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 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
67%
15438
27%
46901
24%
15402
71%
15402
40%
25542

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%
716
19%
2369
19%
716
62%
716
29%
1415
January16th to endNight
59%
768
25%
2496
25%
768
62%
768
27%
1537
February1st to 15thNight
56%
699
24%
2283
23%
699
62%
699
27%
1405
February16th to endNight
51%
632
22%
1895
18%
632
58%
632
28%
1201
March1st to 15thNight
54%
720
19%
2004
17%
720
57%
720
28%
1300
March16th to endNight
51%
768
20%
2088
14%
768
56%
768
30%
1345
April1st to 15thNight
49%
719
22%
1799
12%
719
53%
719
32%
1176
April16th to endNight
57%
720
26%
1800
11%
720
62%
720
44%
1032
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%
656
38%
1799
35%
656
84%
656
62%
861
October1st to 15thNight
73%
660
29%
1980
30%
660
76%
660
46%
1017
October16th to endNight
71%
704
32%
2112
33%
704
75%
704
43%
1112
November1st to 15thNight
67%
700
29%
2174
29%
700
71%
700
39%
1179
November16th to endNight
65%
720
26%
2415
27%
720
68%
720
34%
1323
December1st to 15thNight
57%
720
22%
2520
20%
720
64%
720
25%
1523
December16th to endNight
48%
768
19%
2688
17%
768
54%
768
21%
1701

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%
47838
Night
6%424423448%54%
47090
TransparencyDay
26%39%20%24%16%
47527
Night
27%410%20%21%15%
46902
SeeingDay
8%10%24%33%18%5%
6039
Night
8%412%33%33%8%
15449

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
13%7%2432346%7%43%
1566
16th to endDay
15%6%35%324436%43%
1728
February1st to 15thDay
14%8%35%5%24346%40%
1580
16th to endDay
16%9%46%423347%37%
1579
March1st to 15thDay
11%8%36%6%25%45%10%34%
1953
16th to endDay
11%6%36%6%35%5%46%40%
2134
April1st to 15thDay
14%7%35%423347%42%
2149
16th to endDay
7%414323347%56%
2160
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%
1831
October1st to 15thDay
4414314447%60%
1650
16th to endDay
6%5%13323347%58%
1760
November1st to 15thDay
7%5%25%424447%50%
1684
16th to endDay
9%6%35%5%24336%47%
1542
December1st to 15thDay
12%8%26%7%34346%39%
1440
16th to endDay
14%10%45%42335%7%37%
1536
January1st to 15thNight
11%6%25%4245%5%7%43%
2369
16th to endNight
12%7%35%424447%44%
2496
February1st to 15thNight
10%7%26%5%35%5%5%6%40%
2271
16th to endNight
14%7%35%42435%7%40%
1895
March1st to 15thNight
11%8%35%5%24447%42%
2004
16th to endNight
10%7%35%5%245%47%44%
2088
April1st to 15thNight
11%6%2443445%7%44%
1799
16th to endNight
6%32332346%7%57%
1800
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
2211222248%70%
1799
October1st to 15thNight
4313423447%60%
1980
16th to endNight
43235%23346%59%
2112
November1st to 15thNight
6%42442245%7%55%
2174
16th to endNight
6%425%434347%51%
2415
December1st to 15thNight
10%7%36%5%3445%7%42%
2520
16th to endNight
14%7%35%5%34445%41%
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
36%5%11%19%16%10%
1566
16th to endDay
41%5%6%13%19%13%
1728
February1st to 15thDay
42%47%19%20%5%
1580
16th to endDay
45%5%7%22%13%4
1579
March1st to 15thDay
43%5%10%20%14%5%
1953
16th to endDay
43%5%12%18%16%2
2134
April1st to 15thDay
40%411%21%19%2
2149
16th to endDay
27%413%25%22%7%
2160
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%23%30%20%
1831
October1st to 15thDay
22%5%8%19%25%17%
1650
16th to endDay
25%410%22%23%13%
1760
November1st to 15thDay
32%49%25%20%7%
1684
16th to endDay
36%410%17%26%5%
1542
December1st to 15thDay
44%311%15%18%5%
1440
16th to endDay
45%312%15%17%5%
1536
January1st to 15thNight
36%411%20%17%9%
2369
16th to endNight
38%5%8%13%17%16%
2496
February1st to 15thNight
39%48%20%18%8%
2283
16th to endNight
39%5%12%24%14%3
1895
March1st to 15thNight
40%412%20%15%7%
2004
16th to endNight
38%5%12%19%18%5%
2088
April1st to 15thNight
36%5%13%23%17%3
1799
16th to endNight
23%5%14%26%20%9%
1800
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
14%36%19%33%24%
1799
October1st to 15thNight
22%49%21%22%19%
1980
16th to endNight
24%39%21%24%16%
2112
November1st to 15thNight
27%410%25%22%8%
2174
16th to endNight
30%49%22%22%10%
2415
December1st to 15thNight
39%5%9%17%20%8%
2520
16th to endNight
42%5%11%14%17%7%
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
20%13%26%24%14%1
195
16th to endDay
19%7%32%23%13%3
256
February1st to 15thDay
16%13%29%24%13%2
237
16th to endDay
18%15%31%21%11%1
238
March1st to 15thDay
14%20%27%20%13%3
270
16th to endDay
11%17%32%28%7%1
288
April1st to 15thDay
17%21%30%18%10%1
269
16th to endDay
9%25%30%24%8%1
270
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%17%34%26%7%
256
October1st to 15thDay
6%7%25%37%18%4
255
16th to endDay
9%7%27%35%15%4
272
November1st to 15thDay
10%9%14%26%28%10%
190
16th to endDay
5%14%29%31%16%2
77
December1st to 15thDay
10%21%25%30%13%
60
16th to endDay
10%18%14%28%20%7%
64
January1st to 15thNight
13%411%34%27%8%
720
16th to endNight
15%412%31%25%11%
768
February1st to 15thNight
13%5%14%32%25%7%
704
16th to endNight
17%6%17%32%20%5%
632
March1st to 15thNight
14%7%16%27%28%5%
720
16th to endNight
13%7%19%33%23%3
768
April1st to 15thNight
13%9%20%31%21%3
719
16th to endNight
6%8%21%32%22%7%
720
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
318%32%41%12%
656
October1st to 15thNight
6%210%33%35%12%
660
16th to endNight
7%38%29%34%16%
704
November1st to 15thNight
8%114%31%33%10%
700
16th to endNight
8%411%37%29%10%
720
December1st to 15thNight
13%412%35%26%7%
720
16th to endNight
15%8%17%33%21%4
768
Page generated at 2017-09-16 14:07:00. Version ac04

Copyright 2017 A. Danko


Oak Ridge Observatory Clear Sky Chart | Charts Home