Re: [VAL] White Roof Physics

[John Sellers <sellersj@xxxxxxxxxx>]

Gang,

At the risk of excessive pedantry, let me put in my $.02 worth.  This
*is* a confusing subject.

All objects, such as Airstreams, absorb heat from their surroundings by
radiation (like from the sun), conduction (materials in contact with the
object) and convection (like air and water flowing over the object).

The amount of heat that an object radiates and the wavelengths that it
radiates at depend on the object's temperature.  The sun is *very* hot
and radiates a *lot* of energy.  The rough rule of thumb for a clear day
at the equator is incoming solar radiation of one kilowatt per square
meter.  Wow!  That's a lot of heat!

Cooler objects, like an Airstream, don't radiate much, but they do
radiate some.  For example a human body at 98.6 degrees F or thereabouts
doesn't radiate much, but it radiates enough in the far infrared to be
easily seen by thermal night vision devices.

As Dr. J pointed out, the peak of solar radiation is in the visual
spectrum, wavelengths in the neighborhood of 1/2 micron.  Objects around
room temperature radiate at much longer wavelengths, in the neighborhood
of ten microns.

So, what color should you paint your roof?  What you want is a surface
that absorbs poorly at solar wavelengths and radiates well at
room-temperature wavelengths.  A polished aluminum surface absorbs
poorly and radiates poorly at both wavelengths.  In general, white paint
absorbs poorly at solar wavelengths and radiates pretty well at room
temperature wavelengths.  In general, flat paints radiate better at room
temperature wavelength than glossy paints.  But paint manufacturers
don't
usually specify the long-wavelength IR characteristics of their paints,
so the only way to be sure would be to do experiments as Dr. J suggests.

However, painting the roof white may or may not be worth the trouble. 
In general, for room temperature objects, convection carries away a lot
more heat than radiation.  If there's any wind blowing at all, the color
of the roof won't make a noticeable difference.

Spacecraft, being in a vacuum, can _only_ reject heat by radiation, so
the long-wave infrared performance of their paint is of critical
importance.  Maybe we ought to find out what kind of paint NASA uses on
their spacecraft.

Best,

John & Barb Sellers
WBCCI/VAC #1587
1960 Pacer (polished, no air conditioning)
Dayton, Ohio