Lac de la Montagne noire
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lac de la Montagne noire clear sky chart from 2005-11-06 to 2026-01-02.

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% 12% 4 10% 6% 5% 5% 4 5% 6% 28%	127549
	Night	15% 13% 5% 11% 6% 5% 4 3 5% 6% 28%	123807
Transparency	Day	55% 5% 11% 19% 9% 1	130981
	Night	56% 4 12% 18% 9% 1	128462
Seeing	Day	26% 17% 31% 17% 6% 2	15876
	Night	23% 15% 29% 23% 9% 1	24915

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	12% 20% 8% 16% 6% 5% 6% 4 3 5% 16%	3605
	11th to 20th	Day	9% 17% 7% 19% 12% 7% 3 2 3 4 19%	3570
	21st to end	Day	7% 13% 8% 19% 6% 4 6% 4 5% 6% 23%	3960
February	1st to 10th	Day	8% 19% 6% 12% 10% 6% 3 4 4 6% 22%	1763
	11th to 20th	Day	11% 17% 6% 13% 7% 6% 4 3 3 5% 23%	1817
	21st to end	Day	13% 19% 7% 11% 6% 4 5% 3 4 4 23%	1650
March	1st to 10th	Day	16% 18% 6% 13% 8% 5% 3 3 3 4 22%	4476
	11th to 20th	Day	10% 16% 6% 12% 6% 4 4 3 4 6% 28%	4764
	21st to end	Day	16% 14% 4 9% 4 4 4 3 4 5% 32%	5919
April	1st to 10th	Day	18% 17% 5% 9% 4 3 3 3 4 6% 28%	2400
	11th to 20th	Day	22% 10% 4 8% 5% 4 4 3 4 4 32%	2374
	21st to end	Day	20% 13% 4 9% 5% 5% 4 4 4 5% 29%	2569
May	1st to 10th	Day	24% 12% 4 9% 5% 5% 4 4 5% 6% 23%	6346
	11th to 20th	Day	20% 9% 3 8% 5% 5% 5% 4 6% 7% 27%	6318
	21st to end	Day	22% 10% 3 8% 5% 5% 5% 4 5% 6% 27%	6949
June	1st to 10th	Day	23% 9% 3 8% 4 5% 6% 4 6% 6% 26%	2800
	11th to 20th	Day	16% 7% 3 8% 6% 6% 6% 5% 6% 7% 31%	2800
	21st to end	Day	23% 9% 3 7% 7% 5% 5% 4 5% 6% 26%	2800
July	1st to 10th	Day	16% 8% 3 9% 6% 5% 6% 5% 7% 7% 27%	6360
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 5% 7% 9% 36%	6360
	21st to end	Day	10% 6% 2 6% 4 4 5% 5% 7% 10% 41%	6926
August	1st to 10th	Day	15% 7% 3 8% 6% 6% 6% 5% 7% 8% 30%	4563
	11th to 20th	Day	16% 8% 3 7% 5% 4 6% 5% 6% 7% 34%	4165
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 4 7% 8% 35%	4312
September	1st to 10th	Day	16% 6% 3 7% 5% 4 5% 4 6% 7% 39%	2400
	11th to 20th	Day	15% 5% 3 6% 4 4 5% 4 5% 7% 43%	2340
	21st to end	Day	19% 9% 3 6% 4 4 4 3 5% 6% 38%	2200
October	1st to 10th	Day	22% 9% 3 7% 4 3 4 4 5% 6% 33%	2025
	11th to 20th	Day	22% 13% 4 9% 5% 4 4 3 5% 5% 25%	1970
	21st to end	Day	24% 14% 4 9% 5% 3 5% 3 4 5% 24%	1980
November	1st to 10th	Day	17% 15% 5% 9% 5% 4 4 3 4 6% 28%	1851
	11th to 20th	Day	18% 17% 5% 11% 6% 4 3 4 5% 6% 21%	1743
	21st to end	Day	18% 20% 7% 13% 5% 4 4 3 3 4 18%	1680
December	1st to 10th	Day	16% 19% 11% 15% 5% 4 4 3 4 4 14%	2680
	11th to 20th	Day	15% 19% 7% 13% 6% 5% 4 3 4 4 20%	3066
	21st to end	Day	17% 16% 8% 10% 6% 4 4 3 4 5% 23%	4048
January	1st to 10th	Night	11% 22% 9% 16% 6% 5% 4 3 4 4 16%	6364
	11th to 20th	Night	7% 14% 8% 20% 11% 7% 4 3 4 5% 16%	6315
	21st to end	Night	6% 11% 8% 20% 9% 7% 5% 4 5% 5% 20%	6996
February	1st to 10th	Night	10% 15% 6% 13% 8% 6% 5% 4 4 5% 24%	2635
	11th to 20th	Night	10% 15% 6% 14% 7% 6% 4 4 5% 4 25%	2468
	21st to end	Night	12% 16% 5% 11% 6% 5% 5% 4 6% 6% 24%	1980
March	1st to 10th	Night	19% 16% 5% 12% 6% 4 4 3 4 4 24%	5364
	11th to 20th	Night	10% 15% 5% 10% 5% 5% 5% 4 6% 7% 28%	5191
	21st to end	Night	15% 13% 5% 10% 6% 5% 4 4 5% 6% 29%	5013
April	1st to 10th	Night	20% 13% 4 8% 5% 4 4 3 5% 6% 29%	2000
	11th to 20th	Night	18% 10% 3 8% 4 3 5% 4 5% 6% 35%	1975
	21st to end	Night	20% 10% 3 8% 5% 4 4 3 5% 6% 32%	1848
May	1st to 10th	Night	20% 12% 4 8% 5% 5% 5% 4 5% 6% 26%	3594
	11th to 20th	Night	20% 9% 3 7% 4 4 4 3 5% 6% 34%	3575
	21st to end	Night	23% 9% 3 8% 5% 5% 5% 4 6% 6% 27%	3926
June	1st to 10th	Night	18% 10% 3 7% 4 5% 5% 4 6% 8% 31%	1600
	11th to 20th	Night	18% 8% 2 6% 4 4 4 4 5% 7% 37%	1600
	21st to end	Night	21% 7% 2 6% 5% 4 5% 4 5% 6% 35%	1600
July	1st to 10th	Night	15% 7% 3 7% 7% 5% 6% 5% 7% 8% 31%	3600
	11th to 20th	Night	14% 8% 3 7% 4 4 5% 4 6% 7% 40%	3600
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 8% 46%	3926
August	1st to 10th	Night	12% 8% 3 8% 4 4 4 4 6% 7% 39%	2586
	11th to 20th	Night	16% 8% 3 8% 3 4 4 4 5% 6% 39%	2847
	21st to end	Night	18% 6% 3 6% 4 5% 5% 3 6% 6% 39%	3586
September	1st to 10th	Night	13% 7% 2 6% 3 4 4 3 5% 7% 45%	2000
	11th to 20th	Night	16% 6% 2 4 3 4 3 3 5% 7% 48%	2060
	21st to end	Night	17% 8% 2 5% 4 4 3 3 5% 7% 42%	2200
October	1st to 10th	Night	16% 10% 3 6% 5% 4 4 3 4 6% 39%	2371
	11th to 20th	Night	20% 12% 3 8% 4 4 4 3 4 6% 30%	2430
	21st to end	Night	22% 13% 4 8% 5% 5% 4 3 5% 7% 25%	2860
November	1st to 10th	Night	18% 13% 5% 9% 5% 4 5% 3 6% 5% 27%	2667
	11th to 20th	Night	16% 14% 5% 11% 6% 5% 5% 4 6% 7% 22%	2872
	21st to end	Night	17% 19% 6% 11% 6% 4 4 3 5% 5% 19%	2940
December	1st to 10th	Night	15% 15% 7% 17% 9% 7% 4 3 4 5% 14%	4720
	11th to 20th	Night	20% 16% 6% 12% 7% 5% 3 2 3 4 21%	5348
	21st to end	Night	18% 21% 6% 10% 6% 3 3 2 4 4 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	71% 3 9% 14% 4	3606
	11th to 20th	Day	72% 3 4 12% 8% 2	3600
	21st to end	Day	62% 4 8% 13% 12% 2	3960
February	1st to 10th	Day	64% 3 7% 16% 8% 2	1763
	11th to 20th	Day	63% 3 9% 14% 10% 1	1826
	21st to end	Day	64% 2 8% 14% 10% 3	1650
March	1st to 10th	Day	67% 4 5% 10% 11% 3	4479
	11th to 20th	Day	57% 3 14% 12% 11% 3	4764
	21st to end	Day	54% 4 6% 16% 17% 2	5919
April	1st to 10th	Day	58% 3 8% 17% 12% 2	2400
	11th to 20th	Day	55% 4 8% 20% 11% 2	2374
	21st to end	Day	57% 4 8% 16% 11% 3	2569
May	1st to 10th	Day	61% 5% 7% 19% 8% 1	6357
	11th to 20th	Day	54% 5% 14% 17% 7% 2	6322
	21st to end	Day	57% 5% 10% 16% 11% 1	6949
June	1st to 10th	Day	56% 5% 9% 18% 10% 1	2800
	11th to 20th	Day	50% 5% 12% 20% 10% 2	2800
	21st to end	Day	58% 6% 10% 17% 9%	2800
July	1st to 10th	Day	51% 7% 13% 22% 6%	6290
	11th to 20th	Day	40% 7% 17% 29% 6%	6285
	21st to end	Day	36% 7% 18% 31% 8%	6728
August	1st to 10th	Day	48% 7% 17% 22% 6%	4549
	11th to 20th	Day	46% 7% 17% 25% 5%	4207
	21st to end	Day	44% 7% 15% 22% 12% 1	4312
September	1st to 10th	Day	43% 5% 14% 25% 12%	2400
	11th to 20th	Day	40% 6% 14% 29% 11% 1	2340
	21st to end	Day	47% 5% 12% 24% 11%	2200
October	1st to 10th	Day	49% 5% 11% 26% 8% 1	3298
	11th to 20th	Day	51% 6% 13% 22% 7% 1	3186
	21st to end	Day	53% 4 20% 14% 8% 1	3180
November	1st to 10th	Day	58% 4 12% 19% 8% 1	1851
	11th to 20th	Day	64% 3 11% 14% 7% 1	1743
	21st to end	Day	70% 4 9% 12% 5% 1	1680
December	1st to 10th	Day	73% 3 9% 11% 4	2680
	11th to 20th	Day	67% 3 7% 16% 7%	3066
	21st to end	Day	64% 3 8% 15% 10% 1	4048
January	1st to 10th	Night	71% 3 9% 13% 3	6364
	11th to 20th	Night	69% 3 11% 11% 4 1	6360
	21st to end	Night	65% 4 8% 10% 13% 1	6996
February	1st to 10th	Night	60% 3 9% 19% 7% 2	2635
	11th to 20th	Night	60% 3 11% 17% 8% 1	2470
	21st to end	Night	58% 3 10% 18% 9% 2	1980
March	1st to 10th	Night	63% 3 8% 11% 12% 3	5374
	11th to 20th	Night	55% 5% 12% 15% 11% 3	5192
	21st to end	Night	55% 4 8% 21% 10% 3	5013
April	1st to 10th	Night	55% 3 9% 19% 13% 2	2000
	11th to 20th	Night	48% 4 11% 22% 13% 2	1975
	21st to end	Night	52% 4 8% 20% 13% 2	1848
May	1st to 10th	Night	57% 6% 10% 18% 7% 1	3600
	11th to 20th	Night	50% 6% 14% 18% 10% 2	3576
	21st to end	Night	56% 5% 10% 16% 10% 1	3927
June	1st to 10th	Night	50% 4 13% 22% 10%	1600
	11th to 20th	Night	45% 5% 13% 22% 12% 2	1600
	21st to end	Night	49% 6% 13% 20% 12% 1	1600
July	1st to 10th	Night	47% 7% 20% 20% 6%	3560
	11th to 20th	Night	41% 7% 19% 28% 5%	3552
	21st to end	Night	33% 7% 19% 30% 11%	3809
August	1st to 10th	Night	42% 7% 18% 27% 7%	2576
	11th to 20th	Night	44% 5% 21% 23% 7%	2871
	21st to end	Night	44% 6% 16% 22% 10% 2	3586
September	1st to 10th	Night	38% 4 17% 28% 13% 1	2000
	11th to 20th	Night	36% 5% 17% 33% 10% 1	2060
	21st to end	Night	42% 5% 15% 27% 12%	2200
October	1st to 10th	Night	46% 5% 14% 22% 12% 1	3878
	11th to 20th	Night	50% 5% 13% 24% 9%	3953
	21st to end	Night	51% 5% 17% 17% 9% 2	4610
November	1st to 10th	Night	57% 4 14% 19% 6% 1	2667
	11th to 20th	Night	59% 5% 11% 17% 7% 2	2872
	21st to end	Night	65% 4 11% 14% 5%	2940
December	1st to 10th	Night	70% 5% 12% 9% 4	4720
	11th to 20th	Night	68% 3 6% 15% 8% 1	5348
	21st to end	Night	65% 2 6% 12% 14% 1	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	19% 21% 41% 16% 2	202
	11th to 20th	Day	29% 17% 37% 14% 5% 1	200
	21st to end	Day	21% 21% 38% 15% 4 1	220
February	1st to 10th	Day	31% 29% 32% 5% 2	283
	11th to 20th	Day	28% 24% 39% 7% 1	364
	21st to end	Day	30% 24% 32% 12% 2	414
March	1st to 10th	Day	29% 20% 31% 17% 3	500
	11th to 20th	Day	29% 22% 33% 13% 3 1	500
	21st to end	Day	33% 21% 27% 13% 5% 1	550
April	1st to 10th	Day	34% 17% 30% 12% 6% 1	500
	11th to 20th	Day	37% 15% 24% 14% 7% 2	492
	21st to end	Day	33% 19% 32% 11% 4 1	500
May	1st to 10th	Day	31% 19% 31% 15% 2 2	503
	11th to 20th	Day	26% 19% 29% 19% 6% 2	527
	21st to end	Day	28% 18% 29% 18% 5% 1	573
June	1st to 10th	Day	28% 14% 35% 16% 6% 1	530
	11th to 20th	Day	23% 18% 31% 18% 8% 2	530
	21st to end	Day	28% 14% 34% 17% 6% 1	530
July	1st to 10th	Day	18% 11% 38% 24% 7% 2	525
	11th to 20th	Day	17% 11% 40% 26% 6%	524
	21st to end	Day	16% 13% 35% 27% 7% 2	576
August	1st to 10th	Day	15% 12% 30% 30% 11% 2	526
	11th to 20th	Day	17% 9% 31% 27% 11% 6%	519
	21st to end	Day	20% 15% 30% 19% 11% 6%	583
September	1st to 10th	Day	19% 15% 30% 21% 10% 5%	530
	11th to 20th	Day	14% 13% 25% 25% 15% 8%	530
	21st to end	Day	24% 14% 26% 20% 11% 5%	530
October	1st to 10th	Day	27% 15% 26% 19% 9% 3	530
	11th to 20th	Day	32% 19% 26% 16% 6% 1	513
	21st to end	Day	35% 14% 29% 17% 5%	462
November	1st to 10th	Day	35% 17% 28% 15% 4	425
	11th to 20th	Day	30% 22% 29% 14% 6%	283
	21st to end	Day	31% 26% 31% 9% 3	220
December	1st to 10th	Day	30% 18% 30% 17% 3 1	220
	11th to 20th	Day	28% 29% 32% 10% 1	220
	21st to end	Day	34% 21% 29% 11% 3 2	242
January	1st to 10th	Night	24% 22% 34% 15% 5%	805
	11th to 20th	Night	25% 19% 30% 21% 6%	800
	21st to end	Night	16% 16% 38% 22% 7% 1	880
February	1st to 10th	Night	19% 24% 33% 20% 4	800
	11th to 20th	Night	22% 21% 37% 16% 3	783
	21st to end	Night	24% 17% 34% 22% 3	660
March	1st to 10th	Night	27% 18% 32% 19% 4	800
	11th to 20th	Night	25% 20% 32% 17% 5% 1	800
	21st to end	Night	24% 16% 32% 21% 6% 1	880
April	1st to 10th	Night	30% 16% 28% 22% 5%	800
	11th to 20th	Night	26% 16% 28% 22% 6% 1	791
	21st to end	Night	29% 13% 31% 19% 7% 1	648
May	1st to 10th	Night	24% 13% 30% 23% 9% 2	400
	11th to 20th	Night	21% 14% 28% 27% 9% 1	394
	21st to end	Night	20% 12% 35% 25% 8%	433
June	1st to 10th	Night	22% 10% 32% 28% 8%	400
	11th to 20th	Night	21% 10% 26% 30% 13% 1	400
	21st to end	Night	23% 6% 22% 34% 15% 1	400
July	1st to 10th	Night	14% 5% 29% 35% 15%	390
	11th to 20th	Night	14% 5% 26% 39% 15% 1	388
	21st to end	Night	13% 7% 26% 38% 15% 1	438
August	1st to 10th	Night	13% 6% 19% 45% 17% 1	398
	11th to 20th	Night	13% 6% 23% 34% 23% 2	563
	21st to end	Night	18% 8% 27% 28% 16% 3	880
September	1st to 10th	Night	17% 8% 29% 31% 14% 1	800
	11th to 20th	Night	18% 7% 23% 29% 21% 2	800
	21st to end	Night	21% 11% 25% 25% 16% 2	800
October	1st to 10th	Night	22% 14% 25% 24% 14% 1	800
	11th to 20th	Night	27% 14% 29% 20% 9% 1	800
	21st to end	Night	28% 16% 28% 21% 6%	880
November	1st to 10th	Night	27% 18% 29% 19% 6% 1	820
	11th to 20th	Night	25% 18% 29% 17% 9% 1	840
	21st to end	Night	30% 22% 25% 17% 4 1	840
December	1st to 10th	Night	28% 19% 28% 17% 7% 1	840
	11th to 20th	Night	30% 20% 28% 16% 5% 1	840
	21st to end	Night	33% 18% 26% 17% 5%	924

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