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 2025-03-27.

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	18% 12% 4 10% 6% 5% 4 3 5% 6% 27%	108992
	Night	17% 14% 5% 10% 6% 5% 4 3 4 5% 26%	110334
Transparency	Day	57% 4 11% 19% 8% 1	112482
	Night	60% 4 10% 17% 8% 1	115008
Seeing	Day	27% 13% 30% 20% 8% 2	14781
	Night	26% 11% 25% 26% 10% 1	23965

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	13% 18% 9% 23% 8% 7% 4 2 2 3 10%	3597
	11th to 20th	Day	11% 17% 9% 21% 9% 4 3 2 4 5% 15%	3570
	21st to end	Day	9% 16% 10% 15% 5% 4 5% 4 6% 6% 19%	3960
February	1st to 10th	Day	10% 18% 7% 15% 9% 6% 5% 3 4 5% 18%	1799
	11th to 20th	Day	11% 21% 7% 13% 7% 5% 4 3 4 4 22%	1758
	21st to end	Day	17% 18% 5% 10% 7% 5% 4 3 4 4 22%	1650
March	1st to 10th	Day	15% 18% 4 10% 6% 4 4 3 4 4 26%	3216
	11th to 20th	Day	13% 20% 7% 12% 6% 5% 4 3 3 4 24%	3520
	21st to end	Day	13% 10% 4 9% 6% 5% 4 3 5% 6% 35%	4134
April	1st to 10th	Day	22% 16% 4 9% 4 4 3 2 3 4 29%	2280
	11th to 20th	Day	23% 11% 4 7% 5% 4 3 3 4 5% 31%	2254
	21st to end	Day	22% 13% 4 8% 5% 4 4 2 5% 4 30%	2479
May	1st to 10th	Day	22% 10% 3 8% 5% 4 4 4 5% 6% 28%	4426
	11th to 20th	Day	21% 8% 2 7% 5% 4 5% 3 5% 8% 33%	4398
	21st to end	Day	19% 8% 2 7% 5% 5% 5% 4 5% 6% 33%	4837
June	1st to 10th	Day	24% 8% 3 8% 5% 5% 5% 4 6% 6% 27%	2660
	11th to 20th	Day	17% 6% 2 7% 6% 4 5% 4 7% 8% 35%	2660
	21st to end	Day	23% 8% 3 7% 6% 5% 4 5% 6% 7% 27%	2660
July	1st to 10th	Day	16% 8% 3 8% 5% 4 6% 4 7% 9% 29%	4440
	11th to 20th	Day	17% 8% 3 7% 5% 4 5% 4 6% 7% 34%	4440
	21st to end	Day	14% 7% 2 6% 4 4 4 3 6% 8% 42%	4814
August	1st to 10th	Day	17% 7% 3 8% 5% 6% 6% 5% 6% 8% 28%	4423
	11th to 20th	Day	18% 8% 3 7% 4 5% 5% 4 6% 8% 32%	4060
	21st to end	Day	14% 7% 2 8% 5% 4 5% 4 7% 8% 35%	4180
September	1st to 10th	Day	19% 7% 2 6% 4 4 4 3 6% 6% 40%	2280
	11th to 20th	Day	18% 7% 2 7% 4 4 4 3 5% 7% 40%	2261
	21st to end	Day	23% 7% 3 6% 4 4 4 3 5% 7% 35%	2053
October	1st to 10th	Day	23% 11% 2 7% 4 4 4 3 5% 7% 29%	1900
	11th to 20th	Day	27% 15% 3 8% 4 4 3 2 4 5% 23%	1900
	21st to end	Day	28% 16% 3 7% 4 3 4 3 4 6% 22%	1887
November	1st to 10th	Day	19% 16% 3 8% 4 3 4 3 5% 5% 29%	1750
	11th to 20th	Day	20% 19% 7% 11% 5% 5% 3 3 4 4 19%	1600
	21st to end	Day	23% 20% 8% 12% 6% 4 4 2 2 4 15%	1600
December	1st to 10th	Day	20% 22% 10% 14% 7% 5% 3 2 3 4 10%	2600
	11th to 20th	Day	18% 20% 6% 14% 8% 4 5% 3 3 4 15%	2986
	21st to end	Day	17% 18% 7% 14% 7% 6% 4 3 3 4 17%	3960
January	1st to 10th	Night	13% 23% 9% 17% 9% 4 3 2 3 3 14%	6349
	11th to 20th	Night	12% 17% 7% 17% 12% 6% 5% 3 4 4 13%	6315
	21st to end	Night	8% 16% 9% 16% 6% 7% 7% 4 5% 4 19%	6996
February	1st to 10th	Night	12% 16% 6% 13% 9% 5% 5% 4 5% 5% 22%	2600
	11th to 20th	Night	12% 16% 7% 13% 7% 6% 5% 3 4 5% 21%	2526
	21st to end	Night	13% 18% 5% 10% 6% 6% 5% 4 4 6% 24%	1980
March	1st to 10th	Night	19% 15% 4 9% 6% 5% 5% 3 4 4 26%	3844
	11th to 20th	Night	16% 19% 6% 10% 5% 4 4 3 4 5% 25%	3655
	21st to end	Night	13% 10% 4 10% 6% 4 4 4 5% 7% 34%	3636
April	1st to 10th	Night	22% 13% 4 7% 6% 4 4 3 4 5% 29%	1900
	11th to 20th	Night	19% 10% 3 6% 4 3 4 3 5% 5% 37%	1875
	21st to end	Night	22% 10% 3 8% 4 3 4 3 4 5% 33%	1716
May	1st to 10th	Night	21% 10% 3 7% 4 4 4 3 5% 6% 31%	2514
	11th to 20th	Night	21% 8% 3 6% 5% 4 4 4 5% 5% 35%	2495
	21st to end	Night	19% 7% 3 7% 5% 5% 4 3 5% 7% 34%	2738
June	1st to 10th	Night	19% 8% 2 7% 4 5% 5% 3 6% 8% 32%	1520
	11th to 20th	Night	17% 7% 2 6% 5% 3 4 4 6% 7% 39%	1520
	21st to end	Night	20% 8% 3 6% 4 4 4 3 5% 8% 34%	1520
July	1st to 10th	Night	16% 8% 3 8% 5% 6% 6% 4 7% 6% 31%	2520
	11th to 20th	Night	17% 8% 3 6% 4 4 4 4 5% 7% 37%	2520
	21st to end	Night	11% 6% 2 5% 4 4 4 4 6% 7% 46%	2739
August	1st to 10th	Night	13% 8% 3 8% 4 5% 5% 4 6% 7% 37%	2506
	11th to 20th	Night	20% 8% 3 6% 4 4 4 4 6% 6% 36%	2733
	21st to end	Night	20% 7% 3 6% 4 4 4 4 5% 7% 37%	3476
September	1st to 10th	Night	18% 6% 2 7% 3 3 4 3 4 6% 43%	1900
	11th to 20th	Night	18% 6% 2 5% 3 3 4 4 5% 7% 44%	1918
	21st to end	Night	20% 8% 3 6% 4 3 4 3 5% 6% 40%	2123
October	1st to 10th	Night	21% 9% 2 6% 4 4 4 3 5% 6% 36%	2280
	11th to 20th	Night	26% 12% 3 7% 4 4 4 3 5% 4 26%	2280
	21st to end	Night	26% 14% 3 7% 5% 4 4 3 5% 6% 22%	2709
November	1st to 10th	Night	20% 16% 5% 8% 5% 3 4 3 5% 5% 27%	2547
	11th to 20th	Night	19% 16% 6% 11% 7% 4 4 3 4 5% 22%	2800
	21st to end	Night	24% 20% 6% 12% 6% 4 3 3 4 4 15%	2800
December	1st to 10th	Night	17% 21% 8% 18% 8% 5% 4 3 3 4 10%	4580
	11th to 20th	Night	23% 16% 6% 12% 8% 4 3 2 3 4 20%	5208
	21st to end	Night	21% 21% 5% 11% 7% 4 4 3 3 5% 16%	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	82% 3 5% 7% 3	3598
	11th to 20th	Day	73% 2 5% 12% 8%	3600
	21st to end	Day	64% 4 8% 13% 10% 1	3960
February	1st to 10th	Day	70% 3 6% 13% 8% 1	1799
	11th to 20th	Day	67% 3 8% 12% 9% 1	1767
	21st to end	Day	66% 4 7% 13% 10% 1	1650
March	1st to 10th	Day	62% 3 10% 14% 11% 1	3219
	11th to 20th	Day	65% 4 8% 12% 9% 2	3520
	21st to end	Day	49% 5% 8% 22% 15% 2	4134
April	1st to 10th	Day	60% 4 7% 16% 10% 3	2280
	11th to 20th	Day	57% 4 7% 20% 11% 2	2254
	21st to end	Day	58% 3 9% 15% 12% 3	2479
May	1st to 10th	Day	55% 5% 8% 22% 9% 1	4440
	11th to 20th	Day	50% 5% 12% 23% 9% 1	4402
	21st to end	Day	50% 5% 9% 19% 15% 1	4837
June	1st to 10th	Day	55% 6% 10% 18% 10% 1	2660
	11th to 20th	Day	47% 5% 12% 23% 12% 2	2660
	21st to end	Day	56% 5% 12% 18% 8%	2660
July	1st to 10th	Day	48% 6% 15% 25% 6%	4370
	11th to 20th	Day	47% 6% 18% 24% 5%	4365
	21st to end	Day	41% 5% 18% 27% 8%	4661
August	1st to 10th	Day	52% 6% 15% 21% 6%	4409
	11th to 20th	Day	48% 6% 18% 23% 5%	4101
	21st to end	Day	44% 6% 15% 22% 12% 1	4180
September	1st to 10th	Day	45% 5% 15% 24% 11% 1	2280
	11th to 20th	Day	44% 4 16% 26% 9%	2261
	21st to end	Day	49% 5% 12% 25% 8% 1	2053
October	1st to 10th	Day	55% 5% 11% 24% 5%	3160
	11th to 20th	Day	59% 4 12% 20% 5%	3140
	21st to end	Day	62% 4 16% 13% 4 1	3087
November	1st to 10th	Day	57% 5% 12% 19% 6%	1750
	11th to 20th	Day	70% 4 9% 11% 6%	1600
	21st to end	Day	77% 4 7% 8% 4	1600
December	1st to 10th	Day	80% 3 6% 9% 2	2600
	11th to 20th	Day	75% 3 4 13% 4	2986
	21st to end	Day	72% 3 6% 16% 4	3960
January	1st to 10th	Night	78% 2 8% 9% 2	6349
	11th to 20th	Night	75% 4 8% 9% 4	6360
	21st to end	Night	69% 3 7% 11% 10%	6996
February	1st to 10th	Night	65% 3 8% 15% 7% 1	2600
	11th to 20th	Night	68% 5% 6% 14% 6% 1	2528
	21st to end	Night	62% 4 7% 17% 9% 1	1980
March	1st to 10th	Night	63% 4 10% 11% 10% 2	3854
	11th to 20th	Night	62% 3 8% 17% 9% 1	3656
	21st to end	Night	49% 6% 8% 22% 11% 3	3636
April	1st to 10th	Night	58% 4 8% 16% 12% 2	1900
	11th to 20th	Night	50% 4 8% 23% 13% 2	1875
	21st to end	Night	54% 4 8% 18% 14% 3	1716
May	1st to 10th	Night	53% 5% 13% 18% 9% 2	2520
	11th to 20th	Night	51% 6% 12% 19% 10% 2	2496
	21st to end	Night	50% 5% 10% 21% 13% 1	2739
June	1st to 10th	Night	49% 6% 12% 22% 10%	1520
	11th to 20th	Night	45% 4 13% 25% 11% 3	1520
	21st to end	Night	50% 5% 13% 21% 10% 1	1520
July	1st to 10th	Night	50% 6% 16% 20% 8%	2480
	11th to 20th	Night	46% 5% 21% 23% 6%	2472
	21st to end	Night	35% 7% 18% 29% 11%	2651
August	1st to 10th	Night	46% 5% 17% 24% 7%	2496
	11th to 20th	Night	46% 5% 19% 24% 5%	2758
	21st to end	Night	47% 5% 14% 23% 9% 2	3476
September	1st to 10th	Night	42% 4 15% 26% 12%	1900
	11th to 20th	Night	40% 5% 17% 29% 8%	1918
	21st to end	Night	46% 5% 13% 26% 8%	2123
October	1st to 10th	Night	47% 5% 16% 22% 10%	3800
	11th to 20th	Night	57% 4 12% 20% 6%	3779
	21st to end	Night	59% 3 15% 14% 8% 1	4459
November	1st to 10th	Night	61% 4 12% 18% 6%	2547
	11th to 20th	Night	67% 4 9% 12% 6% 1	2800
	21st to end	Night	75% 4 7% 11% 3	2800
December	1st to 10th	Night	80% 3 8% 7% 2	4580
	11th to 20th	Night	72% 3 4 14% 6%	5208
	21st to end	Night	72% 3 4 12% 8%	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% 47% 14% 3	200
	11th to 20th	Day	23% 15% 41% 15% 6% 2	200
	21st to end	Day	22% 16% 36% 18% 7%	220
February	1st to 10th	Day	27% 26% 36% 9% 2	300
	11th to 20th	Day	28% 22% 39% 10% 1	305
	21st to end	Day	33% 19% 32% 13% 4	414
March	1st to 10th	Day	27% 18% 32% 18% 6%	500
	11th to 20th	Day	27% 19% 33% 16% 4 1	500
	21st to end	Day	29% 18% 30% 15% 6% 2	530
April	1st to 10th	Day	33% 14% 27% 18% 8% 1	460
	11th to 20th	Day	35% 13% 25% 15% 8% 3	452
	21st to end	Day	33% 16% 28% 17% 6%	460
May	1st to 10th	Day	30% 14% 27% 20% 6% 2	463
	11th to 20th	Day	26% 13% 28% 21% 9% 3	487
	21st to end	Day	26% 16% 31% 19% 7% 1	529
June	1st to 10th	Day	30% 8% 37% 17% 7% 1	490
	11th to 20th	Day	23% 12% 31% 21% 10% 3	490
	21st to end	Day	28% 9% 28% 22% 10% 2	490
July	1st to 10th	Day	18% 6% 33% 30% 11% 2	485
	11th to 20th	Day	18% 10% 32% 27% 10% 2	484
	21st to end	Day	17% 7% 30% 32% 12% 2	532
August	1st to 10th	Day	21% 7% 27% 31% 11% 4	486
	11th to 20th	Day	22% 5% 25% 27% 14% 7%	479
	21st to end	Day	19% 11% 30% 19% 14% 7%	539
September	1st to 10th	Day	24% 10% 27% 21% 11% 7%	490
	11th to 20th	Day	22% 9% 24% 22% 17% 8%	490
	21st to end	Day	25% 9% 25% 20% 15% 6%	490
October	1st to 10th	Day	30% 12% 23% 19% 13% 3	490
	11th to 20th	Day	35% 14% 26% 18% 6% 2	490
	21st to end	Day	39% 9% 26% 19% 6% 1	432
November	1st to 10th	Day	36% 12% 29% 17% 4 1	384
	11th to 20th	Day	30% 18% 35% 12% 6% 1	200
	21st to end	Day	33% 22% 35% 7% 4 1	200
December	1st to 10th	Day	34% 11% 30% 19% 7%	200
	11th to 20th	Day	32% 21% 30% 13% 4 2	200
	21st to end	Day	32% 15% 31% 16% 4 3	220
January	1st to 10th	Night	30% 13% 31% 21% 5% 1	800
	11th to 20th	Night	30% 13% 26% 23% 7%	800
	21st to end	Night	19% 11% 30% 30% 10% 1	880
February	1st to 10th	Night	20% 16% 30% 27% 6% 1	800
	11th to 20th	Night	22% 15% 35% 22% 5% 1	783
	21st to end	Night	23% 17% 28% 27% 4	660
March	1st to 10th	Night	25% 14% 29% 25% 6%	800
	11th to 20th	Night	26% 16% 26% 25% 7% 1	800
	21st to end	Night	23% 14% 29% 24% 9% 1	863
April	1st to 10th	Night	31% 11% 22% 29% 6%	760
	11th to 20th	Night	27% 11% 29% 22% 10% 1	751
	21st to end	Night	29% 11% 25% 26% 10%	576
May	1st to 10th	Night	27% 6% 28% 28% 11% 1	380
	11th to 20th	Night	24% 8% 29% 29% 11%	374
	21st to end	Night	22% 8% 30% 31% 8% 1	411
June	1st to 10th	Night	24% 5% 30% 32% 9% 1	380
	11th to 20th	Night	21% 6% 22% 36% 14% 1	380
	21st to end	Night	23% 4 22% 36% 14%	380
July	1st to 10th	Night	16% 4 24% 42% 14%	370
	11th to 20th	Night	17% 2 18% 42% 19% 1	368
	21st to end	Night	13% 3 22% 44% 17% 2	416
August	1st to 10th	Night	17% 3 16% 41% 22% 1	378
	11th to 20th	Night	19% 3 15% 33% 28% 2	523
	21st to end	Night	22% 5% 21% 30% 20% 3	836
September	1st to 10th	Night	23% 4 23% 32% 17% 1	760
	11th to 20th	Night	23% 4 20% 29% 22% 2	760
	21st to end	Night	26% 7% 22% 26% 18% 2	760
October	1st to 10th	Night	26% 8% 26% 25% 13% 1	760
	11th to 20th	Night	36% 11% 22% 22% 8% 1	760
	21st to end	Night	36% 10% 24% 24% 6%	836
November	1st to 10th	Night	31% 12% 27% 21% 7% 1	780
	11th to 20th	Night	30% 15% 27% 18% 10% 1	800
	21st to end	Night	37% 16% 20% 20% 6%	800
December	1st to 10th	Night	33% 15% 25% 20% 7% 1	800
	11th to 20th	Night	29% 16% 26% 22% 7% 1	800
	21st to end	Night	36% 14% 23% 21% 6% 1	880

Page generated at 2025-03-24 09:04:29. Version ac07

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