Radiative Heat Transfer: Simple Overview


The linchpin of the global warming debate centers on what the impact is of increased levels of CO2 in the atmosphere.  If there is little to no impact, then there is not a problem.  If it does make a big difference, then there is reason to worry.  That question is answered through the analysis of radiative heat transfer.  This is the most misunderstood science in the entire debate.  This is meant to be a simple overview of radiative heat transfer (RHT).

The first step is to understand that all objects that have a temperature are radiating energy of some type.  How much energy it radiates is primarily determined by its temperature.  A hot object gives off a lot more energy than one that isn’t hot.  Temperature is everything in RHT.  The temperature scale used to determine how much energy is called Kelvin.  That is similar to Celsius, but it’s zero temperature is -273.15 °C.  So 0 °C is 273.15 K.  This is considered an absolute temperature.

To explain the simple version of RHT I am going to use a situation that I hope most people can relate to.  Consider the situation where you are outdoors on a cold night.  You also have a fire which is good because you forgot your gloves.   So for this situation there are 3 different temperatures that are going to be used.

Fire:  500 °C,  773 K  (932 °F)

Hands:  35 °C,  308 K  (95 °F)

Ground:  -10 °C,  263 K  (14 °F)

For anyone that has been in a situation like this, putting your hands near the fire will warm them up.  The primary reason for this is the fire is much hotter than your hands to it is transferring heat to your hands.  Your hands will not only feel warm, but they will actually warm up as they are receiving heat (energy) from the fire.  This is the simple part.

Now for the tricky part.  As I stated earlier, all objects that have temperature radiate heat.  Here are the energy levels for the three objects.

Fire:          20244 W/m2

Hands:     510 W/m2

Ground:   270 W/m2

This makes it easy to see why the fire can warm your hands.  It is radiating lots of energy.  Your hands are radiating more than the ground, but nowhere near the difference between the fire and your hands.  This is why you don’t put your hands too close to the fire.  If your hands are too close, they absorb too much energy and you will get a burn.

The easiest way to compare energy levels is to think about a hill.  The higher the energy, the higher the hill.  Energy flows downhill.  Energy from the fire flows downhill to your hands.  Even though your hands are radiating energy, it is your hands that are warmed by the fire.

Now, if you put your hands near the ground, will the ground warm your hands up?  The answer is of course not, the ground is “down the energy hill” from your hands.  Even though the ground is radiating energy, the flow of energy is from your hands to the ground.  So your hands lose heat if you put them near the ground (without touching it, that changes the situation).

Finally consider two rocks on the ground that are close to each other, but not touching.  They are both the same temperature as the ground.  They are both “radiating” energy at 270 W/m2.  Do they both warm up?  Does one rock warm up the other rock?  Again, the answer is no.  One cannot warm the other.  The energy is leaving each rock and being absorbed by the other rock, but since they are the same temperature, the net energy transferred between the two rocks is zero and there is no warming in either rock.

The often misunderstood 2nd law of Thermodynamics would be violated if one rock caused the other rock to warm up.  It also prevents the rock from warming your hands or the fire.  As explained in this situation, RHT makes sense to most people because it fits with their personal observations of the world.

Believe it or not, but some people are going to strongly disagree with this scenario.  While this might not seem relevant to the topic of global warming, it is the most critical part of the debate.

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Posted in Radiative Heat Transfer and Science Overviews by inconvenientskeptic on November 2nd, 2010 at 10:28 am.

18 comments

This post has 18 comments

  1. You’re missing an “a” in your 2nd sentence: “… then there is not problem.”

  2. Bravo,
    Hope you continue to write on this topic since I undergo an almost daily barrage from folks who believe the colder atmosphere can heat the hotter earth via radiative transfer. I will gladly steer them your way… lol

  3. Richard111 Nov 2nd 2010

    Okay, with you so far, though I would dearly love to know how you calculate the energy levels in W/m2. I did have a scribbled down formulae but managed to overwrite it and lost the full meaning. I know the temperature to the fourth power was used. I can convert powers for areas reduced from one square metre. If you know of a tutorial on the net that would be fine. Thanks.

  4. inconvenientskeptic Nov 2nd 2010

    Thanks Todd. Those pesky letters getting in the way of communicating. 🙂

    Hockey,
    This is just the start. I figured to start with the basics that people can understand and agree upon before hitting the issue where it is more abstract.

  5. Richard C (NZ) Nov 2nd 2010

    TIS – or anyone,

    I’m compiling links to RTM’s for this website in New Zealand http://www.climateconversation.wordshine.co.nz/open-threads/climate/climate-science/#comment-27279 but now have a knowledge gap to cross.

    I have separated GCM’s into 2 categories: the IPCC RF stable is in “Climate” and Non-IPCC Natural RF is in “Climate Science” as in the link above.

    My problem now is that I don’t know how RTM’s are implemented in GCM’s i.e do most modeling centres create in-house RTM modules for their AOGCM’s as for example GISS does (I think)? Or do they uplift an off-the-shelf RTM to insert as a module in their GCM? Or is it a combination of both? Or have I got this all wrong?

    My thinking is that an RTM is untainted by IPCC CO2 driven RF methodology as long as it is outside of the IPCC stable of GCM’s but as soon as an RTM is integrated in an IPCC RF GCM, Hansen’s formulations must come into play somewhere in the code.

    I have left 2 questions at “AER’s Radiative Transfer Working Group A forum to share information, tips and tricks” http://rtweb.aer.com/forums/index.php?sid=74586ad4edc1ccbe428d04c90aff0f78 the home of the LBLRTM but unfortunately nobody’s home there (have they folded?).

    For ease of discussion, I have started a thread at Climate Conversation Group (NZ) “RTM discussion” http://www.climateconversation.wordshine.co.nz/2010/10/open-threads-as-promised/#comment-28251 and would appreciate replies here and then there for continued discussion.

    Our national atmospheric and water institution (NIWA – a warmist enclave) has recently commissioned the SH’s swankyest supercomputer and will be running the UKMO’s UM (IPCC stable – HadGEM family) for regional simulations so I really want to get a handle on Radiative Transfer in both IPCC RF and Non-IPCC RF models.

    BTW, I was drawn to AER by this “The foundation of our research and model development is the validation of line-by-line radiative transfer calculations with accurate high-resolution measurements”

    Cheers,

    Richard Cumming

  6. Richard C (NZ) Nov 2nd 2010

    Can you give us a COMMENT email (preferably) or RSS feed for THIS POST please?

  7. Glenn Tamblyn Nov 2nd 2010

    John

    Good explanation at this basic level.

    Only thing I would add is an even stronger emphasis on NET radiation re the 2nd Law. That your hands are radiating heat INTO the fire but the fire is radiating more heat than that INTO your hands, so the NET is INTO your hands.

    This point often confuses people wrt Downwelling Longwave Radiation – see Hockey Schtick’s comment, I am not sure from HS’s comment where they stand on this.

    Heat can and does radiate from a cold atmosphere down to a warmer surface. But more heat is radiated from the warm surface to the cold atmosphere so that the NET radiation is from Warm to Cold – down hill in your analogy. Some does go up hill, but more goes downhill producing a net downhill.

    For me this subject is one of the touch-stones for assessing the integrity of technically trained Sceptic Scientists & Engineers. If they put out the line that heat can’t flow from cold to hot, rather than that NET Heat can’t, or allow others to express that opinion without correcting them, then they have either totally forgotten all their training, or they are being deliberately deceptive.

    Glenn

  8. inconvenientskeptic Nov 2nd 2010

    Glenn,

    Earlier editions had net involved, but it kept complicating it. The next level will involve more on the net energy transfer.

    I do appreciate your comments. I know we disagree on certain things, but you do call things as you see them. It was an interesting challenge to put the basics of RHT into a non science article.

    Richard,
    I will see if I can get a feed set up on the comments.

    Thanks,
    John

  9. Richard C (NZ) Nov 2nd 2010

    Glenn,

    Are you saying that the fire is warmer due to the presence of the hands than it would be if the hands were not there?

    If so, is the sun hotter due to the earths presence than it would be if the earth was not present?

  10. inconvenientskeptic Nov 2nd 2010

    Richard,

    Before the discussion escalates in this post lets hold off a little bit.

    The is where the discussion goes to net transfer. As I stated before I intentionally avoided net in this one to keep it simple. I can then use the examples in later, more complex discussions can involve net.

    Since I know there will be other comments along the line of Richards I will let the one through. The good thing is that both sides agree that my simple analogy is correct.

    The net effect of a cooler body on a warmer body is more complex and that will be discussed in a later post.

    Lets keep the heat transfer down in the comments at this time. Glenn clearly understands that the hand is not warming the fire, but the hand does reduce the rate of heat loss in the view factor that the hand intersects the heat leaving the fire.

    Lets leave it at that. 😉

  11. Richard C (NZ) Nov 2nd 2010

    “The net effect of a cooler body on a warmer body is more complex and that will be discussed in a later post.”

    OK, I’ll look forward to that post

  12. Richard C (NZ) Nov 3rd 2010

    I have received this response from Mike Iacono, AER in response to my 2 questions up-thread
    ——————————————————————————
    Richard,

    Our approach is to develop and evaluate radiation models in the context of high-resolution measurements and to collaborate with the dynamical modeling groups that choose to apply our radiation code to their global
    model. We do not currently maintain or run our own climate model.

    The only group that I’m aware of that was using one of our radiation codes for AR4 simulations was the Max Planck Institute in their ECHAM5 model. At that time they used only a somewhat early version of our RRTM
    longwave code. This is documented in the Roeckner et al, 2003 reference cited in Chapter 8 of AR4 and in Wild and Roeckner, J. Climate, 2006.

    Our work to evaluate climate models with satellite radiances was also cited in AR4 in Chapter 8 (Iacono et al., JGR, 2003). ECHAM6 will include our latest RRTMG longwave and shortwave codes, and the NCAR CAM5/CESM1 atmosphere and coupled climate models
    have recently been released with RRTMG. It is probable that at least some of the IPCC simulations for AR5 will be completed with these models.

    Best Regards,
    Mike

  13. inconvenientskeptic Nov 3rd 2010

    Richard,

    I will have to look more into this information you have. It will be interesting to see the modeling they are using. There is sure to be some interesting stuff in there.

  14. Malaga View Nov 4th 2010

    Here are the energy levels for the three objects.

    I personally think you made an unexplained logic jump when you introduced the W/m2 values because I think you need to talk about the objects in three dimensions. For example: I put the palms of my hands towards the fire to warm them while the backs of my hand are radiating heat – that is why I rub my hands together to get heat onto the back of my hands. So you really need to introduce the concept of SURFACE AREA and that the heat energy may not be evenly distributed throughout the object and there is CONVECTION and, by the way, the palms of my hands are less than a square metre 🙂

    Other concepts you probably need to introduce (and explain) are MASS and TIME (heating / cooling curve).

    Additionally, the objects are dynamic in that the fire and hands are both generating warmth (at the moment) but all objects would be at EQUILIBRIUM at -10C after a few hours i.e. the fire will have burnt out and the person attached to the hands will have died of hypothermia.

    So I personally think you have jumped in at the deep end with too complex a situation and jumped over far to many concepts… trying to build upon this example will involved far too many logic jumps for me… and I think you also need to explain the situation if you hadn’t forgotten to bring your gloves 🙂

  15. Malaga View Nov 4th 2010

    PS You will also get into circular logic problems with this example because the GROUND is at -10C ( instead of absolute zero ) because of global warming 🙂

  16. Richard C (NZ) Nov 4th 2010

    John,

    “It will be interesting to see the modeling they are using. There is sure to be some interesting stuff in there.”

    This is an understatement.

    I’ve taken this a bit further and have documented some “interesting stuff” here:

    http://www.climateconversation.wordshine.co.nz/2010/10/open-threads-as-promised/#comment-28449

  17. Sundance Jan 19th 2011

    John I thought you might enjoy Dr. Spencer’s article on back radiation. He provides an experiment that can be repeated.

    “Particularly difficult to grasp is the concept of adding a greenhouse gas to a COLD atmosphere, and that causing a temperature increase at the surface of the Earth, which is already WARM. This, of course, is what is expected to happen from adding more carbon dioixde to the atmosphere: “global warming”.” – Roy Spencer

    http://www.drroyspencer.com/2010/08/help-back-radiation-has-invaded-my-backyard/

  18. The problem with the “science” of “climate scientists” is they have an agenda to push.

    1. They ignore the FACT that, in the absence of an external or internal energy source – our body heat for example – things that radiate are losing energy. They come into equilibrium with their environment but absorbing energy by radiation is countered by the emission of radiation.

    So the earth cannot absorb any NET heat from “backradiation” – the constant energy exchanges of radiating , absorbing and reradiating are not increasing total energy supplied by the Sun during the day – they simply provide a mechanism whereby the otherwise rapid loss of heat is slowed.

    This is obvious at night when things cool down.

    2. The other fallacy is the use of one quarter of the solar constant as the insolation to the Earth.

    Will someone please demonstrate how the divide by four “trick” has any validity ?

    A small perfect blackbody may well heat up fairly quickly and radiate equally to the energy influx.

    BUT, we know this doesn’t happen on Earth –

    (a) The Earth never approaches its theoretical blackbody temperature unlike the surface of the moon.

    (b) The Moon demonstrates the theoretical radiation equilibrium is a myth – the day side is hot – the dark side unbelievanbly cold.

    There is no equilibrium temperature or radiation state !

    Quartering the insolation to equilibrate the radiation over a sphere is just wrong !

    (c) GHGs are not the only warm gases in the atmosphere. OK, Nitrogen and Oxygen may not absorb IR like H2O or CO2 but you cannot tell me they do not become heated. As such they will emit IR and the properties of that emission will be primarily determined by the temperature accorbing to Wein and other physicists.

    Warmed by the surface air rises and upon rising encounters cooler air. This warm air will cool and will also radiate.

    Also, solar insolation directly heats the atmosphere – eg oxygen as ozone, water vapour and other gases by either radiation or direct conduction.

    Being higher in the atmosphere these will cool and will radiate.

    Perhaps DLR has little to do with absorbing IR from the surface but is a product of a convecting atmosphere with some direct solar heating.

    I see little place for “backradiation” as described by climate scientists as it seems they ignore the changes in energy during radiation and simply add it up to achieve higher temperatures.

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