GMDRC
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the GMDRC clear sky chart from 2005-11-06 to 2024-01-12.

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
69%
24601
39%
74383
31%
24559
78%
24559
58%
33218

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
59%
757
36%
2457
29%
756
71%
756
46%
1164
January11th to 20thNight
66%
728
39%
2366
34%
728
77%
728
50%
1077
January21st to endNight
56%
792
41%
2446
29%
792
72%
792
46%
1194
February1st to 10thNight
63%
720
40%
2160
33%
720
74%
720
55%
995
February11th to 20thNight
61%
700
40%
2098
31%
700
74%
700
53%
966
February21st to endNight
55%
592
38%
1776
33%
592
67%
592
48%
847
March1st to 10thNight
56%
720
33%
2160
24%
720
71%
720
47%
1056
March11th to 20thNight
60%
720
38%
2007
28%
720
71%
720
53%
979
March21st to endNight
53%
792
39%
2178
20%
792
69%
792
57%
1026
April1st to 10thNight
53%
720
39%
1800
17%
720
66%
720
56%
912
April11th to 20thNight
52%
711
41%
1779
16%
711
65%
711
60%
860
April21st to endNight
59%
644
44%
1724
20%
644
73%
644
68%
751
May1st to 10thNight
62%
540
43%
1620
25%
540
72%
540
64%
658
May11th to 20thNight
63%
531
43%
1595
26%
531
74%
531
71%
623
May21st to endNight
64%
582
39%
1746
24%
582
75%
582
69%
685
June1st to 10thNight
77%
540
37%
1620
29%
540
82%
540
72%
643
June11th to 20thNight
75%
540
39%
1620
31%
540
80%
540
78%
617
June21st to endNight
80%
540
37%
1620
31%
540
84%
540
75%
640
July1st to 10thNight
92%
525
37%
1575
32%
525
93%
525
75%
627
July11th to 20thNight
87%
522
34%
1566
30%
522
88%
522
59%
722
July21st to endNight
84%
591
36%
1674
31%
558
84%
558
50%
829
August1st to 10thNight
87%
598
38%
1663
27%
595
89%
595
68%
758
August11th to 20thNight
91%
712
40%
1780
26%
712
91%
712
74%
843
August21st to endNight
89%
792
44%
1980
31%
792
90%
792
72%
946
September1st to 10thNight
85%
720
41%
1875
32%
720
87%
720
67%
893
September11th to 20thNight
89%
720
48%
1980
42%
720
92%
720
79%
830
September21st to endNight
81%
720
46%
1980
38%
720
88%
720
76%
830
October1st to 10thNight
74%
720
44%
1980
35%
720
82%
720
67%
901
October11th to 20thNight
78%
720
51%
1991
41%
720
87%
720
76%
823
October21st to endNight
74%
792
44%
2490
39%
792
84%
792
69%
964
November1st to 10thNight
74%
740
42%
2405
39%
740
85%
740
65%
950
November11th to 20thNight
75%
760
43%
2470
42%
760
83%
760
63%
1004
November21st to endNight
65%
760
40%
2470
37%
760
76%
760
53%
1092
December1st to 10thNight
65%
760
38%
2470
34%
760
74%
760
48%
1155
December11th to 20thNight
61%
758
36%
2898
32%
758
71%
758
35%
1426
December21st to endNight
56%
822
24%
4364
30%
817
68%
817
28%
1932

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
3212323345%73%
74880
Night
3212323345%72%
74889
TransparencyDay
15%2212%27%42%
74607
Night
15%2313%27%41%
74684
SeeingDay
5%8%21%35%26%6%
14283
Night
45%16%35%33%7%
24626

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
432442335%8%63%
1708
11th to 20thDay
5%314323347%66%
1633
21st to endDay
442442434465%
1908
February1st to 10thDay
4314313345%69%
1800
11th to 20thDay
322441345%7%66%
1750
21st to endDay
5%413423335%67%
1480
March1st to 10thDay
431322345%7%65%
1800
11th to 20thDay
442332335%7%66%
1953
21st to endDay
221332335%6%70%
2178
April1st to 10thDay
3323323347%68%
2155
11th to 20thDay
2212212335%79%
2129
21st to endDay
2112212346%77%
2235
May1st to 10thDay
1112222346%76%
2340
11th to 20thDay
1212112235%81%
2301
21st to endDay
121222224479%
2530
June1st to 10thDay
1111212235%81%
2340
11th to 20thDay
11111123386%
2340
21st to endDay
211221212383%
2340
July1st to 10thDay
3112312335%76%
2340
11th to 20thDay
3212322346%73%
2340
21st to endDay
4323335%5%5%9%58%
2545
August1st to 10thDay
3312213325%75%
2263
11th to 20thDay
211221345%5%74%
2122
21st to endDay
312223336%74%
2376
September1st to 10thDay
312213345%75%
2084
11th to 20thDay
11111223485%
1980
21st to endDay
111121222384%
1980
October1st to 10thDay
321231323476%
1980
11th to 20thDay
111212235%83%
1967
21st to endDay
121112123483%
1864
November1st to 10thDay
1212212335%76%
1674
11th to 20thDay
122232323475%
1710
21st to endDay
3213324346%70%
1710
December1st to 10thDay
44244235%5%7%60%
1710
11th to 20thDay
5%425%5%2447%8%54%
2169
21st to endDay
7%425%4246%6%7%53%
3146
January1st to 10thNight
432442345%7%62%
2457
11th to 20thNight
3313324446%66%
2366
21st to endNight
6%32343333466%
2446
February1st to 10thNight
4422213346%69%
2160
11th to 20thNight
4323323346%68%
2098
21st to endNight
431232345%6%65%
1776
March1st to 10thNight
33234245%5%6%64%
2160
11th to 20thNight
331322445%8%65%
2007
21st to endNight
2223323347%71%
2178
April1st to 10thNight
2223223445%71%
1800
11th to 20thNight
221322234476%
1779
21st to endNight
2111222347%77%
1724
May1st to 10thNight
1211212346%76%
1620
11th to 20thNight
1111113345%80%
1594
21st to endNight
1111111235%82%
1746
June1st to 10thNight
112112235%82%
1620
11th to 20thNight
111121122386%
1620
21st to endNight
111111222385%
1620
July1st to 10thNight
11111223382%
1620
11th to 20thNight
321232345%6%68%
1620
21st to endNight
33234346%5%7%61%
1773
August1st to 10thNight
2212122235%76%
1672
11th to 20thNight
111312345%78%
1780
21st to endNight
321121234476%
1980
September1st to 10thNight
321221224476%
1875
11th to 20thNight
111111222386%
1980
21st to endNight
211121122385%
1980
October1st to 10thNight
321221233477%
1980
11th to 20thNight
11121122385%
1991
21st to endNight
111221223482%
2490
November1st to 10thNight
121221223479%
2405
11th to 20thNight
1312212335%76%
2470
21st to endNight
4312323346%70%
2470
December1st to 10thNight
4324423447%63%
2470
11th to 20thNight
45%245%35%45%9%55%
3041
21st to endNight
4425%5%245%6%7%54%
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
20%25%19%34%20%
1709
11th to 20thDay
20%1317%34%24%
1633
21st to endDay
22%25%15%33%23%
1908
February1st to 10thDay
18%1314%29%34%
1800
11th to 20thDay
17%2215%27%35%
1760
21st to endDay
21%2215%33%27%
1480
March1st to 10thDay
18%2316%30%31%
1800
11th to 20thDay
19%2216%33%28%
1953
21st to endDay
14%2220%39%24%
2178
April1st to 10thDay
18%1216%30%33%
2155
11th to 20thDay
10%2211%32%43%
2132
21st to endDay
10%1212%34%41%
2235
May1st to 10thDay
10%1113%30%44%
2340
11th to 20thDay
9%118%30%50%
2304
21st to endDay
10%1110%31%47%
2530
June1st to 10thDay
9%18%22%60%
2340
11th to 20thDay
6%15%14%74%
2340
21st to endDay
11%1310%76%
2340
July1st to 10thDay
13%117%15%63%
2276
11th to 20thDay
14%2312%25%44%
2271
21st to endDay
23%25%18%19%33%
2404
August1st to 10thDay
15%228%23%51%
2248
11th to 20thDay
13%229%28%46%
2122
21st to endDay
13%3111%24%48%
2376
September1st to 10thDay
13%2310%19%54%
2084
11th to 20thDay
6%116%22%64%
1980
21st to endDay
9%118%22%59%
1980
October1st to 10thDay
13%2110%25%49%
1980
11th to 20thDay
8%128%25%55%
1967
21st to endDay
8%1210%27%53%
1864
November1st to 10thDay
12%1113%31%41%
1674
11th to 20thDay
14%1212%27%43%
1710
21st to endDay
15%2414%30%35%
1710
December1st to 10thDay
22%35%18%23%29%
1710
11th to 20thDay
27%26%23%26%17%
2168
21st to endDay
29%15%24%27%14%
3146
January1st to 10thNight
22%2317%31%24%
2457
11th to 20thNight
19%3315%31%30%
2366
21st to endNight
22%2418%27%27%
2446
February1st to 10thNight
18%15%15%26%35%
2160
11th to 20thNight
18%2314%29%35%
2099
21st to endNight
20%2314%30%30%
1776
March1st to 10thNight
20%3416%31%26%
2160
11th to 20thNight
17%2316%31%30%
2007
21st to endNight
15%2418%35%27%
2178
April1st to 10thNight
17%1315%33%31%
1800
11th to 20thNight
13%2213%31%41%
1779
21st to endNight
9%1213%32%42%
1724
May1st to 10thNight
10%2314%31%41%
1620
11th to 20thNight
7%1212%30%47%
1595
21st to endNight
8%1112%35%43%
1746
June1st to 10thNight
8%118%22%59%
1620
11th to 20thNight
7%15%16%70%
1620
21st to endNight
8%1310%77%
1620
July1st to 10thNight
9%117%15%68%
1575
11th to 20thNight
16%2311%23%44%
1566
21st to endNight
22%2417%23%32%
1674
August1st to 10thNight
13%238%20%54%
1663
11th to 20thNight
10%2211%27%48%
1780
21st to endNight
11%219%24%53%
1980
September1st to 10thNight
13%228%23%53%
1875
11th to 20thNight
7%116%22%63%
1980
21st to endNight
8%117%22%61%
1980
October1st to 10thNight
13%1110%28%47%
1980
11th to 20thNight
7%117%25%59%
1991
21st to endNight
9%129%22%56%
2490
November1st to 10thNight
11%2110%35%42%
2405
11th to 20thNight
13%2111%29%44%
2470
21st to endNight
17%2213%28%38%
2470
December1st to 10thNight
21%2316%24%33%
2470
11th to 20thNight
27%2417%28%21%
3041
21st to endNight
28%16%23%29%14%
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
12%10%28%31%16%3
270
11th to 20thDay
13%5%21%39%18%4
245
21st to endDay
8%12%23%34%21%3
390
February1st to 10thDay
8%7%28%35%19%3
420
11th to 20thDay
10%13%32%31%13%2
415
21st to endDay
9%13%34%32%12%
346
March1st to 10thDay
9%15%33%29%14%
420
11th to 20thDay
8%13%29%31%18%1
420
21st to endDay
5%18%34%30%11%1
462
April1st to 10thDay
9%12%27%29%20%3
420
11th to 20thDay
5%18%32%30%13%3
412
21st to endDay
418%26%30%17%6%
420
May1st to 10thDay
5%11%27%34%18%4
423
11th to 20thDay
39%27%34%22%5%
448
21st to endDay
49%25%39%18%5%
485
June1st to 10thDay
26%22%37%26%7%
450
11th to 20thDay
15%17%41%28%8%
450
21st to endDay
2318%42%27%8%
450
July1st to 10thDay
2110%38%39%11%
446
11th to 20thDay
217%34%41%15%
444
21st to endDay
226%36%41%13%
487
August1st to 10thDay
4110%36%37%12%
447
11th to 20thDay
319%38%37%13%
437
21st to endDay
28%37%40%12%
495
September1st to 10thDay
3314%37%36%8%
450
11th to 20thDay
2217%42%33%4
450
21st to endDay
25%17%34%32%9%
450
October1st to 10thDay
36%23%37%26%5%
450
11th to 20thDay
27%15%44%27%5%
437
21st to endDay
37%19%30%33%7%
297
November1st to 10thDay
28%17%40%29%3
270
11th to 20thDay
45%18%38%29%6%
270
21st to endDay
39%21%35%26%5%
270
December1st to 10thDay
6%423%42%22%3
270
11th to 20thDay
310%26%30%27%4
270
21st to endDay
9%12%29%33%15%2
297
January1st to 10thNight
5%6%19%36%26%7%
760
11th to 20thNight
5%418%42%24%7%
728
21st to endNight
8%7%22%34%23%5%
792
February1st to 10thNight
5%5%21%37%24%7%
720
11th to 20thNight
5%8%21%38%22%5%
704
21st to endNight
7%9%24%40%16%4
592
March1st to 10thNight
5%9%25%38%21%2
720
11th to 20thNight
410%21%34%23%8%
720
21st to endNight
311%28%35%19%4
792
April1st to 10thNight
411%27%35%21%3
720
11th to 20thNight
5%12%27%31%20%4
711
21st to endNight
211%26%30%26%5%
644
May1st to 10thNight
49%22%41%22%2
540
11th to 20thNight
18%25%29%31%5%
531
21st to endNight
38%22%35%29%4
584
June1st to 10thNight
1414%44%35%1
540
11th to 20thNight
1319%38%37%3
540
21st to endNight
1215%38%39%5%
540
July1st to 10thNight
1437%51%6%
525
11th to 20thNight
3431%54%6%
522
21st to endNight
41328%58%6%
591
August1st to 10thNight
45%29%55%7%
598
11th to 20thNight
1323%60%13%
712
21st to endNight
3324%54%15%
792
September1st to 10thNight
325%27%53%10%
720
11th to 20thNight
116%43%39%11%
720
21st to endNight
2310%33%41%12%
720
October1st to 10thNight
3316%41%33%5%
720
11th to 20thNight
2413%38%34%9%
720
21st to endNight
25%15%32%33%13%
792
November1st to 10thNight
1415%37%34%9%
740
11th to 20thNight
3313%39%33%10%
760
21st to endNight
5%419%36%28%7%
760
December1st to 10thNight
6%5%16%42%26%6%
760
11th to 20thNight
7%6%16%40%27%4
760
21st to endNight
6%9%20%39%20%5%
836
Page generated at 2024-01-09 13:24:25. Version ac07

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