Boreal Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boreal Skies Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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
29%
22550
13%
74734
8%
22508
36%
22508
15%
53619

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
14%
760
8%
2649
2%
757
21%
757
7%
2159
January11th to 20thNight
18%
729
9%
2549
5%
729
23%
729
8%
2018
January21st to endNight
22%
792
13%
2704
7%
792
30%
792
13%
1959
February1st to 10thNight
16%
720
9%
2340
2%
720
24%
720
10%
1829
February11th to 20thNight
17%
703
13%
2271
5%
700
28%
700
13%
1668
February21st to endNight
24%
592
14%
1776
7%
592
34%
592
14%
1317
March1st to 10thNight
20%
720
14%
2160
7%
720
28%
720
14%
1618
March11th to 20thNight
20%
720
16%
1980
6%
720
30%
720
16%
1459
March21st to endNight
27%
792
21%
2062
8%
792
35%
792
20%
1459
April1st to 10thNight
22%
720
14%
1800
4%
720
27%
720
19%
1342
April11th to 20thNight
25%
711
16%
1775
5%
711
31%
711
20%
1302
April21st to endNight
28%
602
16%
1682
7%
602
34%
602
18%
1219
May1st to 10thNight
34%
360
16%
1440
14%
360
43%
360
17%
937
May11th to 20thNight
33%
354
20%
1416
16%
354
48%
354
18%
888
May21st to endNight
35%
389
12%
1551
10%
388
46%
388
21%
933
June1st to 10thNight
38%
360
10%
1440
10%
360
46%
360
17%
941
June11th to 20thNight
40%
360
12%
1440
14%
360
48%
360
20%
872
June21st to endNight
39%
360
10%
1440
13%
360
47%
360
17%
939
July1st to 10thNight
51%
350
14%
1400
18%
350
59%
350
24%
805
July11th to 20thNight
53%
348
15%
1392
22%
348
57%
348
24%
805
July21st to endNight
54%
394
14%
1486
17%
372
59%
372
23%
871
August1st to 10thNight
52%
385
15%
1449
14%
383
57%
383
22%
898
August11th to 20thNight
56%
538
17%
1602
12%
538
60%
538
30%
1020
August21st to endNight
48%
792
17%
1980
8%
792
51%
792
31%
1328
September1st to 10thNight
47%
720
17%
1800
7%
720
50%
720
32%
1199
September11th to 20thNight
50%
720
21%
1800
12%
720
53%
720
33%
1187
September21st to endNight
42%
720
20%
2037
15%
720
47%
720
26%
1343
October1st to 10thNight
34%
720
16%
2160
11%
720
42%
720
20%
1494
October11th to 20thNight
26%
720
14%
2160
7%
720
35%
720
16%
1604
October21st to endNight
25%
792
12%
2523
7%
792
33%
792
13%
1908
November1st to 10thNight
28%
740
14%
2407
9%
740
36%
740
17%
1685
November11th to 20thNight
21%
760
14%
2655
9%
760
29%
760
12%
1945
November21st to endNight
19%
760
9%
2660
4%
760
26%
760
9%
2068
December1st to 10thNight
18%
760
7%
2660
3%
760
22%
760
7%
2157
December11th to 20thNight
18%
759
11%
3243
6%
759
26%
759
8%
2460
December21st to endNight
14%
828
5%
4845
2%
817
21%
817
4%
3983

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%9%37%6%35%5%5%7%33%
74731
Night
18%10%47%6%35%45%6%32%
74989
TransparencyDay
50%8%12%19%10%1
74436
Night
52%7%11%19%10%1
74818
SeeingDay
22%16%30%20%9%3
13348
Night
22%12%30%26%9%1
22559

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
17%17%6%10%6%445%45%21%
1517
11th to 20thDay
19%14%5%10%7%45%45%423%
1456
21st to endDay
14%14%5%10%6%46%5%5%6%25%
1647
February1st to 10thDay
14%15%5%12%8%45%5%45%24%
1620
11th to 20thDay
13%15%5%9%6%445%5%5%28%
1575
21st to endDay
19%13%5%8%7%45%5%5%5%24%
1480
March1st to 10thDay
15%11%5%7%8%45%5%5%6%28%
1797
11th to 20thDay
16%11%48%7%45%446%30%
1976
21st to endDay
19%8%46%7%3345%5%36%
2294
April1st to 10thDay
22%10%46%5%34446%32%
2160
11th to 20thDay
22%7%45%5%34445%37%
2134
21st to endDay
22%10%38%5%34445%31%
2291
May1st to 10thDay
20%8%36%6%345%5%6%34%
2520
11th to 20thDay
17%6%36%6%35%45%6%39%
2478
21st to endDay
16%6%36%6%45%46%8%36%
2725
June1st to 10thDay
21%7%36%7%35%5%5%7%30%
2520
11th to 20thDay
14%5%35%6%36%6%6%9%38%
2520
21st to endDay
18%6%36%6%36%6%7%8%30%
2520
July1st to 10thDay
12%5%35%5%45%6%7%8%40%
2520
11th to 20thDay
12%5%36%6%36%5%7%9%39%
2520
21st to endDay
12%6%25%6%35%6%7%10%39%
2744
August1st to 10thDay
11%6%36%6%35%7%7%8%39%
2476
11th to 20thDay
11%6%26%6%45%5%7%9%39%
2281
21st to endDay
13%5%25%6%45%46%9%41%
2376
September1st to 10thDay
14%5%25%5%35%6%5%8%43%
2160
11th to 20thDay
13%5%245%35%46%7%46%
2160
21st to endDay
17%6%25%5%345%5%9%38%
1921
October1st to 10thDay
21%8%36%5%3446%6%35%
1800
11th to 20thDay
24%8%37%5%45%5%5%6%27%
1800
21st to endDay
26%9%46%6%45%446%27%
1832
November1st to 10thDay
16%8%46%6%36%6%6%6%34%
1671
11th to 20thDay
21%11%37%6%46%5%5%6%26%
1522
21st to endDay
22%14%47%9%45%5%45%21%
1520
December1st to 10thDay
23%15%6%10%7%25%45%6%17%
1520
11th to 20thDay
21%12%5%9%7%35%445%24%
1906
21st to endDay
28%18%5%9%5%3432418%
2772
January1st to 10thNight
18%17%6%12%8%35%445%19%
2649
11th to 20thNight
18%17%5%9%7%45%44423%
2549
21st to endNight
12%15%5%9%7%35%446%30%
2704
February1st to 10thNight
15%12%6%11%8%46%5%5%5%23%
2340
11th to 20thNight
14%12%6%9%8%34446%29%
2269
21st to endNight
17%12%48%6%445%5%5%30%
1776
March1st to 10thNight
16%12%5%8%7%46%445%29%
2150
11th to 20thNight
17%11%5%8%6%3444433%
1979
21st to endNight
18%8%46%6%24345%39%
2062
April1st to 10thNight
21%9%46%5%3445%5%34%
1800
11th to 20thNight
19%7%46%6%35%5%46%36%
1775
21st to endNight
21%9%36%5%35%45%7%33%
1682
May1st to 10thNight
20%7%25%5%3445%7%36%
1440
11th to 20thNight
18%7%445%34447%40%
1415
21st to endNight
13%5%36%5%345%5%8%44%
1550
June1st to 10thNight
16%6%27%6%36%6%6%8%34%
1440
11th to 20thNight
14%6%26%5%3446%8%42%
1440
21st to endNight
17%7%35%445%5%6%8%36%
1440
July1st to 10thNight
11%425%5%36%5%6%7%46%
1440
11th to 20thNight
12%6%36%425%5%6%8%43%
1440
21st to endNight
9%435%6%45%6%6%9%43%
1574
August1st to 10thNight
9%7%25%5%35%5%6%10%42%
1459
11th to 20thNight
10%6%26%5%345%6%8%45%
1602
21st to endNight
11%5%25%6%445%6%7%44%
1980
September1st to 10thNight
12%5%25%5%45%5%5%7%46%
1800
11th to 20thNight
15%5%25%42445%7%48%
1800
21st to endNight
17%6%25%6%33447%44%
2037
October1st to 10thNight
19%8%25%6%34446%38%
2160
11th to 20thNight
22%8%25%7%35%5%5%6%31%
2160
21st to endNight
24%9%36%7%35%5%46%28%
2523
November1st to 10thNight
20%9%35%5%3436%6%34%
2407
11th to 20thNight
19%10%47%6%45%6%46%29%
2655
21st to endNight
23%11%48%7%45%45%6%24%
2660
December1st to 10thNight
22%16%5%10%7%35%5%5%5%18%
2660
11th to 20thNight
23%14%5%8%7%34335%27%
3288
21st to endNight
30%17%48%5%3433418%
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
65%7%9%13%5%
1518
11th to 20thDay
64%5%10%11%10%1
1456
21st to endDay
59%8%12%12%8%1
1647
February1st to 10thDay
62%5%10%17%42
1620
11th to 20thDay
56%7%11%16%9%1
1584
21st to endDay
60%7%11%13%9%1
1480
March1st to 10thDay
56%8%10%17%9%1
1800
11th to 20thDay
53%7%12%14%11%3
1976
21st to endDay
49%6%9%18%13%5%
2294
April1st to 10thDay
53%6%8%19%11%3
2160
11th to 20thDay
49%5%8%22%14%2
2134
21st to endDay
55%6%8%13%14%4
2291
May1st to 10thDay
49%7%8%19%14%3
2520
11th to 20thDay
45%6%9%20%16%3
2482
21st to endDay
45%8%12%20%12%3
2725
June1st to 10thDay
52%9%10%18%11%1
2520
11th to 20thDay
41%10%12%23%13%2
2520
21st to endDay
48%10%14%18%10%1
2520
July1st to 10thDay
37%10%15%25%13%1
2450
11th to 20thDay
38%9%15%26%11%
2445
21st to endDay
39%10%16%24%10%
2591
August1st to 10thDay
38%11%16%24%9%
2462
11th to 20thDay
39%10%17%25%8%1
2281
21st to endDay
39%8%16%26%11%1
2376
September1st to 10thDay
38%9%16%26%11%
2160
11th to 20thDay
36%8%13%30%12%1
2160
21st to endDay
43%8%15%26%8%
1921
October1st to 10thDay
48%8%13%21%10%1
1800
11th to 20thDay
54%10%16%14%6%1
1800
21st to endDay
58%7%12%14%8%
1832
November1st to 10thDay
48%8%16%21%6%1
1671
11th to 20thDay
56%8%13%14%8%1
1522
21st to endDay
64%8%11%10%6%1
1520
December1st to 10thDay
67%8%13%9%31
1520
11th to 20thDay
61%9%13%13%4
1906
21st to endDay
71%5%7%13%3
2772
January1st to 10thNight
68%7%9%11%5%
2649
11th to 20thNight
66%6%10%10%8%1
2549
21st to endNight
55%6%10%17%11%1
2704
February1st to 10thNight
62%7%10%14%6%1
2340
11th to 20thNight
56%6%11%18%7%1
2271
21st to endNight
55%6%10%18%10%1
1776
March1st to 10thNight
56%6%10%16%9%2
2160
11th to 20thNight
53%5%11%18%11%2
1980
21st to endNight
48%5%8%20%15%5%
2062
April1st to 10thNight
52%6%10%17%14%2
1800
11th to 20thNight
49%7%8%20%16%1
1775
21st to endNight
52%6%8%16%15%4
1682
May1st to 10thNight
47%6%8%19%15%5%
1440
11th to 20thNight
44%6%8%21%18%3
1416
21st to endNight
38%9%15%18%17%2
1551
June1st to 10thNight
45%8%15%22%8%1
1440
11th to 20thNight
38%9%12%23%16%2
1440
21st to endNight
44%10%13%20%13%1
1440
July1st to 10thNight
34%9%13%26%16%2
1400
11th to 20thNight
36%8%14%30%12%
1392
21st to endNight
35%10%16%25%13%
1486
August1st to 10thNight
37%10%17%22%12%1
1449
11th to 20thNight
36%8%16%28%11%
1602
21st to endNight
36%9%16%28%11%
1980
September1st to 10thNight
37%7%12%28%14%1
1800
11th to 20thNight
35%9%13%32%9%1
1800
21st to endNight
42%6%13%30%10%
2037
October1st to 10thNight
46%8%11%22%12%1
2160
11th to 20thNight
53%8%12%19%8%
2160
21st to endNight
56%8%13%15%8%
2523
November1st to 10thNight
50%6%15%20%8%
2407
11th to 20thNight
55%7%12%16%8%1
2655
21st to endNight
60%9%12%14%5%
2660
December1st to 10thNight
67%7%12%11%3
2660
11th to 20thNight
61%6%10%16%7%1
3288
21st to endNight
71%6%7%12%3
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
25%21%30%20%4
180
11th to 20thDay
30%15%34%18%21
164
21st to endDay
24%17%36%17%6%
204
February1st to 10thDay
35%24%30%10%1
240
11th to 20thDay
29%26%37%7%1
242
21st to endDay
31%20%32%15%2
346
March1st to 10thDay
24%19%35%18%4
420
11th to 20thDay
26%20%34%16%31
420
21st to endDay
29%21%27%16%6%1
462
April1st to 10thDay
28%23%28%15%6%
420
11th to 20thDay
31%19%22%16%9%2
412
21st to endDay
30%19%31%12%6%1
420
May1st to 10thDay
25%16%32%18%7%2
423
11th to 20thDay
17%20%34%18%9%2
447
21st to endDay
19%16%31%21%11%1
485
June1st to 10thDay
24%12%34%20%8%2
450
11th to 20thDay
16%16%29%26%11%3
450
21st to endDay
19%13%31%23%12%3
450
July1st to 10thDay
11%7%29%35%13%4
445
11th to 20thDay
12%9%34%27%15%3
444
21st to endDay
12%7%33%31%13%4
488
August1st to 10thDay
14%8%28%30%16%4
446
11th to 20thDay
13%8%24%30%17%8%
439
21st to endDay
13%12%31%23%14%7%
495
September1st to 10thDay
16%9%32%19%16%8%
450
11th to 20thDay
12%11%27%22%18%10%
450
21st to endDay
18%13%29%21%14%5%
450
October1st to 10thDay
24%14%25%21%12%4
450
11th to 20thDay
24%20%30%17%7%1
450
21st to endDay
33%15%26%21%5%
421
November1st to 10thDay
24%18%29%20%8%1
365
11th to 20thDay
29%23%32%12%41
182
21st to endDay
28%23%31%13%41
180
December1st to 10thDay
31%17%36%14%2
180
11th to 20thDay
29%24%34%11%11
180
21st to endDay
33%20%35%10%21
198
January1st to 10thNight
26%15%34%21%3
760
11th to 20thNight
26%18%29%21%5%1
729
21st to endNight
19%12%38%25%5%
792
February1st to 10thNight
23%16%36%23%3
720
11th to 20thNight
21%16%40%18%4
703
21st to endNight
25%17%28%26%4
592
March1st to 10thNight
23%15%36%23%3
720
11th to 20thNight
23%19%33%20%5%
720
21st to endNight
23%17%28%24%8%
792
April1st to 10thNight
26%14%32%23%4
720
11th to 20thNight
24%16%30%21%8%
711
21st to endNight
25%14%29%24%7%1
602
May1st to 10thNight
24%9%26%30%9%2
360
11th to 20thNight
22%12%27%29%10%
354
21st to endNight
16%8%37%27%12%
389
June1st to 10thNight
21%431%34%10%
360
11th to 20thNight
17%8%28%33%13%2
360
21st to endNight
21%5%29%31%14%1
360
July1st to 10thNight
10%5%28%40%17%
350
11th to 20thNight
11%5%22%44%16%1
348
21st to endNight
8%427%44%14%2
394
August1st to 10thNight
13%421%41%21%1
385
11th to 20thNight
11%419%40%23%2
538
21st to endNight
13%6%25%35%18%2
792
September1st to 10thNight
14%7%29%32%18%1
720
11th to 20thNight
17%7%22%33%19%3
720
21st to endNight
19%9%25%26%19%2
720
October1st to 10thNight
23%10%27%24%15%1
720
11th to 20thNight
25%12%30%25%7%1
720
21st to endNight
27%13%29%26%5%
792
November1st to 10thNight
26%13%27%24%8%1
740
11th to 20thNight
24%17%33%16%10%1
760
21st to endNight
29%17%27%22%6%
760
December1st to 10thNight
30%16%31%21%3
760
11th to 20thNight
28%16%30%21%5%1
760
21st to endNight
34%15%30%17%3
836
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