1981 Flagstaff Nighttime Cloudiness Summary Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Yr (nights) Photometric 11 7 10 9 13 21 6 11 13 20 12 9 142 Monthly Average: 11.8 Partial 2 4 1 6 2 1 1 3 6 2 0 2 30 Monthly Average: 2.5 Spectroscopic 0 4 2 4 3 1 0 4 1 0 1 6 26 Monthly Average: 2.2 Cloudy 18 13 18 11 13 7 24 13 10 9 17 14 167 Monthly Average: 13.9 -------------------------------------------------------------------------------- Clear Hours 137 92 94 104 104 131 46 96 108 192 124 128 1356 Monthly Average:113.0 Clear+Dark Hours 82 45 44 58 57 46 21 66 80 111 62 67 739 Monthly Average: 61.6 Mean Consec 1.8 1.8 2.0 3.0 2.6 7.0 1.5 2.2 1.7 6.7 1.7 1.8 Monthly Average: 2.7 Max Consec 4 4 3 7 4 16 2 4 4 12 4 4 Monthly Average: 7.0 -------------------------------------------------------------------------------- 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.