AIG public observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the AIG public observatory clear sky chart from 2005-11-06 to 2024-03-01.

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
40%
24210
12%
80134
9%
24107
44%
24107
11%
62624

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
20%
942
8%
4386
3%
926
24%
926
4%
3571
January11th to 20thNight
16%
843
8%
4210
4%
824
20%
824
4%
3453
January21st to endNight
19%
832
6%
4491
4%
814
25%
814
4%
3674
February1st to 10thNight
22%
760
9%
2342
5%
760
26%
760
9%
1920
February11th to 20thNight
19%
742
10%
2219
5%
740
24%
740
10%
1778
February21st to endNight
19%
628
9%
1884
4%
628
24%
628
9%
1551
March1st to 10thNight
27%
723
13%
2167
8%
723
31%
723
11%
1665
March11th to 20thNight
25%
720
10%
1983
6%
720
29%
720
11%
1614
March21st to endNight
29%
792
10%
2178
5%
792
33%
792
11%
1772
April1st to 10thNight
34%
568
14%
1828
9%
568
41%
568
13%
1381
April11th to 20thNight
38%
533
16%
1745
10%
533
46%
533
13%
1310
April21st to endNight
32%
540
12%
1620
6%
540
38%
540
13%
1269
May1st to 10thNight
36%
540
9%
1620
4%
540
40%
540
11%
1315
May11th to 20thNight
45%
531
12%
1592
9%
531
47%
531
12%
1276
May21st to endNight
60%
583
13%
1745
11%
582
61%
582
16%
1311
June1st to 10thNight
60%
540
13%
1589
10%
540
64%
540
18%
1168
June11th to 20thNight
61%
540
16%
1440
11%
540
65%
540
20%
1053
June21st to endNight
60%
540
14%
1440
10%
540
63%
540
20%
1063
July1st to 10thNight
64%
525
11%
1400
9%
525
66%
525
17%
1081
July11th to 20thNight
69%
522
13%
1446
11%
522
71%
522
19%
1070
July21st to endNight
66%
591
15%
1672
13%
558
68%
558
18%
1234
August1st to 10thNight
70%
537
16%
1600
15%
534
71%
534
20%
1148
August11th to 20thNight
70%
534
18%
1606
16%
534
72%
534
20%
1154
August21st to endNight
72%
594
16%
1980
17%
594
73%
594
19%
1366
September1st to 10thNight
64%
617
16%
1877
14%
617
66%
617
18%
1378
September11th to 20thNight
65%
720
22%
1980
16%
720
69%
720
20%
1441
September21st to endNight
52%
720
15%
1980
11%
720
55%
720
17%
1514
October1st to 10thNight
58%
720
18%
1980
14%
720
62%
720
21%
1421
October11th to 20thNight
44%
720
15%
2112
11%
720
49%
720
14%
1632
October21st to endNight
34%
792
14%
2528
11%
792
40%
792
12%
1966
November1st to 10thNight
43%
740
16%
2406
14%
740
48%
740
15%
1741
November11th to 20thNight
32%
760
16%
2470
10%
760
40%
760
11%
1915
November21st to endNight
33%
760
12%
2470
9%
760
37%
760
9%
1976
December1st to 10thNight
23%
760
8%
2470
4%
760
28%
760
8%
2050
December11th to 20thNight
25%
759
10%
2985
6%
759
29%
759
7%
2381
December21st to endNight
18%
942
4%
4693
1%
931
23%
931
3%
4012

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
15%9%46%6%35%5%5%7%37%
78855
Night
15%10%46%5%3445%7%37%
80560
TransparencyDay
32%36%7%14%9%1
78568
Night
33%35%7%14%9%2
80367
SeeingDay
18%11%25%24%15%7%
14347
Night
19%9%26%29%16%2
24234

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
15%18%6%7%5%34446%28%
2527
11th to 20thDay
17%13%7%10%6%5%5%43425%
2689
21st to endDay
29%15%8%6%5%2433420%
3124
February1st to 10thDay
19%12%5%8%42435%5%32%
1836
11th to 20thDay
16%11%5%9%6%3435%6%33%
1848
21st to endDay
19%13%7%7%5%23445%32%
1570
March1st to 10thDay
20%9%37%5%3445%6%36%
1800
11th to 20thDay
20%10%46%6%3345%5%33%
1976
21st to endDay
20%12%36%6%35%45%5%29%
2178
April1st to 10thDay
14%9%36%5%35%5%5%8%36%
2146
11th to 20thDay
17%8%36%6%3445%6%39%
2164
21st to endDay
19%7%26%6%35%45%7%35%
2340
May1st to 10thDay
21%7%36%6%445%5%6%34%
2340
11th to 20thDay
20%8%36%6%3445%6%34%
2301
21st to endDay
9%5%37%7%47%7%7%10%34%
2531
June1st to 10thDay
13%5%25%5%35%6%6%8%42%
2365
11th to 20thDay
9%5%25%6%36%5%6%9%43%
2520
21st to endDay
9%5%37%5%46%6%6%9%40%
2520
July1st to 10thDay
12%5%26%6%45%5%7%9%39%
2520
11th to 20thDay
8%5%25%6%45%6%7%10%41%
2462
21st to endDay
10%5%25%7%45%6%6%8%42%
2547
August1st to 10thDay
8%5%26%6%36%5%7%9%44%
2327
11th to 20thDay
9%5%36%6%46%6%6%10%39%
2291
21st to endDay
8%5%25%5%36%6%6%9%46%
2376
September1st to 10thDay
14%435%4345%6%7%45%
2063
11th to 20thDay
9%435%4345%6%9%46%
1980
21st to endDay
14%5%245%33446%50%
1980
October1st to 10thDay
11%5%25%6%35%45%450%
1980
11th to 20thDay
14%6%26%5%3346%7%45%
1846
21st to endDay
20%8%35%43445%6%38%
1824
November1st to 10thDay
9%6%35%424436%54%
1672
11th to 20thDay
14%9%47%5%2445%6%39%
1710
21st to endDay
18%9%46%5%23346%39%
1710
December1st to 10thDay
21%13%48%6%34445%28%
1710
11th to 20thDay
17%14%5%8%6%34445%31%
2153
21st to endDay
22%20%5%7%7%34445%20%
2929
January1st to 10thNight
18%17%6%9%6%35%5%45%23%
4428
11th to 20thNight
15%16%6%14%8%4433423%
4269
21st to endNight
26%18%5%7%5%2333424%
4532
February1st to 10thNight
21%11%5%8%5%35%45%5%29%
2342
11th to 20thNight
17%10%49%6%345%5%6%33%
2218
21st to endNight
21%13%5%7%5%24446%30%
1884
March1st to 10thNight
18%9%36%5%4345%5%38%
2167
11th to 20thNight
19%8%36%43445%7%36%
1983
21st to endNight
19%9%36%5%345%47%34%
2178
April1st to 10thNight
14%7%35%5%35%5%6%7%39%
1828
11th to 20thNight
14%9%36%5%3346%7%40%
1745
21st to endNight
18%7%35%6%3445%7%38%
1620
May1st to 10thNight
18%7%35%7%345%5%8%34%
1620
11th to 20thNight
16%6%37%5%445%5%8%38%
1592
21st to endNight
7%5%246%35%7%7%12%41%
1744
June1st to 10thNight
8%5%245%335%6%10%49%
1589
11th to 20thNight
8%5%25%43446%11%48%
1440
21st to endNight
6%325%5%35%6%7%10%47%
1440
July1st to 10thNight
11%6%25%43446%9%45%
1440
11th to 20thNight
6%4245%445%7%9%51%
1494
21st to endNight
9%5%25%5%345%5%8%49%
1771
August1st to 10thNight
7%5%25%5%345%6%10%48%
1611
11th to 20thNight
7%425%5%345%7%10%47%
1606
21st to endNight
6%42443345%10%54%
1980
September1st to 10thNight
13%5%2343445%6%51%
1877
11th to 20thNight
9%4243245%5%8%53%
1980
21st to endNight
13%6%345%24436%50%
1980
October1st to 10thNight
8%5%145%335%47%55%
1980
11th to 20thNight
15%6%35%4345%5%7%44%
2112
21st to endNight
19%8%25%434446%40%
2528
November1st to 10thNight
13%6%35%5%2445%6%49%
2406
11th to 20thNight
16%7%37%5%2445%7%40%
2470
21st to endNight
19%10%36%5%34446%35%
2470
December1st to 10thNight
21%11%5%7%43445%6%30%
2470
11th to 20thNight
18%12%5%8%5%3435%6%31%
3042
21st to endNight
20%19%6%8%7%34445%19%
4724

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
46%26%10%11%7%
2528
11th to 20thDay
49%26%7%12%6%
2689
21st to endDay
59%22%6%12%1
3124
February1st to 10thDay
37%34%6%14%8%1
1836
11th to 20thDay
36%35%8%13%7%1
1858
21st to endDay
40%33%6%15%5%1
1570
March1st to 10thDay
32%37%7%17%7%
1800
11th to 20thDay
36%37%6%12%8%1
1976
21st to endDay
39%36%7%13%4
2178
April1st to 10thDay
28%38%7%16%10%1
2146
11th to 20thDay
32%35%6%14%11%2
2164
21st to endDay
34%36%6%15%8%1
2340
May1st to 10thDay
35%38%6%13%6%1
2340
11th to 20thDay
34%37%7%11%10%1
2305
21st to endDay
27%40%10%13%9%1
2531
June1st to 10thDay
25%38%8%17%10%2
2365
11th to 20thDay
22%38%11%15%13%1
2520
21st to endDay
25%38%9%15%10%4
2520
July1st to 10thDay
27%40%8%12%12%2
2451
11th to 20thDay
25%40%8%15%11%2
2387
21st to endDay
25%40%11%14%8%2
2405
August1st to 10thDay
23%39%9%17%11%1
2314
11th to 20thDay
24%37%8%18%12%1
2291
21st to endDay
22%38%8%15%11%6%
2376
September1st to 10thDay
26%38%7%14%12%4
2063
11th to 20thDay
22%37%7%16%15%2
1980
21st to endDay
25%38%5%15%15%2
1980
October1st to 10thDay
22%38%6%13%19%2
1980
11th to 20thDay
26%37%6%18%11%1
1846
21st to endDay
32%37%6%15%9%1
1824
November1st to 10thDay
17%40%5%19%16%3
1672
11th to 20thDay
30%39%5%13%9%2
1710
21st to endDay
29%39%5%14%11%1
1710
December1st to 10thDay
38%39%8%12%3
1710
11th to 20thDay
39%32%6%13%9%
2150
21st to endDay
55%26%6%7%4
2929
January1st to 10thNight
48%29%7%11%5%
4428
11th to 20thNight
52%25%7%10%6%
4269
21st to endNight
55%22%5%13%5%
4532
February1st to 10thNight
39%34%7%13%5%1
2342
11th to 20thNight
35%35%7%13%8%1
2219
21st to endNight
39%34%8%12%6%1
1884
March1st to 10thNight
30%36%5%17%11%1
2167
11th to 20thNight
34%37%8%13%6%1
1983
21st to endNight
35%36%7%16%5%1
2178
April1st to 10thNight
26%38%7%18%10%2
1828
11th to 20thNight
28%36%8%11%13%4
1745
21st to endNight
30%37%7%14%9%2
1620
May1st to 10thNight
34%38%7%14%6%1
1620
11th to 20thNight
32%38%7%11%11%2
1593
21st to endNight
22%40%10%15%10%3
1745
June1st to 10thNight
21%39%9%16%13%2
1589
11th to 20thNight
19%39%10%17%13%2
1440
21st to endNight
20%39%10%15%12%5%
1440
July1st to 10thNight
24%39%10%12%11%4
1400
11th to 20thNight
20%40%9%15%13%3
1446
21st to endNight
20%40%8%21%10%1
1672
August1st to 10thNight
19%38%9%20%12%1
1600
11th to 20thNight
19%38%7%21%14%1
1606
21st to endNight
18%38%9%17%14%5%
1980
September1st to 10thNight
23%37%7%15%12%5%
1877
11th to 20thNight
20%38%6%17%17%2
1980
21st to endNight
26%37%5%14%16%2
1980
October1st to 10thNight
20%36%6%17%18%3
1980
11th to 20thNight
28%37%5%16%12%2
2112
21st to endNight
30%37%6%13%11%2
2528
November1st to 10thNight
20%41%6%14%17%3
2406
11th to 20thNight
28%40%6%13%12%2
2470
21st to endNight
32%39%5%13%9%1
2470
December1st to 10thNight
36%40%7%13%4
2470
11th to 20thNight
39%33%6%14%7%
3044
21st to endNight
55%27%7%8%4
4724

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
22%21%33%18%5%
270
11th to 20thDay
29%24%31%14%2
270
21st to endDay
31%21%31%15%3
297
February1st to 10thDay
28%21%33%15%3
270
11th to 20thDay
27%25%35%10%21
270
21st to endDay
30%19%40%8%3
225
March1st to 10thDay
21%24%41%13%2
240
11th to 20thDay
26%16%36%15%6%1
416
21st to endDay
29%18%29%17%6%1
462
April1st to 10thDay
20%18%28%19%12%2
420
11th to 20thDay
21%18%24%23%10%4
412
21st to endDay
24%14%34%17%8%4
420
May1st to 10thDay
25%12%31%20%8%4
423
11th to 20thDay
22%8%24%26%13%6%
447
21st to endDay
14%7%20%28%18%12%
485
June1st to 10thDay
13%8%19%30%22%8%
475
11th to 20thDay
11%5%23%33%20%7%
630
21st to endDay
10%6%24%30%20%9%
630
July1st to 10thDay
13%414%25%25%19%
621
11th to 20thDay
8%5%19%32%24%13%
560
21st to endDay
11%415%33%24%14%
488
August1st to 10thDay
10%314%27%28%18%
446
11th to 20thDay
13%315%32%23%13%
439
21st to endDay
9%6%13%27%30%15%
495
September1st to 10thDay
13%5%15%26%30%11%
450
11th to 20thDay
10%316%25%29%16%
450
21st to endDay
17%7%20%24%24%9%
450
October1st to 10thDay
11%5%24%33%20%7%
450
11th to 20thDay
16%15%30%24%12%2
450
21st to endDay
25%11%37%19%8%
339
November1st to 10thDay
14%14%33%26%11%3
270
11th to 20thDay
21%17%31%21%7%2
270
21st to endDay
25%12%34%20%8%
270
December1st to 10thDay
21%23%33%16%7%1
270
11th to 20thDay
20%18%37%18%7%
270
21st to endDay
30%19%30%16%4
297
January1st to 10thNight
22%17%33%23%5%
950
11th to 20thNight
29%16%32%19%3
844
21st to endNight
26%15%34%22%3
836
February1st to 10thNight
28%13%33%24%2
760
11th to 20thNight
24%18%35%22%2
743
21st to endNight
29%14%34%23%1
628
March1st to 10thNight
23%15%30%27%5%
723
11th to 20thNight
24%12%34%22%7%1
720
21st to endNight
23%11%32%27%8%
792
April1st to 10thNight
17%10%34%29%8%
568
11th to 20thNight
20%11%28%30%10%1
533
21st to endNight
20%8%35%28%9%1
540
May1st to 10thNight
21%6%31%29%11%1
540
11th to 20thNight
17%6%22%35%18%3
531
21st to endNight
9%319%38%27%4
584
June1st to 10thNight
10%220%43%23%2
540
11th to 20thNight
9%415%40%28%3
540
21st to endNight
6%222%41%26%3
540
July1st to 10thNight
12%212%32%33%10%
525
11th to 20thNight
7%312%37%36%5%
522
21st to endNight
12%210%38%34%4
591
August1st to 10thNight
9%28%36%41%4
537
11th to 20thNight
9%111%39%34%5%
534
21st to endNight
7%210%35%38%8%
594
September1st to 10thNight
15%212%32%34%5%
617
11th to 20thNight
9%314%30%38%5%
720
21st to endNight
16%322%31%25%3
720
October1st to 10thNight
10%5%22%36%23%5%
720
11th to 20thNight
18%9%24%33%14%2
720
21st to endNight
23%9%30%26%11%1
792
November1st to 10thNight
16%8%28%31%16%1
740
11th to 20thNight
20%14%28%26%11%1
760
21st to endNight
25%11%26%29%8%1
760
December1st to 10thNight
27%10%34%23%6%
760
11th to 20thNight
24%12%33%24%7%1
760
21st to endNight
29%14%30%22%4
950
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