Re: Micro in Noosa

>Yeah, there is a phenomenon here that I don't understand. What you describe is also true of unpainted aluminum. You >would think the highly reflective surfaces would stay cool, but they don't.

>On maybe, the metal just transfers heat to your hand faster than a painted wood surface.

>We need some science here.

Let me try to help and keep it as plain english as possible....

You can make black all nice and super shiny too, but it's still going to trap tons of photons and get extraordinarily hot. It's not just how shiny it is, the color represents which frequencies are absorbed or reflected. Shades of grey trap all colors of light photons (frequency), just to varying degrees...black traps more photons more effectively. Titanium white has extraordinarily high albedo. It traps the least number of photons, and therefore stays the coolest.

The reason why metal get so damn hot is because not only is the surface trapping photons, but the heat that is generated travels through the conductive material. Since the surface get's cooler because of this heat sink, it means it can absorb even more heat energy from the sun, and transfer more of it through the metal....i.e. the metal stores the heat. An insulator gets just as hot on the surface, but instead of sinking the heat down into the material, the heat radiates out into the air, so the cooler background behind the surface of the insulated material means it appears very cool. The hot layer is so very thin that as soon as you touch it, your skin cools it down and not enough heat is transfered through your skin to your nerve endings to register the hotness as HOT, though it may feel initially warm for a few moments if the energy heat was enough to reach nerve endings before being completely disappated off through your skin.

If you painted styrofoam with a thin layer of black, you can test this yourself...Experiment, use spray paint to make a very thin layer of black paint, a brush to make a thicker one, and even a few coats on a third stripe to make a nice thick covering. Lay it out in the sun, the paint will get hot really fast, but when you touch it, it will cool very quickly....The thicker layers will cool slower and will feel hot for a few seconds more.....

Do the same with a metal rod...paint it black, thin with spray paint and thick with brush as before....You will not notice any difference between the paint jobs...they'll all get very hot because the metal is the heat sink and it's storing the heat energy.

Do the same experiment with white....oboviously sytrofoam is usually white to begin with....

The metal rod paint white with different thicknesses of white....it'll still get hot....not as hot as the black and it will take longer, but it will still get hot because the metal sinks the heat and stores it allowing the surface to capture more photons and covert it to heat. The styrofoam will get warm too, but it will cool so quickly when you touch it, it may not register because the stryofoam is an insulator not a conductor..the heat stays on the surface and radiates away.

I hope this helps explain how color and materials can interact to either augment or reduce heat from solar gain.

Scot McPherson, CISSP, MCSA
McPherson Family Farms
Le Claire, IA, USA

On Thu, Nov 3, 2011 at 9:07 AM, Peter <pvanderwaart@...> wrote:

>  As far as aluminum color goes stick that in the sun along side white and you will see a huge different in heat

The original color mentioned was "aluminum white" not "aluminum color", but I think he meant Titanium White. Titanium white has a significantly higher albedo than other white colors and across a great frequency range. In fact it also reflects ultraviolet, which is an additional source of solar heat. It would have as noticable a difference in heating between itself and regular whites as regular white does to light grey or aluminum color.

Scot McPherson, CISSP, MCSA
McPherson Family Farms
Le Claire, IA, USA