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 2024-02-10.

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% 4 7% 6% 3 5% 5% 5% 7% 33%	78093
	Night	18% 10% 4 7% 6% 3 4 4 5% 6% 32%	80874
Transparency	Day	51% 3 12% 22% 11% 1	77798
	Night	54% 3 11% 21% 10% 1	80703
Seeing	Day	22% 8% 27% 26% 13% 4	13520
	Night	23% 6% 27% 31% 11% 1	22656

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% 6% 11% 7% 4 5% 4 4 4 24%	2517
	11th to 20th	Day	15% 12% 6% 9% 6% 3 4 4 6% 7% 28%	2520
	21st to end	Day	14% 14% 5% 8% 7% 4 6% 6% 5% 6% 24%	2772
February	1st to 10th	Day	15% 14% 5% 9% 7% 3 5% 5% 4 6% 28%	1656
	11th to 20th	Day	15% 12% 5% 9% 7% 3 6% 6% 5% 6% 28%	1666
	21st to end	Day	18% 15% 6% 8% 6% 3 4 4 5% 4 28%	1480
March	1st to 10th	Day	16% 12% 4 9% 6% 4 6% 4 4 6% 29%	1797
	11th to 20th	Day	17% 11% 4 7% 6% 3 5% 4 4 6% 33%	1862
	21st to end	Day	19% 9% 4 7% 6% 2 4 4 4 5% 36%	2374
April	1st to 10th	Day	22% 10% 4 6% 7% 3 4 4 4 5% 32%	2160
	11th to 20th	Day	20% 7% 3 6% 6% 4 3 4 4 5% 38%	2134
	21st to end	Day	22% 10% 4 7% 5% 2 5% 4 4 5% 32%	2307
May	1st to 10th	Day	19% 7% 3 6% 5% 3 4 5% 5% 6% 35%	2484
	11th to 20th	Day	18% 6% 2 6% 5% 3 5% 4 5% 7% 37%	2478
	21st to end	Day	17% 6% 3 6% 6% 3 4 5% 6% 8% 35%	2725
June	1st to 10th	Day	21% 8% 3 6% 5% 3 5% 5% 6% 7% 31%	2520
	11th to 20th	Day	14% 6% 3 6% 6% 3 5% 5% 5% 9% 38%	2520
	21st to end	Day	19% 7% 3 7% 6% 4 6% 4 6% 7% 31%	2520
July	1st to 10th	Day	12% 5% 2 4 6% 4 6% 6% 6% 8% 41%	2520
	11th to 20th	Day	13% 4 3 6% 5% 4 6% 6% 8% 10% 37%	2520
	21st to end	Day	11% 5% 2 6% 5% 3 5% 6% 8% 10% 38%	2744
August	1st to 10th	Day	12% 6% 3 6% 7% 4 5% 6% 6% 9% 37%	2506
	11th to 20th	Day	10% 5% 3 6% 6% 3 6% 5% 7% 8% 41%	2289
	21st to end	Day	12% 5% 3 5% 5% 3 4 5% 7% 8% 43%	2376
September	1st to 10th	Day	11% 6% 2 6% 5% 3 5% 5% 6% 9% 41%	2160
	11th to 20th	Day	14% 5% 2 3 4 2 5% 4 5% 7% 49%	2084
	21st to end	Day	16% 6% 2 5% 6% 3 4 4 3 9% 42%	1980
October	1st to 10th	Day	21% 6% 3 6% 6% 3 5% 4 5% 6% 36%	1859
	11th to 20th	Day	19% 7% 4 6% 8% 4 6% 5% 6% 7% 28%	1760
	21st to end	Day	25% 9% 3 7% 6% 3 4 4 5% 5% 30%	1782
November	1st to 10th	Day	17% 9% 4 6% 5% 3 4 4 5% 7% 35%	1671
	11th to 20th	Day	18% 11% 3 7% 6% 4 5% 4 6% 6% 30%	1632
	21st to end	Day	22% 11% 3 6% 6% 4 6% 5% 5% 6% 27%	1520
December	1st to 10th	Day	25% 11% 7% 10% 5% 3 4 4 4 5% 21%	1520
	11th to 20th	Day	22% 13% 4 7% 6% 3 4 4 4 4 28%	1906
	21st to end	Day	27% 16% 4 9% 6% 3 4 3 4 4 21%	2772
January	1st to 10th	Night	17% 14% 6% 9% 7% 4 5% 4 4 6% 24%	4429
	11th to 20th	Night	12% 13% 5% 9% 9% 4 6% 5% 5% 6% 25%	4440
	21st to end	Night	16% 16% 6% 9% 8% 4 5% 4 4 5% 25%	4884
February	1st to 10th	Night	14% 14% 6% 11% 7% 4 6% 4 5% 6% 23%	2507
	11th to 20th	Night	15% 11% 4 9% 6% 4 4 6% 5% 5% 31%	2180
	21st to end	Night	16% 14% 5% 8% 6% 3 5% 4 4 5% 32%	1776
March	1st to 10th	Night	17% 13% 5% 8% 6% 3 4 4 4 6% 30%	2150
	11th to 20th	Night	18% 13% 4 6% 6% 3 4 4 4 6% 32%	2094
	21st to end	Night	17% 10% 4 6% 6% 3 4 3 4 5% 38%	1981
April	1st to 10th	Night	25% 8% 4 6% 5% 3 4 4 5% 5% 32%	1800
	11th to 20th	Night	22% 8% 2 6% 5% 3 4 3 4 6% 37%	1775
	21st to end	Night	22% 9% 3 6% 5% 2 5% 4 5% 4 35%	1652
May	1st to 10th	Night	20% 8% 4 7% 5% 2 5% 4 5% 6% 35%	1490
	11th to 20th	Night	19% 6% 3 7% 5% 3 5% 3 6% 6% 37%	1415
	21st to end	Night	16% 6% 3 6% 5% 3 6% 5% 5% 7% 40%	1550
June	1st to 10th	Night	21% 7% 3 6% 5% 3 5% 4 6% 8% 32%	1440
	11th to 20th	Night	17% 6% 3 5% 5% 2 5% 5% 6% 8% 39%	1440
	21st to end	Night	18% 5% 3 6% 6% 3 5% 5% 6% 8% 35%	1440
July	1st to 10th	Night	11% 5% 3 5% 4 3 5% 5% 6% 10% 43%	1440
	11th to 20th	Night	11% 5% 2 5% 5% 5% 5% 6% 7% 9% 40%	1440
	21st to end	Night	12% 4 2 5% 5% 4 5% 5% 7% 8% 42%	1574
August	1st to 10th	Night	13% 5% 2 5% 5% 3 5% 5% 5% 9% 43%	1432
	11th to 20th	Night	13% 5% 3 6% 5% 3 5% 5% 5% 8% 43%	1592
	21st to end	Night	14% 4 3 5% 6% 3 5% 4 5% 8% 43%	1980
September	1st to 10th	Night	12% 5% 3 6% 5% 3 4 5% 5% 8% 45%	1800
	11th to 20th	Night	15% 5% 3 4 4 3 3 3 5% 7% 49%	1876
	21st to end	Night	16% 5% 2 6% 5% 3 4 4 4 7% 44%	1980
October	1st to 10th	Night	18% 6% 3 5% 5% 3 5% 5% 5% 6% 39%	2098
	11th to 20th	Night	21% 7% 4 7% 6% 3 4 3 4 6% 35%	2200
	21st to end	Night	23% 8% 4 7% 5% 4 5% 4 5% 5% 31%	2574
November	1st to 10th	Night	19% 8% 4 6% 5% 3 5% 5% 5% 6% 33%	2407
	11th to 20th	Night	19% 9% 4 7% 6% 2 4 5% 5% 5% 34%	2546
	21st to end	Night	22% 12% 4 7% 6% 3 3 4 4 5% 28%	2660
December	1st to 10th	Night	21% 14% 6% 8% 7% 3 4 4 5% 5% 22%	2660
	11th to 20th	Night	23% 14% 5% 8% 5% 3 4 3 4 5% 27%	3288
	21st to end	Night	29% 15% 5% 8% 5% 2 3 3 4 4 23%	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	60% 2 10% 20% 6% 2	1656
	11th to 20th	Day	58% 4 11% 17% 9% 1	1675
	21st to end	Day	61% 2 9% 17% 10% 2	1480
March	1st to 10th	Day	57% 3 9% 18% 10% 2	1800
	11th to 20th	Day	53% 3 11% 16% 14% 3	1862
	21st to end	Day	51% 3 9% 19% 14% 3	2374
April	1st to 10th	Day	56% 3 7% 19% 13% 2	2160
	11th to 20th	Day	50% 2 7% 23% 16% 2	2134
	21st to end	Day	55% 3 8% 18% 12% 4	2307
May	1st to 10th	Day	50% 3 7% 22% 16% 2	2484
	11th to 20th	Day	47% 4 9% 21% 16% 3	2482
	21st to end	Day	46% 5% 13% 20% 14% 2	2725
June	1st to 10th	Day	52% 4 9% 21% 12% 2	2520
	11th to 20th	Day	44% 4 10% 25% 14% 3	2520
	21st to end	Day	52% 4 12% 20% 11% 1	2520
July	1st to 10th	Day	39% 5% 13% 28% 13% 1	2450
	11th to 20th	Day	40% 4 16% 30% 10%	2445
	21st to end	Day	41% 4 17% 28% 9%	2591
August	1st to 10th	Day	43% 5% 17% 27% 9%	2492
	11th to 20th	Day	39% 4 18% 28% 11%	2289
	21st to end	Day	38% 4 15% 28% 15%	2376
September	1st to 10th	Day	39% 4 14% 30% 13% 1	2160
	11th to 20th	Day	36% 3 12% 33% 15% 1	2084
	21st to end	Day	43% 4 13% 28% 11% 1	1980
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	63% 3 9% 19% 6% 1	2507
	11th to 20th	Night	56% 2 10% 22% 9% 1	2182
	21st to end	Night	57% 2 8% 18% 14% 1	1776
March	1st to 10th	Night	57% 2 9% 18% 12% 2	2160
	11th to 20th	Night	55% 3 9% 17% 14% 3	2095
	21st to end	Night	51% 2 7% 21% 15% 4	1981
April	1st to 10th	Night	55% 3 6% 20% 15% 2	1800
	11th to 20th	Night	51% 2 5% 23% 15% 2	1775
	21st to end	Night	52% 3 7% 20% 14% 5%	1652
May	1st to 10th	Night	52% 1 6% 20% 15% 5%	1490
	11th to 20th	Night	48% 3 7% 20% 18% 4	1416
	21st to end	Night	44% 5% 12% 20% 17% 2	1551
June	1st to 10th	Night	51% 4 9% 23% 13% 1	1440
	11th to 20th	Night	43% 4 11% 24% 15% 3	1440
	21st to end	Night	47% 6% 12% 20% 14%	1440
July	1st to 10th	Night	37% 5% 14% 27% 17% 1	1400
	11th to 20th	Night	38% 4 16% 29% 12%	1392
	21st to end	Night	39% 5% 18% 26% 12%	1486
August	1st to 10th	Night	39% 4 18% 26% 12%	1422
	11th to 20th	Night	39% 4 18% 26% 12%	1592
	21st to end	Night	40% 4 15% 28% 13% 1	1980
September	1st to 10th	Night	39% 3 13% 28% 15% 1	1800
	11th to 20th	Night	37% 3 13% 34% 13% 1	1876
	21st to end	Night	42% 4 11% 29% 15%	1980
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	33% 15% 30% 19% 3	246
	11th to 20th	Day	24% 9% 37% 25% 5%	333
	21st to end	Day	29% 9% 31% 26% 5%	346
March	1st to 10th	Day	25% 10% 31% 25% 9%	420
	11th to 20th	Day	22% 10% 33% 27% 7% 1	420
	21st to end	Day	28% 8% 31% 21% 9% 3	462
April	1st to 10th	Day	28% 12% 26% 22% 10% 3	420
	11th to 20th	Day	27% 12% 25% 22% 11% 3	412
	21st to end	Day	28% 10% 28% 21% 9% 3	420
May	1st to 10th	Day	23% 9% 31% 25% 9% 3	423
	11th to 20th	Day	21% 6% 31% 25% 12% 5%	447
	21st to end	Day	18% 9% 32% 25% 13% 3	485
June	1st to 10th	Day	25% 8% 25% 26% 13% 3	450
	11th to 20th	Day	18% 8% 22% 31% 16% 4	450
	21st to end	Day	20% 6% 24% 28% 18% 4	450
July	1st to 10th	Day	14% 4 20% 33% 21% 7%	445
	11th to 20th	Day	14% 4 26% 33% 19% 5%	444
	21st to end	Day	14% 3 22% 35% 20% 6%	488
August	1st to 10th	Day	15% 3 20% 32% 21% 8%	446
	11th to 20th	Day	12% 4 18% 33% 21% 13%	439
	21st to end	Day	15% 6% 24% 25% 21% 10%	495
September	1st to 10th	Day	13% 4 25% 29% 19% 10%	450
	11th to 20th	Day	15% 5% 24% 24% 20% 12%	450
	21st to end	Day	18% 6% 26% 23% 19% 7%	450
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	759
	11th to 20th	Night	23% 8% 34% 29% 6% 1	703
	21st to end	Night	24% 7% 32% 33% 5%	592
March	1st to 10th	Night	25% 8% 30% 31% 6%	720
	11th to 20th	Night	27% 8% 31% 27% 8%	720
	21st to end	Night	23% 6% 29% 33% 8%	792
April	1st to 10th	Night	27% 9% 27% 31% 6%	720
	11th to 20th	Night	27% 8% 28% 29% 7% 1	711
	21st to end	Night	28% 7% 29% 27% 9%	572
May	1st to 10th	Night	24% 5% 31% 30% 8% 1	410
	11th to 20th	Night	23% 4 30% 34% 8% 1	354
	21st to end	Night	19% 3 33% 34% 10% 1	389
June	1st to 10th	Night	28% 3 23% 33% 13%	360
	11th to 20th	Night	19% 4 26% 36% 14% 1	360
	21st to end	Night	20% 3 23% 37% 16% 1	360
July	1st to 10th	Night	12% 3 26% 42% 17% 1	350
	11th to 20th	Night	12% 2 21% 47% 18% 1	348
	21st to end	Night	12% 3 17% 47% 20% 1	394
August	1st to 10th	Night	16% 3 12% 43% 25% 1	358
	11th to 20th	Night	15% 2 18% 37% 26% 3	528
	21st to end	Night	17% 2 21% 36% 23% 2	792
September	1st to 10th	Night	15% 2 21% 43% 18% 1	720
	11th to 20th	Night	19% 3 16% 38% 22% 2	720
	21st to end	Night	18% 3 26% 33% 18% 2	720
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 2024-02-07 06:11:58. Version ac07

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