Behlen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Behlen Observatory clear sky chart from 2005-11-05 to 2017-04-28.

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
42%
15568
22%
45725
13%
15523
50%
15523
32%
27253

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
38%
900
21%
2327
10%
895
46%
895
31%
1493
January16th to endNight
30%
960
25%
2496
10%
960
39%
960
31%
1608
February1st to 15thNight
28%
880
13%
2288
4%
876
33%
876
23%
1625
February16th to endNight
34%
669
25%
1931
8%
669
46%
669
32%
1158
March1st to 15thNight
37%
569
23%
2009
15%
569
47%
569
28%
1146
March16th to endNight
28%
576
16%
2112
11%
576
37%
576
21%
1332
April1st to 15thNight
32%
539
21%
1864
14%
539
45%
539
25%
1099
April16th to endNight
35%
534
21%
1602
16%
534
45%
534
26%
1026
May1st to 15thNight
36%
494
19%
1485
13%
494
44%
494
26%
950
May16th to endNight
42%
513
14%
1519
7%
513
48%
513
32%
925
June1st to 15thNight
53%
495
18%
1320
8%
495
57%
495
36%
802
June16th to endNight
63%
495
23%
1323
10%
495
67%
495
52%
685
July1st to 15thNight
66%
465
17%
1395
11%
465
69%
465
43%
746
July16th to endNight
77%
522
24%
1468
21%
489
80%
489
56%
701
August1st to 15thNight
71%
494
23%
1475
24%
491
74%
491
42%
805
August16th to endNight
61%
525
25%
1591
26%
525
66%
525
38%
904
September1st to 15thNight
56%
495
27%
1758
25%
495
65%
495
36%
911
September16th to endNight
55%
492
30%
1815
26%
492
65%
492
39%
860
October1st to 15thNight
41%
615
28%
1935
18%
615
49%
615
31%
1119
October16th to endNight
46%
704
26%
2112
16%
704
54%
704
38%
1171
November1st to 15thNight
44%
872
29%
2272
13%
872
53%
872
42%
1291
November16th to endNight
36%
900
25%
2477
12%
900
45%
900
34%
1484
December1st to 15thNight
30%
900
18%
2520
8%
900
38%
900
26%
1656
December16th to endNight
34%
960
21%
2631
9%
960
43%
960
26%
1756

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
11%6%25%42445%7%43%
45964
Night
10%6%24423447%49%
45920
Cloud CoverDay
11%6%25%5%2445%7%43%
45962
Night
10%6%24423447%49%
45920
TransparencyDay
39%314%26%14%1
45662
Night
35%313%27%17%2
45725
SeeingDay
14%5%27%31%17%3
5328
Night
13%7%30%36%12%
15571

By Month

Percentage Cloud Cover SW (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
8%6%26%7%34447%43%
1611
16th to endDay
10%9%28%5%34446%40%
1728
February1st to 15thDay
11%9%48%5%35%46%7%33%
1575
16th to endDay
11%7%36%5%25%45%7%40%
1551
March1st to 15thDay
10%8%46%433446%43%
1950
16th to endDay
17%9%35%5%23446%36%
2112
April1st to 15thDay
16%7%35%424335%41%
2082
16th to endDay
21%8%25%424346%35%
2305
May1st to 15thDay
14%6%35%5%35%46%7%36%
2136
16th to endDay
13%6%26%5%25%5%5%8%36%
2241
June1st to 15thDay
11%5%25%5%25%5%5%8%41%
2310
16th to endDay
6%414325%5%5%9%51%
2304
July1st to 15thDay
8%3244245%6%11%46%
2145
16th to endDay
5%313325%5%6%10%52%
2284
August1st to 15thDay
5%4245%35%5%7%9%44%
2145
16th to endDay
9%3244345%5%8%48%
2259
September1st to 15thDay
13%5%1442435%5%47%
1870
16th to endDay
8%413423246%58%
1815
October1st to 15thDay
12%5%24423435%50%
1683
16th to endDay
12%5%24423345%50%
1760
November1st to 15thDay
8%423413435%57%
1586
16th to endDay
12%8%345%33437%44%
1482
December1st to 15thDay
11%11%36%6%24446%38%
1440
16th to endDay
13%10%48%6%34436%34%
1590
January1st to 15thNight
7%6%36%5%4446%7%45%
2327
16th to endNight
10%9%37%422245%48%
2496
February1st to 15thNight
12%10%47%6%34446%36%
2278
16th to endNight
8%6%24325%45%7%47%
1931
March1st to 15thNight
10%7%35%313346%49%
2009
16th to endNight
15%8%35%42445%7%38%
2112
April1st to 15thNight
13%5%35%423447%46%
1864
16th to endNight
16%7%24423345%44%
1602
May1st to 15thNight
14%6%25%5%2445%7%41%
1484
16th to endNight
11%5%244135%47%46%
1517
June1st to 15thNight
8%42342445%8%49%
1320
16th to endNight
221232445%10%59%
1323
July1st to 15thNight
5%32342445%9%54%
1485
16th to endNight
211231335%10%62%
1576
August1st to 15thNight
33135%245%6%9%53%
1485
16th to endNight
9%5%14323336%55%
1591
September1st to 15thNight
10%41332335%6%55%
1758
16th to endNight
8%312223346%60%
1815
October1st to 15thNight
15%314323445%51%
1935
16th to endNight
12%413312346%56%
2112
November1st to 15thNight
6%414323345%58%
2272
16th to endNight
10%5%24313346%52%
2477
December1st to 15thNight
13%8%35%5%23447%41%
2520
16th to endNight
11%10%37%414347%41%
2631

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
8%7%25%8%34447%43%
1611
16th to endDay
10%9%38%5%24347%39%
1728
February1st to 15thDay
11%9%48%5%35%45%7%34%
1575
16th to endDay
11%8%26%5%25%447%40%
1551
March1st to 15thDay
9%9%46%424347%43%
1950
16th to endDay
17%9%45%5%2345%6%35%
2110
April1st to 15thDay
16%7%345%34345%41%
2082
16th to endDay
21%8%35%424446%35%
2305
May1st to 15thDay
14%6%36%5%35%46%7%36%
2136
16th to endDay
13%6%27%5%25%5%6%7%36%
2241
June1st to 15thDay
11%6%35%5%25%46%8%41%
2310
16th to endDay
6%314425%45%9%51%
2304
July1st to 15thDay
8%4244245%6%11%46%
2145
16th to endDay
5%3133345%6%10%52%
2284
August1st to 15thDay
6%4245%35%6%7%10%44%
2145
16th to endDay
9%42343445%8%48%
2259
September1st to 15thDay
13%5%145%24445%47%
1870
16th to endDay
8%414422346%58%
1815
October1st to 15thDay
12%5%25%423445%49%
1683
16th to endDay
12%5%24423345%49%
1760
November1st to 15thDay
8%424423345%56%
1586
16th to endDay
12%8%345%23346%44%
1482
December1st to 15thDay
11%11%36%5%33447%37%
1440
16th to endDay
12%10%48%6%34446%34%
1590
January1st to 15thNight
7%6%26%5%3446%7%44%
2327
16th to endNight
10%9%37%422345%47%
2496
February1st to 15thNight
12%10%47%6%34447%36%
2278
16th to endNight
8%6%24425%46%7%47%
1931
March1st to 15thNight
10%7%35%422346%49%
2009
16th to endNight
15%8%35%42345%8%38%
2112
April1st to 15thNight
12%6%34413447%46%
1864
16th to endNight
16%8%24423345%44%
1602
May1st to 15thNight
14%6%25%5%2445%7%42%
1484
16th to endNight
11%5%344235%46%47%
1517
June1st to 15thNight
7%5%145%2445%8%49%
1320
16th to endNight
221232446%9%59%
1323
July1st to 15thNight
432432435%10%54%
1485
16th to endNight
211231345%10%62%
1576
August1st to 15thNight
33145%25%5%6%9%53%
1485
16th to endNight
9%424423336%55%
1591
September1st to 15thNight
9%42332435%6%55%
1758
16th to endNight
8%312213446%60%
1815
October1st to 15thNight
15%413323445%51%
1935
16th to endNight
12%413313346%55%
2112
November1st to 15thNight
6%424323446%58%
2272
16th to endNight
10%5%24323346%52%
2477
December1st to 15thNight
13%8%26%5%24447%41%
2520
16th to endNight
11%10%37%5%13347%41%
2631

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
41%219%24%11%
1612
16th to endDay
46%216%21%12%
1728
February1st to 15thDay
49%416%21%7%
1576
16th to endDay
43%413%23%13%2
1551
March1st to 15thDay
42%313%25%13%1
1950
16th to endDay
49%415%21%7%1
2112
April1st to 15thDay
45%312%25%12%1
2082
16th to endDay
50%211%22%10%1
2305
May1st to 15thDay
46%411%24%13%
2136
16th to endDay
45%416%22%10%
2245
June1st to 15thDay
39%416%24%12%1
2310
16th to endDay
29%316%32%13%3
2304
July1st to 15thDay
33%316%30%13%3
2006
16th to endDay
25%416%32%20%1
2129
August1st to 15thDay
32%5%18%26%15%1
2131
16th to endDay
33%410%30%16%5%
2259
September1st to 15thDay
37%39%26%18%3
1870
16th to endDay
29%212%33%22%1
1815
October1st to 15thDay
37%29%24%21%2
1683
16th to endDay
36%212%26%19%2
1760
November1st to 15thDay
30%213%27%24%1
1586
16th to endDay
41%213%29%12%
1482
December1st to 15thDay
47%314%24%9%1
1440
16th to endDay
51%314%19%10%
1590
January1st to 15thNight
38%316%26%14%
2327
16th to endNight
40%215%27%13%1
2496
February1st to 15thNight
50%214%22%9%1
2288
16th to endNight
35%314%28%15%3
1931
March1st to 15thNight
36%211%28%17%3
2009
16th to endNight
44%414%24%10%1
2112
April1st to 15thNight
38%39%29%17%1
1864
16th to endNight
42%312%24%15%1
1602
May1st to 15thNight
41%312%25%15%2
1485
16th to endNight
36%419%24%14%1
1519
June1st to 15thNight
32%5%16%28%14%2
1320
16th to endNight
19%414%34%21%4
1323
July1st to 15thNight
26%318%30%17%4
1395
16th to endNight
17%316%39%20%2
1468
August1st to 15thNight
26%5%16%31%18%2
1475
16th to endNight
31%39%26%20%8%
1591
September1st to 15thNight
29%311%27%21%7%
1758
16th to endNight
25%210%34%22%4
1815
October1st to 15thNight
35%29%23%24%4
1935
16th to endNight
30%211%29%23%2
2112
November1st to 15thNight
27%213%26%26%4
2272
16th to endNight
34%214%28%18%1
2477
December1st to 15thNight
43%315%26%10%
2520
16th to endNight
44%315%23%11%1
2631

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
22%6%33%28%7%1
90
16th to endDay
20%438%28%7%1
96
February1st to 15thDay
18%14%42%18%5%
90
16th to endDay
13%12%33%24%11%4
81
March1st to 15thDay
11%14%46%21%42
90
16th to endDay
29%335%23%8%
96
April1st to 15thDay
17%8%34%27%10%
201
16th to endDay
26%9%32%21%8%1
437
May1st to 15thDay
18%10%31%24%12%1
393
16th to endDay
17%5%29%28%16%3
437
June1st to 15thDay
13%427%32%19%3
420
16th to endDay
6%225%37%24%3
420
July1st to 15thDay
11%121%34%25%6%
400
16th to endDay
5%117%42%28%3
445
August1st to 15thDay
5%16%45%26%5%
418
16th to endDay
11%319%34%26%4
427
September1st to 15thDay
15%126%37%16%2
145
16th to endDay
10%336%23%25%1
90
October1st to 15thDay
11%6%22%35%13%11%
90
16th to endDay
8%5%45%30%10%
96
November1st to 15thDay
7%5%32%35%11%7%
90
16th to endDay
41%5%27%21%4
90
December1st to 15thDay
24%11%37%18%5%2
90
16th to endDay
23%13%37%20%4
96
January1st to 15thNight
10%11%33%34%8%
900
16th to endNight
15%7%37%32%5%
960
February1st to 15thNight
15%8%38%30%6%
880
16th to endNight
12%8%39%32%6%
669
March1st to 15thNight
14%9%31%33%11%
569
16th to endNight
20%7%37%28%6%
576
April1st to 15thNight
14%12%34%33%5%
539
16th to endNight
20%8%30%31%8%
534
May1st to 15thNight
15%9%32%29%11%
495
16th to endNight
13%5%31%36%13%1
515
June1st to 15thNight
12%424%45%13%
495
16th to endNight
4226%48%17%
495
July1st to 15thNight
6%118%45%26%1
465
16th to endNight
313%53%27%1
522
August1st to 15thNight
4214%53%23%2
494
16th to endNight
11%115%40%28%2
525
September1st to 15thNight
11%224%36%21%3
495
16th to endNight
8%328%36%21%1
492
October1st to 15thNight
18%6%30%30%11%2
615
16th to endNight
14%5%28%39%11%
704
November1st to 15thNight
9%9%32%36%10%1
872
16th to endNight
14%10%34%31%9%
900
December1st to 15thNight
18%10%33%30%6%
900
16th to endNight
17%7%31%35%7%
960
Page generated at 2017-04-29 08:44:55. Version ac04

Copyright 2017 A. Danko


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