L'Observatoire des Lacs
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the L'Observatoire des Lacs clear sky chart from 2005-11-06 to 2026-05-02.

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	17% 8% 3 7% 4 4 4 4 5% 7% 36%	130778
	Night	17% 9% 3 7% 4 4 4 4 5% 7% 36%	131722
Transparency	Day	48% 2 10% 23% 14% 2	134285
	Night	49% 2 10% 23% 14% 2	136303
Seeing	Day	25% 13% 31% 20% 8% 2	17165
	Night	24% 11% 31% 26% 7% 1	25926

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% 8% 3 10% 5% 5% 6% 5% 5% 6% 34%	4661
	11th to 20th	Day	14% 11% 4 9% 4 4 4 3 4 6% 38%	4650
	21st to end	Day	9% 9% 4 7% 4 4 5% 4 5% 6% 44%	5124
February	1st to 10th	Day	15% 10% 3 7% 5% 4 5% 4 6% 6% 36%	1846
	11th to 20th	Day	11% 10% 3 8% 6% 5% 5% 4 6% 6% 35%	2022
	21st to end	Day	14% 11% 4 7% 5% 5% 4 3 6% 5% 36%	1730
March	1st to 10th	Day	22% 11% 3 9% 5% 4 4 3 5% 4 30%	4576
	11th to 20th	Day	17% 11% 3 8% 4 5% 4 4 4 5% 35%	5013
	21st to end	Day	26% 9% 3 6% 3 3 3 3 5% 6% 32%	5713
April	1st to 10th	Day	20% 10% 3 7% 5% 4 4 3 4 6% 35%	2520
	11th to 20th	Day	19% 7% 2 6% 5% 4 4 3 6% 6% 38%	2622
	21st to end	Day	21% 8% 3 7% 4 4 4 3 4 5% 35%	2730
May	1st to 10th	Day	30% 10% 3 7% 4 4 4 4 5% 6% 24%	5899
	11th to 20th	Day	18% 8% 3 9% 5% 5% 5% 4 5% 7% 31%	5988
	21st to end	Day	19% 7% 3 6% 5% 5% 5% 5% 6% 8% 32%	6949
June	1st to 10th	Day	22% 8% 2 6% 4 4 6% 4 6% 8% 31%	2915
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 4 5% 8% 38%	3000
	21st to end	Day	24% 6% 2 6% 5% 4 5% 4 5% 7% 33%	2899
July	1st to 10th	Day	15% 7% 3 7% 6% 5% 5% 5% 7% 10% 30%	6360
	11th to 20th	Day	14% 7% 3 6% 4 4 5% 4 7% 9% 37%	6360
	21st to end	Day	14% 6% 2 6% 4 4 4 3 6% 8% 42%	6926
August	1st to 10th	Day	15% 5% 2 8% 5% 5% 6% 4 6% 8% 35%	4273
	11th to 20th	Day	15% 9% 4 8% 4 4 4 4 6% 7% 35%	4175
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 5% 9% 46%	4524
September	1st to 10th	Day	11% 5% 2 5% 4 4 5% 3 5% 7% 48%	2348
	11th to 20th	Day	12% 5% 2 4 3 4 4 3 5% 6% 53%	2200
	21st to end	Day	17% 5% 2 5% 3 4 4 3 5% 7% 45%	2200
October	1st to 10th	Day	18% 6% 2 7% 4 4 5% 3 4 6% 41%	2195
	11th to 20th	Day	20% 6% 2 5% 4 4 4 2 5% 7% 41%	1920
	21st to end	Day	23% 8% 3 7% 4 4 3 3 4 6% 35%	1980
November	1st to 10th	Day	17% 6% 2 5% 4 5% 5% 4 4 6% 41%	1851
	11th to 20th	Day	19% 6% 1 5% 4 4 4 4 6% 6% 41%	1890
	21st to end	Day	21% 8% 3 6% 4 3 3 2 4 6% 41%	1774
December	1st to 10th	Day	17% 11% 2 6% 4 4 4 4 5% 6% 35%	2363
	11th to 20th	Day	19% 8% 2 5% 3 3 3 3 5% 6% 43%	2688
	21st to end	Day	20% 11% 3 6% 4 4 4 3 5% 5% 33%	3894
January	1st to 10th	Night	11% 9% 4 9% 5% 4 4 4 5% 6% 38%	8234
	11th to 20th	Night	14% 12% 4 11% 6% 6% 5% 4 6% 6% 27%	8235
	21st to end	Night	11% 8% 3 7% 5% 5% 5% 4 6% 6% 39%	9057
February	1st to 10th	Night	14% 10% 3 9% 5% 4 4 4 5% 6% 35%	2772
	11th to 20th	Night	12% 10% 3 7% 5% 4 5% 4 5% 6% 39%	2482
	21st to end	Night	14% 10% 3 7% 5% 4 4 3 4 6% 39%	2076
March	1st to 10th	Night	26% 10% 3 7% 5% 4 5% 4 4 5% 29%	5484
	11th to 20th	Night	17% 11% 4 7% 4 4 4 4 6% 7% 33%	5165
	21st to end	Night	24% 9% 2 7% 4 4 4 3 6% 6% 30%	5458
April	1st to 10th	Night	21% 12% 3 6% 4 4 3 3 3 5% 35%	2100
	11th to 20th	Night	19% 7% 2 7% 5% 5% 5% 3 5% 6% 36%	1946
	21st to end	Night	24% 8% 2 7% 4 4 3 3 5% 6% 34%	1890
May	1st to 10th	Night	30% 11% 4 6% 4 3 3 3 5% 6% 25%	4064
	11th to 20th	Night	20% 9% 3 8% 5% 4 5% 4 6% 7% 29%	3947
	21st to end	Night	17% 7% 3 7% 5% 4 5% 4 6% 7% 35%	3926
June	1st to 10th	Night	22% 9% 2 6% 5% 4 4 3 5% 9% 32%	1476
	11th to 20th	Night	17% 6% 2 6% 4 4 3 5% 6% 9% 38%	1400
	21st to end	Night	24% 8% 2 5% 3 3 4 5% 6% 7% 32%	1492
July	1st to 10th	Night	12% 7% 2 7% 5% 5% 5% 4 7% 10% 35%	3600
	11th to 20th	Night	15% 8% 2 8% 5% 5% 5% 3 6% 8% 36%	3600
	21st to end	Night	11% 5% 2 5% 3 4 5% 4 6% 9% 46%	3926
August	1st to 10th	Night	20% 8% 2 8% 5% 5% 5% 4 5% 7% 31%	2842
	11th to 20th	Night	15% 9% 3 8% 4 4 5% 4 6% 7% 35%	2872
	21st to end	Night	11% 6% 2 7% 4 4 5% 3 6% 8% 45%	3371
September	1st to 10th	Night	14% 5% 1 5% 3 4 4 3 5% 7% 49%	2051
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 6% 53%	2200
	21st to end	Night	16% 6% 2 5% 3 4 4 3 4 7% 45%	2200
October	1st to 10th	Night	18% 6% 2 6% 4 5% 4 3 5% 6% 41%	2205
	11th to 20th	Night	17% 7% 2 5% 4 4 4 4 5% 6% 41%	2478
	21st to end	Night	24% 8% 2 5% 4 4 4 3 5% 6% 35%	2860
November	1st to 10th	Night	18% 7% 2 6% 4 4 4 3 4 6% 42%	2667
	11th to 20th	Night	17% 7% 2 6% 4 3 4 3 4 7% 42%	2730
	21st to end	Night	17% 8% 2 6% 5% 5% 4 3 5% 7% 38%	2840
December	1st to 10th	Night	17% 11% 2 7% 4 4 5% 4 6% 7% 33%	5044
	11th to 20th	Night	23% 8% 2 5% 4 4 4 3 4 5% 38%	5741
	21st to end	Night	21% 10% 3 6% 3 4 4 3 5% 5% 36%	7291

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	51% 1 10% 25% 13% 1	4662
	11th to 20th	Day	49% 1 8% 26% 12% 3	4680
	21st to end	Day	43% 1 10% 22% 20% 5%	5124
February	1st to 10th	Day	48% 3 9% 22% 15% 3	1846
	11th to 20th	Day	49% 3 10% 21% 14% 3	2031
	21st to end	Day	49% 3 10% 20% 13% 5%	1730
March	1st to 10th	Day	59% 2 6% 10% 15% 7%	4579
	11th to 20th	Day	52% 2 12% 19% 12% 2	5013
	21st to end	Day	54% 2 4 18% 18% 4	5713
April	1st to 10th	Day	53% 2 7% 18% 16% 5%	2520
	11th to 20th	Day	46% 3 8% 24% 16% 3	2622
	21st to end	Day	52% 2 6% 21% 15% 4	2730
May	1st to 10th	Day	64% 2 9% 16% 7% 1	5910
	11th to 20th	Day	55% 3 12% 19% 8% 3	5992
	21st to end	Day	49% 3 11% 23% 13% 1	6949
June	1st to 10th	Day	52% 3 11% 23% 10%	2915
	11th to 20th	Day	45% 3 11% 23% 16% 2	3000
	21st to end	Day	52% 4 11% 21% 11% 2	2899
July	1st to 10th	Day	50% 2 14% 24% 10%	6290
	11th to 20th	Day	44% 3 13% 25% 14% 1	6285
	21st to end	Day	41% 2 12% 26% 17% 1	6728
August	1st to 10th	Day	48% 3 13% 28% 8%	4259
	11th to 20th	Day	49% 3 14% 25% 8%	4214
	21st to end	Day	36% 2 13% 28% 19% 2	4524
September	1st to 10th	Day	35% 3 11% 29% 20% 1	2348
	11th to 20th	Day	33% 2 9% 31% 23% 2	2200
	21st to end	Day	39% 3 10% 30% 17% 1	2200
October	1st to 10th	Day	53% 2 8% 23% 13% 1	3572
	11th to 20th	Day	37% 2 9% 31% 20% 1	3110
	21st to end	Day	39% 1 12% 24% 20% 3	3180
November	1st to 10th	Day	43% 3 12% 23% 16% 2	1851
	11th to 20th	Day	43% 3 9% 23% 19% 4	1890
	21st to end	Day	48% 2 14% 24% 12% 1	1774
December	1st to 10th	Day	49% 2 13% 23% 13% 1	2363
	11th to 20th	Day	43% 3 10% 20% 17% 7%	2688
	21st to end	Day	55% 1 8% 18% 14% 3	3894
January	1st to 10th	Night	49% 1 10% 26% 13% 1	8234
	11th to 20th	Night	57% 3 10% 16% 11% 3	8280
	21st to end	Night	46% 1 6% 23% 18% 5%	9057
February	1st to 10th	Night	50% 3 10% 20% 14% 3	2772
	11th to 20th	Night	47% 2 9% 22% 17% 3	2484
	21st to end	Night	48% 2 8% 22% 16% 4	2076
March	1st to 10th	Night	60% 2 5% 13% 14% 6%	5494
	11th to 20th	Night	51% 3 15% 20% 9% 2	5166
	21st to end	Night	56% 1 5% 16% 19% 3	5458
April	1st to 10th	Night	54% 2 7% 18% 16% 3	2100
	11th to 20th	Night	50% 3 7% 22% 17% 1	1946
	21st to end	Night	52% 2 10% 17% 15% 3	1890
May	1st to 10th	Night	61% 3 12% 15% 8% 1	4070
	11th to 20th	Night	55% 3 11% 22% 8% 1	3948
	21st to end	Night	50% 2 14% 23% 10% 1	3927
June	1st to 10th	Night	51% 3 11% 24% 10% 1	1476
	11th to 20th	Night	44% 3 15% 23% 13% 2	1400
	21st to end	Night	50% 4 13% 21% 9% 2	1492
July	1st to 10th	Night	46% 2 17% 25% 10%	3560
	11th to 20th	Night	48% 2 13% 26% 11%	3552
	21st to end	Night	35% 3 16% 33% 12% 1	3809
August	1st to 10th	Night	54% 4 13% 22% 7%	2832
	11th to 20th	Night	47% 4 16% 22% 11%	2899
	21st to end	Night	39% 3 13% 27% 16% 2	3371
September	1st to 10th	Night	35% 3 13% 31% 18%	2051
	11th to 20th	Night	33% 3 10% 34% 19% 2	2200
	21st to end	Night	40% 3 9% 30% 15% 2	2200
October	1st to 10th	Night	47% 2 12% 24% 15% 1	3608
	11th to 20th	Night	36% 2 10% 31% 21% 1	4027
	21st to end	Night	41% 2 11% 29% 16% 2	4610
November	1st to 10th	Night	44% 2 10% 26% 16% 1	2667
	11th to 20th	Night	43% 3 9% 25% 17% 3	2730
	21st to end	Night	46% 3 14% 22% 14% 2	2840
December	1st to 10th	Night	50% 1 7% 30% 10% 1	5044
	11th to 20th	Night	50% 2 9% 18% 19% 2	5742
	21st to end	Night	52% 2 9% 19% 17% 1	7291

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	31% 17% 32% 15% 5%	220
	11th to 20th	Day	32% 15% 33% 15% 4 1	220
	21st to end	Day	22% 21% 36% 19% 2	242
February	1st to 10th	Day	28% 17% 31% 19% 4	386
	11th to 20th	Day	26% 20% 36% 15% 2	522
	21st to end	Day	25% 19% 35% 18% 4	446
March	1st to 10th	Day	31% 18% 33% 14% 4	540
	11th to 20th	Day	29% 22% 32% 12% 4	540
	21st to end	Day	31% 20% 29% 12% 6% 2	594
April	1st to 10th	Day	34% 17% 31% 12% 5% 1	540
	11th to 20th	Day	33% 18% 30% 13% 4 2	532
	21st to end	Day	31% 17% 30% 15% 5% 3	540
May	1st to 10th	Day	35% 14% 26% 20% 3 2	507
	11th to 20th	Day	24% 15% 32% 19% 7% 3	527
	21st to end	Day	24% 14% 35% 20% 7% 1	573
June	1st to 10th	Day	24% 11% 31% 23% 10% 2	645
	11th to 20th	Day	23% 12% 28% 21% 12% 3	730
	21st to end	Day	24% 10% 30% 24% 10% 2	629
July	1st to 10th	Day	19% 9% 33% 27% 10% 2	525
	11th to 20th	Day	16% 6% 38% 23% 13% 4	524
	21st to end	Day	16% 6% 36% 27% 12% 3	576
August	1st to 10th	Day	13% 6% 30% 31% 15% 4	526
	11th to 20th	Day	17% 7% 28% 26% 16% 6%	519
	21st to end	Day	15% 8% 33% 26% 14% 5%	583
September	1st to 10th	Day	16% 8% 31% 24% 17% 3	530
	11th to 20th	Day	18% 10% 25% 23% 14% 10%	530
	21st to end	Day	20% 9% 30% 23% 13% 5%	530
October	1st to 10th	Day	23% 12% 25% 23% 13% 5%	527
	11th to 20th	Day	28% 11% 30% 17% 10% 3	420
	21st to end	Day	33% 10% 31% 19% 7% 1	462
November	1st to 10th	Day	28% 12% 30% 20% 9% 2	425
	11th to 20th	Day	28% 15% 30% 19% 6% 1	430
	21st to end	Day	31% 16% 27% 19% 6% 1	430
December	1st to 10th	Day	30% 14% 34% 17% 6%	233
	11th to 20th	Day	38% 15% 36% 8% 4	220
	21st to end	Day	33% 14% 30% 16% 7% 1	242
January	1st to 10th	Night	23% 15% 35% 24% 3	840
	11th to 20th	Night	27% 15% 31% 22% 4	840
	21st to end	Night	19% 18% 37% 22% 4	924
February	1st to 10th	Night	22% 16% 32% 25% 4	840
	11th to 20th	Night	22% 18% 36% 20% 3	823
	21st to end	Night	22% 12% 38% 24% 4	692
March	1st to 10th	Night	28% 14% 37% 20% 2	840
	11th to 20th	Night	25% 15% 33% 21% 6%	840
	21st to end	Night	27% 15% 31% 22% 5% 1	924
April	1st to 10th	Night	30% 16% 31% 20% 3	840
	11th to 20th	Night	26% 14% 33% 21% 5%	702
	21st to end	Night	31% 11% 32% 21% 5%	630
May	1st to 10th	Night	29% 9% 29% 27% 6%	604
	11th to 20th	Night	24% 10% 35% 25% 5%	558
	21st to end	Night	21% 7% 32% 34% 5% 1	433
June	1st to 10th	Night	26% 4 32% 30% 7% 1	400
	11th to 20th	Night	19% 6% 29% 36% 10%	400
	21st to end	Night	27% 4 29% 32% 9%	400
July	1st to 10th	Night	18% 2 30% 36% 13% 1	390
	11th to 20th	Night	20% 2 26% 40% 12% 1	388
	21st to end	Night	16% 3 27% 37% 16% 1	438
August	1st to 10th	Night	16% 3 26% 41% 13% 1	550
	11th to 20th	Night	16% 5% 26% 34% 17% 1	592
	21st to end	Night	17% 7% 22% 37% 15% 2	743
September	1st to 10th	Night	17% 5% 33% 29% 15% 1	800
	11th to 20th	Night	17% 7% 26% 28% 19% 4	800
	21st to end	Night	20% 8% 29% 29% 12% 2	800
October	1st to 10th	Night	21% 9% 26% 29% 14% 1	800
	11th to 20th	Night	22% 11% 29% 26% 10% 1	800
	21st to end	Night	31% 11% 29% 24% 6%	880
November	1st to 10th	Night	25% 10% 32% 25% 6% 1	820
	11th to 20th	Night	23% 13% 33% 23% 8% 1	840
	21st to end	Night	25% 16% 30% 23% 5%	898
December	1st to 10th	Night	28% 12% 33% 21% 6%	950
	11th to 20th	Night	27% 17% 32% 20% 4	950
	21st to end	Night	29% 13% 32% 23% 3	957

Page generated at 2026-04-29 02:21:41. Version ac07

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