EKU Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the EKU Observatory clear sky chart from 2005-11-05 to 2017-09-16.

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
45%
15421
20%
46922
15%
15377
51%
15377
25%
30419

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
20%
720
15%
2327
8%
716
29%
716
14%
1717
January16th to endNight
23%
768
18%
2496
10%
768
33%
768
17%
1738
February1st to 15thNight
23%
704
14%
2144
7%
701
31%
701
17%
1533
February16th to endNight
19%
631
16%
1893
7%
631
27%
631
15%
1380
March1st to 15thNight
27%
720
17%
2082
9%
720
34%
720
19%
1469
March16th to endNight
36%
768
15%
2112
10%
768
40%
768
22%
1483
April1st to 15thNight
38%
593
22%
1851
14%
593
48%
593
28%
1139
April16th to endNight
41%
540
22%
1657
14%
540
49%
540
26%
1069
May1st to 15thNight
42%
540
14%
1620
10%
540
47%
540
21%
1125
May16th to endNight
60%
562
19%
1684
15%
561
63%
561
30%
1060
June1st to 15thNight
71%
540
18%
1620
16%
540
74%
540
38%
918
June16th to endNight
71%
540
18%
1620
15%
540
73%
540
35%
936
July1st to 15thNight
66%
510
13%
1530
12%
510
67%
510
28%
963
July16th to endNight
74%
564
21%
1595
18%
531
77%
531
38%
913
August1st to 15thNight
76%
539
23%
1609
21%
536
77%
536
37%
933
August16th to endNight
76%
573
23%
1857
25%
573
76%
573
35%
1042
September1st to 15thNight
65%
676
24%
1936
21%
676
67%
676
32%
1200
September16th to endNight
60%
661
27%
1826
22%
661
63%
661
34%
1121
October1st to 15thNight
56%
660
32%
1847
28%
660
61%
660
38%
1082
October16th to endNight
43%
704
26%
2164
20%
704
52%
704
29%
1337
November1st to 15thNight
49%
700
29%
2276
24%
700
58%
700
32%
1292
November16th to endNight
36%
720
22%
2340
16%
720
46%
720
24%
1490
December1st to 15thNight
30%
720
15%
2340
11%
720
36%
720
16%
1665
December16th to endNight
25%
768
17%
2496
9%
768
36%
768
15%
1814

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
14%7%35%5%3445%7%37%
47823
Night
13%6%35%5%2445%7%40%
47115
Cloud CoverDay
14%7%35%5%3445%7%37%
47833
Night
13%6%35%5%2445%7%40%
47119
TransparencyDay
45%5%11%21%13%2
47534
Night
42%411%22%15%3
46922
SeeingDay
17%6%18%23%21%13%
5288
Night
17%7%22%29%19%2
15422

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
21%9%49%6%34345%27%
1611
16th to endDay
17%12%5%6%423345%32%
1728
February1st to 15thDay
16%11%6%8%5%25%345%30%
1717
16th to endDay
15%11%37%7%44447%29%
1580
March1st to 15thDay
23%8%236%23345%33%
1867
16th to endDay
19%9%36%5%2445%6%32%
2112
April1st to 15thDay
14%6%27%5%24446%40%
2106
16th to endDay
15%6%26%43445%7%38%
2301
May1st to 15thDay
19%7%35%6%35%45%7%31%
2331
16th to endDay
8%6%36%6%35%6%6%8%39%
2431
June1st to 15thDay
8%425%6%46%6%6%9%39%
2340
16th to endDay
7%435%5%35%6%6%9%41%
2340
July1st to 15thDay
13%5%25%5%45%5%6%9%34%
2340
16th to endDay
10%5%26%5%36%7%8%10%34%
2465
August1st to 15thDay
9%5%25%5%45%6%6%9%38%
2340
16th to endDay
8%426%6%35%5%7%9%38%
2342
September1st to 15thDay
14%5%26%435%45%7%39%
2011
16th to endDay
15%6%2442345%6%45%
1824
October1st to 15thDay
11%425%423346%51%
1782
16th to endDay
20%6%23213335%46%
1705
November1st to 15thDay
13%6%145%13345%49%
1582
16th to endDay
17%8%34423345%42%
1620
December1st to 15thDay
23%10%35%5%2444431%
1620
16th to endDay
18%12%47%433446%30%
1728
January1st to 15thNight
18%10%5%9%6%34435%28%
2327
16th to endNight
18%9%47%5%24435%34%
2496
February1st to 15thNight
18%9%48%5%23447%31%
2142
16th to endNight
18%12%37%6%3435%5%30%
1893
March1st to 15thNight
17%9%35%6%24445%35%
2070
16th to endNight
15%8%46%5%34346%36%
2112
April1st to 15thNight
12%5%2442445%7%45%
1851
16th to endNight
12%6%36%4346%47%41%
1657
May1st to 15thNight
15%7%36%5%35%5%5%6%33%
1620
16th to endNight
8%325%6%346%6%10%41%
1682
June1st to 15thNight
5%3144245%8%11%47%
1620
16th to endNight
5%324425%5%9%13%43%
1620
July1st to 15thNight
7%5%26%5%35%46%9%42%
1620
16th to endNight
5%224426%6%6%11%46%
1703
August1st to 15thNight
5%3145%245%6%10%49%
1620
16th to endNight
4325%335%6%5%11%48%
1857
September1st to 15thNight
11%42442446%8%45%
1936
16th to endNight
9%5%25%5%3345%6%48%
1826
October1st to 15thNight
10%5%13423447%51%
1847
16th to endNight
16%424423445%46%
2164
November1st to 15thNight
13%6%24323246%51%
2276
16th to endNight
19%6%34323346%42%
2340
December1st to 15thNight
21%9%47%423435%31%
2340
16th to endNight
20%11%46%5%24445%29%
2496

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%9%39%6%24445%27%
1611
16th to endDay
18%12%5%5%43334433%
1728
February1st to 15thDay
16%11%5%8%5%34345%30%
1717
16th to endDay
16%11%38%6%44446%29%
1580
March1st to 15thDay
23%8%335%33445%33%
1878
16th to endDay
18%9%35%5%24446%33%
2111
April1st to 15thDay
14%6%26%6%2445%6%40%
2106
16th to endDay
15%5%35%5%34448%38%
2301
May1st to 15thDay
19%7%35%5%35%45%7%31%
2331
16th to endDay
9%6%26%6%35%5%5%7%39%
2431
June1st to 15thDay
8%415%5%46%6%7%9%39%
2340
16th to endDay
7%425%5%46%5%7%9%41%
2340
July1st to 15thDay
13%6%25%6%45%5%5%9%35%
2340
16th to endDay
10%5%25%5%36%7%7%10%34%
2465
August1st to 15thDay
10%5%16%5%45%6%7%8%38%
2340
16th to endDay
8%425%6%45%5%7%10%38%
2342
September1st to 15thDay
14%6%25%435%45%7%39%
2011
16th to endDay
16%5%2432345%6%44%
1824
October1st to 15thDay
11%424423447%50%
1782
16th to endDay
20%6%23213335%46%
1705
November1st to 15thDay
13%6%23413345%50%
1582
16th to endDay
16%9%25%423345%42%
1620
December1st to 15thDay
22%10%35%42433432%
1620
16th to endDay
18%12%46%434446%30%
1728
January1st to 15thNight
18%10%5%8%6%3344429%
2327
16th to endNight
18%9%47%6%24435%34%
2496
February1st to 15thNight
18%10%48%423437%32%
2135
16th to endNight
18%12%38%5%34345%30%
1893
March1st to 15thNight
18%9%36%5%2434436%
2082
16th to endNight
15%8%47%5%3335%6%36%
2112
April1st to 15thNight
13%5%2442435%6%46%
1851
16th to endNight
12%5%36%5%2445%7%41%
1657
May1st to 15thNight
16%6%46%5%25%5%6%6%33%
1619
16th to endNight
8%3345%25%6%7%10%41%
1682
June1st to 15thNight
5%4144345%7%12%47%
1620
16th to endNight
5%224335%5%9%12%44%
1620
July1st to 15thNight
8%5%35%6%35%5%5%8%43%
1620
16th to endNight
6%324335%5%7%10%46%
1703
August1st to 15thNight
6%3145%345%5%10%49%
1620
16th to endNight
5%3244345%6%11%48%
1857
September1st to 15thNight
12%42442346%8%46%
1936
16th to endNight
10%425%5%24445%48%
1826
October1st to 15thNight
10%5%2342345%7%51%
1847
16th to endNight
16%424423445%46%
2164
November1st to 15thNight
13%5%24313346%51%
2276
16th to endNight
18%7%24323445%42%
2340
December1st to 15thNight
21%9%46%423445%31%
2340
16th to endNight
21%11%47%5%2335%430%
2496

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
58%37%17%10%
1612
16th to endDay
53%310%24%8%1
1728
February1st to 15thDay
56%37%22%8%1
1719
16th to endDay
55%311%19%8%1
1580
March1st to 15thDay
52%310%18%12%1
1878
16th to endDay
51%411%22%8%
2112
April1st to 15thDay
43%49%24%14%2
2106
16th to endDay
44%410%23%13%4
2301
May1st to 15thDay
50%5%12%19%10%1
2331
16th to endDay
41%5%15%23%11%1
2435
June1st to 15thDay
38%7%14%24%13%1
2340
16th to endDay
35%8%14%19%18%2
2340
July1st to 15thDay
45%6%13%16%14%2
2201
16th to endDay
40%8%14%20%12%2
2310
August1st to 15thDay
40%8%13%21%14%1
2327
16th to endDay
38%7%12%23%15%3
2342
September1st to 15thDay
43%5%10%18%16%4
2011
16th to endDay
39%48%20%24%2
1824
October1st to 15thDay
34%48%24%24%4
1782
16th to endDay
40%28%27%18%2
1705
November1st to 15thDay
37%37%23%21%5%
1582
16th to endDay
45%26%24%19%1
1620
December1st to 15thDay
54%310%20%9%2
1620
16th to endDay
55%312%21%7%
1728
January1st to 15thNight
58%38%15%12%1
2327
16th to endNight
52%39%22%10%1
2496
February1st to 15thNight
54%29%21%9%1
2144
16th to endNight
56%310%19%9%
1893
March1st to 15thNight
51%38%20%13%2
2082
16th to endNight
48%411%24%9%
2112
April1st to 15thNight
38%310%22%20%4
1851
16th to endNight
42%49%23%14%5%
1657
May1st to 15thNight
48%5%12%20%10%2
1620
16th to endNight
35%8%15%24%13%2
1684
June1st to 15thNight
27%6%18%27%15%3
1620
16th to endNight
29%7%17%24%16%4
1620
July1st to 15thNight
36%7%13%21%15%4
1530
16th to endNight
26%8%18%26%16%3
1595
August1st to 15thNight
29%6%16%23%20%3
1609
16th to endNight
28%8%13%26%20%3
1857
September1st to 15thNight
35%5%12%21%19%5%
1936
16th to endNight
36%47%21%27%2
1826
October1st to 15thNight
30%48%26%22%5%
1847
16th to endNight
40%27%24%21%3
2164
November1st to 15thNight
34%26%24%25%6%
2276
16th to endNight
43%37%24%18%1
2340
December1st to 15thNight
55%310%18%10%2
2340
16th to endNight
56%49%18%9%1
2496

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
30%14%41%6%5%2
90
16th to endDay
23%17%44%10%3
96
February1st to 15thDay
17%21%42%13%5%
90
16th to endDay
30%11%30%20%6%
81
March1st to 15thDay
29%15%31%21%2
168
16th to endDay
22%10%26%27%11%1
288
April1st to 15thDay
17%12%23%27%17%2
270
16th to endDay
18%8%25%24%14%8%
270
May1st to 15thDay
22%9%22%26%11%8%
288
16th to endDay
14%318%26%22%13%
331
June1st to 15thDay
9%113%23%36%16%
315
16th to endDay
8%418%26%25%16%
315
July1st to 15thDay
19%213%22%22%19%
305
16th to endDay
11%25%19%29%31%
326
August1st to 15thDay
11%5%20%32%28%
313
16th to endDay
12%6%26%30%24%
332
September1st to 15thDay
18%110%15%30%23%
315
16th to endDay
18%210%20%28%19%
257
October1st to 15thDay
13%318%27%22%13%
255
16th to endDay
17%5%22%29%21%2
217
November1st to 15thDay
14%5%23%21%25%10%
90
16th to endDay
27%10%26%28%6%
90
December1st to 15thDay
18%11%34%26%6%2
90
16th to endDay
22%16%44%12%3
96
January1st to 15thNight
26%15%31%20%5%
720
16th to endNight
25%12%34%26%2
768
February1st to 15thNight
24%11%35%27%1
704
16th to endNight
26%12%36%21%2
631
March1st to 15thNight
24%11%30%27%6%
720
16th to endNight
19%9%27%32%10%
768
April1st to 15thNight
17%9%32%28%12%
593
16th to endNight
14%6%32%30%14%2
540
May1st to 15thNight
19%425%30%17%2
540
16th to endNight
10%215%35%29%6%
563
June1st to 15thNight
7%112%40%34%3
540
16th to endNight
5%212%40%35%3
540
July1st to 15thNight
10%19%35%38%3
510
16th to endNight
7%17%30%42%9%
564
August1st to 15thNight
7%5%33%45%7%
539
16th to endNight
6%16%35%43%7%
573
September1st to 15thNight
14%29%26%40%6%
676
16th to endNight
12%412%28%37%4
661
October1st to 15thNight
13%5%19%29%27%5%
660
16th to endNight
19%5%26%29%17%1
704
November1st to 15thNight
17%5%24%29%20%2
700
16th to endNight
22%9%26%29%10%
720
December1st to 15thNight
28%10%24%27%7%
720
16th to endNight
27%11%29%25%5%
768
Page generated at 2017-09-17 12:05:42. Version ac04

Copyright 2017 A. Danko


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