Heat radiation - silver vs black?

[slim_boy_fat]

So why would the indigineous people of those countries typically have darker skin tones?

I can only assume that the genes for that developed/evolved over the millenia, before clothes were worn?
But I'm not Professor Alice Roberts [an expert in such things.]

Didn't mean to take this so far fftopic: - apologies to @julianf

 

[Kent]

Duuno the paint comes off when i get stuff hot either way lol

 

[brightspark]

Could be why people in hot countries tend to wear white/light coloured shirts?

that's why arabs wear loose white robes and not dark ones. mind you there not bothered about letting the missus cook in there black gear. hot stuff

 

[DennisCA]

They actually wear black robes too.
https://www.nature.com/articles/283373a0

 

[fizzy]

I can only assume that the genes for that developed/evolved over the millenia, before clothes were worn?
But I'm not Professor Alice Roberts [an expert in such things.]

Didn't mean to take this so far fftopic: - apologies to @julianf

Gorgeous but not always such an expert. I remember in her out of Africa series she ridiculed the idea of Neanderthals and modern humans inter breeding. She obviously hasn't been out in Swansea late at night! Seriously though they have now discovered there was plenty of inter breeding between species.

Anyway back on topic.
Heat buildup in cities is a serious problem due to black roads absorbing heat during the day and radiating it at night. Buildings tend to restrict airflow as well so the roads just get hotter and hotter and release all that heat through the night.
I can never understand why so many buildings have a dark roof either.

 

[slim_boy_fat]

She obviously hasn't been out in Swansea late at night!

 

[mr haynes]

Gorgeous but not always such an expert. I remember in her out of Africa series she ridiculed the idea of Neanderthals and modern humans inter breeding. She obviously hasn't been out in Swansea late at night! Seriously though they have now discovered there was plenty of inter breeding between species.

Anyway back on topic.
Heat buildup in cities is a serious problem due to black roads absorbing heat during the day and radiating it at night. Buildings tend to restrict airflow as well so the roads just get hotter and hotter and release all that heat through the night.
I can never understand why so many buildings have a dark roof either.

...and still going on, just look up a certain Captain James T Kirk!
Spaceships tend to be white/silver too so must be something in it

 

[Burdekin]

It's down to the emissivity of the finish, in the end. White radiators generally emit heat quite well, because the emissivity of white paint is not far off black body. There's an easy test to do if you have one of those infra red thermometers and a thermocouple thermometer. Infra red thermometers are normally calibrated for an emissivity of 0.95 (i.e. 95% of a perfect black body). Measure the temperature of the same spot on a white radiator using infra red and thermocouple, and compare the two readings. They normally don't differ by much. I pointed an IR thermometer at our kettle the other day (cream paint finish). Just as it came to the boil, indicated temperature was 100degC. Did the same using a thermal camera (emissivity set to 0.95), and the kettle surface measured 100degC. Silver paint, on the other hand, tends to reflect more than radiate (i.e. emissivity is low). Most of what is generally seen when pointing a thermal camera at silver paint, is reflections from other hot objects, rather than the temperature of the silver paint. However, not all silver paints are the same, especially if discoloured.

As above.

 

[Burdekin]

So why would the indigineous people of those countries typically have darker skin tones?

Watched a doc a while back regarding Europeans and being white. Europeans came from Africa and were originally black and scientists think they became whiter to adapt to be able to make more vitamin D in the less sunny areas of Europe.

Being ginger I can say I am not made for sunny climates, we would have frazzled and died running around Africa in our Tarzan pants. Black folk have more tolerence to the sun. ;-)

 

[500e]

" She obviously hasn't been out in Swansea late at night! Seriously though they have now discovered there was plenty of inter breeding between species."
And that's only the visitors.
Now back to Julians question, fftopic: as white is a reflector both sides do the same ?
As black is a heat absorber \ radiator both sides do the same ?, confused my self now will leave now with hat & coat

 

[rtbcomp]

If you have a sheet painted black on both sides heat will be absorbed/radiated from both sides.

Google "leslie cube"

 

[julianf]

A lot of (well, pretty much most) people here are talking about absorption - but is that the same as radiation?

(I know one side of the sheet will be absorbing and one side radiating, but, whatever i do, the internal side, right next to the element, will be silver)

 

[Burdekin]

A lot of (well, pretty much most) people here are talking about absorption - but is that the same as radiation?

(I know one side of the sheet will be absorbing and one side radiating, but, whatever i do, the internal side, right next to the element, will be silver)

See post 12. There's a lot of misconceptions regarding this topic. Have a read here: https://en.wikipedia.org/wiki/Emissivity

 

[rtbcomp]

A lot of (well, pretty much most) people here are talking about absorption - but is that the same as radiation?

...

It depends on the temperature difference. If a black surface is hotter than ambient it will radiate, if it is colder it will absorb heat.

 

[Burdekin]

It depends on the temperature difference. If a black surface is hotter than ambient it will radiate, if it is colder it will absorb heat.

That is true for anything. Heat only travels from hot to cold, it never does the opposite.

https://en.wikipedia.org/wiki/Second_law_of_thermodynamics


Kirchhoff’s Law of thermal radiation:

For an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, the emissivity is equal to the absorptivity.

This law must be also valid in order to satisfy the Second Law of Thermodynamics. As was written, all bodies above absolute zero temperature radiate some heat. Two objects radiate heat toward each other. But what if a colder object with high emissivity radiates toward a hotter object with very low emissivity? This seems to violate the Second Law of Thermodynamics, which states that heat cannot spontaneously flow from cold system to hot system without external work being performed on the system. The paradox is resolved by the fact that each body must be in direct line of sight of the other to receive radiation from it. Therefore, whenever the cool body is radiating heat to the hot body, the hot body must also be radiating heat to the cool body. Moreover, the hot body will radiate more energy than cold body. The case of different emissivities is solved by the Kirchhoff’s Law of thermal radiation, which states that object with low emissivity have also low absorptivity. As a result, heat cannot spontaneously flow from cold system to hot system and the second law is still satisfied.

 

[indy4x]

Just to throw into the mix, was looking for a towel rail type radiator for my bathroom to plumb into the central heating

Like for like the chrome finish had a lower BTU or kW rating than white, from memory it was around 30%

 

[Screwdriver]

It's down to the emissivity of the finish, in the end. White radiators generally emit heat quite well, because the emissivity of white paint is not far off black body. There's an easy test to do if you have one of those infra red thermometers and a thermocouple thermometer.

That doesn't sound right. You are only measuring in the infra red with an IR thermometer so your data will be skewed. White painted surfaces do emit a fair bit more in the infra red but because we can't see that, it still looks white.

 

[Eric the chicken]

I built a solar wax melter for rendering down beeswax after the honey had been spun off. The inside is painted matt black and it works very well even in moderate sunshine. The top is a piece of gash double glazed window pane. I'm now wondering if I should have painted the inside white. Maybe if I had more time I'd repaint it but SABLE looms it's ugly head!

 

[Screwdriver]

I built a solar wax melter for rendering down beeswax after the honey had been spun off. The inside is painted matt black and it works very well even in moderate sunshine. The top is a piece of gash double glazed window pane. I'm now wondering if I should have painted the inside white. Maybe if I had more time I'd repaint it but SABLE looms it's ugly head!

Crikey, that takes me back.

Ten years ago!

Found a scrap double glazing unit

That thing used to blow hot air like a hairdryer.

 

[Burdekin]

That doesn't sound right. You are only measuring in the infra red with an IR thermometer so your data will be skewed. White painted surfaces do emit a fair bit more in the infra red but because we can't see that, it still looks white.

He's correct.