Laval
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Laval 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	17% 9% 3 8% 5% 5% 5% 4 5% 7% 32%	138061
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	134330
Transparency	Day	51% 4 11% 22% 10% 1	141508
	Night	54% 3 10% 20% 11% 1	138972
Seeing	Day	22% 9% 28% 25% 12% 4	16605
	Night	23% 7% 28% 31% 11% 1	25575

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% 16% 6% 13% 5% 4 4 3 4 5% 25%	4661
	11th to 20th	Day	11% 16% 6% 14% 6% 5% 5% 3 5% 6% 23%	4650
	21st to end	Day	10% 13% 5% 11% 8% 7% 6% 5% 5% 5% 25%	5124
February	1st to 10th	Day	14% 15% 4 10% 7% 4 5% 4 3 6% 29%	1841
	11th to 20th	Day	13% 12% 5% 11% 6% 5% 5% 5% 5% 6% 27%	1952
	21st to end	Day	16% 17% 4 9% 6% 4 4 4 4 5% 28%	1730
March	1st to 10th	Day	19% 12% 5% 11% 6% 5% 4 3 4 5% 26%	4576
	11th to 20th	Day	11% 14% 4 9% 6% 5% 4 3 6% 7% 32%	4797
	21st to end	Day	19% 9% 4 9% 5% 4 4 3 4 4 34%	6100
April	1st to 10th	Day	22% 12% 3 7% 6% 4 4 3 4 5% 31%	2520
	11th to 20th	Day	21% 9% 3 7% 5% 4 4 3 4 5% 36%	2494
	21st to end	Day	21% 10% 3 8% 5% 3 5% 3 4 6% 33%	2692
May	1st to 10th	Day	23% 9% 3 9% 5% 5% 5% 4 6% 7% 25%	8131
	11th to 20th	Day	20% 7% 3 8% 5% 4 5% 4 6% 7% 31%	8221
	21st to end	Day	22% 8% 3 6% 4 4 5% 4 5% 7% 33%	9060
June	1st to 10th	Day	20% 9% 3 7% 4 5% 5% 4 6% 7% 31%	2940
	11th to 20th	Day	14% 6% 3 7% 5% 5% 5% 4 6% 9% 37%	2940
	21st to end	Day	19% 7% 3 7% 6% 5% 5% 4 6% 8% 31%	2940
July	1st to 10th	Day	13% 5% 3 8% 6% 6% 6% 5% 6% 9% 33%	6360
	11th to 20th	Day	11% 5% 2 7% 4 5% 5% 4 7% 9% 41%	6360
	21st to end	Day	10% 6% 2 5% 3 4 4 4 6% 9% 46%	6926
August	1st to 10th	Day	13% 6% 3 7% 5% 5% 6% 5% 7% 9% 34%	4563
	11th to 20th	Day	13% 6% 3 7% 5% 4 5% 3 6% 8% 39%	4128
	21st to end	Day	15% 5% 2 6% 4 4 4 4 6% 9% 40%	4312
September	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 6% 10% 42%	2400
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 5% 7% 50%	2315
	21st to end	Day	16% 6% 2 6% 5% 4 4 4 4 8% 41%	2200
October	1st to 10th	Day	19% 6% 3 7% 4 5% 4 3 5% 6% 38%	2065
	11th to 20th	Day	18% 8% 3 8% 6% 6% 5% 4 6% 7% 30%	1955
	21st to end	Day	24% 10% 2 7% 5% 4 4 3 5% 6% 31%	1980
November	1st to 10th	Day	17% 11% 3 7% 5% 4 4 4 5% 7% 34%	1851
	11th to 20th	Day	18% 12% 3 8% 4 4 5% 3 5% 7% 31%	1803
	21st to end	Day	23% 13% 3 7% 5% 5% 5% 3 5% 6% 26%	1680
December	1st to 10th	Day	20% 18% 7% 12% 5% 4 4 3 4 5% 19%	2680
	11th to 20th	Day	18% 13% 4 9% 5% 5% 5% 4 5% 4 27%	3066
	21st to end	Day	23% 13% 3 10% 5% 4 5% 3 5% 6% 22%	4048
January	1st to 10th	Night	13% 17% 7% 16% 6% 4 4 3 4 5% 22%	8234
	11th to 20th	Night	11% 17% 6% 16% 8% 6% 5% 3 5% 5% 19%	8235
	21st to end	Night	11% 13% 5% 13% 8% 6% 5% 3 5% 5% 27%	9057
February	1st to 10th	Night	13% 15% 5% 12% 6% 5% 6% 3 5% 6% 24%	2777
	11th to 20th	Night	14% 12% 4 10% 5% 4 4 4 5% 5% 31%	2554
	21st to end	Night	15% 15% 4 9% 6% 4 4 3 4 5% 32%	2076
March	1st to 10th	Night	21% 13% 3 8% 5% 4 4 3 4 5% 30%	5484
	11th to 20th	Night	14% 14% 4 8% 5% 4 4 4 6% 7% 30%	5378
	21st to end	Night	16% 11% 3 10% 5% 5% 5% 3 5% 5% 32%	5073
April	1st to 10th	Night	24% 10% 3 6% 5% 4 4 4 5% 5% 32%	2100
	11th to 20th	Night	22% 9% 2 7% 5% 4 4 3 4 5% 35%	2075
	21st to end	Night	22% 9% 2 6% 4 4 4 3 5% 4 37%	1927
May	1st to 10th	Night	22% 8% 3 9% 5% 5% 6% 4 6% 7% 25%	4822
	11th to 20th	Night	22% 8% 3 8% 4 5% 4 4 5% 7% 31%	4639
	21st to end	Night	20% 8% 3 7% 4 4 5% 4 5% 6% 32%	5114
June	1st to 10th	Night	20% 8% 2 7% 5% 4 6% 3 6% 7% 33%	1680
	11th to 20th	Night	17% 6% 3 6% 4 4 5% 4 6% 7% 39%	1680
	21st to end	Night	19% 6% 3 7% 5% 4 5% 4 6% 8% 34%	1680
July	1st to 10th	Night	13% 6% 3 6% 5% 5% 5% 5% 7% 10% 34%	3600
	11th to 20th	Night	14% 6% 2 6% 4 5% 4 4 6% 8% 41%	3600
	21st to end	Night	10% 5% 2 6% 5% 4 5% 3 7% 7% 45%	3926
August	1st to 10th	Night	14% 7% 3 8% 5% 4 4 4 5% 8% 38%	2586
	11th to 20th	Night	14% 6% 3 8% 5% 4 4 4 5% 7% 39%	2886
	21st to end	Night	16% 5% 3 8% 5% 5% 5% 3 5% 6% 39%	3586
September	1st to 10th	Night	11% 5% 3 7% 5% 4 4 4 5% 8% 46%	2000
	11th to 20th	Night	13% 6% 2 5% 3 3 2 3 5% 7% 51%	2085
	21st to end	Night	16% 5% 2 7% 4 5% 4 3 4 6% 44%	2200
October	1st to 10th	Night	18% 6% 3 6% 4 4 5% 4 5% 6% 41%	2332
	11th to 20th	Night	19% 7% 3 8% 4 4 4 2 4 6% 37%	2445
	21st to end	Night	22% 8% 4 7% 4 5% 4 3 5% 5% 32%	2860
November	1st to 10th	Night	20% 9% 3 7% 5% 4 4 4 5% 6% 33%	2667
	11th to 20th	Night	19% 10% 3 8% 5% 3 4 4 5% 5% 34%	2814
	21st to end	Night	21% 14% 4 9% 5% 4 3 3 4 5% 28%	2940
December	1st to 10th	Night	17% 17% 6% 10% 7% 6% 5% 4 4 5% 19%	4720
	11th to 20th	Night	23% 13% 4 9% 5% 4 4 3 6% 5% 25%	5348
	21st to end	Night	25% 15% 5% 9% 4 3 3 3 5% 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	64% 2 11% 19% 4	4662
	11th to 20th	Day	63% 3 8% 15% 10% 1	4680
	21st to end	Day	60% 3 10% 14% 11% 1	5124
February	1st to 10th	Day	58% 2 9% 21% 7% 2	1841
	11th to 20th	Day	58% 4 11% 18% 9% 1	1961
	21st to end	Day	60% 2 9% 16% 11% 2	1730
March	1st to 10th	Day	62% 2 7% 14% 11% 4	4579
	11th to 20th	Day	53% 3 16% 15% 11% 2	4797
	21st to end	Day	54% 2 6% 17% 19% 2	6100
April	1st to 10th	Day	56% 3 7% 19% 13% 2	2520
	11th to 20th	Day	52% 2 6% 22% 16% 2	2494
	21st to end	Day	54% 3 8% 19% 13% 3	2692
May	1st to 10th	Day	59% 3 6% 21% 10% 1	8143
	11th to 20th	Day	52% 4 13% 21% 9% 1	8225
	21st to end	Day	51% 3 9% 24% 11% 1	9061
June	1st to 10th	Day	52% 4 9% 21% 13% 2	2940
	11th to 20th	Day	44% 4 10% 25% 14% 3	2940
	21st to end	Day	51% 4 12% 21% 10% 1	2940
July	1st to 10th	Day	46% 5% 13% 27% 9%	6290
	11th to 20th	Day	38% 5% 14% 31% 12%	6285
	21st to end	Day	34% 4 14% 38% 9%	6728
August	1st to 10th	Day	45% 5% 17% 24% 8%	4549
	11th to 20th	Day	43% 5% 17% 28% 7%	4170
	21st to end	Day	41% 5% 16% 23% 15% 1	4312
September	1st to 10th	Day	38% 4 14% 32% 12% 1	2400
	11th to 20th	Day	34% 3 12% 34% 15% 1	2315
	21st to end	Day	43% 4 13% 27% 12% 1	2200
October	1st to 10th	Day	44% 4 14% 28% 10%	3364
	11th to 20th	Day	45% 4 16% 28% 7%	3158
	21st to end	Day	44% 4 19% 24% 9% 1	3180
November	1st to 10th	Day	50% 3 12% 25% 9% 1	1851
	11th to 20th	Day	54% 3 11% 20% 12% 1	1803
	21st to end	Day	61% 3 13% 16% 6%	1680
December	1st to 10th	Day	70% 3 9% 15% 3	2680
	11th to 20th	Day	59% 4 9% 18% 10%	3066
	21st to end	Day	65% 3 11% 14% 7%	4048
January	1st to 10th	Night	67% 2 11% 15% 5% 1	8234
	11th to 20th	Night	68% 3 8% 14% 6%	8280
	21st to end	Night	61% 3 8% 15% 13%	9057
February	1st to 10th	Night	61% 3 9% 19% 8% 1	2777
	11th to 20th	Night	56% 2 10% 22% 10%	2556
	21st to end	Night	56% 2 9% 19% 13% 1	2076
March	1st to 10th	Night	59% 1 7% 15% 12% 5%	5494
	11th to 20th	Night	54% 3 12% 15% 13% 2	5379
	21st to end	Night	55% 2 5% 22% 14% 2	5073
April	1st to 10th	Night	55% 3 6% 20% 14% 2	2100
	11th to 20th	Night	54% 2 5% 22% 14% 2	2075
	21st to end	Night	50% 2 7% 20% 16% 4	1927
May	1st to 10th	Night	59% 3 10% 18% 9% 1	4828
	11th to 20th	Night	53% 3 9% 24% 9% 1	4640
	21st to end	Night	52% 3 9% 19% 16% 1	5115
June	1st to 10th	Night	51% 3 8% 22% 14% 1	1680
	11th to 20th	Night	44% 4 11% 24% 15% 3	1680
	21st to end	Night	48% 5% 11% 20% 13% 1	1680
July	1st to 10th	Night	44% 5% 18% 23% 10%	3560
	11th to 20th	Night	40% 5% 16% 28% 10%	3552
	21st to end	Night	37% 4 16% 29% 13%	3809
August	1st to 10th	Night	46% 4 13% 27% 10%	2576
	11th to 20th	Night	44% 5% 18% 24% 9%	2910
	21st to end	Night	45% 5% 13% 23% 14% 1	3586
September	1st to 10th	Night	38% 4 13% 29% 16% 1	2000
	11th to 20th	Night	35% 3 14% 34% 13% 1	2085
	21st to end	Night	42% 4 11% 28% 16%	2200
October	1st to 10th	Night	44% 4 13% 27% 11% 1	3813
	11th to 20th	Night	43% 3 12% 32% 10%	3981
	21st to end	Night	47% 4 16% 20% 12% 2	4610
November	1st to 10th	Night	52% 3 11% 24% 8% 1	2667
	11th to 20th	Night	53% 3 12% 21% 10% 1	2814
	21st to end	Night	61% 3 11% 19% 7%	2940
December	1st to 10th	Night	68% 4 11% 14% 4	4720
	11th to 20th	Night	61% 3 8% 16% 12%	5348
	21st to end	Night	64% 2 8% 15% 11%	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	27% 7% 29% 27% 10%	220
	11th to 20th	Day	29% 15% 29% 21% 4 2	220
	21st to end	Day	19% 15% 30% 27% 7% 2	242
February	1st to 10th	Day	27% 16% 33% 20% 4	305
	11th to 20th	Day	22% 14% 35% 25% 4	439
	21st to end	Day	26% 12% 33% 25% 4	446
March	1st to 10th	Day	26% 13% 32% 22% 8%	540
	11th to 20th	Day	24% 11% 35% 24% 6% 1	540
	21st to end	Day	27% 12% 32% 19% 7% 2	594
April	1st to 10th	Day	29% 14% 27% 20% 8% 2	540
	11th to 20th	Day	30% 13% 25% 21% 9% 2	532
	21st to end	Day	27% 12% 30% 19% 9% 3	540
May	1st to 10th	Day	24% 10% 31% 24% 8% 3	543
	11th to 20th	Day	20% 9% 29% 25% 13% 4	567
	21st to end	Day	21% 10% 30% 24% 12% 3	617
June	1st to 10th	Day	23% 8% 27% 27% 13% 3	570
	11th to 20th	Day	18% 10% 25% 30% 14% 3	570
	21st to end	Day	20% 8% 25% 27% 16% 4	570
July	1st to 10th	Day	14% 5% 24% 32% 18% 6%	525
	11th to 20th	Day	14% 5% 27% 32% 18% 4	524
	21st to end	Day	13% 4 25% 34% 19% 5%	576
August	1st to 10th	Day	14% 4 22% 32% 21% 7%	526
	11th to 20th	Day	13% 4 18% 32% 22% 11%	519
	21st to end	Day	15% 6% 24% 25% 21% 8%	583
September	1st to 10th	Day	13% 6% 25% 28% 19% 9%	530
	11th to 20th	Day	13% 5% 20% 26% 23% 13%	530
	21st to end	Day	19% 6% 26% 24% 18% 6%	530
October	1st to 10th	Day	19% 9% 27% 24% 15% 6%	530
	11th to 20th	Day	21% 10% 33% 23% 12% 3	505
	21st to end	Day	32% 10% 24% 25% 8% 1	462
November	1st to 10th	Day	29% 12% 25% 23% 10% 1	425
	11th to 20th	Day	25% 11% 34% 21% 8% 1	343
	21st to end	Day	35% 10% 27% 20% 6% 1	220
December	1st to 10th	Day	29% 10% 24% 26% 9% 2	220
	11th to 20th	Day	26% 12% 38% 19% 4 1	220
	21st to end	Day	29% 13% 31% 19% 7% 1	242
January	1st to 10th	Night	26% 9% 32% 26% 6%	840
	11th to 20th	Night	25% 10% 31% 27% 7%	840
	21st to end	Night	20% 7% 34% 30% 8% 1	924
February	1st to 10th	Night	21% 6% 36% 29% 7% 1	840
	11th to 20th	Night	22% 9% 36% 28% 5%	823
	21st to end	Night	24% 7% 32% 33% 4	692
March	1st to 10th	Night	25% 10% 30% 29% 6%	840
	11th to 20th	Night	27% 9% 31% 25% 8%	840
	21st to end	Night	23% 10% 29% 31% 7%	924
April	1st to 10th	Night	27% 10% 28% 29% 5%	840
	11th to 20th	Night	28% 8% 29% 28% 7% 1	831
	21st to end	Night	28% 7% 30% 26% 8%	667
May	1st to 10th	Night	25% 5% 31% 30% 7% 1	478
	11th to 20th	Night	23% 5% 28% 35% 8% 1	414
	21st to end	Night	20% 5% 34% 32% 9%	455
June	1st to 10th	Night	25% 4 23% 35% 13%	420
	11th to 20th	Night	19% 7% 25% 36% 13%	420
	21st to end	Night	21% 4 23% 37% 15% 1	420
July	1st to 10th	Night	12% 3 26% 42% 16% 1	390
	11th to 20th	Night	12% 3 22% 45% 17% 1	388
	21st to end	Night	12% 3 18% 47% 19% 1	438
August	1st to 10th	Night	15% 3 13% 44% 24% 1	398
	11th to 20th	Night	14% 2 18% 37% 26% 3	589
	21st to end	Night	17% 2 21% 38% 22% 2	880
September	1st to 10th	Night	15% 3 22% 42% 18% 1	800
	11th to 20th	Night	17% 3 15% 38% 24% 2	800
	21st to end	Night	18% 3 25% 34% 18% 2	800
October	1st to 10th	Night	20% 5% 27% 33% 14% 1	800
	11th to 20th	Night	24% 5% 26% 33% 11% 1	800
	21st to end	Night	27% 8% 28% 30% 7%	880
November	1st to 10th	Night	25% 10% 27% 29% 9%	820
	11th to 20th	Night	24% 8% 31% 26% 10%	840
	21st to end	Night	30% 10% 28% 23% 8%	840
December	1st to 10th	Night	30% 8% 29% 26% 7%	840
	11th to 20th	Night	28% 11% 32% 22% 6% 1	840
	21st to end	Night	34% 10% 27% 22% 6% 1	924

Page generated at 2026-06-28 02:10:43. Version ac07

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