GuideStar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the GuideStar Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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
62%
23628
31%
74263
25%
23578
70%
23578
43%
37724

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 10thNight
62%
760
32%
2457
28%
756
69%
756
42%
1234
January11th to 20thNight
60%
734
33%
2385
31%
734
68%
734
42%
1214
January21st to endNight
57%
792
34%
2574
28%
792
67%
792
39%
1341
February1st to 10thNight
60%
720
32%
2340
28%
720
71%
720
49%
1109
February11th to 20thNight
49%
701
28%
2245
21%
700
60%
700
37%
1208
February21st to endNight
49%
592
40%
1628
27%
592
62%
592
48%
871
March1st to 10thNight
51%
720
35%
1980
22%
720
63%
720
49%
1021
March11th to 20thNight
56%
720
35%
1980
22%
720
66%
720
48%
1044
March21st to endNight
51%
792
32%
2178
18%
792
62%
792
44%
1185
April1st to 10thNight
51%
639
33%
1899
18%
639
66%
639
51%
909
April11th to 20thNight
47%
533
35%
1777
22%
533
64%
533
46%
811
April21st to endNight
49%
540
36%
1800
22%
540
69%
540
49%
806
May1st to 10thNight
56%
540
35%
1652
24%
540
68%
540
51%
780
May11th to 20thNight
57%
531
32%
1594
23%
531
65%
531
50%
779
May21st to endNight
57%
583
33%
1746
23%
582
67%
582
57%
782
June1st to 10thNight
66%
540
32%
1620
27%
540
68%
540
53%
769
June11th to 20thNight
67%
540
35%
1620
29%
540
72%
540
67%
679
June21st to endNight
70%
540
27%
1620
26%
540
72%
540
49%
805
July1st to 10thNight
68%
525
17%
1575
15%
525
69%
525
33%
924
July11th to 20thNight
69%
522
13%
1566
11%
522
69%
522
26%
991
July21st to endNight
68%
591
12%
1674
12%
558
68%
558
18%
1208
August1st to 10thNight
73%
537
17%
1602
15%
534
74%
534
30%
972
August11th to 20thNight
70%
534
20%
1646
20%
534
70%
534
26%
1034
August21st to endNight
74%
594
22%
1980
23%
594
75%
594
30%
1158
September1st to 10thNight
68%
540
27%
1800
28%
540
70%
540
30%
1049
September11th to 20thNight
78%
712
36%
1972
29%
712
81%
712
52%
1041
September21st to endNight
70%
720
35%
1980
22%
720
78%
720
53%
1017
October1st to 10thNight
66%
720
32%
2155
22%
720
74%
720
50%
1060
October11th to 20thNight
73%
720
42%
2160
32%
720
81%
720
66%
928
October21st to endNight
70%
792
43%
2376
31%
792
79%
792
61%
1050
November1st to 10thNight
71%
740
41%
2220
31%
740
79%
740
57%
1018
November11th to 20thNight
73%
760
43%
2280
34%
760
80%
760
61%
1018
November21st to endNight
60%
760
36%
2470
30%
760
69%
760
47%
1184
December1st to 10thNight
63%
760
36%
2470
32%
760
70%
760
46%
1210
December11th to 20thNight
57%
759
31%
2924
28%
759
65%
759
32%
1581
December21st to endNight
56%
825
21%
4318
25%
817
63%
817
26%
1934

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
342443445%7%60%
75006
Night
442443445%7%60%
74789
TransparencyDay
23%47%17%28%21%
74732
Night
23%48%18%27%20%
74584
SeeingDay
6%8%22%28%23%12%
13485
Night
6%421%40%26%3
23644

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 10thDay
45%1433446%6%59%
1708
11th to 20thDay
443433335%6%61%
1647
21st to endDay
47%24423349%58%
1782
February1st to 10thDay
26%25%433346%62%
1620
11th to 20thDay
47%345%2345%7%55%
1598
21st to endDay
47%35%5%34446%57%
1628
March1st to 10thDay
35%34435%45%8%56%
1980
11th to 20thDay
35%25%43445%6%57%
1980
21st to endDay
342433445%6%61%
2178
April1st to 10thDay
331342445%7%65%
2060
11th to 20thDay
3414434345%65%
2131
21st to endDay
2323324346%68%
2160
May1st to 10thDay
221332345%7%66%
2304
11th to 20thDay
332343445%6%63%
2301
21st to endDay
3323323345%71%
2530
June1st to 10thDay
111222345%7%71%
2340
11th to 20thDay
211221333479%
2340
21st to endDay
22134345%5%6%64%
2340
July1st to 10thDay
3335%5%45%7%7%9%49%
2340
11th to 20thDay
4425%7%48%7%8%9%43%
2340
21st to endDay
5%437%7%5%8%8%9%10%34%
2545
August1st to 10thDay
5%5%38%6%37%6%7%7%42%
2328
11th to 20thDay
5%425%5%47%7%8%9%44%
2255
21st to endDay
6%315%6%36%8%7%8%48%
2376
September1st to 10thDay
5%435%6%45%5%6%8%52%
2160
11th to 20thDay
331342345%7%64%
1982
21st to endDay
331332345%5%67%
1980
October1st to 10thDay
4323323245%67%
1803
11th to 20thDay
321221123479%
1800
21st to endDay
2312322335%73%
1980
November1st to 10thDay
2311213338%72%
1859
11th to 20thDay
2213323335%71%
1896
21st to endDay
4424323335%65%
1710
December1st to 10thDay
45%14333446%63%
1710
11th to 20thDay
47%47%6%3445%5%50%
2169
21st to endDay
47%35%5%3445%5%56%
3146
January1st to 10thNight
45%2443445%6%60%
2457
11th to 20thNight
5%6%24323345%61%
2385
21st to endNight
6%6%24424446%58%
2574
February1st to 10thNight
2424423347%65%
2340
11th to 20thNight
47%35%434446%56%
2244
21st to endNight
46%24423436%62%
1628
March1st to 10thNight
45%2432335%8%62%
1980
11th to 20thNight
5%5%23323346%63%
1980
21st to endNight
44244245%46%60%
2178
April1st to 10thNight
332332345%6%65%
1899
11th to 20thNight
3423323447%65%
1777
21st to endNight
3323323346%68%
1800
May1st to 10thNight
231342335%6%67%
1652
11th to 20thNight
332343335%8%64%
1594
21st to endNight
3313222335%73%
1746
June1st to 10thNight
431332335%6%66%
1620
11th to 20thNight
2212212235%77%
1620
21st to endNight
1314426%46%7%61%
1620
July1st to 10thNight
4336%7%36%7%8%7%45%
1620
11th to 20thNight
4339%7%5%7%8%7%11%36%
1620
21st to endNight
6%6%38%7%6%9%9%9%11%26%
1773
August1st to 10thNight
35%27%6%46%7%8%9%44%
1611
11th to 20thNight
6%425%8%48%7%8%9%39%
1646
21st to endNight
5%425%6%36%7%7%11%43%
1980
September1st to 10thNight
45%27%7%46%7%7%8%42%
1800
11th to 20thNight
33145%2445%6%63%
1972
21st to endNight
432342335%5%66%
1980
October1st to 10thNight
431332445%5%65%
2155
11th to 20thNight
221331223479%
2160
21st to endNight
1212322336%75%
2376
November1st to 10thNight
3213323345%71%
2220
11th to 20thNight
231231234474%
2280
21st to endNight
45%23333345%65%
2470
December1st to 10thNight
6%5%23323347%62%
2470
11th to 20thNight
5%6%25%5%3445%7%52%
3041
21st to endNight
46%244345%5%6%56%
4521

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 10thDay
25%48%24%30%9%
1709
11th to 20thDay
25%38%23%35%7%
1647
21st to endDay
26%38%20%34%10%
1782
February1st to 10thDay
24%38%20%30%15%
1620
11th to 20thDay
29%45%22%23%18%
1607
21st to endDay
28%35%17%29%18%
1628
March1st to 10thDay
25%35%15%34%17%
1980
11th to 20thDay
26%45%15%31%19%
1980
21st to endDay
22%36%17%34%17%
2178
April1st to 10thDay
20%35%16%35%22%
2060
11th to 20thDay
22%2414%33%26%
2134
21st to endDay
17%35%14%37%23%
2160
May1st to 10thDay
16%35%14%34%27%
2304
11th to 20thDay
21%46%13%30%27%
2304
21st to endDay
17%3310%32%35%
2530
June1st to 10thDay
12%25%9%31%40%
2340
11th to 20thDay
11%236%24%54%
2340
21st to endDay
19%46%10%22%38%
2340
July1st to 10thDay
29%6%10%16%14%25%
2276
11th to 20thDay
33%10%15%19%12%10%
2271
21st to endDay
39%11%18%19%9%4
2404
August1st to 10thDay
37%9%13%12%14%15%
2313
11th to 20thDay
32%9%11%19%16%13%
2255
21st to endDay
30%7%12%19%23%10%
2376
September1st to 10thDay
29%5%9%16%21%20%
2160
11th to 20thDay
20%47%16%27%27%
1982
21st to endDay
18%36%13%29%31%
1980
October1st to 10thDay
20%37%19%26%25%
1803
11th to 20thDay
12%2311%34%38%
1800
21st to endDay
15%35%17%39%21%
1980
November1st to 10thDay
12%35%23%38%20%
1859
11th to 20thDay
17%3418%39%19%
1896
21st to endDay
23%35%19%33%18%
1710
December1st to 10thDay
23%47%24%28%14%
1710
11th to 20thDay
35%48%21%23%9%
2168
21st to endDay
30%46%23%27%10%
3146
January1st to 10thNight
25%47%25%31%8%
2457
11th to 20thNight
26%35%25%33%8%
2385
21st to endNight
28%37%19%33%10%
2574
February1st to 10thNight
21%38%22%30%16%
2340
11th to 20thNight
30%35%21%22%18%
2245
21st to endNight
23%35%17%34%18%
1628
March1st to 10thNight
21%35%21%32%18%
1980
11th to 20thNight
24%25%16%36%17%
1980
21st to endNight
24%5%6%19%32%15%
2178
April1st to 10thNight
20%26%19%34%20%
1899
11th to 20thNight
19%35%14%36%23%
1777
21st to endNight
18%36%14%36%23%
1800
May1st to 10thNight
18%36%16%32%25%
1652
11th to 20thNight
20%39%16%28%23%
1595
21st to endNight
15%35%11%33%33%
1746
June1st to 10thNight
19%3310%25%40%
1620
11th to 20thNight
11%237%23%54%
1620
21st to endNight
20%5%7%13%20%36%
1620
July1st to 10thNight
31%8%13%13%11%23%
1575
11th to 20thNight
37%10%17%18%10%8%
1566
21st to endNight
45%14%19%13%7%2
1674
August1st to 10thNight
30%12%18%13%14%14%
1602
11th to 20thNight
35%10%18%13%11%12%
1646
21st to endNight
31%10%15%19%16%10%
1980
September1st to 10thNight
34%7%13%15%15%17%
1800
11th to 20thNight
20%5%8%18%25%24%
1972
21st to endNight
20%46%12%27%31%
1980
October1st to 10thNight
19%5%8%18%23%27%
2155
11th to 20thNight
12%2313%31%38%
2160
21st to endNight
13%35%16%38%25%
2376
November1st to 10thNight
16%3420%36%21%
2220
11th to 20thNight
15%26%20%35%23%
2280
21st to endNight
22%35%18%33%20%
2470
December1st to 10thNight
24%26%27%27%14%
2470
11th to 20thNight
31%48%20%27%10%
3041
21st to endNight
27%47%21%25%16%
4521

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 10thDay
8%7%26%35%21%3
270
11th to 20thDay
9%7%23%34%18%8%
249
21st to endDay
49%22%36%19%9%
264
February1st to 10thDay
38%22%36%23%8%
240
11th to 20thDay
11%13%25%34%17%1
263
21st to endDay
12%18%33%22%12%3
346
March1st to 10thDay
8%20%29%26%13%3
420
11th to 20thDay
9%17%23%28%17%5%
420
21st to endDay
10%22%35%18%10%3
462
April1st to 10thDay
7%15%30%26%15%6%
420
11th to 20thDay
11%14%37%21%13%4
412
21st to endDay
6%15%34%25%14%6%
420
May1st to 10thDay
7%10%34%30%16%4
423
11th to 20thDay
6%12%29%24%21%8%
448
21st to endDay
7%13%27%28%19%6%
485
June1st to 10thDay
36%22%32%27%10%
450
11th to 20thDay
26%26%30%23%13%
450
21st to endDay
3223%35%27%10%
450
July1st to 10thDay
6%211%27%33%21%
446
11th to 20thDay
7%18%23%28%33%
444
21st to endDay
7%18%20%30%34%
487
August1st to 10thDay
6%28%23%35%26%
447
11th to 20thDay
8%211%23%33%23%
437
21st to endDay
6%18%26%33%27%
495
September1st to 10thDay
6%214%23%33%23%
450
11th to 20thDay
4216%30%33%14%
450
21st to endDay
36%19%31%27%14%
450
October1st to 10thDay
7%5%21%27%23%17%
273
11th to 20thDay
6%418%30%27%16%
270
21st to endDay
45%17%30%30%14%
297
November1st to 10thDay
5%5%23%36%25%7%
270
11th to 20thDay
35%16%31%32%13%
270
21st to endDay
5%9%24%32%21%9%
270
December1st to 10thDay
7%5%22%30%28%7%
270
11th to 20thDay
47%26%36%19%7%
270
21st to endDay
12%9%24%34%18%2
297
January1st to 10thNight
8%321%43%22%4
760
11th to 20thNight
7%6%21%42%20%4
734
21st to endNight
10%423%42%18%3
792
February1st to 10thNight
4427%42%20%3
720
11th to 20thNight
8%7%30%40%14%1
704
21st to endNight
7%7%33%40%12%
592
March1st to 10thNight
7%8%30%38%17%
720
11th to 20thNight
7%7%25%39%19%2
720
21st to endNight
7%8%30%41%14%1
792
April1st to 10thNight
5%6%33%41%14%1
639
11th to 20thNight
5%10%34%39%11%1
533
21st to endNight
5%9%33%38%13%2
540
May1st to 10thNight
45%32%44%14%1
540
11th to 20thNight
5%428%42%19%2
531
21st to endNight
5%5%29%41%20%1
584
June1st to 10thNight
5%321%50%20%
540
11th to 20thNight
2325%49%20%1
540
21st to endNight
3117%46%32%1
540
July1st to 10thNight
5%112%39%41%2
525
11th to 20thNight
6%17%30%51%5%
522
21st to endNight
9%26%22%53%9%
591
August1st to 10thNight
7%15%25%55%7%
537
11th to 20thNight
7%18%31%48%4
534
21st to endNight
6%7%29%54%4
594
September1st to 10thNight
6%211%39%41%1
540
11th to 20thNight
5%19%42%40%4
712
21st to endNight
4317%38%33%4
720
October1st to 10thNight
4321%45%24%3
720
11th to 20thNight
3218%46%28%3
720
21st to endNight
2420%40%29%5%
792
November1st to 10thNight
5%318%43%27%4
740
11th to 20thNight
3417%46%25%4
760
21st to endNight
8%5%23%35%27%3
760
December1st to 10thNight
8%420%43%23%3
760
11th to 20thNight
10%5%21%43%20%1
760
21st to endNight
8%6%23%45%15%3
836
Page generated at 2024-01-11 02:22:43. Version ac07

Copyright 2024 Attilla Danko


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