Observatoire de Val-Des-Bois
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire de Val-Des-Bois clear sky chart from 2005-11-06 to 2025-09-04.

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% 11% 4 9% 5% 5% 4 3 5% 7% 30%	126487
	Night	16% 13% 4 10% 6% 5% 4 3 5% 6% 29%	122232
Transparency	Day	54% 3 11% 21% 10% 1	129939
	Night	56% 3 11% 19% 10% 1	126854
Seeing	Day	26% 16% 30% 18% 7% 2	15473
	Night	24% 13% 28% 25% 9% 1	24473

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	13% 18% 8% 18% 9% 7% 4 3 3 3 15%	3597
	11th to 20th	Day	10% 17% 8% 17% 9% 5% 4 3 4 6% 17%	3570
	21st to end	Day	9% 16% 8% 16% 7% 6% 5% 3 4 5% 21%	4070
February	1st to 10th	Day	11% 18% 6% 15% 7% 5% 4 4 5% 5% 21%	1800
	11th to 20th	Day	11% 19% 6% 12% 7% 6% 3 3 3 5% 25%	1758
	21st to end	Day	17% 16% 6% 10% 7% 5% 4 3 4 5% 24%	1650
March	1st to 10th	Day	17% 16% 5% 13% 6% 4 4 3 4 5% 23%	4476
	11th to 20th	Day	11% 15% 6% 10% 6% 5% 5% 3 5% 7% 28%	4950
	21st to end	Day	16% 13% 4 7% 6% 5% 3 3 4 5% 33%	5786
April	1st to 10th	Day	22% 14% 3 7% 4 4 3 3 4 6% 30%	2400
	11th to 20th	Day	21% 11% 4 9% 3 3 3 3 5% 5% 33%	2374
	21st to end	Day	21% 12% 4 8% 5% 3 4 4 4 5% 30%	2569
May	1st to 10th	Day	21% 12% 4 9% 5% 5% 4 4 5% 6% 25%	6346
	11th to 20th	Day	17% 9% 3 9% 6% 5% 5% 4 5% 7% 31%	6318
	21st to end	Day	22% 9% 3 7% 4 4 4 3 5% 6% 32%	6949
June	1st to 10th	Day	22% 9% 3 8% 5% 5% 5% 3 5% 6% 29%	2800
	11th to 20th	Day	15% 6% 3 6% 5% 5% 5% 4 7% 8% 37%	2800
	21st to end	Day	20% 8% 2 7% 5% 4 5% 4 7% 8% 30%	2800
July	1st to 10th	Day	15% 6% 2 8% 5% 6% 6% 5% 7% 8% 32%	6360
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	6360
	21st to end	Day	10% 6% 2 5% 4 4 4 4 6% 10% 46%	6926
August	1st to 10th	Day	14% 6% 3 8% 5% 5% 5% 4 6% 9% 33%	4543
	11th to 20th	Day	17% 7% 3 6% 4 4 4 4 6% 8% 36%	4122
	21st to end	Day	14% 6% 2 6% 4 4 5% 3 7% 9% 39%	4312
September	1st to 10th	Day	15% 7% 2 6% 5% 4 4 3 5% 8% 41%	2316
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 5% 8% 43%	2275
	21st to end	Day	21% 7% 2 6% 4 3 4 4 5% 8% 38%	2037
October	1st to 10th	Day	20% 10% 3 7% 4 5% 5% 3 5% 6% 32%	1900
	11th to 20th	Day	25% 12% 3 8% 5% 5% 4 3 4 6% 25%	1900
	21st to end	Day	25% 14% 3 8% 4 4 4 4 5% 6% 24%	1918
November	1st to 10th	Day	15% 14% 4 8% 4 4 4 4 5% 7% 32%	1759
	11th to 20th	Day	18% 16% 5% 10% 5% 6% 5% 4 4 5% 22%	1600
	21st to end	Day	20% 19% 6% 12% 6% 4 5% 3 4 4 17%	1600
December	1st to 10th	Day	19% 23% 9% 12% 7% 4 4 3 4 5% 10%	2600
	11th to 20th	Day	16% 17% 7% 14% 6% 5% 4 3 4 4 20%	2986
	21st to end	Day	19% 20% 5% 11% 6% 5% 5% 3 3 3 20%	3960
January	1st to 10th	Night	12% 21% 9% 16% 8% 5% 3 2 4 4 17%	6349
	11th to 20th	Night	10% 15% 7% 17% 9% 6% 6% 3 4 4 19%	6315
	21st to end	Night	9% 17% 8% 15% 7% 8% 5% 3 4 4 20%	6882
February	1st to 10th	Night	11% 16% 6% 13% 8% 5% 5% 4 5% 5% 23%	2600
	11th to 20th	Night	11% 16% 6% 13% 7% 6% 4 3 4 4 26%	2528
	21st to end	Night	12% 17% 6% 11% 5% 5% 4 4 4 6% 26%	1980
March	1st to 10th	Night	19% 17% 5% 10% 5% 4 4 3 4 4 26%	5364
	11th to 20th	Night	11% 15% 5% 9% 5% 5% 4 4 6% 7% 28%	5007
	21st to end	Night	16% 11% 4 11% 6% 4 4 3 5% 5% 30%	5146
April	1st to 10th	Night	20% 14% 3 7% 5% 4 4 3 5% 6% 29%	2000
	11th to 20th	Night	18% 10% 3 7% 5% 4 4 3 5% 5% 36%	1975
	21st to end	Night	19% 12% 4 9% 3 3 4 3 4 5% 33%	1846
May	1st to 10th	Night	19% 12% 4 9% 6% 5% 5% 4 6% 7% 25%	3594
	11th to 20th	Night	15% 8% 3 8% 5% 5% 5% 3 6% 6% 37%	3575
	21st to end	Night	19% 9% 3 9% 5% 5% 5% 4 5% 5% 31%	3926
June	1st to 10th	Night	17% 9% 2 6% 4 4 5% 5% 6% 9% 33%	1600
	11th to 20th	Night	15% 7% 2 7% 5% 4 4 3 5% 7% 41%	1600
	21st to end	Night	19% 7% 2 6% 5% 5% 5% 4 5% 8% 34%	1600
July	1st to 10th	Night	14% 8% 3 6% 5% 5% 5% 4 7% 8% 35%	3600
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	3600
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 6% 8% 48%	3926
August	1st to 10th	Night	11% 7% 4 8% 5% 5% 5% 4 6% 7% 38%	2603
	11th to 20th	Night	14% 7% 3 7% 4 3 4 4 5% 6% 42%	2893
	21st to end	Night	17% 6% 3 6% 4 5% 4 3 5% 7% 40%	3586
September	1st to 10th	Night	14% 6% 2 7% 5% 3 4 4 5% 5% 45%	1931
	11th to 20th	Night	15% 6% 2 5% 4 4 2 3 5% 7% 48%	1905
	21st to end	Night	17% 7% 2 7% 4 3 3 3 5% 5% 43%	2141
October	1st to 10th	Night	19% 9% 2 7% 4 4 4 3 4 7% 37%	2280
	11th to 20th	Night	20% 13% 3 7% 5% 5% 4 3 4 6% 30%	2280
	21st to end	Night	23% 13% 3 9% 5% 4 4 3 5% 5% 27%	2679
November	1st to 10th	Night	18% 13% 5% 7% 5% 4 4 4 5% 6% 29%	2537
	11th to 20th	Night	18% 14% 5% 12% 5% 5% 4 3 4 5% 26%	2800
	21st to end	Night	20% 17% 5% 11% 5% 6% 5% 3 4 4 20%	2800
December	1st to 10th	Night	17% 19% 6% 14% 6% 5% 5% 3 5% 5% 14%	4580
	11th to 20th	Night	21% 16% 5% 11% 5% 4 4 2 4 4 24%	5208
	21st to end	Night	21% 21% 5% 11% 5% 3 4 3 4 5% 18%	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	74% 3 6% 12% 5%	3598
	11th to 20th	Day	69% 3 7% 12% 9%	3600
	21st to end	Day	67% 2 8% 13% 9% 1	4070
February	1st to 10th	Day	65% 2 8% 16% 7% 2	1800
	11th to 20th	Day	64% 2 9% 14% 9% 1	1767
	21st to end	Day	64% 3 9% 13% 11% 1	1650
March	1st to 10th	Day	65% 1 7% 11% 12% 3	4479
	11th to 20th	Day	58% 3 14% 14% 10% 1	4950
	21st to end	Day	55% 2 5% 19% 18% 2	5786
April	1st to 10th	Day	57% 2 6% 18% 13% 3	2400
	11th to 20th	Day	54% 2 6% 21% 15% 2	2374
	21st to end	Day	58% 2 7% 16% 13% 3	2569
May	1st to 10th	Day	61% 2 6% 21% 8% 2	6357
	11th to 20th	Day	54% 3 12% 20% 10% 1	6322
	21st to end	Day	54% 3 10% 19% 13% 1	6949
June	1st to 10th	Day	55% 4 8% 20% 12% 1	2800
	11th to 20th	Day	44% 4 12% 27% 12% 2	2800
	21st to end	Day	51% 4 14% 21% 10%	2800
July	1st to 10th	Day	47% 6% 12% 28% 7%	6290
	11th to 20th	Day	40% 5% 13% 32% 9%	6285
	21st to end	Day	35% 5% 15% 36% 9%	6728
August	1st to 10th	Day	47% 5% 17% 24% 8%	4529
	11th to 20th	Day	45% 4 15% 29% 7% 1	4161
	21st to end	Day	41% 4 16% 26% 12% 1	4312
September	1st to 10th	Day	42% 3 13% 29% 12%	2316
	11th to 20th	Day	40% 3 13% 32% 12% 1	2275
	21st to end	Day	45% 4 14% 27% 10%	2037
October	1st to 10th	Day	52% 3 10% 29% 6%	3160
	11th to 20th	Day	53% 5% 11% 22% 8%	3140
	21st to end	Day	54% 4 19% 17% 6%	3130
November	1st to 10th	Day	51% 4 14% 23% 7% 1	1759
	11th to 20th	Day	63% 4 10% 14% 8% 1	1600
	21st to end	Day	70% 3 12% 9% 6% 1	1600
December	1st to 10th	Day	75% 4 9% 10% 2	2600
	11th to 20th	Day	69% 2 8% 15% 7%	2986
	21st to end	Day	68% 3 7% 17% 5%	3960
January	1st to 10th	Night	72% 2 12% 11% 3	6349
	11th to 20th	Night	70% 2 10% 13% 5%	6360
	21st to end	Night	68% 2 8% 9% 12%	6882
February	1st to 10th	Night	63% 3 8% 17% 8% 1	2600
	11th to 20th	Night	62% 2 8% 19% 8% 1	2530
	21st to end	Night	60% 2 8% 19% 10% 1	1980
March	1st to 10th	Night	63% 1 8% 13% 12% 3	5374
	11th to 20th	Night	55% 4 13% 17% 8% 3	5008
	21st to end	Night	56% 3 6% 20% 12% 2	5146
April	1st to 10th	Night	57% 2 8% 18% 14% 2	2000
	11th to 20th	Night	51% 2 6% 23% 16% 1	1975
	21st to end	Night	53% 3 8% 19% 14% 3	1846
May	1st to 10th	Night	59% 3 8% 20% 8% 2	3600
	11th to 20th	Night	48% 4 11% 23% 12% 2	3576
	21st to end	Night	56% 3 10% 19% 12% 1	3927
June	1st to 10th	Night	47% 5% 12% 24% 11% 1	1600
	11th to 20th	Night	43% 4 11% 28% 13% 2	1600
	21st to end	Night	47% 6% 12% 22% 13% 1	1600
July	1st to 10th	Night	46% 5% 18% 22% 8%	3560
	11th to 20th	Night	39% 5% 15% 32% 9%	3552
	21st to end	Night	34% 5% 15% 34% 13%	3809
August	1st to 10th	Night	43% 5% 16% 27% 9%	2593
	11th to 20th	Night	43% 4 16% 30% 8%	2920
	21st to end	Night	42% 4 15% 25% 12% 2	3586
September	1st to 10th	Night	40% 4 11% 30% 14% 1	1931
	11th to 20th	Night	37% 3 13% 36% 10% 1	1905
	21st to end	Night	43% 2 12% 31% 11%	2141
October	1st to 10th	Night	43% 4 13% 29% 12%	3800
	11th to 20th	Night	48% 6% 13% 24% 9%	3779
	21st to end	Night	52% 4 18% 16% 9%	4404
November	1st to 10th	Night	56% 3 14% 20% 8%	2537
	11th to 20th	Night	61% 4 10% 15% 9% 1	2800
	21st to end	Night	66% 4 11% 14% 4	2800
December	1st to 10th	Night	70% 4 11% 12% 3	4580
	11th to 20th	Night	66% 2 5% 15% 11% 1	5208
	21st to end	Night	69% 2 6% 13% 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	28% 16% 37% 18% 3	200
	11th to 20th	Day	28% 19% 37% 13% 4 1	200
	21st to end	Day	22% 22% 34% 18% 3	243
February	1st to 10th	Day	34% 26% 30% 8% 2	300
	11th to 20th	Day	32% 29% 32% 8%	305
	21st to end	Day	33% 21% 33% 12% 2	414
March	1st to 10th	Day	29% 20% 33% 13% 5%	500
	11th to 20th	Day	32% 20% 31% 13% 4 1	500
	21st to end	Day	34% 21% 26% 14% 5% 1	550
April	1st to 10th	Day	35% 19% 28% 14% 4 1	500
	11th to 20th	Day	36% 21% 21% 13% 8% 1	492
	21st to end	Day	33% 22% 28% 12% 5% 1	500
May	1st to 10th	Day	30% 17% 33% 12% 6% 2	503
	11th to 20th	Day	25% 17% 31% 18% 6% 2	527
	21st to end	Day	25% 17% 31% 19% 8% 1	573
June	1st to 10th	Day	29% 13% 33% 19% 5% 1	530
	11th to 20th	Day	20% 16% 32% 21% 9% 2	530
	21st to end	Day	25% 13% 33% 20% 8% 2	530
July	1st to 10th	Day	17% 9% 36% 27% 8% 3	525
	11th to 20th	Day	15% 12% 34% 27% 9% 2	524
	21st to end	Day	15% 10% 36% 27% 10% 2	576
August	1st to 10th	Day	17% 6% 28% 35% 11% 2	526
	11th to 20th	Day	20% 7% 25% 28% 12% 7%	519
	21st to end	Day	18% 13% 30% 22% 11% 6%	583
September	1st to 10th	Day	17% 12% 32% 20% 13% 6%	502
	11th to 20th	Day	15% 10% 25% 23% 17% 9%	490
	21st to end	Day	23% 14% 25% 22% 11% 6%	490
October	1st to 10th	Day	27% 15% 22% 23% 11% 2	490
	11th to 20th	Day	32% 20% 28% 12% 7% 1	490
	21st to end	Day	33% 13% 32% 17% 4	448
November	1st to 10th	Day	31% 17% 30% 17% 5% 1	393
	11th to 20th	Day	33% 20% 33% 10% 3 2	200
	21st to end	Day	33% 22% 31% 12% 2 1	200
December	1st to 10th	Day	32% 15% 34% 16% 4	200
	11th to 20th	Day	34% 26% 29% 10% 1 1	200
	21st to end	Day	34% 21% 29% 13% 2 1	220
January	1st to 10th	Night	29% 15% 34% 17% 4	800
	11th to 20th	Night	30% 16% 27% 21% 6%	800
	21st to end	Night	19% 14% 36% 23% 7% 1	880
February	1st to 10th	Night	21% 20% 32% 22% 4	800
	11th to 20th	Night	23% 20% 35% 18% 4 1	783
	21st to end	Night	26% 19% 27% 25% 3 1	660
March	1st to 10th	Night	26% 17% 30% 21% 5%	800
	11th to 20th	Night	25% 20% 31% 21% 4 1	800
	21st to end	Night	27% 16% 29% 22% 7%	880
April	1st to 10th	Night	28% 17% 25% 25% 5%	800
	11th to 20th	Night	27% 16% 29% 20% 7% 1	791
	21st to end	Night	28% 15% 27% 23% 7%	646
May	1st to 10th	Night	28% 11% 25% 27% 9% 2	400
	11th to 20th	Night	23% 9% 30% 28% 9%	394
	21st to end	Night	18% 11% 34% 25% 11%	433
June	1st to 10th	Night	20% 6% 31% 34% 8% 1	400
	11th to 20th	Night	18% 9% 28% 32% 14% 1	400
	21st to end	Night	22% 5% 26% 36% 12% 1	400
July	1st to 10th	Night	11% 5% 29% 40% 14% 1	390
	11th to 20th	Night	16% 4 22% 42% 16%	388
	21st to end	Night	10% 5% 27% 41% 15% 2	438
August	1st to 10th	Night	11% 4 22% 42% 20%	402
	11th to 20th	Night	14% 3 19% 37% 24% 4	599
	21st to end	Night	17% 6% 25% 32% 18% 2	880
September	1st to 10th	Night	18% 5% 28% 29% 18% 1	773
	11th to 20th	Night	19% 6% 24% 28% 21% 4	760
	21st to end	Night	21% 8% 25% 26% 18% 2	760
October	1st to 10th	Night	24% 11% 27% 23% 14% 1	760
	11th to 20th	Night	28% 13% 27% 25% 6% 1	760
	21st to end	Night	30% 14% 29% 22% 6%	836
November	1st to 10th	Night	29% 13% 26% 23% 8% 1	780
	11th to 20th	Night	28% 18% 29% 15% 10% 1	800
	21st to end	Night	31% 20% 23% 21% 5%	800
December	1st to 10th	Night	29% 17% 28% 22% 4	800
	11th to 20th	Night	30% 20% 26% 19% 5% 1	800
	21st to end	Night	35% 15% 27% 18% 5% 1	880

Page generated at 2025-09-01 01:51:02. Version ac07

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