Every now and then we get enough new MAPUG members that it's time to trot
out an exposition on dew formation. This is the 2001 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 any attention to the relative
humidity (which everyone seems to like to quote - I think it's a conspiracy
by TV "meteorologists" (to use the term loosely)) but look at the dew point.
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 rarely
find dew points higher than the mid 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.

The atmosphere will not get cooler than the dew point. Once the temperature
of the atmosphere reaches the dew point, fog forms (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 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.

So what's a high dew point and what's a low dew point? Here on the east
coast, we'll have dew points around 70 F or a bit above 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. I've been backpacking in the Wind River Range in Wyoming in late
summer when, at around 11,000 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 probably around -5 F
or so and surfaces couldn't cool that far - the temperature losses due to
radiation were compensated by conduction from the atmosphere.

Temperature losses 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 as I
recall, 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 (and in particular, the thin correcting
lens of an SCT) *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 thin SCT corrector (which has very little thermal inertia so
ordinarily cools very rapidly) is radiating. This can substantially increase
the length of time before the onset of dewing. Nonetheless, dewing will
usually occur eventually. (For the observant, you'll note it usually starts
at the center of the corrector when a dew shield is in place. This is
because this area of the corrector 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 -
conduction from the atmosphere is sufficient to maintain temps so dew
doesn't form.

So do the dew. 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

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