Planétarium Rio Tinto Alcan de Montréal
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Planétarium Rio Tinto Alcan de Montréal clear sky chart from 2005-11-06 to 2025-01-07.

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 Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(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

By Month

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 Forecast	Data Points
Cloud Cover	Day	16% 9% 3 8% 5% 5% 5% 4 6% 7% 32%	105149
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 31%	103879
Transparency	Day	49% 7% 12% 21% 9% 1	108612
	Night	53% 6% 11% 19% 10% 1	108505
Seeing	Day	21% 13% 28% 24% 12% 3	14723
	Night	23% 9% 29% 28% 10% 1	23771

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	17% 15% 5% 12% 6% 6% 5% 4 3 5% 22%	2565
	11th to 20th	Day	16% 13% 5% 9% 5% 5% 4 4 5% 7% 25%	2520
	21st to end	Day	15% 16% 4 10% 5% 5% 6% 4 5% 6% 24%	2772
February	1st to 10th	Day	15% 16% 4 10% 7% 5% 5% 4 4 5% 26%	1657
	11th to 20th	Day	13% 12% 5% 11% 7% 5% 5% 4 6% 5% 28%	1769
	21st to end	Day	16% 18% 3 8% 6% 5% 3 3 5% 6% 27%	1570
March	1st to 10th	Day	19% 12% 4 10% 6% 5% 5% 3 5% 5% 26%	3116
	11th to 20th	Day	13% 16% 5% 11% 6% 5% 5% 3 5% 5% 26%	3274
	21st to end	Day	15% 8% 3 9% 5% 4 4 3 5% 6% 38%	4166
April	1st to 10th	Day	22% 10% 3 8% 5% 5% 4 3 4 5% 31%	2280
	11th to 20th	Day	20% 9% 3 6% 5% 4 3 3 4 6% 37%	2254
	21st to end	Day	21% 9% 3 9% 5% 4 5% 3 5% 5% 32%	2441
May	1st to 10th	Day	19% 9% 3 8% 5% 5% 5% 4 6% 7% 29%	4352
	11th to 20th	Day	17% 9% 3 8% 5% 4 5% 4 6% 7% 32%	4398
	21st to end	Day	14% 7% 2 7% 5% 4 5% 5% 6% 9% 36%	4837
June	1st to 10th	Day	20% 8% 2 6% 5% 5% 5% 4 6% 8% 30%	2660
	11th to 20th	Day	14% 6% 3 7% 5% 4 4 4 6% 8% 39%	2660
	21st to end	Day	20% 8% 3 7% 6% 5% 5% 3 5% 7% 31%	2660
July	1st to 10th	Day	11% 7% 2 7% 5% 5% 6% 4 7% 9% 37%	4440
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 4 7% 9% 38%	4440
	21st to end	Day	11% 6% 2 6% 4 4 4 4 6% 8% 45%	4814
August	1st to 10th	Day	14% 6% 3 8% 5% 5% 5% 5% 7% 9% 33%	4423
	11th to 20th	Day	13% 6% 3 8% 5% 4 5% 3 6% 9% 38%	3997
	21st to end	Day	15% 6% 2 6% 4 5% 4 4 6% 8% 39%	4180
September	1st to 10th	Day	11% 5% 3 6% 5% 4 5% 5% 6% 7% 43%	2280
	11th to 20th	Day	14% 6% 2 5% 3 3 4 3 5% 8% 48%	2194
	21st to end	Day	15% 6% 2 6% 5% 3 4 4 6% 8% 41%	2090
October	1st to 10th	Day	19% 7% 2 7% 6% 5% 4 4 6% 6% 35%	1971
	11th to 20th	Day	18% 8% 3 9% 6% 5% 5% 5% 7% 6% 29%	1851
	21st to end	Day	25% 10% 3 7% 4 4 4 3 5% 5% 31%	1881
November	1st to 10th	Day	18% 12% 3 6% 5% 4 5% 4 6% 7% 33%	1761
	11th to 20th	Day	17% 10% 3 8% 5% 5% 5% 5% 6% 5% 30%	1730
	21st to end	Day	22% 13% 3 8% 5% 4 6% 4 5% 6% 25%	1600
December	1st to 10th	Day	21% 20% 6% 11% 5% 4 4 3 4 5% 18%	2600
	11th to 20th	Day	18% 14% 4 10% 4 5% 5% 4 5% 4 26%	2986
	21st to end	Day	23% 13% 3 11% 6% 5% 6% 5% 5% 5% 18%	3960
January	1st to 10th	Night	17% 16% 5% 11% 6% 5% 5% 3 5% 5% 23%	4514
	11th to 20th	Night	12% 13% 5% 12% 7% 7% 6% 4 5% 6% 22%	4440
	21st to end	Night	17% 17% 5% 10% 7% 4 4 3 4 4 24%	4884
February	1st to 10th	Night	14% 16% 5% 13% 6% 5% 5% 4 5% 6% 21%	2519
	11th to 20th	Night	15% 11% 5% 10% 5% 6% 5% 5% 5% 5% 30%	2297
	21st to end	Night	16% 14% 4 10% 4 5% 4 3 4 5% 32%	1884
March	1st to 10th	Night	21% 11% 3 7% 5% 4 4 3 4 5% 33%	3724
	11th to 20th	Night	17% 19% 4 9% 5% 4 4 2 3 5% 27%	3668
	21st to end	Night	12% 11% 4 9% 5% 5% 4 3 4 5% 38%	3466
April	1st to 10th	Night	23% 10% 3 7% 5% 4 4 4 4 6% 30%	1900
	11th to 20th	Night	23% 9% 3 6% 4 3 4 3 4 6% 34%	1875
	21st to end	Night	21% 9% 3 7% 4 4 4 3 5% 4 35%	1739
May	1st to 10th	Night	22% 9% 3 7% 5% 5% 5% 3 5% 6% 31%	2618
	11th to 20th	Night	23% 8% 3 7% 4 4 4 3 5% 6% 34%	2495
	21st to end	Night	15% 9% 2 7% 5% 4 5% 4 5% 7% 37%	2738
June	1st to 10th	Night	22% 7% 3 7% 4 4 4 5% 5% 7% 32%	1520
	11th to 20th	Night	16% 6% 2 6% 4 5% 4 4 6% 8% 39%	1520
	21st to end	Night	20% 6% 2 8% 5% 5% 5% 4 5% 8% 32%	1520
July	1st to 10th	Night	14% 7% 4 8% 5% 5% 5% 4 6% 7% 34%	2520
	11th to 20th	Night	14% 5% 2 6% 5% 4 5% 4 6% 9% 40%	2520
	21st to end	Night	11% 6% 2 6% 5% 4 5% 4 7% 8% 42%	2739
August	1st to 10th	Night	16% 8% 3 7% 5% 5% 5% 3 5% 7% 36%	2506
	11th to 20th	Night	15% 8% 3 8% 4 4 5% 4 6% 6% 38%	2783
	21st to end	Night	16% 6% 2 7% 5% 4 5% 4 6% 6% 39%	3476
September	1st to 10th	Night	12% 5% 2 7% 5% 4 4 3 5% 7% 44%	1900
	11th to 20th	Night	15% 5% 2 5% 4 3 2 3 4 6% 51%	1985
	21st to end	Night	17% 6% 2 6% 4 4 3 3 5% 7% 42%	2090
October	1st to 10th	Night	18% 6% 3 7% 4 5% 4 3 5% 7% 38%	2205
	11th to 20th	Night	21% 8% 3 7% 4 4 4 3 4 6% 34%	2328
	21st to end	Night	23% 9% 3 9% 5% 4 4 3 4 5% 31%	2717
November	1st to 10th	Night	19% 10% 3 7% 5% 5% 4 3 4 6% 32%	2537
	11th to 20th	Night	19% 10% 3 8% 5% 5% 3 3 5% 6% 32%	2668
	21st to end	Night	22% 14% 4 8% 5% 5% 4 3 4 5% 26%	2800
December	1st to 10th	Night	18% 19% 6% 12% 6% 5% 4 3 5% 4 18%	4580
	11th to 20th	Night	23% 14% 4 9% 5% 4 4 3 6% 5% 24%	5208
	21st to end	Night	25% 15% 5% 8% 4 3 4 4 6% 5% 21%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	64% 8% 11% 13% 4	2566
	11th to 20th	Day	57% 8% 12% 16% 7%	2520
	21st to end	Day	59% 8% 13% 13% 5%	2772
February	1st to 10th	Day	61% 5% 9% 18% 6% 1	1657
	11th to 20th	Day	57% 5% 11% 17% 8% 1	1778
	21st to end	Day	59% 4 11% 15% 10% 1	1570
March	1st to 10th	Day	59% 6% 8% 18% 7% 1	3119
	11th to 20th	Day	57% 7% 11% 12% 10% 2	3274
	21st to end	Day	46% 6% 9% 20% 17% 2	4166
April	1st to 10th	Day	56% 5% 7% 19% 11% 2	2280
	11th to 20th	Day	49% 5% 8% 23% 14% 2	2254
	21st to end	Day	53% 5% 9% 18% 12% 3	2441
May	1st to 10th	Day	51% 8% 8% 22% 9% 1	4366
	11th to 20th	Day	48% 9% 14% 18% 9% 2	4402
	21st to end	Day	41% 9% 12% 24% 13% 2	4837
June	1st to 10th	Day	50% 7% 11% 19% 11% 2	2660
	11th to 20th	Day	42% 6% 11% 24% 14% 3	2660
	21st to end	Day	51% 6% 13% 19% 10% 1	2660
July	1st to 10th	Day	41% 8% 13% 28% 10% 1	4370
	11th to 20th	Day	39% 8% 15% 31% 7%	4365
	21st to end	Day	36% 8% 13% 33% 9%	4661
August	1st to 10th	Day	45% 8% 16% 23% 8%	4409
	11th to 20th	Day	42% 7% 18% 26% 7%	4039
	21st to end	Day	40% 7% 17% 21% 15%	4180
September	1st to 10th	Day	37% 8% 14% 29% 12%	2280
	11th to 20th	Day	35% 6% 13% 32% 14% 1	2194
	21st to end	Day	41% 7% 14% 27% 11% 1	2090
October	1st to 10th	Day	44% 8% 13% 27% 8%	3270
	11th to 20th	Day	43% 7% 18% 26% 6%	3054
	21st to end	Day	44% 6% 18% 23% 8% 1	3081
November	1st to 10th	Day	49% 7% 13% 20% 9% 1	1761
	11th to 20th	Day	53% 8% 10% 17% 12%	1730
	21st to end	Day	60% 8% 13% 14% 5%	1600
December	1st to 10th	Day	70% 5% 7% 14% 3	2600
	11th to 20th	Day	58% 7% 9% 17% 9%	2986
	21st to end	Day	66% 7% 11% 11% 6%	3960
January	1st to 10th	Night	63% 7% 13% 12% 4	4514
	11th to 20th	Night	59% 9% 12% 15% 5% 1	4440
	21st to end	Night	63% 6% 11% 13% 6%	4884
February	1st to 10th	Night	64% 5% 9% 15% 6% 1	2519
	11th to 20th	Night	56% 6% 10% 19% 9% 1	2299
	21st to end	Night	55% 5% 9% 17% 13% 2	1884
March	1st to 10th	Night	53% 5% 8% 20% 9% 3	3734
	11th to 20th	Night	60% 5% 7% 13% 13% 2	3669
	21st to end	Night	48% 5% 5% 23% 15% 3	3466
April	1st to 10th	Night	55% 4 7% 18% 14% 2	1900
	11th to 20th	Night	52% 4 6% 21% 14% 2	1875
	21st to end	Night	51% 5% 7% 19% 13% 4	1739
May	1st to 10th	Night	53% 7% 10% 19% 8% 3	2624
	11th to 20th	Night	48% 8% 10% 20% 11% 3	2496
	21st to end	Night	45% 7% 9% 20% 18% 1	2739
June	1st to 10th	Night	50% 6% 8% 21% 12% 1	1520
	11th to 20th	Night	43% 7% 11% 22% 14% 3	1520
	21st to end	Night	49% 7% 11% 19% 13% 1	1520
July	1st to 10th	Night	46% 8% 13% 20% 13% 1	2480
	11th to 20th	Night	39% 8% 17% 27% 10%	2472
	21st to end	Night	38% 9% 16% 24% 13%	2651
August	1st to 10th	Night	47% 6% 12% 26% 9%	2496
	11th to 20th	Night	44% 8% 18% 22% 8%	2807
	21st to end	Night	43% 7% 12% 23% 14% 1	3476
September	1st to 10th	Night	38% 7% 11% 27% 16% 1	1900
	11th to 20th	Night	35% 6% 13% 32% 14% 1	1985
	21st to end	Night	41% 7% 10% 26% 15%	2090
October	1st to 10th	Night	44% 6% 14% 26% 10%	3686
	11th to 20th	Night	45% 6% 10% 31% 9%	3864
	21st to end	Night	48% 6% 14% 18% 12% 2	4467
November	1st to 10th	Night	53% 5% 11% 21% 9% 1	2537
	11th to 20th	Night	53% 5% 10% 19% 11% 2	2668
	21st to end	Night	61% 6% 10% 17% 6%	2800
December	1st to 10th	Night	70% 6% 10% 11% 3	4580
	11th to 20th	Night	61% 6% 8% 15% 11%	5208
	21st to end	Night	64% 4 9% 14% 9%	6996

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	27% 17% 28% 20% 9%	192
	11th to 20th	Day	25% 15% 34% 19% 6% 1	180
	21st to end	Day	22% 14% 34% 20% 9% 1	198
February	1st to 10th	Day	30% 18% 36% 16% 1	245
	11th to 20th	Day	21% 18% 37% 19% 3 1	377
	21st to end	Day	26% 15% 31% 24% 4	382
March	1st to 10th	Day	25% 13% 34% 22% 7%	460
	11th to 20th	Day	21% 18% 32% 22% 6% 1	460
	21st to end	Day	26% 16% 29% 19% 8% 2	506
April	1st to 10th	Day	29% 15% 28% 19% 8% 2	460
	11th to 20th	Day	28% 21% 22% 16% 10% 3	452
	21st to end	Day	26% 15% 29% 19% 10% 2	460
May	1st to 10th	Day	23% 11% 31% 22% 11% 2	463
	11th to 20th	Day	16% 11% 31% 26% 12% 3	487
	21st to end	Day	18% 11% 33% 24% 11% 2	529
June	1st to 10th	Day	21% 14% 25% 28% 10% 3	490
	11th to 20th	Day	15% 12% 27% 31% 13% 3	490
	21st to end	Day	22% 11% 26% 25% 14% 2	490
July	1st to 10th	Day	12% 9% 26% 29% 19% 5%	485
	11th to 20th	Day	14% 7% 29% 30% 16% 4	484
	21st to end	Day	13% 7% 25% 32% 19% 4	532
August	1st to 10th	Day	15% 6% 24% 30% 21% 5%	486
	11th to 20th	Day	12% 6% 22% 32% 19% 9%	479
	21st to end	Day	13% 10% 24% 26% 17% 10%	539
September	1st to 10th	Day	12% 10% 27% 28% 17% 8%	490
	11th to 20th	Day	15% 9% 19% 27% 19% 12%	490
	21st to end	Day	17% 8% 28% 24% 17% 6%	490
October	1st to 10th	Day	20% 11% 29% 22% 14% 3	490
	11th to 20th	Day	21% 17% 29% 20% 11% 2	463
	21st to end	Day	32% 14% 24% 23% 6%	429
November	1st to 10th	Day	26% 16% 26% 24% 7% 1	395
	11th to 20th	Day	24% 19% 33% 16% 8% 1	330
	21st to end	Day	30% 16% 31% 18% 6% 1	200
December	1st to 10th	Day	33% 14% 21% 24% 7% 2	200
	11th to 20th	Day	26% 23% 35% 13% 4 1	200
	21st to end	Day	29% 19% 26% 19% 5% 3	220
January	1st to 10th	Night	26% 13% 32% 23% 5%	785
	11th to 20th	Night	25% 11% 35% 23% 6%	760
	21st to end	Night	21% 10% 35% 26% 8% 1	836
February	1st to 10th	Night	23% 10% 35% 27% 6% 1	760
	11th to 20th	Night	21% 11% 38% 24% 5%	743
	21st to end	Night	25% 11% 32% 28% 4	628
March	1st to 10th	Night	25% 11% 32% 26% 6%	760
	11th to 20th	Night	26% 11% 32% 24% 7%	760
	21st to end	Night	22% 10% 32% 27% 8%	836
April	1st to 10th	Night	28% 11% 28% 28% 6%	760
	11th to 20th	Night	27% 13% 29% 25% 6% 1	751
	21st to end	Night	25% 12% 30% 25% 8%	599
May	1st to 10th	Night	23% 8% 32% 27% 8% 2	445
	11th to 20th	Night	23% 5% 32% 29% 9% 1	374
	21st to end	Night	17% 11% 31% 32% 9%	411
June	1st to 10th	Night	24% 6% 26% 32% 11%	380
	11th to 20th	Night	18% 5% 29% 34% 13% 1	380
	21st to end	Night	21% 4 27% 33% 14% 1	380
July	1st to 10th	Night	13% 7% 26% 36% 17%	370
	11th to 20th	Night	11% 3 26% 44% 15% 1	368
	21st to end	Night	12% 4 23% 44% 16% 1	416
August	1st to 10th	Night	16% 3 15% 47% 18% 1	378
	11th to 20th	Night	13% 5% 20% 36% 24% 2	559
	21st to end	Night	15% 3 23% 36% 20% 2	836
September	1st to 10th	Night	15% 5% 24% 39% 16% 1	760
	11th to 20th	Night	17% 6% 18% 35% 21% 3	760
	21st to end	Night	19% 7% 26% 30% 16% 2	760
October	1st to 10th	Night	20% 9% 28% 29% 13% 1	760
	11th to 20th	Night	25% 9% 27% 29% 9%	760
	21st to end	Night	27% 11% 29% 28% 5%	836
November	1st to 10th	Night	26% 12% 30% 24% 8%	780
	11th to 20th	Night	23% 13% 31% 22% 9% 1	800
	21st to end	Night	30% 15% 28% 20% 8%	800
December	1st to 10th	Night	30% 11% 30% 23% 5%	800
	11th to 20th	Night	31% 12% 32% 18% 6% 1	800
	21st to end	Night	33% 12% 30% 20% 5%	880

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