Big "A" Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big "A" Observatory clear sky chart from 2005-11-05 to 2017-09-16.

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
47%
15443
23%
46938
15%
15400
56%
15400
28%
28755

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
26%
720
23%
2327
9%
716
43%
716
21%
1432
January16th to endNight
25%
768
24%
2414
11%
768
40%
768
21%
1545
February1st to 15thNight
27%
704
25%
2112
9%
701
43%
701
26%
1286
February16th to endNight
24%
631
23%
1893
7%
631
38%
631
23%
1188
March1st to 15thNight
29%
720
23%
2082
10%
720
40%
720
26%
1294
March16th to endNight
37%
768
22%
2112
12%
768
45%
768
29%
1307
April1st to 15thNight
41%
605
25%
1863
16%
605
52%
605
32%
1073
April16th to endNight
47%
540
26%
1731
18%
540
56%
540
31%
983
May1st to 15thNight
51%
540
22%
1620
12%
540
60%
540
31%
985
May16th to endNight
63%
561
22%
1682
17%
561
67%
561
32%
1024
June1st to 15thNight
71%
540
17%
1620
15%
540
73%
540
32%
1001
June16th to endNight
77%
540
17%
1620
14%
540
78%
540
34%
982
July1st to 15thNight
70%
510
14%
1530
12%
510
71%
510
28%
991
July16th to endNight
74%
564
16%
1595
15%
531
76%
531
29%
1024
August1st to 15thNight
72%
539
18%
1684
19%
536
73%
536
28%
1076
August16th to endNight
66%
573
19%
1910
21%
573
67%
573
23%
1249
September1st to 15thNight
64%
687
20%
1947
19%
687
65%
687
29%
1262
September16th to endNight
59%
661
24%
1826
17%
661
64%
661
31%
1166
October1st to 15thNight
54%
660
33%
1815
25%
660
61%
660
37%
1069
October16th to endNight
50%
704
33%
2103
25%
704
61%
704
38%
1150
November1st to 15thNight
47%
700
30%
2276
24%
700
60%
700
35%
1203
November16th to endNight
39%
720
26%
2340
16%
720
52%
720
28%
1350
December1st to 15thNight
33%
720
24%
2340
14%
720
45%
720
24%
1438
December16th to endNight
28%
768
20%
2496
13%
768
38%
768
18%
1677

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
13%5%25%5%35%5%5%7%40%
47817
Night
11%42442445%7%46%
47137
TransparencyDay
40%6%11%18%17%4
47518
Night
36%5%10%20%19%6%
46940
SeeingDay
15%7%19%21%21%14%
5330
Night
14%8%23%27%21%4
15445

By Month

Percentage Cloud Cover SE (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
16%7%24423435%44%
1611
16th to endDay
17%5%25%413445%44%
1809
February1st to 15thDay
14%6%35%423445%44%
1750
16th to endDay
17%5%245%24445%42%
1580
March1st to 15thDay
15%7%26%5%335%3442%
1876
16th to endDay
16%6%2442445%7%41%
2111
April1st to 15thDay
12%6%245%34346%45%
2094
16th to endDay
11%4135%3445%6%48%
2229
May1st to 15thDay
11%5%25%5%4445%6%44%
2331
16th to endDay
8%5%25%6%45%6%6%9%40%
2431
June1st to 15thDay
8%5%27%7%46%7%7%10%32%
2340
16th to endDay
5%2145%47%8%7%11%39%
2340
July1st to 15thDay
10%5%36%7%48%8%8%10%27%
2340
16th to endDay
6%436%7%410%8%9%9%28%
2465
August1st to 15thDay
8%415%7%5%8%8%8%11%30%
2262
16th to endDay
14%7%36%8%37%6%7%8%26%
2292
September1st to 15thDay
13%6%25%5%36%6%6%7%35%
2001
16th to endDay
16%5%15%5%2435%5%41%
1824
October1st to 15thDay
14%422313245%52%
1815
16th to endDay
13%31332233457%
1766
November1st to 15thDay
13%413413345%53%
1582
16th to endDay
15%5%24424345%49%
1620
December1st to 15thDay
16%7%25%42435%6%40%
1620
16th to endDay
23%8%345%3333435%
1728
January1st to 15thNight
17%5%2432435%5%44%
2327
16th to endNight
17%6%35%434345%42%
2414
February1st to 15thNight
14%6%24423446%46%
2103
16th to endNight
16%5%24433446%45%
1893
March1st to 15thNight
13%5%24324347%46%
2082
16th to endNight
11%5%25%324446%48%
2112
April1st to 15thNight
12%41342435%7%51%
1863
16th to endNight
10%42432445%6%52%
1731
May1st to 15thNight
8%424426%5%6%8%47%
1619
16th to endNight
7%3244245%7%9%47%
1682
June1st to 15thNight
4325%436%5%7%12%43%
1620
16th to endNight
3225%437%5%7%13%43%
1620
July1st to 15thNight
6%435%6%37%6%7%11%38%
1620
16th to endNight
5%325%6%37%8%9%11%36%
1703
August1st to 15thNight
6%325%5%5%5%7%8%11%38%
1695
16th to endNight
13%425%5%345%7%10%36%
1910
September1st to 15thNight
9%425%6%35%5%6%8%42%
1947
16th to endNight
12%5%24434447%45%
1826
October1st to 15thNight
11%5%23313346%55%
1815
16th to endNight
10%42242223460%
2103
November1st to 15thNight
11%322312235%58%
2276
16th to endNight
13%423424345%51%
2340
December1st to 15thNight
14%6%26%423346%45%
2340
16th to endNight
21%7%15%43435%7%35%
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
42%39%14%24%4
1612
16th to endDay
41%36%18%20%7%
1809
February1st to 15thDay
41%37%19%23%3
1752
16th to endDay
43%36%18%21%6%
1580
March1st to 15thDay
44%49%16%19%5%
1876
16th to endDay
42%5%10%20%17%3
2112
April1st to 15thDay
40%5%8%20%19%6%
2094
16th to endDay
34%5%9%21%23%4
2229
May1st to 15thDay
39%5%12%18%20%3
2331
16th to endDay
38%7%13%24%14%2
2435
June1st to 15thDay
43%9%15%20%9%1
2340
16th to endDay
32%11%17%22%14%2
2340
July1st to 15thDay
44%12%18%16%6%1
2201
16th to endDay
42%13%18%16%8%1
2310
August1st to 15thDay
41%12%16%18%8%1
2249
16th to endDay
50%10%14%14%9%1
2292
September1st to 15thDay
43%8%12%16%14%3
2001
16th to endDay
43%5%9%18%19%3
1824
October1st to 15thDay
33%48%18%26%8%
1815
16th to endDay
30%36%16%31%11%
1766
November1st to 15thDay
33%35%16%30%10%
1582
16th to endDay
37%45%15%28%8%
1620
December1st to 15thDay
44%48%20%15%7%
1620
16th to endDay
52%37%16%14%5%
1728
January1st to 15thNight
40%37%18%23%5%
2327
16th to endNight
42%45%18%20%8%
2414
February1st to 15thNight
38%37%20%20%10%
2112
16th to endNight
40%36%18%24%6%
1893
March1st to 15thNight
37%410%20%19%8%
2082
16th to endNight
36%410%23%20%4
2112
April1st to 15thNight
32%48%23%22%8%
1863
16th to endNight
32%5%9%24%20%8%
1731
May1st to 15thNight
33%6%13%20%22%2
1620
16th to endNight
31%9%12%27%15%3
1684
June1st to 15thNight
30%11%19%26%11%
1620
16th to endNight
30%9%19%20%17%2
1620
July1st to 15thNight
36%10%20%22%8%2
1530
16th to endNight
33%11%22%19%10%1
1595
August1st to 15thNight
35%10%16%23%11%2
1684
16th to endNight
40%9%17%18%12%1
1910
September1st to 15thNight
37%8%13%19%15%4
1947
16th to endNight
37%6%10%21%17%6%
1826
October1st to 15thNight
30%5%8%20%25%8%
1815
16th to endNight
28%35%15%32%14%
2103
November1st to 15thNight
29%26%15%31%13%
2276
16th to endNight
35%37%16%26%10%
2340
December1st to 15thNight
40%38%20%18%7%
2340
16th to endNight
48%47%16%18%5%
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
20%18%36%17%33
90
16th to endDay
17%27%44%7%3
96
February1st to 15thDay
12%30%41%14%2
90
16th to endDay
22%18%44%14%
81
March1st to 15thDay
20%16%36%18%6%1
166
16th to endDay
18%14%30%21%14%1
288
April1st to 15thDay
16%17%26%16%21%1
270
16th to endDay
13%7%28%24%21%5%
270
May1st to 15thDay
15%9%26%19%18%10%
288
16th to endDay
11%419%29%18%16%
331
June1st to 15thDay
12%12%19%26%27%
315
16th to endDay
6%13%25%26%27%
315
July1st to 15thDay
8%16%24%29%29%
305
16th to endDay
8%16%19%34%27%
326
August1st to 15thDay
12%18%23%32%22%
313
16th to endDay
18%17%24%27%21%
332
September1st to 15thDay
14%413%19%26%21%
315
16th to endDay
20%116%27%22%11%
257
October1st to 15thDay
19%421%25%21%8%
255
16th to endDay
16%6%19%24%26%6%
261
November1st to 15thDay
30%15%16%21%13%3
90
16th to endDay
12%13%34%25%12%2
90
December1st to 15thDay
20%24%33%17%4
90
16th to endDay
31%16%29%15%6%1
96
January1st to 15thNight
21%17%30%23%6%
720
16th to endNight
19%18%34%24%3
768
February1st to 15thNight
17%15%34%27%4
704
16th to endNight
17%18%36%23%4
631
March1st to 15thNight
16%15%35%24%7%
720
16th to endNight
14%13%30%27%14%
768
April1st to 15thNight
14%11%30%25%16%1
605
16th to endNight
14%7%27%32%16%1
540
May1st to 15thNight
10%6%26%29%23%3
540
16th to endNight
10%116%31%34%5%
563
June1st to 15thNight
5%111%33%34%13%
540
16th to endNight
310%30%42%12%
540
July1st to 15thNight
7%26%26%41%15%
510
16th to endNight
6%7%25%45%15%
564
August1st to 15thNight
8%6%24%48%11%
539
16th to endNight
14%16%26%39%11%
573
September1st to 15thNight
12%19%23%41%11%
687
16th to endNight
16%114%33%29%3
661
October1st to 15thNight
14%422%29%23%6%
660
16th to endNight
11%8%24%29%21%4
704
November1st to 15thNight
14%8%25%28%20%2
700
16th to endNight
16%13%28%27%13%
720
December1st to 15thNight
16%12%31%26%11%
720
16th to endNight
25%13%29%24%6%
768
Page generated at 2017-09-17 12:05:45. Version ac04

Copyright 2017 A. Danko


Big "A" Observatory Clear Sky Chart | Charts Home