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 2025-02-25.

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% 4 5% 4 5% 7% 33%	108655
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	110029
Transparency	Day	51% 3 12% 23% 10% 1	112148
	Night	54% 3 11% 20% 11% 1	114700
Seeing	Day	22% 9% 27% 26% 13% 4	14839
	Night	23% 6% 28% 31% 11% 1	23960

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% 18% 7% 13% 5% 4 4 3 4 4 23%	3597
	11th to 20th	Day	12% 15% 6% 12% 6% 5% 4 3 5% 5% 25%	3570
	21st to end	Day	12% 15% 5% 10% 5% 6% 5% 4 5% 5% 28%	3960
February	1st to 10th	Day	15% 15% 4 10% 6% 4 5% 4 3 6% 28%	1753
	11th to 20th	Day	14% 12% 5% 11% 6% 5% 5% 5% 5% 5% 27%	1857
	21st to end	Day	17% 16% 4 9% 5% 5% 4 4 5% 5% 28%	1610
March	1st to 10th	Day	20% 12% 4 9% 6% 5% 4 3 4 5% 26%	3116
	11th to 20th	Day	13% 17% 5% 10% 5% 5% 4 3 4 5% 28%	3278
	21st to end	Day	15% 7% 3 9% 5% 4 4 3 5% 5% 39%	4164
April	1st to 10th	Day	22% 11% 3 7% 6% 4 4 3 4 5% 32%	2280
	11th to 20th	Day	20% 8% 3 7% 5% 4 4 3 4 5% 37%	2254
	21st to end	Day	21% 10% 3 8% 5% 3 5% 3 4 5% 33%	2436
May	1st to 10th	Day	19% 9% 3 8% 5% 4 5% 4 6% 7% 30%	4376
	11th to 20th	Day	18% 8% 3 9% 5% 4 5% 4 6% 7% 32%	4398
	21st to end	Day	15% 6% 2 6% 5% 4 5% 4 6% 9% 37%	4837
June	1st to 10th	Day	20% 9% 3 7% 4 5% 5% 4 6% 7% 30%	2660
	11th to 20th	Day	14% 7% 3 6% 5% 5% 5% 4 6% 9% 37%	2660
	21st to end	Day	20% 7% 3 7% 6% 5% 5% 4 6% 7% 30%	2660
July	1st to 10th	Day	12% 5% 3 7% 6% 6% 6% 5% 6% 9% 36%	4440
	11th to 20th	Day	13% 5% 2 6% 4 5% 5% 5% 8% 9% 38%	4440
	21st to end	Day	11% 6% 2 6% 4 4 4 4 6% 8% 45%	4814
August	1st to 10th	Day	14% 6% 3 7% 6% 5% 6% 5% 7% 9% 33%	4423
	11th to 20th	Day	14% 6% 3 7% 5% 4 5% 4 6% 8% 39%	3999
	21st to end	Day	15% 5% 2 6% 4 4 4 4 6% 9% 39%	4180
September	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 6% 10% 41%	2280
	11th to 20th	Day	14% 5% 2 4 3 3 4 3 5% 7% 49%	2199
	21st to end	Day	16% 6% 2 6% 5% 4 4 4 4 8% 41%	2090
October	1st to 10th	Day	20% 6% 3 7% 4 5% 4 3 5% 6% 36%	1962
	11th to 20th	Day	18% 8% 3 7% 6% 6% 5% 4 6% 7% 30%	1857
	21st to end	Day	24% 10% 2 7% 5% 4 3 3 5% 6% 31%	1881
November	1st to 10th	Day	17% 10% 3 6% 5% 4 4 4 5% 7% 35%	1761
	11th to 20th	Day	17% 11% 3 8% 4 4 5% 3 5% 7% 32%	1717
	21st to end	Day	22% 12% 3 7% 5% 5% 5% 4 5% 6% 26%	1600
December	1st to 10th	Day	21% 17% 7% 12% 5% 4 4 3 4 4 19%	2600
	11th to 20th	Day	19% 13% 4 9% 5% 4 4 4 5% 4 28%	2986
	21st to end	Day	23% 13% 3 10% 5% 4 5% 3 5% 6% 22%	3960
January	1st to 10th	Night	14% 19% 7% 15% 6% 4 4 3 3 4 21%	6349
	11th to 20th	Night	10% 16% 7% 15% 8% 6% 5% 3 5% 5% 21%	6315
	21st to end	Night	13% 15% 5% 12% 7% 5% 5% 3 5% 5% 26%	6996
February	1st to 10th	Night	13% 15% 5% 12% 6% 5% 5% 3 5% 6% 24%	2645
	11th to 20th	Night	14% 12% 4 10% 6% 4 4 5% 5% 5% 30%	2429
	21st to end	Night	16% 15% 4 9% 5% 4 4 3 4 5% 32%	1939
March	1st to 10th	Night	20% 11% 3 7% 5% 4 4 3 4 5% 33%	3724
	11th to 20th	Night	18% 18% 4 10% 5% 4 3 3 4 4 28%	3678
	21st to end	Night	13% 10% 3 9% 5% 5% 4 3 4 5% 39%	3467
April	1st to 10th	Night	24% 9% 3 6% 5% 4 4 4 5% 5% 32%	1900
	11th to 20th	Night	22% 9% 2 7% 5% 4 4 3 4 5% 35%	1875
	21st to end	Night	22% 9% 3 6% 4 3 4 3 6% 4 36%	1743
May	1st to 10th	Night	21% 9% 3 8% 5% 4 5% 3 5% 6% 31%	2592
	11th to 20th	Night	22% 8% 3 7% 4 4 4 3 5% 6% 35%	2495
	21st to end	Night	16% 8% 2 8% 4 5% 5% 4 5% 8% 37%	2738
June	1st to 10th	Night	20% 8% 2 7% 4 4 6% 3 6% 7% 32%	1520
	11th to 20th	Night	16% 6% 3 6% 4 3 5% 3 7% 7% 39%	1520
	21st to end	Night	20% 6% 3 7% 5% 4 5% 3 6% 7% 33%	1520
July	1st to 10th	Night	14% 7% 3 7% 5% 5% 5% 5% 6% 8% 35%	2520
	11th to 20th	Night	14% 6% 2 6% 4 5% 4 4 6% 8% 42%	2520
	21st to end	Night	11% 5% 2 6% 5% 4 5% 4 7% 7% 44%	2739
August	1st to 10th	Night	15% 7% 3 8% 5% 5% 5% 3 5% 7% 37%	2506
	11th to 20th	Night	15% 7% 3 8% 5% 4 4 4 5% 7% 38%	2796
	21st to end	Night	16% 5% 3 8% 5% 5% 5% 3 5% 6% 39%	3476
September	1st to 10th	Night	11% 5% 3 7% 5% 4 4 4 5% 8% 45%	1900
	11th to 20th	Night	14% 6% 2 5% 3 3 2 3 5% 7% 50%	1981
	21st to end	Night	16% 5% 2 7% 4 4 4 3 4 7% 44%	2090
October	1st to 10th	Night	18% 6% 3 6% 4 4 5% 4 5% 6% 39%	2215
	11th to 20th	Night	20% 7% 3 8% 4 4 4 2 4 6% 36%	2323
	21st to end	Night	22% 9% 4 8% 4 5% 4 4 5% 5% 32%	2717
November	1st to 10th	Night	19% 9% 3 7% 5% 4 4 4 5% 6% 33%	2537
	11th to 20th	Night	19% 10% 3 8% 5% 4 4 4 5% 5% 34%	2680
	21st to end	Night	22% 14% 3 8% 5% 4 3 3 4 5% 28%	2800
December	1st to 10th	Night	18% 17% 6% 10% 7% 6% 5% 4 4 5% 19%	4580
	11th to 20th	Night	23% 13% 4 9% 5% 4 3 3 6% 5% 26%	5208
	21st to end	Night	25% 15% 5% 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	67% 2 9% 18% 4	3598
	11th to 20th	Day	61% 3 10% 17% 8% 1	3600
	21st to end	Day	57% 3 12% 16% 11% 1	3960
February	1st to 10th	Day	59% 2 10% 20% 7% 2	1753
	11th to 20th	Day	58% 4 11% 17% 9% 1	1866
	21st to end	Day	60% 2 10% 17% 10% 2	1610
March	1st to 10th	Day	62% 2 8% 19% 8% 1	3119
	11th to 20th	Day	60% 2 10% 14% 12% 3	3278
	21st to end	Day	48% 2 7% 21% 19% 2	4164
April	1st to 10th	Day	56% 3 7% 19% 13% 2	2280
	11th to 20th	Day	51% 2 7% 23% 15% 2	2254
	21st to end	Day	55% 3 8% 18% 13% 3	2436
May	1st to 10th	Day	54% 2 7% 25% 10% 1	4390
	11th to 20th	Day	52% 3 14% 20% 9% 2	4402
	21st to end	Day	45% 3 11% 26% 14% 2	4837
June	1st to 10th	Day	52% 4 9% 21% 12% 2	2660
	11th to 20th	Day	44% 4 11% 24% 14% 3	2660
	21st to end	Day	52% 4 12% 20% 10% 1	2660
July	1st to 10th	Day	43% 6% 13% 28% 11% 1	4370
	11th to 20th	Day	40% 5% 15% 31% 8%	4365
	21st to end	Day	36% 5% 14% 35% 10%	4661
August	1st to 10th	Day	46% 5% 17% 23% 8%	4409
	11th to 20th	Day	43% 5% 18% 28% 7%	4041
	21st to end	Day	41% 5% 16% 22% 15% 1	4180
September	1st to 10th	Day	39% 4 13% 31% 13% 1	2280
	11th to 20th	Day	35% 3 12% 34% 15% 1	2199
	21st to end	Day	43% 4 13% 28% 11% 1	2090
October	1st to 10th	Day	45% 3 14% 28% 9%	3261
	11th to 20th	Day	45% 4 17% 28% 7%	3060
	21st to end	Day	43% 4 19% 24% 10% 1	3081
November	1st to 10th	Day	49% 3 13% 25% 9% 1	1761
	11th to 20th	Day	53% 3 10% 20% 12% 1	1717
	21st to end	Day	60% 4 14% 16% 6% 1	1600
December	1st to 10th	Day	70% 2 9% 15% 3	2600
	11th to 20th	Day	59% 4 8% 19% 10%	2986
	21st to end	Day	65% 3 11% 14% 7%	3960
January	1st to 10th	Night	69% 2 11% 14% 4	6349
	11th to 20th	Night	65% 3 10% 16% 5%	6360
	21st to end	Night	62% 3 9% 13% 13% 1	6996
February	1st to 10th	Night	62% 3 9% 18% 8% 1	2645
	11th to 20th	Night	56% 2 10% 22% 10%	2431
	21st to end	Night	57% 2 9% 18% 13% 1	1939
March	1st to 10th	Night	56% 1 9% 20% 10% 4	3734
	11th to 20th	Night	61% 2 8% 14% 13% 2	3679
	21st to end	Night	50% 2 5% 24% 16% 3	3467
April	1st to 10th	Night	55% 3 7% 20% 15% 2	1900
	11th to 20th	Night	53% 2 5% 23% 15% 2	1875
	21st to end	Night	51% 2 7% 20% 14% 4	1743
May	1st to 10th	Night	56% 1 10% 22% 9% 3	2598
	11th to 20th	Night	52% 2 11% 21% 12% 3	2496
	21st to end	Night	47% 3 9% 21% 18% 1	2739
June	1st to 10th	Night	51% 4 9% 22% 13% 1	1520
	11th to 20th	Night	43% 4 12% 24% 15% 3	1520
	21st to end	Night	49% 5% 12% 20% 14% 1	1520
July	1st to 10th	Night	46% 4 15% 21% 13%	2480
	11th to 20th	Night	40% 4 19% 28% 9%	2472
	21st to end	Night	38% 5% 19% 25% 13%	2651
August	1st to 10th	Night	47% 4 13% 26% 10%	2496
	11th to 20th	Night	45% 5% 18% 24% 8%	2820
	21st to end	Night	45% 5% 13% 22% 14% 1	3476
September	1st to 10th	Night	38% 3 13% 28% 16% 1	1900
	11th to 20th	Night	36% 3 14% 33% 13% 1	1981
	21st to end	Night	42% 4 11% 28% 15%	2090
October	1st to 10th	Night	44% 4 13% 27% 10% 1	3696
	11th to 20th	Night	44% 3 12% 32% 9%	3859
	21st to end	Night	47% 4 16% 19% 12% 2	4467
November	1st to 10th	Night	52% 3 11% 24% 9% 1	2537
	11th to 20th	Night	52% 3 12% 21% 11% 1	2680
	21st to end	Night	60% 2 11% 19% 7%	2800
December	1st to 10th	Night	68% 4 11% 13% 4	4580
	11th to 20th	Night	61% 3 8% 16% 12%	5208
	21st to end	Night	64% 2 8% 15% 11%	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	28% 7% 29% 28% 10%	200
	11th to 20th	Day	28% 13% 30% 23% 5% 2	200
	21st to end	Day	20% 14% 30% 28% 7% 1	220
February	1st to 10th	Day	30% 16% 34% 17% 3	277
	11th to 20th	Day	23% 14% 36% 23% 4	404
	21st to end	Day	27% 10% 32% 26% 4	398
March	1st to 10th	Day	25% 11% 30% 25% 9%	460
	11th to 20th	Day	22% 10% 35% 25% 6% 1	460
	21st to end	Day	27% 10% 33% 20% 8% 3	506
April	1st to 10th	Day	29% 12% 26% 22% 9% 2	460
	11th to 20th	Day	29% 13% 26% 20% 10% 3	452
	21st to end	Day	28% 12% 29% 20% 9% 3	460
May	1st to 10th	Day	22% 9% 31% 26% 9% 3	463
	11th to 20th	Day	20% 6% 30% 26% 13% 5%	487
	21st to end	Day	19% 9% 31% 26% 13% 3	529
June	1st to 10th	Day	24% 8% 25% 27% 13% 3	490
	11th to 20th	Day	17% 9% 24% 32% 15% 4	490
	21st to end	Day	21% 8% 24% 27% 16% 4	490
July	1st to 10th	Day	14% 5% 22% 33% 20% 6%	485
	11th to 20th	Day	14% 5% 26% 32% 18% 4	484
	21st to end	Day	14% 3 23% 36% 19% 5%	532
August	1st to 10th	Day	15% 4 21% 32% 21% 7%	486
	11th to 20th	Day	13% 4 18% 32% 22% 11%	479
	21st to end	Day	15% 6% 24% 26% 20% 9%	539
September	1st to 10th	Day	13% 6% 25% 29% 19% 9%	490
	11th to 20th	Day	14% 5% 22% 25% 21% 13%	490
	21st to end	Day	19% 6% 26% 25% 19% 7%	490
October	1st to 10th	Day	20% 9% 27% 23% 14% 6%	490
	11th to 20th	Day	22% 9% 32% 22% 12% 3	467
	21st to end	Day	32% 10% 25% 25% 8% 1	429
November	1st to 10th	Day	29% 10% 25% 24% 11% 1	395
	11th to 20th	Day	24% 12% 35% 21% 8% 1	317
	21st to end	Day	34% 10% 27% 22% 7% 2	200
December	1st to 10th	Day	30% 10% 24% 27% 9% 2	200
	11th to 20th	Day	27% 13% 36% 19% 5% 1	200
	21st to end	Day	30% 13% 30% 19% 8% 1	220
January	1st to 10th	Night	27% 9% 32% 27% 6%	800
	11th to 20th	Night	25% 9% 32% 28% 7%	800
	21st to end	Night	20% 7% 34% 31% 8% 1	880
February	1st to 10th	Night	22% 6% 37% 29% 6% 1	800
	11th to 20th	Night	22% 9% 36% 27% 5% 1	783
	21st to end	Night	24% 7% 31% 33% 5%	647
March	1st to 10th	Night	25% 8% 29% 31% 6%	760
	11th to 20th	Night	27% 8% 30% 26% 8%	760
	21st to end	Night	23% 7% 30% 32% 8%	836
April	1st to 10th	Night	27% 9% 27% 31% 6%	760
	11th to 20th	Night	27% 8% 29% 28% 7% 1	751
	21st to end	Night	28% 7% 30% 27% 8%	603
May	1st to 10th	Night	25% 5% 31% 30% 8% 1	432
	11th to 20th	Night	23% 4 28% 35% 9% 1	374
	21st to end	Night	18% 4 34% 34% 9%	411
June	1st to 10th	Night	26% 3 22% 34% 13%	380
	11th to 20th	Night	18% 5% 25% 37% 14% 1	380
	21st to end	Night	22% 4 22% 37% 15% 1	380
July	1st to 10th	Night	12% 3 25% 42% 16% 1	370
	11th to 20th	Night	12% 2 21% 46% 17% 1	368
	21st to end	Night	12% 3 17% 46% 20% 1	416
August	1st to 10th	Night	16% 3 12% 44% 24% 1	378
	11th to 20th	Night	15% 2 18% 36% 25% 3	559
	21st to end	Night	16% 2 21% 37% 22% 2	836
September	1st to 10th	Night	15% 2 21% 42% 18% 1	760
	11th to 20th	Night	18% 3 15% 38% 24% 2	760
	21st to end	Night	18% 3 25% 33% 18% 2	760
October	1st to 10th	Night	21% 5% 27% 33% 14% 1	760
	11th to 20th	Night	25% 5% 26% 32% 11% 1	760
	21st to end	Night	27% 8% 28% 30% 7%	836
November	1st to 10th	Night	25% 10% 26% 29% 9%	780
	11th to 20th	Night	24% 8% 32% 26% 10% 1	800
	21st to end	Night	30% 10% 27% 24% 9% 1	800
December	1st to 10th	Night	30% 7% 29% 26% 7%	800
	11th to 20th	Night	29% 10% 32% 23% 6% 1	800
	21st to end	Night	34% 10% 27% 22% 6% 1	880

Page generated at 2025-02-22 06:51:35. Version ac07

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