Re: THERMO

["John S. Denker" <jsd@MONMOUTH.COM>]

David Abineri wrote:

> How does one explain the concept of its being perhaps 700 degrees C
> high in the atmosphere but that an person would freeze to death (if
> not succumb to some other fate) if exposed up there?

Suppose you are standing near a roaring fire,
but there's a thin piece of transparent glass
between you and the fire.  Suppose the glass
is very cold.  It doesn't matter how cold
the glass is;  you're gonna get cooked by
the radiative heat from the fire.  Your
energy budget isn't determined by the nearest
things (air and glass) because they are
transparent;  other effects dominate.

To illustrate the other side of the same
coin, the sparkles coming off a 4th of July
sparkler are at a few thousand degrees, but
they don't burn your hand, because they are
so few and far between.  The energy budget
is dominated by other effects.

At the top of the atmosphere, you couple to
300K on one side (the earth's surface) and
3K on the other side (outer space).  You
would be losing heat by blackbody radiation
from your skin (65 mW per cm^2).  You would
be roughly getting this right back on the
earth side, but it would be a total loss on
the sky side.  This
wouldn't be too terribly hard to deal with;
it would be roughly comparable to walking
around in the high desert at night, for
the same reason:  you've got 2pi steradians
of warm earth and 2pi steradians of cold
sky.  Given some decent clothes you'd be
fine.  The temperature of the air high in
the atmosphere is irrelevant because there's
not enough of it.  It's like sparkles.  The
energy budget is dominated by radiative transfer,
not air conduction.

If you had a reeeeally thick blanket of air
at 700K (optically thick) then we would need
to have a different conversation.

I don't much care about the temperature of
an object unless there's some process that's
trying to bring me into equilibrium with
that object.

You can freeze water in the high desert in
summer by radiative cooling.  Just insulate
it from the air and give it a line of sight
to the sky (and not to the ground).  Been
there, done that.  Impresses the Muggles.
Don't try it in Florida;  only works in dry
air.  H2O in the atmosphere couples to the
radiation.  N2 and O2 don't couple.

> What would it mean to have a temperature of 700 degrees C in empty
> space?

Does "empty" mean devoid of matter?
Then the meaning is clear:  the space would
be filled with blackbody radiation at 700K.
An ordinary object sitting in that space would
fairly quickly come into equilibrium at 700K.