Ville-Marie
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ville-Marie 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
27%
23421
10%
81346
7%
23334
31%
23334
12%
62019

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
15%
944
4%
4737
3%
926
18%
926
4%
3909
January11th to 20thNight
18%
946
7%
4724
3%
930
22%
930
5%
3782
January21st to endNight
26%
828
6%
4577
7%
814
31%
814
5%
3887
February1st to 10thNight
22%
760
9%
2470
5%
760
26%
760
11%
1949
February11th to 20thNight
18%
743
9%
2405
4%
740
24%
740
10%
1878
February21st to endNight
17%
628
10%
2041
4%
628
24%
628
10%
1595
March1st to 10thNight
21%
760
13%
2125
6%
760
26%
760
14%
1648
March11th to 20thNight
21%
760
12%
2090
6%
760
26%
760
13%
1637
March21st to endNight
25%
821
16%
2256
7%
821
32%
821
20%
1622
April1st to 10thNight
24%
720
13%
1809
7%
720
27%
720
17%
1414
April11th to 20thNight
28%
587
16%
1651
9%
587
32%
587
19%
1186
April21st to endNight
31%
540
15%
1620
8%
540
37%
540
19%
1147
May1st to 10thNight
35%
426
14%
1506
12%
426
43%
426
17%
1040
May11th to 20thNight
32%
354
18%
1416
17%
354
44%
354
16%
934
May21st to endNight
40%
388
12%
1533
11%
388
48%
388
19%
952
June1st to 10thNight
38%
360
9%
1260
8%
360
43%
360
19%
852
June11th to 20thNight
38%
360
12%
1260
10%
360
43%
360
23%
781
June21st to endNight
43%
360
9%
1260
9%
360
47%
360
19%
824
July1st to 10thNight
45%
350
12%
1225
12%
350
49%
350
25%
764
July11th to 20thNight
48%
348
15%
1218
15%
348
54%
348
25%
740
July21st to endNight
46%
394
13%
1453
15%
372
50%
372
20%
901
August1st to 10thNight
47%
525
12%
1589
9%
523
51%
523
23%
1077
August11th to 20thNight
50%
534
15%
1598
11%
534
53%
534
26%
1050
August21st to endNight
39%
620
15%
1808
9%
620
42%
620
21%
1248
September1st to 10thNight
42%
720
15%
1800
7%
720
45%
720
28%
1242
September11th to 20thNight
38%
720
17%
1878
9%
720
41%
720
25%
1300
September21st to endNight
40%
720
17%
2028
12%
720
42%
720
23%
1391
October1st to 10thNight
30%
720
15%
2160
11%
720
36%
720
17%
1571
October11th to 20thNight
22%
720
9%
2160
6%
720
26%
720
12%
1703
October21st to endNight
19%
792
10%
2449
6%
792
25%
792
10%
1982
November1st to 10thNight
19%
740
11%
2592
7%
740
26%
740
11%
1960
November11th to 20thNight
15%
760
8%
2660
5%
760
19%
760
7%
2201
November21st to endNight
14%
760
4%
2660
2%
760
17%
760
6%
2270
December1st to 10thNight
14%
760
6%
2660
2%
760
19%
760
5%
2292
December11th to 20thNight
17%
916
6%
3485
3%
915
21%
915
7%
2860
December21st to endNight
17%
1037
5%
5183
3%
1026
20%
1026
4%
4430

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%11%5%8%7%45%5%5%6%28%
78398
Night
17%13%5%9%7%45%446%26%
81616
TransparencyDay
57%6%11%18%8%1
78092
Night
60%6%10%16%7%1
81457
SeeingDay
22%14%31%22%9%3
14231
Night
24%12%30%25%8%1
23448

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
13%20%9%14%7%5%5%5%5%5%12%
2208
11th to 20thDay
11%13%6%16%12%5%5%6%45%16%
2210
21st to endDay
15%18%10%12%8%45%45%414%
3069
February1st to 10thDay
8%15%7%12%12%6%7%5%5%6%18%
1710
11th to 20thDay
11%18%5%9%10%46%5%5%7%20%
1663
21st to endDay
14%16%5%9%9%45%6%45%22%
1413
March1st to 10thDay
14%12%7%11%7%445%5%6%26%
2047
11th to 20thDay
13%12%6%10%9%4445%5%29%
2090
21st to endDay
15%12%5%8%6%44445%33%
2255
April1st to 10thDay
19%13%6%8%6%345%46%26%
2150
11th to 20thDay
20%10%38%5%34445%33%
2274
21st to endDay
21%11%36%5%35%346%34%
2340
May1st to 10thDay
19%8%46%5%35%5%47%35%
2452
11th to 20thDay
20%11%47%5%34446%33%
2478
21st to endDay
17%7%35%6%35%447%39%
2739
June1st to 10thDay
20%7%36%6%35%6%5%7%32%
2700
11th to 20thDay
14%6%35%6%35%6%7%7%37%
2700
21st to endDay
19%7%36%6%46%5%5%7%33%
2700
July1st to 10thDay
16%5%25%5%345%6%9%39%
2700
11th to 20thDay
16%5%35%435%6%6%9%37%
2700
21st to endDay
13%6%36%6%46%6%7%8%36%
2779
August1st to 10thDay
16%7%37%6%45%5%7%8%32%
2337
11th to 20thDay
17%5%35%6%35%5%7%7%37%
2303
21st to endDay
17%6%37%7%445%5%8%34%
2529
September1st to 10thDay
18%6%26%6%36%45%7%36%
2160
11th to 20thDay
21%6%36%5%445%47%35%
2080
21st to endDay
24%7%27%5%4445%6%33%
1931
October1st to 10thDay
21%12%36%5%3445%5%32%
1800
11th to 20thDay
26%15%5%8%6%445%45%21%
1800
21st to endDay
26%20%49%6%34345%16%
1904
November1st to 10thDay
20%15%7%8%7%34345%25%
1486
11th to 20thDay
20%14%6%13%9%4445%417%
1520
21st to endDay
23%18%8%12%8%45%33410%
1520
December1st to 10thDay
16%20%9%12%7%445%3415%
1520
11th to 20thDay
17%17%7%11%8%36%445%19%
1700
21st to endDay
19%18%8%17%9%3433313%
2431
January1st to 10thNight
14%16%8%14%9%46%5%45%15%
4748
11th to 20thNight
11%14%6%14%11%5%7%7%5%5%17%
4760
21st to endNight
12%21%9%11%9%5%6%5%4413%
4582
February1st to 10thNight
11%13%6%11%11%5%5%5%5%6%20%
2470
11th to 20thNight
11%15%8%11%9%45%5%45%23%
2403
21st to endNight
12%15%6%11%8%46%45%6%23%
2041
March1st to 10thNight
12%13%7%10%7%46%35%5%27%
2115
11th to 20thNight
14%14%5%10%8%45%345%26%
2089
21st to endNight
15%12%47%6%45%445%35%
2256
April1st to 10thNight
20%13%48%7%34346%28%
1809
11th to 20thNight
19%11%37%5%34446%33%
1651
21st to endNight
19%10%47%5%3435%5%35%
1620
May1st to 10thNight
16%10%38%6%45%445%35%
1506
11th to 20thNight
20%7%36%5%345%6%6%35%
1415
21st to endNight
18%435%5%345%6%7%40%
1532
June1st to 10thNight
16%6%37%5%46%6%6%8%34%
1260
11th to 20thNight
20%42442436%8%43%
1260
21st to endNight
17%5%36%5%35%5%6%8%37%
1260
July1st to 10thNight
13%4345%35%5%6%8%42%
1260
11th to 20thNight
15%5%3443445%8%45%
1260
21st to endNight
11%437%6%46%5%6%8%41%
1539
August1st to 10thNight
14%5%25%7%46%46%8%38%
1599
11th to 20thNight
16%5%344345%6%7%42%
1598
21st to endNight
18%6%26%6%25%447%39%
1808
September1st to 10thNight
17%5%245%4445%8%42%
1800
11th to 20thNight
20%7%244345%46%41%
1878
21st to endNight
21%7%34434447%38%
2028
October1st to 10thNight
22%11%35%5%3445%6%32%
2160
11th to 20thNight
26%14%36%5%345%45%24%
2160
21st to endNight
24%17%47%6%4444422%
2449
November1st to 10thNight
19%14%5%8%6%34446%27%
2592
11th to 20thNight
18%15%7%12%9%35%445%18%
2660
21st to endNight
23%17%7%11%7%45%5%3415%
2660
December1st to 10thNight
17%22%7%12%7%45%44314%
2660
11th to 20thNight
17%18%6%11%10%45%44417%
3492
21st to endNight
20%20%7%14%9%3433413%
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
73%7%12%7%1
2209
11th to 20thDay
71%5%8%12%4
2210
21st to endDay
72%5%9%10%4
3069
February1st to 10thDay
65%8%8%12%7%
1710
11th to 20thDay
64%6%9%12%9%1
1672
21st to endDay
64%6%7%15%8%
1413
March1st to 10thDay
59%5%9%16%9%1
2050
11th to 20thDay
58%5%8%20%8%1
2090
21st to endDay
55%39%18%14%2
2255
April1st to 10thDay
60%5%9%17%9%1
2150
11th to 20thDay
53%48%23%10%2
2274
21st to endDay
54%5%8%16%15%2
2340
May1st to 10thDay
50%5%11%20%12%3
2452
11th to 20thDay
53%5%8%18%14%1
2482
21st to endDay
46%5%13%22%13%1
2739
June1st to 10thDay
51%7%11%22%9%1
2700
11th to 20thDay
44%7%12%25%12%1
2700
21st to endDay
50%7%11%23%9%1
2700
July1st to 10thDay
41%8%15%27%10%1
2626
11th to 20thDay
42%9%13%27%9%
2619
21st to endDay
44%9%16%26%5%
2625
August1st to 10thDay
49%9%16%21%5%
2323
11th to 20thDay
44%9%18%21%8%1
2303
21st to endDay
49%8%13%22%7%1
2529
September1st to 10thDay
47%8%14%22%9%
2160
11th to 20thDay
49%7%16%22%6%
2080
21st to endDay
54%6%14%22%5%
1931
October1st to 10thDay
55%6%11%19%8%1
1800
11th to 20thDay
67%5%9%14%41
1800
21st to endDay
71%6%9%10%4
1904
November1st to 10thDay
64%413%14%5%
1486
11th to 20thDay
70%6%9%10%5%
1520
21st to endDay
80%5%7%6%2
1520
December1st to 10thDay
74%6%8%9%3
1520
11th to 20thDay
68%7%7%15%2
1700
21st to endDay
77%5%6%10%2
2431
January1st to 10thNight
71%7%11%8%2
4748
11th to 20thNight
68%7%9%13%4
4760
21st to endNight
74%5%7%8%5%
4582
February1st to 10thNight
64%6%10%13%7%
2470
11th to 20thNight
64%411%14%6%
2405
21st to endNight
62%6%9%14%8%1
2041
March1st to 10thNight
60%5%9%14%10%2
2125
11th to 20thNight
61%5%10%14%9%1
2090
21st to endNight
52%49%16%15%3
2256
April1st to 10thNight
59%48%15%12%2
1809
11th to 20thNight
52%5%8%19%14%2
1651
21st to endNight
51%5%10%15%17%2
1620
May1st to 10thNight
52%5%8%19%14%2
1506
11th to 20thNight
49%5%7%22%15%2
1416
21st to endNight
43%6%13%20%16%3
1533
June1st to 10thNight
47%7%12%22%10%2
1260
11th to 20thNight
41%6%12%24%15%2
1260
21st to endNight
45%7%12%22%13%
1260
July1st to 10thNight
37%10%13%26%14%1
1225
11th to 20thNight
39%6%14%28%13%
1218
21st to endNight
41%8%18%27%7%
1453
August1st to 10thNight
43%9%15%25%8%
1589
11th to 20thNight
39%9%16%26%8%1
1598
21st to endNight
46%8%14%24%9%1
1808
September1st to 10thNight
40%8%16%24%9%1
1800
11th to 20thNight
44%7%15%27%7%
1878
21st to endNight
45%7%15%26%7%
2028
October1st to 10thNight
52%7%11%19%11%
2160
11th to 20thNight
63%5%13%16%4
2160
21st to endNight
66%6%10%13%6%
2449
November1st to 10thNight
59%7%12%17%4
2592
11th to 20thNight
69%7%10%9%5%
2660
21st to endNight
73%7%10%8%2
2660
December1st to 10thNight
76%5%7%9%3
2660
11th to 20thNight
71%5%9%12%3
3491
21st to endNight
77%46%11%3
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
27%16%38%13%5%1
180
11th to 20thDay
21%14%37%19%7%2
180
21st to endDay
20%11%36%21%10%1
285
February1st to 10thDay
21%24%34%16%5%
270
11th to 20thDay
21%21%39%16%3
270
21st to endDay
28%25%34%12%
225
March1st to 10thDay
20%17%38%20%5%
438
11th to 20thDay
17%18%37%22%5%1
460
21st to endDay
26%16%35%15%7%1
490
April1st to 10thDay
24%16%31%21%6%1
420
11th to 20thDay
26%18%23%23%7%3
412
21st to endDay
27%19%28%16%8%3
420
May1st to 10thDay
26%11%31%21%9%2
423
11th to 20thDay
22%15%28%21%11%3
447
21st to endDay
21%13%34%21%10%2
499
June1st to 10thDay
21%13%32%24%8%1
630
11th to 20thDay
19%12%28%26%11%3
630
21st to endDay
20%10%27%29%12%3
630
July1st to 10thDay
14%11%31%32%10%1
621
11th to 20thDay
18%11%29%25%14%3
618
21st to endDay
12%10%29%31%14%4
523
August1st to 10thDay
19%9%26%26%15%4
446
11th to 20thDay
17%5%25%26%17%10%
439
21st to endDay
21%9%30%20%13%6%
495
September1st to 10thDay
21%8%30%25%12%4
450
11th to 20thDay
22%9%24%31%9%5%
450
21st to endDay
26%12%25%22%13%3
450
October1st to 10thDay
27%14%26%19%12%2
450
11th to 20thDay
31%20%29%15%41
450
21st to endDay
34%16%31%16%31
432
November1st to 10thDay
32%18%39%9%2
180
11th to 20thDay
26%16%37%16%41
180
21st to endDay
33%18%36%10%21
180
December1st to 10thDay
26%16%34%20%31
180
11th to 20thDay
27%16%37%13%6%2
180
21st to endDay
24%20%29%19%6%2
198
January1st to 10thNight
26%14%34%21%5%
950
11th to 20thNight
25%14%31%24%6%
950
21st to endNight
19%11%31%27%11%1
836
February1st to 10thNight
19%15%34%25%7%
760
11th to 20thNight
21%14%37%22%5%
743
21st to endNight
22%18%33%21%5%
628
March1st to 10thNight
20%18%34%23%5%1
760
11th to 20thNight
23%16%32%22%6%1
760
21st to endNight
20%12%37%24%6%1
821
April1st to 10thNight
26%15%29%24%6%1
720
11th to 20thNight
26%13%27%24%9%1
587
21st to endNight
24%11%28%29%7%1
540
May1st to 10thNight
21%9%28%34%8%1
426
11th to 20thNight
22%8%32%27%9%1
354
21st to endNight
21%8%27%38%6%
389
June1st to 10thNight
18%10%28%36%8%
360
11th to 20thNight
21%8%27%31%13%
360
21st to endNight
15%8%27%36%14%1
360
July1st to 10thNight
15%5%31%39%9%1
350
11th to 20thNight
16%6%24%40%14%
348
21st to endNight
12%5%29%37%16%1
394
August1st to 10thNight
15%5%25%36%17%2
525
11th to 20thNight
18%5%23%32%20%2
534
21st to endNight
20%8%27%26%17%2
620
September1st to 10thNight
18%8%27%29%17%1
720
11th to 20thNight
23%9%27%25%16%1
720
21st to endNight
25%6%24%30%13%2
720
October1st to 10thNight
28%9%28%25%9%1
720
11th to 20thNight
33%12%28%19%7%1
720
21st to endNight
33%12%28%22%31
792
November1st to 10thNight
26%15%32%20%6%1
740
11th to 20thNight
25%17%35%17%6%
760
21st to endNight
33%16%28%19%4
760
December1st to 10thNight
31%16%29%18%5%1
760
11th to 20thNight
27%14%32%20%5%1
916
21st to endNight
29%16%25%23%6%1
1045
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