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 2005-11-05 to 2017-09-23.

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
37%
15036
18%
46922
11%
15005
46%
15005
23%
30300

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%
720
18%
2506
8%
716
38%
716
18%
1635
January16th to endNight
23%
768
20%
2553
7%
768
39%
768
21%
1617
February1st to 15thNight
20%
704
16%
2269
5%
701
34%
701
18%
1511
February16th to endNight
21%
631
25%
1893
9%
631
40%
631
21%
1217
March1st to 15thNight
23%
720
18%
2105
7%
720
36%
720
21%
1400
March16th to endNight
26%
768
21%
2025
8%
768
37%
768
26%
1348
April1st to 15thNight
25%
718
16%
1796
5%
718
31%
718
24%
1272
April16th to endNight
34%
720
17%
1800
6%
720
39%
720
29%
1221
May1st to 15thNight
37%
468
18%
1548
11%
468
45%
468
19%
1039
May16th to endNight
47%
374
15%
1497
13%
374
54%
374
20%
937
June1st to 15thNight
47%
360
15%
1440
15%
360
54%
360
19%
911
June16th to endNight
61%
360
15%
1440
20%
360
66%
360
26%
815
July1st to 15thNight
60%
340
12%
1360
17%
340
62%
340
20%
832
July16th to endNight
65%
376
15%
1418
18%
354
69%
354
27%
792
August1st to 15thNight
64%
564
17%
1635
13%
562
66%
562
31%
1037
August16th to endNight
60%
764
20%
1910
12%
764
62%
764
33%
1272
September1st to 15thNight
50%
720
22%
1800
12%
720
54%
720
34%
1175
September16th to endNight
56%
689
25%
1861
20%
689
60%
689
34%
1164
October1st to 15thNight
43%
660
20%
1978
17%
660
49%
660
26%
1262
October16th to endNight
35%
704
21%
2164
12%
704
47%
704
27%
1345
November1st to 15thNight
37%
700
20%
2277
12%
700
48%
700
24%
1403
November16th to endNight
33%
720
19%
2439
12%
720
44%
720
20%
1561
December1st to 15thNight
27%
720
14%
2520
7%
720
37%
720
16%
1727
December16th to endNight
23%
768
16%
2688
5%
768
37%
768
16%
1807

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
15%6%25%5%3445%7%38%
47987
Night
16%5%2442445%6%42%
47092
Cloud CoverDay
15%6%25%5%3445%7%38%
47996
Night
16%5%2442445%6%42%
47098
TransparencyDay
44%6%12%20%14%1
47674
Night
41%6%13%22%15%1
46922
SeeingDay
19%9%24%25%14%5%
5729
Night
18%8%29%30%11%
15037

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
13%7%36%5%35%5%46%38%
1432
16th to endDay
13%7%35%6%35%5%6%7%37%
1668
February1st to 15thDay
14%7%37%5%35%5%6%6%34%
1592
16th to endDay
14%7%36%7%3444438%
1580
March1st to 15thDay
16%7%36%5%3445%6%35%
1844
16th to endDay
16%7%35%5%24346%40%
2199
April1st to 15thDay
18%7%34423446%39%
2152
16th to endDay
14%5%245%2445%6%42%
2160
May1st to 15thDay
21%6%25%5%35%446%35%
2409
16th to endDay
17%5%36%426%45%7%35%
2618
June1st to 15thDay
16%425%6%35%46%9%33%
2520
16th to endDay
13%6%25%5%36%45%7%37%
2520
July1st to 15thDay
11%5%25%6%36%6%6%9%35%
2520
16th to endDay
11%425%6%45%6%7%9%37%
2654
August1st to 15thDay
14%426%5%46%6%7%6%34%
2299
16th to endDay
13%5%24434348%43%
2292
September1st to 15thDay
15%425%434447%43%
2160
16th to endDay
15%5%25%5%34446%44%
1935
October1st to 15thDay
17%5%24423447%42%
1650
16th to endDay
20%5%235%34446%40%
1708
November1st to 15thDay
16%8%24422235%46%
1581
16th to endDay
20%42442245%6%42%
1518
December1st to 15thDay
17%8%35%435%45%6%35%
1440
16th to endDay
19%9%36%43345%7%32%
1536
January1st to 15thNight
16%6%2444445%6%39%
2506
16th to endNight
14%7%2442445%6%42%
2553
February1st to 15thNight
13%7%36%6%24447%38%
2268
16th to endNight
15%5%35%42445%5%43%
1893
March1st to 15thNight
17%6%35%32435%5%42%
2092
16th to endNight
16%6%25%324446%42%
2025
April1st to 15thNight
17%5%25%5%2446%6%38%
1796
16th to endNight
18%5%2432446%6%43%
1800
May1st to 15thNight
19%6%245%235%5%5%38%
1548
16th to endNight
16%5%245%3445%7%42%
1495
June1st to 15thNight
18%4234245%5%7%41%
1440
16th to endNight
10%3134245%7%9%45%
1440
July1st to 15thNight
11%425%5%35%5%7%9%41%
1440
16th to endNight
9%324425%5%7%10%44%
1514
August1st to 15thNight
11%5%2433347%8%44%
1645
16th to endNight
13%414425%46%9%43%
1910
September1st to 15thNight
14%6%2442335%6%47%
1800
16th to endNight
13%5%24422347%49%
1861
October1st to 15thNight
19%424423435%44%
1978
16th to endNight
20%5%2331235%6%45%
2164
November1st to 15thNight
14%6%34324345%46%
2277
16th to endNight
17%5%24425%35%7%40%
2439
December1st to 15thNight
19%6%35%445%45%6%34%
2520
16th to endNight
18%7%24435%45%5%37%
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
13%7%35%5%36%446%38%
1432
16th to endDay
13%7%35%5%35%5%6%6%37%
1668
February1st to 15thDay
15%7%37%5%35%5%5%7%34%
1592
16th to endDay
15%6%46%7%25%335%38%
1580
March1st to 15thDay
16%7%35%5%3445%6%35%
1854
16th to endDay
16%7%26%5%24336%40%
2198
April1st to 15thDay
19%7%34323445%40%
2152
16th to endDay
14%5%25%5%3345%6%42%
2160
May1st to 15thDay
21%6%15%5%345%46%34%
2409
16th to endDay
17%6%26%436%45%8%35%
2618
June1st to 15thDay
16%425%6%45%5%6%9%33%
2520
16th to endDay
13%5%25%7%35%45%7%38%
2520
July1st to 15thDay
11%6%25%6%45%6%6%9%35%
2520
16th to endDay
11%425%6%36%5%7%9%37%
2654
August1st to 15thDay
13%6%25%6%36%6%7%6%34%
2299
16th to endDay
13%5%25%5%25%35%7%43%
2292
September1st to 15thDay
15%5%25%434347%43%
2160
16th to endDay
15%425%424447%43%
1935
October1st to 15thDay
17%5%15%423447%42%
1650
16th to endDay
19%5%23434446%40%
1708
November1st to 15thDay
16%8%24423235%46%
1581
16th to endDay
19%42442335%6%42%
1518
December1st to 15thDay
17%8%34435%46%6%35%
1440
16th to endDay
19%9%45%43445%7%32%
1536
January1st to 15thNight
17%6%2443445%7%38%
2506
16th to endNight
15%7%3442445%6%41%
2553
February1st to 15thNight
13%7%37%5%34446%38%
2261
16th to endNight
15%5%35%42445%5%43%
1893
March1st to 15thNight
18%6%35%324346%42%
2105
16th to endNight
16%6%24423446%42%
2025
April1st to 15thNight
17%5%345%2446%5%38%
1796
16th to endNight
18%42432445%5%43%
1800
May1st to 15thNight
20%5%344245%5%5%38%
1548
16th to endNight
16%5%245%3445%7%41%
1495
June1st to 15thNight
18%42343445%7%41%
1440
16th to endNight
10%4134236%6%9%45%
1440
July1st to 15thNight
11%4245%45%5%6%9%40%
1440
16th to endNight
9%4144345%7%10%44%
1514
August1st to 15thNight
11%425%3335%7%7%45%
1645
16th to endNight
13%4135%245%5%8%43%
1910
September1st to 15thNight
14%5%2442335%6%47%
1800
16th to endNight
13%5%14422447%48%
1861
October1st to 15thNight
19%424423335%45%
1978
16th to endNight
19%5%23213346%45%
2164
November1st to 15thNight
14%6%34424345%46%
2277
16th to endNight
17%6%2442445%7%40%
2439
December1st to 15thNight
20%6%25%5%35%45%5%35%
2520
16th to endNight
19%7%24434445%37%
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
46%6%11%23%9%2
1433
16th to endDay
45%5%14%21%11%1
1668
February1st to 15thDay
48%6%12%20%11%1
1594
16th to endDay
48%5%10%23%10%1
1580
March1st to 15thDay
47%5%15%19%9%1
1854
16th to endDay
46%410%19%16%2
2199
April1st to 15thDay
45%5%9%22%12%4
2152
16th to endDay
40%5%10%20%19%3
2160
May1st to 15thDay
48%6%10%18%14%2
2409
16th to endDay
47%7%12%18%13%1
2622
June1st to 15thDay
46%8%14%20%9%
2520
16th to endDay
44%7%13%23%10%1
2520
July1st to 15thDay
43%7%12%20%14%1
2371
16th to endDay
40%8%13%22%12%1
2487
August1st to 15thDay
45%7%13%18%14%
2285
16th to endDay
39%5%9%24%18%2
2292
September1st to 15thDay
41%5%11%20%20%1
2160
16th to endDay
40%5%8%21%21%3
1935
October1st to 15thDay
41%6%13%21%14%1
1650
16th to endDay
44%5%11%23%13%1
1708
November1st to 15thDay
42%314%19%17%3
1581
16th to endDay
42%412%24%13%1
1518
December1st to 15thDay
48%6%16%18%10%
1440
16th to endDay
50%5%15%17%11%
1536
January1st to 15thNight
44%6%14%20%11%2
2506
16th to endNight
42%413%24%13%1
2553
February1st to 15thNight
45%5%13%20%12%2
2269
16th to endNight
40%5%9%25%15%2
1893
March1st to 15thNight
43%6%14%21%12%2
2105
16th to endNight
41%5%12%21%16%2
2025
April1st to 15thNight
44%6%10%20%15%2
1796
16th to endNight
39%6%9%20%21%2
1800
May1st to 15thNight
45%7%9%19%16%1
1548
16th to endNight
41%7%15%17%14%3
1497
June1st to 15thNight
42%7%11%22%14%
1440
16th to endNight
32%9%15%29%12%
1440
July1st to 15thNight
38%9%13%23%13%1
1360
16th to endNight
31%8%19%23%13%2
1418
August1st to 15thNight
34%9%15%22%16%1
1635
16th to endNight
36%7%11%21%20%2
1910
September1st to 15thNight
37%5%12%22%20%1
1800
16th to endNight
35%49%21%26%2
1861
October1st to 15thNight
41%412%23%15%2
1978
16th to endNight
38%414%26%14%
2164
November1st to 15thNight
39%413%21%16%4
2277
16th to endNight
43%5%13%23%12%1
2439
December1st to 15thNight
49%6%14%18%10%
2520
16th to endNight
47%5%14%20%10%
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
20%25%35%16%3
60
16th to endDay
18%17%33%25%4
86
February1st to 15thDay
16%25%42%14%
108
16th to endDay
17%15%38%24%4
239
March1st to 15thDay
20%21%27%25%5%
270
16th to endDay
18%16%31%22%7%2
288
April1st to 15thDay
20%13%30%22%11%1
270
16th to endDay
19%10%29%22%12%5%
270
May1st to 15thDay
34%15%17%15%12%4
288
16th to endDay
20%7%22%26%17%4
331
June1st to 15thDay
19%9%25%24%16%4
315
16th to endDay
20%7%25%27%15%2
315
July1st to 15thDay
17%222%28%23%4
305
16th to endDay
14%220%29%24%7%
326
August1st to 15thDay
18%315%28%22%11%
313
16th to endDay
12%215%30%25%14%
332
September1st to 15thDay
16%6%20%26%17%13%
315
16th to endDay
18%216%25%19%17%
283
October1st to 15thDay
21%5%17%29%20%6%
255
16th to endDay
18%11%31%28%9%
263
November1st to 15thDay
24%6%24%27%13%3
235
16th to endDay
19%14%31%24%8%1
138
December1st to 15thDay
20%26%25%16%8%3
60
16th to endDay
39%15%26%17%1
64
January1st to 15thNight
19%13%36%26%3
720
16th to endNight
18%12%40%26%1
768
February1st to 15thNight
17%14%43%24%
704
16th to endNight
17%17%38%24%2
631
March1st to 15thNight
20%14%36%25%3
720
16th to endNight
18%11%38%23%7%
768
April1st to 15thNight
19%12%38%25%4
718
16th to endNight
20%9%30%28%10%
720
May1st to 15thNight
22%6%28%32%8%
468
16th to endNight
18%324%39%14%
375
June1st to 15thNight
20%223%32%20%
360
16th to endNight
11%119%51%16%
360
July1st to 15thNight
14%215%45%21%
340
16th to endNight
10%216%44%23%2
376
August1st to 15thNight
14%111%43%26%2
564
16th to endNight
16%113%36%29%2
764
September1st to 15thNight
16%320%35%20%1
720
16th to endNight
16%218%30%26%5%
689
October1st to 15thNight
21%5%23%30%17%1
660
16th to endNight
23%7%30%30%7%
704
November1st to 15thNight
18%9%29%29%12%1
700
16th to endNight
20%11%30%28%9%
720
December1st to 15thNight
24%10%31%26%6%
720
16th to endNight
22%12%38%24%2
768
Page generated at 2017-09-24 09:15:54. Version ac04

Copyright 2017 A. Danko


Bel Tor Observatory Clear Sky Chart | Charts Home