Birmingham Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Birmingham Astronomical 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
44%
15760
22%
46828
15%
15714
51%
15714
28%
29493

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
24%
900
19%
2506
7%
895
34%
895
21%
1715
January16th to endNight
25%
960
19%
2532
7%
960
32%
960
24%
1759
February1st to 15thNight
23%
744
17%
2152
6%
741
31%
741
19%
1510
February16th to endNight
24%
631
19%
1893
7%
631
33%
631
20%
1274
March1st to 15thNight
25%
720
17%
2160
8%
720
32%
720
20%
1472
March16th to endNight
35%
588
17%
1996
12%
588
41%
588
19%
1315
April1st to 15thNight
34%
539
22%
1797
13%
539
48%
539
26%
1088
April16th to endNight
43%
540
21%
1800
16%
540
52%
540
25%
1136
May1st to 15thNight
45%
539
19%
1646
13%
539
51%
539
25%
1076
May16th to endNight
58%
563
19%
1564
15%
561
60%
561
30%
1020
June1st to 15thNight
65%
540
22%
1440
17%
540
68%
540
41%
856
June16th to endNight
72%
540
25%
1440
18%
540
75%
540
44%
813
July1st to 15thNight
70%
510
20%
1360
17%
510
71%
510
40%
819
July16th to endNight
74%
564
25%
1429
21%
531
77%
531
43%
833
August1st to 15thNight
80%
539
23%
1763
27%
536
80%
536
40%
928
August16th to endNight
74%
573
26%
1910
30%
573
76%
573
37%
1030
September1st to 15thNight
65%
540
27%
1800
29%
540
68%
540
32%
1034
September16th to endNight
61%
652
30%
1812
22%
652
65%
652
37%
1072
October1st to 15thNight
57%
660
31%
1897
23%
660
63%
660
36%
1102
October16th to endNight
39%
704
27%
2173
19%
704
49%
704
30%
1304
November1st to 15thNight
45%
700
31%
2276
24%
700
56%
700
34%
1264
November16th to endNight
32%
720
24%
2340
16%
720
43%
720
26%
1432
December1st to 15thNight
29%
834
17%
2454
10%
834
36%
834
19%
1723
December16th to endNight
23%
960
18%
2688
7%
960
34%
960
21%
1918

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
12%5%25%5%3445%7%42%
47918
Night
13%5%24424446%45%
47005
TransparencyDay
41%5%11%24%14%2
47598
Night
39%410%23%17%3
46830
SeeingDay
13%5%21%25%22%11%
5798
Night
16%7%26%30%17%2
15761

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
19%7%47%5%35%34435%
1432
16th to endDay
12%9%37%5%24345%39%
1689
February1st to 15thDay
15%9%47%5%23445%38%
1708
16th to endDay
12%7%47%6%35%445%37%
1580
March1st to 15thDay
19%7%35%424345%39%
1800
16th to endDay
16%8%46%6%3445%6%33%
2233
April1st to 15thDay
13%6%2442445%7%45%
2151
16th to endDay
18%5%35%5%2445%6%37%
2160
May1st to 15thDay
18%5%245%35%5%5%8%35%
2304
16th to endDay
10%6%26%6%345%6%9%38%
2548
June1st to 15thDay
7%3146%35%5%5%9%45%
2520
16th to endDay
5%3245%36%6%6%9%45%
2520
July1st to 15thDay
10%3235%36%5%7%9%41%
2520
16th to endDay
42236%36%6%8%11%43%
2639
August1st to 15thDay
6%425%6%35%5%5%9%45%
2186
16th to endDay
7%4245%35%6%6%10%44%
2292
September1st to 15thDay
11%5%35%5%25%447%43%
2152
16th to endDay
10%5%25%5%235%47%47%
1815
October1st to 15thDay
12%42442245%6%50%
1731
16th to endDay
14%413314336%52%
1697
November1st to 15thDay
11%424425%345%49%
1582
16th to endDay
20%6%25%423335%43%
1620
December1st to 15thDay
18%9%35%5%34346%35%
1503
16th to endDay
17%10%45%5%24347%35%
1536
January1st to 15thNight
17%7%36%5%33345%39%
2506
16th to endNight
14%8%35%424446%40%
2532
February1st to 15thNight
17%8%47%423335%37%
2143
16th to endNight
13%8%36%5%34435%40%
1893
March1st to 15thNight
17%7%35%43444440%
2160
16th to endNight
17%6%35%435%35%7%38%
1996
April1st to 15thNight
12%6%145%24445%47%
1797
16th to endNight
17%5%24433447%42%
1800
May1st to 15thNight
15%5%36%5%24447%42%
1646
16th to endNight
6%426%6%36%5%6%9%42%
1563
June1st to 15thNight
6%3244245%49%51%
1440
16th to endNight
42133345%6%12%51%
1440
July1st to 15thNight
8%31442436%7%53%
1440
16th to endNight
6%214435%5%5%10%50%
1526
August1st to 15thNight
5%21332446%10%55%
1773
16th to endNight
5%4135%2345%10%52%
1910
September1st to 15thNight
9%5%245%2445%6%50%
1800
16th to endNight
11%314314436%53%
1812
October1st to 15thNight
10%324433345%54%
1897
16th to endNight
18%5%2331223450%
2173
November1st to 15thNight
11%423313446%55%
2276
16th to endNight
16%6%25%322336%46%
2340
December1st to 15thNight
21%8%35%423346%35%
2454
16th to endNight
19%9%345%24345%36%
2688

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
51%49%21%11%1
1433
16th to endDay
47%411%27%9%
1689
February1st to 15thDay
48%410%23%11%1
1710
16th to endDay
49%312%20%12%1
1580
March1st to 15thDay
49%48%22%12%1
1800
16th to endDay
51%6%13%20%7%
2235
April1st to 15thDay
38%411%25%17%1
2151
16th to endDay
45%5%10%25%11%1
2160
May1st to 15thDay
46%7%10%23%10%
2304
16th to endDay
41%7%15%20%12%3
2552
June1st to 15thDay
33%7%12%26%18%2
2520
16th to endDay
32%7%14%21%20%3
2520
July1st to 15thDay
38%6%12%22%14%5%
2371
16th to endDay
30%8%15%28%14%1
2473
August1st to 15thDay
35%8%12%25%15%2
2172
16th to endDay
34%6%12%23%17%5%
2292
September1st to 15thDay
39%5%8%22%18%4
2152
16th to endDay
35%5%7%24%22%4
1815
October1st to 15thDay
33%49%27%20%5%
1731
16th to endDay
34%38%30%19%3
1697
November1st to 15thDay
37%39%21%20%7%
1582
16th to endDay
44%27%27%14%2
1620
December1st to 15thDay
50%411%20%11%
1503
16th to endDay
50%312%22%10%1
1536
January1st to 15thNight
47%39%22%16%1
2506
16th to endNight
44%5%12%22%13%1
2532
February1st to 15thNight
50%48%23%12%1
2152
16th to endNight
46%410%23%13%1
1893
March1st to 15thNight
47%49%19%15%4
2160
16th to endNight
47%414%21%11%1
1996
April1st to 15thNight
38%49%24%19%3
1797
16th to endNight
41%5%7%26%15%3
1800
May1st to 15thNight
42%5%10%24%15%1
1647
16th to endNight
37%7%14%20%15%4
1565
June1st to 15thNight
29%5%13%27%22%2
1440
16th to endNight
24%6%14%25%22%6%
1440
July1st to 15thNight
31%413%27%16%7%
1360
16th to endNight
27%5%13%33%17%2
1429
August1st to 15thNight
23%7%16%25%23%3
1763
16th to endNight
28%5%13%24%22%5%
1910
September1st to 15thNight
34%46%27%19%7%
1800
16th to endNight
32%37%22%26%7%
1812
October1st to 15thNight
32%37%21%27%6%
1897
16th to endNight
38%26%28%18%5%
2173
November1st to 15thNight
31%48%23%25%7%
2276
16th to endNight
40%38%25%20%2
2340
December1st to 15thNight
51%39%20%13%1
2454
16th to endNight
50%410%21%11%2
2688

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
28%11%38%16%22
90
16th to endDay
15%17%47%14%4
96
February1st to 15thDay
20%18%42%15%3
90
16th to endDay
20%17%33%23%4
81
March1st to 15thDay
28%11%41%13%41
90
16th to endDay
20%10%33%22%12%
222
April1st to 15thDay
15%15%28%26%13%1
270
16th to endDay
22%5%27%24%15%4
270
May1st to 15thDay
18%10%28%24%13%5%
288
16th to endDay
15%425%25%19%9%
448
June1st to 15thDay
9%218%24%32%12%
495
16th to endDay
6%423%35%21%7%
495
July1st to 15thDay
14%114%32%22%14%
475
16th to endDay
5%17%27%35%22%
515
August1st to 15thDay
12%18%18%33%25%
339
16th to endDay
12%7%24%35%18%
332
September1st to 15thDay
10%310%22%35%18%
315
16th to endDay
15%310%24%28%17%
254
October1st to 15thDay
9%121%32%23%10%
171
16th to endDay
14%5%23%34%21%
96
November1st to 15thDay
47%26%24%23%13%
90
16th to endDay
40%10%22%16%10%1
90
December1st to 15thDay
15%8%38%22%10%4
90
16th to endDay
21%17%46%10%3
96
January1st to 15thNight
23%16%33%22%4
900
16th to endNight
19%12%39%25%3
960
February1st to 15thNight
22%13%35%26%2
744
16th to endNight
18%14%38%26%1
631
March1st to 15thNight
21%12%37%24%4
720
16th to endNight
19%8%31%28%11%
588
April1st to 15thNight
16%11%34%26%10%
539
16th to endNight
20%5%26%32%14%1
540
May1st to 15thNight
17%424%37%14%1
540
16th to endNight
9%122%38%25%3
563
June1st to 15thNight
8%117%39%30%3
540
16th to endNight
5%213%46%30%2
540
July1st to 15thNight
10%9%38%36%4
510
16th to endNight
7%8%31%41%9%
564
August1st to 15thNight
7%5%32%45%8%
539
16th to endNight
7%8%33%42%7%
573
September1st to 15thNight
13%11%27%37%9%
540
16th to endNight
13%215%30%30%8%
652
October1st to 15thNight
11%419%32%25%6%
660
16th to endNight
20%5%30%28%13%
704
November1st to 15thNight
12%8%29%34%14%1
700
16th to endNight
22%9%31%24%11%
720
December1st to 15thNight
25%12%28%25%6%1
834
16th to endNight
24%9%37%24%3
960
Page generated at 2017-09-16 16:06:58. Version ac04

Copyright 2017 A. Danko


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