USNO Flagstaff Station
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the USNO Flagstaff Station clear sky chart from 2005-11-05 to 2017-09-18.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
55%
15595
33%
46924
22%
15554
66%
15554
44%
23175

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 15thNight
45%
720
31%
2327
21%
716
60%
716
38%
1163
January16th to endNight
44%
768
34%
2496
22%
768
61%
768
35%
1292
February1st to 15thNight
49%
700
33%
2220
23%
700
65%
700
43%
1073
February16th to endNight
39%
632
36%
1892
23%
632
56%
632
37%
980
March1st to 15thNight
44%
720
37%
1980
20%
720
60%
720
45%
1010
March16th to endNight
48%
768
34%
2112
18%
768
62%
768
47%
1064
April1st to 15thNight
36%
689
29%
1948
12%
689
51%
689
39%
991
April16th to endNight
42%
540
41%
1671
19%
540
64%
540
46%
728
May1st to 15thNight
47%
540
35%
1619
20%
539
61%
539
52%
719
May16th to endNight
46%
561
34%
1684
19%
561
58%
561
47%
755
June1st to 15thNight
50%
540
32%
1620
22%
540
59%
540
59%
694
June16th to endNight
67%
540
34%
1620
27%
540
72%
540
65%
674
July1st to 15thNight
73%
510
20%
1530
15%
510
75%
510
41%
824
July16th to endNight
74%
564
17%
1593
15%
531
74%
531
32%
956
August1st to 15thNight
79%
539
26%
1611
21%
536
80%
536
40%
855
August16th to endNight
80%
605
33%
1817
29%
605
81%
605
43%
961
September1st to 15thNight
73%
720
28%
1980
21%
720
78%
720
45%
1099
September16th to endNight
71%
667
43%
1847
28%
667
81%
667
64%
829
October1st to 15thNight
58%
660
38%
1903
22%
660
72%
660
54%
894
October16th to endNight
61%
704
41%
2112
28%
704
73%
704
52%
974
November1st to 15thNight
53%
700
36%
2166
23%
700
68%
700
49%
1017
November16th to endNight
51%
720
38%
2340
27%
720
67%
720
47%
1051
December1st to 15thNight
51%
720
33%
2340
23%
720
66%
720
39%
1162
December16th to endNight
44%
768
30%
2496
23%
768
56%
768
30%
1410

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
45%24424446%57%
47872
Night
2413323346%63%
47120
TransparencyDay
25%46%19%27%16%
47567
Night
20%37%20%29%18%
46926
SeeingDay
8%20%28%27%12%3
5411
Night
413%21%30%23%6%
15601

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
27%13312247%63%
1611
16th to endDay
48%23313346%56%
1728
February1st to 15thDay
19%43313346%58%
1640
16th to endDay
28%23424347%55%
1582
March1st to 15thDay
25%23313446%60%
1980
16th to endDay
35%12313447%61%
2112
April1st to 15thDay
2423333446%60%
2000
16th to endDay
35%23312235%65%
2286
May1st to 15thDay
4414313346%61%
2331
16th to endDay
431432344463%
2429
June1st to 15thDay
211212335%74%
2340
16th to endDay
211232333471%
2340
July1st to 15thDay
6%326%7%47%6%7%7%39%
2340
16th to endDay
9%6%37%8%47%7%8%9%26%
2467
August1st to 15thDay
9%5%26%7%5%6%6%5%6%38%
2340
16th to endDay
7%425%6%5%6%6%6%7%41%
2382
September1st to 15thDay
7%5%15%5%36%7%5%7%44%
1980
16th to endDay
3213313335%67%
1838
October1st to 15thDay
4313313335%66%
1727
16th to endDay
322212345%69%
1760
November1st to 15thDay
2212113346%69%
1691
16th to endDay
4412212135%70%
1620
December1st to 15thDay
38%24322346%58%
1620
16th to endDay
39%45%323446%50%
1728
January1st to 15thNight
26%14323335%62%
2327
16th to endNight
5%7%23313345%58%
2496
February1st to 15thNight
17%3221235%6%63%
2209
16th to endNight
27%2322435%6%59%
1892
March1st to 15thNight
2423313346%65%
1980
16th to endNight
2222213346%66%
2112
April1st to 15thNight
1423213346%65%
1948
16th to endNight
112322235%73%
1671
May1st to 15thNight
1112223346%71%
1619
16th to endNight
221221333472%
1684
June1st to 15thNight
111211124476%
1620
16th to endNight
11121222478%
1620
July1st to 15thNight
3314425%6%7%9%52%
1620
16th to endNight
42245%47%8%9%10%38%
1701
August1st to 15thNight
423335%6%7%9%52%
1620
16th to endNight
3213425%5%6%10%54%
1817
September1st to 15thNight
43144245%5%8%55%
1980
16th to endNight
211221235%77%
1847
October1st to 15thNight
321213346%69%
1903
16th to endNight
1312212335%71%
2112
November1st to 15thNight
2322213345%67%
2166
16th to endNight
241221234470%
2340
December1st to 15thNight
26%23313337%60%
2340
16th to endNight
39%35%324446%52%
2496

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
21%37%24%33%9%
1612
16th to endDay
27%37%19%29%11%
1728
February1st to 15thDay
26%37%27%19%16%
1640
16th to endDay
27%36%23%29%10%
1582
March1st to 15thDay
22%4422%33%12%
1980
16th to endDay
21%46%26%28%12%
2112
April1st to 15thDay
22%35%22%32%13%
2000
16th to endDay
22%3319%35%15%
2286
May1st to 15thDay
23%3419%35%15%
2331
16th to endDay
22%3416%37%14%
2432
June1st to 15thDay
12%3211%30%39%
2340
16th to endDay
15%318%27%42%
2340
July1st to 15thDay
36%6%11%13%12%18%
2201
16th to endDay
45%10%16%14%11%1
2312
August1st to 15thDay
41%6%9%14%17%10%
2325
16th to endDay
36%7%9%19%20%6%
2382
September1st to 15thDay
34%7%9%15%17%15%
1980
16th to endDay
18%4317%34%22%
1838
October1st to 15thDay
21%3319%29%22%
1727
16th to endDay
16%2418%38%19%
1760
November1st to 15thDay
16%25%25%32%18%
1691
16th to endDay
17%2421%33%20%
1620
December1st to 15thDay
25%410%26%24%8%
1620
16th to endDay
35%38%23%22%6%
1728
January1st to 15thNight
23%4423%31%11%
2327
16th to endNight
27%35%19%31%13%
2496
February1st to 15thNight
21%37%24%25%17%
2220
16th to endNight
24%36%23%29%13%
1892
March1st to 15thNight
19%3423%36%12%
1980
16th to endNight
17%48%25%31%12%
2112
April1st to 15thNight
20%25%21%35%13%
1948
16th to endNight
13%3422%36%20%
1671
May1st to 15thNight
13%36%25%31%19%
1620
16th to endNight
14%3418%40%17%
1685
June1st to 15thNight
12%1110%30%42%
1620
16th to endNight
10%226%25%52%
1620
July1st to 15thNight
24%6%15%14%17%21%
1530
16th to endNight
31%8%22%19%13%4
1593
August1st to 15thNight
24%6%13%20%22%12%
1611
16th to endNight
22%6%12%23%25%9%
1817
September1st to 15thNight
23%5%11%20%22%15%
1980
16th to endNight
9%3318%35%29%
1847
October1st to 15thNight
15%35%23%28%24%
1903
16th to endNight
15%2416%36%24%
2112
November1st to 15thNight
18%35%23%30%18%
2166
16th to endNight
17%25%19%32%22%
2340
December1st to 15thNight
22%48%26%24%13%
2340
16th to endNight
32%38%23%24%7%
2496

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
10%31%26%22%7%2
90
16th to endDay
6%30%26%21%14%1
96
February1st to 15thDay
7%26%27%25%10%3
155
16th to endDay
16%36%27%15%3
238
March1st to 15thDay
8%27%34%18%8%1
270
16th to endDay
10%26%36%18%5%2
288
April1st to 15thDay
11%33%31%11%9%1
269
16th to endDay
8%42%26%15%41
270
May1st to 15thDay
9%31%34%16%6%1
288
16th to endDay
7%29%30%24%6%1
330
June1st to 15thDay
323%36%23%9%3
315
16th to endDay
39%35%36%13%1
315
July1st to 15thDay
6%8%26%42%13%2
305
16th to endDay
12%414%39%21%7%
327
August1st to 15thDay
9%318%40%20%7%
315
16th to endDay
7%321%37%21%8%
332
September1st to 15thDay
8%8%28%33%16%4
315
16th to endDay
6%14%27%31%14%5%
264
October1st to 15thDay
5%25%27%25%10%4
167
16th to endDay
9%16%31%23%15%3
96
November1st to 15thDay
5%13%28%30%15%6%
90
16th to endDay
8%30%24%28%5%2
90
December1st to 15thDay
416%35%25%14%3
90
16th to endDay
14%25%27%25%6%2
96
January1st to 15thNight
7%14%25%26%16%9%
720
16th to endNight
7%18%25%27%16%5%
768
February1st to 15thNight
416%25%32%17%4
704
16th to endNight
6%22%29%26%12%2
632
March1st to 15thNight
5%19%26%28%15%4
720
16th to endNight
417%26%32%16%2
768
April1st to 15thNight
5%24%28%27%11%2
689
16th to endNight
223%28%28%13%3
540
May1st to 15thNight
217%30%34%13%1
540
16th to endNight
417%26%27%20%3
563
June1st to 15thNight
211%33%33%18%2
540
16th to endNight
1421%41%27%2
540
July1st to 15thNight
5%10%36%40%5%
510
16th to endNight
415%34%44%9%
564
August1st to 15thNight
415%28%48%10%
539
16th to endNight
31430%44%15%
605
September1st to 15thNight
339%26%45%10%
720
16th to endNight
38%13%25%37%10%
667
October1st to 15thNight
410%23%32%22%6%
660
16th to endNight
210%19%32%24%10%
704
November1st to 15thNight
311%25%30%22%5%
700
16th to endNight
5%15%24%27%18%9%
720
December1st to 15thNight
6%15%22%29%22%3
720
16th to endNight
7%16%26%29%14%4
768
Page generated at 2017-09-19 04:32:23. Version ac04

Copyright 2017 A. Danko


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