Ellen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ellen Observatory clear sky chart from 2005-11-05 to 2017-10-06.

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
57%
16055
20%
47134
16%
16015
62%
16015
29%
29948

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
43%
719
20%
2327
14%
716
52%
716
24%
1449
January16th to endNight
40%
768
23%
2487
16%
768
52%
768
27%
1516
February1st to 15thNight
41%
701
24%
2105
15%
700
51%
700
30%
1262
February16th to endNight
41%
631
24%
1815
13%
631
53%
631
32%
1106
March1st to 15thNight
50%
720
29%
1980
17%
720
60%
720
42%
1116
March16th to endNight
50%
768
25%
2112
17%
768
56%
768
40%
1220
April1st to 15thNight
52%
718
23%
1976
16%
718
57%
718
41%
1136
April16th to endNight
57%
720
27%
1815
17%
720
61%
720
42%
1085
May1st to 15thNight
67%
589
26%
1669
18%
589
71%
589
41%
935
May16th to endNight
72%
561
21%
1682
17%
561
73%
561
31%
1047
June1st to 15thNight
67%
540
13%
1620
11%
540
68%
540
23%
1117
June16th to endNight
70%
540
8%
1620
7%
540
70%
540
21%
1182
July1st to 15thNight
72%
510
8%
1530
6%
510
72%
510
21%
1107
July16th to endNight
73%
564
9%
1595
8%
531
74%
531
21%
1128
August1st to 15thNight
77%
539
8%
1609
7%
536
77%
536
24%
1127
August16th to endNight
64%
734
10%
2003
9%
734
64%
734
22%
1448
September1st to 15thNight
66%
720
10%
1980
9%
720
66%
720
18%
1465
September16th to endNight
60%
717
13%
1980
13%
717
60%
717
19%
1482
October1st to 15thNight
66%
684
25%
1963
25%
684
69%
684
30%
1240
October16th to endNight
62%
704
26%
2112
26%
704
67%
704
33%
1262
November1st to 15thNight
57%
700
26%
2101
21%
700
65%
700
32%
1264
November16th to endNight
51%
720
27%
2217
19%
720
61%
720
32%
1298
December1st to 15thNight
50%
720
26%
2340
23%
720
58%
720
27%
1433
December16th to endNight
50%
768
22%
2496
20%
768
57%
768
27%
1523

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
9%5%36%6%46%6%6%8%35%
48047
Night
8%425%5%35%5%6%8%44%
47341
TransparencyDay
43%9%13%15%12%5%
47748
Night
35%9%15%19%13%6%
47144
SeeingDay
9%316%22%23%23%
5896
Night
9%319%31%27%7%
16062

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 15thDay
12%6%36%435%5%6%7%38%
1611
16th to endDay
11%5%25%42445%8%45%
1735
February1st to 15thDay
11%6%26%4435%46%44%
1750
16th to endDay
9%435%6%35%45%6%44%
1656
March1st to 15thDay
7%325%6%35%45%8%47%
1980
16th to endDay
9%425%6%4445%6%46%
2111
April1st to 15thDay
7%425%435%5%6%8%45%
1972
16th to endDay
6%425%6%35%5%6%9%43%
2144
May1st to 15thDay
7%4245%35%6%6%7%46%
2289
16th to endDay
9%5%25%7%46%6%6%10%34%
2431
June1st to 15thDay
10%7%38%8%5%8%6%6%9%25%
2340
16th to endDay
10%6%38%8%5%9%7%7%8%24%
2340
July1st to 15thDay
10%7%48%9%6%8%7%7%8%21%
2340
16th to endDay
9%7%49%8%5%7%7%7%8%22%
2465
August1st to 15thDay
6%6%38%9%5%8%8%8%12%21%
2340
16th to endDay
12%7%48%9%46%6%8%8%21%
2183
September1st to 15thDay
10%7%38%8%6%7%7%8%9%22%
1980
16th to endDay
12%7%310%9%5%6%7%7%8%20%
1980
October1st to 15thDay
11%7%36%5%46%5%6%7%34%
1795
16th to endDay
10%5%25%5%26%5%5%6%43%
1760
November1st to 15thDay
6%435%5%35%5%5%7%45%
1757
16th to endDay
11%5%35%5%35%5%48%42%
1740
December1st to 15thDay
11%6%36%5%335%6%7%39%
1620
16th to endDay
8%5%25%5%45%45%8%43%
1728
January1st to 15thNight
11%6%344345%5%8%42%
2327
16th to endNight
13%5%3442346%6%44%
2487
February1st to 15thNight
11%424423445%51%
2103
16th to endNight
8%425%6%24448%48%
1815
March1st to 15thNight
6%31342345%7%56%
1980
16th to endNight
6%42442345%5%57%
2112
April1st to 15thNight
6%41332446%7%54%
1976
16th to endNight
7%31343435%8%53%
1815
May1st to 15thNight
5%21342445%9%55%
1669
16th to endNight
7%215%435%6%7%9%45%
1682
June1st to 15thNight
9%425%5%46%5%8%11%35%
1620
16th to endNight
6%5%47%7%36%6%8%10%32%
1620
July1st to 15thNight
7%5%26%5%47%6%8%11%33%
1620
16th to endNight
5%446%7%36%7%7%11%33%
1703
August1st to 15thNight
5%416%6%47%6%9%12%34%
1620
16th to endNight
10%6%36%6%35%5%7%12%32%
2003
September1st to 15thNight
9%5%26%6%46%7%8%11%30%
1980
16th to endNight
11%7%37%7%47%7%7%10%27%
1980
October1st to 15thNight
7%5%25%435%5%6%9%43%
1963
16th to endNight
9%5%2442445%7%49%
2112
November1st to 15thNight
6%426%6%25%46%7%47%
2101
16th to endNight
8%425%325%45%8%48%
2217
December1st to 15thNight
10%5%24435%447%45%
2340
16th to endNight
9%5%245%3446%8%44%
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
41%7%9%17%16%8%
1612
16th to endDay
36%5%9%16%19%11%
1735
February1st to 15thDay
40%6%7%12%19%13%
1752
16th to endDay
38%5%10%15%19%10%
1656
March1st to 15thDay
33%5%11%23%18%8%
1980
16th to endDay
37%6%11%17%18%8%
2112
April1st to 15thDay
33%8%11%18%21%7%
1972
16th to endDay
34%6%12%20%17%8%
2144
May1st to 15thDay
33%8%11%20%17%8%
2289
16th to endDay
41%9%15%17%13%3
2435
June1st to 15thDay
52%10%15%17%4
2340
16th to endDay
52%12%19%11%3
2340
July1st to 15thDay
55%15%16%11%1
2201
16th to endDay
55%13%17%12%1
2310
August1st to 15thDay
50%16%22%9%1
2327
16th to endDay
55%14%17%11%1
2183
September1st to 15thDay
53%15%17%10%3
1980
16th to endDay
55%12%13%12%42
1980
October1st to 15thDay
44%8%10%12%14%8%
1795
16th to endDay
38%6%8%14%21%10%
1760
November1st to 15thDay
35%6%9%18%21%7%
1757
16th to endDay
40%6%9%15%18%10%
1740
December1st to 15thDay
42%6%12%18%14%5%
1620
16th to endDay
37%6%14%18%13%8%
1728
January1st to 15thNight
38%6%10%20%15%7%
2327
16th to endNight
37%410%17%21%8%
2487
February1st to 15thNight
33%6%8%16%17%16%
2112
16th to endNight
33%7%13%17%20%8%
1815
March1st to 15thNight
25%5%13%28%19%6%
1980
16th to endNight
28%5%12%23%19%9%
2112
April1st to 15thNight
26%6%15%23%18%8%
1976
16th to endNight
27%5%12%26%19%7%
1815
May1st to 15thNight
25%7%15%25%17%7%
1670
16th to endNight
31%10%15%24%16%1
1684
June1st to 15thNight
39%12%19%23%3
1620
16th to endNight
41%15%21%16%4
1620
July1st to 15thNight
41%15%26%13%2
1530
16th to endNight
39%16%26%15%2
1595
August1st to 15thNight
34%17%34%11%
1609
16th to endNight
42%13%26%15%1
2003
September1st to 15thNight
42%18%18%15%5%
1980
16th to endNight
47%12%17%16%41
1980
October1st to 15thNight
34%9%12%19%17%7%
1963
16th to endNight
34%5%10%16%23%10%
2112
November1st to 15thNight
34%5%11%19%20%7%
2101
16th to endNight
32%6%12%18%18%11%
2217
December1st to 15thNight
37%5%12%24%12%6%
2340
16th to endNight
36%7%12%22%12%7%
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
18%10%34%22%10%4
90
16th to endDay
14%18%32%23%7%3
96
February1st to 15thDay
8%22%30%17%17%3
90
16th to endDay
13%12%39%25%8%
157
March1st to 15thDay
6%10%32%31%15%4
270
16th to endDay
9%7%26%29%23%5%
288
April1st to 15thDay
6%10%25%24%25%8%
270
16th to endDay
7%220%27%22%20%
270
May1st to 15thDay
6%417%22%28%19%
288
16th to endDay
7%28%23%31%26%
331
June1st to 15thDay
13%11%20%22%30%
315
16th to endDay
7%7%12%26%45%
315
July1st to 15thDay
5%112%26%54%
305
16th to endDay
11%415%23%43%
326
August1st to 15thDay
7%6%16%24%44%
313
16th to endDay
13%6%13%19%47%
332
September1st to 15thDay
14%2416%26%35%
315
16th to endDay
10%5%22%27%33%
314
October1st to 15thDay
14%116%26%29%11%
278
16th to endDay
10%322%29%25%9%
272
November1st to 15thDay
6%222%28%31%8%
265
16th to endDay
10%331%31%18%4
210
December1st to 15thDay
8%8%44%30%7%
90
16th to endDay
45%27%42%16%4
96
January1st to 15thNight
13%7%30%32%15%1
720
16th to endNight
14%8%30%34%10%
768
February1st to 15thNight
12%6%35%31%12%1
704
16th to endNight
9%7%36%39%6%
631
March1st to 15thNight
6%8%31%40%12%
720
16th to endNight
6%6%31%36%17%1
768
April1st to 15thNight
7%430%38%17%
718
16th to endNight
7%325%41%20%
720
May1st to 15thNight
6%116%45%27%1
590
16th to endNight
7%110%31%41%7%
563
June1st to 15thNight
10%8%27%42%10%
540
16th to endNight
8%419%45%20%
540
July1st to 15thNight
10%314%45%25%
510
16th to endNight
41217%52%22%
564
August1st to 15thNight
4316%51%24%
539
16th to endNight
12%6%16%42%20%
734
September1st to 15thNight
12%5%20%42%18%
720
16th to endNight
13%6%25%41%12%
717
October1st to 15thNight
10%211%34%35%6%
684
16th to endNight
11%314%35%30%4
704
November1st to 15thNight
6%223%40%24%2
700
16th to endNight
8%428%36%19%1
720
December1st to 15thNight
11%5%26%33%21%2
720
16th to endNight
10%7%25%35%20%1
768
Page generated at 2017-10-07 14:07:50. Version ac04

Copyright 2017 A. Danko


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