Dark Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dark Sky Observatory clear sky chart from 2005-11-05 to 2017-09-24.

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
37%
16054
18%
46970
10%
16007
44%
16007
26%
30444

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
27%
900
14%
2475
5%
895
34%
895
22%
1723
January16th to endNight
23%
960
16%
2496
5%
960
31%
960
19%
1831
February1st to 15thNight
23%
880
12%
2288
3%
876
29%
876
20%
1676
February16th to endNight
28%
735
19%
1997
6%
735
38%
735
25%
1342
March1st to 15thNight
32%
552
19%
1992
12%
552
42%
552
23%
1200
March16th to endNight
26%
576
12%
2112
7%
576
35%
576
16%
1456
April1st to 15thNight
28%
539
16%
1797
11%
539
38%
539
22%
1183
April16th to endNight
29%
540
18%
1707
11%
540
42%
540
22%
1146
May1st to 15thNight
39%
540
17%
1562
12%
540
45%
540
26%
1040
May16th to endNight
43%
563
17%
1497
7%
561
49%
561
28%
1008
June1st to 15thNight
51%
540
14%
1440
6%
540
54%
540
32%
935
June16th to endNight
55%
540
19%
1440
9%
540
59%
540
42%
851
July1st to 15thNight
63%
510
17%
1360
7%
510
66%
510
45%
777
July16th to endNight
73%
564
24%
1518
19%
531
75%
531
48%
810
August1st to 15thNight
66%
539
21%
1610
21%
536
69%
536
36%
933
August16th to endNight
59%
573
21%
1815
21%
573
64%
573
34%
1038
September1st to 15thNight
58%
540
26%
1800
23%
540
65%
540
37%
956
September16th to endNight
48%
555
25%
1920
18%
555
59%
555
34%
1055
October1st to 15thNight
37%
660
25%
1980
14%
660
45%
660
31%
1186
October16th to endNight
34%
704
20%
2112
10%
704
42%
704
26%
1326
November1st to 15thNight
34%
784
25%
2324
12%
784
45%
784
30%
1388
November16th to endNight
28%
900
16%
2520
7%
900
35%
900
24%
1675
December1st to 15thNight
21%
900
11%
2520
3%
900
28%
900
17%
1857
December16th to endNight
21%
960
14%
2688
5%
960
30%
960
15%
2052

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
13%7%36%5%3445%7%37%
47956
Night
12%7%35%424447%43%
47149
Cloud CoverDay
13%7%36%5%3445%7%37%
47965
Night
12%7%35%424447%43%
47153
TransparencyDay
44%6%14%22%11%1
47652
Night
40%5%13%23%14%1
46971
SeeingDay
15%7%29%29%15%3
6069
Night
15%8%32%30%11%1
16054

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
7%10%6%7%8%46%45%8%29%
1462
16th to endDay
12%10%7%10%6%36%5%5%8%24%
1728
February1st to 15thDay
11%11%5%9%6%34447%30%
1575
16th to endDay
12%9%47%5%345%6%5%34%
1476
March1st to 15thDay
12%9%47%6%3446%7%32%
1966
16th to endDay
18%11%5%6%5%3435%6%28%
2112
April1st to 15thDay
18%8%35%5%24346%34%
2151
16th to endDay
23%8%37%5%24445%29%
2251
May1st to 15thDay
19%8%35%5%3445%7%32%
2388
16th to endDay
17%6%25%5%3445%6%36%
2617
June1st to 15thDay
14%5%25%5%2346%9%38%
2520
16th to endDay
8%425%5%35%5%6%9%43%
2520
July1st to 15thDay
6%424425%5%5%9%47%
2520
16th to endDay
6%3145%35%5%6%9%46%
2543
August1st to 15thDay
8%425%6%35%5%6%8%43%
2340
16th to endDay
11%425%5%245%5%8%43%
2387
September1st to 15thDay
10%5%23324447%51%
2160
16th to endDay
11%5%25%43435%6%46%
1886
October1st to 15thDay
15%6%246%24245%46%
1650
16th to endDay
16%8%25%42235%7%40%
1760
November1st to 15thDay
10%6%4441446%6%47%
1528
16th to endDay
15%9%35%423448%38%
1440
December1st to 15thDay
16%12%48%6%36%447%27%
1440
16th to endDay
15%15%5%9%7%35%437%22%
1536
January1st to 15thNight
9%7%47%5%36%5%6%7%36%
2475
16th to endNight
11%12%5%7%6%3435%6%33%
2496
February1st to 15thNight
11%11%5%10%5%24447%32%
2285
16th to endNight
11%8%36%6%24346%42%
1997
March1st to 15thNight
14%6%35%423337%45%
1980
16th to endNight
16%10%5%6%5%24346%33%
2112
April1st to 15thNight
15%7%35%5%34446%41%
1797
16th to endNight
20%8%245%23346%39%
1707
May1st to 15thNight
15%424425%46%7%41%
1562
16th to endNight
12%5%246%245%5%8%42%
1496
June1st to 15thNight
10%325%4245%7%10%43%
1440
16th to endNight
6%22442445%11%51%
1440
July1st to 15thNight
42134345%5%9%56%
1440
16th to endNight
321332446%10%57%
1623
August1st to 15thNight
7%2244245%5%7%53%
1620
16th to endNight
10%5%33423236%54%
1815
September1st to 15thNight
10%413332236%57%
1800
16th to endNight
9%41332435%6%54%
1920
October1st to 15thNight
12%5%25%423245%50%
1980
16th to endNight
14%6%35%423337%45%
2112
November1st to 15thNight
11%5%24423346%50%
2324
16th to endNight
14%8%46%323347%42%
2520
December1st to 15thNight
14%11%47%5%35%45%5%30%
2520
16th to endNight
18%11%5%7%6%34445%29%
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
7%10%6%6%8%46%45%8%29%
1462
16th to endDay
12%9%7%9%7%35%5%5%7%24%
1728
February1st to 15thDay
11%11%5%9%6%24447%30%
1575
16th to endDay
12%9%47%5%345%6%5%34%
1476
March1st to 15thDay
13%9%47%6%2445%7%32%
1977
16th to endDay
18%12%46%5%3345%7%28%
2110
April1st to 15thDay
19%8%35%5%24346%34%
2151
16th to endDay
23%8%37%5%24445%29%
2251
May1st to 15thDay
19%7%36%5%3445%7%31%
2388
16th to endDay
17%7%25%5%3445%6%36%
2617
June1st to 15thDay
14%5%25%5%2445%9%38%
2520
16th to endDay
8%425%435%5%6%9%42%
2520
July1st to 15thDay
6%5%24425%5%6%9%47%
2520
16th to endDay
6%3245%35%5%7%9%46%
2543
August1st to 15thDay
8%425%6%35%5%6%8%43%
2340
16th to endDay
12%435%5%2445%8%43%
2387
September1st to 15thDay
10%5%23324447%51%
2160
16th to endDay
12%5%15%43335%6%47%
1886
October1st to 15thDay
15%5%246%24245%46%
1650
16th to endDay
16%7%25%42335%6%40%
1760
November1st to 15thDay
10%6%3441436%6%47%
1528
16th to endDay
15%9%35%424448%38%
1440
December1st to 15thDay
16%12%48%5%36%346%27%
1440
16th to endDay
16%14%5%10%7%34437%22%
1536
January1st to 15thNight
10%7%47%6%35%5%6%7%35%
2475
16th to endNight
11%12%5%7%5%2435%7%33%
2496
February1st to 15thNight
11%11%5%9%5%34446%32%
2278
16th to endNight
11%8%36%6%24346%42%
1997
March1st to 15thNight
14%6%25%423347%44%
1992
16th to endNight
16%10%5%6%5%24347%33%
2112
April1st to 15thNight
15%7%25%5%24446%41%
1797
16th to endNight
20%8%245%13337%39%
1707
May1st to 15thNight
15%4244245%5%7%42%
1561
16th to endNight
12%5%246%245%5%8%42%
1496
June1st to 15thNight
9%324425%5%6%11%43%
1440
16th to endNight
6%214435%36%10%51%
1440
July1st to 15thNight
42134335%5%9%56%
1440
16th to endNight
311432346%9%57%
1623
August1st to 15thNight
7%224415%45%8%52%
1620
16th to endNight
10%424423236%54%
1815
September1st to 15thNight
10%413422336%57%
1800
16th to endNight
9%5%1332345%6%54%
1920
October1st to 15thNight
12%5%25%324245%50%
1980
16th to endNight
14%6%35%323337%45%
2112
November1st to 15thNight
11%5%25%423346%50%
2324
16th to endNight
14%8%35%324347%41%
2520
December1st to 15thNight
14%11%5%7%5%4445%6%30%
2520
16th to endNight
18%11%5%7%6%34345%29%
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
51%5%14%16%10%
1463
16th to endDay
56%7%15%15%5%
1728
February1st to 15thDay
52%5%14%20%6%
1576
16th to endDay
47%7%15%17%9%1
1476
March1st to 15thDay
48%7%16%18%8%
1977
16th to endDay
56%5%13%18%6%
2112
April1st to 15thDay
50%5%12%22%8%
2151
16th to endDay
56%410%18%9%
2251
May1st to 15thDay
50%6%13%21%7%
2388
16th to endDay
47%6%14%18%11%2
2621
June1st to 15thDay
41%7%15%21%13%
2520
16th to endDay
36%8%13%25%14%1
2520
July1st to 15thDay
33%7%16%26%13%3
2371
16th to endDay
30%9%17%26%14%2
2385
August1st to 15thDay
37%8%15%25%12%1
2326
16th to endDay
37%7%12%25%14%2
2387
September1st to 15thDay
32%6%13%28%17%1
2160
16th to endDay
36%413%29%14%
1886
October1st to 15thDay
40%411%23%19%1
1650
16th to endDay
43%5%13%25%12%
1760
November1st to 15thDay
36%5%13%25%17%1
1528
16th to endDay
45%5%18%22%7%
1440
December1st to 15thDay
57%5%15%14%6%1
1440
16th to endDay
62%5%11%12%7%
1536
January1st to 15thNight
45%6%16%18%12%
2475
16th to endNight
50%6%14%19%9%
2496
February1st to 15thNight
50%5%14%20%9%
2288
16th to endNight
43%414%23%12%2
1997
March1st to 15thNight
40%414%27%11%1
1992
16th to endNight
51%5%14%19%7%
2112
April1st to 15thNight
43%5%11%24%12%1
1797
16th to endNight
46%412%20%12%2
1707
May1st to 15thNight
40%8%13%24%13%
1562
16th to endNight
38%8%12%23%14%2
1498
June1st to 15thNight
33%9%15%24%13%2
1440
16th to endNight
28%5%14%27%19%4
1440
July1st to 15thNight
24%7%16%31%16%4
1360
16th to endNight
20%7%17%32%20%2
1518
August1st to 15thNight
28%7%12%31%15%2
1610
16th to endNight
33%413%26%18%4
1815
September1st to 15thNight
29%310%29%24%2
1800
16th to endNight
30%5%12%29%20%1
1920
October1st to 15thNight
36%411%23%21%3
1980
16th to endNight
39%5%14%23%15%1
2112
November1st to 15thNight
34%5%12%26%19%2
2324
16th to endNight
43%415%25%9%1
2520
December1st to 15thNight
53%5%14%18%8%
2520
16th to endNight
58%411%15%8%
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
21%11%36%25%32
90
16th to endDay
18%9%42%21%43
96
February1st to 15thDay
5%24%43%20%6%
90
16th to endDay
22%8%39%14%12%2
81
March1st to 15thDay
18%15%42%20%3
90
16th to endDay
33%13%27%23%2
96
April1st to 15thDay
20%10%34%24%10%
270
16th to endDay
31%9%30%18%8%2
361
May1st to 15thDay
22%10%33%21%9%1
468
16th to endDay
21%7%29%27%12%1
517
June1st to 15thDay
12%5%29%33%16%2
495
16th to endDay
9%6%35%31%14%3
495
July1st to 15thDay
7%226%34%19%9%
475
16th to endDay
7%124%37%23%5%
515
August1st to 15thDay
9%218%37%26%4
493
16th to endDay
14%5%18%31%22%7%
427
September1st to 15thDay
8%324%35%20%8%
315
16th to endDay
13%9%30%30%13%3
143
October1st to 15thDay
6%14%27%34%13%3
90
16th to endDay
19%8%37%22%11%
96
November1st to 15thDay
5%440%33%11%5%
90
16th to endDay
34%15%30%17%2
90
December1st to 15thDay
18%22%31%20%7%
90
16th to endDay
29%15%39%13%2
96
January1st to 15thNight
13%12%39%25%7%
900
16th to endNight
16%11%39%27%5%
960
February1st to 15thNight
17%10%37%25%7%
880
16th to endNight
16%11%38%25%7%1
735
March1st to 15thNight
17%11%33%25%9%1
552
16th to endNight
19%11%35%23%8%1
576
April1st to 15thNight
19%11%35%27%5%
539
16th to endNight
23%9%33%24%8%
540
May1st to 15thNight
17%8%31%34%7%
540
16th to endNight
14%6%30%36%11%
563
June1st to 15thNight
11%5%27%42%13%
540
16th to endNight
7%229%47%13%
540
July1st to 15thNight
5%23%41%26%2
510
16th to endNight
4114%52%25%2
564
August1st to 15thNight
7%18%44%26%2
539
16th to endNight
10%321%35%24%4
573
September1st to 15thNight
12%321%35%24%3
540
16th to endNight
12%430%30%20%1
555
October1st to 15thNight
16%6%35%27%12%1
660
16th to endNight
18%8%34%29%8%
704
November1st to 15thNight
14%8%38%29%7%
784
16th to endNight
17%14%37%22%7%
900
December1st to 15thNight
20%12%38%23%5%
900
16th to endNight
24%8%35%24%6%
960
Page generated at 2017-09-25 08:02:21. Version ac04

Copyright 2017 A. Danko


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