Below is a report on the weather at Flagstaff, compiled and posted to the newsgroup sci.astro.amateur by Brian Skiff. At the bottom of the message, I make a brief comparison between the weather records from Flagstaff (compiled by Skiff) and the records from APO (compiled by me, from the APO 3.5-m Night Logs). So, let me first turn you over to Brian. +++++++++++++++++++++++ Since folks have been complaining lately about cloudiness, here is the final tally of 1996 data for Flagstaff, Arizona. This is the eighteenth year for which I have similar data. 1996 Flagstaff Nighttime Cloudiness Summary Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Yr (nights) Photometric 3 2 3 2 10 8 0 4 6 12 7 4 61 Monthly Average: 5.1 Partial 9 3 10 9 6 7 0 5 5 7 9 8 78 Monthly Average: 6.5 Spectroscopic 7 7 6 12 10 6 6 6 4 3 3 4 74 Monthly Average: 6.2 Cloudy 12 17 12 7 5 9 25 16 15 9 11 15 152 Monthly Average: 12.7 -------------------------------------------------------------------------------- Clear Hours 97 36 72 64 102 86 0 54 85 160 128 100 984 Monthly Average: 82.0 Clear+Dark Hours 55 22 41 24 30 42 0 24 24 102 75 71 510 Monthly Average: 42.5 Mean Consec 1.5 1.0 1.0 1.0 1.4 1.6 0 2.0 2.0 3.0 1.4 1.3 Monthly Average: 1.4 Max Consec 2 1 1 1 3 3 0 3 3 7 2 2 Monthly Average: 2.3 -------------------------------------------------------------------------------- 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. 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 18-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 and standard deviations 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 1996 61 984 Oct/12 <95> <1087> +/-23 +/-130 The average number of clear nights is about 100, but with wide annual variation (1996 was a -1.5 sigma deviation, however). The total number of clear hours is rather more stable: those partial nights are important! In thirteen of the eighteen years, May or June is our best month (or one or them), with around 16 clear nights. These numbers may seem low compared to numbers that Chambers of Commerce would like to advertise, and it is almost certainly less than you expect if you do not live in the region---it's sunny every day in Arizona, que no? The reason for this is that my standards are quite strict and conservative: it's really clear when I say it's clear. The thin, scattered cirrus so common throughout the Southwest, leaving the days brilliant and sunny, instantly puts a night into the "spectroscopic" category, with zero hours added to the tally of clear ones. (A gorgeous "Arizona sunset" means it's cloudy!) A better estimate of the number of "usable" nights can be obtained by simply summing the "photometric", "partial", and "spectroscopic" nights: for 1996 this was 213, or about 60% of the nights. \Brian Skiff (bas@lowell.edu) +++++++++++++++++++++++++++++++++++++++++++++++++++ Okay, this is Michael again. Let's compare the Flagstaff and APO records, keeping in mind that the criteria are different at each site. APO Flagstaff ----------------------------------------------------------------- Photometric nights 1994 27 (of 157) 83 (of 365) 1995 78 (of 311) 80 (of 365) 1996 47 (of 329) 61 (of 366) It appears that the weather at APO and Flagstaff isn't too different, by this measure; one might then conclude that all of Brian's numbers might be reasonably close to the APO values. Michael Richmond APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 111 in the apo35-general archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-general/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-general@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO