To: atm{at}shore.net
From: mdholm{at}telerama.com
Reply-To: mdholm{at}telerama.com


I would like to be able to be quantitative about this topic, but am not
prepared for that at this time.  (and probably not any time soon.)

Air, especially the dry, slow moving kind astronomers prefer, is a pretty
good insulator.  Around any object is a thin layer of air, known as the
boundary layer, that does not move very quickly unless there is really some
serious wind blowing.  The boundary layer exists because, although the
viscosity of air is low, it is not zero.  Viscosity is the transfer of
momentum through a fluid. At the exact boundary between air and a solid,
the air molecules collide with the surface, and become nearly stationary
(regarding bulk motion, not Brownian).  The viscosity of air tends to
transfer this stationary condition out into the surrounding air.  Hence the
boundary layer.

Since the boundary layer experiences little bulk motion, little convective
heat transfer takes place.  Conductive heat transfer is strongly dependent
on the number of molecules per fixed volume.  Air has rather few molecules
per unit volume compared to liquids or solids, as a result, conductive heat
transfer is also rather low.  Air also has rather low heat capacity, so you
have to have heat transfer with a lot of it to move much heat.

Since conductive and convective heat transfer are low in dry still air,
radiative heat transfer becomes relatively much more important.

Some important facts about radiative heat transfer.

1. Everything radiates. (except vacuums).  Everthing also absorbs heat radiation.

2. Different substances vary widely in the effectivnes of radiation (called
emissivity) and the efficiency of absorbtion (called absorbtivity).
Absorbtivity is, at least roughly, the reciprocal of reflectivity: shiny
objects reflect more and absorb less.  A very important point is that shiny
objects also tend to have low emissivity: they radiate less heat out.

3. Emissivity and absorbtivity often vary quite strongly with wavelength. 
This is similar to saying that, over the range of the electromagnetic
spectrum, objects are often strongly colored.  Something that appears quite
white at one wavelength range might well be nearly black in another range.

4. Amount of heat radiated is strongly dependent on temperture.  Cold
objects radiate little, warm objects radiate a lot more.

5. At temperatures characteristic of Earth's surface, most radiative heat
transfer occurs in the near, mid and far infrared wavelength range.

6. Dry air is fairly transparent to radiation in the infrared range (low
absorbtivity), not perfectly so, but enough so that significant radiative
heat transfer can occur right through Earth's atmosphere, especially when
it is nice and dry.  Dry air also has quite low emissivity, so that even
though it is a lot warmer than outer space, it doesn't radiate much heat.

7. The effective temperature of a dry, cloudless sky is quite low, I forget
the exact value, but it is significantly cold, i.e. cryogenic.  This is
true even when the air temperature is balmy.  Compared to the sky
temperature, nearly everything on Earth's surface is pretty warm, this
includes your telescope.

There are several consequences of the above. I can't begin to go into all
the ones I know here and there are undoubtably more that I don't know. 
Here are a few.

Your telescope on a clear, dry night with low wind is in poor thermal
contact with it's immediate surroundings, but in pretty good contact with
fridgidly cold outer space.  It will cool significanty below ambient air
temperature (if the ambient air temperature doesn't fall faster).  Portions
of your telescope that are not shiny metal and are more exposed to the sky
will cool faster.  In many telescopes, the primary mirror is one of the
best shielded parts, and it has a very low emissivity shiny metal coating. 
It tends to stay warmer than the rest.  Black objects tend to have quite
high emissivity as well as high absorbtivity.  At night, there is no hot
Sun to absorb heat from, so black objects tend to radiate much more away
than they absorb, they tend to cool off.  In the infrared, the same is
pretty much true of most white surfaces. Shiny metal surfaces will stay
warmer than most white or black ones at night.

During the day, when there is one very hot, small (in angular extent),
copious source of infrared radiation in the sky, things are changed.  White
stays pretty cool because it absorbs little and emits a lot.  Black gets
hotter because it absorbs more while emitting a similar amount.  Shiny
metal often gets hottest of all, because even though it absorbs little, it
emits even less.

An umbrella, or parasol, cools during the day because it absorbs a great
deal of the Sun's incoming radiation, loses a great deal of it to the air,
and by radiation to the sky, so it is much cooler than the Sun and emits
much less radiation toward you.  At night, an umbrElla actually keeps one
warmer, because it blocks part of the very cold sky.  The umbrella absorbs
heat from the air and it's surroundings, as well as radiating it to the
sky, so it stays considerably warmer than outer space.  The umbrella
radiates more heat toward you than the equivalent area of sky would.  One
problem, the sky is a lot bigger area than the Sun.  An effective night
umbrella has to be much larger than an effective day umbrella so that it
can block all or most of the sky, and we often call it a tent.

At night, a polished metal telescope outside will probably stay warmer than
any other surface finish.  Spider vanes, near the open end of a telescope
are more exposed to the sky than the main mirror.  Radiative effects have
more bearing on them than on the primary.  Shiny spider vanes might not be
so crazy as they at first seem, especially when you consider that most
black surfaces actually have fairly high reflectivity at low angles.  Black
doesn't really gain much in reduced low angle reflection.  A small, shiny
umbrella close to and just above the secondary mounting might keep it
significanlty warmer, possibly delaying dewing.  A shiny outside dewcap
over a Telrad or similar device can keep it warmer and ward off dewing,
even without added electrical heat.  (This has been verified by amateur
astronomers).

Mark Holm
mdholm{at}telerama.com

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