Dear Solar Guy,

        The other day I picked up a wrench that was lying in the sun, and it
'burned' my hand.  The wrench is very shiny, so I know that it reflects the
sun.  If it reflects the sun, why is does it get so hot ?
Ray Caliente, Shiner, TX.

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Dear Ray,

        Our sense of temperature is not absolute, as it depends on the type
of material touched.  Consider two materials of the same true temperature.
If our finger comes in contact with a metal and a non-metal (i.e. plastic),
the metal will feel hotter.  This is because the metal's high thermal
conductivity readily draws heat from its interior to the surface ..... and
it therefore feels hot ... and what's more is hot.  If one were to do the
same thing with a non-metal (plastic), the surface in contact with our
finger cools off to a temperature between its true (interior) temperature
and that of our finger, since it has a much lower thermal conductivity that
the metal. We 'sense' the temperature as not so hot as the metal.  However,
if for the two cases, we were to use a thermometer and let it come to
'equilibrium', it would sense the 'true' temperatures and they would be the
same.

        It is true that clean metals are shiny and tend to reflect the sun's
rays fairly effectively.  Well polished stainless steel has a solar
reflectance of about 60 percent and a typical 'clean' metal may have a
reflectance of 40-50 percent, so about half is reflected and half absorbed.
Thus, it still absorbs significant radiation.  A black garden hose on the
other hand will reflect about 10% and absorb about 90% of incident solar
radiation.  Yet if we touch the two, the metal 'feels' hotter, even though
it is cooler.

        I guess the best analogy to this is the shower room floor.  We know
that if we have a tile floor, we go to Sears and buy a rug ... not a
stainless steel mat.  They will be the same temperature, but the rug sure is
a lot more comfortable.

The Solar Guy.