C.E.K. Mees Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the C.E.K. Mees Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
28%
14743
12%
46519
8%
14708
34%
14708
15%
34056

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
11%
719
7%
2506
4%
716
16%
716
6%
2100
January16th to endNight
12%
768
7%
2514
3%
768
17%
768
6%
2085
February1st to 15thNight
10%
704
6%
2287
1%
701
15%
701
6%
1883
February16th to endNight
14%
631
9%
1968
3%
631
21%
631
9%
1572
March1st to 15thNight
17%
720
11%
2024
4%
720
24%
720
11%
1600
March16th to endNight
22%
768
15%
2082
7%
768
29%
768
16%
1561
April1st to 15thNight
21%
718
13%
1796
5%
718
25%
718
18%
1360
April16th to endNight
33%
666
17%
1746
7%
666
38%
666
26%
1208
May1st to 15thNight
40%
435
15%
1515
10%
435
47%
435
16%
1038
May16th to endNight
42%
374
16%
1497
16%
374
52%
374
21%
908
June1st to 15thNight
41%
360
12%
1440
12%
360
49%
360
18%
914
June16th to endNight
47%
360
12%
1440
15%
360
53%
360
21%
856
July1st to 15thNight
58%
340
14%
1360
18%
340
63%
340
26%
782
July16th to endNight
54%
426
15%
1464
14%
400
59%
400
26%
864
August1st to 15thNight
54%
527
15%
1575
11%
524
57%
524
30%
979
August16th to endNight
55%
638
19%
1688
12%
638
58%
638
34%
1080
September1st to 15thNight
49%
660
20%
1650
10%
660
53%
660
31%
1098
September16th to endNight
48%
657
20%
1814
15%
657
51%
657
27%
1214
October1st to 15thNight
33%
660
15%
1980
12%
660
39%
660
19%
1387
October16th to endNight
18%
704
8%
2117
3%
704
24%
704
12%
1643
November1st to 15thNight
24%
700
16%
2328
11%
700
32%
700
16%
1621
November16th to endNight
20%
720
11%
2520
7%
720
26%
720
10%
1921
December1st to 15thNight
12%
720
5%
2520
2%
720
17%
720
6%
2069
December16th to endNight
11%
768
4%
2688
1%
768
15%
768
5%
2313

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%10%48%7%35%45%6%28%
47454
Night
14%10%48%7%35%45%6%29%
46694
Cloud CoverDay
14%11%5%8%7%35%45%6%27%
47464
Night
13%10%48%7%35%45%6%29%
46699
TransparencyDay
55%7%11%16%8%
47147
Night
53%7%12%16%8%
46519
SeeingDay
19%16%27%20%10%4
5350
Night
20%17%28%22%9%1
14745

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
12%15%8%13%12%5%6%335%13%
1432
16th to endDay
8%15%8%16%13%47%5%3312%
1707
February1st to 15thDay
14%15%9%14%11%45%33314%
1575
16th to endDay
14%16%7%12%9%4443418%
1503
March1st to 15thDay
14%16%7%10%7%3434421%
1924
16th to endDay
14%13%6%9%6%3435%6%26%
2142
April1st to 15thDay
16%13%5%8%6%24345%28%
2152
16th to endDay
15%11%36%424445%36%
2223
May1st to 15thDay
15%9%37%6%345%46%32%
2436
16th to endDay
14%6%25%6%35%45%9%35%
2618
June1st to 15thDay
20%6%36%5%35%46%6%31%
2520
16th to endDay
14%5%16%6%45%5%6%8%34%
2520
July1st to 15thDay
10%5%25%6%45%7%7%8%36%
2520
16th to endDay
12%5%26%7%47%5%7%9%31%
2602
August1st to 15thDay
12%6%37%6%46%5%5%9%32%
2288
16th to endDay
13%7%26%6%5%5%5%7%7%33%
2149
September1st to 15thDay
18%5%35%5%45%446%36%
1980
16th to endDay
16%6%36%5%35%5%5%6%37%
1813
October1st to 15thDay
20%9%36%6%2345%6%31%
1650
16th to endDay
23%14%47%5%33435%21%
1755
November1st to 15thDay
12%16%36%6%44445%30%
1529
16th to endDay
15%16%6%9%5%445%46%21%
1440
December1st to 15thDay
14%21%7%13%7%2334416%
1440
16th to endDay
15%15%11%14%9%35%5%349%
1536
January1st to 15thNight
13%16%6%14%10%45%445%14%
2506
16th to endNight
9%14%7%13%13%6%6%5%5%414%
2514
February1st to 15thNight
12%14%7%14%9%35%5%4417%
2285
16th to endNight
11%11%7%14%11%35%35%6%18%
1968
March1st to 15thNight
14%12%6%10%8%34445%24%
2012
16th to endNight
15%11%5%8%7%34345%30%
2082
April1st to 15thNight
16%13%45%6%25%445%31%
1796
16th to endNight
14%8%27%424446%39%
1746
May1st to 15thNight
16%6%25%6%35%5%5%7%33%
1515
16th to endNight
10%6%26%4245%5%7%43%
1495
June1st to 15thNight
15%5%25%5%25%5%6%7%37%
1440
16th to endNight
11%424425%5%6%8%42%
1440
July1st to 15thNight
5%5%345%25%6%5%11%44%
1440
16th to endNight
10%4245%35%6%7%8%43%
1564
August1st to 15thNight
9%415%5%345%5%8%45%
1586
16th to endNight
9%5%25%43345%7%47%
1688
September1st to 15thNight
16%425%424447%45%
1650
16th to endNight
15%5%24335%5%6%7%40%
1814
October1st to 15thNight
18%8%36%5%2435%6%34%
1980
16th to endNight
23%11%37%5%335%45%24%
2117
November1st to 15thNight
15%12%26%5%34447%33%
2328
16th to endNight
15%15%5%8%6%4445%5%24%
2520
December1st to 15thNight
16%16%6%10%7%36%5%46%16%
2520
16th to endNight
15%14%10%13%9%45%45%412%
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
12%15%7%15%11%5%6%43413%
1432
16th to endDay
8%15%9%15%13%47%44412%
1707
February1st to 15thDay
13%15%10%13%11%46%43314%
1575
16th to endDay
15%15%7%12%10%3443319%
1503
March1st to 15thDay
14%16%7%11%6%2435%5%22%
1935
16th to endDay
14%13%6%9%5%4445%6%25%
2141
April1st to 15thDay
16%13%6%8%6%2335%5%28%
2152
16th to endDay
14%12%35%5%24346%36%
2223
May1st to 15thDay
14%9%47%6%35%446%32%
2436
16th to endDay
13%7%36%6%35%5%5%7%34%
2618
June1st to 15thDay
19%7%36%6%35%47%6%30%
2520
16th to endDay
12%5%26%7%46%5%6%9%32%
2520
July1st to 15thDay
8%436%7%46%6%7%9%34%
2520
16th to endDay
11%5%36%7%47%6%7%9%30%
2602
August1st to 15thDay
11%7%36%7%46%5%6%8%30%
2288
16th to endDay
13%6%36%7%36%6%5%8%31%
2149
September1st to 15thDay
17%7%25%6%4436%6%35%
1980
16th to endDay
16%7%25%5%35%5%5%6%37%
1813
October1st to 15thDay
19%10%47%5%23347%30%
1650
16th to endDay
22%16%5%6%6%4344421%
1755
November1st to 15thDay
11%16%37%7%45%45%5%29%
1529
16th to endDay
15%15%6%9%6%34446%21%
1440
December1st to 15thDay
13%20%9%13%7%3434316%
1440
16th to endDay
15%15%10%14%9%45%5%35%9%
1536
January1st to 15thNight
13%14%7%13%10%5%5%5%5%5%13%
2506
16th to endNight
9%14%8%11%12%7%7%5%5%5%14%
2514
February1st to 15thNight
11%15%7%13%10%46%5%4417%
2278
16th to endNight
11%11%6%14%11%5%5%446%19%
1968
March1st to 15thNight
14%12%5%11%7%35%34425%
2024
16th to endNight
15%10%5%8%8%34435%30%
2082
April1st to 15thNight
16%12%45%6%3445%6%30%
1796
16th to endNight
13%8%37%5%24446%39%
1746
May1st to 15thNight
15%7%26%6%46%6%5%7%32%
1515
16th to endNight
10%6%25%5%35%46%7%42%
1495
June1st to 15thNight
14%6%25%5%35%5%5%8%38%
1440
16th to endNight
10%5%25%425%5%7%8%42%
1440
July1st to 15thNight
5%5%25%5%35%5%6%9%45%
1440
16th to endNight
9%3245%35%6%6%8%43%
1564
August1st to 15thNight
8%415%5%345%6%7%44%
1586
16th to endNight
10%425%5%3445%6%47%
1688
September1st to 15thNight
15%5%1442445%6%45%
1650
16th to endNight
14%6%15%4346%5%7%40%
1814
October1st to 15thNight
17%9%26%5%3435%6%34%
1980
16th to endNight
22%12%36%6%3445%5%24%
2117
November1st to 15thNight
14%11%36%6%235%46%33%
2328
16th to endNight
14%14%5%8%7%35%5%5%5%24%
2520
December1st to 15thNight
17%15%6%10%7%46%5%5%5%15%
2520
16th to endNight
14%14%9%14%9%5%5%44412%
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
73%6%7%8%4
1433
16th to endDay
72%7%9%8%2
1707
February1st to 15thDay
73%5%8%8%3
1577
16th to endDay
68%49%12%4
1503
March1st to 15thDay
63%5%11%12%5%
1935
16th to endDay
57%6%9%16%8%
2142
April1st to 15thDay
58%49%15%10%1
2152
16th to endDay
48%5%8%18%16%2
2223
May1st to 15thDay
51%6%10%19%10%1
2436
16th to endDay
46%8%12%18%12%1
2622
June1st to 15thDay
50%7%12%18%10%
2520
16th to endDay
45%9%13%20%11%
2520
July1st to 15thDay
42%9%11%23%12%
2371
16th to endDay
46%9%14%18%9%1
2439
August1st to 15thDay
47%9%13%18%10%1
2275
16th to endDay
47%7%12%21%9%1
2149
September1st to 15thDay
47%6%11%22%11%
1980
16th to endDay
46%7%10%20%13%1
1813
October1st to 15thDay
53%5%11%16%12%1
1650
16th to endDay
64%414%12%3
1755
November1st to 15thDay
55%6%12%15%8%2
1529
16th to endDay
61%6%12%14%3
1440
December1st to 15thDay
71%6%11%8%1
1440
16th to endDay
75%7%9%5%1
1536
January1st to 15thNight
69%7%9%7%5%
2506
16th to endNight
67%8%10%10%3
2514
February1st to 15thNight
66%7%10%10%4
2287
16th to endNight
64%5%12%11%5%
1968
March1st to 15thNight
60%6%11%15%5%
2024
16th to endNight
55%5%10%15%12%
2082
April1st to 15thNight
53%5%11%17%11%
1796
16th to endNight
44%5%11%20%15%2
1746
May1st to 15thNight
47%8%11%19%12%1
1515
16th to endNight
38%8%15%22%13%1
1497
June1st to 15thNight
42%8%15%21%12%
1440
16th to endNight
35%10%13%23%15%1
1440
July1st to 15thNight
32%9%15%22%18%1
1360
16th to endNight
34%8%15%25%13%2
1464
August1st to 15thNight
34%8%17%27%11%
1575
16th to endNight
34%7%13%29%13%1
1688
September1st to 15thNight
37%6%14%25%13%2
1650
16th to endNight
40%7%13%23%14%
1814
October1st to 15thNight
49%6%12%18%13%
1980
16th to endNight
58%7%17%12%4
2117
November1st to 15thNight
49%7%13%17%9%3
2328
16th to endNight
59%7%13%15%4
2520
December1st to 15thNight
67%7%13%9%2
2520
16th to endNight
73%6%9%8%1
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
13%41%30%6%6%1
60
16th to endDay
19%32%39%7%1
93
February1st to 15thDay
25%44%23%6%
90
16th to endDay
25%23%35%12%1
162
March1st to 15thDay
27%22%27%17%4
270
16th to endDay
22%22%31%14%7%1
288
April1st to 15thDay
27%21%29%14%41
270
16th to endDay
19%19%27%21%9%3
270
May1st to 15thDay
26%18%24%16%11%3
288
16th to endDay
14%13%36%19%10%5%
331
June1st to 15thDay
20%11%30%25%10%2
315
16th to endDay
15%12%32%26%9%3
315
July1st to 15thDay
10%5%29%35%13%5%
305
16th to endDay
13%9%27%30%14%5%
326
August1st to 15thDay
12%7%26%24%17%11%
297
16th to endDay
13%5%20%23%17%19%
268
September1st to 15thDay
18%9%17%20%22%12%
255
16th to endDay
16%11%21%22%17%10%
254
October1st to 15thDay
20%14%23%20%16%4
255
16th to endDay
31%20%26%17%3
271
November1st to 15thDay
17%17%32%18%13%
183
16th to endDay
16%33%28%20%1
60
December1st to 15thDay
21%31%23%18%31
60
16th to endDay
23%43%28%4
64
January1st to 15thNight
22%29%31%12%2
720
16th to endNight
19%26%33%16%2
768
February1st to 15thNight
21%27%35%13%1
704
16th to endNight
20%28%31%17%2
631
March1st to 15thNight
24%26%29%16%2
720
16th to endNight
22%19%30%20%6%
768
April1st to 15thNight
23%19%31%21%2
718
16th to endNight
18%12%32%25%9%1
666
May1st to 15thNight
19%8%24%34%11%
435
16th to endNight
12%7%31%31%16%
375
June1st to 15thNight
15%8%26%33%15%
360
16th to endNight
11%6%29%40%10%
360
July1st to 15thNight
8%223%42%21%1
340
16th to endNight
11%325%36%20%3
426
August1st to 15thNight
11%323%35%22%4
527
16th to endNight
12%422%31%27%2
638
September1st to 15thNight
18%7%19%29%22%3
660
16th to endNight
16%8%23%23%22%5%
657
October1st to 15thNight
23%11%24%22%15%1
660
16th to endNight
29%16%29%19%4
704
November1st to 15thNight
21%15%33%20%6%1
700
16th to endNight
23%24%26%17%6%1
720
December1st to 15thNight
27%25%30%12%3
720
16th to endNight
26%26%31%13%2
768
Page generated at 2017-08-13 14:16:27. Version ac04

Copyright 2017 A. Danko


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