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 2024-02-05.

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% 11% 5% 8% 6% 3 4 5% 5% 7% 30%	77815
	Night	16% 14% 6% 10% 7% 3 5% 4 5% 6% 25%	81074
Transparency	Day	37% 36% 9% 12% 5% 1	77512
	Night	42% 34% 8% 11% 4	80912
Seeing	Day	21% 11% 28% 26% 11% 2	14484
	Night	22% 11% 29% 27% 11% 1	24078

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	11% 23% 14% 14% 9% 4 5% 4 5% 5% 7%	2346
	11th to 20th	Day	10% 12% 7% 16% 10% 5% 5% 5% 7% 7% 17%	2530
	21st to end	Day	8% 18% 12% 15% 10% 4 6% 6% 6% 6% 10%	2783
February	1st to 10th	Day	8% 14% 6% 11% 13% 7% 8% 5% 4 7% 17%	1539
	11th to 20th	Day	12% 16% 7% 10% 9% 4 4 5% 6% 6% 22%	1656
	21st to end	Day	8% 14% 8% 14% 9% 4 5% 5% 5% 6% 23%	1480
March	1st to 10th	Day	11% 12% 6% 10% 7% 3 6% 3 5% 6% 32%	1798
	11th to 20th	Day	12% 13% 4 7% 6% 3 5% 4 4 7% 34%	1946
	21st to end	Day	16% 10% 4 7% 6% 4 5% 4 5% 6% 34%	2228
April	1st to 10th	Day	18% 13% 4 9% 5% 2 5% 4 5% 7% 29%	2160
	11th to 20th	Day	20% 10% 4 7% 5% 2 3 5% 5% 5% 34%	2289
	21st to end	Day	16% 10% 4 7% 5% 2 4 5% 6% 8% 34%	2340
May	1st to 10th	Day	16% 7% 3 6% 6% 3 4 5% 6% 7% 36%	2340
	11th to 20th	Day	19% 7% 3 5% 4 3 4 4 5% 7% 40%	2440
	21st to end	Day	16% 6% 2 5% 5% 3 5% 5% 6% 8% 39%	2918
June	1st to 10th	Day	18% 6% 2 5% 4 5% 5% 6% 8% 8% 34%	2700
	11th to 20th	Day	15% 7% 3 5% 5% 3 5% 5% 6% 10% 36%	2700
	21st to end	Day	17% 7% 3 5% 4 2 5% 5% 5% 9% 38%	2700
July	1st to 10th	Day	14% 6% 3 5% 4 3 3 5% 5% 10% 42%	2700
	11th to 20th	Day	15% 7% 2 4 4 3 4 5% 6% 9% 42%	2563
	21st to end	Day	17% 6% 3 4 4 2 3 4 5% 10% 41%	2743
August	1st to 10th	Day	12% 6% 3 5% 5% 3 5% 5% 7% 9% 41%	2450
	11th to 20th	Day	15% 6% 2 4 5% 3 4 4 5% 8% 44%	2303
	21st to end	Day	19% 6% 3 6% 4 2 4 4 4 8% 41%	2566
September	1st to 10th	Day	21% 7% 2 6% 6% 3 3 3 5% 7% 39%	2124
	11th to 20th	Day	22% 6% 2 3 4 3 5% 5% 6% 8% 37%	1980
	21st to end	Day	25% 8% 3 5% 5% 3 4 4 5% 7% 33%	1980
October	1st to 10th	Day	22% 12% 4 6% 4 3 3 5% 4 6% 31%	1940
	11th to 20th	Day	28% 16% 4 7% 7% 3 3 5% 4 5% 17%	1693
	21st to end	Day	25% 16% 4 9% 6% 4 5% 4 4 4 19%	1782
November	1st to 10th	Day	21% 22% 4 7% 5% 3 4 4 5% 6% 19%	1672
	11th to 20th	Day	18% 17% 8% 11% 6% 4 5% 4 6% 5% 16%	1592
	21st to end	Day	19% 25% 7% 10% 7% 4 4 5% 4 6% 9%	1402
December	1st to 10th	Day	12% 23% 12% 15% 9% 2 4 3 4 3 13%	1330
	11th to 20th	Day	15% 19% 7% 15% 8% 4 5% 5% 5% 5% 11%	1671
	21st to end	Day	19% 21% 7% 13% 8% 3 4 4 4 4 13%	2431
January	1st to 10th	Night	11% 22% 10% 15% 8% 4 4 4 5% 4 13%	4606
	11th to 20th	Night	8% 16% 10% 16% 11% 4 6% 5% 5% 5% 14%	4440
	21st to end	Night	10% 17% 13% 17% 10% 5% 6% 5% 4 4 10%	4884
February	1st to 10th	Night	8% 17% 7% 15% 15% 5% 5% 5% 5% 4 14%	2515
	11th to 20th	Night	11% 13% 8% 13% 9% 3 6% 6% 6% 6% 18%	2188
	21st to end	Night	11% 16% 8% 12% 9% 3 6% 4 4 6% 21%	1776
March	1st to 10th	Night	10% 10% 6% 10% 7% 4 5% 4 4 6% 33%	2149
	11th to 20th	Night	14% 11% 4 8% 6% 4 4 5% 5% 6% 34%	2012
	21st to end	Night	12% 10% 4 7% 6% 4 5% 4 5% 7% 36%	2142
April	1st to 10th	Night	20% 10% 4 9% 6% 3 5% 4 4 6% 28%	1800
	11th to 20th	Night	18% 11% 4 7% 5% 3 4 5% 4 6% 33%	1619
	21st to end	Night	17% 9% 3 6% 5% 2 4 4 5% 6% 38%	1620
May	1st to 10th	Night	16% 8% 4 6% 5% 3 5% 4 6% 8% 35%	1620
	11th to 20th	Night	18% 8% 3 5% 5% 3 5% 4 6% 7% 36%	1452
	21st to end	Night	14% 8% 3 4 5% 4 4 5% 5% 5% 43%	1357
June	1st to 10th	Night	17% 6% 2 5% 7% 3 3 5% 5% 8% 39%	1260
	11th to 20th	Night	19% 8% 3 7% 4 2 3 4 6% 6% 38%	1260
	21st to end	Night	15% 7% 3 5% 6% 3 5% 5% 5% 8% 38%	1260
July	1st to 10th	Night	11% 6% 4 5% 6% 3 5% 6% 5% 8% 40%	1260
	11th to 20th	Night	15% 6% 3 6% 6% 3 5% 4 5% 7% 40%	1394
	21st to end	Night	18% 7% 2 5% 5% 2 4 4 5% 8% 40%	1575
August	1st to 10th	Night	13% 7% 2 5% 4 3 4 5% 7% 7% 43%	1487
	11th to 20th	Night	11% 5% 2 5% 5% 2 4 4 5% 7% 49%	1598
	21st to end	Night	18% 6% 3 5% 5% 3 4 4 4 6% 42%	1787
September	1st to 10th	Night	17% 7% 2 5% 5% 2 4 4 4 6% 44%	1818
	11th to 20th	Night	22% 6% 3 5% 5% 3 4 4 5% 8% 37%	1980
	21st to end	Night	23% 7% 3 5% 5% 3 3 4 4 7% 36%	1980
October	1st to 10th	Night	23% 10% 3 6% 5% 3 4 3 4 6% 33%	2019
	11th to 20th	Night	27% 15% 4 7% 5% 3 4 4 4 5% 22%	2263
	21st to end	Night	27% 18% 4 7% 5% 3 3 4 4 5% 19%	2574
November	1st to 10th	Night	21% 19% 6% 8% 6% 3 4 4 3 4 22%	2406
	11th to 20th	Night	22% 19% 8% 10% 6% 3 5% 4 4 4 14%	2587
	21st to end	Night	18% 22% 8% 11% 7% 3 6% 5% 4 4 12%	2774
December	1st to 10th	Night	13% 22% 10% 14% 8% 3 4 5% 4 5% 11%	2850
	11th to 20th	Night	15% 21% 7% 14% 7% 3 3 5% 5% 5% 15%	3526
	21st to end	Night	15% 19% 7% 13% 9% 4 5% 4 5% 6% 13%	5236

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	62% 27% 6% 5%	2347
	11th to 20th	Day	55% 26% 10% 8% 1	2530
	21st to end	Day	61% 23% 11% 3 2	2783
February	1st to 10th	Day	46% 36% 7% 7% 4	1539
	11th to 20th	Day	43% 37% 8% 7% 3 1	1665
	21st to end	Day	40% 36% 8% 9% 6% 1	1480
March	1st to 10th	Day	37% 37% 7% 11% 8% 1	1800
	11th to 20th	Day	33% 36% 9% 14% 6% 1	1946
	21st to end	Day	37% 35% 9% 11% 6% 1	2228
April	1st to 10th	Day	36% 36% 8% 13% 6% 1	2160
	11th to 20th	Day	36% 34% 7% 15% 7%	2289
	21st to end	Day	32% 36% 7% 12% 9% 3	2340
May	1st to 10th	Day	30% 37% 7% 17% 8% 1	2340
	11th to 20th	Day	30% 36% 7% 16% 9% 1	2444
	21st to end	Day	27% 37% 9% 15% 9% 2	2918
June	1st to 10th	Day	32% 37% 9% 14% 7% 1	2700
	11th to 20th	Day	27% 37% 10% 17% 8%	2700
	21st to end	Day	31% 37% 10% 15% 5% 1	2700
July	1st to 10th	Day	26% 40% 10% 17% 7%	2626
	11th to 20th	Day	26% 38% 10% 19% 7% 1	2485
	21st to end	Day	28% 40% 12% 15% 5%	2591
August	1st to 10th	Day	26% 38% 13% 17% 7%	2435
	11th to 20th	Day	25% 36% 12% 21% 7%	2303
	21st to end	Day	28% 36% 10% 18% 7% 1	2566
September	1st to 10th	Day	29% 36% 11% 17% 7%	2124
	11th to 20th	Day	32% 37% 14% 16% 2	1980
	21st to end	Day	35% 36% 11% 14% 3	1980
October	1st to 10th	Day	36% 36% 12% 14% 3	1940
	11th to 20th	Day	47% 37% 8% 7% 1	1693
	21st to end	Day	45% 37% 6% 8% 3	1782
November	1st to 10th	Day	42% 40% 7% 8% 3	1672
	11th to 20th	Day	43% 41% 7% 7% 2	1592
	21st to end	Day	48% 41% 6% 4 1	1402
December	1st to 10th	Day	49% 40% 5% 6% 1	1330
	11th to 20th	Day	55% 33% 6% 6% 1	1671
	21st to end	Day	58% 25% 7% 5% 4	2431
January	1st to 10th	Night	57% 27% 6% 7% 3	4606
	11th to 20th	Night	57% 28% 8% 7% 1	4440
	21st to end	Night	65% 24% 5% 5% 1	4884
February	1st to 10th	Night	49% 35% 6% 7% 3 1	2515
	11th to 20th	Night	47% 38% 7% 6% 3	2190
	21st to end	Night	42% 35% 8% 9% 5%	1776
March	1st to 10th	Night	35% 37% 8% 11% 9%	2160
	11th to 20th	Night	35% 37% 8% 14% 5%	2013
	21st to end	Night	34% 36% 8% 12% 8% 2	2142
April	1st to 10th	Night	36% 36% 8% 12% 8%	1800
	11th to 20th	Night	35% 34% 7% 16% 7%	1619
	21st to end	Night	31% 35% 8% 13% 10% 2	1620
May	1st to 10th	Night	32% 35% 8% 14% 9% 2	1620
	11th to 20th	Night	32% 36% 6% 17% 8% 1	1453
	21st to end	Night	27% 37% 7% 15% 10% 3	1358
June	1st to 10th	Night	29% 37% 9% 14% 9% 1	1260
	11th to 20th	Night	28% 38% 8% 15% 10% 2	1260
	21st to end	Night	30% 38% 11% 15% 6% 1	1260
July	1st to 10th	Night	24% 39% 10% 19% 8%	1225
	11th to 20th	Night	28% 38% 8% 17% 9%	1349
	21st to end	Night	32% 38% 10% 15% 6%	1487
August	1st to 10th	Night	27% 35% 14% 18% 6%	1478
	11th to 20th	Night	21% 35% 13% 22% 7%	1598
	21st to end	Night	27% 36% 10% 19% 8%	1787
September	1st to 10th	Night	26% 36% 10% 20% 8% 1	1818
	11th to 20th	Night	32% 36% 11% 17% 3	1980
	21st to end	Night	34% 35% 12% 15% 4	1980
October	1st to 10th	Night	35% 36% 10% 14% 4 1	2019
	11th to 20th	Night	45% 36% 8% 10% 2	2263
	21st to end	Night	45% 38% 6% 9% 2	2574
November	1st to 10th	Night	41% 41% 6% 10% 3	2406
	11th to 20th	Night	46% 40% 6% 6% 3	2587
	21st to end	Night	47% 41% 7% 5%	2774
December	1st to 10th	Night	48% 42% 4 5% 1	2850
	11th to 20th	Night	51% 34% 5% 6% 3 1	3525
	21st to end	Night	56% 26% 8% 9% 1	5236

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	22% 18% 30% 19% 10% 2	221
	11th to 20th	Day	16% 15% 35% 24% 8% 1	270
	21st to end	Day	16% 10% 38% 27% 7%	297
February	1st to 10th	Day	17% 18% 37% 21% 6% 2	252
	11th to 20th	Day	22% 20% 36% 17% 4 2	240
	21st to end	Day	21% 24% 40% 14% 1	198
March	1st to 10th	Day	17% 17% 39% 22% 6%	240
	11th to 20th	Day	21% 14% 32% 24% 7% 2	386
	21st to end	Day	22% 15% 32% 23% 7% 1	462
April	1st to 10th	Day	22% 11% 34% 27% 6% 1	420
	11th to 20th	Day	25% 11% 31% 19% 11% 2	412
	21st to end	Day	21% 11% 32% 23% 10% 3	420
May	1st to 10th	Day	21% 11% 31% 24% 10% 3	423
	11th to 20th	Day	20% 7% 32% 26% 11% 3	586
	21st to end	Day	21% 7% 29% 29% 11% 2	679
June	1st to 10th	Day	19% 7% 32% 30% 10% 1	630
	11th to 20th	Day	18% 9% 29% 31% 12% 2	630
	21st to end	Day	19% 6% 25% 31% 15% 3	630
July	1st to 10th	Day	14% 6% 26% 37% 15% 2	621
	11th to 20th	Day	17% 7% 21% 35% 17% 3	618
	21st to end	Day	20% 4 23% 34% 16% 4	684
August	1st to 10th	Day	12% 8% 23% 33% 21% 4	569
	11th to 20th	Day	17% 5% 16% 33% 23% 6%	439
	21st to end	Day	26% 7% 20% 29% 13% 4	495
September	1st to 10th	Day	24% 10% 24% 21% 17% 4	450
	11th to 20th	Day	23% 8% 27% 28% 12% 2	450
	21st to end	Day	28% 10% 19% 26% 16% 1	450
October	1st to 10th	Day	28% 12% 21% 26% 10% 2	450
	11th to 20th	Day	35% 14% 28% 20% 3	343
	21st to end	Day	34% 13% 34% 15% 4	297
November	1st to 10th	Day	30% 15% 30% 20% 6%	270
	11th to 20th	Day	22% 26% 33% 14% 5%	214
	21st to end	Day	26% 22% 37% 13% 2	180
December	1st to 10th	Day	24% 24% 26% 17% 9% 1	180
	11th to 20th	Day	21% 19% 39% 14% 6% 1	180
	21st to end	Day	26% 21% 34% 12% 7% 1	198
January	1st to 10th	Night	21% 16% 36% 21% 5%	950
	11th to 20th	Night	19% 15% 34% 25% 6% 1	950
	21st to end	Night	19% 10% 34% 29% 8% 1	1045
February	1st to 10th	Night	16% 12% 41% 25% 5% 1	855
	11th to 20th	Night	17% 13% 38% 23% 8% 1	703
	21st to end	Night	19% 13% 34% 28% 4 1	592
March	1st to 10th	Night	14% 10% 38% 29% 8% 1	720
	11th to 20th	Night	20% 11% 37% 25% 7% 1	720
	21st to end	Night	15% 10% 34% 30% 10% 1	756
April	1st to 10th	Night	26% 8% 28% 28% 8% 2	540
	11th to 20th	Night	24% 7% 26% 32% 10% 1	533
	21st to end	Night	19% 9% 27% 28% 16% 1	540
May	1st to 10th	Night	19% 6% 23% 39% 11% 1	540
	11th to 20th	Night	22% 6% 24% 28% 16% 3	531
	21st to end	Night	17% 7% 24% 35% 17% 1	389
June	1st to 10th	Night	17% 5% 23% 39% 16% 1	360
	11th to 20th	Night	20% 8% 19% 38% 15% 1	360
	21st to end	Night	15% 5% 22% 32% 23% 3	360
July	1st to 10th	Night	14% 3 20% 39% 23% 2	350
	11th to 20th	Night	17% 4 16% 37% 24% 2	479
	21st to end	Night	21% 4 14% 36% 22% 4	591
August	1st to 10th	Night	17% 5% 17% 34% 25% 2	537
	11th to 20th	Night	13% 4 14% 37% 28% 3	534
	21st to end	Night	21% 4 23% 31% 18% 3	594
September	1st to 10th	Night	18% 6% 24% 28% 18% 5%	558
	11th to 20th	Night	23% 7% 25% 29% 14% 2	720
	21st to end	Night	27% 8% 19% 29% 14% 2	720
October	1st to 10th	Night	29% 7% 27% 23% 13% 1	720
	11th to 20th	Night	36% 13% 24% 20% 6%	720
	21st to end	Night	35% 13% 26% 22% 5%	792
November	1st to 10th	Night	31% 14% 31% 19% 5%	740
	11th to 20th	Night	29% 17% 30% 18% 6% 1	760
	21st to end	Night	28% 16% 32% 19% 4	874
December	1st to 10th	Night	25% 15% 34% 20% 6% 1	950
	11th to 20th	Night	22% 17% 37% 17% 6% 1	950
	21st to end	Night	27% 16% 32% 19% 6%	1045

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