Observatoire astronomique du bois de Belle-Rivière
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire astronomique du bois de Belle-Rivière clear sky chart from 2005-11-06 to 2026-09-29.

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	16% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	149800
	Night	16% 11% 4 9% 5% 5% 4 4 5% 6% 31%	141636
Transparency	Day	49% 7% 13% 21% 10% 1	153247
	Night	52% 7% 12% 19% 10% 1	146278
Seeing	Day	20% 14% 30% 23% 10% 3	16941
	Night	21% 10% 31% 28% 9% 1	25840

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	14% 17% 6% 13% 5% 4 4 3 5% 5% 24%	4661
	11th to 20th	Day	11% 17% 7% 12% 7% 5% 4 3 5% 6% 23%	4650
	21st to end	Day	8% 15% 6% 12% 8% 6% 6% 4 5% 6% 24%	5124
February	1st to 10th	Day	13% 15% 5% 10% 6% 5% 5% 4 5% 6% 27%	1861
	11th to 20th	Day	13% 12% 6% 9% 7% 6% 4 5% 6% 6% 27%	1932
	21st to end	Day	18% 15% 5% 9% 6% 4 4 4 4 5% 27%	1730
March	1st to 10th	Day	19% 14% 5% 11% 6% 5% 4 3 4 5% 26%	4576
	11th to 20th	Day	11% 14% 4 10% 5% 5% 4 4 6% 6% 32%	4848
	21st to end	Day	19% 9% 4 9% 5% 4 4 3 4 4 34%	6070
April	1st to 10th	Day	21% 11% 3 8% 5% 4 4 4 4 5% 33%	2520
	11th to 20th	Day	21% 8% 3 6% 5% 5% 4 3 5% 5% 35%	2494
	21st to end	Day	22% 10% 3 7% 5% 4 4 3 5% 6% 32%	2672
May	1st to 10th	Day	24% 9% 3 8% 5% 5% 5% 4 6% 7% 24%	8209
	11th to 20th	Day	19% 9% 3 8% 5% 4 4 4 6% 7% 31%	8221
	21st to end	Day	21% 9% 2 6% 4 4 4 4 5% 6% 33%	9060
June	1st to 10th	Day	20% 8% 2 7% 5% 5% 5% 4 6% 7% 31%	2940
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 5% 6% 7% 37%	2940
	21st to end	Day	19% 8% 3 7% 6% 5% 5% 4 7% 8% 29%	2940
July	1st to 10th	Day	11% 5% 3 7% 6% 6% 6% 4 6% 8% 38%	8280
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 4 7% 9% 44%	8280
	21st to end	Day	11% 6% 2 6% 4 5% 5% 4 7% 10% 42%	8993
August	1st to 10th	Day	14% 7% 3 8% 5% 6% 6% 5% 6% 8% 32%	6483
	11th to 20th	Day	13% 6% 2 8% 4 5% 5% 5% 6% 9% 37%	5886
	21st to end	Day	13% 5% 2 7% 5% 5% 5% 4 7% 10% 38%	6080
September	1st to 10th	Day	12% 5% 1 6% 4 5% 5% 4 7% 10% 41%	2520
	11th to 20th	Day	11% 5% 3 4 3 5% 4 3 4 7% 51%	2447
	21st to end	Day	16% 5% 2 7% 4 4 3 4 5% 7% 43%	2288
October	1st to 10th	Day	19% 7% 3 6% 4 4 4 4 5% 6% 39%	2040
	11th to 20th	Day	19% 7% 3 8% 6% 5% 6% 4 5% 6% 31%	1970
	21st to end	Day	23% 9% 3 7% 5% 4 4 4 6% 6% 29%	1980
November	1st to 10th	Day	15% 10% 3 8% 4 5% 5% 3 5% 7% 33%	1851
	11th to 20th	Day	18% 9% 5% 7% 5% 5% 6% 4 5% 6% 30%	1780
	21st to end	Day	22% 12% 3 7% 5% 7% 5% 4 5% 6% 24%	1680
December	1st to 10th	Day	20% 18% 6% 12% 4 4 4 4 5% 4 19%	2680
	11th to 20th	Day	18% 15% 5% 10% 5% 4 4 3 5% 4 27%	3066
	21st to end	Day	23% 13% 5% 9% 5% 4 5% 4 4 4 24%	4048
January	1st to 10th	Night	13% 18% 7% 15% 5% 5% 4 3 4 5% 22%	8234
	11th to 20th	Night	11% 17% 6% 17% 7% 5% 5% 3 4 5% 19%	8235
	21st to end	Night	10% 14% 5% 12% 8% 6% 5% 4 5% 6% 26%	9057
February	1st to 10th	Night	13% 13% 5% 11% 6% 5% 5% 4 6% 6% 26%	2756
	11th to 20th	Night	13% 11% 4 9% 6% 6% 5% 4 6% 6% 30%	2573
	21st to end	Night	13% 15% 5% 9% 5% 4 4 3 4 5% 32%	2076
March	1st to 10th	Night	22% 14% 3 7% 5% 4 4 3 4 5% 29%	5484
	11th to 20th	Night	14% 14% 4 8% 5% 5% 5% 3 6% 7% 30%	5329
	21st to end	Night	15% 11% 4 10% 6% 5% 5% 3 5% 4 33%	5103
April	1st to 10th	Night	23% 10% 3 6% 5% 3 4 3 5% 6% 32%	2100
	11th to 20th	Night	21% 9% 3 6% 5% 4 3 3 4 6% 35%	2075
	21st to end	Night	21% 8% 3 7% 3 3 4 4 4 6% 36%	1946
May	1st to 10th	Night	21% 9% 3 9% 5% 6% 5% 4 6% 7% 25%	4744
	11th to 20th	Night	21% 8% 3 7% 4 4 5% 3 5% 7% 33%	4639
	21st to end	Night	20% 9% 3 7% 5% 4 4 4 5% 7% 32%	5114
June	1st to 10th	Night	19% 7% 2 7% 4 6% 5% 3 8% 8% 32%	1680
	11th to 20th	Night	16% 8% 2 6% 5% 4 4 4 5% 7% 38%	1680
	21st to end	Night	19% 7% 2 6% 5% 5% 5% 4 6% 6% 36%	1680
July	1st to 10th	Night	12% 5% 2 7% 5% 6% 6% 4 8% 9% 34%	4680
	11th to 20th	Night	12% 6% 2 6% 4 4 5% 4 6% 8% 45%	4680
	21st to end	Night	13% 6% 2 6% 4 5% 4 4 6% 8% 41%	5084
August	1st to 10th	Night	16% 8% 3 8% 5% 4 5% 4 6% 7% 35%	3666
	11th to 20th	Night	15% 6% 3 7% 4 4 5% 4 5% 7% 41%	4109
	21st to end	Night	15% 6% 2 7% 5% 4 5% 4 5% 7% 40%	5045
September	1st to 10th	Night	10% 6% 3 7% 4 4 4 4 6% 8% 44%	2100
	11th to 20th	Night	13% 6% 2 4 4 3 3 3 5% 8% 50%	2173
	21st to end	Night	16% 5% 2 6% 4 4 4 4 5% 7% 44%	2289
October	1st to 10th	Night	18% 7% 3 5% 4 4 5% 3 5% 5% 42%	2355
	11th to 20th	Night	20% 8% 3 6% 5% 4 4 3 4 6% 37%	2428
	21st to end	Night	21% 8% 3 8% 5% 5% 4 4 4 5% 32%	2860
November	1st to 10th	Night	20% 9% 3 7% 4 5% 4 4 6% 7% 33%	2667
	11th to 20th	Night	17% 11% 3 8% 5% 4 5% 4 5% 6% 34%	2837
	21st to end	Night	20% 14% 4 9% 5% 4 3 4 4 5% 28%	2940
December	1st to 10th	Night	17% 16% 6% 10% 7% 6% 5% 4 5% 5% 20%	4720
	11th to 20th	Night	23% 14% 5% 10% 5% 4 4 2 4 4 26%	5348
	21st to end	Night	25% 16% 4 9% 4 3 3 3 5% 5% 24%	7150

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	60% 7% 11% 17% 4	4662
	11th to 20th	Day	61% 6% 9% 14% 9% 1	4680
	21st to end	Day	60% 7% 10% 12% 11% 1	5124
February	1st to 10th	Day	58% 5% 10% 19% 6% 2	1861
	11th to 20th	Day	57% 6% 12% 16% 8% 1	1941
	21st to end	Day	59% 5% 9% 15% 10% 2	1730
March	1st to 10th	Day	60% 6% 9% 12% 10% 3	4579
	11th to 20th	Day	49% 9% 18% 13% 10% 2	4848
	21st to end	Day	51% 6% 8% 15% 18% 2	6070
April	1st to 10th	Day	53% 6% 9% 18% 12% 2	2520
	11th to 20th	Day	49% 6% 8% 21% 14% 2	2494
	21st to end	Day	53% 5% 9% 17% 12% 3	2672
May	1st to 10th	Day	55% 8% 8% 18% 10% 1	8221
	11th to 20th	Day	49% 8% 13% 20% 8% 1	8225
	21st to end	Day	49% 7% 11% 21% 11% 1	9061
June	1st to 10th	Day	49% 9% 10% 19% 12% 1	2940
	11th to 20th	Day	43% 8% 12% 21% 13% 3	2940
	21st to end	Day	49% 9% 13% 19% 9% 1	2940
July	1st to 10th	Day	41% 7% 14% 26% 11%	8210
	11th to 20th	Day	33% 7% 14% 32% 13%	8205
	21st to end	Day	37% 7% 16% 31% 8%	8795
August	1st to 10th	Day	48% 7% 16% 18% 9% 2	6469
	11th to 20th	Day	41% 8% 18% 26% 7%	5928
	21st to end	Day	40% 7% 19% 23% 10%	6080
September	1st to 10th	Day	36% 8% 16% 28% 12%	2520
	11th to 20th	Day	33% 7% 14% 32% 14% 1	2447
	21st to end	Day	41% 6% 14% 25% 12% 1	2288
October	1st to 10th	Day	44% 7% 13% 26% 10%	3326
	11th to 20th	Day	43% 6% 17% 26% 7%	3186
	21st to end	Day	44% 6% 20% 21% 9% 1	3180
November	1st to 10th	Day	49% 7% 14% 21% 7% 1	1851
	11th to 20th	Day	52% 8% 13% 17% 10%	1780
	21st to end	Day	59% 10% 12% 14% 5%	1680
December	1st to 10th	Day	69% 6% 9% 14% 3	2680
	11th to 20th	Day	60% 6% 10% 16% 9%	3066
	21st to end	Day	62% 6% 10% 15% 7%	4048
January	1st to 10th	Night	64% 6% 10% 14% 5% 1	8234
	11th to 20th	Night	65% 8% 10% 11% 5%	8280
	21st to end	Night	58% 6% 9% 13% 13%	9057
February	1st to 10th	Night	58% 7% 11% 16% 7% 1	2756
	11th to 20th	Night	52% 7% 12% 20% 8% 1	2575
	21st to end	Night	54% 6% 10% 17% 12% 1	2076
March	1st to 10th	Night	57% 6% 8% 14% 12% 4	5494
	11th to 20th	Night	50% 8% 14% 16% 10% 2	5330
	21st to end	Night	52% 6% 7% 21% 12% 2	5103
April	1st to 10th	Night	52% 6% 10% 18% 13% 2	2100
	11th to 20th	Night	51% 6% 8% 19% 13% 2	2075
	21st to end	Night	49% 6% 7% 19% 15% 4	1946
May	1st to 10th	Night	55% 7% 12% 16% 9% 1	4750
	11th to 20th	Night	49% 8% 10% 22% 9% 1	4640
	21st to end	Night	50% 7% 10% 18% 15% 1	5115
June	1st to 10th	Night	48% 9% 11% 19% 13% 1	1680
	11th to 20th	Night	43% 7% 11% 22% 15% 2	1680
	21st to end	Night	46% 9% 11% 20% 13% 1	1680
July	1st to 10th	Night	43% 7% 16% 21% 12% 1	4640
	11th to 20th	Night	35% 7% 16% 31% 11%	4632
	21st to end	Night	40% 7% 15% 27% 11%	4967
August	1st to 10th	Night	47% 6% 15% 21% 9% 1	3656
	11th to 20th	Night	42% 7% 17% 26% 8%	4133
	21st to end	Night	41% 8% 17% 22% 11% 1	5045
September	1st to 10th	Night	37% 8% 13% 27% 15% 1	2100
	11th to 20th	Night	34% 6% 13% 33% 13% 1	2173
	21st to end	Night	39% 8% 12% 25% 16%	2289
October	1st to 10th	Night	43% 6% 14% 25% 11% 1	3849
	11th to 20th	Night	43% 6% 13% 29% 9%	3951
	21st to end	Night	45% 6% 18% 18% 11% 1	4610
November	1st to 10th	Night	51% 6% 13% 21% 8% 1	2667
	11th to 20th	Night	51% 7% 14% 17% 10% 1	2837
	21st to end	Night	59% 7% 12% 16% 6%	2940
December	1st to 10th	Night	65% 6% 12% 13% 4	4720
	11th to 20th	Night	61% 6% 7% 15% 10%	5348
	21st to end	Night	63% 5% 8% 14% 11%	7150

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	21% 16% 35% 23% 5%	220
	11th to 20th	Day	27% 22% 29% 17% 5%	220
	21st to end	Day	19% 20% 31% 22% 8%	242
February	1st to 10th	Day	27% 23% 33% 16% 2	316
	11th to 20th	Day	21% 19% 39% 19% 2	419
	21st to end	Day	27% 16% 33% 21% 2	446
March	1st to 10th	Day	25% 16% 33% 22% 4	540
	11th to 20th	Day	22% 19% 35% 18% 5% 1	540
	21st to end	Day	26% 18% 30% 16% 8% 1	594
April	1st to 10th	Day	28% 18% 28% 18% 6% 2	540
	11th to 20th	Day	27% 20% 26% 17% 9% 2	532
	21st to end	Day	26% 18% 30% 17% 7% 1	540
May	1st to 10th	Day	24% 13% 32% 23% 7% 1	543
	11th to 20th	Day	19% 13% 33% 23% 11% 2	567
	21st to end	Day	21% 13% 32% 23% 10% 2	617
June	1st to 10th	Day	21% 12% 29% 27% 10% 1	570
	11th to 20th	Day	15% 14% 29% 29% 12% 2	570
	21st to end	Day	21% 9% 31% 24% 12% 3	570
July	1st to 10th	Day	11% 8% 31% 32% 14% 4	565
	11th to 20th	Day	13% 11% 28% 31% 13% 3	564
	21st to end	Day	14% 8% 27% 30% 16% 4	620
August	1st to 10th	Day	14% 6% 24% 34% 19% 4	566
	11th to 20th	Day	10% 8% 25% 31% 18% 9%	559
	21st to end	Day	14% 9% 28% 26% 17% 7%	627
September	1st to 10th	Day	14% 8% 28% 28% 14% 7%	570
	11th to 20th	Day	10% 8% 26% 24% 22% 10%	570
	21st to end	Day	17% 10% 30% 21% 17% 5%	562
October	1st to 10th	Day	19% 12% 27% 24% 15% 3	530
	11th to 20th	Day	19% 16% 31% 23% 10% 1	513
	21st to end	Day	29% 15% 27% 22% 7%	462
November	1st to 10th	Day	28% 17% 25% 20% 8% 1	425
	11th to 20th	Day	23% 18% 35% 18% 5%	320
	21st to end	Day	27% 23% 26% 18% 4 1	220
December	1st to 10th	Day	29% 14% 28% 22% 7% 1	220
	11th to 20th	Day	28% 15% 38% 15% 3 1	220
	21st to end	Day	30% 18% 30% 16% 5% 2	242
January	1st to 10th	Night	24% 13% 34% 25% 4	840
	11th to 20th	Night	25% 15% 31% 23% 5%	840
	21st to end	Night	17% 11% 38% 26% 6% 1	924
February	1st to 10th	Night	19% 13% 39% 24% 5%	840
	11th to 20th	Night	20% 12% 41% 22% 4	823
	21st to end	Night	22% 11% 35% 28% 3	692
March	1st to 10th	Night	23% 14% 34% 25% 3	840
	11th to 20th	Night	23% 16% 32% 23% 5%	840
	21st to end	Night	23% 11% 32% 28% 6%	924
April	1st to 10th	Night	27% 13% 28% 27% 5%	840
	11th to 20th	Night	24% 13% 32% 26% 5%	831
	21st to end	Night	26% 10% 31% 24% 8%	686
May	1st to 10th	Night	24% 8% 33% 26% 7% 2	452
	11th to 20th	Night	22% 7% 29% 33% 9%	414
	21st to end	Night	17% 7% 38% 29% 8%	455
June	1st to 10th	Night	23% 6% 28% 33% 9%	420
	11th to 20th	Night	19% 7% 30% 33% 11%	420
	21st to end	Night	20% 6% 27% 31% 16%	420
July	1st to 10th	Night	10% 6% 30% 38% 16%	410
	11th to 20th	Night	12% 4 24% 43% 16%	408
	21st to end	Night	12% 4 25% 43% 16% 1	460
August	1st to 10th	Night	13% 4 17% 45% 18% 1	418
	11th to 20th	Night	14% 4 21% 36% 23% 2	622
	21st to end	Night	14% 5% 25% 35% 20% 2	924
September	1st to 10th	Night	12% 5% 25% 41% 15% 1	840
	11th to 20th	Night	15% 5% 20% 36% 21% 2	840
	21st to end	Night	17% 5% 27% 31% 17% 2	833
October	1st to 10th	Night	20% 8% 27% 31% 13% 1	800
	11th to 20th	Night	21% 9% 31% 28% 9% 1	800
	21st to end	Night	26% 10% 33% 26% 5%	880
November	1st to 10th	Night	25% 14% 28% 26% 7%	820
	11th to 20th	Night	23% 13% 33% 21% 9%	840
	21st to end	Night	28% 16% 32% 19% 6%	840
December	1st to 10th	Night	28% 12% 33% 22% 5%	840
	11th to 20th	Night	27% 16% 33% 20% 4 1	840
	21st to end	Night	33% 13% 31% 18% 5%	924

Page generated at 2026-09-26 04:18:38. Version ac07

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