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 2005-11-05 to 2017-09-21.

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%
14659
13%
46894
8%
14627
37%
14627
16%
33636

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
17%
720
7%
2506
3%
716
22%
716
7%
2043
January16th to endNight
22%
768
10%
2643
6%
768
27%
768
11%
1976
February1st to 15thNight
19%
704
10%
2287
4%
701
26%
701
11%
1765
February16th to endNight
23%
631
13%
1949
6%
631
33%
631
13%
1462
March1st to 15thNight
20%
720
13%
2100
7%
720
27%
720
14%
1585
March16th to endNight
29%
768
20%
2034
9%
768
37%
768
21%
1412
April1st to 15thNight
24%
718
15%
1796
5%
718
29%
718
20%
1331
April16th to endNight
30%
640
17%
1720
8%
640
36%
640
21%
1217
May1st to 15thNight
35%
360
16%
1440
15%
360
44%
360
16%
954
May16th to endNight
36%
375
12%
1497
11%
374
46%
374
18%
969
June1st to 15thNight
35%
360
9%
1440
8%
360
44%
360
15%
964
June16th to endNight
41%
360
8%
1440
11%
360
46%
360
17%
927
July1st to 15thNight
54%
340
14%
1360
20%
340
59%
340
26%
760
July16th to endNight
57%
376
13%
1418
17%
354
61%
354
25%
803
August1st to 15thNight
52%
438
14%
1509
12%
436
57%
436
23%
962
August16th to endNight
53%
708
19%
1854
10%
708
57%
708
35%
1161
September1st to 15thNight
47%
720
15%
1800
7%
720
49%
720
28%
1252
September16th to endNight
47%
681
22%
1856
14%
681
51%
681
31%
1190
October1st to 15thNight
32%
660
17%
1980
11%
660
41%
660
19%
1395
October16th to endNight
26%
704
11%
2203
6%
704
33%
704
13%
1689
November1st to 15thNight
28%
700
16%
2334
12%
700
36%
700
16%
1657
November16th to endNight
20%
720
10%
2520
6%
720
26%
720
9%
1966
December1st to 15thNight
19%
720
6%
2520
3%
720
24%
720
8%
2026
December16th to endNight
14%
768
9%
2688
3%
768
22%
768
8%
2170

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%8%36%5%35%45%6%32%
47976
Night
18%9%36%5%34446%32%
47063
Cloud CoverDay
19%7%36%5%3445%6%34%
47985
Night
19%9%36%5%34445%34%
47069
TransparencyDay
50%7%12%19%9%1
47663
Night
52%6%11%19%9%
46894
SeeingDay
21%12%28%22%11%3
5486
Night
22%10%29%26%10%
14659

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%13%6%9%5%33435%21%
1432
16th to endDay
15%15%5%8%6%35%45%6%24%
1578
February1st to 15thDay
13%13%5%12%7%45%5%35%24%
1575
16th to endDay
17%13%5%7%6%345%5%426%
1522
March1st to 15thDay
17%10%46%7%46%5%45%27%
1847
16th to endDay
15%7%35%6%2335%5%39%
2190
April1st to 15thDay
23%8%45%423345%33%
2152
16th to endDay
20%7%36%43444437%
2248
May1st to 15thDay
20%7%36%5%3445%5%33%
2511
16th to endDay
18%6%36%6%34447%33%
2618
June1st to 15thDay
20%6%35%6%35%45%7%29%
2520
16th to endDay
18%5%25%5%36%6%6%7%31%
2520
July1st to 15thDay
13%424435%6%6%8%37%
2520
16th to endDay
12%5%25%5%35%6%6%9%36%
2654
August1st to 15thDay
11%5%25%6%45%6%6%8%37%
2439
16th to endDay
11%415%5%35%46%8%42%
2335
September1st to 15thDay
16%5%15%5%25%446%41%
2160
16th to endDay
15%5%2442335%8%44%
1897
October1st to 15thDay
21%7%26%43445%6%34%
1650
16th to endDay
27%7%46%5%44436%25%
1669
November1st to 15thDay
15%8%36%5%35%5%6%6%32%
1523
16th to endDay
25%12%36%6%34435%24%
1440
December1st to 15thDay
23%13%410%7%26%45%5%17%
1440
16th to endDay
23%15%5%8%6%34335%19%
1536
January1st to 15thNight
19%15%6%10%7%34445%18%
2506
16th to endNight
13%16%5%8%6%34435%26%
2643
February1st to 15thNight
14%11%6%11%7%36%45%424%
2285
16th to endNight
17%11%5%8%6%345%45%28%
1949
March1st to 15thNight
19%10%48%6%3433429%
2087
16th to endNight
16%6%36%6%2333441%
2034
April1st to 15thNight
23%7%44424435%35%
1796
16th to endNight
18%7%35%425%346%37%
1720
May1st to 15thNight
22%7%35%424446%35%
1440
16th to endNight
16%6%35%5%245%47%37%
1495
June1st to 15thNight
17%5%26%6%25%5%5%7%33%
1440
16th to endNight
16%5%25%435%5%6%7%37%
1440
July1st to 15thNight
10%324425%5%6%7%47%
1440
16th to endNight
9%423435%5%5%8%45%
1514
August1st to 15thNight
10%6%25%5%345%6%10%40%
1519
16th to endNight
9%41443445%7%49%
1854
September1st to 15thNight
15%5%25%5%34446%42%
1800
16th to endNight
17%424423235%49%
1856
October1st to 15thNight
20%7%25%5%34445%36%
1980
16th to endNight
25%8%35%6%34446%27%
2203
November1st to 15thNight
20%7%36%5%3435%6%32%
2334
16th to endNight
23%11%37%7%34435%24%
2520
December1st to 15thNight
22%15%5%8%5%24445%20%
2520
16th to endNight
24%15%5%8%6%2433420%
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
22%13%6%8%5%24445%21%
1432
16th to endDay
15%15%48%6%45%45%5%25%
1578
February1st to 15thDay
13%13%5%11%6%45%44425%
1575
16th to endDay
17%13%5%7%7%3445%426%
1522
March1st to 15thDay
22%7%45%6%36%45%329%
1857
16th to endDay
21%6%35%433345%39%
2189
April1st to 15thDay
22%8%35%423345%37%
2152
16th to endDay
19%6%35%5%2344440%
2248
May1st to 15thDay
23%6%25%5%24445%35%
2511
16th to endDay
21%5%25%5%24445%37%
2618
June1st to 15thDay
20%5%35%6%34447%33%
2520
16th to endDay
16%6%245%26%6%6%6%35%
2520
July1st to 15thDay
13%425%436%5%6%9%38%
2520
16th to endDay
12%425%6%35%5%6%9%37%
2654
August1st to 15thDay
21%425%5%345%6%6%35%
2439
16th to endDay
18%415%42445%7%41%
2335
September1st to 15thDay
15%4245%245%5%6%42%
2160
16th to endDay
15%5%2442335%7%45%
1897
October1st to 15thDay
20%8%26%43435%6%34%
1650
16th to endDay
27%6%36%6%44345%26%
1669
November1st to 15thDay
16%7%45%5%35%5%5%5%35%
1523
16th to endDay
23%11%36%5%34435%26%
1440
December1st to 15thDay
22%14%49%7%35%45%5%18%
1440
16th to endDay
24%14%5%8%7%3433420%
1536
January1st to 15thNight
19%15%6%9%7%4444419%
2506
16th to endNight
13%16%5%8%6%34436%26%
2643
February1st to 15thNight
15%11%6%10%7%35%435%24%
2278
16th to endNight
18%11%48%6%34445%28%
1949
March1st to 15thNight
25%8%46%5%3333431%
2100
16th to endNight
21%6%35%5%1333342%
2034
April1st to 15thNight
22%7%34423335%38%
1796
16th to endNight
18%6%25%423445%40%
1720
May1st to 15thNight
22%7%34324345%36%
1440
16th to endNight
17%6%25%5%3345%6%38%
1495
June1st to 15thNight
17%5%35%6%35%5%5%7%35%
1440
16th to endNight
15%5%25%425%6%5%6%39%
1440
July1st to 15thNight
10%324425%5%6%7%46%
1440
16th to endNight
9%424425%6%5%8%46%
1514
August1st to 15thNight
15%6%2432436%8%40%
1519
16th to endNight
15%314424446%48%
1854
September1st to 15thNight
15%5%26%5%2435%5%44%
1800
16th to endNight
16%425%413335%49%
1856
October1st to 15thNight
20%7%25%424345%36%
1980
16th to endNight
25%8%35%6%35%446%26%
2203
November1st to 15thNight
20%7%35%5%33345%37%
2334
16th to endNight
21%10%37%5%3434430%
2520
December1st to 15thNight
23%15%47%5%24445%21%
2520
16th to endNight
25%15%5%7%6%2433421%
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
65%5%10%12%5%
1433
16th to endDay
60%6%12%11%7%
1578
February1st to 15thDay
61%5%11%16%3
1577
16th to endDay
58%6%10%13%8%1
1522
March1st to 15thDay
58%7%11%14%7%1
1857
16th to endDay
45%5%10%19%14%5%
2190
April1st to 15thDay
52%47%19%12%2
2152
16th to endDay
49%5%7%18%15%3
2248
May1st to 15thDay
51%6%8%18%13%2
2511
16th to endDay
48%8%11%18%11%1
2622
June1st to 15thDay
52%8%10%19%8%
2520
16th to endDay
47%8%13%19%10%
2520
July1st to 15thDay
37%9%15%25%11%
2371
16th to endDay
41%9%17%21%9%
2487
August1st to 15thDay
41%10%16%21%9%
2425
16th to endDay
37%7%18%27%9%
2335
September1st to 15thDay
40%7%13%26%11%
2160
16th to endDay
39%7%13%30%8%
1897
October1st to 15thDay
50%6%14%19%8%
1650
16th to endDay
60%7%12%13%6%
1669
November1st to 15thDay
49%7%14%19%7%
1523
16th to endDay
62%6%11%12%5%1
1440
December1st to 15thDay
67%8%11%10%2
1440
16th to endDay
67%5%10%9%5%
1536
January1st to 15thNight
68%6%9%10%4
2506
16th to endNight
59%5%11%15%7%
2643
February1st to 15thNight
61%5%11%15%5%
2287
16th to endNight
57%6%10%17%8%
1949
March1st to 15thNight
57%49%18%7%1
2100
16th to endNight
46%49%19%15%4
2034
April1st to 15thNight
52%48%18%15%
1796
16th to endNight
47%5%7%18%16%3
1720
May1st to 15thNight
49%5%7%18%14%4
1440
16th to endNight
44%7%12%19%13%2
1497
June1st to 15thNight
48%6%13%21%8%1
1440
16th to endNight
43%8%12%19%14%
1440
July1st to 15thNight
33%7%13%30%14%
1360
16th to endNight
33%8%16%26%14%
1418
August1st to 15thNight
37%10%16%22%11%
1509
16th to endNight
32%7%16%31%11%
1854
September1st to 15thNight
42%7%11%27%11%
1800
16th to endNight
38%411%34%9%
1856
October1st to 15thNight
48%6%10%22%11%
1980
16th to endNight
58%7%12%15%5%
2203
November1st to 15thNight
51%6%13%16%10%
2334
16th to endNight
62%6%11%15%4
2520
December1st to 15thNight
65%6%13%11%2
2520
16th to endNight
67%5%9%11%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%21%21%21%5%
60
16th to endDay
40%9%32%11%5%
71
February1st to 15thDay
32%22%31%14%
90
16th to endDay
32%20%23%19%4
181
March1st to 15thDay
23%14%35%21%4
270
16th to endDay
22%16%30%18%10%2
288
April1st to 15thDay
25%23%25%17%7%
270
16th to endDay
23%16%28%17%10%2
270
May1st to 15thDay
27%12%30%17%9%3
288
16th to endDay
18%17%31%19%12%1
331
June1st to 15thDay
24%8%32%23%8%1
315
16th to endDay
19%10%30%25%13%1
315
July1st to 15thDay
14%5%30%30%13%5%
305
16th to endDay
12%329%30%18%5%
326
August1st to 15thDay
13%6%23%33%16%6%
313
16th to endDay
11%5%26%27%18%10%
332
September1st to 15thDay
17%7%23%18%20%12%
315
16th to endDay
17%8%22%22%18%9%
275
October1st to 15thDay
21%12%25%23%14%2
255
16th to endDay
29%16%24%20%7%
255
November1st to 15thDay
17%12%31%24%11%2
177
16th to endDay
33%15%26%23%1
60
December1st to 15thDay
23%18%38%18%1
60
16th to endDay
42%17%35%4
64
January1st to 15thNight
25%17%30%21%4
720
16th to endNight
21%12%35%24%5%
768
February1st to 15thNight
20%14%37%23%4
704
16th to endNight
24%14%32%23%5%
631
March1st to 15thNight
24%14%34%24%2
720
16th to endNight
20%16%27%24%10%
768
April1st to 15thNight
26%13%32%23%4
718
16th to endNight
23%14%28%24%8%
640
May1st to 15thNight
25%8%25%29%9%1
360
16th to endNight
21%6%31%27%12%
375
June1st to 15thNight
21%6%29%31%9%
360
16th to endNight
20%227%35%12%1
360
July1st to 15thNight
10%225%42%17%
340
16th to endNight
9%224%45%16%1
376
August1st to 15thNight
14%319%38%22%1
438
16th to endNight
11%422%37%22%2
708
September1st to 15thNight
17%5%24%31%18%2
720
16th to endNight
18%6%22%26%22%2
681
October1st to 15thNight
25%9%26%26%11%1
660
16th to endNight
28%12%26%24%7%
704
November1st to 15thNight
24%11%31%20%11%
700
16th to endNight
29%16%25%19%8%
720
December1st to 15thNight
29%13%31%21%3
720
16th to endNight
33%12%31%17%3
768
Page generated at 2017-09-22 07:35:34. Version ac04

Copyright 2017 A. Danko


Boreal Skies Observatory Clear Sky Chart | Charts Home