[AZ-Observing] Cloudy May?

  • From: Brian Skiff <bas@xxxxxxxxxx>
  • To: az-observing@xxxxxxxxxxxxx
  • Date: Sun, 27 May 2001 16:18:24 -0700 (MST)

     Andrew Cooper asked rhetorically "what happened to a dry and clear May?"
Since I have kept nighttime cloudiness records for 20+ years, I can tell you
the answer:  "it didn't go anywhere in particular", or maybe more specifically
"it wasn't clear on Saturdays this May".  Here's a list of the sum of nights
I called "photometric" (cloudless from dusk to dawn) plus "partial" (at least
three consecutive hours cloudless) for the last 23 Mays.  The last entry
assumes tonight (27th) will be cloudy, a near-certainty.

year  nights
1979  14
1980  13
1981  15
1982  20
1983  26
1984  12
1985  10
1986  12
1987   9
1988  17
1989  11
1990  15
1991  21
1992   4
1993  15
1994  14
1995   8
1996  16
1997   2
1998  15
1999  13
2000  15
2001 (10 thusfar from 27 nights)

     Excluding the current month, the 22 other Mays average 13.5 "good" nights.
The rms scatter is large, however, about 5.  Thus two-thirds of the time we get
between 8 and 19 workable nights during May.  Notice some really suck-y years,
such as May 1997, which is probably too far back for anyone to remember.
     So far in 2001 we've had 10 good nights by my quite conservative criteria,
well within the typical range of the previous 22 years.  These figures are
for Flagstaff, but the overall trend would doubtless be very similar for
Phoenix or Tucson.  If you were to go through my calendric logs and count up
just the Friday or Saturday nights that were clear (the only nights most
amateurs observe), the small-number statistics might get you, and give much
slimmer pickings.
     This month I discovered three near-Earth asteroids and a comet, so I'm
not complaining, especially since it's taken me two whole days to recover from
the last observing run.

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