Laval-West
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Laval-West clear sky chart from 2005-11-06 to 2026-08-30.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	144351
	Night	16% 11% 4 9% 5% 5% 5% 4 5% 6% 30%	137868
Transparency	Day	49% 6% 12% 22% 10% 1	147798
	Night	53% 5% 11% 19% 10% 1	142510
Seeing	Day	21% 10% 28% 25% 13% 4	16839
	Night	22% 8% 28% 31% 10% 1	25732

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	14% 17% 5% 14% 5% 4 4 3 5% 6% 23%	4661
	11th to 20th	Day	11% 15% 6% 13% 6% 6% 5% 3 5% 5% 23%	4650
	21st to end	Day	10% 12% 4 12% 9% 7% 6% 4 5% 5% 24%	5124
February	1st to 10th	Day	13% 15% 4 11% 6% 5% 5% 4 5% 6% 27%	1852
	11th to 20th	Day	14% 11% 5% 11% 6% 5% 5% 5% 6% 6% 27%	1945
	21st to end	Day	17% 15% 4 10% 6% 4 4 3 5% 5% 27%	1730
March	1st to 10th	Day	19% 12% 5% 11% 6% 5% 4 3 4 5% 25%	4576
	11th to 20th	Day	10% 12% 4 10% 6% 5% 5% 4 6% 6% 31%	4808
	21st to end	Day	18% 10% 3 8% 5% 5% 5% 3 4 5% 34%	6093
April	1st to 10th	Day	21% 11% 3 8% 4 4 4 3 4 5% 32%	2520
	11th to 20th	Day	20% 9% 3 7% 5% 4 4 3 4 5% 36%	2494
	21st to end	Day	20% 10% 3 7% 5% 4 4 3 5% 6% 33%	2683
May	1st to 10th	Day	21% 9% 3 9% 5% 5% 5% 4 6% 8% 24%	8153
	11th to 20th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 7% 32%	8221
	21st to end	Day	21% 9% 2 7% 4 4 5% 3 6% 6% 33%	9060
June	1st to 10th	Day	19% 8% 3 7% 5% 5% 5% 4 6% 7% 31%	2940
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 8% 37%	2940
	21st to end	Day	18% 8% 3 7% 5% 5% 5% 4 6% 8% 31%	2940
July	1st to 10th	Day	10% 5% 2 8% 6% 5% 6% 5% 7% 8% 38%	8280
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 7% 9% 46%	8280
	21st to end	Day	10% 6% 2 6% 4 4 5% 4 6% 10% 42%	8993
August	1st to 10th	Day	13% 6% 2 8% 5% 5% 6% 5% 7% 9% 34%	4703
	11th to 20th	Day	12% 6% 2 8% 5% 5% 5% 4 6% 8% 39%	4248
	21st to end	Day	15% 4 2 6% 5% 4 5% 4 6% 9% 39%	4420
September	1st to 10th	Day	11% 6% 2 5% 5% 5% 5% 4 7% 10% 42%	2400
	11th to 20th	Day	12% 5% 2 5% 3 4 4 3 6% 7% 50%	2320
	21st to end	Day	16% 6% 3 6% 4 4 4 3 6% 7% 41%	2200
October	1st to 10th	Day	18% 8% 3 6% 4 5% 4 4 5% 6% 38%	2055
	11th to 20th	Day	17% 8% 3 7% 7% 6% 5% 4 6% 6% 31%	1962
	21st to end	Day	23% 10% 3 6% 4 4 4 3 5% 7% 30%	1980
November	1st to 10th	Day	16% 11% 3 7% 5% 4 5% 4 6% 7% 33%	1851
	11th to 20th	Day	17% 11% 4 9% 4 5% 5% 4 6% 6% 30%	1795
	21st to end	Day	22% 13% 3 7% 5% 5% 5% 4 6% 6% 24%	1680
December	1st to 10th	Day	20% 18% 6% 11% 5% 4 4 4 5% 5% 17%	2680
	11th to 20th	Day	17% 13% 4 10% 6% 5% 5% 3 5% 4 27%	3066
	21st to end	Day	23% 12% 4 10% 6% 4 6% 4 4 5% 22%	4048
January	1st to 10th	Night	13% 18% 7% 14% 5% 5% 4 3 4 5% 21%	8234
	11th to 20th	Night	11% 16% 6% 17% 7% 6% 6% 3 5% 5% 17%	8235
	21st to end	Night	11% 13% 5% 12% 9% 6% 5% 4 5% 6% 26%	9057
February	1st to 10th	Night	13% 14% 5% 11% 7% 5% 5% 4 5% 6% 24%	2767
	11th to 20th	Night	14% 11% 4 9% 6% 5% 6% 4 6% 7% 29%	2559
	21st to end	Night	15% 15% 4 9% 5% 4 4 3 4 5% 32%	2076
March	1st to 10th	Night	21% 13% 3 9% 4 4 4 3 4 5% 30%	5484
	11th to 20th	Night	14% 14% 4 8% 5% 5% 4 4 6% 8% 29%	5355
	21st to end	Night	16% 10% 4 10% 5% 5% 5% 4 5% 5% 32%	5080
April	1st to 10th	Night	23% 9% 4 6% 4 4 4 4 5% 6% 31%	2100
	11th to 20th	Night	22% 9% 3 7% 5% 4 3 3 4 6% 34%	2075
	21st to end	Night	21% 9% 2 7% 4 4 4 3 5% 6% 35%	1937
May	1st to 10th	Night	21% 9% 3 9% 5% 5% 5% 4 6% 8% 25%	4788
	11th to 20th	Night	20% 9% 2 8% 4 4 4 4 6% 7% 31%	4639
	21st to end	Night	18% 9% 3 8% 5% 4 4 4 5% 8% 32%	5114
June	1st to 10th	Night	19% 7% 3 7% 5% 4 5% 4 6% 8% 32%	1680
	11th to 20th	Night	15% 7% 2 8% 4 3 4 3 6% 7% 39%	1680
	21st to end	Night	18% 7% 2 9% 5% 4 5% 4 6% 6% 35%	1680
July	1st to 10th	Night	12% 6% 3 7% 5% 5% 6% 5% 7% 10% 35%	4680
	11th to 20th	Night	11% 5% 2 6% 4 4 4 3 6% 9% 44%	4680
	21st to end	Night	12% 6% 3 7% 4 5% 5% 4 7% 8% 41%	5084
August	1st to 10th	Night	14% 7% 3 8% 4 4 4 3 6% 7% 39%	2666
	11th to 20th	Night	14% 7% 3 7% 4 4 4 3 6% 7% 40%	2982
	21st to end	Night	16% 6% 2 7% 5% 4 4 4 6% 7% 39%	3677
September	1st to 10th	Night	11% 4 3 7% 4 5% 4 4 6% 8% 44%	2000
	11th to 20th	Night	13% 6% 3 5% 3 3 2 3 4 7% 50%	2078
	21st to end	Night	16% 5% 2 6% 4 4 4 3 5% 7% 43%	2200
October	1st to 10th	Night	18% 7% 2 6% 3 5% 5% 3 5% 7% 40%	2341
	11th to 20th	Night	19% 8% 3 7% 4 5% 4 3 4 6% 37%	2436
	21st to end	Night	21% 9% 3 7% 5% 4 5% 3 5% 6% 32%	2860
November	1st to 10th	Night	20% 10% 3 6% 5% 4 4 4 5% 6% 32%	2667
	11th to 20th	Night	18% 10% 3 9% 4 4 4 4 6% 6% 32%	2819
	21st to end	Night	21% 13% 4 9% 6% 5% 4 3 4 5% 26%	2940
December	1st to 10th	Night	17% 17% 5% 11% 6% 6% 6% 4 5% 5% 19%	4720
	11th to 20th	Night	22% 14% 4 10% 5% 4 4 3 6% 5% 24%	5348
	21st to end	Night	24% 16% 4 9% 4 3 3 3 6% 5% 23%	7150

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	63% 6% 10% 17% 4	4662
	11th to 20th	Day	61% 5% 10% 13% 10% 1	4680
	21st to end	Day	60% 6% 10% 13% 11% 1	5124
February	1st to 10th	Day	58% 4 9% 21% 6% 2	1852
	11th to 20th	Day	56% 7% 10% 16% 9% 1	1954
	21st to end	Day	60% 3 9% 16% 10% 2	1730
March	1st to 10th	Day	60% 5% 7% 14% 11% 3	4579
	11th to 20th	Day	51% 7% 16% 13% 11% 2	4808
	21st to end	Day	52% 5% 7% 16% 18% 2	6093
April	1st to 10th	Day	55% 4 8% 18% 13% 2	2520
	11th to 20th	Day	50% 5% 7% 21% 15% 2	2494
	21st to end	Day	52% 5% 9% 18% 13% 3	2683
May	1st to 10th	Day	56% 5% 8% 19% 10% 1	8165
	11th to 20th	Day	50% 7% 13% 21% 8% 1	8225
	21st to end	Day	50% 5% 10% 23% 11% 1	9061
June	1st to 10th	Day	50% 7% 10% 19% 13% 1	2940
	11th to 20th	Day	43% 7% 10% 23% 13% 3	2940
	21st to end	Day	50% 7% 11% 21% 10% 1	2940
July	1st to 10th	Day	41% 6% 13% 27% 11%	8210
	11th to 20th	Day	34% 5% 14% 33% 14%	8205
	21st to end	Day	37% 6% 16% 32% 8%	8795
August	1st to 10th	Day	45% 7% 17% 23% 8%	4689
	11th to 20th	Day	41% 6% 17% 28% 7%	4290
	21st to end	Day	40% 6% 17% 22% 15% 1	4420
September	1st to 10th	Day	36% 7% 15% 30% 12% 1	2400
	11th to 20th	Day	33% 6% 13% 32% 15% 1	2320
	21st to end	Day	41% 6% 14% 27% 12% 1	2200
October	1st to 10th	Day	44% 5% 13% 28% 10%	3341
	11th to 20th	Day	43% 5% 17% 27% 7%	3178
	21st to end	Day	44% 4 20% 23% 9% 1	3180
November	1st to 10th	Day	49% 5% 14% 22% 8% 1	1851
	11th to 20th	Day	53% 6% 12% 18% 11% 1	1795
	21st to end	Day	59% 8% 13% 14% 5%	1680
December	1st to 10th	Day	69% 4 9% 15% 3	2680
	11th to 20th	Day	60% 5% 11% 17% 8%	3066
	21st to end	Day	65% 5% 11% 13% 6%	4048
January	1st to 10th	Night	66% 5% 10% 14% 5% 1	8234
	11th to 20th	Night	67% 5% 9% 12% 6%	8280
	21st to end	Night	60% 4 9% 14% 13%	9057
February	1st to 10th	Night	60% 5% 10% 17% 7% 1	2767
	11th to 20th	Night	54% 5% 11% 21% 9%	2561
	21st to end	Night	55% 4 8% 18% 12% 1	2076
March	1st to 10th	Night	58% 4 7% 15% 12% 4	5494
	11th to 20th	Night	52% 5% 13% 16% 12% 2	5356
	21st to end	Night	54% 4 6% 21% 13% 2	5080
April	1st to 10th	Night	54% 4 8% 19% 14% 2	2100
	11th to 20th	Night	53% 3 7% 21% 14% 2	2075
	21st to end	Night	50% 4 9% 18% 15% 4	1937
May	1st to 10th	Night	57% 6% 11% 17% 8% 2	4794
	11th to 20th	Night	51% 6% 10% 23% 9% 1	4640
	21st to end	Night	51% 5% 9% 19% 16% 1	5115
June	1st to 10th	Night	49% 7% 9% 21% 13% 1	1680
	11th to 20th	Night	43% 6% 12% 22% 15% 2	1680
	21st to end	Night	48% 7% 11% 20% 13% 1	1680
July	1st to 10th	Night	43% 6% 17% 21% 13% 1	4640
	11th to 20th	Night	36% 5% 16% 31% 12%	4632
	21st to end	Night	40% 6% 16% 27% 11%	4967
August	1st to 10th	Night	45% 6% 14% 26% 10%	2656
	11th to 20th	Night	43% 6% 18% 24% 9%	3006
	21st to end	Night	43% 7% 14% 21% 13% 1	3677
September	1st to 10th	Night	37% 6% 13% 28% 16% 1	2000
	11th to 20th	Night	35% 4 14% 33% 13% 1	2078
	21st to end	Night	41% 6% 12% 26% 15%	2200
October	1st to 10th	Night	43% 5% 14% 27% 11% 1	3835
	11th to 20th	Night	43% 4 13% 31% 9%	3959
	21st to end	Night	47% 4 16% 19% 12% 1	4610
November	1st to 10th	Night	52% 4 12% 22% 8% 1	2667
	11th to 20th	Night	52% 5% 13% 19% 10% 1	2819
	21st to end	Night	60% 5% 12% 17% 6%	2940
December	1st to 10th	Night	68% 5% 11% 12% 4	4720
	11th to 20th	Night	62% 4 8% 15% 11%	5348
	21st to end	Night	64% 4 8% 15% 10%	7150

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	25% 12% 29% 23% 10%	220
	11th to 20th	Day	27% 19% 27% 20% 5% 1	220
	21st to end	Day	20% 14% 31% 26% 7% 1	242
February	1st to 10th	Day	27% 17% 34% 18% 4	311
	11th to 20th	Day	22% 15% 38% 22% 4	432
	21st to end	Day	26% 11% 33% 27% 2	446
March	1st to 10th	Day	25% 13% 32% 23% 7%	540
	11th to 20th	Day	23% 11% 35% 24% 6% 1	540
	21st to end	Day	26% 13% 31% 20% 8% 2	594
April	1st to 10th	Day	26% 16% 29% 19% 8% 2	540
	11th to 20th	Day	27% 16% 25% 19% 10% 2	532
	21st to end	Day	27% 13% 30% 19% 9% 2	540
May	1st to 10th	Day	24% 10% 30% 25% 9% 3	543
	11th to 20th	Day	20% 8% 30% 25% 12% 4	567
	21st to end	Day	21% 9% 31% 23% 12% 3	617
June	1st to 10th	Day	22% 9% 26% 28% 12% 3	570
	11th to 20th	Day	17% 10% 26% 31% 14% 3	570
	21st to end	Day	20% 7% 26% 28% 16% 4	570
July	1st to 10th	Day	10% 7% 27% 32% 17% 6%	565
	11th to 20th	Day	12% 7% 30% 29% 17% 5%	564
	21st to end	Day	14% 6% 22% 33% 20% 5%	620
August	1st to 10th	Day	14% 3 22% 33% 21% 7%	566
	11th to 20th	Day	11% 6% 20% 30% 23% 10%	559
	21st to end	Day	14% 7% 25% 24% 22% 8%	619
September	1st to 10th	Day	12% 7% 24% 28% 18% 10%	530
	11th to 20th	Day	13% 6% 19% 25% 23% 14%	530
	21st to end	Day	19% 7% 25% 24% 19% 6%	530
October	1st to 10th	Day	20% 9% 28% 22% 15% 6%	530
	11th to 20th	Day	19% 12% 31% 23% 12% 2	508
	21st to end	Day	32% 11% 24% 23% 10%	462
November	1st to 10th	Day	29% 12% 24% 23% 10% 1	425
	11th to 20th	Day	22% 14% 35% 21% 7% 1	335
	21st to end	Day	31% 15% 27% 21% 5% 1	220
December	1st to 10th	Day	28% 10% 26% 26% 8% 1	220
	11th to 20th	Day	25% 13% 39% 18% 4 1	220
	21st to end	Day	28% 15% 29% 19% 7% 1	242
January	1st to 10th	Night	25% 11% 32% 27% 5%	840
	11th to 20th	Night	25% 11% 33% 26% 5%	840
	21st to end	Night	19% 8% 35% 29% 8% 1	924
February	1st to 10th	Night	20% 7% 37% 28% 7%	840
	11th to 20th	Night	21% 9% 37% 27% 5%	823
	21st to end	Night	23% 9% 33% 32% 4	692
March	1st to 10th	Night	25% 11% 31% 28% 5%	840
	11th to 20th	Night	27% 9% 31% 26% 7%	840
	21st to end	Night	23% 9% 28% 31% 7%	924
April	1st to 10th	Night	28% 9% 28% 29% 5%	840
	11th to 20th	Night	27% 10% 30% 27% 6%	831
	21st to end	Night	26% 8% 30% 26% 9%	677
May	1st to 10th	Night	23% 7% 31% 30% 7% 1	470
	11th to 20th	Night	23% 5% 27% 36% 8% 1	414
	21st to end	Night	19% 6% 33% 33% 9%	455
June	1st to 10th	Night	23% 5% 23% 36% 12%	420
	11th to 20th	Night	19% 6% 25% 36% 13%	420
	21st to end	Night	20% 5% 24% 36% 15% 1	420
July	1st to 10th	Night	12% 3 27% 41% 16% 1	410
	11th to 20th	Night	12% 4 21% 46% 17% 1	408
	21st to end	Night	13% 3 18% 45% 20% 1	460
August	1st to 10th	Night	16% 2 13% 46% 22% 1	418
	11th to 20th	Night	14% 3 19% 37% 25% 2	625
	21st to end	Night	15% 3 20% 37% 22% 2	917
September	1st to 10th	Night	13% 4 22% 42% 18% 1	800
	11th to 20th	Night	17% 4 16% 36% 24% 3	800
	21st to end	Night	19% 4 25% 33% 18% 2	800
October	1st to 10th	Night	20% 6% 26% 34% 14% 1	800
	11th to 20th	Night	23% 7% 28% 31% 11% 1	800
	21st to end	Night	26% 8% 30% 30% 6%	880
November	1st to 10th	Night	25% 10% 29% 28% 9%	820
	11th to 20th	Night	24% 9% 33% 24% 10% 1	840
	21st to end	Night	29% 11% 29% 23% 7% 1	840
December	1st to 10th	Night	29% 9% 31% 24% 6%	840
	11th to 20th	Night	28% 11% 33% 21% 6% 1	840
	21st to end	Night	33% 11% 29% 22% 5% 1	924

Page generated at 2026-08-27 04:07:16. Version ac07

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