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Montreal
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montreal 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
33%
14611
13%
46768
9%
14580
39%
14580
17%
33571

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
19%
720
9%
2506
5%
716
25%
716
9%
1981
January16th to endNight
24%
768
11%
2688
7%
768
31%
768
11%
2020
February1st to 15thNight
22%
704
8%
2310
5%
701
27%
701
11%
1837
February16th to endNight
26%
631
13%
1893
6%
631
34%
631
14%
1428
March1st to 15thNight
27%
720
14%
2160
10%
720
32%
720
15%
1632
March16th to endNight
33%
768
19%
1998
10%
768
40%
768
23%
1423
April1st to 15thNight
29%
718
16%
1796
6%
718
34%
718
20%
1366
April16th to endNight
30%
618
17%
1698
6%
618
37%
618
21%
1213
May1st to 15thNight
33%
401
17%
1481
14%
401
42%
401
18%
976
May16th to endNight
37%
374
13%
1497
14%
374
45%
374
17%
990
June1st to 15thNight
35%
360
9%
1440
9%
360
43%
360
13%
1011
June16th to endNight
45%
360
8%
1440
11%
360
49%
360
19%
917
July1st to 15thNight
53%
340
13%
1360
19%
340
57%
340
23%
799
July16th to endNight
56%
376
12%
1418
17%
354
59%
354
23%
831
August1st to 15thNight
60%
368
15%
1439
18%
366
61%
366
21%
882
August16th to endNight
56%
756
20%
1902
12%
756
59%
756
35%
1229
September1st to 15thNight
51%
700
15%
1750
8%
700
52%
700
29%
1218
September16th to endNight
48%
657
20%
1809
13%
657
53%
657
30%
1167
October1st to 15thNight
34%
660
19%
1939
11%
660
43%
660
22%
1360
October16th to endNight
28%
704
11%
2260
8%
704
34%
704
14%
1681
November1st to 15thNight
27%
700
17%
2277
12%
700
36%
700
16%
1635
November16th to endNight
25%
720
14%
2499
9%
720
32%
720
12%
1862
December1st to 15thNight
19%
720
8%
2520
4%
720
24%
720
9%
1980
December16th to endNight
13%
768
10%
2688
3%
768
22%
768
9%
2133

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
18%8%36%5%3445%6%33%
47876
Night
19%9%36%5%34445%32%
46937
Cloud CoverDay
18%8%36%5%3445%7%32%
47886
Night
19%9%46%5%34446%31%
46942
TransparencyDay
50%5%12%20%9%1
47563
Night
53%5%10%19%10%1
46768
SeeingDay
21%7%25%25%14%4
5564
Night
23%7%27%29%11%
14612

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
21%14%6%8%6%45%32421%
1432
16th to endDay
14%13%47%5%46%45%7%26%
1536
February1st to 15thDay
14%12%5%10%7%45%445%24%
1552
16th to endDay
17%12%5%7%7%3435%427%
1580
March1st to 15thDay
17%10%5%8%7%4445%5%27%
1790
16th to endDay
18%8%46%424345%37%
2225
April1st to 15thDay
22%8%37%5%3433432%
2152
16th to endDay
20%6%36%5%24345%37%
2261
May1st to 15thDay
21%6%35%435%5%46%33%
2481
16th to endDay
19%6%35%5%345%5%7%33%
2618
June1st to 15thDay
22%6%345%3445%7%31%
2520
16th to endDay
18%5%35%5%36%45%8%32%
2520
July1st to 15thDay
12%5%345%45%5%6%9%37%
2520
16th to endDay
11%5%25%5%445%7%10%37%
2654
August1st to 15thDay
11%5%37%6%35%5%6%9%35%
2499
16th to endDay
9%5%14435%5%6%9%45%
2299
September1st to 15thDay
16%5%25%5%235%5%7%40%
2098
16th to endDay
14%5%1441335%7%46%
1821
October1st to 15thDay
20%6%36%5%3345%6%33%
1691
16th to endDay
23%9%36%5%34445%27%
1612
November1st to 15thDay
17%11%4443446%7%30%
1581
16th to endDay
23%10%37%5%24345%29%
1458
December1st to 15thDay
26%13%5%7%5%3445%6%18%
1440
16th to endDay
27%15%36%6%24335%20%
1536
January1st to 15thNight
18%15%5%9%6%3444422%
2506
16th to endNight
14%13%5%7%7%45%5%45%26%
2688
February1st to 15thNight
14%14%6%11%7%45%445%21%
2308
16th to endNight
18%11%5%8%5%45%43428%
1893
March1st to 15thNight
21%12%36%6%24335%28%
2147
16th to endNight
17%10%35%5%2433438%
1998
April1st to 15thNight
26%8%45%5%24335%31%
1796
16th to endNight
22%7%245%24345%36%
1698
May1st to 15thNight
21%6%36%5%33345%36%
1481
16th to endNight
19%7%25%435%45%6%34%
1495
June1st to 15thNight
26%6%25%5%245%47%29%
1440
16th to endNight
17%425%4346%7%8%35%
1440
July1st to 15thNight
11%325%5%45%5%7%9%39%
1440
16th to endNight
11%425%5%3448%8%41%
1514
August1st to 15thNight
13%425%6%35%5%5%8%39%
1449
16th to endNight
12%42343345%8%46%
1902
September1st to 15thNight
17%5%25%6%23446%41%
1750
16th to endNight
19%323322346%47%
1809
October1st to 15thNight
18%6%35%5%33446%36%
1939
16th to endNight
26%8%37%5%23335%29%
2260
November1st to 15thNight
21%9%26%6%34445%31%
2277
16th to endNight
23%12%46%5%3434427%
2499
December1st to 15thNight
23%15%5%8%423345%22%
2520
16th to endNight
25%16%5%7%5%2333422%
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
21%14%49%7%3433321%
1432
16th to endDay
14%13%48%5%45%6%5%7%24%
1536
February1st to 15thDay
15%12%5%10%9%36%435%23%
1552
16th to endDay
17%13%5%7%7%34445%26%
1580
March1st to 15thDay
17%11%47%8%35%445%26%
1800
16th to endDay
18%9%45%5%13446%35%
2225
April1st to 15thDay
22%9%35%5%34335%32%
2152
16th to endDay
20%6%35%5%2345%5%36%
2261
May1st to 15thDay
20%7%25%5%35%45%6%33%
2481
16th to endDay
19%5%35%5%345%5%7%34%
2618
June1st to 15thDay
22%6%345%35%448%31%
2520
16th to endDay
18%6%25%6%35%5%5%8%33%
2520
July1st to 15thDay
12%5%346%45%5%5%9%38%
2520
16th to endDay
12%425%6%346%7%10%36%
2654
August1st to 15thDay
11%5%26%6%35%6%6%9%35%
2499
16th to endDay
10%4144245%6%8%46%
2299
September1st to 15thDay
15%5%2442445%6%42%
2098
16th to endDay
13%5%24424348%46%
1821
October1st to 15thDay
20%7%26%5%3435%6%33%
1691
16th to endDay
24%9%35%6%4444427%
1612
November1st to 15thDay
18%10%36%5%25%5%5%7%30%
1581
16th to endDay
22%10%46%6%24445%27%
1458
December1st to 15thDay
26%12%5%8%6%3445%6%16%
1440
16th to endDay
27%15%47%6%2443419%
1536
January1st to 15thNight
18%15%6%9%6%345%45%21%
2506
16th to endNight
13%13%5%8%6%45%5%4426%
2688
February1st to 15thNight
15%13%6%11%8%45%44420%
2301
16th to endNight
18%11%5%8%5%45%43428%
1893
March1st to 15thNight
21%12%36%7%24345%28%
2160
16th to endNight
17%10%35%5%33345%37%
1997
April1st to 15thNight
27%8%35%5%33336%30%
1796
16th to endNight
21%7%35%425%445%36%
1698
May1st to 15thNight
20%7%46%5%24345%36%
1481
16th to endNight
20%5%35%435%447%34%
1495
June1st to 15thNight
26%6%35%424446%30%
1440
16th to endNight
16%5%25%5%25%6%7%8%35%
1440
July1st to 15thNight
12%3245%35%5%7%9%39%
1440
16th to endNight
11%434435%5%6%10%40%
1514
August1st to 15thNight
13%5%245%35%5%6%9%38%
1449
16th to endNight
12%42433445%7%46%
1902
September1st to 15thNight
17%5%25%5%24446%40%
1750
16th to endNight
19%423323435%47%
1809
October1st to 15thNight
18%7%26%6%24446%36%
1939
16th to endNight
26%8%36%424345%28%
2260
November1st to 15thNight
21%8%36%5%35%346%30%
2277
16th to endNight
23%11%37%6%2434427%
2499
December1st to 15thNight
24%14%5%7%434346%20%
2520
16th to endNight
24%17%6%7%5%2333421%
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
67%48%13%5%
1433
16th to endDay
57%8%11%15%7%
1536
February1st to 15thDay
63%310%18%3
1554
16th to endDay
59%311%15%9%1
1580
March1st to 15thDay
58%412%15%7%1
1800
16th to endDay
48%5%11%17%14%3
2225
April1st to 15thDay
55%37%19%11%2
2152
16th to endDay
49%48%19%14%3
2261
May1st to 15thDay
50%5%9%18%14%1
2481
16th to endDay
47%6%11%20%11%1
2622
June1st to 15thDay
50%6%11%19%10%1
2520
16th to endDay
49%5%12%22%9%1
2520
July1st to 15thDay
40%7%13%26%11%
2371
16th to endDay
40%8%17%24%8%
2487
August1st to 15thDay
44%8%16%23%8%
2485
16th to endDay
33%6%18%27%13%
2299
September1st to 15thDay
41%5%14%26%12%
2098
16th to endDay
37%5%12%31%12%
1821
October1st to 15thDay
49%7%11%22%8%
1691
16th to endDay
58%412%17%6%
1612
November1st to 15thDay
51%7%12%18%9%
1581
16th to endDay
57%6%11%16%7%
1458
December1st to 15thDay
67%5%11%12%3
1440
16th to endDay
67%5%11%9%6%
1536
January1st to 15thNight
64%5%9%14%4
2506
16th to endNight
59%5%9%16%8%
2688
February1st to 15thNight
65%49%15%4
2310
16th to endNight
59%49%15%10%
1893
March1st to 15thNight
58%48%17%9%1
2160
16th to endNight
49%38%18%15%3
1998
April1st to 15thNight
57%2419%14%1
1796
16th to endNight
50%47%18%15%4
1698
May1st to 15thNight
50%37%18%15%4
1481
16th to endNight
48%6%9%20%13%2
1497
June1st to 15thNight
54%5%8%20%9%
1440
16th to endNight
43%7%13%20%14%1
1440
July1st to 15thNight
37%7%12%26%15%
1360
16th to endNight
37%7%17%23%13%
1418
August1st to 15thNight
40%7%16%23%11%
1439
16th to endNight
35%5%15%29%13%
1902
September1st to 15thNight
43%5%11%25%13%
1750
16th to endNight
38%5%10%30%14%
1809
October1st to 15thNight
47%6%9%24%10%
1939
16th to endNight
57%411%18%7%
2260
November1st to 15thNight
54%410%18%8%2
2277
16th to endNight
59%49%18%7%
2499
December1st to 15thNight
65%411%15%3
2520
16th to endNight
67%310%12%6%
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
23%16%18%26%15%
60
16th to endDay
35%428%21%7%1
64
February1st to 15thDay
29%15%36%15%4
93
16th to endDay
27%7%30%26%7%
239
March1st to 15thDay
24%10%30%28%6%
270
16th to endDay
24%7%28%22%13%3
288
April1st to 15thDay
29%12%27%19%8%3
270
16th to endDay
18%12%25%22%15%3
270
May1st to 15thDay
26%6%27%20%13%4
288
16th to endDay
20%8%31%20%16%3
331
June1st to 15thDay
27%8%21%27%9%4
315
16th to endDay
19%8%24%29%16%2
315
July1st to 15thDay
12%6%27%28%19%6%
305
16th to endDay
14%218%34%24%5%
326
August1st to 15thDay
14%220%30%23%8%
313
16th to endDay
9%5%21%28%20%14%
332
September1st to 15thDay
17%321%26%18%12%
294
16th to endDay
16%7%28%16%18%11%
254
October1st to 15thDay
21%8%25%23%18%2
255
16th to endDay
32%9%22%24%9%
245
November1st to 15thDay
23%10%28%22%13%1
235
16th to endDay
35%8%25%19%10%
78
December1st to 15thDay
30%5%31%26%33
60
16th to endDay
37%15%32%14%
64
January1st to 15thNight
24%12%31%24%6%
720
16th to endNight
21%8%35%25%8%
768
February1st to 15thNight
22%7%33%28%7%
704
16th to endNight
24%8%32%27%6%
631
March1st to 15thNight
26%8%29%27%6%
720
16th to endNight
22%5%30%30%10%
768
April1st to 15thNight
29%8%25%30%5%
718
16th to endNight
24%10%29%25%9%
618
May1st to 15thNight
23%430%29%10%1
401
16th to endNight
22%5%28%33%10%
375
June1st to 15thNight
28%422%32%11%
360
16th to endNight
17%226%38%14%
360
July1st to 15thNight
11%424%37%21%
340
16th to endNight
11%121%46%17%1
376
August1st to 15thNight
15%213%45%21%1
368
16th to endNight
13%221%36%24%2
756
September1st to 15thNight
20%218%40%16%1
700
16th to endNight
21%5%20%31%19%1
657
October1st to 15thNight
22%6%28%29%11%1
660
16th to endNight
30%8%25%27%7%
704
November1st to 15thNight
26%7%30%24%11%
700
16th to endNight
28%10%27%20%12%
720
December1st to 15thNight
31%9%30%23%5%
720
16th to endNight
34%9%30%18%5%1
768
Page generated at 2017-09-11 19:35:19. Version ac04

Copyright 2017 A. Danko


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