As most readers know, clouds are still poorly understood and under-represented in climate models. This new research may help.
From: DOE/Pacific Northwest National Laboratory
Effect of cloud-scattered sunlight on earth’s energy balance depends on wavelength of light
Accounting for wavelength effects will likely improve climate models
RICHLAND, Wash. — Atmospheric scientists trying to pin down how clouds curb the amount of sunlight available to warm the earth have found that it depends on the wavelength of sunlight being measured. This unexpected result will help researchers improve how they portray clouds in climate models.
Additionally, the researchers found that sunlight scattered by clouds — the reason why beachgoers can get sunburned on overcast days — is an important component of cloud contributions to the earth’s energy balance. Capturing such contributions will increase the accuracy of climate models, the team from the Department of Energy’s Pacific Northwest National Laboratory reported in Geophysical Research Letters earlier this month.
“The amount of the sun’s energy that reaches the earth’s surface is the main driver of the earth’s temperature. Clouds are one of the least understood aspects of climate change. They can block the sun, but light can also bounce off one cloud into another cloud’s shadow and increase the solar energy hitting earth,” said PNNL atmospheric scientist Evgueni Kassianov.
White clouds
Clouds both cool down and warm up the earth’s surface. They cool the earth by reflecting some sunlight up into outer space, and they warm it by bouncing some sunlight down to the surface. Overall, most clouds have a net cooling effect, but atmospheric scientists need to accurately measure when they cool and warm to produce better climate models that incorporate clouds faithfully.
But it’s a hard number to get. Fair-weather clouds are big puffy white objects that bounce a lot of light around. They can make the sky around them look brighter when they’re there, but they float about and reform constantly. Cloud droplets and aerosol particles in the sky — tiny bits of dirt and water in the air that cause haziness — scatter light in three dimensions, even into cloud shadows.
To determine the net cloud effect, researchers need two numbers. First they need to measure the total amount of sunlight in a cloudy sky. Then they need to determine how bright that sky would be without the clouds, imagining that same sky to be blue and cloudless, when aerosols are in charge of a sky’s brightness. The difference between those numbers is the net cloud effect.
Rainbow energy
Researchers have traditionally estimated the net cloud effect by measuring a broad spectrum of sunlight that makes it to the earth’s surface, from ultraviolet to infrared. But clouds are white — that’s because the large water droplets within them scatter light of all colors almost equally in the visible spectrum, the part of the electromagnetic spectrum that includes the colors of the rainbow.
On the other hand, aerosols — both within clouds and in the open sky — bounce different-colored light unequally. Broadband measurements that fail to distinguish color differences might be covering up important details, the researchers thought.
Instead of taking one broadband measurement that covers everything from ultraviolet to infrared, Kassianov and crew wanted to determine how individual wavelengths contribute to the net cloud effect. To do so, the team used an instrument that can measure brightness at four different wavelengths of color — violet, green, orange, red — and two of infrared.
In addition, this instrument, a spectral radiometer at DOE’s Atmospheric Radiation Measurement Climate Research Facility located on the southern Great Plains in Oklahoma, allowed the team to calculate what the brightness would be if the day sported a cloudless, blue sky. The spectral measurements taken by the radiometer can be converted into the amount and properties of aerosols. Then aerosol properties can be used to calculate clear blue sky brightness.
Clouds Gone Wild
Comparing measured values for cloudy sky to the calculated values for clear sky, the researchers found that, on average, puffy fair-weather clouds cool down the earth’s surface by several percent on a summer day. Although clouds cool overall, two components that the researchers looked at — from direct and scattered sunlight — had opposite effects.
The direct component accounts for the shade provided by clouds and cools the earth. The second component accounts for the sunlight scattered between and under clouds, which makes the sky brighter, warming the earth.
“The sunlight scattered by clouds can heat the surface,” said Kassianov. “We all know that we can still get sunburned on cloudy days. This explains why.”
In the Oklahoma summer, the scattered-light effect measured by the researchers could be quite large. For example, if a cloud passed over the instrument, the measured cloudy sky brightness exceeded calculated clear sky value by up to 30 percent. Kassianov attributes that large difference to scattered sunlight being “caught on tape” by the radiometer.
“Sunlight scattered by three-dimensional, irregular clouds is responsible for the observed large difference. The one-dimensional cloud simulations currently used in large-scale climate models don’t capture this diffuse light,” said Kassianov.
Aerosols’ Day in the Sky
The team also found that the effect changed depending on the measured visible-spectrum wavelength, and whether the light was direct or scattered.
With direct light, the cooling caused by clouds was weakest on the violet end of the spectrum and strongest at infrared. With scattered light, warming caused by clouds was also weakest at violet and the strongest at infrared. Overall, the least cooling and warming occurred at violet, and the most cooling and warming occurred at infrared.
Because large droplets in clouds scatter sunlight almost uniformly across the spectrum, the clouds themselves can’t be the reason why different wavelengths contribute differently to the net cloud effect. Compared to cloud droplets, aerosols are more than 100 times smaller and scatter wavelengths differently. These results suggest that aerosols — which not only cause haziness but contribute to cloud formation as well — are responsible for the wavelength differences, something researchers need to be aware of as they study clouds in the sky.
“If you want to study how aerosols and clouds interact,” said Kassianov, “you need to look in the region of the spectrum where aerosol effects are significant. If you want to fish, you go where the fish are biting.”
This work was supported by the U.S. Department of Energy Office of Science.
Pacific Northwest National Laboratory is a Department of Energy Office of Science national laboratory where interdisciplinary teams advance science and technology and deliver solutions to America’s most intractable problems in energy, national security and the environment. PNNL employs 4,900 staff, has an annual budget of nearly $1.1 billion, and has been managed by Ohio-based Battelle since the lab’s inception in 1965. Follow PNNL on Facebook, LinkedIn and Twitter.
Reference: Kassianov E., Barnard J., Berg L.K., Long C.N., and C. Flynn, Shortwave Spectral Radiative Forcing of Cumulus Clouds from Surface Observations, Geophys Res Lett, April 2, 2011, DOI 10.1029/2010GL046282 (http://www.agu.org/pubs/crossref/2011/2010GL046282.shtml).
Abstract:
The spectral changes of the shortwave total, direct and diffuse cloud radiative forcing (CRF) at surface are examined for the first time using spectrally resolved all-sky flux observations and clear-sky fluxes. The latter are computed applying a physically based approach, which accounts for the spectral changes of aerosol optical properties and surface albedo. Application of this approach to 13 summertime days with single-layer continental cumuli demonstrates: (i) the substantial contribution of the diffuse component to the total CRF, (ii) the well-defined spectral variations of total CRF in the visible spectral region, and (iii) the strong statistical relationship between spectral (500 nm) and shortwave broadband values of total CRF. Our results suggest that the framework based on the visible narrowband fluxes can provide important radiative quantities for rigorous evaluation of radiative transfer parameterizations and also can be applied for estimation of the shortwave broadband CRF.
Huh? How can you “increase the accuracy” of something which has no accuracy to begin with? If they increase it 1000%, it will still have zero predictive value.
I am so TIRED of pseudoscience. I only want real science.
At least the results of the study on wavelengths is actual data and actual science.
So, in other words, it’s back to the basics. No one really understands even one of the simplest factors in our energy balance.
Kind of makes Trenberth’s simplistic picture look pretty stupid.
This seems to be a step in the right direction, but one wonders why it took them so long to get around to taking it. Instrumentation to do broad spectrum sweeps of the radiation involved has been around for quite awhile. like maybe 15 years or more.
I’m concerned that after all the new studies on the properties of clouds are done, it will be determined that they are too complex to model. So the modelers will just use the value “42” for clouds as a simplification and off we go down the rabbit hole… again.
Interesting,
now we need to study the effect on the completely different cloud formations produced at other latitudes.
Whoa, who knew?
But at least NOW the science is settled, right?
Right?
It’s a shame they didn’t test at UV frequencies too.
Atmospheric scientists trying to pin down how clouds curb the amount of sunlight available to warm the earth have found that it depends on the wavelength of sunlight being measured. This unexpected result will help researchers improve how they portray clouds in climate models.
How can this be “unexpected”? This is well understood physics surely?
It is encouraging that at least some real science with data is being published in peer review journals. However I wonder how useful 6 points across a spectrum might be in describing the reality and how can “a number” (presumeably derived from an average of some sort) be employed in a mathematical model to characterize a chaotic system such as cloud effects on climate?
They are still using models /sarc… At least they might be slightly more realistic ones – but who can tell :-)?
The fact that CO2 and H2O absorb different wavelengths of light has been known for a while now. Did they just now figure out that clouds are actually water vapor?
Imagine if they measured sunlight at weather stations … and they found out there was 10 % more bright sunshine than in 1900. And that the up and down pattern matched the global temperature record.
http://www.jstage.jst.go.jp/article/jmsj/86/1/57/_pdf
“Overall, most clouds have a net cooling effect, but atmospheric scientists need to accurately measure when they cool and warm to produce better climate models that incorporate clouds faithfully.”
Translated: we have to find a way in which clouds can be assumed to still have a positive feedback in GCM’s so the IPCC can maintain a projection of significant warming
BTW – I have NEVER been sunburnt on a cloudy day, I assume that must only happen to people with very pale skin?!
Any photographer worthy of the name, whether amateur or professional, could tell you that the spectal balance of natural light varies with conditions. So, why is any of this such a total surprise to the scientists who are finally starting to notice this sort of thing? Perhaps if they were to stop discounting anything told to them by someone without the magic three letters following their name things would improve. Lots of time in classes but no trace of any common sense. God help us all if this is the best we can get.
These people should get outdoors more often, play golf or go sailing, and all their questions will be answered. My unscientific observation is that any cloud, type or amount, will reduce the temp during the day. Also, I have yet to see any cloud increase the output of my solar panels but if I could get a grant I would be willing to do some research to prove the obvious.
stumpy says: ” ‘Overall, most clouds have a net cooling effect, but atmospheric scientists need to accurately measure when they cool and warm to produce better climate models that incorporate clouds faithfully.’ Translated: we have to find a way in which clouds can be assumed to still have a positive feedback in GCM’s so the IPCC can maintain a projection of significant warming.
Yeah, it looks like that to me, too.
BTW – I have NEVER been sunburnt on a cloudy day, I assume that must only happen to people with very pale skin?!
Actually, as I recall it, you can get sunburnt on an overcast day. I’ve never heard of anybody being burnt on a cloudy day. The mechanism would be completely different.
From the post: “Clouds both cool down and warm up the earth’s surface. They cool the earth by reflecting some sunlight up into outer space, and they warm it by bouncing some sunlight down to the surface. “
Hogwash. The net effect of clouds is clearly cooling. If the clouds weren’t there, 100% of the solar radiation would hit the surface. Essentially all the sunlight hitting the top of the clouds goes back into space. Only a small portion of the edge of the clouds might reflect a little sunlight to ground. /sarc on: Clouds are about as smooth and shiny as mashed potatoes. That’s why so many heliostats are made of mashed potatoes. /sarc off
Oh come on ! UVB (wavelength 315-280 nm) can cause sunburn on exposed skin, but this is entirely different and significantly less than IR heating the ground.
How could an argument like this pass the censors ?
So the sun heats the Earth and water vapour regulates our temperature, silly me I thought CO2 heated the Earth. What ever will science think of next, what ever it is they will try and confuse me.
I anticipate that it will be found that there is little difference in any of the parameters averaged globally over time save for total cloud quantities. The purpose of the research will be to try and establish the net energy budget effect for any given quantity of global cloud as a starting point for assessing the likely scale of the energy budget effect from a change in such quantity.
The question then is as to what most affects total cloud quantities because that directly affects global albedo and the amount of solar energy available at the surface to be absorbed by the oceans.
There are currently three suggested candidates:
i) Lindzen’s ‘iris’ theory
ii) Svensmark’s cosmic ray theory
iii) My suggestion that what matters most is the length of the air mass boundaries and/or the latitudinal positioning of the main cloud bands which separate the main air circulation systems. Generally more meridional jets and/or more equatorward jets will result in higher global cloud quantities and higher global albedo.
Stumpy, you have obviously never spent any time on a Southern California beach on a foggy day. From painful experience, I can assure you that it happens.
Getting sunburnt on overcast days can happen as your body does not feel any ‘heat’ from UV, and, as a result you spend more time exposed and this can result in sunburn. This becomes a problem as the latitude decreases and two hours before and after noon, in Australia UV indexes are issued.
BOM has a good summary
Are they completely mad? Clouds can never have a net warming effect on a sunny day, because of albedo. The only real question is this: When the earth is viewed from space on the sunny side of the planet, is the net reflected light greater or smaller than it would be if illuminated without the clouds?
White clouds are almost invariably brighter than the earth or water underneath. More light energy reflected up means less delivered down to the surface, out of a total conserved energy budget in the incoming light. You can’t “win” by bouncing some light “under the clouds” as long as a lot more light is being reflected right back up out of the atmosphere before it ever reaches the earth.
I do agree that studying the effect of clouds on the Earth’s energy budget is strongly advised, because as greenhouse gases go they are the most important one by orders of magnitude and are a highly variable and chaotic one with multiple factors driving their formation and effect. Clouds at night often trap heat; clouds in the daytime are almost invariably cooling. Clouds in the polar zones are often net warming; clouds in the temperate through tropical zones are net cooling. Clouds release heat into the air as they rain, and the rainwater cools the ground it falls on as it evaporates. All of this occurs in a complex pattern of global circulation, and under a variable sun that causes major fluctuations in the flux of e.g. solar wind and galactic cosmic rays (which may or may not directly influence the rate of cloud formation, adding yet another nonlinear and highly complex feedback loop affecting heating or cooling).
So I applaud the desire to understand clouds and climate better, but if anyone is actually trying to argue that straight up white fluffy little clouds like the ones in the photograph above could ever produce net warming, they are batshit crazy. All you have to do is count the “energy” as seen above in the pixels of the picture. The dark ones are the ground — that’s where light is mostly being absorbed, which is why the ground is dark. The white fluffy clouds are where the RGB values for the pixels are all peaked, because most of the visible light is being reflected by the clouds! This energy does not, actually, loop around or somehow get “focused” by multiple reflections to deliver more energy to the ground than would get there on a perfectly clear, sunny day.
rgb
quote
[A] UK project – funded by NCAS, the Natural Environment Research Council (Nerc) and the UK Met Office – is one part of an international three-year project called VOCALS, which is exploring how complex interactions between clouds, oceans and land affect the world’s climate.
unquote
the results from that should be interesting.
Re the above article, they are perhaps also addressing something only mildly related to GW, but give some support to one of those ‘no it doesn’t!’ responses to things we are told.
You know sunrays round clouds? People say, ‘oh, it’s just the light from the sun shining through’. I don’t think it is, I think it’s reflected light from the cloud tops shining through gaps and round edges. I’ve even dusted off my A level trig and found that the light origin seems to be at about the right height.
If I’m right I’d like the effect to be called ‘the Julian Flood sunray effect’. If I’m wrong, of course, I’d like everyone to forget I said anything….
JF
It’s clouds’ illusions I recall,
I really don’t know clouds at all…
– Joni Mitchell
Best,
Frank
I allways assumed (…yes I know ass u me…) that the GCM’s took the aerosol size distribution into account when they considered cloud scattering. It is basic undergrad level spectroscopy that scattering is a function of wavelength and aerodynamic diameter, that also has big implications for directional scattering. I am staggered that this is not the case. However at least they will now be looking at it using the proper fundamentals.
It is great to see that clouds are finally being considered in the effect on our climate, but as many astute readers have noticed some glaring problems with this article.
1. Clouds warming??? You must be joking. They really need to get out more. The only way clouds will “appear” to warm is by preventing re-radiated earth heat from escaping.
2. Getting sunburnt on cloudy days is from UV penetrating the clouds. I have even been sunburnt on a day with drizzly rain. I’d say that was impirical evidence, woul’d you?
3. And finally I’d just like to commend the other RGB on his “batshit crazy” comment. Just too funny!!!!
Accounting for wavelength effects will likely improve climate models.
Oh good Lord! They have to qualify that statement with “likely?” Are they saying that the modelers may or may not shape up? Or, more likely, the models are completely hopeless and no amount of added accuracy will make the slightest difference.
“Fair-weather clouds are big puffy white objects …” – are these serious scientists? Or do they think we’re too stupid to understand what they are purporting to study, and need to be told in baby language?
Words fail me …
I’ve done alot of work on Albedo (which is how much sunlight is reflected back to space by clouds, ground, water and Ice).
Something which is not that commonly understood is that clouds only reflect about 25% on average of the sunlight that hits them back to space.
A large high thunderstorm which is deep blue underneath will reflect as much as 75% of the sunlight (letting UV through at least), but the thin high cirrus clouds let most of the light through or reflect much of it back to the Earth’s surface at some angle.
On average the Earth is about 65% cloud covered (although this number varies widely depending on the method used to estimate it) but the clouds only contribute about 16% to 17% to the Earth’s Albedo of 29.83%.
Most surfaces, including cloud-covered ones are in the same relative range (8% to 35%) reflection – enough similarity that it doesn’t make much difference.
The surface that is the most important, the big make or break surface, is “Ice and Snow” which can be as high as 85% (cloud covered or not). The Ice and Snow extent on the planet is the big driver of Albedo (and in essence, the climate, in my opinion). It is the surface which varies the most from the Earth average.
Why did it get so cold in the Ice Ages, when the actual sunlight hitting the surface of the Earth was about the same as today and even a little higher. It is because all the extra Ice and Snow, starting out initially only in the high latitudes, reflects just that little extra sunlight.
In a less scientific and more subjective manner, I too wonder over how light bounces about. You can get a worse sunburn when working in the sun when there are white clouds about, than you can on a cloudless day. I suppose the sunlight reflects off the sides of the clouds the same way it bounces and burn you when it reflects off shimmering water.
I once stood behind a landscape painter, and he pointed out that the underside of a puffy cloud over distant pines had the slightest tint of green, while the underside of a similar cloud over distant wheat had the slightest tint of yellow.
Light is bouncing all over the place. If you could mark down all the vectors they would resemble swirling straws, especially as clouds are always moving and changing their shapes.
I’m glad I’m not mathematically inclined, and have no need to mark down all the vectors. It leaves me free to simply stand in awe.
Bill Illis said
“The surface that is the most important, the big make or break surface, is “Ice and Snow” which can be as high as 85% (cloud covered or not). The Ice and Snow extent on the planet is the big driver of Albedo (and in essence, the climate, in my opinion). It is the surface which varies the most from the Earth average.”
I’m not so sure because one has to change the temperature trend BEFORE the changes in snow and ice cover can occur. Certainly once present the snow and ice has a strong positive feedback effect on any increase in albedo from another cause for the reason you say but that something else has to kick start the process.
So I’ll continue with the view that total global cloud quantities are the initial driving agent for albedo and everything else follows.
I did not see anything about day and night. Its pretty simple, clouds cool during the day and keep the heat in at night.
When you are a climate researcher looking only at temperatures averaged over a day, and then averaged over a month I guess you forget little things like night and day.
Yep, more junk science. It is amazing that, even in made-up pseudo-science, climate scientists remain firmly “a priori” in their reasoning. The climate scientist says “Assume a spherical cloud made of the finest white stuff, assume the standard numbers for radiation from the sun, and calculate what happens as the radiation passes through the cloud.” Apparently, when they try empirical research their minds remained trapped by their Tinker-Toy imaginations. And the stuff about clouds increasing the radiation that reaches Earth’s surface is nonsense. I guess they are talking about the clouds that are out to the left or right of the Earth, as seen from the sun.
The actual empirical research has to answer two big questions: One is “Where are the clouds?” and Two is “Under what conditions are the clouds there?” In other words, what are the natural regularities that constitute cloud phenomena on Earth? A good starting place would be Central Florida because we have this fabulous cloud show every summer. Every afternoon, you can see the Thunder Heads rolling in. If you are in Orlando, they come from the East. If you are in Tampa, they come from the West. Sometime between three and five PM the showers begin. The temperature drops by as much as ten degrees and the evening is very pleasant. Will the Warmista someday account for cloud behavior over Central Florida? It is highly unlikely, as it would constitute more empirical research than the sum total of all empirical research they have done so far.
And I am so sick of the excuse that Earth’s climate is so complicated. Give me a break. Is it more complicated than energy transfers in the nucleus of an Uranium atom? I don’t think so. Yet those matters were mastered by a bunch of guys who were still reeling from the suffering of the Great Depression, all of whom chain-smoked and enjoyed good whiskey. The reason we do not understand the natural regularities that drive cloud phenomena on the surface of the Earth is that we have not done the research and the little research that has been submitted has been done by bozos. As proof, I bet not one of them recognizes the universally know observational term “Thunder Head.” My mother had taught it to me by the time I was three.
Anthony is far too kind in his title to this post. Instead of “still tweaking,” I suggest “still Tinker-Toying.”
Nonegatives says:
April 23, 2011 at 7:53 pm
The fact that CO2 and H2O absorb different wavelengths of light has been known for a while now. Did they just now figure out that clouds are actually water vapor?
Clouds are droplets. vapor is individual molecules. where ever you’ve got droplets, you’ve got additional degrees of freedom capable of absorbing and emitting continuum like a solid or liquid black body radiator versus a discrete spectrum like an atom or molecule.
They’ve “discovered” that different wavelengths interact in different ways with different kinds of clouds?
What will the “discover” next? The freezing point of water?
Bill Illis says:
April 24, 2011 at 6:02 am
“Something which is not that commonly understood is that clouds only reflect about 25% on average of the sunlight that hits them back to space.
…
On average the Earth is about 65% cloud covered (although this number varies widely depending on the method used to estimate it) but the clouds only contribute about 16% to 17% to the Earth’s Albedo of 29.83%.
Most surfaces, including cloud-covered ones are in the same relative range (8% to 35%) reflection – enough similarity that it doesn’t make much difference.
”
Bill,
I think you’re way off base. Not even K&T97, trenberth’s tinkertoy effort are that off the mark. liquid h2o is going to run an albedo of under 0.04 and that’s over 70% of the surface. Most of your ice & snow are at high latitudes that have much less incoming insolation. The numbers are more like 24% of the incoming tsi is reflected by the clouds, not 24% of the incoming tsi impinging on the clouds or of the total albedo. We’re talking around 0.24*341 = 82 w/m^2 of cloud reflection. To that, add around 24w/m^2 of atmospheric scattering and a ground contribution of 14 w/m^2 for a total of around 120 w/m^2 of albedo reflection or an albedo value of 0.35 or 35%. These numbers are values from 1957, J. London and are discussed in Introduction to Theoretical Meteorology by S.L. Hess, 1959.
Your number of 29.83% albedo is also interesting and wrong. Albedo is a variable, almost totally dependent upon cloud cover and upon the makeup of cloud particulates. Over two recent decades, it is known to have varied by nearly 10%. While it would seem that an albedo measurement of 35% from 1957 would indicate serious accuracy problems, that is not necessarily the case. It well could have been 35.0% back in 1957, 30.5% in 1996, and 29.8% in 2008, averaged over a year or two for each.
As for trenberth and K&T97, they used 0.62 cloud cover fraction for their simple model composed of two layers of opaque clouds, referred to by emissivity = 1 – meaning optically thick, and by high clouds with low coverage area and emissivity = 0.6. Unfortunately, in their paper, they used the term clear sky for the condition of no clouds, yet used the cloudy condition to mean 62% cloud cover rather than to mean total cloud cover in normal convention which can be combined with clear sky to yield the net result with a simple weighted average of 0.38 clear and 0.62 cloudy.
“unexpected result”
Press-release writers always paint scientists as lacking intuition about reality.
–
“To determine the net cloud effect, researchers need two numbers. First they need to measure the total amount of sunlight in a cloudy sky. Then they need to determine how bright that sky would be without the clouds, imagining that same sky to be blue and cloudless, when aerosols are in charge of a sky’s brightness. The difference between those numbers is the net cloud effect.”
Need info about the PATTERN too, as that affects flow… Hint…
–
“clouds in white reveal landmass shapes”
…which is why folks should start cluing in to widespread misconceptions (including at WUWT) about AMO.
Lot of (seemingly exclusive?) focus on albedo ….doesn’t cloud do anything else besides reflect light?!…
With an eye for continental-maritime contrast, the distribution of continents on the globe, and the position of the [magically powerful in some minds] North Atlantic in the context of the global distribution of continents, carefully compare:
1) http://icecap.us/images/uploads/AMOTEMPS.jpg
2) Figure 10:
Carvalho, L.M.V.; Tsonis, A.A.; Jones, C.; Rocha, H.R.; & Polito, P.S. (2007). Anti-persistence in the global temperature anomaly field. Nonlinear Processes in Geophysics 14, 723-733.
http://www.uwm.edu/~aatsonis/npg-14-723-2007.pdf
http://www.icess.ucsb.edu/gem/papers/npg-14-723-2007.pdf
Hydrology is a function of absolutes, NOT anomalies. I suggest blinking between the upper & lower panels of figure 6 here for related insight:
Trenberth, K.E. (2010). Changes in precipitation with climate change.
http://www.cgd.ucar.edu/cas/Trenberth/trenberth.papers/ClimateChangeWaterCycle-rev.pdf
Temperature-precipitation relations seasonally reverse sign over large portions of the globe.
High amplitude regional variance (think continental-maritime heat-capacity contrast) leverages “global” averages, particularly during periods of interannual synchronicity [which show nonrandom relations with solar variables across a range of timescales from semi-annual through interannual to multidecadal].
Sorry for the bad link. Trenberth’s 2010 draft appears to have been removed from the directory, but here’s the recently published article — see figure 6:
Trenberth, K.E. (2011). Changes in precipitation with climate change. Climate Research 47, 123-138. doi: 10.3354/cr00953.
http://www.int-res.com/articles/cr_oa/c047p123.pdf
Just curious. It seems there should be more water transported out of the lakes and oceans and up to the clouds if things heat up. The phase change to gas will cool the surface and deposit heat wherever the clouds form as they phase change from gas to liquid. Then the heat is a bit closer to exiting the earth than it was when it was on the earth’s surface.
Thinking simply, it would seem if the earth were to heat up, there would be more water evaporation, as the atmosphere heats up the clouds will be higher, etc.
I have no idea whether this is an “in the noise” effect or actually makes a difference. Anyone know?
For example, if a cloud passed over the instrument, the measured cloudy sky brightness exceeded calculated clear sky value by up to 30 percent. Kassianov attributes that large difference to scattered sunlight being “caught on tape” by the radiometer.
“Sunlight scattered by three-dimensional, irregular clouds is responsible for the observed large difference. The one-dimensional cloud simulations currently used in large-scale climate models don’t capture this diffuse light,” said Kassianov.
Incredible! They are still modelling clouds in one dimension. I thought that was where they were 30 years ago. I could do that on my PC.
It has been calculated that a change of 2% in the effect of clouds could outweigh all that is being attributed to CO2. They now realise that in some areas they are off by 30% . Well that’s solved AGW, we can all go home now.
Unexpected ?! They have just worked out that reflection and diffraction are wavelength dependant. I learnt that at the age of 14 in my first year of physics lessons. Have climate modellers never seen a prism??
I suppose if you are only doing 1D modelling you cannot account for either so it never mattered to them.
>>
cba says:
April 24, 2011 at 9:46 am
Unfortunately, in their paper [KT97], they used the term clear sky for the condition of no clouds, yet used the cloudy condition to mean 62% cloud cover rather than to mean total cloud cover in normal convention which can be combined with clear sky to yield the net result with a simple weighted average of 0.38 clear and 0.62 cloudy.
<<
After KT97 calculates the 62% cloud cover value, the term “cloudy” is ambiguous in the rest of the paper. The diagram (Fig. 7) probably should state that 62% cloudiness is assumed. However, they don’t make it that easy for us.
Notice how they calculate the IR window:
“The estimate of the amount leaving via the atmospheric window is somewhat ad hoc. In the clear sky case, the radiation in the window amounts to 99 W m-2, while in the cloudy case the amount decreases to 80 W m-2, showing that there is considerable absorption and re-emission at wavelengths in the so-called window by clouds. The value assigned in Fig. 7 of 40 W m-2 is simply 38% of the clear sky case, corresponding to the observed cloudiness of about 62%.”
Did you notice the subtle error in the math? If “cloudy” here means 62%, then the value in Fig. 7 should be 80 W m-2. If, instead, they mean “cloudy” is 100% (and that’s implied because they proceed with an interpolation), then they should interpolate between 99 W m-2 and 80 W m-2 which gives us about 87 W m-2. They actually interpolate between 99 W m-2 and 0 W m-2. In that case, the value should be about 38 W m-2. Why did they round up to 40 W m-2? If you’re looking for a missing heat flow, then here is about 2.38 W m-2 that they lost through rounding error.
The cloudy case of 80 W m-2 is apparently some other cloudiness value (not 100% and not 62%).
Jim
Amongst other flaws in this paper, and in answer to some of the comments in this thread, one cannot quantify the net effect of clouds the way they measured it one way or the other regardless of the accuracy of their measurements.
To arrive at something meaningful, the same cloud has to be evaluated for net effect under different circumstances. For example, I saw several comments to the effect that a low altitude cloud has an obvious cooling effect as anyone standing outside when a cloud crosses the sun can tell you. This is highly misleading but it is only a single context.
That same cloud, should it hand around until after sunset, will have a warming effect on the surface below. Perhaps more accurately, it will slow the rate of cooling of the surface below. Similarily, cloud cover in the depths of winter in high latitude has a net warming effect (or less cooling depending on how you want to view it) during both day and night. As anyone from a harsh winter climate can tell you, a bright blue sky on a July morning means it will be a very hot day. A bright blue sky on a January morning means go back to bed because it is -40 degrees outside.
The “net effect” of cloud cover not only is governed by the properties of the cloud, but also by the environment the cloud is in. Sometimes it warms and sometimes it cools despite being the exact same cloud. As a result, you can’t point your fancy meters at the clouds in one spot on earth in one season and come to any meaningfull conclusions about net effect on a global and annual basis.
Worst of all, nothing I wrote just now is anything that hasn’t been known and understood for decades. While I welcome the fact that what they are talking about may now actually get included in climate models as it should have from day one, their own research amounts to putting lipstick on a pig and calling it a discovery of something new. The only thing that is new is the lipstick.
Wyatt, M.G.; Kravtsov, S.; & Tsonis, A.A. (2011). Atlantic Multidecadal Oscillation and Northern Hemisphere’s climate variability. Climate Dynamics. doi: 10.1007/s00382-011-1071-8.
…haven’t yet found a free link, but having read this recent paper fairly carefully, I would say it will eventually stimulate a lot of discussion.
Some connected material is available:
Wyatt, M.G.; Kravtsov, S.; & Tsonis, A.A. (2011). Poster: Atlantic Multidecadal Oscillation and Northern Hemisphere’s climate variability.
https://pantherfile.uwm.edu/kravtsov/www/downloads/WKT_poster.pdf
Also:
http://pielkeclimatesci.wordpress.com/2011/04/21/guest-post-atlantic-multidecadal-oscillation-and-northern-hemisphere%E2%80%99s-climate-variability-by-marcia-glaze-wyatt-sergey-kravtsov-and-anastasios-a-tsonis/
Important Highlight:
Mainstream acknowledgement of NPI pivot:
“PNA participates in all synchronizations.” – Wyatt, Kravtsov, & Tsonis
(more on why this is significant at a later date…)
Dr. Judith Curry mentioned the article at Climate Etc. [ http://judithcurry.com/2011/04/21/week-in-review-42211/ ] and it received (quite interestingly) minimal attention, but she has not yet introduced a technical thread devoted specifically to the article. Certainly the current absence of a free version of the article puts a serious chill on the potential for open public discussion, so I support her restraint as sensibly prudent if this is the rationale for delay.
Jim Masterson says:
April 24, 2011 at 1:46 pm
jim,
I’m a bit distracted this afternoon so I’m having trouble following some of your comments. I don’t buy anything KT claims unless I can duplicate it. I haven’t gone over KT97 for a while – not since I discovered at least some of their numbers for cloudy skies are already weighted for 62%. It may well be that some of them are not.
I’m using a 1d model for looking at the spectral content and total power. I’ve just done a very simple cloud model for LWIR that looks like it might actually work. It simply assumes that an optically thick cloud in the LWIR will result in total absorption and a continuum thermal emission at the cloud top (and bottom). I’ve got a lot of digging to find LWIR spectral graphs from the TOA for cloudy conditions again. I think it’s in the ballpark for total output emission at the TOA (or tropopause) .
One of the KT97 problems is the overestimate of surface reflections escaping to the TOA. It’s high by over a factor of 2 from some other sources and this permits them to underestimate cloud and atmospheric effects. They also seem to ignore atmospheric scattering separately from clouds – actually, it’s probably about as much now as the overestimate of the ground reflection. If the 30w/m^2 were the 38% clear sky portion and with 70% of the surface being h2o at about 0.04 reflectivity, the result would give the average surface reflectivity for land as being around 0.78 – which could only happen with fresh snow over all land, lol. 0.085 albedo average is for oceans being about 0.04 albedo and land mass being 0.19 average albedo and that provides 30w/m^2 reflection in clear skies. for 38% clear skies, that reduces down to about 11 w/m^2 which is more in line with the 13.6w/m^2 for J. London and for some other references I’ve seen.
davidmhoffer says:
April 24, 2011 at 2:15 pm
“Amongst other flaws in this paper, and in answer to some of the comments in this thread, one cannot quantify the net effect of clouds the way they measured it one way or the other regardless of the accuracy of their measurements.”
The clouds that we so love in summer here in Central Florida appear on the horizon about 11:00 AM or so and arrive about 3:00 PM or so. They are bearing rain which drops the temperature as much as ten degrees and produces a very pleasant evening. The clouds hang around but do not cause the temperature to increase.
Do you think any of this will ever get addressed by climate scientists studying the effects of clouds on climate and temperature? I have a feeling that it is just way too real for them to even consider addressing it. It is just a hornet’s nest for them. The darn clouds are not spherical, not fluffy, not white, low flying, dripping with moisture, and repeat this process all summer, which is May 1 to October 1 in Central Florida.
Terry says:
April 24, 2011 at 2:32 am
It is basic undergrad level spectroscopy that scattering is a function of wavelength and aerodynamic diameter, that also has big implications for directional scattering. I am staggered that this is not the case.
==========================================================
Me too. I don’t see how you can talk about light scattering in the atmosphere without some reference to Mie theory, which to me includes both Mie scattering and Rayleigh scattering. After all, Rayleigh scattering of visible sunlight off nitrogen molecules is why the sky is blue. The wavelength in relation to areodynamic diameter of the particle is all important in the amount and direction of scattered light.
FYI: Here is the abstract of the actual paper:
GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L07801, 5 PP., 2011
doi:10.1029/2010GL046282
Shortwave spectral radiative forcing of cumulus clouds from surface observations
E. Kassianov
Pacific Northwest National Laboratory, Richland, Washington, USA
J. Barnard
Pacific Northwest National Laboratory, Richland, Washington, USA
L. K. Berg
Pacific Northwest National Laboratory, Richland, Washington, USA
C. N. Long
Pacific Northwest National Laboratory, Richland, Washington, USA
C. Flynn
Pacific Northwest National Laboratory, Richland, Washington, USA
The spectral changes of the shortwave total, direct and diffuse cloud radiative forcing (CRF) at surface are examined for the first time using spectrally resolved all-sky flux observations and clear-sky fluxes. The latter are computed applying a physically based approach, which accounts for the spectral changes of aerosol optical properties and surface albedo. Application of this approach to 13 summertime days with single-layer continental cumuli demonstrates: (i) the substantial contribution of the diffuse component to the total CRF, (ii) the well-defined spectral variations of total CRF in the visible spectral region, and (iii) the strong statistical relationship between spectral (500 nm) and shortwave broadband values of total CRF. Our results suggest that the framework based on the visible narrowband fluxes can provide important radiative quantities for rigorous evaluation of radiative transfer parameterizations and also can be applied for estimation of the shortwave broadband CRF.
>>
cba says:
April 24, 2011 at 5:02 pm
. . . I’m having trouble following some of your comments.
<<
That’s about par for most of my posts.
>>
I’m using a 1d model for looking at the spectral content and total power.
<<
Interesting. I, too, have a 1d model, but it’s duplicating the KT97 heat flows.
Jim
This is crazy. Photographers have known this since before the First World War (that’s why they used to carry so many slightly different yellow and orangeish filters), and the lowered contrast on cloudy days was often utilized for studio portraits. This is bleeding-edge science? A 1930s-era Weston photographic meter would have told the same story — though not, of course, peer-reviewed or tax-funded…
RE: Ed Barbar says:
April 24, 2011 at 11:53 am
The transport of warmth, as latent heat, from the surface to altitudes twice as high as Mount Everest, where the latent heat is released as vapor becomes liquid, and more heat is released as liquid becomes solid, is an obvious and huge “safety valve,” allowing the cooling of over-heated places. Think about how thunderstorms bloom up over heated plains, and how hurricanes swirl up over heated oceans.
Surely this is included in models. It is likely the reason some expected there to be a huge increase in big hurricanes. The simplistic model had to do something with all the extra heat it was creating.
The problem is that reality is far more complex than anything our puny minds can create. Compared to reality, models are like Haiku.
I have seen complex discussions concerning where exactly, in a cloud, the latent heat is released. In the middle? Where humidity is 100%? Or at the very “skin” of a cloud, where humidity is less than 100%? (It makes a difference, for latent heat released at the “skin” of a cloud escapes upwards to outer space more easily.)
Any one of these complex discussions can make a big difference in a model. A little pebble can start a big avalanche.
Not that we shouldn’t try to model our atmosphere. However people need to be a lot more humble while doing so. Also, when people do discover some tiny aspect of how the atmosphere works, they need to stop making grand pronouncements, as if they have discovered the Rosetta Stone, or proven the world will end next Tuesday.
it depends on the wavelength of sunlight being measured. This unexpected result will help researchers improve how they portray clouds in climate models.
Unexpected? There’s the problem… climate researchers know very little about the underlying science.
Aside from the reflectance/trapping effects discussion with regard to clouds, I have seen something that appears to be a lens-focusing effect. I have a 1KW PV system that has a typical maximum output of 8-900 watts on a clear day. We experience lenticular clouds here in Colorado, and I have seen on a few occasions where the PV output goes to 1.2-1.3KW when the sun is behind a solitary lenticular cloud. Of course this would only be a local concentrating of light, not affecting the average, but I find it interesting that this appears to happen.
Ed Barbar says:
April 24, 2011 at 11:53 am
….
Thinking simply, it would seem if the earth were to heat up, there would be more water evaporation, as the atmosphere heats up the clouds will be higher, etc.
I have no idea whether this is an “in the noise” effect or actually makes a difference. Anyone know?
Yes, please read “The Thermostat Hypothesis”, an essay by Willis Eschenbach, June 14 2009. At http://www.oarval.org/Thermostat.htm
“”””” cba says:
April 24, 2011 at 8:57 am
Nonegatives says:
April 23, 2011 at 7:53 pm
The fact that CO2 and H2O absorb different wavelengths of light has been known for a while now. Did they just now figure out that clouds are actually water vapor?
Clouds are droplets. vapor is individual molecules. where ever you’ve got droplets, you’ve got additional degrees of freedom capable of absorbing and emitting continuum like a solid or liquid black body radiator versus a discrete spectrum like an atom or molecule. “””””
Well some clouds are water droplets, and soem are probably ice crystals; presumably the high ones ar emostly the latter.
But the liquid droplet ones are NOT interracting with the sunlight, in any simple “reflectance” mode. They are lenses, with very definable optical properties, and it is trivial to show that a spherical droplet of water, illuminated by a collimated parallel beam of light, such as sunlight (1/2 degree divergence), simply refratcs the beam into a broadly diverging cone of rays. Well the droplet of course is a “Converging” lens, but the focus is less than the dop diameter, behind the drop, and from there on the light diverges into a cone that has most of its energy inside about a 45 degree cone half angle, but some light is scattered to very wide angles beyond 90 degrees from the sun. So the fluffy white light you see, is scattered light, not reflected light. Two or three droplets in succession, and you have a totally diffused isotropic radiation pattern. We can’t post pictures here, or else I could show exactly what the refracted output from a spherical droplet looks like.
My gut feel for the albedo component of clouds versus polar ice, is more in tune with your view cba, that it would be with Bill Illis picture. Fresh snow, less than a few hours old can have high reflectance, but after just a few hours, it becomes somewhat transparent, due to surface melting, and once the light gets into the snow/ice, it is largely trapped by TIR, so the reflectance drops to where it is no more reflective than a lot of rocks or greenery.
But the key indicator of the net effect of clouds, is simply the total reflectance of a cloud covered earth compared to a cloudless earth. If it wasn’t for the blue sky scattering; the blue planet, would be the black planet, as the oceans have about 3% max sunlight reflectance; and they are over 70% of the surface, and an even larger fraction of the earth area that receives the bulk of the solar energy. The area inside the Antarctic circle contains only 8.3% of the Southern Hemisphere surface area (1-cos 23.5 deg).
My quarrle with the authors of this study/paper/whatever, would be that the sides of those fluffy clouds, that they say are scattering light down to the surface (that would otherwise miss the surface), are also going to scatter light that was headed for the surface anyway, and redirect it out to the rest of the universe. I doubt that they can prove any significant bias towards, an increase, downwards.
I don’t have a quarrel with the statement that the underside of thin white clouds is radiometrically brighter than the clear blue sky. After all, the normal BB spectrum contains only 25% of its total energy at wavelengths below the spectral peak wavelength, and the dominantly blue color of the sky is well below the solar spectral peak wavelength. So the blue sky adreesses less than 25% of solar energy, and it is isotropic, so no more than half of it can be directed downwards.
Liquid water droplets on the other hand are highly transparent to the bulk of the solar energy spectrum; including that same blue light, and therefore those water droplets, can optically scatter a much broader spectrum, than the Blue Sky Raleigh scattering.
But don’t try to josh us, that because the cloud is brighter than the sky, then it must warm the earth. Try pointing your radiaometer to the approximately half of one degree direction that contains that local area star known as “the sun”, and then tell us the clouds are brighter than that. That half of one degree of the sky is what is warming the earth; not the blue skylight, nor the clouds.
NOBODY, ever observed the Temperature to increase, (signifying the effect of warming), when a cloud (ANY cloud) passes between the observer and the sun. Iy ALWAYS cools down to a LOWER Temperature; no matter what.
“”””” Craig Goodrich says:
April 24, 2011 at 11:09 pm
… if a cloud passed over the instrument, the measured cloudy sky brightness exceeded calculated clear sky value by up to 30 percent. Kassianov attributes that large difference to scattered sunlight being “caught on tape” by the radiometer.
This is crazy. Photographers have known this since before the First World War (that’s why they used to carry so many slightly different yellow and orangeish filters), and the lowered contrast on cloudy days was often utilized for studio portraits. This is bleeding-edge science? A 1930s-era Weston photographic meter would have told the same story — though not, of course, peer-reviewed or tax-funded… “””””
The clear blue sky, is NOT the source of earth’s energy budget. On a perfectly cloudless day, when there are no clouds around to increase the sky brightness, by 30%, a careful inspection of the entire sky, will generally find one particular direction encompassing about one half of one degree of solid angle (cone full diameter), where the brightness suddenly rises to a value many thousands of times brighter than either the clear blue sky; or any cloud that might happen on the scene.
Experimental evidence suggests that this very high brightness radiant energy is coming from a nearby star in our galaxy.
George E. Smith says:
April 25, 2011 at 11:11 am
George, it looks like we’re in pretty close agreement. BTW, there is a little program I found that will do Mie scattering calculations and show various things like a fogbow. Some of the reflections do split up wavelengths and some do not. The Mie scattering is far stronger than wavelength dependent clear sky scattering and it produces nonuniform results by direction for a single scattering. Suffice to say one could use a supercomputer just to tabulate this stuff. It appears that a fogbow intensifies the apparent optical thickness and increases the reflectance at around 180 deg from the direction or within 60 deg. of that point. Note that I use the term reflectivity to refer to the scattering of incoming light by all those millions of water droplets as it really is not going to be calculable or distinguishable in any meaningful way.
Of course at high enough altitudes, there will be ice rather than liquid. That leads to the possibility of non spherical particles also. Combine that with the tiny size of many of the ice or liquid particles and the potential for seeding particulates being close in size to that of the very small droplets and you’ve got a devils brew that cannot be calculated realistically.
It’s interesting that J London, 1957 as reference in the physical meteorology book I referenced above covered these various aspects that you don’t find in KT97. Scattering (rayleigh) was determined to be about 17% of incoming with only 7% escaping the atmosphere. Clouds tended to transmit about 10% or 20% or more and absorb only 1 or 2% (if I recall the numbers properly).
Note that the modelers of warming think they can imitate the stuff assuming 1 scattering event.
Somewhere I came across the notion that total overcast conditions tended to allow around only 10% of the incoming power to reach the surface.
As for your 3% water surface reflection, I’d put that closer to 4 or 3.5 %. If the sun is at a high zenith angle, it tends to be quite low. As that angle increases towards 90, the surface becomes quite reflective – but then again, this is not where most of the TSI is going and in fact, there is much smaller amounts per unit area there and thicker atmospheric pathlengths. While this occurs at higher latitudes always, but not that much towards the equator – except that early morning and late afternoon gives one a daily equivalent to that higher latitude zenith condition. Realistic average land conditions including ice and snow provide albedo reflectivity in the order of 17 to 19 % which leaves surface albedo to be less than 10% and 62% almost totally opaque cloud cover drops that down towards 3.5% or about 11 w/m^2 surface reflection that makes it back to the TOA. It seems to me that the KT97 errors were quite convenient to trenberth’s position on the subject. However, it does put the atmospheric scattering/cloud reflectivity (scattering too) up in the 27-28% albedo realm rather than merely the 24% range and their proclaimed factors of land use change and snow/ice cover loss into the realm of the ludicrous.
what I find interesting is that of the LWIR cloud situation. Regardless of ice or liquid, one now has solids or liquids (or sort of supermolecules) capable of continuum emissions rather than spectral emissions.
I just did a crude model test of this sort of thing assuming that a cloud top at 3km would have the same T as ambient and that it was opaque in the LWIR – leading to a full BB emission at temperature T. The result was a somewhat familiar spectral pattern with a BB temperature at 3km ambient (using the 1976 std atm value). I got around 235 w/m^2 power at the TOA (instead of almost 270w/m^2 for the clear sky 288.2k case). In the cloudy case, the co2 doubling had an effect of less than 3 w/m^2 versus the 3.7w/m^2 clear sky co2 doubling case. In fact, assuming the case of a 30% h2o vapor increase along with the co2 doubling, the result was hardly over 4w/m^2 total, again significantly less than the clear sky version. Of course, the 30% h2o increase would require a 5 deg C rise in the whole atmospheric column if one assumes constant relative humidity as a constraint. One is only missing the vast majority of w/m^2 necessary to cause such a 5 deg C increase.
“”””” As for your 3% water surface reflection, I’d put that closer to 4 or 3.5 %. If the sun is at a high zenith angle, it tends to be quite low. As that angle increases towards 90, the surface becomes quite reflective – “””””
Water has a refractive index over most of the solar energy spectrum of 1.333. The normal incidence Fresnel reflection coefficient is ((n-1/(n+1))^2 =2.037% The total reflectance (both polarisations) remains almost constant up to the Brewster angle which is arctan (n) = 53.1 deg for water; that is the angle from the zenith; not the elevation angle. Cosine of 53 deg is 0.6 ( large angle of a 3-4-5 triangle is 53 deg 8 minutes). So at 53 degrees, your have 1/0.6 = 1.666 air mass, so quite a lot more atmospheric absorption than the zenith air mass one value. And don’t forget that as that sun elevation angle drops, to get those higher reflection coefficients, the obliquity factor is spreading the light out over a much larger earth surface area; so the resultant reflected energy per unit area isn’t as large as you would think. I believe a full integration using the full Fresnel Polarisation equations, would show that the full hemispherical surface reflectance does not exceed 3% (off the oceans). That’s for a uniform diffuse (whiteout) irradiance.
People try to argue that the 3% reflectanvce from the oceans, is only valid for a perfectly flat (optical) ocena, and that waves will tip the surface away from the sun creating a higherr reflectance. Unfortunately, they forget, that evbery wave has a back side too, and for every surface element tilted to higher incidence angles, there is another tilted to lower incidence angles. The net result is that the reflectance off at least a mildly rough sea, is no different from a perfectly flat sea.
Glass with a Refractive index of 1.5 has a normal reflectance coefficient of 4%; and you only get that low a refractive index for glasses that are almost pure silicon dioxide.
Schott BK-7 which is a Boro-slicate crown glass has a nominal refractive index of 1.517. 5% reflectance (normal) is more typical of things like ordinary plate glass, which are higher index, than Borosilicate crowns.
I don’t have a lot of problem with the clouds being quite high absorbers of the LWIR surface emissions. But whereas the incoming blockage is of the direct, almost colimated sun beam, the outgoign LWIR is at least Lambertian, and most likely isotropic, since the surfaces aren’t optically flat. Even if it was Lambertian, then the irradiance of the cloud bottome, would go as cosine^4 of the obliquity angle (from surface to cloud), and also drop as inverse square of the cloud height.
So those nice puffy cotton ball clouds, only get to intercept a miniscule fraction of the LWIR emissions from their shadow zone; yet they block ALL (times their absorptance) of the incoming solar beam.
If the extra scattered light off the bottom of the cloud exceeded, the amount of solar energy incoming that they block; then the Temperature would go up in the shadow zone. It doesn’t; it always goes down; no matter what.
George E. Smith says:
April 25, 2011 at 1:56 pm
George,
I think we’ve got a disagreement there. According to the old 1957 London paper, actual absorption of visible is only a couple of percent of total incoming as I recall. A big portion of the incoming TSI is ‘reflected’ away as albedo and the remaining is going to be scattered downward, granted significantly less than clear sky. I think a typical number is around 10% for total overcast conditions. I don’t see LWIR penetrating the cloud to any depth. What comes out the top is going to be BB radiation from the top small fraction of thickness that constitutes one optical path length. If it were dust, I’d believe there would be substantial penetration. This though has got to be close to the equivalent of a surface of water or clear ice that yields the low albedo, even at visible wavelengths. I think that low reflectivity gets worse at longer wavelengths too.
Generally, the reality follows the theory pretty well. There’s nothing significant starting to happen with visible light reflectivity of water by the time you hit 60 deg from the zenith. It’s only the last little bit where the reflectivity shoots up and by that time, the atm. thickness is getting thicker than 1.6 x zenith angle = 0 and the surface area of a square meter of insolation is spread out over more than that.
It all still leaves KT97 numbers at around 0.033 for average surface albedo and all the rest of the roughly 0.3 albedo from atmospheric scattering (small potatoes) and clouds (the 800 pound gorilla in the room). This also tends to leave hansen, lacis, dessler, trenberth and the like grasping at exagerations and grasping at straws, apparently hoping no one has noticed their little errors.
cba says:
April 25, 2011 at 6:01 pm
What comes out the top is going to be BB radiation from the top small fraction of thickness that constitutes one optical path length.
———————————————————–;
Assuming BB denotes Black Body radiation, the Stefan-Boltzmann law (or BB) results from the integral of the Planck radiation law.
Planck law requires a continuous frequency distribution, e.g., from a solid or a liquid.
Would you happen to have the frequency or wavelength spectrum of a cloud?
And by one optical length, are we talking about 10 microns – or maybe 20 microns?
Agile,
you are right about BB, wrong about optical path length. It’s not a wavelength but rather a thickness associated with absorption.
Since the droplets are either ice or liquid, there should be a continuum absorption and emission and not a molecular spectral response like a single molecule or a gas exhibits. It’s energy quantum states existing due to the fact that many molecules are interacting as a solid or as a liquid. Hence, apply Planck’s law for the temperature present. Then, apply the absorption spectrum to the curve to deal with the gas between the cloud and the ‘measurement’ location because it’s a log function and there’s going to be h2o molecules present that will exhibit the molecular spectral absorption/emission response.
How close this is to reality may depend somewhat on droplet size. I doubt it can really be actually measured very well either, except from the surface or TOA. I’ve made a synthetic spectrum sample using a line by line database but I haven’t been able to compare this to any TOA measurements yet.
“”””” cba says:
April 25, 2011 at 6:01 pm
George E. Smith says:
April 25, 2011 at 1:56 pm
George,
I think we’ve got a disagreement there. According to the old 1957 London paper, actual absorption of visible is only a couple of percent of total incoming as I recall. A big portion of the incoming TSI is ‘reflected’ away as albedo and the remaining is going to be scattered downward, granted significantly less than clear sky. I think a typical number is around 10% for total overcast conditions. I don’t see LWIR penetrating the cloud to any depth. What comes out the top is going to be BB radiation from the top small fraction of thickness that constitutes one optical path length. “””””
Well I can’t disagree with you; or agree, for that matter, since I can’t determine just what it is you are saying:- “”””” According to the old 1957 London paper, actual absorption of visible is only a couple of percent of total incoming as I recall. “”””” So what are they (the old 1957 london paper) saying is absorbing what part of the “visible” portion of TSI.
I’m still using, and still seeing published, the exact same solar spectral graphs for air mass zero (TSI) and ground level air mass one (Zenith sun) that used in school 55 years ago; well even before that old 1957 London paper came out. The AM-1 curve always assumes some standard average water vapor content for the atmosphere, since it includes the water absorption bands that start at about 700 nm. But before you get to 700 nm, you already have a significant loss even at the spectral peak. Well the curve I can most easily get my hands on today, is from the Aero-Jet General Infra-Red Engineering Handbook, 25 August 1961, and it is republished in The Infrared Handbook from ERIM., in their chapter 3 on the sun. For those graphs they plot a best fit black body radiation peak spectral irradiance of 2.0 kW/m^2/micron, and the actual anomalous AM-0 sun is about 2.25 around 480 nm. The AM-1 sea level peak, is at the4 same 480 bn but is only1.5 kW/m^2/micron, so that is a 25% loss at the center of the spectrum. Now that isn’t atmospheric absorption or water vapor absorption; it is mostly Raleigh Scattering, that produces the blue sky. I haven’t done the actual calculation of the blue scattered energy, but one thing I do know from actual observation, is that the blue sky looks exactly the same looking down as it does looking up, so the Raleigh scattered blue source is quite isotropic, which means that about half of the short wavelengths that are scattered, ends up returning to space (scattered, not reflected), and the other half reaches the ground. If you look down from 36,000 feet in a cloudless sky over the ocean, you see the same blue sky because the ocean looks quite black (in the visible). Of course if you happen to look in the direction of the specular reflection of the sun, you will get 2% of the sunlight, but away from the sun, you get black from the ocean. At LWIR wavelengths the (calm) ocean is going to be a Lambertian thermal emitter, when seen from altitude.
Ozone has a weak absorption in the visible from about 500 to 650 nm and O2 even has a band around 780nm but I can’t separate that from the coincident water band. Then a number of H2O bands occur, and CO2 doesn’t kick in until about1.4 microns.
But actual water droplets in clouds are quite different, because they are large enough to behave optically. Ordinary geometrical optics predicts that a spherical droplet, will convert colimated sunlight into a strongly divergent beam (after focussing) that puts most of the energy into about a 90 degree full cone angle; but is still reflracting loight out to 90 degrees from the sun. So in a cloud, just a few consecutive droplet refractions, and you have a perfectly isotropic distribution of scattered light.
I’ve never said that the LWIR can penetrate the cloud to any depth; of course it is absorbed. But that simply results in subsequent emission of a thermal spectrum from the cloud, which also gets absorbed, and then re-emitted. So the original ground sourced LWIR may not come out the top of the cloud, but the cloud itself will become a source of isotropic LWIR emissions.
My point was that because the surface emission is totally diffuse, the cloud can only intercept that fraction of the diffuse energy that actually hits the lcoud, and the higher those puff balls are the smaller that fraction is. Yes of course with a total overcast, you have a different result, including that a lot less solar energy reaches the surface to heat it.
But as to the polar ice albedo, you can’t have it both ways. You can’t combine a non-existent zenith sun irradiance, with a near grazing incidence high surface reflectance, and claim a large albedo due to the ice, or the open sea water either.
George,
The 1-2 % absorption was referring to cloud absorption and I recall that the number is for the total TSI absorbed by clouds over the cloud fraction present. The London paper is quite different in cloud reflectivity and cloud fraction from that of the KT97 crowd. The amount turns out similar but the two values are virtually swapped. KT97 assumes a 0.62 cloud fraction and a reflectivity back to space of around 35%. London has a cloud fraction assumed to be around 35% and an average cloud reflectivity of about 60% +. These all refer to the the visible spectrum or at least to the TSI.
I think the London paper assumes a rayleigh scattering in clear sky of around 17% with about 7% escaping the atmosphere as albedo. The rest gets absorbed by the atmosphere and the surface.
What I use is the 1976 std atmosphere values and I assume a true BB spectrum from the Sun. Note that the BB spectrum is close but not actually correct for what comes from the sun. Dealing with LWIR means though that I am not using a solar spectrum. As for the atmosphere, it has plenty of visible light absorption lines present. These are telluric lines found in spectral imaging of stars. Of course plenty of lines doesn’t mean plenty of power absorption in this case.
I’m not sure of the reference to snow and water albedo at angles far from the zenith. Ice and snow tend to be at high latitude. These do not require large zenith angles to be large. Ice cannot be clear ice though. Otherwise, it’s going to be similar to water in reflectivity. It’s mostly fresh snow that offers high albedo. Of course, at the greater angles from the zenith plain old water is going to increase in reflectivity so there will be little advantage in having the fresh snow and ice instead of open water. Hence, those claiming the loss of snow and ice having a significant effect on albedo is totally bogus for the world today.
your comment on the ground lwir not making through but rather the top emitting at its characteristic temperature was the point I was trying to make with the other gentleman earlier. That should be true as long as the optical thickness in the LWIR qualifies as thick or that it isn’t transparent to LWIR.
The net result for my little model I’m playing with is that the spectrum seen for cloud cover should be the top of the cloud temperature BB attenuated by the presence of the atmospheric contents above that surface.
I seem to recall seeing spectrums for TOA looking down (and surface looking up) that came from ARM. First attempts to find my links or to find them on a site though has not been going well. Until then, I cannot compare my model assumptions to what is being measured.
It would seem we have less in disagreement than I originally thought
“”””” I seem to recall seeing spectrums for TOA looking down (and surface looking up) that came from ARM. First attempts to find my links or to find them on a site though has not been going well. Until then, I cannot compare my model assumptions to what is being measured.
It would seem we have less in disagreement than I originally thought “””””
I’m sure we are not in great disagreement. For example, your “London Paper talks of a 17% Raleigh Scattering with about 7% lost as albedo. And I mentioned that the Raleigh scattered blue light (amount unknown) was essenitally isotropic, so about half of it escapes to space, with half coming down as blue sky. I do believe 7% is half of 17%; give or take some error bands. And yes I did simply ignore the atomic spectral lines of the atmosphere, on the premise that though they may be many, they are also narrow compared to molecular bands. I’m not sure of the origin of the 500-650 band of Ozone. Assuming that the ozone is in a high cold thin layer, then the broadening of molecular lines shouldn’t be too large; but I don’t quite see how such a shrt wavelength band occurs. I’m not a chemist; so pretty shaky on molecular spectroscopy. I’d tend to believe Phil’s interpretation of what causes that absorption, although it is not a prominent dip.
As for the thermal continuum emission from water/ice (in clouds), I’m quite interested in what real observed spectra are since I am quite uncomfortable with the notion that only ghg molecular species, are emitting LWIR from the atmosphere. I don’t see why gas molecules in collision (acceleration events) can’t emit BB like radiation; but such emission from a single molecule presumably changes the Temperature; since the heat capacity of a single molecule is small. So I’m not sold on the notion that gases don’t emit thermal continuum spectra; but of course I wouldn’t expect the emissivity to be very high because of the lower molecular density. Thermal emission from solids (at earth environmental Temperatures) also lowers the Temperature; but in that case, the heat capacity is very much higher so the Temperature inertia is more robust.
I haven’t looked into Mie scattering much, because I get the impression that it is a consequence of crud in the atmosphere; whereas Raleigh scattering occurs even in pollution free atmosphere.
George,
gas molecules are colliding at tremendous rates. Sometimes they raise a molecule to a higher state and sometimes they lower it from a higher state. Also photons being absorbed and emitted will change the state. You have to have a transition of the right energy to absorb or emit a given photon. Also, times for a state to decay and emit a photon may be longer than typical collision interaction times and all these go into the nature of a strong or weak line.
A state change will change the energy of a molecule, but then it won’t take long for it to come back to ‘normal’ Of course normal is quite a range of possible energies. If enough energy is coming in to balance what is going out, the temperature wont change overall. If not, then the T will change until the energy transfer rates balance again.
Ionized gases can emit continuum. The sun’s photosphere is emitting a continuum, mostly due to the ‘metals’ and not the H or He. A typical response would be that sure a gas will emit a continuum if you’re talking about an optically thick amount at the wavelengths of interest. And, it’s true. It’s a bit of the old ‘catch 22’. Thick means it can absorb as well as emit. That won’t happen where the gas is transparent. Ultimately that might mean that the wings of the lines get smeared out due to all the molecules bouncing around at different velocities in difference directions and red shifts and blue shifts and just plain old statistical liklihoods of being on the fringes.
mie scattering is you cloud droplets and also potentially your dust particles. rayleigh is individual molecules or atoms.
other than to see it, you might search for Iris, a Mie scattering display program. I doubt it can produce much information of interest beyond the graphs.
“”””” cba says:
April 26, 2011 at 4:37 pm
George,
gas molecules are colliding at tremendous rates. Sometimes they raise a molecule to a higher state and sometimes they lower it from a higher state. Also photons being absorbed and emitted will change the state. You have to have a transition of the right energy to absorb or emit a given photon. “””””
As I understand the continuum emission of ionised gases; it is a simple consequence of the fact that an arriving electron that “drops in” to one of the atom’s available (and permitted) energy levels, can have any value of energy, so the energy change in the reaction is a continuous function; in a sense the vast plasma doesn’t have discrete energy levels for its free electrons.
When it comes to the Black Body spectrum, and the Planck Radiation formula; which is the best known continuum spectrum, I’m under the impression that the derivation is purely one of classical Physics involving “particles” having a certain number of degrees of freedom to which an amount of energy can be assigned. There aren’t any atomic energy levels involved; and Planck’s contribution to remove the Ultra-Violet catastrophe of the Raleigh Jeans formula, was simply to require that the allowable energy assigned to each degree of freedom, under the equi-partition Principle had to be an integral multiple of a discrete or quantized amount (kT0 rather than any possible continuous value, that Raleigh Jeans assumed, in their derivation.
The purported origin of the radiation was simply the Classical Maxwellian Electromagnetism requirement, that an an accelerated electric charge; being thus a varying electric current, must radiate electro-magnetic waves. The whole purpose of the Stanford two mile linear accelerator, was to eliminate the charge acceleration due to electrons travelling in a circle in a magnetic field. Even in this post Quantum Mechanical world, Maxwell’s equations still insist that accelerated charges radiate; it’s just an electric current in an antenna, situation.
So how does this apply to BB radiation
“”””” cba says:
April 26, 2011 at 4:37 pm
George,
gas molecules are colliding at tremendous rates. Sometimes they raise a molecule to a higher state and sometimes they lower it from a higher state. Also photons being absorbed and emitted will change the state. You have to have a transition of the right energy to absorb or emit a given photon. “””””
As I understand the continuum emission of ionised gases; it is a simple consequence of the fact that an arriving electron that “drops in” to one of the atom’s available (and permitted) energy levels, can have any value of energy, so the energy change in the reaction is a continuous function; in a sense the vast plasma doesn’t have discrete energy levels for its free electrons.
When it comes to the Black Body spectrum, and the Planck Radiation formula; which is the best known continuum spectrum, I’m under the impression that the derivation is purely one of classical Physics involving “particles” having a certain number of degrees of freedom to which an amount of energy can be assigned. There aren’t any atomic energy levels involved; and Planck’s contribution to remove the Ultra-Violet catastrophe of the Raleigh Jeans formula, was simply to require that the allowable energy assigned to each degree of freedom, under the equi-partition Principle had to be an integral multiple of a discrete or quantized amount (kT0 rather than any possible continuous value, that Raleigh Jeans assumed, in their derivation.
The purported origin of the radiation was simply the Classical Maxwellian Electromagnetism requirement, that an an accelerated electric charge; being thus a varying electric current, must radiate electro-magnetic waves. The whole purpose of the Stanford two mile linear accelerator, was to eliminate the charge acceleration due to electrons travelling in a circle in a magnetic field. Even in this post Quantum Mechanical world, Maxwell’s equations still insist that accelerated charges radiate; it’s just an electric current in an antenna, situation.
So how does this apply to BB radiation “DUE SOLELY TO TEMPERATURE” ? Well Temperature is characterized by a collection of molecules/atoms having a Maxwell Boltzmann distribution of kinetic energies ,with the mean energy being Temperature dependent; square root of T, as I recall. As a result of this, the molecules/atoms are in constant collisions covering the whole range of collision energies due to the MB distribution. In those collisions, you have electric charges in free flight (constant velocity( neglecting gravity)) entering into collisions that result in a follwing free flight (constant velocity) at some totally different and unpredictable velocities; and for the duration of the interraction time of the two colliding atoms/molecules, those electric charges are undergoing acceleration, and therefore must radiate EM waves during the collision time. Now of course for non-polar atoms/molecules, such as Ar, or O2, or N2, etc the charge centre of the nuclear protons, and the orbital electrons, is essentially a single point; so what is this NET charge acceleration ?
Well the -ve charge of course is due to the orbital electrons, and the +ve charge is due to the nuclear protons; which are nearly a thousand times more massive. so when the particles collide, the acceleration of the electron cloud, and the nucleus are not the same, so the atom/molecule gets deformed during the collision time; and that is why there is a net charge acceleration because the more massive nucleus, is alot more sluggish tna the elctrons; so for all practical purposes, it is only the electron acceleration that matters.
So BB radiation from gases is explainable without quantum mechanics, using just Maxwell’s equations of Electro-Magnetism. Planck only needed to have the equipartitioned energies assigned to the degrees of freedom to be multiples of kT, rather than a continuous energy spectrum, to eliminate the UV catastrophe of Raleigh-Jeans. Other than that the derivation was just statistical mechanics.
It is pretty hard to argue that the Classical Physics of Maxwell’s Equations of Electro-magnetism was destroyed by the Quantum Theory of atomic structure; when those equations are enshrined via (c), (mu-naught) (epsilon-naught) as the only fundamental physical constants to have absolute values; with zero uncertainty.
But your point about the optically thick gaseous source is just a different way of expressing my comment that the Temperature must drop with the emission due to the low thermal mass associated with the gas sample, and that due to the low total number of radiating particles (density), the emissivity is low, which is why the gaseous continuum emission is of low intensity; but I don’t believe it is absent; it just takes a hell of a lot of gas to get any intensity.
Someone else referred to the thermal emission from gases, as being collision induced radiation. I like that description; because I believe it is simply the transient acceleration of the electron cloud charge during however many femtosecond (or maybe attoseconds) is involved in the atomic/molecular collision event, that is the physical origin of that emission.
Just remember that theoretically, the spectrum of (BB) thermal emissions goes from zero frequency to infinite frequency. You can’t explain that with any sort of energy level structure even in the solid state.
Niels Bohr, sort of screwed things up, by declaring that Maxwell’s insistence that acelerated charges radiate, was null and void, and that they didn’t have to radiate, if they were part of an energy level orbital structure; only when they jumped from one state to another. It was quite arbitrary, and quite without foundation, and if it were accepted today, then Maxwell’s electromagnetism would be dead. Of course it isn’t because the quantum mechanical picture was changed so that the atomic energy level structure was not one of planetary orbits of moving electrons; but simply a probability distribution of electron location (orbitals). Get rid of the planetary orbits with their constant acceleration, and Maxwells is resurrected to become King of the heap with his derivation of the Velocity of light.
Anyway; that is the way I view it; but your exposition does not create any trauma for me; I can adapt to your imagery.
George,
well you’re pretty much describing stuff. Maybe the question should be why do electrons in these various states – like the ground state – not radiate even though they seem to be in motion with continual acceleration. Note, I seem to recall one hypothesis somewhere about maybe it was actually the change in acceleration rather than the acceleration that caused the radiation.
Of course, for IR we’re not talking electron states but rotation and vibrational molecular activities and for the continuum of solids and liquids, one must even add in the interactions between molecules.
The boltzman (or planck curve) distribution of energy states defines the classical BB. It’s not the only continuum distribution possible although it’s the most likely one. Synchrotron radiation is another and it can be found apparently as the dominant emission from galaxies with active galactic nuclei. But that has nothing to do with the topic at hand.
cba says:
April 26, 2011 at 5:29 am
————————————————————;
How large is a water molecule?
How large is a CO2 molecule?
How large is a raindrop?
What is the density of water vapor?
What is the density of water?
Since the Earth’s surface temperature is roughly 1/3 radiative and 2/3 thermodynamic – and the radiative part has a small variation, then IMHO, any explanation which isn’t based fluid properites (interacting with external forces) is doomed to failure.
What needs to be explained is why the Earth is acting like a refrigerator.
“”””” cba says:
April 26, 2011 at 7:06 pm
George,
well you’re pretty much describing stuff. Maybe the question should be why do electrons in these various states – like the ground state – not radiate even though they seem to be in motion with continual acceleration. Note, I seem to recall one hypothesis somewhere about maybe it was actually the change in acceleration rather than the acceleration that caused the radiation. “””””
Well cba, that would seem to be the whole point. Bohr in his “planetary” description of the atom simply assumed that the electrons are in motion; and therefore must be accelerating if in circular orbits; later generalized to elliptical orbits by Arnold Sommerfeld, and that picture enabled Bohr to calculate the (presumed) kinetic energy of the orbiting electron, and in the end explain the Balmer and other series of atomic spectral lines (for Hydrogen atom at least). That discovery was one of the truly great achievements of the golden age of Physics. I used to think that Atomic spectra were the greatest thing since long before sliced bread.
But in the modern quantum view of the atom; there isn’t any need to assign movement and hence acceleration to the elctrons; the “Orbital” shapes are simply probability maps of where the electron is likely to be found. Bohr’s original atom doesn’t allow the Principal quantum number (n) to be zero, since that implied in his model, an orbit that went right through the center of the nucleus. Today the probability of that being the location of the electron, is not considered a problem.
Maxwells EM radiation of course arises naturally from applying his four equations of the EM field to varying (sinusoidal) currents in a “dipole” antenna; which implies separated electric charges (dipole) and movement of those charges (current) which alters the dipole moment, and hence the EM field. The force of electromagnetism is one of the two forces of nature that has infinite range; the other being gravity, so that implies that the EM field, like the gravitation field, can spread infinitely far from the source (antenna) . But once the field is way out there, and the source current is varying rapidly enough, the finite value of the velocity of propagation of the field, prevents the entire field from collapsing back on the antenna, and it starts to disconnect from the antenna, and create closed fields that now propagate away at the speed of the EM radiation which is (c). So it is not unlike blowing soap bubbles. The bubble stays attached to the wire ring, so long as you don’t blow too hard. But when you blow hard enough, the soap film tears off the ring, and if the surface tension can close the gaping hole in the film (natural surface area minimisation) faster than the air can rush out of the bubble through the hole (velicity of sound limitation), then a disconnected closed bubble is generated, and can propagate away.
Somewhat analagously the :bubbles of electromagnetic field disconnect from the antenna and close up both the electric field lines, and the magnetic field lines so both are continuous, and the bubble escapes at 3E8 m/s. To achieve that disconnect and propagation, the current in the antenna has to increase, to establish the field, and then rapidly decrease, to reverse the field before the established field can collapse back onto the antenna. That’s why the requirement for charge acceleration (or current variation) which is the same thing.
My Radio-Physics Major comes in handy, when trying to understand how EM radiation works.
The key distinction between the continuum thermal radiation, and the absorption band spectra of molecules, is that the former is a consequence entirely of the Temperature of the assemblage of particles, and somewhat independent of what those particles are; Planck’s formula is a consequence of statistical mechanics, and involves no assumptions about the energy level structure of atoms or molecules. It does require the assumption of only quantised energy values being allowed for the kinetic energies of the degrees of freedom of the energetic particles; but needs no knowledge of atomic numbers or electron masses or anything like that.
The LWIR emissions from molecules (or absorptions) are “resonance” phenomena, that are entirely dictated by the structure of those molecules, and things like the strength of chemical bonds, and the possible modes of oscillation. All the common GHGs are known to have molecular structures that either have a non zero electric charge dipole moment, as H2O does (CO2 doesn’t); or are capable of exhibiting a non zero electric dipole moment, when oscillating in some of the possible modes of oscillation permitted by the physical structure of the molecule.
In the case of CO2, we have the “symmetrical stretch” mode, where the two Oxygens maintain equal and opposite vibrations along the linear axis of the molecule while the C remains stationary (well it is the origin of the local co-ordinate system). In that case, the dipole moment remains identically zero, so that modfe is not IR active. The assymmetrical stretch mode, where all three atoms move along the molecule axis of symmetry, and only their center of mass remains stationary, the electric dipole moment now is not always zero so this mode is IR active at 4 microns. The important mode for the GHG phenomenon, is the degenerate bending mode, where two identical modes at right angles are possible (the degeneracy), and the molecule can do an elbow bend about the carbon atom. Since that sort of oscillation involves a higher moment of inertia for the components, the resulting resonance frequency is lower, so we get the common 15 micron band of CO2 (or is it 14.74 or some such number).
But you see those are discrete resonance oscilations that are a consequence of atomic or molecular structure; and are pretty much independent of Temperature. The thermal continuum radiation is entirely a consequence of Temperature because of the inter molecular collisions; which distort the atomic or molecular electric charge dipole moment during the collision and permit the radiation of EM radiant energy.
Well that’s a somewhat stick in the sand description of how I see it; which takes a lot longer to describe than to understand.
The molecular band spectra cannot absorb or emit EM radiation from down to but non including DC, and up to beyond the upper reaches of the gamma ray frequencies; but a heated body can.
George,
”
Just remember that theoretically, the spectrum of (BB) thermal emissions goes from zero frequency to infinite frequency. You can’t explain that with any sort of energy level structure even in the solid state.
”
there’s lots of other stuff though other than molecular or atomic. Once you go beyond a single molecule, you’ve got molecular interactions plus broadeing due to the doppler effects.
“The LWIR emissions from molecules (or absorptions) are “resonance” phenomena, that are entirely dictated by the structure of those molecules, and things like the strength of chemical bonds, and the possible modes of oscillation. All the common GHGs are known to have molecular structures that either have a non zero electric charge dipole moment, as H2O does (CO2 doesn’t); or are capable of exhibiting a non zero electric dipole moment, when oscillating in some of the possible modes of oscillation permitted by the physical structure of the molecule.
”
Here is where we are not in agreement exactly. I think you’re missing a little tidbit.
The BB spectrum curve at a temperature is also the boltzman distributions of energy states. What one has with something like the Hitran database information is the absorption coefficient for a given line under the defined circumstances used. This is not directly dependent on temperature for the absorption. However, this is the liklihood of a molecular interaction at the particular wavelength (or wavelengths given the line width). Multiplying the liklihood of the number of molecules in a given state (the BB curve) by the liklihood of a molecule undergoing an absorption transition gives us our emission spectrum, which depends both on the nature of the materials and upon the temperature of those molecules. After all, if there are no molecules in the higher energy excited states there can’t be any emissions from them. This combination gives us the emission spectrum for those molecules and it is temperature dependent even though the absorption portion of this concept is almost independent of temperature. Net result is that a gas blocking a bb emitter at the same temperature will result line emission rates = line absorption rates and the gas will be essentially invisible. It also can’t happen for llong because of energy balance considerations.