This page made possible by the sponsorship of: Jim & Lisa McKay.

Thimble Islands Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thimble Islands Observatory clear sky chart from 2005-11-05 to 2017-09-10.

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
26%
14958
18%
46759
8%
14926
38%
14926
18%
30551

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
15%
720
20%
2506
7%
716
33%
716
14%
1583
January16th to endNight
15%
768
21%
2657
5%
768
35%
768
14%
1664
February1st to 15thNight
14%
704
18%
2180
4%
701
31%
701
15%
1452
February16th to endNight
15%
631
24%
1893
7%
631
37%
631
18%
1226
March1st to 15thNight
17%
720
17%
2160
5%
720
31%
720
15%
1443
March16th to endNight
19%
768
23%
2006
6%
768
33%
768
21%
1324
April1st to 15thNight
19%
718
17%
1796
4%
718
26%
718
19%
1277
April16th to endNight
22%
651
17%
1731
3%
651
31%
651
19%
1191
May1st to 15thNight
23%
540
16%
1620
6%
540
32%
540
14%
1149
May16th to endNight
29%
386
12%
1508
9%
385
37%
385
14%
974
June1st to 15thNight
32%
360
13%
1440
11%
360
43%
360
15%
956
June16th to endNight
44%
360
13%
1440
12%
360
54%
360
22%
845
July1st to 15thNight
44%
340
11%
1360
15%
340
47%
340
17%
864
July16th to endNight
47%
376
14%
1418
13%
354
56%
354
20%
878
August1st to 15thNight
46%
523
16%
1594
10%
521
51%
521
24%
1001
August16th to endNight
53%
764
18%
1910
9%
764
56%
764
31%
1283
September1st to 15thNight
39%
700
21%
1775
11%
700
44%
700
29%
1173
September16th to endNight
44%
657
24%
1815
15%
657
51%
657
28%
1161
October1st to 15thNight
29%
660
22%
1878
11%
660
41%
660
23%
1241
October16th to endNight
22%
704
17%
2244
5%
704
36%
704
19%
1456
November1st to 15thNight
25%
700
20%
2277
10%
700
39%
700
19%
1445
November16th to endNight
21%
720
20%
2343
8%
720
35%
720
16%
1508
December1st to 15thNight
20%
720
16%
2520
6%
720
33%
720
13%
1657
December16th to endNight
12%
768
18%
2688
4%
768
30%
768
12%
1800

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%5%2442445%7%42%
47874
Night
17%6%24424446%41%
46929
Cloud CoverDay
16%5%2442445%7%42%
47884
Night
17%6%24424446%41%
46934
TransparencyDay
42%6%12%21%15%2
47562
Night
43%5%11%21%15%2
46759
SeeingDay
18%20%28%20%9%2
5832
Night
18%24%26%22%7%
14958

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
17%6%1342345%6%43%
1432
16th to endDay
16%7%25%41345%6%41%
1566
February1st to 15thDay
16%7%244345%46%39%
1681
16th to endDay
18%4245%2445%6%40%
1580
March1st to 15thDay
19%7%25%424347%38%
1790
16th to endDay
18%6%24423345%42%
2214
April1st to 15thDay
20%6%23323446%41%
2152
16th to endDay
17%5%23424346%45%
2228
May1st to 15thDay
23%6%24323246%37%
2332
16th to endDay
17%5%2442445%7%41%
2616
June1st to 15thDay
22%5%2433435%8%35%
2520
16th to endDay
11%425%5%345%5%9%41%
2520
July1st to 15thDay
12%5%24435%5%5%10%39%
2520
16th to endDay
12%424425%5%7%8%41%
2654
August1st to 15thDay
13%424425%5%6%9%42%
2341
16th to endDay
11%4245%2436%7%46%
2292
September1st to 15thDay
13%424424446%47%
2073
16th to endDay
15%413424346%48%
1815
October1st to 15thDay
18%6%24423346%42%
1749
16th to endDay
15%5%2442446%6%44%
1628
November1st to 15thDay
20%6%3342333445%
1581
16th to endDay
18%413325%347%44%
1614
December1st to 15thDay
18%6%15%333346%41%
1440
16th to endDay
22%8%24423245%38%
1536
January1st to 15thNight
19%6%23323336%43%
2506
16th to endNight
16%8%24323435%44%
2657
February1st to 15thNight
15%9%35%5%2435%5%41%
2179
16th to endNight
17%6%24433345%44%
1893
March1st to 15thNight
21%6%2442435%5%40%
2147
16th to endNight
16%7%34313346%45%
2006
April1st to 15thNight
19%7%24424447%39%
1796
16th to endNight
21%5%25%43334441%
1731
May1st to 15thNight
21%7%34434447%34%
1620
16th to endNight
20%7%2342436%8%36%
1506
June1st to 15thNight
22%5%25%324445%38%
1440
16th to endNight
14%31342445%8%45%
1440
July1st to 15thNight
13%5%245%35%6%5%8%38%
1440
16th to endNight
12%5%245%35%6%6%9%37%
1514
August1st to 15thNight
10%424425%5%7%8%42%
1604
16th to endNight
12%424435%45%8%43%
1910
September1st to 15thNight
15%5%2433435%5%46%
1775
16th to endNight
15%5%2442433449%
1815
October1st to 15thNight
16%7%245%2333445%
1878
16th to endNight
18%5%24435%445%42%
2244
November1st to 15thNight
17%6%24314345%45%
2277
16th to endNight
19%5%34424446%41%
2343
December1st to 15thNight
22%6%2442445%6%38%
2520
16th to endNight
21%5%25%424346%38%
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
17%6%1442436%6%42%
1432
16th to endDay
16%7%25%32435%7%41%
1566
February1st to 15thDay
16%5%244235%5%7%40%
1681
16th to endDay
16%4245%34448%40%
1580
March1st to 15thDay
19%5%245%34448%38%
1800
16th to endDay
18%5%24423345%43%
2214
April1st to 15thDay
20%5%23324446%42%
2152
16th to endDay
16%5%14423446%45%
2228
May1st to 15thDay
22%6%2432335%7%38%
2332
16th to endDay
16%5%25%325%45%9%40%
2616
June1st to 15thDay
20%5%24424348%37%
2520
16th to endDay
11%5%244345%5%9%43%
2520
July1st to 15thDay
11%5%24425%5%6%10%41%
2520
16th to endDay
12%3244245%6%8%43%
2654
August1st to 15thDay
12%42432446%8%45%
2341
16th to endDay
11%5%1443345%7%48%
2292
September1st to 15thDay
12%42442345%6%48%
2073
16th to endDay
14%5%2342345%6%48%
1815
October1st to 15thDay
17%6%25%42335%7%42%
1749
16th to endDay
14%5%2342445%7%43%
1628
November1st to 15thDay
20%5%2442333444%
1581
16th to endDay
17%5%13424447%44%
1614
December1st to 15thDay
18%5%24433447%39%
1440
16th to endDay
22%7%15%423346%38%
1536
January1st to 15thNight
19%6%24323346%42%
2506
16th to endNight
16%9%24223346%43%
2657
February1st to 15thNight
14%8%35%5%24445%41%
2172
16th to endNight
17%5%24434346%43%
1893
March1st to 15thNight
20%6%24324446%40%
2160
16th to endNight
17%7%23414336%45%
2005
April1st to 15thNight
18%7%245%23337%39%
1796
16th to endNight
20%5%35%433336%41%
1731
May1st to 15thNight
20%7%3443445%6%35%
1620
16th to endNight
20%6%2443445%8%36%
1506
June1st to 15thNight
22%414424446%38%
1440
16th to endNight
13%4135%24449%45%
1440
July1st to 15thNight
12%5%245%36%5%6%8%38%
1440
16th to endNight
11%4246%45%5%7%8%38%
1514
August1st to 15thNight
9%4144336%8%9%42%
1604
16th to endNight
12%415%43446%7%44%
1910
September1st to 15thNight
15%5%1442445%6%45%
1775
16th to endNight
14%5%25%424336%47%
1815
October1st to 15thNight
15%7%25%5%23335%44%
1878
16th to endNight
18%5%25%5%24446%41%
2244
November1st to 15thNight
18%6%24424335%45%
2277
16th to endNight
19%5%2442445%6%40%
2343
December1st to 15thNight
21%6%2442445%7%37%
2520
16th to endNight
21%6%25%423446%38%
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
42%5%14%21%12%3
1433
16th to endDay
42%411%24%14%2
1566
February1st to 15thDay
45%6%12%20%14%1
1684
16th to endDay
44%411%23%14%2
1580
March1st to 15thDay
46%315%21%10%2
1800
16th to endDay
44%410%17%18%3
2214
April1st to 15thDay
43%5%10%20%15%3
2152
16th to endDay
40%5%9%21%18%3
2228
May1st to 15thDay
48%5%10%19%13%2
2332
16th to endDay
42%7%13%20%13%2
2620
June1st to 15thDay
47%7%11%20%10%1
2520
16th to endDay
38%9%13%23%13%1
2520
July1st to 15thDay
40%8%11%22%16%1
2371
16th to endDay
37%8%15%21%13%2
2487
August1st to 15thDay
37%9%14%21%15%
2327
16th to endDay
36%7%10%24%18%1
2292
September1st to 15thDay
37%7%12%20%20%1
2073
16th to endDay
36%6%7%22%24%2
1815
October1st to 15thDay
42%410%22%16%2
1749
16th to endDay
39%6%11%24%15%1
1628
November1st to 15thDay
43%5%13%17%18%2
1581
16th to endDay
40%412%25%14%2
1614
December1st to 15thDay
44%416%18%14%
1440
16th to endDay
49%414%18%12%
1536
January1st to 15thNight
43%412%22%14%2
2506
16th to endNight
43%410%23%16%2
2657
February1st to 15thNight
44%5%11%22%12%2
2180
16th to endNight
42%310%22%17%2
1893
March1st to 15thNight
44%5%14%19%13%3
2160
16th to endNight
41%310%20%18%4
2006
April1st to 15thNight
46%410%20%15%2
1796
16th to endNight
46%48%19%16%3
1731
May1st to 15thNight
48%7%9%16%14%2
1620
16th to endNight
45%7%17%16%11%1
1508
June1st to 15thNight
47%6%11%18%14%1
1440
16th to endNight
36%7%15%24%15%
1440
July1st to 15thNight
40%10%11%21%14%1
1360
16th to endNight
39%8%15%22%11%2
1418
August1st to 15thNight
35%9%14%23%15%2
1594
16th to endNight
38%6%12%24%17%
1910
September1st to 15thNight
38%5%10%25%17%
1775
16th to endNight
38%46%21%26%2
1815
October1st to 15thNight
42%5%9%21%16%3
1878
16th to endNight
43%5%15%20%14%
2244
November1st to 15thNight
42%411%20%18%3
2277
16th to endNight
44%411%24%12%2
2343
December1st to 15thNight
46%5%14%18%14%
2520
16th to endNight
48%5%12%20%12%1
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%33%35%15%1
60
16th to endDay
27%24%37%10%
69
February1st to 15thDay
20%26%37%13%2
197
16th to endDay
15%29%33%16%5%
239
March1st to 15thDay
22%33%25%14%4
270
16th to endDay
20%29%32%13%21
288
April1st to 15thDay
22%27%28%15%6%
270
16th to endDay
21%20%28%19%7%2
270
May1st to 15thDay
29%25%22%12%7%2
288
16th to endDay
21%18%28%20%9%1
331
June1st to 15thDay
20%19%27%19%11%2
315
16th to endDay
15%20%26%25%11%1
315
July1st to 15thDay
16%10%28%26%14%3
305
16th to endDay
11%14%27%30%12%3
326
August1st to 15thDay
15%9%24%31%13%5%
313
16th to endDay
12%9%27%28%15%7%
332
September1st to 15thDay
14%14%26%22%13%6%
294
16th to endDay
18%9%27%20%14%9%
254
October1st to 15thDay
20%14%24%24%11%3
255
16th to endDay
14%24%31%21%6%1
248
November1st to 15thDay
18%23%25%18%10%2
235
16th to endDay
14%29%27%19%8%
234
December1st to 15thDay
20%25%38%10%6%
60
16th to endDay
34%21%31%10%1
64
January1st to 15thNight
20%33%28%15%2
720
16th to endNight
19%32%29%16%1
768
February1st to 15thNight
17%30%33%17%1
704
16th to endNight
19%30%31%15%2
631
March1st to 15thNight
23%28%26%18%2
720
16th to endNight
17%29%31%17%4
768
April1st to 15thNight
20%27%30%17%3
718
16th to endNight
19%26%27%20%5%
651
May1st to 15thNight
19%25%27%18%7%
540
16th to endNight
20%20%23%28%6%
386
June1st to 15thNight
20%20%24%26%8%
360
16th to endNight
14%18%20%36%10%
360
July1st to 15thNight
15%13%20%34%15%
340
16th to endNight
11%18%17%38%13%1
376
August1st to 15thNight
13%10%22%37%14%
523
16th to endNight
12%10%17%36%19%3
764
September1st to 15thNight
16%12%26%27%15%2
700
16th to endNight
17%12%19%26%19%3
657
October1st to 15thNight
19%16%29%22%11%1
660
16th to endNight
19%20%34%18%6%
704
November1st to 15thNight
17%27%26%17%9%1
700
16th to endNight
18%30%25%17%6%
720
December1st to 15thNight
24%27%23%18%5%
720
16th to endNight
23%32%26%14%2
768
Page generated at 2017-09-11 19:35:15. Version ac04

Copyright 2017 A. Danko


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