Bell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bell Observatory clear sky chart from 2005-11-06 to 2024-03-23.

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%
24221
20%
80227
15%
24109
51%
24109
24%
51376

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
24%
945
15%
4356
8%
926
33%
926
13%
2892
January11th to 20thNight
20%
908
17%
4250
8%
888
27%
888
10%
3001
January21st to endNight
20%
831
13%
4262
8%
814
32%
814
11%
2874
February1st to 10thNight
22%
760
17%
2280
7%
760
30%
760
17%
1649
February11th to 20thNight
19%
743
17%
2219
8%
740
27%
740
18%
1573
February21st to endNight
17%
628
16%
1884
7%
628
26%
628
17%
1351
March1st to 10thNight
28%
760
20%
2280
10%
760
36%
760
21%
1529
March11th to 20thNight
27%
760
18%
2168
10%
760
34%
760
22%
1480
March21st to endNight
32%
718
15%
2118
9%
718
37%
718
20%
1450
April1st to 10thNight
39%
540
25%
1800
15%
540
53%
540
29%
1045
April11th to 20thNight
37%
533
24%
1775
17%
533
48%
533
25%
1082
April21st to endNight
41%
540
20%
1723
15%
540
48%
540
24%
1106
May1st to 10thNight
40%
540
18%
1620
11%
540
47%
540
26%
1041
May11th to 20thNight
53%
531
17%
1593
12%
531
57%
531
25%
1045
May21st to endNight
64%
582
18%
1744
14%
582
67%
582
31%
1076
June1st to 10thNight
61%
540
19%
1571
15%
540
65%
540
35%
939
June11th to 20thNight
73%
540
27%
1440
21%
540
75%
540
46%
818
June21st to endNight
74%
540
21%
1440
16%
540
75%
540
40%
863
July1st to 10thNight
70%
525
17%
1400
14%
525
70%
525
34%
885
July11th to 20thNight
71%
522
20%
1411
18%
522
72%
522
36%
868
July21st to endNight
70%
591
21%
1672
20%
558
72%
558
34%
987
August1st to 10thNight
74%
537
23%
1639
24%
534
75%
534
37%
948
August11th to 20thNight
73%
534
19%
1776
21%
534
74%
534
31%
1037
August21st to endNight
76%
594
25%
1980
29%
594
77%
594
37%
1064
September1st to 10thNight
67%
558
24%
1818
25%
558
70%
558
30%
1080
September11th to 20thNight
65%
720
28%
1980
24%
720
68%
720
36%
1219
September21st to endNight
61%
720
31%
1980
22%
720
66%
720
39%
1153
October1st to 10thNight
62%
720
34%
1980
27%
720
67%
720
43%
1099
October11th to 20thNight
46%
720
29%
2024
19%
720
54%
720
32%
1247
October21st to endNight
41%
792
26%
2574
21%
792
50%
792
28%
1544
November1st to 10thNight
50%
740
29%
2406
25%
740
59%
740
33%
1342
November11th to 20thNight
39%
760
28%
2470
19%
760
52%
760
29%
1443
November21st to endNight
38%
760
24%
2470
18%
760
48%
760
25%
1545
December1st to 10thNight
28%
760
17%
2470
10%
760
37%
760
17%
1742
December11th to 20thNight
29%
759
20%
2968
13%
758
39%
758
18%
1966
December21st to endNight
24%
970
12%
4686
9%
954
33%
954
10%
3393

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
14%7%36%6%35%5%5%7%39%
79064
Night
15%7%35%5%3445%7%42%
80834
TransparencyDay
43%6%11%22%15%3
78777
Night
43%5%10%22%17%3
80641
SeeingDay
16%10%23%24%18%8%
14331
Night
17%9%24%29%18%2
24237

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 10thDay
14%11%7%8%7%5%5%5%4430%
2527
11th to 20thDay
19%9%49%7%35%33432%
2591
21st to endDay
24%12%8%5%424446%29%
3320
February1st to 10thDay
19%11%47%6%2435%6%34%
1900
11th to 20thDay
20%9%37%5%3445%6%34%
1848
21st to endDay
19%10%46%5%2345%6%36%
1570
March1st to 10thDay
16%10%35%5%3445%5%39%
1900
11th to 20thDay
21%9%46%5%34445%36%
2010
21st to endDay
17%8%47%6%45%45%5%33%
2298
April1st to 10thDay
12%7%35%5%25%5%5%7%44%
2160
11th to 20thDay
15%7%35%5%34446%42%
2134
21st to endDay
16%6%36%6%3445%8%39%
2236
May1st to 10thDay
16%6%26%6%35%5%5%8%39%
2340
11th to 20thDay
15%7%36%7%35%5%5%6%38%
2301
21st to endDay
8%5%36%7%5%6%6%7%10%38%
2531
June1st to 10thDay
9%5%27%5%36%6%6%7%43%
2384
11th to 20thDay
6%4145%46%7%8%11%45%
2520
21st to endDay
6%435%5%46%7%8%11%42%
2520
July1st to 10thDay
10%5%36%7%5%6%6%7%10%34%
2520
11th to 20thDay
8%5%25%7%37%6%7%10%39%
2498
21st to endDay
9%5%36%5%47%7%7%11%37%
2547
August1st to 10thDay
7%6%26%7%46%7%8%10%38%
2284
11th to 20thDay
10%5%36%7%46%7%6%9%38%
2125
21st to endDay
8%4345%45%7%7%9%43%
2376
September1st to 10thDay
12%7%26%6%45%5%6%8%40%
2124
11th to 20thDay
12%5%35%5%35%5%5%6%46%
1980
21st to endDay
11%5%25%42445%6%52%
1980
October1st to 10thDay
10%5%245%335%46%54%
1980
11th to 20thDay
14%4345%33445%52%
1930
21st to endDay
19%5%26%42335%5%45%
1782
November1st to 10thDay
11%426%423347%55%
1672
11th to 20thDay
15%6%25%5%2345%5%47%
1710
21st to endDay
17%8%35%43435%5%45%
1710
December1st to 10thDay
22%9%35%43345%6%36%
1710
11th to 20thDay
16%9%5%6%6%3445%6%35%
2153
21st to endDay
18%13%46%6%35%5%5%7%28%
2893
January1st to 10thNight
17%11%47%5%3445%6%34%
4428
11th to 20thNight
19%11%48%7%4434432%
4372
21st to endNight
24%12%45%424346%33%
4331
February1st to 10thNight
23%9%48%434346%33%
2280
11th to 20thNight
22%8%36%5%34445%37%
2218
21st to endNight
23%9%37%42445%6%32%
1884
March1st to 10thNight
18%8%35%5%35%45%6%40%
2280
11th to 20thNight
19%7%36%5%33447%38%
2168
21st to endNight
17%8%36%6%35%5%47%36%
2118
April1st to 10thNight
11%6%35%4345%5%8%47%
1800
11th to 20thNight
16%5%36%5%2445%7%44%
1775
21st to endNight
13%7%35%5%35%46%8%40%
1723
May1st to 10thNight
14%6%26%5%35%5%5%6%42%
1620
11th to 20thNight
10%7%35%6%46%5%7%9%38%
1592
21st to endNight
4425%7%46%7%6%12%43%
1744
June1st to 10thNight
9%6%3442446%9%48%
1571
11th to 20thNight
432443447%10%54%
1440
21st to endNight
32135%47%7%7%13%49%
1440
July1st to 10thNight
8%5%246%346%7%10%44%
1440
11th to 20thNight
7%425%5%25%5%7%10%47%
1459
21st to endNight
8%425%5%36%6%7%9%45%
1771
August1st to 10thNight
7%425%6%35%45%8%52%
1650
11th to 20thNight
5%5%25%5%45%6%7%10%46%
1776
21st to endNight
6%4245%25%46%11%51%
1980
September1st to 10thNight
12%425%4345%5%6%50%
1818
11th to 20thNight
10%5%36%435%46%8%49%
1980
21st to endNight
12%42442445%6%55%
1980
October1st to 10thNight
9%5%14322346%60%
1980
11th to 20thNight
15%6%25%424336%50%
2024
21st to endNight
15%6%35%42435%5%47%
2574
November1st to 10thNight
11%5%2443335%6%54%
2406
11th to 20thNight
14%5%23434446%49%
2470
21st to endNight
17%7%34434346%46%
2470
December1st to 10thNight
21%11%45%5%4345%6%33%
2470
11th to 20thNight
19%8%47%5%2345%6%38%
3042
21st to endNight
18%11%5%8%7%45%45%5%27%
4760

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
57%37%21%10%1
2528
11th to 20thDay
57%37%21%11%1
2591
21st to endDay
58%47%23%7%1
3320
February1st to 10thDay
52%48%21%14%2
1900
11th to 20thDay
52%47%20%13%3
1858
21st to endDay
49%410%17%14%5%
1570
March1st to 10thDay
47%47%22%18%3
1900
11th to 20thDay
51%410%20%12%3
2010
21st to endDay
52%5%10%20%10%2
2298
April1st to 10thDay
38%6%11%24%18%3
2160
11th to 20thDay
43%48%22%19%3
2134
21st to endDay
44%5%11%24%14%2
2236
May1st to 10thDay
44%6%12%25%12%1
2340
11th to 20thDay
45%6%11%23%14%1
2305
21st to endDay
40%8%16%23%11%2
2531
June1st to 10thDay
38%8%11%22%18%2
2384
11th to 20thDay
30%8%17%24%18%2
2520
21st to endDay
32%9%16%22%16%4
2520
July1st to 10thDay
42%10%13%19%14%3
2451
11th to 20thDay
38%9%16%20%13%3
2423
21st to endDay
37%9%16%24%11%3
2405
August1st to 10thDay
37%9%17%23%14%
2271
11th to 20thDay
41%8%10%23%16%2
2125
21st to endDay
35%9%12%24%16%4
2376
September1st to 10thDay
41%7%10%20%18%3
2124
11th to 20thDay
38%6%9%24%21%3
1980
21st to endDay
33%49%26%23%4
1980
October1st to 10thDay
31%5%8%22%29%4
1980
11th to 20thDay
36%47%21%26%7%
1930
21st to endDay
41%47%23%21%4
1782
November1st to 10thDay
32%38%26%26%5%
1672
11th to 20thDay
38%5%8%23%21%6%
1710
21st to endDay
43%38%24%19%3
1710
December1st to 10thDay
49%410%23%12%1
1710
11th to 20thDay
50%410%21%14%2
2150
21st to endDay
55%5%11%19%9%
2893
January1st to 10thNight
50%5%8%22%13%1
4428
11th to 20thNight
58%27%16%13%3
4372
21st to endNight
55%38%21%12%1
4331
February1st to 10thNight
56%39%19%13%1
2280
11th to 20thNight
52%46%20%15%3
2219
21st to endNight
54%410%14%15%3
1884
March1st to 10thNight
46%47%18%18%6%
2280
11th to 20thNight
47%410%24%12%3
2168
21st to endNight
48%5%11%23%11%2
2118
April1st to 10thNight
35%411%26%20%3
1800
11th to 20thNight
40%410%18%22%6%
1775
21st to endNight
42%6%9%24%17%2
1723
May1st to 10thNight
41%5%11%24%15%2
1620
11th to 20thNight
41%8%14%21%15%1
1593
21st to endNight
32%10%17%23%15%2
1745
June1st to 10thNight
33%6%14%27%17%4
1571
11th to 20thNight
24%6%16%29%22%3
1440
21st to endNight
26%10%19%25%14%7%
1440
July1st to 10thNight
33%8%17%25%14%3
1400
11th to 20thNight
31%9%15%24%16%5%
1411
21st to endNight
31%8%17%28%15%2
1672
August1st to 10thNight
30%5%16%27%19%1
1639
11th to 20thNight
30%10%17%24%17%2
1776
21st to endNight
27%7%14%29%19%5%
1980
September1st to 10thNight
34%7%10%23%21%5%
1818
11th to 20thNight
34%5%8%26%22%4
1980
21st to endNight
30%47%25%26%7%
1980
October1st to 10thNight
27%48%23%31%7%
1980
11th to 20thNight
37%47%21%25%6%
2024
21st to endNight
39%37%24%23%5%
2574
November1st to 10thNight
32%48%25%26%6%
2406
11th to 20thNight
37%37%23%23%7%
2470
21st to endNight
42%47%25%19%4
2470
December1st to 10thNight
53%5%8%20%12%2
2470
11th to 20thNight
48%48%21%16%3
3044
21st to endNight
58%49%18%9%1
4760

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
24%20%31%20%41
270
11th to 20thDay
32%23%29%11%5%
270
21st to endDay
22%22%38%13%4
297
February1st to 10thDay
26%20%36%14%3
270
11th to 20thDay
31%22%31%11%3
270
21st to endDay
31%20%37%10%3
225
March1st to 10thDay
26%23%34%14%3
270
11th to 20thDay
24%19%35%16%6%1
380
21st to endDay
24%19%32%19%7%
470
April1st to 10thDay
17%19%26%29%9%1
420
11th to 20thDay
19%16%28%22%13%2
412
21st to endDay
20%14%33%19%10%4
420
May1st to 10thDay
19%13%27%26%10%4
423
11th to 20thDay
16%9%26%28%13%8%
447
21st to endDay
10%5%19%30%20%16%
485
June1st to 10thDay
11%5%18%26%26%15%
494
11th to 20thDay
5%418%31%29%13%
630
21st to endDay
7%420%34%23%12%
630
July1st to 10thDay
12%313%23%29%20%
621
11th to 20thDay
8%414%27%30%18%
596
21st to endDay
11%213%26%31%18%
488
August1st to 10thDay
11%311%25%34%16%
446
11th to 20thDay
13%313%31%30%10%
439
21st to endDay
11%37%25%34%21%
495
September1st to 10thDay
13%214%26%31%15%
450
11th to 20thDay
12%312%25%34%13%
450
21st to endDay
14%6%19%24%27%10%
450
October1st to 10thDay
11%422%28%27%8%
450
11th to 20thDay
15%14%30%24%12%5%
419
21st to endDay
26%11%26%26%8%2
297
November1st to 10thDay
16%9%32%26%16%2
270
11th to 20thDay
17%17%33%25%8%
270
21st to endDay
24%12%30%22%11%1
270
December1st to 10thDay
23%22%27%19%9%1
270
11th to 20thDay
23%17%30%22%8%
270
21st to endDay
26%20%32%18%4
297
January1st to 10thNight
22%17%32%25%4
950
11th to 20thNight
28%15%31%21%4
908
21st to endNight
23%15%38%21%2
836
February1st to 10thNight
28%13%34%21%4
760
11th to 20thNight
28%16%34%20%2
743
21st to endNight
28%14%38%18%2
628
March1st to 10thNight
24%13%32%26%5%
760
11th to 20thNight
22%14%33%24%6%1
760
21st to endNight
21%12%29%30%7%
718
April1st to 10thNight
14%12%33%32%9%
540
11th to 20thNight
18%10%32%27%12%1
533
21st to endNight
16%9%30%33%10%1
540
May1st to 10thNight
17%6%31%29%15%2
540
11th to 20thNight
12%6%22%37%19%4
531
21st to endNight
5%217%40%31%5%
584
June1st to 10thNight
12%315%37%30%4
540
11th to 20thNight
5%212%42%36%3
540
21st to endNight
4215%39%35%6%
540
July1st to 10thNight
10%29%27%42%10%
525
11th to 20thNight
7%39%27%46%7%
522
21st to endNight
8%210%35%38%7%
591
August1st to 10thNight
9%16%35%42%7%
537
11th to 20thNight
7%19%42%36%5%
534
21st to endNight
8%16%31%47%8%
594
September1st to 10thNight
13%29%31%40%5%
558
11th to 20thNight
12%311%30%39%5%
720
21st to endNight
13%318%31%31%5%
720
October1st to 10thNight
11%418%33%29%4
720
11th to 20thNight
19%7%23%34%16%2
720
21st to endNight
18%9%29%31%14%1
792
November1st to 10thNight
13%8%26%34%17%2
740
11th to 20thNight
17%11%31%29%12%
760
21st to endNight
21%12%26%29%11%1
760
December1st to 10thNight
26%11%30%27%6%
760
11th to 20thNight
23%11%33%27%7%
760
21st to endNight
28%14%29%25%4
973
Page generated at 2024-03-19 22:31:07. Version ac07

Copyright © 2024 Attilla Danko.


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