Last updated on
4/11/2006

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You can use some of the stars in the constellation Ursa Minor as an indication of the quality of the observing conditions (atmospheric turbulence and haze) for a given evening.  Ursa Minor (the Little Dipper) is a good constellation for this since it is a circumpolar constellation and thus usually visible from the northern hemisphere year-round. Credit: the technique described below was suggested by Mike Inglis in his (highly recommended) book Field Guide to the Deep Sky Objects, Springer, 2001.

Look for Theta UMi, a magnitude 5.2 star next to the (brighter) Zeta UMi. If Theta UMi is visible, viewing conditions are decent for deep-sky objects.

If Theta UMi is not visible but Delta, Epsilon, and Zeta UMi (mag ~4.3) are visible, viewing conditions are not good for deep-sky objects but are acceptable for casual observing.

If Delta, Epsilon, and Zeta UMi are not visible, viewing conditions are not good for any observing. Curl up and read a good book on observing.

There is merit in separating the concepts of the transparency (clarity or lack of haze) of the night sky, and its “seeing” (lack of turbulence). For example, a cold and clear but very windy winter night can provide atmospheric conditions with good transparency but lousy seeing; both must be good to observe faint objects. There are systems that rate both transparency and seeing on a scale of 1 to 10, and these are discussed in Steven Coe’s book Deep Sky Observing – The Astronomical Tourist (Springer, 2000). Whereas the rating system for seeing, proposed by W. H. Pickering at Harvard Observatory, may be more complex than you want to delve into, there is a fairly simple system for rating transparency .  It was formulated by members of the Saguaro Astronomy Club and I have it here for your download, courtesy of Steve Coe:

SAC Transparency Scale.html

Note that the position of the Jet Stream has a big impact on seeing conditions - if it is overhead at your observing location your seeing conditions will not be good.  The Clear Sky Clock predictive model does include the Jet Stream position; also you can see its predicted position directly from Weatherimages.org, or a more detailed view from the San Francisco State University web site.
 

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