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

["Steve Walters" <s-walters@comcast.net>]

Hi Greg,

This is an excellent exposition but I have a question. You said:

> 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.

Can you explain why this is true? And what are the other factors that will
cause changes?

Thanks!

Steve...


----- Original Message ----- 
From: "Greg Hartke" <ghartke@adelphia.net>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Tuesday, September 30, 2003 5:18 PM
Subject: RE: [APML] dew formation. Was: Film Creep


>
> 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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