Green Bank Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Green Bank Observatory clear sky chart from 2005-11-06 to 2024-03-23.

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
35%
24139
17%
80510
10%
24034
43%
24034
17%
54476

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
12%
756
16%
4057
4%
737
28%
737
5%
3010
January11th to 20thNight
19%
760
22%
4004
9%
740
30%
740
7%
2650
January21st to endNight
13%
832
11%
4458
5%
814
28%
814
7%
3022
February1st to 10thNight
20%
760
17%
2470
9%
760
31%
760
13%
1693
February11th to 20thNight
10%
741
13%
2401
3%
740
22%
740
8%
1720
February21st to endNight
12%
628
19%
1814
4%
628
27%
628
12%
1311
March1st to 10thNight
21%
760
20%
2090
8%
760
32%
760
16%
1455
March11th to 20thNight
20%
760
16%
2090
6%
760
30%
760
14%
1487
March21st to endNight
21%
801
15%
2201
7%
801
28%
801
15%
1582
April1st to 10thNight
21%
720
15%
1894
4%
720
30%
720
18%
1345
April11th to 20thNight
31%
711
16%
1775
7%
711
37%
711
23%
1225
April21st to endNight
31%
594
19%
1674
7%
594
38%
594
20%
1142
May1st to 10thNight
36%
540
13%
1620
6%
540
42%
540
19%
1130
May11th to 20thNight
37%
531
12%
1593
8%
531
40%
531
18%
1151
May21st to endNight
52%
582
12%
1745
7%
582
55%
582
24%
1177
June1st to 10thNight
56%
392
17%
1472
17%
392
61%
392
24%
891
June11th to 20thNight
53%
360
14%
1440
14%
360
58%
360
21%
861
June21st to endNight
56%
465
18%
1545
15%
465
62%
465
28%
927
July1st to 10thNight
57%
525
12%
1575
10%
525
60%
525
26%
1022
July11th to 20thNight
68%
522
15%
1566
13%
522
69%
522
32%
950
July21st to endNight
63%
591
17%
1672
13%
558
67%
558
30%
1072
August1st to 10thNight
60%
537
15%
1600
12%
534
62%
534
25%
1073
August11th to 20thNight
62%
534
17%
1598
13%
534
64%
534
27%
1038
August21st to endNight
60%
738
18%
1926
12%
738
61%
738
31%
1281
September1st to 10thNight
55%
720
21%
1800
11%
720
59%
720
32%
1192
September11th to 20thNight
57%
720
25%
1978
19%
720
60%
720
32%
1245
September21st to endNight
42%
720
19%
1981
15%
720
47%
720
22%
1390
October1st to 10thNight
48%
720
19%
2160
17%
720
53%
720
25%
1407
October11th to 20thNight
36%
720
21%
2160
13%
720
48%
720
21%
1407
October21st to endNight
34%
792
24%
2376
16%
792
44%
792
19%
1594
November1st to 10thNight
38%
740
24%
2221
16%
740
48%
740
25%
1400
November11th to 20thNight
31%
760
22%
2467
15%
760
43%
760
17%
1637
November21st to endNight
29%
760
19%
2660
13%
760
40%
760
15%
1736
December1st to 10thNight
19%
760
14%
2660
7%
760
28%
760
9%
1955
December11th to 20thNight
24%
760
17%
3218
10%
759
35%
759
11%
2155
December21st to endNight
23%
827
12%
4549
7%
817
35%
817
8%
3143

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
15%9%47%6%45%5%5%7%34%
78753
Night
13%8%47%6%35%5%5%8%36%
81103
TransparencyDay
50%5%10%20%13%3
78474
Night
48%5%11%20%14%3
80901
SeeingDay
26%14%25%19%11%5%
14053
Night
24%13%21%26%14%2
24160

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 10thDay
14%12%5%9%6%35%45%6%31%
2836
11th to 20thDay
11%12%8%14%7%44445%27%
2840
21st to endDay
18%12%5%8%5%34446%32%
3124
February1st to 10thDay
12%13%48%5%45%46%7%33%
1710
11th to 20thDay
15%9%49%6%35%5%5%8%31%
1665
21st to endDay
16%11%5%7%5%35%446%33%
1638
March1st to 10thDay
13%7%47%5%35%5%5%8%39%
2090
11th to 20thDay
14%11%48%6%35%5%5%6%34%
2090
21st to endDay
17%11%46%6%35%447%32%
2200
April1st to 10thDay
12%9%47%6%3445%7%37%
2065
11th to 20thDay
14%8%45%5%35%45%7%40%
2134
21st to endDay
19%8%46%6%45%5%5%5%34%
2299
May1st to 10thDay
17%10%38%7%45%5%5%6%30%
2340
11th to 20thDay
20%9%47%6%35%5%5%7%29%
2301
21st to endDay
14%8%38%7%47%5%7%8%29%
2531
June1st to 10thDay
15%6%35%5%45%6%6%7%37%
2479
11th to 20thDay
13%7%37%7%5%6%6%7%9%32%
2520
21st to endDay
11%6%47%6%47%6%6%9%35%
2406
July1st to 10thDay
13%7%47%6%46%8%6%9%31%
2340
11th to 20thDay
9%5%36%8%47%7%8%10%33%
2340
21st to endDay
12%8%5%7%7%47%7%7%8%28%
2547
August1st to 10thDay
14%8%37%8%5%7%7%6%10%26%
2327
11th to 20thDay
15%6%47%8%5%7%6%6%8%27%
2303
21st to endDay
12%7%36%8%47%6%7%9%30%
2410
September1st to 10thDay
15%7%36%6%45%5%6%7%38%
2159
11th to 20thDay
13%7%36%6%35%5%5%7%41%
1980
21st to endDay
22%7%36%5%3445%6%35%
1978
October1st to 10thDay
16%7%35%5%4445%7%40%
1800
11th to 20thDay
18%9%25%6%33445%42%
1800
21st to endDay
17%9%36%42445%6%42%
1980
November1st to 10thDay
10%8%35%5%3445%5%48%
1857
11th to 20thDay
15%7%35%425%5%5%7%42%
1704
21st to endDay
15%8%45%5%45%45%5%40%
1520
December1st to 10thDay
16%13%5%7%7%35%46%7%28%
1520
11th to 20thDay
16%8%46%6%44347%38%
1910
21st to endDay
18%10%5%8%7%45%5%6%7%26%
3010
January1st to 10thNight
10%13%6%11%8%5%6%5%5%6%26%
4109
11th to 20thNight
13%12%48%8%44346%35%
4120
21st to endNight
20%11%47%43445%6%32%
4532
February1st to 10thNight
11%9%47%7%35%5%6%8%34%
2470
11th to 20thNight
15%8%48%7%45%6%6%7%30%
2399
21st to endNight
15%9%49%6%25%5%5%7%33%
1814
March1st to 10thNight
13%8%46%5%45%6%6%6%39%
2090
11th to 20thNight
14%9%38%6%35%45%7%35%
2090
21st to endNight
14%9%47%6%35%5%6%7%34%
2201
April1st to 10thNight
13%10%5%7%6%35%5%5%7%36%
1894
11th to 20thNight
11%7%35%7%45%46%9%39%
1775
21st to endNight
14%7%36%5%45%5%6%7%38%
1674
May1st to 10thNight
15%8%36%7%45%5%5%7%37%
1620
11th to 20thNight
18%10%46%6%25%5%5%8%32%
1592
21st to endNight
12%8%246%36%5%7%9%37%
1744
June1st to 10thNight
10%6%345%35%5%7%11%42%
1472
11th to 20thNight
12%5%35%6%345%7%9%41%
1440
21st to endNight
10%4245%346%7%11%42%
1545
July1st to 10thNight
10%6%25%6%35%5%6%11%41%
1620
11th to 20thNight
7%4245%35%5%7%12%47%
1620
21st to endNight
8%5%36%6%37%6%7%10%39%
1771
August1st to 10thNight
11%7%26%7%46%6%6%9%37%
1611
11th to 20thNight
13%6%25%5%35%5%7%8%40%
1598
21st to endNight
10%6%36%6%347%5%10%40%
1926
September1st to 10thNight
14%7%25%5%35%45%7%44%
1800
11th to 20thNight
13%6%25%435%45%8%46%
1978
21st to endNight
18%8%26%5%45%45%6%37%
1981
October1st to 10thNight
16%8%244345%5%7%42%
2160
11th to 20thNight
13%9%45%434447%41%
2160
21st to endNight
15%7%45%5%26%5%5%7%40%
2376
November1st to 10thNight
9%8%36%5%345%5%6%45%
2221
11th to 20thNight
13%7%46%436%6%5%7%39%
2467
21st to endNight
14%7%25%6%46%5%6%7%37%
2660
December1st to 10thNight
14%12%5%8%8%35%5%5%7%27%
2660
11th to 20thNight
13%8%49%6%35%5%5%7%34%
3285
21st to endNight
16%8%6%7%6%45%6%5%8%28%
4628

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
55%5%419%14%3
2837
11th to 20thDay
61%6%5%13%12%3
2840
21st to endDay
57%36%17%15%2
3124
February1st to 10thDay
52%48%20%13%3
1710
11th to 20thDay
53%47%17%15%3
1674
21st to endDay
54%48%17%13%4
1638
March1st to 10thDay
44%5%9%23%16%2
2090
11th to 20thDay
52%5%7%19%13%4
2090
21st to endDay
55%48%19%10%4
2200
April1st to 10thDay
48%411%18%15%4
2065
11th to 20thDay
46%39%21%17%3
2134
21st to endDay
53%410%20%11%2
2299
May1st to 10thDay
55%5%10%18%10%2
2340
11th to 20thDay
55%5%14%18%6%2
2305
21st to endDay
52%5%14%20%8%
2531
June1st to 10thDay
46%5%13%23%12%
2479
11th to 20thDay
49%5%15%18%12%1
2520
21st to endDay
45%6%14%23%11%1
2406
July1st to 10thDay
50%5%12%20%12%1
2275
11th to 20thDay
44%7%15%21%12%1
2271
21st to endDay
51%7%9%19%12%1
2405
August1st to 10thDay
52%6%15%19%9%
2314
11th to 20thDay
54%6%12%19%8%1
2303
21st to endDay
49%6%13%20%10%2
2410
September1st to 10thDay
47%410%19%15%4
2159
11th to 20thDay
44%37%24%18%4
1980
21st to endDay
51%37%21%16%3
1978
October1st to 10thDay
44%410%23%15%4
1800
11th to 20thDay
46%38%22%18%2
1800
21st to endDay
45%38%22%18%4
1980
November1st to 10thDay
39%38%24%21%5%
1857
11th to 20thDay
43%47%18%22%5%
1704
21st to endDay
48%38%20%19%2
1520
December1st to 10thDay
58%48%17%12%3
1520
11th to 20thDay
49%45%20%17%5%
1906
21st to endDay
58%5%8%16%13%1
3010
January1st to 10thNight
57%9%413%13%4
4109
11th to 20thNight
54%37%17%15%4
4120
21st to endNight
55%46%18%16%2
4532
February1st to 10thNight
49%5%11%19%13%2
2470
11th to 20thNight
55%5%7%17%12%4
2401
21st to endNight
53%410%18%9%5%
1814
March1st to 10thNight
47%48%23%15%3
2090
11th to 20thNight
50%5%10%21%12%2
2090
21st to endNight
51%5%9%18%13%4
2201
April1st to 10thNight
48%5%10%20%14%2
1894
11th to 20thNight
43%5%15%19%14%3
1775
21st to endNight
45%5%13%23%12%1
1674
May1st to 10thNight
49%5%15%19%10%2
1620
11th to 20thNight
51%6%14%20%7%1
1593
21st to endNight
43%7%22%23%5%1
1745
June1st to 10thNight
38%7%16%28%11%
1472
11th to 20thNight
39%7%19%23%10%2
1440
21st to endNight
35%8%19%27%11%1
1545
July1st to 10thNight
40%8%17%21%13%2
1575
11th to 20thNight
33%7%20%25%12%2
1566
21st to endNight
37%8%18%25%13%
1672
August1st to 10thNight
43%7%17%23%10%
1600
11th to 20thNight
42%5%19%23%10%1
1598
21st to endNight
39%6%17%25%10%2
1926
September1st to 10thNight
41%317%20%17%3
1800
11th to 20thNight
37%5%10%27%18%2
1978
21st to endNight
48%5%10%22%15%1
1981
October1st to 10thNight
42%5%10%24%15%3
2160
11th to 20thNight
44%410%21%18%3
2160
21st to endNight
46%47%20%19%4
2376
November1st to 10thNight
41%410%18%20%6%
2221
11th to 20thNight
46%5%6%18%20%5%
2467
21st to endNight
48%49%20%16%3
2660
December1st to 10thNight
59%48%16%11%3
2660
11th to 20thNight
51%48%15%19%2
3288
21st to endNight
54%6%9%18%11%1
4628

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
44%21%23%7%31
270
11th to 20thDay
41%23%27%7%1
270
21st to endDay
41%23%27%8%2
297
February1st to 10thDay
42%21%26%7%3
270
11th to 20thDay
49%23%22%5%
270
21st to endDay
45%22%23%9%1
293
March1st to 10thDay
36%22%24%15%3
460
11th to 20thDay
35%24%23%13%5%1
460
21st to endDay
32%21%27%14%6%
470
April1st to 10thDay
30%24%28%12%6%
420
11th to 20thDay
28%18%26%15%11%2
412
21st to endDay
33%19%26%12%7%3
420
May1st to 10thDay
34%16%25%14%8%4
423
11th to 20thDay
25%14%28%19%9%4
447
21st to endDay
17%10%25%24%16%8%
485
June1st to 10thDay
21%11%24%22%16%5%
450
11th to 20thDay
22%9%28%18%15%8%
450
21st to endDay
14%14%27%28%13%4
450
July1st to 10thDay
16%5%24%23%20%11%
445
11th to 20thDay
7%5%29%33%16%10%
444
21st to endDay
14%423%29%20%9%
488
August1st to 10thDay
17%7%20%25%20%11%
446
11th to 20thDay
18%421%26%18%13%
439
21st to endDay
14%424%27%19%13%
495
September1st to 10thDay
18%5%19%27%20%12%
450
11th to 20thDay
14%6%19%30%22%11%
450
21st to endDay
28%11%20%22%15%4
450
October1st to 10thDay
22%9%22%24%18%6%
450
11th to 20thDay
26%15%22%23%11%2
450
21st to endDay
29%15%29%16%9%2
495
November1st to 10thDay
21%18%27%21%10%3
455
11th to 20thDay
36%14%26%17%6%
272
21st to endDay
31%14%34%17%21
180
December1st to 10thDay
33%22%27%14%21
180
11th to 20thDay
39%17%27%14%31
180
21st to endDay
35%23%24%12%5%
267
January1st to 10thNight
34%22%26%15%3
760
11th to 20thNight
34%17%24%23%2
760
21st to endNight
32%21%30%16%2
836
February1st to 10thNight
28%20%27%23%3
760
11th to 20thNight
36%25%24%13%1
743
21st to endNight
34%21%29%14%1
628
March1st to 10thNight
27%23%25%22%3
760
11th to 20thNight
34%20%21%19%5%1
760
21st to endNight
26%22%25%21%6%1
801
April1st to 10thNight
28%19%28%21%4
720
11th to 20thNight
22%17%24%25%12%1
711
21st to endNight
24%12%26%30%9%1
594
May1st to 10thNight
24%8%22%29%16%1
540
11th to 20thNight
24%9%20%31%15%1
531
21st to endNight
16%5%18%33%23%5%
584
June1st to 10thNight
14%420%33%27%3
392
11th to 20thNight
15%5%18%40%21%2
360
21st to endNight
12%419%42%20%3
465
July1st to 10thNight
14%214%33%30%7%
525
11th to 20thNight
9%212%37%33%7%
522
21st to endNight
11%312%35%35%4
591
August1st to 10thNight
12%212%38%33%3
537
11th to 20thNight
15%39%35%33%5%
534
21st to endNight
15%212%33%33%6%
738
September1st to 10thNight
18%313%30%31%6%
720
11th to 20thNight
16%413%31%32%4
720
21st to endNight
22%6%19%27%24%2
720
October1st to 10thNight
24%9%14%27%21%5%
720
11th to 20thNight
23%14%20%27%15%1
720
21st to endNight
26%14%21%26%12%1
792
November1st to 10thNight
18%13%24%31%13%2
740
11th to 20thNight
27%16%21%26%9%2
760
21st to endNight
28%16%23%26%6%1
760
December1st to 10thNight
31%22%22%21%4
760
11th to 20thNight
27%19%25%25%4
760
21st to endNight
28%15%26%27%4
836
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