Thunder Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thunder Bay clear sky chart from 2005-11-06 to 2024-03-28.

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%
25123
12%
81238
7%
25026
39%
25026
17%
57758

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
23%
943
12%
4381
4%
926
32%
926
11%
2935
January11th to 20thNight
21%
945
7%
4413
3%
930
28%
930
9%
3394
January21st to endNight
21%
1032
7%
4878
3%
1023
29%
1023
9%
3721
February1st to 10thNight
27%
950
13%
2572
3%
950
33%
950
19%
1854
February11th to 20thNight
30%
910
18%
2385
6%
906
38%
906
22%
1690
February21st to endNight
27%
628
15%
1884
5%
628
36%
628
17%
1354
March1st to 10thNight
32%
758
17%
2280
9%
758
40%
758
19%
1575
March11th to 20thNight
31%
687
16%
2207
9%
687
39%
687
19%
1481
March21st to endNight
34%
615
19%
2069
13%
615
45%
615
19%
1404
April1st to 10thNight
33%
540
12%
1800
8%
540
40%
540
16%
1278
April11th to 20thNight
37%
533
14%
1775
11%
533
42%
533
17%
1240
April21st to endNight
41%
540
14%
1553
8%
540
45%
540
21%
1096
May1st to 10thNight
39%
540
14%
1440
8%
540
43%
540
23%
1028
May11th to 20thNight
42%
531
14%
1416
8%
531
45%
531
25%
995
May21st to endNight
44%
584
10%
1552
5%
582
47%
582
23%
1110
June1st to 10thNight
45%
540
7%
1440
5%
540
47%
540
24%
1017
June11th to 20thNight
46%
360
8%
1260
8%
360
49%
360
21%
813
June21st to endNight
50%
524
8%
1424
6%
524
51%
524
24%
978
July1st to 10thNight
55%
525
9%
1400
6%
525
57%
525
33%
905
July11th to 20thNight
54%
522
12%
1392
6%
522
56%
522
34%
888
July21st to endNight
58%
591
14%
1487
8%
558
58%
558
34%
953
August1st to 10thNight
53%
537
16%
1423
9%
534
57%
534
34%
910
August11th to 20thNight
58%
534
19%
1458
12%
534
63%
534
34%
904
August21st to endNight
49%
594
16%
1980
14%
594
53%
594
26%
1229
September1st to 10thNight
53%
540
20%
1800
17%
540
59%
540
30%
1055
September11th to 20thNight
45%
571
16%
1831
12%
571
50%
571
26%
1155
September21st to endNight
39%
720
12%
1980
9%
720
42%
720
22%
1421
October1st to 10thNight
33%
720
13%
2148
7%
720
38%
720
19%
1513
October11th to 20thNight
30%
720
14%
2220
7%
720
38%
720
17%
1575
October21st to endNight
26%
792
9%
2574
5%
792
32%
792
12%
1923
November1st to 10thNight
27%
759
13%
2425
8%
759
34%
759
14%
1782
November11th to 20thNight
20%
950
10%
2660
3%
950
27%
950
12%
2081
November21st to endNight
18%
950
9%
2660
4%
950
23%
950
12%
2050
December1st to 10thNight
20%
950
9%
2827
2%
950
27%
950
13%
2104
December11th to 20thNight
20%
949
11%
3423
4%
948
26%
948
10%
2437
December21st to endNight
15%
1039
6%
4821
2%
1026
22%
1026
6%
3910

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%47%6%45%5%6%8%35%
78361
Night
13%10%48%7%35%5%5%7%32%
81651
TransparencyDay
46%8%14%21%9%1
78066
Night
50%7%14%20%8%1
81480
SeeingDay
17%13%35%24%9%2
15048
Night
18%10%30%29%12%1
25157

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
9%12%5%8%7%46%6%6%7%29%
2527
11th to 20thDay
6%10%7%13%10%445%7%8%25%
2530
21st to endDay
11%14%7%13%9%45%5%5%6%21%
2783
February1st to 10thDay
8%10%410%7%35%46%6%36%
1605
11th to 20thDay
8%10%37%7%5%5%6%5%6%38%
1683
21st to endDay
7%10%5%8%8%45%5%5%8%35%
1570
March1st to 10thDay
9%9%48%6%45%46%6%40%
1898
11th to 20thDay
12%9%47%5%5%5%5%7%6%37%
1990
21st to endDay
15%8%37%6%45%5%6%7%35%
2441
April1st to 10thDay
14%8%36%7%46%6%6%8%33%
2160
11th to 20thDay
17%8%39%5%36%5%6%7%31%
2134
21st to endDay
16%9%37%6%34446%36%
2405
May1st to 10thDay
16%8%37%7%445%6%6%34%
2520
11th to 20thDay
15%7%35%5%46%5%6%8%37%
2479
21st to endDay
16%7%36%6%345%5%7%37%
2725
June1st to 10thDay
15%5%26%5%35%6%6%9%39%
2520
11th to 20thDay
13%6%35%5%35%5%6%9%40%
2690
21st to endDay
15%7%35%5%35%6%6%9%36%
2524
July1st to 10thDay
9%425%436%6%6%10%44%
2520
11th to 20thDay
9%6%36%6%36%5%7%9%40%
2520
21st to endDay
8%425%6%36%6%6%10%43%
2743
August1st to 10thDay
8%5%25%5%47%6%7%10%42%
2507
11th to 20thDay
11%3245%35%6%6%9%46%
2438
21st to endDay
11%5%25%5%3446%8%46%
2376
September1st to 10thDay
12%5%35%6%45%5%5%7%44%
2160
11th to 20thDay
13%6%37%5%35%5%6%9%40%
2111
21st to endDay
15%8%46%6%45%5%6%8%33%
1980
October1st to 10thDay
17%9%37%5%36%5%6%6%33%
1810
11th to 20thDay
17%10%36%7%446%7%8%27%
1739
21st to endDay
21%11%46%7%46%5%46%27%
1782
November1st to 10thDay
13%9%5%9%8%46%6%5%8%27%
1650
11th to 20thDay
17%13%49%7%45%5%6%6%24%
1520
21st to endDay
16%13%47%6%46%5%7%9%24%
1520
December1st to 10thDay
11%10%7%7%9%5%6%5%8%8%25%
1348
11th to 20thDay
12%13%6%10%8%445%6%9%23%
1671
21st to endDay
26%20%48%5%2434420%
2782
January1st to 10thNight
9%16%47%7%45%46%6%32%
4428
11th to 20thNight
6%10%6%15%15%5%5%5%5%7%21%
4440
21st to endNight
9%16%8%11%9%5%45%46%23%
4884
February1st to 10thNight
8%10%5%9%7%5%6%5%5%7%33%
2572
11th to 20thNight
10%8%48%7%5%7%5%5%6%36%
2384
21st to endNight
8%11%6%10%8%35%6%6%7%30%
1884
March1st to 10thNight
9%7%5%7%7%5%7%5%5%6%36%
2269
11th to 20thNight
12%10%46%7%345%5%7%36%
2206
21st to endNight
14%8%47%7%345%5%7%35%
2069
April1st to 10thNight
16%9%47%7%45%5%5%7%31%
1800
11th to 20thNight
18%11%35%5%25%6%5%7%33%
1775
21st to endNight
17%10%36%6%3446%6%34%
1553
May1st to 10thNight
21%7%35%5%345%6%7%35%
1440
11th to 20thNight
14%7%35%6%45%5%6%7%36%
1416
21st to endNight
18%8%46%5%345%5%8%35%
1551
June1st to 10thNight
16%7%36%5%35%5%6%7%37%
1440
11th to 20thNight
13%6%36%6%45%5%5%7%39%
1260
21st to endNight
14%7%37%435%5%6%9%37%
1424
July1st to 10thNight
8%6%25%6%35%5%7%8%44%
1440
11th to 20thNight
9%5%26%5%35%5%7%7%45%
1440
21st to endNight
8%3245%35%6%7%9%47%
1575
August1st to 10thNight
9%5%246%35%5%6%9%48%
1432
11th to 20thNight
10%5%245%245%5%8%50%
1458
21st to endNight
14%6%2443445%7%45%
1980
September1st to 10thNight
12%5%244345%5%7%50%
1800
11th to 20thNight
13%6%35%43445%8%44%
1831
21st to endNight
17%8%36%5%345%5%8%35%
1980
October1st to 10thNight
17%8%25%5%45%5%5%7%35%
2148
11th to 20thNight
15%10%47%6%35%5%5%8%33%
2220
21st to endNight
18%11%38%6%46%5%5%6%28%
2574
November1st to 10thNight
16%8%5%7%7%46%5%7%6%29%
2425
11th to 20thNight
17%12%5%9%7%46%6%5%6%24%
2660
21st to endNight
15%14%5%8%7%45%5%6%8%24%
2660
December1st to 10thNight
10%11%5%11%8%45%5%8%8%25%
2827
11th to 20thNight
11%12%7%10%6%36%6%5%8%27%
3522
21st to endNight
24%21%6%8%5%2333419%
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%7%14%20%6%
2528
11th to 20thDay
54%7%12%25%2
2530
21st to endDay
63%9%11%12%5%1
2783
February1st to 10thDay
47%7%15%20%10%1
1605
11th to 20thDay
47%6%14%23%9%2
1692
21st to endDay
47%7%15%16%14%2
1570
March1st to 10thDay
44%9%13%20%13%3
1900
11th to 20thDay
46%9%11%21%12%2
1990
21st to endDay
47%10%12%15%14%2
2441
April1st to 10thDay
48%9%11%19%11%2
2160
11th to 20thDay
51%8%11%20%9%1
2134
21st to endDay
48%6%12%17%15%1
2405
May1st to 10thDay
48%8%13%21%10%1
2520
11th to 20thDay
43%7%15%22%11%1
2483
21st to endDay
45%8%13%19%14%1
2725
June1st to 10thDay
39%9%15%21%14%1
2520
11th to 20thDay
40%8%14%24%13%1
2690
21st to endDay
43%8%17%23%9%1
2524
July1st to 10thDay
33%10%15%27%14%1
2451
11th to 20thDay
37%9%14%28%11%1
2445
21st to endDay
33%10%16%29%11%1
2591
August1st to 10thDay
34%11%20%26%8%1
2492
11th to 20thDay
33%11%18%27%12%
2438
21st to endDay
35%9%16%28%11%
2376
September1st to 10thDay
38%8%16%29%9%
2160
11th to 20thDay
41%11%22%22%41
2111
21st to endDay
48%10%17%19%5%
1980
October1st to 10thDay
49%10%15%22%4
1810
11th to 20thDay
51%11%14%19%5%1
1739
21st to endDay
58%8%12%18%4
1782
November1st to 10thDay
53%8%17%15%6%1
1650
11th to 20thDay
60%7%13%16%4
1520
21st to endDay
55%8%17%15%5%
1520
December1st to 10thDay
55%9%16%16%4
1348
11th to 20thDay
58%7%14%12%5%4
1671
21st to endDay
69%47%17%3
2782
January1st to 10thNight
51%6%17%21%5%
4428
11th to 20thNight
62%7%11%17%3
4440
21st to endNight
60%8%14%13%4
4884
February1st to 10thNight
50%5%14%20%10%1
2572
11th to 20thNight
47%7%12%22%11%1
2385
21st to endNight
50%7%13%18%11%1
1884
March1st to 10thNight
47%7%12%20%13%2
2280
11th to 20thNight
46%6%12%22%12%2
2207
21st to endNight
47%7%11%20%12%3
2069
April1st to 10thNight
51%7%9%20%13%
1800
11th to 20thNight
50%6%10%20%11%2
1775
21st to endNight
48%6%10%21%13%2
1553
May1st to 10thNight
47%7%12%20%13%1
1440
11th to 20thNight
44%8%12%22%12%2
1417
21st to endNight
45%9%10%22%13%1
1552
June1st to 10thNight
44%9%12%20%13%2
1440
11th to 20thNight
43%7%12%21%16%2
1260
21st to endNight
44%8%15%21%11%2
1424
July1st to 10thNight
33%10%13%27%16%1
1400
11th to 20thNight
34%7%16%30%12%1
1392
21st to endNight
30%11%17%28%12%1
1487
August1st to 10thNight
32%9%19%29%9%2
1423
11th to 20thNight
32%8%16%33%11%1
1458
21st to endNight
38%8%14%28%12%1
1980
September1st to 10thNight
33%7%16%30%13%
1800
11th to 20thNight
38%8%20%30%3
1831
21st to endNight
46%9%16%25%4
1980
October1st to 10thNight
47%9%17%22%6%
2148
11th to 20thNight
48%9%15%22%5%1
2220
21st to endNight
55%8%14%16%6%
2574
November1st to 10thNight
53%9%13%19%6%
2425
11th to 20thNight
59%7%13%16%4
2660
21st to endNight
57%8%15%15%5%
2660
December1st to 10thNight
54%8%18%17%3
2827
11th to 20thNight
54%6%15%19%6%
3521
21st to endNight
69%5%9%15%2
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
14%13%38%29%6%
270
11th to 20thDay
16%10%41%26%6%2
270
21st to endDay
15%13%41%25%5%
297
February1st to 10thDay
14%15%43%19%7%2
270
11th to 20thDay
11%20%43%20%41
270
21st to endDay
14%23%47%15%2
225
March1st to 10thDay
10%20%43%22%41
270
11th to 20thDay
13%22%38%19%7%1
270
21st to endDay
19%13%40%20%7%2
471
April1st to 10thDay
17%16%33%27%6%
420
11th to 20thDay
23%15%27%23%11%1
412
21st to endDay
23%14%29%23%10%1
485
May1st to 10thDay
18%15%38%20%7%2
603
11th to 20thDay
16%10%38%25%8%3
625
21st to endDay
19%11%36%23%8%2
679
June1st to 10thDay
15%11%39%23%10%2
630
11th to 20thDay
16%10%37%25%11%2
630
21st to endDay
16%8%32%29%13%2
630
July1st to 10thDay
9%7%37%32%11%3
621
11th to 20thDay
12%9%36%27%11%5%
618
21st to endDay
8%6%36%31%13%6%
684
August1st to 10thDay
12%7%37%31%10%4
626
11th to 20thDay
12%7%30%30%15%5%
605
21st to endDay
17%11%28%28%12%3
495
September1st to 10thDay
13%11%30%27%15%4
450
11th to 20thDay
19%10%31%24%13%2
450
21st to endDay
19%15%31%22%12%2
450
October1st to 10thDay
23%16%27%26%8%1
280
11th to 20thDay
19%18%39%17%6%1
270
21st to endDay
31%22%33%11%21
297
November1st to 10thDay
20%24%28%23%31
270
11th to 20thDay
27%21%36%11%4
270
21st to endDay
28%19%30%19%4
270
December1st to 10thDay
23%12%36%20%7%2
188
11th to 20thDay
22%24%36%13%41
180
21st to endDay
22%19%34%21%4
297
January1st to 10thNight
16%13%39%26%6%
950
11th to 20thNight
15%13%39%25%6%1
950
21st to endNight
18%8%40%26%8%1
1045
February1st to 10thNight
14%12%37%28%8%1
950
11th to 20thNight
13%13%34%29%10%1
910
21st to endNight
13%11%36%32%8%
628
March1st to 10thNight
12%12%34%33%9%1
760
11th to 20thNight
17%12%32%29%9%1
687
21st to endNight
16%11%30%27%14%2
615
April1st to 10thNight
19%9%29%32%10%1
540
11th to 20thNight
21%11%23%30%14%1
533
21st to endNight
21%6%24%33%14%2
540
May1st to 10thNight
24%7%24%31%13%2
540
11th to 20thNight
17%6%27%34%14%3
531
21st to endNight
20%8%21%35%16%1
584
June1st to 10thNight
19%6%21%37%15%2
540
11th to 20thNight
15%6%22%42%15%1
360
21st to endNight
17%319%40%20%2
524
July1st to 10thNight
11%323%40%21%2
525
11th to 20thNight
11%325%36%23%3
522
21st to endNight
9%323%36%26%3
591
August1st to 10thNight
11%225%39%21%2
537
11th to 20thNight
12%418%32%29%5%
534
21st to endNight
17%5%22%34%19%3
594
September1st to 10thNight
14%5%23%33%20%5%
540
11th to 20thNight
17%6%27%33%15%2
571
21st to endNight
21%10%23%29%15%1
720
October1st to 10thNight
21%11%29%22%15%2
720
11th to 20thNight
20%15%29%26%9%1
720
21st to endNight
24%13%29%27%7%1
792
November1st to 10thNight
19%12%33%26%8%1
759
11th to 20thNight
23%15%32%20%8%1
950
21st to endNight
24%18%31%22%5%
950
December1st to 10thNight
17%16%36%23%7%1
950
11th to 20thNight
19%16%35%21%8%1
950
21st to endNight
26%14%32%23%41
1045
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