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.
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.