Folks in the US Southwest have been enjoying a remarkably long string
of warm, sunny days and clear nights since late September.  Based on my
nighttime cloudiness data since 1978, the present spell is about as good as
it gets.  The 30-day interval from 24 September through 23 October had 27
"photometric" nights, i.e. cloudfree from dusk-to-dawn, amounting to nearly
300 clear hours.  The 16-day stretch from 7-22 October was cloudless.  Several
of the days/nights were marred to some degree by smoke, first from California
brush fires (caused by stupid humans) over 500km distant, then local
"prescribed burns" on the forest near Flagstaff, which is done to prevent
wildfires.  Although this latter sort of burning is very much necessary, it is
unfortunate that the best weather for it (cool, little wind) coincides with
good weather for astronomy.  The smoke from these nearby fires was highly
localized, such that for instance Mars Hill and the USNO-Flagstaff site were
often noticeably affected, while Lowell's Anderson Mesa site was nearly always
smoke-free.  Apart from these episodes, when the extinction was up around
0.25 mag/airmass at V, we had mostly winter-like extinction of about 0.12-0.13,
with onyx-blue daytime sky right down to the horizon.  Gorgeous!
     The current clear spell sets the local "Chilean" clear-weather record
since 1978.  Other similar periods include 14 Sep - 10 Oct 1980, a 27-day
stretch that included 24 photometric nights, of which 14 were consecutive.
There was another 14-day string 20 Sep - 3 Oct 1993; the 23-day interval
16 June - 8 July 1997 included 19 photometric nights with 11 consecutive.
You can get a hint from the four sets of dates that our statistically most
consistent clear weather occurs in May/June and September/October.
     It appears that in the coming week this cloudfree period will be broken
by varying amounts of cirrus, which nevertheless will make the sky
nonphotometric, and this major block of clear sky is finished for now.  The
present 90-day climate forecast suggests generally dry weather will continue
through the winter (see http://www.wrcc.sage.dri.edu/longrang.html).
     My nighttime cloudiness list is appended.  Each night is categorized by
a single term, defined below.  The values in parentheses are the number of
clear hours followed by the number of clear, Moonless hours, all reckoned
using the interval between nautical twilights, and any Moon presence ignored
when it is less than 15 percent illuminated.  Data for other periods (including
lots of cloudy nights!) are available via the Lowell Web site starting at:

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

\Brian

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photom = "photometric", cloudfree dusk-to-dawn
partial = at least 3 consecutive photometric hours
spectro = "spectroscopic", less than ~1 mag. extinction [didn't occur here]
cloudy = cloudy


September

23   cloudy  [Winterstorm 3]
24   photom (10,0)  [end monsoon]
25   photom (10,0)
26   photom (10,0)
27   photom (10,1)
28   photom (10,2)
29   photom (10,2)
30   photom (10,3)

October
1    photom (10,4)
2    photom (10,5)  [smoky]
3    photom (10,6)  [smoky]
4    partial (8,7)
5    photom (10,10)
6    cloudy  [Winterstorm 4]
7    photom (10,10)
8    photom (10,10)
9    photom (10,10)
10   photom (10,10)
11   photom (10,10)
12   photom (10,10)
13   photom (11,8)  [smoky]
14   photom (11,7)  [smoky]
15   photom (11.6)  [smoky]
16   photom (11,5)
17   photom (11,4)
18   photom (11,4)
19   photom (11,3)
20   photom (11,2)
21   photom (11,1)
22   photom (11,0)
23   partial (6,0)