Arkansas Sky Observatory

NOTES ON REGIONAL, LOCAL AND GPS-SPECIFIC SITE SEEING FORECASTS AND
PREDICTION

A Brief Overview by P. Clay Sherrod

We have posted here on the Arkansas Sky Observatory a link to the National
Oceanic and Atmospheric Administration's forecast center for seeing
conditions for North American.

I was asked by an editor at Sky and Telescope magazine whether I believed
that such forecasts were reliable and could be depended upon in astronomical
planning and observing.

At the Arkansas Sky Observatory, nearly continuous and daily logs of seeing
(steadiness of the air) and transparency (clarity of the air) have been
conducted since 1975 and there is a wealth of data available that supports
such forecasts.

For my complete overview of air steadiness and sky transparency as it
relates to astronomical observing, see:
http://www.weasner.com/etx/buyer-newuser-tips/seeing.html.

From the ASO logs, an overview of seeing conditions vs. wave fronts
(meteorological) has been on-going and thus far we have found the following:
(NOTE that "seeing" refers to all bodies of astronomical reference,
including visual, photographic, and photometric)

1) there are regional (i.e., noted through the NOAA forecasts) changes that
can, indeed, be assumed base on incoming/ongoing weather patterns; these are
discussed below;

2) local (i.e., within 100-mile radius) conditions are much more prone to be
overriding of #1) above and are the keys to good astronomical seeing
conditions;

3) immediate (i.e., geographical) conditions are the ultimate key to success
with seeing and will stand alone, REGARDLESS OF #s 1) and 2) above.

To clarify:

We have been keeping sky transparency/seeing logs at the Observatory since
1975 and have found considerable seasonal trends that will vary with #2)
above; there is NO trend that can be accounted for by #1 nor by #3).

Expounding on #1): (no big surprises here for anyone who has been into
astronomy for a while!)

1. Extremely POOR seeing will typically be associated with any locale after
the passage of a strong cold front in which temperature drops are exceeding
10-degrees F. at any specific geographical location;

2. Extremely GOOD conditions will NOT necessarily occur after the passage
of a warm front (regardless of temperature change, up or down) for a
specific geographical change, BUT will be seen improved region-wide (per
#2);

3. BEST seeing is always accompanied by STAGNANT, non-changing air; in
other words no front whatsoever for a period of 48-hours or more.

Regarding # 2):

1. Local seeing conditions can be accurately assumed through the NOAA
forecasts, as they can be through some common sense. Local seeing will be
adversely affected by the passage of any cold front with a temperature drop
of 10 degrees or more; local seeing will also be adversely affect by the
passage of any trough or front in which a humidity factory of greater than
65% is realized from a percentage lower than that number;

2) Humidity will be the driving key for seeing "changes" in local
conditions; an increase in humidity in local areas, regardless of frontal
passage, will result in overall better astronomical seeing conditions; a
DROP in humidity/dew point will result in WORSE seeing conditions for the
same local area.

Now, pulling all that together as it relates to # 3):

1. Specific geographical (i.e., GPS-specific) locations used for
astronomical observing are definitely dictated by the conditions of #s 1)
and 2) above, but are independent as noted following:

2. Geographical topography is the key to local seeing conditions and can be
defined per the following list:

    a) In high humidity/dew point conditions with nearby large bodies of
water (i.e., fresh water lakes and larger rivers) present, an observing
location with an elevation of better than 200 meters ABOVE that body of
water will always benefit from superior seeing conditions to any lower than
that elevation;

    b) during stagnant air periods (i.e., summertime inversions), there
will little advantage to an elevated observing location (such as specified
in "a)" above) over lower positions....in fact, we have actually seen
improved seeing and image steadiness, albeit with decreased limiting
magnitudes, at lower elevations and those closest to the elevation of the
body of water;

    c) if specific GPS position is located within irregular (i.e.,
mountainous with sharp declines in elevation nearby) the seeing conditions
are always more difficult to predict and "good seeing" always occurs
sporadically, rather than for long durations of time;

    d) if specific GPS position is located within high mesa or low flatland
elevations (regardless) in which the terrain is more predictable, the NOAA
forecasts are quite accurate and conditions of seeing (astronomically) will
be prolonged for long periods of time, whether "good seeing" or "bad
seeing"; such locations are advantageous for PLANNED events in which good
steady seeing conditions are required, and thus this is a good tool to use
for planning special events as such;

    e) high humidity (even dew) in GPS specific locations is desired for
steady seeing; low humidity/low dew point, typically results in very poor
seeing;

    f) high temperature, regardless of meteorological events, such as
frontal passages, will oftentimes be accompanied by periods of excellently
steady steering; low temperatures are the converse to that;

    g) clouds moving IN a GPS specific location will be heralded by
increasingly poor seeing; clouds moving OUT of the same area will result in
increasingly good seeing.

    h) perhaps most interesting and curious of all of this is that GPS
specific areas that are within zones (i.e., "local #2) above) in which very
high cumulo-nimbus formations exceed 25,000 feet (as in area severe
thunderstorm development) appear to actually be afforded a "window" of
excellent and remarkably steady seeing conditions, even though lightening
from such thunderstorms might even fill the night sky being observed!

There is a considerable amount more information that is involved with all
this, but this pretty much sums up what we have learned in 25 years of
monitoring the sky from four (4) specific locations at the Arkansas Sky
Observatory....it helps to stay in one place after all!