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 2026-05-02.

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% 10% 3 8% 5% 5% 5% 4 5% 7% 32%	131751
	Night	17% 11% 4 9% 5% 5% 4 3 5% 6% 30%	130736
Transparency	Day	50% 7% 12% 21% 10% 1	135196
	Night	54% 6% 10% 19% 10% 1	135378
Seeing	Day	21% 13% 29% 23% 11% 3	16378
	Night	23% 10% 29% 28% 9% 1	25461

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	15% 17% 6% 14% 6% 5% 4 3 3 5% 23%	4661
	11th to 20th	Day	13% 15% 6% 12% 7% 6% 4 4 5% 6% 22%	4650
	21st to end	Day	10% 13% 4 11% 8% 8% 7% 4 5% 5% 24%	5124
February	1st to 10th	Day	14% 15% 4 10% 7% 5% 5% 4 4 6% 27%	1832
	11th to 20th	Day	13% 12% 6% 11% 7% 5% 5% 4 5% 6% 27%	1961
	21st to end	Day	16% 18% 3 9% 6% 5% 4 3 5% 6% 26%	1730
March	1st to 10th	Day	17% 12% 4 11% 6% 5% 5% 3 4 5% 26%	4576
	11th to 20th	Day	12% 13% 4 10% 5% 5% 5% 4 6% 6% 30%	4779
	21st to end	Day	18% 10% 4 10% 5% 4 4 3 5% 5% 33%	6102
April	1st to 10th	Day	22% 10% 3 8% 5% 5% 4 3 5% 5% 31%	2520
	11th to 20th	Day	20% 9% 3 7% 5% 4 3 3 4 5% 36%	2494
	21st to end	Day	21% 9% 3 8% 4 4 5% 3 5% 6% 33%	2698
May	1st to 10th	Day	24% 10% 4 10% 5% 5% 5% 4 5% 7% 22%	6257
	11th to 20th	Day	20% 8% 3 8% 5% 4 5% 4 6% 7% 30%	6318
	21st to end	Day	21% 9% 3 7% 4 4 4 5% 6% 8% 31%	6949
June	1st to 10th	Day	20% 8% 3 6% 5% 5% 5% 4 6% 7% 31%	2800
	11th to 20th	Day	14% 6% 3 6% 5% 4 4 4 6% 8% 39%	2800
	21st to end	Day	19% 8% 3 7% 6% 5% 5% 3 6% 7% 31%	2800
July	1st to 10th	Day	12% 7% 3 7% 6% 6% 6% 4 7% 9% 34%	6360
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 8% 41%	6360
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 9% 46%	6926
August	1st to 10th	Day	13% 6% 3 8% 5% 5% 6% 4 7% 9% 34%	4563
	11th to 20th	Day	13% 6% 3 8% 5% 4 5% 3 6% 9% 39%	4126
	21st to end	Day	15% 5% 2 6% 4 5% 4 4 6% 8% 39%	4312
September	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 5% 7% 8% 44%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 8% 49%	2309
	21st to end	Day	16% 6% 2 6% 5% 3 4 4 6% 8% 41%	2200
October	1st to 10th	Day	18% 7% 2 6% 6% 5% 4 4 6% 6% 37%	2075
	11th to 20th	Day	17% 8% 3 9% 6% 5% 5% 5% 6% 7% 30%	1948
	21st to end	Day	24% 10% 3 7% 4 4 4 3 5% 5% 31%	1980
November	1st to 10th	Day	17% 12% 2 7% 5% 4 5% 4 6% 7% 31%	1851
	11th to 20th	Day	17% 11% 3 8% 5% 5% 5% 5% 6% 5% 30%	1816
	21st to end	Day	23% 13% 3 8% 5% 4 5% 3 4 6% 25%	1680
December	1st to 10th	Day	20% 20% 6% 11% 5% 4 4 3 4 5% 18%	2680
	11th to 20th	Day	18% 14% 4 10% 4 5% 5% 4 5% 4 26%	3066
	21st to end	Day	23% 13% 3 11% 6% 5% 6% 5% 5% 5% 18%	4048
January	1st to 10th	Night	13% 19% 7% 15% 6% 4 4 3 4 4 21%	8234
	11th to 20th	Night	12% 16% 7% 17% 7% 6% 5% 4 5% 5% 17%	8235
	21st to end	Night	11% 13% 5% 12% 9% 7% 5% 4 4 5% 25%	9057
February	1st to 10th	Night	13% 15% 5% 13% 6% 5% 5% 4 5% 6% 23%	2783
	11th to 20th	Night	14% 12% 5% 10% 5% 5% 5% 5% 5% 5% 29%	2545
	21st to end	Night	16% 14% 4 9% 5% 5% 4 3 4 5% 32%	2076
March	1st to 10th	Night	21% 13% 3 8% 5% 5% 4 3 4 4 30%	5484
	11th to 20th	Night	14% 14% 3 8% 5% 5% 5% 3 5% 7% 30%	5383
	21st to end	Night	15% 12% 4 9% 6% 6% 4 4 5% 4 32%	5072
April	1st to 10th	Night	23% 10% 3 7% 5% 4 4 4 4 6% 30%	2100
	11th to 20th	Night	23% 9% 3 7% 5% 3 4 3 4 6% 33%	2075
	21st to end	Night	21% 8% 3 7% 5% 4 4 3 4 5% 35%	1921
May	1st to 10th	Night	25% 10% 4 9% 5% 4 5% 3 5% 6% 25%	3750
	11th to 20th	Night	23% 8% 3 8% 5% 5% 4 3 5% 6% 30%	3575
	21st to end	Night	18% 10% 3 8% 5% 5% 5% 4 5% 6% 32%	3926
June	1st to 10th	Night	22% 7% 3 7% 4 4 4 5% 5% 7% 32%	1600
	11th to 20th	Night	16% 6% 2 7% 5% 5% 4 4 6% 8% 39%	1600
	21st to end	Night	19% 6% 2 8% 5% 5% 5% 4 5% 8% 33%	1600
July	1st to 10th	Night	13% 6% 4 8% 5% 5% 5% 5% 7% 9% 34%	3600
	11th to 20th	Night	14% 6% 2 6% 5% 4 5% 4 6% 8% 40%	3600
	21st to end	Night	9% 5% 2 7% 5% 4 5% 4 7% 8% 43%	3926
August	1st to 10th	Night	15% 8% 3 7% 5% 5% 5% 3 5% 7% 37%	2586
	11th to 20th	Night	14% 7% 3 8% 4 4 5% 4 6% 6% 39%	2873
	21st to end	Night	16% 6% 2 7% 5% 4 5% 4 6% 7% 39%	3586
September	1st to 10th	Night	12% 5% 2 7% 5% 4 4 3 5% 7% 45%	2000
	11th to 20th	Night	15% 5% 2 5% 4 3 2 3 4 6% 51%	2090
	21st to end	Night	17% 6% 2 6% 4 4 3 3 5% 7% 42%	2200
October	1st to 10th	Night	17% 6% 3 6% 4 4 4 3 5% 7% 40%	2321
	11th to 20th	Night	20% 7% 3 7% 4 4 4 3 5% 6% 35%	2451
	21st to end	Night	23% 9% 3 9% 5% 4 4 4 4 5% 31%	2860
November	1st to 10th	Night	20% 10% 3 7% 5% 5% 4 3 4 6% 32%	2667
	11th to 20th	Night	19% 11% 3 8% 5% 5% 3 3 5% 6% 32%	2802
	21st to end	Night	22% 14% 4 8% 5% 5% 4 3 4 5% 26%	2940
December	1st to 10th	Night	17% 19% 6% 12% 6% 5% 4 3 5% 4 18%	4720
	11th to 20th	Night	22% 14% 4 9% 5% 4 4 3 6% 5% 24%	5348
	21st to end	Night	25% 15% 5% 8% 4 3 4 4 6% 5% 21%	7150

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	65% 6% 10% 16% 3	4662
	11th to 20th	Day	62% 7% 8% 13% 9% 1	4680
	21st to end	Day	59% 7% 9% 13% 11%	5124
February	1st to 10th	Day	59% 5% 9% 19% 6% 2	1832
	11th to 20th	Day	57% 5% 11% 18% 8% 1	1970
	21st to end	Day	59% 4 10% 16% 10% 1	1730
March	1st to 10th	Day	59% 6% 8% 15% 10% 3	4579
	11th to 20th	Day	51% 7% 16% 14% 10% 2	4779
	21st to end	Day	53% 5% 7% 17% 17% 1	6102
April	1st to 10th	Day	55% 5% 8% 19% 12% 2	2520
	11th to 20th	Day	50% 4 8% 22% 14% 1	2494
	21st to end	Day	52% 5% 9% 19% 12% 3	2698
May	1st to 10th	Day	60% 7% 7% 18% 7% 1	6268
	11th to 20th	Day	50% 8% 13% 18% 9% 2	6322
	21st to end	Day	49% 7% 11% 20% 12% 1	6949
June	1st to 10th	Day	50% 7% 11% 19% 12% 2	2800
	11th to 20th	Day	42% 6% 11% 25% 14% 3	2800
	21st to end	Day	51% 6% 12% 20% 10% 1	2800
July	1st to 10th	Day	45% 8% 12% 26% 8%	6290
	11th to 20th	Day	38% 7% 13% 30% 12%	6285
	21st to end	Day	34% 7% 13% 37% 9%	6728
August	1st to 10th	Day	44% 8% 16% 24% 8%	4549
	11th to 20th	Day	42% 7% 17% 27% 7%	4168
	21st to end	Day	40% 7% 16% 21% 14%	4312
September	1st to 10th	Day	36% 8% 14% 31% 12%	2400
	11th to 20th	Day	34% 6% 13% 33% 15% 1	2309
	21st to end	Day	41% 7% 13% 27% 12% 1	2200
October	1st to 10th	Day	43% 8% 13% 27% 9%	3374
	11th to 20th	Day	43% 7% 17% 26% 6%	3151
	21st to end	Day	45% 6% 18% 23% 8% 1	3180
November	1st to 10th	Day	51% 7% 12% 20% 9% 1	1851
	11th to 20th	Day	54% 7% 10% 17% 11%	1816
	21st to end	Day	60% 8% 12% 14% 5%	1680
December	1st to 10th	Day	70% 5% 7% 15% 3	2680
	11th to 20th	Day	58% 7% 9% 17% 9%	3066
	21st to end	Day	65% 7% 10% 12% 6%	4048
January	1st to 10th	Night	66% 5% 10% 13% 5%	8234
	11th to 20th	Night	67% 7% 8% 12% 6%	8280
	21st to end	Night	60% 5% 8% 14% 12%	9057
February	1st to 10th	Night	62% 5% 9% 15% 7% 1	2783
	11th to 20th	Night	56% 5% 10% 19% 9% 1	2547
	21st to end	Night	55% 4 9% 18% 12% 1	2076
March	1st to 10th	Night	57% 4 7% 16% 12% 4	5494
	11th to 20th	Night	53% 6% 12% 16% 12% 1	5384
	21st to end	Night	54% 5% 6% 21% 12% 2	5072
April	1st to 10th	Night	55% 4 7% 19% 14% 2	2100
	11th to 20th	Night	53% 5% 6% 20% 14% 2	2075
	21st to end	Night	51% 5% 7% 19% 14% 4	1921
May	1st to 10th	Night	60% 6% 9% 17% 6% 2	3756
	11th to 20th	Night	53% 7% 9% 20% 9% 2	3576
	21st to end	Night	52% 7% 9% 17% 15% 1	3927
June	1st to 10th	Night	51% 6% 8% 21% 13% 1	1600
	11th to 20th	Night	43% 7% 11% 22% 14% 3	1600
	21st to end	Night	49% 7% 11% 19% 13% 1	1600
July	1st to 10th	Night	44% 8% 17% 21% 10%	3560
	11th to 20th	Night	39% 8% 15% 27% 11%	3552
	21st to end	Night	37% 8% 15% 28% 13%	3809
August	1st to 10th	Night	46% 6% 12% 27% 9%	2576
	11th to 20th	Night	43% 8% 17% 22% 9%	2897
	21st to end	Night	43% 7% 12% 23% 14% 1	3586
September	1st to 10th	Night	38% 7% 11% 27% 16% 1	2000
	11th to 20th	Night	35% 6% 13% 32% 14% 1	2090
	21st to end	Night	42% 7% 10% 26% 16%	2200
October	1st to 10th	Night	43% 6% 13% 26% 10% 1	3802
	11th to 20th	Night	44% 6% 11% 31% 9%	3987
	21st to end	Night	48% 6% 14% 19% 12% 2	4610
November	1st to 10th	Night	53% 5% 11% 22% 8% 1	2667
	11th to 20th	Night	53% 5% 10% 19% 10% 1	2802
	21st to end	Night	61% 6% 10% 17% 6%	2940
December	1st to 10th	Night	70% 6% 10% 11% 3	4720
	11th to 20th	Night	61% 6% 8% 14% 11%	5348
	21st to end	Night	64% 4 9% 14% 9%	7150

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	26% 17% 28% 20% 9%	220
	11th to 20th	Day	27% 17% 33% 17% 5% 1	220
	21st to end	Day	21% 17% 33% 20% 8%	242
February	1st to 10th	Day	26% 19% 36% 16% 2	300
	11th to 20th	Day	21% 18% 36% 21% 3	449
	21st to end	Day	25% 17% 33% 23% 3	446
March	1st to 10th	Day	24% 15% 35% 19% 6%	540
	11th to 20th	Day	24% 18% 31% 21% 6% 1	540
	21st to end	Day	27% 17% 30% 18% 7% 2	594
April	1st to 10th	Day	30% 16% 28% 17% 7% 1	540
	11th to 20th	Day	29% 21% 22% 16% 9% 2	532
	21st to end	Day	26% 15% 31% 18% 9% 1	540
May	1st to 10th	Day	23% 13% 31% 21% 10% 2	507
	11th to 20th	Day	18% 10% 31% 26% 12% 3	527
	21st to end	Day	20% 11% 32% 23% 11% 2	573
June	1st to 10th	Day	21% 15% 26% 27% 9% 2	530
	11th to 20th	Day	16% 12% 27% 30% 12% 3	530
	21st to end	Day	21% 11% 28% 25% 13% 2	530
July	1st to 10th	Day	11% 10% 28% 29% 18% 5%	525
	11th to 20th	Day	13% 7% 30% 30% 15% 4	524
	21st to end	Day	13% 7% 27% 31% 18% 3	576
August	1st to 10th	Day	13% 5% 25% 30% 21% 5%	526
	11th to 20th	Day	12% 6% 22% 33% 19% 9%	519
	21st to end	Day	13% 10% 24% 25% 18% 9%	583
September	1st to 10th	Day	12% 10% 26% 26% 18% 8%	530
	11th to 20th	Day	14% 8% 18% 28% 20% 12%	530
	21st to end	Day	17% 9% 28% 24% 16% 6%	530
October	1st to 10th	Day	19% 11% 30% 22% 15% 4	530
	11th to 20th	Day	20% 17% 30% 21% 11% 2	500
	21st to end	Day	32% 14% 24% 23% 6%	462
November	1st to 10th	Day	27% 17% 26% 23% 7%	425
	11th to 20th	Day	24% 19% 33% 16% 8% 1	356
	21st to end	Day	31% 15% 31% 16% 5%	220
December	1st to 10th	Day	32% 14% 23% 22% 7% 2	220
	11th to 20th	Day	26% 20% 38% 12% 3	220
	21st to end	Day	28% 19% 29% 19% 4 2	242
January	1st to 10th	Night	26% 13% 33% 23% 5%	840
	11th to 20th	Night	25% 12% 34% 23% 6%	840
	21st to end	Night	20% 11% 35% 26% 7% 1	924
February	1st to 10th	Night	21% 10% 34% 28% 6% 1	840
	11th to 20th	Night	22% 12% 38% 24% 5%	823
	21st to end	Night	25% 12% 33% 27% 4	692
March	1st to 10th	Night	25% 12% 32% 25% 6%	840
	11th to 20th	Night	26% 13% 32% 23% 7%	840
	21st to end	Night	24% 12% 31% 26% 7%	924
April	1st to 10th	Night	28% 12% 28% 27% 5%	840
	11th to 20th	Night	28% 13% 29% 25% 5%	831
	21st to end	Night	25% 12% 30% 25% 8%	661
May	1st to 10th	Night	24% 9% 32% 26% 8% 1	472
	11th to 20th	Night	23% 6% 33% 28% 9% 1	394
	21st to end	Night	18% 12% 30% 31% 9%	433
June	1st to 10th	Night	25% 6% 27% 32% 10%	400
	11th to 20th	Night	18% 6% 29% 34% 12% 1	400
	21st to end	Night	20% 5% 27% 34% 14% 1	400
July	1st to 10th	Night	13% 7% 26% 36% 17%	390
	11th to 20th	Night	11% 3 26% 43% 15% 1	388
	21st to end	Night	12% 5% 23% 44% 16% 1	438
August	1st to 10th	Night	15% 3 16% 46% 18% 1	398
	11th to 20th	Night	12% 5% 20% 36% 24% 2	589
	21st to end	Night	15% 4 23% 36% 20% 2	880
September	1st to 10th	Night	15% 5% 25% 39% 17% 1	800
	11th to 20th	Night	16% 6% 18% 35% 22% 3	800
	21st to end	Night	19% 8% 26% 30% 15% 2	800
October	1st to 10th	Night	19% 9% 28% 29% 14% 1	800
	11th to 20th	Night	24% 9% 28% 30% 9%	800
	21st to end	Night	27% 11% 29% 28% 5%	880
November	1st to 10th	Night	26% 12% 30% 24% 8%	820
	11th to 20th	Night	24% 13% 31% 22% 9% 1	840
	21st to end	Night	30% 15% 28% 20% 8%	840
December	1st to 10th	Night	30% 12% 30% 23% 5%	840
	11th to 20th	Night	31% 13% 33% 18% 6% 1	840
	21st to end	Night	34% 12% 30% 19% 5%	924

Page generated at 2026-04-29 02:21:41. Version ac07

Copyright © 2026 Attilla Danko.