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 2024-09-29.

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% 4 5% 4 5% 7% 33%	101191
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 33%	97060
Transparency	Day	51% 3 12% 23% 10% 1	100952
	Night	54% 3 11% 21% 10% 1	96919
Seeing	Day	22% 8% 27% 26% 13% 4	14443
	Night	23% 6% 27% 32% 11% 1	23365

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	16% 15% 5% 13% 6% 5% 5% 3 4 4 23%	2517
	11th to 20th	Day	15% 13% 6% 10% 6% 4 4 3 6% 6% 28%	2520
	21st to end	Day	14% 15% 5% 10% 5% 6% 5% 4 6% 6% 24%	2772
February	1st to 10th	Day	15% 15% 4 10% 6% 4 4 4 3 6% 28%	1665
	11th to 20th	Day	14% 12% 5% 10% 6% 5% 5% 5% 5% 6% 27%	1761
	21st to end	Day	17% 17% 4 9% 5% 4 4 3 5% 4 28%	1570
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	17% 6% 2 6% 5% 4 4 4 4 8% 41%	2070
October	1st to 10th	Day	21% 6% 3 7% 4 5% 4 3 5% 6% 36%	1859
	11th to 20th	Day	19% 8% 3 8% 7% 6% 5% 4 6% 7% 28%	1760
	21st to end	Day	25% 10% 3 8% 5% 4 3 3 5% 5% 30%	1782
November	1st to 10th	Day	17% 10% 3 6% 5% 4 4 4 5% 7% 35%	1671
	11th to 20th	Day	18% 12% 3 8% 4 5% 4 3 5% 6% 30%	1632
	21st to end	Day	22% 11% 3 6% 5% 5% 6% 3 5% 6% 27%	1520
December	1st to 10th	Day	24% 13% 6% 11% 5% 4 4 3 4 5% 21%	1520
	11th to 20th	Day	22% 14% 4 8% 5% 4 4 4 5% 4 28%	1906
	21st to end	Day	27% 16% 4 11% 5% 4 3 2 4 4 21%	2772
January	1st to 10th	Night	17% 15% 5% 11% 6% 5% 4 3 4 5% 24%	4429
	11th to 20th	Night	11% 14% 5% 11% 7% 6% 6% 4 5% 6% 25%	4440
	21st to end	Night	15% 17% 5% 11% 6% 5% 4 3 4 5% 25%	4884
February	1st to 10th	Night	14% 15% 5% 12% 7% 5% 5% 3 5% 6% 23%	2513
	11th to 20th	Night	14% 12% 4 10% 6% 4 4 4 5% 6% 31%	2305
	21st to end	Night	16% 15% 4 9% 5% 4 4 3 4 5% 32%	1884
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%	2078
October	1st to 10th	Night	18% 6% 3 6% 4 4 5% 4 5% 6% 39%	2098
	11th to 20th	Night	21% 8% 3 8% 5% 4 4 2 4 6% 35%	2200
	21st to end	Night	23% 9% 4 8% 4 5% 4 3 5% 5% 31%	2574
November	1st to 10th	Night	19% 9% 3 7% 5% 4 4 4 5% 6% 33%	2407
	11th to 20th	Night	19% 10% 3 8% 5% 3 4 4 5% 5% 34%	2546
	21st to end	Night	22% 14% 4 8% 5% 4 3 3 4 5% 28%	2660
December	1st to 10th	Night	21% 15% 5% 9% 6% 4 5% 3 5% 5% 22%	2660
	11th to 20th	Night	23% 15% 4 8% 5% 4 3 3 4 5% 27%	3288
	21st to end	Night	28% 15% 5% 9% 4 3 3 3 4 4 22%	4884

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	66% 2 12% 15% 5% 1	2518
	11th to 20th	Day	58% 2 14% 18% 8%	2520
	21st to end	Day	61% 3 15% 15% 6%	2772
February	1st to 10th	Day	59% 2 10% 21% 6% 2	1665
	11th to 20th	Day	57% 4 11% 17% 10% 1	1770
	21st to end	Day	60% 2 10% 16% 11% 2	1570
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	44% 4 13% 27% 11% 1	2070
October	1st to 10th	Day	49% 3 12% 24% 12%	1859
	11th to 20th	Day	55% 4 13% 21% 7%	1760
	21st to end	Day	56% 2 12% 20% 8% 1	1782
November	1st to 10th	Day	49% 2 13% 25% 9% 1	1671
	11th to 20th	Day	55% 3 11% 19% 11%	1632
	21st to end	Day	59% 4 14% 17% 6% 1	1520
December	1st to 10th	Day	67% 2 12% 14% 5%	1520
	11th to 20th	Day	60% 3 11% 19% 7%	1906
	21st to end	Day	70% 2 12% 10% 6%	2772
January	1st to 10th	Night	64% 2 15% 14% 5%	4429
	11th to 20th	Night	61% 3 12% 18% 5% 1	4440
	21st to end	Night	64% 3 12% 14% 7% 1	4884
February	1st to 10th	Night	62% 3 9% 19% 6% 1	2513
	11th to 20th	Night	55% 2 10% 22% 10% 1	2307
	21st to end	Night	56% 2 9% 18% 13% 1	1884
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%	2078
October	1st to 10th	Night	46% 4 9% 27% 13% 1	2098
	11th to 20th	Night	52% 3 11% 26% 9%	2200
	21st to end	Night	56% 3 12% 19% 9% 1	2574
November	1st to 10th	Night	52% 3 11% 24% 9% 1	2407
	11th to 20th	Night	53% 3 12% 21% 10% 1	2546
	21st to end	Night	60% 2 11% 19% 7%	2660
December	1st to 10th	Night	65% 3 12% 16% 4	2660
	11th to 20th	Night	62% 2 9% 19% 8%	3288
	21st to end	Night	68% 2 10% 16% 5%	4884

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% 7% 27% 30% 11%	180
	11th to 20th	Day	29% 11% 31% 23% 5% 2	180
	21st to end	Day	20% 12% 30% 29% 8% 2	198
February	1st to 10th	Day	31% 15% 33% 18% 4	249
	11th to 20th	Day	23% 12% 37% 24% 4	368
	21st to end	Day	28% 10% 31% 26% 4	382
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% 24% 18% 7%	483
October	1st to 10th	Day	21% 9% 26% 23% 15% 6%	450
	11th to 20th	Day	22% 9% 32% 22% 12% 3	430
	21st to end	Day	34% 9% 24% 25% 8% 1	396
November	1st to 10th	Day	28% 10% 25% 25% 12% 1	365
	11th to 20th	Day	25% 11% 34% 20% 8% 1	292
	21st to end	Day	32% 8% 26% 24% 7% 2	180
December	1st to 10th	Day	30% 9% 24% 27% 8% 2	180
	11th to 20th	Day	26% 12% 37% 19% 4 1	180
	21st to end	Day	32% 14% 29% 17% 8% 1	198
January	1st to 10th	Night	28% 8% 30% 28% 6%	760
	11th to 20th	Night	25% 9% 32% 28% 6%	760
	21st to end	Night	20% 7% 34% 30% 8% 1	836
February	1st to 10th	Night	22% 6% 36% 29% 6% 1	760
	11th to 20th	Night	22% 8% 35% 29% 5% 1	743
	21st to end	Night	24% 7% 32% 33% 5%	628
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	756
October	1st to 10th	Night	21% 5% 27% 34% 13% 1	720
	11th to 20th	Night	26% 5% 25% 32% 12% 1	720
	21st to end	Night	28% 7% 28% 30% 7%	792
November	1st to 10th	Night	25% 9% 27% 30% 10%	740
	11th to 20th	Night	24% 8% 32% 25% 11% 1	760
	21st to end	Night	30% 9% 27% 24% 9% 1	760
December	1st to 10th	Night	31% 8% 28% 26% 7%	760
	11th to 20th	Night	28% 9% 32% 23% 6% 1	760
	21st to end	Night	34% 10% 28% 22% 6%	836

Page generated at 2024-09-26 09:52:19. Version ac07

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