ONU Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ONU Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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
36%
24269
16%
74665
10%
24211
42%
24211
20%
52288

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 10thNight
16%
950
11%
2648
4%
946
22%
946
11%
2129
January11th to 20thNight
13%
880
9%
2590
3%
880
19%
880
10%
2111
January21st to endNight
17%
820
10%
2664
5%
820
24%
820
10%
2115
February1st to 10thNight
17%
720
9%
2322
5%
720
22%
720
11%
1852
February11th to 20thNight
20%
701
12%
2099
6%
700
26%
700
13%
1575
February21st to endNight
16%
592
13%
1776
3%
592
25%
592
12%
1379
March1st to 10thNight
25%
720
14%
2129
8%
720
31%
720
18%
1540
March11th to 20thNight
28%
720
19%
1980
10%
720
36%
720
23%
1361
March21st to endNight
29%
792
16%
2178
8%
792
36%
792
19%
1592
April1st to 10thNight
28%
617
15%
1877
7%
617
36%
617
21%
1276
April11th to 20thNight
31%
533
18%
1673
9%
533
40%
533
20%
1135
April21st to endNight
30%
540
16%
1620
5%
540
39%
540
22%
1123
May1st to 10thNight
34%
540
12%
1620
6%
540
38%
540
17%
1190
May11th to 20thNight
39%
531
17%
1593
10%
531
45%
531
23%
1071
May21st to endNight
55%
583
15%
1745
11%
582
59%
582
31%
1073
June1st to 10thNight
55%
540
15%
1546
12%
540
59%
540
33%
950
June11th to 20thNight
56%
540
19%
1440
13%
540
62%
540
34%
924
June21st to endNight
56%
540
18%
1440
13%
540
61%
540
35%
898
July1st to 10thNight
66%
525
16%
1400
15%
525
67%
525
32%
906
July11th to 20thNight
65%
522
21%
1420
17%
522
68%
522
38%
864
July21st to endNight
65%
591
19%
1672
16%
558
67%
558
35%
983
August1st to 10thNight
69%
537
20%
1600
17%
534
71%
534
34%
946
August11th to 20thNight
65%
534
22%
1598
19%
534
68%
534
34%
963
August21st to endNight
67%
594
21%
1872
18%
594
69%
594
35%
1069
September1st to 10thNight
61%
644
22%
1904
17%
644
65%
644
32%
1162
September11th to 20thNight
65%
720
29%
1980
23%
720
68%
720
38%
1184
September21st to endNight
52%
720
24%
1980
18%
720
57%
720
32%
1247
October1st to 10thNight
52%
720
22%
1993
17%
720
57%
720
33%
1238
October11th to 20thNight
33%
720
19%
2160
12%
720
42%
720
22%
1467
October21st to endNight
27%
792
19%
2506
12%
792
37%
792
19%
1722
November1st to 10thNight
35%
740
20%
2406
16%
740
42%
740
25%
1478
November11th to 20thNight
27%
760
19%
2470
11%
760
38%
760
18%
1688
November21st to endNight
24%
760
15%
2470
9%
760
32%
760
14%
1838
December1st to 10thNight
19%
760
11%
2470
5%
760
26%
760
11%
1912
December11th to 20thNight
18%
759
10%
3011
4%
758
26%
758
10%
2258
December21st to endNight
14%
1012
5%
4813
3%
997
19%
997
5%
4069

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
14%10%47%6%35%5%5%7%33%
75129
Night
15%10%47%6%3445%7%35%
74907
TransparencyDay
49%7%12%19%11%2
74842
Night
48%7%12%19%12%2
74714
SeeingDay
18%11%27%23%15%7%
14375
Night
19%10%29%28%13%1
24277

By Month

Percentage Cloud Cover SE (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 10thDay
18%14%6%12%9%5%5%445%18%
1517
11th to 20thDay
16%18%8%11%9%45%34417%
1584
21st to endDay
15%16%8%12%10%44446%18%
1836
February1st to 10thDay
19%15%6%11%8%35%445%20%
1636
11th to 20thDay
15%12%7%14%9%5%4446%21%
1748
21st to endDay
17%14%5%9%6%3445%7%26%
1480
March1st to 10thDay
17%12%7%10%8%345%45%26%
1831
11th to 20thDay
17%12%6%7%7%345%46%29%
1980
21st to endDay
22%13%5%7%5%35%45%5%25%
2178
April1st to 10thDay
13%13%5%6%6%35%5%5%8%31%
2098
11th to 20thDay
17%9%37%6%35%5%5%7%34%
2235
21st to endDay
19%8%47%5%35%5%5%7%33%
2340
May1st to 10thDay
16%10%47%7%445%6%8%30%
2340
11th to 20thDay
17%8%46%6%346%5%6%34%
2301
21st to endDay
9%5%36%6%37%6%6%10%38%
2531
June1st to 10thDay
12%6%36%7%36%5%6%7%39%
2409
11th to 20thDay
10%5%35%6%36%5%6%9%41%
2520
21st to endDay
10%5%25%6%46%6%9%9%38%
2520
July1st to 10thDay
10%6%25%5%36%6%7%10%39%
2520
11th to 20thDay
8%425%5%35%6%7%9%45%
2489
21st to endDay
8%5%35%5%36%6%8%9%40%
2547
August1st to 10thDay
8%425%5%446%6%10%46%
2327
11th to 20thDay
8%5%37%6%36%6%8%9%38%
2303
21st to endDay
9%5%36%5%35%6%6%9%42%
2481
September1st to 10thDay
12%7%35%5%3445%8%44%
2036
11th to 20thDay
9%5%35%5%45%5%6%9%44%
1980
21st to endDay
15%6%25%43445%8%44%
1980
October1st to 10thDay
12%7%36%6%3445%7%42%
1965
11th to 20thDay
16%8%25%6%35%5%5%7%37%
1800
21st to endDay
19%10%36%7%5%445%6%30%
1848
November1st to 10thDay
11%8%35%434446%48%
1672
11th to 20thDay
16%13%5%6%6%34446%33%
1710
21st to endDay
18%14%5%8%5%33446%29%
1710
December1st to 10thDay
16%17%9%10%8%4444420%
1710
11th to 20thDay
13%16%7%10%7%46%45%7%21%
2153
21st to endDay
21%20%10%11%7%45%33410%
2814
January1st to 10thNight
16%14%7%11%8%45%445%23%
2648
11th to 20thNight
17%17%7%11%7%44445%20%
2590
21st to endNight
16%17%8%11%7%34345%22%
2664
February1st to 10thNight
20%13%6%10%6%45%445%23%
2322
11th to 20thNight
14%13%6%8%8%35%45%6%27%
2098
21st to endNight
20%13%49%7%34445%26%
1776
March1st to 10thNight
15%10%5%8%7%45%446%32%
2129
11th to 20thNight
13%10%45%6%3446%8%38%
1980
21st to endNight
19%10%47%5%445%5%5%33%
2178
April1st to 10thNight
14%10%47%4345%6%7%36%
1877
11th to 20thNight
17%7%46%6%35%6%6%7%35%
1673
21st to endNight
17%9%36%5%3445%7%36%
1620
May1st to 10thNight
19%8%36%7%36%5%6%6%31%
1620
11th to 20thNight
19%7%45%43445%8%38%
1592
21st to endNight
8%5%245%46%5%7%12%41%
1744
June1st to 10thNight
9%5%25%436%5%5%7%48%
1546
11th to 20thNight
9%5%25%425%6%6%10%46%
1440
21st to endNight
9%426%6%345%6%11%45%
1440
July1st to 10thNight
9%5%26%5%35%36%9%47%
1440
11th to 20thNight
7%4245%35%5%6%10%49%
1468
21st to endNight
8%425%5%35%5%7%8%49%
1771
August1st to 10thNight
6%425%435%6%7%10%47%
1611
11th to 20thNight
8%6%35%5%345%6%8%48%
1598
21st to endNight
5%425%6%345%6%8%51%
1872
September1st to 10thNight
12%5%344345%5%7%49%
1904
11th to 20thNight
10%5%2332446%7%53%
1980
21st to endNight
11%6%35%6%335%5%5%48%
1980
October1st to 10thNight
10%6%25%5%35%45%7%49%
1993
11th to 20thNight
17%8%36%5%3445%6%39%
2160
21st to endNight
22%8%35%5%3445%6%36%
2506
November1st to 10thNight
12%8%35%5%3445%6%44%
2406
11th to 20thNight
15%10%5%8%6%34447%35%
2470
21st to endNight
17%12%5%8%7%35%446%27%
2470
December1st to 10thNight
17%15%6%9%7%4435%6%23%
2470
11th to 20thNight
17%14%7%9%6%35%5%45%26%
3042
21st to endNight
23%19%8%9%7%34445%14%
4829

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 10thDay
68%6%8%13%5%
1518
11th to 20thDay
71%6%11%9%4
1584
21st to endDay
69%6%9%11%5%
1836
February1st to 10thDay
68%410%11%6%1
1636
11th to 20thDay
65%5%8%14%7%
1758
21st to endDay
58%6%12%15%8%2
1480
March1st to 10thDay
59%6%10%18%6%
1831
11th to 20thDay
55%6%11%16%9%2
1980
21st to endDay
61%5%10%18%5%1
2178
April1st to 10thDay
51%7%12%17%12%
2098
11th to 20thDay
49%6%12%19%12%2
2235
21st to endDay
50%7%13%18%10%2
2340
May1st to 10thDay
52%8%13%17%9%1
2340
11th to 20thDay
49%9%11%20%11%1
2305
21st to endDay
39%11%15%22%11%2
2531
June1st to 10thDay
42%8%11%23%13%2
2409
11th to 20thDay
37%9%14%23%15%2
2520
21st to endDay
37%11%16%19%14%4
2520
July1st to 10thDay
37%10%13%21%16%3
2451
11th to 20thDay
33%8%16%26%16%1
2414
21st to endDay
35%9%16%23%14%3
2405
August1st to 10thDay
32%9%16%26%16%1
2314
11th to 20thDay
37%10%15%25%12%1
2303
21st to endDay
34%10%14%21%16%4
2481
September1st to 10thDay
38%7%13%21%17%4
2036
11th to 20thDay
35%8%11%25%19%2
1980
21st to endDay
39%6%11%23%20%1
1980
October1st to 10thDay
42%6%11%24%15%2
1965
11th to 20thDay
43%8%10%22%15%2
1800
21st to endDay
53%7%13%17%10%1
1848
November1st to 10thDay
37%6%13%24%19%1
1672
11th to 20thDay
53%5%10%20%11%2
1710
21st to endDay
57%5%11%18%8%1
1710
December1st to 10thDay
68%6%9%13%4
1710
11th to 20thDay
60%8%11%17%5%
2150
21st to endDay
78%6%6%6%4
2814
January1st to 10thNight
64%5%8%15%8%
2648
11th to 20thNight
66%5%11%13%4
2590
21st to endNight
65%5%10%13%6%1
2664
February1st to 10thNight
63%7%9%14%6%1
2322
11th to 20thNight
57%6%12%15%9%1
2099
21st to endNight
60%6%9%14%10%2
1776
March1st to 10thNight
53%6%12%18%11%1
2129
11th to 20thNight
45%6%12%22%12%3
1980
21st to endNight
52%6%9%21%11%1
2178
April1st to 10thNight
46%6%13%20%13%1
1877
11th to 20thNight
46%7%12%15%17%3
1673
21st to endNight
47%6%13%18%13%3
1620
May1st to 10thNight
52%8%11%16%10%3
1620
11th to 20thNight
44%7%12%20%15%2
1593
21st to endNight
32%10%17%24%14%3
1745
June1st to 10thNight
34%7%14%23%18%3
1546
11th to 20thNight
32%10%15%25%15%3
1440
21st to endNight
33%8%18%23%16%3
1440
July1st to 10thNight
35%8%12%23%17%4
1400
11th to 20thNight
29%9%14%28%20%1
1420
21st to endNight
29%9%17%26%16%3
1672
August1st to 10thNight
30%9%16%26%19%1
1600
11th to 20thNight
33%6%15%28%17%1
1598
21st to endNight
27%10%17%21%20%5%
1872
September1st to 10thNight
33%8%12%25%16%6%
1904
11th to 20thNight
29%7%11%26%25%2
1980
21st to endNight
36%6%12%23%20%3
1980
October1st to 10thNight
34%7%13%24%21%1
1993
11th to 20thNight
45%6%9%23%15%1
2160
21st to endNight
48%6%11%22%12%2
2506
November1st to 10thNight
40%5%12%23%18%2
2406
11th to 20thNight
49%5%10%20%13%2
2470
21st to endNight
57%6%11%18%8%
2470
December1st to 10thNight
62%5%10%16%7%
2470
11th to 20thNight
58%8%13%16%6%1
3044
21st to endNight
72%7%9%8%3
4829

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 10thDay
24%19%33%19%31
270
11th to 20thDay
23%19%42%12%41
270
21st to endDay
28%20%32%15%41
282
February1st to 10thDay
33%19%32%15%1
240
11th to 20thDay
26%19%36%15%4
240
21st to endDay
25%22%44%8%2
198
March1st to 10thDay
24%20%36%16%4
271
11th to 20thDay
25%14%36%17%5%2
420
21st to endDay
30%16%32%18%5%
462
April1st to 10thDay
23%15%30%23%8%2
420
11th to 20thDay
23%16%28%18%10%4
412
21st to endDay
25%14%28%22%8%2
420
May1st to 10thDay
25%12%30%23%8%3
423
11th to 20thDay
23%9%26%24%13%5%
447
21st to endDay
9%8%25%27%20%10%
485
June1st to 10thDay
12%7%27%29%19%6%
519
11th to 20thDay
14%6%26%26%22%6%
630
21st to endDay
13%7%26%30%18%6%
630
July1st to 10thDay
13%417%28%21%16%
621
11th to 20thDay
7%5%23%30%22%13%
587
21st to endDay
11%318%27%27%15%
488
August1st to 10thDay
12%312%28%28%17%
446
11th to 20thDay
13%417%28%26%11%
439
21st to endDay
9%417%27%27%17%
495
September1st to 10thDay
16%316%26%25%13%
450
11th to 20thDay
10%317%22%31%17%
450
21st to endDay
16%11%20%24%18%11%
450
October1st to 10thDay
12%8%24%28%20%8%
450
11th to 20thDay
18%16%30%23%12%1
450
21st to endDay
24%14%38%19%41
363
November1st to 10thDay
16%14%35%22%9%4
270
11th to 20thDay
24%21%28%16%10%1
270
21st to endDay
26%17%36%19%2
270
December1st to 10thDay
22%21%35%14%7%1
270
11th to 20thDay
22%19%37%19%3
270
21st to endDay
25%24%35%14%3
297
January1st to 10thNight
22%17%36%20%4
950
11th to 20thNight
26%17%36%16%41
880
21st to endNight
23%17%35%21%3
820
February1st to 10thNight
28%12%37%21%3
720
11th to 20thNight
23%15%36%23%3
703
21st to endNight
27%14%38%18%3
592
March1st to 10thNight
20%13%34%25%7%1
720
11th to 20thNight
18%14%36%26%5%1
720
21st to endNight
25%11%30%26%8%1
792
April1st to 10thNight
19%13%36%28%5%1
617
11th to 20thNight
19%14%33%24%8%1
533
21st to endNight
21%8%37%23%10%1
540
May1st to 10thNight
22%9%30%29%9%1
540
11th to 20thNight
24%429%29%14%
531
21st to endNight
11%423%36%23%3
584
June1st to 10thNight
10%325%44%18%
540
11th to 20thNight
11%423%41%20%1
540
21st to endNight
10%422%39%24%2
540
July1st to 10thNight
11%213%42%28%4
525
11th to 20thNight
8%218%39%29%4
522
21st to endNight
9%215%42%30%2
591
August1st to 10thNight
8%213%42%30%5%
537
11th to 20thNight
10%213%39%32%4
534
21st to endNight
6%314%38%34%5%
594
September1st to 10thNight
12%415%34%30%4
644
11th to 20thNight
10%315%35%32%5%
720
21st to endNight
14%6%22%33%23%2
720
October1st to 10thNight
14%5%24%35%18%4
720
11th to 20thNight
22%12%29%27%10%1
720
21st to endNight
26%12%31%25%6%
792
November1st to 10thNight
16%11%34%25%12%1
740
11th to 20thNight
22%16%30%24%8%1
760
21st to endNight
24%13%32%25%6%
760
December1st to 10thNight
23%16%35%22%5%
760
11th to 20thNight
25%13%38%19%5%
760
21st to endNight
29%17%33%19%2
1016
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