An opaque object that is subject to radiant heat may show an increase in temperature.

Like a reporter near a Hawaiian lava flow.
That reporter then referred to the radiant heat as "up to some amount of °C". (What pleasantly caught my attention first was that he specified the unit used.)
He felt it's hotter the nearer he got, so it naively makes sense to assume an increase in temperature.
Note I refer to the sensory input of the person, not his experience of temperature, which may be pain for example. But part of that sensation was the effect of the IR radiation on his skin, or even the heat receptors of his skin.

I do not see how this "hot" can be expressed as a temperature.
One could wait until the temperature of the object comes to an equilibrium with available cooling, and then measure some well defined temperature, like the maximum of the surface temperature. But that is very different from the situation at hand, and depends a lot on properties of the object.

  • Is it indeed possible to specify a meaningful amount of °C?
  • If not, what would be best used instead?
  • Or is the situation of the reporter just too ill-defined, even though he validly has the sensation of increased temperature?
  • $\begingroup$ Possible duplicate of How does a radiometric infrared camera estimate an objects temperature? $\endgroup$ – sammy gerbil May 12 '18 at 5:05
  • $\begingroup$ @sammygerbil That seems related, but not duplicate. $\endgroup$ – Volker Siegel May 12 '18 at 5:08
  • $\begingroup$ Why do you think so? $\endgroup$ – sammy gerbil May 12 '18 at 5:13
  • $\begingroup$ While that question certainly handles central effect, I think this has some side aspects that make it less clear to me. For example, the person does not experience only the radiation. $\endgroup$ – Volker Siegel May 12 '18 at 5:22
  • $\begingroup$ What the person experiences is subjective and physiological. Questions on this site are required to ask about physics. $\endgroup$ – sammy gerbil May 12 '18 at 5:26

Well, there is local heating of the air--and I am going to leave that as "it is what is is". Regarding the wall of lava, what counts is the surface temperature and the solid angle is subtends. Let's say the surface is 1000C. If you are in a bubble ($4\pi$ solid angle), you are at 1000C whether the bubble diameter is 10 feet or 10 miles--that is the nature of radiant heat. If it is a wall of lava, then the closer you get, the greater the solid angle, so the hotter it feels. If there is absorption in the air, then you get hotter air and less exponential decay--so the apparent temp goes up faster than the solid angle.

Note that this effect occurs in forrest fires too: if the wall of flames is high enough, and wide enough--radiant heat can ignite structures from a surprising distance.

The material question is also a factor--but it just delays the inevitable. Volcanologist don't wear black, they wear insulating reflective suits, but that just buys them time. If they don't leave the scene, they will eventual be the same temperature as a Goth kid wearing all black.


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