Observatoire de Lac Creux
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire de Lac Creux clear sky chart from 2005-11-06 to 2026-07-31.

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% 12% 4 10% 5% 5% 5% 3 5% 6% 27%	138572
	Night	17% 14% 5% 11% 6% 5% 4 3 5% 5% 26%	134436
Transparency	Day	57% 4 11% 19% 8% 1	142016
	Night	60% 4 10% 17% 8% 1	139081
Seeing	Day	27% 13% 30% 19% 8% 2	16540
	Night	26% 11% 25% 27% 10% 1	25508

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% 16% 9% 22% 8% 7% 5% 3 3 3 12%	4661
	11th to 20th	Day	10% 22% 9% 21% 8% 4 3 2 3 4 14%	4650
	21st to end	Day	7% 13% 10% 17% 7% 6% 6% 5% 6% 6% 17%	5124
February	1st to 10th	Day	10% 18% 7% 15% 9% 6% 5% 3 4 5% 19%	1889
	11th to 20th	Day	10% 21% 6% 13% 7% 5% 4 3 4 4 23%	1849
	21st to end	Day	17% 18% 5% 10% 7% 5% 5% 3 4 4 22%	1730
March	1st to 10th	Day	16% 19% 5% 12% 6% 4 3 3 5% 4 22%	4576
	11th to 20th	Day	15% 16% 6% 11% 6% 5% 5% 3 4 5% 25%	4992
	21st to end	Day	17% 13% 5% 9% 5% 4 4 3 4 5% 32%	5956
April	1st to 10th	Day	22% 15% 4 9% 4 4 3 2 3 4 28%	2520
	11th to 20th	Day	23% 11% 4 7% 5% 4 3 3 4 5% 30%	2494
	21st to end	Day	21% 13% 3 8% 4 4 4 3 4 4 31%	2741
May	1st to 10th	Day	21% 12% 4 8% 5% 5% 5% 4 5% 7% 23%	8237
	11th to 20th	Day	20% 9% 3 9% 5% 5% 5% 4 5% 7% 29%	8221
	21st to end	Day	22% 10% 3 7% 5% 5% 5% 4 5% 6% 30%	9060
June	1st to 10th	Day	23% 8% 3 7% 5% 5% 5% 4 6% 6% 28%	2940
	11th to 20th	Day	17% 7% 2 7% 6% 4 5% 4 7% 8% 33%	2940
	21st to end	Day	22% 8% 3 7% 6% 5% 4 4 6% 8% 28%	2940
July	1st to 10th	Day	18% 7% 3 7% 5% 5% 6% 4 7% 9% 28%	6500
	11th to 20th	Day	14% 6% 3 7% 5% 4 5% 4 6% 8% 37%	6500
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 7% 10% 40%	7066
August	1st to 10th	Day	16% 7% 3 8% 5% 6% 6% 5% 7% 8% 30%	4563
	11th to 20th	Day	18% 8% 3 7% 4 5% 5% 4 6% 8% 32%	4191
	21st to end	Day	14% 7% 2 8% 5% 4 5% 4 7% 8% 35%	4312
September	1st to 10th	Day	19% 7% 2 6% 4 4 4 3 6% 6% 40%	2400
	11th to 20th	Day	17% 7% 2 7% 4 4 4 3 5% 7% 41%	2380
	21st to end	Day	23% 7% 3 6% 4 4 4 3 5% 7% 36%	2161
October	1st to 10th	Day	22% 10% 2 7% 4 4 4 3 5% 7% 31%	2000
	11th to 20th	Day	27% 14% 3 8% 4 4 4 3 4 5% 24%	2000
	21st to end	Day	27% 16% 3 7% 5% 3 4 3 4 6% 22%	1986
November	1st to 10th	Day	19% 17% 4 8% 4 3 3 3 5% 5% 28%	1839
	11th to 20th	Day	20% 19% 7% 11% 5% 5% 3 3 4 4 18%	1680
	21st to end	Day	23% 21% 8% 13% 6% 4 4 2 2 4 15%	1680
December	1st to 10th	Day	19% 22% 9% 14% 7% 5% 3 2 3 4 10%	2680
	11th to 20th	Day	18% 19% 6% 14% 8% 4 5% 3 3 4 15%	3066
	21st to end	Day	17% 18% 7% 14% 7% 6% 4 3 3 4 17%	4048
January	1st to 10th	Night	12% 20% 9% 17% 9% 5% 4 3 4 4 15%	8234
	11th to 20th	Night	11% 21% 8% 18% 11% 5% 4 3 3 4 11%	8235
	21st to end	Night	6% 13% 9% 16% 9% 8% 7% 4 6% 4 17%	9057
February	1st to 10th	Night	11% 15% 6% 13% 9% 6% 5% 4 5% 5% 22%	2730
	11th to 20th	Night	12% 16% 7% 14% 7% 6% 5% 4 4 5% 21%	2655
	21st to end	Night	12% 17% 5% 11% 6% 6% 5% 4 4 5% 24%	2076
March	1st to 10th	Night	19% 17% 5% 11% 6% 5% 4 3 4 4 23%	5484
	11th to 20th	Night	13% 15% 5% 11% 6% 5% 4 3 5% 6% 26%	5183
	21st to end	Night	15% 13% 4 9% 6% 5% 5% 3 5% 6% 28%	5217
April	1st to 10th	Night	22% 13% 4 8% 5% 4 4 3 4 5% 29%	2100
	11th to 20th	Night	19% 11% 4 7% 4 3 4 3 5% 5% 36%	2075
	21st to end	Night	22% 10% 3 8% 4 3 3 3 4 5% 34%	1896
May	1st to 10th	Night	19% 12% 4 9% 4 5% 4 4 6% 7% 27%	4660
	11th to 20th	Night	20% 9% 3 7% 5% 5% 5% 4 5% 6% 31%	4639
	21st to end	Night	22% 10% 3 8% 5% 5% 4 3 5% 6% 31%	5114
June	1st to 10th	Night	19% 9% 2 7% 4 5% 5% 3 6% 8% 33%	1680
	11th to 20th	Night	17% 7% 2 6% 5% 4 4 4 6% 7% 38%	1680
	21st to end	Night	20% 8% 3 7% 4 5% 4 3 5% 8% 34%	1680
July	1st to 10th	Night	15% 8% 3 8% 6% 6% 6% 4 7% 8% 29%	3680
	11th to 20th	Night	15% 7% 3 6% 4 5% 4 4 5% 8% 40%	3680
	21st to end	Night	12% 6% 2 6% 4 4 4 4 5% 7% 46%	4006
August	1st to 10th	Night	13% 8% 3 7% 4 5% 5% 4 6% 7% 37%	2586
	11th to 20th	Night	20% 8% 3 6% 4 4 4 4 6% 6% 37%	2821
	21st to end	Night	20% 7% 3 6% 4 4 4 4 5% 7% 36%	3586
September	1st to 10th	Night	17% 6% 2 7% 4 3 4 3 5% 6% 43%	2000
	11th to 20th	Night	17% 6% 2 5% 3 3 4 3 5% 7% 45%	2019
	21st to end	Night	20% 8% 3 6% 4 3 4 3 5% 6% 40%	2235
October	1st to 10th	Night	20% 9% 2 6% 4 4 4 3 5% 6% 38%	2400
	11th to 20th	Night	25% 12% 3 7% 5% 5% 5% 3 5% 4 27%	2400
	21st to end	Night	26% 15% 3 7% 5% 4 4 3 4 6% 23%	2852
November	1st to 10th	Night	20% 17% 5% 8% 5% 3 4 3 5% 5% 26%	2678
	11th to 20th	Night	19% 16% 6% 11% 7% 4 3 3 4 5% 21%	2940
	21st to end	Night	23% 21% 6% 12% 5% 4 3 3 4 4 14%	2940
December	1st to 10th	Night	17% 21% 8% 18% 8% 5% 4 3 3 4 10%	4720
	11th to 20th	Night	23% 16% 6% 13% 8% 4 3 2 3 4 19%	5348
	21st to end	Night	20% 21% 5% 11% 7% 4 4 3 3 5% 16%	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	78% 3 6% 10% 3	4662
	11th to 20th	Day	76% 2 4 10% 8%	4680
	21st to end	Day	66% 4 7% 12% 10%	5124
February	1st to 10th	Day	69% 3 6% 14% 8% 1	1889
	11th to 20th	Day	66% 3 8% 13% 10% 1	1858
	21st to end	Day	66% 4 7% 13% 10% 1	1730
March	1st to 10th	Day	66% 3 8% 10% 12% 2	4579
	11th to 20th	Day	62% 4 12% 11% 10% 1	4992
	21st to end	Day	55% 4 6% 18% 15% 2	5956
April	1st to 10th	Day	61% 3 7% 16% 10% 2	2520
	11th to 20th	Day	57% 3 7% 19% 11% 2	2494
	21st to end	Day	58% 3 9% 16% 12% 3	2741
May	1st to 10th	Day	57% 5% 9% 20% 9% 1	8249
	11th to 20th	Day	53% 5% 14% 21% 7% 1	8225
	21st to end	Day	54% 4 9% 18% 13% 1	9061
June	1st to 10th	Day	54% 6% 10% 19% 11% 1	2940
	11th to 20th	Day	48% 5% 12% 23% 11% 1	2940
	21st to end	Day	54% 5% 13% 20% 8%	2940
July	1st to 10th	Day	50% 6% 14% 24% 6%	6430
	11th to 20th	Day	43% 6% 19% 28% 5%	6425
	21st to end	Day	39% 6% 19% 29% 8%	6868
August	1st to 10th	Day	50% 6% 16% 22% 6%	4549
	11th to 20th	Day	48% 6% 18% 23% 5%	4232
	21st to end	Day	44% 6% 15% 22% 12% 1	4312
September	1st to 10th	Day	45% 5% 15% 24% 11% 1	2400
	11th to 20th	Day	43% 4 15% 27% 9%	2380
	21st to end	Day	49% 5% 12% 24% 9% 1	2161
October	1st to 10th	Day	54% 5% 11% 24% 6%	3260
	11th to 20th	Day	58% 4 12% 20% 5%	3240
	21st to end	Day	62% 4 16% 13% 4 1	3186
November	1st to 10th	Day	59% 4 12% 18% 6%	1839
	11th to 20th	Day	71% 4 9% 11% 6%	1680
	21st to end	Day	78% 4 7% 8% 4	1680
December	1st to 10th	Day	80% 3 6% 9% 2	2680
	11th to 20th	Day	75% 3 4 13% 4	3066
	21st to end	Day	72% 2 6% 16% 4	4048
January	1st to 10th	Night	75% 2 9% 11% 2	8234
	11th to 20th	Night	79% 3 6% 8% 4	8280
	21st to end	Night	69% 3 6% 12% 9%	9057
February	1st to 10th	Night	65% 3 8% 16% 7% 1	2730
	11th to 20th	Night	67% 5% 6% 15% 7% 1	2657
	21st to end	Night	63% 4 7% 17% 8% 1	2076
March	1st to 10th	Night	67% 3 8% 9% 11% 2	5494
	11th to 20th	Night	59% 4 11% 16% 8% 2	5184
	21st to end	Night	56% 6% 7% 21% 9% 2	5217
April	1st to 10th	Night	59% 4 7% 16% 12% 2	2100
	11th to 20th	Night	51% 4 8% 23% 13% 2	2075
	21st to end	Night	53% 3 8% 19% 15% 2	1896
May	1st to 10th	Night	55% 5% 10% 19% 9% 2	4666
	11th to 20th	Night	53% 6% 10% 21% 8% 2	4640
	21st to end	Night	54% 4 9% 19% 12% 1	5115
June	1st to 10th	Night	48% 6% 11% 23% 11% 1	1680
	11th to 20th	Night	46% 4 13% 24% 11% 2	1680
	21st to end	Night	50% 5% 14% 21% 10% 1	1680
July	1st to 10th	Night	50% 6% 17% 20% 7%	3640
	11th to 20th	Night	42% 5% 20% 27% 5%	3632
	21st to end	Night	37% 6% 17% 31% 10%	3889
August	1st to 10th	Night	45% 5% 18% 24% 7%	2576
	11th to 20th	Night	46% 5% 19% 24% 6%	2846
	21st to end	Night	48% 5% 14% 23% 9% 2	3586
September	1st to 10th	Night	41% 4 16% 26% 12%	2000
	11th to 20th	Night	39% 5% 17% 30% 8%	2019
	21st to end	Night	46% 5% 13% 26% 9%	2235
October	1st to 10th	Night	46% 5% 16% 22% 10% 1	3920
	11th to 20th	Night	56% 4 12% 20% 6%	3899
	21st to end	Night	59% 3 15% 14% 8% 1	4602
November	1st to 10th	Night	62% 4 12% 17% 5%	2678
	11th to 20th	Night	67% 4 9% 12% 6% 1	2940
	21st to end	Night	76% 4 7% 11% 3	2940
December	1st to 10th	Night	80% 3 8% 7% 2	4720
	11th to 20th	Night	72% 3 4 14% 6%	5348
	21st to end	Night	73% 3 4 12% 8%	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	20% 15% 48% 13% 4	220
	11th to 20th	Day	24% 17% 39% 13% 5% 1	220
	21st to end	Day	21% 17% 34% 21% 7%	242
February	1st to 10th	Day	25% 23% 38% 11% 3	330
	11th to 20th	Day	26% 22% 38% 12% 2	336
	21st to end	Day	32% 19% 32% 13% 3	446
March	1st to 10th	Day	27% 19% 32% 17% 5%	540
	11th to 20th	Day	29% 19% 33% 15% 4 1	540
	21st to end	Day	28% 20% 30% 15% 5% 2	594
April	1st to 10th	Day	35% 15% 26% 16% 7% 1	540
	11th to 20th	Day	36% 15% 24% 15% 8% 3	532
	21st to end	Day	34% 16% 28% 15% 6% 1	540
May	1st to 10th	Day	30% 15% 28% 20% 5% 2	543
	11th to 20th	Day	27% 14% 28% 20% 9% 2	567
	21st to end	Day	27% 17% 31% 18% 7% 1	617
June	1st to 10th	Day	29% 9% 36% 18% 7% 1	570
	11th to 20th	Day	24% 13% 31% 21% 9% 2	570
	21st to end	Day	26% 11% 29% 22% 10% 2	570
July	1st to 10th	Day	18% 7% 35% 28% 10% 1	565
	11th to 20th	Day	18% 11% 35% 25% 9% 2	564
	21st to end	Day	16% 9% 31% 30% 12% 2	616
August	1st to 10th	Day	19% 6% 27% 31% 13% 4	526
	11th to 20th	Day	21% 5% 26% 28% 14% 7%	519
	21st to end	Day	20% 11% 30% 19% 14% 7%	583
September	1st to 10th	Day	24% 10% 27% 21% 12% 6%	530
	11th to 20th	Day	20% 9% 23% 22% 17% 8%	530
	21st to end	Day	25% 9% 25% 20% 15% 6%	530
October	1st to 10th	Day	28% 12% 25% 20% 13% 3	530
	11th to 20th	Day	34% 15% 25% 18% 6% 2	530
	21st to end	Day	38% 9% 26% 20% 7% 1	465
November	1st to 10th	Day	36% 13% 29% 17% 4 1	413
	11th to 20th	Day	30% 18% 32% 14% 5%	220
	21st to end	Day	35% 21% 34% 6% 3	220
December	1st to 10th	Day	32% 11% 32% 19% 6%	220
	11th to 20th	Day	31% 20% 32% 12% 3 1	220
	21st to end	Day	32% 15% 31% 16% 4 2	242
January	1st to 10th	Night	30% 12% 31% 21% 5% 1	840
	11th to 20th	Night	30% 13% 26% 23% 7%	840
	21st to end	Night	19% 11% 30% 29% 10% 1	924
February	1st to 10th	Night	19% 15% 30% 28% 7% 1	840
	11th to 20th	Night	22% 15% 35% 22% 5% 1	823
	21st to end	Night	23% 16% 28% 27% 5%	692
March	1st to 10th	Night	26% 15% 29% 25% 6%	840
	11th to 20th	Night	27% 16% 26% 24% 6% 1	840
	21st to end	Night	23% 14% 30% 24% 8% 1	924
April	1st to 10th	Night	31% 12% 22% 28% 6%	840
	11th to 20th	Night	27% 10% 29% 23% 10% 1	831
	21st to end	Night	29% 11% 25% 25% 9%	636
May	1st to 10th	Night	26% 7% 26% 28% 11% 1	420
	11th to 20th	Night	23% 8% 29% 29% 11%	414
	21st to end	Night	23% 8% 30% 31% 8% 1	455
June	1st to 10th	Night	24% 5% 29% 33% 9%	420
	11th to 20th	Night	21% 6% 22% 36% 13% 1	420
	21st to end	Night	23% 4 23% 36% 14%	420
July	1st to 10th	Night	15% 4 25% 43% 13%	410
	11th to 20th	Night	17% 4 19% 42% 18%	408
	21st to end	Night	13% 5% 22% 43% 16% 2	458
August	1st to 10th	Night	16% 3 17% 41% 22% 1	398
	11th to 20th	Night	19% 3 16% 34% 28% 2	551
	21st to end	Night	22% 5% 20% 30% 20% 3	880
September	1st to 10th	Night	23% 4 23% 33% 17% 1	800
	11th to 20th	Night	22% 4 20% 30% 22% 2	800
	21st to end	Night	26% 7% 22% 26% 18% 2	800
October	1st to 10th	Night	25% 8% 27% 26% 14% 1	800
	11th to 20th	Night	35% 11% 23% 23% 8% 1	800
	21st to end	Night	35% 10% 24% 25% 6%	880
November	1st to 10th	Night	32% 12% 27% 21% 7% 1	820
	11th to 20th	Night	30% 15% 26% 18% 10% 1	840
	21st to end	Night	37% 16% 20% 20% 6%	840
December	1st to 10th	Night	32% 15% 25% 20% 7% 1	840
	11th to 20th	Night	28% 16% 27% 21% 7% 1	840
	21st to end	Night	36% 14% 23% 21% 6% 1	924

Page generated at 2026-07-28 03:25:25. Version ac07

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