Thunder Bay Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thunder Bay Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
33%
16123
13%
46601
6%
16076
39%
16076
19%
33037

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
22%
900
12%
2506
4%
895
30%
895
17%
1846
January16th to endNight
20%
960
12%
2688
2%
960
28%
960
13%
2095
February1st to 15thNight
28%
879
14%
2339
3%
875
35%
875
18%
1688
February16th to endNight
31%
684
14%
1946
6%
684
36%
684
16%
1426
March1st to 15thNight
33%
720
15%
2160
10%
720
38%
720
19%
1496
March16th to endNight
37%
579
18%
2001
13%
579
47%
579
19%
1324
April1st to 15thNight
37%
539
13%
1797
10%
539
44%
539
19%
1243
April16th to endNight
40%
540
15%
1634
9%
540
46%
540
20%
1138
May1st to 15thNight
37%
540
11%
1440
5%
540
41%
540
20%
1062
May16th to endNight
41%
563
9%
1498
4%
561
44%
561
22%
1098
June1st to 15thNight
40%
540
6%
1440
3%
540
42%
540
23%
1040
June16th to endNight
44%
540
6%
1440
4%
540
46%
540
23%
1023
July1st to 15thNight
49%
510
8%
1360
5%
510
52%
510
31%
885
July16th to endNight
55%
564
14%
1417
6%
531
56%
531
34%
890
August1st to 15thNight
53%
527
18%
1400
9%
524
59%
524
37%
858
August16th to endNight
48%
525
15%
1669
12%
525
53%
525
29%
1008
September1st to 15thNight
50%
495
19%
1650
15%
495
57%
495
32%
937
September16th to endNight
41%
616
14%
1777
11%
616
45%
616
24%
1198
October1st to 15thNight
33%
660
18%
1980
10%
660
40%
660
21%
1342
October16th to endNight
29%
704
9%
2263
4%
704
35%
704
15%
1659
November1st to 15thNight
29%
778
12%
2354
7%
778
35%
778
15%
1721
November16th to endNight
23%
900
12%
2520
6%
900
30%
900
13%
1939
December1st to 15thNight
20%
900
11%
2631
3%
900
28%
900
13%
1950
December16th to endNight
18%
960
12%
2691
4%
960
25%
960
11%
2171

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
14%9%36%5%35%45%7%33%
47363
Night
13%9%46%5%3445%6%34%
46771
Cloud CoverDay
14%9%36%5%3445%7%34%
47373
Night
13%9%46%5%3445%6%34%
46776
Cloud CoverDay
14%8%36%5%3445%7%34%
47371
Night
13%9%46%5%35%45%6%34%
46776
TransparencyDay
48%6%14%20%9%
47048
Night
48%6%12%22%8%
46604
SeeingDay
18%12%32%21%10%3
6044
Night
18%10%28%28%12%1
16124

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%11%5%8%5%3445%7%31%
1432
16th to endDay
13%11%410%7%45%45%8%24%
1536
February1st to 15thDay
9%11%48%6%25%5%5%6%33%
1518
16th to endDay
10%12%5%9%7%34346%30%
1527
March1st to 15thDay
13%10%46%6%34445%37%
1796
16th to endDay
15%10%47%7%345%46%30%
2226
April1st to 15thDay
12%8%48%6%35%5%5%7%32%
2151
16th to endDay
20%11%45%433445%32%
2322
May1st to 15thDay
18%10%37%6%35%445%29%
2511
16th to endDay
17%8%35%43446%6%34%
2617
June1st to 15thDay
16%6%36%42445%7%37%
2520
16th to endDay
15%6%35%5%35%5%6%9%32%
2520
July1st to 15thDay
11%6%25%5%36%5%6%8%38%
2520
16th to endDay
9%5%35%5%35%5%5%9%41%
2655
August1st to 15thDay
9%5%245%46%5%6%9%41%
2465
16th to endDay
11%424425%5%7%9%42%
2176
September1st to 15thDay
11%5%26%5%35%46%7%40%
1980
16th to endDay
18%8%35%42345%7%37%
1842
October1st to 15thDay
16%10%37%5%345%5%6%31%
1650
16th to endDay
20%12%47%6%34435%25%
1609
November1st to 15thDay
15%10%49%6%345%45%27%
1501
16th to endDay
17%13%5%7%5%25%446%26%
1440
December1st to 15thDay
11%11%48%7%3445%6%30%
1322
16th to endDay
17%19%6%9%6%34345%19%
1527
January1st to 15thNight
12%12%5%7%5%35%5%5%6%30%
2506
16th to endNight
12%13%6%10%7%45%5%36%25%
2688
February1st to 15thNight
8%9%38%6%46%6%5%7%33%
2339
16th to endNight
9%10%5%7%7%35%6%6%7%28%
1946
March1st to 15thNight
12%11%36%5%245%45%36%
2147
16th to endNight
12%10%36%5%2445%6%37%
2001
April1st to 15thNight
13%10%36%5%35%45%6%34%
1797
16th to endNight
17%10%45%5%24445%34%
1634
May1st to 15thNight
19%8%46%5%24446%32%
1440
16th to endNight
17%7%36%5%25%45%7%33%
1496
June1st to 15thNight
16%8%36%435%446%35%
1440
16th to endNight
14%7%36%6%245%5%8%36%
1440
July1st to 15thNight
10%5%145%35%45%7%45%
1440
16th to endNight
7%3244246%7%9%47%
1513
August1st to 15thNight
9%313325%5%5%8%50%
1409
16th to endNight
12%42433346%7%48%
1669
September1st to 15thNight
10%32442446%7%49%
1650
16th to endNight
14%6%25%6%2445%7%40%
1777
October1st to 15thNight
16%8%246%3335%6%39%
1980
16th to endNight
16%11%48%6%34447%29%
2263
November1st to 15thNight
16%10%37%6%35%5%5%6%30%
2354
16th to endNight
17%12%5%8%7%34346%27%
2520
December1st to 15thNight
13%12%6%8%5%33447%30%
2631
16th to endNight
15%17%6%9%6%34345%23%
2691

Percentage Cloud Cover SW (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%5%7%6%4345%7%31%
1432
16th to endDay
13%10%410%7%45%5%5%8%24%
1536
February1st to 15thDay
8%11%47%6%25%5%6%6%33%
1518
16th to endDay
10%12%5%8%8%35%346%30%
1527
March1st to 15thDay
13%9%47%44444437%
1806
16th to endDay
16%10%5%6%6%35%446%30%
2226
April1st to 15thDay
12%8%48%7%35%5%5%7%32%
2151
16th to endDay
20%10%35%4335%36%32%
2322
May1st to 15thDay
19%10%37%6%35%435%30%
2511
16th to endDay
18%7%35%43445%7%34%
2617
June1st to 15thDay
16%6%35%42445%7%38%
2520
16th to endDay
15%6%35%5%35%5%6%9%33%
2520
July1st to 15thDay
12%5%25%5%36%5%6%7%38%
2520
16th to endDay
10%5%25%5%35%5%5%9%41%
2655
August1st to 15thDay
9%5%245%35%5%6%8%41%
2465
16th to endDay
11%4244245%7%9%43%
2176
September1st to 15thDay
11%5%26%6%35%5%6%7%40%
1980
16th to endDay
18%8%345%23447%38%
1842
October1st to 15thDay
16%10%27%6%245%5%7%31%
1650
16th to endDay
20%12%47%6%35%345%25%
1609
November1st to 15thDay
15%10%49%6%35%5%46%27%
1501
16th to endDay
17%13%5%7%5%35%337%26%
1440
December1st to 15thDay
11%12%48%6%25%46%6%31%
1322
16th to endDay
17%18%6%8%6%3444420%
1527
January1st to 15thNight
12%12%5%7%5%35%45%6%30%
2506
16th to endNight
12%13%6%10%7%45%445%25%
2688
February1st to 15thNight
8%9%37%6%46%6%5%6%33%
2332
16th to endNight
9%10%47%7%36%6%5%8%28%
1946
March1st to 15thNight
12%10%37%5%35%5%45%37%
2160
16th to endNight
13%10%36%5%2445%6%37%
2001
April1st to 15thNight
14%10%35%6%35%35%6%35%
1797
16th to endNight
18%9%45%5%245%45%34%
1634
May1st to 15thNight
19%8%45%6%34446%33%
1439
16th to endNight
17%7%35%5%35%45%7%34%
1496
June1st to 15thNight
16%8%35%5%35%446%35%
1440
16th to endNight
14%6%27%5%245%5%9%35%
1440
July1st to 15thNight
10%5%145%35%45%7%45%
1440
16th to endNight
7%2145%245%7%8%48%
1513
August1st to 15thNight
9%313425%5%47%51%
1409
16th to endNight
12%42433346%7%48%
1669
September1st to 15thNight
10%4135%235%6%8%49%
1650
16th to endNight
15%6%25%5%2445%7%40%
1777
October1st to 15thNight
16%8%245%3435%5%39%
1980
16th to endNight
16%11%47%6%345%46%29%
2263
November1st to 15thNight
16%10%37%6%35%5%5%6%29%
2354
16th to endNight
17%12%5%8%6%34445%27%
2520
December1st to 15thNight
13%11%6%8%6%23446%30%
2631
16th to endNight
15%17%6%10%6%3443423%
2691

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%5%7%6%3445%7%31%
1432
16th to endDay
12%11%410%7%35%45%9%24%
1536
February1st to 15thDay
9%10%47%6%35%5%6%5%33%
1518
16th to endDay
10%13%48%8%35%337%30%
1527
March1st to 15thDay
13%10%36%435%44437%
1806
16th to endDay
16%10%46%7%3445%6%30%
2224
April1st to 15thDay
12%9%38%7%345%5%7%32%
2151
16th to endDay
20%10%35%433446%32%
2322
May1st to 15thDay
19%9%46%6%34445%30%
2511
16th to endDay
17%7%36%5%2345%7%34%
2617
June1st to 15thDay
16%6%25%4335%5%7%38%
2520
16th to endDay
16%5%25%5%25%5%6%9%33%
2520
July1st to 15thDay
12%5%246%35%5%6%8%38%
2520
16th to endDay
10%425%5%35%5%6%8%42%
2655
August1st to 15thDay
10%5%145%35%6%6%8%42%
2465
16th to endDay
11%4144245%7%9%43%
2176
September1st to 15thDay
11%6%26%435%5%5%8%41%
1980
16th to endDay
17%8%35%5%2335%7%37%
1842
October1st to 15thDay
16%10%36%5%235%5%6%31%
1650
16th to endDay
19%13%46%7%34436%25%
1609
November1st to 15thDay
14%11%49%5%35%5%5%5%28%
1501
16th to endDay
17%12%5%7%6%34447%26%
1440
December1st to 15thDay
11%11%5%8%6%3445%7%31%
1322
16th to endDay
18%18%5%9%7%3344420%
1527
January1st to 15thNight
12%12%5%7%5%35%5%5%6%30%
2506
16th to endNight
11%13%6%10%8%35%445%25%
2688
February1st to 15thNight
9%9%37%7%47%6%5%7%32%
2332
16th to endNight
9%10%5%7%7%46%5%6%7%28%
1946
March1st to 15thNight
13%10%36%5%35%5%45%36%
2160
16th to endNight
13%10%35%5%3435%6%37%
2001
April1st to 15thNight
14%9%36%6%35%35%6%35%
1797
16th to endNight
17%9%45%5%25%445%34%
1634
May1st to 15thNight
20%8%35%5%25%345%34%
1439
16th to endNight
18%6%35%5%345%6%6%34%
1496
June1st to 15thNight
17%8%26%5%35%447%35%
1440
16th to endNight
14%6%36%5%25%45%9%35%
1440
July1st to 15thNight
10%6%245%35%5%5%8%44%
1440
16th to endNight
7%3244245%7%10%47%
1513
August1st to 15thNight
9%314325%46%7%51%
1409
16th to endNight
12%4243145%5%7%47%
1669
September1st to 15thNight
9%41343346%8%49%
1650
16th to endNight
15%6%345%2445%7%40%
1777
October1st to 15thNight
16%8%245%24445%39%
1980
16th to endNight
16%11%37%5%45%446%29%
2263
November1st to 15thNight
16%9%47%6%35%45%6%29%
2354
16th to endNight
17%13%48%6%34446%27%
2520
December1st to 15thNight
13%11%5%9%6%24445%31%
2631
16th to endNight
15%17%6%10%6%3434423%
2691

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
51%5%11%23%7%
1433
16th to endDay
56%7%9%19%6%
1536
February1st to 15thDay
49%5%13%20%9%
1519
16th to endDay
55%411%19%8%
1527
March1st to 15thDay
48%6%11%19%11%2
1806
16th to endDay
53%6%13%16%8%2
2226
April1st to 15thDay
49%6%11%17%12%2
2151
16th to endDay
52%411%19%11%1
2322
May1st to 15thDay
54%411%18%9%
2511
16th to endDay
47%7%12%21%11%
2621
June1st to 15thDay
43%6%14%20%13%
2520
16th to endDay
45%8%17%20%8%
2520
July1st to 15thDay
38%7%14%24%14%
2371
16th to endDay
37%9%14%28%10%1
2488
August1st to 15thDay
35%8%17%26%10%1
2450
16th to endDay
33%8%19%26%11%
2176
September1st to 15thDay
39%7%18%28%6%
1980
16th to endDay
46%6%18%22%4
1842
October1st to 15thDay
50%6%16%21%4
1650
16th to endDay
60%6%11%17%5%
1609
November1st to 15thDay
55%6%14%16%7%
1501
16th to endDay
56%7%14%13%7%
1440
December1st to 15thDay
52%7%15%18%5%
1322
16th to endDay
66%5%9%11%5%
1527
January1st to 15thNight
52%6%12%22%6%
2506
16th to endNight
60%5%9%18%6%
2688
February1st to 15thNight
49%6%13%20%9%
2342
16th to endNight
51%8%9%22%7%
1946
March1st to 15thNight
48%5%12%20%11%2
2160
16th to endNight
46%6%13%20%10%3
2001
April1st to 15thNight
49%5%10%19%14%1
1797
16th to endNight
50%48%23%12%1
1634
May1st to 15thNight
51%6%9%20%12%
1440
16th to endNight
47%8%9%21%11%
1498
June1st to 15thNight
49%410%19%14%1
1440
16th to endNight
44%9%14%19%11%1
1440
July1st to 15thNight
36%7%12%29%13%
1360
16th to endNight
30%10%16%28%13%1
1417
August1st to 15thNight
30%7%17%31%11%1
1400
16th to endNight
34%7%17%28%11%
1669
September1st to 15thNight
31%6%19%34%8%
1650
16th to endNight
42%7%16%28%4
1777
October1st to 15thNight
45%6%15%25%6%
1980
16th to endNight
54%6%15%18%4
2263
November1st to 15thNight
53%6%14%18%6%
2354
16th to endNight
58%6%11%16%7%
2520
December1st to 15thNight
53%7%14%19%5%
2631
16th to endNight
64%5%9%16%4
2691

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
14%14%42%24%4
90
16th to endDay
21%12%31%22%9%2
96
February1st to 15thDay
16%18%47%11%41
90
16th to endDay
18%24%34%18%3
81
March1st to 15thDay
13%16%42%17%7%2
90
16th to endDay
22%13%26%20%11%6%
210
April1st to 15thDay
17%12%27%27%13%1
270
16th to endDay
28%13%23%21%9%3
321
May1st to 15thDay
23%15%31%18%7%3
468
16th to endDay
22%11%32%21%8%3
517
June1st to 15thDay
19%13%34%19%7%4
495
16th to endDay
17%14%30%23%11%2
495
July1st to 15thDay
14%9%34%27%9%5%
475
16th to endDay
10%8%40%20%13%6%
515
August1st to 15thDay
14%9%35%22%12%5%
474
16th to endDay
14%11%30%24%14%4
320
September1st to 15thDay
10%6%29%22%23%8%
255
16th to endDay
25%11%29%17%12%3
251
October1st to 15thDay
16%17%33%18%12%1
90
16th to endDay
34%10%43%10%1
96
November1st to 15thDay
23%17%36%14%5%2
90
16th to endDay
44%8%14%22%8%1
90
December1st to 15thDay
25%8%40%20%41
70
16th to endDay
22%26%35%12%21
95
January1st to 15thNight
17%15%36%24%5%
900
16th to endNight
17%11%38%24%7%1
960
February1st to 15thNight
15%12%33%27%9%1
880
16th to endNight
14%11%30%31%10%1
684
March1st to 15thNight
16%15%25%30%10%1
720
16th to endNight
17%10%24%30%14%3
579
April1st to 15thNight
16%10%25%32%13%
539
16th to endNight
21%6%23%32%13%2
540
May1st to 15thNight
23%8%24%30%11%1
540
16th to endNight
19%6%24%33%14%1
563
June1st to 15thNight
20%7%22%33%14%1
540
16th to endNight
16%6%21%37%17%
540
July1st to 15thNight
12%5%25%35%17%1
510
16th to endNight
7%425%35%23%2
564
August1st to 15thNight
11%324%35%22%1
527
16th to endNight
14%427%32%17%3
525
September1st to 15thNight
12%5%28%27%21%4
495
16th to endNight
19%7%23%31%14%2
616
October1st to 15thNight
21%13%27%23%13%1
660
16th to endNight
21%12%27%25%11%
704
November1st to 15thNight
21%12%29%24%9%1
778
16th to endNight
24%16%25%23%9%1
900
December1st to 15thNight
19%16%33%21%7%1
900
16th to endNight
24%13%30%23%7%2
960
Page generated at 2017-08-13 14:16:28. Version ac04

Copyright 2017 A. Danko


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