Subject: weather statistics at Flagstaff, 1979-1995
From: Stupendous Man
Submitted: Wed, 14 Feb 1996 10:59:57 -0500
Message number: 49
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Brian Skiff of Lowell Observatory has been keeping track
of the weather at Lowell for many years. He recently posted
a summary of the past 17 years of weather there, which might
be useful as a check/comparison for APO.
Here's Brian Skiff's message:
------------------ Brian Skiff's analysis ------------------------------
Just to finish off the discussion about quality of observing sites, here's
some yak-yak about cloudiness. The following is adapted from a note I prepared
for the Saguaro Astronomy Club (Phoenix) newsletter a few years ago. Members
of the "Friends of Lowell Observatory" will also hear some echoes from a past
newsletter. Unlike the previous general discussions, this one deals more
specifically with Arizona weather.
I have been noting the nighttime cloudiness in Flagstaff since June 1978,
and from 1979 I have daily data on hand. (I regret now having not started
this business the day I arrived in 1973.) I categorize each night as follows:
photometric - cloud-free dusk to dawn
partial - more than 3 consecutive photometric hours during the night
spectroscopic - less than about 1 magnitude of obscuration by thin clouds
cloudy - cloudy
Since clouds absorb light at all visible wavelengths equally, it is usuallynot necessary when taking spectra of astronomical objects for the sky to
actually be cloudfree. It merely means the observer must expose longer to get
enough signal. Thus spectroscopists are going full blast on nights that those
of us who do photometry are out of business merely from a few cirrus clouds.
Thus it is worth noting those nights for statistical purposes, since our
telescopes are working.
The common definition of a "clear" night at most observatories is one
with at least six consecutive cloudfree hours. This means that on a 12-hour
winter night it can be cloudy for half the night, yet it still gets counted as
a fully "clear" night. My "photometric" definition requires that the entire
night be clear, which makes our numbers look less impressive. But the
"partial" nights are accounted for in the tally, so the total clear hours for
any period can be estimated accurately.
For what it's worth, I reckon the duration of the night to be the
interval between _nautical_ twilights minus 30 minutes, which corresponds to
a solar elevation of -15 degrees at our latitude. This is about the time when
the rate of change in the twilight sky brightness slows from its exponential
decline (or starts up in the morning). It is also about the time when the
twilight sky toward the Sun loses its blue color (or gains it at dawn). When
doing CCD observing, one is often taking twilight flat-field frames within a
few minutes after sunset, so if you consider this to be "data" (you gotta have
it get decent CCD results, so why not?), we're observing well before/after the
traditional time of astronomical twilight (-18 deg solar elevation).
Some highlights from the 17-year dataset are given in the table below.
This shows the total number of "photometric" nights (completely cloudfree
dusk-to-dawn) for each year along with the total number clear hours, which
includes time from partly clear nights. In addition, the best month(s) for
each year is noted along with the number of clear nights in that month(s).
The averages for photometric nights/hours are given at the bottom.
Year Photometric Best
nights hours Month/# clear nights
1979 121 1060 Jun/16
1980 141 1320 Oct/20
1981 142 1356 Jun/21
1982 108 1076 May/18
1983 99 969 May/19
1984 95 1025 Feb,Jun/13
1985 83 953 Jun/14
1986 72 997 Jun/11
1987 61 910 Jun/10
1988 88 1062 May/15
1989 106 1288 Nov,Dec/14
1990 91 1223 Oct/19
1991 94 1079 May/17
1992 82 1036 Jun/15
1993 109 1156 Sep/21
1994 83 1030 Jun/12
1995 80 1036 Jun,Oct/15
<97> <1093>
----------------------- end of Brian Skiff's message ---------------------
It is an interesting coincidence that, in 1994-1995, Skiff reports
that 22 percent of the nights were photometric at Lowell; and
during the period Nov 7, 1994 - Feb 13, 1996, the weather
logs from APO also show 22 percent of the nights as photometric.
We might take solace in Brian's tally: it appears that
over the long term, we can expect slightly more photometric
nights.
Michael Richmond
APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO
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