GuideStar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the GuideStar Observatory clear sky chart from 2005-11-05 to 2017-09-23.

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
64%
15377
31%
46946
25%
15335
71%
15335
44%
24224

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
67%
720
30%
2327
30%
716
71%
716
42%
1171
January16th to endNight
61%
768
34%
2496
31%
768
69%
768
41%
1284
February1st to 15thNight
62%
701
30%
2288
28%
700
69%
700
45%
1125
February16th to endNight
54%
632
38%
1836
28%
632
66%
632
47%
967
March1st to 15thNight
58%
720
36%
1980
25%
720
67%
720
48%
1034
March16th to endNight
60%
768
35%
2112
23%
768
69%
768
52%
1070
April1st to 15thNight
48%
605
32%
1864
17%
605
64%
605
48%
902
April16th to endNight
50%
540
34%
1800
22%
540
68%
540
46%
823
May1st to 15thNight
56%
539
31%
1640
21%
539
66%
539
48%
810
May16th to endNight
58%
562
31%
1684
22%
561
67%
561
53%
787
June1st to 15thNight
66%
540
31%
1620
27%
540
69%
540
59%
729
June16th to endNight
77%
540
28%
1620
27%
540
78%
540
55%
762
July1st to 15thNight
69%
510
15%
1530
13%
510
70%
510
29%
942
July16th to endNight
69%
564
12%
1593
11%
531
69%
531
21%
1099
August1st to 15thNight
73%
539
17%
1611
16%
536
73%
536
28%
1016
August16th to endNight
74%
573
22%
1881
24%
573
74%
573
27%
1168
September1st to 15thNight
70%
596
26%
1856
24%
596
72%
596
31%
1126
September16th to endNight
74%
688
39%
1903
27%
688
81%
688
57%
934
October1st to 15thNight
69%
660
36%
1977
25%
660
77%
660
58%
910
October16th to endNight
74%
704
40%
2112
30%
704
82%
704
60%
959
November1st to 15thNight
73%
700
39%
2100
31%
700
79%
700
58%
958
November16th to endNight
67%
720
38%
2280
33%
720
74%
720
52%
1066
December1st to 15thNight
64%
720
33%
2340
30%
720
71%
720
45%
1162
December16th to endNight
58%
768
30%
2496
26%
768
64%
768
33%
1420

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
3313424446%59%
47959
Night
4413323446%60%
47142
TransparencyDay
24%47%17%27%19%
47654
Night
23%47%18%27%17%
46948
SeeingDay
8%9%22%28%22%10%
5248
Night
6%319%38%28%3
15382

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
4414333346%60%
1611
16th to endDay
5%6%23312247%60%
1728
February1st to 15thDay
27%34322345%60%
1575
16th to endDay
46%25%423346%56%
1635
March1st to 15thDay
35%23424346%59%
1980
16th to endDay
3312224346%63%
2112
April1st to 15thDay
3313315%346%62%
2084
16th to endDay
2313213235%69%
2160
May1st to 15thDay
221332345%6%63%
2307
16th to endDay
3223313335%66%
2429
June1st to 15thDay
112213246%74%
2340
16th to endDay
1112323445%70%
2340
July1st to 15thDay
4325%6%46%6%6%8%45%
2340
16th to endDay
6%436%6%48%7%8%10%31%
2467
August1st to 15thDay
6%5%27%6%37%6%7%7%39%
2340
16th to endDay
6%315%6%36%7%5%7%44%
2321
September1st to 15thDay
5%3246%345%6%8%47%
2101
16th to endDay
322223245%69%
1893
October1st to 15thDay
311231223471%
1652
16th to endDay
222212335%72%
1760
November1st to 15thDay
1212213337%70%
1759
16th to endDay
3313223325%69%
1677
December1st to 15thDay
45%13323445%61%
1620
16th to endDay
6%6%25%32345%5%54%
1728
January1st to 15thNight
45%14323335%61%
2327
16th to endNight
6%5%24323345%59%
2496
February1st to 15thNight
2423323246%62%
2277
16th to endNight
45%23313347%60%
1836
March1st to 15thNight
35%23222246%63%
1980
16th to endNight
3223213336%66%
2112
April1st to 15thNight
332332245%6%62%
1864
16th to endNight
3313213335%68%
1800
May1st to 15thNight
232332435%6%63%
1640
16th to endNight
3313213235%70%
1684
June1st to 15thNight
222212235%71%
1620
16th to endNight
121231445%6%66%
1620
July1st to 15thNight
4427%7%36%7%7%9%40%
1620
16th to endNight
45%37%6%5%9%8%9%10%28%
1701
August1st to 15thNight
4416%6%36%7%9%9%40%
1620
16th to endNight
5%5%246%36%7%8%9%39%
1881
September1st to 15thNight
4425%7%46%6%7%7%42%
1856
16th to endNight
3113312345%68%
1903
October1st to 15thNight
3212213345%70%
1977
16th to endNight
11231233473%
2112
November1st to 15thNight
2212213345%69%
2100
16th to endNight
341222123469%
2280
December1st to 15thNight
5%5%13213336%61%
2340
16th to endNight
7%7%24423446%51%
2496

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
24%48%24%30%6%
1612
16th to endDay
24%28%22%31%10%
1728
February1st to 15thDay
25%38%25%23%14%
1575
16th to endDay
28%4422%28%12%
1635
March1st to 15thDay
23%3416%36%15%
1980
16th to endDay
20%35%18%34%18%
2112
April1st to 15thDay
21%2417%33%19%
2084
16th to endDay
18%2414%36%23%
2160
May1st to 15thDay
18%35%15%30%25%
2307
16th to endDay
21%3410%32%27%
2432
June1st to 15thDay
12%238%27%45%
2340
16th to endDay
15%248%22%45%
2340
July1st to 15thDay
33%7%11%15%12%19%
2201
16th to endDay
41%11%17%19%8%2
2312
August1st to 15thDay
39%8%10%15%15%10%
2325
16th to endDay
34%6%11%17%20%9%
2321
September1st to 15thDay
31%5%10%18%20%13%
2101
16th to endDay
17%35%14%30%29%
1893
October1st to 15thDay
17%2417%33%23%
1652
16th to endDay
15%35%15%37%22%
1760
November1st to 15thDay
15%25%23%35%16%
1759
16th to endDay
19%2421%31%21%
1677
December1st to 15thDay
24%48%29%25%8%
1620
16th to endDay
30%310%29%21%3
1728
January1st to 15thNight
24%36%23%33%7%
2327
16th to endNight
27%26%23%30%9%
2496
February1st to 15thNight
23%27%25%24%16%
2288
16th to endNight
23%35%23%32%11%
1836
March1st to 15thNight
22%3418%37%14%
1980
16th to endNight
18%35%18%37%16%
2112
April1st to 15thNight
20%2421%32%18%
1864
16th to endNight
19%35%14%37%20%
1800
May1st to 15thNight
21%36%18%28%22%
1641
16th to endNight
18%27%13%32%25%
1685
June1st to 15thNight
15%228%22%48%
1620
16th to endNight
16%45%10%19%44%
1620
July1st to 15thNight
34%9%14%14%11%16%
1530
16th to endNight
41%12%21%15%7%1
1593
August1st to 15thNight
33%11%17%12%13%10%
1611
16th to endNight
34%10%16%15%13%9%
1881
September1st to 15thNight
33%7%12%16%16%12%
1856
16th to endNight
17%35%15%28%29%
1903
October1st to 15thNight
15%37%19%28%25%
1977
16th to endNight
13%3414%35%27%
2112
November1st to 15thNight
15%35%22%35%17%
2100
16th to endNight
19%2420%30%23%
2280
December1st to 15thNight
24%28%28%26%10%
2340
16th to endNight
33%49%26%21%4
2496

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
12%10%28%30%11%7%
90
16th to endDay
411%17%35%18%12%
96
February1st to 15thDay
45%12%34%35%7%
90
16th to endDay
11%22%26%20%15%3
195
March1st to 15thDay
7%20%24%25%15%6%
270
16th to endDay
8%19%31%23%12%3
288
April1st to 15thDay
9%17%29%22%14%5%
269
16th to endDay
10%21%33%21%9%2
270
May1st to 15thDay
8%15%31%27%13%3
288
16th to endDay
9%12%30%26%15%5%
330
June1st to 15thDay
37%30%28%21%7%
315
16th to endDay
4217%33%28%12%
315
July1st to 15thDay
8%115%30%28%14%
305
16th to endDay
12%11%26%25%22%
327
August1st to 15thDay
12%9%26%33%17%
315
16th to endDay
10%10%31%31%15%
332
September1st to 15thDay
7%19%32%26%12%
315
16th to endDay
6%6%20%34%22%9%
284
October1st to 15thDay
3216%32%27%18%
92
16th to endDay
8%212%32%30%14%
96
November1st to 15thDay
4420%28%31%11%
90
16th to endDay
413%27%31%20%3
90
December1st to 15thDay
5%328%28%24%8%
90
16th to endDay
11%13%17%32%20%4
96
January1st to 15thNight
7%315%42%25%6%
720
16th to endNight
10%219%40%22%5%
768
February1st to 15thNight
5%224%39%24%3
704
16th to endNight
6%5%29%42%14%
632
March1st to 15thNight
7%6%23%36%24%
720
16th to endNight
45%25%42%19%2
768
April1st to 15thNight
47%33%37%14%1
605
16th to endNight
5%8%32%37%14%2
540
May1st to 15thNight
4430%42%16%1
540
16th to endNight
5%427%37%23%1
563
June1st to 15thNight
3222%47%22%
540
16th to endNight
3112%47%33%1
540
July1st to 15thNight
7%8%37%42%2
510
16th to endNight
7%6%25%50%9%
564
August1st to 15thNight
8%423%54%8%
539
16th to endNight
7%6%29%51%4
573
September1st to 15thNight
5%9%36%43%3
596
16th to endNight
4213%36%36%6%
688
October1st to 15thNight
4120%41%27%4
660
16th to endNight
1117%39%33%5%
704
November1st to 15thNight
5%216%40%30%5%
700
16th to endNight
6%418%33%31%5%
720
December1st to 15thNight
8%318%39%26%3
720
16th to endNight
10%419%42%19%4
768
Page generated at 2017-09-24 06:15:51. Version ac04

Copyright 2017 A. Danko


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