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 2025-04-26.

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% 32%	113570
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 30%	115031
Transparency	Day	49% 6% 12% 21% 10% 1	117063
	Night	53% 5% 11% 19% 10% 1	119702
Seeing	Day	21% 10% 28% 25% 13% 4	15074
	Night	23% 7% 28% 31% 10% 1	24207

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	15% 19% 5% 14% 5% 4 4 3 5% 5% 22%	3597
	11th to 20th	Day	12% 14% 7% 12% 6% 6% 5% 3 5% 5% 25%	3570
	21st to end	Day	11% 14% 5% 11% 5% 6% 6% 4 5% 5% 27%	3960
February	1st to 10th	Day	14% 15% 4 11% 5% 5% 5% 4 4 6% 26%	1764
	11th to 20th	Day	14% 11% 4 11% 6% 5% 5% 5% 6% 5% 27%	1850
	21st to end	Day	18% 15% 4 10% 6% 4 4 3 5% 5% 27%	1650
March	1st to 10th	Day	19% 12% 5% 11% 6% 5% 4 3 4 5% 25%	4476
	11th to 20th	Day	10% 12% 4 10% 6% 5% 5% 4 6% 6% 31%	4704
	21st to end	Day	18% 10% 3 8% 5% 5% 5% 3 4 5% 34%	5961
April	1st to 10th	Day	21% 11% 3 8% 5% 4 4 3 5% 5% 32%	2400
	11th to 20th	Day	20% 9% 3 7% 5% 4 4 3 4 5% 36%	2374
	21st to end	Day	21% 11% 3 7% 5% 4 4 4 4 6% 32%	2490
May	1st to 10th	Day	19% 9% 3 8% 5% 5% 5% 4 6% 7% 28%	4384
	11th to 20th	Day	17% 9% 3 8% 5% 4 5% 4 5% 7% 32%	4398
	21st to end	Day	15% 6% 2 7% 5% 4 5% 3 7% 8% 37%	4837
June	1st to 10th	Day	20% 8% 3 7% 5% 5% 5% 4 6% 7% 31%	2660
	11th to 20th	Day	14% 6% 3 7% 5% 5% 5% 4 6% 8% 38%	2660
	21st to end	Day	19% 8% 3 7% 6% 5% 5% 4 6% 7% 31%	2660
July	1st to 10th	Day	11% 6% 2 8% 6% 6% 6% 4 6% 9% 37%	4440
	11th to 20th	Day	12% 6% 2 6% 5% 4 6% 5% 8% 9% 38%	4440
	21st to end	Day	11% 6% 2 5% 4 4 5% 4 7% 8% 45%	4814
August	1st to 10th	Day	13% 6% 3 8% 5% 5% 6% 4 7% 9% 33%	4423
	11th to 20th	Day	13% 6% 3 8% 5% 5% 4 3 6% 9% 39%	3990
	21st to end	Day	15% 4 2 6% 5% 4 5% 4 6% 9% 39%	4180
September	1st to 10th	Day	11% 6% 2 5% 5% 5% 5% 4 6% 10% 41%	2280
	11th to 20th	Day	13% 5% 2 5% 3 4 4 3 6% 6% 49%	2204
	21st to end	Day	16% 6% 3 6% 4 4 4 3 6% 7% 41%	2090
October	1st to 10th	Day	18% 8% 3 7% 4 5% 4 4 5% 6% 36%	1952
	11th to 20th	Day	18% 8% 3 7% 7% 6% 5% 4 6% 6% 30%	1864
	21st to end	Day	23% 10% 3 6% 4 4 4 3 5% 7% 30%	1881
November	1st to 10th	Day	16% 11% 3 6% 4 4 5% 4 6% 7% 34%	1761
	11th to 20th	Day	17% 10% 4 8% 4 5% 4 4 7% 6% 31%	1710
	21st to end	Day	22% 12% 3 7% 6% 5% 5% 4 6% 6% 24%	1600
December	1st to 10th	Day	21% 18% 6% 11% 5% 4 4 4 5% 5% 17%	2600
	11th to 20th	Day	17% 13% 4 10% 6% 5% 5% 4 5% 4 27%	2986
	21st to end	Day	24% 12% 4 10% 5% 4 6% 4 4 5% 22%	3960
January	1st to 10th	Night	14% 20% 8% 14% 5% 4 4 3 4 5% 20%	6349
	11th to 20th	Night	10% 15% 6% 16% 8% 6% 5% 3 5% 6% 20%	6315
	21st to end	Night	13% 15% 5% 11% 8% 5% 5% 4 5% 6% 25%	6996
February	1st to 10th	Night	14% 15% 5% 11% 7% 5% 5% 4 5% 6% 24%	2635
	11th to 20th	Night	14% 11% 4 9% 6% 5% 6% 4 6% 7% 29%	2434
	21st to end	Night	15% 15% 4 9% 5% 4 4 3 4 5% 32%	1980
March	1st to 10th	Night	21% 13% 3 9% 4 4 4 3 4 5% 30%	5364
	11th to 20th	Night	14% 13% 4 8% 5% 5% 4 4 6% 8% 30%	5239
	21st to end	Night	16% 10% 4 10% 5% 5% 5% 4 5% 5% 33%	4970
April	1st to 10th	Night	23% 9% 4 6% 4 4 4 3 5% 6% 31%	2000
	11th to 20th	Night	22% 9% 3 6% 6% 4 3 3 4 6% 34%	1975
	21st to end	Night	21% 9% 2 7% 4 4 4 3 5% 6% 34%	1807
May	1st to 10th	Night	21% 10% 3 8% 4 4 4 3 6% 7% 30%	2586
	11th to 20th	Night	23% 8% 2 6% 4 4 4 3 5% 6% 36%	2495
	21st to end	Night	15% 8% 3 7% 5% 4 4 4 5% 9% 36%	2738
June	1st to 10th	Night	20% 8% 3 7% 5% 4 4 4 6% 8% 31%	1520
	11th to 20th	Night	15% 7% 3 7% 4 3 4 3 6% 8% 40%	1520
	21st to end	Night	18% 7% 2 8% 5% 4 5% 4 6% 6% 34%	1520
July	1st to 10th	Night	13% 7% 3 8% 5% 5% 5% 5% 7% 8% 34%	2520
	11th to 20th	Night	13% 5% 2 7% 5% 4 4 3 6% 9% 41%	2520
	21st to end	Night	11% 6% 3 6% 4 4 4 4 7% 8% 43%	2739
August	1st to 10th	Night	14% 8% 3 8% 4 4 5% 3 5% 7% 38%	2506
	11th to 20th	Night	15% 7% 3 8% 4 4 4 3 6% 7% 39%	2802
	21st to end	Night	16% 6% 2 7% 5% 4 4 4 6% 7% 39%	3476
September	1st to 10th	Night	11% 4 3 7% 5% 5% 4 4 6% 8% 44%	1900
	11th to 20th	Night	14% 6% 3 6% 3 3 2 2 5% 7% 49%	1974
	21st to end	Night	17% 5% 2 6% 4 4 4 3 5% 7% 43%	2090
October	1st to 10th	Night	18% 7% 2 7% 4 5% 5% 3 5% 6% 38%	2224
	11th to 20th	Night	20% 8% 3 7% 4 5% 4 3 4 6% 36%	2314
	21st to end	Night	21% 9% 3 8% 5% 4 5% 3 5% 5% 31%	2717
November	1st to 10th	Night	19% 10% 3 6% 5% 4 4 4 6% 6% 32%	2537
	11th to 20th	Night	19% 9% 3 8% 4 4 4 4 6% 6% 33%	2685
	21st to end	Night	22% 13% 4 8% 6% 4 4 3 4 5% 27%	2800
December	1st to 10th	Night	17% 17% 5% 11% 7% 5% 6% 4 5% 5% 19%	4580
	11th to 20th	Night	23% 14% 4 9% 4 4 4 3 6% 5% 24%	5208
	21st to end	Night	25% 16% 4 9% 4 3 3 3 6% 5% 23%	6996

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% 7% 9% 16% 3	3598
	11th to 20th	Day	59% 6% 11% 15% 7% 1	3600
	21st to end	Day	57% 6% 12% 15% 10% 1	3960
February	1st to 10th	Day	59% 4 9% 19% 6% 2	1764
	11th to 20th	Day	57% 7% 11% 15% 9% 1	1859
	21st to end	Day	59% 4 10% 16% 10% 2	1650
March	1st to 10th	Day	60% 5% 7% 14% 11% 3	4479
	11th to 20th	Day	50% 7% 16% 14% 11% 2	4704
	21st to end	Day	52% 5% 7% 16% 18% 2	5961
April	1st to 10th	Day	54% 5% 7% 19% 13% 2	2400
	11th to 20th	Day	49% 5% 7% 21% 16% 2	2374
	21st to end	Day	53% 5% 9% 17% 12% 3	2490
May	1st to 10th	Day	51% 7% 9% 22% 10% 1	4398
	11th to 20th	Day	48% 9% 14% 18% 9% 2	4402
	21st to end	Day	42% 7% 12% 24% 14% 2	4837
June	1st to 10th	Day	50% 7% 11% 19% 12% 2	2660
	11th to 20th	Day	43% 7% 11% 23% 14% 3	2660
	21st to end	Day	51% 7% 11% 20% 10% 1	2660
July	1st to 10th	Day	42% 7% 13% 27% 11% 1	4370
	11th to 20th	Day	39% 7% 15% 30% 8%	4365
	21st to end	Day	36% 6% 14% 34% 10%	4661
August	1st to 10th	Day	45% 7% 17% 22% 9%	4409
	11th to 20th	Day	42% 6% 17% 28% 7%	4032
	21st to end	Day	40% 6% 17% 21% 15% 1	4180
September	1st to 10th	Day	37% 7% 15% 28% 13% 1	2280
	11th to 20th	Day	34% 6% 13% 31% 15% 1	2204
	21st to end	Day	41% 6% 14% 27% 11% 1	2090
October	1st to 10th	Day	45% 5% 13% 28% 9%	3238
	11th to 20th	Day	44% 5% 17% 27% 7%	3080
	21st to end	Day	44% 4 20% 23% 9% 1	3081
November	1st to 10th	Day	48% 5% 14% 23% 8% 1	1761
	11th to 20th	Day	52% 6% 12% 18% 12% 1	1710
	21st to end	Day	58% 8% 13% 15% 5%	1600
December	1st to 10th	Day	69% 4 9% 15% 3	2600
	11th to 20th	Day	59% 5% 10% 17% 9%	2986
	21st to end	Day	64% 5% 11% 13% 6%	3960
January	1st to 10th	Night	67% 5% 11% 13% 4	6349
	11th to 20th	Night	64% 6% 12% 14% 4	6360
	21st to end	Night	61% 5% 10% 12% 13% 1	6996
February	1st to 10th	Night	61% 5% 10% 16% 7% 1	2635
	11th to 20th	Night	54% 5% 12% 20% 9%	2436
	21st to end	Night	56% 5% 8% 18% 12% 1	1980
March	1st to 10th	Night	58% 4 7% 15% 12% 4	5374
	11th to 20th	Night	52% 5% 14% 16% 12% 2	5240
	21st to end	Night	54% 4 6% 21% 13% 2	4970
April	1st to 10th	Night	54% 4 8% 19% 14% 2	2000
	11th to 20th	Night	53% 3 7% 21% 14% 2	1975
	21st to end	Night	50% 4 9% 19% 14% 4	1807
May	1st to 10th	Night	52% 7% 10% 20% 9% 3	2592
	11th to 20th	Night	48% 7% 12% 19% 11% 2	2496
	21st to end	Night	45% 6% 9% 20% 18% 1	2739
June	1st to 10th	Night	50% 7% 10% 21% 12% 1	1520
	11th to 20th	Night	42% 6% 12% 22% 15% 3	1520
	21st to end	Night	48% 8% 11% 19% 13% 1	1520
July	1st to 10th	Night	46% 6% 14% 20% 13%	2480
	11th to 20th	Night	39% 7% 18% 27% 9%	2472
	21st to end	Night	37% 7% 18% 25% 13%	2651
August	1st to 10th	Night	46% 6% 13% 26% 9%	2496
	11th to 20th	Night	44% 7% 18% 23% 8%	2826
	21st to end	Night	43% 7% 14% 21% 14% 1	3476
September	1st to 10th	Night	38% 6% 13% 26% 16% 1	1900
	11th to 20th	Night	36% 4 14% 32% 13% 1	1974
	21st to end	Night	41% 6% 12% 26% 15%	2090
October	1st to 10th	Night	44% 5% 14% 27% 10%	3718
	11th to 20th	Night	44% 4 13% 31% 9%	3837
	21st to end	Night	47% 4 17% 19% 12% 1	4467
November	1st to 10th	Night	52% 5% 12% 22% 8% 1	2537
	11th to 20th	Night	52% 5% 13% 19% 11% 1	2685
	21st to end	Night	60% 5% 12% 17% 6%	2800
December	1st to 10th	Night	68% 5% 12% 12% 4	4580
	11th to 20th	Night	62% 4 8% 16% 11%	5208
	21st to end	Night	63% 4 8% 14% 10%	6996

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% 12% 29% 24% 10%	200
	11th to 20th	Day	28% 15% 29% 22% 6% 2	200
	21st to end	Day	21% 13% 33% 25% 7% 1	220
February	1st to 10th	Day	29% 17% 35% 16% 3	283
	11th to 20th	Day	22% 15% 38% 21% 3	397
	21st to end	Day	27% 11% 32% 27% 2	414
March	1st to 10th	Day	25% 12% 31% 24% 8%	500
	11th to 20th	Day	21% 11% 36% 24% 6% 1	500
	21st to end	Day	26% 13% 31% 20% 8% 3	550
April	1st to 10th	Day	26% 16% 28% 20% 8% 2	500
	11th to 20th	Day	27% 16% 25% 19% 10% 2	492
	21st to end	Day	27% 13% 29% 20% 9% 3	480
May	1st to 10th	Day	22% 9% 31% 25% 10% 3	463
	11th to 20th	Day	19% 7% 30% 27% 12% 5%	487
	21st to end	Day	18% 9% 32% 25% 13% 3	529
June	1st to 10th	Day	23% 10% 25% 28% 12% 3	490
	11th to 20th	Day	16% 9% 23% 33% 15% 3	490
	21st to end	Day	21% 7% 25% 27% 16% 4	490
July	1st to 10th	Day	11% 7% 24% 33% 19% 6%	485
	11th to 20th	Day	13% 6% 27% 32% 18% 5%	484
	21st to end	Day	13% 5% 21% 35% 21% 5%	532
August	1st to 10th	Day	15% 3 21% 33% 21% 7%	486
	11th to 20th	Day	11% 6% 19% 29% 23% 11%	479
	21st to end	Day	14% 7% 24% 25% 21% 9%	539
September	1st to 10th	Day	12% 7% 24% 29% 18% 10%	490
	11th to 20th	Day	14% 7% 20% 25% 21% 14%	490
	21st to end	Day	18% 7% 25% 24% 19% 6%	490
October	1st to 10th	Day	21% 9% 28% 22% 14% 6%	490
	11th to 20th	Day	21% 11% 31% 22% 13% 2	470
	21st to end	Day	32% 11% 24% 22% 10%	429
November	1st to 10th	Day	28% 11% 25% 24% 11% 1	395
	11th to 20th	Day	21% 15% 35% 21% 7% 1	310
	21st to end	Day	30% 14% 28% 22% 6% 1	200
December	1st to 10th	Day	29% 10% 26% 27% 8% 2	200
	11th to 20th	Day	25% 15% 37% 19% 4 1	200
	21st to end	Day	29% 15% 28% 20% 7% 1	220
January	1st to 10th	Night	25% 11% 31% 28% 5%	800
	11th to 20th	Night	25% 10% 33% 27% 6%	800
	21st to end	Night	19% 7% 36% 30% 8%	880
February	1st to 10th	Night	21% 8% 37% 28% 6% 1	800
	11th to 20th	Night	22% 10% 37% 27% 4	783
	21st to end	Night	23% 8% 32% 32% 4	660
March	1st to 10th	Night	25% 10% 31% 29% 6%	800
	11th to 20th	Night	26% 9% 31% 26% 7%	800
	21st to end	Night	23% 9% 28% 32% 8%	880
April	1st to 10th	Night	28% 9% 28% 30% 5%	800
	11th to 20th	Night	26% 10% 30% 27% 7% 1	791
	21st to end	Night	27% 8% 30% 26% 9%	633
May	1st to 10th	Night	23% 6% 31% 30% 8% 2	426
	11th to 20th	Night	23% 5% 26% 36% 9% 1	374
	21st to end	Night	17% 6% 33% 35% 9%	411
June	1st to 10th	Night	24% 5% 22% 36% 12% 1	380
	11th to 20th	Night	18% 4 26% 37% 14% 1	380
	21st to end	Night	19% 6% 23% 36% 16% 1	380
July	1st to 10th	Night	12% 4 25% 42% 17% 1	370
	11th to 20th	Night	12% 3 20% 48% 17% 1	368
	21st to end	Night	13% 3 18% 45% 20% 1	416
August	1st to 10th	Night	17% 2 12% 45% 23% 1	378
	11th to 20th	Night	15% 2 18% 36% 26% 2	565
	21st to end	Night	16% 3 20% 37% 23% 2	836
September	1st to 10th	Night	14% 4 23% 42% 17% 1	760
	11th to 20th	Night	18% 4 17% 36% 23% 3	760
	21st to end	Night	19% 3 25% 33% 19% 2	760
October	1st to 10th	Night	21% 6% 26% 33% 13% 1	760
	11th to 20th	Night	23% 7% 27% 31% 11% 1	760
	21st to end	Night	27% 8% 30% 29% 6%	836
November	1st to 10th	Night	24% 10% 29% 28% 9%	780
	11th to 20th	Night	24% 9% 33% 25% 10% 1	800
	21st to end	Night	30% 11% 28% 23% 8% 1	800
December	1st to 10th	Night	30% 8% 31% 24% 7%	800
	11th to 20th	Night	29% 10% 32% 22% 6% 1	800
	21st to end	Night	32% 11% 29% 22% 5% 1	880

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