Bell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bell Observatory clear sky chart from 2005-11-05 to 2018-02-14.

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
46%
16174
22%
48655
16%
16130
52%
16130
27%
30793

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
25%
975
17%
2716
8%
970
33%
970
20%
1913
January16th to endNight
25%
868
20%
2655
10%
868
34%
868
20%
1792
February1st to 15thNight
22%
760
16%
2280
7%
757
30%
757
18%
1613
February16th to endNight
22%
631
18%
1893
9%
631
30%
631
19%
1318
March1st to 15thNight
29%
720
19%
2149
11%
720
35%
720
21%
1460
March16th to endNight
34%
712
17%
2056
11%
712
39%
712
21%
1404
April1st to 15thNight
40%
539
24%
1797
16%
539
53%
539
29%
1045
April16th to endNight
42%
540
22%
1747
16%
540
50%
540
26%
1089
May1st to 15thNight
46%
539
17%
1619
11%
539
51%
539
24%
1079
May16th to endNight
64%
561
18%
1683
15%
561
66%
561
31%
1031
June1st to 15thNight
69%
540
22%
1527
17%
540
73%
540
41%
887
June16th to endNight
77%
540
22%
1440
18%
540
78%
540
41%
860
July1st to 15thNight
68%
510
16%
1360
13%
510
69%
510
34%
855
July16th to endNight
74%
564
21%
1550
20%
531
76%
531
37%
895
August1st to 15thNight
76%
539
22%
1696
23%
536
77%
536
38%
958
August16th to endNight
75%
573
24%
1910
27%
573
76%
573
36%
1031
September1st to 15thNight
64%
612
23%
1872
23%
612
66%
612
31%
1136
September16th to endNight
64%
717
30%
1980
25%
717
67%
717
36%
1199
October1st to 15thNight
58%
720
33%
1980
25%
720
63%
720
37%
1174
October16th to endNight
47%
768
29%
2406
23%
768
55%
768
32%
1401
November1st to 15thNight
48%
760
29%
2471
24%
760
57%
760
32%
1409
November16th to endNight
40%
780
26%
2535
21%
780
50%
780
26%
1535
December1st to 15thNight
30%
780
17%
2535
12%
780
37%
780
18%
1755
December16th to endNight
25%
926
18%
2798
9%
926
35%
926
19%
1954

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%6%25%5%345%5%7%40%
49408
Night
13%6%25%42445%7%43%
48843
TransparencyDay
42%5%11%21%15%2
49098
Night
40%410%22%17%3
48657
SeeingDay
15%6%20%23%21%12%
6102
Night
17%7%23%29%20%2
16177

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
21%7%36%5%44445%32%
1552
16th to endDay
15%8%36%5%33336%40%
1913
February1st to 15thDay
17%10%47%5%2435%6%34%
1883
16th to endDay
15%10%37%42345%7%34%
1580
March1st to 15thDay
20%8%345%2344436%
1809
16th to endDay
16%8%36%5%45%45%5%34%
2177
April1st to 15thDay
12%6%35%424447%43%
2151
16th to endDay
15%5%25%6%34447%40%
2211
May1st to 15thDay
17%5%25%5%3445%7%36%
2331
16th to endDay
9%5%36%6%45%6%6%8%38%
2431
June1st to 15thDay
7%316%437%6%6%8%42%
2430
16th to endDay
5%3246%46%7%7%10%40%
2520
July1st to 15thDay
11%426%6%36%6%6%8%35%
2520
16th to endDay
8%425%5%47%7%8%10%36%
2510
August1st to 15thDay
7%5%26%7%45%6%7%9%39%
2250
16th to endDay
8%4246%35%6%7%9%40%
2292
September1st to 15thDay
12%6%26%6%45%5%5%6%37%
2076
16th to endDay
11%5%25%324446%49%
1980
October1st to 15thDay
12%4234335%35%51%
1980
16th to endDay
15%415%413335%51%
1814
November1st to 15thDay
13%425%413346%48%
1717
16th to endDay
16%6%33323245%48%
1755
December1st to 15thDay
21%7%35%423345%36%
1755
16th to endDay
19%10%35%5%23346%34%
1771
January1st to 15thNight
17%9%36%433345%36%
2716
16th to endNight
18%8%36%424335%38%
2655
February1st to 15thNight
21%8%37%423345%35%
2271
16th to endNight
18%8%36%42445%5%35%
1893
March1st to 15thNight
18%7%35%5%34346%38%
2149
16th to endNight
16%7%36%5%24447%36%
2056
April1st to 15thNight
12%5%24323449%47%
1797
16th to endNight
12%5%345%35%45%7%42%
1747
May1st to 15thNight
13%7%36%5%35%5%5%7%38%
1619
16th to endNight
5%3247%37%6%6%11%43%
1682
June1st to 15thNight
5%5%2432446%10%51%
1527
16th to endNight
3113436%5%8%12%47%
1440
July1st to 15thNight
9%3245%245%7%9%45%
1440
16th to endNight
6%214436%6%6%9%46%
1657
August1st to 15thNight
5%3244345%5%8%52%
1707
16th to endNight
5%4135%25%45%11%50%
1910
September1st to 15thNight
13%425%323447%47%
1872
16th to endNight
11%425%414346%52%
1980
October1st to 15thNight
11%5%24323336%54%
1980
16th to endNight
15%5%1432334451%
2406
November1st to 15thNight
13%5%13323346%51%
2471
16th to endNight
16%6%22324345%48%
2535
December1st to 15thNight
20%9%35%5%23345%34%
2535
16th to endNight
22%9%34422345%36%
2798

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
54%37%20%11%1
1553
16th to endDay
46%49%26%11%1
1913
February1st to 15thDay
52%39%21%11%2
1885
16th to endDay
48%312%20%12%2
1580
March1st to 15thDay
49%39%20%13%2
1809
16th to endDay
50%5%10%23%8%1
2178
April1st to 15thDay
40%5%9%23%18%2
2151
16th to endDay
43%5%9%23%15%2
2211
May1st to 15thDay
46%5%12%20%13%
2331
16th to endDay
42%8%12%22%11%2
2435
June1st to 15thDay
36%8%13%22%17%1
2430
16th to endDay
33%9%14%20%17%3
2520
July1st to 15thDay
42%9%13%17%13%3
2371
16th to endDay
37%9%18%21%10%2
2354
August1st to 15thDay
38%8%14%19%16%1
2237
16th to endDay
37%8%10%21%18%3
2292
September1st to 15thDay
44%6%11%18%15%3
2076
16th to endDay
34%49%23%23%3
1980
October1st to 15thDay
33%48%24%24%4
1980
16th to endDay
36%36%23%25%4
1814
November1st to 15thDay
36%47%22%22%6%
1717
16th to endDay
39%27%26%20%2
1755
December1st to 15thDay
49%312%20%12%2
1755
16th to endDay
51%312%21%9%1
1771
January1st to 15thNight
50%48%18%16%2
2716
16th to endNight
49%29%22%14%1
2655
February1st to 15thNight
53%29%21%11%1
2280
16th to endNight
50%410%20%12%1
1893
March1st to 15thNight
47%48%20%13%4
2149
16th to endNight
47%411%24%10%1
2056
April1st to 15thNight
35%410%24%20%4
1797
16th to endNight
39%5%8%24%17%4
1747
May1st to 15thNight
44%5%12%22%14%1
1620
16th to endNight
33%8%17%20%16%3
1684
June1st to 15thNight
28%6%15%28%18%2
1527
16th to endNight
25%8%16%25%18%5%
1440
July1st to 15thNight
33%7%15%23%15%4
1360
16th to endNight
29%7%18%25%16%3
1550
August1st to 15thNight
27%7%16%24%20%2
1696
16th to endNight
27%6%14%26%20%3
1910
September1st to 15thNight
36%410%23%18%5%
1872
16th to endNight
33%46%22%26%6%
1980
October1st to 15thNight
32%37%21%28%5%
1980
16th to endNight
36%26%24%24%5%
2406
November1st to 15thNight
34%36%22%25%7%
2471
16th to endNight
38%36%25%21%3
2535
December1st to 15thNight
51%39%18%13%3
2535
16th to endNight
51%311%20%11%1
2798

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%14%35%8%8%1
135
16th to endDay
17%18%45%13%4
144
February1st to 15thDay
27%14%40%14%3
130
16th to endDay
23%16%44%11%4
81
March1st to 15thDay
30%18%33%15%3
99
16th to endDay
18%13%33%20%13%
288
April1st to 15thDay
18%12%26%27%14%1
270
16th to endDay
17%5%29%22%18%5%
270
May1st to 15thDay
19%9%24%23%13%8%
288
16th to endDay
13%420%29%17%14%
331
June1st to 15thDay
8%114%23%31%20%
405
16th to endDay
6%217%30%28%14%
495
July1st to 15thDay
13%214%28%26%14%
475
16th to endDay
10%16%19%29%32%
370
August1st to 15thDay
15%5%20%32%25%
313
16th to endDay
12%7%26%30%23%
332
September1st to 15thDay
15%29%20%32%18%
315
16th to endDay
14%115%20%33%14%
314
October1st to 15thDay
15%319%23%26%11%
315
16th to endDay
18%7%26%25%16%5%
183
November1st to 15thDay
17%11%23%24%19%4
135
16th to endDay
20%10%29%25%12%1
135
December1st to 15thDay
15%16%38%17%11%1
135
16th to endDay
29%15%40%13%
144
January1st to 15thNight
22%14%31%25%5%
975
16th to endNight
23%11%35%25%3
868
February1st to 15thNight
26%13%33%22%3
760
16th to endNight
24%11%38%22%2
631
March1st to 15thNight
25%11%30%28%5%
720
16th to endNight
20%10%28%29%10%
712
April1st to 15thNight
16%10%30%29%12%
539
16th to endNight
14%6%32%31%14%
540
May1st to 15thNight
18%425%34%16%2
540
16th to endNight
5%216%38%30%6%
563
June1st to 15thNight
8%112%38%35%3
540
16th to endNight
3110%38%41%4
540
July1st to 15thNight
11%19%29%43%4
510
16th to endNight
7%17%30%41%11%
564
August1st to 15thNight
8%5%31%43%9%
539
16th to endNight
7%6%33%45%6%
573
September1st to 15thNight
15%19%25%42%5%
612
16th to endNight
13%213%29%36%5%
717
October1st to 15thNight
14%417%29%29%5%
720
16th to endNight
17%5%25%30%18%1
768
November1st to 15thNight
15%6%26%28%20%2
760
16th to endNight
20%11%24%30%12%1
780
December1st to 15thNight
24%10%28%26%8%
780
16th to endNight
27%9%31%25%4
926
Page generated at 2018-02-15 09:58:40. Version ac04

Copyright 2018 A. Danko


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