In previous posts on the myth of of backradiation we have discovered that a cold blackbody B can get heated by absorbing radiation from a warmer blackbody A. We have seen that B can "read" the spectrum of the incoming radiation and thereby "understand" that it has a lower temperature than A, without literally measuring the temperature of A, thus allowing to get heated by A because A "is stronger".
But how does A get to "know" that B has a lower temperature? Is it necessary?
Let us compare with making yourself heard over a the background noise of a party after a couple of drinks: You read the pitch of the background noise and modulate your screaming pitch so that it gets across. The body A thus can read the pitch of the background noise, that is the temperature of B, and thus understand what level/pitch of screaming will be necessary.
We understand that the cold B reads the coherent high frequency fine detail spectrum of A and converts it to incoherent heat, while A only has to read the overall spectrum of B or its cut-off level, to set up its effective emission.
In other words, the emission of A is relative to the background given by the colder B.
But B does not warm A in some form of two-way heating with backradiation.
Similarly, a blogger has to overpower a certain level of background noise, if you want to get listened to. The temperature/pitch of the blog must be bigger than that of the background.
Notice that it is the pitch that counts, not the amplitude: A small hot blogger (or Sun at distance) can heat a big lukewarm blogosphere (or big Earth), but not the other way around.
A high pitch soprano can get heard over a big orchestra. A melody on top can be heard over
a powerful bass line.
Insulation works by establishing (a sequence of) relative temperature/pitch differences which add up to a total temperature difference. This is the way a cloud layer helps to keep the Earth surface warm during night. Not by backradiation!
Computational Blackbody Radiation gives the math.
Compare with the following comment by Stephen Wilde to Spencer's "Greenhouse Effect":
- A cooler object near a warmer object will slow the rate of cooling of the warmer object because of the exchange of energy to and fro between the two objects before that energy departs the combined system.
- If there is a constant energy input throughout then the slowing down of the rate of cooling will cause the warmer object to settle at a higher equilibrium temperature than would otherwise have been the case.
- Thus the presence of the cooler object does indeed result in the warmer object becoming warmer than it otherwise would have done.
- That is the essence of the greenhouse effect which is a concept I have always accepted despite the misleading nomenclature.
- Any sceptical viewpoints that rely on denying those simple facts must be rejected because they weaken the sceptical cause.
This is (more or less) correct, but has nothing to do with backradiation and to call it a "greenhouse effect" is severely misleading. As misleading as saying that an insulating blanket on your bed warms because of a mystical "greenhouse effect" based on mystical "backradiation". In science you are not allowed to deliberately use misleading terminology, only in politics, and maybe not even there in the long run...
PS "Slowing of cooling" is a bit misleading because the cooling is not slowed; it is the temperature which is "lifted" to a higher relative level.
Inlägg
Dear Claes,
SvaraRaderaI struggle to comprehend your reasoning, but I would like to suggest the following experiment:
A large tube, say 1m long and 25 cm in diameter from perspex. Immersed in a temperature controlled bath at say 0 C. In this evacuated tube, a coaxial filament of a material with a high positive temperature coefficient. This filament is coaxically surrounded, inside the tube by: nothing (20cm), an internally and externally black coated cylinder (20 cm long, radius 15 cm) and an internally reflecting, externally blackened cylinder (20cm long, radius 15 cm). The cylinders are spaced 10 cm from each other. The filament is tapped at the intersections with the cylinders so as to allow to measure the voltage across the segment of filament inside each cylinder. The cylinders are tapped with Pt100 probes. A constant current is passed through the filament. When the voltage across the segments inside the two cylinders is measured and compared to the temperature of the cylinders, this experiment could prove whether the backradiation exists or not.
I think. But maybe this has already been performed. Was not able to find any such thing.
Here is a very useful comment by Peter Benkendorff offering experimental support that backradiation is non-physical.
SvaraRaderaClaes, On this I have to agree.
I have experience with flames and combustion in furnaces. One can calculate the theoretical flame temperature, measure the flame temperature and measure the heat transfer at various parts of a furnace and heat exchanger. Chemical engineers have been doing that for more than 70 years. Chemical engineers especially the late Prof Hoyt Hottel at MIT were the ones that determined the emissivities and absorptivities of CO2 and H2O including correction factors for overlap at various temperatures (radiation wavelengths). The flame temperature is not affected by back radiation from the walls. The flame temperature will be reduced by heat flux to cool walls of a furnace. Insulating a furnace will reduce the heat flux from the flame so the flame is higher temperature than in a situation in a water wall boiler. However, the flame temperature can never be higher than the theoretical flame temperature which occurs if there is no heat loss ie back radiation does not occur. This has actually been measured.
On another subject I have some problems with your concepts of conduction and convection but that may be differences in definitions and language. In (engineering) books on heat transfer conduction is defined to occur in solids and is expressed by Fourier's Law. Heat transfer by convection occurs in liquids and gases. There is a boundary layer at surfaces which is affected by fluid dynamics. Books refer to natural and forced convection. Various dimensionless numbers such as Reynolds, Nusselt and Prandtl are used in calculation.
The use of forcing factors (such as CO2 forcing)in climate have no logic or scientific/technical basis. People who use the "forcing" terms for other than forced convection do not understand heat transfer and fluid dynamics and also probably thermodynamics.