Stellar Skies Dark Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stellar Skies Dark Sky Site clear sky chart from 2005-11-05 to 2017-12-20.

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
53%
15865
29%
48095
20%
15822
62%
15822
40%
26167

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
41%
895
29%
2327
13%
895
50%
895
39%
1384
January16th to endNight
41%
864
34%
2400
15%
864
53%
864
40%
1359
February1st to 15thNight
36%
704
29%
2106
14%
698
50%
698
36%
1163
February16th to endNight
30%
632
31%
1894
10%
632
46%
632
37%
1031
March1st to 15thNight
37%
597
27%
2037
14%
597
51%
597
29%
1144
March16th to endNight
36%
576
28%
2112
17%
576
51%
576
33%
1074
April1st to 15thNight
30%
539
26%
1959
13%
539
49%
539
32%
1039
April16th to endNight
32%
540
29%
1731
13%
540
53%
540
36%
954
May1st to 15thNight
35%
539
25%
1620
16%
539
50%
539
36%
938
May16th to endNight
47%
562
25%
1685
18%
561
59%
561
36%
980
June1st to 15thNight
70%
540
23%
1620
23%
540
75%
540
51%
791
June16th to endNight
73%
540
25%
1620
29%
540
76%
540
44%
869
July1st to 15thNight
79%
510
23%
1530
26%
510
81%
510
47%
810
July16th to endNight
85%
564
30%
1594
36%
531
87%
531
55%
763
August1st to 15thNight
90%
539
31%
1704
38%
536
90%
536
57%
769
August16th to endNight
79%
573
25%
1931
31%
573
79%
573
37%
1035
September1st to 15thNight
77%
540
22%
1980
27%
540
79%
540
34%
1052
September16th to endNight
71%
537
29%
1980
29%
537
75%
537
33%
1060
October1st to 15thNight
72%
540
34%
1980
34%
540
76%
540
43%
938
October16th to endNight
58%
749
37%
2285
24%
749
66%
749
48%
1135
November1st to 15thNight
55%
851
28%
2371
17%
851
61%
851
42%
1335
November16th to endNight
52%
975
35%
2535
17%
975
61%
975
45%
1427
December1st to 15thNight
48%
975
30%
2535
16%
975
54%
975
37%
1551
December16th to endNight
40%
984
28%
2559
13%
984
49%
984
36%
1566

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
8%42442445%6%51%
48734
Night
8%423323347%57%
48296
Cloud CoverDay
8%42442445%6%51%
48732
Night
7%313323347%57%
48296
TransparencyDay
31%5%9%20%21%10%
48432
Night
27%411%21%22%13%
48101
SeeingDay
10%420%31%26%7%
5717
Night
10%5%27%38%17%1
15872

By Month

Percentage Cloud Cover SW (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
12%6%2431323454%
1611
16th to endDay
13%312223345%57%
1833
February1st to 15thDay
10%6%24213236%56%
1750
16th to endDay
11%423323345%55%
1580
March1st to 15thDay
13%6%24313335%51%
1923
16th to endDay
11%5%34323345%53%
2112
April1st to 15thDay
8%5%25%423447%52%
1986
16th to endDay
10%5%35%42344451%
2226
May1st to 15thDay
11%6%36%425%5%5%7%42%
2331
16th to endDay
10%6%25%5%25%46%7%41%
2430
June1st to 15thDay
421332446%8%57%
2340
16th to endDay
7%423435%5%7%9%46%
2340
July1st to 15thDay
6%3245%36%5%6%9%45%
2340
16th to endDay
43135%36%5%7%9%49%
2466
August1st to 15thDay
12134345%6%9%57%
2242
16th to endDay
7%426%6%46%5%6%9%40%
2270
September1st to 15thDay
9%415%6%35%45%5%46%
1980
16th to endDay
9%4245%245%5%6%48%
1980
October1st to 15thDay
6%424323346%58%
1980
16th to endDay
6%31221233368%
1926
November1st to 15thDay
10%42432325%7%52%
1814
16th to endDay
8%5%13311235%63%
1755
December1st to 15thDay
12%7%24323335%51%
1755
16th to endDay
11%7%34323447%48%
1764
January1st to 15thNight
11%424313235%56%
2327
16th to endNight
12%423213245%57%
2400
February1st to 15thNight
10%5%24312235%59%
2102
16th to endNight
10%423222346%57%
1894
March1st to 15thNight
11%5%13324346%51%
2037
16th to endNight
8%333313447%56%
2112
April1st to 15thNight
6%424324446%54%
1959
16th to endNight
7%42432435%7%55%
1731
May1st to 15thNight
9%32442345%7%52%
1619
16th to endNight
7%41432446%10%50%
1683
June1st to 15thNight
1113323349%64%
1620
16th to endNight
421332435%9%58%
1620
July1st to 15thNight
321332335%11%57%
1620
16th to endNight
3122233414%59%
1702
August1st to 15thNight
2122245%8%67%
1714
16th to endNight
5%41442337%10%50%
1931
September1st to 15thNight
7%3134345%6%9%51%
1980
16th to endNight
8%4244145%5%7%51%
1980
October1st to 15thNight
6%312323347%62%
1980
16th to endNight
6%21221333467%
2285
November1st to 15thNight
7%414323345%59%
2371
16th to endNight
8%413212335%61%
2535
December1st to 15thNight
12%5%34223345%53%
2535
16th to endNight
11%5%24423346%51%
2559

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
12%5%2441223454%
1611
16th to endDay
13%312223345%57%
1833
February1st to 15thDay
9%6%24213336%56%
1750
16th to endDay
11%423323335%55%
1580
March1st to 15thDay
13%5%24313435%50%
1923
16th to endDay
12%434313345%54%
2110
April1st to 15thDay
8%5%24423447%52%
1986
16th to endDay
10%5%35%32344451%
2226
May1st to 15thDay
11%6%26%425%5%5%7%42%
2331
16th to endDay
10%6%25%5%25%46%7%42%
2430
June1st to 15thDay
421332446%7%57%
2340
16th to endDay
7%4235%25%5%6%9%47%
2340
July1st to 15thDay
5%425%436%6%5%9%45%
2340
16th to endDay
43135%36%5%7%9%49%
2466
August1st to 15thDay
121333446%8%58%
2242
16th to endDay
7%426%6%46%5%6%9%41%
2270
September1st to 15thDay
9%415%5%45%5%46%47%
1980
16th to endDay
9%5%14425%5%5%7%47%
1980
October1st to 15thDay
6%424323336%58%
1980
16th to endDay
6%32221222368%
1926
November1st to 15thDay
10%42432335%7%52%
1814
16th to endDay
8%413312235%63%
1755
December1st to 15thDay
12%7%14333335%51%
1755
16th to endDay
11%7%24323446%48%
1764
January1st to 15thNight
11%424213335%57%
2327
16th to endNight
12%423213245%58%
2400
February1st to 15thNight
9%5%24212235%59%
2102
16th to endNight
10%423222336%57%
1894
March1st to 15thNight
11%5%13324446%51%
2037
16th to endNight
8%323413447%57%
2112
April1st to 15thNight
6%42432445%6%54%
1959
16th to endNight
7%42332436%7%55%
1731
May1st to 15thNight
9%21442345%8%52%
1619
16th to endNight
7%32343446%10%50%
1683
June1st to 15thNight
111332335%9%64%
1620
16th to endNight
421332445%9%58%
1620
July1st to 15thNight
321332335%12%57%
1620
16th to endNight
3121233414%59%
1702
August1st to 15thNight
2222345%8%67%
1714
16th to endNight
5%41442437%10%50%
1931
September1st to 15thNight
7%31333445%9%52%
1980
16th to endNight
8%42442345%8%51%
1980
October1st to 15thNight
6%212323337%62%
1980
16th to endNight
6%21221333467%
2285
November1st to 15thNight
7%314323335%60%
2371
16th to endNight
8%413213336%61%
2535
December1st to 15thNight
12%5%24323345%53%
2535
16th to endNight
10%5%25%423345%52%
2559

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
34%3417%26%12%
1612
16th to endDay
29%37%14%27%17%
1833
February1st to 15thDay
29%26%17%21%22%
1751
16th to endDay
31%2418%25%16%
1580
March1st to 15thDay
35%5%7%13%21%16%
1923
16th to endDay
32%48%19%22%12%
2112
April1st to 15thDay
32%49%16%19%16%
1986
16th to endDay
35%46%19%21%12%
2226
May1st to 15thDay
40%6%9%14%18%10%
2331
16th to endDay
40%6%10%18%19%4
2434
June1st to 15thDay
22%6%14%23%24%7%
2340
16th to endDay
32%7%13%22%17%5%
2340
July1st to 15thDay
31%7%15%26%17%2
2201
16th to endDay
25%9%18%28%17%1
2311
August1st to 15thDay
20%7%15%34%20%2
2228
16th to endDay
36%8%13%25%12%3
2270
September1st to 15thDay
36%7%11%19%19%6%
1980
16th to endDay
34%5%10%20%20%8%
1980
October1st to 15thDay
28%410%21%21%13%
1980
16th to endDay
21%3415%31%23%
1926
November1st to 15thDay
32%38%23%20%11%
1814
16th to endDay
25%35%18%25%22%
1755
December1st to 15thDay
36%36%17%22%13%
1755
16th to endDay
36%27%17%24%11%
1764
January1st to 15thNight
31%25%18%23%19%
2327
16th to endNight
29%25%16%26%19%
2400
February1st to 15thNight
29%36%13%23%23%
2112
16th to endNight
29%27%15%23%20%
1894
March1st to 15thNight
33%49%12%23%16%
2037
16th to endNight
27%410%19%23%12%
2112
April1st to 15thNight
30%49%18%18%19%
1959
16th to endNight
29%5%10%19%21%13%
1731
May1st to 15thNight
30%5%11%22%19%10%
1620
16th to endNight
29%7%13%27%17%4
1685
June1st to 15thNight
18%5%14%30%25%5%
1620
16th to endNight
23%6%14%27%22%5%
1620
July1st to 15thNight
20%7%19%32%17%2
1530
16th to endNight
15%5%24%33%18%1
1594
August1st to 15thNight
14%5%19%34%24%2
1704
16th to endNight
26%6%20%28%13%4
1931
September1st to 15thNight
27%6%15%22%20%7%
1980
16th to endNight
29%6%12%21%20%10%
1980
October1st to 15thNight
22%412%26%19%14%
1980
16th to endNight
21%35%15%29%24%
2285
November1st to 15thNight
27%48%22%22%14%
2371
16th to endNight
25%37%15%25%22%
2535
December1st to 15thNight
33%38%16%23%14%
2535
16th to endNight
33%38%14%25%14%
2559

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%6%28%34%12%1
90
16th to endDay
12%10%42%23%8%2
96
February1st to 15thDay
11%12%28%34%12%1
90
16th to endDay
9%6%45%28%9%
81
March1st to 15thDay
21%10%38%17%12%
90
16th to endDay
36%41%34%14%
96
April1st to 15thDay
12%17%33%30%5%
105
16th to endDay
12%13%43%23%6%
336
May1st to 15thDay
16%9%37%27%8%
468
16th to endDay
17%5%28%34%13%1
517
June1st to 15thDay
4215%45%28%4
495
16th to endDay
7%112%36%33%8%
495
July1st to 15thDay
417%42%36%8%
475
16th to endDay
45%26%48%14%
515
August1st to 15thDay
31428%44%17%
396
16th to endDay
11%10%27%35%15%
310
September1st to 15thDay
5%219%45%25%
135
16th to endDay
16%221%41%17%
135
October1st to 15thDay
28%30%48%10%
135
16th to endDay
13%5%18%29%29%4
144
November1st to 15thDay
25%224%31%11%4
135
16th to endDay
15%13%23%26%14%6%
135
December1st to 15thDay
18%329%33%11%2
135
16th to endDay
19%13%36%25%3
108
January1st to 15thNight
14%8%30%36%9%
900
16th to endNight
14%7%33%37%7%
864
February1st to 15thNight
12%8%38%33%7%
704
16th to endNight
14%9%44%30%2
632
March1st to 15thNight
14%7%37%35%4
597
16th to endNight
9%9%42%34%4
576
April1st to 15thNight
8%8%50%29%3
539
16th to endNight
8%7%51%25%7%
540
May1st to 15thNight
10%7%42%32%6%
540
16th to endNight
8%336%34%15%
563
June1st to 15thNight
221%55%18%
540
16th to endNight
3115%53%24%1
540
July1st to 15thNight
418%49%34%2
510
16th to endNight
5%445%41%2
564
August1st to 15thNight
2238%51%4
539
16th to endNight
8%440%42%4
573
September1st to 15thNight
8%6%40%39%3
540
16th to endNight
11%8%40%35%3
537
October1st to 15thNight
8%115%44%28%2
540
16th to endNight
7%327%36%22%2
749
November1st to 15thNight
9%5%25%39%19%1
851
16th to endNight
10%7%26%38%16%1
975
December1st to 15thNight
16%6%26%41%9%
975
16th to endNight
14%8%32%35%8%
984
Page generated at 2017-12-21 06:04:53. Version ac04

Copyright 2017 A. Danko


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