Ville-Marie
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ville-Marie clear sky chart from 2005-11-06 to 2025-09-04.

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% 4 9% 5% 5% 4 4 5% 6% 30%	126570
	Night	16% 14% 5% 11% 6% 5% 4 3 4 5% 27%	122256
Transparency	Day	55% 5% 11% 20% 8% 1	130074
	Night	59% 5% 10% 17% 8% 1	126840
Seeing	Day	22% 14% 31% 21% 9% 2	16229
	Night	23% 12% 30% 25% 8% 1	25134

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% 19% 8% 18% 11% 8% 5% 3 4 4 10%	3158
	11th to 20th	Day	10% 18% 6% 19% 8% 7% 4 4 4 5% 17%	3133
	21st to end	Day	11% 17% 8% 14% 7% 6% 5% 3 5% 5% 18%	4397
February	1st to 10th	Day	7% 16% 6% 14% 11% 8% 6% 4 5% 5% 18%	1800
	11th to 20th	Day	10% 20% 5% 10% 8% 6% 5% 4 5% 7% 20%	1753
	21st to end	Day	15% 17% 4 11% 8% 5% 5% 4 4 5% 22%	1485
March	1st to 10th	Day	18% 15% 5% 11% 5% 4 4 4 5% 6% 24%	4853
	11th to 20th	Day	15% 14% 5% 13% 7% 4 4 3 5% 6% 24%	4980
	21st to end	Day	14% 15% 6% 12% 5% 5% 4 3 3 4 28%	5465
April	1st to 10th	Day	18% 14% 5% 9% 5% 4 4 4 5% 6% 26%	2390
	11th to 20th	Day	21% 11% 3 9% 4 4 3 3 4 5% 33%	2530
	21st to end	Day	21% 12% 3 7% 4 4 4 3 4 5% 33%	2600
May	1st to 10th	Day	18% 11% 4 7% 4 4 4 3 4 6% 36%	6195
	11th to 20th	Day	13% 8% 3 7% 5% 5% 5% 4 5% 7% 39%	6318
	21st to end	Day	20% 10% 3 6% 4 4 3 3 4 6% 37%	6991
June	1st to 10th	Day	19% 8% 3 7% 5% 5% 5% 4 6% 7% 32%	3000
	11th to 20th	Day	14% 6% 2 6% 6% 4 5% 5% 7% 7% 38%	3000
	21st to end	Day	19% 8% 3 7% 5% 5% 5% 4 6% 7% 32%	3000
July	1st to 10th	Day	16% 7% 2 7% 5% 4 6% 5% 7% 9% 33%	6819
	11th to 20th	Day	15% 5% 2 6% 4 4 5% 4 6% 9% 39%	6820
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 7% 9% 41%	7017
August	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 7% 8% 36%	4244
	11th to 20th	Day	18% 6% 3 6% 4 4 5% 4 6% 7% 37%	4174
	21st to end	Day	13% 6% 2 8% 4 4 4 4 5% 7% 41%	4585
September	1st to 10th	Day	17% 7% 2 7% 5% 4 5% 3 5% 7% 37%	2316
	11th to 20th	Day	19% 7% 2 6% 5% 4 4 4 4 7% 38%	2195
	21st to end	Day	24% 8% 2 8% 4 5% 4 3 4 6% 33%	2038
October	1st to 10th	Day	21% 13% 3 7% 4 4 4 3 5% 5% 32%	1900
	11th to 20th	Day	24% 16% 4 8% 5% 5% 3 4 4 5% 22%	1900
	21st to end	Day	26% 20% 3 10% 5% 4 3 3 4 5% 17%	2009
November	1st to 10th	Day	20% 16% 7% 8% 6% 4 4 2 4 5% 25%	1566
	11th to 20th	Day	19% 16% 5% 14% 7% 5% 4 3 5% 4 17%	1600
	21st to end	Day	23% 20% 7% 13% 7% 6% 4 2 4 4 10%	1600
December	1st to 10th	Day	13% 25% 11% 19% 7% 5% 4 3 3 3 9%	2600
	11th to 20th	Day	15% 17% 6% 15% 8% 4 5% 2 3 4 21%	2663
	21st to end	Day	19% 18% 7% 17% 7% 4 3 4 4 4 11%	3476
January	1st to 10th	Night	11% 17% 9% 18% 9% 6% 5% 3 4 4 13%	6808
	11th to 20th	Night	10% 17% 6% 17% 8% 6% 5% 4 5% 5% 17%	6772
	21st to end	Night	9% 19% 8% 14% 7% 7% 6% 4 5% 5% 18%	6554
February	1st to 10th	Night	11% 14% 5% 14% 9% 6% 5% 4 5% 6% 20%	2600
	11th to 20th	Night	11% 17% 7% 13% 8% 5% 4 4 4 5% 22%	2533
	21st to end	Night	11% 16% 4 13% 8% 6% 5% 4 5% 6% 23%	2145
March	1st to 10th	Night	16% 17% 5% 11% 5% 4 5% 4 4 5% 26%	4982
	11th to 20th	Night	15% 17% 5% 12% 7% 5% 5% 3 5% 4 21%	4979
	21st to end	Night	16% 14% 5% 10% 5% 5% 4 3 3 5% 30%	5467
April	1st to 10th	Night	19% 15% 3 9% 6% 4 4 3 4 5% 28%	2009
	11th to 20th	Night	19% 11% 3 8% 4 4 4 3 5% 6% 34%	1837
	21st to end	Night	19% 11% 3 8% 4 4 4 2 5% 6% 34%	1800
May	1st to 10th	Night	16% 10% 3 8% 5% 5% 4 3 4 5% 36%	3744
	11th to 20th	Night	16% 6% 2 6% 4 4 3 2 5% 7% 43%	3575
	21st to end	Night	19% 7% 2 6% 5% 4 4 4 5% 7% 36%	3882
June	1st to 10th	Night	16% 8% 2 8% 5% 5% 5% 5% 6% 7% 34%	1400
	11th to 20th	Night	19% 5% 2 4 4 4 3 3 6% 8% 42%	1400
	21st to end	Night	16% 6% 2 8% 4 4 5% 4 6% 8% 36%	1400
July	1st to 10th	Night	16% 7% 3 7% 5% 5% 5% 5% 5% 6% 37%	3160
	11th to 20th	Night	14% 7% 2 6% 4 5% 5% 4 5% 7% 41%	3160
	21st to end	Night	13% 5% 2 6% 4 4 3 4 6% 8% 46%	3839
August	1st to 10th	Night	11% 5% 2 6% 5% 5% 5% 4 6% 8% 42%	2903
	11th to 20th	Night	16% 7% 3 6% 4 4 4 4 6% 8% 39%	2875
	21st to end	Night	16% 6% 2 7% 5% 4 4 3 5% 7% 43%	3279
September	1st to 10th	Night	17% 5% 2 5% 4 4 4 3 5% 7% 43%	1931
	11th to 20th	Night	19% 7% 2 5% 3 4 4 4 4 6% 43%	1983
	21st to end	Night	21% 8% 3 5% 3 3 3 3 4 7% 39%	2141
October	1st to 10th	Night	21% 11% 3 6% 4 4 4 3 5% 6% 33%	2280
	11th to 20th	Night	25% 15% 3 7% 5% 4 4 4 4 5% 24%	2280
	21st to end	Night	23% 17% 4 9% 5% 5% 4 3 4 4 22%	2586
November	1st to 10th	Night	19% 15% 5% 9% 5% 4 4 3 4 6% 26%	2732
	11th to 20th	Night	18% 17% 6% 13% 7% 4 4 3 4 4 18%	2800
	21st to end	Night	22% 19% 6% 12% 6% 5% 5% 4 4 4 14%	2800
December	1st to 10th	Night	14% 24% 9% 16% 8% 6% 5% 3 3 3 9%	4580
	11th to 20th	Night	15% 18% 5% 13% 9% 6% 5% 3 4 4 18%	5538
	21st to end	Night	20% 21% 6% 15% 6% 4 4 3 3 4 14%	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	79% 5% 9% 6% 2	3159
	11th to 20th	Day	71% 4 7% 10% 7%	3160
	21st to end	Day	68% 4 8% 14% 6%	4397
February	1st to 10th	Day	66% 7% 8% 12% 7%	1800
	11th to 20th	Day	64% 5% 10% 12% 8% 1	1762
	21st to end	Day	64% 6% 6% 16% 8%	1485
March	1st to 10th	Day	61% 5% 6% 14% 12% 1	4856
	11th to 20th	Day	63% 4 11% 14% 8% 1	4980
	21st to end	Day	61% 3 7% 19% 9% 1	5465
April	1st to 10th	Day	59% 5% 8% 19% 9% 1	2390
	11th to 20th	Day	54% 4 7% 22% 11% 1	2530
	21st to end	Day	55% 4 7% 17% 15% 2	2600
May	1st to 10th	Day	49% 6% 8% 19% 15% 4	6205
	11th to 20th	Day	44% 7% 13% 21% 12% 2	6322
	21st to end	Day	50% 5% 10% 20% 14% 1	6991
June	1st to 10th	Day	51% 7% 10% 21% 10% 1	3000
	11th to 20th	Day	44% 7% 12% 25% 12% 1	3000
	21st to end	Day	51% 7% 11% 22% 9% 1	3000
July	1st to 10th	Day	46% 5% 18% 27% 5%	6746
	11th to 20th	Day	40% 5% 19% 30% 6%	6739
	21st to end	Day	39% 6% 16% 31% 8%	6818
August	1st to 10th	Day	44% 6% 16% 28% 6%	4230
	11th to 20th	Day	44% 6% 15% 25% 9% 1	4213
	21st to end	Day	43% 6% 13% 29% 8% 2	4585
September	1st to 10th	Day	46% 8% 15% 22% 9%	2316
	11th to 20th	Day	47% 7% 16% 24% 6%	2195
	21st to end	Day	53% 5% 13% 22% 7%	2038
October	1st to 10th	Day	55% 4 10% 24% 6%	3160
	11th to 20th	Day	58% 5% 11% 19% 7%	3140
	21st to end	Day	68% 4 15% 8% 5%	3287
November	1st to 10th	Day	64% 4 13% 14% 5%	1566
	11th to 20th	Day	71% 6% 8% 11% 4	1600
	21st to end	Day	80% 5% 7% 6% 2	1600
December	1st to 10th	Day	83% 3 6% 6% 2	2600
	11th to 20th	Day	69% 5% 6% 17% 2	2663
	21st to end	Day	77% 4 5% 10% 4	3476
January	1st to 10th	Night	75% 5% 8% 8% 3	6808
	11th to 20th	Night	69% 5% 8% 10% 7%	6820
	21st to end	Night	69% 4 5% 12% 10%	6554
February	1st to 10th	Night	64% 6% 10% 13% 8%	2600
	11th to 20th	Night	65% 4 10% 14% 6%	2535
	21st to end	Night	62% 6% 9% 15% 8% 1	2145
March	1st to 10th	Night	62% 5% 7% 13% 10% 4	4992
	11th to 20th	Night	66% 5% 10% 13% 6% 1	4980
	21st to end	Night	58% 3 7% 17% 11% 3	5467
April	1st to 10th	Night	59% 4 8% 16% 12% 1	2009
	11th to 20th	Night	51% 4 8% 19% 15% 2	1837
	21st to end	Night	52% 5% 10% 15% 17% 2	1800
May	1st to 10th	Night	50% 6% 5% 19% 18% 2	3750
	11th to 20th	Night	41% 5% 13% 22% 16% 3	3576
	21st to end	Night	47% 5% 10% 20% 15% 2	3883
June	1st to 10th	Night	47% 7% 11% 22% 11% 2	1400
	11th to 20th	Night	41% 5% 12% 24% 15% 2	1400
	21st to end	Night	46% 7% 11% 22% 14%	1400
July	1st to 10th	Night	47% 5% 12% 27% 9%	3125
	11th to 20th	Night	43% 4 16% 29% 9%	3118
	21st to end	Night	37% 4 16% 30% 13%	3724
August	1st to 10th	Night	39% 6% 16% 30% 9%	2893
	11th to 20th	Night	42% 6% 17% 28% 7%	2902
	21st to end	Night	42% 5% 14% 27% 10% 2	3279
September	1st to 10th	Night	40% 8% 17% 25% 9% 1	1931
	11th to 20th	Night	42% 7% 15% 28% 9%	1983
	21st to end	Night	45% 7% 14% 26% 8%	2141
October	1st to 10th	Night	51% 5% 10% 24% 10%	3800
	11th to 20th	Night	57% 4 14% 22% 4	3779
	21st to end	Night	60% 6% 17% 11% 6%	4258
November	1st to 10th	Night	60% 6% 12% 17% 4	2732
	11th to 20th	Night	69% 7% 10% 10% 4	2800
	21st to end	Night	74% 6% 10% 8% 2	2800
December	1st to 10th	Night	81% 4 5% 8% 2	4580
	11th to 20th	Night	70% 4 8% 11% 7%	5537
	21st to end	Night	76% 3 5% 13% 3	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	26% 16% 40% 14% 5% 1	200
	11th to 20th	Day	23% 13% 37% 19% 7% 3	200
	21st to end	Day	20% 13% 36% 20% 9% 1	317
February	1st to 10th	Day	22% 24% 34% 14% 5%	300
	11th to 20th	Day	21% 20% 41% 14% 3	300
	21st to end	Day	29% 23% 34% 14%	249
March	1st to 10th	Day	22% 16% 38% 18% 5%	477
	11th to 20th	Day	20% 18% 36% 21% 4 1	500
	21st to end	Day	26% 17% 35% 16% 6% 1	550
April	1st to 10th	Day	25% 17% 30% 19% 7% 2	500
	11th to 20th	Day	27% 18% 24% 20% 7% 3	492
	21st to end	Day	29% 19% 27% 15% 8% 3	500
May	1st to 10th	Day	25% 11% 34% 19% 9% 2	503
	11th to 20th	Day	20% 15% 29% 22% 10% 3	527
	21st to end	Day	23% 11% 32% 22% 10% 2	589
June	1st to 10th	Day	20% 14% 33% 25% 7% 1	730
	11th to 20th	Day	19% 13% 30% 26% 10% 3	730
	21st to end	Day	21% 10% 28% 28% 10% 2	730
July	1st to 10th	Day	15% 11% 33% 29% 10% 2	721
	11th to 20th	Day	18% 11% 30% 25% 13% 3	718
	21st to end	Day	13% 10% 30% 31% 14% 4	615
August	1st to 10th	Day	17% 9% 28% 26% 15% 5%	526
	11th to 20th	Day	18% 6% 25% 26% 17% 9%	519
	21st to end	Day	20% 9% 31% 20% 14% 6%	583
September	1st to 10th	Day	21% 8% 30% 25% 12% 4	502
	11th to 20th	Day	20% 9% 23% 31% 11% 6%	490
	21st to end	Day	25% 11% 25% 21% 14% 3	490
October	1st to 10th	Day	27% 14% 27% 18% 12% 2	490
	11th to 20th	Day	31% 19% 29% 16% 3 1	490
	21st to end	Day	34% 16% 31% 16% 3 1	471
November	1st to 10th	Day	33% 18% 39% 10% 2	200
	11th to 20th	Day	27% 16% 38% 15% 4 1	200
	21st to end	Day	34% 18% 34% 11% 3 1	200
December	1st to 10th	Day	26% 15% 35% 20% 4 1	200
	11th to 20th	Day	27% 15% 37% 14% 6% 2	200
	21st to end	Day	25% 19% 29% 20% 5% 2	220
January	1st to 10th	Night	26% 13% 35% 21% 5%	1000
	11th to 20th	Night	25% 14% 31% 24% 6%	1000
	21st to end	Night	19% 11% 32% 27% 11% 1	880
February	1st to 10th	Night	18% 15% 35% 24% 7%	800
	11th to 20th	Night	21% 14% 38% 22% 5%	783
	21st to end	Night	21% 17% 34% 22% 5%	660
March	1st to 10th	Night	21% 18% 34% 22% 5% 1	800
	11th to 20th	Night	24% 16% 33% 21% 6% 1	800
	21st to end	Night	21% 13% 36% 23% 6% 1	880
April	1st to 10th	Night	26% 16% 29% 24% 6% 1	800
	11th to 20th	Night	26% 13% 27% 24% 9% 1	653
	21st to end	Night	24% 11% 29% 28% 7% 1	600
May	1st to 10th	Night	20% 10% 30% 33% 7% 1	472
	11th to 20th	Night	20% 9% 32% 29% 9% 1	394
	21st to end	Night	22% 8% 26% 37% 7%	433
June	1st to 10th	Night	18% 10% 28% 36% 8%	400
	11th to 20th	Night	21% 8% 28% 32% 12%	400
	21st to end	Night	16% 8% 28% 36% 13% 1	400
July	1st to 10th	Night	15% 5% 30% 39% 9% 1	390
	11th to 20th	Night	16% 6% 26% 40% 13%	388
	21st to end	Night	12% 5% 29% 37% 15% 1	438
August	1st to 10th	Night	15% 4 24% 37% 18% 2	585
	11th to 20th	Night	18% 6% 24% 31% 20% 2	594
	21st to end	Night	20% 7% 28% 26% 17% 2	690
September	1st to 10th	Night	19% 8% 27% 29% 17% 1	773
	11th to 20th	Night	21% 8% 26% 26% 17% 2	760
	21st to end	Night	25% 6% 23% 31% 13% 2	760
October	1st to 10th	Night	27% 9% 29% 26% 9% 1	760
	11th to 20th	Night	33% 12% 28% 20% 7% 1	760
	21st to end	Night	33% 13% 29% 22% 3 1	836
November	1st to 10th	Night	28% 14% 32% 20% 6% 1	780
	11th to 20th	Night	25% 17% 35% 17% 6%	800
	21st to end	Night	33% 16% 28% 19% 4	800
December	1st to 10th	Night	30% 16% 29% 19% 6% 1	800
	11th to 20th	Night	27% 14% 33% 21% 5% 1	965
	21st to end	Night	30% 15% 25% 23% 6% 1	1100

Page generated at 2025-09-01 01:51:06. Version ac07

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