The Difference between “Forcing” and Heat Transfer


This is a continuation of the series on Radiative Heat Transfer (RHT).  The purpose of the series is to use normal life experiences to explain RHT.  In this article I will explain the difference between “forcing” and the transfer of energy.  They seem similar, but they are different.

A useful situation to explain the difference is a cool afternoon with a partly cloudy day.  The temperature is 10 °C (50 °F).  Based on the previous article that temperature means that the air has a “forcing,” or radiative flux of 364 W/m2.  As a normal human your surface temperature is 35 °C and your “forcing” level is 511 W/m2.

What is happening?  Based on previous discussions your are losing energy to the air around you.  Depending on the person, it could be a little chilly.  Fortunately it is a partly cloudy day.  When the sun is out, it feels much warmer.

Interestingly enough, the suns energy is much less than the “forcing” from the air around you.  For a cool autumn day the sun is likely providing 100 W/m2 of energy.  The difference is the sun is transferring energy to the objects that it reaches.   Warming can only happen when energy is transferred.  So the sun can warm you up, even though the energy value for the sun is less than the “forcing” of the air around you.

How long does it take to notice if a cloud blocks the sun?  I know I feel the difference instantly.  That is  because RHT is instant (speed of light, but for humans that is close enough).  When the sun is behind the cloud, your body changes from getting energy from the sun, to losing energy to the air around you.  From receiving 100 W/m2 to losing about that much (depending on what you are wearing).

That is why when the cloud finally moves out of the way, you feel warmer instantly.  Once again you are gaining energy instead of losing it.  It is all about the heat transfer.  “Forcing” cannot warm anything, only the positive flow of energy can cause something to warm up.

That is the difference between energy transfer and “forcing.”  One is simple and direct.  Energy is flowing and the temperature of something is changing.  The other is the potential for heat transfer.  If a person were placed in space, that potential heat transfer would become an actual heat transfer of 511 W/m2.  That would cool a person down very quickly.

That is why the 364 W/m2 of potential heat transfer from air that is 10 °C will not warm you up, but the actual heat transfer of 100 W/m2 from the sun will.  The effect from the forcing is that without it you would lose heat even faster.  It does not warm you, but it limits the amount of energy that you lose.

According to the energy balance of the IPCC (by way of Kiehle and Trenberth) the atmosphere provides the surface of the Earth with 324 W/m2, but that value is a “forcing” and not an energy transfer.  That is equivalent to a temperature of 1.8 °C.  On average the sun provides 168 W/m2 of energy to the surface of the Earth.  Based on appearances, the atmosphere is providing more energy to the surface of the Earth than the Sun.

If you are cold and outside, would you rather get 324 W/m2 of “forcing” or 168 W/m2 of energy transfer?

The Inconvenient Skeptic

In the cold only heat transfer will warm you up.

—————————————————–

Part 1:  Radiative Heat Transfer – Overview

Part 2:  Radiative Heat Transfer – Medium 1/2

Part 3:  Radiative Heat Transfer – Medium 2/2

Posted in Radiative Heat Transfer by inconvenientskeptic on November 21st, 2010 at 12:49 pm.

16 comments

This post has 16 comments

  1. Richard111 Nov 21st 2010

    Forcing. Something that has always baffled me. Must think about this for a bit. But I think I see what’s coming. 🙂

  2. Richard C (NZ) Nov 22nd 2010

    The RHT acronym is misleading – should be RET.

    I think there needs to be a distinction between radiation and heat. The main difference being the speed of propagation: radiation at the speed of light; heat varying between stationary and maybe the speed of jetstreams (400km/hr max?) or even the speed of the rotation of the earth (1669.8 km/h at the equator) depending on the medium. Both are forms of energy (E in RET) but heat is only manifest when radiation encounters matter (molecules) i.e.heat requires a medium, radiation doesn’t.

    I wonder if forcing in this case is just another word for conduction or convection that renders this statement incorrect: ““Forcing” cannot warm anything, only the positive flow of energy can cause something to warm up”. Forcing by heat conduction will certainly cause something to warm up.if there is a thermal gradient (potential difference) but only if the thing being warmed up is at the cooler end of the gradient in terms of radiation or heat (the “positive flow of energy”).

    The entire universe is seeking thermal equilibrium so although the sun receives earths re-emitted radiation there’s no heating effect on the sun (no rise in temperature) because the thermal gradient is hot sun to cooler earth . The earth then, receives energy via radiation from a heat source but dissipates it via radiation, conduction and convection (gas and hydrological process for the latter two mediums). The earth is also giving up internal energy by the same means (hot lava “radiates”).

    Also the sensation of heat felt in the skin is misleading. The heat generated by radiation, conduction or convection is from excitation of molecules but the sensation is from nerve endings. This is why people suffering from leprosy burn themselves because the disease attacks nerves. I we leave aside human nerve sensation, we are basically talking about different methods of cooking meat – radiative, conductive and convective.

    Radiation intensity varies with angle of incidence, concentration etc but heat is only concentrated in a superheated enclosure that I can think of. The experiment by Wood reiterated in Gerlich and Tscheuschner 2009 documents how greenhouses don’t work unless enclosed and that the greenhouse gas analogy is inappropriate for the atmosphere.

    “If you are cold and outside, would you rather get 324 W/m2 of “forcing” or 168 W/m2 of energy transfer?”

    Moot, because the forcing is unavailable – the ambient temperature is cold so no heat to do any forcing that is felt via conduction or convection. An option then is to remove clothes if there is no wind (think bikini skiing in the alps) to maximize radiative energy transfer. Obviously there is an ambient threshold where this option ceases.

    I’m guessing that we will move on to blanket vs greenhouse discussion and how a blanket slows heat loss (but doesn’t do any heating in my view).

    I disagree with the photo caption BTW – should read: “In the cold only energy transfer will warm you up” (because there’s no heat to transfer).

  3. Malaga View Nov 22nd 2010

    Thanks for an interesting series of posts… I was generally ok with the first three parts… but as soon as you introduced Kiehle and Trenberth into the mix then everything descended into confusion and muddle… perhaps that was their intent.

    For example: you write That is why the 364 W/m2 of potential heat transfer from air that is 10 °C will not warm you up but I know if I take an ice cube out of the freezer it will melt in the shade during your cool afternoon air… so it is all about the net balances… and the introduction of the word forcing seems to introduce a totally spurious concept that can only confuse.

    For example: you write For a cool autumn day the sun is likely providing 100 W/m2 of energy.… that is almost OK… but in your example a lot depends upon my position relative to the sun… if I lie down and sunbathe I will probably feel cool… but if I stand up in the sun I might feel warm… so if my shadow covers 2m2 on the ground then I guess I might actually be getting 200 W/m2 from the sun… your snowy mountain picture illustrates this well because the trees have long shadows.

    For example: you ask How long does it take to notice if a cloud blocks the sun? I know I feel the difference instantly.… that is almost OK… but I know if I am standing at the top of a mountain then the wind is usually very strong… thus the wind chill factor kicks in (which I assume is adding heat loss by convection) and I feel colder and may not notice the sun coming out from behind a cloud… and if I am sweating after a long climb up the mountain then I will feel very cold very quickly – regardless of the sun.

    Having written my comments I guess what I am trying to say is that the science of the real world gets very complicated very quickly… there is Radiative & Convective Heat Transfer… and it seems that Kiehle and Trenberth only add confusion.

  4. inconvenientskeptic Nov 22nd 2010

    Malaga,

    That forcing and RHT have the same units is part of what makes this tricky to explain.

    That is especially why intermixing forcing into a discussion about energy transfer is so misleading. In the next week I will do more to additionally simplify everything.

    One other example to compare to forcing….

    Gravity exerts a force down on everything. Since gravity is pulling you down, the ground pushes back (called the normal force, NF). The NF rarely matters because it is always balanced by gravity, but it is always there. Forcing is very similar to the NF. In a situation where most objects are close in temperature, there is very little heat transfer, but there is lots of forcing.

    Just like each object on the surface of the Earth has the force of gravity pulling and the NF pushing back. Objects stay in the same place unless an additional force gets involved.

    Keep reading. Much more interesting stuff to come. 🙂

  5. inconvenientskeptic Nov 22nd 2010

    Richard,

    The sun provides energy in the form of radiation constantly to the Earth. A room full of objects at 500K have lots of forcing, but no heat transfer.

    If there is a difference in “forcing,” then heat (energy) transfer will occur. The amount of heat transfer is determined primarily by the difference in forcing from different objects.

    Convection and conduction are forms of heat transfer. So is latent heat transfer. All of these are forms of heat transfer.

    “Forcing” by itself is NOT a measure of heat transfer. That is why mixing in forcing with energy transfer is so mis-leading.

    Ground that is 1.8 °C does provide 324 W/m2 of “forcing.” That is clearly stated by the Stefan–Boltzmann law. I am assuming blackbody, but that is reasonable for this discussion.

    j = σ T^4
    j = 5.6704E-8 * (274.95)^4
    j= 324 W/m2

    Please read the previous articles that explain this more fully.

  6. Malaga View Nov 22nd 2010

    Keep reading. Much more interesting stuff to come.
    I will… thanks for the clarification… I look forward to next weeks simplifications 🙂

    The experiment by Wood reiterated in Gerlich and Tscheuschner 2009 documents how greenhouses don’t work unless enclosed and that the greenhouse gas analogy is inappropriate for the atmosphere.

    Going back to my school days I learnt: Hot air rises.

    So starting with water vapour as a Green House Gas:
    1) The water vapour in the air (near the ground) is heated by the ground and the sun.
    2) The warmed water vapour rises up in the air… and as it rises it cools… and frequently forms clouds at altitude.
    3) Cooler, (usually) drier air descends to replace the heated water vapour that has risen up to form clouds.

    Now if CO2 was a really effective Green House Gas I would expect to see similar results:
    a) Clouds of (invisible) CO2 at altitude – as per 2) above – or at least increased concentrations.
    b) Depletion of (invisible) CO2 near the ground – as per 3) above – because heated CO2 is replaced by air.

    Now I reckon this would best be tested in a Hot Desert where there is little ground water and few plants. So should I book myself a holiday to the Sahara or has someone beaten me to it?

  7. Malaga View Nov 22nd 2010

    PS
    Just revisited Ernst-Georg Beck’s Vertical CO2 Profiles on page 4 at http://www.biokurs.de/treibhaus/CO2_versus_windspeed-review-1-FM.pdf

    The Annual Mean graph shows there is slightly more CO2 near the ground… and levels decrease slightly until you reach 4 km… so it is back to the drawing board… either CO2 sinks because it is heavier than air and/or near ground CO2 doesn’t heat up enough to rise in the air column.

  8. inconvenientskeptic Nov 22nd 2010

    Malga,

    CO2 is effective, but in a narrow band. So it absorbs energy, but water vapor is a different beast. It changes the density of the air as the concentration changes. More moist air is less dense. When it warms is becomes even less dense and rises. When the water vapor condenses, it increases the density of the air.

    CO2 will not cause the same type of behavior (density and concentration variation) and it absorbs much less overall energy than water vapor does.

  9. Richard111 Nov 23rd 2010

    “”When the water vapor condenses, it increases the density of the air.””

    Umm.. typo? 🙂

  10. Richard111 Nov 23rd 2010

    John, thanks for the input. I managed to put that ‘sum’ through my on board calculator and got the right answer for the very first time! Don’t know what I was doing wrong previously. Anyway will go back through your earlier posts and practice number crunching.

    So far all your examples refer to radiation from a surface. A gas, to my layman’s mind, has an infinite surface? i.e. every molecule in a gas is capable of radiating, if and only if, it has aquired sufficient kinetic energy, (Maxwell-Boltzmann distribution), and those molecules are evenly distributed through the volume of the gas.

  11. inconvenientskeptic Nov 23rd 2010

    Richard,

    The density of air is influenced by the molecular weight of the molecules. Water vapor has a lower molecular weight than the other components. So when water vapor changes from gas to solid, the density of the air actually increases.

    This is part of why afternoon thunderstorms are common. The rising warm, moist air rises and then cools. As the water vapor condenses it provides energy, liquid water and pressure gradients (wind). The combination creates a storm where none existed.

    Glad to hear that you are getting the hand of it. A gas is still a surface. There are two normal methods of dealing with it. Treating it like a column or just treating it like a normal flat surface. It depends on what you are looking at.

  12. Malaga View Nov 23rd 2010

    John, thank you for being so helpful… much appreciated.

  13. Richard111 Nov 24th 2010

    John, I have been using Patrick J. Tyson’s site at http://www.climates.com to educate myself in things climatological.

    The fllowing statement makes intuitive and logical sense to me

    “”Condensation has just the opposite effect of vaporization. Condensation diminishes the volume of the atmosphere in the same proportion as vaporization increases it.””
    See page 4 of the pdf:

    http://www.climates.com/KA/ATMOSPHERIC%20WATER/volumetricchanges.pdf

    I have made simple calculations with a water vapour content of 1% and find that liquid water has less volume than the equivalent number of H2O molecules in gas form. This seems to support the above statement.

  14. Glenn Tamblyn Nov 25th 2010

    John

    Your previous posts on RHT were technically correct but this one is gobbledeegook. You seem to be falling into the trap of simplifying something, in the process tending to introduce small innaccuracies in peoples understanding. Then you are building simplification on simplification, a process that compounds the inaccuracies until they start to matter. We have discussed this before.

    Firstly, where did you get this term ‘forcing’ from. I have never come across this in thermodynamics before. Unless you are leading towards the usage of Forcing in Climate Science, in which case you look like you are building a strawman. For the record, here is the definition of ‘Radiative Forcing’ from the FAQ for the 4th IPCC Report:

    “What is radiative forcing? The influence of a factor that can cause climate change, such as a greenhouse gas, is often evaluated in terms of its radiative forcing. Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system is influenced when factors that affect climate are altered. The word radiative arises because these factors change the balance between incoming solar radiation and outgoing infrared radiation within the Earth’s atmosphere. This radiative balance controls the Earth’s surface temperature. The term forcing is used to indicate that Earth’s radiative balance is being pushed away from its normal state.
    Radiative forcing is usually quantified as the ‘rate of energy change per unit area of the globe as measured at the top of the atmosphere’, and is expressed in units of ‘Watts per square metre’ (see Figure 2). When radiative forcing from a factor or group of factors is evaluated as positive, the energy of the Earth-atmosphere system will ultimately increase, leading to a warming of the system. In contrast, for a negative radiative forcing, the energy will ultimately decrease, leading to a cooling of the system. Important challenges for climate scientists are to identify all the factors that affect climate and the mechanisms by which they exert a forcing, to quantify the radiative forcing of each factor and to evaluate the total radiative forcing from the group of factors.”

    So, what is actually happening in your example, expressed a little less simply, but more accurately, but still trying to stay within the simplification limits of your example.

    Firstly, the distinction between heat and energy is spurious. Heat is just a more colloquial term for energy.

    Your body is generating energy internally through its metabolism which it needs to loose to stay in thermal equilibrium. It looses this through radiation to its surroundings, through convection over the skin and also through evaporation of sweat off the skin. At the same time it is receiving energy from its surrounds, some from convection but most from radiation. So it IS absorbing the 364 W/M^2 from the atmosphere. When the Sun is out (in you example) it is also absorbing an additional 100 W/M^2. When the Sun is behind the clouds it is not receiving the 100 (those are amazing clouds by the way, to be 100% efficient).

    So with the Sun out
    Energy out (Eo) is 511 W/M^2 + Convective Loss + Evaporation Loss
    Energy in (Ei) is 364 W/M^2 + 100 W/M^2 + small convective gain.

    With the Sun hidden
    Energy out (Eo) is 511 W/M^2 + Convective Loss + Evaporation Loss
    Energy in (Ei) is 364 W/M^2 + small convective gain.

    NET Energy flow in/out of you body is
    En = Internally Generated Energy (Eg) + Ei – Eo where a +ve value means your body is gaining energy

    So when the Sun comes out from behind the clouds the NET energy flow in/out of your body changes, but NOT because the 364 is a ‘potential’ energy flow. That 364 ABSOLUTELY is flowing INTO your body, AS IS the 100 when the Sun is out, AS IS the 511 flowing OUT (you seemed to drop that from consideration after only mentioning it once).

    They are all energy flows. The problem with your example is that because the Sun’s radiation is the only thing you can vary, it makes it seem like the other flows aren’t there. If we could magically turn that 364 flow IN off, Oh Boy, would you notice the change then. As you would if we turned the 511 OUT off.

    Your argument about potential flows or forcings or whatever is like saying that if a locomotive pulls 100 wagons along a line, drops one at the end and returns pulling 99 wagons then only one wagon moved. Yes, 1 wagon moved NET, but actually there were 199 actual wagon movements. Your avoidance of strongly emphasising the NET nature of something, the balance of multiple flows, and that all the flows ACTUALLY exist makes it possible to fall into this trap.

    To then try and argue that these other flows are just ‘potential’ is bizarre logic – when that Loco arrived at the first end of the line the 100 wagons really were there. Not just 1 real and 99 potential.

    So, lets dissect this paragraph of yours
    “According to the energy balance of the IPCC (by way of Kiehle and Trenberth) the atmosphere provides the surface of the Earth with 324 W/m2, but that value is a “forcing” and not an energy transfer. That is equivalent to a temperature of 1.8 °C. On average the sun provides 168 W/m2 of energy to the surface of the Earth. Based on appearances, the atmosphere is providing more energy to the surface of the Earth than the Sun.”

    I don’t have the older KT paper but I do have a copy of their more recent one:
    “EARTH’S GLOBAL ENERGY BUDGET by Kevin E. Trenberth, John T. Fasullo, and Jeffrey Kieh” 2009

    From Figure 1, looking at just the energy flows into and out of the earths surface and ignoring the higher fluxes just in the atmosphere we have (all in W/M^2 and rounded off)

    Radiation absorbed by the Surface from the Sun +161
    Convection Up from the Surface -17
    EvapoTranspiration Up from the Surface -80
    Radiation Up from the Surface -396
    Radiation absorbed by the Surface from the Atmosphere +333

    NET flow +1 into the Surface

    NONE of these flows are potential, ‘forcing’ or any such thing. They are all REAL energy flows. So your statement “…324 W/m2, but that value is a “forcing” and not an energy transfer”. Totally Wrong. And just as well, since that flow is needed to counterbalance the flows leaving the surface.

    So, “Based on appearances, the atmosphere is providing more energy to the surface of the Earth than the Sun”. Yes. The size of the energy flows bouncing around inside the lower atmosphere/surface is greater than the size of the flow reaching the lower atmosphere/surface from the Sun.

    So this statement “That is why the 364 W/m2 of potential heat transfer from air that is 10 °C will not warm you up, but the actual heat transfer of 100 W/m2 from the sun will” is gobbledeegook. ‘warm you up’ means that your body is in a state of energy imbalance and is gaining heat. Like the last straw that broke the camels back, adding one more energy flow can switch you to a +ve energy balance. But that straw only breaks the camels back because of all the other straws that are there as well.

    Come on John. Your old lecturers would expect better of you than this!

  15. inconvenientskeptic Nov 25th 2010

    Glenn,

    Radiative flux is the proper term, but since “forcing” is what is used by most climate change people, that is what I must use. Heat and energy are the same thing. Radiative flux is different.

    Radiative flux is meaningless by itself in most situations. Much like the Normal Force is meaningless without the gravity. When any object is surrounded by an environment that has a comparable temperature, there is very little heat (energy) transfer between them.

    That is why two rocks at 1.8 °C don’t warm each other up. They both have 324 W/m2 of radiative forcing, but no energy is transferred.

    When insulating a house, stopping convection is always more important than stopping RHT. Why is that? In every real case that people deal with, RHT is not a critical method of energy transfer.

    Warming (change in temperature) happens only when energy is transferred. If you think a professor will disagree with that….

  16. inconvenientskeptic Nov 25th 2010

    Glenn,

    While you disagree. Sunday’s article will be interesting for you as I will use the Kiehl papers. Old and New. Doesn’t matter as they both agree with me. Only energy transfer matters.

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