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 2024-01-22.

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	17% 9% 3 7% 6% 3 5% 5% 5% 7% 33%	74836
	Night	19% 10% 4 7% 6% 3 4 4 5% 6% 32%	75098
Transparency	Day	50% 6% 12% 20% 10% 1	74541
	Night	53% 6% 10% 20% 10% 1	74927
Seeing	Day	21% 12% 28% 24% 12% 3	13489
	Night	23% 9% 29% 28% 10% 1	22590

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	19% 16% 6% 11% 8% 4 5% 4 2 4 21%	1517
	11th to 20th	Day	20% 13% 6% 9% 7% 4 4 4 4 5% 24%	1520
	21st to end	Day	12% 14% 3 7% 7% 4 7% 5% 5% 7% 28%	1592
February	1st to 10th	Day	16% 15% 5% 9% 7% 4 5% 4 4 5% 25%	1570
	11th to 20th	Day	14% 11% 5% 9% 7% 4 5% 5% 5% 6% 28%	1674
	21st to end	Day	17% 16% 5% 7% 7% 3 4 4 5% 5% 27%	1480
March	1st to 10th	Day	15% 11% 5% 9% 8% 3 6% 4 5% 5% 29%	1797
	11th to 20th	Day	16% 12% 4 7% 6% 3 5% 4 5% 5% 32%	1858
	21st to end	Day	19% 10% 4 8% 5% 2 4 4 4 5% 35%	2376
April	1st to 10th	Day	22% 9% 3 7% 6% 4 4 3 4 5% 31%	2160
	11th to 20th	Day	20% 8% 4 6% 5% 3 3 4 4 6% 38%	2134
	21st to end	Day	21% 9% 4 8% 5% 3 5% 4 4 5% 32%	2312
May	1st to 10th	Day	20% 7% 3 6% 5% 3 4 5% 5% 7% 34%	2474
	11th to 20th	Day	18% 7% 3 6% 5% 3 4 5% 5% 7% 37%	2478
	21st to end	Day	17% 6% 3 6% 5% 3 4 6% 5% 8% 36%	2725
June	1st to 10th	Day	21% 7% 3 5% 5% 4 6% 5% 6% 8% 30%	2520
	11th to 20th	Day	15% 6% 3 5% 6% 3 4 5% 6% 8% 39%	2520
	21st to end	Day	19% 7% 3 6% 6% 4 6% 4 5% 8% 31%	2520
July	1st to 10th	Day	12% 6% 3 5% 5% 4 5% 5% 7% 8% 41%	2520
	11th to 20th	Day	12% 5% 3 4 6% 4 5% 6% 8% 10% 38%	2520
	21st to end	Day	12% 5% 2 6% 5% 4 5% 7% 6% 10% 38%	2744
August	1st to 10th	Day	12% 6% 3 6% 6% 4 5% 6% 6% 9% 38%	2506
	11th to 20th	Day	9% 5% 2 7% 5% 3 6% 5% 6% 9% 42%	2287
	21st to end	Day	12% 6% 2 4 5% 3 5% 5% 6% 9% 42%	2376
September	1st to 10th	Day	12% 5% 3 5% 6% 3 4 6% 7% 7% 42%	2160
	11th to 20th	Day	15% 6% 2 4 4 2 3 4 5% 8% 48%	2079
	21st to end	Day	15% 6% 2 5% 6% 2 4 4 6% 8% 41%	1980
October	1st to 10th	Day	20% 6% 3 6% 7% 4 4 5% 6% 6% 35%	1868
	11th to 20th	Day	18% 8% 3 8% 7% 4 5% 6% 7% 7% 28%	1754
	21st to end	Day	26% 9% 3 6% 5% 3 4 4 5% 5% 30%	1782
November	1st to 10th	Day	18% 10% 4 5% 5% 3 5% 5% 5% 7% 33%	1671
	11th to 20th	Day	18% 10% 4 7% 6% 3 6% 6% 6% 6% 29%	1644
	21st to end	Day	22% 11% 3 7% 7% 3 7% 4 4 6% 26%	1520
December	1st to 10th	Day	25% 14% 6% 9% 6% 3 3 4 4 6% 19%	1520
	11th to 20th	Day	22% 14% 5% 7% 4 2 4 5% 4 6% 26%	1906
	21st to end	Day	27% 15% 4 10% 7% 3 5% 3 4 4 19%	2772
January	1st to 10th	Night	18% 17% 5% 10% 7% 4 5% 4 4 4 21%	2649
	11th to 20th	Night	17% 15% 6% 10% 7% 4 5% 5% 5% 5% 23%	2660
	21st to end	Night	14% 11% 6% 8% 8% 4 5% 5% 4 5% 30%	2798
February	1st to 10th	Night	15% 15% 6% 12% 7% 4 5% 4 5% 6% 21%	2386
	11th to 20th	Night	16% 10% 5% 9% 5% 4 5% 5% 5% 5% 30%	2172
	21st to end	Night	17% 13% 5% 9% 5% 3 5% 3 4 5% 32%	1776
March	1st to 10th	Night	18% 14% 4 7% 6% 3 5% 4 4 6% 29%	2150
	11th to 20th	Night	18% 13% 4 6% 6% 3 5% 4 4 6% 31%	2098
	21st to end	Night	17% 10% 4 6% 5% 3 4 4 4 4 38%	1980
April	1st to 10th	Night	24% 9% 4 6% 5% 3 4 5% 4 6% 30%	1800
	11th to 20th	Night	23% 7% 3 6% 4 2 4 4 4 7% 35%	1775
	21st to end	Night	22% 9% 3 7% 5% 2 5% 4 4 4 34%	1648
May	1st to 10th	Night	21% 7% 4 6% 5% 4 4 4 4 6% 35%	1502
	11th to 20th	Night	19% 7% 3 6% 5% 3 5% 4 6% 6% 37%	1415
	21st to end	Night	16% 7% 3 5% 5% 3 5% 5% 5% 6% 40%	1550
June	1st to 10th	Night	23% 7% 3 5% 5% 3 3 6% 5% 7% 32%	1440
	11th to 20th	Night	16% 6% 3 6% 5% 4 5% 5% 6% 8% 39%	1440
	21st to end	Night	18% 5% 2 6% 7% 4 5% 5% 5% 9% 33%	1440
July	1st to 10th	Night	11% 5% 3 6% 6% 4 4 6% 6% 8% 42%	1440
	11th to 20th	Night	10% 5% 2 6% 5% 4 6% 6% 7% 10% 39%	1440
	21st to end	Night	12% 4 3 5% 6% 3 5% 4 8% 10% 40%	1574
August	1st to 10th	Night	13% 5% 2 5% 5% 3 6% 5% 6% 9% 42%	1432
	11th to 20th	Night	13% 5% 3 5% 5% 3 5% 5% 6% 7% 42%	1593
	21st to end	Night	13% 6% 2 5% 6% 3 4 4 6% 8% 42%	1980
September	1st to 10th	Night	13% 5% 3 6% 6% 3 4 5% 5% 7% 44%	1800
	11th to 20th	Night	16% 5% 2 4 4 2 2 4 4 6% 49%	1880
	21st to end	Night	17% 5% 3 5% 5% 3 4 4 5% 7% 42%	1980
October	1st to 10th	Night	18% 6% 3 6% 5% 4 4 4 5% 7% 38%	2088
	11th to 20th	Night	22% 7% 4 6% 5% 3 4 4 4 6% 33%	2205
	21st to end	Night	24% 8% 3 8% 6% 3 4 4 4 5% 30%	2574
November	1st to 10th	Night	19% 10% 3 6% 6% 3 4 5% 5% 6% 33%	2407
	11th to 20th	Night	19% 10% 4 7% 6% 3 4 4 5% 6% 32%	2534
	21st to end	Night	23% 13% 4 7% 6% 3 5% 4 4 5% 26%	2660
December	1st to 10th	Night	22% 16% 6% 9% 5% 3 4 4 5% 5% 20%	2660
	11th to 20th	Night	23% 15% 5% 8% 5% 3 3 4 4 5% 25%	3288
	21st to end	Night	29% 15% 6% 8% 5% 2 3 3 4 4 22%	4884

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	68% 5% 8% 14% 5%	1518
	11th to 20th	Day	63% 6% 10% 12% 10%	1520
	21st to end	Day	54% 8% 11% 17% 10% 1	1592
February	1st to 10th	Day	62% 4 9% 17% 6% 1	1570
	11th to 20th	Day	58% 5% 11% 17% 8% 1	1683
	21st to end	Day	60% 4 10% 15% 10% 1	1480
March	1st to 10th	Day	57% 6% 9% 17% 9% 2	1800
	11th to 20th	Day	52% 5% 13% 15% 13% 3	1858
	21st to end	Day	51% 5% 10% 18% 13% 3	2376
April	1st to 10th	Day	56% 5% 7% 19% 11% 2	2160
	11th to 20th	Day	48% 4 9% 23% 14% 2	2134
	21st to end	Day	54% 5% 9% 18% 12% 3	2312
May	1st to 10th	Day	49% 6% 8% 20% 15% 2	2474
	11th to 20th	Day	45% 6% 10% 20% 16% 3	2482
	21st to end	Day	45% 8% 13% 19% 13% 2	2725
June	1st to 10th	Day	50% 7% 11% 19% 11% 2	2520
	11th to 20th	Day	42% 6% 10% 24% 14% 3	2520
	21st to end	Day	51% 6% 13% 19% 10% 1	2520
July	1st to 10th	Day	38% 8% 13% 28% 12% 1	2450
	11th to 20th	Day	40% 8% 15% 29% 9%	2445
	21st to end	Day	40% 9% 15% 26% 9%	2591
August	1st to 10th	Day	41% 9% 16% 26% 8%	2492
	11th to 20th	Day	37% 8% 18% 26% 11% 1	2287
	21st to end	Day	38% 7% 15% 26% 14% 1	2376
September	1st to 10th	Day	38% 8% 14% 28% 12%	2160
	11th to 20th	Day	35% 6% 12% 32% 14% 1	2079
	21st to end	Day	41% 7% 14% 27% 10% 1	1980
October	1st to 10th	Day	49% 8% 11% 22% 10%	1868
	11th to 20th	Day	53% 7% 13% 19% 7%	1754
	21st to end	Day	57% 5% 11% 19% 8% 1	1782
November	1st to 10th	Day	49% 7% 13% 20% 9% 1	1671
	11th to 20th	Day	55% 8% 11% 16% 11%	1644
	21st to end	Day	59% 8% 13% 14% 6%	1520
December	1st to 10th	Day	69% 5% 9% 12% 4	1520
	11th to 20th	Day	59% 7% 11% 17% 5%	1906
	21st to end	Day	69% 6% 11% 9% 5%	2772
January	1st to 10th	Night	67% 5% 9% 14% 5%	2649
	11th to 20th	Night	63% 6% 10% 13% 7% 1	2660
	21st to end	Night	56% 5% 9% 17% 11% 1	2798
February	1st to 10th	Night	65% 5% 8% 15% 6% 1	2386
	11th to 20th	Night	56% 5% 10% 19% 8% 1	2174
	21st to end	Night	56% 4 9% 17% 13% 1	1776
March	1st to 10th	Night	57% 5% 9% 17% 11% 2	2160
	11th to 20th	Night	55% 4 9% 16% 13% 3	2099
	21st to end	Night	50% 4 7% 20% 15% 4	1980
April	1st to 10th	Night	55% 4 7% 18% 14% 2	1800
	11th to 20th	Night	51% 4 6% 21% 15% 2	1775
	21st to end	Night	52% 5% 7% 19% 13% 4	1648
May	1st to 10th	Night	52% 4 6% 18% 15% 5%	1502
	11th to 20th	Night	46% 5% 8% 19% 17% 4	1416
	21st to end	Night	44% 7% 11% 20% 16% 3	1551
June	1st to 10th	Night	51% 6% 9% 22% 12% 1	1440
	11th to 20th	Night	43% 7% 10% 22% 14% 3	1440
	21st to end	Night	48% 7% 12% 20% 13%	1440
July	1st to 10th	Night	38% 8% 12% 25% 16% 1	1400
	11th to 20th	Night	37% 8% 15% 27% 13%	1392
	21st to end	Night	38% 9% 16% 25% 11%	1486
August	1st to 10th	Night	38% 8% 16% 26% 11%	1422
	11th to 20th	Night	37% 8% 17% 24% 13% 1	1593
	21st to end	Night	39% 7% 13% 27% 13% 1	1980
September	1st to 10th	Night	39% 7% 11% 27% 15% 1	1800
	11th to 20th	Night	36% 6% 13% 32% 13% 1	1880
	21st to end	Night	41% 7% 10% 27% 14%	1980
October	1st to 10th	Night	45% 6% 10% 25% 12% 1	2088
	11th to 20th	Night	53% 6% 10% 24% 8%	2205
	21st to end	Night	57% 6% 11% 16% 9% 1	2574
November	1st to 10th	Night	52% 5% 11% 21% 9% 1	2407
	11th to 20th	Night	53% 5% 11% 19% 10% 1	2534
	21st to end	Night	60% 6% 10% 17% 7%	2660
December	1st to 10th	Night	67% 5% 10% 14% 4	2660
	11th to 20th	Night	63% 5% 9% 17% 7%	3288
	21st to end	Night	67% 5% 10% 14% 4	4884

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% 26% 21% 10%	180
	11th to 20th	Day	25% 14% 35% 19% 5% 1	180
	21st to end	Day	22% 13% 36% 19% 10% 1	178
February	1st to 10th	Day	33% 19% 36% 12%	218
	11th to 20th	Day	23% 16% 37% 20% 4 1	342
	21st to end	Day	27% 13% 30% 25% 4	346
March	1st to 10th	Day	24% 13% 34% 23% 6%	420
	11th to 20th	Day	21% 17% 31% 23% 7% 1	420
	21st to end	Day	26% 15% 28% 20% 9% 2	462
April	1st to 10th	Day	28% 15% 29% 19% 9% 2	420
	11th to 20th	Day	28% 20% 21% 17% 11% 3	412
	21st to end	Day	26% 13% 29% 20% 10% 2	420
May	1st to 10th	Day	23% 11% 32% 21% 11% 2	423
	11th to 20th	Day	17% 11% 32% 25% 11% 4	447
	21st to end	Day	18% 11% 34% 23% 12% 2	485
June	1st to 10th	Day	22% 14% 24% 28% 10% 3	450
	11th to 20th	Day	16% 11% 26% 30% 13% 3	450
	21st to end	Day	20% 10% 25% 26% 16% 3	450
July	1st to 10th	Day	11% 9% 24% 30% 20% 5%	445
	11th to 20th	Day	13% 6% 28% 31% 17% 5%	444
	21st to end	Day	13% 7% 25% 33% 19% 4	488
August	1st to 10th	Day	15% 4 25% 29% 22% 5%	446
	11th to 20th	Day	11% 5% 22% 33% 18% 10%	439
	21st to end	Day	13% 10% 24% 25% 18% 11%	495
September	1st to 10th	Day	11% 8% 26% 28% 18% 8%	450
	11th to 20th	Day	16% 10% 20% 26% 16% 12%	450
	21st to end	Day	17% 8% 28% 23% 17% 7%	450
October	1st to 10th	Day	21% 12% 28% 21% 15% 4	450
	11th to 20th	Day	20% 18% 29% 20% 12% 2	426
	21st to end	Day	33% 14% 24% 23% 6%	396
November	1st to 10th	Day	25% 16% 25% 25% 8% 1	365
	11th to 20th	Day	25% 18% 32% 15% 8% 1	304
	21st to end	Day	29% 14% 32% 19% 6% 1	180
December	1st to 10th	Day	34% 13% 21% 23% 7% 2	180
	11th to 20th	Day	26% 22% 34% 14% 3 1	180
	21st to end	Day	30% 20% 25% 18% 5% 2	198
January	1st to 10th	Night	27% 13% 31% 24% 6%	760
	11th to 20th	Night	25% 11% 35% 23% 6%	760
	21st to end	Night	20% 10% 35% 27% 8% 1	800
February	1st to 10th	Night	23% 9% 35% 27% 6% 1	720
	11th to 20th	Night	22% 10% 38% 24% 6%	703
	21st to end	Night	25% 10% 32% 28% 4	592
March	1st to 10th	Night	25% 11% 31% 27% 7%	720
	11th to 20th	Night	26% 10% 33% 24% 7%	720
	21st to end	Night	23% 9% 32% 28% 8% 1	792
April	1st to 10th	Night	28% 10% 28% 28% 6%	720
	11th to 20th	Night	27% 11% 29% 26% 6% 1	711
	21st to end	Night	25% 11% 30% 25% 9%	568
May	1st to 10th	Night	22% 9% 32% 27% 9% 2	422
	11th to 20th	Night	23% 5% 33% 30% 9% 1	354
	21st to end	Night	18% 10% 30% 32% 10%	389
June	1st to 10th	Night	25% 5% 27% 32% 11%	360
	11th to 20th	Night	18% 5% 29% 33% 13% 1	360
	21st to end	Night	18% 4 28% 35% 15% 1	360
July	1st to 10th	Night	12% 7% 27% 36% 18%	350
	11th to 20th	Night	11% 2 25% 45% 16% 1	348
	21st to end	Night	12% 3 24% 43% 17% 1	394
August	1st to 10th	Night	15% 3 14% 48% 19% 1	358
	11th to 20th	Night	13% 4 20% 37% 24% 2	529
	21st to end	Night	15% 3 23% 36% 21% 2	792
September	1st to 10th	Night	15% 5% 23% 39% 17% 1	720
	11th to 20th	Night	18% 6% 19% 35% 19% 2	720
	21st to end	Night	19% 7% 26% 29% 16% 3	720
October	1st to 10th	Night	20% 9% 28% 29% 13% 1	720
	11th to 20th	Night	25% 9% 27% 29% 9%	720
	21st to end	Night	27% 11% 29% 28% 5%	792
November	1st to 10th	Night	25% 11% 31% 24% 9%	740
	11th to 20th	Night	24% 13% 31% 22% 10% 1	760
	21st to end	Night	30% 14% 28% 20% 8%	760
December	1st to 10th	Night	31% 11% 29% 23% 5%	760
	11th to 20th	Night	31% 12% 33% 18% 6% 1	760
	21st to end	Night	34% 12% 31% 19% 4	836

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