Guest Post by Willis Eschenbach
This is the third in a series ( Part 1, Part 2 ) of occasional posts regarding my somewhat peripatetic analysis of the data from the TAO moored buoys in the Western Pacific. I’m doing construction work these days, and so in between pounding nails into the frame of a building I continue to pound on the TAO dataset. I noticed that a few of the buoys collect data on both shortwave (solar) radiation and longwave (infrared or greenhouse) radiation at two-minute intervals. For a data junkie like myself, two-minute intervals is heaven. I decided to look at the data from one of those buoys, one located on the Equator. at 165° East.
Figure 1. Location of the buoy (red square) which recorded the data used in this study. Solid blue squares show which of all the buoys have the two-minute data. DATA SOURCE
It was a fascinating wander through the data, and I found that it strongly supports my contention, which is that the net effect of clouds in the tropics is one of strong cooling (negative feedback).
To start with, I looked (as always) at a number of the individual records. I began with the shortwave records. Here is a typical day’s record of the sun hitting the buoy, taken at two-minute intervals:
Figure 2. A typical day showing the effect of clouds on the incoming solar (shortwave) radiation.
In Figure 2 we can see that when clouds come over the sun, there is an immediate and large reduction in the incoming solar energy. On the other hand, Figure 3 shows that clouds have the opposite effect on the downwelling longwave radiation (DLR, also called downwelling infrared or “greenhouse” radiation). Clouds increase the DLR. Clouds are black-body absorbers for longwave radiation. After they absorb the radiation coming up from the ground, they radiate about half of it back towards the ground, while the other half is radiated upwards The effect is very perceptible on a cold winter night. Clear nights are the coldest, the radiation from the ground is freer to escape to space. With clouds the nights are warmer, because clouds increase the DLR. Figure 3 shows a typical 24 hour record, showing periods of increased DLR when clouds pass over the buoy sensors.
Figure 3. A typical day showing the effect of clouds on the downwelling longwave radiation (DLR).
Once again we see the sudden changes in the radiation when the clouds pass overhead. In the longwave case, however, the changes are in the other direction. Clouds cause an increase in the DLR.
So, here was my plan of attack. Consider the solar (shortwave) data, a typical day of which is shown in Figure 2. I averaged the data for every 2-minute interval over the 24 hours, to give me the average changes in solar radiation on a typical day, clouds and all. This is shown in gray in Figure 4.
Then, in addition to averaging the data for each time of day, I also took the highest value for that time of day. This maximum value gives me the strength of the solar radiation when the sky is as clear as it gets. Figure 4 shows those two curves, one for the maximum solar clear-sky conditions, and the second one the all-sky values.
Figure 4. The clear-sky (blue line) and all-sky (gray line) solar radiation for all days of the record (2214 days).
As expected, the clouds cut down the amount of solar radiation by a large amount. On a 24-hour basis, the reduction in solar radiation is about 210 watts per square metre.
However, that’s just the shortwave radiation. Figure 5 shows the comparable figures for the longwave radiation at the same scale, with the difference discussed above that the clear-sky numbers are the minimum rather than the maximum values.
Figure 5. The clear-sky (blue line) and all-sky (gray line) downwelling longwave radiation (DLR) for all days of the record.
As you can see, the longwave doesn’t vary much from clouds. Looking at Figure 3, there’s only about a 40 W/m2 difference between cloud and no cloud conditions, and we find the same in the averages, a difference of 36 W/m2 on a 24-hour basis between the clear-sky and all-sky conditions.
DISCUSSION
At this location, clouds strongly cool the surface via reflection of solar radiation (- 210 W/m2) and only weakly warm the surface through increased downwelling longwave radiation (+ 36 W/m2). The net effect of clouds on radiation at this location, therefore, is a strong cooling of – 174 W/m2.
This likely slightly overstates the radiation contribution of the clouds. This is because, although unraveling the effect on shortwave is simple, the effect on longwave is more complex. In addition to the clouds, the water vapor itself affects the downwelling longwave radiation. However, we can get an idea of the size of this effect by looking at the daily variation of longwave with and without clouds in more detail. Figure 6 shows the same data as in Figure 5, except the scale is different.
Figure 6. As in Figure 5 but with a different scale, the clear-sky (blue line) and all-sky (gray line) solar radiation for all days of the record.
Note that the minimum (clear-sky) DLR varies by about 10 W/m2 during the 24 hours of the day. Presumably, this variation is from changes in water vapor. (The data is there in the TAO dataset to confirm or falsify that presumption, another challenge for the endless list. So many musicians … so little time …). Curiously, the effect of the clouds is to reduce the underlying variations in the DLR.
This warming due to water vapor, of course, reduces the warming effect of the clouds by about half the swings, or 5 W/m2, to something on the order of 30 W/m2.
Finally, to the perplexing question of the so-called “cloud feedback”. Here’s the problem, a long-time issue of mine, the question of averages. Averages conceal as much as they reveal. For example, suppose we know that the average cloud cover for one 24 hour period was forty percent, and for the next 24 hours it was fifty percent. Since there were more clouds, would we expect less net radiation?
The difficulty is, the value and even the sign of the change in radiation is determined by the time of day when the clouds are present. At night, increasing clouds warm the planet, while during the day, increasing clouds have the opposite effect. Unfortunately, when we take a daily average of cloud cover, that information is lost. This means that averages, even daily averages, must be treated with great caution. For example, the average cloud cover could stay exactly the same, say 40%, but if the timing of the clouds shifts, the net radiation can vary greatly. How greatly? Figure 7 show the change in net radiation caused by clouds.
Figure 7. Net cloud forcing (all-sky minus clear-sky). Net night-time forcing is positive (average 36 W/m2), showing the warming effect.
In this location, the clouds are most common at the time they reduce the net radiation the most (mid-day to evening). At night, when they have a warming effect, the clouds die away. This temporal dependence is lost if we use a daily average.
So I’m not sure that some kind of 24-hour average feedback value is going to tell us a lot. I need to think about this question some more. I’ll likely look next at splitting the dataset in two, warm dawns versus cool dawns, as I did before. This should reveal something about the cloud feedback question … although I’m not sure what.
In any case, the net cloud radiative forcing in this area is strongly negative, and we know that increasing cloud coverage and earlier time of cloud onset are functions of temperature. So my expectation is that I’ll find that the average cloud feedback (whatever that means) to be strongly negative as well … but in the meantime, my day job is calling.
A final note. This is a calculation of the variation in incoming radiation. As such, we are looking at the throttle of the huge heat engine which is the climate. This throttle controls the incoming energy that enters the system. As shown in Figure 7, in the tropics it routinely varies the incoming energy by up to half a kilowatt … but it’s just the throttle. It cools the surface by cutting down incoming fuel.
The other parts of the system are the tropical thunderstorms, which further cool the surface in a host of other ways detailed elsewhere. So the analysis above, which is strictly about radiation, actually underestimates the cooling effect of tropical clouds on surface temperature.
All the best, please don’t bother questioning my motives, I sometimes bite back when bitten, or I’ll simply ignore your post. I’m just a fool like you, trying to figure this all out. I don’t have time to respond to every question and statement. Your odds of getting a reply go way up if you are supportive, on topic, provide citations, and stick to the science. And yes, I know I don’t always practice that, I’m learning too …
w.
PS — Here’s a final bonus chart and digression. Figure 8 shows the average of the actual, observed, measured variation in total downwelling radiation of both types, solar (also called shortwave) radiation and longwave (also called infrared or “greenhouse”) radiation.
Figure 8. Changes in average total forcing (solar plus longwave) over the 24 hours of the day.
Here’s the digression. I find it useful to divide forcings into three kinds, “first order”, “second order”, and “third order”. Variations in first order forcings have an effect greater than 10% of the average forcing of the system. For the system above, this would be something with an effect greater than about seventy W/m2. Figure 7 shows that the cooling from clouds is a first order forcing during the daytime.
Variations in second order forcings have an effect between 1% and 10% of the average. For Figure 8 that would be between say seven and seventy W/m2. They are smaller, but too big to be ignored in a serious analysis. With an average value of 36 W/m2, the warming from night-time clouds is an example of a second order forcing.
Finally, variations from third order forcings are less than 1%, or less than about seven W/m2 for this system. These can often be ignored. As an example of why a third order forcing can be ignored in an overall analysis, I have overlaid the Total Radiation (red line in Figure 8) with what total radiation would look like with an additional 7 W/m2 of radiation from some hypothetical CO2 increase (black line in Figure 8). This seven watts is about 1% of the 670 W/m2 average energy flowing through the system. The lines are one pixel wide, and you can scarcely see the difference.
Which is why I say that the natural governing mechanisms that have controlled the tropical temperatures for millions of years will have no problem adjusting for a change in CO2 forcing. Compared to the temperature-controlled cloud forcing, which averages more than one hundred and fifty W/m2, the CO2 change is trivial.
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Willis Eschenbach says:
September 15, 2011 at 5:12 pm
“Part of the problem is the issue of causality. Suppose the day warms, and at some point a thunderstorm forms. That thunderstorm wanders over the ocean, and it leaves the ocean cooler, often cooler than it was when the thunderstorm formed.
What would you say is the cause of that cooling? The thunderstorm? The high temperature that precipitated the thunderstorm?”
The cooling from the thunderstorm is a feedback response.
Thank you, Willis, for this excellent ‘Third Instalment’.
Using observational data, thinking about them, and finding this leads to further questions and debates: well, that is how science used to be done.
I am very glad to learn from you and from the debates following your essays.
For me, it’s like being back in the research lab, brainstorming at the end of a productive day.
Re: Averaging
Let us focus on figures 2 and 4.
In Figure 2 you show a “typical day”. You don’t say what a “typical” day is so I will assume that you just randomly choose a day which is neither more nor less typical than any other day.
However this figure clearly shows when there were clouds.
Between sunrise and about 11:00 the curve follows approximately the clear sky curve with very short jumps downwards.
The distance between the downward jumps decreases as one approaches 11:00 what means that the morning began cloudless and then more and more clouds passed over the detector.
At 11:00 the sky is almost wholly covered and the clear sky curve joins the cloudy sky curve and stays there untill 13:00.
Afterwards it jumps again up and joins the clear sky curve where it stays without interruption untill sunset.
So clearly during this “typical day” the clouds gathered in the morning, reached a full cover around 11:00 and then moved away at 13:00.
From the cooling “efficiency” point of view the clouds acted “maximally” – they removed SW at a time of the day when it is maximal.
The Figure 4 tells another story. The grey all sky curve which is an average of every 2 minute interval over 2214 days is perfectly smooth and symmetrical. At least it looks like that.
If there is any assymmetry, it could only be detected by computing the differences F(12+t) – F(12-t) for all t between 2 min and 12 hours where F is the SW all sky flux.
But if the curve is perfectly symmetrical then it means that the clouds distribution in day time is perfectly random.
In other words if Fclear(t) = k.Fallsky(t) with k some constant for all t as Figure 4 seems to say, then Fallsky(t)/Fclear (t) which represents the fraction of SW removed by the clouds is independent of time.
It is easy to check with the data – just compute Fallsky(t)/Fclear (t) for all 12×30 2 minute day time intervals. If this ratio is approximately constant then clouds are random and make the same “cooling” work on any time of the day. From that follows that averages can be used because there is no privileged time where clouds would preferentially work.
Of course if this ratio was not constant or if you amused yourself by computing the grey curves for shorter time intervals (f.ex 3 months averages) and observed that they are not identical, then the time would matter and averages couldn’t be used.
P.S
Like you deduced the clear sky curve by taking the maximum value for every 2 min interval, you could also deduce the fully cloud covered sky by taking the minimum value for every 2 min interval.
On an updated Figure 4, you would then see that the allsky grey curve would be between the clear sky and the fully clouded sky curves (as expected :)) and from there you could deduce the cloudiness for every interval if you wished to do so.
Willis
“After they absorb the radiation coming up from the ground, they radiate about half of it back towards the ground, while the other half is radiated upwards “
What ground are we talking about out there over the Pacific ocean?
Over the ocean, any so called DLR would be that from incoming Solar (at least 50% LW). This appears to be supported by figure 3 where DLR seems to pick up most around the hours of daylight.
Clouds can only block the shortwave portion of the incoming.
I thought that was a pretty good article Willis. Very illuminating.
Willis
An interesting post and nice to see an analysis performed on empirical observational data. For telling you what is going on in the real world, it trumps model projections anytime.
No surprise that cloud formation around midday reduces incoming solar radiation by a substancial extent whereas cloud cover at night increases DWLWIR by only a relatively small extent. Most people have experienced the effect of clouds passing over on a bright sunny summer day and know how this can cause temperature on your skin to drop by 20deg or so. Most people have witnessed the difference between clear night and cloudy night with the latter being a couple or so degrees warmer. .
I consider that it would be useful to see the data plots from data taken from the SAME buoy at (or around) the turn of each season so that we can see the differing strength of the input radiation.
On a related point, would an ocean receiving between 1000 to 1400 w per sqm for 12 hours a day freeze even without the benefit of DWLWIR? As you are aware 1000 to 1400 w per sqm is a lot more than the Trenberth aveage figure he uses for solar irradiance in his cartoon.
I look forward to hearing from you on that question.
That bonus chart (figure 8) is the pièce de résistance!
“George E. Smith says:
September 15, 2011 at 7:26 pm
“”””” Once again we see the sudden changes in the radiation when the clouds pass overhead. In the longwave case, however, the changes are in the other direction. Clouds cause an increase in the DLR. “””””
And by inference those same clouds would also be increasing the LWIR to space, since as you say, half of the cloud absorbed radiation is re-emitted to space, so the cloud which you say is an efficient black body absorber (and emitter) whereas the atmosphere is not.
”
Such things should not be a suprise. However, cloud radiation downward is at the bottom of the cloud and radiation upward is at the top and there are temperature differences. At the top, typically there is little to no h2o vapor above to absorb the new continuum but it’s still less than what radiates downward. Note also that it must be less than what escapes to space from the surface radiation. Otherwise Earth would have to be somewhat colder on average.
Great post, Mr Eschenbach.
In reply to
dcfl51
September 15, 2011 at 2:38 pm
“Willis, I am not a scientist so forgive me if this is a silly question. I thought that just under 50% of the radiation emitted by the sun was long wave. So why do the instruments not detect more DLR during the day than during the night ?”
I suppose in theory there IS more DLR during the day, by about 1/2%.
I did a rough calculation of daily cooling of
the atmosphere.
mass atmosphere = 5* 10^18 kg=5*10^21gm
temp atmosphere 255K (effective radiating temp to space- underestimates heat content)
specific heat 1.01 joules/gm C
5* 10^21*1.01*255= 1.288 * 10^24 joules
radius earth = 6400km= 6.4*10^6 meters.
area earth = 4 pi r^2 =514,718,540,364,021.76
240 watts/sq meter = 240 joules/sec per square meter
60 sec/min*60 min/hr*24hr/day=86,400 secs per day
5.147* 10^14 sq meters*240 joules/sec/sq meter *8.64*10^4 secs/day= 1.067*10^22 joules per day radiated away
1.067*10^22/1.288*10^24 = 0.83%
So the atmosphere as a whole cools by less than 1% over the course of a day. That figure makes sense when you figure that the earth’s surface temperature my change by 10 C or more overnight far more than average changes over a week, but weather patterns persist for several days, and that’s why meteorologists can predict daily highs out a week or so. That cooling is obviously mostly from the
earth’s surface and air near the surface ,leaving most of the atmosphere unchanged.
Willis writes “The issue is not where the radiation is intercepted on its way to space. It is how many times, on average, it is intercepted. This is because every time the radiation is absorbed, half of it goes back towards the surface.”
My understanding is that the average time between collisions of molecules (near sea level) is much less than the average time to re-radiate. Consequently at sea level the energy isn’t re-radiated “up or down” but rather its conducted to the N2 and O2.
Of course collisions can go the other way and give the CO2 molecule sufficient energy to radiate and this does happen sometimes…its just that when thinking about CO2’s role at sea level its more of a transference of energy to the atmosphere effect.
Further up in the atmosphere where there are fewer molecules and lower rates of collision, the radiation “up and down” becomes more prominent.
Mark says:
September 15, 2011 at 6:53 pm
“If nearly all of the LW-IR is absorbed within 100 m of the surface, how does it matter what happens at high altitude or the top of the atmosphere?”
You have to work this out wavelength by wavelength in the IR. For wavelengths for which CO2 is highly absorbing (e.g. the main C02 absorption line), the TOA radiation comes from the stratosphere, yielding a negative greenhouse effect change with increasing CO2. For most of the central part of the spectrum, the TOA radiation comes from the tropopause, i.e. zero greenhouse effect change. For the tails of the band and some wavelengths between lines, the TOA comes from the troposphere and produces the greenhouse effect change through a rise of the effective radiation altitude with increasing CO2 concentration. These wavelengths dominate the concentration dependence of TOA radiation, not the TOA radiation itself. The overall concentration dependence corresponds to a positive greenhouse effect change. Downward radiation variations, on the other hand can come from a variety of wavelengths regions outside the main CO2 band where the absorption is not saturated, and be due to multiple causes.
Willis – there is no way you have added value to the conversation. You haven’t used a computer model to verify and amplify your result 🙂
R. Gates says:
September 15, 2011 at 8:58 pm
Thanks, R. When there is emission from the atmosphere, it is emitted spherically. Half of it is going up, and half going down.
As you note, the horizon is not horizontal, and this increases as you go upwards. But the majority of the absorption and emission is taking place at lower elevations. In addition, even from a height of say 15 kilometres, the depression of the visual horizon is only about four degrees.
So if the horizon depression is zero at the ground, and it is 4° at 15 kilometres, the average is about 2°.
So we could say that rather than 50% going up and 50% going down, that 92/180 goes toward space and 88/180 goes towards the earth … which is 51% going up and 49% going down.
In other words, R., you’ve busted me about a 1% error, in a system where it is very hard to measure just about any variable to better than 5% …
Yes, I knew going in it wasn’t exactly 50/50 … but that is the usual assumption made, to simplify calculation, since the error is so small.
w.
PS – You are correct that 50% (or 51%) going upwards doesn’t mean that 51% goes directly to space.
PPS – This hightlights an important issue, that of size. There are many things that look like they should be variables in the climate mix, until one looks at them closely and realizes that they are too small to make a difference. Run your numbers first, folks, I’m often ignoring these third order effects because I’ve run the numbers and I know they are small.
Bart says:
September 16, 2011 at 1:41 am
Thanks, Bart, but that is a naming without explanation. If I’m mean to somebody and they punch me in the nose, you could describe that as a “feedback response” as well … but like with your claim, it leaves a whole lot out of the explanation.
In any case, since the thunderstorm can leave the local ocean much cooler than when it started, are you saying that that part of the ocean is cooling because it is warming? This is a recurring problem with self-organized criticality …
w.
Willis Eschenbach says:
September 16, 2011 at 9:05 am
R. Gates says:
September 15, 2011 at 8:58 pm
Interesting as usual Willis. This comment however, may be a bit of an oversimplification:
“After they absorb the radiation coming up from the ground, they radiate about half of it back towards the ground, while the other half is radiated upwards.”
How did you compute this percentage? And even if correct (50% seems way too high), it doesn’t of course mean that 50% is lost to space.
Thanks, R. When there is emission from the atmosphere, it is emitted spherically. Half of it is going up, and half going down.
______
If you agree that it is essentially a spherical transimission pattern for LW being re-emitted from a greenhouse gas molecule, then I think we may differ in our definition of what “up” means. If by “up” you mean 180 degrees from the ground, then of course far less than 50% goes “up”, and this is important as the majority of the other angles, would either be back to the ground or in some other angle tangential to the ground (but not 180 degrees from the ground). These tangential angles and ground directed angles give much more opportunity for the the continually absorption and re-emission by either the ground or other greenhouse gas molecules. Thus, of course, the greater amount of greenhouse gas molecules present, the greater the opportunity for the absorption and re-emission of LW.
In short, maybe only something less than 5% actually goes “up” and this could make a difference in the long-run in how you calculate the effects of both water vapor and other greenhouse gases.
Mark:
“If the difference is that increased CO2 raises the altitude where the radiation finally “escapes to space”, and therefore the temperature at high altitude increases, how does that heat get back to the surface? . . . If the difference is that increased CO2 raises the altitude where the radiation finally “escapes to space”, and therefore the temperature at high altitude increases, how does that heat get back to the surface?”
I don’t profess to be knowledgeable in the field, but one comment of Richard Lindzen’s here: http://www-eaps.mit.edu/faculty/lindzen/230_TakingGr.pdf, p.940, regarding what he feels is misnamed the “greenhouse” effect may supplement what those who answered above said. My no doubt inaccurate paraphrase is that CO2 enrichment raises the altitude at which, seen from space, a given optical depth is reached. Given the lapse rate, that means that the earth’s effective radiation temperature for a given surface temperature decreases: there is less radiation into space.
Perhaps you will find that helpful, at least when taken together with the comments above regarding differing opacities at different wavelengths.
@ur momisugly Willis Eschenbach says:September 16, 2011 at 9:05 am
in the course of his comment in response to R. Gates says:September 15, 2011 at 8:58 pm
“…PS – You are correct that 50% (or 51%) going upwards doesn’t mean that 51% goes directly to space….”
/////////////////////////////////////////////////////////////
Much may depend upon the meaning attributed to the word “directly”, but, Willis, to the extnt that your observation is correct, it equally follows that of the 50% (or 49%) DWLWIR going downwards this does not go directly to the ground.
Do you not consider that your assessment of 51% up/49% down might be a little bit of an under-assessment of the odds of the LWR radiating upwards? I say this, since, if you consider the path of a downward radiated photon from high in the atmosphere, each time it is involved in a collision on its downwrd path it either gives up its energy in heat (which may not necessarily result in another photon being radiated), or if colliding with a GHG molecule the resultant photon then radiated has a 51% prospect of radiating upwards and only a 49% prospect of being radiated downwards. Consider the effect of this in a scenario where there a re multiple collisions on the downward path. Do you not consider that the odds are more adversely stacked against the DWLWIR photon reaching the ground (or ocean) than you are actually suggesting.
Some consider that the role that CO2 may play, in the overall workings of the atmosphere when convection, conduction, energy transfer consequent upon molecular collsions etc are taken into account, is predominantly in delaying energy finding its way out to space.
Further to my last post, I accept that nearer to the ground, the curvature/horizon point becomes less of an issue such that the odds of a re-radiated photon going downwards tends towards becoming more equally balanced.
Mr. Eschenbach …
Thanks for the informative article and the discussion it generated.
As a non-scientist, I have an observation and query regarding your comments at 7:32pm on 9/15/11:
“It is how many times, on average, it is intercepted. This is because every time the radiation is absorbed, half of it goes back towards the surface. This is true even if 100% of the radiation is intercepted just off the deck.”
Observation: As a person with many hours flying aircraft, I know that oftentimes weather system clouds are layered, perhaps 3 or 4 layers (with clear air in between) in 20 – 25 thousand feet of altitude.
Query: What, if any, effect do the several layers have on the absorptiion and return properties of cloud layers as opposed to your apparently simple illustration?
Keep up the great work … the truth is out there, but it takes open minds like yours and some honest initiative to find it.
Pete
Willis Eschenbach says:
September 16, 2011 at 9:21 am
“…but like with your claim, it leaves a whole lot out of the explanation.”
In a feedback loop, such as depicted here, there is perfect symmetry if you ignore the input. Resolution of the ambiguity is found by where the outside forcing comes in. I explained this to Nick Stokes here.
“In any case, since the thunderstorm can leave the local ocean much cooler than when it started, are you saying that that part of the ocean is cooling because it is warming?”
You have to look at the system in its entirety to see where all the energy ends up. But, yes, it is entirely possible to have a sensitivity function which attenuates an input in one particular variable in the system.
Mr Gates
I would appreciate you answering a question arising out of your comment:
R. Gates says:
September 16, 2011 at 10:15 am
“…If you agree that it is essentially a spherical transimission pattern for LW being re-emitted from a greenhouse gas molecule, then I think we may differ in our definition of what “up” means. If by “up” you mean 180 degrees from the ground, then of course far less than 50% goes “up”, and this is important as the majority of the other angles, would either be back to the ground or in some other angle tangential to the ground (but not 180 degrees from the ground)…”
///////////////////////////////////////////////////////
Do you not accept that this comment applies equally to the ‘down’ scenario? If not, why not?
It appears to me that on your reasoning,something less than 5% would go down. Should you disagree please explain why you disagree and what percentage goes ‘down’ and why that is so.
As I see matters, the logical conclusion of your comment is that when one also takes into account the effect of the horizon/curvature point (which slightly favours the upward direction), the less than 5% going ‘down’ would be less than the less than 5% going ‘up’. Should you disagrre, I would appreciation your full explanation/reasoning. .
After they absorb the radiation coming up from the ground, they radiate about half of it back towards the ground, while the other half is radiated upwards The effect is very perceptible on a cold winter night. Clear nights are the coldest, the radiation from the ground is freer to escape to space.
——–
Half and half? Thats simply wrong. The amount radiated down through the window depends only and only on cloud’s own temperature. If we to disregard changes of emissivity with temperature, got this: for a stratus cloud 1 km aloft, with 6.5 *C/km lapse rate and +20C ground surface temperature, the percentage of down welling radiation from the cloud would be ((273+20-6.5*1)/(273+20))**4 = 91%, for cloud at 2 km it would be 83%; for altostratus at 5 km it would be 63%, and for high cirrus at 12 rm it would be a mere 29%. See, high clouds cool earth more than low clouds.
To me, all this shit about ‘clouds warm earth at night’ is preposterous. They don’t. The only thing low clouds do is prevent to some degree formation nighttime inversions – but that can only cool planet, as warmer surface of cloud emits more efficiently than cold ground. High clouds however don’t really warm the earth underneath them. They do warm the planet as a whole, though.
To add, Even more preposterous is the popular fable of ‘water vapor at night keeps earth warmer’. What a bullshit. Locally, more wv would decrease depth of inversion, and therefore make night colder but make earth warm up quicker in the morning, thus warming the earth slightly – but not locally. The common perception of warm nights being moist is from confusion of cause and effect. The cause is advection of warm, moist air. The effect is warmer nights – because the air is already warm.
Pete says:
September 16, 2011 at 11:17 am
Multiple cloud layers mean that photons will perforce be absorbed and emitted by each cloud on the way up, increasing the number of “shells” in the greenhouse.
w.
Re R Gates’ issue:
Try the following as a thought experiment:
Draw a circle and draw a line tangent to the circle. This models the Earth and an IR and CO2 molecule interaction. An IR photon meeting a CO2 molecule near the surface of the Earth will be absorbed and then, assuming spherical pattern of re-radiaton, has a 50% chance of being radiated outside of that tangential line—ie away from the Earth.
Then draw a second line, parallel to the first, but further from the circle. This represents the next interaction of that original photon’s energy with a second CO2 molecule, which 50% of the time will be further out from the Earth. Again, 50% of the time the photon will be re-radiated outside the line, ie away from the Earth and 50% of the time toward the Earth.
Those photons (initially 50%) that are re-radiated back toward the Earth have as their next encounter either the Earth’s surface or another CO2 molecule nearer to the Earth. The 50% rule continues to apply for all CO2 encounters near the surface. The photon’s that end back up at the surface are also re-radiated ultimately, though they may spend some time as kinetic energy (ie heat) at the surface.
For the photon’s that do get reradiated away from the Earth, two changes in their chances of escaping the Earth occur:
1. With altitude, the CO2 concentration rapidly diminshes, and the chances of getting to space without another encounter increases.
2. The further the Photon/CO2 encounter occurs from the surface, the lower the chance of hitting the Earth even if reradiated below the line parallel to the tangent line.
Thus as the photon “bounces around” from CO2 encounter to CO2 encounter, there is an inevitable bias for the photon to escape to space. Yet compared to a “no greenhouse gas” atmosphere, that escape takes longer when greenhouse gases are present and therefore the energy content of that atmosphere—-for the same temperature at the surface—-will be higher—-ie the temperature will be higher—-ie warmer.
Any “saturation” effect from risng CO2 concentration (ie black cannot get any blacker than black) does not address the issue that rising atmospheric CO2 results in a taller column of CO2 above the gound, and therefore CO2 heating effects continue to increase, though not linearly, because to photons trying to escape have further to do before getting free of CO2 interactions.
Thus the CO2 greenhouse effect is more like running through a minefield than just shining through a single window of variable opacity. The longer the minefield, the harder to get through free of getting blown up.
So the CO2 Greenhouse effect is real. Fortunately it seems to be countered by negative feedback related to clouds, storms, etc. and certainly is not an apocalyptic concern.