Observatoire Isis
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Isis clear sky chart from 2005-11-05 to 2017-08-12.

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
27%
14493
13%
46489
8%
14461
34%
14461
15%
33204

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
16%
720
7%
2506
3%
716
22%
716
7%
2029
January16th to endNight
19%
768
11%
2649
6%
768
26%
768
10%
1987
February1st to 15thNight
16%
704
11%
2287
4%
701
24%
701
10%
1763
February16th to endNight
20%
631
14%
1949
6%
631
31%
631
12%
1470
March1st to 15thNight
17%
720
13%
2103
7%
720
25%
720
12%
1586
March16th to endNight
25%
768
19%
2031
8%
768
34%
768
19%
1407
April1st to 15thNight
21%
718
15%
1796
5%
718
27%
718
19%
1324
April16th to endNight
24%
636
17%
1716
5%
636
33%
636
18%
1206
May1st to 15thNight
30%
360
17%
1440
13%
360
41%
360
12%
957
May16th to endNight
28%
375
13%
1497
8%
374
41%
374
16%
959
June1st to 15thNight
31%
360
9%
1440
8%
360
41%
360
14%
968
June16th to endNight
37%
360
8%
1440
8%
360
44%
360
15%
914
July1st to 15thNight
50%
340
14%
1360
19%
340
56%
340
23%
774
July16th to endNight
48%
376
13%
1418
15%
354
58%
354
24%
806
August1st to 15thNight
50%
419
14%
1468
13%
417
55%
417
21%
914
August16th to endNight
48%
649
18%
1699
9%
649
53%
649
32%
1069
September1st to 15thNight
43%
660
15%
1650
6%
660
46%
660
25%
1155
September16th to endNight
44%
657
21%
1793
14%
657
49%
657
30%
1155
October1st to 15thNight
30%
660
18%
1980
11%
660
39%
660
19%
1382
October16th to endNight
25%
704
11%
2205
7%
704
32%
704
13%
1661
November1st to 15thNight
26%
700
16%
2334
11%
700
34%
700
15%
1638
November16th to endNight
18%
720
10%
2520
5%
720
25%
720
9%
1926
December1st to 15thNight
16%
720
7%
2520
3%
720
21%
720
8%
2010
December16th to endNight
13%
768
10%
2688
3%
768
21%
768
7%
2144

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
19%8%36%5%3445%6%31%
47487
Night
18%9%36%5%34445%32%
46658
Cloud CoverDay
21%7%36%5%34445%32%
47496
Night
20%9%36%5%24445%33%
46664
TransparencyDay
51%6%12%19%9%1
47174
Night
52%5%11%19%9%
46489
SeeingDay
20%17%25%21%11%3
5323
Night
20%21%24%23%9%
14493

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 15thDay
23%15%5%8%6%3333420%
1432
16th to endDay
16%15%49%7%33445%24%
1572
February1st to 15thDay
13%14%5%12%8%35%335%24%
1575
16th to endDay
18%11%6%9%7%3343425%
1523
March1st to 15thDay
17%13%47%7%45%44427%
1844
16th to endDay
16%7%36%5%3435%5%38%
2193
April1st to 15thDay
24%9%36%423235%32%
2152
16th to endDay
21%8%36%5%24335%35%
2253
May1st to 15thDay
22%7%27%5%34345%32%
2511
16th to endDay
19%7%35%5%45%45%6%33%
2618
June1st to 15thDay
22%6%36%5%35%5%48%28%
2520
16th to endDay
19%5%24446%5%6%7%31%
2520
July1st to 15thDay
14%5%245%36%5%7%8%36%
2520
16th to endDay
13%5%25%5%35%6%7%8%35%
2654
August1st to 15thDay
12%5%25%7%45%5%6%8%35%
2393
16th to endDay
12%5%15%5%345%6%8%41%
2140
September1st to 15thDay
18%425%5%245%47%38%
1980
16th to endDay
18%5%24323347%44%
1833
October1st to 15thDay
23%8%245%24346%33%
1650
16th to endDay
28%8%37%6%3434424%
1667
November1st to 15thDay
15%8%46%5%35%46%6%32%
1521
16th to endDay
25%12%38%5%24345%23%
1440
December1st to 15thDay
22%14%7%9%7%35%435%16%
1440
16th to endDay
22%17%5%8%6%3333419%
1536
January1st to 15thNight
19%17%6%9%7%3434418%
2506
16th to endNight
13%16%6%7%6%35%435%26%
2649
February1st to 15thNight
12%13%6%10%7%45%44424%
2285
16th to endNight
17%12%5%7%6%35%435%28%
1949
March1st to 15thNight
19%11%5%8%5%3443429%
2090
16th to endNight
14%8%35%6%3433441%
2031
April1st to 15thNight
24%8%35%433335%34%
1796
16th to endNight
19%8%26%42425%5%37%
1716
May1st to 15thNight
23%7%35%32335%5%36%
1440
16th to endNight
16%6%26%4345%5%5%37%
1495
June1st to 15thNight
20%5%25%5%35%5%5%6%34%
1440
16th to endNight
17%4245%35%5%5%7%36%
1440
July1st to 15thNight
11%31443446%7%46%
1440
16th to endNight
9%5%23435%5%7%8%45%
1514
August1st to 15thNight
11%5%245%3347%9%41%
1478
16th to endNight
10%315%42445%8%49%
1699
September1st to 15thNight
15%6%26%5%245%5%5%40%
1650
16th to endNight
18%425%313235%48%
1793
October1st to 15thNight
22%6%26%5%24346%36%
1980
16th to endNight
25%9%26%6%34446%27%
2205
November1st to 15thNight
19%9%35%5%35%445%32%
2334
16th to endNight
23%11%48%6%35%43425%
2520
December1st to 15thNight
22%15%5%7%6%34445%20%
2520
16th to endNight
24%15%6%8%6%2434420%
2688

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 15thDay
23%14%6%8%5%2443421%
1432
16th to endDay
16%15%5%9%5%45%345%25%
1572
February1st to 15thDay
14%12%6%10%8%35%345%25%
1575
16th to endDay
18%12%5%9%6%3344426%
1523
March1st to 15thDay
24%9%36%7%35%34327%
1854
16th to endDay
24%6%35%42334437%
2192
April1st to 15thDay
23%9%24423235%36%
2152
16th to endDay
21%7%35%5%2433439%
2253
May1st to 15thDay
27%6%35%42434433%
2511
16th to endDay
24%6%25%5%24345%35%
2618
June1st to 15thDay
27%6%25%5%35%447%27%
2520
16th to endDay
23%425%435%5%6%7%29%
2520
July1st to 15thDay
14%425%5%25%5%8%7%37%
2520
16th to endDay
13%5%25%5%35%6%6%8%36%
2654
August1st to 15thDay
24%414435%35%7%34%
2393
16th to endDay
23%21342445%6%41%
2140
September1st to 15thDay
18%4245%2335%8%39%
1980
16th to endDay
18%424323346%45%
1833
October1st to 15thDay
21%8%35%344445%35%
1650
16th to endDay
28%8%27%5%35%33426%
1667
November1st to 15thDay
19%9%35%5%3445%5%33%
1521
16th to endDay
27%11%36%6%24335%24%
1440
December1st to 15thDay
22%13%5%9%7%3444419%
1440
16th to endDay
23%15%5%7%7%3423420%
1536
January1st to 15thNight
20%16%5%8%6%4434419%
2506
16th to endNight
13%15%6%7%6%35%435%27%
2649
February1st to 15thNight
13%13%5%10%7%35%44426%
2278
16th to endNight
17%12%46%6%2444429%
1949
March1st to 15thNight
27%8%46%5%2333329%
2103
16th to endNight
22%6%3442332441%
2031
April1st to 15thNight
24%6%24423435%37%
1796
16th to endNight
19%7%35%424336%39%
1716
May1st to 15thNight
27%5%24423346%35%
1440
16th to endNight
20%5%26%4245%45%38%
1495
June1st to 15thNight
22%5%445%346%5%6%32%
1440
16th to endNight
18%5%25%4245%5%8%36%
1440
July1st to 15thNight
11%3244245%5%8%46%
1440
16th to endNight
9%5%144346%6%8%44%
1514
August1st to 15thNight
16%315%32345%7%45%
1478
16th to endNight
18%31332335%6%48%
1699
September1st to 15thNight
16%4246%2445%6%41%
1650
16th to endNight
19%5%14422235%48%
1793
October1st to 15thNight
21%6%25%5%24345%37%
1980
16th to endNight
24%8%36%5%34345%29%
2205
November1st to 15thNight
24%7%26%5%2345%5%32%
2334
16th to endNight
26%10%46%6%23435%25%
2520
December1st to 15thNight
21%15%5%7%5%3444423%
2520
16th to endNight
24%14%5%7%6%2333423%
2688

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
66%48%13%5%
1433
16th to endDay
60%5%11%13%8%
1572
February1st to 15thDay
60%5%12%16%4
1577
16th to endDay
60%5%10%14%9%1
1523
March1st to 15thDay
58%5%11%15%7%1
1854
16th to endDay
46%49%18%15%5%
2193
April1st to 15thDay
53%38%18%13%2
2152
16th to endDay
51%36%19%14%3
2253
May1st to 15thDay
52%5%7%19%13%2
2511
16th to endDay
49%6%11%18%11%1
2622
June1st to 15thDay
52%6%11%20%8%
2520
16th to endDay
48%7%13%19%9%
2520
July1st to 15thDay
39%9%13%26%11%
2371
16th to endDay
43%7%17%20%9%
2487
August1st to 15thDay
43%8%16%22%9%
2379
16th to endDay
36%7%18%26%10%
2140
September1st to 15thDay
42%5%13%27%10%
1980
16th to endDay
39%6%13%30%9%
1833
October1st to 15thDay
50%5%12%20%10%
1650
16th to endDay
61%5%12%13%6%
1667
November1st to 15thDay
49%6%15%19%8%
1521
16th to endDay
61%6%12%13%5%
1440
December1st to 15thDay
68%5%11%12%1
1440
16th to endDay
69%5%8%11%5%
1536
January1st to 15thNight
68%5%9%11%4
2506
16th to endNight
59%5%10%16%7%
2649
February1st to 15thNight
62%410%17%5%
2287
16th to endNight
58%5%8%18%9%
1949
March1st to 15thNight
59%39%18%8%1
2103
16th to endNight
46%48%20%15%4
2031
April1st to 15thNight
51%48%18%16%1
1796
16th to endNight
48%48%18%17%3
1716
May1st to 15thNight
49%5%6%18%15%4
1440
16th to endNight
44%6%12%20%13%2
1497
June1st to 15thNight
49%6%11%22%8%1
1440
16th to endNight
43%7%13%19%14%
1440
July1st to 15thNight
33%8%12%29%15%
1360
16th to endNight
34%7%16%26%15%
1418
August1st to 15thNight
37%9%17%23%11%
1468
16th to endNight
33%6%15%31%11%
1699
September1st to 15thNight
42%8%10%27%10%
1650
16th to endNight
39%310%34%9%
1793
October1st to 15thNight
48%5%10%23%11%
1980
16th to endNight
57%5%13%17%5%
2205
November1st to 15thNight
51%5%14%17%10%
2334
16th to endNight
61%5%11%16%4
2520
December1st to 15thNight
64%6%12%12%3
2520
16th to endNight
66%49%12%5%
2688

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
30%13%26%23%5%1
60
16th to endDay
32%7%37%17%41
70
February1st to 15thDay
34%17%35%11%1
90
16th to endDay
30%25%19%18%5%
182
March1st to 15thDay
23%21%28%20%5%
270
16th to endDay
21%23%25%19%7%3
288
April1st to 15thDay
24%29%23%15%6%
270
16th to endDay
19%24%27%16%10%1
270
May1st to 15thDay
27%19%24%14%10%3
288
16th to endDay
18%21%28%18%10%2
331
June1st to 15thDay
22%17%29%20%8%1
315
16th to endDay
20%17%24%24%11%1
315
July1st to 15thDay
12%11%27%27%15%4
305
16th to endDay
10%13%23%31%16%4
326
August1st to 15thDay
11%13%17%31%19%6%
297
16th to endDay
10%10%22%29%18%9%
268
September1st to 15thDay
16%10%21%18%20%11%
255
16th to endDay
18%13%20%22%17%7%
254
October1st to 15thDay
22%14%24%23%14%1
255
16th to endDay
29%18%22%22%6%
255
November1st to 15thDay
18%17%26%22%13%1
175
16th to endDay
33%13%23%28%1
60
December1st to 15thDay
20%15%35%26%3
60
16th to endDay
40%18%32%7%
64
January1st to 15thNight
23%27%26%18%4
720
16th to endNight
19%24%29%22%5%
768
February1st to 15thNight
17%26%29%21%4
704
16th to endNight
21%25%25%22%4
631
March1st to 15thNight
23%26%25%20%2
720
16th to endNight
18%26%23%23%7%
768
April1st to 15thNight
23%25%26%20%3
718
16th to endNight
20%27%24%20%7%
636
May1st to 15thNight
23%21%19%25%8%1
360
16th to endNight
16%21%28%24%8%
375
June1st to 15thNight
16%20%26%28%7%
360
16th to endNight
17%19%18%29%13%1
360
July1st to 15thNight
9%13%22%37%16%
340
16th to endNight
8%17%18%37%17%1
376
August1st to 15thNight
14%11%15%33%23%1
419
16th to endNight
10%12%19%34%20%1
649
September1st to 15thNight
15%13%23%27%19%1
660
16th to endNight
18%13%19%25%19%1
657
October1st to 15thNight
22%16%24%23%11%1
660
16th to endNight
26%19%23%23%7%
704
November1st to 15thNight
20%20%28%20%11%1
700
16th to endNight
23%29%21%17%7%
720
December1st to 15thNight
26%26%24%18%2
720
16th to endNight
28%23%26%16%41
768
Page generated at 2017-08-13 14:16:50. Version ac04

Copyright 2017 A. Danko


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