Bailyhill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bailyhill Observatory clear sky chart from 2005-11-05 to 2017-09-19.

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
31%
14786
14%
46851
8%
14754
38%
14754
17%
32201

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
22%
720
13%
2506
6%
716
31%
716
12%
1732
January16th to endNight
17%
768
12%
2688
3%
768
28%
768
11%
1860
February1st to 15thNight
15%
704
11%
2248
3%
701
25%
701
10%
1646
February16th to endNight
21%
631
15%
1893
6%
631
32%
631
13%
1349
March1st to 15thNight
19%
720
14%
2160
7%
720
29%
720
14%
1542
March16th to endNight
26%
768
20%
2017
7%
768
36%
768
19%
1392
April1st to 15thNight
24%
718
16%
1796
5%
718
30%
718
18%
1315
April16th to endNight
26%
591
16%
1671
6%
591
33%
591
19%
1141
May1st to 15thNight
28%
515
15%
1595
9%
515
36%
515
16%
1135
May16th to endNight
37%
375
10%
1497
11%
374
43%
374
14%
1022
June1st to 15thNight
33%
360
9%
1440
7%
360
42%
360
12%
1030
June16th to endNight
47%
360
9%
1440
11%
360
51%
360
17%
950
July1st to 15thNight
55%
340
11%
1360
17%
340
58%
340
22%
821
July16th to endNight
53%
376
14%
1418
16%
354
60%
354
24%
835
August1st to 15thNight
51%
455
14%
1526
11%
453
54%
453
22%
973
August16th to endNight
53%
720
19%
1866
12%
720
55%
720
33%
1202
September1st to 15thNight
47%
720
17%
1842
9%
720
50%
720
31%
1209
September16th to endNight
52%
673
23%
1858
17%
673
56%
673
33%
1136
October1st to 15thNight
37%
660
18%
1869
10%
660
45%
660
19%
1313
October16th to endNight
26%
704
12%
2288
5%
704
35%
704
14%
1619
November1st to 15thNight
34%
700
18%
2277
12%
700
43%
700
18%
1542
November16th to endNight
27%
720
16%
2388
9%
720
36%
720
15%
1629
December1st to 15thNight
19%
720
10%
2520
4%
720
26%
720
11%
1821
December16th to endNight
14%
768
10%
2688
3%
768
24%
768
8%
1987

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%8%36%6%35%45%6%31%
47972
Night
16%8%36%5%35%45%6%32%
47021
Cloud CoverDay
17%7%36%6%35%5%5%7%32%
47982
Night
16%7%35%5%3445%7%35%
47026
TransparencyDay
49%6%12%19%10%1
47660
Night
47%6%14%20%10%1
46851
SeeingDay
22%15%25%20%11%3
5779
Night
22%15%25%25%9%1
14786

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 15thDay
15%8%47%7%46%6%8%7%23%
1432
16th to endDay
13%9%47%8%35%6%6%7%28%
1536
February1st to 15thDay
13%11%5%9%6%46%5%5%5%24%
1609
16th to endDay
13%12%5%9%7%345%46%27%
1580
March1st to 15thDay
17%9%49%6%25%6%46%26%
1790
16th to endDay
15%9%38%6%24445%34%
2204
April1st to 15thDay
18%9%46%5%24346%33%
2152
16th to endDay
18%8%36%6%34446%34%
2289
May1st to 15thDay
23%11%45%5%23335%29%
2366
16th to endDay
19%7%36%5%25%45%7%32%
2618
June1st to 15thDay
23%8%36%5%35%5%5%7%26%
2520
16th to endDay
19%7%37%6%35%45%7%29%
2520
July1st to 15thDay
14%7%26%6%46%6%6%8%31%
2520
16th to endDay
15%6%25%5%36%6%6%9%32%
2654
August1st to 15thDay
15%6%25%6%5%6%6%6%7%30%
2412
16th to endDay
12%436%5%35%6%6%8%37%
2334
September1st to 15thDay
15%6%245%34446%40%
2118
16th to endDay
14%5%14323345%49%
1848
October1st to 15thDay
21%8%37%5%23335%34%
1760
16th to endDay
20%10%47%5%35%445%27%
1584
November1st to 15thDay
15%7%46%6%25%448%32%
1581
16th to endDay
19%8%37%6%24445%32%
1569
December1st to 15thDay
20%11%37%6%45%446%25%
1440
16th to endDay
20%12%5%8%7%34445%22%
1536
January1st to 15thNight
13%10%38%7%36%5%6%7%26%
2506
16th to endNight
13%10%48%7%46%46%7%27%
2688
February1st to 15thNight
13%9%6%8%8%46%6%6%5%24%
2247
16th to endNight
13%11%37%7%46%5%6%6%27%
1893
March1st to 15thNight
17%9%37%6%35%46%6%29%
2147
16th to endNight
15%7%35%435%45%5%37%
2017
April1st to 15thNight
19%9%36%5%24445%33%
1796
16th to endNight
20%6%36%5%33346%37%
1671
May1st to 15thNight
20%9%45%5%34446%31%
1595
16th to endNight
18%8%36%4345%6%8%31%
1495
June1st to 15thNight
25%6%35%5%2445%7%28%
1440
16th to endNight
15%6%25%5%445%6%7%36%
1440
July1st to 15thNight
15%5%25%3346%5%8%38%
1440
16th to endNight
13%5%35%42446%8%41%
1514
August1st to 15thNight
15%5%25%435%45%8%39%
1536
16th to endNight
13%43442345%9%44%
1866
September1st to 15thNight
15%5%25%5%24337%44%
1842
16th to endNight
13%5%24423347%49%
1858
October1st to 15thNight
19%8%26%434446%35%
1869
16th to endNight
20%10%36%5%3445%5%29%
2288
November1st to 15thNight
14%7%46%6%35%45%6%33%
2277
16th to endNight
18%7%36%6%35%45%6%31%
2388
December1st to 15thNight
20%10%47%6%35%45%6%24%
2520
16th to endNight
18%12%5%8%6%45%46%6%21%
2688

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
15%7%35%6%46%7%7%7%27%
1432
16th to endDay
12%9%36%6%45%5%8%8%30%
1536
February1st to 15thDay
12%10%49%5%46%5%5%6%27%
1609
16th to endDay
13%10%47%8%35%45%6%29%
1580
March1st to 15thDay
16%11%47%6%45%447%28%
1800
16th to endDay
15%8%36%6%34445%36%
2204
April1st to 15thDay
18%7%45%5%34446%34%
2152
16th to endDay
18%6%36%5%45%445%35%
2289
May1st to 15thDay
23%9%47%6%33345%29%
2366
16th to endDay
20%7%25%5%35%36%7%32%
2618
June1st to 15thDay
23%6%36%6%36%45%7%26%
2520
16th to endDay
18%7%36%6%45%5%48%29%
2520
July1st to 15thDay
14%6%26%6%46%6%6%8%31%
2520
16th to endDay
15%426%6%37%6%7%8%32%
2654
August1st to 15thDay
14%7%25%7%46%7%7%6%30%
2412
16th to endDay
11%435%5%36%5%5%9%38%
2334
September1st to 15thDay
15%425%5%245%5%7%40%
2118
16th to endDay
13%5%14423336%50%
1848
October1st to 15thDay
20%9%26%5%33336%34%
1760
16th to endDay
19%8%37%6%5%4445%29%
1584
November1st to 15thDay
16%6%36%6%34448%35%
1581
16th to endDay
20%7%35%6%35%346%34%
1569
December1st to 15thDay
21%8%35%6%45%447%27%
1440
16th to endDay
21%10%37%5%45%6%45%25%
1536
January1st to 15thNight
13%9%36%5%36%6%6%8%31%
2506
16th to endNight
13%9%37%6%35%5%5%8%30%
2688
February1st to 15thNight
12%9%38%7%46%6%5%7%27%
2240
16th to endNight
13%10%345%35%5%7%8%30%
1893
March1st to 15thNight
17%8%37%5%34447%32%
2160
16th to endNight
15%7%35%4245%6%6%38%
2016
April1st to 15thNight
18%7%35%5%34445%35%
1796
16th to endNight
21%6%245%24347%38%
1671
May1st to 15thNight
21%8%25%5%34446%33%
1595
16th to endNight
20%7%35%4345%5%8%32%
1495
June1st to 15thNight
24%6%346%15%45%8%29%
1440
16th to endNight
16%5%25%5%345%6%6%37%
1440
July1st to 15thNight
14%5%244245%6%8%40%
1440
16th to endNight
14%5%16%42446%8%41%
1514
August1st to 15thNight
15%5%25%4245%5%7%40%
1536
16th to endNight
13%42433435%8%46%
1866
September1st to 15thNight
15%5%24423447%46%
1842
16th to endNight
12%5%1332335%8%49%
1858
October1st to 15thNight
19%6%26%5%33446%36%
1869
16th to endNight
20%7%36%4245%5%7%31%
2288
November1st to 15thNight
15%5%35%5%35%5%5%7%35%
2277
16th to endNight
18%6%346%3445%7%35%
2388
December1st to 15thNight
20%8%46%5%3445%7%29%
2520
16th to endNight
18%10%37%6%35%46%7%25%
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%8%14%17%7%
1433
16th to endDay
47%6%17%19%8%
1536
February1st to 15thDay
55%6%13%16%7%
1612
16th to endDay
53%6%11%14%11%1
1580
March1st to 15thDay
54%5%14%14%8%1
1800
16th to endDay
48%5%11%16%14%3
2204
April1st to 15thDay
50%5%9%18%13%2
2152
16th to endDay
48%5%9%20%12%2
2289
May1st to 15thDay
56%48%17%11%1
2366
16th to endDay
49%7%11%18%10%1
2622
June1st to 15thDay
55%7%12%16%7%
2520
16th to endDay
51%7%11%17%10%
2520
July1st to 15thDay
46%9%12%19%11%
2371
16th to endDay
45%8%14%21%9%
2487
August1st to 15thDay
48%8%14%19%7%
2398
16th to endDay
40%6%15%23%12%
2334
September1st to 15thDay
41%7%10%25%14%1
2118
16th to endDay
35%410%30%16%1
1848
October1st to 15thDay
52%49%21%9%1
1760
16th to endDay
54%5%13%16%7%1
1584
November1st to 15thDay
47%5%15%18%10%1
1581
16th to endDay
51%5%14%16%10%1
1569
December1st to 15thDay
56%5%16%14%7%
1440
16th to endDay
59%6%12%12%8%
1536
January1st to 15thNight
49%7%16%17%8%1
2506
16th to endNight
50%6%15%17%9%1
2688
February1st to 15thNight
53%6%14%14%8%
2248
16th to endNight
49%7%13%19%8%1
1893
March1st to 15thNight
51%5%13%19%8%1
2160
16th to endNight
43%6%12%17%15%4
2017
April1st to 15thNight
51%49%19%13%1
1796
16th to endNight
45%5%11%21%13%2
1671
May1st to 15thNight
52%5%8%16%15%1
1595
16th to endNight
49%7%14%17%9%1
1497
June1st to 15thNight
53%7%13%16%8%
1440
16th to endNight
44%8%13%21%10%
1440
July1st to 15thNight
37%8%18%23%11%
1360
16th to endNight
38%6%17%24%12%1
1418
August1st to 15thNight
40%8%14%24%11%
1526
16th to endNight
35%6%14%28%14%
1866
September1st to 15thNight
38%5%13%28%13%
1842
16th to endNight
33%5%11%30%17%
1858
October1st to 15thNight
48%6%11%20%11%2
1869
16th to endNight
50%7%13%20%7%
2288
November1st to 15thNight
46%6%15%19%10%1
2277
16th to endNight
48%6%15%20%8%1
2388
December1st to 15thNight
54%7%16%17%4
2520
16th to endNight
56%7%14%14%6%
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
31%16%28%16%5%1
60
16th to endDay
25%21%31%17%4
64
February1st to 15thDay
27%23%28%17%2
170
16th to endDay
20%27%25%20%6%
239
March1st to 15thDay
29%19%25%20%5%
270
16th to endDay
24%23%23%15%8%3
288
April1st to 15thDay
31%15%24%19%7%2
270
16th to endDay
23%21%28%12%8%4
270
May1st to 15thDay
34%19%20%12%7%5%
288
16th to endDay
25%14%30%22%6%1
331
June1st to 15thDay
26%15%29%18%9%
315
16th to endDay
20%15%28%22%11%2
315
July1st to 15thDay
19%8%33%21%13%3
305
16th to endDay
14%8%31%29%12%3
326
August1st to 15thDay
16%14%24%22%16%5%
313
16th to endDay
14%9%22%23%19%10%
332
September1st to 15thDay
16%9%25%20%20%7%
315
16th to endDay
18%11%18%19%16%15%
266
October1st to 15thDay
22%15%19%20%15%5%
254
16th to endDay
25%18%20%26%8%
240
November1st to 15thDay
20%14%26%24%12%1
235
16th to endDay
28%19%23%16%11%
189
December1st to 15thDay
23%21%33%18%3
60
16th to endDay
40%15%32%10%
64
January1st to 15thNight
22%20%29%23%3
720
16th to endNight
22%20%30%22%3
768
February1st to 15thNight
20%22%33%21%2
704
16th to endNight
23%21%27%22%4
631
March1st to 15thNight
26%19%29%21%3
720
16th to endNight
23%19%26%22%8%1
768
April1st to 15thNight
22%21%26%24%4
718
16th to endNight
24%19%26%20%8%
591
May1st to 15thNight
24%13%26%26%7%
515
16th to endNight
21%9%25%31%10%1
375
June1st to 15thNight
27%10%21%30%9%
360
16th to endNight
18%424%40%11%
360
July1st to 15thNight
15%418%37%22%1
340
16th to endNight
14%420%39%20%1
376
August1st to 15thNight
18%319%38%18%1
455
16th to endNight
15%419%30%25%3
720
September1st to 15thNight
18%8%20%31%19%2
720
16th to endNight
15%9%17%30%21%5%
673
October1st to 15thNight
24%13%19%25%14%1
660
16th to endNight
25%17%26%23%6%
704
November1st to 15thNight
19%15%25%28%9%1
700
16th to endNight
23%19%24%22%9%1
720
December1st to 15thNight
28%18%28%20%4
720
16th to endNight
28%22%28%17%2
768
Page generated at 2017-09-20 04:49:54. Version ac04

Copyright 2017 A. Danko


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