[vsnet-chat 1529] 1998 Flagstaff nighttime cloudiness

[bas@lowell.Lowell.Edu (Brian Skiff)]

Folks,
     We had a pretty good year in Flagstaff as far as observing weather goes.
The total number of completely cloudfree nights, 96, was typical, but the
number of clear _hours_, about 1100 by my reckoning, was above average at the
+1-sigma level.  All these numbers were helped by persistently dry weather
during autumn (which looks likely to continue through the winter) and our usual
Arizona dry'za-bone June.  Folks wanting to know the number of "usable nights"
for astronomy can get a good value by simply adding up the 'photometric',
'partial', and 'spectroscopic' nights.  Even this might be considered
conservative---this isn't Chile, but there aren't very many nights when
absolutely no data of any sort can be obtained.
     I started keeping this record of the nighttime cloudiness in June 1978,
so this marks the twentieth full year for which I have results.  The statistics
might actually start to mean something now!  Annual summaries for each year,
such as the one below, are available at the Lowell Web site, starting at:

http://vsnet.lowell.edu/cgi-bin/www/clouds/clouds.cgi

The nightly listings for the last few years are provided there as well.

\Brian

===============================================================================

1998 Flagstaff Nighttime Cloudiness Summary


                 Jan  Feb  Mar  Apr  May  Jun  Jul  Aug  Sep  Oct  Nov  Dec  Yr
                              
                                        (nights)
           
Photometric        3    4    5    6   10   19    8    3   14    9    8    7   96
                                                           Monthly Average:  8.0

Partial            2    2    4    5    5    1    2    2    2    6    8    8   47
                                                           Monthly Average:  3.9

Spectroscopic      8    3    6    8    4    5    5    7    2    4    7    6   65
                                                           Monthly Average:  5.4

Cloudy            18   19   16   11   12    5   16   19   12   12    7   10  157
                                                           Monthly Average: 13.1

-------------------------------------------------------------------------------

Clear Hours       51   53   71   78   75  136   64   34  145  126  131  139 1103
                                                           Monthly Average: 91.9

Clear+Dark Hours  30   38   35   35   49   83   24   24  116   56   98   81  669
                                                           Monthly Average: 55.8

Mean Consec      1.0  1.3  1.3  1.0  2.0  3.2  2.7  1.0  3.5  1.8  1.6  2.3
                                                           Monthly Average:  1.9

Max Consec         1    2    2    1    3    6    4    1    5    3    3    3
                                                           Monthly Average:  2.8
-------------------------------------------------------------------------------

Definitions:

Photometric  -  cloudfree dusk-to-dawn.
Partial  -  at least 3 consecutive cloudfree hours during the night.
Spectroscopic  -  less than about 1 mag. extinction throughout the night.
Cloudy  -  cloudy.

Clear+Dark Hours = photometric hours with moon absent.
Mean Consec = mean number of consecutive photometric nights during the month.
Max Consec = maximum number of consecutive photometric nights during the month.

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