RE: [APML] dew formation. Was: Film Creep

["Greg Hartke" <ghartke@adelphia.net>]

Every now and then I get motivated enough to trot out an exposition on dew
formation. This is the late-2003 version. ;)

First off, the dew point is the temperature at which the air (with it's
current amount of moisture) would be completely saturated. If you really
want to know how humid it is, don't pay too much attention to the relative
humidity but look at the dew point. When you understand what the dew point
is telling you, you'll know much more than if you only know the relative
humidity. At a given tempertaure, the air can only hold so much moisture;
when completely saturated the relative humidity is 100%. The amount of
moisture that the air can hold varies dramatically with temperature so that
70% relative humidity is very high humidity in the summer when it's warm and
very moderate (and possibly low) humidity in the winter when it's really
cold. So relative humidity doesn't tell you much because you have to know
the temperature, too. If you know the dew point, you know immediately if
there's a lot of moisture in the atmosphere. Here in the USA, you don't find
dew points higher than the mid to upper 70's and that only in the southern
(especially coastal) regions. Be thankful. Tropical areas can have dew
points considerably higher and that would *really* be uncomfortable. BTW, if
you ever hear anyone saying, "Yessir, we had 90+ F temperatures and 90%+
humidity," you know immediately they don't know what they're talking about.
A relative humidity of 90% at 90 F requires a dew point of 88 F and that
just doesn't happen in the US.

The atmosphere will not get cooler than the dew point. Once the temperature
of the atmosphere reaches the dew point, fog will form (a relative humidity
of 100%, of course) and the temperature remains pretty stable until other
factors cause changes. In most areas, if you know the dew point in the
evening you can predict rather closely how low the temperature will drop
overnight: It won't drop any lower than the dew point. If the dew point is
*very* low, the temperature will never drop that far but it will likely get
very chilly. Most of us live in areas where the relative humidity is
routinely in the 90+% range. Nothing strange there. The energetics of the
atmosphere dictate that the air temperature at night will usually drop to
just above the dew point resulting in such relative humidity values. There
are also may locations where the dew point is sufficiently low that the
temperature normally doesn't approach the dew point at night. I wish I lived
in a place like that!

So what's a high dew point and what's a low dew point? Here on the east
coast, we'll have dew points in the low 70's F or maybe mid-70's F when it's
really yucky weather. In the summer, a dew point in the 50's F here means
very comfortable conditions and you know darned well you'll have highly
transparent conditions at night. Great stuff for the summer here, but a bit
uncommon. Libby and I have been backpacking in the Wind River Range in
Wyoming in late summer when, encamped at 12,000 ft, the dew point was so low
that the temp dropped well below freezing at night yet there was no frost
(frozen dew, of course) that could form on the tent. The dew point was well
below 0 F and surfaces couldn't cool to temperatures that low;  the thermal
energy losses due to radiation were compensated by conduction from the
atmosphere.

Thermal energy loss (hence low temperature) due to radiation? Yup, that's
what actually causes surfaces to cool below the ambient temperature and drop
below the dew point.

Dew formation is a radiation problem, not a conduction/convection problem as
some people occasionally seem to think. If it were conduction/convection,
the scope (and other surfaces) would never cool much below the dew point
which would be above ambient. Instead, the surfaces exposed to the sky
undergo radiation exchange as they work to reach radiative equilibrium with
the sky. This can cause surfaces to cool significantly below the ambient
temperature and if the dew point is high enough, the surface will try to
cool below that and dew will form. If there were nothing to mitigate this
radiational cooling, surfaces would continue to cool all the way down to the
radiation temperature of the sky.

The clear, dry sky has a radiation temperature of approx 180-200 K if I
recall correctly, NOT the 2.73 K of the cosmic microwave background as some
would have you think. There are some spectral line features in the radiation
of the sky, as you would expect, but it can (to zeroth order) be considered
a blackbody at the above radiation temperature. Anyway, the reason that
surfaces such as our exposed scopes *don't* cool to the radiation
temperature of the sky is because conduction from the surrounding air serves
to continually warm the surfaces. A dew shield operates by signifcantly
reducing the solid angle into which the exposed optic (which might have very
little thermal inertia so ordinarily would cool very rapidly) is radiating.
This can substantially increase the length of time before the onset of
dewing. Nonetheless, dewing will usually occur eventually. The observant
will note that it usually starts at the center of the optic when a dew
shield is in place. This is because this area of the optic sees the most
solid angle of the sky, hence radiates and cools the most. For anyone who
will be out for long periods with their scope, some form of active dew
control is usually necessary to replenish the thermal energy lost to the
sky.

As many have no doubt noticed, dew is much less of  a problem when there's a
bit of a breeze. The breeze serves 2 purposes: First of course it helps to
evaporate the dew as it forms. Somewhat surprisingly, it also provides a
replenishable reservoir of warmth (relative to the temperature that would be
reached if the surface could continue to cool) to help to maintain surfaces
at a slightly higher temperature. It's hardly important to the amateur
astronomer but I'm sure many have noticed that dew doesn't form when it's
cloudy. The underside of the clouds have a much higher radiation temperature
than the clear sky so the radiation losses of surfaces exposed to clouds is
much less than to clear sky. The radiational cooling rate under cloudy skies
is sufficiently low that surfaces don't drop below the dew point  since
conduction from the atmosphere is sufficient to maintain the surface
temperature to the point that dew doesn't form.

So do dew mitigation. Use a due shield and a do-zapper so you can dew your
observing. Or something like that. I dew get mixed up occasionally. Don't
due?

Greg Hartke
Sykesville, MD

PS. BTW, aren't all of us on APML "film creeps"??

> -----Original Message-----
> From: astro-photo-bounces@seds.org
> [mailto:astro-photo-bounces@seds.org]On Behalf Of George Anderson
> Sent: Monday, September 29, 2003 11:34 PM
> To: Discussion of Film Astrophotography
> Subject: Re: [APML] Film Creep
>
>
> That is a simple and sweet circuit, and the dewpoint chart is very
> useful.
>
> George Anderson
> Montreal Canada


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