Bel Tor Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bel Tor Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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
36%
23114
12%
74711
7%
23071
42%
23071
12%
57371

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
22%
760
12%
2649
4%
757
32%
757
10%
2043
January11th to 20thNight
25%
736
10%
2570
5%
736
32%
736
8%
2072
January21st to endNight
22%
792
13%
2574
4%
792
34%
792
12%
1954
February1st to 10thNight
21%
720
10%
2340
3%
720
31%
720
9%
1831
February11th to 20thNight
17%
703
12%
2157
3%
700
29%
700
9%
1700
February21st to endNight
19%
592
16%
1776
6%
592
31%
592
10%
1400
March1st to 10thNight
24%
720
12%
2160
5%
720
33%
720
13%
1627
March11th to 20thNight
25%
720
13%
1989
5%
720
33%
720
11%
1588
March21st to endNight
24%
792
14%
2048
5%
792
31%
792
14%
1608
April1st to 10thNight
23%
720
10%
1800
2%
720
28%
720
13%
1465
April11th to 20thNight
30%
711
12%
1775
4%
711
33%
711
14%
1440
April21st to endNight
32%
720
12%
1800
4%
720
36%
720
15%
1417
May1st to 10thNight
31%
525
11%
1605
7%
525
35%
525
11%
1289
May11th to 20thNight
35%
354
15%
1416
10%
354
44%
354
10%
1059
May21st to endNight
52%
388
12%
1551
13%
388
56%
388
12%
1142
June1st to 10thNight
49%
360
12%
1440
13%
360
54%
360
11%
1073
June11th to 20thNight
51%
360
10%
1440
10%
360
56%
360
12%
1070
June21st to endNight
62%
360
10%
1440
13%
360
65%
360
13%
1031
July1st to 10thNight
59%
350
8%
1400
9%
350
61%
350
10%
1078
July11th to 20thNight
64%
348
10%
1392
14%
348
65%
348
12%
1021
July21st to endNight
61%
394
12%
1486
13%
372
63%
372
13%
1051
August1st to 10thNight
65%
529
11%
1593
9%
527
66%
527
16%
1203
August11th to 20thNight
55%
667
10%
1731
6%
667
56%
667
15%
1390
August21st to endNight
56%
792
10%
1980
4%
792
58%
792
15%
1595
September1st to 10thNight
50%
720
13%
1800
6%
720
53%
720
17%
1404
September11th to 20thNight
54%
720
19%
1827
13%
720
58%
720
20%
1371
September21st to endNight
48%
720
15%
1980
10%
720
53%
720
17%
1515
October1st to 10thNight
42%
720
11%
2156
9%
720
46%
720
14%
1649
October11th to 20thNight
40%
720
12%
2160
7%
720
47%
720
14%
1652
October21st to endNight
32%
792
13%
2461
8%
792
39%
792
13%
1901
November1st to 10thNight
40%
740
14%
2407
8%
740
48%
740
14%
1763
November11th to 20thNight
31%
760
16%
2470
9%
760
41%
760
11%
1896
November21st to endNight
31%
760
12%
2626
7%
760
39%
760
11%
1999
December1st to 10thNight
24%
760
10%
2660
4%
760
33%
760
8%
2102
December11th to 20thNight
28%
760
12%
3224
5%
759
36%
759
8%
2390
December21st to endNight
23%
829
6%
4828
3%
817
31%
817
5%
3582

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
16%6%36%5%35%5%5%7%38%
74746
Night
17%6%35%5%3445%7%41%
75002
TransparencyDay
29%38%8%14%9%1
74451
Night
28%38%9%14%9%1
74831
SeeingDay
21%13%27%23%12%4
13775
Night
20%9%30%30%11%1
23122

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
12%9%46%5%36%5%5%7%39%
1517
11th to 20thDay
15%9%46%6%45%6%6%6%32%
1464
21st to endDay
12%7%37%6%45%5%6%7%38%
1782
February1st to 10thDay
16%7%38%6%45%5%6%6%34%
1620
11th to 20thDay
14%8%37%6%35%5%5%7%38%
1690
21st to endDay
21%9%45%6%34445%36%
1480
March1st to 10thDay
14%7%46%6%3445%6%41%
1797
11th to 20thDay
15%8%36%7%35%46%7%36%
1970
21st to endDay
20%7%36%5%34446%38%
2308
April1st to 10thDay
17%8%36%5%345%5%6%38%
2160
11th to 20thDay
17%7%36%5%3445%6%40%
2134
21st to endDay
18%5%25%6%35%5%6%6%39%
2160
May1st to 10thDay
21%7%26%5%35%46%7%34%
2366
11th to 20thDay
21%6%36%5%35%5%47%35%
2478
21st to endDay
16%6%35%5%35%45%8%40%
2725
June1st to 10thDay
16%5%26%6%36%5%6%8%37%
2520
11th to 20thDay
16%6%26%6%46%5%6%9%35%
2520
21st to endDay
14%5%25%6%45%5%6%8%41%
2520
July1st to 10thDay
11%5%26%6%46%6%6%9%38%
2520
11th to 20thDay
10%425%6%35%7%7%9%41%
2520
21st to endDay
13%6%35%6%46%6%7%9%36%
2744
August1st to 10thDay
12%5%26%6%45%7%8%8%35%
2331
11th to 20thDay
12%6%36%5%5%7%5%6%9%37%
2150
21st to endDay
15%6%345%35%5%6%8%41%
2376
September1st to 10thDay
17%5%25%5%345%5%7%41%
2160
11th to 20thDay
15%42443445%7%49%
2131
21st to endDay
19%6%35%5%345%5%6%40%
1980
October1st to 10thDay
21%5%35%5%24448%40%
1800
11th to 20thDay
17%6%245%34447%44%
1800
21st to endDay
22%6%245%35%447%37%
1895
November1st to 10thDay
15%7%24433236%52%
1671
11th to 20thDay
19%5%25%4335%5%5%44%
1710
21st to endDay
18%5%35%4335%6%7%41%
1549
December1st to 10thDay
19%9%46%5%35%45%6%35%
1520
11th to 20thDay
21%8%25%5%3435%6%37%
1906
21st to endDay
19%10%5%9%7%45%45%6%26%
2772
January1st to 10thNight
16%6%25%5%3446%7%42%
2649
11th to 20thNight
19%8%35%5%4445%6%37%
2570
21st to endNight
12%8%35%5%345%5%7%42%
2574
February1st to 10thNight
17%8%36%6%2445%8%38%
2340
11th to 20thNight
15%8%46%6%345%5%6%39%
2155
21st to endNight
19%6%36%6%3445%5%40%
1776
March1st to 10thNight
15%6%36%42446%6%44%
2150
11th to 20thNight
17%7%34435%5%36%43%
1988
21st to endNight
17%7%36%5%3445%6%40%
2048
April1st to 10thNight
18%7%36%5%35%46%7%37%
1800
11th to 20thNight
17%6%26%435%5%5%7%40%
1775
21st to endNight
21%6%2432446%6%41%
1800
May1st to 10thNight
22%8%26%6%34445%35%
1605
11th to 20thNight
19%6%345%345%6%6%40%
1415
21st to endNight
16%6%25%5%335%5%7%43%
1550
June1st to 10thNight
17%5%245%346%5%7%44%
1440
11th to 20thNight
14%5%24435%5%7%10%41%
1440
21st to endNight
13%323435%6%7%9%46%
1440
July1st to 10thNight
12%5%25%5%46%5%7%9%41%
1440
11th to 20thNight
11%425%5%35%7%8%9%43%
1440
21st to endNight
11%5%25%435%5%7%9%44%
1574
August1st to 10thNight
11%5%25%435%5%7%10%42%
1603
11th to 20thNight
14%6%37%6%35%5%7%8%38%
1731
21st to endNight
16%424425%5%6%9%42%
1980
September1st to 10thNight
16%6%25%5%35%35%7%45%
1800
11th to 20thNight
13%5%24423447%52%
1827
21st to endNight
18%5%25%423446%46%
1980
October1st to 10thNight
23%5%24434436%43%
2156
11th to 20thNight
18%6%245%2445%5%47%
2160
21st to endNight
23%7%23323446%43%
2461
November1st to 10thNight
14%5%24325%45%5%51%
2407
11th to 20thNight
18%6%34423446%45%
2470
21st to endNight
15%5%24435%45%8%44%
2626
December1st to 10thNight
19%7%36%5%45%45%6%36%
2660
11th to 20thNight
19%8%245%35%5%5%6%38%
3288
21st to endNight
21%10%47%5%34445%34%
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
28%42%7%18%5%
1518
11th to 20thDay
31%44%9%10%5%1
1464
21st to endDay
29%38%8%15%8%1
1782
February1st to 10thDay
33%36%8%14%8%1
1620
11th to 20thDay
32%38%7%14%7%1
1699
21st to endDay
34%37%6%15%7%2
1480
March1st to 10thDay
28%37%9%17%8%2
1800
11th to 20thDay
32%37%8%13%9%1
1970
21st to endDay
33%36%7%13%10%2
2308
April1st to 10thDay
29%38%7%14%9%3
2160
11th to 20thDay
31%37%8%14%8%3
2134
21st to endDay
28%38%8%13%10%4
2160
May1st to 10thDay
30%38%7%12%10%3
2366
11th to 20thDay
31%38%6%12%10%2
2482
21st to endDay
27%38%9%14%10%1
2725
June1st to 10thDay
28%39%10%14%8%1
2520
11th to 20thDay
31%39%10%13%5%2
2520
21st to endDay
27%39%11%17%6%
2520
July1st to 10thDay
28%40%7%15%8%1
2450
11th to 20thDay
23%39%11%17%9%1
2445
21st to endDay
28%41%7%12%11%1
2591
August1st to 10thDay
29%39%10%12%9%
2317
11th to 20thDay
28%38%8%15%10%1
2150
21st to endDay
30%38%7%12%10%2
2376
September1st to 10thDay
29%37%8%13%12%2
2160
11th to 20thDay
21%36%9%15%16%2
2131
21st to endDay
31%37%6%12%14%1
1980
October1st to 10thDay
28%39%9%14%10%
1800
11th to 20thDay
27%37%9%18%8%1
1800
21st to endDay
31%39%7%13%8%1
1895
November1st to 10thDay
20%40%9%16%13%2
1671
11th to 20thDay
28%40%7%15%7%1
1710
21st to endDay
28%41%9%15%6%1
1549
December1st to 10thDay
32%41%10%12%6%
1520
11th to 20thDay
32%34%13%15%5%
1906
21st to endDay
44%30%9%12%5%
2772
January1st to 10thNight
27%41%9%16%7%
2649
11th to 20thNight
30%43%9%13%41
2570
21st to endNight
27%38%8%16%10%1
2574
February1st to 10thNight
31%37%8%14%9%1
2340
11th to 20thNight
30%38%8%13%8%2
2157
21st to endNight
31%37%5%16%9%2
1776
March1st to 10thNight
24%39%10%18%7%1
2160
11th to 20thNight
31%38%8%14%8%2
1989
21st to endNight
28%38%8%12%12%3
2048
April1st to 10thNight
31%38%8%12%9%3
1800
11th to 20thNight
30%38%7%12%12%1
1775
21st to endNight
27%38%8%13%11%3
1800
May1st to 10thNight
32%38%6%11%11%2
1605
11th to 20thNight
28%38%8%12%13%2
1416
21st to endNight
24%40%11%13%10%3
1551
June1st to 10thNight
27%39%8%15%10%1
1440
11th to 20thNight
25%40%11%16%7%1
1440
21st to endNight
21%41%11%20%7%
1440
July1st to 10thNight
27%42%11%12%7%1
1400
11th to 20thNight
23%42%11%16%8%1
1392
21st to endNight
20%41%13%14%11%1
1486
August1st to 10thNight
23%40%12%14%11%
1593
11th to 20thNight
29%39%9%12%10%1
1731
21st to endNight
28%39%8%12%11%1
1980
September1st to 10thNight
27%37%9%15%12%1
1800
11th to 20thNight
23%37%7%15%17%2
1827
21st to endNight
26%36%8%15%14%1
1980
October1st to 10thNight
28%37%8%17%10%1
2156
11th to 20thNight
28%37%9%17%8%1
2160
21st to endNight
28%38%9%16%9%1
2461
November1st to 10thNight
20%42%8%14%14%2
2407
11th to 20thNight
26%40%8%15%9%2
2470
21st to endNight
26%40%9%17%8%
2626
December1st to 10thNight
29%42%9%14%6%1
2660
11th to 20thNight
31%35%11%13%10%1
3288
21st to endNight
40%28%11%16%5%
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
26%23%28%19%4
180
11th to 20thDay
27%25%29%16%31
166
21st to endDay
20%28%32%13%6%1
264
February1st to 10thDay
27%28%34%9%2
240
11th to 20thDay
26%19%40%14%1
358
21st to endDay
28%21%31%18%2
346
March1st to 10thDay
23%23%28%22%4
420
11th to 20thDay
22%21%35%17%41
420
21st to endDay
28%19%26%18%6%2
462
April1st to 10thDay
22%23%30%18%6%1
420
11th to 20thDay
24%17%29%18%11%2
412
21st to endDay
26%16%33%17%6%2
420
May1st to 10thDay
29%20%22%16%9%3
423
11th to 20thDay
23%13%30%21%9%3
447
21st to endDay
20%7%24%25%20%4
485
June1st to 10thDay
18%13%26%25%14%3
450
11th to 20thDay
21%10%26%27%14%3
450
21st to endDay
16%10%28%28%16%3
450
July1st to 10thDay
13%423%28%23%9%
445
11th to 20thDay
12%421%32%25%7%
444
21st to endDay
15%7%27%30%16%5%
488
August1st to 10thDay
14%5%20%33%21%8%
446
11th to 20thDay
14%419%34%21%9%
439
21st to endDay
13%6%25%27%18%10%
495
September1st to 10thDay
21%6%21%26%17%9%
450
11th to 20thDay
15%5%25%26%16%13%
450
21st to endDay
22%8%24%25%14%6%
450
October1st to 10thDay
26%7%23%22%17%5%
450
11th to 20thDay
19%14%28%27%10%2
450
21st to endDay
25%14%31%21%8%1
453
November1st to 10thDay
21%12%26%27%12%3
365
11th to 20thDay
24%17%29%23%6%1
370
21st to endDay
22%19%33%20%5%
209
December1st to 10thDay
24%24%31%13%6%1
180
11th to 20thDay
32%19%27%18%4
180
21st to endDay
27%18%33%16%5%2
198
January1st to 10thNight
20%13%41%23%3
760
11th to 20thNight
24%13%33%27%3
736
21st to endNight
16%14%43%26%2
792
February1st to 10thNight
22%12%39%27%1
720
11th to 20thNight
21%17%41%20%1
703
21st to endNight
23%16%36%22%2
592
March1st to 10thNight
21%14%37%25%3
720
11th to 20thNight
21%13%37%24%5%
720
21st to endNight
19%13%39%23%6%
792
April1st to 10thNight
21%14%38%23%5%
720
11th to 20thNight
20%11%35%25%9%
711
21st to endNight
24%9%29%30%7%
720
May1st to 10thNight
25%9%29%28%8%1
525
11th to 20thNight
19%7%33%33%8%
354
21st to endNight
19%418%42%18%
389
June1st to 10thNight
18%322%35%21%1
360
11th to 20thNight
14%326%40%16%
360
21st to endNight
14%216%51%17%
360
July1st to 10thNight
13%218%42%23%2
350
11th to 20thNight
11%413%48%22%1
348
21st to endNight
13%416%44%20%2
394
August1st to 10thNight
13%211%45%26%3
529
11th to 20thNight
15%215%37%28%2
667
21st to endNight
17%316%37%26%2
792
September1st to 10thNight
19%319%36%21%2
720
11th to 20thNight
14%5%18%35%23%5%
720
21st to endNight
20%422%33%20%2
720
October1st to 10thNight
24%5%23%27%19%2
720
11th to 20thNight
20%8%27%36%9%
720
21st to endNight
27%7%32%26%8%1
792
November1st to 10thNight
17%7%31%33%11%1
740
11th to 20thNight
22%13%29%28%8%1
760
21st to endNight
18%12%35%28%7%
760
December1st to 10thNight
24%11%34%25%6%1
760
11th to 20thNight
23%12%33%29%4
760
21st to endNight
23%13%37%26%2
836
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