Frog Island Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Frog Island Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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%
24855
14%
74752
8%
24797
41%
24797
20%
52467

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
21%
950
10%
2648
3%
946
27%
946
17%
1969
January11th to 20thNight
21%
920
10%
2575
3%
920
28%
920
14%
1982
January21st to endNight
25%
990
10%
2772
4%
990
30%
990
16%
2110
February1st to 10thNight
25%
900
13%
2462
4%
900
32%
900
20%
1806
February11th to 20thNight
25%
711
17%
2107
7%
708
34%
708
18%
1491
February21st to endNight
24%
592
15%
1776
6%
592
32%
592
15%
1319
March1st to 10thNight
31%
719
17%
2159
10%
719
38%
719
23%
1429
March11th to 20thNight
30%
720
19%
2098
10%
720
39%
720
24%
1384
March21st to endNight
37%
635
19%
2021
14%
635
45%
635
21%
1344
April1st to 10thNight
29%
540
11%
1800
9%
540
35%
540
16%
1308
April11th to 20thNight
37%
533
18%
1736
12%
533
44%
533
19%
1195
April21st to endNight
37%
540
15%
1620
8%
540
44%
540
21%
1125
May1st to 10thNight
38%
540
15%
1580
9%
540
43%
540
21%
1107
May11th to 20thNight
37%
531
15%
1416
10%
531
41%
531
22%
1042
May21st to endNight
53%
583
13%
1552
8%
582
57%
582
31%
1012
June1st to 10thNight
52%
540
13%
1440
7%
540
54%
540
31%
936
June11th to 20thNight
50%
540
12%
1440
8%
540
51%
540
27%
982
June21st to endNight
53%
540
13%
1440
7%
540
56%
540
32%
928
July1st to 10thNight
68%
525
15%
1400
9%
525
70%
525
41%
826
July11th to 20thNight
56%
522
17%
1392
10%
522
60%
522
38%
849
July21st to endNight
59%
591
18%
1487
11%
558
62%
558
40%
891
August1st to 10thNight
63%
537
20%
1423
14%
534
65%
534
37%
885
August11th to 20thNight
64%
534
21%
1585
16%
534
69%
534
37%
913
August21st to endNight
54%
594
18%
1960
15%
594
60%
594
30%
1157
September1st to 10thNight
53%
540
20%
1800
17%
540
59%
540
30%
1054
September11th to 20thNight
50%
658
22%
1918
19%
658
54%
658
31%
1184
September21st to endNight
42%
720
17%
1980
11%
720
46%
720
25%
1367
October1st to 10thNight
37%
720
16%
2009
10%
720
43%
720
24%
1373
October11th to 20thNight
28%
720
13%
2269
6%
720
36%
720
17%
1597
October21st to endNight
25%
792
11%
2574
6%
792
31%
792
15%
1886
November1st to 10thNight
27%
740
18%
2406
11%
740
37%
740
19%
1620
November11th to 20thNight
20%
760
13%
2470
6%
760
29%
760
13%
1818
November21st to endNight
22%
940
11%
2650
4%
940
27%
940
15%
2007
December1st to 10thNight
18%
950
10%
2674
2%
950
26%
950
13%
2034
December11th to 20thNight
18%
950
10%
3243
2%
948
25%
948
11%
2475
December21st to endNight
14%
1038
7%
4870
2%
1026
22%
1026
6%
4062

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
13%8%37%6%45%5%6%8%36%
74711
Night
14%10%47%5%35%5%5%7%35%
74980
TransparencyDay
44%9%15%21%10%1
74418
Night
47%8%14%21%10%1
74808
SeeingDay
15%11%32%28%12%3
14799
Night
18%9%31%29%12%1
24865

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
10%11%6%10%7%5%5%6%6%8%27%
1517
11th to 20thDay
16%12%6%9%8%45%5%5%8%24%
1464
21st to endDay
13%13%410%7%5%5%46%6%28%
1584
February1st to 10thDay
9%9%49%8%46%6%7%7%30%
1494
11th to 20thDay
10%11%5%6%7%5%6%6%5%6%33%
1738
21st to endDay
13%9%411%7%45%6%7%7%26%
1480
March1st to 10thDay
13%8%5%7%5%35%6%6%9%33%
1798
11th to 20thDay
10%10%47%7%45%5%6%7%37%
1860
21st to endDay
15%10%47%7%45%6%6%7%30%
2349
April1st to 10thDay
16%10%47%8%45%5%6%7%28%
2160
11th to 20thDay
18%9%47%6%45%5%5%6%33%
2173
21st to endDay
17%8%35%5%35%6%5%8%35%
2340
May1st to 10thDay
14%7%47%6%45%45%7%36%
2376
11th to 20thDay
14%7%36%6%35%6%5%7%38%
2479
21st to endDay
10%5%36%6%35%5%6%9%41%
2725
June1st to 10thDay
11%425%5%45%5%7%9%43%
2520
11th to 20thDay
12%5%26%5%346%7%8%41%
2520
21st to endDay
12%436%5%35%6%7%9%40%
2520
July1st to 10thDay
7%3245%35%6%7%11%48%
2520
11th to 20thDay
6%425%5%35%5%5%12%48%
2520
21st to endDay
8%4245%35%5%8%9%47%
2743
August1st to 10thDay
6%5%25%5%36%6%7%10%45%
2507
11th to 20thDay
10%5%25%5%35%6%6%8%46%
2315
21st to endDay
11%5%26%435%5%6%8%46%
2394
September1st to 10thDay
12%5%25%6%35%5%5%8%43%
2160
11th to 20thDay
11%5%245%35%5%7%8%43%
2023
21st to endDay
13%7%35%7%45%6%6%8%37%
1980
October1st to 10thDay
16%7%26%6%25%5%6%9%36%
1949
11th to 20thDay
20%10%47%6%45%5%6%7%25%
1690
21st to endDay
19%11%47%6%45%5%6%6%25%
1782
November1st to 10thDay
13%10%47%6%35%6%5%7%36%
1672
11th to 20thDay
15%10%5%10%7%46%6%5%6%27%
1710
21st to endDay
17%12%48%5%35%6%7%7%25%
1529
December1st to 10thDay
14%11%47%8%5%6%6%6%7%26%
1461
11th to 20thDay
16%14%5%9%7%5%5%5%5%6%22%
1876
21st to endDay
23%20%5%9%7%345%4416%
2783
January1st to 10thNight
13%12%5%9%8%3445%7%30%
2648
11th to 20thNight
15%14%6%10%5%35%45%6%26%
2575
21st to endNight
12%13%5%9%7%46%5%6%7%26%
2772
February1st to 10thNight
10%12%6%8%7%35%5%5%8%31%
2462
11th to 20thNight
11%11%47%7%46%5%6%8%32%
2107
21st to endNight
14%12%5%10%6%45%5%6%5%27%
1776
March1st to 10thNight
12%10%36%6%3445%7%38%
2149
11th to 20thNight
12%9%37%5%35%5%47%40%
2098
21st to endNight
16%6%36%5%35%6%5%7%38%
2021
April1st to 10thNight
18%11%48%6%36%45%7%30%
1800
11th to 20thNight
20%8%46%5%345%5%6%36%
1736
21st to endNight
16%8%36%6%35%5%5%7%35%
1620
May1st to 10thNight
16%9%36%5%35%6%6%8%33%
1580
11th to 20thNight
17%8%38%6%45%5%5%7%32%
1416
21st to endNight
12%5%25%435%5%5%9%43%
1551
June1st to 10thNight
12%5%25%5%35%5%47%46%
1440
11th to 20thNight
14%6%35%6%46%46%8%40%
1440
21st to endNight
13%6%244235%5%8%46%
1440
July1st to 10thNight
7%314335%6%6%9%53%
1440
11th to 20thNight
9%5%25%42445%8%51%
1440
21st to endNight
8%5%2443446%9%53%
1575
August1st to 10thNight
7%425%4346%5%10%48%
1432
11th to 20thNight
8%324426%46%7%53%
1585
21st to endNight
12%5%25%43335%6%51%
1960
September1st to 10thNight
9%6%26%43345%6%52%
1800
11th to 20thNight
12%6%345%2446%5%49%
1918
21st to endNight
15%7%36%6%35%45%7%39%
1980
October1st to 10thNight
16%8%26%4345%5%7%39%
2009
11th to 20thNight
17%8%36%6%45%6%6%6%33%
2269
21st to endNight
21%11%46%435%5%6%7%29%
2574
November1st to 10thNight
14%8%36%6%35%5%5%7%36%
2406
11th to 20thNight
16%12%47%6%46%5%5%6%29%
2470
21st to endNight
15%11%5%7%6%35%5%6%7%29%
2650
December1st to 10thNight
13%12%5%8%7%36%45%8%28%
2674
11th to 20thNight
14%14%5%11%6%4445%6%26%
3283
21st to endNight
25%18%6%10%5%3433418%
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
53%8%13%19%6%1
1518
11th to 20thDay
59%8%11%16%6%
1464
21st to endDay
54%8%13%15%7%1
1584
February1st to 10thDay
48%9%14%19%10%
1494
11th to 20thDay
48%9%15%18%9%1
1748
21st to endDay
53%8%13%16%9%1
1480
March1st to 10thDay
45%9%15%18%11%2
1800
11th to 20thDay
44%8%14%23%9%2
1860
21st to endDay
49%11%13%20%8%1
2349
April1st to 10thDay
53%9%12%18%8%1
2160
11th to 20thDay
50%7%10%22%10%1
2173
21st to endDay
46%8%11%19%14%2
2340
May1st to 10thDay
45%8%13%21%11%2
2376
11th to 20thDay
43%8%11%23%13%1
2483
21st to endDay
37%10%14%25%12%2
2725
June1st to 10thDay
34%9%16%26%14%2
2520
11th to 20thDay
38%9%15%21%16%1
2520
21st to endDay
36%10%18%22%13%1
2520
July1st to 10thDay
27%9%17%30%15%1
2451
11th to 20thDay
28%9%17%28%15%3
2445
21st to endDay
30%10%19%24%17%1
2591
August1st to 10thDay
31%11%19%26%11%2
2492
11th to 20thDay
33%9%16%28%14%
2315
21st to endDay
34%9%18%24%13%1
2394
September1st to 10thDay
37%8%16%27%11%1
2160
11th to 20thDay
36%11%22%26%6%
2023
21st to endDay
42%9%18%24%7%
1980
October1st to 10thDay
42%9%16%24%8%
1949
11th to 20thDay
55%9%13%19%41
1690
21st to endDay
57%8%14%15%6%
1782
November1st to 10thDay
46%8%15%21%10%
1672
11th to 20thDay
56%7%15%14%7%
1710
21st to endDay
55%8%19%14%31
1529
December1st to 10thDay
54%9%15%16%5%
1461
11th to 20thDay
61%8%14%10%6%
1877
21st to endDay
69%8%9%10%4
2783
January1st to 10thNight
54%7%15%17%8%1
2648
11th to 20thNight
58%7%13%17%5%
2575
21st to endNight
55%8%13%15%8%1
2772
February1st to 10thNight
52%6%12%19%10%1
2462
11th to 20thNight
49%7%13%20%10%2
2108
21st to endNight
55%7%11%19%7%
1776
March1st to 10thNight
44%6%15%24%10%1
2160
11th to 20thNight
42%6%13%23%14%2
2099
21st to endNight
44%8%13%22%13%1
2021
April1st to 10thNight
54%6%10%16%12%2
1800
11th to 20thNight
49%6%9%22%12%3
1736
21st to endNight
48%6%12%19%13%2
1620
May1st to 10thNight
47%7%11%20%12%2
1580
11th to 20thNight
49%8%8%21%12%2
1417
21st to endNight
36%8%16%24%13%2
1552
June1st to 10thNight
37%7%12%28%14%2
1440
11th to 20thNight
41%8%14%18%17%2
1440
21st to endNight
34%9%14%26%15%2
1440
July1st to 10thNight
25%9%15%32%18%1
1400
11th to 20thNight
31%7%15%28%16%3
1392
21st to endNight
28%7%18%29%14%3
1487
August1st to 10thNight
31%8%18%30%11%2
1423
11th to 20thNight
28%8%15%33%14%2
1585
21st to endNight
33%8%14%26%17%1
1960
September1st to 10thNight
33%7%14%29%16%1
1800
11th to 20thNight
35%7%17%32%9%1
1918
21st to endNight
43%8%15%25%8%
1980
October1st to 10thNight
43%7%14%26%8%1
2009
11th to 20thNight
49%8%15%20%8%
2269
21st to endNight
53%9%14%17%7%
2574
November1st to 10thNight
45%8%14%23%10%1
2406
11th to 20thNight
55%7%14%15%9%
2470
21st to endNight
54%8%17%17%31
2650
December1st to 10thNight
54%9%16%17%4
2674
11th to 20thNight
57%8%14%16%5%
3281
21st to endNight
68%9%10%10%4
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
15%14%39%23%10%
270
11th to 20thDay
23%13%35%22%6%1
249
21st to endDay
20%9%36%28%6%
264
February1st to 10thDay
16%13%46%22%3
240
11th to 20thDay
12%20%43%21%41
240
21st to endDay
20%15%48%16%1
198
March1st to 10thDay
14%15%44%21%7%
240
11th to 20thDay
14%15%36%25%8%2
300
21st to endDay
21%14%31%25%9%1
462
April1st to 10thDay
22%11%35%24%7%1
420
11th to 20thDay
20%16%28%23%11%1
412
21st to endDay
18%14%32%24%10%2
420
May1st to 10thDay
19%12%36%22%10%2
459
11th to 20thDay
16%9%36%26%10%2
625
21st to endDay
14%8%30%31%13%4
679
June1st to 10thDay
11%8%35%33%12%2
630
11th to 20thDay
16%8%28%33%12%2
630
21st to endDay
13%6%29%35%15%2
630
July1st to 10thDay
7%427%39%18%5%
621
11th to 20thDay
8%6%28%39%15%5%
618
21st to endDay
7%5%28%35%20%6%
684
August1st to 10thDay
7%424%40%20%5%
626
11th to 20thDay
11%420%33%25%7%
451
21st to endDay
13%9%26%26%22%5%
495
September1st to 10thDay
13%7%28%28%17%6%
450
11th to 20thDay
12%5%30%36%13%4
450
21st to endDay
14%14%27%26%15%4
450
October1st to 10thDay
19%11%32%23%12%3
419
11th to 20thDay
19%19%40%19%4
270
21st to endDay
27%20%30%19%3
297
November1st to 10thDay
14%16%39%23%8%1
270
11th to 20thDay
20%20%40%15%5%1
270
21st to endDay
21%18%39%16%5%
270
December1st to 10thDay
20%19%36%17%9%
241
11th to 20thDay
25%17%38%15%41
252
21st to endDay
28%18%33%17%4
297
January1st to 10thNight
17%11%40%27%6%
950
11th to 20thNight
21%10%37%23%8%
920
21st to endNight
19%6%36%28%10%1
990
February1st to 10thNight
16%7%40%29%6%1
900
11th to 20thNight
17%11%38%27%6%1
712
21st to endNight
20%11%36%29%4
592
March1st to 10thNight
17%7%37%29%9%1
720
11th to 20thNight
15%10%40%27%7%
720
21st to endNight
18%8%30%33%10%1
635
April1st to 10thNight
21%8%33%30%7%1
540
11th to 20thNight
23%7%28%29%12%1
533
21st to endNight
17%9%29%36%9%1
540
May1st to 10thNight
20%7%30%32%11%1
540
11th to 20thNight
20%5%29%34%11%1
531
21st to endNight
14%421%40%19%2
584
June1st to 10thNight
13%425%43%16%1
540
11th to 20thNight
17%421%39%16%2
540
21st to endNight
15%321%39%19%1
540
July1st to 10thNight
7%318%44%26%2
525
11th to 20thNight
9%322%39%26%2
522
21st to endNight
8%322%40%23%3
591
August1st to 10thNight
8%220%41%27%2
537
11th to 20thNight
10%120%37%29%4
534
21st to endNight
12%425%36%23%1
594
September1st to 10thNight
11%5%24%31%23%5%
540
11th to 20thNight
14%5%26%35%16%3
658
21st to endNight
17%9%26%26%19%3
720
October1st to 10thNight
19%8%30%26%15%2
720
11th to 20thNight
22%12%32%24%9%
720
21st to endNight
26%12%33%22%6%1
792
November1st to 10thNight
18%13%36%26%7%
740
11th to 20thNight
21%17%34%21%6%1
760
21st to endNight
22%17%33%22%6%
940
December1st to 10thNight
21%16%37%19%6%
950
11th to 20thNight
21%14%38%20%7%1
950
21st to endNight
27%11%36%22%4
1045
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Copyright 2024 Attilla Danko


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