Comments

1. #1 5ecular

   England

   June 11, 2012

   I was thinking visibility, rather than heat.
   Good to know all those years in education weren’t wasted on the Cycle Gear Dude.

2. #2 Optimus Primate

   June 11, 2012

   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.)

3. #3 Mu

   June 11, 2012

   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).

4. #4 James

   June 11, 2012

   Mu,

   Is black really better than other colors at reradiating IR? Can you give an explanation?

5. #5 Mu

   June 11, 2012

   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.

6. #6 Bill Door

   June 11, 2012

   @Mu
   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.

7. #7 Mu

   June 11, 2012

   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.

8. #8 Bill Door

   June 11, 2012

   Well shit, no grant for me.
   Thanks, though.

9. #9 John

   June 21, 2012

   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.

10. #10 John

    June 22, 2012

    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?

    http://www.psmag.com/environment/new-yorks-white-roofs-prove-theyre-cool-40312/

    http://green.blogs.nytimes.com/2012/03/09/in-an-urban-roof-contest-white-trumps-black/

    http://www.nasa.gov/topics/earth/features/ny-roofs.html

11. #11 Ysanne

    Oz

    June 25, 2012

    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.

12. #12 Dominik Hussl

    Houston TX

    September 30, 2012

    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!