Empirical Refutation of Back-radiation
Looking up at a platform holding a lit candle and ice on a pedestal, a typical thermal imaging camera shows that the ice is appreciably warmer than the clear blue background sky. Assuming a 0.95 emissivity, ice at 0°C would be radiating about 299.88 watts per square meter. This means that a typical thermal imaging camera is capable of detecting 300 W/m² of thermal radiation.
Now examine this record from an ARM (Atmospheric Radiation Measurement) station.
The blue profile and scale refer to “downwelling longwave irradiance,” which is thought mainly due to thermal emissions from greenhouse gases in the atmosphere.
Although the worldwide downwelling average is adjudged to be 333 W/m², the ARM chart for Oklahoma shows a diurnal swing between about 380 and 445 W/m², well within the range of a thermal camera. But this whole-sky irradiance is detected by curious devices called pyrgeometers.
If you look up ‘pyrgeometer calibration’ on the internet you will find information like this, from a 2005 government report:
Pyrgeometer Calibrations: Important Considerations
- There is currently no accepted international measurement reference for longwave irradiance.
- Temperature-controlled blackbodies are central to the calibration and operation of longwave radiation measuring instruments.
- Pyrgeometer measurements and blackbody calibration standards provide no information about uncertainty of the absolute value of atmospheric longwave radiation measurements (Philipona et al., 2001)….
- Pyrgeometer blackbody calibration differences are greatest during clear-sky conditions (high levels of net radiation) and least during cloudy sky periods (low levels of net radiation)
And this: “pyrgeometers are calibrated during the nighttime only, because no consensus reference has yet been established for the daytime longwave irradiance.”
The most recent reference I’ve found is in the Journal of Atmospheric and Solar-Terrestrial Physics, July 2015. The header cites “Lack of a daytime reference for longwave radiometers, i.e. pyrgeometers.”
Yet these devices are the backbone of back-radiation estimates.
We’ve seen above that a typical thermal imaging camera is able to detect at least 300 W/m² of radiation, i.e., below the 333 back-radiation average. But what are its limits? Here I borrow a daytime thermal image posted by Anthony Watts.
The reticle on the sky fairly close to the horizon can only report that the temperature is somewhere below minus 20°C. More sensitive equipment, though, is able to identify a clear sky’s temperature as minus 50° to minus 60°C.
Thermographers call this phenomenon “cold diffuse celestial radiation,” and it’s much the same both day and night. Given the low radiating power of gases, I would imagine that this temperature is due to ice crystals and other frigid particles in the air.
Conclusion: There is no reason to assume that the magnitude of back-radiation from greenhouse gases reported by pyrgeometers is valid. Observing the rich blackness of a clear sky in a thermal imaging camera refutes it.
A pdf of this article is located at:
http://tech-know-group.com/essays/Empirical_Refutation_Back-radiation.pdf
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jerry krause
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Hi Alan,
You conclude: “Observing the rich blackness of a clear sky in a thermal imaging camera refutes it.”
I ask: What in a clear sky is there to image?
The thermal imaging camera is not a pyrgeometer. So why do you expect a thermal imaging camera to function as a pyreometer? However, now I contradict myself as I ask you to carefully study (compare) your visible image with your infrared image. Focus on the flame of the candle. It seems I can see the IR image of air that is being locally heated by the hot flame as I cannot see an image of the hot flame. From both images I must conclude there is a slight breeze from right to left.
I do not question what you quoted about the pyrgeometer. For this is the conventional wisdom about the use of their measurement of the radiation flux being used to measure temperature via the Stefan-Boltzmann radiation law which clearly is only valid for radiation from a surface of which the atmosphere has none unless it is of the “ice crystals and other frigid particles in the air”. And this condensed matter (cloud droplets and condensation nuclei) does not necessary have to be frigid in the lower atmosphere.
The reason the conventional wisdom is what it is seems to be no one (except myself) has attempted to use the data of the SURFRAD and USCRN projects to field test the ability of downward facing pyrgeometers to measure the ‘ground’ surface temperature.
I am working on Part 2 of Part 1(https://principia-scientific.com/a-natural-laboratory-and-an-eastern-south-dakota-blizzard-part-1/). As Part 1 ended with a problem which needs resolution. Just as you need to explain how the upward facing pyrgeometers seem so responsive to rapid changes of ‘something’ incident upon them. (The ARM figure)
Have a good day, Jerry
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tom0mason
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Well said Alan,
Indeed these things have been looked at by others (see https://tallbloke.wordpress.com/2013/04/26/pyrgeometers-untangled/ ), they look at real instruments and say —
So, with over 80% of whatever is coming from “the sky” is in fact coming from only a few hundred meters above the ground a measurement by pyrgeometers tells us little about what is truly happening.
(It is so much cooler at or above cloud level compared to sea-level at the same latitude, etc.)
Also noted is some commentator’s on ‘Tallbloke’ indicate that climate models, when utilizing such averaged pyrgeometers figures, assume calculated S-B of the air/ground with an average emissivity. IMO this is just plain wrong.
Also of interest is http://claesjohnson.blogspot.co.uk/search/label/pyrgeometer ,and the ensuing discussion between Claes Johnson and the teacher Gavin Cawley.
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Alan Siddons
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Thanks for the background info,Tom0mason. Here’s a recent YouTube of professor Johnson’s that touches on pyrgeometers, too.
Don’t miss where he calls ‘em Ghost Detectors!
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Photoncounter
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Treating the sky as a blackbody – emissivity =1.00 – is certainly valid, if Earth had no atmosphere! That would suck for the observer, naturally but be great for the IR instrument.
The microbolometer in the FLIR camera is sensitive betweern 8 and 12 microns, in that waveband there is absorption by water vapor, little else. Pointing at a horizon on a clear day and using a high contrast color palette will yield different layers in the atmosphere. I’ve observed dry lines, moisture differences, cloud formation. However, without a surface to measure, any temperature measurements would be invalid, just for info. That particular camera could be calibrated to -40 but is generally calibrated to -20 C. Deep space is much colder.
A pyrgeometer is an integrating device, broad band and as pointed out incapable of being calibrated. I consulted with a firm monitoring the growth of cotton plants and they set up both an IR camera and a pyrgeometer. The reflected radiation measured by the two instruments were vastly different, nearly an order of magnitude off. After several weeks the pyrgeometer was shut off, all data was taken with a FLIR camera, verifiable and reliable.
I believe the pyrgeometer has the capability of measuring insolation – yes, energy flows from the Sun to the ground. However, downwelling radiation from high in our atmosphere where it is very cold to the warm surface of Earth violates the Second Law of Thermodynamics and like much of “climate science” is pure fantasy and wishful thinking.
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Alan Siddons
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I believe you’ve lodged an arrow into the energy budget’s Achilles Heel there, Photoncounter. If some “thing” whose emissivity is unknown is radiating 333 W/m², then… it is radiating 333 W/m². There’s no way around that fact. Imagine this “thing” is a low-emissivity greenhouse gas, however, whose emissivity is whatever, say 0.01. Then its temperature would have to be 602°C at least. Because a body whose temperature and emissivity are 602°C and 0.01 respectively will radiate 333 W/m².
Even assuming a 0.5 emissivity for a gas radiating 333 W/m², its temperature would have to be 56°C. I don’t recall any weather balloon recording such a temperature. So it cannot even have a 0.5 emissivity. Regarding that gas as a blackbody, though, brings its temperature down to 4°, easily detectable to a typical thermal imaging camera, being well within an 8 to 12 micron range.
With a 333 W/m² emission…
If it’s a low-emissivity gas, its temperature has to be impossibly high.
If it’s a blackbody, a thermal camera will see it anyway.
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jerry krause
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Hi Alan and others,
I concluded my comment with: ” Just as you need to explain how the upward facing pyrgeometers seem so responsive to rapid changes of ‘something’ incident upon them.” (The ARM figure)
The issue is what is the pyrgeometer responding to if it is not downingwelling IR radiation?
Each of you propose why the pyrgeometer cannot be measuring what that those who use it as an instrument claim it does but you have to explain the abrupt changes in its values that can be commonly observed unless the sky appears cloudless.
Have a good day, Jerry
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jerry krause
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Hi Alan,
“With a 333 W/m² emission…
If it’s a low-emissivity gas, its temperature has to be impossibly high.
If it’s a blackbody, a thermal camera will see it anyway.”
In your IR image of the candle I do not see the flame but I see the image of a larger (than that of the flame in the visible light image) volume of the air, a gas. Is the temperature of this gas impossibly high because of the heat of the flame or is this volume of gas a blackbody? The problem is the atmosphere does not have a surface which has only one temperature.
I have to draw to your attention that there is only natural earth surface which is atomistically smooth as the surface of plate glass; it is a liquid water surface. So what? you may ask. Rough surfaces have a greater surface area than their cross-sectional area. Can a glass surface have an emissivity greater than 1 if it molded with ridges or bumps so its actual surface is 1.2 times its cross-sectional area? My answer is NO. But might it have an emissivity of 1 when that of the smooth plate glass is a little less than 1? My answer is possibly or even probably. For this seems to explain how the downward facing pyrgeometers seem to measure the actual temperatures of a great variety of natural land surfaces which are rough and maybe not quite the actual temperatures of water surfaces. But there is this term–skin temperature. It seems that observed skin temperatures can change during the daytime almost as rapidly as the ‘output’ of upward facing pyrgeometers can because of the influence of scattered clouds.
Previously I wrote: Just as you need to explain how the upward facing pyrgeometers seem so responsive to rapid changes of ‘something’ incident upon them. (The ARM figure)
You seem not have done this; or did I miss it?
Have a good day, Jerry
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