Sudbury
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sudbury clear sky chart from 2005-11-06 to 2024-01-15.

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
30%
23393
14%
74860
8%
23342
36%
23342
17%
53318

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
19%
949
10%
2838
4%
946
25%
946
11%
2251
January11th to 20thNight
21%
923
9%
2677
4%
923
25%
923
11%
2124
January21st to endNight
28%
859
13%
2641
8%
859
35%
859
14%
1991
February1st to 10thNight
19%
720
11%
2340
5%
720
27%
720
12%
1756
February11th to 20thNight
21%
702
14%
2273
5%
699
31%
699
14%
1668
February21st to endNight
20%
592
13%
1852
4%
592
29%
592
13%
1385
March1st to 10thNight
23%
720
16%
2051
8%
720
31%
720
19%
1425
March11th to 20thNight
24%
720
16%
1980
7%
720
32%
720
19%
1432
March21st to endNight
28%
792
17%
2178
7%
792
37%
792
23%
1492
April1st to 10thNight
26%
707
15%
1857
7%
707
31%
707
20%
1375
April11th to 20thNight
32%
533
20%
1597
11%
533
39%
533
22%
1105
April21st to endNight
31%
540
15%
1620
8%
540
36%
540
19%
1148
May1st to 10thNight
35%
532
14%
1612
8%
532
42%
532
21%
1094
May11th to 20thNight
35%
354
19%
1415
18%
354
48%
354
19%
876
May21st to endNight
36%
389
14%
1479
12%
388
47%
388
24%
866
June1st to 10thNight
34%
360
11%
1260
10%
360
41%
360
22%
795
June11th to 20thNight
38%
360
15%
1260
12%
360
45%
360
25%
754
June21st to endNight
43%
360
12%
1260
11%
360
49%
360
24%
760
July1st to 10thNight
49%
350
17%
1225
15%
350
56%
350
34%
667
July11th to 20thNight
47%
348
18%
1218
16%
348
56%
348
31%
691
July21st to endNight
49%
514
15%
1516
12%
486
53%
486
31%
917
August1st to 10thNight
49%
537
14%
1600
11%
534
53%
534
30%
1013
August11th to 20thNight
51%
534
17%
1598
11%
534
57%
534
31%
994
August21st to endNight
45%
594
17%
1782
10%
594
50%
594
27%
1150
September1st to 10thNight
44%
702
19%
1782
9%
702
49%
702
32%
1138
September11th to 20thNight
45%
720
19%
1969
13%
720
48%
720
32%
1261
September21st to endNight
42%
720
20%
1980
12%
720
47%
720
26%
1348
October1st to 10thNight
34%
720
17%
2150
13%
720
40%
720
22%
1484
October11th to 20thNight
25%
720
16%
2160
9%
720
35%
720
16%
1598
October21st to endNight
22%
792
12%
2479
6%
792
29%
792
11%
1924
November1st to 10thNight
25%
740
15%
2523
9%
740
35%
740
16%
1732
November11th to 20thNight
25%
760
13%
2660
9%
760
33%
760
12%
1990
November21st to endNight
21%
760
9%
2660
5%
760
26%
760
10%
2078
December1st to 10thNight
16%
783
10%
2683
3%
783
25%
783
9%
2097
December11th to 20thNight
18%
950
11%
3488
4%
948
26%
948
10%
2626
December21st to endNight
18%
1037
5%
5197
2%
1026
22%
1026
5%
4313

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
16%10%47%6%35%5%5%7%32%
74675
Night
16%11%48%6%35%45%6%31%
75101
TransparencyDay
52%412%21%10%1
74373
Night
54%412%20%10%1
74939
SeeingDay
21%12%30%24%10%3
14093
Night
22%10%30%28%9%1
23403

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
15%19%7%10%7%35%5%4421%
1328
11th to 20thDay
15%16%7%11%7%3446%5%22%
1375
21st to endDay
16%15%7%9%8%45%45%5%23%
1714
February1st to 10thDay
10%15%6%9%9%5%6%5%5%7%23%
1620
11th to 20thDay
13%16%49%7%3445%6%29%
1573
21st to endDay
14%16%5%10%7%46%5%5%5%23%
1400
March1st to 10thDay
14%12%37%6%3445%7%33%
1905
11th to 20thDay
12%11%5%8%8%34446%35%
1980
21st to endDay
15%11%38%5%34446%37%
2178
April1st to 10thDay
19%10%47%6%35%5%5%7%28%
2115
11th to 20thDay
20%10%35%6%3445%7%35%
2312
21st to endDay
22%10%37%5%24346%35%
2340
May1st to 10thDay
16%8%36%7%35%5%6%8%34%
2348
11th to 20thDay
17%9%36%5%3445%7%35%
2478
21st to endDay
16%6%25%5%345%5%9%39%
2795
June1st to 10thDay
16%8%36%6%45%6%6%8%33%
2700
11th to 20thDay
13%6%36%6%45%5%5%8%39%
2700
21st to endDay
17%8%36%7%35%6%6%8%33%
2700
July1st to 10thDay
11%5%36%6%45%5%6%9%42%
2700
11th to 20thDay
12%6%36%6%46%6%6%7%37%
2700
21st to endDay
12%6%26%7%36%5%7%9%37%
2708
August1st to 10thDay
14%7%36%5%47%5%7%9%34%
2327
11th to 20thDay
14%6%35%5%35%5%7%8%39%
2303
21st to endDay
13%7%25%6%45%5%6%9%35%
2574
September1st to 10thDay
13%8%36%6%4445%8%39%
2160
11th to 20thDay
15%5%25%6%35%5%6%8%41%
1989
21st to endDay
22%7%36%5%345%5%6%34%
1980
October1st to 10thDay
19%8%46%6%35%446%34%
1810
11th to 20thDay
25%11%39%7%45%45%6%22%
1800
21st to endDay
26%13%48%6%345%5%5%21%
1875
November1st to 10thDay
18%12%5%8%6%445%6%6%27%
1554
11th to 20thDay
17%13%5%11%7%45%45%6%23%
1520
21st to endDay
23%19%5%9%8%4433418%
1520
December1st to 10thDay
15%20%6%10%6%46%445%19%
1492
11th to 20thDay
17%18%6%10%7%34345%22%
1671
21st to endDay
23%21%6%13%6%2333414%
2431
January1st to 10thNight
16%15%7%11%8%45%446%21%
2838
11th to 20thNight
17%18%6%9%7%35%345%22%
2677
21st to endNight
14%12%7%9%8%46%5%6%6%24%
2641
February1st to 10thNight
11%14%6%10%10%46%5%5%6%23%
2340
11th to 20thNight
12%14%5%10%8%45%45%5%27%
2273
21st to endNight
14%15%6%8%7%445%5%5%27%
1852
March1st to 10thNight
13%9%5%9%6%3445%7%35%
2041
11th to 20thNight
15%10%49%7%34446%34%
1979
21st to endNight
15%9%46%6%345%5%6%38%
2178
April1st to 10thNight
18%11%5%7%5%3445%7%31%
1857
11th to 20thNight
19%8%35%6%34347%37%
1597
21st to endNight
19%9%36%6%45%45%6%33%
1620
May1st to 10thNight
14%8%46%6%46%5%5%5%36%
1612
11th to 20thNight
14%8%36%6%45%5%6%7%37%
1415
21st to endNight
12%6%36%425%5%48%45%
1478
June1st to 10thNight
12%6%36%6%45%6%7%8%37%
1260
11th to 20thNight
14%6%25%5%35%47%10%40%
1260
21st to endNight
13%5%346%35%46%9%42%
1260
July1st to 10thNight
8%3245%35%6%6%9%48%
1260
11th to 20thNight
9%5%26%5%35%46%9%47%
1260
21st to endNight
9%5%36%4247%6%8%46%
1604
August1st to 10thNight
10%5%36%7%35%45%7%44%
1611
11th to 20thNight
12%7%245%2445%8%45%
1598
21st to endNight
15%6%25%5%35%5%5%7%42%
1782
September1st to 10thNight
12%6%25%4345%5%7%47%
1782
11th to 20thNight
13%6%25%5%3345%6%47%
1969
21st to endNight
18%7%35%5%345%48%39%
1980
October1st to 10thNight
21%8%35%5%34446%37%
2150
11th to 20thNight
26%9%46%6%3446%6%28%
2160
21st to endNight
26%12%48%5%45%345%23%
2479
November1st to 10thNight
17%12%46%5%345%5%8%31%
2523
11th to 20thNight
19%12%5%8%7%45%45%5%25%
2660
21st to endNight
22%15%5%7%6%35%446%21%
2660
December1st to 10thNight
18%18%6%9%7%35%45%5%21%
2683
11th to 20thNight
18%16%5%9%7%3445%6%24%
3526
21st to endNight
22%21%5%12%6%3433415%
5236

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
67%311%15%4
1329
11th to 20thDay
64%38%16%8%2
1375
21st to endDay
64%26%17%9%1
1714
February1st to 10thDay
61%312%17%6%1
1620
11th to 20thDay
58%410%16%11%1
1582
21st to endDay
62%38%17%10%1
1400
March1st to 10thDay
51%215%16%14%1
1908
11th to 20thDay
52%29%24%11%2
1980
21st to endDay
50%210%21%14%3
2178
April1st to 10thDay
56%412%18%10%1
2115
11th to 20thDay
50%38%26%12%1
2312
21st to endDay
52%48%19%14%3
2340
May1st to 10thDay
49%311%20%14%3
2348
11th to 20thDay
48%39%21%17%2
2482
21st to endDay
42%414%24%14%2
2795
June1st to 10thDay
49%412%22%13%1
2700
11th to 20thDay
43%5%11%23%17%1
2700
21st to endDay
49%5%11%23%11%2
2700
July1st to 10thDay
39%413%27%16%1
2626
11th to 20thDay
43%5%11%29%12%1
2619
21st to endDay
43%5%13%28%10%
2557
August1st to 10thDay
44%6%16%24%9%1
2314
11th to 20thDay
43%5%18%26%7%1
2303
21st to endDay
44%5%15%27%8%1
2574
September1st to 10thDay
45%315%26%10%1
2160
11th to 20thDay
41%417%28%9%
1989
21st to endDay
51%416%24%6%
1980
October1st to 10thDay
52%412%20%11%
1810
11th to 20thDay
63%410%15%7%
1800
21st to endDay
65%312%12%7%
1875
November1st to 10thDay
57%414%18%7%
1554
11th to 20thDay
62%312%16%6%
1520
21st to endDay
72%310%10%5%
1520
December1st to 10thDay
67%310%14%5%
1492
11th to 20thDay
66%38%18%41
1671
21st to endDay
77%26%12%3
2431
January1st to 10thNight
66%310%15%6%
2838
11th to 20thNight
66%39%13%8%1
2677
21st to endNight
60%47%19%10%1
2641
February1st to 10thNight
61%310%17%7%1
2340
11th to 20thNight
59%312%18%7%1
2275
21st to endNight
61%210%16%10%1
1852
March1st to 10thNight
50%312%18%15%2
2051
11th to 20thNight
54%311%18%13%2
1980
21st to endNight
47%411%18%15%5%
2178
April1st to 10thNight
55%38%19%14%1
1857
11th to 20thNight
50%35%24%16%2
1597
21st to endNight
53%310%15%16%3
1620
May1st to 10thNight
49%410%21%13%3
1612
11th to 20thNight
45%48%24%16%2
1416
21st to endNight
39%5%13%24%17%3
1479
June1st to 10thNight
44%5%12%24%13%3
1260
11th to 20thNight
39%5%13%23%19%2
1260
21st to endNight
40%5%17%23%15%2
1260
July1st to 10thNight
30%5%15%30%17%2
1225
11th to 20thNight
35%415%32%14%1
1218
21st to endNight
36%419%28%12%1
1516
August1st to 10thNight
40%419%23%12%1
1600
11th to 20thNight
37%5%17%28%11%2
1598
21st to endNight
42%415%26%11%1
1782
September1st to 10thNight
37%5%14%30%11%3
1782
11th to 20thNight
39%415%33%9%
1969
21st to endNight
44%5%15%28%7%
1980
October1st to 10thNight
49%413%22%12%
2150
11th to 20thNight
58%311%22%5%
2160
21st to endNight
64%310%15%7%1
2479
November1st to 10thNight
51%5%14%22%8%
2523
11th to 20thNight
61%411%16%8%1
2660
21st to endNight
65%5%12%13%5%
2660
December1st to 10thNight
66%411%16%4
2683
11th to 20thNight
62%310%15%8%1
3525
21st to endNight
75%29%10%4
5236

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
24%21%27%21%7%1
180
11th to 20thDay
18%10%36%27%8%2
207
21st to endDay
27%6%29%23%11%4
264
February1st to 10thDay
24%20%30%20%41
240
11th to 20thDay
22%15%38%23%3
240
21st to endDay
24%23%37%15%2
198
March1st to 10thDay
21%14%36%24%6%
348
11th to 20thDay
21%15%32%24%8%1
420
21st to endDay
24%13%31%25%6%2
462
April1st to 10thDay
23%10%30%25%10%2
420
11th to 20thDay
30%12%23%18%11%5%
412
21st to endDay
30%13%23%20%11%3
420
May1st to 10thDay
23%9%28%25%12%3
423
11th to 20thDay
21%14%27%20%12%5%
447
21st to endDay
20%13%32%23%10%3
555
June1st to 10thDay
18%14%35%25%7%1
630
11th to 20thDay
18%14%32%24%8%3
630
21st to endDay
21%8%32%26%10%3
630
July1st to 10thDay
10%11%32%30%14%3
621
11th to 20thDay
15%10%31%26%14%5%
618
21st to endDay
13%6%29%31%14%6%
576
August1st to 10thDay
19%7%23%25%20%7%
446
11th to 20thDay
15%5%20%28%20%11%
439
21st to endDay
18%6%28%24%16%8%
495
September1st to 10thDay
16%11%26%27%15%6%
450
11th to 20thDay
16%6%28%31%14%5%
450
21st to endDay
27%11%22%24%12%4
450
October1st to 10thDay
24%14%25%23%12%3
450
11th to 20thDay
28%16%33%17%5%1
450
21st to endDay
31%14%30%21%41
390
November1st to 10thDay
29%12%37%18%4
214
11th to 20thDay
18%20%36%21%5%1
180
21st to endDay
31%15%29%17%8%1
180
December1st to 10thDay
24%12%31%22%9%1
180
11th to 20thDay
32%13%35%13%7%
180
21st to endDay
30%11%38%15%6%
198
January1st to 10thNight
24%11%31%26%7%1
950
11th to 20thNight
27%11%28%26%7%
923
21st to endNight
21%8%28%31%10%1
859
February1st to 10thNight
18%13%39%26%3
720
11th to 20thNight
19%13%35%25%7%1
703
21st to endNight
21%16%33%25%4
592
March1st to 10thNight
20%15%33%26%6%
720
11th to 20thNight
21%13%33%24%7%1
720
21st to endNight
21%10%34%27%8%1
792
April1st to 10thNight
24%11%30%29%5%
707
11th to 20thNight
23%11%28%29%8%1
533
21st to endNight
25%8%28%28%10%
540
May1st to 10thNight
18%7%32%34%9%
532
11th to 20thNight
18%9%31%34%8%1
354
21st to endNight
15%6%38%32%9%1
389
June1st to 10thNight
15%6%37%32%9%
360
11th to 20thNight
19%7%29%36%9%
360
21st to endNight
15%7%27%35%15%1
360
July1st to 10thNight
8%434%44%9%1
350
11th to 20thNight
11%432%39%15%
348
21st to endNight
10%330%41%14%2
514
August1st to 10thNight
12%425%37%20%1
537
11th to 20thNight
15%422%35%21%3
534
21st to endNight
17%425%34%19%1
594
September1st to 10thNight
16%429%31%20%1
702
11th to 20thNight
16%8%26%31%16%2
720
21st to endNight
22%5%26%29%16%3
720
October1st to 10thNight
26%7%28%28%10%1
720
11th to 20thNight
32%10%28%23%7%1
720
21st to endNight
32%13%26%24%41
792
November1st to 10thNight
26%11%30%25%7%1
740
11th to 20thNight
27%12%29%25%7%
760
21st to endNight
30%11%28%24%6%
760
December1st to 10thNight
29%11%31%22%5%1
783
11th to 20thNight
28%12%32%22%6%
950
21st to endNight
30%12%29%22%6%
1045
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