I was just derping around at Cycle Gear this afternoon, when alovasudden, SCIENCE!!!
Couple: *debating about what color helmet to buy - its summer, should they get the white/yellow one instead of black/blue*
Cycle Gear Dude: "The material that makes helmets work as helmets insulates you from the 'heat' of the sun. We did an experiment last summer-- we put a white helmet and a black helmet out in the summer sun for four hours, with a cup of water underneath. There was like, not even a degree difference between the temperature of the water between them."
Me, from across the store: "HAAA! SCIENCE!!!!"
Love the Cycle Gear guys!!
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I was thinking visibility, rather than heat.
Good to know all those years in education weren't wasted on the Cycle Gear Dude.
HA! Love it!
Meanwhile, I had exactly the opposite experience yesterday afternoon. Went to see Prometheus.
Me, from the front row of the riser section: "Yargh! Anti-science!"
(Actually, that's not true. I suffered in silence. But only because I think people who talk in cinemas should have unfortunate experiences with porcupines.)
Actually, it's science, but they got their conclusions wrong. Both black and white make great heat rejecting surfaces, white because it doesn't absorb much, black because it's very good at radiating it back off. Would be an interesting experiment to compare that to a tin foil coated helmet (most metal surfaces seem shiny, but are lousy at radiating infrared).
Is black really better than other colors at reradiating IR? Can you give an explanation?
Your thermal performance is determined by two factors, alpha, the solar absorptivity, and epsilon, the emissivity. A good white coating has an alpha of under 0.2, so it absorbs less than 20% of the incoming radiation. Epsilon typically is 0.95. A good black coating has an epsilon of .99+, so nearly everything absorbed gets radiated off.
For comparison, a polished aluminum sheet has an epsilon of 0.3, which is why you burn your hand on a sheet of metal lying in the sun.
I assume in the case of the white helmet, the radiation is mostly scattered away. But for the black one, wouldn't some of the IR radiation be emitted towards the interior of the helmet, eventually heating the interior to a (slightly) greater degree than with white?
This is important, because I'm designing an earth-sized bike helmet to stop Global Warming.
Bill (great TP reference), the IR radiated to the inside is directly related to the temperature of the material. So if the bulk of the white helmet has the same temperature as the black, the IR inside is the same. As for your earth side helmet, now you're getting into convective heat transfer, and that is usually an order of magnitude higher than radiative. Due to the typical open structure of a bike helmet, you get a lot of localized heating of the atmosphere in the open section and nice temperature gradients to the shaded section. Temperature gradient leads to storm, and all your calculation about radiative effects become a footnote.
Well shit, no grant for me.
Mu, if that's the case (and if it has any real world applicability as opposed to simply being a theoretically sound construct with major confounding factors which render it irrelevant) then could you please offer an explanation as to why our house's propane tank was re-painted silver when it started to become black on top due to dirt and age and such, with the effect of the darkening being that the tank heated up in the sun so much that the propane was expanding and escaping into the air, and when the tank was re-painted silver the tank cooled down to the point that the propane was no longer leaking out?
Also, I think yellow is the most visible color, which is why school buses and taxis are painted yellow.
Also, if all things are equal in black and white coverings, then why is NYC sponsoring a drive to paint all the roofs in the city white in order to lower utility costs as a result of improved energy effeciency based on buildings with white roofs being cooler in the summer than those with dark roofs?
See Mu's comment above about the temperature at the inside.
Plus, heat conduction shouldn't be forgotten.
Bicycle helmets are made of material that has quite low thermal conductivity, so it's quite good at thermal insulation: When you hold something hot to the outside, it takes a lot of time for that heat to spread to the inside. This would be really different with a metal, e.g. copper: Then a lot of the heat would just get conducted straight to the inside.
Plus, on the shady inside of the helmet, there's a nice flow of air that lets the tiny bit warmed air escape through the slits.
Now a propane tank is usually made of metal and doesn't have slits for the warm propane to escape either. So while a black tank surface radiates heat away better than a shiny/white one at the same temperature, in practice a lot of the absorbed heat gets conducted to the inside, heating the contents.
That's great I work at a motorcycle gear place called chaotic motorsports and we have that argument all the time with customers when they are picking out a lid (helmet). Next time I will point them here for the science!