Evolution of Venus Temperature & Climate

[wogoga]

Think also about the poles where the solar flux is virtually zero all the time. Ground temperature there is essentially the same as on the equator.

It's close to equatorial temperatures, because of convection. You can't get that kind of temperature homogeneity across the entire planet without it. Why on earth did you think that this piece of evidence undermined rather than supported what I've been saying?

Even if we start with the highly questionable claim that as much as 2.6 percent of the incoming radiation reaches the crust, on the equator at midday we get only around 70 W/m2 in order to heat up a ground of 470°C.

Ever tried to heat up e.g. melted Zinc of 470°C with weak daylight?
(Adding 70 W/m2 to a blackbody radiation of 470°C, i.e. from 17,000 W/m2 to 17,070 W/m2, increases temperature not even by half a degree C.)

Horizontal winds near surface are slower than 1 m/s, i.e. less than 100 km/day. Because the distance from the midday point to the midnight point on the equator is around 20,000 km, more than 200 days would be needed for temperature homogeneity to be achieved by near-surface convection.

By the way, the atmospheres of both Earth and Mars are heated up by sunlight from the crust surface, and an obvious logical consequence of such a heating process is an unhomogeneous temperature near surface.

Every source I can find says that near-surface vertical air motion is somewhere in the 1mm/s or 1cm/s ballpark.

A horizontal wind of 0.2 m/s and a slope of 5% are enough to entail a vertical air motion of 10 mm/s.

Cheers, Wolfgang
www.pandualism.com

 

[Ziggurat]

Even if we start with the highly questionable claim that as much as 2.6 percent of the incoming radiation reaches the crust

Why is that highly questionable?

Ever tried to heat up e.g. melted Zinc of 470°C with weak daylight?

Nonsensical comparison, since I don't have a Venus-like environment to try that experiment in. Ever heated something to high temperatures with low power? Oh, that's easy.

(Adding 70 W/m2 to a blackbody radiation of 470°C, i.e. from 17,000 W/m2 to 17,070 W/m2, increases temperature not even by half a degree C.)

Your point?

Horizontal winds near surface are slower than 1 m/s, i.e. less than 100 km/day.

Wow, you've discovered shear viscosity. Your point?

Because the distance from the midday point to the midnight point on the equator is around 20,000 km, more than 200 days would be needed for temperature homogeneity to be achieved by near-surface convection.

But the convection is NOT simply near-surface. Why would it be?

By the way, the atmospheres of both Earth and Mars are heated up by sunlight from the crust surface, and an obvious logical consequence of such a heating process is an unhomogeneous temperature near surface.

Venus's surface is only relatively homogeneous. And it's relatively homogeneous because it's got a much thicker atmosphere than Mars or Earth, just like Earth has a much more homogeneous surface temperature than Mars because it's got a thicker atmosphere. So what? That doesn't validate your idea of sub-surface heating being the driving force for the temperature on Mars.

Seriously, try it out. You're arguing that the amount of power incident on the surface from solar radiation is not sufficient to account for its temperature. OK, then how much power is needed? How large a temperature gradient do you need through rock in order to get that power? How thin a crust can Venus have and still create that large a temperature gradient and thermal output? If you're not satisfied by the consistency of the standard picture, why don't you test your own ideas for consistency?

Or are you just another one of the endless string of internet cranks?

 

[Trakar]

Sounding more and more like "DenialDepot."

 

[wogoga]

Your hypothesis does not predict an isothermal atmosphere, nor a non-circulating atmosphere. Your hypothesis, "Venus's surface is heated from below", predicts that the lower atmosphere is circulating like crazy due to convection. That's what happens when you heat something from below.

Am I actually the only one here, being capable (not without effort) of recognizing how mistaken such statements are?
If you heat up an area on Earth to 470°C, then the atmosphere near that area will "circulating like crazy due to convection". The reason is obvious: The air near the hot spot is heated up and its density (mass divided by volume) becomes lower than the density of the surrounding air. The heated air moves upwards, and is replaced by not yet heated air.

The situation on Venus however is not such a dynamic one. The hottest air (i.e. the one next to the ground) also has the highest density. And because not higher temperature but lower density ultimately causes such vertical convection, the air near the surface will simply remain where it is.

Cheers, Wolfgang

Medieval superstition in modern society

 

[Trakar]

The situation on Venus however is not such a dynamic one. The hottest air (i.e. the one next to the ground) also has the highest density. And because not higher temperature but lower density ultimately causes such vertical convection, the air near the surface will simply remain where it is.

you seem to be describing a violation of physical law and scientific principle, care to reword or provide support and verifiable evidences to confirm your allegations?

 

[ben m]

The situation on Venus however is not such a dynamic one. The hottest air (i.e. the one next to the ground) also has the highest density. And because not higher temperature but lower density ultimately causes such vertical convection, the air near the surface will simply remain where it is.

Wogoga, this is complete nonsense. On Earth, the hottest air is also closest to the ground and also has the highest density---it still convects. Likewise in the Sun---the interior is at higher density and higher temperature than the surface, but there's still convection.

What this shows is that you don't understand convection well enough. It's not just a matter of comparing two densities, as though you had two equal-volume jars of air on a laboratory balance. You need to compare the densities at equal pressures, and the pressure-change needs to be tracked adiabatically. When you work it out, under a reasonable set of assumptions, you find that convection occurs when the temperature lapse rate is lower than the adiabatic lapse rate; this is called the Schwartzchild criterion. (Hansen & Kawaler, "Stellar Interiors", pg 180)

You can't understand atmospheres without knowing any thermodynamics, Wogoga.

 

[wogoga]

The situation on Venus however is not such a dynamic one. The hottest air (i.e. the one next to the ground) also has the highest density. And because not higher temperature but lower density ultimately causes such vertical convection, the air near the surface will simply remain where it is.

On Earth, the hottest air is also closest to the ground and also has the highest density---it still convects.

The air closest to the ground is the hottest with the highest density on average.

Likewise in the Sun---the interior is at higher density and higher temperature than the surface, but there's still convection.

Also correct, if we assume "on average".

You need to compare the densities at equal pressures, and the pressure-change needs to be tracked adiabatically.

That we "need to compare the densities at equal pressures" seems quite wrong to me. If the air at a higher altitude is less dense, then no downward force arises in the first place, and without movement no adiabatic changes.

On Venus, a surface air layer of 67 kg/m3 is not replaced by a superior layer of 66 kg/m3, even if the colder superior layer would turn out denser "at equal pressures".

When you work it out, under a reasonable set of assumptions, you find that convection occurs when the temperature lapse rate is lower than the adiabatic lapse rate;

This statement seems not unreasonable to my Kantian synthetic a priori reasoning.

Let us make a rough estimate:

• Venus atmosphere is around 100 metric tons per square meter.
• The lowest 150 m atmosphere result in 67 kg/m3 * 150 m = 1 ton/m2.
• So at a height of 150 m, we have 99% of ground level pressure.
• Assuming ideal gas law, we conclude that absolute temperature of air at 150 m height must be at least 1% lower than at ground level, in order to sink due to higher density.
• One percent of ground level temperature (740°K) is 7.4 K.
• If actual lapse rate is less than 7.4 K / 150 m = 50°C/km near surface, then convection cannot start.

Cheers, Wolfgang

An inconvenient truth: Rising sea levels tend to moderate global climate

 

[lomiller]

If you heat up an area on Earth to 470°C, then the atmosphere near that area will "circulating like crazy due to convection".

Not if the “atmosphere near the area” is already at 469.9 deg…

 

[ben m]

On Venus, a surface air layer of 67 kg/m3 is not replaced by a superior layer of 66 kg/m3, even if the colder superior layer would turn out denser "at equal pressures".

Bzzt. Sorry, Wogoga, you're simply wrong; you are probably used to thinking about buoyancy only for incompressible objects and incompressible fluids.

Perhaps the answer is more obvious if you treat it as a Hamiltonian system---does the system's total gravitational potential energy increase, or decrease, as the *net* effect (including adiabatic heating, hydrostatic pressure increase, etc.) of swapping Air Parcel #1 with Air Parcel #2? If the answer is "decrease", then there's a buoyancy force working to carry out that swap. The calculation I cited (Hansen & Kawaler) does this correctly. You do not.

 

[wogoga]

Sorry, Wogoga, you're simply wrong; you are probably used to thinking about buoyancy only for incompressible objects and incompressible fluids.

Ben, I've changed my mind several times. Just after having convinced myself once again that I'm rather wrong than right, I noticed in the table posted by Ziggurat in post #19 a column with the name Autoconvective Lapse Rate. Its value is 47 K/Km for Venus, which comes quite close to my above rough calculation (showing that vertical convection on Venus near ground only starts if actual lapse rate is higher than around 50°K/Km).

Then I found this quote from Basic Convection (for students of meteorology):

In many situations, surface heating is so intense that even convection is not enough to transfer the heat. This condition can happen near the surface in quite shallow layers, at locations where the vertical motions generated by convection are limited by the shallowness of the layer. Then it is possible for extremely high lapse rates to develop. The lapse rate can become sufficiently high so that density increases with height. It can be shown that a hydrostatic atmosphere in which density doesn’t change with height has a lapse rate of g/R. This rate turns out to be about 34 C km-1 [on Earth] and is known as the autoconvective lapse rate. If the environmental lapse rate exceeds this value, not even a butterfly’s flapping is necessary to initiate overturning; overturning is spontaneous in the same way that a brick held and then released in the air falls spontaneously!​

Perhaps the answer is more obvious if you treat it as a Hamiltonian system---does the system's total gravitational potential energy increase, or decrease, as the *net* effect (including adiabatic heating, hydrostatic pressure increase, etc.) of swapping Air Parcel #1 with Air Parcel #2?

Sounds reasonable.

If the answer is "decrease", then there's a buoyancy force working to carry out that swap.

It is rather the contrary: If actual lapse rate is between adiabatic (10.5°C/Km on Venus) and autoconvective (47°C/Km) lapse rate, then only a forced vertical convection opposing buoyancy can start a movement. Buoyancy itself has a stabilizing effect.

Only after a movement in the right direction has started, it is more and more reinforced by buoyancy, because adiabatic temperature changes (in function of changing height) are weaker than corresponding volume changes. In the case of sinking air parcels, this means that, as long as they sink, they become heavier and heavier in comparison with their environment.

Thus, an actual lapse rate between the adiabatic and the autoconvective one leads rather to what can be called a meta-stable atmosphere. I do not exclude that actual lapse rate on Venus may have been in the lower meta-stable range over hundreds of millions of years.

A further quote from Basic Convection:

Unless the lapse rate exceeds the autoconvective, the parcels will not rise spontaneously.​

Anyway, as actual lapse rate on Venus is even lower than the adiabatic one, your claim of post #75 that heating the atmosphere from the crust would predict "that the lower atmosphere is circulating like crazy due to convection" is simply untenable.

Cheers, Wolfgang

 

[ben m]

Anyway, as actual lapse rate on Venus is even lower than the adiabatic one, your claim of post #75 that heating the atmosphere from the crust would predict "that the lower atmosphere is circulating like crazy due to convection" is simply untenable.

You've forgotten---I said that in the context of the (hypothetical) "isothermal atmosphere" which you had latched onto for some reason. In an isothermal atmosphere, *any* heating from the bottom will result in convection. Venus isn't isothermal, so who cares?

I've long since lost track of your point, if there was one. No, Venus's lower atmosphere is not convecting strongly. It's convecting slowly, a little, with equatorial air welling up and polar air sinking, which provides (a) the observed large single Hadley cell and (b) is apparently fast enough to make the isothermal solution impossible, i.e. you have to pay attention to the compression of the air. Is that clear?

Now that you understand that, we get back to your first post.

Q) Does the high temperature at Venus's surface, despite the fact that it doesn't receive much sunlight, mean that there's something wrong with the idea of the greenhouse effect??

A) No. Venus's upper atmosphere heats up and absorbs (most) of the direct sunlight; if you want to talk about an Earth-like greenhouse effect you need to do it up there. Additional temperature rise towards the surface is (largely) due to adiabatic compression of hot, sinking upper-atmosphere air. (Why doesn't this compression-heated air just cool radiatively? That is indeed the greenhouse effect.)

Q) Could Venus's primordial/geological heat explain the surface temperature "without the greenhouse effect"?

A) No. First, "without the greenhouse effect" seems to mean "ignoring some of the laws of radiation, heat, and thermodynamics" and therefore can't explain anything. Second, no, Venus's atmosphere is not THAT good an insulator.

 

[wogoga]

Even if we start with the highly questionable claim that as much as 2.6 percent of the incoming radiation reaches the crust, on the equator at midday we get only around 70 W/m2 in order to heat up a ground of 470°C.

Why is that highly questionable?

Venus atmosphere is around 100 times denser (mass/surface) than our atmosphere, and contains around 67 times more molecules per surface. (On average, atmosphere molecules on Venus have 50% more mass than on Earth.)

Average solar radiation arriving at the top of the Earth's atmosphere is roughly 1,366 watts per square meter. However, as the Sun's rays are attenuated be the atmosphere, surface insolation is reduced to approximately 1,000 watts per square meter for a surface perpendicular to the Sun's rays at sea level on a clear day (see).

Thus, the effect of our atmosphere is a reduction of solar radiation by a factor of 1,000 W / 1,366 W = 0.73. According to the Beer-Lambert law, we would expect for an atmosphere which is only 15 times denser, a reduction of radiation energy to 0.01 (and for one 30 times denser, a reduction to 0.0001).

Venus atmosphere however is 67 or 100 times denser, and in addition to that has a consistent thick cloud deck. On Earth thick clouds easily reduce the energy of insolation reaching the surface to 1% (i.e. by a factor of 0.01) (see).

So if Venus crust surface were actually heated up by the sun, then we would have to attribute this fact rather to an incredible transparency of Venus atmosphere for solar radiation than to an untransparency for thermal radiation.
I'm glad to see that I'm not alone with my assessment. Relevant quotes from Questionable Science, by Robert Clemenzi:

"Notice that both Earth and Venus have surface temperatures that are hotter than their respective black body temperatures - this difference IS the Greenhouse Effect. In both cases, this is caused by the atmosphere holding heat at the surface.

Also notice that the Venusian surface temperature is near constant day and night - diurnal (daily) temperature change is about zero."

"The only important item is that the atmosphere of Venus has about 100 times more gas than Earth between the surface and space."

"The Venus lapse rate and surface temperature appear to be constant day or night (by itself, this implies that the Sun has little effect on the planet's temperature)."

"The point of this is that to cool the Earth, hot air only needs to rise about 2 km (an easy task) to get above most of the green house gases - on Venus, it has to rise 55 km. In my opinion, this is the reason Venus is so hot - in fact, if the Venusian atmosphere had the same composition as Earth, it would be even hotter (because water vapor is a much better green house gas than CO2).

The fact that the Venusian surface temperature is the same after 58 days of sun light and 58 days of darkness is really the main reason I claim that the Sun does not heat the surface. (Venus rotates with respect to the Sun once every 116.75 Earth days.)

On Earth, the minimum expected difference would be 50°C for a 2,800 hour day, on Venus, no difference is reported.

The official explanation for Venus having a constant surface temperature is strong winds - I am not convinced and I have not seen any evidence to support that position.

From NASA – Venusian wind speeds: 0.3 to 1.0 m/s (surface)."

"Based on the number of visible craters, it is believed that the surface of the planet was liquid rock 500 million years ago. If that is true, then it is safe to assume that the crust is much thinner than on Earth. As a result, the internal core heat is perhaps the most important reason that Venus is so hot.

It is frequently stated that because of the high albedo and thick atmosphere, almost no solar energy reaches the surface of Venus. If this is true, then how come the surface temperature is given as over 900F? Obviously, all this heat is coming from the planet itself, and not the Sun.

Coupled with the thicker atmosphere, and the fact that the surface temperature is near constant day and night, this is why Venus is hot."

"To be fair, Earth's oceanic crust is fairly young (still being created) ... and very thin. Yet the oceans are not boiling.

So, maybe the crust thickness is irrelevant - just the atmospheric thickness is important."​

Cheers, Wolfgang

Maybe a good example of a greenhouse-gas establishment's insidious fight against its oppenents – Fake opponent, little grey rat: "I am far more interested in holocaust history than global warming. I fully expect to die in a world that still believes in fantastical homicidal gas chambers even though I am certain it is absolute bollocks."

 

[dasmiller]

So if Venus crust surface were actually heated up by the sun, then we would have to attribute this fact rather to an incredible transparency of Venus atmosphere for solar radiation than to an untransparency for thermal radiation.

so, are you assuming that the surface is not heated by heat radiated from the clouds? I'd think that would dominate.

 

[Trakar]

I'm glad to see that I'm not alone with my assessment. Relevant quotes from Questionable Science, by Robert Clemenzi:

EUREKA!

You have managed to find another unqualified, nonscientist writing about his politically inspired and informed impressions of science without much of a clue as to what he's really talking about. At least he had the courtesy to more appropriately label his confused ramblings "Questionable Science."

 

[Ziggurat]

Thus, the effect of our atmosphere is a reduction of solar radiation by a factor of 1,000 W / 1,366 W = 0.73. According to the Beer-Lambert law, we would expect for an atmosphere which is only 15 times denser, a reduction of radiation energy to 0.01 (and for one 30 times denser, a reduction to 0.0001).

You're treating a spectrum-averaged value as if it were the value for every wavelength in the entire spectrum. It isn't, which means you can't use those equations in the manner you are trying to use them. Let me give you a simple example: suppose that the atmosphere is absorptive across half the spectrum, and not absorptive across the other half. At some depth, the radiation from the absorbed part is cut in half, for a total transmission of 0.75. At twice the depth, what's the absorption? Well, the absorbed part gets cut down to 0.25, but that's half the spectrum, so the total transmission is 0.625. But according to your naive interpretation, it should be 0.5625. Go really far, and the transmission should approach 0.5, while your naive interpretation indicates it should approach 0.

So, you're wrong.

So if Venus crust surface were actually heated up by the sun, then we would have to attribute this fact rather to an incredible transparency of Venus atmosphere for solar radiation than to an untransparency for thermal radiation.

Here's a simple test. If the surface of Venus is heated by the sun, then light from the sun is hitting the surface (and vice versa). If we were to put a camera on the surface of Venus, it should be able to take pictures with visible light. In contrast, if no light is reaching the surface from the sun, the only light should be blackbody radiation, which (like looking inside a hot kiln) won't reveal any detail to our images. Now, what do you think will happen? Or, should I say, already happened.

 

[wogoga]

You're treating a spectrum-averaged value as if it were the value for every wavelength in the entire spectrum. It isn't, which means you can't use those equations in the manner you are trying to use them. Let me give you a simple example: suppose that the atmosphere is absorptive across half the spectrum, and not absorptive across the other half. At some depth, the radiation from the absorbed part is cut in half, for a total transmission of 0.75. At twice the depth, what's the absorption? Well, the absorbed part gets cut down to 0.25, but that's half the spectrum, so the total transmission is 0.625. But according to your naive interpretation, it should be 0.5625. Go really far, and the transmission should approach 0.5, while your naive interpretation indicates it should approach 0.

I fully agree. However, I never had the intention (post #92) to show that atmosphere and clouds of Venus attenuate incoming radiation to

(1000W/1366W)^67 * 0.01 = 10^-11 = 0.000,000,001%​

I only wanted to demonstrate in very rough, simple, and transparent way that the assumption of attenuation to a value as high as 2.6% is a priori rather unlikely.

Already when I started this thread, I doubted the statement "The cloud cover is such that very little sunlight can penetrate down to the surface, and the light level is only around 5,000–10,000 lux with a visibility of three kilometres".

According to this source, on Earth, illumination (more or less proportional to radiation energy) of an overcast day is around 1,000 Lux and of a very dark overcast one around 100 Lux.

So if Venus crust surface were actually heated up by the sun, then we would have to attribute this fact rather to an incredible transparency of Venus atmosphere for solar radiation than to an untransparency for thermal radiation.

Here's a simple test. If the surface of Venus is heated by the sun, then light from the sun is hitting the surface (and vice versa). If we were to put a camera on the surface of Venus, it should be able to take pictures with visible light.

Even on a very dark overcast day, we can take pictures with visible light. So this cannot be considered evidence that atmospheric attenuation of incoming radiation on Venus crust surface is only to around 2.6%.

In contrast, if no light is reaching the surface from the sun, the only light should be blackbody radiation, which (like looking inside a hot kiln) won't reveal any detail to our images. Now, what do you think will happen? Or, should I say, already happened.

Because Venus ground temperature (730°K) is not far away from the Draper point (798°K, above which almost all solid materials glow as a result of blackbody radiation), radiation in the near infrared could actually have contributed to the picture of Venus' surface (if the picture was taken without infrared filter).

And because emission spectra correspond to their absorption spectra, thermal radiation should reveal details in a similar way as external illumination (see also infrared photography).

Cheers,
Wolfgang

Exposed to Facts, the Misinformed Believe Lies More Strongly – And what if the authoritative majority has been misinformed in the first place?

 

[ben m]

Wolfgang, once again: what's your point? You think there's something wrong with the way we discuss Venus. You are wrong. Let's go back to your original point.

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.​

At this point, you already missed 90% of the truth. Most of the incident radiation is absorbed directly by the atmosphere, thus heating the atmosphere. The lower the atmosphere heats further due to adiabatic compression of the already-hot upper atmosphere. The bottom of the atmosphere is in contact with the ground, so they're both hot, and this high temperature does *not* require lots of solar radiation to go straight to the ground. (It is not vaguely "somehow"; we are not making it up. The atmosphere heats the ground via perfectly standard gas behaviors, which you seem to want to ignore.)

The "greenhouse" aspect of it is that the atmosphere is opaque enough to allow this whole thing to actually be adiabatic. If the atmosphere were transparent, the lower heights would indeed heat up due to compression and also (rapidly) cool due to radiation. Since the atmosphere is not transparent, the adiabatic temperature increases are not fighting with radiative losses, and the temperature therefore gets very high.

You have not disputed one word of this as far as I can tell.

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.​

And you have provided no evidence whatsoever for any of this.

Want to start over?

 

[Ziggurat]

I fully agree. However, I never had the intention (post #92) to show that atmosphere and clouds of Venus attenuate incoming radiation to

(1000W/1366W)^67 * 0.01 = 10^-11 = 0.000,000,001%​

I only wanted to demonstrate in very rough, simple, and transparent way that the assumption of attenuation to a value as high as 2.6% is a priori rather unlikely.

It's not an assumed value, there's nothing "a priori" about it, and your estimates of likelihood are worthless.

Even on a very dark overcast day, we can take pictures with visible light. So this cannot be considered evidence that atmospheric attenuation of incoming radiation on Venus crust surface is only to around 2.6%.

Way to miss the point. The 2.6% number doesn't come from that observation, the fact that the surface is still heated by the sun does. And guess what: it's still heated by the sun even on an overcast day here on earth.

Exposed to Facts, the Misinformed Believe Lies More Strongly – And what if the authoritative majority has been misinformed in the first place?

The irony, it burns.

 

[wogoga]

A compilation from my posts of this thread, highlighting the most convincing arguments: Discussion on Evolution of Venus Temperature & Climate in the Context of Global Warming

The discussion of 2010 has shown that the extremely high temperature on Venus' surface is at most marginally influenced by a greenhouse effect.

Unexpected for me, lapse rate (temperate decrease with increasing altitude) has become the central point of the discussion. There are three types of lapse rate:

• Actual lapse rate of ~8°Kelvin per km (Source).
• Adiabatic lapse rate of ~10.5°K/km
• Autoconductive lapse rate of ~47°K/km (Source)

A lapse rate lower than adiabatic (10.5°K/km on Venus) leads to a stable atmosphere, i.e. without vertical convection. If we transfer an atmospheric volume-sample to a lower altitude then the original sample gets compressed to the pressure of the lower altitude. Compression leads to temperature increase. Yet this adiabatic temperature increase of our volume-sample is bigger than the actual temperature increase of the environment (from higher to lower altitude). Therefore the sample's lower density leads by buoyancy to an opposite force (upwards, back to the original altitude).

If we transfer an atmospheric volume-sample to a higher altitude then the original sample gets decompressed leading to temperature decrease. Yet this adiabatic temperature decrease is bigger than the actual temperature decrease of the environment. Therefore the sample's higher density leads to a downwards force.

A lapse rate between adiabatic and autoconvective leads to a metastable atmosphere. Air at lower altitude still has a higher density, but if a given sample-volume is somehow forced downwards then adiabatic heating is less than heating of the new environment. As the lowered volume-sample turns out be colder and thus denser than its new environment, the downwards movement is reinforced by buoyancy (and weakened by heat transfer via conduction and mixing). In case of upwards movement, reinforcement works analogously.

A lapse rate higher than autoconvective (47°K/km on Venus) would imply that atmospheric layers at higher altitudes, due to substantially lower temperatures, have higher densities than layers below. Buoyancy alone is in principle enough to start vertical convection. (A stabilizing effect results from the fact that the layer of a given altitude cannot sink as a whole at the same time; see #90.)

Summary: Since on Venus actual lapse rate of 8°K/km is lower than adiabatic of 10.5°K/km, no vertical convection can arise in the first place on the isothermal crust surface. Thus the lower atmospheric layers act as excellent heat insulators keeping crust surface at a temperature which can be found on Earth only at a depth of 15 - 20 km below crust surface (Source). Nevertheless, Venus as a whole has (maybe apart from exceptional events) always been radiating away more energy than receiving from the Sun.

Quote from NASA climate modeling suggests Venus may have been habitable, 2016 (emphasis mine):

Venus may have had a shallow liquid-water ocean and habitable surface temperatures for up to 2 billion years of its early history, according to computer modeling of the planet's ancient climate by scientists at NASA's Goddard Institute for Space Studies (GISS) in New York.

The findings, published this week in the journal Geophysical Research Letters, were obtained with a model similar to the type used to predict future climate change on Earth.​

It is rather unlikely that a model entailing such unreasonable findings for Venus' past could lead to reasonable findings for the Earth's future. In any case, a runaway greenhouse effect (implying a much colder Venus in the past) is rather the result of ideology than of science.

Cheers, Wolfgang

A fundamental scientific discussion relevant to the evolution of the Earth-Moon system relegated (or promoted?) to the conspiracy forum due to scientific taboo: Kinetic energy at atmospheric reentry from lunar mission

 

[bobdroege7]

What you are doing is the same as using the greenhouse effect to prove there is no lapse rate.

The lapse rate and the greenhouse effect are two sides of the same coin.