Observatoire Aster
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Aster clear sky chart from 2005-11-06 to 2024-02-25.

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
22%
23022
10%
81001
5%
22925
29%
22925
11%
60044

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
10%
827
5%
4701
1%
807
16%
807
3%
3907
January11th to 20thNight
15%
759
9%
4408
3%
740
23%
740
4%
3348
January21st to endNight
16%
828
9%
4834
5%
814
29%
814
6%
3301
February1st to 10thNight
16%
760
12%
2571
5%
760
26%
760
9%
1922
February11th to 20thNight
11%
743
8%
2359
1%
740
19%
740
7%
1870
February21st to endNight
16%
611
14%
1831
4%
611
28%
611
14%
1355
March1st to 10thNight
19%
720
14%
2160
7%
720
29%
720
12%
1642
March11th to 20thNight
19%
720
12%
1993
6%
720
24%
720
13%
1515
March21st to endNight
22%
792
14%
2156
5%
792
29%
792
14%
1625
April1st to 10thNight
18%
713
13%
1793
3%
713
24%
713
14%
1402
April11th to 20thNight
24%
534
16%
1598
8%
534
29%
534
15%
1126
April21st to endNight
23%
540
15%
1620
7%
540
31%
540
14%
1181
May1st to 10thNight
24%
540
12%
1620
7%
540
31%
540
14%
1202
May11th to 20thNight
27%
408
15%
1407
9%
408
39%
408
17%
940
May21st to endNight
26%
389
11%
1357
5%
388
36%
388
12%
971
June1st to 10thNight
31%
360
8%
1260
6%
360
35%
360
15%
884
June11th to 20thNight
38%
360
11%
1260
9%
360
43%
360
19%
823
June21st to endNight
28%
360
6%
1260
4%
360
31%
360
13%
928
July1st to 10thNight
33%
350
9%
1225
6%
350
40%
350
18%
811
July11th to 20thNight
37%
348
10%
1275
8%
348
44%
348
19%
832
July21st to endNight
40%
394
9%
1486
11%
372
46%
372
15%
1000
August1st to 10thNight
39%
430
11%
1494
8%
428
45%
428
19%
995
August11th to 20thNight
35%
532
11%
1596
6%
532
40%
532
20%
1113
August21st to endNight
37%
594
17%
1782
9%
594
43%
594
23%
1191
September1st to 10thNight
33%
684
14%
1818
6%
684
38%
684
25%
1241
September11th to 20thNight
33%
720
12%
1980
5%
720
39%
720
22%
1357
September21st to endNight
30%
720
19%
1980
11%
720
37%
720
24%
1320
October1st to 10thNight
29%
720
19%
2044
9%
720
37%
720
20%
1409
October11th to 20thNight
25%
720
11%
2238
5%
720
32%
720
14%
1647
October21st to endNight
22%
792
10%
2574
4%
792
29%
792
12%
1913
November1st to 10thNight
21%
740
12%
2407
5%
740
29%
740
10%
1798
November11th to 20thNight
20%
843
12%
2645
6%
843
26%
843
10%
1991
November21st to endNight
17%
850
8%
2759
3%
850
22%
850
7%
2214
December1st to 10thNight
10%
850
7%
2850
2%
850
16%
850
7%
2351
December11th to 20thNight
15%
850
6%
3481
2%
848
19%
848
5%
2872
December21st to endNight
11%
921
5%
5179
2%
907
18%
907
3%
4047

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
17%10%48%7%45%5%5%6%28%
77951
Night
15%11%48%7%46%5%6%7%28%
81361
TransparencyDay
55%7%12%17%8%1
77644
Night
55%8%12%16%8%1
81203
SeeingDay
26%20%30%16%6%1
14758
Night
24%18%29%21%7%1
23057

By Month

Percentage Cloud Cover NE (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
12%14%6%11%11%6%8%5%6%6%14%
2238
11th to 20thDay
11%11%414%14%5%7%7%7%6%15%
2520
21st to endDay
7%8%412%9%8%7%7%6%8%25%
2772
February1st to 10thDay
8%14%6%9%10%7%8%6%6%7%19%
1607
11th to 20thDay
12%15%6%10%8%5%6%6%6%7%19%
1709
21st to endDay
12%10%5%8%10%5%8%5%5%6%25%
1520
March1st to 10thDay
14%14%6%9%8%47%5%5%6%23%
1797
11th to 20thDay
15%13%48%9%45%6%45%26%
1966
21st to endDay
16%15%6%9%7%35%445%27%
2200
April1st to 10thDay
21%13%48%8%45%45%6%22%
2166
11th to 20thDay
18%11%47%7%3445%6%32%
2312
21st to endDay
22%13%36%5%3435%5%31%
2340
May1st to 10thDay
21%11%37%6%4445%5%30%
2340
11th to 20thDay
19%11%37%6%3445%434%
2501
21st to endDay
21%8%45%5%35%5%47%33%
2919
June1st to 10thDay
21%8%36%6%46%5%6%7%28%
2700
11th to 20thDay
21%7%26%5%35%45%8%34%
2700
21st to endDay
24%9%37%6%35%46%7%25%
2700
July1st to 10thDay
16%7%36%6%46%5%6%9%30%
2700
11th to 20thDay
14%6%36%7%46%6%7%8%33%
2640
21st to endDay
14%7%38%6%46%5%6%7%33%
2744
August1st to 10thDay
17%7%37%8%46%6%7%8%28%
2417
11th to 20thDay
14%7%47%6%35%6%6%7%34%
2304
21st to endDay
14%8%46%5%35%46%8%36%
2574
September1st to 10thDay
16%8%25%6%3445%8%40%
2140
11th to 20thDay
16%7%26%5%3435%6%44%
1980
21st to endDay
18%7%36%5%34447%39%
1980
October1st to 10thDay
17%10%46%6%44446%35%
1915
11th to 20thDay
21%12%38%6%35%446%26%
1720
21st to endDay
24%13%48%6%345%45%24%
1782
November1st to 10thDay
17%13%6%9%7%34445%27%
1671
11th to 20thDay
18%11%5%11%7%46%5%6%6%22%
1532
21st to endDay
18%15%412%8%45%5%45%21%
1416
December1st to 10thDay
19%15%5%12%8%5%7%5%45%14%
1330
11th to 20thDay
17%16%6%10%6%5%6%5%4422%
1668
21st to endDay
19%12%410%10%36%45%423%
2431
January1st to 10thNight
9%14%6%12%12%8%9%7%6%6%12%
4713
11th to 20thNight
11%13%5%9%11%6%7%7%7%7%19%
4440
21st to endNight
6%10%6%10%8%5%7%6%6%7%28%
4884
February1st to 10thNight
9%12%5%9%10%5%7%6%6%7%26%
2571
11th to 20thNight
10%13%6%12%9%47%6%6%6%21%
2356
21st to endNight
10%14%5%9%8%45%45%7%30%
1831
March1st to 10thNight
13%11%5%9%7%38%5%5%6%27%
2150
11th to 20thNight
12%13%6%8%7%46%5%5%7%27%
1992
21st to endNight
13%13%5%8%8%45%445%31%
2156
April1st to 10thNight
22%12%5%7%6%345%5%5%27%
1793
11th to 20thNight
20%7%36%5%45%5%6%6%33%
1598
21st to endNight
23%11%35%445%5%36%31%
1620
May1st to 10thNight
21%10%48%5%35%445%30%
1620
11th to 20thNight
21%9%35%42345%6%37%
1406
21st to endNight
18%9%26%6%36%46%6%32%
1356
June1st to 10thNight
21%7%46%6%346%6%6%31%
1260
11th to 20thNight
16%7%345%45%5%5%8%38%
1260
21st to endNight
23%8%36%6%35%5%6%7%28%
1260
July1st to 10thNight
16%6%37%435%6%6%8%37%
1260
11th to 20thNight
14%6%35%6%36%6%9%8%36%
1317
21st to endNight
14%7%26%6%36%7%7%8%34%
1574
August1st to 10thNight
12%5%36%7%35%5%7%9%37%
1504
11th to 20thNight
14%7%27%6%35%7%6%9%34%
1596
21st to endNight
12%6%36%6%445%5%9%40%
1782
September1st to 10thNight
16%5%25%5%345%7%6%42%
1818
11th to 20thNight
18%7%35%4345%46%41%
1980
21st to endNight
15%6%26%5%245%5%7%43%
1980
October1st to 10thNight
17%7%46%5%2435%7%40%
2044
11th to 20thNight
22%10%25%5%45%46%6%30%
2238
21st to endNight
21%11%37%5%345%5%7%29%
2574
November1st to 10thNight
17%10%5%8%6%35%6%6%6%28%
2407
11th to 20thNight
17%11%47%6%35%5%6%7%27%
2645
21st to endNight
16%12%5%11%8%5%6%5%6%6%21%
2759
December1st to 10thNight
19%14%6%11%8%45%5%5%5%18%
2850
11th to 20thNight
19%13%5%10%10%5%6%6%6%6%15%
3531
21st to endNight
16%13%5%9%11%6%6%5%45%21%
5236

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
66%10%13%9%2
2239
11th to 20thDay
65%10%10%11%31
2520
21st to endDay
56%9%11%11%12%
2772
February1st to 10thDay
62%7%11%11%8%1
1607
11th to 20thDay
63%8%12%12%5%
1717
21st to endDay
58%6%11%17%8%1
1520
March1st to 10thDay
62%5%10%13%8%2
1800
11th to 20thDay
60%7%10%13%9%2
1966
21st to endDay
60%5%8%16%9%2
2200
April1st to 10thDay
61%6%11%13%7%2
2166
11th to 20thDay
52%6%10%19%11%2
2312
21st to endDay
57%5%7%17%12%2
2340
May1st to 10thDay
55%5%10%16%12%1
2340
11th to 20thDay
53%6%10%19%12%1
2505
21st to endDay
51%8%10%21%11%
2919
June1st to 10thDay
53%9%10%17%10%1
2700
11th to 20thDay
49%8%12%19%11%1
2700
21st to endDay
58%7%13%15%8%
2700
July1st to 10thDay
49%8%17%19%7%
2625
11th to 20thDay
47%8%15%23%7%
2559
21st to endDay
50%9%15%20%7%
2591
August1st to 10thDay
51%11%16%17%5%
2403
11th to 20thDay
47%9%15%22%6%
2304
21st to endDay
45%9%16%24%7%
2574
September1st to 10thDay
43%7%14%26%10%
2140
11th to 20thDay
42%6%15%24%13%1
1980
21st to endDay
46%7%13%23%11%
1980
October1st to 10thDay
50%7%11%22%10%
1915
11th to 20thDay
60%8%12%17%3
1720
21st to endDay
62%6%12%13%6%1
1782
November1st to 10thDay
58%8%13%17%4
1671
11th to 20thDay
63%8%12%13%5%
1532
21st to endDay
65%8%11%13%3
1416
December1st to 10thDay
72%5%11%8%3
1330
11th to 20thDay
66%6%9%12%8%
1668
21st to endDay
65%7%12%11%41
2431
January1st to 10thNight
69%9%12%8%2
4713
11th to 20thNight
60%10%12%11%6%
4440
21st to endNight
52%9%10%17%11%1
4884
February1st to 10thNight
56%7%11%14%10%1
2571
11th to 20thNight
60%10%11%12%7%
2359
21st to endNight
54%6%10%19%10%1
1831
March1st to 10thNight
56%7%8%19%9%1
2160
11th to 20thNight
57%5%12%15%10%1
1993
21st to endNight
55%5%11%18%8%2
2156
April1st to 10thNight
58%5%9%13%12%3
1793
11th to 20thNight
50%7%10%18%15%1
1598
21st to endNight
54%6%8%18%13%2
1620
May1st to 10thNight
55%6%9%15%13%1
1620
11th to 20thNight
48%6%11%23%11%1
1407
21st to endNight
51%9%8%18%12%1
1357
June1st to 10thNight
51%7%12%20%9%1
1260
11th to 20thNight
44%7%13%20%14%2
1260
21st to endNight
54%9%13%16%8%
1260
July1st to 10thNight
42%10%20%21%6%
1225
11th to 20thNight
41%10%18%23%7%1
1275
21st to endNight
45%11%18%19%7%
1486
August1st to 10thNight
42%11%18%24%4
1494
11th to 20thNight
44%11%20%18%6%1
1596
21st to endNight
40%9%17%27%7%
1782
September1st to 10thNight
39%10%16%24%10%1
1818
11th to 20thNight
42%9%17%21%10%1
1980
21st to endNight
40%8%13%27%12%1
1980
October1st to 10thNight
45%7%12%25%10%1
2044
11th to 20thNight
53%8%14%20%5%
2238
21st to endNight
55%7%15%15%7%1
2574
November1st to 10thNight
54%9%14%17%5%1
2407
11th to 20thNight
55%9%15%13%7%
2645
21st to endNight
63%9%14%11%31
2759
December1st to 10thNight
67%7%11%10%4
2850
11th to 20thNight
66%9%11%11%3
3532
21st to endNight
66%7%11%11%5%
5236

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
29%23%32%13%21
180
11th to 20thDay
33%22%31%8%41
180
21st to endDay
21%29%28%14%7%2
198
February1st to 10thDay
22%28%34%10%6%1
267
11th to 20thDay
26%26%31%13%4
388
21st to endDay
24%29%28%12%6%
362
March1st to 10thDay
29%23%31%14%4
420
11th to 20thDay
28%22%30%12%7%1
420
21st to endDay
32%23%24%13%6%1
462
April1st to 10thDay
33%24%25%12%5%1
420
11th to 20thDay
27%21%34%12%42
412
21st to endDay
31%21%32%11%31
420
May1st to 10thDay
31%21%31%13%4
423
11th to 20thDay
27%20%33%14%6%1
508
21st to endDay
29%21%27%15%6%1
680
June1st to 10thDay
27%18%30%18%6%
630
11th to 20thDay
26%20%25%20%8%1
630
21st to endDay
26%15%29%22%8%
630
July1st to 10thDay
21%16%32%23%7%1
620
11th to 20thDay
17%15%36%22%8%2
558
21st to endDay
17%14%40%21%7%1
488
August1st to 10thDay
19%15%35%21%8%2
446
11th to 20thDay
18%12%35%28%6%1
439
21st to endDay
22%18%29%18%11%3
495
September1st to 10thDay
19%22%29%17%11%3
450
11th to 20thDay
22%19%33%17%7%3
450
21st to endDay
25%19%30%15%6%5%
450
October1st to 10thDay
21%22%28%18%10%2
417
11th to 20thDay
34%20%22%13%9%1
360
21st to endDay
36%18%28%13%4
396
November1st to 10thDay
34%20%26%16%31
365
11th to 20thDay
28%22%28%14%5%2
370
21st to endDay
33%26%27%11%3
266
December1st to 10thDay
29%24%28%14%32
180
11th to 20thDay
29%29%32%8%2
180
21st to endDay
36%22%26%13%21
198
January1st to 10thNight
22%22%36%17%3
839
11th to 20thNight
25%26%28%17%5%
760
21st to endNight
16%21%36%22%5%
836
February1st to 10thNight
18%26%30%19%5%1
760
11th to 20thNight
23%23%33%16%4
743
21st to endNight
23%19%35%19%5%
611
March1st to 10thNight
23%17%33%22%4
720
11th to 20thNight
23%22%31%16%7%1
720
21st to endNight
23%22%27%20%7%1
792
April1st to 10thNight
31%19%26%18%5%1
713
11th to 20thNight
25%18%28%22%7%
534
21st to endNight
34%14%25%20%6%1
540
May1st to 10thNight
28%12%30%23%6%1
540
11th to 20thNight
24%13%30%24%8%1
408
21st to endNight
24%17%28%23%8%1
389
June1st to 10thNight
24%9%26%34%7%1
360
11th to 20thNight
20%9%27%33%11%1
360
21st to endNight
27%9%28%28%8%1
360
July1st to 10thNight
16%12%33%27%11%1
350
11th to 20thNight
14%10%32%32%13%
348
21st to endNight
18%7%28%34%12%1
394
August1st to 10thNight
17%7%29%33%13%
430
11th to 20thNight
18%8%31%30%12%1
532
21st to endNight
12%14%32%29%12%2
594
September1st to 10thNight
18%15%30%25%11%1
684
11th to 20thNight
23%10%29%24%11%2
720
21st to endNight
19%15%31%22%12%1
720
October1st to 10thNight
22%17%27%24%8%1
720
11th to 20thNight
27%17%26%18%10%
720
21st to endNight
29%20%24%21%5%
792
November1st to 10thNight
27%21%26%20%6%
740
11th to 20thNight
27%23%26%18%6%1
843
21st to endNight
28%21%28%17%5%
850
December1st to 10thNight
28%20%32%13%5%1
850
11th to 20thNight
29%22%27%17%4
850
21st to endNight
30%22%26%17%5%
935
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