CAC
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CAC 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%
23720
34%
74230
28%
23674
76%
23674
47%
36553

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
66%
758
33%
2457
33%
756
72%
756
45%
1201
January11th to 20thNight
65%
724
36%
2353
34%
724
75%
724
50%
1102
January21st to endNight
58%
792
36%
2574
30%
792
70%
792
41%
1317
February1st to 10thNight
65%
720
36%
2340
33%
720
75%
720
51%
1088
February11th to 20thNight
56%
700
33%
2112
25%
700
67%
700
46%
1075
February21st to endNight
51%
592
40%
1628
28%
592
63%
592
49%
867
March1st to 10thNight
53%
720
38%
1980
25%
720
66%
720
50%
1026
March11th to 20thNight
61%
720
38%
1980
24%
720
72%
720
55%
983
March21st to endNight
61%
792
38%
2178
23%
792
72%
792
54%
1093
April1st to 10thNight
64%
680
40%
1940
26%
680
76%
680
61%
888
April11th to 20thNight
59%
533
42%
1777
31%
533
76%
533
56%
746
April21st to endNight
63%
540
45%
1800
35%
540
80%
540
65%
696
May1st to 10thNight
65%
540
39%
1760
31%
540
77%
540
62%
719
May11th to 20thNight
70%
531
39%
1594
33%
531
77%
531
63%
686
May21st to endNight
74%
582
44%
1746
37%
582
80%
582
75%
678
June1st to 10thNight
78%
540
36%
1620
31%
540
80%
540
62%
709
June11th to 20thNight
80%
540
35%
1620
30%
540
83%
540
63%
708
June21st to endNight
77%
540
25%
1620
23%
540
78%
540
43%
843
July1st to 10thNight
77%
525
19%
1575
17%
525
77%
525
35%
900
July11th to 20thNight
74%
522
12%
1566
11%
522
74%
522
24%
1031
July21st to endNight
74%
591
10%
1674
9%
558
74%
558
22%
1155
August1st to 10thNight
77%
537
13%
1602
12%
534
77%
534
28%
995
August11th to 20thNight
76%
534
21%
1704
22%
534
76%
534
30%
1020
August21st to endNight
77%
594
22%
1980
25%
594
77%
594
29%
1174
September1st to 10thNight
75%
598
24%
1858
24%
598
75%
598
30%
1143
September11th to 20thNight
83%
720
37%
1980
31%
720
84%
720
52%
1039
September21st to endNight
78%
720
36%
1980
26%
720
82%
720
59%
964
October1st to 10thNight
77%
720
37%
2132
29%
720
82%
720
58%
1005
October11th to 20thNight
79%
720
41%
2160
31%
720
86%
720
65%
936
October21st to endNight
80%
792
44%
2376
35%
792
85%
792
68%
1000
November1st to 10thNight
75%
740
45%
2220
36%
740
83%
740
66%
948
November11th to 20thNight
75%
760
42%
2280
34%
760
82%
760
60%
1022
November21st to endNight
58%
760
39%
2346
29%
760
68%
760
47%
1165
December1st to 10thNight
65%
760
38%
2470
34%
760
72%
760
46%
1193
December11th to 20thNight
59%
760
33%
2926
31%
758
68%
758
36%
1500
December21st to endNight
57%
823
23%
4322
30%
817
66%
817
28%
1938

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
432342445%7%61%
74995
Night
432332445%7%63%
74751
TransparencyDay
23%35%13%27%27%
74722
Night
22%36%14%28%27%
74546
SeeingDay
6%7%18%29%27%12%
13591
Night
5%317%39%30%6%
23741

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
6%5%1332446%7%58%
1708
11th to 20thDay
6%32332445%7%61%
1629
21st to endDay
7%5%24313346%62%
1782
February1st to 10thDay
6%41232235%6%65%
1620
11th to 20thDay
6%5%23424445%61%
1730
21st to endDay
7%5%24322235%66%
1628
March1st to 10thDay
5%42342445%8%60%
1980
11th to 20thDay
4422313346%69%
1980
21st to endDay
2313223346%69%
2178
April1st to 10thDay
3212223446%71%
2020
11th to 20thDay
321111223478%
2131
21st to endDay
221221223479%
2160
May1st to 10thDay
1113212336%75%
2196
11th to 20thDay
2212223346%74%
2301
21st to endDay
2212212235%80%
2530
June1st to 10thDay
2212224447%71%
2340
11th to 20thDay
1212212345%76%
2340
21st to endDay
33145%45%6%6%7%55%
2340
July1st to 10thDay
4325%7%48%7%8%9%42%
2340
11th to 20thDay
4425%8%5%8%9%10%12%32%
2340
21st to endDay
8%6%46%7%5%7%9%10%11%27%
2545
August1st to 10thDay
6%5%35%6%5%7%8%9%11%37%
2328
11th to 20thDay
5%446%7%5%7%7%8%11%37%
2195
21st to endDay
5%325%6%5%7%6%8%11%41%
2376
September1st to 10thDay
6%5%25%6%46%5%7%7%46%
2098
11th to 20thDay
3214425%5%6%7%59%
1980
21st to endDay
321332446%7%64%
1980
October1st to 10thDay
211342445%6%69%
1827
11th to 20thDay
221222334475%
1800
21st to endDay
1212213335%76%
1980
November1st to 10thDay
2113222348%73%
1859
11th to 20thDay
2213323446%71%
1900
21st to endDay
6%4235%24346%62%
1829
December1st to 10thDay
7%31422445%7%61%
1710
11th to 20thDay
7%8%25%423348%54%
2169
21st to endDay
5%5%23434445%61%
3146
January1st to 10thNight
6%5%2332445%6%61%
2457
11th to 20thNight
6%423312346%66%
2353
21st to endNight
7%5%24324445%60%
2574
February1st to 10thNight
4313323445%67%
2340
11th to 20thNight
6%5%24313346%63%
2112
21st to endNight
45%24323345%64%
1628
March1st to 10thNight
432432346%6%63%
1980
11th to 20thNight
5%412323345%69%
1980
21st to endNight
4313223347%67%
2178
April1st to 10thNight
2212313346%73%
1940
11th to 20thNight
221231244475%
1777
21st to endNight
2211212235%80%
1800
May1st to 10thNight
2113222346%75%
1760
11th to 20thNight
1112213345%76%
1594
21st to endNight
111121223484%
1746
June1st to 10thNight
2312223448%70%
1620
11th to 20thNight
2212313335%76%
1620
21st to endNight
23245%45%6%7%8%56%
1620
July1st to 10thNight
3425%6%46%6%8%9%46%
1620
11th to 20thNight
3437%7%48%10%10%14%29%
1620
21st to endNight
5%5%36%9%6%8%11%11%12%25%
1773
August1st to 10thNight
3335%7%5%8%7%9%12%37%
1611
11th to 20thNight
5%436%6%47%7%8%13%39%
1704
21st to endNight
5%325%6%47%7%9%10%40%
1980
September1st to 10thNight
7%5%25%6%46%6%8%9%43%
1858
11th to 20thNight
321433446%7%64%
1980
21st to endNight
3213323446%70%
1980
October1st to 10thNight
221332335%6%70%
2132
11th to 20thNight
2112223345%76%
2160
21st to endNight
211121233480%
2376
November1st to 10thNight
2111212246%76%
2220
11th to 20thNight
221232235%6%71%
2280
21st to endNight
5%424323346%63%
2346
December1st to 10thNight
7%4133235%48%61%
2470
11th to 20thNight
45%25%5%35%45%6%56%
3041
21st to endNight
7%6%23423445%59%
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%3415%32%20%
1709
11th to 20thDay
24%2312%35%23%
1629
21st to endDay
26%1216%30%24%
1782
February1st to 10thDay
22%2214%26%35%
1620
11th to 20thDay
28%3214%26%27%
1740
21st to endDay
24%119%27%38%
1628
March1st to 10thDay
24%2211%31%30%
1980
11th to 20thDay
19%1210%34%33%
1980
21st to endDay
17%2211%37%30%
2178
April1st to 10thDay
16%227%40%34%
2020
11th to 20thDay
13%117%30%48%
2134
21st to endDay
12%217%35%43%
2160
May1st to 10thDay
13%218%25%51%
2196
11th to 20thDay
13%3310%30%41%
2304
21st to endDay
10%116%30%52%
2530
June1st to 10thDay
15%227%30%45%
2340
11th to 20thDay
13%216%21%57%
2340
21st to endDay
26%46%11%24%29%
2340
July1st to 10thDay
34%6%13%15%16%16%
2276
11th to 20thDay
38%9%20%21%8%3
2271
21st to endDay
44%11%20%16%8%1
2404
August1st to 10thDay
37%8%17%19%15%4
2313
11th to 20thDay
37%7%18%18%15%5%
2195
21st to endDay
34%8%12%21%18%8%
2376
September1st to 10thDay
36%5%10%16%20%14%
2098
11th to 20thDay
23%46%18%28%21%
1980
21st to endDay
19%3419%35%21%
1980
October1st to 10thDay
17%37%18%28%28%
1827
11th to 20thDay
14%2413%32%35%
1800
21st to endDay
13%2213%32%37%
1980
November1st to 10thDay
13%2215%37%30%
1859
11th to 20thDay
16%2316%34%29%
1900
21st to endDay
24%2217%28%26%
1829
December1st to 10thDay
24%2419%31%21%
1710
11th to 20thDay
31%1420%25%18%
2168
21st to endDay
26%2312%39%18%
3146
January1st to 10thNight
24%25%15%32%21%
2457
11th to 20thNight
21%2314%34%27%
2353
21st to endNight
28%2215%31%22%
2574
February1st to 10thNight
20%2314%26%35%
2340
11th to 20thNight
24%2315%28%28%
2113
21st to endNight
25%128%28%35%
1628
March1st to 10thNight
22%2313%30%30%
1980
11th to 20thNight
19%1210%32%35%
1980
21st to endNight
19%2411%39%25%
2178
April1st to 10thNight
14%3213%40%30%
1940
11th to 20thNight
15%1110%31%41%
1777
21st to endNight
11%2210%34%41%
1800
May1st to 10thNight
12%238%29%46%
1760
11th to 20thNight
13%2412%30%39%
1595
21st to endNight
7%110%34%47%
1746
June1st to 10thNight
15%227%32%42%
1620
11th to 20thNight
14%215%23%55%
1620
21st to endNight
25%46%12%21%32%
1620
July1st to 10thNight
31%6%15%19%14%15%
1575
11th to 20thNight
39%11%22%19%7%2
1566
21st to endNight
43%11%26%12%5%2
1674
August1st to 10thNight
35%9%19%18%14%5%
1602
11th to 20thNight
34%8%18%18%17%5%
1704
21st to endNight
35%7%18%19%13%9%
1980
September1st to 10thNight
36%7%12%15%17%13%
1858
11th to 20thNight
21%46%20%28%21%
1980
21st to endNight
17%27%18%37%19%
1980
October1st to 10thNight
17%28%16%28%29%
2132
11th to 20thNight
12%2415%28%39%
2160
21st to endNight
11%2314%32%39%
2376
November1st to 10thNight
12%2315%36%32%
2220
11th to 20thNight
16%2317%32%30%
2280
21st to endNight
24%2415%29%26%
2346
December1st to 10thNight
23%2419%29%22%
2470
11th to 20thNight
29%15%18%28%19%
3041
21st to endNight
28%2312%33%22%
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%9%20%36%20%3
270
11th to 20thDay
11%422%35%22%6%
243
21st to endDay
6%10%22%36%22%4
264
February1st to 10thDay
7%6%20%34%29%5%
240
11th to 20thDay
12%19%29%29%10%
351
21st to endDay
12%22%29%26%8%2
346
March1st to 10thDay
9%16%31%30%10%3
420
11th to 20thDay
9%16%25%25%20%5%
420
21st to endDay
5%21%29%28%13%3
462
April1st to 10thDay
413%31%29%17%5%
420
11th to 20thDay
5%17%33%28%13%4
412
21st to endDay
417%29%28%17%6%
420
May1st to 10thDay
5%13%29%29%16%8%
423
11th to 20thDay
49%25%35%22%6%
448
21st to endDay
37%29%37%19%5%
485
June1st to 10thDay
4321%36%27%10%
450
11th to 20thDay
25%18%33%32%11%
450
21st to endDay
5%111%34%35%14%
450
July1st to 10thDay
6%5%26%43%19%
446
11th to 20thDay
4420%42%29%
444
21st to endDay
7%2418%36%33%
487
August1st to 10thDay
7%1415%36%36%
447
11th to 20thDay
5%15%20%44%25%
437
21st to endDay
5%1318%38%36%
495
September1st to 10thDay
9%16%29%37%18%
450
11th to 20thDay
426%28%42%18%
450
21st to endDay
5%412%31%33%15%
450
October1st to 10thDay
2416%30%31%17%
297
11th to 20thDay
4415%29%32%17%
270
21st to endDay
25%15%31%32%14%
297
November1st to 10thDay
5%6%19%36%28%6%
270
11th to 20thDay
3317%31%41%6%
270
21st to endDay
12%10%19%34%23%3
270
December1st to 10thDay
11%6%20%33%25%4
270
11th to 20thDay
8%6%24%40%20%2
270
21st to endDay
17%10%21%32%17%3
297
January1st to 10thNight
7%319%45%22%4
760
11th to 20thNight
7%320%43%22%4
724
21st to endNight
10%323%41%20%3
792
February1st to 10thNight
5%422%41%23%4
720
11th to 20thNight
9%7%24%42%17%2
704
21st to endNight
8%6%32%39%13%2
592
March1st to 10thNight
5%7%31%40%14%3
720
11th to 20thNight
7%7%22%35%25%4
720
21st to endNight
46%24%42%20%3
792
April1st to 10thNight
2426%48%19%2
680
11th to 20thNight
26%29%43%19%2
533
21st to endNight
2428%46%17%3
540
May1st to 10thNight
3426%46%20%1
540
11th to 20thNight
2320%47%25%2
531
21st to endNight
1319%49%27%1
584
June1st to 10thNight
4112%52%29%2
540
11th to 20thNight
2112%51%32%3
540
21st to endNight
319%40%44%3
540
July1st to 10thNight
45%31%50%10%
525
11th to 20thNight
6%1322%58%9%
522
21st to endNight
5%2320%58%13%
591
August1st to 10thNight
41315%59%17%
537
11th to 20thNight
41320%56%15%
534
21st to endNight
6%1318%58%14%
594
September1st to 10thNight
9%15%25%51%8%
598
11th to 20thNight
415%32%48%9%
720
21st to endNight
3211%38%39%7%
720
October1st to 10thNight
2214%43%33%6%
720
11th to 20thNight
3214%43%32%6%
720
21st to endNight
2213%39%33%10%
792
November1st to 10thNight
3216%40%32%7%
740
11th to 20thNight
3216%44%30%4
760
21st to endNight
7%425%37%22%5%
760
December1st to 10thNight
9%318%42%24%4
760
11th to 20thNight
6%7%21%42%20%3
760
21st to endNight
11%420%41%19%5%
836
Page generated at 2024-01-08 20:31:27. Version ac07

Copyright 2024 Attilla Danko


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