Jim Lux wrote:

There are also complications about temperature when referring to solids,
liquids, and gases. The "temperature" of even a weekly ionized plasma
is quite high (e.g. 11000 K per eV), but that more to do with the
velocity of the ions and the mean free path. There's not much mass
there, so the "heat" is small. That is, even though the ionosphere is
"hot" in a temperature sense, it's not very "hot" in a sensible transfer
of heat sense.


BTW, I think the sunburn is not from thermal absorption, but from
photons with enough energy to make the reaction go. The total energy
in the UV of sunlight is MUCH lower than the total energy in the visible
range. The power spectrum of sunlight is pretty close to the spectral
sensitivity of your eyes (which evolved that way to match, I would think).

A good graph of sunlight power density vs wavelength can be found at
http://en.wikipedia.org/wiki/File:Solar_Spectrum.png. Comparing areas of
various graph sections shows that the UV part of the spectrum contains
maybe 1/5 the amount of energy as the visible part -- plenty enough to
embrittle plastics and fabrics and sunburn skin. But the infrared energy
-- invisible to our eyes -- looks to be at least equal to the visible
energy.

At least one reference says that sunburn is a direct reaction to DNA
damage from UV photons. Melanin protects because it absorbs the UV and
turns it into heat.

http://www.scientificamerican.com/ar...s-when-you-get


Some people like to view electromagnetic waves as photons. I find waves
easier to understand, but each to his own.

My explanation was simplified. There's also latent heat or heat of
change of state. For example, if you apply heat to ice, it'll warm up to
0C, but stay at that temperature in spite of the heat input until it
melts. The heat (energy) goes into converting the ice to water instead
of raising the temperature. After it all melts, continued heat
application will of course raise the temperature of the water.(*) Until
it reaches the boiling point, that is. Then the same thing happens again
-- it stays at 100C until it all boils. If you confine the resulting
steam, adding heat will raise both its temperature and pressure after
the water is all converted.

(*) That's why people experienced in cold weather outdoor activities
never eat unmelted snow for water when there's any danger of hypothermia
-- it takes about twice as much heat just to melt 0C snow into 0C water
as it does to raise the temperature of 0C water to body temperature. In
other words, you use up 3 times the energy eating 0C snow as you do
drinking 0C water.

Roy Lewallen, W7EL