Black Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black 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
28%
14804
12%
46616
7%
14770
33%
14770
14%
34414

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
9%
761
4%
2540
1%
759
13%
759
4%
2187
January16th to endNight
18%
768
8%
2496
4%
768
22%
768
7%
2073
February1st to 15thNight
13%
701
6%
2287
1%
701
17%
701
6%
1894
February16th to endNight
18%
631
10%
2027
4%
631
25%
631
10%
1575
March1st to 15thNight
18%
720
12%
1983
4%
720
25%
720
14%
1476
March16th to endNight
29%
768
19%
2112
11%
768
36%
768
22%
1458
April1st to 15thNight
22%
717
14%
1819
5%
717
26%
717
19%
1354
April16th to endNight
30%
540
18%
1620
10%
540
37%
540
21%
1133
May1st to 15thNight
39%
474
15%
1554
12%
474
45%
474
21%
1024
May16th to endNight
38%
374
14%
1497
11%
374
49%
374
21%
875
June1st to 15thNight
39%
360
10%
1386
11%
360
45%
360
19%
859
June16th to endNight
44%
360
12%
1260
11%
360
51%
360
23%
796
July1st to 15thNight
55%
340
16%
1219
19%
340
61%
340
33%
648
July16th to endNight
49%
464
15%
1499
14%
435
53%
435
26%
890
August1st to 15thNight
56%
539
16%
1609
12%
536
59%
536
30%
980
August16th to endNight
52%
609
18%
1755
11%
609
56%
609
30%
1114
September1st to 15thNight
45%
720
15%
1815
7%
720
48%
720
28%
1244
September16th to endNight
46%
681
19%
1900
13%
681
50%
681
26%
1262
October1st to 15thNight
29%
660
14%
1980
10%
660
36%
660
16%
1449
October16th to endNight
16%
704
9%
2112
4%
704
23%
704
8%
1711
November1st to 15thNight
25%
700
14%
2413
12%
700
30%
700
13%
1774
November16th to endNight
15%
720
7%
2520
4%
720
21%
720
7%
2077
December1st to 15thNight
8%
720
3%
2520
1%
720
11%
720
3%
2208
December16th to endNight
7%
773
6%
2693
1%
773
12%
773
4%
2353

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
16%9%46%5%3445%6%32%
48226
Night
16%11%47%6%34446%29%
46786
Cloud CoverDay
16%9%46%5%34446%32%
48236
Night
16%11%47%6%34446%29%
46791
TransparencyDay
51%6%12%19%9%
47914
Night
55%6%11%16%9%
46616
SeeingDay
21%13%29%21%9%4
5688
Night
23%13%28%23%9%1
14810

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
20%14%6%11%8%44335%17%
1384
16th to endDay
13%17%8%11%9%46%43418%
1728
February1st to 15thDay
12%15%7%11%10%45%44418%
1575
16th to endDay
14%16%6%10%7%3444422%
1443
March1st to 15thDay
15%10%46%6%44336%31%
1966
16th to endDay
13%8%45%6%24446%39%
2112
April1st to 15thDay
20%10%35%5%35%335%33%
2140
16th to endDay
19%8%24324446%39%
2340
May1st to 15thDay
17%7%35%43445%6%37%
2397
16th to endDay
13%5%2443446%7%42%
2618
June1st to 15thDay
18%6%2432445%8%37%
2571
16th to endDay
14%5%25%4345%5%9%38%
2700
July1st to 15thDay
10%425%33446%8%45%
2667
16th to endDay
10%426%5%35%5%6%8%40%
2565
August1st to 15thDay
12%5%25%42445%8%43%
2340
16th to endDay
11%5%25%5%35%45%8%43%
2435
September1st to 15thDay
17%6%25%5%2345%7%38%
2143
16th to endDay
16%5%25%434446%40%
1853
October1st to 15thDay
21%8%37%5%34346%30%
1650
16th to endDay
30%11%47%6%33445%18%
1760
November1st to 15thDay
20%12%47%5%3445%6%25%
1444
16th to endDay
21%17%6%8%7%45%445%14%
1440
December1st to 15thDay
19%20%6%9%8%35%445%11%
1440
16th to endDay
19%17%8%9%8%45%44412%
1515
January1st to 15thNight
19%18%7%10%8%3444413%
2540
16th to endNight
14%16%6%10%10%4446%6%15%
2496
February1st to 15thNight
11%19%8%11%8%46%44316%
2285
16th to endNight
17%12%6%10%7%34345%24%
2027
March1st to 15thNight
17%10%5%7%6%34335%31%
1971
16th to endNight
13%10%37%5%2334439%
2112
April1st to 15thNight
22%10%36%423345%33%
1819
16th to endNight
16%9%35%5%3435%6%36%
1620
May1st to 15thNight
17%7%345%3435%6%38%
1554
16th to endNight
12%4245%3446%7%43%
1495
June1st to 15thNight
15%5%25%5%25%46%8%37%
1386
16th to endNight
12%5%25%4245%6%9%39%
1260
July1st to 15thNight
7%32443345%7%53%
1291
16th to endNight
9%5%15%4345%6%8%44%
1602
August1st to 15thNight
8%424435%46%8%47%
1620
16th to endNight
11%5%25%5%23446%46%
1755
September1st to 15thNight
15%5%25%5%24447%41%
1815
16th to endNight
14%7%25%5%24346%42%
1900
October1st to 15thNight
21%9%35%5%34435%32%
1980
16th to endNight
28%12%47%5%33435%20%
2112
November1st to 15thNight
18%14%46%5%34445%28%
2413
16th to endNight
22%15%6%8%6%34435%17%
2520
December1st to 15thNight
19%22%7%11%7%33335%11%
2520
16th to endNight
21%18%7%12%7%3433411%
2693

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
20%15%6%9%8%4444417%
1384
16th to endDay
13%17%8%11%8%35%44417%
1728
February1st to 15thDay
12%16%7%12%9%46%34418%
1575
16th to endDay
15%14%7%10%7%3443321%
1443
March1st to 15thDay
15%11%37%7%44445%30%
1977
16th to endDay
13%9%36%5%25%445%38%
2111
April1st to 15thDay
21%10%46%5%34345%32%
2140
16th to endDay
19%9%24423446%38%
2340
May1st to 15thDay
17%7%35%6%25%45%5%37%
2397
16th to endDay
14%5%245%245%5%7%41%
2618
June1st to 15thDay
19%5%1442445%8%36%
2571
16th to endDay
15%5%25%4345%5%9%37%
2700
July1st to 15thDay
11%5%244345%6%8%43%
2667
16th to endDay
11%436%5%25%5%5%8%40%
2565
August1st to 15thDay
12%5%25%43445%8%42%
2340
16th to endDay
12%5%245%3445%8%41%
2435
September1st to 15thDay
18%6%26%424447%38%
2143
16th to endDay
16%5%25%5%35%347%40%
1853
October1st to 15thDay
22%8%37%5%3345%6%30%
1650
16th to endDay
30%11%47%6%34445%18%
1760
November1st to 15thDay
20%13%47%6%33447%25%
1444
16th to endDay
20%18%5%9%7%45%445%14%
1440
December1st to 15thDay
19%19%7%10%7%34446%11%
1440
16th to endDay
19%18%8%10%7%45%44411%
1515
January1st to 15thNight
19%17%7%10%9%35%44413%
2540
16th to endNight
14%16%6%11%10%45%45%5%15%
2496
February1st to 15thNight
11%18%8%11%9%46%43316%
2278
16th to endNight
16%12%5%10%7%35%345%24%
2027
March1st to 15thNight
17%10%5%7%7%33335%31%
1983
16th to endNight
14%10%47%5%3344439%
2112
April1st to 15thNight
22%10%45%5%1334433%
1819
16th to endNight
16%10%26%434347%36%
1620
May1st to 15thNight
17%8%245%3435%6%37%
1554
16th to endNight
12%425%5%35%46%7%43%
1495
June1st to 15thNight
16%5%26%5%2446%8%36%
1386
16th to endNight
12%5%346%245%6%9%39%
1260
July1st to 15thNight
7%32442445%7%53%
1291
16th to endNight
10%424435%5%6%8%44%
1602
August1st to 15thNight
9%325%435%5%6%8%47%
1620
16th to endNight
10%5%15%5%24446%47%
1755
September1st to 15thNight
14%5%25%43445%7%41%
1815
16th to endNight
14%6%26%433447%43%
1900
October1st to 15thNight
20%8%36%5%34446%32%
1980
16th to endNight
27%12%5%7%5%34336%20%
2112
November1st to 15thNight
18%12%46%5%34446%28%
2413
16th to endNight
21%14%7%8%6%35%44418%
2520
December1st to 15thNight
19%20%8%10%8%25%345%11%
2520
16th to endNight
21%18%8%12%7%4434311%
2693

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
69%7%11%8%3
1385
16th to endDay
71%38%10%5%
1728
February1st to 15thDay
69%6%8%11%2
1577
16th to endDay
66%5%9%12%6%
1443
March1st to 15thDay
55%6%14%16%6%
1977
16th to endDay
46%6%10%18%16%2
2112
April1st to 15thDay
55%49%19%10%
2140
16th to endDay
46%5%9%22%13%1
2340
May1st to 15thDay
47%6%10%21%12%1
2397
16th to endDay
39%8%12%23%13%1
2622
June1st to 15thDay
44%7%12%24%10%
2571
16th to endDay
42%8%15%21%11%
2700
July1st to 15thDay
34%8%15%25%15%1
2510
16th to endDay
41%8%11%26%10%
2405
August1st to 15thDay
38%8%17%21%13%
2327
16th to endDay
39%8%15%26%9%
2435
September1st to 15thDay
44%6%14%24%9%
2143
16th to endDay
44%7%13%27%7%
1853
October1st to 15thDay
54%5%11%17%9%
1650
16th to endDay
68%6%12%8%4
1760
November1st to 15thDay
59%6%12%14%6%
1444
16th to endDay
72%6%10%8%1
1440
December1st to 15thDay
73%8%10%5%1
1440
16th to endDay
75%5%8%7%1
1515
January1st to 15thNight
73%7%9%6%3
2540
16th to endNight
68%6%9%9%4
2496
February1st to 15thNight
70%5%9%11%2
2287
16th to endNight
62%5%11%14%5%
2027
March1st to 15thNight
55%5%13%17%7%
1983
16th to endNight
48%39%20%16%1
2112
April1st to 15thNight
53%48%19%12%1
1819
16th to endNight
48%5%8%17%18%2
1620
May1st to 15thNight
46%411%17%16%2
1554
16th to endNight
38%6%14%23%15%1
1497
June1st to 15thNight
44%412%24%13%
1386
16th to endNight
40%7%13%23%14%
1260
July1st to 15thNight
28%6%14%27%21%1
1219
16th to endNight
36%6%12%28%14%
1499
August1st to 15thNight
33%7%17%26%14%
1609
16th to endNight
36%6%14%25%14%
1755
September1st to 15thNight
41%6%15%23%11%
1815
16th to endNight
42%5%13%27%10%
1900
October1st to 15thNight
52%7%10%18%10%
1980
16th to endNight
65%6%10%12%4
2112
November1st to 15thNight
56%6%13%16%6%1
2413
16th to endNight
67%7%11%10%1
2520
December1st to 15thNight
76%6%10%42
2520
16th to endNight
77%47%8%1
2693

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
16%31%37%13%1
61
16th to endDay
38%13%34%11%2
96
February1st to 15thDay
26%36%35%1
90
16th to endDay
33%23%38%4
81
March1st to 15thDay
22%20%35%17%4
267
16th to endDay
22%18%29%18%10%1
288
April1st to 15thDay
27%16%33%12%8%1
270
16th to endDay
24%17%30%13%10%4
270
May1st to 15thDay
27%9%28%20%8%5%
288
16th to endDay
16%13%34%23%8%3
331
June1st to 15thDay
25%10%26%27%8%1
366
16th to endDay
17%9%37%24%9%1
495
July1st to 15thDay
13%9%29%29%12%5%
444
16th to endDay
11%8%32%27%13%5%
326
August1st to 15thDay
15%424%30%12%11%
313
16th to endDay
16%6%25%28%13%9%
332
September1st to 15thDay
15%7%24%25%17%10%
315
16th to endDay
18%10%20%25%15%9%
275
October1st to 15thDay
24%15%27%18%10%2
255
16th to endDay
36%17%27%13%4
272
November1st to 15thDay
21%18%28%26%4
69
16th to endDay
33%18%35%13%
60
December1st to 15thDay
25%35%25%8%6%
60
16th to endDay
29%48%20%1
64
January1st to 15thNight
31%23%27%12%3
763
16th to endNight
25%15%31%21%5%
768
February1st to 15thNight
22%21%33%18%3
704
16th to endNight
24%17%31%21%5%
631
March1st to 15thNight
25%20%30%19%3
720
16th to endNight
21%13%30%25%8%
768
April1st to 15thNight
29%14%30%21%4
717
16th to endNight
22%11%31%22%11%
540
May1st to 15thNight
22%9%23%30%13%1
474
16th to endNight
13%7%33%28%16%
375
June1st to 15thNight
16%7%27%36%11%
360
16th to endNight
14%431%34%14%
360
July1st to 15thNight
7%5%25%41%18%1
340
16th to endNight
11%428%34%20%1
464
August1st to 15thNight
10%5%20%36%23%2
539
16th to endNight
14%423%29%25%2
609
September1st to 15thNight
17%6%25%25%18%5%
720
16th to endNight
18%6%23%31%16%3
681
October1st to 15thNight
26%11%28%22%9%1
660
16th to endNight
34%18%25%16%4
704
November1st to 15thNight
26%17%27%20%8%1
700
16th to endNight
32%16%29%17%4
720
December1st to 15thNight
35%23%27%10%2
720
16th to endNight
32%20%30%12%2
773
Page generated at 2017-09-22 07:35:30. Version ac04

Copyright 2017 A. Danko


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