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 2005-11-06 to 2024-01-15.

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
27%
22929
19%
74591
8%
22883
40%
22883
20%
48030

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 10thNight
17%
760
21%
2649
6%
757
35%
757
17%
1648
January11th to 20thNight
16%
739
17%
2583
4%
739
32%
739
13%
1722
January21st to endNight
17%
792
21%
2772
7%
792
37%
792
17%
1735
February1st to 10thNight
15%
720
17%
2286
5%
720
30%
720
16%
1537
February11th to 20thNight
13%
703
21%
2100
4%
700
32%
700
16%
1394
February21st to endNight
16%
592
25%
1776
7%
592
37%
592
17%
1173
March1st to 10thNight
19%
720
22%
2160
7%
720
37%
720
18%
1417
March11th to 20thNight
16%
720
18%
2074
5%
720
30%
720
16%
1438
March21st to endNight
20%
792
22%
2044
6%
792
33%
792
20%
1408
April1st to 10thNight
21%
720
20%
1800
4%
720
31%
720
20%
1271
April11th to 20thNight
23%
707
18%
1771
4%
707
31%
707
22%
1223
April21st to endNight
20%
540
20%
1620
6%
540
31%
540
18%
1089
May1st to 10thNight
22%
540
13%
1620
6%
540
29%
540
16%
1155
May11th to 20thNight
21%
488
16%
1550
7%
488
30%
488
18%
1025
May21st to endNight
31%
389
13%
1551
9%
388
40%
388
17%
978
June1st to 10thNight
31%
360
14%
1440
10%
360
46%
360
18%
931
June11th to 20thNight
33%
360
13%
1440
12%
360
42%
360
17%
927
June21st to endNight
37%
360
13%
1440
11%
360
47%
360
23%
857
July1st to 10thNight
45%
350
13%
1400
14%
350
53%
350
22%
841
July11th to 20thNight
48%
348
13%
1392
13%
348
53%
348
20%
833
July21st to endNight
48%
394
20%
1486
19%
372
57%
372
25%
856
August1st to 10thNight
50%
472
17%
1536
12%
470
54%
470
28%
936
August11th to 20thNight
45%
568
17%
1632
10%
568
50%
568
26%
1064
August21st to endNight
49%
792
22%
1980
10%
792
53%
792
36%
1260
September1st to 10thNight
43%
720
21%
1800
8%
720
49%
720
35%
1160
September11th to 20thNight
45%
720
25%
1973
14%
720
54%
720
35%
1192
September21st to endNight
40%
720
25%
1980
15%
720
49%
720
30%
1267
October1st to 10thNight
33%
720
18%
1980
9%
720
41%
720
23%
1329
October11th to 20thNight
33%
720
22%
2204
9%
720
48%
720
27%
1342
October21st to endNight
23%
792
17%
2574
7%
792
36%
792
18%
1726
November1st to 10thNight
32%
740
23%
2407
12%
740
47%
740
24%
1468
November11th to 20thNight
23%
760
24%
2470
10%
760
42%
760
21%
1516
November21st to endNight
21%
760
23%
2470
7%
760
40%
760
20%
1526
December1st to 10thNight
17%
760
20%
2645
6%
760
33%
760
15%
1728
December11th to 20thNight
21%
760
18%
3194
5%
759
37%
759
15%
1954
December21st to endNight
16%
831
15%
4792
5%
817
35%
817
10%
3104

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
17%6%35%5%345%5%7%40%
74820
Night
17%7%35%5%3445%6%41%
74949
TransparencyDay
43%5%11%22%15%3
74525
Night
43%5%11%23%16%2
74778
SeeingDay
24%14%31%20%8%2
13900
Night
22%13%32%25%7%1
22934

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 10thDay
14%8%36%6%33445%43%
1517
11th to 20thDay
18%7%25%5%45%6%5%5%38%
1472
21st to endDay
13%7%246%35%46%6%44%
1584
February1st to 10thDay
20%8%36%435%5%5%7%34%
1670
11th to 20thDay
13%8%36%5%35%45%7%39%
1748
21st to endDay
19%10%36%6%34345%37%
1480
March1st to 10thDay
17%8%25%435%46%6%39%
1797
11th to 20thDay
18%6%36%435%446%39%
1885
21st to endDay
19%8%36%5%24345%41%
2309
April1st to 10thDay
18%8%35%5%34447%38%
2160
11th to 20thDay
17%6%245%35%5%6%7%39%
2138
21st to endDay
22%6%35%6%44446%35%
2340
May1st to 10thDay
25%8%35%434447%33%
2340
11th to 20thDay
18%6%36%5%34446%41%
2358
21st to endDay
14%5%26%5%3445%8%44%
2725
June1st to 10thDay
19%6%35%5%35%5%6%9%34%
2520
11th to 20thDay
17%5%245%35%6%6%8%37%
2520
21st to endDay
18%5%26%6%35%5%6%9%34%
2520
July1st to 10thDay
15%5%35%5%35%5%5%7%42%
2520
11th to 20thDay
12%5%25%7%45%5%6%9%40%
2520
21st to endDay
12%5%35%5%35%5%6%10%41%
2744
August1st to 10thDay
9%6%36%6%46%6%6%9%40%
2373
11th to 20thDay
13%5%25%5%47%5%5%8%40%
2264
21st to endDay
13%325%5%35%5%6%7%45%
2376
September1st to 10thDay
14%6%244346%47%46%
2160
11th to 20thDay
16%5%2432345%5%50%
1986
21st to endDay
16%6%25%433447%46%
1980
October1st to 10thDay
24%5%25%434445%40%
1980
11th to 20thDay
14%6%244345%5%7%46%
1753
21st to endDay
22%7%25%5%335%45%40%
1782
November1st to 10thDay
16%6%345%3334449%
1671
11th to 20thDay
19%6%25%324445%47%
1710
21st to endDay
18%6%35%43445%5%43%
1710
December1st to 10thDay
18%8%36%5%345%5%6%38%
1530
11th to 20thDay
22%6%25%423345%45%
1906
21st to endDay
22%11%445%3435%6%34%
2772
January1st to 10thNight
16%8%35%435%446%44%
2649
11th to 20thNight
18%9%35%6%3445%7%36%
2583
21st to endNight
13%8%46%6%345%5%6%41%
2772
February1st to 10thNight
17%9%35%5%3445%6%38%
2286
11th to 20thNight
15%7%45%5%4445%5%40%
2098
21st to endNight
17%8%36%5%23447%40%
1776
March1st to 10thNight
16%8%35%43445%6%41%
2150
11th to 20thNight
16%8%46%5%35%5%45%38%
2074
21st to endNight
17%9%35%434446%42%
2044
April1st to 10thNight
18%8%35%7%35%36%6%37%
1800
11th to 20thNight
19%5%35%5%3445%7%40%
1771
21st to endNight
22%7%35%425%347%39%
1620
May1st to 10thNight
24%7%36%5%3445%6%32%
1620
11th to 20thNight
19%6%346%245%6%7%38%
1549
21st to endNight
17%5%36%5%345%5%7%40%
1550
June1st to 10thNight
19%7%35%5%3445%7%38%
1440
11th to 20thNight
21%6%335%35%5%5%7%38%
1440
21st to endNight
15%6%34435%5%6%10%40%
1440
July1st to 10thNight
14%5%35%5%445%5%8%41%
1440
11th to 20thNight
12%6%35%6%445%7%7%40%
1440
21st to endNight
12%6%25%5%25%5%5%8%46%
1574
August1st to 10thNight
9%5%245%45%5%6%9%46%
1546
11th to 20thNight
12%5%347%5%6%7%7%9%37%
1632
21st to endNight
15%42443445%9%47%
1980
September1st to 10thNight
14%5%245%2445%6%49%
1800
11th to 20thNight
14%5%24313436%53%
1973
21st to endNight
16%6%2443445%6%47%
1980
October1st to 10thNight
22%7%245%2345%443%
1980
11th to 20thNight
16%5%1443446%6%46%
2204
21st to endNight
23%8%25%5%34446%37%
2574
November1st to 10thNight
16%6%24434445%49%
2407
11th to 20thNight
19%7%244235%45%47%
2470
21st to endNight
16%7%2443445%7%44%
2470
December1st to 10thNight
19%8%36%5%3435%6%38%
2645
11th to 20thNight
19%7%25%42445%6%42%
3288
21st to endNight
20%10%35%6%3435%6%34%
4884

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 10thDay
44%39%27%14%3
1518
11th to 20thDay
46%7%12%19%13%3
1472
21st to endDay
39%413%25%16%3
1584
February1st to 10thDay
50%412%19%12%3
1670
11th to 20thDay
44%6%10%22%14%3
1757
21st to endDay
50%47%20%15%3
1480
March1st to 10thDay
45%412%21%13%5%
1800
11th to 20thDay
45%5%10%21%16%4
1885
21st to endDay
47%47%19%17%6%
2309
April1st to 10thDay
47%49%19%17%4
2160
11th to 20thDay
42%5%11%21%18%3
2138
21st to endDay
49%5%8%18%17%4
2340
May1st to 10thDay
53%38%20%13%2
2340
11th to 20thDay
44%6%9%20%18%3
2362
21st to endDay
40%6%13%25%14%3
2725
June1st to 10thDay
46%7%13%23%11%1
2520
11th to 20thDay
43%7%12%20%16%2
2520
21st to endDay
46%7%12%22%13%1
2520
July1st to 10thDay
41%6%11%24%16%2
2450
11th to 20thDay
39%6%14%27%13%1
2445
21st to endDay
38%5%15%24%15%3
2591
August1st to 10thDay
40%7%14%24%13%1
2359
11th to 20thDay
41%6%13%25%12%2
2264
21st to endDay
36%6%10%26%18%3
2376
September1st to 10thDay
38%6%11%26%18%2
2160
11th to 20thDay
36%48%26%23%3
1986
21st to endDay
40%38%23%23%2
1980
October1st to 10thDay
46%5%9%22%14%4
1980
11th to 20thDay
37%5%12%30%15%2
1753
21st to endDay
47%411%20%14%3
1782
November1st to 10thDay
40%312%26%17%2
1671
11th to 20thDay
40%49%23%21%3
1710
21st to endDay
43%414%24%12%2
1710
December1st to 10thDay
47%414%16%17%1
1530
11th to 20thDay
43%413%19%18%4
1906
21st to endDay
51%5%11%20%11%1
2772
January1st to 10thNight
42%5%10%23%16%3
2649
11th to 20thNight
48%5%14%20%9%3
2583
21st to endNight
43%410%21%19%3
2772
February1st to 10thNight
48%5%12%21%13%3
2286
11th to 20thNight
46%49%21%16%4
2100
21st to endNight
45%48%23%16%3
1776
March1st to 10thNight
44%5%8%24%15%4
2160
11th to 20thNight
48%410%16%17%4
2075
21st to endNight
46%48%17%20%5%
2044
April1st to 10thNight
47%5%9%20%16%3
1800
11th to 20thNight
44%49%22%19%2
1771
21st to endNight
46%5%10%19%15%5%
1620
May1st to 10thNight
52%5%9%18%13%3
1620
11th to 20thNight
44%7%12%18%16%3
1550
21st to endNight
45%6%14%21%13%1
1551
June1st to 10thNight
47%6%11%24%12%1
1440
11th to 20thNight
46%6%12%21%14%2
1440
21st to endNight
37%9%15%24%15%1
1440
July1st to 10thNight
39%7%14%23%16%1
1400
11th to 20thNight
40%7%16%26%11%1
1392
21st to endNight
36%6%15%29%12%2
1486
August1st to 10thNight
34%9%14%25%16%1
1536
11th to 20thNight
40%8%16%23%13%
1632
21st to endNight
35%5%12%27%20%1
1980
September1st to 10thNight
37%413%26%19%1
1800
11th to 20thNight
33%410%23%27%2
1973
21st to endNight
39%48%26%21%3
1980
October1st to 10thNight
44%5%11%23%13%4
1980
11th to 20thNight
37%414%28%15%1
2204
21st to endNight
49%412%21%14%1
2574
November1st to 10thNight
39%5%10%22%20%4
2407
11th to 20thNight
40%411%22%20%3
2470
21st to endNight
40%5%13%27%14%2
2470
December1st to 10thNight
47%412%23%13%1
2645
11th to 20thNight
43%415%18%18%2
3288
21st to endNight
50%5%9%25%10%1
4884

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 10thDay
28%22%31%15%41
180
11th to 20thDay
35%19%31%11%4
168
21st to endDay
24%26%32%14%31
176
February1st to 10thDay
36%20%30%12%2
324
11th to 20thDay
29%25%33%11%2
416
21st to endDay
32%20%32%12%4
346
March1st to 10thDay
30%19%34%14%2
420
11th to 20thDay
27%25%31%14%31
420
21st to endDay
30%17%34%12%5%2
462
April1st to 10thDay
32%20%27%15%6%1
420
11th to 20thDay
29%21%31%11%7%1
412
21st to endDay
31%16%32%15%5%1
420
May1st to 10thDay
35%15%30%14%5%2
423
11th to 20thDay
24%13%42%16%42
447
21st to endDay
19%12%35%23%9%2
485
June1st to 10thDay
19%11%37%20%10%2
450
11th to 20thDay
20%12%37%22%8%2
450
21st to endDay
19%11%36%22%9%4
450
July1st to 10thDay
19%8%27%28%14%4
445
11th to 20thDay
16%5%32%31%13%3
444
21st to endDay
13%7%34%33%10%2
488
August1st to 10thDay
11%9%29%35%14%3
446
11th to 20thDay
18%7%29%29%14%4
439
21st to endDay
14%8%31%27%15%6%
495
September1st to 10thDay
19%9%26%23%17%6%
450
11th to 20thDay
22%7%29%21%12%9%
450
21st to endDay
19%10%31%23%11%5%
450
October1st to 10thDay
29%10%24%20%12%5%
450
11th to 20thDay
18%16%29%24%11%2
405
21st to endDay
30%13%29%19%9%1
396
November1st to 10thDay
22%13%33%19%10%3
365
11th to 20thDay
31%19%24%21%5%1
370
21st to endDay
28%16%29%19%7%1
370
December1st to 10thDay
20%28%30%17%41
190
11th to 20thDay
33%22%33%9%21
180
21st to endDay
27%21%31%16%41
198
January1st to 10thNight
23%17%38%19%3
760
11th to 20thNight
28%17%35%19%2
739
21st to endNight
20%19%36%21%3
792
February1st to 10thNight
25%17%38%19%2
720
11th to 20thNight
23%19%39%17%1
703
21st to endNight
26%19%33%18%21
592
March1st to 10thNight
25%17%36%19%3
720
11th to 20thNight
23%20%36%17%3
720
21st to endNight
26%18%32%20%5%
792
April1st to 10thNight
25%19%32%21%3
720
11th to 20thNight
25%17%32%20%6%
707
21st to endNight
24%18%34%21%2
540
May1st to 10thNight
30%11%31%23%4
540
11th to 20thNight
21%15%37%22%5%
488
21st to endNight
21%7%36%29%7%
389
June1st to 10thNight
24%436%27%9%
360
11th to 20thNight
22%6%33%31%7%1
360
21st to endNight
17%435%36%8%1
360
July1st to 10thNight
15%428%41%11%1
350
11th to 20thNight
13%325%45%14%
348
21st to endNight
13%428%44%10%1
394
August1st to 10thNight
12%327%43%14%1
472
11th to 20thNight
13%5%27%40%14%1
568
21st to endNight
17%323%38%17%2
792
September1st to 10thNight
15%5%30%34%15%1
720
11th to 20thNight
19%7%24%30%18%3
720
21st to endNight
20%7%27%31%14%2
720
October1st to 10thNight
25%9%26%26%11%2
720
11th to 20thNight
20%10%32%24%12%1
720
21st to endNight
30%11%31%22%6%
792
November1st to 10thNight
21%12%30%27%9%1
740
11th to 20thNight
25%16%33%19%6%1
760
21st to endNight
24%17%33%20%5%1
760
December1st to 10thNight
27%17%34%18%4
760
11th to 20thNight
25%17%32%22%3
760
21st to endNight
27%15%36%19%3
836
Page generated at 2024-01-12 07:31:52. Version ac07

Copyright 2024 Attilla Danko


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