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 2024-03-28.

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	18% 11% 4 8% 6% 3 5% 4 5% 6% 30%	78581
	Night	17% 12% 5% 8% 6% 3 4 4 4 6% 30%	81416
Transparency	Day	56% 3 10% 20% 10% 1	78286
	Night	57% 3 10% 20% 9% 1	81245
Seeing	Day	26% 16% 30% 19% 7% 2	13574
	Night	24% 13% 28% 25% 9% 1	22815

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% 18% 8% 15% 9% 4 4 3 3 3 18%	2517
	11th to 20th	Day	13% 14% 7% 15% 10% 4 4 3 3 5% 20%	2520
	21st to end	Day	12% 17% 8% 14% 8% 4 6% 5% 4 4 18%	2840
February	1st to 10th	Day	12% 17% 6% 12% 8% 4 4 4 5% 6% 21%	1710
	11th to 20th	Day	12% 17% 7% 11% 7% 4 4 3 3 6% 26%	1667
	21st to end	Day	17% 14% 6% 8% 8% 3 5% 4 4 5% 25%	1570
March	1st to 10th	Day	16% 12% 6% 9% 7% 4 4 4 4 5% 28%	1897
	11th to 20th	Day	14% 15% 5% 8% 8% 4 5% 4 4 6% 28%	2074
	21st to end	Day	18% 10% 5% 7% 5% 3 3 3 5% 6% 34%	2384
April	1st to 10th	Day	23% 13% 4 6% 6% 3 3 3 3 6% 29%	2160
	11th to 20th	Day	21% 10% 4 8% 4 2 3 3 5% 5% 34%	2134
	21st to end	Day	22% 11% 4 7% 6% 3 4 5% 3 5% 30%	2311
May	1st to 10th	Day	21% 9% 4 7% 6% 3 4 4 5% 6% 33%	2520
	11th to 20th	Day	18% 8% 3 6% 6% 3 4 4 5% 6% 36%	2478
	21st to end	Day	18% 7% 3 7% 5% 4 5% 5% 5% 7% 35%	2725
June	1st to 10th	Day	22% 9% 3 7% 6% 4 6% 4 5% 6% 29%	2520
	11th to 20th	Day	16% 6% 3 5% 5% 3 5% 5% 7% 9% 37%	2520
	21st to end	Day	20% 7% 3 5% 7% 3 5% 5% 7% 8% 30%	2520
July	1st to 10th	Day	13% 6% 3 6% 5% 4 6% 6% 7% 8% 37%	2520
	11th to 20th	Day	14% 6% 3 6% 5% 3 6% 6% 8% 8% 34%	2520
	21st to end	Day	13% 7% 3 5% 5% 3 5% 6% 6% 9% 37%	2744
August	1st to 10th	Day	14% 6% 3 6% 7% 4 5% 6% 6% 8% 34%	2501
	11th to 20th	Day	15% 6% 3 6% 5% 4 5% 5% 5% 8% 37%	2281
	21st to end	Day	16% 7% 2 5% 5% 3 5% 4 6% 8% 38%	2376
September	1st to 10th	Day	15% 7% 3 5% 5% 3 4 4 5% 8% 41%	2160
	11th to 20th	Day	15% 6% 3 5% 5% 3 4 5% 5% 7% 42%	2156
	21st to end	Day	21% 7% 2 5% 4 3 3 4 5% 8% 38%	1930
October	1st to 10th	Day	20% 9% 3 7% 5% 4 5% 4 5% 6% 32%	1800
	11th to 20th	Day	27% 11% 4 7% 6% 3 4 4 4 6% 24%	1800
	21st to end	Day	27% 13% 3 7% 5% 2 4 5% 5% 5% 23%	1818
November	1st to 10th	Day	16% 13% 4 7% 5% 3 4 4 5% 8% 33%	1670
	11th to 20th	Day	19% 15% 6% 9% 6% 4 6% 5% 4 5% 21%	1520
	21st to end	Day	21% 17% 7% 10% 7% 3 5% 4 4 5% 18%	1520
December	1st to 10th	Day	22% 19% 8% 11% 7% 4 4 3 3 5% 14%	1520
	11th to 20th	Day	18% 15% 8% 12% 7% 3 4 4 5% 5% 19%	1906
	21st to end	Day	23% 21% 7% 10% 7% 3 3 3 2 3 18%	2772
January	1st to 10th	Night	16% 18% 8% 11% 8% 3 4 4 4 5% 20%	4429
	11th to 20th	Night	12% 14% 7% 13% 10% 4 6% 4 4 5% 21%	4440
	21st to end	Night	13% 17% 10% 13% 8% 5% 6% 4 3 4 19%	4812
February	1st to 10th	Night	12% 15% 7% 11% 9% 4 5% 5% 4 5% 22%	2470
	11th to 20th	Night	12% 15% 6% 11% 8% 4 4 4 4 4 27%	2399
	21st to end	Night	13% 16% 6% 9% 6% 4 5% 5% 4 6% 26%	1884
March	1st to 10th	Night	17% 13% 5% 8% 7% 4 6% 4 4 4 28%	2270
	11th to 20th	Night	16% 14% 5% 7% 6% 3 4 5% 4 5% 31%	2103
	21st to end	Night	15% 11% 4 7% 6% 2 4 4 4 5% 38%	2127
April	1st to 10th	Night	21% 13% 4 6% 5% 3 4 4 4 7% 30%	1800
	11th to 20th	Night	19% 8% 3 6% 6% 2 4 5% 5% 5% 36%	1775
	21st to end	Night	19% 11% 4 8% 4 2 4 4 4 6% 33%	1662
May	1st to 10th	Night	20% 9% 3 6% 5% 3 4 3 5% 6% 36%	1440
	11th to 20th	Night	18% 8% 3 5% 5% 3 4 4 5% 5% 39%	1415
	21st to end	Night	14% 6% 3 6% 5% 4 4 6% 5% 6% 42%	1550
June	1st to 10th	Night	19% 7% 2 6% 4 3 5% 6% 6% 10% 33%	1440
	11th to 20th	Night	15% 7% 2 5% 6% 3 5% 4 5% 7% 41%	1440
	21st to end	Night	17% 6% 3 5% 5% 4 6% 5% 6% 8% 35%	1440
July	1st to 10th	Night	10% 7% 3 5% 4 3 4 5% 6% 8% 46%	1440
	11th to 20th	Night	13% 6% 3 5% 4 3 4 5% 5% 8% 44%	1440
	21st to end	Night	9% 5% 2 5% 6% 3 5% 6% 7% 9% 43%	1574
August	1st to 10th	Night	9% 5% 3 6% 6% 4 4 6% 7% 8% 41%	1435
	11th to 20th	Night	10% 6% 3 6% 6% 3 4 5% 6% 7% 45%	1602
	21st to end	Night	14% 6% 3 5% 5% 3 4 4 6% 7% 43%	1980
September	1st to 10th	Night	14% 6% 3 6% 6% 2 4 4 5% 6% 44%	1800
	11th to 20th	Night	15% 6% 2 4 4 3 3 4 5% 7% 46%	1804
	21st to end	Night	17% 7% 2 6% 4 2 4 4 5% 5% 43%	2028
October	1st to 10th	Night	20% 8% 3 6% 5% 3 4 5% 4 7% 37%	2160
	11th to 20th	Night	21% 12% 4 6% 6% 4 5% 4 4 6% 29%	2160
	21st to end	Night	24% 12% 3 8% 7% 3 4 4 5% 5% 26%	2538
November	1st to 10th	Night	18% 12% 5% 7% 6% 3 4 5% 5% 6% 30%	2407
	11th to 20th	Night	19% 13% 5% 10% 7% 4 4 4 4 5% 26%	2660
	21st to end	Night	21% 16% 6% 10% 6% 4 5% 4 4 4 20%	2660
December	1st to 10th	Night	20% 19% 7% 10% 7% 4 4 4 4 5% 17%	2660
	11th to 20th	Night	21% 16% 6% 9% 6% 3 4 3 4 4 23%	3288
	21st to end	Night	24% 21% 6% 10% 7% 3 4 3 3 3 17%	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	72% 3 8% 12% 5%	2518
	11th to 20th	Day	69% 2 8% 14% 7%	2520
	21st to end	Day	71% 2 9% 12% 6% 1	2840
February	1st to 10th	Day	65% 2 7% 17% 7% 1	1710
	11th to 20th	Day	63% 2 10% 14% 10% 1	1676
	21st to end	Day	63% 3 9% 14% 11% 1	1570
March	1st to 10th	Day	60% 2 8% 18% 10% 2	1900
	11th to 20th	Day	60% 3 9% 14% 11% 3	2074
	21st to end	Day	52% 2 7% 19% 15% 4	2384
April	1st to 10th	Day	58% 2 6% 18% 12% 3	2160
	11th to 20th	Day	53% 2 6% 22% 15% 2	2134
	21st to end	Day	59% 2 6% 16% 13% 4	2311
May	1st to 10th	Day	54% 3 5% 21% 14% 3	2520
	11th to 20th	Day	50% 3 6% 22% 16% 3	2482
	21st to end	Day	48% 4 11% 20% 14% 2	2725
June	1st to 10th	Day	55% 4 9% 20% 12% 1	2520
	11th to 20th	Day	44% 3 11% 27% 12% 2	2520
	21st to end	Day	52% 4 13% 21% 10%	2520
July	1st to 10th	Day	43% 4 12% 30% 12%	2450
	11th to 20th	Day	44% 4 14% 27% 11%	2445
	21st to end	Day	44% 4 15% 27% 10%	2591
August	1st to 10th	Day	47% 4 16% 25% 8%	2487
	11th to 20th	Day	45% 4 15% 26% 9% 1	2281
	21st to end	Day	44% 4 13% 27% 11% 1	2376
September	1st to 10th	Day	43% 3 13% 29% 13%	2160
	11th to 20th	Day	42% 3 12% 31% 11% 1	2156
	21st to end	Day	46% 4 14% 28% 8%	1930
October	1st to 10th	Day	53% 3 11% 23% 10% 1	1800
	11th to 20th	Day	62% 3 11% 18% 6% 1	1800
	21st to end	Day	63% 3 10% 16% 6% 1	1818
November	1st to 10th	Day	50% 4 14% 24% 7% 1	1670
	11th to 20th	Day	65% 4 11% 13% 7% 1	1520
	21st to end	Day	69% 3 12% 9% 6% 1	1520
December	1st to 10th	Day	73% 3 10% 10% 4	1520
	11th to 20th	Day	68% 3 10% 14% 6%	1906
	21st to end	Day	74% 2 5% 15% 5%	2772
January	1st to 10th	Night	68% 2 15% 12% 3	4429
	11th to 20th	Night	67% 2 12% 14% 5%	4440
	21st to end	Night	71% 2 9% 10% 7%	4812
February	1st to 10th	Night	63% 3 8% 17% 7% 1	2470
	11th to 20th	Night	61% 2 8% 19% 9% 1	2401
	21st to end	Night	59% 2 8% 18% 11% 1	1884
March	1st to 10th	Night	60% 3 7% 19% 10% 2	2280
	11th to 20th	Night	57% 2 10% 17% 11% 3	2104
	21st to end	Night	50% 3 6% 21% 15% 4	2127
April	1st to 10th	Night	56% 2 8% 18% 14% 2	1800
	11th to 20th	Night	50% 3 6% 24% 16% 1	1775
	21st to end	Night	54% 3 7% 18% 14% 3	1662
May	1st to 10th	Night	51% 2 6% 20% 16% 4	1440
	11th to 20th	Night	46% 3 5% 24% 18% 3	1416
	21st to end	Night	43% 3 13% 22% 16% 3	1551
June	1st to 10th	Night	47% 5% 13% 25% 10% 1	1440
	11th to 20th	Night	43% 4 11% 27% 13% 2	1440
	21st to end	Night	46% 6% 13% 23% 13% 1	1440
July	1st to 10th	Night	37% 4 14% 29% 15% 1	1400
	11th to 20th	Night	39% 4 12% 32% 12%	1392
	21st to end	Night	37% 5% 17% 28% 13%	1486
August	1st to 10th	Night	39% 5% 19% 26% 11%	1425
	11th to 20th	Night	38% 4 17% 30% 11%	1602
	21st to end	Night	40% 3 17% 29% 10%	1980
September	1st to 10th	Night	41% 4 11% 29% 14% 1	1800
	11th to 20th	Night	39% 3 13% 35% 9% 1	1804
	21st to end	Night	44% 2 11% 33% 10%	2028
October	1st to 10th	Night	48% 4 9% 25% 13% 1	2160
	11th to 20th	Night	57% 4 11% 20% 8%	2160
	21st to end	Night	61% 3 11% 17% 7% 1	2538
November	1st to 10th	Night	55% 2 14% 20% 8%	2407
	11th to 20th	Night	61% 4 10% 16% 8% 2	2660
	21st to end	Night	66% 4 11% 14% 4	2660
December	1st to 10th	Night	70% 3 8% 14% 5%	2660
	11th to 20th	Night	65% 2 7% 16% 8% 1	3288
	21st to end	Night	74% 2 5% 14% 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	29% 15% 34% 19% 3	180
	11th to 20th	Day	29% 16% 37% 13% 4 1	180
	21st to end	Day	23% 20% 34% 19% 4	218
February	1st to 10th	Day	34% 25% 31% 8% 3	270
	11th to 20th	Day	31% 28% 33% 8%	274
	21st to end	Day	33% 21% 31% 12% 2	382
March	1st to 10th	Day	29% 19% 33% 14% 5%	460
	11th to 20th	Day	30% 20% 32% 13% 4 1	460
	21st to end	Day	34% 20% 25% 16% 5% 1	490
April	1st to 10th	Day	34% 18% 30% 14% 4 1	420
	11th to 20th	Day	34% 19% 23% 14% 9% 1	412
	21st to end	Day	32% 21% 27% 13% 5% 1	420
May	1st to 10th	Day	30% 16% 31% 14% 7% 2	423
	11th to 20th	Day	26% 17% 32% 17% 6% 2	447
	21st to end	Day	23% 18% 31% 18% 9%	485
June	1st to 10th	Day	28% 12% 35% 18% 6% 1	450
	11th to 20th	Day	21% 15% 30% 21% 9% 3	450
	21st to end	Day	23% 11% 33% 22% 9% 2	450
July	1st to 10th	Day	16% 8% 34% 30% 8% 4	445
	11th to 20th	Day	15% 11% 33% 28% 10% 2	444
	21st to end	Day	16% 8% 36% 28% 10% 2	488
August	1st to 10th	Day	19% 6% 28% 34% 11% 3	446
	11th to 20th	Day	21% 5% 26% 28% 13% 8%	439
	21st to end	Day	19% 12% 28% 23% 11% 7%	495
September	1st to 10th	Day	17% 12% 32% 20% 13% 6%	450
	11th to 20th	Day	17% 11% 28% 22% 15% 8%	450
	21st to end	Day	23% 15% 25% 20% 11% 6%	450
October	1st to 10th	Day	27% 15% 22% 22% 11% 3	450
	11th to 20th	Day	34% 20% 28% 12% 7% 1	450
	21st to end	Day	34% 13% 31% 18% 4	414
November	1st to 10th	Day	29% 17% 30% 18% 5% 1	364
	11th to 20th	Day	34% 21% 33% 8% 3 2	180
	21st to end	Day	32% 22% 31% 13% 2 1	180
December	1st to 10th	Day	33% 14% 34% 16% 3	180
	11th to 20th	Day	35% 26% 28% 10% 1 1	180
	21st to end	Day	35% 22% 28% 11% 3 1	198
January	1st to 10th	Night	30% 15% 33% 17% 4	760
	11th to 20th	Night	30% 16% 27% 21% 6%	760
	21st to end	Night	19% 13% 36% 24% 7% 1	836
February	1st to 10th	Night	22% 19% 32% 22% 4	760
	11th to 20th	Night	22% 20% 35% 18% 4 1	743
	21st to end	Night	26% 20% 26% 25% 3 1	628
March	1st to 10th	Night	26% 17% 31% 22% 5%	760
	11th to 20th	Night	25% 20% 30% 21% 4 1	760
	21st to end	Night	26% 16% 28% 23% 7%	821
April	1st to 10th	Night	28% 18% 24% 25% 5%	720
	11th to 20th	Night	27% 16% 29% 20% 8% 1	711
	21st to end	Night	28% 15% 26% 23% 8% 1	582
May	1st to 10th	Night	27% 11% 26% 26% 9% 2	360
	11th to 20th	Night	24% 9% 29% 29% 9%	354
	21st to end	Night	16% 12% 35% 25% 11% 1	389
June	1st to 10th	Night	21% 6% 31% 34% 7% 1	360
	11th to 20th	Night	18% 8% 27% 32% 14% 1	360
	21st to end	Night	20% 4 25% 36% 14% 1	360
July	1st to 10th	Night	11% 5% 30% 39% 15% 1	350
	11th to 20th	Night	16% 4 22% 41% 17%	348
	21st to end	Night	10% 4 27% 41% 15% 2	394
August	1st to 10th	Night	11% 4 23% 41% 20% 1	361
	11th to 20th	Night	14% 3 19% 38% 23% 4	538
	21st to end	Night	17% 7% 25% 31% 18% 2	792
September	1st to 10th	Night	18% 6% 29% 28% 18% 1	720
	11th to 20th	Night	20% 6% 25% 28% 18% 3	720
	21st to end	Night	22% 8% 25% 25% 18% 2	720
October	1st to 10th	Night	24% 11% 27% 23% 13% 1	720
	11th to 20th	Night	29% 12% 27% 25% 6% 1	720
	21st to end	Night	30% 14% 28% 22% 6%	792
November	1st to 10th	Night	29% 13% 26% 23% 8% 1	740
	11th to 20th	Night	28% 18% 29% 15% 10% 1	760
	21st to end	Night	31% 19% 23% 21% 5%	760
December	1st to 10th	Night	30% 17% 28% 21% 4	760
	11th to 20th	Night	29% 20% 26% 19% 5% 1	760
	21st to end	Night	36% 15% 27% 17% 4 1	836

Page generated at 2024-03-24 23:37:04. Version ac07

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