Fan Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fan Mountain Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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%
23792
22%
74691
14%
23745
54%
23745
27%
45718

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
29%
757
26%
2459
11%
757
47%
757
26%
1451
January11th to 20thNight
30%
739
25%
2399
13%
739
45%
739
22%
1505
January21st to endNight
28%
792
27%
2574
13%
792
47%
792
27%
1520
February1st to 10thNight
30%
720
20%
2340
11%
720
44%
720
24%
1440
February11th to 20thNight
20%
701
21%
2273
7%
699
39%
699
21%
1431
February21st to endNight
27%
592
28%
1703
12%
592
44%
592
25%
1086
March1st to 10thNight
32%
720
28%
1980
13%
720
46%
720
31%
1183
March11th to 20thNight
30%
720
22%
1980
11%
720
41%
720
25%
1294
March21st to endNight
31%
792
21%
2178
11%
792
40%
792
26%
1421
April1st to 10thNight
29%
720
21%
1844
6%
720
39%
720
30%
1188
April11th to 20thNight
40%
711
24%
1775
9%
711
47%
711
37%
1111
April21st to endNight
40%
702
23%
1782
10%
702
47%
702
32%
1143
May1st to 10thNight
40%
540
19%
1620
8%
540
47%
540
26%
1036
May11th to 20thNight
46%
531
15%
1593
10%
531
50%
531
24%
1069
May21st to endNight
60%
488
16%
1650
15%
488
63%
488
26%
1043
June1st to 10thNight
67%
360
21%
1440
24%
360
70%
360
28%
795
June11th to 20thNight
61%
360
16%
1440
18%
360
65%
360
25%
840
June21st to endNight
67%
360
20%
1440
22%
360
71%
360
28%
803
July1st to 10thNight
68%
497
15%
1547
15%
497
69%
497
28%
988
July11th to 20thNight
77%
522
16%
1566
15%
522
77%
522
37%
910
July21st to endNight
68%
591
18%
1672
16%
558
69%
558
32%
1041
August1st to 10thNight
67%
537
17%
1600
13%
534
68%
534
28%
1038
August11th to 20thNight
62%
552
19%
1616
13%
552
64%
552
27%
1065
August21st to endNight
64%
792
17%
1980
11%
792
66%
792
31%
1347
September1st to 10thNight
63%
720
24%
1800
16%
720
65%
720
34%
1196
September11th to 20thNight
62%
720
27%
1885
21%
720
65%
720
34%
1214
September21st to endNight
47%
720
21%
2034
17%
720
52%
720
26%
1353
October1st to 10thNight
53%
720
23%
2160
22%
720
59%
720
31%
1338
October11th to 20thNight
48%
720
27%
2160
21%
720
60%
720
31%
1290
October21st to endNight
47%
792
30%
2376
22%
792
60%
792
35%
1365
November1st to 10thNight
43%
740
29%
2221
19%
740
55%
740
32%
1305
November11th to 20thNight
40%
760
27%
2463
17%
760
54%
760
29%
1424
November21st to endNight
41%
760
26%
2660
17%
760
55%
760
30%
1473
December1st to 10thNight
32%
760
23%
2660
13%
760
47%
760
22%
1634
December11th to 20thNight
39%
759
25%
3180
16%
758
53%
758
22%
1829
December21st to endNight
36%
825
16%
4641
14%
817
49%
817
15%
2549

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%5%25%5%35%5%6%8%43%
74631
Night
14%5%25%4345%5%7%45%
75142
TransparencyDay
39%5%12%24%17%4
74352
Night
39%411%23%18%5%
74940
SeeingDay
19%9%25%25%16%6%
13910
Night
17%7%24%34%16%2
23811

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%6%36%5%34445%46%
1706
11th to 20thDay
17%6%35%5%25%5%46%41%
1656
21st to endDay
15%7%25%5%35%5%5%6%44%
1782
February1st to 10thDay
11%7%37%6%35%6%5%6%41%
1620
11th to 20thDay
17%6%35%43345%6%43%
1573
21st to endDay
18%7%35%5%35%45%6%39%
1549
March1st to 10thDay
13%5%25%434437%51%
1980
11th to 20thDay
15%7%35%445%46%6%41%
1980
21st to endDay
15%7%36%6%3445%6%39%
2178
April1st to 10thDay
10%5%245%3446%8%49%
2114
11th to 20thDay
12%5%245%3446%7%48%
2134
21st to endDay
17%5%35%6%3445%6%42%
2193
May1st to 10thDay
15%5%25%5%36%5%6%9%39%
2340
11th to 20thDay
18%6%25%6%35%6%6%8%36%
2301
21st to endDay
14%5%35%5%35%5%6%9%40%
2621
June1st to 10thDay
10%425%6%35%6%6%9%44%
2520
11th to 20thDay
11%425%6%46%6%7%10%39%
2520
21st to endDay
7%325%6%36%7%8%12%42%
2520
July1st to 10thDay
10%5%35%5%46%6%6%10%40%
2361
11th to 20thDay
5%325%6%47%7%9%12%40%
2340
21st to endDay
9%437%6%46%7%9%10%34%
2547
August1st to 10thDay
11%6%27%6%36%6%7%9%36%
2327
11th to 20thDay
12%436%6%5%6%7%8%9%35%
2267
21st to endDay
10%5%25%5%35%6%7%10%41%
2376
September1st to 10thDay
12%5%25%435%45%7%48%
2160
11th to 20thDay
12%5%345%345%6%8%46%
2070
21st to endDay
22%5%245%345%46%39%
1926
October1st to 10thDay
15%424424447%48%
1800
11th to 20thDay
16%414324346%52%
1800
21st to endDay
14%6%24423345%53%
1980
November1st to 10thDay
13%5%13323445%56%
1857
11th to 20thDay
15%5%24424445%53%
1711
21st to endDay
14%42442345%7%52%
1520
December1st to 10thDay
16%5%26%5%345%5%8%41%
1520
11th to 20thDay
16%5%23324345%54%
1910
21st to endDay
20%5%25%5%3445%7%39%
2872
January1st to 10thNight
14%5%35%43445%6%47%
2459
11th to 20thNight
16%7%35%425%5%5%6%42%
2399
21st to endNight
14%5%34435%45%6%48%
2574
February1st to 10thNight
14%6%245%45%5%5%7%42%
2340
11th to 20thNight
20%6%3442445%5%43%
2273
21st to endNight
15%6%35%43445%6%45%
1703
March1st to 10thNight
12%42343445%7%52%
1980
11th to 20thNight
19%5%2443445%8%42%
1980
21st to endNight
15%6%35%5%34446%45%
2178
April1st to 10thNight
13%5%345%345%5%7%46%
1844
11th to 20thNight
12%5%3442335%9%50%
1775
21st to endNight
16%425%435%45%8%45%
1782
May1st to 10thNight
16%6%245%345%5%8%42%
1620
11th to 20thNight
18%6%25%5%45%5%7%9%35%
1592
21st to endNight
15%5%25%5%36%45%9%41%
1650
June1st to 10thNight
10%425%426%5%7%10%45%
1440
11th to 20thNight
10%435%5%36%5%5%9%44%
1440
21st to endNight
9%324435%5%7%12%45%
1440
July1st to 10thNight
11%436%5%45%6%6%9%42%
1592
11th to 20thNight
5%4245%45%7%8%11%45%
1620
21st to endNight
7%4246%46%7%8%11%40%
1771
August1st to 10thNight
10%5%26%5%5%6%6%6%10%39%
1611
11th to 20thNight
12%6%36%6%446%7%9%37%
1616
21st to endNight
10%6%36%6%46%6%7%9%37%
1980
September1st to 10thNight
14%6%244345%5%7%46%
1800
11th to 20thNight
12%5%25%5%435%5%7%46%
1885
21st to endNight
23%6%2432445%5%41%
2034
October1st to 10thNight
17%5%2442445%6%46%
2160
11th to 20thNight
15%5%24424446%50%
2160
21st to endNight
15%32333345%6%53%
2376
November1st to 10thNight
12%6%35%43433454%
2221
11th to 20thNight
13%5%25%423446%51%
2463
21st to endNight
13%42442445%6%51%
2660
December1st to 10thNight
15%7%35%5%3445%6%43%
2660
11th to 20thNight
15%6%245%34346%48%
3285
21st to endNight
17%5%24435%5%6%7%42%
4779

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
40%49%22%19%6%
1707
11th to 20thDay
43%49%22%19%3
1656
21st to endDay
41%38%23%19%6%
1782
February1st to 10thDay
43%411%24%14%4
1620
11th to 20thDay
42%48%21%20%5%
1582
21st to endDay
47%310%17%18%6%
1549
March1st to 10thDay
35%39%23%23%6%
1980
11th to 20thDay
43%410%18%17%7%
1980
21st to endDay
46%5%9%19%16%6%
2178
April1st to 10thDay
34%411%23%19%9%
2114
11th to 20thDay
36%311%22%21%6%
2134
21st to endDay
43%411%23%14%5%
2193
May1st to 10thDay
41%6%15%20%15%3
2340
11th to 20thDay
44%6%15%22%11%2
2305
21st to endDay
41%5%16%27%12%
2621
June1st to 10thDay
35%6%16%27%15%1
2520
11th to 20thDay
38%7%17%21%15%2
2520
21st to endDay
33%7%16%28%15%2
2520
July1st to 10thDay
39%6%15%21%16%3
2296
11th to 20thDay
33%7%18%26%15%2
2271
21st to endDay
40%7%14%24%14%1
2405
August1st to 10thDay
42%6%16%26%9%1
2314
11th to 20thDay
41%7%15%24%12%1
2267
21st to endDay
36%6%18%24%13%2
2376
September1st to 10thDay
37%415%19%21%4
2160
11th to 20thDay
37%5%7%28%20%4
2070
21st to endDay
46%5%9%20%18%3
1926
October1st to 10thDay
36%49%28%20%4
1800
11th to 20thDay
34%49%25%24%4
1800
21st to endDay
35%37%26%23%6%
1980
November1st to 10thDay
31%39%24%28%5%
1857
11th to 20thDay
35%29%22%23%8%
1711
21st to endDay
33%39%26%25%4
1520
December1st to 10thDay
42%311%23%15%5%
1520
11th to 20thDay
35%28%30%17%8%
1906
21st to endDay
44%311%26%13%2
2872
January1st to 10thNight
38%39%22%20%7%
2459
11th to 20thNight
42%49%22%18%5%
2399
21st to endNight
38%38%25%22%4
2574
February1st to 10thNight
41%411%24%17%3
2340
11th to 20thNight
43%37%20%19%8%
2275
21st to endNight
41%310%21%18%6%
1703
March1st to 10thNight
34%39%26%22%6%
1980
11th to 20thNight
42%38%22%21%4
1980
21st to endNight
42%410%22%16%6%
2178
April1st to 10thNight
38%411%18%23%7%
1844
11th to 20thNight
34%213%21%24%6%
1775
21st to endNight
38%311%25%17%6%
1782
May1st to 10thNight
42%413%24%13%5%
1620
11th to 20thNight
44%6%13%22%12%2
1593
21st to endNight
41%5%15%25%12%1
1651
June1st to 10thNight
33%7%14%29%15%1
1440
11th to 20thNight
38%6%18%22%14%2
1440
21st to endNight
32%8%15%24%18%4
1440
July1st to 10thNight
39%7%17%18%15%4
1547
11th to 20thNight
29%7%22%27%12%2
1566
21st to endNight
35%7%18%26%14%
1672
August1st to 10thNight
40%7%17%24%11%2
1600
11th to 20thNight
42%6%16%23%13%1
1616
21st to endNight
40%7%17%22%11%3
1980
September1st to 10thNight
38%5%12%21%18%5%
1800
11th to 20thNight
38%410%27%18%3
1885
21st to endNight
45%310%22%17%4
2034
October1st to 10thNight
38%49%26%19%3
2160
11th to 20thNight
36%46%25%23%5%
2160
21st to endNight
33%38%24%26%6%
2376
November1st to 10thNight
36%37%22%25%7%
2221
11th to 20thNight
36%36%23%24%8%
2463
21st to endNight
35%39%26%23%6%
2660
December1st to 10thNight
43%38%25%17%4
2660
11th to 20thNight
39%39%22%21%7%
3288
21st to endNight
40%411%26%16%2
4779

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
23%22%32%19%4
270
11th to 20thDay
31%19%33%13%4
252
21st to endDay
28%20%34%12%41
264
February1st to 10thDay
26%22%39%11%2
240
11th to 20thDay
35%23%34%8%
240
21st to endDay
31%18%28%20%4
332
March1st to 10thDay
24%18%30%22%6%
420
11th to 20thDay
23%18%31%21%6%1
420
21st to endDay
25%14%31%23%5%1
462
April1st to 10thDay
24%15%30%20%9%2
420
11th to 20thDay
19%11%28%24%13%4
412
21st to endDay
25%10%32%20%11%2
420
May1st to 10thDay
23%14%30%21%10%3
423
11th to 20thDay
22%6%26%27%15%4
447
21st to endDay
20%320%30%21%7%
485
June1st to 10thDay
12%6%24%29%22%8%
450
11th to 20thDay
13%6%24%28%18%11%
450
21st to endDay
9%425%33%21%7%
450
July1st to 10thDay
12%213%30%30%12%
445
11th to 20thDay
8%215%34%25%15%
444
21st to endDay
12%217%32%24%13%
488
August1st to 10thDay
13%416%27%26%14%
446
11th to 20thDay
13%16%28%27%16%
439
21st to endDay
11%215%27%27%18%
495
September1st to 10thDay
16%313%26%25%18%
450
11th to 20thDay
16%313%28%29%10%
450
21st to endDay
29%419%25%17%6%
450
October1st to 10thDay
21%5%18%32%18%7%
450
11th to 20thDay
16%12%25%27%17%3
450
21st to endDay
18%8%30%30%10%3
495
November1st to 10thDay
19%7%27%29%15%4
455
11th to 20thDay
20%11%31%20%15%2
329
21st to endDay
16%16%34%22%11%2
180
December1st to 10thDay
21%12%39%22%6%
180
11th to 20thDay
25%10%32%23%9%1
180
21st to endDay
29%14%30%19%8%1
227
January1st to 10thNight
17%12%36%31%4
760
11th to 20thNight
21%11%31%33%4
739
21st to endNight
17%11%38%29%4
792
February1st to 10thNight
19%11%36%32%3
720
11th to 20thNight
25%13%35%25%1
703
21st to endNight
22%12%35%28%4
592
March1st to 10thNight
16%13%34%32%4
720
11th to 20thNight
24%12%30%27%7%1
720
21st to endNight
19%10%33%31%6%
792
April1st to 10thNight
17%13%36%27%7%
720
11th to 20thNight
15%10%29%32%13%1
711
21st to endNight
20%6%28%37%9%
702
May1st to 10thNight
20%6%24%34%14%1
540
11th to 20thNight
20%320%37%18%2
531
21st to endNight
17%12%37%29%4
490
June1st to 10thNight
10%114%44%29%2
360
11th to 20thNight
12%113%41%31%2
360
21st to endNight
12%111%48%26%2
360
July1st to 10thNight
13%26%35%38%7%
497
11th to 20thNight
6%15%38%43%8%
522
21st to endNight
9%110%37%36%5%
591
August1st to 10thNight
13%16%40%36%5%
537
11th to 20thNight
15%7%33%40%5%
552
21st to endNight
13%28%35%37%6%
792
September1st to 10thNight
17%29%33%32%8%
720
11th to 20thNight
15%310%35%31%7%
720
21st to endNight
27%215%31%22%3
720
October1st to 10thNight
21%316%31%25%5%
720
11th to 20thNight
17%6%23%35%17%2
720
21st to endNight
17%6%26%38%13%1
792
November1st to 10thNight
17%5%26%36%14%2
740
11th to 20thNight
17%8%27%36%11%1
760
21st to endNight
16%7%30%36%10%1
760
December1st to 10thNight
20%8%31%34%7%1
760
11th to 20thNight
16%8%31%39%6%
760
21st to endNight
19%8%29%40%4
836
Page generated at 2024-01-12 07:31:49. Version ac07

Copyright 2024 Attilla Danko


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