Guest Post by Willis Eschenbach
I stumbled across a lovely article about the Saharan silver ant over at phys.org. These ants have special hairs that reflect strongly in the visual and radiate strongly in the infrared. They show a photo of the ant hairs under a couple different amounts of magnification:
Figure 1. Photograph from the phys.org article on the Saharan silver ants and their hair.
The article says:
Saharan silver ants (Cataglyphis bombycina) forage in the Saharan Desert in the full midday sun when surface temperatures reach up to 70°C (158°F), and they must keep their body temperature below their critical thermal maximum of 53.6°C (128.48°F) most of the time. In their wide-ranging foraging journeys, the ants search for corpses of insects and other arthropods that have succumbed to the thermally harsh desert conditions, which they are able to endure more successfully. Being most active during the hottest moment of the day also allows these ants to avoid predatory desert lizards. Researchers have long wondered how these tiny insects (about 10 mm, or 3/8″ long) can survive under such thermally extreme and stressful conditions.
Using electron microscopy and ion beam milling, Yu’s group discovered that the ants are covered on the top and sides of their bodies with a coating of uniquely shaped hairs with triangular cross-sections that keep them cool in two ways. These hairs are highly reflective under the visible and near-infrared light, i.e., in the region of maximal solar radiation (the ants run at a speed of up to 0.7 meters per second and look like droplets of mercury on the desert surface). The hairs are also highly emissive in the mid-infrared portion of the electromagnetic spectrum, where they serve as an antireflection layer that enhances the ants’ ability to offload excess heat via thermal radiation, which is emitted from the hot body of the ants to the cold sky. This passive cooling effect works under the full sun whenever the insects are exposed to the clear sky.
They describe how the hairs “keep [the ants] cool in two ways”—by reflecting the visible light, and by strongly emitting in the thermal infrared.
Curiously, however, nowhere do they mention the importance of a third cooling method that I noticed as soon as I looked at their photograph—the shape of the hairs ensures that more energy is radiated upwards than is radiated downwards. I had never considered that such a thing might be possible. The silver ants have a layer of hairs above their skin which selectively radiate more thermal energy away from the skin than towards the skin. Amazing.
The hairs can do this because, as shown in the right half of Figure 1 and as described in their caption to Figure 1,
a) the hairs have a roughly triangular shape in cross-section and
b) the flat side of the triangular cross-section of the hairs is towards the skin and
c) the two upper sides of the hair are “corrugated”, increasing the surface area facing skywards.
The net result of all of these acting together is to minimize the surface area of the side of the hair facing the skin, and to maximize the surface area of the sides facing the sky. Energy will be radiated from the hair surfaces at some rate per square unit of surface area (e.g. watts/square metre). So the larger the proportion of the hairs’ surface area facing the sky, the greater the proportion of energy radiated skywards versus back towards the ant.
How large is the imbalance in radiation likely to be? Well, the triangular cross-section of the hairs in the picture are about equilateral (three sides the same length). This would mean twice the area pointing skywards as is pointing towards the ant’s skin.
However, there would still be some loss back to the ant’s skin from some portion of the radiation from the tilted upper surfaces of the hairs. Some of that sideways/downwards radiation would be absorbed by the adjacent hairs, however. And some of that back-radiation would be offset by the increased skyward-facing surface area resulting from the corrugation of the upper surfaces of the hairs.
So overall those lesser effects might cancel out in whole or in part, and thus it seems like the layer of ant hairs will emit something like up to twice as much radiation out towards the sky as it does towards the ant’s skin. As is often the case, nature shows the way … what an ingenious cooling method.
And what, you might ask, do Saharan silver ants have to do with climate science?
Well, looking at the cross-sections of the hairs making up the layer shown in the right half of Figure 1, I was reminded of the shape of a cross-section through a layer of tropical cumulus clouds. In particular, I realized that:
a) tropical cumulus clouds have a roughly triangular shape in cross-section and
b) the flat side of the roughly triangular cross-section of the clouds is towards the surface and
c) the upper sides of the clouds are “corrugated”, increasing the surface area facing skywards.
Just sayin’ … it’s something I wouldn’t have guessed was possible, that an absorptive atmospheric layer of clouds could radiate perhaps up to twice as much thermal radiation upwards as it radiates downwards.
I do so enjoy climate science, there are so many amazing things for me to learn about.
w.
PS: My usual request—if you disagree with someone, please quote their exact words that you disagree with. That way, we can all understand exactly what you object to.
Thanks Willis , my wife and I have often wondered about that , the flat bottom of T-storms or rising cumulus. Your observations are terrific and to us is way more important!! Why ? your thoughts are LOGICAL. Thanks you made our day! We are observers for the Can Gov and so always look for info, this is great stuff as is your tropical clouds theory you have proposed the last few weeks, as we say LOGICAL! And keep it up! You are very clear and understandable ( hopefully even to ” climate scientists” ).
“Curiously, however, nowhere do they mention the importance of a third cooling method that I noticed as soon as I looked at their photograph—the shape of the hairs ensures that more energy is radiated upwards than is radiated downwards. “
It actually isn’t true. As a matter of geometry, corrugation doesn’t help. The surface has more area to radiate from, but the view factor drops. The extra radiation impinges on surfaces rather than escaping to space.
It’s not clear to me that the high infrared emissivity is unusual. Many materials are pretty close to 1. It seems the main thing going for them is the SW reflectivity.
I wondered the same; however, these kind of questions are often trick questions from the mathematical point of view. It could be, and probably is, important for some reason of what shape the silvery hairs are. Exactly how, escapes me.
It’s a result of Lambert’s Cosine Law. If you are looking from outside, you see a component of the radiation proportional to the area of your view field that it occupies. Corrugations don’t change that, unless they increase the area subtended by the object. That’s why, if you look at a white hot body of uniform temperature, you don’t see any surface features.
The total radiative output is the sum of what such observers see. If they can’t see corrugations, then the radiation is the same with or without.
The ants could increase the radiation by having an outer shell of greater area, at the same temperature. But then they would be living in a “steel greenhouse”.
Nick, stop thinking professionally, please. Let us assume that we’re considering isosceles triangles, close enough to the image and simple. And let us formally note your unstated assumption that we’re dealing with a Lambertian surface, just to avoid confusion. Then certainly if I view such a single triangle from any angle around its circumference I will see a single value for radiative power. That is, watts per meter squared, observed will be the same everywhere.
But Willis’ point was not about changing the ratio of watts per meter squared, it was solely about doubling the meters squared that the surface radiates from.
I’ll grant you the possibility that you could, perhaps, carry your argument about a perfectly flat and perfectly corrugated sheet with respect to the total radiated flux. But even then we get nowhere if you don’t acknowledge that a silver ant isn’t perfectly flat and clouds are not perfectly corrugated.
Maybe like stealth aircraft surface angles.
Radiating fins to disperse excess heat on a spacecraft would have no meaning, if being long and thin did not increase the surface area from which to radiate. Just saying, more surface area, more energy at a surface, more radiation in all directions upward makes sense, so the corrugation works.
Nick Stokes June 22, 2015 at 4:43 am
“If you are looking from outside, you see a component of the radiation proportional to the area of your view field that it occupies. Corrugations don’t change that, unless they increase the area subtended by the object.”
But the important view is from inside the hair. From inside the radiation is more away from the ant than down toward the ant corrugation increases surface area _and_ has the same effect as a triangular surface. An outside single viewpoint is immaterial precisely because the radiation of heat from the ant is in multiple directions.
“Maybe like stealth aircraft surface angles.”
oh boy.
Let’s start with the F-117.
The F-117 has facets for one reason that most folks dont know. At the time the only codes for estimating reflections were codes developed from an obscure Russian paper. Using that code we could predict the reflection and backscatter from flat surfaces only. So to build a plane where we could make predictions that we could test in chambers and in scale feild tests the design had to be of flat plates. Manufacturing curces was also a bear.
The angles are designed to reflect returns away from an attacker coming at the front of the plane. The biggest airborne threat is a head on attacker so the lines and angles reflect returns away. There are still huge returns if you are looking at such a design from a differnt angle.
The break through in this approach happened at Northrop where we solved the problem for certain forms of curves ( like an Ogive ). Plus there were manufacturing breakthroughs in making curved surfaces.
What Nick says is correct.
“The biggest airborne threat is a head on attacker”
Sorry, but no. The biggest airborne threat is from behind. Compare the number of frontal hits with hits “up the tailpipe”.
Jquip wrote: “Nick, stop thinking professionally, please. Let us assume that we’re considering isosceles triangles, close enough to the image and simple. And let us formally note your unstated assumption that we’re dealing with a Lambertian surface, just to avoid confusion. Then certainly if I view such a single triangle from any angle around its circumference I will see a single value for radiative power. That is, watts per meter squared, observed will be the same everywhere. But Willis’ point was not about changing the ratio of watts per meter squared, it was solely about doubling the meters squared that the surface radiates from.”
Doubling the surface area doesn’t help at all, since one surface of the the hair points back towards the ant’s body, returning just as much radiation to the ant in that direction as it radiates away from the ant in the other direction. As you correctly point out, it doesn’t make any different whether one triangular side is facing the ant’s body or two sides and one vertex face the ants body, the flux in all directions will be the same.
Furthermore, unless these ants are – like Willis and Higley7 lost in outer space with radiating fins – any surface from which they emit LWR is a surface that will also absorb LWR (emissivity equals absorptivity). That includes DLR from the atmosphere and OLR from the surface of the desert.
This time, at least, Nicky is right and Willis wrong about the basic physics. The shape of the hair is irrelevant, as is whether a triangular hair has its vertex pointing at the sky, the ground, away from the body or towards the body. Evolution might have supplemented low absorptivity for SWR by arranged for a low LWR emissivity/absorptivity surfaces facing the hot ground and high emissivity/absorptivity surfaces facing the sky.
Frank June 22, 2015 at 11:05 pm
Thanks, Frank, but before you declare winners I’d be interested in your comments on my illustration of the question below …
Regards,
w.
As long as we’re going to focus on the ant itself — chitin is not Lambertian, full stop. So no, the physics aren’t wrong Just Because. But focusing on the construction of the ant itself is completely meaningless. Willis can be as barking mad and Not Even Wrong as he likes on the analogy that led to the point of this mental excursion. To wit:
The bulk of his post was simply color commentary laying out his through process that led to the hypothesis about the clouds themselves. With respect to which he stated the only meaningful thing that needs to be stated here:
Assuming we have one lambertian ‘cloud’ that is nicely isosceles and lonely, hanging by it’s little old self in the middle of a big sky — this is exactly and precisely what basic geometry requires. We needn’t even focus on Lambertian or non-Lambertian anything. His entire notion is predicated as:
Presuming that our lonesome cloud is opaque, Lambertian, and has it’s very own temperature it radiates at, then this is precisely the case. Right up until some smart bean wants to tell me how Lambert’s law permits an observer to observe the radiation emitting off the face not facing him this will remain the case. Of historically curious humor, Euclid’s 5th proposition — the one about the interior angles of an isosceles triangle — is fondly known as the Bridge of Asses.
Now with respect to a corrugation, where there is an end to end sheet of isosceles configuration, I won’t even dare say what will or won’t be the case with respect to total emitted flux when considering the entire surface area. I will state regardless that the observed flux will be equal in all observed angles as we would expect — subject to the constraint that we’re not dealing with the obvious temperature gradient that would occur in real materials from peak to trough versus the magic instantaneity of black bodies upon which Lambert’s math is based.
And while that latter is a very intersting notion — and horribly relevant to this site as it’s an issue of ‘back radiation’ — it is not something I feel inclined to don a gimp suit and grab a slide rule over.
@Steven Mosher
Head on from the perspective of the radar is exactly what I’m saying…From the suns perspective.
Some years ago Japanese researchers developed nano-etched surfaces that allow photons to diffract over adjacent sides rather than be absorbed. These hairs may, or may not, act in a similar fashion.
With this method emissivities of more than 2 have been claimed
This is correct, of course. But clouds are modeled as Lambertian. And while there are discrepancies from prediction they are considered to be a result of optical thickness of the clouds. This is my best understanding on the subject, but it is terribly old; so take it for what it’s worth.
Ahem … not exactly? Corrugation wouldn’t help if one were receiving radiation from a point or near point source. e.g. Corrugating solar cell surfaces probably wouldn’t make much difference in cell efficiency. My cocktail napkin here says it also won’t help if the corrugated elements are tightly packed. As you point out, with tight packing, the view factor will decrease by the same percentage as the “radiating surface area” increases. and will presumably exactly offset the gains from greater radiating surface. But if there is significant distance between the radiating elements, the “view factor” will increase while the radiating surface area stays constant, resulting in some gain in radiation efficiency. I doubt the effect is of much use for the ants which seem to have fairly densely packed, layered hair. But it might be significant for Willis’ cumulus clouds?
Nick, you’re looking at this exclusively as a radiation process. Actually, this kind of extended surface is a kind of “fin”, often employed in living creatures for cooling by enhancing the transferring internal heat to the air via radiation, convection and conduction:
https://en.wikipedia.org/wiki/Fin_%28extended_surface%29
For example, it has been hypothesized that Stegosaurus’ fin was used to transfer heat from its massive body.
The longer the fin the better is the heat transfer, but these ants would be rather clumsy sporting longer fins. So the extra length, in effect, is distributed along the axis provided additional surface area shielding as well. Would be interesting to apply calculus of variations, given conductivity and total heat vs mobility requirements to generate the the optimal geometry for this animal.
Why do you think that the ant hairs are Lambertian surfaces?#/media/File:SEM_image_of_a_Peacock_wing,_slant_view_4.JPG)
Me, I don’t know much physics. But, if the surfaces of the corrugations have highly asymmetric properties—highly emissive at right angles to the surface and progressively less emissive and more reflective as the obliqueness of the angle of incidence increases, then it seems to the corrugated surface would radiate more strongly (at the relevant frequency) than would a flat surface. Think of the limiting case–highly emissive normal to the surface, perfectly reflective at all other angles.
It almost has to be the case that the surface becomes more reflective as the angle of incidence becomes closer to the grazing angle.
10 million years of evolution can do a lot to optimize optical properties of organisms. I am pretty confident that butterfly wings are not Lambertian surfaces. This image
shows periodic structures in a bird’s wing that have dimensions of about 500 nm. Wouldn’t a few mm of such a wing have highly directional properties in the IR?
Bob
More triangles in rows. What a coinkidink!
Nick Stokes what you do not appreciate (and that applies to all who do not understand heat transfer) is that there is also convection and evaporation. Increased surface area increases heat loss by convection. The so-called radiator on a car or at the back of a fridge loses heat mainly by convection and not radiation. From a surface at about 50C natural convection and radiation are about equal. Forced convection ( when there is a wind can cause convection to be much greater than radiation. A wind also helps evaporation.
With more surface area you can pass heat to cooler air. But that is in short supply here. That is the difference from a car radiator. The ant’s problem is heat coming from the environment, not the heat it generates. Increasing exposure to the environment brings in more heat than it loses.
I would expect the air temp a meter above the sun-exposed surface to be a bit cooler than the surface. The ants, however, live in the air right next to the surface, where I would expect the air temp to be nearly the temp of the surface. I recall when I was young, I was fascinated by mirages of water puddles on the hot surface of the highway ahead, and how they would disappear as we drove closer.
If the triangular corrugated geometry of the hairs is irrelevant to radiative cooling and of little use in convective cooling, perhaps the reason for the shape is mainly structural? Or does a thick triangular hair conduct body heat to the radiating surface better than a thin flat one?
One other thought: During previous discussions with certain “doubters” of the so-called “greenhouse effect”, WUWT commenters have noted that a thermocouple immersed in a hot gas stream will register a lower temperature than the actual temp of the gas, due to radiative heat loss. The silver ant would seem to be a living analog of the thermocouple.
http://facstaff.cbu.edu/~jdavila/Heat%20Transfer%206th/Chapter%2001/sm1-086f.pdf
I think you are simply missing the fact there is one downward surface but two upward surfaces. It has nothing to do with what any particular viewer sees based on a viewer’s angle–that’s a red herring. It’s that there are more upwards facing viewing angles all with the same radiative flux than there are downwards facing viewing angles.
Nick Stokes wrote: “The surface has more area to radiate from, but the view factor drops.”
I am pretty sure Nick is correct. If Willis is right, the outer surface cross section looks like a triangle wave. Now draw an imaginary surface that just touches all the peaks of the triangles. It seems to me that it would violate the Second Law of Thermodynamics to have the upward radiative flux through the imaginary surface exceed the blackbody flux for at the temperature of the hairs.
But the shape may help with convective heat transfer, as suggested by johanus.
I can confirm what Nick is writing from my own experience in optics. Lambertian emitters work the same no matter what the surface roughness is. All that matters to the observer is how much surface area is in the projected direction of the observer.
Thanks Jeff
The point though is with this arrangement, the surface area remains fairly flat from any viewing angle above the ant, moving light away in the critical wavelengths.
From the vantage point of the ant the view angle is much smaller as the flat of the hair is at or near 90 degrees while the solar energy is reflected through about 240 degrees to miss the ant’s body from above. Really if the material has the proper molecular structure the reflectance can be optimized to be very wavelength specific as well. It might be fun to play with such a material.
Jeff, Mosh, Nick, I’ve provided an illustrated example below. Your comments are welcome.
As to the question of corrugation, if we are talking about black bodies you are all correct … but we’re not talking about blackbodies. We’re talking bodies with non-zero reflection.
What is at question is whether the radiation from an object is the same in all directions, or whether it is directional in nature. Let me give two examples to show what I mean.
Consider first a blackbody with a hemispherical recess in it. Let’s insert an omnidirectional LED every 5 mm or so all over the blackbody including the hemispherical recess to simulate the emission of thermal radiation …
Would such a setup make a directional light? Other things being equal, not in the slightest. As you all point out, in such a situation all that matters is how much surface area is in the projected cross-section in the direction of the observer.
Now, consider what happens if we replace the blackbody with a piece of silver, same hemispherical recess. Is there now a non-uniform, directional light? Of course, because the light inside the hemispherical recess is no longer absorbed as it was by the blackbody. Instead, it merely reflects around until it escapes. This makes the light directional, despite each of the LEDs being omnidirectional.
The same thing is true for thermal radiation provided that the reflectivity of the substance is non-zero. And as I showed below, for some substances the reflection of thermal radiation is definitely non-zero.
Anyhow, your comments on my drawing at the end of the thread would be appreciated.
w.
Corrugated surfaces do improve conduction to air, though. I expect the ants are using air for moderating their temperature as well as reflection and radiation.
Nick Stokes, June 22, 2015 at 1:15 am
Then why do we work so hard to increase the radiative surface area of heat sinks such as cool the cpu? Or your car’s radiator for that matter?
It seems to me your argument describes absorption from a point source, where radiation is proportional to the surface area with a near 180° view. The two are orthogonal.
“Then why do we work so hard to increase the radiative surface area of heat sinks such as cool the cpu?”<
Because we can. The air is the heat sink.
But for the ant, it's the problem, not the solution. The ant wants to stay cooler than the air. The only place cooler than the ant is the sky (away from the sun). So the strategy is to block as much incoming as possible (silver) and radiate upward.
But for that, surface corrugations don't make any difference..
The corrugation acts like an absorber which converts the absorbed radiation into a sort of jamming device on other frequencies of incoming EM.
So Nick, doesn’t that also give them more area to absorb downwelling or Back Radiation??
snicker…
[Willis Eschenbach ]
It is of my opinion that it is the “corrugation”, rather than the increased surface area, that permits more thermal energy to radiate away from the skin.
Thermal (heat) energy will migrate across a smooth surface …. but if it encounters a projection or “outward” bend in/on said surface …. it will more easily radiate (per se ‘jump off’) into the atmosphere (space).
OH my, my, …. in that no one responded to my above comment I began to wonder iffen they just thought it was utterly silly ….. and was just being nice by not criticizing it …… so I figured I had better do some “checking” just to see how silly it was.
And I found the following which confirms part of my statement about …. “thermal (heat) energy migrating across a smooth surface”, ….. to wit:
But that did nothing to support the 2nd part of my statement whereby I claimed that ….. “if it (migrating heat) encounters a projection or “outward” bend in/on said surface …. it will more easily radiate (per se ‘jump off’) into the atmosphere (space)” …… so I kept on a looking ….. and found the following
Which, …. on a macroscopic scale, ….. kinda, sorta supports and/or confirms the 2nd part of my statement.
Whenever I read something like this Shakespeare’s lines spoken by Hamlet always come to mind.
As true now as in the 16th Century
“(the ants run at a speed of up to 0.7 meters per second and look like droplets of mercury on the desert surface)”
These must surely be the fastest ants in the world – perhaps not surprising is they are treading on ground surfaces of up to 158 C. Rather reminiscent of the guest in a Canadian hotel in winter who complained about the peculiar noises from the steam pipes at night. Hotel manager sat up with guest next night in the dark, and they heard the peculiar sounds “pitter, patter, phef, phef, pitter, patter, phef, phef”, so switched on the lights, and saw a mouse running along the pipes and stopping every second or so to blow on his feet to cool them off. (Enough of that!)
But notice that humans are similarly biologically organized. Stand out in the sun and you will absorb energy in the ultra violet, visible and near infra-red bands, but will radiate in the far infra-red, at a black body temperature of 98.4 Fahrenheit.
BTW, have you ever seen an unconstrained droplet of mercury about 10 mm long and 1 mm wide?
Willis for President
I once saw footage of a scorpion , frozen in a block of ice, that was then put in a very hot oven. The ice melted away and the scorpion re-animated. Apart from having to do a dance to try to keep it’s feet off the hot surface, the scorpion survived undamaged. It’s truly amazing what extremes nature can cope with.
Another means of coping:
http://www.luerzersarchive.net/en/magazine/commercial-detail/25765.html
I try to be a rational person but I think the scorpion is actually proof of the existence of Satan rather than anything natural.
(If quoted or questioned I will claim that was just a joke!)
I disagree on several points
* you make the hypothesis that ants somehow manage to keep the hair in order, “the flat side of the triangular cross-section of the hairs is towards the skin”. Actually the 50 µm image, and even the 10 µm image when you look where hairs are not cut, seem to hint at the hypothesis that they DON’T. Hairs twist.
* hair seems (and for sure, should be) multi layered. This would make the bottom far from flat, and contradict your argument. It would, however, surely be a more efficient arrangement to prevent over-heating.
* about your clouds analogy : ant hairs are small enough to keep the same temperature on all three side, this is obviously NOT true for clouds.
I am still fascinated by these ants and still don’t understand how they can stay colder than the very air in which they move. I suspect that their speed is the key, because of some venturi effect that affect temperature of the air next to them. Pretty much like airplanes can experience freezing even in above 0°C air. I am not sure, though. Some computing required.
You’d probably have to include the effects of air viscosity too. At these sizes it may be dominant and kill any venturi effect with the result that the ant carries around its own little air bubble. Dunno though, some calculation required.
you’re right. I forgot “boundary layer”. Moreover, hair configuration has much effect on it.
Looks like a pretty hard problem.
That’s why it’s interesting.
Don’t mistake the ‘Aha!’ process of analogy for the necessity of the hypothesis it provoked. It’s color commentary. Likewise, it’s also the case that chitin — and assumptively the ‘hairs’ are made of such — is not Lambertian. Not a little not-such, but a whole lot.
But yes, the ‘twisting’ makes for an interesting statistical exercise for the masochistic.
paqyfelyc,
“don’t understand how they can stay colder than the very air in which they move.”
I don’t think that is the case. They stay cooler than the surface temperature, that is, the temperature of the sand they are running on.
Willis,
From having been flown in a sailplane once up to the base of a cloud on the top of a thermal – inside actually.
The base isn’t flat, it is dome shaped, so we were inside the cloud but in clear air.
I’ll leave it to you to decide if this shape concentrates something going down.
Dome pointing down, that is roughly the centre of the cloud at the lowest altitude, or dome upwards; that is roughly the centre of the base of the cloud is at the greatest altitude for the base?
@AnotherIan
> The base isn’t flat, it is dome shaped.
Hmm, the base of a cumulus cloud certainly is not perfectly flat, but tends to be less dome-shaped (i.e. flatter) than the top of the cloud.
http://www.birdmen.co.za/blog/wp-content/uploads/2011/12/dom-26-pv-4.jpg
I believe that is because there are two different mechanisms involved in the formation of these visible layers.
Cumulus clouds are manifestations of convection. Moist warm air rises and cooled by adiabatic lifting, causing the relative humidity to rise because cool air can hold less moisture than warmer air.
The height at which the relative humidity reaches 100% is exactly when the bottom of the cloud becomes visible. So the air just below the cloud tends to have the same absolute humidity as the bottom of the cloud, but is slightly warmer so remains invisible until it reaches a higher level. This level, called the lifting condensation level (LCL), is a function of the adiabatic lapse rate, which is determined mainly by thermodynamics (Boyle’s Law etc), which evidently is a more uniform process than convection itself.
But air at high altitudes is dryer than air at the surface. So the tops of cumulus clouds are manifestations of the vertical extent of convected moist air. The air above the cloud top is cooler than the bottom but much dryer and has less moisture to condense. This tends to be more random.
So the visible boundaries of clouds are due to the variation in relative humidity (bottom) and absolute humidity (top). The former has less variance and so looks flatter, the latter has higher variance so looks more fluffy.
Would the most latent heat be released at the bottom of the cloud, at the point where gas turns to liquid (albeit in a state of very small cloud-particles)?
Sometimes the edge of the top of the cloud is very crisp, clear, and expanding upwards at an impressive rate, and I have the sense that is where the gas is turning to liquid, so that is where the latent heat would be released.
However it does seem that, in the middle of the cloud, less latent head would be released.
(Someone else will have to do the math for me, for I did most of my cloud-studying during math classes.)
You’ve got it backwards. You can see, literally, where the condensation begins: at the bottom of the cloud. The boundary at the top is where moist rising air meets dry upper air, where condensation stops.
Yes, latent heat is released by this condensation, but that further warms the air, enhancing convection, so carrying this whole process even higher. Even more heat is released when raindrops freeze into hail. Until it hits the stratosphere which, by definition, is where all vertical convection stops. Then it spreads out horizontally, creating those peculiar anvil-shaped thunder heads.
Clouds are complex entities, mixtures of air, water and other aerosols, having both cooling and warming behavior. Clouds can absorb terrestrial radiation, in effect re-radiation part of that back to warm the Earth. But clouds also reflect a lot of sunlight back into space. And remember that all of that released latent heat represents energy expended at the surface by evaporation forming, in effect, a heat pump transferring surface energy to higher altitudes where it can more easily escape to space.
So overall a cooling effect.
Why aren’t we using this design to keep houses, cars etc cool in hot climates?
Material woven with similar shaped fibres could be used on rooftops etc to keep buildings cooler and reducing energy requirements. Hats, clothing etc could also benefit.
Cost.
Yes, cost, now, but give clever people a chance. I’m sure everyone is familiar with the creation of velcro.
I find the cross-fertilization of ideas by people of different disciplines discissing their fields to be extremely interesting. That seems to be how lithotripsy was developed. If would be inspiring if one of you WUWT readers had knowledge in material science and was thinking, wait, I know how to create that type surface cheaply. We would all be witnessing the birth of a new industry.
Well, heat-reflecting windows do this: they are double-pane windows with a heat-reflective metal-oxide layer on the outer layer, and a gas layer trapped between the two panes. As far as cost, the liners are available at Walmart, so not very expensive. It would be interesting to know if the hairs are hollow. Even if they cast a shadow they’d still need to be heat-reflectors.
Always interesting, Willis.
Ants can read as well. In times long gone by, someone wrote in capital letters, the word ADAPTATION into the sand of the dunes. The ants followed suit and they still live and prosper.
The hot priests of the carbon dioxide church cannot even read and they dont understand the meaning of that word….
Thanks, Willis, fascinating!
Unless I missed it I didn’t see a discussion of what is on the underside protecting the ant from the158°F surface temperature. In the spirit of adaptation, suggest a line of clothes based on this principle to protect us from CAGW. Always enjoy your Posts.
pyrotect
fire suits
Apparently, it’s just naked chiton on the (shaded) underside of the ant’s exoskeleton. If the setae (hairs) on the Sahara silver ants were being used primarily for radiative cooling, I’d think these hairs would be all over the ant’s body, including its ventre, or belly.
For a living creature, cooling off just means that the heat is moving away from the body.
I know this from hanging my legs and arms off the side of my bed during very hot weather like we are having now, so that a greater amount of skin is exposed to the air. It is for the same reason that ladies tie up their long hair in the summertime so that it is not hanging on the back of the neck.
Great stuff! No linear thinking in this exploration of nature. Another feature visible in the left hand photo (50microns view) is short ‘pins’ sticking out at right angles to the hairs or is this something in photography. Possibly these are spacers(?) or ties (?) that keep the hairs separated with distance from the body. The engineer in me sees some practical design applications here.
Very interesting stuff, but the scientists conducting this study likely will get no funding, for they forgot to splice some connection to Global Warming into the final paragraph.
They need to add that the range of these heat-loving, lizard-loathing ants “might have perhaps” expanded north 100 yards over the past three years.
3 other adaptations are at work:
This is a very ‘furry’ ant with an air gap between flat fur bottom and body. This would make for really calm air against body, providing excellent insulation.
Very straight triangular outer surfaces are like fins assisting with laminar flow at speed. Not sure what this means for conduction but laminar flow is way different than turbulent flow isn’t it?
Finally high speed means minimal ground contact; better pain management, and less conduction through their little feet. It might be that speed enables fewer feet touching.
Willis, do you have any high frame rate video so we can check this out?
“…hairs above their skin”
Willis, ants don’t have “skin.”
They have a chitin exoskeleton.
In regard to hear exchange the two function differently.
Just sayin.
Bert: Say “chitin exoskeleton” to the average “man-in-the-street” and you will get a look that says “WTF are you talkin’ about?” If you say “chitin exoskeleton” is “kinda-like” a skin, the look will say “Oh… OK”. Unless you happen to talk someone who has stayed awake in biology class or has had some invertebrate paleo. who will comprehend exactly what you are saying.
It’s true that the basic rigid element of insects and other arthropods is the exoskeleton which isn’t sheathed in a skin. But hairlike structures used for defense or sensing the environment seem to be found in a number of arthropods. Best known would be the defensive uricating hairs on spiders and caterpillars.
You should probably look up the definition of skin. All animals have a skin, made of various materials of course. Humans have a dermis, ants have an exoskeleton.
I would accept “cuticle”, or even “integument” as general terms for the outer covering of all animals, but I don’t believe “skin” includes the exoskeleton of arthropods.
Interesting that chitin is translucent, perhaps more so in these ants, so perhaps a prismatic or reflective effect is occurring to the visible light in those triangular hairs.
If anyone is aware of “skin” els ware applied to arthropods please give the reference.
Skin on an ant?
Think of it like- chicken lips.
Just like the skin of a jet.
Or the skin on a rice pudding
cool had never seen these before.
some google images of them
http://tinyurl.com/p6rr5x4
used tinyurl due to length of url it leads to a google search of silver ants and shows the images section
nothign nefarious
The hairs appear to be hollow. Open the image in a new window and expand a few times. What appears to be the tube wall is visible in the bottom portion of the triangle on several of the cut hairs. The tubes seem to have a very thin wall. Combined with the small size of perhaps 1 – 2 um. they must be very good insulators. Like a fireman in a burning building, the protective outer suit serves well for a while, but then it is time to get out and cool off.
Yes, the hairs definitely appear to be hollow. I suppose Buckminster Fuller would call them tetrahedrons – very elongated ones, at that – a geometrical form that fascinated Bucky.
Insects inhale O₂ and exhale CO₂ through spiracles or holes in the exoskeleton, thereby feeding cells directly through a system of tracheal tubes, which are then also used to route the exhaust CO₂ back out through the spiracles.
I wonder what gas is in the hairs?
I also wonder why the ant’s ventral surface (underside) is not hairy. Apparently the little creature’s hairs are there primarily as direct protection from the sun, rather than as an efficient way to shed heat, else we’d see the hairs all over the wee critter.
Were the ants sampled dead or alive as their condition at the time of the photos could change everything? How do you get a speedster ant to stand still for the microscopic camera?
Another inane thought, how many ants were sampled? Could be that a freak ant was looked at or that there are variations within the species or that some simply comb their hair differently. Do I need to note “humor” for the statisticians?
Wouldn’t that make it a Fabulous Furry Freak Ant?
“And some of that back radiation would be…..” What back radiation? Unless you mean radiation from the ant’s back! It is absorbed radiation of the hair, not back radiation as described in CO2 sending back heat to increase the energy in the Earths surface. If that was applicable to the ant, the heat leaving the ant would have to be absorbed by a hair in the sky and redirected back to the ant to boil it alive.
Instead the shape of the hair inhibits the speed of heat transfer from the sun, via the hair to the ant, by transferring a majority of what the hair absorbs to the air and not the ant.
The reflective parts should be self explanatory.
This got me thinking about the1974 movie “Phase IV”. A great underrated SF film. Check it out if you get a chance.
P.S. anybody got video links to these fast little critters?
And you got me thinking about Them!
Here is a nice one.
Cheers.
Thank you!
What amazes me is the reported maximum speed – 0.7 m/s – that’s about 10x faster than any ant I’ve ever seen!
But watching the video above, I totally believe it. Those little buggers can move.
“Largely due to the extreme high temperatures of their habitat, but also due to the threat of predators, the ants are active outside their nest for only about ten minutes per day.”
“They have longer legs than other ants. This keeps their body away from the hot sand,[2] and when traveling at full speed, they use only four of their six legs. This quadrupedal gait is achieved by raising the front pair of legs.[4]”
“A few scouts keep watch and alert the colony when the ant lizards take shelter in their burrows. Then the whole colony, hundreds of ants, leave to search for food. They must hurry before the temperature reaches 53 degrees Celsius (128 degrees Fahrenheit), a temperature capable of killing them.”
http://www.discovery.com/tv-shows/africa/videos/ants-in-silver-space-suits/
One incredible video, seems like something out of a Vin Diesel movie…
Noteworthy element of this video:
Only 3 or 4 guys doing all the work with the fly. ‘Hard to to tell with the editing, but in any case, when the surviving 3 ants arrive with fly, many more ants down in the shadows pull the dewinged fly into the chow hall. ‘Must be some kind of ant bureaucracy in that ant colony, where a few ants do most of the hard and dangerous work, and the rest of them do something else, like coddle eggs, or consort with the queen.
It’s also very interesting that these Silver ants do not have the magic hairs on the underside of their exoskeletons. I would expect the surface of the sand to be much warmer than the ant’s bodies, but apparently the greater danger is from above, and not from IR beaming from the burning sand at their feet.
Unavailable in Canada. Balzac.
According to natureisanythingbutsimple.wordpress.com
“This abundance of ants around the world – which outnumber us 1.5 million to one – makes them a dominant force in nature.”
Think of the picnics! It’s worse than we thought!
Some years ago I saw a detailed analysis of the effect of ocean wave shape at different wind speeds and the resulting variation in thermal radiative output. Unfortunately, I cannot find the paper now, but I do remember that they found a significant difference.
Most of the paper was detailed consideration of things like the “view factors” that Nick mentions, but they concluded that the added surface area of waves did provide a significant additional radiative output. The stat I remember is that this additional output was enough to lower the steady-state surface by 2C for 15 m/s wind, other things being equal.
This both supports Willis’ conjecture about the ants, and adds another cooling mechanism to Willis’ thunderstorm thermostat hypothesis.
Ocean waves are different from these hairs in one important respect. A rough ocean surface has segments that can see other segments of the same surface, and these exchange IR radiation. An individual triangular piece of hair has segments that cannot see one another. Look, if a triangular cross-section would radiate more strongly upward by virtue of shape alone, then these triangular pieces could propel themselves in an isothermal enclosure…perpetual motion of the second kind.
I don’t believe you’re correct. Take the limiting case of a single hair in an enclosure, said enclosure in free fall so there are no gravitational effects. In this case, there is no “skin” being shaded, so the radiation effect is the same for all surfaces. If you do a force diagram, you will find that the vectors from the two sides (for isosceles) partially cancel, and the force from the base cancels the residual.
Hawkins: you have misread what I wrote. I never spoke of shading at all. With regard to there being no vector result in radiation or force, you and I are saying the same thing.
It just makes me marvel at Gods creation.
@Willis
Willis, after digging some more on this phys.org paper that you are referencing I believe you are missing the main idea of these tiny hairs. Nanfang Yu, the lead author of the phys.org article, does give credit to the unique geometry of these hairs, in the sense that their width is on the order of one micron. This width lies between the wavelength of the peak solar “short-wave” radiation (0.5 micron) and the peak “long-wave” terrestrial radiation (~12 microns).
http://www.scopii.com/news-collection/2015/06/18/using-saharan-silver-ants-as-an-inspiration-for-cooling-surface-coatings/
So its size is bigger than optical light and thus can reflect short-wave radiation from the Sun back to where it came from. Otherwise it would tend to absorb short-waves and convert it to heat (like the Earth’s surface does). Furthermore the triangular shape is important because of the “flat” surfaces exposing two flat optical surfaces which enhance this reflective action.
But ground around the ant does absorb this solar radiation and tries heats up the environment (including the ant). But the hairs are smaller than the upwelling long-wave so are unable to reflect this heat back to the ant. This is good. It can’t reflect so it is acting like a ‘blackbody’ at these longwave frequencies and emitting 100% to space.
“But a blackbody would also absorb sunlight and heat up the ants again” one might object. But it doesn’t work that way here, because it’s only a black body at longwave, with 100% emittance and highly reflective (i.e. non-blackbody at short wavelengths).
So these ants are reaping the best benefits of both worlds: longwave and shortwave. I don’t think your “upward vs. downward” idea is any different in this broad spectrum sense.
In another article just published, Dr. Yu estimates this results in a 5C-10C cooling effect, which is the crucial effect needed for the ant’s survival in the Saharan desert.
http://www.newscientist.com/article/dn27748-silver-coat-lets-saharan-ants-withstand-scorching-desert-heat.html
So it’s a combination of longwave “microfins” radiating excess thermal heat, coupled the shielding effect of mirror-like shortwave optical reflectors, all wrapped up in a nice shiny-fuzzy ball.
The triangular cross-sectional shape is immaterial, as Nick Stokes pointed out early in this thread, what matters is the view factor so long as we are assuming same temperature to all surface segments. Another thing to consider is that the hairs form a mass with openings that allow visible light to enter; and, even though the individual hairs are reflective, with multiple reflections absorbs for visible light than measurements on an individual hair would suggest. Note the occasional dark areas between hairs. The figure of merit for this ant cooling system would be solar absorptivity (absorption coefficient integrated over the solar spectrum for (0.28 to 3 micrometers) divided by infrared emissivity integrated over 5 to 20 micrometers or so. Aluminum coated with white epoxy paint has a figure of merit of about one-sixth. Does the ant cooler do better than that?
Cripes. Not “for visible light” but “more visible light”
Final sentence in the article
As they had the ability to do so, I wish the authors would have stated the figure of merit for the surface. Their paper was about biomimicry in engineered materials after all.
Where is my tax credit for ant hairs?
For those who think this feature or that doesn’t work, I would suggest that many thousands of years of evolution guarantee that every single feature on this ant contributes to its success and what ‘doesn’t work’ might be just in your way of looking at the feature.
I don’t agree that any of its features are designed to enhance conduction to the external environment like what worked for dinosaurs. Quite the contrary! These little guys don’t want any of the external heat conducted to their (relatively) cold little bodies. It’s insulation all the way down and radiation all the way up, eh?
The thing I object to is anyone who can’t see that these ants scream engineering, and especially those who require that there can be no designer or you are mocked and banned. The alternative is beyond a miracle, it is that these amazingly effective and precise systems (and every one in all biology) happened by random, unguided, purposeless copying errors.
The “copying errors” may be random, but they are not unguided. They are guided very strongly:
– Errors that confer a survival advantage are reproduced and refined through repeated generations.
– Errors that don’t, die out.
Given 100 million years or so, I’m not surprised at all that an ant species could adapt to Saharan conditions.
Well, as a very lazy and most patient engineer, i found evolution a perfect engineering tool. I plant a single seed and, lo, behold : a living, growing tree result. Of course many time the result is pretty stupid (such like when a ant got 6 legs when it use only 4. Or when the lungs are connected to the stomach, when a separate entrance — and exit ! — for each would be so more convenient and so less dangerous ), and it takes very long time (billion of years), but the results DO work. Most of the time. Well, not most of the time, just some time, 1 out of 10 or 100, maybe ; but enough, anyway.
Not only the working results that work are pretty amazing, but the failures and the process itself are most funny.
So please, do not deny the existence of my favorite engineering tool. evolution.
Evolution is great at conserving vital features, not so good at creating them. Take, for instance, the separate blood circulation system of cetaceans, which is designed to cool the blood in the testes by running it through the fins. Otherwise, the temperatures under the blubber would be too high for the sperm.
Any cetacean born without such a circulatory system, or born with a defective one, would be quickly eliminated from the gene pool, since it could not reproduce.
However, since every part of the system must work or the organism cannot reproduce, a gradual, one part at a time change from a cetacean’s purported ancestors, cow like critters, doesn’t work. Either everything works together, or reproduction doesn’t work at all.
Same thing is true in many other biological features, which is why doctors as a group tend to be skeptical about neo-Darwinian notions that unguided processes created what they observe in biology.
These hairs don not seem to contain blood vessels, so the way heat gets in them is either a radiation or a thermal conduction (via the air; at this scale there should be no convection.) They are probably just a high quality thermal insulator.
This would break the second law of thermodynamics if it was true.
To see it, imagine for instance a container with uniform temperature and separated in two parts by a membrane consisting of this triangular hairs.
One flat side to one part of the container and two flat sides to the other half.
If this resulted in an energy transport from one side to another, you would have a conflict with the second law of thermodynamics.
You could then have a small power plant powered by the temperature difference between the two halves and get a perpetuum mobile.
Therefore it is unfortunately untrue
/Jan
Hmmm … although you may be right, I don’t think so … but I couldn’t say why. I’ll think about that.
w.
OK, I think I know where the problem is. I’m talking about increasing the rate of heat loss. Take as an example fins. Fins increase the heat loss from something that is warm.
Now, if you put a divider in a container and put fins on one side of the divider, it doesn’t “result in an energy transport from one side to another”. That would indeed be a violation of the second law.
But that doesn’t stop fins from being an effective way to increase the cooling rate.
Similarly, a membrane of triangular hairs could increase the cooling rate but like fins, it would not “result in an energy transport from one side to another”.
Your situation starts from equal energy on both sides of the membrane, that is to say equilibrium.
The ants’ situation involves needing to increase the rate of heat loss from an ongoing heat source (the ant itself plus absorbed downwelling shortwave and longwave radiation). As such, your analogy doesn’t hold.
w.
Well, which parts do you not agree in?
Firstly, if the geometry of the hair on the ants results in more radiation going outward than inward, it would be possible to create a membrane such that “out” was to one of the halves in the container, right?
Secondly, if the membrane emits more radiation to one side then to another it would
result in energy transport across the membrane
Thirdly, the energy transport would lead to temperature difference between the two halves
Then lastly, the temperature difference could be used to power a heat engine.
/Jan
I stated the same thing not far above this comment, Willis. If it were possible to produce a net flux in some direction by shape of the radiator alone, then you could have that shape propel itself in an isothermal environment–i.e. violate the second law.
Willis, as I commented earlier, cetaceans use fins to cool the blood going to their testes. Their other circulatory system does not send blood through the fins since it can run hotter without creating a problem.
Please explain in a little more detail why you think that an object must have equal emissivity on all sides? Consider a large disk of silver, polished on one side, covered with black paint on the other, and heated to 100 C. One would expect that the back side would be highly emissive, the polished, not so much. No second law problem here. Indeed, if the highly reflective side had equal emissivity to the black side, you could probably build some sort of perpetual motion machine. Just put two of these disks next to each other and with the smooth sides parallel. Watch the space between them fill up with photons. Extract energy from the photons. Voila.
All this posited different emissivity could do, if it does exist—a question on which I take no position—is slow the ant’s progress to thermal equilibrium. A half-shiny, half-black disk can be in equilibrium with the temperature inside a closed box. But, as it gets there it absorbs (radiates) more heat on the black side than on the silver side.
Radiometer?
Non-metallic things look metallic for 2 reasons – diffraction and total internal reflections. Since diffraction generally produces metallic colors (think butterflies), and since prisms are used in optical systems as reflectors, and since the “hairs” are triangular (like prisms), I think “total internal reflections” is the more likely explanation. As a result, the hairs act like mirrors simply reflecting the solar radiation.
About clouds… and observing real life.
It is continually repeated that the condensation of moist air to form what we see as clouds releases heat. In the thread above Johanus (who seems to know a lot about clouds) states: “Yes, latent heat is released by this condensation, but that further warms the air, enhancing convection, so carrying this whole process even higher. Even more heat is released when raindrops freeze into hail. ” So standing in that boundary air mass where humidity becomes visible cloud I ought to feel a warm, albeit wet, fuzzy feeling. Right!
Now I hike mountains in the Pacific NW, where there are lots of clouds forming all the time. I hike below them, in them and above them. I have spent many hours, days and even weeks moving up and down through those boundary layers. Every time it was cold, sometimes profoundly cold. Never once, ever, did I get a warm feeling from all that latent heat that was being released. On any given day I could tell you precisely how thick that boundary layer is (it is well-defined in most cases) with a transition zone of about 10-20 vertical meters, both top and bottom. I could also tell you how intense the process is – all based on the stunning temperature drop, not latent heat driven temperature rise.
Recently one of those “latent heat releasing” transition zones from humidity to visible cloud dropped the temperature over 10 degrees celcius with visible cloud formation sucking the heat out of the air and one small intrepid band of hikers. So where is all that latent heat? and what sucked up all the ambient heat?
Comments?
The basic physics of a latent heat release by condensation is not disputed. However, meteorological phenomena are rather complex (look for a reliable 100-hour weather forecast; let me know if you find it).
Back to clouds, moisture in the air starts condensing once the air temperature dives under a dew point. It will not heat the air back above dew point; it simply means that the air temperature in the cloud is slightly higher than if there were no condensation – let’s say 35 F instead of 33 F. Both are rather chilly. When you get out of the cloud into sunshine, it will feel (and be) much warmer.
So… above the cloud 19 degrees celcius (air temp) and as predicted low humidity. Below the cloud a consistent 25C, 80% humidity and rising as you approach the cloud forming layer. In the cloud forming layer we had air temperature of 8C…
And this is not an air mass moving in off the coast. It is a single cloud forming over a single mountain ridge. With sunny warm air all around it. With tons and tons of water condensing, where has the ambient heat gone? Where is the latent heat gone?
Les, a good question. One thing that I’d say is that in general the “surface” of the cloud is doing one of two things. It is either condensing or it is evaporating. Typically (but not always) it is condensing at the bottom surface and evaporating at the top surface. At other times it may be evaporating at the leading edge, like when clouds spill over a mountain pass and evaporate on the downwind slope.
Now, when you hear the word “evaporation” it brings to mind evaporative cooling, because that is what happens. It takes energy to evaporate water, and that energy has to come from somewhere, so the air cools in response.
Often there is a wind that blows across the top of some flat layer of clouds. Since the top layer is evaporating, that wind can be quite cold. We get it at our place from the wind that often blows over the top of the offshore marine fog layer. The fog evaporates as it comes onshore so it often doesn’t reach our house, but the cold wind continues to flow inland. It can leave a clear sunny day quite chilly.
The other reason that clouds feel so dang cold to us is that when the air is at the dew point water is constantly condensing on the surface of our faces, hands, and bodies. Of course it evaporates, often almost instantly, but it takes energy to evaporate it and in this case the energy comes at a cost to our body surface temperature.
Finally, as Curious George points out, we’re talking a difference in the rate at which the temperature drops. As mountaineers like you know, for every 100 metres of additional altitude, it will likely cool by about 1°C (or 5.5°F per 1,000 feet of altitude, for those living in the US. Or in Liberia.) Or if you prefer, it cools by about 10°C per vertical kilometer.
Now, that’s what’s called the “dry adiabatic lapse rate”, meaning that the water vapor is NOT condensing. If it is condensing, air temperature doesn’t drop by 1°C per hundred metres. Instead it only drops by about 0.55°C per 100 metres vertical (3°F per 1,000 feet). This is because the air is slightly warmed (0.45°C) by the heat of condensation.
So it’s not “warmer” inside a cloud, it is “less cold than a theoretical alternative” …
w.
Thanks for the explanation Willis… all my life I’ve had the 3K/1000 feet as a rule of thumb, but was unaware of the dry adiabatic lapse rate. That amount of difference is really impressive. Still, there must be some other processes like a huge venturi effect that kicks it off and maintains the cold dew-point temperature in the 100 -200 meter section just above the cloud-open air interface. Once past that section, going up, things begin to warm slowly until you break out into sunshine.
Les, one more effect .. at a bottom of a cloud you are in a convection zone. Not only there is no direct sunshine, but also a “wind chill effect” kicks in.
Willis, Les, and Curious
There is a third element at play in cloud formation that is being overlooked by most people when we fixate on temperature and dew point. That would be air pressure drop triggering condensation. Wind driven turbulence causes evanescent cloud formation/dissipation by creating standing wave pressure gradients. Convection creates turbulent pressure changes which spawn cloud formation. It is not just aerosol condensation nucleation that is at play.
This is illustrated during aircraft flight. Condensation forms in the low pressure region above wing surfaces.
https://youtu.be/dBjTnS-X8ik
Willis, thanks for the brain tease.
Dang … I wrote this, went off to sleep, and when I get back there’s 80 comments … that’ll teach me, gotta learn how to give that sleeping stuff up someday.
First, my thanks to all for a host of good comments. Next, my thanks to Nick Stokes for bringing up an interesting issue, although as is sometimes the case he confuses things a bit:
Nick Stokes June 22, 2015 at 1:15 am
There are two questions here that Nick is conflating.
First, does the triangular shape ensure that more radiation goes upwards than downwards?
Second, do the “corrugations” in the upper faces of the hairs increase radiative heat loss?
As to the first, I say definitely yes.
As to the second, I say I thought so when I went to sleep … now I’m not as sure. The question is whether in the real world both the thermal radiation of a planar surface drops of exactly as the cosine of the angle of view … and whether there is no dependence of thermal absorption on the angle of incidence.
Part of the issue relates to reflectivity in the thermal IR range. We don’t think of it often, but objects can reflect thermal radiation (longwave IR) quite well. There’s an outstanding discussion of reflection of thermal radiation here.
The reference says:
Here’s an example they give, showing an old brass plate.
In visible light it doesn’t reflect the man’s image at all … but in thermal infrared the reflection from the brass plate is quite clear.
it seems to me that as long as the reflectivity of the silver hairs is non-zero, the corrugations would result in an increase in the thermal radiative heat loss. Although as Nick says the field of view is reduced for corrugated ares, some of the impinging thermal radiation would reflect to the sky and thus would not be re-absorbed by the hair.
In addition, the above cited article shows that like light, the reflectivity of a surface for thermal radiation increases as the impinging angle goes from perpendicular to just grazing the surface. This would also increase the “focused” nature of the thermal radiation by reflecting low grazing-angle radiation away from the surface.
So my conclusion is that while Nick is right for perfect blackbodies, for real substances thermal radiation would be increased by the corrugations.
However, evolution tends to favor “dual use” solutions. In this case, the “corrugation” of the upper surface would definitely increase the heat loss by conduction from the surface to the atmosphere. This is the common concept of “fins” used to cool a variety of living and non-living things.
Anyhow, I strongly recommend that interested folks read the citation above. It is a very readable discussion of the issue of the reflection of thermal radiation.
w.
PS—Hey, I just thought of another reason that the corrugations would increase radiative heat loss. This is that some of the radiated thermal IR which otherwise would be re-absorbed by the hair itself will be absorbed by a GHG before the radiated IR makes it across the air space and hits the hair again …
Willis,
There is a thermodynamic constraint. We’re talking about the ants getting rid of their heat that they acquire from the environment. Any geometric modification that emits more heat exposes them to more incoming heat. You can’t win that way.
Imagine a black body in a closed room, all at uniform temperature. It gets radiant heat from the walls, according to its optical cross section. Roughness doesn’t change that. If roughness could increase its emissivity, then the object would remain cooler than its environment. You could run a heat engine on that.
Your reflection argument is a deviation from black body. But Kirchhoff says you can’t win that way either. Their capacity to deflect heat is balanced by reduced ability to emit it in the first place.
roughness doesn’t change emissivity? You sure.
look at http://www.engineeringtoolbox.com/radiation-heat-emissivity-aluminum-d_433.html
compare emissivity of highly polished aluminum (0.09) with that of roughly polished (0.18). Other sources give somewhat similar numbers.
There’s some general problem here. The hairs appear to be about 3 um wide. The relevant IR spectrum peaks at something like 8 or 9 um. Whatever is going on here is going to be a long ways from geometric optics. I don’t know enough about near-field optics to have much of an opinion regarding how absorbing the adjacent triangular surfaces will be.
It seems to me that the purpose of these hairs is to reflect the sunlight and to radiate and convect away the non-reflected energy from the sunlight.
Sure glad you folk didn’t design the radiator in my car and on behalf of the local rabbits I pass on their gratification that you weren’t responsible for the evolution of their ears.
Willis:
I just spent my lunch hour fiddling in Excel to test Nick’s claim, and my numerical “finite element” model completely verifies his assertion. For a Lambertian distribution of radiation (intensity proportional to the cosine of the angle from zenith of the surface), the increased surface area of a sloped surface in a “V”-shaped trough is exactly canceled out by the percentage of radiation captured at the edge of the distribution.
I verified this for angles of 10, 20, 30, 40, 50, and 60 degrees.
And of course, absorptivity equals emissivity, so you don’t get anything there.
One possibly interesting issue comes up. This analysis is all using geometric optics. But the spacing of the hairs is a few microns, right at the peak wavelength of longwave infrared. Could there be some interferometric diffraction grating effect here?
Thanks, Curt. I do admire a man who actually takes the time to run the numbers. The problem is the assumption of the Lambertian distribution. In the real world we have both reflection and a dependence of reflectivity on grazing angle. Try it again with a semi-reflective surface.
w.
I haven’t had a chance to run actual numbers, but I can confirm that if the surface emits more toward the zone near the zenith than a Lambertian distribution would, this corrugation would lead to increased radiative losses. If the surface emits more away from the zenith than Lambertian, this corrugation would lead to decreased radiative losses.
My engineering heat transfer text from the 1970s says that the emission from conductors is usually of the first type, and emissions from non-conductors is usually of the second type. I wonder what the properties of this biological material are.
Again, the issue that fascinates me is the size of these features and the possibility for effects like diffraction, which could overwhelm the “geometric optical” properties I talk about above.
If preferential reflectance/emission sorted by wavelength is possible using this geometry, you can be sure the ant has evolved to exploit it. Indeed, the hairs may constitute an “existence proof”!
Jan:
“This would break the second law of thermodynamics if it was true.”
I disagree. You are leaving out the energy source (incoming SWR). A complete picture could be 900 watts of SW per square meter in, 300 down of LWIR, 600 up of LWIR. No laws broken and complete energy balance.
Right?
What is the energy source in a silver ant? It the hair is to radiate heat, it must have a source of that heat.
The energy source is the SW hitting his little furry body. The hair isn’t necessarily their to expel body heat. It could be there solely to manage the radiation environment.
Paulatmisterbees,
I think you bring in a topic not discussed in Willis article.
Quote from Willy’s article:
I read this as the discussion is about long wave thermal radiation which originates from the temperature of the hairs, and not short wave reflection.
Based on this condition I state that it is not possible to have a layer/ membrane to radiate more in one direction than the other without breaking the second law.
It would of course be possible if we were discussing reflection.
/Jan
Hi Jan:
To make it simpler, take a 1 square meter, very thin silver plate at 58 C. One side is highly polished. One side is painted flat black. Does each side radiate the same flux? I say one side has very low emissivity and the other, very high. This would mean different flux, wouldn’t it?
I believe this is why the radiometer you buy to amuse your children (A.K.A. Crookes radiometer)spins in sunlight.
As to corrugations…
Perhaps they have to do with conduction. Straight fins would promote laminar flow which is not good for conduction. Corrugations introduce turbulence, as the little guy screams across the sand, which enhances mixing/conduction. Even with high albedo his ‘fur’ would still be mighty hot.
First question to answer is this… Has the fur evolved to reject external energy, emit internal energy, or some of both?
It’s interesting that time alone has solved the physical conundrum better than we can know. I conclude that it is better to be ‘in’ the physics than to be outside looking in, provided that you have an infinite amount of time to work out a solution.
I have a question re: condensation and latent heat….
Since the latent heat is the heat required to break the molecular bonds at the surface, isn’t it necessary for condensation to ‘replace’ that energy in the newly formed bonds of condensation? And, if that is true, should there even be an expectation that you would ‘feel’ anything sensible in a condensing cloud other than the cold it requires to make those bonds to begin with?
After watching the video of the little buggers running I believe the airflow through those hairs, as someone noted above, might, indeed, have significant cooling effects, as in Willis’ wind in the clouds example above, irrespective of any other cooling going on. These critters can move!
They seem to almost generate some lift. It looks like they go so fast their load of carrion floats off the sand.
Flying over the tropical forest of Brazil, I saw high rising clouds wide from each other. This means that there was plenty space for the corrugated clouds to radiate to all directions slightly upwards – to space.
“in the full midday sun when surface temperatures reach up to 70°C (158°F)”
Nope. Never once in human history has the temperature ever reached 70C.
58C is the world-wide record… set in Libya, 1922.
You are talking air temperature just above the surface. They are talking about the temperature of the solid surface itself, which on a sunny day can be substantially higher than the air temperature a meter or so up.
https://en.wikipedia.org/wiki/List_of_weather_records#Highest_temperatures_ever_recorded
If polar bear hairs evolved to conduct heat in and down, how hard could it be to make hairs that do the opposite? Heliophobic instead of heliophillic are mere ant-onyms.
Note also: I live at 5000′ at the foot of a 13000 ft mountain and spend a great deal of time in the mountains and can tell you the average 1 degree drop in temp per 1000′ of elevation is wrong more often than not. Many times it is warmer up higher (inversion) or no change. Beware of the ‘drowning in a river of average depth of three feet’ syndrome.
It appears their spacing is in the neighborhood of the wavelength of the IR. Due to this. could the corrugation (or even the hairs laying together, them selves) act like the flat corrugated magnifying lenses you see today? Or a more effective reflector at this frequency? And thus the “Silver” color.
Ants and insects DO NOT have “hair”, a defining characteristic of mammals.
The proper term is “setae” plural or “seta” singular, which are extensions of the exoskeleton.
BioBob
June 22, 2015 at 2:35 pm
Yes, strictly speaking, you are right.
Insect setae are often called hairs or chaetae. They are unicellular and formed by the outgrowth of a single epidermal cell (trichogen). They are generally hollow and project through a secondary or accessory (tormogen) cell as it develops. The setal membrane is not cuticularized and movement is possible. This serves to protect the body.
https://en.wikipedia.org/wiki/Seta
Interesting to see that the insect hairs, or setae if you prefer, are movable. I note that cats fluff their fur when it is cool, but during hot weather, their fur lies flat against the cat’s body, which does reduce the airspace between hairs, and it is the air, after all, which provides the insulation. ‘Not saying that’s what’s going on with the ants, but again, no ventral setae argues against a radiative function for these hollow, tetrahedonal bristles, and argues for simple insulation against the overhead sun.
tetrahedronal
Whatever they are called, they look and act like hairs. If their job is to reflect heat, it is interesting to consider that they move. Some solar buildings have louvers that adjust to accept or shade the building from the sun’s rays. They can be operated mechanically by a computer program to tune their angle relative to the sun as its orientation to the earth changes. I was curious about the ants’ circling movements as they forage. The moderator of the video above suggests it is a maneuver intended to orient them to the sun’s location as they move further and further from the protection of their underground nests. I wondered if such circling might also allow the ants to expel heat from the sun side of their bodies, a thermal burden that it is shown killing some of the foragers in a matter of minutes. Anyway, it’s a very curious maneuver for ants in a temperate climate. Leaf-cutter ants can be observed following their own scent trails – drag a foot through a column on the march and they follow the newly formed furrow of dirt even if it takes them ninety degrees away from the rest of the column. I always thought most ants used the same navigational system.
About 33 years ago I built a house in New Orleans with a steeply pitched galvanized roof over an air space shaped like an equilateral triangle with an insulated attic “floor” above the living space below. I remember being shocked that the air space under the roof remained just about outside air temperature on even the hottest sunniest days. From the outside it was apparent that the roof was reflecting most of the sunlight, at times projecting light strongly on nearby structures. Convection carried heat within the airspace upwards away from the living area below, released the heated air at the peak and drew outside air into bottom of the airspace, which then washed over the insulation. The metal roofing readily conducted heat from the warmer side to the cooler.
At the time I congratulated myself for being so smart, but I guess mother nature had beaten me by a few million years, at least.
That’s where the missing energy is! The ants are reflecting it back into space! But… oh, noooooooooo…. the ants are dying. I found one dead one on one day and four dead ones the day after. I fitted an exponential curve to the data and concluded that at this rate the ants will become extinct within a decade. Nature Paper is pending! It has been pre-approved by my mates who do the reviews. No probs!
This will cause a global catastrophe. Only one thing can save us now. I must get more funding for my research which will include 50 trips abroad each year to enjoy dinner and wine.. er, I mean, to present my results .. well look, don’t worry about all of that, just remember that if you don’t give me more money you are a hater.
You wouldn’t want to be called a nasty name, would you? No! I didn’t think so. So there you go. Give me the money!
Let me see if I can’t explain what I mean by example. Imagine a very small star hanging in space. It has a vertical pillar next to it which is three sided. A cross section of the situation looks like the left side of this figure:
Let me compare that to the star whose cross-section is shown in the right side of the picture, which has a vertical pillar next to it with two nearly flat and roughly parallel sides.
Now the question is, will the temperature of the pillars be different in the two cases? Assume that the pillars are the same temperature throughout the cross-section, such as if they are superconducting or very small in dimension.
I say yes, the columns will have different temperatures. Both are absorbing the exact same amount of energy from the star. But they have very different total surface areas. Absorbing the same amount but having different surface areas means that the equilibrium radiation per square metre must be less, and thus the temperature must be less.
However, note that this does NOT mean that if placed in an isothermal container it would force a thermal differential. It does NOT violate any laws of thermodynamics.
Despite that, the two columns end up at different temperatures … which in turn implies a different cooling rate for the two situations, a difference based entirely on the shape of the pillar.
Which is what I’ve been saying all along, but folks said was impossible. The radiation away from the star is different than the radiation back towards the star.
w.
Willis: Your ant hairs are not triangular columns in space. Although they reflect most SWR, the ants are surrounded on all sides by incoming LWR. Absorptivity = emissivity at LWR wavelengths. While they are “cooling” themselves by emitting LWR, they are “warming” themselves by absorbing DLR from the atmosphere and OLR from the ground.
One can maximize the AREA exposed to the weaker DLR by slanting surfaces facing the sky, but the radiative flux is given by Lamberts cosine law (area times cosine of the angle) and is constant no matter how much you slant the surface.
Thanks, Frank, but that’s changing the goalposts. What I was told was that the shape of the hairs didn’t matter at all in a purely radiative situation. Not one bit. It was solely a matter of the cross-sectional area, that was all that counted.
So now, if I understand it, you agree with me that the columns would take up different temperatures specifically because of their different shapes. And while you are right that there are other things happening with the ant, my point remains.
The shape of the columns/hairs affects the rate of radiational heat loss.
The shape affects the rate of heat loss by doing what folks told me upstream was impossible—the triangular shape in my figure just above has a larger radiating area facing away from the heat source than does the flat shape. As a result, it is able to lose heat faster by radiation alone than if it were a flat shape.
w.
Willis,
You can do some arithmetic on this. Suppose the triangle is equilateral. Then yes, it radiates 3/2 times the flat shape at the same temp – 3 sides instead of two.
But suppose there are 3 triangles in a row. Radiation is the same as the convex hull, and the perimeter is 7; 3+2+2. Flat would be 6=3+3. So the radiation is 7/6.
And suppose there were 100. Ratio is (100+2+99=201/200). Getting close to 1. If you are concerned only about upward radiation, the ratio is 101/100. Once you are talking about a surface material, it is virtually 1.
Willis – it’s complete OT here … but … given your recent look at UAH data I cannot help thinking that there’s another satellite dataset that could do with a bit of an exposé given that the normal mission eye candy is extremely sparse (and what has been released is seemingly almost wholly at odds with in particular- NASA models) It honestly looks like an opportunity for WUWT folk to get a jump on the herd….
I’ve looked and there are calibration issues – but – nitpicking absolute values cannot detract from what look like dynamic features and distributions that are new …
Willis: When I wrote at 12:11 am, I had missed the part of your reply near my original comment. Near your original reply, I wrote: “Doubling the surface area doesn’t help at all, since one surface of the the hair points back towards the ant’s body, returning just as much radiation to the ant in that direction as it radiates away from the ant in the other direction.”
Your figure with a star and a flat or triangular column isn’t relevant to this situation.
I also wrote: “Furthermore, unless these ants are – like Willis and Higley7 lost in outer space with radiating fins – any surface from which they emit LWR is a surface that will also absorb LWR (emissivity equals absorptivity). That includes DLR from the atmosphere and OLR from the surface of the desert.”
Your figure with a star and a flat or triangular column isn’t relevant to this situation either. In fact, you have illustrated a situation in outer space – the “unless” that qualified my statement. (Higley7’s comment discussed space.) Your illustration may be relevant to comments others have made, but not mine.
Before we can tell whether any change or modification will warm, cool or leave the temperature of an object unchanged, we need to consider all of the routes by which energy can enter and leave that object. The current discussion seems to be restricted to radiative transfer of energy. Question: If I cut a sphere into two hemispheres, their total surface area will increase by 50%. If I move them far apart compared with their size, will their increased surface area cause them to cool off more quickly? Answer: You don’t know, until you specify the surroundings. If the sphere and hemispheres had been in the freezer and were now in an ordinary room at 20 degC, the two hemispheres will WARM up faster than the whole sphere. In empty space, the two hemispheres will cool off faster than the whole sphere – unless they are already at the same temperature as or colder than the cosmic microwave background (2.7 degK).
The subject to this post – the goal post – was triangular hair on an ant in a desert. The ant radiates infrared proportional to the fourth power of its body temperature. The top half of the ant receives weaker DLR from the cooler sky above. The bottom half of the ant receives stronger OLR from the hotter ground. Whatever emissivity may be, it effects both incoming and outgoing fluxes equally. I don’t believe the orientation of triangular hair or a change to round hair will make any difference in THIS situation.
Consider a figure with two infinite planes at temperatures T1 and T3 with a triangular column in between at an equilibrium temperature T2. Point the vertex towards or away from the warmer plane (T3). Does T2 change? Now make the column circular. Actually, even a simple situation like this one may require a long calculation involving “view factors”.
http://webserver.dmt.upm.es/~isidoro/tc3/Radiation%20View%20factors.pdf
The view factors for “Patch to infinite plate” may make the problem easier.
Nick Stokes June 23, 2015 at 3:16 am
Nick, you are a piece of work. This is why discussing things with you is so unpleasant.
You never, ever admit that you were wrong.
In this case, you claimed over and over that geometry (the shape of the hairs) makes absolutely no difference. None.
Now, having noticed that you were 100% wrong, you’re trying to to get people to not notice your error by coming back to lecture me on the subtle differences based on the number of hairs … nice try.
I think I’ll wait to discuss this with you further until you actually admit that you were totally wrong, that what counts is NOT just the cross-sectional area. I don’t think you are capable of actually admitting you were wrong.
But I could be wrong, and I’d be happy for you to prove me so …
w.
Willis:
And that is absolutely correct when talking about the radiative power as would be observed by a point in space hanging out anywhere around the exterior of the enclosed volume or area. So long as the material is Lambertian, or Lambertian with respect to the wave lengths we’re interested in, this remains true. As you noted, of course, not all materials are Lambertian with respect to all wavelengths. And not noted is that emitters need not be Lambertian either; the sun is not for example. But keeping with simplicity we’ll simply call it Lambertian just for the envelope sketch you’re after.
Yep, obviously. This is a common bit of garble that arises in physics when everything is treated as a dimensionless point, even when it should not be. By sheer obviousness, if a body is radiating x photons per second per meter squared, then doubling the meters squared doubles the total x. That makes absolutely no difference to the observed radiative power of course.
And now that Nick has stepped up to the plate, let me finish some of his notes for you. His golfball example from StackExchange is valid, but not germane. The strict point we are interested in here is how many photons per second go which way with respect to a horizon line. How many go up and how many go down. This is not relevant for any sphere as it is perfectly rotationally symmetric with respect to any plane; and the putative triangle is not.
So assume we posit our nifty isosceles hanging in space above a surface, and let us only look at how many photons go which way with respect to a horizon line parallel to the base, one edge, of the triangle. For convenience we’ll simply state that each point on the circumference radiates 1 photon per second per nominal degree. As all sides are equal length, we’re only concerned about what fraction of any single 180 degree sweep per face cross that horizon line. As the base is parallel, then we just call it 180 photons. The other two sides are equal but opposite with respect to the horizon line. If the faces were perpendicular to the horizon line we’d get 90 up and 90 down. But as they as canted back 30 degrees from perpendicular, each will send 120 up and 60 down. (with respect to the base.) Your sums here are obvious as 300 down and 240 up.
So can a Lambertian emitter with respect to a horizon line show differential in what quantity of radiation goes where with respect to that horizon line. Obviously. Geometry matters when geometry matters. And so long as we’re talking about up and down radiation, geometry matters. You can offset some of the detriment that would occur with the ‘pointy bit up’ if we note that we don’t have a horizon line, but a triangle above a cylinder (cross sectionally) but I highly doubt you’ll find the elevation you need with clouds to make this go away. (The ant analogy being understood here as the pointy bit being the up direction.) But flip it upside down and you’re on track for interesting things.
Happy hunting. And a healthy kudos for Nick and everyone else that stepped up to the plate.
Jquip June 23, 2015 at 10:43 am Edit
Mmm … I pointed out not once but several times that I was NOT “talking about the radiative power as would be observed by a point in space”. I reiterated that instead of radiative power, I was talking about a flow situation involving a solid object interrupting the radiative heat loss from a warmed object.
As a result, I fail to understand what your statement has to do with anything under discussion.
I did much more than note that all materials are not Lambertian. I pointed out that some materials are specularly reflective in the frequencies of interest. This means that all bets are off regarding the uniform radiation that everyone (including you) assumes is happening, and that the idea of “Lambertian” goes out the window entirely. Does the thermal radiation coming off the brass plate in the right half of the picture below look Lambertian to you?
So no, we can’t just assume for simplicity that the surfaces are Lambertian, and I’ve said that several times as well. As long as the reflectivity is non-zero we can have unevenly distributed radiation coming off of a solid object, something that y’all keep saying doesn’t happen. You’re 100% right … but only in Lambertia, which near as I can tell is a country in BlackBodyLand.
Here on this planet nothing is truly Lambertian, everything is at least somewhat reflective even in thermal radiation, and from the looks of the photomicrographs, the ant hairs are likely to be reflective as well.
The difference between an imaginary horizon and the horizon of the actual earth when looking from 10 km up is a couple of degrees, from memory. Small enough to ignore in first cut analyses at any rate.
Thanks, jquip, for your clear post. I’d have more kudos for Nick if anyone at any time had ever heard him admit he was wrong. But I thanked him for raising the issue when he first raised it.
w.
Willis,
“In this case, you claimed over and over that geometry (the shape of the hairs) makes absolutely no difference. None.”
I think you should follow “My usual request—if you disagree with someone, please quote their exact words that you disagree with.”. I said, rather carefully:
” Corrugations don’t change that, unless they increase the area subtended by the object. “
You are increasing the area subtended by an object, from line segment to triangle. I am calculating the effect of that.
You said
“So overall those lesser effects might cancel out in whole or in part, and thus it seems like the layer of ant hairs will emit something like up to twice as much radiation out towards the sky as it does towards the ant’s skin. “
I am pointing out that the view factor issue is dominant, and the factor is nothing like twice. With n parallel hairs, it’s 1+1/n. The ants have a lot of hairs.
Nick Stokes June 23, 2015 at 12:24 pm
Glad to, Nick. Here was your first reply to the issue.
Nick Stokes June 22, 2015 at 1:15 am
Regards,
w.
Willis,
You wrote: “Despite that, the two columns end up at different temperatures … which in turn implies a different cooling rate for the two situations, a difference based entirely on the shape of the pillar.”
That is true for this diagram.
“Which is what I’ve been saying all along”
No, it isn’t. An ant does not have just one hair, so you also have to consider the interaction between the hairs. So change your diagram into two walls extending to infinity horizontally for the diagram oriented as shown. Both walls present a flat surface toward the star, but one presents a corrugated surface “upward” (away from the star). The two walls will be at the same temperature. Half the radiation emitted from the upper surface will hit another part of the upper surface and be re-adsorbed, so the net emission will be the same.
The issue is not the number of hairs an ant has. It is that people have been saying over and over that the shape of the hair didn’t matter for the radiation.
Now that I have shown that the shape DOES matter for the radiation, now you and others want to claim that it’s not the shape, it’s the number of hairs, or it’s the arrangement of the hairs, or something else.
Look, Mike, you and Mosh and Nick were wrong about whether the shape affects the radiation. It is NOT, as you and others claimed, merely a matter of the cross-sectional area perpendicular to the field of view.
Now, I’m willing to discuss this further. But not with folks who won’t admit that they were wrong. Here’s you, Jeff Id, and Mosh:
Mike M. June 22, 2015 at 8:00 am Edit
So you were 100% wrong, the shape is involved in radiative as well as convective heat transfer, and it would NOT violate the Second Law.
Here’s Jeff Id, with more of the same nonsense about how the shape makes absolutely no difference, just the projected surface area. And he was 100% wrong:
Jeff Id June 22, 2015 at 11:43 am
And here is the “me too” from Mosh, who also was 100% wrong ..
Steven Mosher June 22, 2015 at 12:52 pm
So I’m waiting for you guys to man up and admit you were wrong. Nick Stokes won’t do it, but I had hoped for better from you. You accused my claims of violating the Second Law … you gonna man up, or hide with Nick?
w.
Willis,
I will not admit to being wrong for the most fundamental of reasons: I AM RIGHT. And so is Nick, and Mosher, and Jeff. I wrote “If Willis is right, the outer surface cross section looks like a triangle wave. Now draw an imaginary surface that just touches all the peaks of the triangles.” Obviously talking about a collection of closely spaced objects, not a single hair. And when you wrote “The silver ants have a layer of hairs above their skin which selectively radiate more thermal energy away from the skin than towards the skin” you were pretty obviously not talking about a single hair.
It is one thing for us to be unable to convince you or for you to be unable to convince us. But for you to insult us because we won’t bow down before the Great and Mighty Willis is beneath you. Or should be.
Willis,
Yes, the triangle will radiate more effectively and stay cooler in the configuration you show. You have expanded the outline. The body is convex, and there is no view obstruction. But in the triangle case, some of the radiation goes back below the base line.
There is a discussion here that is on point.
The more precise version of my claim that corrugation doesn’t help is that a body of uniform temperature absorbs and emits in just the same way as its radiative profile. That is, the envelope of all straight lines in space that touch the surface. In 2D that is the convex hull; in 3D it is a bit more complicated. It’s what a CAT scan would make of a totally absorbing body.
The thing about 2LoT, which Jan also referred to, is that a complex surface is subject to the same sort of 2LoT restriction as Kirchhoff’s Law. If something improves its emissivity, then the absorptivity has to follow. But a black body already absorbs every ray that encounters its radiative profile, so the emissivity can’t improve.
Of course, if you do expand the profile, then you are likely to absorb more as well as emit. In your case, you have made it one way, and that is probably more applicable to clouds. A big isolated cloud will radiate more than a small one. But if you are looking at a whole bank of clouds, a bumpy surface doesn’t make a difference.
Here’s a bit of reasoning on the triangle. The total radiation is what observers can pick up. Say the triangle is equilateral. An observer vertically above, or up to 30° each side, sees an outline equal to the base, and could not tell the difference. But from 30 to 90°, he sees a larger profile – ie receives more radiation. Total radiation above the plane is greater.
But suppose there were three adjacent triangles. It’s only when the observer is so low in the sky that he sees the closest peak in line above the farthest base point that he can distinguish the triangles from the flattened version. So the radiation discrepancy is much less. And when it starts to look like a ripply surface, virtually none.
Like I said, Nick, I doubted greatly that you had the blanquillos to admit you were wrong. So far you’re proving me right.
Before, you were claiming that the geometric shape of the hairs made no difference at all. None. It was a matter of basic physics.
Now, you say it makes a difference, but that the difference depends on the surroundings … well, duh. Thanks for telling me what I already know.
I’m still waiting, however, for you to admit that you were 100% wrong about how the geometric shape was immaterial. All I’ve gotten so far from you is tap dancing. Impressive tap dancing to be sure … but no admission of error.
w.
Willis,
“Before, you were claiming that the geometric shape of the hairs made no difference at all. None.”
Again, please follow your request and quote my exact words.
Duh – OCO-2 data being sat on until Paris is done and dusted ?
Willis, if you have vertical angle of incidence of the impinging suns light (almost in the Sahara), from an equilateral triangular surface, reflection is very strong by bouncing horizontally from one hair surface to the adjacent hair surface and straight back up into a clear sky. That they are silver is a measure of the efficiency of this effect. I would say that this surely is by far the major (90%[?]) effect. It is the icing on the cake that the hairs also serve to reduce heat further and the source of nature’s detailed wonder that she experimented on improvements possibly over millions of years to get this model.
It makes me think that not only do we wonder about phenomena and incessantly inquire by experimentation to find out what makes things tick, but even nature doesn’t know what works without constant experimental adjustment until they get the model (almost) ‘right’. Humbling to think that’s how we ourselves got to the here and now. Moreover, nature does it with millions of different species and she’s still rejecting her work for better – will we ever reach a perfect ecology?
Willis:
You stated “As a result, it is able to lose heat faster by radiation alone than if it were a flat shape.”
Isn’t it more correct to say that the rate of energy lost is independent of shape (LW out will always equal radiation adsorbed)? What IS dependent is the temperature of the emitter. More radiating area for LW reduces T required to drive the flux
Paul, it seems like you’re talking about the rate of energy loss per square meter, and I’m talking about total energy loss … but it’s not clear.
w.
I am talking about total energy loss. In your example of equilateral triangles imagine 900 watts per square meter in from SW, then each surface radiates 300 watts per square meter of LW. If it was a flat profile it would be 450 watts per square meter of LW per surface. If it was a square it would be 225 of LW per surface. Total energy rate out isn’t any more or less regardless of surface configuration. Sum up all the surfaces and they have to total to 900. Surface rate out changes
Temperature of the hair is what changes dramatically. It goes down in proportion to number of surfaces doesn’t it? Temperature at any arbitrary area of surface will be only high enough to drive the flux out of that arbitrary area.
From the ant’s body perspective, a triangle generates the least ‘down welling LW’ flux to its exoskeleton. That’s the ant’s goal, right?
[assuming anything greater than triangle create new issues]
Frank: you said…
“The ant radiates infrared proportional to the fourth power of its body temperature.”
Body temperature is most likely not the ‘hair’ temperature. The body is only connected to each hair by its attachment point and it is probably not a great conductor. The ‘hair’ is effectively a surface that is mostly isolated from the body. The ant is radiating LW from its body of course but I’d wager that this is a very small proportion of what is coming from its radiating surface.
Ever pet a cat that’s been laying in the sun? Fur gets real hot. Kitty is just toasty warm. Same principle.
paulatmisterbees commented: “Body temperature is most likely not the ‘hair’ temperature. The body is only connected to each hair by its attachment point and it is probably not a great conductor. The ‘hair’ is effectively a surface that is mostly isolated from the body. The ant is radiating LW from its body of course but I’d wager that this is a very small proportion of what is coming from its radiating surface.”
I agree with you that I over-simplified this part of the problem to focus on Lambert’s cosine law and other factors that control radiative heat transfer under these circumstances: reflection of SWR, OLR from below, DLR from above, emission from ant itself. Distinguishing between the ant’s triangular hair and the ant itself – which is triangular in Willis’s latest scheme – doesn’t change Willis’s incorrect reasoning.
Hi Frank:
If you think about the totality of the little guys life, he actually spends most of his time in the sand, not on the sand. So it might well be that most of his adaptations have to do with shedding heat while underground. He seems to manage life above ground, in part, by screaming around to get his/her work done before his radiation budget blows up.
I think his reasoning is fine but perhaps semantics have tripped us up. His triangle example contains this…
“Despite that, the two columns end up at different temperatures … which in turn implies a different cooling rate for the two situations, a difference based entirely on the shape of the pillar.”
It seems like a conflation of coolness (meaning reduction in temperature) with cooling rate. I take ‘cooling rate’ meaning as identical to loss of energy. He explicitly says in the example the correct implications of differing surfaces will effect surface loss rates resulting in reduction in temperature.
I agree completely with this but suggest that ‘cooling rate’, at least as I take it to mean energy rate, is not a function of surfaces. In fact total energy loss rate can’t have anything to do with surface configuration. It has to balance the incoming.
What is the energy loss rate of an infinite plate of superconductor? It’s the same as the energy rate being fed to it. What is its temperature rise? Nada. What is its cooling rate? Same as its heating rate. But is it cool? Yes, cooler than it would be if it was as big as the ant.Is it cooling? No. It never heats up.
Damn English anyway. That’s why these things are better communicated with equations.
Interesting side note:
I did a project once with a brilliant officer from the South Korean Navy. He couldn’t speak a lick of English and my Korean was limited to kimchee. We were modeling dredge spoil flows for the Army Corps of Engineers; heavy duty math and programming was involved. You don’t want the flow to be back to the channel you just dug out, right?
We did the whole project by ‘talking’ in FORTRAN on a blackboard. Fun times and no mistakes (fingers crossed here).
“Saharan silver ants (Cataglyphis bombycina) forage in the Saharan Desert in the full midday sun when surface temperatures reach up to 70°C (158°F), and they must keep their body temperature below their critical thermal maximum of 53.6°C (128.48°F) most of the time.”
If the air temperature of the surroundings is 70 °C the ant will also have this temperature after some time by thermal convection. The hairs are only delaying this process of establishing thermal equilibrium. As the emissivity in the IR of the surroundings is near one radiative cooling of the ant cannot be better than the surrounding desert.
I am surprised no one has brought up the aspect of this being a bit like Willis’s Steel Shelled Planet example, besides the reflectivity issue… only in reverse. That is, not keeping heat in by multiple layers between the ants body surface and the outside but this time the constant energy source being from the outside inward.
Willis, do you see some parallel there?
Sorry to come to this thread so late.
Interesting, but the first thing that occurs to me is- what clues do we get from the described lifestyle of these ants? Seems likely that the ants are designed to survive relatively short forays in the most extreme heat. Therefore likely that whatever physical adaptations we are looking at, are designed to slow up the acquisition of an untimately unsurviveable air temperature. So the physical properties we are examining will be to do with reflectivity/insulation rather than any equilibrium radiative properties.
Not sure Willis’s links to cloud shapes stand up, but intriguing nonetheless
Fascinating subjects and I have not read all the comments but does anyone know what the hairs on the bottom of the ant look like along with the feet “pads”? It would seem like that if the mechanism is correct then the hairs on the bottom would different in configuration that what is on the top.
Seems likely the triangular hairs evolved for a reason and quite likely Willis has arrived at a correct explanation for an unusual situation. Frank and Nick just want to argue for the sake of doing so.
Larry: Since Willis writes so compellingly and occasionally brilliantly, I’d prefer to believe that everything Willis writes is right (probably unlike Nick). Unfortunately, none of us is always right and spreading incorrect information is not good for the skeptical cause.
https://en.wikipedia.org/wiki/Lambert%27s_cosine_law
https://en.wikipedia.org/wiki/View_factor
OK, here is an experiment that doesn’t involve outer space. Two identical rooms separated by a wall with a radiator built into it such that one side radiates into room A, the other into room B. The room A side of the radiator is ‘corrugated’ and the room B side is flat – to simulate the upper and lower sides of the silver ant-hair. If the radiator is left to run for a couple of hours, will room A be warmer than room B? If so, doesn’t Willis have a point?
mcdodwell,
Nice thought experiment.
“If the radiator is left to run for a couple of hours, will room A be warmer than room B? If so, doesn’t Willis have a point?”
To answer your second question: Absolutely. That is how I understood Willis’s original claim.
To answer your first question: If heat transfer from the radiator is by radiation only, then the two rooms will end up at the same temperature.
He didn’t specify reaching equilibrium. If Room A reaches that point faster, and the time required for Room B to match is greater than a couple of hours ….
mcdodwell,
Your nice, clear thought experiment deserves a more careful answer than my previous post.
To understand the issue, we need the concept of convexity. An object is convex if a straight line between any two points on its surface lies entirely within the object. The extension of such a line outside the object will not intersect the object.
If we have a convex object, a photon emitted from the surface will travel in a straight line and will not strike the surface of the object. In that case, more surface area means more emission, assuming that other factors, such as temperature and emissivity, are the same.
If an object is non-convex, then there will be some pairs of points on the surface such that a line between those points lies outside the object. Some photons emitted from the surface will travel along such lines and will be re-adsorbed by the object. Thus, the net emission from the object will be less than would be calculated from its surface area.
How much less? I am not absolutely certain of this, but I think the answer is that you draw the convex object of minimum surface area that just encloses the non-convex object.
The wall in your thought experiment is non-convex. The convex object that just contains it has flat surfaces on both sides: on side B, matching the flat surface that is already there; on side A, just touching the peaks of the corrugation. Both flat surfaces have the same area, so the net emission is the same on both sides.
The ant hairs are individually convex, so in isolation they emit more than, say, a circular hair of the same cross section. But Willis postulated that the shape assists the radiative cooling of the ant as a whole and the ant is most certainly not convex. So what matters is the surface that encloses the ant as a whole. That is independent of the shape of the hairs.
“If the radiator is left to run for a couple of hours, will room A be warmer than room B?”
The thought experiment can be used to show the thermodynamic issue. Suppose you didn’t apply power to the radiator, but it conducts internally. That is actually what we have here; the hairs don’t have their own power source. It still radiates; just less because it is cooler. Would room A be warmer than room B? If so, you could run a heat engine using the difference.
You might say, well, you could easily silver one side so it radiates less. But that is where Kirchhoff comes in; absorptivity also has to drop, so there is no difference in net flow. So if it was a black surface, and you corrugate one side, if that increases the emissivity the absorptivity has to rise. But it can’t. It was already totally absorbing all incident radiation.
Interesting adaptation.
Reminds me of Caribou/Reindeer living on the tundra and boreal forests (summer>winter).
Their hair is hollow which provides some insulation.
I will jump in with another possible use for corrugated hollow “hairs” which are highly reflective and triangular in cross section.
Suppose that a live ant could nest two layers of the hair before going out in the sun, bottom layer with apexes pointed “up” and top layer with apexes pointed “down”. They would form a largely hollow sheet with diagonal interior walls. This should be highly resistant to heat flow, almost like a thermal bottle, yet somewhat flexible as each triangular element could slide past is neighbors in two directions, and the corrugations would allow each hair side to bulge to allow movement in a third direction.
Eventually, however the ant would still become hot and race back to the cool inside the nest. Once inside, if the ant could slightly lift one layer of hairs, or cross one layer in relation to the other, the diagonals would open up and allow for the circulation of air from the hot body of the ant to the cooler surroundings.
As soon as the ant had cooled it could again seal its thermal insulation and return outside.
Of course this is simply the wildest speculation…. like climate science in miniature.
I’m not sure why the hairs’ turning or adjusting to face the sun requires a power source. Does heliotropism require it? The Aristotelean theory was that the sunflower’s head is making passive adjustments… maybe this has been disproven.
Willis, you say: “Curiously, however, nowhere do they mention the importance of a third cooling method that I noticed as soon as I looked at their photograph—the shape of the hairs ensures that more energy is radiated upwards than is radiated downwards.”
Gee, you don’t think the reason why these scientists didn’t mention the importance of your third cooling method is simply because … it’s not an issue.
You even quoted the correct point from the article: “c) the two upper sides of the hair are “corrugated”, increasing the surface area facing skywards.”
Indeed, what’s important from an evolusionary perspective is to increase the surface area “facing skywards”. Because that’s the way the radiation cooling the ant is moving. It’s about maximising its radiant heat loss. Conversely, the undersides of the hairs are flat so that they lie parallel to the ant’s body, making the transfer of energy by radiation from the body to the hairs as effective as possible.
The hair layer (and the air gap between it and the ant’s body) would also insulate against the searing heat being radiated and conducted>convected from the ground and up towards the ant. Working much like the loose garments worn by desert people during the day.
“Yu’s group discovered that the ants are covered on the top and sides of their bodies with a coating of uniquely shaped hairs with triangular cross-sections…”
Presumably, there are no hairs on the ant’s underside, which is exposed to the searing heat upwelling from the burning sand, but which is also in shadow. Whatever is going on with the shiny hairs on the ant’s back, something else is happening on its underside, where conditions are different.
But it would be nice to have a good image of the critter’s ventral surface, and a peek at the various structures of the exoskeleton there, before saying much more.
Yeah, that’s actually true. So no hairy insulation against searing surface heat going on after all. Interesting.
Air is not a good thermal conductor, but there are other parts to the puzzle. Note my bold below in this short excerpt from a NY Times article about the Sahara silver ants, from 1992. Dr. Wehner’s work includes peeling the eyeballs of these ants, but my question is: are the ants really climbing the vegetation to cool off? Or are they just getting their bearings?
Undaunted by their task, these desert ants literally rise to the occasion, hauling themselves up out of their burrows on limbs nearly a quarter of inch long, a great length for an ant. Just by raising themselves a quarter of an inch above the ground, the ants can cool their bodies by nearly 30 degrees Fahrenheit, since even this small distance affords a measure of protection from the surface’s intense heat.
The silver ant, which of all the desert ant species endures the highest temperatures, must periodically find and climb small stalks on which to cool off before continuing its hunt. On the top of a flower stalk, the ants can often be seen stretching their front legs skyward to reach the cooler air. The Swiss biologists have found that the cooling-off spells on these refuges are vital if the ant is survive the heat on its long hunting expeditions.
The silver ant’s search for cooler vantage points can complicate the biologist’s task, since the ants rush to climb anyone who comes close to them. “We have to be quite careful of the ants when we do our measurements,” Dr. Wehner said. “They will run towards you and climb up onto you as the tallesest thing around.”
NY Times
Science
Life at the Extremes: Ants Defy Desert Heat
By CAROL KAESUK YOON
Published: June 30, 1992
http://www.nytimes.com/1992/06/30/science/life-at-the-extremes-ants-defy-desert-heat.html?pagewanted=1
Well, I dunno. I’m not an ant, but I do live in a desert. This is the first I’ve heard that reaching for the sky aids in cooling off, but I’ll give it a try next time I start to overheat. Then I’ll prolly make an antline for the nearest shade, let the hot air out of my ballcap, and have a big glug of water.
It is interesting to note that the photograph reveals (with a little imaging) what appears to be protein striations that wrap around the folicle in a spiral. Their width appears to be around 318 nm and are embedded in a semi-clear sheath. Submicron studies on a single follicle would seem to be something that might reveal some interesting results.
On reflection from clouds. Light reflection from clouds is still not well understood. Even knowledge about water molecule size, mix, altitude, evaporation, temperature, sunlight distribution, bandwidth absorption/reflection, atmospherics etc. when mixed with system dynamics yields a fairly empty set between the braces. So little time so much to know…,
Willis, here is another incredible evolutionary story. The corals in the Red Sea fluoresce at night.
Quite magical
http://www.washingtonpost.com/news/speaking-of-science/wp/2015/06/24/scientists-discover-an-unexpectedly-beautiful-rainbow-of-fluorescent-corals-in-the-red-sea/
One hypothesis was that they might create these pigments for sunscreen, but very little light reaches these depths in the Red Sea.
http://www.nature.com/news/radiant-reefs-found-deep-in-the-red-sea-1.17840
Joanne Ballard
Willis: I’ll make one more try explaining why various commenters don’t believe the shape of the hair on the ants or the shape of any other part of the ant makes any difference for radiative cooling.
Your ants do not live in space. In your illustration, the triangle receives radiation from a POINT-SOURCE and increasing the surface area pointing away from that source increases radiative cooling. This doesn’t happen on the surface of the earth. The ants receive thermal IR from all directions in the environment and emit in all directions. The ants can reflect SWR; but they can’t emit LWR without absorbing it, because emissivity = absorptivity at all wavelengths.
The ant emits eoTb^4 in all directions, where Ta is the surface temperature of the bug and e is the emissivity of the ant’s surface. Let’s first simplify the environment that is radiating towards the ant. From the ant’s perspective, 50% of the environment consists of hot desert sand (blackbody equivalent temperature of Ts) emitting OLR and the other 50% is from the cooler atmosphere emitting DLR (with a blackbody equivalent temperature of Ta). From a mathematical perspective, we can say that the ants live between two infinite planes: one at Ts and the other at Ta. Since these planes are infinite, the distance from the ant to the plane turns out to be irrelevant.
“Flat ants”: Let’s first imagine that our ant is effectively flat and horizontal. In the absence of other mechanism of heat transfer (no conduction or convection, 100% reflection of SWR), conservation of energy demands that the temperature (Tb) of the ant in radiative equilibrium with its environment satisfy the following equation:
eTb^4 = 0.5*Ta^4 + 0.5*Ts^4
“Triangular Ants”: Now let’s imagine that our ant is an equilateral triangular or covered with equilateral triangular hairs. Will the ant receive less flux from the hot surface and more flux from the cool atmosphere if we point the vertex of the triangles at the cooler sky. The view factors for a small patch of plane surface in the vicinity of an infinite plan can be found at the link below and it helps to look at the diagram there. When the angle between the two planes is B, the front side (Ff) and backside (Fb) view factors are
Ff = 0.5*(1+cos(B))
Fb = 0.5*(1-cos(B))
http://webserver.dmt.upm.es/~isidoro/tc3/Radiation%20View%20factors.pdf
For radiation from the ground, the face of the triangle parallel to the ground has B = 0 and receives all radiation on the “front side” and none on the “back side”. The other two faces have B = 60 deg and receive radiation on the “back side”. Remember: The ground is an infinite plane, not a point source, and the “upward facing” sides of the triangle receive some radiation from the ground on their “back side”! The bottom side of the triangle has a view factor from the ground (infinite plane) of 1 and the two top sides of the triangle have view factor of 1/4 each. By expanding the above flat ant into a triangle, it receives 50% more radiation from below when the radiation source is an infinite plane (but no more radiation when the source is one point)!
If the triangle is rotated so that the vertex faces the ground, there will be two faces with view factors of 3/4 and one face with a view factor of 0. Rotating the triangle doesn’t change the total view factor from the ground (or the sky) when the sources are infinite planes (not single points).
Rotating “Flat Ants”: If we go back to the above “flat ant” model and rotate the ant 90 degrees so it is perpendicular to the ground, it will receive half as much radiation from the ground on each of two faces compared with how much it received on one face when it was horizontal. (IF the ground were a point source, a “flat ant” would receive zero on both faces when vertical.) No matter what angle you rotate the “flat ant”, it receives the same amount of radiation from a source that is an infinite plane. This example most clearly illustrates the point Nick and others have been trying to make. You can’t escape expose to radiation by changing the angle of a face when the radiation is coming from all directions or from an infinite plane.
IF the face or faces of the ant that are facing the hot ground have low emissivity/absorptivity and the faces or faces of the ant facing the cool sky have high emissivity/absorptivity, THEN the ant can gain an advantage. However, this advantage can be gained by both flat and triangular ants.
Any other examples of triangular hairs anywhere? Only on these extremophile ants? That is a very expensive genetic trick which must have an adaptive thermodynamic purpose. Likewise the corrugation. However it works–it obviously works. –AGF