Bailyhill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bailyhill Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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
29%
22751
14%
74674
8%
22707
37%
22707
16%
51421

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
19%
760
12%
2649
5%
757
28%
757
11%
1823
January11th to 20thNight
17%
729
10%
2549
3%
729
25%
729
8%
1875
January21st to endNight
16%
792
13%
2772
4%
792
28%
792
11%
1890
February1st to 10thNight
13%
720
11%
2364
3%
720
24%
720
11%
1708
February11th to 20thNight
15%
703
14%
2100
3%
700
30%
700
13%
1472
February21st to endNight
21%
592
16%
1776
5%
592
33%
592
14%
1238
March1st to 10thNight
18%
720
16%
2160
7%
720
30%
720
15%
1521
March11th to 20thNight
22%
720
14%
2089
6%
720
30%
720
14%
1499
March21st to endNight
24%
792
20%
2018
6%
792
34%
792
18%
1428
April1st to 10thNight
23%
720
16%
1800
4%
720
29%
720
17%
1322
April11th to 20thNight
26%
698
15%
1762
5%
698
32%
698
19%
1264
April21st to endNight
22%
540
15%
1620
6%
540
29%
540
16%
1138
May1st to 10thNight
25%
540
12%
1620
7%
540
30%
540
15%
1186
May11th to 20thNight
31%
407
17%
1469
13%
407
41%
407
16%
982
May21st to endNight
40%
389
12%
1551
13%
388
47%
388
17%
1006
June1st to 10thNight
36%
360
11%
1440
10%
360
43%
360
14%
987
June11th to 20thNight
40%
360
11%
1440
12%
360
47%
360
16%
947
June21st to endNight
44%
360
10%
1440
13%
360
49%
360
17%
956
July1st to 10thNight
50%
350
12%
1400
16%
350
55%
350
21%
856
July11th to 20thNight
52%
348
13%
1392
18%
348
57%
348
23%
827
July21st to endNight
51%
394
17%
1486
18%
372
58%
372
24%
875
August1st to 10thNight
56%
411
16%
1475
14%
409
60%
409
23%
905
August11th to 20thNight
45%
553
16%
1617
11%
553
47%
553
25%
1076
August21st to endNight
50%
792
19%
1980
9%
792
52%
792
31%
1318
September1st to 10thNight
47%
720
17%
1800
7%
720
51%
720
34%
1152
September11th to 20thNight
46%
720
22%
1952
14%
720
51%
720
30%
1246
September21st to endNight
44%
720
22%
1980
14%
720
50%
720
29%
1280
October1st to 10thNight
39%
720
16%
1980
9%
720
45%
720
20%
1417
October11th to 20thNight
33%
720
16%
2245
8%
720
43%
720
18%
1540
October21st to endNight
24%
792
12%
2574
7%
792
32%
792
13%
1878
November1st to 10thNight
31%
740
18%
2407
12%
740
41%
740
18%
1594
November11th to 20thNight
26%
760
19%
2470
11%
760
36%
760
16%
1636
November21st to endNight
23%
760
14%
2546
6%
760
33%
760
14%
1759
December1st to 10thNight
15%
760
10%
2660
3%
760
23%
760
9%
1995
December11th to 20thNight
18%
760
10%
3238
4%
759
26%
759
9%
2258
December21st to endNight
14%
829
7%
4853
2%
817
24%
817
5%
3567

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
17%9%47%6%35%5%5%7%32%
74792
Night
17%9%47%6%35%5%5%7%32%
74926
TransparencyDay
48%7%13%19%11%1
74497
Night
48%7%14%20%10%1
74755
SeeingDay
24%19%28%19%8%2
13734
Night
23%16%26%26%9%1
22758

By Month

Percentage Cloud Cover NE (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
14%11%5%9%8%35%6%7%7%25%
1517
11th to 20thDay
17%10%47%8%46%5%6%6%26%
1456
21st to endDay
11%10%6%9%8%35%6%7%6%29%
1584
February1st to 10thDay
16%12%5%8%7%5%7%5%5%7%25%
1589
11th to 20thDay
11%11%5%11%7%45%6%5%6%28%
1748
21st to endDay
16%12%49%7%45%5%46%29%
1480
March1st to 10thDay
13%10%49%7%46%5%5%6%30%
1797
11th to 20thDay
16%11%5%7%6%346%46%32%
1870
21st to endDay
17%10%48%7%35%445%33%
2335
April1st to 10thDay
19%11%5%7%5%35%446%31%
2160
11th to 20thDay
16%9%36%6%35%5%5%6%35%
2146
21st to endDay
22%10%48%7%35%45%6%27%
2340
May1st to 10thDay
22%11%46%6%34346%30%
2340
11th to 20thDay
20%8%36%5%345%46%36%
2440
21st to endDay
15%7%36%6%35%5%6%7%37%
2725
June1st to 10thDay
20%8%46%7%46%6%5%7%28%
2520
11th to 20thDay
18%7%35%5%35%5%6%8%35%
2520
21st to endDay
20%7%38%7%46%5%5%7%29%
2520
July1st to 10thDay
16%7%36%5%35%5%7%8%35%
2520
11th to 20thDay
14%5%26%7%46%6%5%9%35%
2520
21st to endDay
15%6%25%5%5%6%6%7%9%34%
2744
August1st to 10thDay
14%6%25%7%46%7%7%7%34%
2435
11th to 20thDay
14%7%36%6%45%6%7%8%32%
2280
21st to endDay
13%435%5%35%6%6%9%40%
2376
September1st to 10thDay
13%5%346%46%5%6%7%42%
2160
11th to 20thDay
15%6%25%433345%49%
2008
21st to endDay
16%6%25%43445%8%42%
1980
October1st to 10thDay
23%9%36%6%34446%32%
1980
11th to 20thDay
15%9%47%6%35%446%37%
1711
21st to endDay
26%11%47%5%35%445%26%
1782
November1st to 10thDay
13%8%47%7%35%35%8%37%
1671
11th to 20thDay
18%8%47%6%35%5%5%6%32%
1710
21st to endDay
20%9%38%7%35%5%46%29%
1630
December1st to 10thDay
21%12%410%7%5%5%446%22%
1520
11th to 20thDay
21%13%47%324346%30%
1906
21st to endDay
24%12%49%8%44435%22%
2772
January1st to 10thNight
14%10%49%8%46%6%6%8%25%
2649
11th to 20thNight
15%13%49%8%46%46%6%25%
2549
21st to endNight
11%10%5%9%8%46%6%6%7%29%
2772
February1st to 10thNight
15%10%5%8%8%45%6%6%7%25%
2364
11th to 20thNight
12%9%48%8%5%7%6%7%7%27%
2098
21st to endNight
12%11%49%8%45%6%6%7%29%
1776
March1st to 10thNight
13%11%5%8%7%46%5%6%7%29%
2150
11th to 20thNight
17%10%47%7%46%45%7%29%
2088
21st to endNight
17%9%36%5%345%5%6%36%
2018
April1st to 10thNight
20%9%47%7%35%445%33%
1800
11th to 20thNight
20%9%37%5%35%5%47%34%
1762
21st to endNight
20%9%48%6%43346%33%
1620
May1st to 10thNight
22%11%46%5%345%47%29%
1620
11th to 20thNight
18%7%35%5%45%5%5%6%35%
1468
21st to endNight
16%7%36%5%345%7%8%36%
1550
June1st to 10thNight
21%7%37%6%4447%8%30%
1440
11th to 20thNight
19%7%36%5%35%45%6%37%
1440
21st to endNight
16%8%36%6%45%6%6%7%36%
1440
July1st to 10thNight
16%6%25%435%6%6%8%39%
1440
11th to 20thNight
15%5%35%5%245%6%8%41%
1440
21st to endNight
14%6%35%43446%9%42%
1574
August1st to 10thNight
15%5%25%435%5%6%9%42%
1485
11th to 20thNight
14%8%25%6%445%6%8%39%
1617
21st to endNight
16%6%35%43445%9%43%
1980
September1st to 10thNight
13%5%35%5%2345%7%47%
1800
11th to 20thNight
15%6%25%5%23447%47%
1952
21st to endNight
17%7%2443445%7%44%
1980
October1st to 10thNight
22%7%36%5%3445%6%35%
1980
11th to 20thNight
16%11%36%6%35%5%5%6%34%
2245
21st to endNight
24%11%36%6%35%45%6%27%
2574
November1st to 10thNight
14%8%48%6%35%5%5%5%36%
2407
11th to 20thNight
17%7%5%7%7%4445%7%32%
2470
21st to endNight
17%9%47%7%36%5%5%6%30%
2546
December1st to 10thNight
19%12%5%10%7%46%5%5%6%22%
2660
11th to 20thNight
21%12%5%7%5%35%46%6%25%
3288
21st to endNight
23%13%5%10%6%4445%420%
4884

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
51%7%14%19%8%
1518
11th to 20thDay
53%7%14%16%8%2
1456
21st to endDay
49%6%14%19%10%2
1584
February1st to 10thDay
55%6%13%16%8%2
1589
11th to 20thDay
52%6%12%18%9%2
1757
21st to endDay
54%7%11%15%12%1
1480
March1st to 10thDay
51%6%15%16%10%3
1800
11th to 20thDay
49%6%13%15%13%3
1870
21st to endDay
51%5%10%17%14%4
2335
April1st to 10thDay
52%5%10%19%11%3
2160
11th to 20thDay
46%7%12%20%14%1
2146
21st to endDay
55%6%9%16%11%2
2340
May1st to 10thDay
55%5%8%20%11%2
2340
11th to 20thDay
47%5%11%20%15%2
2444
21st to endDay
44%9%14%21%11%2
2725
June1st to 10thDay
53%8%13%17%9%
2520
11th to 20thDay
45%8%12%19%14%2
2520
21st to endDay
52%9%12%17%9%1
2520
July1st to 10thDay
43%9%12%21%15%1
2450
11th to 20thDay
44%9%15%23%8%1
2445
21st to endDay
43%9%14%23%11%1
2591
August1st to 10thDay
44%9%16%21%10%
2421
11th to 20thDay
45%9%15%22%9%
2280
21st to endDay
38%7%16%23%14%1
2376
September1st to 10thDay
37%7%13%28%14%1
2160
11th to 20thDay
36%6%9%29%18%2
2008
21st to endDay
38%6%14%26%13%2
1980
October1st to 10thDay
52%6%10%19%11%2
1980
11th to 20thDay
46%6%14%25%7%1
1711
21st to endDay
58%6%11%16%8%2
1782
November1st to 10thDay
43%6%15%21%14%1
1671
11th to 20thDay
47%6%14%18%13%2
1710
21st to endDay
50%7%20%17%5%
1630
December1st to 10thDay
61%5%14%14%5%1
1520
11th to 20thDay
52%7%12%16%12%1
1906
21st to endDay
61%7%11%13%8%
2772
January1st to 10thNight
49%7%17%17%8%1
2649
11th to 20thNight
56%7%15%15%7%1
2549
21st to endNight
48%7%14%18%10%2
2772
February1st to 10thNight
53%7%14%16%9%2
2364
11th to 20thNight
48%7%15%19%10%1
2100
21st to endNight
48%7%15%19%9%2
1776
March1st to 10thNight
48%7%14%19%10%2
2160
11th to 20thNight
50%7%13%18%10%3
2089
21st to endNight
46%6%10%19%16%4
2018
April1st to 10thNight
51%5%11%19%12%2
1800
11th to 20thNight
47%6%10%23%13%1
1762
21st to endNight
49%6%12%18%12%3
1620
May1st to 10thNight
54%5%10%17%13%1
1620
11th to 20thNight
48%6%10%19%14%3
1469
21st to endNight
44%8%15%20%11%2
1551
June1st to 10thNight
50%8%13%19%10%
1440
11th to 20thNight
46%8%14%19%12%2
1440
21st to endNight
45%9%14%21%11%
1440
July1st to 10thNight
39%9%17%21%14%1
1400
11th to 20thNight
37%8%18%24%12%
1392
21st to endNight
37%7%16%27%11%2
1486
August1st to 10thNight
37%10%15%26%12%1
1475
11th to 20thNight
41%8%16%22%12%
1617
21st to endNight
38%7%14%27%14%
1980
September1st to 10thNight
34%6%17%28%14%1
1800
11th to 20thNight
36%7%12%28%16%1
1952
21st to endNight
38%6%13%26%15%1
1980
October1st to 10thNight
48%7%11%21%10%2
1980
11th to 20thNight
46%8%14%23%8%1
2245
21st to endNight
53%7%13%17%8%1
2574
November1st to 10thNight
44%7%15%21%11%2
2407
11th to 20thNight
45%6%15%21%12%1
2470
21st to endNight
48%8%17%20%7%
2546
December1st to 10thNight
57%8%14%16%4
2660
11th to 20thNight
52%7%15%16%9%1
3288
21st to endNight
61%6%12%16%4
4884

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
28%19%33%16%31
180
11th to 20thDay
32%27%27%11%21
164
21st to endDay
20%28%34%13%5%1
176
February1st to 10thDay
35%28%28%7%2
254
11th to 20thDay
29%27%28%13%2
416
21st to endDay
32%25%25%15%3
346
March1st to 10thDay
27%20%35%14%4
420
11th to 20thDay
33%26%24%12%41
420
21st to endDay
30%23%24%15%6%3
462
April1st to 10thDay
35%20%22%15%5%2
420
11th to 20thDay
30%24%22%15%7%2
412
21st to endDay
32%23%27%11%5%2
420
May1st to 10thDay
33%23%24%12%5%3
423
11th to 20thDay
22%21%32%16%6%2
447
21st to endDay
21%16%31%22%8%1
485
June1st to 10thDay
23%15%36%17%9%1
450
11th to 20thDay
19%19%30%18%10%3
450
21st to endDay
19%18%31%20%10%2
450
July1st to 10thDay
18%12%33%22%12%3
445
11th to 20thDay
15%9%32%30%12%2
444
21st to endDay
14%11%36%27%9%3
488
August1st to 10thDay
12%14%30%26%15%3
446
11th to 20thDay
17%12%28%23%14%6%
439
21st to endDay
13%13%29%25%14%5%
495
September1st to 10thDay
17%10%27%24%17%5%
450
11th to 20thDay
17%15%26%20%15%7%
450
21st to endDay
17%17%26%22%10%7%
450
October1st to 10thDay
27%15%22%21%12%4
450
11th to 20thDay
24%19%22%23%10%2
403
21st to endDay
37%17%21%19%7%
396
November1st to 10thDay
23%18%24%24%11%1
365
11th to 20thDay
28%21%26%18%6%1
370
21st to endDay
31%21%26%17%4
290
December1st to 10thDay
29%24%27%16%3
180
11th to 20thDay
36%21%32%9%2
180
21st to endDay
33%22%26%16%31
198
January1st to 10thNight
24%21%29%23%3
760
11th to 20thNight
27%20%26%24%3
729
21st to endNight
17%25%33%21%4
792
February1st to 10thNight
23%21%34%19%2
720
11th to 20thNight
22%24%32%20%2
703
21st to endNight
25%20%28%24%3
592
March1st to 10thNight
25%18%32%22%3
720
11th to 20thNight
26%21%25%22%5%
720
21st to endNight
27%18%25%22%7%1
792
April1st to 10thNight
27%20%27%23%4
720
11th to 20thNight
22%22%26%23%7%
698
21st to endNight
28%17%29%22%5%
540
May1st to 10thNight
29%14%26%24%7%
540
11th to 20thNight
18%16%29%29%7%
407
21st to endNight
21%8%24%35%11%1
389
June1st to 10thNight
24%9%23%33%10%1
360
11th to 20thNight
20%9%25%33%12%
360
21st to endNight
17%7%24%41%11%1
360
July1st to 10thNight
15%5%22%41%17%
350
11th to 20thNight
16%421%37%20%2
348
21st to endNight
14%5%22%41%17%1
394
August1st to 10thNight
16%318%44%19%
411
11th to 20thNight
15%6%26%32%18%2
553
21st to endNight
17%7%19%31%23%2
792
September1st to 10thNight
15%8%23%32%20%2
720
11th to 20thNight
17%11%21%28%20%3
720
21st to endNight
18%12%20%30%17%2
720
October1st to 10thNight
25%11%21%27%15%2
720
11th to 20thNight
22%15%24%27%11%1
720
21st to endNight
29%16%25%24%6%
792
November1st to 10thNight
20%15%26%29%9%1
740
11th to 20thNight
23%20%26%23%7%1
760
21st to endNight
23%20%26%24%6%1
760
December1st to 10thNight
26%20%31%19%4
760
11th to 20thNight
29%19%28%20%3
760
21st to endNight
28%22%28%19%3
836
Page generated at 2024-01-10 01:25:00. Version ac07

Copyright 2024 Attilla Danko


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