Observatoire de La Sérénité
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire de La Sérénité clear sky chart from 2005-11-06 to 2026-09-14.

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% 10% 3 8% 5% 5% 5% 4 6% 7% 31%	149050
	Night	18% 12% 4 9% 5% 5% 4 3 5% 6% 30%	142094
Transparency	Day	52% 5% 14% 20% 9% 1	152494
	Night	55% 4 12% 19% 9% 1	146740
Seeing	Day	26% 13% 30% 21% 9% 2	17591
	Night	25% 10% 28% 28% 9% 1	26021

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	15% 17% 6% 12% 5% 4 3 3 5% 5% 26%	4661
	11th to 20th	Day	10% 18% 8% 17% 7% 5% 4 3 4 4 20%	4650
	21st to end	Day	6% 12% 5% 11% 6% 4 5% 5% 7% 8% 32%	5124
February	1st to 10th	Day	10% 15% 5% 11% 8% 6% 4 5% 4 5% 27%	1680
	11th to 20th	Day	13% 16% 6% 12% 5% 4 4 3 5% 5% 27%	2032
	21st to end	Day	12% 14% 5% 11% 7% 5% 5% 3 4 6% 27%	1730
March	1st to 10th	Day	22% 14% 5% 10% 6% 4 4 2 4 4 26%	4576
	11th to 20th	Day	15% 15% 5% 8% 5% 5% 5% 4 5% 6% 29%	4846
	21st to end	Day	18% 14% 4 8% 4 4 4 3 4 4 33%	5956
April	1st to 10th	Day	22% 13% 3 8% 6% 4 4 3 4 5% 29%	2520
	11th to 20th	Day	22% 7% 3 7% 5% 4 3 3 5% 6% 35%	2590
	21st to end	Day	24% 8% 3 7% 6% 4 4 3 4 6% 31%	2730
May	1st to 10th	Day	26% 10% 3 7% 4 4 4 3 5% 6% 28%	7851
	11th to 20th	Day	20% 7% 2 8% 5% 5% 5% 4 6% 7% 32%	8221
	21st to end	Day	20% 8% 3 8% 5% 5% 5% 4 6% 6% 29%	9115
June	1st to 10th	Day	19% 7% 3 7% 6% 5% 6% 5% 7% 8% 27%	3150
	11th to 20th	Day	19% 7% 3 7% 4 5% 5% 4 6% 7% 33%	3150
	21st to end	Day	23% 9% 3 8% 5% 5% 5% 4 6% 7% 26%	3150
July	1st to 10th	Day	15% 7% 3 7% 5% 5% 6% 4 6% 9% 33%	8344
	11th to 20th	Day	15% 6% 3 7% 4 4 5% 4 7% 9% 35%	8280
	21st to end	Day	12% 5% 3 7% 5% 5% 6% 4 7% 9% 37%	8993
August	1st to 10th	Day	15% 6% 2 7% 6% 5% 6% 4 7% 9% 32%	6466
	11th to 20th	Day	18% 6% 3 7% 5% 6% 5% 5% 7% 9% 28%	5953
	21st to end	Day	19% 6% 2 6% 4 4 4 4 5% 8% 38%	6291
September	1st to 10th	Day	17% 7% 3 6% 4 3 4 4 7% 8% 37%	2520
	11th to 20th	Day	18% 5% 2 5% 3 4 3 3 5% 6% 47%	2262
	21st to end	Day	21% 7% 3 6% 4 3 4 3 5% 7% 38%	2200
October	1st to 10th	Day	21% 7% 3 6% 4 5% 5% 4 5% 6% 35%	2190
	11th to 20th	Day	24% 8% 3 8% 6% 5% 5% 4 6% 6% 25%	1835
	21st to end	Day	28% 11% 2 7% 5% 4 4 3 5% 5% 25%	1980
November	1st to 10th	Day	20% 12% 3 7% 6% 5% 5% 2 5% 5% 30%	1851
	11th to 20th	Day	21% 14% 5% 10% 5% 4 4 3 4 4 26%	1885
	21st to end	Day	22% 17% 5% 9% 5% 4 4 4 4 4 21%	1519
December	1st to 10th	Day	18% 16% 5% 9% 6% 5% 5% 3 5% 6% 22%	2350
	11th to 20th	Day	20% 14% 5% 11% 5% 4 3 3 3 4 28%	2688
	21st to end	Day	20% 17% 4 8% 4 4 4 4 5% 5% 26%	3711
January	1st to 10th	Night	14% 18% 6% 14% 6% 5% 4 3 4 4 22%	8234
	11th to 20th	Night	11% 22% 9% 16% 6% 5% 4 3 4 4 17%	8235
	21st to end	Night	6% 11% 5% 13% 8% 6% 5% 4 7% 7% 27%	9057
February	1st to 10th	Night	11% 13% 5% 11% 7% 5% 4 4 4 5% 29%	2940
	11th to 20th	Night	13% 17% 5% 11% 6% 5% 4 3 5% 5% 27%	2471
	21st to end	Night	14% 15% 5% 10% 5% 4 3 4 4 6% 30%	2076
March	1st to 10th	Night	22% 13% 4 10% 5% 4 6% 3 4 5% 25%	5484
	11th to 20th	Night	16% 14% 3 10% 6% 6% 6% 4 5% 6% 24%	5332
	21st to end	Night	18% 15% 4 8% 5% 4 4 3 4 5% 30%	5215
April	1st to 10th	Night	22% 14% 4 7% 5% 4 3 3 5% 4 29%	2100
	11th to 20th	Night	20% 9% 3 7% 5% 4 4 4 5% 6% 32%	1978
	21st to end	Night	23% 9% 3 8% 3 3 3 2 4 6% 35%	1890
May	1st to 10th	Night	24% 8% 3 7% 4 5% 4 3 5% 5% 30%	5104
	11th to 20th	Night	24% 7% 3 7% 5% 5% 4 4 5% 6% 32%	4639
	21st to end	Night	20% 7% 3 7% 5% 4 5% 3 5% 7% 34%	5058
June	1st to 10th	Night	21% 7% 3 7% 6% 4 4 4 7% 7% 30%	1470
	11th to 20th	Night	21% 5% 3 6% 4 4 4 4 7% 7% 35%	1470
	21st to end	Night	22% 7% 3 8% 5% 4 4 4 5% 8% 30%	1470
July	1st to 10th	Night	15% 7% 3 8% 5% 5% 5% 4 6% 7% 35%	4613
	11th to 20th	Night	17% 6% 2 6% 4 4 5% 4 6% 8% 38%	4680
	21st to end	Night	13% 6% 2 8% 5% 5% 5% 4 6% 8% 40%	5084
August	1st to 10th	Night	19% 8% 3 7% 5% 4 4 3 6% 7% 33%	3666
	11th to 20th	Night	17% 6% 3 7% 5% 5% 6% 4 7% 7% 35%	4064
	21st to end	Night	21% 7% 3 7% 5% 4 4 4 4 6% 35%	4831
September	1st to 10th	Night	18% 6% 2 6% 4 4 3 3 5% 7% 41%	2100
	11th to 20th	Night	18% 5% 2 6% 4 4 3 3 5% 6% 45%	2204
	21st to end	Night	20% 7% 2 6% 3 4 3 3 4 6% 41%	2200
October	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 6% 5% 39%	2209
	11th to 20th	Night	23% 9% 2 7% 5% 5% 4 4 5% 5% 31%	2565
	21st to end	Night	26% 10% 3 6% 4 5% 4 3 5% 5% 30%	2860
November	1st to 10th	Night	21% 11% 3 8% 4 5% 4 3 4 6% 30%	2667
	11th to 20th	Night	20% 11% 4 8% 5% 4 4 3 4 6% 31%	2732
	21st to end	Night	22% 16% 5% 9% 6% 4 4 2 4 4 23%	3098
December	1st to 10th	Night	17% 15% 6% 11% 8% 6% 5% 3 4 4 21%	5060
	11th to 20th	Night	22% 14% 4 9% 5% 4 4 3 4 5% 24%	5741
	21st to end	Night	20% 18% 5% 10% 4 4 3 3 3 3 27%	7497

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% 2 9% 17% 10%	4662
	11th to 20th	Day	67% 4 8% 14% 6% 1	4680
	21st to end	Day	47% 4 10% 23% 14%	5124
February	1st to 10th	Day	57% 4 10% 17% 10% 2	1680
	11th to 20th	Day	58% 3 11% 16% 10% 1	2041
	21st to end	Day	58% 3 8% 20% 9% 2	1730
March	1st to 10th	Day	62% 3 10% 13% 10% 3	4579
	11th to 20th	Day	55% 4 14% 18% 8% 1	4846
	21st to end	Day	54% 2 8% 17% 15% 4	5956
April	1st to 10th	Day	58% 2 11% 16% 10% 3	2520
	11th to 20th	Day	50% 2 10% 24% 11% 2	2590
	21st to end	Day	54% 4 11% 18% 12% 2	2730
May	1st to 10th	Day	55% 4 10% 18% 12% 1	7863
	11th to 20th	Day	50% 5% 15% 21% 6% 2	8225
	21st to end	Day	53% 6% 11% 18% 12% 2	9116
June	1st to 10th	Day	52% 5% 12% 21% 8% 1	3150
	11th to 20th	Day	48% 5% 14% 21% 11% 1	3150
	21st to end	Day	56% 5% 13% 17% 8% 1	3150
July	1st to 10th	Day	46% 6% 15% 23% 10%	8274
	11th to 20th	Day	42% 7% 18% 26% 7%	8205
	21st to end	Day	41% 8% 21% 23% 7%	8795
August	1st to 10th	Day	46% 7% 19% 19% 7% 1	6452
	11th to 20th	Day	48% 8% 20% 19% 4	5992
	21st to end	Day	43% 6% 17% 23% 10%	6291
September	1st to 10th	Day	44% 4 16% 26% 10%	2520
	11th to 20th	Day	38% 3 18% 31% 10%	2262
	21st to end	Day	47% 3 14% 25% 10%	2200
October	1st to 10th	Day	52% 7% 12% 23% 6%	3567
	11th to 20th	Day	52% 6% 18% 23% 2	2960
	21st to end	Day	53% 7% 17% 16% 7%	3180
November	1st to 10th	Day	56% 4 12% 21% 7%	1851
	11th to 20th	Day	61% 4 9% 19% 7% 1	1885
	21st to end	Day	66% 5% 10% 13% 6% 1	1519
December	1st to 10th	Day	63% 5% 11% 13% 8%	2350
	11th to 20th	Day	61% 3 7% 17% 11%	2688
	21st to end	Day	60% 3 11% 15% 10% 2	3711
January	1st to 10th	Night	66% 3 9% 14% 8%	8234
	11th to 20th	Night	72% 3 6% 15% 4	8280
	21st to end	Night	53% 4 11% 18% 13%	9057
February	1st to 10th	Night	57% 3 8% 21% 10% 1	2940
	11th to 20th	Night	60% 3 9% 16% 11% 1	2473
	21st to end	Night	57% 2 7% 19% 13% 2	2076
March	1st to 10th	Night	63% 3 9% 12% 12% 1	5494
	11th to 20th	Night	60% 5% 12% 14% 8% 2	5333
	21st to end	Night	58% 3 5% 21% 11% 2	5215
April	1st to 10th	Night	58% 3 8% 15% 14% 3	2100
	11th to 20th	Night	51% 4 7% 23% 13% 2	1978
	21st to end	Night	52% 3 8% 20% 14% 2	1890
May	1st to 10th	Night	54% 4 10% 18% 13% 1	5110
	11th to 20th	Night	53% 5% 11% 21% 10% 1	4640
	21st to end	Night	49% 4 11% 17% 16% 3	5059
June	1st to 10th	Night	51% 4 12% 22% 10% 1	1470
	11th to 20th	Night	47% 5% 12% 21% 11% 3	1470
	21st to end	Night	54% 4 14% 17% 11%	1470
July	1st to 10th	Night	46% 5% 18% 20% 11%	4573
	11th to 20th	Night	42% 7% 19% 26% 6%	4632
	21st to end	Night	42% 7% 19% 24% 8%	4967
August	1st to 10th	Night	50% 6% 17% 21% 6%	3656
	11th to 20th	Night	45% 7% 22% 21% 4	4091
	21st to end	Night	48% 6% 13% 22% 10%	4831
September	1st to 10th	Night	44% 4 14% 26% 11% 1	2100
	11th to 20th	Night	41% 3 18% 29% 8% 1	2204
	21st to end	Night	45% 4 11% 26% 13%	2200
October	1st to 10th	Night	48% 5% 12% 26% 8% 1	3612
	11th to 20th	Night	52% 4 15% 26% 4	4179
	21st to end	Night	50% 5% 18% 17% 10%	4610
November	1st to 10th	Night	55% 4 12% 21% 7% 1	2667
	11th to 20th	Night	55% 5% 12% 19% 10% 1	2732
	21st to end	Night	65% 4 9% 15% 7% 1	3098
December	1st to 10th	Night	66% 4 12% 14% 4	5060
	11th to 20th	Night	62% 4 6% 20% 7%	5742
	21st to end	Night	63% 2 8% 16% 10% 1	7497

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	27% 15% 33% 19% 4 1	220
	11th to 20th	Day	28% 21% 31% 13% 5%	220
	21st to end	Day	20% 21% 33% 21% 4 1	242
February	1st to 10th	Day	25% 20% 35% 15% 3 1	220
	11th to 20th	Day	24% 21% 35% 16% 3 1	523
	21st to end	Day	22% 20% 31% 20% 5%	446
March	1st to 10th	Day	27% 19% 31% 16% 6% 1	540
	11th to 20th	Day	27% 16% 33% 17% 6% 1	540
	21st to end	Day	26% 18% 24% 21% 9% 1	594
April	1st to 10th	Day	31% 14% 30% 17% 8%	540
	11th to 20th	Day	29% 13% 28% 20% 8% 1	532
	21st to end	Day	31% 13% 31% 18% 6% 1	540
May	1st to 10th	Day	30% 17% 33% 15% 5% 1	543
	11th to 20th	Day	23% 14% 32% 21% 8% 1	567
	21st to end	Day	27% 18% 31% 17% 6% 1	633
June	1st to 10th	Day	23% 12% 31% 25% 9% 1	780
	11th to 20th	Day	24% 12% 28% 24% 11% 1	780
	21st to end	Day	29% 9% 27% 22% 10% 2	780
July	1st to 10th	Day	19% 14% 31% 23% 11% 2	590
	11th to 20th	Day	20% 13% 32% 25% 9% 2	564
	21st to end	Day	18% 11% 32% 26% 11% 2	620
August	1st to 10th	Day	20% 6% 33% 24% 13% 4	566
	11th to 20th	Day	18% 5% 27% 30% 17% 3	559
	21st to end	Day	23% 10% 24% 23% 13% 7%	627
September	1st to 10th	Day	21% 11% 27% 25% 13% 3	570
	11th to 20th	Day	20% 11% 27% 23% 15% 5%	542
	21st to end	Day	24% 11% 25% 23% 12% 5%	530
October	1st to 10th	Day	23% 12% 29% 23% 11% 3	530
	11th to 20th	Day	33% 9% 27% 20% 10% 1	439
	21st to end	Day	37% 13% 27% 18% 5%	462
November	1st to 10th	Day	34% 13% 24% 21% 7%	425
	11th to 20th	Day	31% 13% 28% 19% 8% 1	425
	21st to end	Day	32% 19% 30% 15% 5%	220
December	1st to 10th	Day	30% 11% 29% 22% 5% 2	220
	11th to 20th	Day	32% 20% 28% 16% 3 1	220
	21st to end	Day	30% 19% 30% 15% 5%	242
January	1st to 10th	Night	26% 15% 30% 24% 5%	840
	11th to 20th	Night	27% 14% 27% 25% 7% 1	840
	21st to end	Night	16% 14% 32% 30% 7% 1	924
February	1st to 10th	Night	18% 13% 33% 28% 7% 1	840
	11th to 20th	Night	23% 15% 35% 22% 4 1	823
	21st to end	Night	25% 11% 31% 28% 5%	692
March	1st to 10th	Night	23% 12% 34% 25% 6%	840
	11th to 20th	Night	26% 14% 31% 22% 6% 1	840
	21st to end	Night	24% 10% 29% 29% 7% 1	924
April	1st to 10th	Night	30% 12% 27% 25% 6%	840
	11th to 20th	Night	27% 8% 34% 25% 6%	734
	21st to end	Night	29% 10% 32% 23% 6%	630
May	1st to 10th	Night	26% 8% 33% 27% 6%	578
	11th to 20th	Night	24% 8% 28% 33% 7%	414
	21st to end	Night	26% 5% 31% 29% 9%	455
June	1st to 10th	Night	25% 5% 28% 36% 6%	420
	11th to 20th	Night	21% 5% 24% 37% 13%	420
	21st to end	Night	25% 4 22% 38% 10% 1	420
July	1st to 10th	Night	19% 4 27% 40% 10% 1	410
	11th to 20th	Night	19% 6% 20% 41% 14%	408
	21st to end	Night	18% 3 17% 45% 15% 1	460
August	1st to 10th	Night	18% 4 15% 46% 17%	418
	11th to 20th	Night	18% 3 21% 41% 15% 1	607
	21st to end	Night	20% 5% 22% 33% 18% 2	824
September	1st to 10th	Night	22% 3 25% 34% 15% 1	840
	11th to 20th	Night	20% 5% 22% 32% 19% 2	813
	21st to end	Night	24% 6% 26% 30% 14% 2	800
October	1st to 10th	Night	24% 7% 26% 33% 9% 1	800
	11th to 20th	Night	29% 7% 25% 28% 10% 1	800
	21st to end	Night	32% 10% 28% 25% 5%	880
November	1st to 10th	Night	30% 12% 24% 26% 8%	820
	11th to 20th	Night	26% 13% 25% 27% 8% 1	840
	21st to end	Night	30% 14% 28% 20% 7% 1	923
December	1st to 10th	Night	30% 11% 27% 23% 8% 1	950
	11th to 20th	Night	27% 17% 27% 22% 6% 1	950
	21st to end	Night	31% 14% 29% 20% 5% 1	1004

Page generated at 2026-09-11 04:11:57. Version ac07

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