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 2005-11-05 to 2017-09-23.

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
31%
16156
14%
46902
7%
16112
38%
16112
19%
32705

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
18%
900
9%
2506
2%
895
24%
895
13%
1910
January16th to endNight
19%
960
12%
2688
3%
960
27%
960
14%
2029
February1st to 15thNight
21%
829
12%
2319
4%
827
27%
827
15%
1708
February16th to endNight
26%
631
14%
1893
7%
631
33%
631
13%
1423
March1st to 15thNight
28%
720
16%
2160
8%
720
35%
720
20%
1437
March16th to endNight
33%
663
19%
2025
12%
663
42%
663
20%
1360
April1st to 15thNight
28%
539
13%
1797
8%
539
36%
539
16%
1278
April16th to endNight
30%
540
17%
1653
7%
540
40%
540
18%
1128
May1st to 15thNight
31%
540
13%
1533
8%
540
36%
540
17%
1105
May16th to endNight
40%
562
14%
1498
6%
561
45%
561
26%
999
June1st to 15thNight
38%
540
11%
1440
5%
540
42%
540
21%
978
June16th to endNight
39%
540
11%
1440
5%
540
42%
540
23%
978
July1st to 15thNight
56%
510
14%
1360
8%
510
59%
510
37%
803
July16th to endNight
50%
564
17%
1417
8%
531
59%
531
38%
823
August1st to 15thNight
59%
539
21%
1482
16%
536
63%
536
36%
869
August16th to endNight
52%
573
19%
1838
14%
573
59%
573
29%
1078
September1st to 15thNight
47%
559
20%
1819
15%
559
56%
559
28%
1081
September16th to endNight
45%
689
18%
1903
12%
689
50%
689
27%
1249
October1st to 15thNight
33%
660
17%
1900
10%
660
40%
660
20%
1303
October16th to endNight
26%
704
10%
2288
5%
704
33%
704
14%
1656
November1st to 15thNight
24%
700
15%
2276
10%
700
32%
700
15%
1584
November16th to endNight
20%
834
10%
2454
5%
834
25%
834
12%
1900
December1st to 15thNight
15%
900
9%
2525
2%
900
21%
900
11%
1931
December16th to endNight
17%
960
11%
2688
3%
960
24%
960
12%
2095

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%7%36%6%35%5%5%7%35%
47956
Night
14%8%36%5%3445%7%36%
47070
Cloud CoverDay
13%7%36%6%35%5%5%7%35%
47965
Night
14%8%36%5%3445%7%36%
47075
TransparencyDay
45%8%14%21%9%
47642
Night
47%6%13%21%9%1
46902
SeeingDay
15%15%27%26%11%3
6127
Night
16%20%25%25%11%1
16158

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 15thDay
11%12%6%8%6%45%5%5%7%25%
1432
16th to endDay
14%11%49%7%4345%7%27%
1536
February1st to 15thDay
11%10%5%7%8%35%6%6%6%29%
1539
16th to endDay
12%9%49%7%46%6%7%6%25%
1580
March1st to 15thDay
13%9%5%7%5%25%5%5%7%32%
1790
16th to endDay
13%9%37%7%3445%6%32%
2207
April1st to 15thDay
17%9%47%5%4445%7%30%
2151
16th to endDay
18%8%245%25%447%35%
2307
May1st to 15thDay
16%7%36%6%35%446%34%
2415
16th to endDay
9%6%25%6%25%5%5%8%41%
2617
June1st to 15thDay
13%5%25%435%46%8%39%
2520
16th to endDay
11%436%5%35%6%7%8%36%
2520
July1st to 15thDay
7%4145%35%6%6%10%44%
2520
16th to endDay
7%314435%5%7%10%46%
2655
August1st to 15thDay
7%4245%35%6%6%9%45%
2467
16th to endDay
12%415%4245%6%8%43%
2363
September1st to 15thDay
10%425%6%35%5%5%7%43%
2128
16th to endDay
11%425%6%446%6%8%40%
1894
October1st to 15thDay
17%8%25%5%2445%7%35%
1728
16th to endDay
21%9%47%6%5%5%5%5%6%23%
1584
November1st to 15thDay
14%9%37%6%35%5%46%32%
1582
16th to endDay
18%12%48%6%35%6%7%5%22%
1506
December1st to 15thDay
17%11%46%6%5%4446%27%
1379
16th to endDay
18%14%5%8%6%36%5%5%5%17%
1536
January1st to 15thNight
15%13%5%8%6%35%35%6%26%
2506
16th to endNight
13%11%5%8%6%45%5%5%6%28%
2688
February1st to 15thNight
10%12%5%7%6%35%5%5%7%30%
2317
16th to endNight
12%12%5%8%6%45%45%6%26%
1893
March1st to 15thNight
13%9%36%5%24347%39%
2147
16th to endNight
14%7%35%425%6%47%39%
2025
April1st to 15thNight
19%8%36%425%446%33%
1797
16th to endNight
16%8%35%5%24447%37%
1653
May1st to 15thNight
18%7%36%6%25%5%6%8%31%
1533
16th to endNight
11%5%25%435%5%5%9%41%
1496
June1st to 15thNight
15%5%25%435%446%43%
1440
16th to endNight
12%6%35%5%245%5%8%40%
1440
July1st to 15thNight
8%3133245%6%7%52%
1440
16th to endNight
7%424323348%54%
1513
August1st to 15thNight
7%31442445%8%53%
1491
16th to endNight
11%41432335%7%51%
1838
September1st to 15thNight
8%5%25%43345%5%51%
1819
16th to endNight
13%5%245%2435%7%43%
1903
October1st to 15thNight
16%7%36%4335%5%5%39%
1900
16th to endNight
19%8%35%5%35%5%6%7%30%
2288
November1st to 15thNight
16%9%35%5%3445%6%33%
2276
16th to endNight
17%10%47%6%36%5%45%26%
2454
December1st to 15thNight
17%11%48%5%34447%27%
2525
16th to endNight
18%14%5%7%5%34345%26%
2688

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 15thDay
11%11%7%8%8%35%5%5%6%25%
1432
16th to endDay
14%12%49%8%4335%7%27%
1536
February1st to 15thDay
11%10%5%7%8%36%5%6%6%28%
1539
16th to endDay
12%9%39%7%5%6%6%6%6%25%
1580
March1st to 15thDay
13%9%5%7%6%245%5%7%31%
1800
16th to endDay
14%8%47%7%3445%7%32%
2206
April1st to 15thDay
17%9%46%6%44446%31%
2151
16th to endDay
18%8%245%2445%6%35%
2307
May1st to 15thDay
17%6%36%6%35%446%34%
2415
16th to endDay
10%5%25%6%35%46%8%41%
2617
June1st to 15thDay
14%5%35%5%3445%8%39%
2520
16th to endDay
12%426%6%25%6%6%8%37%
2520
July1st to 15thDay
7%4245%25%5%6%10%44%
2520
16th to endDay
7%3145%26%47%9%47%
2655
August1st to 15thDay
7%4245%246%6%9%45%
2467
16th to endDay
12%4244245%6%8%43%
2363
September1st to 15thDay
11%425%6%26%46%7%42%
2128
16th to endDay
11%4345%346%6%8%40%
1894
October1st to 15thDay
17%9%35%5%2445%7%35%
1728
16th to endDay
22%9%37%6%46%46%6%22%
1584
November1st to 15thDay
14%9%47%6%345%47%31%
1582
16th to endDay
18%13%5%7%6%346%6%6%21%
1506
December1st to 15thDay
18%11%5%7%6%5%4435%27%
1379
16th to endDay
19%14%5%8%6%35%5%6%6%17%
1536
January1st to 15thNight
15%13%6%8%5%35%45%6%27%
2506
16th to endNight
12%12%5%8%6%445%5%5%28%
2688
February1st to 15thNight
10%12%6%8%6%245%5%8%29%
2310
16th to endNight
12%12%5%8%6%5%5%5%5%6%26%
1893
March1st to 15thNight
13%8%36%6%24447%39%
2160
16th to endNight
14%7%344345%47%39%
2025
April1st to 15thNight
19%8%36%5%25%445%33%
1797
16th to endNight
16%8%35%5%34446%37%
1653
May1st to 15thNight
17%6%36%6%345%5%8%31%
1532
16th to endNight
12%5%15%5%35%5%5%8%41%
1496
June1st to 15thNight
14%5%25%5%34446%42%
1440
16th to endNight
13%6%25%5%245%5%7%40%
1440
July1st to 15thNight
7%31432446%7%53%
1440
16th to endNight
7%423423349%53%
1513
August1st to 15thNight
6%3143345%48%53%
1491
16th to endNight
12%414323347%51%
1838
September1st to 15thNight
8%5%25%424445%51%
1819
16th to endNight
13%6%25%5%1445%7%43%
1903
October1st to 15thNight
16%7%36%5%235%46%38%
1900
16th to endNight
19%8%36%5%35%46%7%29%
2288
November1st to 15thNight
16%9%36%5%3445%6%33%
2276
16th to endNight
17%11%5%6%6%46%5%45%25%
2454
December1st to 15thNight
18%11%48%43435%7%27%
2525
16th to endNight
18%14%5%7%5%24445%26%
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
57%6%10%17%7%
1433
16th to endDay
54%7%14%15%7%
1536
February1st to 15thDay
52%8%15%17%6%
1541
16th to endDay
53%9%14%14%7%
1580
March1st to 15thDay
48%8%15%18%7%1
1800
16th to endDay
48%9%13%20%5%1
2207
April1st to 15thDay
52%6%11%20%8%
2151
16th to endDay
47%7%10%19%13%1
2307
May1st to 15thDay
49%6%12%18%11%1
2415
16th to endDay
38%8%12%25%14%1
2621
June1st to 15thDay
40%7%15%22%12%
2520
16th to endDay
40%10%17%19%10%
2520
July1st to 15thDay
31%7%16%29%12%1
2371
16th to endDay
30%9%17%23%16%1
2488
August1st to 15thDay
31%9%16%29%12%1
2452
16th to endDay
36%8%18%24%11%
2363
September1st to 15thDay
38%7%15%28%9%
2128
16th to endDay
39%8%20%24%6%
1894
October1st to 15thDay
46%7%13%25%7%
1728
16th to endDay
59%8%12%14%4
1584
November1st to 15thDay
51%5%14%19%8%
1582
16th to endDay
60%7%15%12%3
1506
December1st to 15thDay
58%7%14%13%6%
1379
16th to endDay
65%6%13%10%4
1536
January1st to 15thNight
57%5%12%16%6%
2506
16th to endNight
54%7%13%18%5%
2688
February1st to 15thNight
52%6%13%17%8%
2319
16th to endNight
56%6%12%16%7%1
1893
March1st to 15thNight
45%5%13%23%11%1
2160
16th to endNight
43%7%13%23%11%1
2025
April1st to 15thNight
52%5%8%21%10%1
1797
16th to endNight
46%5%11%19%13%2
1653
May1st to 15thNight
50%6%9%18%12%1
1533
16th to endNight
38%7%13%24%14%
1498
June1st to 15thNight
42%6%11%22%16%1
1440
16th to endNight
39%8%14%23%12%1
1440
July1st to 15thNight
27%7%14%34%15%1
1360
16th to endNight
27%5%18%30%14%3
1417
August1st to 15thNight
28%6%16%35%12%1
1482
16th to endNight
32%7%15%26%15%2
1838
September1st to 15thNight
34%6%12%31%14%
1819
16th to endNight
40%5%16%29%7%
1903
October1st to 15thNight
45%6%13%24%8%1
1900
16th to endNight
51%9%15%17%6%
2288
November1st to 15thNight
49%7%13%20%9%
2276
16th to endNight
58%6%15%13%4
2454
December1st to 15thNight
55%7%15%16%4
2525
16th to endNight
59%5%13%14%5%
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
15%11%44%25%3
90
16th to endDay
29%7%26%31%5%1
96
February1st to 15thDay
17%23%35%21%11
90
16th to endDay
22%17%35%19%4
81
March1st to 15thDay
15%14%41%20%8%
90
16th to endDay
21%17%23%25%9%2
269
April1st to 15thDay
21%17%27%21%11%
270
16th to endDay
21%20%28%17%9%2
270
May1st to 15thDay
22%18%27%20%9%2
372
16th to endDay
14%20%29%23%8%3
517
June1st to 15thDay
15%18%29%28%7%1
495
16th to endDay
14%17%26%27%11%2
495
July1st to 15thDay
10%13%23%35%13%4
475
16th to endDay
7%15%27%28%16%5%
515
August1st to 15thDay
8%10%20%36%17%6%
440
16th to endDay
16%9%22%24%20%6%
332
September1st to 15thDay
9%9%26%28%16%9%
315
16th to endDay
14%10%25%27%16%4
283
October1st to 15thDay
20%14%28%22%9%3
181
16th to endDay
31%14%33%18%2
96
November1st to 15thDay
6%12%45%23%11%1
90
16th to endDay
28%18%25%16%10%
90
December1st to 15thDay
34%12%37%8%5%1
79
16th to endDay
26%22%33%17%
96
January1st to 15thNight
16%24%31%20%6%
900
16th to endNight
16%22%30%21%7%1
960
February1st to 15thNight
15%21%30%25%6%
830
16th to endNight
15%21%25%29%6%
631
March1st to 15thNight
13%23%30%25%7%
720
16th to endNight
15%21%23%29%8%1
663
April1st to 15thNight
18%21%27%24%7%
539
16th to endNight
18%19%27%23%10%
540
May1st to 15thNight
17%20%24%28%8%
540
16th to endNight
11%19%21%31%13%1
563
June1st to 15thNight
15%20%19%32%11%
540
16th to endNight
16%17%18%30%13%2
540
July1st to 15thNight
7%13%18%36%22%1
510
16th to endNight
8%17%17%34%19%2
564
August1st to 15thNight
7%10%17%35%26%2
539
16th to endNight
11%10%20%31%23%2
573
September1st to 15thNight
9%12%23%28%20%5%
559
16th to endNight
15%13%20%27%18%3
689
October1st to 15thNight
18%15%25%24%14%1
660
16th to endNight
22%18%28%22%8%
704
November1st to 15thNight
18%24%29%20%7%
700
16th to endNight
19%27%26%18%6%
834
December1st to 15thNight
22%29%28%14%4
900
16th to endNight
21%25%26%20%5%1
960
Page generated at 2017-09-24 09:15:53. Version ac04

Copyright 2017 A. Danko


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