FreeLists / az-observing / [AZ-Observing] Re: Flagstaff nighttime cloudiness/sky brightness

["Dan Heim" <dan@xxxxxxxxxxxxx>]

Brian & Tom:

Thanks for the info and graphs.  I ran a linear regression on the yearly
data for "photometric nights."  You can see the results here:

http://www.heimhenge.com/downloads/regression.jpg

On the horizontal axis, year zero is 1980.  The multiplier of -0.67 implies
a trend of about one less clear day every 16 months.  Granted, the
correlation is a shaky 0.09, in accordance with your observation that the
amount of scatter obscures any clear trend.  Certainly a longer data run
than 30 years would be better.  But I swear I'm canceling more observing
these days than when I moved here in 79.

Interestingly, if you extrapolate this trend forward, it predicts no
photometric nights (scatter notwithstanding) by the year 2131.  Perish the
thought!  :)

Dan Heim
DFAC President
http://www.dfacaz.org




----- Original Message ----- 
From: "Brian Skiff" <bas@xxxxxxxxxx>
To: <az-observing@xxxxxxxxxxxxx>
Sent: Wednesday, June 13, 2007 12:25 PM
Subject: [AZ-Observing] Re: Flagstaff nighttime cloudiness/sky brightness


>      Further to Dan Heim's queries, Tom Polakis has kindly posted
> three relevant plots from my dataset:
>
> Yearly:
> http://members.cox.net/tpolakis/astro/clouds1.jpg
>
> Monthly:
> http://members.cox.net/tpolakis/astro/clouds2.jpg
>
> Monthly with scatter:
> http://members.cox.net/tpolakis/astro/clouds3.jpg
>
>      The first of these shows the run of the number of clear+partial
> nights since 1980 (there is somewhat fragmentary data for 1978 and 1979).
> The decline through the early 80s I think is real, and from the
> late-80s onward you see only the wide annual variation.
>      The second plot shows the clear+partial nights averaged over the
> whole interval and split up by month.  This shows what we already knew:
> the May/June interval is statistically good, as is the Sep-Oct regime.
>      The third graph shows the same data as the second but with
> one-sigma error bars.  The horizontal lines for each month are the
> means, while the vertical lines show the range within which roughly
> two-thirds of the individual points lie.These are always something
> like one-third of the mean value itself, which tells you that
> good/bad months can occur any time of year, and that the "good" months
> are only one-sigma better than the "bad" months (notice that the
> scatter for February, say, overlaps a lot with the scatter for May).
>      Another way of considering this is that there's plenty of clear
> weather any time of year, and there's no excuse for waiting until June
> (or whatever) for observing.  An example is the unparallelled success
> of the mid-March Messier Marathon in Arizona, which has produced
> by far the greatest number of "110s" recorded by Harmut Frommert
> at the SEDS MM site.  That's cuz it's nearly always cloudy everywhere
> else, but even snowy Flagstaff averages 10 clear+partial nights
> in that month.
>
> \Brian
> --
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