Blue Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Sky Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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
30%
22849
12%
74552
5%
22805
37%
22805
10%
57677

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%
760
13%
2649
4%
757
30%
757
8%
2042
January11th to 20thNight
17%
728
8%
2546
3%
728
23%
728
6%
2097
January21st to endNight
16%
792
13%
2772
4%
792
29%
792
9%
2060
February1st to 10thNight
16%
720
9%
2320
2%
720
25%
720
8%
1818
February11th to 20thNight
16%
703
13%
2100
2%
700
28%
700
8%
1646
February21st to endNight
21%
592
15%
1776
5%
592
34%
592
10%
1382
March1st to 10thNight
18%
720
14%
2160
4%
720
30%
720
10%
1665
March11th to 20thNight
21%
720
11%
2047
4%
720
28%
720
10%
1644
March21st to endNight
22%
792
14%
2081
4%
792
30%
792
12%
1635
April1st to 10thNight
22%
720
12%
1800
4%
720
27%
720
13%
1446
April11th to 20thNight
27%
668
13%
1732
5%
668
32%
668
14%
1376
April21st to endNight
25%
540
13%
1620
4%
540
31%
540
10%
1282
May1st to 10thNight
25%
540
10%
1620
4%
540
29%
540
10%
1301
May11th to 20thNight
28%
505
10%
1567
5%
505
34%
505
8%
1262
May21st to endNight
37%
389
10%
1551
11%
388
41%
388
10%
1159
June1st to 10thNight
35%
360
9%
1440
7%
360
44%
360
8%
1135
June11th to 20thNight
42%
360
9%
1440
11%
360
45%
360
9%
1118
June21st to endNight
41%
360
7%
1440
8%
360
44%
360
9%
1131
July1st to 10thNight
52%
350
6%
1400
8%
350
54%
350
8%
1103
July11th to 20thNight
56%
348
8%
1392
10%
348
58%
348
11%
1027
July21st to endNight
58%
394
12%
1486
14%
372
62%
372
12%
1106
August1st to 10thNight
55%
488
9%
1552
7%
486
58%
486
13%
1194
August11th to 20thNight
48%
532
12%
1596
9%
532
50%
532
13%
1236
August21st to endNight
52%
767
12%
1955
5%
767
55%
767
17%
1527
September1st to 10thNight
48%
720
14%
1817
5%
720
53%
720
19%
1400
September11th to 20thNight
44%
720
18%
1980
9%
720
51%
720
18%
1451
September21st to endNight
43%
720
15%
1980
9%
720
48%
720
16%
1516
October1st to 10thNight
35%
720
11%
1980
6%
720
40%
720
13%
1567
October11th to 20thNight
35%
720
13%
2210
6%
720
43%
720
11%
1739
October21st to endNight
26%
792
11%
2574
5%
792
33%
792
10%
1988
November1st to 10thNight
31%
740
13%
2407
7%
740
38%
740
12%
1827
November11th to 20thNight
25%
760
17%
2470
7%
760
38%
760
11%
1883
November21st to endNight
25%
760
13%
2470
5%
760
35%
760
10%
1908
December1st to 10thNight
19%
760
11%
2635
3%
760
29%
760
7%
2104
December11th to 20thNight
21%
760
13%
3191
2%
759
32%
759
7%
2328
December21st to endNight
18%
829
8%
4796
2%
817
27%
817
4%
3574

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%6%35%5%3445%7%40%
74810
Night
18%7%25%5%3445%6%40%
74909
TransparencyDay
29%38%8%15%9%2
74515
Night
30%37%7%15%9%1
74738
SeeingDay
22%15%32%21%8%2
13887
Night
22%12%30%27%9%1
22856

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
15%7%35%42445%7%42%
1517
11th to 20thDay
20%6%35%5%3446%7%38%
1454
21st to endDay
13%8%25%5%445%5%6%43%
1584
February1st to 10thDay
18%8%36%5%35%5%5%7%36%
1638
11th to 20thDay
14%7%35%5%345%6%7%41%
1748
21st to endDay
15%11%46%6%23446%38%
1480
March1st to 10thDay
17%8%25%5%35%46%7%37%
1797
11th to 20thDay
17%7%35%42445%7%41%
1911
21st to endDay
19%7%36%5%34445%42%
2272
April1st to 10thDay
19%7%26%6%44346%39%
2160
11th to 20thDay
17%6%35%5%35%5%6%8%38%
2177
21st to endDay
21%7%36%6%35%446%35%
2340
May1st to 10thDay
23%7%35%5%33447%34%
2340
11th to 20thDay
19%6%25%5%3445%6%40%
2342
21st to endDay
16%5%25%5%345%6%8%41%
2725
June1st to 10thDay
21%7%36%5%345%6%7%32%
2520
11th to 20thDay
16%5%345%3446%8%40%
2520
21st to endDay
19%6%36%5%45%5%6%8%33%
2520
July1st to 10thDay
14%5%35%445%5%6%10%40%
2520
11th to 20thDay
13%6%26%5%35%6%7%10%38%
2520
21st to endDay
12%5%35%6%35%6%7%9%39%
2744
August1st to 10thDay
11%6%25%6%45%6%7%9%39%
2357
11th to 20thDay
12%6%35%6%45%6%6%8%39%
2304
21st to endDay
12%5%235%35%5%6%9%45%
2398
September1st to 10thDay
13%4245%345%6%7%46%
2142
11th to 20thDay
14%425%433345%53%
1980
21st to endDay
16%6%24424446%48%
1980
October1st to 10thDay
21%7%3443445%5%41%
1980
11th to 20thDay
15%5%26%5%3435%7%45%
1745
21st to endDay
23%6%244235%5%6%39%
1782
November1st to 10thDay
17%5%15%423336%49%
1671
11th to 20thDay
20%5%24323446%47%
1710
21st to endDay
21%6%34423446%43%
1710
December1st to 10thDay
20%7%27%5%35%446%38%
1544
11th to 20thDay
22%7%24423445%43%
1906
21st to endDay
25%9%25%5%3345%5%33%
2772
January1st to 10thNight
16%8%35%43345%7%42%
2649
11th to 20thNight
18%9%36%6%345%5%7%34%
2546
21st to endNight
13%9%35%5%345%6%6%40%
2772
February1st to 10thNight
16%10%37%5%345%5%6%37%
2320
11th to 20thNight
15%8%36%6%345%5%6%40%
2098
21st to endNight
17%9%34435%45%6%42%
1776
March1st to 10thNight
15%9%35%434446%42%
2150
11th to 20thNight
18%7%36%5%35%5%5%6%38%
2047
21st to endNight
18%8%35%433345%43%
2081
April1st to 10thNight
20%7%25%6%35%45%6%37%
1800
11th to 20thNight
19%7%26%4345%46%41%
1732
21st to endNight
23%6%45%43446%6%36%
1620
May1st to 10thNight
22%8%36%4345%5%7%32%
1620
11th to 20thNight
19%7%36%5%45%6%5%6%34%
1566
21st to endNight
18%7%36%5%345%5%7%38%
1550
June1st to 10thNight
20%7%46%6%35%35%6%35%
1440
11th to 20thNight
21%5%25%5%35%47%8%37%
1440
21st to endNight
19%7%26%5%25%45%9%36%
1440
July1st to 10thNight
16%6%25%5%35%5%7%8%39%
1440
11th to 20thNight
13%6%25%5%36%5%7%10%39%
1440
21st to endNight
14%5%24335%5%6%8%45%
1574
August1st to 10thNight
11%5%145%35%5%6%8%45%
1562
11th to 20thNight
13%7%26%5%46%6%7%9%36%
1596
21st to endNight
15%425%4245%5%8%46%
1955
September1st to 10thNight
13%6%245%2345%8%48%
1817
11th to 20thNight
14%6%1432335%7%51%
1980
21st to endNight
15%5%24424347%48%
1980
October1st to 10thNight
22%6%35%43445%5%40%
1980
11th to 20thNight
16%6%25%445%5%5%6%43%
2210
21st to endNight
24%6%35%5%3345%5%38%
2574
November1st to 10thNight
15%5%35%5%24446%46%
2407
11th to 20thNight
18%6%24424436%46%
2470
21st to endNight
17%6%34434447%45%
2470
December1st to 10thNight
20%9%36%5%3345%5%38%
2635
11th to 20thNight
20%7%2442445%6%40%
3288
21st to endNight
24%9%26%5%34445%35%
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
26%41%8%16%9%
1518
11th to 20thDay
29%43%9%11%6%1
1454
21st to endDay
25%38%7%19%9%1
1584
February1st to 10thDay
31%37%8%14%8%2
1638
11th to 20thDay
26%39%10%15%8%3
1757
21st to endDay
34%36%7%13%8%2
1480
March1st to 10thDay
30%37%9%12%8%3
1800
11th to 20thDay
30%37%7%14%9%2
1911
21st to endDay
28%37%414%12%4
2272
April1st to 10thDay
31%37%6%12%12%2
2160
11th to 20thDay
30%37%7%14%10%2
2177
21st to endDay
32%38%5%12%10%3
2340
May1st to 10thDay
34%37%5%13%10%2
2340
11th to 20thDay
30%37%6%13%12%3
2346
21st to endDay
26%39%9%16%9%2
2725
June1st to 10thDay
35%37%8%13%6%
2520
11th to 20thDay
28%40%9%13%8%2
2520
21st to endDay
33%39%9%12%6%1
2520
July1st to 10thDay
29%40%9%15%7%1
2450
11th to 20thDay
27%40%9%16%7%
2445
21st to endDay
25%41%9%16%8%2
2591
August1st to 10thDay
28%39%11%14%7%
2343
11th to 20thDay
28%38%9%16%8%1
2304
21st to endDay
24%39%8%16%11%2
2398
September1st to 10thDay
24%38%8%17%13%
2142
11th to 20thDay
22%36%7%19%15%2
1980
21st to endDay
28%37%6%15%13%2
1980
October1st to 10thDay
29%37%7%15%11%1
1980
11th to 20thDay
28%37%6%20%7%1
1745
21st to endDay
28%38%8%15%11%2
1782
November1st to 10thDay
24%40%7%17%12%1
1671
11th to 20thDay
23%41%8%15%12%2
1710
21st to endDay
28%40%11%14%7%
1710
December1st to 10thDay
31%40%9%11%8%1
1544
11th to 20thDay
31%32%8%14%11%4
1906
21st to endDay
42%27%9%15%6%2
2772
January1st to 10thNight
28%40%8%16%8%
2649
11th to 20thNight
33%43%7%11%5%1
2546
21st to endNight
27%38%7%15%13%1
2772
February1st to 10thNight
31%38%8%13%9%1
2320
11th to 20thNight
30%38%8%14%8%2
2100
21st to endNight
29%37%7%16%8%2
1776
March1st to 10thNight
29%36%6%15%11%2
2160
11th to 20thNight
32%37%9%12%8%1
2048
21st to endNight
29%36%7%11%14%3
2081
April1st to 10thNight
31%37%7%14%9%3
1800
11th to 20thNight
30%36%6%14%12%1
1732
21st to endNight
33%36%6%12%10%2
1620
May1st to 10thNight
34%37%6%13%9%3
1620
11th to 20thNight
33%39%6%11%7%4
1567
21st to endNight
28%38%9%14%8%2
1551
June1st to 10thNight
35%37%7%13%7%1
1440
11th to 20thNight
31%39%7%11%9%3
1440
21st to endNight
33%38%10%12%6%1
1440
July1st to 10thNight
32%40%10%13%5%1
1400
11th to 20thNight
26%39%11%17%7%
1392
21st to endNight
25%40%9%18%6%1
1486
August1st to 10thNight
28%39%8%18%7%
1552
11th to 20thNight
29%38%9%14%9%1
1596
21st to endNight
26%38%8%16%11%1
1955
September1st to 10thNight
24%38%7%17%13%1
1817
11th to 20thNight
22%37%7%18%15%1
1980
21st to endNight
26%37%6%17%13%2
1980
October1st to 10thNight
31%37%7%14%9%1
1980
11th to 20thNight
29%39%6%17%7%
2210
21st to endNight
30%38%8%15%9%1
2574
November1st to 10thNight
24%41%7%14%12%1
2407
11th to 20thNight
26%39%6%16%11%1
2470
21st to endNight
27%39%8%16%8%1
2470
December1st to 10thNight
31%41%7%14%6%1
2635
11th to 20thNight
30%34%9%13%14%
3288
21st to endNight
43%26%5%18%8%
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
26%24%29%18%3
180
11th to 20thDay
25%23%34%13%41
164
21st to endDay
18%24%38%15%41
176
February1st to 10thDay
29%22%33%13%2
320
11th to 20thDay
25%25%33%14%3
416
21st to endDay
26%21%33%17%3
346
March1st to 10thDay
27%20%35%15%3
420
11th to 20thDay
25%25%33%13%4
420
21st to endDay
26%21%32%13%6%2
462
April1st to 10thDay
28%22%30%12%6%2
420
11th to 20thDay
27%22%27%17%6%1
412
21st to endDay
30%14%35%14%6%2
420
May1st to 10thDay
32%20%29%14%41
423
11th to 20thDay
20%17%40%17%5%1
447
21st to endDay
20%14%34%24%7%1
485
June1st to 10thDay
24%12%36%19%8%1
450
11th to 20thDay
17%16%37%22%7%1
450
21st to endDay
20%10%36%26%6%3
450
July1st to 10thDay
18%6%34%28%11%3
445
11th to 20thDay
18%5%30%34%11%2
444
21st to endDay
12%6%36%32%12%2
488
August1st to 10thDay
14%7%29%31%15%4
446
11th to 20thDay
18%6%31%28%11%6%
439
21st to endDay
16%7%32%26%15%4
495
September1st to 10thDay
19%6%27%29%12%7%
450
11th to 20thDay
21%7%32%20%12%7%
450
21st to endDay
18%11%32%25%8%6%
450
October1st to 10thDay
26%11%26%22%11%4
450
11th to 20thDay
18%14%31%24%10%3
386
21st to endDay
27%14%30%22%6%
396
November1st to 10thDay
19%16%30%23%10%2
365
11th to 20thDay
29%21%25%19%5%1
370
21st to endDay
27%18%27%20%6%2
370
December1st to 10thDay
21%23%33%17%5%1
204
11th to 20thDay
30%21%33%13%3
180
21st to endDay
29%21%29%16%5%1
198
January1st to 10thNight
21%19%35%22%3
760
11th to 20thNight
25%16%35%21%3
728
21st to endNight
20%20%37%19%4
792
February1st to 10thNight
23%18%35%22%2
720
11th to 20thNight
21%19%39%18%3
703
21st to endNight
22%18%34%22%3
592
March1st to 10thNight
26%17%35%19%3
720
11th to 20thNight
23%19%33%20%6%
720
21st to endNight
24%15%32%22%6%
792
April1st to 10thNight
25%17%32%22%4
720
11th to 20thNight
23%12%32%24%8%
668
21st to endNight
26%13%32%25%5%
540
May1st to 10thNight
26%9%31%24%9%1
540
11th to 20thNight
23%9%31%29%7%1
505
21st to endNight
23%7%26%32%11%1
389
June1st to 10thNight
24%5%27%31%12%1
360
11th to 20thNight
23%424%35%13%
360
21st to endNight
21%326%38%12%
360
July1st to 10thNight
18%218%44%17%1
350
11th to 20thNight
15%217%43%22%1
348
21st to endNight
14%317%47%17%2
394
August1st to 10thNight
14%320%41%21%1
488
11th to 20thNight
15%5%21%38%18%2
532
21st to endNight
17%321%35%22%2
767
September1st to 10thNight
15%325%34%21%1
720
11th to 20thNight
18%6%26%27%20%3
720
21st to endNight
18%428%29%19%2
720
October1st to 10thNight
24%8%24%27%15%2
720
11th to 20thNight
19%10%29%28%13%1
720
21st to endNight
27%11%30%24%7%
792
November1st to 10thNight
19%11%31%29%8%1
740
11th to 20thNight
22%16%31%23%7%1
760
21st to endNight
20%19%30%26%4
760
December1st to 10thNight
25%16%35%20%5%
760
11th to 20thNight
23%19%31%23%31
760
21st to endNight
25%16%34%22%3
836
Page generated at 2024-01-09 16:24:27. Version ac07

Copyright 2024 Attilla Danko


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