ONF Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ONF 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
58%
15959
21%
47143
16%
15921
63%
15921
30%
29544

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
42%
717
21%
2327
15%
716
52%
716
26%
1428
January16th to endNight
41%
768
23%
2460
17%
768
51%
768
27%
1500
February1st to 15thNight
42%
701
24%
2107
15%
700
53%
700
32%
1272
February16th to endNight
41%
631
23%
1848
13%
631
51%
631
29%
1157
March1st to 15thNight
50%
720
28%
1980
18%
720
59%
720
39%
1155
March16th to endNight
53%
768
26%
2112
17%
768
59%
768
41%
1209
April1st to 15thNight
54%
718
25%
1976
16%
718
60%
718
41%
1144
April16th to endNight
60%
711
27%
1830
17%
711
65%
711
43%
1069
May1st to 15thNight
69%
539
26%
1619
19%
539
74%
539
42%
881
May16th to endNight
75%
561
23%
1682
19%
561
76%
561
35%
987
June1st to 15thNight
67%
540
13%
1620
11%
540
68%
540
24%
1124
June16th to endNight
76%
540
9%
1620
7%
540
76%
540
24%
1133
July1st to 15thNight
75%
510
9%
1530
8%
510
75%
510
23%
1078
July16th to endNight
77%
564
8%
1595
7%
531
78%
531
24%
1107
August1st to 15thNight
76%
539
7%
1609
7%
536
76%
536
22%
1136
August16th to endNight
67%
699
11%
2013
11%
699
67%
699
21%
1433
September1st to 15thNight
64%
720
10%
1980
9%
720
64%
720
20%
1447
September16th to endNight
58%
717
15%
1980
15%
717
59%
717
20%
1425
October1st to 15thNight
65%
684
25%
1941
25%
684
67%
684
33%
1195
October16th to endNight
62%
704
28%
2112
27%
704
67%
704
34%
1232
November1st to 15thNight
59%
700
28%
2101
24%
700
66%
700
38%
1182
November16th to endNight
50%
720
28%
2265
20%
720
60%
720
31%
1330
December1st to 15thNight
49%
720
27%
2340
24%
720
56%
720
28%
1405
December16th to endNight
52%
768
24%
2496
20%
768
59%
768
27%
1515

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%5%6%8%36%
48039
Night
8%425%5%35%5%6%8%44%
47348
TransparencyDay
42%8%13%16%13%6%
47740
Night
35%8%15%19%15%6%
47151
SeeingDay
9%312%21%25%26%
5815
Night
10%318%32%27%7%
15968

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%25%425%45%8%41%
1611
16th to endDay
13%5%25%424446%46%
1761
February1st to 15thDay
14%5%25%5%1433448%
1750
16th to endDay
10%5%335%25%347%47%
1622
March1st to 15thDay
9%5%13423447%50%
1980
16th to endDay
9%5%26%43445%6%47%
2111
April1st to 15thDay
7%4245%45%5%5%9%45%
1972
16th to endDay
7%4245%35%6%5%7%47%
2138
May1st to 15thDay
6%3245%35%5%6%8%47%
2331
16th to endDay
7%426%6%48%6%6%9%37%
2431
June1st to 15thDay
10%5%37%8%48%6%7%9%28%
2340
16th to endDay
9%5%37%7%5%8%7%8%11%24%
2340
July1st to 15thDay
9%6%38%7%5%9%7%9%9%23%
2340
16th to endDay
7%6%48%9%6%9%8%7%10%20%
2465
August1st to 15thDay
9%5%39%9%6%9%9%8%10%18%
2340
16th to endDay
12%7%39%9%47%8%8%8%20%
2173
September1st to 15thDay
11%6%38%9%5%7%7%7%8%22%
1980
16th to endDay
13%7%28%8%5%7%7%6%7%24%
1980
October1st to 15thDay
10%7%37%7%46%5%46%37%
1816
16th to endDay
10%425%5%44447%47%
1760
November1st to 15thDay
7%5%145%25%45%8%48%
1757
16th to endDay
9%4346%34347%47%
1693
December1st to 15thDay
11%6%35%5%24448%41%
1620
16th to endDay
8%5%25%5%35%5%6%9%41%
1728
January1st to 15thNight
11%5%25%5%25%45%7%42%
2327
16th to endNight
15%425%42345%8%44%
2460
February1st to 15thNight
11%5%25%423335%51%
2103
16th to endNight
9%5%25%6%3435%7%46%
1848
March1st to 15thNight
7%324324348%54%
1980
16th to endNight
7%315%42345%6%56%
2112
April1st to 15thNight
7%423324347%54%
1976
16th to endNight
6%31342345%8%55%
1830
May1st to 15thNight
4312325%46%9%56%
1619
16th to endNight
5%314435%46%8%51%
1682
June1st to 15thNight
11%4245%46%6%7%11%34%
1620
16th to endNight
7%425%6%37%7%8%14%32%
1620
July1st to 15thNight
5%5%25%6%47%7%9%11%34%
1620
16th to endNight
3425%6%46%9%10%11%32%
1703
August1st to 15thNight
45%26%6%46%8%9%14%30%
1620
16th to endNight
9%6%26%6%35%6%7%11%33%
2013
September1st to 15thNight
9%5%26%6%46%6%9%10%31%
1980
16th to endNight
11%7%36%6%35%6%7%9%30%
1980
October1st to 15thNight
9%5%2443445%8%47%
1941
16th to endNight
9%5%245%23446%51%
2112
November1st to 15thNight
6%32442445%8%51%
2101
16th to endNight
8%424434447%49%
2265
December1st to 15thNight
10%5%35%42435%7%47%
2340
16th to endNight
9%5%36%425%5%6%6%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
39%5%9%17%18%9%
1612
16th to endDay
38%47%17%20%10%
1761
February1st to 15thDay
40%37%12%20%15%
1752
16th to endDay
36%411%14%20%12%
1622
March1st to 15thDay
32%49%21%21%10%
1980
16th to endDay
35%5%11%19%17%10%
2112
April1st to 15thDay
34%6%10%17%22%8%
1972
16th to endDay
33%6%11%21%18%8%
2138
May1st to 15thDay
32%7%12%20%18%9%
2331
16th to endDay
39%8%15%18%14%3
2435
June1st to 15thDay
49%10%15%17%5%
2340
16th to endDay
48%13%19%14%2
2340
July1st to 15thDay
51%14%20%12%1
2201
16th to endDay
53%13%20%11%1
2310
August1st to 15thDay
53%15%20%8%1
2327
16th to endDay
56%12%17%11%1
2173
September1st to 15thDay
53%13%17%10%5%
1980
16th to endDay
53%10%12%13%7%2
1980
October1st to 15thDay
45%6%8%14%13%11%
1816
16th to endDay
36%48%12%23%13%
1760
November1st to 15thDay
32%410%20%20%11%
1757
16th to endDay
36%5%8%17%21%11%
1693
December1st to 15thDay
41%5%10%20%16%5%
1620
16th to endDay
36%7%12%21%13%9%
1728
January1st to 15thNight
39%410%19%17%8%
2327
16th to endNight
38%5%9%16%22%9%
2460
February1st to 15thNight
35%48%15%19%16%
2112
16th to endNight
36%7%10%15%20%10%
1848
March1st to 15thNight
29%5%12%24%19%9%
1980
16th to endNight
27%5%12%23%20%10%
2112
April1st to 15thNight
28%5%12%23%21%9%
1976
16th to endNight
26%6%13%25%20%8%
1830
May1st to 15thNight
24%6%16%25%20%6%
1620
16th to endNight
28%9%15%26%18%2
1684
June1st to 15thNight
40%12%20%22%5%
1620
16th to endNight
38%14%24%17%5%
1620
July1st to 15thNight
39%14%28%15%2
1530
16th to endNight
36%16%30%13%3
1595
August1st to 15thNight
36%18%32%12%
1609
16th to endNight
41%14%23%17%2
2013
September1st to 15thNight
42%16%21%13%6%
1980
16th to endNight
45%11%17%16%7%1
1980
October1st to 15thNight
33%7%12%18%19%9%
1941
16th to endNight
33%410%15%24%11%
2112
November1st to 15thNight
28%5%12%21%21%9%
2101
16th to endNight
33%5%8%19%17%14%
2265
December1st to 15thNight
37%410%25%15%6%
2340
16th to endNight
38%5%12%21%14%9%
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
11%10%31%27%11%8%
90
16th to endDay
12%17%33%18%15%2
96
February1st to 15thDay
6%21%27%23%16%4
90
16th to endDay
9%8%32%34%13%
123
March1st to 15thDay
10%6%26%35%17%3
270
16th to endDay
11%324%27%26%6%
288
April1st to 15thDay
10%6%21%27%25%8%
270
16th to endDay
6%216%26%25%22%
270
May1st to 15thDay
7%114%25%29%21%
288
16th to endDay
8%6%16%35%31%
331
June1st to 15thDay
13%35%13%28%35%
315
16th to endDay
6%1311%28%48%
315
July1st to 15thDay
7%6%24%59%
305
16th to endDay
7%1310%23%53%
326
August1st to 15thDay
9%5%12%22%49%
313
16th to endDay
11%312%19%53%
332
September1st to 15thDay
14%26%13%24%39%
315
16th to endDay
12%416%32%32%
314
October1st to 15thDay
11%211%31%29%12%
278
16th to endDay
11%115%30%33%8%
272
November1st to 15thDay
9%218%30%30%8%
265
16th to endDay
7%326%32%25%4
163
December1st to 15thDay
10%235%36%13%2
90
16th to endDay
10%216%41%22%6%
96
January1st to 15thNight
13%7%29%33%15%
720
16th to endNight
17%6%28%36%10%
768
February1st to 15thNight
13%5%32%36%11%1
704
16th to endNight
11%7%34%39%7%
631
March1st to 15thNight
8%6%29%41%12%
720
16th to endNight
8%329%40%17%
768
April1st to 15thNight
8%427%40%17%
718
16th to endNight
6%323%43%22%
711
May1st to 15thNight
5%216%45%26%2
540
16th to endNight
6%18%33%41%8%
563
June1st to 15thNight
12%26%25%41%12%
540
16th to endNight
9%119%46%23%
540
July1st to 15thNight
8%1212%41%33%
510
16th to endNight
41114%56%21%
564
August1st to 15thNight
7%213%53%23%
539
16th to endNight
11%1418%42%21%
699
September1st to 15thNight
12%15%25%41%13%
720
16th to endNight
15%6%26%41%9%
717
October1st to 15thNight
11%211%36%34%3
684
16th to endNight
11%314%38%28%2
704
November1st to 15thNight
9%222%38%24%2
700
16th to endNight
9%427%38%19%1
720
December1st to 15thNight
12%5%26%32%23%
720
16th to endNight
11%5%23%36%20%
768
Page generated at 2017-10-07 14:07:53. Version ac04

Copyright 2017 A. Danko


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