Dr Roy Spencer has done an interesting experiment using two containers with water above which one has a silver foil which partly obscures the container from the cold sky.
This shows that water exposed to a higher radiant temperature is warmer. However, reading the comments I found some people arguing that this didn’t prove IR could heat water.
The controversy
For those who do not know, the controversy arises because IR is absorbed in the top few um of the water which evaporates off and/or because some people don’t believe in IR. This is all part of the argument that “back radiation” either doesn’t exist or cannot heat the sea. I usually keep away from the arguments, because back radiation exists. But this concept comes from greenhouse warming models which I describe as “noddy science”. They are just bad physics, not because of back radiation but for other reasons. So I have a lot of sympathy for those who don’t like these “backradiation” models of how greenhouse gases work and try to find fault with them.
Also, I don’t think the argument about heating water is completely daft – but I wish those who argue above water heating spent more time doing practical experiments and less time arguing the theory.
The Story
As I drank my whisky last night I thought “This is easy to prove”
I filled a tray ~1cm deep with cold water (at 18.6C using IR thermometer – which is possibly a few degrees higher than I expect, but perhaps it was sitting in a pipe). I put it under the grill which I then turned on as per this exhaustively researched experimental set up:
Pan of water under a grill
I set the timer to 5 minutes and then after five recorded the surface temperature without moving it – so when still irradiated. The IR thermometer read 63C. I then removed it – which inevitably stirred it, the IR thermometer read around 50C, I then gave it a good shiggle to ensure it was mixed up and it still read around 50C.
The 63C was at a low angle with IR still present and probably raised by reflected IR – but it might be interesting to repeat it with deeper water to see if there was noticeable surface warming. But the key measurement of bulk temperature had definitely increased by a good 30C.
For interest I measured the various surfaces inside the grill and they varied from 30 to 60C. The 30C was at the far end which had much less radiant heat, but was in much the same air temperatures. So, this is the worst case scenario of “air heating”. Air heating cannot explain the increase in the water.
The 60C was the surface closest to the tray – so it was being heated, but only to a very similar temperature as the water.This shows that IR was heating both the side surface and water in a similar way. And clearly there was no significant temperature gradient that could explain the rise in temperature of the tray from the surrounding surface.
I then touched the grill base and confirmed it was much cooler than the heated water (so no conductive transfer from the base – probably the reverse).
And whilst I did not measure it directly, the hotter air leaving the grill remained pretty cool and certainly not high enough to heat the water this quickly. (It would be like trying to make toast on a central heating radiator!)
Heating was not from visible!
Finally, to avoid some comments on visible heating, the grill was so dark I had to use a very long expose (~10seconds). The element was actually much darker than the picture suggests.
This tells me that the intensity of the visible was less than if I had left the tray exposed to the kitchen light.
Having worked with LED lights, I can be very confident the light level was much lower than a 20ma LED. This means the available visible heating was much less than 20mw and probably less than 1mw. So the visible was around 1millionth that of the IR and if anyone even suggests the heating was visible red, …
Conclusion
IR certainly heats water from above: and if the effect is just at the surface, then thermal conductivity rapidly transfers that heat into the bulk of the water.
It might be worth trying this with deeper water. But I can probably calculate it quicker (see: Caterpillar III) than the ~20min experiment would take with deeper water ~5cm deep.
Well, I notice a few things here.
One, you are not from around here. My clue is all that talk of “C” rather than “F”. 🙂 My second clue was the “Scottish” in your nom de plume. Nothing wrong with that as I am Scots-irish myself. (plus other things — we Americans are such a mongrel race)
Two — Your experiment is not using just the radiation that is supposedly coming back down due to CO2, so that is a little off. (but I like the idea none the less)
Three — Can the container itself be doing any of the heating via conduction? Or the air at all?
Four — Can the closed oven be effecting the convection and evaporation rates due to cutting of the air flow?
——–
I would suggest another experiment.
Take two identical “kiddy pools” which are very small swimming pools made of plastic and fill them with water. Let them sit outside all day and they should have identical temperatures. — if only Scotland were close to an ocean you could use sea water 🙂
Then after the sun goes down, aim an IR source at one of them. Let that source “heat” the water over night and check the temperature of both pools just before dawn. (drink whiskey as you sit and wait if you want)
What would be great would be to get an IR source that can be adjusted to where it radiates in one of three narrow bands of wavelengths, which are 2.7, 4.3 and 15 micrometers (µM). Even better would be if you had three “guns” so you could do all three at once — but we can’t have everything now can we?
Of course, if sources of these bands are not available or too expensive, then I guess one would have to hobble along with just a plain IR source that you could aim at one of the pools. Does such a device exist inexpensively?
Note that even in my experiment we are not using “back-radiation” which is said to allow cooler things to heat up warmer things in violation of the laws of thermodynamics.
Anyway, thanks for the post. I enjoyed it. I hope my little addition to your thinking is not taken as criticism, it was not intended as such.
~ Mark
Some warming would come from the air, but it really isn’t hot enough. The container will be heated slightly as will the mesh on which the grill sits, but iron is a surprisingly bad conductor (try putting an old piece of cutlery in a flame – you can hold the other end for several minutes even when the other end is glowing red).
On the IR source, when I started work it was on an IR body heat detector. The person I worked for had this idea we needed to use quartz to block out all but one part of the spectrum. If I remember right it was something like (???) 2-3um. Black plastics also have some interesting IR properties.
Thanks for the comment
As Mark says your experiment does not reflect what happens in Nature.
1. The water is “still”, oceans & seas are not.
2. Your oven restricts convection.
3. It also has no winds.
3. If you put your hand under the grill I am sure that you will feel “heat”, with “back Radiation” you do not, in fact it cools items, just look up Solar Stills used as Night time Refrigerators.
Just give it up. It’s obvious to anyone with any sort of background and understanding in this that you can heat water from above as you have demonstrated (with an experiment that has some flaws/limitations but gets the general stuff correct). And yes, that’s what it comes down to…heating from above. The IR wavelength under discussion does only penetrate a few microns, but that’s orders of magnitude more than the penetration depth of warm gas molecules bouncing off the top surface, so their argument means no heating from above at all…absurd.
I ran a conclusive experiment on this–used a CO2 laser to heat the water from above. Worked just fine…same IR wavelength of interest and everything. They won’t accept it…say it’s different but won’t specify how. I’ve covered at least half a dozen mechanisms covering a wide range of spatial scales and they still don’t get it. Never will. If I have the time/energy I’ll cover it one more time tonight at Roy’s, but I don’t expect them to get it.
Just for the record, the same crowd ignores the CO2 band at ~4.25 um. Water has little absorbance here, about 1 M-1 cm-1. At that molar absorptivity, the 4.25-um photons travel ~240 microns before 95% of them are absorbed by the water. This is much deeper than their proposed “surface layer”, but I’m sure they’ll just handwave it away too.
-Scott
Whereas your heating element may emit Long wave IR, it will emit far more Shortwave IR light.
Shortwave IR will go thru about a meter or two of clear water and does have some heating the top cm or so.
If you want a greater intensity of longwave IR, Have element first heat a chunk metal which will give a more uniform heat. Or small surface area of element ensures one get less long wave vs shortwave IR- if more uniformly heat over larger area, you still will get SW IR, but at least you increase the longwave IR.
Or people [some people] can see Near IR as right next to visible light. Si Near is:
0.75–1.4 µm. So can see ,75 light particularly if intense enough [or everything else is dark]
And though Near IR is shortwave, the catagory of IR is Shortwave IR: 1.4-3 µm
see:
https://en.wikipedia.org/wiki/Infrared
Now if look at Blackbody graph:
https://www.google.com/search?q=blackbody+spectrum+of+100+C&biw=1143&bih=709&tbm=isch&imgil=rP9bAf20K8QgjM%253A%253BzRJ-XrVtMHU3VM%253Bhttp%25253A%25252F%25252Fwww.scitec.uk.com%25252Firsources%25252Fir-50&source=iu&pf=m&fir=rP9bAf20K8QgjM%253A%252CzRJ-XrVtMHU3VM%252C_&dpr=0.9&usg=__5MEiDbv6TOK3kwt-u93i-iaGbAk%3D&ved=0CDYQyjc&ei=6oiUVbeqBZTsoASLgqzwCQ#imgrc=rP9bAf20K8QgjM%3A&usg=__5MEiDbv6TOK3kwt-u93i-iaGbAk%3D
Or let’s try this:
http://www.scitec.uk.com/irsources/ir-50
[near bottom of page]
It has blackbody for 25 C and one for 250 C
The 25 one peaks at 10 µm
And longwave IR is according to wiki: 8–15 µm
But the 25 C also radiated at shorter wavelength than 8 µm
And then the 250 C peaks at about point time the other 25 C ends, but spectrum
extends almost into Near IR, but has large amount of emitted energy in the Shortwave
IR range.
One also see the 250 blackbody emit far more energy at 10 µm, then 25 C blackbody emits.
And I believe how IR sensor works is by detecting spectrum that can only belong to object of a certain temperature and how much. Or frequency and intensity is a fingerprint of temperature.
And another aspect is warmed water rises
“As I drank my whisky last night I thought “This is easy to prove”
Glenfiddich I suppose! Does improve clarity.
Your conclusions are correct Any matter can absorb any wavelength for which it has absorptivity/emissivity greater than zero, and a radiance less than the emitter mass, at that wavelength! However, if your heating element were at a lower temperature than the water the water surface would radiate the “proper” EMR flux to that colder heating element. EMR flux can be only unidirectional at each frequency and in each direction. I did visit Spencer’s blog on this topic. I am amazed at the incompetence of Dr. Roy! He obviously has no education in electromagnetic field theory. EMR is never heat!
A good try but I think you do need a few more different heat source inputs to sustain your model. Intriguing is the concept that global warming is the only event and that cause is solely AGW. What if the opposite is true and that global cooling takes place.
The suggestion of a solar activity pause for the Middle Ages cooling is rather interesting. But the other cause could be a pause in submarine volcanic actiivity which tends to be cyclic on some scales. A pause in submarine vocanic activity would result in far less energy entereing into the oceans and seriouly disrupt current flow.
Recently while flying across the Pacific to the USA and just to the south of Pago Pago on the new Dreamliner with its much improved back of the seat nav display I picked up a thought while the new plane was bouncing around in what appeared to be CAT. The pilot chappy went hunting smoother air but no success. It occurred to me, having had some knowledge of sea floor volcanic activity in the area that, if there was no ‘weather influence’ on the turbulence then what was its cause. While flying over bush fires in Australia a few years back I experienced a similar choppy ride at about 8,500m a little below that of the cruising altitude of the Dreamliner. Seeing that we were about to pass over the volcanic arc and then onto being above a deep ocean trench it occurred to me that the turbulence should cease on the cross over. Sure enough it did. One tick for geologists in the climate stakes.
On the way back a more northerly route was followed and not as much CAT except over seamounts. I have experienced a similar reaction on an A380 while going across the Indian Ocean mid ocean ridge on the way to the 1820 celebrations. My thought is that submarine volcanic activity does contribute a significant heat source along with many greenhouse gases such as CO2 and CH4 not to mention abundant SO2. So the tantalising question is what is the influence of submarine volcanic activity in the climate debate bearing in mind that there is very little known about sea floor activity of the type i have just described.
Enjoy ypur blog
Of course you feel back radiation. Go out on a cloudless night and feel how cold the sky is. The Romans used just the phenomenon to produce ice in the desert. They dug huge pits and put water in the bottom. These pits were opened at night and then covered during the day with insulation.
But this is not possible when there is cloud. This shows that clouds produce backradiation which you don’t need a thermometer to know is there.
As for all the rest, they are irrelevant because the experiment was set up to test if IR radiation can warm water from above and it unequivocally proves that water heats when subject to IR radiation from above.
Thanks Scott. Real scientists can’t just dismiss an alternative interpretation of the facts without data, so I did want to check.
Of course the whole thing is really quite absurd, because when I put a pan on the stove, it only heats on the very surface, and if I used the same argument I would be able to say “therefore it cannot heat the bulk of the pan”.
The simple fact is that all heating to an object occurs at the surface!!
Interesting. I’ve been exploring what I call the “leaky atmosphere hypothesis” which is that the atmosphere “leaks away” by around 30% over 100,000 years and causes ice-ages [check previous leak rates!!] .. sorry totally lost my train of thought as I’ve just realised I can test the hypothesis.
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