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 2025-05-11.

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% 4 8% 5% 4 4 4 5% 6% 30%	113210
	Night	18% 12% 4 9% 5% 5% 4 3 5% 5% 29%	115768
Transparency	Day	53% 4 13% 19% 9% 1	116700
	Night	56% 4 11% 19% 9% 1	120443
Seeing	Day	26% 13% 29% 21% 9% 2	15766
	Night	25% 10% 27% 28% 9% 1	24450

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	16% 20% 7% 13% 6% 4 3 3 3 4 22%	3597
	11th to 20th	Day	9% 16% 8% 18% 8% 5% 4 3 4 4 23%	3570
	21st to end	Day	7% 12% 5% 12% 6% 4 5% 4 5% 6% 34%	3960
February	1st to 10th	Day	10% 16% 5% 12% 8% 6% 4 5% 4 5% 25%	1600
	11th to 20th	Day	13% 16% 6% 12% 5% 4 4 4 5% 5% 25%	1932
	21st to end	Day	13% 14% 5% 11% 7% 5% 5% 3 4 6% 27%	1650
March	1st to 10th	Day	22% 15% 5% 10% 6% 4 4 2 4 4 26%	4476
	11th to 20th	Day	15% 15% 5% 8% 5% 5% 5% 4 5% 6% 29%	4741
	21st to end	Day	18% 14% 4 8% 4 4 4 3 4 4 33%	5827
April	1st to 10th	Day	21% 13% 3 8% 6% 4 3 3 4 5% 29%	2400
	11th to 20th	Day	21% 7% 3 7% 5% 4 4 3 5% 6% 35%	2465
	21st to end	Day	24% 9% 3 7% 6% 4 4 3 4 6% 31%	2600
May	1st to 10th	Day	22% 8% 2 7% 5% 4 4 3 5% 6% 35%	4356
	11th to 20th	Day	18% 7% 2 7% 5% 5% 5% 4 6% 8% 32%	4398
	21st to end	Day	19% 6% 3 7% 5% 5% 6% 4 6% 7% 32%	4864
June	1st to 10th	Day	20% 7% 3 7% 6% 5% 6% 5% 7% 8% 26%	2850
	11th to 20th	Day	19% 7% 3 6% 4 5% 5% 5% 6% 7% 34%	2850
	21st to end	Day	23% 9% 3 8% 6% 5% 5% 4 5% 7% 26%	2850
July	1st to 10th	Day	15% 7% 3 9% 6% 5% 6% 4 6% 8% 30%	4476
	11th to 20th	Day	17% 6% 3 8% 5% 5% 5% 5% 7% 9% 30%	4440
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 7% 8% 39%	4814
August	1st to 10th	Day	16% 5% 2 8% 6% 5% 6% 5% 8% 10% 30%	4406
	11th to 20th	Day	21% 6% 3 7% 5% 5% 5% 5% 7% 9% 28%	4045
	21st to end	Day	21% 6% 2 7% 4 4 4 4 4 7% 37%	4320
September	1st to 10th	Day	17% 7% 3 6% 4 4 4 4 6% 8% 37%	2280
	11th to 20th	Day	18% 5% 2 5% 3 4 3 3 5% 7% 46%	2117
	21st to end	Day	22% 7% 2 6% 3 3 4 3 5% 7% 37%	2090
October	1st to 10th	Day	22% 7% 3 7% 5% 5% 5% 4 5% 6% 33%	2081
	11th to 20th	Day	25% 8% 3 8% 6% 5% 4 4 6% 6% 24%	1743
	21st to end	Day	28% 11% 2 7% 5% 4 4 3 5% 5% 25%	1881
November	1st to 10th	Day	21% 11% 3 7% 6% 4 5% 2 5% 5% 31%	1761
	11th to 20th	Day	22% 14% 4 9% 4 4 4 3 4 5% 27%	1795
	21st to end	Day	23% 17% 5% 9% 5% 4 4 4 4 4 21%	1447
December	1st to 10th	Day	19% 16% 5% 9% 6% 6% 5% 3 5% 6% 21%	2280
	11th to 20th	Day	20% 14% 5% 11% 5% 4 3 3 3 4 28%	2618
	21st to end	Day	20% 17% 4 8% 4 4 4 4 5% 5% 26%	3630
January	1st to 10th	Night	16% 22% 6% 13% 6% 4 3 2 3 3 21%	6349
	11th to 20th	Night	10% 20% 8% 16% 6% 5% 4 3 4 4 19%	6315
	21st to end	Night	7% 12% 4 12% 7% 6% 5% 4 6% 6% 29%	6996
February	1st to 10th	Night	11% 14% 5% 11% 7% 5% 4 4 4 5% 28%	2800
	11th to 20th	Night	13% 18% 5% 11% 6% 5% 4 3 5% 5% 26%	2351
	21st to end	Night	14% 15% 5% 10% 5% 4 3 4 4 6% 29%	1980
March	1st to 10th	Night	22% 13% 4 10% 5% 4 6% 3 4 5% 25%	5364
	11th to 20th	Night	16% 14% 3 10% 6% 6% 6% 4 5% 6% 25%	5217
	21st to end	Night	19% 15% 4 8% 5% 4 4 3 4 5% 31%	5102
April	1st to 10th	Night	21% 14% 4 7% 5% 4 3 3 5% 4 29%	2000
	11th to 20th	Night	20% 9% 3 7% 5% 4 4 4 5% 6% 33%	1883
	21st to end	Night	24% 9% 3 8% 3 3 3 3 3 6% 35%	1800
May	1st to 10th	Night	23% 7% 3 6% 4 5% 5% 4 5% 5% 33%	2835
	11th to 20th	Night	23% 7% 3 7% 5% 4 4 3 5% 6% 34%	2501
	21st to end	Night	20% 5% 2 6% 5% 5% 5% 4 6% 8% 35%	2708
June	1st to 10th	Night	21% 7% 3 7% 6% 4 4 5% 6% 7% 29%	1330
	11th to 20th	Night	21% 5% 3 6% 4 4 4 4 6% 7% 36%	1330
	21st to end	Night	23% 8% 3 8% 5% 4 4 4 5% 7% 29%	1330
July	1st to 10th	Night	15% 7% 3 8% 6% 6% 5% 5% 6% 7% 33%	2479
	11th to 20th	Night	20% 7% 2 7% 5% 4 5% 4 5% 7% 34%	2520
	21st to end	Night	14% 5% 2 7% 4 4 5% 4 5% 7% 43%	2739
August	1st to 10th	Night	18% 7% 3 7% 5% 5% 5% 3 6% 7% 35%	2506
	11th to 20th	Night	19% 7% 3 8% 5% 5% 6% 3 6% 6% 32%	2769
	21st to end	Night	21% 7% 3 7% 5% 4 4 4 5% 6% 34%	3334
September	1st to 10th	Night	19% 6% 2 6% 4 4 3 3 5% 7% 41%	1900
	11th to 20th	Night	19% 5% 2 6% 4 4 3 3 5% 6% 44%	2062
	21st to end	Night	20% 7% 2 6% 3 4 3 3 4 6% 41%	2090
October	1st to 10th	Night	20% 7% 2 7% 5% 4 5% 3 6% 5% 37%	2098
	11th to 20th	Night	24% 9% 3 7% 5% 4 3 4 5% 5% 31%	2437
	21st to end	Night	26% 10% 3 6% 4 4 4 3 5% 5% 30%	2717
November	1st to 10th	Night	20% 11% 3 8% 4 5% 4 3 4 6% 31%	2537
	11th to 20th	Night	20% 11% 3 8% 5% 3 4 3 4 6% 32%	2602
	21st to end	Night	23% 16% 5% 8% 6% 4 4 2 4 4 24%	2950
December	1st to 10th	Night	17% 15% 6% 11% 8% 6% 5% 3 4 4 20%	4910
	11th to 20th	Night	23% 14% 4 9% 5% 4 4 3 5% 5% 24%	5591
	21st to end	Night	21% 18% 4 10% 4 3 3 3 3 3 27%	7336

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	67% 2 8% 14% 8% 1	3598
	11th to 20th	Day	64% 3 9% 17% 6% 1	3600
	21st to end	Day	51% 3 9% 21% 16% 1	3960
February	1st to 10th	Day	59% 4 10% 16% 9% 2	1600
	11th to 20th	Day	60% 3 12% 15% 9% 1	1941
	21st to end	Day	58% 3 9% 19% 9% 2	1650
March	1st to 10th	Day	62% 2 9% 13% 10% 3	4479
	11th to 20th	Day	54% 4 15% 18% 8% 1	4741
	21st to end	Day	54% 2 8% 18% 15% 4	5827
April	1st to 10th	Day	58% 2 11% 15% 11% 3	2400
	11th to 20th	Day	49% 2 10% 24% 11% 2	2465
	21st to end	Day	55% 4 10% 17% 12% 2	2600
May	1st to 10th	Day	50% 3 13% 19% 14% 1	4370
	11th to 20th	Day	50% 3 18% 21% 7% 1	4402
	21st to end	Day	51% 4 10% 21% 13% 1	4864
June	1st to 10th	Day	53% 5% 13% 20% 8% 2	2850
	11th to 20th	Day	48% 5% 15% 20% 11% 1	2850
	21st to end	Day	57% 4 12% 17% 9%	2850
July	1st to 10th	Day	49% 7% 16% 21% 7%	4406
	11th to 20th	Day	47% 7% 21% 21% 4	4365
	21st to end	Day	42% 5% 21% 24% 8%	4661
August	1st to 10th	Day	46% 8% 21% 20% 6%	4392
	11th to 20th	Day	50% 8% 20% 18% 4	4084
	21st to end	Day	46% 5% 16% 19% 13%	4320
September	1st to 10th	Day	45% 3 16% 26% 10%	2280
	11th to 20th	Day	39% 3 18% 30% 10%	2117
	21st to end	Day	48% 3 15% 25% 9%	2090
October	1st to 10th	Day	53% 7% 11% 22% 6%	3458
	11th to 20th	Day	53% 6% 18% 22% 2	2868
	21st to end	Day	53% 7% 17% 16% 7%	3081
November	1st to 10th	Day	56% 4 12% 21% 7%	1761
	11th to 20th	Day	60% 3 9% 20% 7% 1	1795
	21st to end	Day	65% 4 11% 14% 5% 1	1447
December	1st to 10th	Day	64% 5% 11% 13% 8%	2280
	11th to 20th	Day	61% 3 7% 18% 11%	2618
	21st to end	Day	60% 3 11% 14% 10% 2	3630
January	1st to 10th	Night	70% 3 8% 12% 6%	6349
	11th to 20th	Night	69% 3 7% 17% 4 1	6360
	21st to end	Night	54% 3 10% 19% 13% 1	6996
February	1st to 10th	Night	58% 3 8% 19% 10% 1	2800
	11th to 20th	Night	61% 3 10% 15% 10% 1	2353
	21st to end	Night	57% 2 7% 18% 13% 2	1980
March	1st to 10th	Night	63% 3 9% 12% 12% 1	5374
	11th to 20th	Night	59% 5% 12% 14% 8% 2	5218
	21st to end	Night	58% 3 5% 21% 11% 2	5102
April	1st to 10th	Night	58% 2 8% 16% 14% 3	2000
	11th to 20th	Night	51% 4 8% 23% 13% 2	1883
	21st to end	Night	53% 3 8% 19% 14% 2	1800
May	1st to 10th	Night	52% 3 11% 21% 12% 1	2841
	11th to 20th	Night	52% 4 12% 20% 11% 1	2502
	21st to end	Night	46% 4 13% 19% 17% 1	2709
June	1st to 10th	Night	53% 4 13% 20% 9% 1	1330
	11th to 20th	Night	46% 5% 12% 22% 12% 3	1330
	21st to end	Night	56% 4 12% 17% 11%	1330
July	1st to 10th	Night	49% 5% 20% 20% 7%	2439
	11th to 20th	Night	48% 6% 19% 22% 4	2472
	21st to end	Night	39% 6% 23% 22% 9%	2651
August	1st to 10th	Night	48% 5% 16% 23% 7%	2496
	11th to 20th	Night	50% 8% 20% 16% 5%	2796
	21st to end	Night	49% 6% 13% 22% 10%	3334
September	1st to 10th	Night	44% 4 14% 26% 12% 1	1900
	11th to 20th	Night	42% 3 18% 29% 8% 1	2062
	21st to end	Night	46% 4 11% 26% 12%	2090
October	1st to 10th	Night	50% 5% 11% 26% 8%	3501
	11th to 20th	Night	52% 4 14% 25% 4	4051
	21st to end	Night	49% 5% 18% 17% 10%	4467
November	1st to 10th	Night	54% 4 13% 21% 7% 1	2537
	11th to 20th	Night	54% 4 12% 19% 10% 1	2602
	21st to end	Night	64% 3 9% 16% 7% 1	2950
December	1st to 10th	Night	66% 4 12% 14% 4	4910
	11th to 20th	Night	62% 4 6% 20% 7%	5592
	21st to end	Night	63% 2 8% 16% 10% 1	7336

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	29% 15% 32% 20% 4 2	200
	11th to 20th	Day	26% 22% 33% 14% 6% 1	200
	21st to end	Day	20% 20% 33% 21% 5% 1	220
February	1st to 10th	Day	28% 21% 36% 15% 1 1	200
	11th to 20th	Day	24% 21% 35% 15% 3 1	483
	21st to end	Day	23% 21% 29% 21% 6%	414
March	1st to 10th	Day	26% 19% 31% 17% 6% 1	500
	11th to 20th	Day	26% 15% 33% 18% 7% 1	500
	21st to end	Day	26% 17% 24% 23% 9% 1	550
April	1st to 10th	Day	31% 13% 30% 17% 8%	500
	11th to 20th	Day	29% 13% 28% 21% 8% 1	492
	21st to end	Day	32% 13% 31% 17% 5% 1	500
May	1st to 10th	Day	29% 16% 33% 15% 5% 1	503
	11th to 20th	Day	22% 12% 33% 23% 9% 1	487
	21st to end	Day	27% 18% 31% 17% 6% 1	543
June	1st to 10th	Day	24% 12% 30% 24% 9% 1	680
	11th to 20th	Day	23% 11% 28% 25% 12% 1	680
	21st to end	Day	30% 8% 26% 22% 11% 2	680
July	1st to 10th	Day	20% 11% 31% 24% 12% 1	508
	11th to 20th	Day	20% 9% 33% 26% 10% 2	484
	21st to end	Day	19% 9% 32% 28% 11% 2	532
August	1st to 10th	Day	21% 6% 32% 24% 13% 3	486
	11th to 20th	Day	20% 5% 25% 30% 17% 3	479
	21st to end	Day	23% 9% 24% 22% 13% 8%	539
September	1st to 10th	Day	22% 10% 26% 26% 13% 3	490
	11th to 20th	Day	21% 9% 28% 23% 15% 4	490
	21st to end	Day	24% 11% 23% 23% 12% 6%	490
October	1st to 10th	Day	24% 11% 27% 23% 11% 3	490
	11th to 20th	Day	35% 9% 25% 20% 11% 1	407
	21st to end	Day	38% 12% 27% 18% 5%	429
November	1st to 10th	Day	33% 13% 24% 22% 7% 1	395
	11th to 20th	Day	31% 13% 28% 19% 8% 1	395
	21st to end	Day	33% 18% 30% 15% 5%	200
December	1st to 10th	Day	33% 9% 29% 22% 6% 3	200
	11th to 20th	Day	32% 20% 28% 16% 4 2	200
	21st to end	Day	31% 17% 30% 15% 6%	220
January	1st to 10th	Night	27% 16% 30% 23% 5%	800
	11th to 20th	Night	26% 14% 28% 25% 7% 1	800
	21st to end	Night	16% 13% 33% 31% 7% 1	880
February	1st to 10th	Night	19% 13% 34% 27% 6%	800
	11th to 20th	Night	24% 15% 36% 21% 5% 1	783
	21st to end	Night	26% 11% 31% 27% 5%	660
March	1st to 10th	Night	23% 11% 33% 27% 6%	800
	11th to 20th	Night	26% 13% 32% 22% 6% 1	800
	21st to end	Night	25% 9% 29% 29% 7% 1	880
April	1st to 10th	Night	30% 12% 26% 25% 6% 1	800
	11th to 20th	Night	28% 7% 33% 25% 6%	699
	21st to end	Night	30% 10% 32% 22% 6%	600
May	1st to 10th	Night	26% 9% 33% 26% 6%	550
	11th to 20th	Night	24% 9% 27% 34% 7%	376
	21st to end	Night	26% 5% 31% 29% 9%	411
June	1st to 10th	Night	25% 6% 28% 34% 7% 1	380
	11th to 20th	Night	20% 4 25% 37% 13%	380
	21st to end	Night	26% 4 22% 37% 10% 1	380
July	1st to 10th	Night	19% 3 26% 39% 11% 1	370
	11th to 20th	Night	19% 5% 20% 41% 15%	368
	21st to end	Night	18% 3 17% 45% 16% 1	416
August	1st to 10th	Night	19% 4 14% 46% 17%	378
	11th to 20th	Night	19% 3 21% 40% 16% 2	549
	21st to end	Night	20% 4 22% 33% 19% 2	746
September	1st to 10th	Night	22% 3 25% 33% 15% 1	760
	11th to 20th	Night	21% 5% 22% 31% 19% 2	760
	21st to end	Night	24% 6% 25% 29% 14% 2	760
October	1st to 10th	Night	24% 8% 26% 33% 9% 1	760
	11th to 20th	Night	30% 7% 24% 28% 10% 1	760
	21st to end	Night	32% 10% 28% 25% 5%	836
November	1st to 10th	Night	29% 11% 24% 27% 8% 1	780
	11th to 20th	Night	26% 13% 25% 27% 8% 1	800
	21st to end	Night	31% 13% 29% 20% 6% 1	875
December	1st to 10th	Night	31% 10% 27% 22% 8% 1	900
	11th to 20th	Night	28% 17% 26% 23% 6% 1	900
	21st to end	Night	32% 14% 28% 20% 6% 1	953

Page generated at 2025-05-08 09:34:00. Version ac07

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