Montreal River Harbour
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montreal River Harbour clear sky chart from 2005-11-06 to 2026-07-01.

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	15% 12% 4 9% 5% 4 4 3 5% 7% 31%	137788
	Night	14% 14% 5% 11% 6% 5% 4 4 5% 6% 26%	134855
Transparency	Day	51% 6% 13% 20% 9% 1	141234
	Night	56% 7% 11% 17% 8% 1	139497
Seeing	Day	21% 12% 29% 25% 10% 2	17776
	Night	22% 11% 29% 26% 10% 1	27166

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	7% 20% 11% 14% 12% 5% 6% 4 6% 7% 9%	4372
	11th to 20th	Day	8% 16% 7% 18% 8% 6% 5% 4 7% 7% 15%	4679
	21st to end	Day	6% 15% 9% 16% 8% 8% 6% 4 6% 7% 14%	5156
February	1st to 10th	Day	8% 17% 5% 14% 11% 8% 7% 4 4 6% 16%	1758
	11th to 20th	Day	11% 17% 6% 11% 9% 6% 5% 4 6% 6% 20%	1940
	21st to end	Day	8% 15% 7% 15% 7% 6% 4 4 5% 6% 23%	1730
March	1st to 10th	Day	14% 12% 4 10% 6% 5% 5% 3 5% 7% 30%	4578
	11th to 20th	Day	14% 16% 4 8% 6% 4 5% 3 5% 7% 26%	5012
	21st to end	Day	18% 15% 7% 12% 5% 4 4 3 4 4 24%	5721
April	1st to 10th	Day	17% 14% 4 10% 4 4 4 3 5% 6% 30%	2520
	11th to 20th	Day	19% 12% 4 9% 4 3 3 3 5% 5% 33%	2676
	21st to end	Day	16% 10% 4 8% 4 3 4 4 6% 8% 33%	2730
May	1st to 10th	Day	14% 9% 3 7% 4 4 4 3 5% 6% 42%	7642
	11th to 20th	Day	18% 7% 2 6% 3 3 4 3 4 7% 42%	8101
	21st to end	Day	13% 8% 2 7% 4 4 4 3 5% 8% 43%	9715
June	1st to 10th	Day	18% 7% 2 5% 4 6% 5% 5% 7% 8% 34%	3150
	11th to 20th	Day	15% 7% 3 6% 5% 5% 5% 4 6% 9% 34%	3150
	21st to end	Day	18% 8% 3 6% 4 4 4 4 5% 9% 35%	3150
July	1st to 10th	Day	16% 9% 3 7% 4 4 4 3 6% 11% 34%	6820
	11th to 20th	Day	15% 8% 2 6% 4 3 3 3 6% 10% 40%	6459
	21st to end	Day	14% 7% 2 5% 4 3 4 3 6% 10% 43%	6926
August	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 7% 11% 41%	4447
	11th to 20th	Day	14% 7% 2 5% 4 3 4 3 5% 8% 45%	4174
	21st to end	Day	19% 7% 3 6% 3 3 3 2 4 7% 44%	4652
September	1st to 10th	Day	21% 7% 2 6% 4 4 3 2 5% 7% 38%	2360
	11th to 20th	Day	21% 6% 2 3 3 4 4 3 5% 8% 40%	2200
	21st to end	Day	23% 8% 2 6% 4 4 4 3 5% 7% 35%	2200
October	1st to 10th	Day	22% 11% 3 6% 4 4 4 3 5% 6% 33%	2155
	11th to 20th	Day	25% 17% 4 9% 6% 4 3 4 4 6% 19%	1881
	21st to end	Day	24% 17% 4 11% 4 5% 4 3 3 4 20%	1980
November	1st to 10th	Day	21% 22% 3 8% 4 4 4 3 5% 6% 20%	1852
	11th to 20th	Day	18% 19% 7% 13% 5% 5% 5% 3 6% 5% 16%	1759
	21st to end	Day	19% 27% 7% 12% 6% 5% 4 3 4 5% 9%	1549
December	1st to 10th	Day	12% 24% 12% 18% 7% 4 3 3 4 4 10%	2350
	11th to 20th	Day	10% 20% 7% 17% 9% 6% 5% 4 6% 5% 11%	2691
	21st to end	Day	19% 21% 7% 13% 6% 5% 4 3 3 4 16%	3553
January	1st to 10th	Night	9% 17% 8% 16% 9% 7% 5% 4 5% 5% 14%	8549
	11th to 20th	Night	6% 21% 10% 18% 7% 5% 5% 4 6% 8% 10%	8236
	21st to end	Night	6% 13% 10% 19% 9% 7% 7% 5% 7% 7% 11%	9058
February	1st to 10th	Night	8% 18% 6% 17% 13% 6% 5% 4 4 4 14%	2861
	11th to 20th	Night	10% 14% 7% 15% 9% 5% 5% 5% 6% 5% 17%	2564
	21st to end	Night	10% 17% 7% 13% 8% 5% 5% 4 5% 6% 21%	2076
March	1st to 10th	Night	15% 12% 4 10% 4 4 3 3 5% 8% 31%	5482
	11th to 20th	Night	14% 15% 5% 9% 5% 4 4 3 5% 7% 27%	5166
	21st to end	Night	18% 15% 7% 12% 5% 4 3 2 4 5% 24%	5495
April	1st to 10th	Night	18% 12% 4 9% 6% 4 4 3 5% 5% 29%	2100
	11th to 20th	Night	20% 12% 4 8% 4 4 3 4 4 5% 32%	1892
	21st to end	Night	18% 10% 3 7% 4 3 4 3 5% 6% 37%	1890
May	1st to 10th	Night	13% 9% 3 7% 4 3 3 3 5% 7% 43%	5257
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 6% 8% 40%	4782
	21st to end	Night	11% 8% 3 8% 4 4 4 3 5% 7% 43%	4492
June	1st to 10th	Night	17% 6% 2 6% 5% 3 4 3 5% 9% 39%	1470
	11th to 20th	Night	18% 9% 3 7% 4 3 4 4 6% 7% 36%	1470
	21st to end	Night	15% 8% 3 6% 5% 5% 5% 4 6% 7% 37%	1470
July	1st to 10th	Night	15% 6% 4 7% 5% 4 4 4 6% 10% 37%	3160
	11th to 20th	Night	16% 6% 2 7% 5% 4 4 3 5% 7% 41%	3518
	21st to end	Night	14% 7% 2 7% 4 4 4 3 5% 7% 43%	3948
August	1st to 10th	Night	14% 8% 2 7% 5% 4 4 4 6% 8% 36%	2707
	11th to 20th	Night	14% 5% 2 6% 4 3 4 3 5% 8% 46%	2875
	21st to end	Night	15% 6% 3 5% 3 3 3 3 4 6% 49%	3243
September	1st to 10th	Night	17% 7% 2 6% 4 3 4 3 4 6% 43%	2020
	11th to 20th	Night	20% 6% 2 6% 4 3 4 3 5% 8% 40%	2200
	21st to end	Night	22% 7% 3 6% 4 3 3 3 4 6% 37%	2200
October	1st to 10th	Night	22% 10% 3 6% 4 4 4 3 4 6% 34%	2244
	11th to 20th	Night	25% 15% 4 8% 4 4 3 3 4 5% 23%	2515
	21st to end	Night	25% 18% 4 9% 5% 4 4 3 4 5% 20%	2860
November	1st to 10th	Night	21% 20% 5% 9% 5% 4 4 3 3 4 22%	2666
	11th to 20th	Night	20% 22% 8% 11% 5% 4 5% 3 4 4 14%	2860
	21st to end	Night	17% 24% 8% 12% 6% 5% 5% 4 4 4 12%	3066
December	1st to 10th	Night	13% 25% 7% 14% 7% 5% 5% 4 4 5% 11%	5060
	11th to 20th	Night	10% 21% 6% 21% 7% 5% 4 3 4 5% 13%	5736
	21st to end	Night	16% 21% 7% 15% 6% 5% 4 3 5% 7% 13%	7667

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	71% 7% 9% 9% 3 2	4373
	11th to 20th	Day	65% 6% 9% 12% 8%	4710
	21st to end	Day	64% 8% 8% 12% 7%	5156
February	1st to 10th	Day	66% 8% 10% 10% 5%	1758
	11th to 20th	Day	64% 7% 11% 10% 7% 1	1949
	21st to end	Day	62% 6% 10% 13% 9% 1	1730
March	1st to 10th	Day	52% 6% 14% 12% 13% 3	4580
	11th to 20th	Day	58% 5% 12% 15% 9% 1	5012
	21st to end	Day	63% 4 10% 12% 9% 2	5721
April	1st to 10th	Day	54% 6% 11% 18% 9% 1	2520
	11th to 20th	Day	53% 4 11% 20% 10% 1	2676
	21st to end	Day	48% 6% 11% 19% 13% 3	2730
May	1st to 10th	Day	41% 5% 9% 29% 14% 2	7651
	11th to 20th	Day	44% 6% 10% 23% 14% 4	8105
	21st to end	Day	39% 5% 11% 26% 15% 3	9716
June	1st to 10th	Day	46% 8% 12% 20% 12% 2	3150
	11th to 20th	Day	44% 8% 14% 22% 11% 1	3150
	21st to end	Day	45% 8% 14% 23% 10% 1	3150
July	1st to 10th	Day	44% 6% 14% 26% 9%	6747
	11th to 20th	Day	41% 5% 20% 26% 8% 1	6381
	21st to end	Day	37% 7% 16% 34% 7%	6728
August	1st to 10th	Day	35% 8% 21% 29% 7%	4432
	11th to 20th	Day	38% 6% 17% 31% 8%	4213
	21st to end	Day	43% 6% 15% 25% 11% 1	4651
September	1st to 10th	Day	47% 6% 16% 22% 9%	2360
	11th to 20th	Day	43% 7% 20% 26% 4	2200
	21st to end	Day	50% 6% 16% 21% 7% 1	2200
October	1st to 10th	Day	43% 6% 17% 23% 11%	3506
	11th to 20th	Day	57% 6% 15% 18% 4	3045
	21st to end	Day	67% 5% 11% 12% 5%	3180
November	1st to 10th	Day	66% 6% 11% 13% 4	1852
	11th to 20th	Day	68% 9% 11% 9% 3	1759
	21st to end	Day	77% 8% 7% 6% 2	1549
December	1st to 10th	Day	76% 6% 10% 7% 2	2350
	11th to 20th	Day	71% 7% 10% 9% 3	2691
	21st to end	Day	72% 6% 7% 10% 4	3553
January	1st to 10th	Night	66% 8% 9% 10% 4 2	8549
	11th to 20th	Night	70% 6% 8% 10% 7%	8280
	21st to end	Night	66% 9% 8% 11% 5% 1	9058
February	1st to 10th	Night	71% 7% 9% 9% 4	2861
	11th to 20th	Night	64% 8% 12% 11% 4	2566
	21st to end	Night	62% 7% 11% 13% 7%	2076
March	1st to 10th	Night	51% 6% 14% 14% 13% 2	5493
	11th to 20th	Night	56% 5% 16% 16% 7% 1	5167
	21st to end	Night	59% 7% 9% 14% 10% 1	5495
April	1st to 10th	Night	54% 6% 10% 18% 11% 1	2100
	11th to 20th	Night	53% 4 10% 21% 11% 2	1892
	21st to end	Night	48% 4 12% 18% 14% 3	1890
May	1st to 10th	Night	41% 5% 9% 25% 17% 3	5264
	11th to 20th	Night	42% 6% 11% 29% 9% 3	4783
	21st to end	Night	40% 5% 10% 23% 16% 5%	4493
June	1st to 10th	Night	42% 7% 12% 20% 18% 1	1470
	11th to 20th	Night	45% 8% 11% 20% 13% 2	1470
	21st to end	Night	43% 9% 14% 24% 10% 1	1470
July	1st to 10th	Night	41% 7% 13% 30% 9%	3125
	11th to 20th	Night	41% 6% 18% 26% 8% 1	3473
	21st to end	Night	40% 6% 13% 28% 12%	3832
August	1st to 10th	Night	45% 7% 14% 27% 8%	2698
	11th to 20th	Night	36% 6% 18% 30% 10%	2902
	21st to end	Night	38% 5% 12% 31% 14%	3243
September	1st to 10th	Night	43% 6% 14% 26% 10% 1	2020
	11th to 20th	Night	41% 9% 17% 26% 7%	2200
	21st to end	Night	47% 7% 17% 22% 7%	2200
October	1st to 10th	Night	42% 6% 14% 28% 10%	3673
	11th to 20th	Night	50% 6% 15% 25% 4	4090
	21st to end	Night	66% 7% 14% 9% 3	4610
November	1st to 10th	Night	65% 6% 10% 14% 5%	2666
	11th to 20th	Night	70% 8% 9% 9% 3	2860
	21st to end	Night	72% 9% 12% 6% 1	3066
December	1st to 10th	Night	72% 7% 12% 7% 2	5060
	11th to 20th	Night	73% 6% 8% 9% 4 1	5735
	21st to end	Night	71% 8% 7% 10% 4	7667

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	20% 18% 35% 18% 8% 1	271
	11th to 20th	Day	17% 15% 35% 24% 7% 1	330
	21st to end	Day	15% 12% 39% 27% 7%	363
February	1st to 10th	Day	21% 21% 33% 20% 5% 1	330
	11th to 20th	Day	19% 19% 39% 17% 5% 2	330
	21st to end	Day	20% 25% 39% 14% 2	273
March	1st to 10th	Day	20% 18% 40% 18% 4	330
	11th to 20th	Day	22% 17% 33% 21% 5% 1	501
	21st to end	Day	24% 19% 30% 20% 6% 1	594
April	1st to 10th	Day	23% 13% 34% 24% 5% 1	540
	11th to 20th	Day	25% 13% 31% 19% 9% 2	532
	21st to end	Day	22% 11% 33% 22% 9% 2	540
May	1st to 10th	Day	20% 12% 31% 25% 10% 3	543
	11th to 20th	Day	21% 10% 31% 26% 9% 3	730
	21st to end	Day	19% 9% 31% 29% 10% 2	844
June	1st to 10th	Day	20% 8% 32% 29% 10% 1	780
	11th to 20th	Day	20% 9% 29% 29% 11% 2	780
	21st to end	Day	21% 7% 27% 29% 14% 3	780
July	1st to 10th	Day	16% 6% 27% 35% 14% 2	722
	11th to 20th	Day	16% 8% 24% 34% 15% 3	718
	21st to end	Day	19% 4 24% 34% 15% 4	794
August	1st to 10th	Day	12% 8% 25% 32% 20% 3	662
	11th to 20th	Day	16% 6% 18% 32% 22% 6%	519
	21st to end	Day	26% 8% 21% 28% 13% 5%	583
September	1st to 10th	Day	25% 12% 24% 21% 15% 4	530
	11th to 20th	Day	21% 7% 25% 28% 15% 3	530
	21st to end	Day	26% 9% 21% 27% 15% 1	530
October	1st to 10th	Day	28% 14% 21% 25% 10% 1	530
	11th to 20th	Day	32% 15% 31% 19% 3	411
	21st to end	Day	33% 13% 32% 15% 6%	363
November	1st to 10th	Day	32% 16% 28% 19% 5%	330
	11th to 20th	Day	27% 25% 30% 13% 5%	261
	21st to end	Day	28% 21% 35% 14% 2	220
December	1st to 10th	Day	23% 25% 27% 14% 10%	220
	11th to 20th	Day	23% 17% 40% 13% 6% 1	220
	21st to end	Day	28% 18% 36% 11% 7%	242
January	1st to 10th	Night	20% 16% 36% 22% 5%	1050
	11th to 20th	Night	19% 15% 36% 24% 5%	1050
	21st to end	Night	18% 11% 35% 28% 7% 1	1155
February	1st to 10th	Night	17% 14% 40% 24% 5% 1	971
	11th to 20th	Night	16% 14% 37% 24% 8% 1	823
	21st to end	Night	18% 13% 35% 27% 5%	692
March	1st to 10th	Night	17% 11% 37% 28% 7% 1	840
	11th to 20th	Night	21% 12% 35% 25% 6% 1	840
	21st to end	Night	17% 11% 34% 29% 9% 1	882
April	1st to 10th	Night	26% 9% 29% 26% 8% 2	630
	11th to 20th	Night	26% 8% 25% 30% 9% 1	623
	21st to end	Night	22% 9% 27% 27% 14% 1	630
May	1st to 10th	Night	19% 7% 24% 38% 11% 1	630
	11th to 20th	Night	21% 7% 26% 28% 15% 3	621
	21st to end	Night	16% 7% 26% 36% 15%	455
June	1st to 10th	Night	16% 5% 25% 39% 15% 1	420
	11th to 20th	Night	20% 8% 20% 38% 14% 1	420
	21st to end	Night	16% 5% 21% 34% 21% 3	420
July	1st to 10th	Night	14% 3 20% 39% 22% 2	390
	11th to 20th	Night	17% 4 15% 38% 24% 2	535
	21st to end	Night	21% 4 15% 35% 22% 3	657
August	1st to 10th	Night	18% 5% 16% 33% 25% 2	597
	11th to 20th	Night	14% 5% 15% 36% 26% 4	594
	21st to end	Night	21% 4 22% 31% 19% 3	660
September	1st to 10th	Night	20% 6% 25% 27% 18% 4	620
	11th to 20th	Night	21% 6% 24% 31% 15% 2	800
	21st to end	Night	26% 8% 20% 30% 14% 2	800
October	1st to 10th	Night	28% 8% 28% 23% 12% 1	800
	11th to 20th	Night	34% 13% 27% 20% 6%	800
	21st to end	Night	34% 13% 26% 23% 5%	880
November	1st to 10th	Night	32% 14% 30% 19% 5%	820
	11th to 20th	Night	30% 17% 29% 18% 6% 1	840
	21st to end	Night	29% 16% 32% 19% 4	966
December	1st to 10th	Night	25% 15% 34% 20% 5% 1	1050
	11th to 20th	Night	22% 17% 37% 17% 6% 1	1050
	21st to end	Night	28% 16% 31% 19% 5%	1155

Page generated at 2026-06-28 02:10:47. Version ac07

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