Transparency vs. Seeing
Transparency refers to the clarity of the sky while seeing concerns the
"steadiness" or stability of the atmosphere.
You can have a clear, deep blue sky
with great transparency, but poor seeing...
or a fairly
clear, but hazy sky with very steady conditions;
or in rare
cases, a very clear sky with a very dry and stable atmosphere.
Transparency is generally best after a surface cold front/low pressure
system moves through the region and surface high pressure builds in.
Typically the air mass accompanying surface high pressure is cooler,
drier, and more stable.
However, many times, especially during the fall, winter, and spring the
upper air pattern with this situation may have the jet stream in close
proximity (within 300 miles), thereby causing poor seeing conditions
due to turbulence in the upper atmosphere.
In addition, the local effects of strong radiational cooling,
especially an hour or two after sunset, can contribute to poor seeing
within the lower atmosphere despite very clear conditions.
Depending on how much moisture is in the air, the formation of dew,
frost, and/or fog can also become a problem for observing. If early
evening dew points are above the forecast lows for a given night, and
clear skies and light winds favor decent radiational cooling, then dew,
frost or fog will likely form.
You
can have a fairly clear, but hazy sky with very steady conditions...
In Michigan, this situation
usually occurs during warm/dry periods in the summer and fall when a
pronounced ridge of high pressure develops over our region in the upper
atmosphere and remains nearly stationary.
This set up can bring us
heat waves in mid summer when air temperatures soar into the 90s to
near 100 degrees and dew points climb to miserable levels above 70.
Huge Thunderstorm complexes with annoying cirrus blow-off can hamper
viewing mainly along the top or outer periphery of the ridge.
During the fall, most
notably in October, a large ridge of high pressure in the upper
atmosphere gives us "Indian Summer" weather characterized by afternoons
of warm/hazy sunshine and mostly clear/cool nights with patchy fog.
When a strong upper level
ridge of high pressure prevails, the first couple of days are usually
the clearest. Strong subsidence (sinking motion), combined with a
gradual return of southerly winds at the surface help to trap moisture
and pollutants in the lower atmosphere. This is a very stable situation
with no jet stream overhead, therefore the atmosphere is typically very
steady.
You
may experience on rare occasions a very clear sky with a very dry and
stable atmosphere...
This unique situation
usually occurs in Michigan during the summer months after the passage
of a backdoor cool front. The front then becomes stationary to our
southwest while a pronounced ridge of high pressure develops overhead.
Although air temperatures may only be a little cooler behind the front,
northeast winds at the surface initially bring in drier air and lower
the dew point. It is during the first couple of days of this event in
which both a very clear and steady atmosphere may preside. Eventually a
southerly flow returns and the lower atmosphere becomes murky again.
Some
other things to keep in mind...
The
atmosphere is most placid shortly after sunset, just before the onset
of strong radiational cooling, and again prior to daybreak when cooling
is complete.
There is no such a thing as a truly dark sky in southern Lower
Michigan. If you seek a very dark sky you must go north into the Upper
Peninsula or Canada.
At higher
elevations (large hills and mountains) the sky tends to be clearer
especially if you can get above the boundary layer of the lower
atmosphere. This may also reduce the effects of light pollution since
the concentration of atmospheric aerosols is usually less above the
boundary layer.
![[Clear Skies]](https://web.archive.org/web/20051225221233im_/http://my.voyager.net/~stargazer/Images/clearskies.jpg)
