Oak Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oak Ridge Observatory clear sky chart from 2005-11-05 to 2018-04-23.

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
67%
16310
27%
49564
24%
16274
71%
16274
40%
26990

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
58%
776
19%
2567
19%
776
61%
776
27%
1566
January16th to endNight
59%
832
24%
2704
24%
832
62%
832
27%
1676
February1st to 15thNight
58%
759
26%
2477
26%
759
64%
759
29%
1477
February16th to endNight
52%
684
23%
2051
19%
684
59%
684
29%
1277
March1st to 15thNight
53%
780
19%
2171
17%
780
56%
780
28%
1427
March16th to endNight
52%
832
20%
2262
15%
832
57%
832
31%
1451
April1st to 15thNight
49%
779
22%
1949
12%
779
53%
779
32%
1274
April16th to endNight
58%
744
27%
1860
12%
744
63%
744
45%
1061
May1st to 15thNight
67%
629
24%
1709
15%
629
70%
629
51%
895
May16th to endNight
70%
408
25%
1531
22%
408
76%
408
41%
708
June1st to 15thNight
75%
360
26%
1440
27%
360
82%
360
46%
593
June16th to endNight
88%
360
35%
1440
44%
360
91%
360
61%
501
July1st to 15thNight
92%
398
33%
1418
38%
398
94%
398
73%
488
July16th to endNight
91%
564
34%
1593
32%
531
91%
531
61%
702
August1st to 15thNight
86%
565
32%
1638
25%
562
86%
562
58%
753
August16th to endNight
88%
764
37%
1910
23%
764
89%
764
70%
917
September1st to 15thNight
88%
720
40%
1800
29%
720
89%
720
71%
858
September16th to endNight
82%
716
41%
1962
37%
716
85%
716
64%
923
October1st to 15thNight
74%
720
29%
2160
30%
720
78%
720
49%
1077
October16th to endNight
73%
768
34%
2304
36%
768
76%
768
45%
1192
November1st to 15thNight
67%
760
28%
2360
28%
760
71%
760
39%
1291
November16th to endNight
64%
780
26%
2616
28%
780
68%
780
33%
1452
December1st to 15thNight
59%
780
23%
2730
22%
780
65%
780
27%
1598
December16th to endNight
48%
832
21%
2912
18%
832
55%
832
21%
1833

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
7%423323347%57%
49947
Night
6%42442345%8%54%
49753
TransparencyDay
26%39%20%24%16%
49636
Night
27%410%20%21%15%
49565
SeeingDay
8%10%23%32%18%5%
6765
Night
8%412%33%33%8%
16321

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
14%7%2443346%7%41%
1697
16th to endDay
14%6%35%324447%42%
1872
February1st to 15thDay
13%7%35%42445%6%42%
1716
16th to endDay
15%8%46%423346%39%
1709
March1st to 15thDay
12%8%36%6%25%5%5%10%33%
2116
16th to endDay
11%6%36%6%35%5%46%39%
2312
April1st to 15thDay
14%6%35%423347%43%
2329
16th to endDay
8%414323347%57%
2214
May1st to 15thDay
3413213339%62%
2238
16th to endDay
5%5%34322347%57%
2581
June1st to 15thDay
5%412212348%61%
2520
16th to endDay
111111236%77%
2520
July1st to 15thDay
111111236%77%
2454
16th to endDay
11112245%77%
2468
August1st to 15thDay
111212238%73%
2313
16th to endDay
1112247%77%
2292
September1st to 15thDay
112111226%77%
2160
16th to endDay
3212212246%70%
1994
October1st to 15thDay
4313313447%62%
1800
16th to endDay
5%413324347%58%
1920
November1st to 15thDay
8%5%25%424447%50%
1828
16th to endDay
9%6%35%5%24447%47%
1670
December1st to 15thDay
11%7%26%6%34346%41%
1560
16th to endDay
13%9%45%43345%7%37%
1664
January1st to 15thNight
12%6%25%5%346%5%7%40%
2567
16th to endNight
12%6%35%424447%43%
2704
February1st to 15thNight
9%7%25%5%35%45%6%43%
2465
16th to endNight
13%7%25%42435%7%43%
2051
March1st to 15thNight
11%8%35%5%245%47%40%
2171
16th to endNight
10%7%35%5%24447%44%
2262
April1st to 15thNight
11%6%2443446%7%43%
1949
16th to endNight
6%32331345%7%57%
1860
May1st to 15thNight
131332435%9%61%
1709
16th to endNight
33133234410%58%
1530
June1st to 15thNight
121321245%10%64%
1440
16th to endNight
1211146%78%
1440
July1st to 15thNight
111348%76%
1503
16th to endNight
121345%10%68%
1701
August1st to 15thNight
1132134412%63%
1647
16th to endNight
2213449%70%
1910
September1st to 15thNight
11213239%73%
1800
16th to endNight
2211212247%71%
1962
October1st to 15thNight
4313323346%63%
2160
16th to endNight
4323423346%60%
2304
November1st to 15thNight
6%42442345%7%54%
2360
16th to endNight
7%425%434347%50%
2616
December1st to 15thNight
9%7%35%43435%8%44%
2730
16th to endNight
13%7%35%5%2445%5%41%
2912

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
38%5%11%19%15%9%
1697
16th to endDay
40%5%7%14%19%12%
1872
February1st to 15thDay
39%47%19%21%7%
1716
16th to endDay
44%5%8%22%15%4
1709
March1st to 15thDay
44%5%10%20%14%5%
2116
16th to endDay
43%5%12%18%17%2
2312
April1st to 15thDay
40%311%22%19%2
2329
16th to endDay
27%413%24%22%7%
2214
May1st to 15thDay
19%49%30%29%7%
2238
16th to endDay
26%413%25%23%6%
2585
June1st to 15thDay
21%412%22%24%13%
2520
16th to endDay
10%27%17%34%27%
2520
July1st to 15thDay
10%15%12%24%45%
2310
16th to endDay
9%26%13%28%39%
2313
August1st to 15thDay
12%25%16%30%33%
2297
16th to endDay
8%16%20%32%29%
2292
September1st to 15thDay
10%16%17%31%31%
2160
16th to endDay
15%26%24%31%18%
1994
October1st to 15thDay
21%47%18%24%22%
1800
16th to endDay
24%410%21%23%15%
1920
November1st to 15thDay
31%5%10%25%20%6%
1828
16th to endDay
35%410%18%26%5%
1670
December1st to 15thDay
42%311%14%18%10%
1560
16th to endDay
44%411%15%18%5%
1664
January1st to 15thNight
38%5%11%19%16%8%
2567
16th to endNight
39%5%9%14%16%15%
2704
February1st to 15thNight
37%48%19%21%9%
2477
16th to endNight
37%5%12%25%15%3
2051
March1st to 15thNight
41%411%20%15%6%
2171
16th to endNight
38%5%12%19%19%5%
2262
April1st to 15thNight
36%5%12%24%17%3
1949
16th to endNight
22%5%14%26%21%8%
1860
May1st to 15thNight
19%5%12%22%28%11%
1710
16th to endNight
20%5%16%26%24%6%
1531
June1st to 15thNight
16%413%25%22%17%
1440
16th to endNight
10%26%16%30%33%
1440
July1st to 15thNight
7%1416%25%45%
1418
16th to endNight
14%28%19%20%34%
1593
August1st to 15thNight
17%27%20%22%30%
1638
16th to endNight
11%19%16%29%30%
1910
September1st to 15thNight
10%27%16%27%35%
1800
16th to endNight
13%26%20%33%23%
1962
October1st to 15thNight
21%39%19%22%24%
2160
16th to endNight
23%39%20%24%18%
2304
November1st to 15thNight
28%411%25%21%7%
2360
16th to endNight
32%49%22%22%9%
2616
December1st to 15thNight
36%49%16%19%12%
2730
16th to endNight
42%5%11%14%19%7%
2912

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%11%24%25%15%1
236
16th to endDay
16%7%30%24%16%3
304
February1st to 15thDay
13%12%28%25%14%4
283
16th to endDay
16%17%31%20%12%2
290
March1st to 15thDay
16%18%27%21%13%2
330
16th to endDay
13%16%31%28%9%1
352
April1st to 15thDay
17%19%30%19%11%1
329
16th to endDay
10%24%28%25%8%3
288
May1st to 15thDay
7%16%32%30%10%2
288
16th to endDay
7%13%21%32%19%5%
330
June1st to 15thDay
5%12%32%33%12%4
315
16th to endDay
323%43%20%7%
315
July1st to 15thDay
2112%47%28%6%
305
16th to endDay
111%50%25%10%
327
August1st to 15thDay
120%39%30%7%
315
16th to endDay
115%45%24%13%
332
September1st to 15thDay
317%44%23%10%
315
16th to endDay
6%7%18%34%25%7%
315
October1st to 15thDay
5%8%26%35%18%5%
315
16th to endDay
8%6%23%33%19%8%
336
November1st to 15thDay
10%7%18%28%25%9%
244
16th to endDay
6%9%26%32%21%2
115
December1st to 15thDay
6%17%18%25%23%7%
90
16th to endDay
10%16%14%23%21%12%
96
January1st to 15thNight
14%411%33%28%7%
780
16th to endNight
15%412%31%26%10%
832
February1st to 15thNight
12%5%13%33%26%7%
764
16th to endNight
16%6%17%33%21%4
684
March1st to 15thNight
14%7%16%28%28%5%
780
16th to endNight
13%6%18%34%23%3
832
April1st to 15thNight
13%8%19%32%22%3
779
16th to endNight
6%8%21%32%22%8%
744
May1st to 15thNight
2320%36%31%6%
630
16th to endNight
3414%38%32%5%
409
June1st to 15thNight
2213%45%31%3
360
16th to endNight
117%46%40%2
360
July1st to 15thNight
332%55%8%
398
16th to endNight
126%55%15%
564
August1st to 15thNight
15%33%51%7%
565
16th to endNight
5%31%50%10%
764
September1st to 15thNight
15%31%48%13%
720
16th to endNight
218%33%40%12%
716
October1st to 15thNight
5%210%35%34%11%
720
16th to endNight
7%28%27%36%17%
768
November1st to 15thNight
8%113%33%33%9%
760
16th to endNight
8%411%37%29%9%
780
December1st to 15thNight
12%412%35%27%7%
780
16th to endNight
15%7%16%32%22%4
832
Page generated at 2018-04-24 07:03:01. Version ac04

Copyright 2018 A. Danko


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