CAC
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CAC clear sky chart from 2005-11-05 to 2017-09-17.

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
70%
15417
34%
46872
29%
15377
77%
15377
49%
22979

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%
718
33%
2327
33%
716
73%
716
47%
1115
January16th to endNight
63%
768
35%
2496
32%
768
72%
768
42%
1276
February1st to 15thNight
68%
698
35%
2262
32%
696
78%
696
52%
1030
February16th to endNight
58%
632
41%
1766
31%
632
68%
632
50%
921
March1st to 15thNight
60%
720
40%
1980
28%
720
70%
720
55%
979
March16th to endNight
66%
768
39%
2112
25%
768
77%
768
61%
999
April1st to 15thNight
60%
632
37%
1891
24%
632
73%
632
53%
898
April16th to endNight
63%
540
44%
1800
34%
540
80%
540
66%
682
May1st to 15thNight
64%
539
37%
1712
28%
539
75%
539
64%
700
May16th to endNight
72%
561
42%
1684
34%
561
78%
561
70%
685
June1st to 15thNight
80%
540
34%
1620
29%
540
82%
540
64%
701
June16th to endNight
80%
540
27%
1620
24%
540
81%
540
49%
804
July1st to 15thNight
77%
510
15%
1530
14%
510
77%
510
32%
910
July16th to endNight
78%
564
11%
1593
10%
531
78%
531
24%
1070
August1st to 15thNight
76%
539
18%
1620
17%
536
76%
536
29%
994
August16th to endNight
79%
573
24%
1910
27%
573
79%
573
33%
1092
September1st to 15thNight
76%
639
25%
1899
22%
639
77%
639
34%
1142
September16th to endNight
80%
664
39%
1837
28%
664
85%
664
60%
887
October1st to 15thNight
77%
660
39%
1963
31%
660
82%
660
62%
892
October16th to endNight
83%
704
42%
2112
35%
704
88%
704
67%
907
November1st to 15thNight
75%
700
43%
2100
34%
700
82%
700
67%
881
November16th to endNight
66%
720
40%
2202
33%
720
74%
720
52%
1055
December1st to 15thNight
68%
720
38%
2340
36%
720
74%
720
49%
1091
December16th to endNight
61%
768
35%
2496
33%
768
67%
768
41%
1268

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
431332345%7%61%
47891
Night
3313323346%63%
47079
TransparencyDay
22%35%13%27%26%
47586
Night
20%36%15%28%25%
46885
SeeingDay
7%8%19%28%26%9%
5294
Night
5%315%35%33%7%
15428

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
4312324447%62%
1611
16th to endDay
7%5%23313346%57%
1728
February1st to 15thDay
5%412221245%67%
1588
16th to endDay
5%41341333463%
1708
March1st to 15thDay
4312213347%66%
1980
16th to endDay
231222335%72%
2112
April1st to 15thDay
3211123345%70%
2057
16th to endDay
2211112380%
2160
May1st to 15thDay
1112112335%75%
2235
16th to endDay
111112235%76%
2429
June1st to 15thDay
111213235%73%
2340
16th to endDay
2233345%5%7%60%
2340
July1st to 15thDay
44247%47%8%7%10%38%
2340
16th to endDay
7%6%37%8%48%8%8%10%26%
2467
August1st to 15thDay
6%435%7%46%7%8%11%34%
2330
16th to endDay
5%325%5%47%6%7%10%40%
2292
September1st to 15thDay
6%4246%45%6%7%8%44%
2058
16th to endDay
421321335%6%64%
1828
October1st to 15thDay
2112323235%72%
1666
16th to endDay
112212335%75%
1760
November1st to 15thDay
1111112237%73%
1759
16th to endDay
4212313235%68%
1755
December1st to 15thDay
6%3211335%7%63%
1620
16th to endDay
7%5%13213337%59%
1728
January1st to 15thNight
5%412313345%64%
2327
16th to endNight
6%423323445%59%
2496
February1st to 15thNight
3212212346%67%
2262
16th to endNight
332432324465%
1766
March1st to 15thNight
3213313245%67%
1980
16th to endNight
3312112235%73%
2112
April1st to 15thNight
321231334469%
1891
16th to endNight
1111222481%
1800
May1st to 15thNight
12112236%75%
1712
16th to endNight
11111123480%
1684
June1st to 15thNight
221112336%74%
1620
16th to endNight
12133245%7%6%61%
1620
July1st to 15thNight
2425%6%36%7%9%12%39%
1620
16th to endNight
4425%7%5%9%10%10%12%27%
1701
August1st to 15thNight
3325%6%47%6%9%12%37%
1629
16th to endNight
43245%36%6%8%10%44%
1910
September1st to 15thNight
6%4145%35%5%8%9%45%
1899
16th to endNight
2212222345%70%
1837
October1st to 15thNight
1112213245%72%
1963
16th to endNight
11111233477%
2112
November1st to 15thNight
1111112235%76%
2100
16th to endNight
431322223468%
2202
December1st to 15thNight
6%212213337%64%
2340
16th to endNight
7%5%23412236%60%
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
21%2415%33%21%
1612
16th to endDay
28%1217%29%20%
1728
February1st to 15thDay
20%1114%31%30%
1588
16th to endDay
24%2114%30%25%
1708
March1st to 15thDay
19%1211%36%30%
1980
16th to endDay
16%1111%36%31%
2112
April1st to 15thDay
16%118%39%32%
2057
16th to endDay
11%17%30%48%
2160
May1st to 15thDay
12%119%29%44%
2235
16th to endDay
11%128%31%44%
2432
June1st to 15thDay
14%125%24%51%
2340
16th to endDay
21%439%23%37%
2340
July1st to 15thDay
36%7%18%15%13%9%
2201
16th to endDay
46%11%20%15%5%1
2312
August1st to 15thDay
40%8%15%17%15%2
2315
16th to endDay
34%8%14%18%15%9%
2292
September1st to 15thDay
34%410%19%20%10%
2058
16th to endDay
19%3317%35%20%
1828
October1st to 15thDay
16%26%16%28%29%
1666
16th to endDay
13%2314%31%35%
1760
November1st to 15thDay
11%1216%36%30%
1759
16th to endDay
19%2215%32%28%
1755
December1st to 15thDay
22%1320%33%17%
1620
16th to endDay
26%26%21%30%14%
1728
January1st to 15thNight
23%1413%32%24%
2327
16th to endNight
27%1217%31%18%
2496
February1st to 15thNight
18%1415%30%29%
2273
16th to endNight
23%1213%34%24%
1766
March1st to 15thNight
18%1212%33%30%
1980
16th to endNight
16%1310%39%29%
2112
April1st to 15thNight
18%2112%39%26%
1891
16th to endNight
10%1110%31%45%
1800
May1st to 15thNight
11%238%33%41%
1713
16th to endNight
9%1213%34%38%
1685
June1st to 15thNight
13%125%26%50%
1620
16th to endNight
20%4310%24%37%
1620
July1st to 15thNight
31%8%21%18%11%8%
1530
16th to endNight
37%12%28%14%41
1593
August1st to 15thNight
33%10%18%18%16%3
1620
16th to endNight
31%7%18%18%14%9%
1910
September1st to 15thNight
31%7%12%20%17%9%
1899
16th to endNight
16%2418%40%18%
1837
October1st to 15thNight
14%28%18%27%29%
1963
16th to endNight
11%1414%28%39%
2112
November1st to 15thNight
11%1318%34%30%
2100
16th to endNight
20%1313%27%32%
2202
December1st to 15thNight
21%1420%33%18%
2340
16th to endNight
27%16%18%30%15%
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
14%5%26%35%16%1
90
16th to endDay
8%8%18%37%16%10%
96
February1st to 15thDay
9%5%15%29%34%4
103
16th to endDay
10%24%27%25%10%2
238
March1st to 15thDay
9%14%28%25%18%4
270
16th to endDay
5%19%27%26%14%5%
288
April1st to 15thDay
8%15%31%26%13%2
269
16th to endDay
5%21%31%26%10%3
270
May1st to 15thDay
5%18%33%26%12%3
288
16th to endDay
310%33%37%12%3
330
June1st to 15thDay
3426%35%21%8%
315
16th to endDay
4113%32%40%8%
315
July1st to 15thDay
10%8%29%42%9%
305
16th to endDay
10%2423%36%22%
327
August1st to 15thDay
12%6%19%39%22%
315
16th to endDay
6%6%19%44%21%
332
September1st to 15thDay
12%5%33%37%9%
315
16th to endDay
6%415%33%31%8%
260
October1st to 15thDay
2219%29%28%16%
106
16th to endDay
1311%29%39%15%
96
November1st to 15thDay
413%24%28%21%7%
90
16th to endDay
8%13%22%30%24%1
90
December1st to 15thDay
6%321%32%28%7%
90
16th to endDay
21%5%22%32%17%
96
January1st to 15thNight
7%216%42%24%5%
720
16th to endNight
10%317%39%25%4
768
February1st to 15thNight
5%5%16%39%26%6%
704
16th to endNight
5%5%26%42%17%2
632
March1st to 15thNight
6%6%22%36%22%5%
720
16th to endNight
45%20%38%27%4
768
April1st to 15thNight
45%24%45%18%1
632
16th to endNight
26%26%43%18%3
540
May1st to 15thNight
2326%41%23%2
540
16th to endNight
1318%43%30%1
563
June1st to 15thNight
3110%49%32%3
540
16th to endNight
27%42%42%3
540
July1st to 15thNight
3429%51%10%
510
16th to endNight
5%1217%56%15%
564
August1st to 15thNight
41213%58%19%
539
16th to endNight
6%1216%55%17%
573
September1st to 15thNight
7%15%20%51%12%
639
16th to endNight
410%31%43%8%
664
October1st to 15thNight
314%40%31%9%
660
16th to endNight
210%35%38%11%
704
November1st to 15thNight
2216%35%34%8%
700
16th to endNight
6%320%32%29%7%
720
December1st to 15thNight
8%415%37%29%5%
720
16th to endNight
10%416%40%22%5%
768
Page generated at 2017-09-18 05:38:19. Version ac04

Copyright 2017 A. Danko


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