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 2025-02-25.

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% 30%	108238
	Night	14% 15% 6% 12% 6% 4 4 3 5% 6% 25%	110609
Transparency	Day	52% 7% 13% 19% 9% 1	111731
	Night	57% 7% 11% 16% 7% 1	115279
Seeing	Day	21% 11% 29% 26% 11% 2	15892
	Night	22% 11% 29% 27% 10% 1	25482

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	8% 18% 10% 14% 9% 5% 6% 4 6% 8% 11%	3366
	11th to 20th	Day	8% 14% 7% 18% 7% 6% 5% 4 7% 8% 15%	3589
	21st to end	Day	7% 18% 11% 18% 7% 6% 5% 4 6% 6% 11%	3982
February	1st to 10th	Day	8% 16% 5% 14% 11% 8% 7% 4 4 7% 16%	1674
	11th to 20th	Day	10% 17% 6% 11% 9% 6% 5% 4 6% 6% 20%	1845
	21st to end	Day	8% 15% 8% 15% 8% 6% 4 4 5% 6% 22%	1610
March	1st to 10th	Day	11% 12% 4 9% 5% 4 5% 3 5% 6% 35%	3118
	11th to 20th	Day	11% 18% 4 8% 7% 4 4 3 5% 6% 30%	3432
	21st to end	Day	14% 15% 9% 13% 6% 4 4 3 4 4 24%	3905
April	1st to 10th	Day	17% 14% 4 9% 4 4 5% 3 5% 6% 30%	2280
	11th to 20th	Day	19% 11% 3 8% 4 3 3 4 5% 5% 33%	2418
	21st to end	Day	16% 10% 4 7% 4 3 4 4 6% 8% 34%	2470
May	1st to 10th	Day	17% 9% 3 7% 4 4 4 3 5% 7% 38%	4108
	11th to 20th	Day	16% 7% 2 6% 4 4 4 3 5% 9% 39%	4332
	21st to end	Day	17% 8% 2 6% 4 4 4 3 6% 7% 39%	5184
June	1st to 10th	Day	18% 7% 2 6% 4 6% 5% 5% 7% 8% 33%	2850
	11th to 20th	Day	16% 7% 3 6% 5% 5% 5% 4 6% 9% 35%	2850
	21st to end	Day	18% 8% 2 6% 4 4 4 4 5% 9% 37%	2850
July	1st to 10th	Day	19% 9% 2 7% 4 4 4 3 5% 9% 33%	4760
	11th to 20th	Day	12% 8% 2 6% 4 3 4 3 6% 9% 42%	4511
	21st to end	Day	14% 7% 2 5% 4 3 4 4 6% 9% 43%	4814
August	1st to 10th	Day	11% 6% 2 6% 3 4 5% 4 7% 11% 40%	4310
	11th to 20th	Day	14% 7% 2 5% 4 3 4 3 5% 8% 45%	4044
	21st to end	Day	19% 7% 3 6% 3 3 3 2 4 6% 44%	4510
September	1st to 10th	Day	20% 7% 2 6% 4 4 3 2 5% 7% 39%	2242
	11th to 20th	Day	21% 6% 2 3 3 4 4 3 6% 8% 39%	2090
	21st to end	Day	23% 8% 2 6% 4 3 4 3 5% 7% 34%	2090
October	1st to 10th	Day	22% 11% 3 6% 4 4 4 4 4 6% 32%	2047
	11th to 20th	Day	26% 17% 3 9% 5% 4 3 4 4 6% 18%	1787
	21st to end	Day	25% 17% 4 11% 4 5% 4 3 4 4 19%	1881
November	1st to 10th	Day	21% 22% 3 8% 4 4 4 3 5% 6% 19%	1762
	11th to 20th	Day	18% 19% 7% 13% 5% 5% 5% 3 6% 5% 16%	1675
	21st to end	Day	19% 26% 7% 11% 6% 5% 4 3 4 5% 9%	1475
December	1st to 10th	Day	12% 25% 12% 18% 6% 4 3 3 4 4 10%	2280
	11th to 20th	Day	10% 20% 7% 17% 9% 6% 5% 4 6% 5% 11%	2621
	21st to end	Day	19% 21% 7% 13% 6% 4 4 3 3 4 16%	3476
January	1st to 10th	Night	8% 19% 9% 16% 8% 6% 5% 4 5% 5% 16%	6596
	11th to 20th	Night	7% 19% 9% 18% 7% 6% 5% 4 6% 7% 12%	6316
	21st to end	Night	8% 17% 12% 20% 8% 6% 5% 4 6% 6% 9%	6996
February	1st to 10th	Night	8% 18% 6% 17% 13% 6% 5% 4 4 4 14%	2725
	11th to 20th	Night	10% 14% 7% 15% 9% 5% 5% 5% 6% 5% 17%	2439
	21st to end	Night	11% 18% 6% 13% 7% 5% 5% 4 4 6% 21%	1939
March	1st to 10th	Night	15% 12% 4 9% 4 4 3 3 6% 8% 33%	3722
	11th to 20th	Night	10% 16% 6% 10% 4 4 4 3 5% 6% 33%	3526
	21st to end	Night	14% 13% 8% 14% 5% 4 4 3 5% 6% 25%	3754
April	1st to 10th	Night	19% 12% 4 9% 6% 4 4 3 4 6% 29%	1900
	11th to 20th	Night	19% 12% 3 8% 4 4 3 4 5% 6% 32%	1710
	21st to end	Night	17% 10% 3 7% 5% 3 4 3 5% 6% 38%	1710
May	1st to 10th	Night	19% 9% 3 6% 4 3 3 2 5% 6% 40%	2833
	11th to 20th	Night	15% 7% 3 7% 5% 4 4 3 6% 8% 39%	2568
	21st to end	Night	17% 9% 3 6% 4 4 4 3 4 5% 39%	2402
June	1st to 10th	Night	17% 7% 2 6% 5% 3 4 4 5% 8% 38%	1330
	11th to 20th	Night	18% 9% 3 7% 4 3 3 3 6% 6% 37%	1330
	21st to end	Night	15% 7% 2 6% 5% 4 4 4 6% 8% 38%	1330
July	1st to 10th	Night	16% 7% 4 7% 5% 4 4 4 5% 9% 34%	2210
	11th to 20th	Night	12% 7% 2 7% 4 3 4 4 5% 7% 45%	2456
	21st to end	Night	15% 7% 2 8% 4 3 3 3 5% 7% 43%	2750
August	1st to 10th	Night	14% 8% 2 7% 5% 4 4 4 6% 8% 36%	2624
	11th to 20th	Night	14% 5% 2 6% 4 3 4 3 5% 8% 47%	2785
	21st to end	Night	15% 6% 3 5% 3 3 3 3 4 6% 49%	3143
September	1st to 10th	Night	17% 7% 2 6% 4 3 4 3 4 6% 44%	1919
	11th to 20th	Night	21% 6% 2 6% 4 3 4 3 5% 9% 39%	2090
	21st to end	Night	23% 7% 3 6% 4 3 3 3 4 6% 37%	2090
October	1st to 10th	Night	23% 10% 3 6% 4 4 4 3 4 6% 33%	2132
	11th to 20th	Night	25% 15% 4 8% 4 4 3 3 4 5% 23%	2389
	21st to end	Night	26% 19% 4 8% 5% 4 3 3 4 5% 19%	2717
November	1st to 10th	Night	21% 20% 5% 9% 5% 4 4 3 3 4 22%	2536
	11th to 20th	Night	21% 21% 7% 11% 5% 4 4 3 4 4 14%	2724
	21st to end	Night	17% 24% 8% 12% 6% 5% 5% 4 4 4 12%	2920
December	1st to 10th	Night	13% 25% 7% 13% 7% 5% 5% 4 4 5% 11%	4910
	11th to 20th	Night	10% 22% 6% 21% 7% 5% 4 3 4 5% 13%	5586
	21st to end	Night	16% 21% 7% 15% 6% 5% 4 3 5% 7% 13%	7502

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	66% 7% 10% 11% 3 3	3367
	11th to 20th	Day	63% 7% 11% 11% 9%	3620
	21st to end	Day	69% 9% 8% 8% 7%	3982
February	1st to 10th	Day	66% 8% 10% 10% 5%	1674
	11th to 20th	Day	63% 7% 11% 10% 7% 1	1854
	21st to end	Day	63% 5% 10% 12% 8% 1	1610
March	1st to 10th	Day	47% 7% 18% 15% 12% 2	3120
	11th to 20th	Day	54% 6% 13% 18% 8% 1	3432
	21st to end	Day	62% 5% 10% 10% 10% 2	3905
April	1st to 10th	Day	54% 6% 12% 18% 9% 1	2280
	11th to 20th	Day	53% 4 11% 21% 11% 1	2418
	21st to end	Day	47% 6% 10% 19% 14% 4	2470
May	1st to 10th	Day	44% 7% 10% 26% 12% 2	4120
	11th to 20th	Day	41% 8% 13% 24% 13% 1	4336
	21st to end	Day	42% 7% 13% 21% 16% 2	5184
June	1st to 10th	Day	46% 8% 13% 20% 11% 2	2850
	11th to 20th	Day	43% 8% 14% 22% 12% 1	2850
	21st to end	Day	44% 8% 14% 22% 10% 1	2850
July	1st to 10th	Day	47% 7% 15% 22% 9% 1	4686
	11th to 20th	Day	38% 6% 21% 25% 9% 1	4433
	21st to end	Day	37% 7% 14% 32% 10%	4662
August	1st to 10th	Day	35% 8% 20% 29% 7%	4295
	11th to 20th	Day	38% 5% 17% 31% 9%	4083
	21st to end	Day	43% 6% 15% 25% 11% 1	4509
September	1st to 10th	Day	46% 6% 16% 23% 10%	2242
	11th to 20th	Day	44% 7% 20% 24% 5%	2090
	21st to end	Day	50% 6% 17% 20% 6% 1	2090
October	1st to 10th	Day	43% 6% 17% 22% 11%	3398
	11th to 20th	Day	57% 6% 15% 18% 4	2951
	21st to end	Day	68% 5% 11% 11% 5%	3081
November	1st to 10th	Day	66% 6% 11% 13% 3	1762
	11th to 20th	Day	67% 9% 11% 9% 3	1675
	21st to end	Day	77% 8% 7% 6% 2	1475
December	1st to 10th	Day	77% 6% 10% 6% 1	2280
	11th to 20th	Day	71% 7% 10% 9% 3	2621
	21st to end	Day	72% 6% 7% 10% 5%	3476
January	1st to 10th	Night	64% 8% 9% 10% 5% 3	6596
	11th to 20th	Night	68% 7% 9% 10% 6%	6360
	21st to end	Night	72% 9% 5% 8% 5% 1	6996
February	1st to 10th	Night	71% 7% 9% 8% 4	2725
	11th to 20th	Night	65% 8% 12% 11% 4	2441
	21st to end	Night	63% 6% 11% 12% 7%	1939
March	1st to 10th	Night	48% 7% 15% 18% 11%	3733
	11th to 20th	Night	49% 5% 16% 21% 8% 1	3527
	21st to end	Night	56% 8% 12% 13% 9% 2	3754
April	1st to 10th	Night	54% 6% 11% 18% 11% 1	1900
	11th to 20th	Night	52% 4 9% 21% 11% 2	1710
	21st to end	Night	48% 4 12% 18% 15% 3	1710
May	1st to 10th	Night	46% 5% 8% 24% 15% 2	2840
	11th to 20th	Night	43% 7% 12% 29% 9% 1	2569
	21st to end	Night	46% 7% 11% 18% 14% 5%	2403
June	1st to 10th	Night	43% 7% 12% 20% 16% 2	1330
	11th to 20th	Night	45% 8% 11% 20% 14% 2	1330
	21st to end	Night	43% 9% 14% 24% 10% 1	1330
July	1st to 10th	Night	45% 6% 14% 26% 8%	2175
	11th to 20th	Night	37% 6% 17% 29% 10% 1	2411
	21st to end	Night	41% 6% 11% 25% 17% 1	2662
August	1st to 10th	Night	45% 7% 13% 27% 8%	2615
	11th to 20th	Night	36% 6% 18% 30% 10%	2812
	21st to end	Night	38% 5% 11% 31% 14%	3143
September	1st to 10th	Night	41% 7% 14% 27% 10% 1	1919
	11th to 20th	Night	43% 9% 17% 24% 7%	2090
	21st to end	Night	47% 7% 17% 22% 7%	2090
October	1st to 10th	Night	42% 6% 14% 28% 10%	3561
	11th to 20th	Night	49% 6% 16% 25% 4	3964
	21st to end	Night	67% 8% 14% 9% 3	4467
November	1st to 10th	Night	65% 6% 10% 14% 4	2536
	11th to 20th	Night	71% 8% 9% 9% 3	2724
	21st to end	Night	72% 10% 12% 5% 1	2920
December	1st to 10th	Night	72% 7% 12% 7% 2	4910
	11th to 20th	Night	73% 6% 8% 9% 4 1	5585
	21st to end	Night	71% 8% 7% 10% 4	7502

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% 17% 33% 20% 9% 2	246
	11th to 20th	Day	16% 15% 36% 24% 7% 1	300
	21st to end	Day	16% 12% 38% 26% 7%	330
February	1st to 10th	Day	20% 21% 34% 19% 5% 1	300
	11th to 20th	Day	19% 18% 40% 17% 5% 1	300
	21st to end	Day	19% 24% 41% 14% 2	237
March	1st to 10th	Day	16% 17% 41% 21% 5%	270
	11th to 20th	Day	20% 15% 32% 24% 6% 1	425
	21st to end	Day	21% 16% 32% 22% 7% 1	506
April	1st to 10th	Day	21% 12% 35% 25% 6% 1	460
	11th to 20th	Day	25% 13% 32% 18% 10% 2	452
	21st to end	Day	22% 12% 31% 23% 10% 3	460
May	1st to 10th	Day	21% 11% 31% 25% 10% 3	463
	11th to 20th	Day	20% 7% 32% 27% 11% 3	634
	21st to end	Day	20% 8% 29% 30% 11% 2	734
June	1st to 10th	Day	20% 8% 32% 30% 10% 1	680
	11th to 20th	Day	19% 9% 29% 29% 12% 2	680
	21st to end	Day	20% 7% 26% 30% 15% 3	680
July	1st to 10th	Day	16% 6% 26% 36% 14% 2	671
	11th to 20th	Day	16% 7% 22% 35% 16% 3	668
	21st to end	Day	19% 4 23% 35% 16% 4	739
August	1st to 10th	Day	12% 9% 24% 32% 20% 4	615
	11th to 20th	Day	16% 5% 17% 33% 23% 6%	479
	21st to end	Day	26% 8% 20% 29% 13% 5%	539
September	1st to 10th	Day	23% 11% 25% 21% 16% 4	490
	11th to 20th	Day	22% 8% 26% 28% 13% 3	490
	21st to end	Day	27% 9% 20% 27% 16% 1	490
October	1st to 10th	Day	29% 14% 21% 25% 10% 1	490
	11th to 20th	Day	33% 14% 31% 19% 4	377
	21st to end	Day	35% 14% 32% 14% 5%	330
November	1st to 10th	Day	31% 16% 29% 19% 5%	300
	11th to 20th	Day	24% 26% 31% 13% 5%	237
	21st to end	Day	26% 22% 36% 15% 2	200
December	1st to 10th	Day	25% 24% 27% 16% 10% 1	200
	11th to 20th	Day	21% 18% 41% 15% 6% 1	200
	21st to end	Day	27% 19% 34% 12% 7%	220
January	1st to 10th	Night	20% 16% 36% 23% 5%	1000
	11th to 20th	Night	19% 15% 35% 25% 6% 1	1000
	21st to end	Night	19% 11% 34% 28% 7% 1	1100
February	1st to 10th	Night	17% 13% 40% 25% 5% 1	925
	11th to 20th	Night	16% 14% 38% 23% 8% 1	783
	21st to end	Night	19% 14% 35% 28% 5%	647
March	1st to 10th	Night	15% 9% 38% 29% 8% 1	760
	11th to 20th	Night	19% 11% 36% 26% 7% 1	760
	21st to end	Night	15% 10% 34% 30% 9% 1	798
April	1st to 10th	Night	26% 8% 29% 27% 8% 2	570
	11th to 20th	Night	25% 8% 25% 31% 9% 1	563
	21st to end	Night	20% 9% 28% 27% 15% 1	570
May	1st to 10th	Night	19% 6% 23% 39% 11% 1	570
	11th to 20th	Night	22% 6% 24% 29% 16% 3	561
	21st to end	Night	17% 7% 25% 35% 16%	411
June	1st to 10th	Night	17% 5% 24% 38% 15% 1	380
	11th to 20th	Night	20% 8% 20% 37% 14% 1	380
	21st to end	Night	16% 5% 22% 32% 22% 3	380
July	1st to 10th	Night	14% 3 21% 38% 22% 2	370
	11th to 20th	Night	16% 4 15% 38% 25% 2	507
	21st to end	Night	21% 4 15% 35% 22% 4	624
August	1st to 10th	Night	18% 5% 17% 34% 24% 2	567
	11th to 20th	Night	14% 4 14% 37% 27% 4	564
	21st to end	Night	21% 4 22% 31% 18% 3	627
September	1st to 10th	Night	18% 6% 25% 28% 18% 4	589
	11th to 20th	Night	22% 6% 24% 31% 14% 2	760
	21st to end	Night	27% 8% 19% 29% 14% 2	760
October	1st to 10th	Night	29% 8% 27% 22% 13% 1	760
	11th to 20th	Night	35% 12% 26% 20% 6%	760
	21st to end	Night	36% 13% 25% 22% 4	836
November	1st to 10th	Night	32% 14% 30% 19% 5%	780
	11th to 20th	Night	30% 17% 29% 18% 6% 1	800
	21st to end	Night	28% 16% 33% 19% 4	920
December	1st to 10th	Night	25% 14% 34% 20% 6% 1	1000
	11th to 20th	Night	22% 17% 37% 17% 6% 1	1000
	21st to end	Night	28% 16% 31% 19% 6%	1100

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