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 2024-11-02.

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 5% 7% 33%	101537
	Night	17% 11% 4 8% 5% 4 4 3 5% 6% 33%	97444
Transparency	Day	48% 8% 13% 20% 9% 1	101298
	Night	51% 8% 13% 18% 9% 1	97303
Seeing	Day	20% 13% 29% 24% 11% 3	14541
	Night	21% 10% 31% 28% 9% 1	23496

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% 5% 14% 6% 5% 5% 3 4 4 23%	2517
	11th to 20th	Day	15% 12% 6% 9% 6% 5% 4 4 6% 7% 26%	2520
	21st to end	Day	13% 18% 5% 9% 5% 5% 6% 4 5% 6% 23%	2772
February	1st to 10th	Day	14% 16% 5% 10% 6% 5% 5% 3 5% 6% 26%	1683
	11th to 20th	Day	14% 12% 5% 9% 6% 6% 4 5% 6% 6% 28%	1743
	21st to end	Day	18% 15% 5% 8% 5% 4 4 4 4 5% 27%	1570
March	1st to 10th	Day	20% 13% 4 9% 6% 5% 4 3 4 5% 26%	3116
	11th to 20th	Day	12% 17% 5% 11% 5% 5% 4 4 5% 4 28%	3313
	21st to end	Day	14% 7% 3 8% 5% 4 5% 4 5% 5% 39%	4149
April	1st to 10th	Day	21% 11% 3 8% 5% 3 4 4 3 5% 33%	2280
	11th to 20th	Day	21% 8% 3 6% 5% 5% 4 3 5% 5% 36%	2254
	21st to end	Day	22% 10% 3 7% 5% 5% 4 3 5% 5% 32%	2418
May	1st to 10th	Day	20% 9% 3 8% 5% 5% 5% 4 6% 7% 28%	4412
	11th to 20th	Day	17% 8% 3 9% 5% 4 5% 4 6% 7% 32%	4398
	21st to end	Day	16% 6% 2 7% 5% 5% 4 4 6% 8% 37%	4837
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 8% 4 5% 5% 3 6% 7% 34%	1680
	11th to 20th	Day	19% 9% 5% 8% 5% 4 6% 4 5% 7% 29%	1611
	21st to end	Day	22% 10% 3 7% 5% 7% 5% 4 6% 7% 25%	1520
December	1st to 10th	Day	24% 13% 5% 10% 5% 5% 4 4 5% 4 20%	1520
	11th to 20th	Day	21% 14% 5% 9% 5% 4 3 3 5% 5% 28%	1906
	21st to end	Day	27% 16% 5% 9% 4 4 4 3 3 4 21%	2772
January	1st to 10th	Night	16% 15% 6% 11% 5% 5% 4 4 5% 5% 23%	4429
	11th to 20th	Night	12% 13% 5% 13% 7% 6% 5% 4 5% 6% 25%	4440
	21st to end	Night	15% 16% 6% 11% 6% 5% 4 4 4 5% 25%	4884
February	1st to 10th	Night	14% 14% 5% 11% 6% 5% 5% 4 6% 6% 25%	2494
	11th to 20th	Night	13% 11% 4 9% 6% 6% 5% 3 6% 6% 30%	2323
	21st to end	Night	14% 16% 5% 9% 5% 4 4 3 4 5% 32%	1884
March	1st to 10th	Night	20% 12% 4 7% 5% 4 4 3 4 5% 33%	3724
	11th to 20th	Night	17% 18% 5% 9% 5% 4 4 3 4 5% 28%	3644
	21st to end	Night	12% 10% 4 9% 6% 5% 4 3 5% 4 39%	3482
April	1st to 10th	Night	23% 10% 3 6% 5% 3 4 3 5% 6% 32%	1900
	11th to 20th	Night	21% 9% 3 6% 5% 4 4 3 5% 6% 35%	1875
	21st to end	Night	21% 8% 4 7% 4 3 4 4 4 6% 35%	1761
May	1st to 10th	Night	22% 9% 3 7% 5% 5% 4 3 5% 6% 31%	2556
	11th to 20th	Night	22% 7% 3 5% 4 4 4 3 5% 6% 37%	2495
	21st to end	Night	16% 7% 3 7% 5% 4 4 3 5% 8% 37%	2738
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% 8% 3 7% 4 5% 4 4 6% 7% 33%	2421
	11th to 20th	Night	17% 11% 3 8% 5% 4 4 4 5% 6% 34%	2566
	21st to end	Night	21% 13% 3 8% 5% 4 3 4 4 5% 29%	2660
December	1st to 10th	Night	20% 14% 5% 9% 6% 5% 4 4 5% 5% 22%	2660
	11th to 20th	Night	22% 15% 4 8% 5% 4 4 3 4 4 27%	3288
	21st to end	Night	28% 17% 5% 9% 4 3 3 2 3 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	60% 11% 11% 12% 5% 1	2518
	11th to 20th	Day	54% 9% 15% 15% 7%	2520
	21st to end	Day	59% 9% 14% 12% 6%	2772
February	1st to 10th	Day	59% 6% 11% 18% 5% 2	1683
	11th to 20th	Day	56% 7% 13% 15% 9% 1	1752
	21st to end	Day	59% 5% 10% 15% 9% 2	1570
March	1st to 10th	Day	58% 7% 10% 16% 8% 1	3119
	11th to 20th	Day	54% 10% 12% 13% 10% 2	3313
	21st to end	Day	43% 8% 10% 19% 17% 2	4149
April	1st to 10th	Day	53% 6% 9% 19% 11% 2	2280
	11th to 20th	Day	48% 7% 9% 21% 14% 2	2254
	21st to end	Day	53% 5% 9% 16% 12% 3	2418
May	1st to 10th	Day	50% 10% 12% 19% 9% 1	4426
	11th to 20th	Day	46% 11% 16% 17% 8% 2	4402
	21st to end	Day	40% 10% 14% 22% 13% 1	4837
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	47% 7% 12% 22% 11%	1938
	11th to 20th	Day	53% 8% 14% 18% 7%	1872
	21st to end	Day	54% 7% 15% 16% 7% 1	1881
November	1st to 10th	Day	47% 8% 16% 21% 7% 1	1680
	11th to 20th	Day	52% 8% 13% 17% 10%	1611
	21st to end	Day	57% 11% 13% 14% 5% 1	1520
December	1st to 10th	Day	67% 8% 10% 12% 4	1520
	11th to 20th	Day	59% 7% 14% 15% 5%	1906
	21st to end	Day	67% 7% 10% 10% 5%	2772
January	1st to 10th	Night	60% 9% 15% 11% 4	4429
	11th to 20th	Night	55% 12% 16% 12% 4 1	4440
	21st to end	Night	61% 9% 13% 10% 6% 1	4884
February	1st to 10th	Night	58% 8% 12% 15% 6% 1	2494
	11th to 20th	Night	52% 8% 13% 20% 7% 1	2325
	21st to end	Night	54% 6% 10% 16% 13% 1	1884
March	1st to 10th	Night	52% 7% 11% 17% 9% 3	3734
	11th to 20th	Night	56% 8% 11% 14% 10% 2	3645
	21st to end	Night	45% 7% 8% 22% 15% 3	3482
April	1st to 10th	Night	52% 6% 10% 18% 13% 2	1900
	11th to 20th	Night	50% 6% 9% 20% 14% 2	1875
	21st to end	Night	50% 6% 8% 18% 14% 4	1761
May	1st to 10th	Night	50% 9% 13% 17% 9% 3	2562
	11th to 20th	Night	44% 10% 13% 18% 11% 2	2496
	21st to end	Night	44% 8% 12% 18% 18% 1	2739
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	46% 7% 11% 24% 12% 1	2237
	11th to 20th	Night	49% 7% 12% 23% 8%	2306
	21st to end	Night	54% 7% 14% 15% 9% 1	2717
November	1st to 10th	Night	50% 7% 14% 20% 8% 1	2421
	11th to 20th	Night	51% 7% 14% 16% 9% 2	2566
	21st to end	Night	58% 8% 12% 17% 6%	2660
December	1st to 10th	Night	63% 8% 12% 13% 4	2660
	11th to 20th	Night	61% 7% 9% 16% 6%	3288
	21st to end	Night	66% 6% 10% 14% 4	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	20% 17% 31% 26% 5% 1	180
	11th to 20th	Day	25% 21% 28% 19% 6% 1	180
	21st to end	Day	19% 17% 32% 23% 8% 1	198
February	1st to 10th	Day	29% 22% 33% 12% 2	258
	11th to 20th	Day	21% 18% 39% 19% 2	350
	21st to end	Day	28% 14% 33% 23% 2	382
March	1st to 10th	Day	24% 13% 33% 25% 5%	460
	11th to 20th	Day	19% 20% 35% 19% 5% 1	460
	21st to end	Day	25% 16% 32% 17% 9% 1	506
April	1st to 10th	Day	27% 16% 29% 19% 7% 2	460
	11th to 20th	Day	25% 20% 25% 18% 9% 2	452
	21st to end	Day	27% 18% 29% 18% 7% 2	460
May	1st to 10th	Day	22% 12% 33% 24% 7% 2	463
	11th to 20th	Day	18% 10% 34% 25% 11% 2	487
	21st to end	Day	17% 13% 33% 24% 10% 2	529
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	26% 16% 26% 22% 9% 1	368
	11th to 20th	Day	24% 18% 36% 17% 5%	271
	21st to end	Day	23% 22% 28% 22% 4 1	180
December	1st to 10th	Day	31% 12% 27% 23% 7% 1	180
	11th to 20th	Day	28% 15% 36% 17% 3 1	180
	21st to end	Day	31% 20% 28% 15% 6% 1	198
January	1st to 10th	Night	24% 13% 33% 26% 4	760
	11th to 20th	Night	25% 14% 33% 23% 5%	760
	21st to end	Night	17% 11% 38% 26% 6% 1	836
February	1st to 10th	Night	20% 12% 39% 24% 5%	760
	11th to 20th	Night	19% 13% 41% 23% 4	743
	21st to end	Night	22% 11% 36% 28% 3	628
March	1st to 10th	Night	23% 12% 34% 27% 4	760
	11th to 20th	Night	23% 16% 32% 23% 6%	760
	21st to end	Night	22% 10% 33% 28% 6%	836
April	1st to 10th	Night	26% 13% 29% 28% 5%	760
	11th to 20th	Night	23% 13% 31% 26% 5%	751
	21st to end	Night	26% 10% 31% 24% 8%	621
May	1st to 10th	Night	24% 8% 34% 26% 7% 2	409
	11th to 20th	Night	22% 6% 30% 33% 9% 1	374
	21st to end	Night	15% 6% 40% 30% 9%	411
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% 10% 33% 26% 5%	836
November	1st to 10th	Night	24% 14% 29% 26% 7%	745
	11th to 20th	Night	23% 14% 34% 20% 10%	760
	21st to end	Night	28% 15% 31% 20% 6%	760
December	1st to 10th	Night	30% 12% 33% 21% 5%	760
	11th to 20th	Night	28% 15% 32% 20% 4 1	760
	21st to end	Night	33% 13% 31% 17% 5%	836

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