John Bryan State Park Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the John Bryan State Park Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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
39%
23938
17%
74342
12%
23883
45%
23883
22%
50740

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
18%
904
11%
2602
5%
900
25%
900
12%
2074
January11th to 20thNight
16%
760
10%
2470
5%
760
23%
760
11%
1945
January21st to endNight
19%
820
12%
2664
8%
820
25%
820
11%
2071
February1st to 10thNight
21%
720
11%
2295
7%
720
27%
720
13%
1750
February11th to 20thNight
21%
700
14%
2099
7%
700
28%
700
14%
1534
February21st to endNight
16%
592
13%
1776
4%
592
24%
592
13%
1345
March1st to 10thNight
27%
720
16%
2124
9%
720
33%
720
20%
1470
March11th to 20thNight
29%
720
18%
1980
8%
720
36%
720
21%
1391
March21st to endNight
29%
792
16%
2178
8%
792
37%
792
20%
1578
April1st to 10thNight
31%
627
17%
1887
9%
627
39%
627
24%
1243
April11th to 20thNight
37%
533
21%
1687
13%
533
45%
533
24%
1088
April21st to endNight
30%
540
18%
1620
7%
540
40%
540
23%
1102
May1st to 10thNight
34%
540
13%
1620
7%
540
39%
540
19%
1156
May11th to 20thNight
45%
531
17%
1593
10%
531
50%
531
24%
1070
May21st to endNight
59%
583
17%
1745
13%
582
62%
582
32%
1059
June1st to 10thNight
57%
540
15%
1620
13%
540
60%
540
33%
975
June11th to 20thNight
62%
540
20%
1463
14%
540
66%
540
36%
901
June21st to endNight
61%
540
19%
1440
13%
540
66%
540
37%
884
July1st to 10thNight
66%
525
18%
1400
16%
525
68%
525
35%
884
July11th to 20thNight
70%
522
19%
1501
17%
522
72%
522
37%
893
July21st to endNight
67%
591
21%
1672
17%
558
69%
558
34%
985
August1st to 10thNight
70%
537
21%
1600
19%
534
71%
534
35%
936
August11th to 20thNight
68%
534
22%
1598
19%
534
71%
534
33%
979
August21st to endNight
69%
594
23%
1896
21%
594
72%
594
36%
1060
September1st to 10thNight
62%
652
23%
1912
18%
652
65%
652
34%
1149
September11th to 20thNight
69%
720
31%
1980
25%
720
72%
720
42%
1130
September21st to endNight
53%
720
28%
1980
22%
720
58%
720
34%
1232
October1st to 10thNight
52%
720
25%
1984
19%
720
58%
720
35%
1204
October11th to 20thNight
39%
720
21%
2160
16%
720
47%
720
26%
1406
October21st to endNight
32%
792
21%
2484
16%
792
41%
792
21%
1675
November1st to 10thNight
41%
740
24%
2406
19%
740
49%
740
28%
1439
November11th to 20thNight
30%
760
22%
2470
13%
760
42%
760
21%
1613
November21st to endNight
29%
760
17%
2470
11%
760
37%
760
16%
1788
December1st to 10thNight
19%
760
12%
2470
7%
760
26%
760
12%
1881
December11th to 20thNight
23%
759
13%
2983
7%
758
31%
758
12%
2169
December21st to endNight
18%
830
7%
4513
6%
817
25%
817
5%
3681

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%35%5%5%7%34%
75401
Night
15%9%46%5%3445%7%37%
74617
TransparencyDay
48%6%12%20%12%2
75116
Night
47%6%11%21%13%2
74422
SeeingDay
19%8%26%24%16%8%
14221
Night
19%8%27%30%15%2
23949

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
18%12%7%10%8%47%445%21%
1544
11th to 20thDay
20%14%7%11%7%3434421%
1710
21st to endDay
17%17%7%10%7%45%446%19%
1836
February1st to 10thDay
20%13%7%10%8%44445%22%
1663
11th to 20thDay
16%13%6%11%8%46%5%36%24%
1748
21st to endDay
19%15%5%9%5%35%436%28%
1480
March1st to 10thDay
18%9%48%7%345%5%6%30%
1836
11th to 20thDay
18%11%48%6%4445%7%29%
1980
21st to endDay
21%11%5%8%6%3445%8%27%
2178
April1st to 10thDay
14%11%5%8%5%35%45%7%34%
2088
11th to 20thDay
16%9%37%6%34447%37%
2221
21st to endDay
18%8%47%6%45%5%6%6%33%
2340
May1st to 10thDay
20%9%36%6%45%45%7%31%
2340
11th to 20thDay
19%8%46%5%35%5%5%7%32%
2301
21st to endDay
10%5%37%6%46%6%8%8%36%
2531
June1st to 10thDay
13%6%36%6%446%6%10%37%
2340
11th to 20thDay
11%5%26%6%45%6%7%9%40%
2491
21st to endDay
10%435%6%36%7%7%10%38%
2520
July1st to 10thDay
12%6%25%6%37%7%7%9%37%
2520
11th to 20thDay
10%425%5%46%6%7%8%42%
2405
21st to endDay
9%425%6%47%6%7%9%41%
2547
August1st to 10thDay
8%425%5%46%6%8%10%42%
2327
11th to 20thDay
9%5%36%6%47%6%7%10%38%
2303
21st to endDay
8%5%25%5%45%5%7%9%45%
2457
September1st to 10thDay
12%5%245%345%5%7%48%
2028
11th to 20thDay
10%5%25%5%3445%9%48%
1980
21st to endDay
14%6%25%5%2445%7%46%
1980
October1st to 10thDay
12%7%25%5%345%6%7%44%
1976
11th to 20thDay
15%7%345%3445%7%44%
1800
21st to endDay
21%9%36%5%3435%7%34%
1869
November1st to 10thDay
10%6%35%43445%6%49%
1672
11th to 20thDay
15%11%49%5%3445%7%35%
1710
21st to endDay
18%10%5%8%6%44346%32%
1710
December1st to 10thDay
21%13%6%9%6%35%5%46%22%
1710
11th to 20thDay
17%15%6%10%6%3445%5%26%
2153
21st to endDay
22%21%7%9%7%34445%13%
3107
January1st to 10thNight
17%14%6%11%8%4445%423%
2602
11th to 20thNight
19%14%7%9%8%44445%21%
2470
21st to endNight
17%14%6%10%8%34345%24%
2664
February1st to 10thNight
22%13%48%6%35%43427%
2295
11th to 20thNight
16%11%5%9%6%45%446%29%
2098
21st to endNight
20%14%6%9%5%35%335%28%
1776
March1st to 10thNight
18%7%47%7%34346%36%
2124
11th to 20thNight
17%10%36%5%35%447%35%
1980
21st to endNight
21%9%46%5%34446%34%
2178
April1st to 10thNight
15%7%45%5%3446%7%39%
1887
11th to 20thNight
15%8%25%5%345%5%8%39%
1687
21st to endNight
17%7%36%5%35%5%6%7%37%
1620
May1st to 10thNight
18%9%36%5%345%6%7%33%
1620
11th to 20thNight
16%8%35%5%3446%8%38%
1592
21st to endNight
7%5%25%5%35%7%7%12%42%
1744
June1st to 10thNight
8%5%245%35%5%5%8%49%
1620
11th to 20thNight
8%4244345%6%11%47%
1463
21st to endNight
7%425%5%35%6%6%11%46%
1440
July1st to 10thNight
8%5%35%5%346%6%9%46%
1440
11th to 20thNight
7%3245%35%5%6%10%49%
1551
21st to endNight
8%5%35%5%35%5%6%9%47%
1771
August1st to 10thNight
6%425%6%347%7%10%47%
1611
11th to 20thNight
7%5%35%5%35%5%5%8%49%
1598
21st to endNight
6%325%5%45%46%9%51%
1896
September1st to 10thNight
13%5%2443345%7%50%
1912
11th to 20thNight
9%4235%3345%6%56%
1980
21st to endNight
13%5%25%423446%50%
1980
October1st to 10thNight
8%5%36%5%3345%7%50%
1984
11th to 20thNight
17%7%25%5%33446%43%
2160
21st to endNight
20%9%35%5%3445%5%38%
2484
November1st to 10thNight
14%7%25%434447%47%
2406
11th to 20thNight
17%8%47%42445%6%38%
2470
21st to endNight
18%12%5%6%7%3445%7%29%
2470
December1st to 10thNight
20%13%7%8%6%3445%5%26%
2470
11th to 20thNight
18%13%7%8%5%34446%27%
3042
21st to endNight
23%19%8%9%7%34435%16%
4532

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
66%5%7%15%7%
1545
11th to 20thDay
68%5%10%13%5%
1710
21st to endDay
67%410%13%5%1
1836
February1st to 10thDay
66%410%12%7%1
1663
11th to 20thDay
63%5%9%15%7%1
1758
21st to endDay
60%5%9%17%8%1
1480
March1st to 10thDay
54%412%21%8%1
1836
11th to 20thDay
56%5%11%18%10%1
1980
21st to endDay
57%5%12%19%6%1
2178
April1st to 10thDay
49%6%12%20%12%1
2088
11th to 20thDay
48%5%12%19%14%3
2221
21st to endDay
50%5%12%20%11%2
2340
May1st to 10thDay
52%6%12%20%9%1
2340
11th to 20thDay
50%7%12%18%11%1
2305
21st to endDay
41%10%15%22%11%1
2531
June1st to 10thDay
41%7%11%24%14%2
2340
11th to 20thDay
38%7%15%23%16%1
2491
21st to endDay
38%8%15%21%14%3
2520
July1st to 10thDay
41%9%12%20%15%3
2451
11th to 20thDay
36%8%12%27%16%1
2332
21st to endDay
35%9%16%24%14%3
2405
August1st to 10thDay
34%9%15%27%14%2
2314
11th to 20thDay
38%8%15%26%13%1
2303
21st to endDay
34%8%13%24%16%5%
2457
September1st to 10thDay
35%6%12%24%20%4
2028
11th to 20thDay
33%5%13%25%21%3
1980
21st to endDay
38%49%24%23%1
1980
October1st to 10thDay
38%6%11%25%18%1
1976
11th to 20thDay
40%5%10%25%18%3
1800
21st to endDay
50%413%20%12%1
1869
November1st to 10thDay
36%5%11%27%19%3
1672
11th to 20thDay
50%410%20%14%2
1710
21st to endDay
55%5%10%19%10%2
1710
December1st to 10thDay
64%5%11%15%4
1710
11th to 20thDay
61%5%9%17%8%
2150
21st to endDay
74%5%7%8%5%1
3107
January1st to 10thNight
65%48%16%7%
2602
11th to 20thNight
65%410%15%5%
2470
21st to endNight
63%48%16%7%1
2664
February1st to 10thNight
62%410%17%7%1
2295
11th to 20thNight
56%410%18%10%1
2099
21st to endNight
61%310%15%9%2
1776
March1st to 10thNight
50%410%22%13%2
2124
11th to 20thNight
49%5%12%21%10%3
1980
21st to endNight
52%5%9%23%10%1
2178
April1st to 10thNight
43%6%12%24%13%2
1887
11th to 20thNight
42%6%12%17%18%4
1687
21st to endNight
45%7%11%23%12%3
1620
May1st to 10thNight
49%7%11%19%9%3
1620
11th to 20thNight
44%6%13%21%13%2
1593
21st to endNight
32%9%16%26%13%3
1745
June1st to 10thNight
33%7%12%25%21%2
1620
11th to 20thNight
31%8%17%24%17%4
1463
21st to endNight
31%8%17%24%17%3
1440
July1st to 10thNight
34%7%14%23%18%4
1400
11th to 20thNight
29%8%14%29%18%2
1501
21st to endNight
32%7%15%28%16%3
1672
August1st to 10thNight
30%8%17%27%18%1
1600
11th to 20thNight
33%8%13%26%19%1
1598
21st to endNight
29%7%15%24%21%5%
1896
September1st to 10thNight
33%6%13%25%18%6%
1912
11th to 20thNight
28%5%11%27%27%3
1980
21st to endNight
35%6%9%23%25%2
1980
October1st to 10thNight
33%6%11%25%23%2
1984
11th to 20thNight
42%5%8%25%18%2
2160
21st to endNight
48%48%23%15%3
2484
November1st to 10thNight
38%412%22%21%3
2406
11th to 20thNight
46%48%23%16%3
2470
21st to endNight
56%410%20%9%1
2470
December1st to 10thNight
61%5%9%18%7%1
2470
11th to 20thNight
56%5%10%20%7%1
3044
21st to endNight
71%5%8%12%4
4532

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
25%14%37%19%41
270
11th to 20thDay
25%17%39%14%4
270
21st to endDay
30%13%35%16%6%
282
February1st to 10thDay
30%15%36%16%2
240
11th to 20thDay
26%23%35%14%2
240
21st to endDay
27%14%44%13%3
198
March1st to 10thDay
25%16%38%19%3
276
11th to 20thDay
24%14%34%20%8%1
420
21st to endDay
30%11%36%17%6%
462
April1st to 10thDay
18%12%35%22%10%2
420
11th to 20thDay
20%15%30%21%10%4
412
21st to endDay
23%10%36%19%9%3
420
May1st to 10thDay
27%12%27%21%8%4
423
11th to 20thDay
22%7%26%27%13%6%
447
21st to endDay
14%5%24%26%18%13%
485
June1st to 10thDay
15%419%30%23%8%
450
11th to 20thDay
13%423%29%22%8%
601
21st to endDay
11%6%26%28%19%9%
630
July1st to 10thDay
14%214%27%23%21%
621
11th to 20thDay
11%220%25%26%16%
505
21st to endDay
11%214%29%27%17%
488
August1st to 10thDay
12%211%24%30%21%
446
11th to 20thDay
14%313%30%27%14%
439
21st to endDay
10%212%29%30%17%
495
September1st to 10thDay
13%5%13%23%32%14%
450
11th to 20thDay
11%213%26%29%19%
450
21st to endDay
18%7%16%25%25%9%
450
October1st to 10thDay
12%6%23%29%21%8%
450
11th to 20thDay
18%11%31%24%14%2
450
21st to endDay
25%9%34%26%5%1
384
November1st to 10thDay
16%9%34%26%10%5%
270
11th to 20thDay
20%15%31%20%12%3
270
21st to endDay
26%11%36%22%5%
270
December1st to 10thDay
21%18%33%19%8%
270
11th to 20thDay
24%12%41%18%5%
270
21st to endDay
34%14%31%16%4
297
January1st to 10thNight
23%15%35%21%5%1
904
11th to 20thNight
25%14%34%22%4
760
21st to endNight
24%12%35%25%4
820
February1st to 10thNight
29%9%34%26%2
720
11th to 20thNight
24%14%36%24%1
703
21st to endNight
26%10%40%21%3
592
March1st to 10thNight
20%12%35%27%6%1
720
11th to 20thNight
22%11%34%25%7%1
720
21st to endNight
26%9%31%26%9%
792
April1st to 10thNight
18%11%35%29%7%
627
11th to 20thNight
18%10%31%28%12%1
533
21st to endNight
18%7%39%26%8%2
540
May1st to 10thNight
22%6%33%26%12%1
540
11th to 20thNight
20%5%25%33%16%2
531
21st to endNight
9%222%37%26%4
584
June1st to 10thNight
10%221%44%22%1
540
11th to 20thNight
9%418%42%25%2
540
21st to endNight
8%320%40%27%2
540
July1st to 10thNight
11%110%35%34%9%
525
11th to 20thNight
8%113%40%32%5%
522
21st to endNight
9%111%40%36%3
591
August1st to 10thNight
6%29%41%37%4
537
11th to 20thNight
10%212%39%33%5%
534
21st to endNight
7%212%36%36%7%
594
September1st to 10thNight
15%313%32%32%6%
652
11th to 20thNight
11%112%35%37%5%
720
21st to endNight
15%5%18%36%23%3
720
October1st to 10thNight
13%424%33%22%5%
720
11th to 20thNight
21%7%28%32%12%1
720
21st to endNight
23%9%31%27%9%
792
November1st to 10thNight
18%8%29%28%16%1
740
11th to 20thNight
20%13%31%25%9%1
760
21st to endNight
25%10%30%28%7%
760
December1st to 10thNight
26%12%34%23%5%
760
11th to 20thNight
25%10%35%25%5%1
760
21st to endNight
29%13%30%23%4
836
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