Observatoire Isis
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Isis 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
28%
22692
13%
80710
8%
22604
36%
22604
14%
57859

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
13%
754
7%
4406
2%
737
22%
737
4%
3367
January11th to 20thNight
16%
756
8%
4391
4%
740
23%
740
5%
3268
January21st to endNight
19%
831
7%
4784
6%
814
29%
814
6%
3775
February1st to 10thNight
17%
760
11%
2470
3%
760
26%
760
10%
1892
February11th to 20thNight
15%
743
13%
2401
4%
740
27%
740
11%
1773
February21st to endNight
21%
611
15%
1831
6%
611
32%
611
13%
1349
March1st to 10thNight
19%
720
14%
2160
7%
720
28%
720
13%
1621
March11th to 20thNight
19%
720
16%
1985
6%
720
29%
720
15%
1464
March21st to endNight
24%
792
21%
2057
7%
792
33%
792
19%
1448
April1st to 10thNight
22%
720
15%
1800
4%
720
28%
720
18%
1334
April11th to 20thNight
24%
711
17%
1775
6%
711
31%
711
19%
1292
April21st to endNight
26%
596
16%
1676
7%
596
33%
596
18%
1209
May1st to 10thNight
33%
360
17%
1440
13%
360
43%
360
16%
931
May11th to 20thNight
32%
354
21%
1416
18%
354
46%
354
18%
887
May21st to endNight
34%
389
13%
1551
10%
388
47%
388
21%
938
June1st to 10thNight
36%
360
10%
1440
10%
360
45%
360
17%
940
June11th to 20thNight
39%
360
12%
1440
14%
360
48%
360
18%
876
June21st to endNight
41%
360
10%
1440
13%
360
47%
360
16%
935
July1st to 10thNight
52%
350
14%
1400
18%
350
61%
350
23%
815
July11th to 20thNight
51%
348
16%
1392
23%
348
55%
348
23%
812
July21st to endNight
53%
394
14%
1486
16%
372
58%
372
23%
882
August1st to 10thNight
55%
375
16%
1439
16%
373
61%
373
22%
867
August11th to 20thNight
57%
538
17%
1602
12%
538
61%
538
29%
1027
August21st to endNight
46%
792
17%
1980
7%
792
50%
792
30%
1325
September1st to 10thNight
44%
720
17%
1800
6%
720
49%
720
30%
1201
September11th to 20thNight
48%
720
22%
1800
12%
720
51%
720
31%
1190
September21st to endNight
40%
720
21%
2034
14%
720
46%
720
26%
1341
October1st to 10thNight
33%
720
17%
2160
10%
720
42%
720
19%
1488
October11th to 20thNight
26%
720
15%
2160
8%
720
35%
720
16%
1588
October21st to endNight
22%
792
13%
2528
6%
792
32%
792
13%
1881
November1st to 10thNight
29%
740
15%
2407
9%
740
37%
740
16%
1674
November11th to 20thNight
20%
760
15%
2656
8%
760
30%
760
12%
1916
November21st to endNight
19%
760
10%
2660
4%
760
26%
760
9%
2040
December1st to 10thNight
17%
760
8%
2660
4%
760
22%
760
7%
2159
December11th to 20thNight
16%
759
13%
3241
5%
759
26%
759
8%
2440
December21st to endNight
11%
827
6%
4842
1%
817
19%
817
4%
3914

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
18%10%47%6%35%45%6%31%
78247
Night
18%11%47%6%3445%6%32%
81042
TransparencyDay
52%7%12%19%10%1
77952
Night
53%6%11%19%10%1
80871
SeeingDay
24%15%31%19%8%2
13458
Night
24%14%28%25%9%1
22717

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
16%18%7%11%8%4444420%
2517
11th to 20thDay
15%13%5%14%10%44446%21%
2520
21st to endDay
14%17%7%11%8%5%6%5%45%18%
2868
February1st to 10thDay
13%15%6%11%8%45%446%23%
1710
11th to 20thDay
13%15%6%11%7%34445%28%
1666
21st to endDay
19%11%6%10%7%445%46%24%
1520
March1st to 10thDay
16%12%49%7%45%45%5%29%
1797
11th to 20thDay
15%14%47%7%46%446%30%
1970
21st to endDay
19%9%47%5%4435%5%36%
2298
April1st to 10thDay
23%11%37%6%34336%31%
2160
11th to 20thDay
22%9%36%5%23445%36%
2134
21st to endDay
23%10%48%5%34345%30%
2298
May1st to 10thDay
22%7%36%6%35%45%5%34%
2520
11th to 20thDay
17%7%37%6%35%45%6%38%
2478
21st to endDay
18%7%25%6%45%5%5%7%36%
2725
June1st to 10thDay
23%7%37%5%35%5%5%7%29%
2520
11th to 20thDay
16%5%35%5%45%5%6%9%38%
2520
21st to endDay
20%7%35%5%46%6%7%8%31%
2520
July1st to 10thDay
12%5%35%5%46%6%7%8%39%
2520
11th to 20thDay
13%6%25%5%46%6%7%8%37%
2520
21st to endDay
13%5%35%6%35%6%7%10%38%
2744
August1st to 10thDay
12%6%26%6%45%5%7%9%38%
2485
11th to 20thDay
14%5%26%6%45%5%7%8%38%
2281
21st to endDay
14%6%25%6%345%5%9%40%
2376
September1st to 10thDay
15%5%25%5%35%5%5%8%42%
2160
11th to 20thDay
14%435%5%345%5%7%45%
2160
21st to endDay
20%6%245%4445%9%38%
1923
October1st to 10thDay
23%7%36%5%3445%6%35%
1800
11th to 20thDay
25%9%36%6%45%45%6%26%
1800
21st to endDay
27%10%47%6%45%445%26%
1828
November1st to 10thDay
15%9%37%5%45%47%7%34%
1671
11th to 20thDay
20%12%5%8%6%35%5%5%6%25%
1520
21st to endDay
23%14%49%8%35%45%5%21%
1520
December1st to 10thDay
22%16%8%10%7%35%446%16%
1520
11th to 20thDay
21%11%5%10%7%34445%24%
1906
21st to endDay
26%20%6%8%6%2333418%
2772
January1st to 10thNight
17%18%6%10%7%34445%21%
4429
11th to 20thNight
12%15%6%11%8%45%45%5%24%
4440
21st to endNight
16%18%7%11%7%5%5%33420%
4784
February1st to 10thNight
13%14%6%11%8%45%5%45%24%
2470
11th to 20thNight
13%13%6%11%7%45%445%29%
2399
21st to endNight
16%12%48%6%45%5%46%30%
1831
March1st to 10thNight
17%12%6%7%7%36%5%4429%
2150
11th to 20thNight
16%13%5%8%6%44445%33%
1984
21st to endNight
16%10%46%6%24345%40%
2057
April1st to 10thNight
22%10%37%5%34346%33%
1800
11th to 20thNight
19%8%36%5%35%45%6%36%
1775
21st to endNight
22%9%37%5%3445%6%33%
1676
May1st to 10thNight
21%6%36%5%3345%7%37%
1440
11th to 20thNight
19%7%35%5%2445%6%40%
1415
21st to endNight
14%6%25%445%5%6%6%43%
1550
June1st to 10thNight
19%5%26%5%35%6%7%7%35%
1440
11th to 20thNight
15%5%36%435%5%6%6%42%
1440
21st to endNight
18%6%25%6%45%5%5%8%36%
1440
July1st to 10thNight
12%5%245%35%5%6%7%46%
1440
11th to 20thNight
13%5%25%5%345%6%8%44%
1440
21st to endNight
9%5%26%5%36%5%6%9%43%
1574
August1st to 10thNight
10%6%36%6%436%6%9%43%
1449
11th to 20thNight
11%6%25%43446%9%46%
1602
21st to endNight
12%5%26%6%345%5%8%44%
1980
September1st to 10thNight
12%6%25%5%345%6%6%46%
1800
11th to 20thNight
16%5%25%4245%5%6%47%
1800
21st to endNight
18%6%35%5%24346%44%
2034
October1st to 10thNight
21%7%26%43445%5%39%
2160
11th to 20thNight
23%8%26%7%445%5%6%31%
2160
21st to endNight
24%10%47%6%3445%6%28%
2528
November1st to 10thNight
19%11%35%5%345%5%6%33%
2407
11th to 20thNight
19%11%48%6%46%345%29%
2656
21st to endNight
21%12%5%9%6%35%6%45%24%
2660
December1st to 10thNight
21%16%6%9%6%45%5%5%5%19%
2660
11th to 20thNight
22%14%5%9%7%23335%27%
3288
21st to endNight
28%19%5%8%6%3333418%
4884

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%8%10%10%5%1
2518
11th to 20thDay
63%6%12%13%6%
2520
21st to endDay
67%8%10%10%5%
2868
February1st to 10thDay
61%5%10%18%5%2
1710
11th to 20thDay
58%412%15%9%1
1675
21st to endDay
60%5%10%13%10%1
1520
March1st to 10thDay
56%6%10%17%10%1
1800
11th to 20thDay
55%5%12%14%11%3
1970
21st to endDay
50%5%8%18%14%5%
2298
April1st to 10thDay
55%48%18%12%3
2160
11th to 20thDay
50%47%22%15%2
2134
21st to endDay
56%48%14%14%4
2298
May1st to 10thDay
51%6%7%19%14%3
2520
11th to 20thDay
46%5%9%21%16%3
2482
21st to endDay
46%7%12%19%14%2
2725
June1st to 10thDay
53%6%11%18%11%1
2520
11th to 20thDay
42%8%11%24%13%2
2520
21st to endDay
48%9%13%20%10%1
2520
July1st to 10thDay
39%9%12%26%14%1
2450
11th to 20thDay
40%8%15%26%11%
2445
21st to endDay
40%9%16%24%11%
2591
August1st to 10thDay
41%9%16%25%9%
2471
11th to 20thDay
40%10%15%26%9%1
2281
21st to endDay
39%7%15%27%12%1
2376
September1st to 10thDay
39%7%14%27%12%
2160
11th to 20thDay
37%7%13%31%12%1
2160
21st to endDay
43%8%15%26%9%
1923
October1st to 10thDay
48%7%12%22%11%1
1800
11th to 20thDay
57%7%14%16%6%1
1800
21st to endDay
59%5%13%15%8%
1828
November1st to 10thDay
48%7%14%22%7%1
1671
11th to 20thDay
57%6%13%15%8%1
1520
21st to endDay
63%8%12%10%6%1
1520
December1st to 10thDay
67%7%12%11%3
1520
11th to 20thDay
64%5%12%14%5%
1906
21st to endDay
71%5%7%14%3
2772
January1st to 10thNight
65%6%14%11%3
4429
11th to 20thNight
61%8%14%13%5%
4440
21st to endNight
67%6%10%10%6%1
4784
February1st to 10thNight
61%5%10%16%7%1
2470
11th to 20thNight
58%5%10%19%8%1
2401
21st to endNight
55%6%9%18%11%1
1831
March1st to 10thNight
58%5%9%17%10%2
2160
11th to 20thNight
55%49%18%11%3
1985
21st to endNight
48%47%20%15%5%
2057
April1st to 10thNight
53%410%17%15%2
1800
11th to 20thNight
49%6%8%20%16%1
1775
21st to endNight
52%5%8%16%16%4
1676
May1st to 10thNight
47%6%7%19%16%5%
1440
11th to 20thNight
44%5%7%21%19%3
1416
21st to endNight
39%7%14%20%17%2
1551
June1st to 10thNight
46%7%13%24%9%1
1440
11th to 20thNight
40%8%11%24%16%2
1440
21st to endNight
45%9%12%19%14%1
1440
July1st to 10thNight
34%10%11%27%17%2
1400
11th to 20thNight
37%8%11%31%13%
1392
21st to endNight
37%8%16%24%15%
1486
August1st to 10thNight
35%11%18%23%12%1
1439
11th to 20thNight
36%8%15%29%11%
1602
21st to endNight
37%9%16%28%11%
1980
September1st to 10thNight
37%8%11%29%14%1
1800
11th to 20thNight
35%9%12%34%9%1
1800
21st to endNight
43%5%12%31%10%
2034
October1st to 10thNight
47%7%10%23%13%1
2160
11th to 20thNight
53%7%11%21%8%
2160
21st to endNight
56%6%13%16%8%
2528
November1st to 10thNight
50%6%16%20%9%
2407
11th to 20thNight
56%6%12%16%9%1
2656
21st to endNight
61%6%13%15%5%
2660
December1st to 10thNight
67%6%10%12%4
2660
11th to 20thNight
61%5%9%17%8%1
3288
21st to endNight
71%5%8%13%4
4884

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%16%33%18%4
180
11th to 20thDay
29%17%38%12%31
180
21st to endDay
23%19%36%17%5%
225
February1st to 10thDay
34%23%31%9%2
270
11th to 20thDay
29%28%36%7%1
273
21st to endDay
32%23%29%14%2
362
March1st to 10thDay
28%20%33%15%5%
420
11th to 20thDay
29%21%31%15%31
420
21st to endDay
32%18%30%14%6%1
462
April1st to 10thDay
31%22%27%14%5%1
420
11th to 20thDay
34%18%24%15%8%1
412
21st to endDay
31%20%30%12%6%1
420
May1st to 10thDay
27%17%30%17%6%3
423
11th to 20thDay
20%18%34%17%7%2
447
21st to endDay
22%15%33%19%9%2
485
June1st to 10thDay
26%11%37%18%6%1
450
11th to 20thDay
19%15%30%22%11%3
450
21st to endDay
23%11%31%22%10%3
450
July1st to 10thDay
14%8%30%33%11%3
445
11th to 20thDay
14%9%35%27%11%3
444
21st to endDay
14%7%36%28%12%3
488
August1st to 10thDay
15%9%27%31%14%4
446
11th to 20thDay
17%428%30%13%8%
439
21st to endDay
17%11%29%24%13%6%
495
September1st to 10thDay
18%10%30%22%13%7%
450
11th to 20thDay
15%11%28%20%17%9%
450
21st to endDay
21%13%27%21%14%4
450
October1st to 10thDay
25%16%23%22%10%4
450
11th to 20thDay
28%18%31%16%6%1
450
21st to endDay
34%15%27%20%4
419
November1st to 10thDay
27%16%31%19%7%1
365
11th to 20thDay
29%26%28%13%3
180
21st to endDay
27%24%30%13%32
180
December1st to 10thDay
36%13%39%11%2
180
11th to 20thDay
28%22%37%11%2
180
21st to endDay
35%20%32%11%21
198
January1st to 10thNight
28%17%32%19%4
760
11th to 20thNight
30%17%26%21%5%
760
21st to endNight
20%15%35%25%5%
836
February1st to 10thNight
22%20%31%24%3
760
11th to 20thNight
22%21%36%18%4
743
21st to endNight
26%19%28%23%4
611
March1st to 10thNight
26%16%32%22%3
720
11th to 20thNight
25%19%32%20%5%
720
21st to endNight
25%17%28%22%7%
792
April1st to 10thNight
29%16%26%24%4
720
11th to 20thNight
24%19%28%20%7%1
711
21st to endNight
27%15%26%23%9%
596
May1st to 10thNight
25%10%26%28%8%2
360
11th to 20thNight
24%9%28%29%10%
354
21st to endNight
18%8%35%27%11%1
389
June1st to 10thNight
20%9%28%34%9%
360
11th to 20thNight
16%9%26%32%15%1
360
21st to endNight
18%7%24%36%15%
360
July1st to 10thNight
12%328%37%19%
350
11th to 20thNight
13%322%43%17%1
348
21st to endNight
11%5%25%41%17%2
394
August1st to 10thNight
12%5%19%41%23%1
375
11th to 20thNight
13%5%17%37%25%3
538
21st to endNight
15%6%25%34%18%2
792
September1st to 10thNight
15%6%29%29%19%1
720
11th to 20thNight
18%7%21%32%19%3
720
21st to endNight
21%8%25%25%18%2
720
October1st to 10thNight
24%12%25%23%15%1
720
11th to 20thNight
27%12%27%25%8%1
720
21st to endNight
29%15%27%24%5%
792
November1st to 10thNight
27%14%23%26%9%1
740
11th to 20thNight
26%17%31%16%10%
760
21st to endNight
29%19%24%22%6%
760
December1st to 10thNight
30%16%29%21%3
760
11th to 20thNight
28%19%27%20%5%1
760
21st to endNight
34%16%31%16%4
836
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