiming anything different. He's not claiming that basic thermo "magically" tells you the surface temperature or the scale height. He's claiming that there's a basic thermo lapse rate which, combined with the hydrostatic equilibrium equation (which includes the total mass of the atmosphere, which differs from planet to planet) and some temperature setpoint (which depends on radiation and whatnot, and differs from planet to planet) tells you that a massive atmosphere has higher lapse rate than a low-mass one. Sounds good to me.

But note that this is not the only way for an atmosphere to behave. An isothermal atmosphere is also a perfectly good solution to the thermodynamics. The stratosphere is another perfectly good solution.
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Old 23rd June 2010, 10:22 AM   #37
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What we are seeing here, AGAIN, is what happens when people with essentially no physics education and transparent political motives try to refute a scientific consensus that took many years to reach.
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Old 23rd June 2010, 10:32 AM   #38
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Originally Posted by lomiller View Post
As I said weíve seen this all before. Atmospheric height is not a constant, itís a function of surface temperature and lapse rate.. The equation you cite is perfectly acceptable to us in that context. You cannot use the way you are attempting and try to pick a fixed atmospheric height and us lapse rate to derive surface temperature.
I'm not picking a fixed atmospheric height. Rather, you are assuming that the surface temperature is the only relevant boundary condition. And it isn't. The convective zone where the adiabatic lapse rate dominates does not extend to the edge of space. Where the adiabatic lapse regions meets the upper atmosphere, the temperatures AND the pressures must be equal. This region will end when the atmosphere gets thick enough, and its temperature will be determined largely by radiative balance between the sun and space. That sets our pressure and temperature for the top of the adiabatic lapse region.

If you try to fix a set surface temperature and use the adiabatic lapse rate, you can calculate an altitude where it will match our upper atmosphere, and it will indeed vary depending on the surface temperature you picked. But since the pressure will also need to match, doing this also constrains the amount of gas that the atmosphere has. But of course, that's wrong: temperature can vary quite a bit, but atmospheric content is essentially a conserved quantity, at least over the time scales needed to reach thermal equilibrium. Pick the wrong surface temperature, and you'll have the wrong amount of gas. Going the other direction, starting from the upper atmosphere at a fixed temperature and pressure, we can use the adiabatic lapse rate PLUS the total atmospheric content to calculate an altitude, which will give us a final surface temperature. And more atmosphere means more altitude, and higher surface temperature. That's a simplification, of course, but it's a far more accurate simplification than what you suggest.

But it's more than a little ironic that you basically ignored radiative transfer in the upper atmosphere in order to defend your understanding of the green house effect.
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Old 23rd June 2010, 10:33 AM   #39
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Originally Posted by BenBurch View Post
What we are seeing here, AGAIN, is what happens when people with essentially no physics education and transparent political motives try to refute a scientific consensus that took many years to reach.
Who, pray tell, are you referring to?
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Old 23rd June 2010, 10:48 AM   #40
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Originally Posted by ben m View Post
...But Zig is not, as far as I can tell, claiming anything different...
Well, then one of us is reading his responses improperly, if it is I, I apologize for the improper charaterization, but I have a hard time reconciling the following statements with that understanding:

"...So the fact that Venus's atmosphere is so much thicker than Earth's atmosphere is the primary cause of its dramatically higher surface temperature. Conversely, Mars has a much thinner atmosphere, and is much colder, despite being primarily CO2..."

"...That's why Death Valley is so damned hot: it's below sea level..."

"...This isn't about a nullification of greenhouse effects. It's about the relative sizes of effects. And atmospheric thickness effects will dominate over greenhouse effects when you want to discuss the difference in surface temperature between, say, Earth and Venus..."
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