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 2025-05-26.

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% 10% 3 8% 5% 5% 5% 4 5% 7% 32%	114009
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	115266
Transparency	Day	49% 8% 13% 20% 10% 1	117502
	Night	52% 7% 12% 18% 10% 1	119937
Seeing	Day	20% 14% 29% 23% 11% 3	15176
	Night	22% 10% 31% 28% 9% 1	24260

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% 6% 14% 5% 4 4 3 4 4 23%	3597
	11th to 20th	Day	13% 15% 6% 11% 7% 5% 4 3 5% 6% 26%	3570
	21st to end	Day	10% 17% 5% 11% 5% 5% 6% 4 5% 6% 26%	3960
February	1st to 10th	Day	13% 16% 5% 10% 6% 5% 6% 3 5% 5% 26%	1772
	11th to 20th	Day	14% 12% 5% 10% 7% 6% 4 5% 6% 6% 27%	1838
	21st to end	Day	18% 15% 5% 8% 5% 4 4 4 4 5% 27%	1650
March	1st to 10th	Day	19% 14% 5% 11% 6% 5% 4 3 4 4 25%	4476
	11th to 20th	Day	10% 14% 4 10% 5% 4 4 4 6% 7% 32%	4743
	21st to end	Day	19% 9% 4 9% 5% 4 4 3 4 4 35%	5939
April	1st to 10th	Day	21% 11% 3 7% 5% 4 4 4 4 5% 33%	2400
	11th to 20th	Day	21% 8% 3 6% 5% 5% 4 3 5% 5% 36%	2374
	21st to end	Day	22% 10% 3 7% 5% 5% 4 3 5% 6% 32%	2545
May	1st to 10th	Day	21% 9% 3 8% 5% 5% 5% 4 5% 7% 28%	4552
	11th to 20th	Day	18% 8% 3 9% 5% 4 4 4 5% 7% 32%	4538
	21st to end	Day	16% 6% 2 7% 5% 5% 4 4 6% 8% 36%	4907
June	1st to 10th	Day	21% 8% 2 7% 5% 5% 5% 4 6% 7% 30%	2660
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 5% 6% 7% 38%	2660
	21st to end	Day	19% 8% 3 7% 6% 5% 5% 5% 6% 8% 29%	2660
July	1st to 10th	Day	11% 6% 3 7% 6% 6% 6% 4 6% 8% 36%	4440
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 5% 7% 9% 37%	4440
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 6% 8% 45%	4814
August	1st to 10th	Day	14% 6% 3 7% 5% 6% 5% 5% 7% 9% 34%	4423
	11th to 20th	Day	14% 7% 3 8% 4 4 5% 4 6% 8% 38%	3994
	21st to end	Day	15% 5% 2 6% 5% 5% 5% 4 6% 9% 39%	4180
September	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 4 6% 9% 41%	2280
	11th to 20th	Day	12% 5% 3 4 3 5% 4 3 4 7% 50%	2214
	21st to end	Day	17% 5% 2 7% 4 4 3 4 5% 7% 42%	2090
October	1st to 10th	Day	19% 7% 3 6% 5% 4 4 4 5% 6% 37%	1938
	11th to 20th	Day	19% 7% 3 8% 7% 5% 6% 4 5% 6% 30%	1872
	21st to end	Day	23% 9% 3 7% 5% 4 4 4 6% 6% 29%	1881
November	1st to 10th	Day	15% 9% 3 7% 5% 5% 5% 3 6% 7% 34%	1761
	11th to 20th	Day	18% 9% 5% 7% 5% 4 6% 4 5% 7% 31%	1695
	21st to end	Day	22% 11% 3 8% 5% 7% 5% 4 6% 7% 24%	1600
December	1st to 10th	Day	20% 18% 6% 12% 4 4 4 4 5% 4 19%	2600
	11th to 20th	Day	18% 15% 5% 10% 5% 4 3 3 5% 4 28%	2986
	21st to end	Day	23% 13% 5% 9% 5% 4 5% 4 4 4 24%	3960
January	1st to 10th	Night	13% 20% 7% 14% 5% 4 4 3 4 5% 21%	6349
	11th to 20th	Night	10% 15% 6% 16% 7% 6% 5% 3 5% 5% 21%	6315
	21st to end	Night	12% 15% 5% 12% 7% 5% 5% 4 5% 5% 25%	6996
February	1st to 10th	Night	14% 14% 5% 11% 6% 5% 5% 4 6% 6% 26%	2625
	11th to 20th	Night	13% 11% 4 9% 6% 6% 5% 4 6% 6% 30%	2448
	21st to end	Night	14% 16% 5% 9% 5% 4 4 3 4 5% 32%	1980
March	1st to 10th	Night	22% 14% 3 7% 5% 4 4 3 4 5% 29%	5364
	11th to 20th	Night	13% 14% 4 8% 5% 5% 5% 3 6% 7% 30%	5214
	21st to end	Night	15% 11% 3 9% 6% 5% 5% 3 5% 4 33%	4992
April	1st to 10th	Night	23% 10% 3 6% 5% 3 4 3 5% 6% 32%	2000
	11th to 20th	Night	21% 9% 3 6% 5% 4 3 3 5% 6% 35%	1975
	21st to end	Night	21% 8% 4 7% 3 3 4 4 4 6% 35%	1853
May	1st to 10th	Night	22% 9% 3 8% 5% 5% 5% 3 5% 6% 30%	2637
	11th to 20th	Night	22% 7% 3 6% 4 4 4 3 5% 6% 37%	2575
	21st to end	Night	17% 8% 3 7% 5% 4 4 3 5% 8% 36%	2784
June	1st to 10th	Night	20% 7% 2 7% 4 6% 5% 3 8% 8% 31%	1520
	11th to 20th	Night	15% 8% 2 6% 5% 3 4 4 5% 8% 39%	1520
	21st to end	Night	19% 7% 3 6% 5% 5% 5% 4 6% 6% 35%	1520
July	1st to 10th	Night	14% 6% 3 8% 5% 6% 6% 4 7% 7% 35%	2520
	11th to 20th	Night	14% 6% 2 6% 5% 4 5% 4 6% 8% 42%	2520
	21st to end	Night	11% 6% 3 6% 4 4 4 4 6% 9% 43%	2739
August	1st to 10th	Night	15% 7% 3 7% 5% 4 4 4 6% 7% 39%	2506
	11th to 20th	Night	15% 7% 3 7% 4 4 4 4 5% 6% 40%	2799
	21st to end	Night	17% 6% 2 7% 4 4 5% 4 5% 6% 40%	3476
September	1st to 10th	Night	10% 5% 3 7% 4 5% 4 4 5% 8% 45%	1900
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 5% 8% 49%	1966
	21st to end	Night	16% 5% 2 6% 3 3 4 4 5% 7% 44%	2090
October	1st to 10th	Night	19% 7% 3 6% 4 4 5% 3 5% 5% 40%	2237
	11th to 20th	Night	21% 8% 3 6% 5% 4 4 3 5% 6% 36%	2306
	21st to end	Night	21% 9% 4 8% 5% 5% 4 4 4 5% 32%	2717
November	1st to 10th	Night	19% 9% 3 7% 4 5% 4 4 6% 7% 33%	2537
	11th to 20th	Night	17% 10% 3 8% 5% 4 5% 4 5% 6% 34%	2702
	21st to end	Night	21% 13% 3 9% 5% 4 3 4 4 5% 28%	2800
December	1st to 10th	Night	17% 15% 5% 10% 7% 6% 5% 4 5% 5% 20%	4580
	11th to 20th	Night	24% 13% 4 9% 5% 4 4 2 4 4 26%	5208
	21st to end	Night	25% 16% 4 9% 4 3 3 3 5% 5% 24%	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	62% 8% 9% 16% 4	3598
	11th to 20th	Day	58% 7% 11% 16% 7% 1	3600
	21st to end	Day	57% 8% 11% 13% 10% 1	3960
February	1st to 10th	Day	59% 6% 10% 18% 6% 2	1772
	11th to 20th	Day	57% 7% 12% 14% 8% 1	1847
	21st to end	Day	59% 5% 10% 15% 9% 2	1650
March	1st to 10th	Day	60% 6% 8% 12% 10% 3	4479
	11th to 20th	Day	48% 9% 18% 13% 10% 2	4743
	21st to end	Day	50% 6% 8% 15% 18% 2	5939
April	1st to 10th	Day	53% 6% 9% 18% 12% 2	2400
	11th to 20th	Day	48% 6% 8% 21% 15% 2	2374
	21st to end	Day	53% 5% 10% 16% 13% 3	2545
May	1st to 10th	Day	51% 9% 11% 19% 9% 1	4566
	11th to 20th	Day	47% 11% 16% 17% 8% 2	4542
	21st to end	Day	41% 9% 14% 21% 13% 1	4907
June	1st to 10th	Day	50% 10% 10% 18% 12% 1	2660
	11th to 20th	Day	42% 8% 13% 20% 13% 3	2660
	21st to end	Day	51% 9% 13% 17% 9% 1	2660
July	1st to 10th	Day	42% 8% 14% 25% 10% 1	4370
	11th to 20th	Day	39% 9% 16% 28% 8%	4365
	21st to end	Day	35% 8% 15% 33% 10%	4661
August	1st to 10th	Day	45% 8% 17% 21% 8%	4409
	11th to 20th	Day	41% 8% 17% 27% 6%	4036
	21st to end	Day	40% 8% 17% 21% 14% 1	4180
September	1st to 10th	Day	37% 8% 16% 26% 13%	2280
	11th to 20th	Day	34% 8% 14% 30% 13% 1	2214
	21st to end	Day	41% 7% 14% 26% 10% 1	2090
October	1st to 10th	Day	45% 7% 13% 26% 9%	3224
	11th to 20th	Day	44% 7% 17% 26% 7%	3088
	21st to end	Day	43% 6% 21% 21% 9% 1	3081
November	1st to 10th	Day	48% 7% 15% 22% 7% 1	1761
	11th to 20th	Day	51% 8% 13% 18% 10%	1695
	21st to end	Day	58% 10% 12% 14% 5% 1	1600
December	1st to 10th	Day	69% 6% 9% 14% 2	2600
	11th to 20th	Day	60% 6% 10% 16% 9%	2986
	21st to end	Day	62% 6% 10% 15% 6%	3960
January	1st to 10th	Night	65% 7% 11% 12% 4	6349
	11th to 20th	Night	61% 9% 12% 12% 4	6360
	21st to end	Night	59% 7% 10% 10% 13% 1	6996
February	1st to 10th	Night	58% 8% 11% 15% 7% 1	2625
	11th to 20th	Night	52% 8% 13% 20% 7% 1	2450
	21st to end	Night	54% 6% 10% 17% 12% 1	1980
March	1st to 10th	Night	57% 6% 8% 13% 12% 4	5374
	11th to 20th	Night	49% 8% 14% 16% 10% 3	5215
	21st to end	Night	52% 6% 7% 21% 12% 2	4992
April	1st to 10th	Night	52% 6% 10% 18% 13% 2	2000
	11th to 20th	Night	51% 6% 8% 19% 14% 2	1975
	21st to end	Night	50% 6% 8% 18% 14% 4	1853
May	1st to 10th	Night	51% 9% 12% 17% 9% 2	2643
	11th to 20th	Night	45% 10% 13% 19% 11% 2	2576
	21st to end	Night	44% 8% 12% 17% 17% 1	2785
June	1st to 10th	Night	48% 10% 12% 18% 12% 1	1520
	11th to 20th	Night	41% 8% 12% 22% 15% 2	1520
	21st to end	Night	47% 9% 12% 19% 13% 1	1520
July	1st to 10th	Night	45% 8% 15% 19% 13%	2480
	11th to 20th	Night	39% 7% 18% 27% 8%	2472
	21st to end	Night	37% 7% 18% 25% 13%	2651
August	1st to 10th	Night	45% 6% 14% 27% 9%	2496
	11th to 20th	Night	42% 8% 18% 24% 7%	2823
	21st to end	Night	43% 8% 15% 21% 13% 1	3476
September	1st to 10th	Night	36% 8% 14% 25% 16% 1	1900
	11th to 20th	Night	35% 6% 14% 33% 12% 1	1966
	21st to end	Night	39% 8% 12% 27% 14%	2090
October	1st to 10th	Night	44% 6% 14% 26% 10% 1	3731
	11th to 20th	Night	43% 6% 13% 29% 9%	3829
	21st to end	Night	46% 6% 18% 17% 12% 1	4467
November	1st to 10th	Night	50% 7% 14% 20% 8% 1	2537
	11th to 20th	Night	51% 7% 14% 17% 10% 2	2702
	21st to end	Night	58% 8% 12% 17% 5%	2800
December	1st to 10th	Night	66% 6% 13% 12% 3	4580
	11th to 20th	Night	61% 6% 7% 15% 10%	5208
	21st to end	Night	62% 5% 8% 14% 11%	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	21% 17% 34% 24% 5% 1	200
	11th to 20th	Day	25% 21% 30% 19% 6% 1	200
	21st to end	Day	19% 19% 32% 22% 7%	220
February	1st to 10th	Day	29% 24% 33% 12% 2	287
	11th to 20th	Day	23% 19% 39% 18% 2	385
	21st to end	Day	27% 15% 33% 22% 2	414
March	1st to 10th	Day	25% 14% 32% 24% 5%	500
	11th to 20th	Day	19% 20% 36% 19% 5% 1	500
	21st to end	Day	26% 17% 31% 17% 9% 1	550
April	1st to 10th	Day	27% 18% 27% 19% 7% 2	500
	11th to 20th	Day	25% 20% 26% 18% 9% 2	492
	21st to end	Day	27% 18% 30% 17% 7% 1	500
May	1st to 10th	Day	24% 13% 32% 23% 7% 2	503
	11th to 20th	Day	19% 10% 33% 23% 11% 2	527
	21st to end	Day	19% 13% 32% 23% 10% 2	549
June	1st to 10th	Day	22% 12% 28% 27% 10% 2	490
	11th to 20th	Day	15% 13% 27% 30% 13% 2	490
	21st to end	Day	23% 10% 29% 23% 13% 3	490
July	1st to 10th	Day	11% 8% 29% 32% 16% 4	485
	11th to 20th	Day	14% 8% 27% 33% 14% 3	484
	21st to end	Day	14% 8% 27% 33% 16% 3	532
August	1st to 10th	Day	15% 6% 23% 34% 19% 3	486
	11th to 20th	Day	11% 7% 24% 31% 18% 9%	479
	21st to end	Day	14% 8% 28% 26% 17% 7%	539
September	1st to 10th	Day	13% 8% 28% 28% 14% 7%	490
	11th to 20th	Day	11% 9% 23% 24% 22% 10%	490
	21st to end	Day	17% 11% 29% 21% 17% 5%	490
October	1st to 10th	Day	20% 13% 27% 23% 14% 3	490
	11th to 20th	Day	19% 17% 29% 23% 11% 1	475
	21st to end	Day	28% 15% 28% 21% 7%	429
November	1st to 10th	Day	27% 16% 26% 21% 9% 1	395
	11th to 20th	Day	23% 18% 36% 18% 5%	295
	21st to end	Day	25% 23% 28% 20% 4 1	200
December	1st to 10th	Day	30% 13% 27% 24% 7% 1	200
	11th to 20th	Day	29% 17% 35% 16% 3 1	200
	21st to end	Day	30% 19% 28% 17% 5% 2	220
January	1st to 10th	Night	24% 13% 34% 25% 4	800
	11th to 20th	Night	24% 15% 32% 24% 5%	800
	21st to end	Night	17% 11% 39% 27% 6% 1	880
February	1st to 10th	Night	20% 13% 39% 23% 5%	800
	11th to 20th	Night	20% 13% 41% 22% 4	783
	21st to end	Night	22% 12% 35% 28% 3	660
March	1st to 10th	Night	23% 13% 34% 26% 4	800
	11th to 20th	Night	23% 15% 32% 24% 6%	800
	21st to end	Night	23% 11% 32% 28% 6%	880
April	1st to 10th	Night	27% 13% 29% 27% 5%	800
	11th to 20th	Night	24% 13% 31% 26% 5%	791
	21st to end	Night	27% 10% 31% 24% 8%	653
May	1st to 10th	Night	25% 8% 34% 25% 7% 2	430
	11th to 20th	Night	23% 6% 29% 32% 9% 1	394
	21st to end	Night	16% 7% 39% 29% 9%	423
June	1st to 10th	Night	23% 6% 28% 33% 9% 1	380
	11th to 20th	Night	18% 6% 30% 33% 12% 1	380
	21st to end	Night	20% 6% 27% 32% 16% 1	380
July	1st to 10th	Night	10% 6% 30% 38% 16% 1	370
	11th to 20th	Night	13% 3 24% 44% 17%	368
	21st to end	Night	11% 4 25% 43% 16% 1	416
August	1st to 10th	Night	14% 4 18% 45% 18% 1	378
	11th to 20th	Night	14% 3 22% 35% 24% 2	562
	21st to end	Night	14% 5% 25% 35% 20% 2	836
September	1st to 10th	Night	12% 5% 26% 41% 16% 1	760
	11th to 20th	Night	16% 5% 20% 35% 21% 2	760
	21st to end	Night	18% 5% 27% 30% 17% 2	760
October	1st to 10th	Night	21% 9% 27% 31% 13% 1	760
	11th to 20th	Night	22% 9% 30% 28% 10% 1	760
	21st to end	Night	26% 11% 32% 26% 5%	836
November	1st to 10th	Night	24% 15% 28% 26% 7%	780
	11th to 20th	Night	23% 13% 34% 21% 9%	800
	21st to end	Night	28% 16% 31% 19% 6%	800
December	1st to 10th	Night	29% 12% 33% 21% 5%	800
	11th to 20th	Night	28% 16% 32% 20% 4 1	800
	21st to end	Night	33% 13% 31% 18% 5%	880

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