Observatoire de La Sitelle
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire de La Sitelle clear sky chart from 2005-11-06 to 2025-11-18.

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	16% 11% 4 9% 5% 5% 5% 4 5% 6% 29%	127178
	Night	15% 12% 5% 10% 6% 5% 4 3 5% 6% 30%	123177
Transparency	Day	52% 7% 12% 19% 9% 1	130610
	Night	51% 8% 12% 18% 10% 1	127832
Seeing	Day	24% 19% 32% 18% 6% 2	15801
	Night	21% 16% 31% 23% 8% 1	24752

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	11% 21% 8% 14% 6% 5% 5% 3 5% 4 18%	3597
	11th to 20th	Day	10% 16% 7% 19% 10% 6% 4 3 3 4 19%	3570
	21st to end	Day	7% 13% 8% 17% 6% 4 6% 4 5% 5% 24%	3960
February	1st to 10th	Day	10% 18% 6% 12% 9% 5% 4 3 4 6% 23%	1765
	11th to 20th	Day	11% 17% 6% 12% 7% 6% 5% 3 5% 5% 24%	1822
	21st to end	Day	14% 19% 6% 10% 7% 5% 4 4 4 5% 24%	1650
March	1st to 10th	Day	16% 18% 6% 13% 8% 5% 4 2 3 4 22%	4476
	11th to 20th	Day	10% 16% 5% 11% 6% 4 5% 4 4 7% 28%	4750
	21st to end	Day	16% 14% 4 9% 5% 4 4 3 4 5% 32%	5921
April	1st to 10th	Day	18% 15% 4 9% 5% 4 3 3 4 6% 29%	2400
	11th to 20th	Day	21% 10% 3 8% 5% 3 4 3 4 5% 33%	2374
	21st to end	Day	21% 11% 4 9% 4 4 4 4 4 5% 30%	2560
May	1st to 10th	Day	25% 11% 3 9% 5% 5% 5% 3 5% 6% 23%	6346
	11th to 20th	Day	20% 9% 3 7% 5% 5% 5% 4 6% 7% 28%	6318
	21st to end	Day	22% 10% 3 7% 5% 5% 5% 4 5% 6% 28%	6949
June	1st to 10th	Day	22% 9% 3 8% 6% 5% 5% 4 6% 7% 27%	2800
	11th to 20th	Day	15% 8% 3 7% 6% 6% 5% 5% 6% 7% 32%	2800
	21st to end	Day	22% 9% 3 7% 6% 5% 5% 4 6% 6% 26%	2800
July	1st to 10th	Day	15% 7% 3 9% 6% 6% 6% 4 7% 8% 28%	6360
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 5% 7% 8% 38%	6360
	21st to end	Day	10% 5% 2 6% 4 5% 5% 4 7% 10% 42%	6926
August	1st to 10th	Day	14% 6% 2 8% 6% 6% 6% 5% 7% 8% 31%	4563
	11th to 20th	Day	15% 7% 3 7% 4 5% 6% 5% 6% 8% 34%	4156
	21st to end	Day	15% 6% 2 7% 5% 5% 5% 4 6% 8% 36%	4312
September	1st to 10th	Day	15% 6% 3 6% 4 4 4 4 6% 7% 40%	2400
	11th to 20th	Day	14% 5% 3 5% 4 5% 4 4 6% 6% 45%	2337
	21st to end	Day	19% 8% 3 6% 4 4 3 4 5% 6% 39%	2200
October	1st to 10th	Day	21% 9% 3 6% 4 4 4 3 6% 7% 34%	2030
	11th to 20th	Day	22% 11% 4 8% 5% 5% 4 4 5% 5% 27%	1970
	21st to end	Day	24% 11% 4 9% 5% 4 4 3 4 5% 25%	1980
November	1st to 10th	Day	15% 14% 5% 8% 6% 5% 4 3 5% 6% 30%	1851
	11th to 20th	Day	18% 15% 5% 10% 6% 5% 3 4 5% 5% 23%	1729
	21st to end	Day	19% 17% 5% 12% 5% 4 5% 4 5% 4 20%	1600
December	1st to 10th	Day	17% 17% 10% 15% 5% 4 4 3 4 4 15%	2600
	11th to 20th	Day	15% 18% 7% 12% 6% 4 4 3 4 4 22%	2986
	21st to end	Day	18% 16% 7% 10% 5% 4 3 3 4 5% 25%	3960
January	1st to 10th	Night	10% 22% 9% 15% 6% 4 4 3 5% 4 18%	6349
	11th to 20th	Night	7% 13% 8% 19% 10% 7% 4 3 4 4 19%	6315
	21st to end	Night	7% 12% 7% 18% 8% 7% 5% 3 5% 6% 22%	6996
February	1st to 10th	Night	10% 15% 5% 12% 8% 5% 5% 4 5% 5% 25%	2633
	11th to 20th	Night	10% 14% 6% 13% 7% 5% 5% 3 6% 6% 26%	2463
	21st to end	Night	11% 16% 6% 11% 5% 5% 5% 4 6% 6% 27%	1980
March	1st to 10th	Night	18% 15% 5% 12% 6% 4 4 3 4 5% 26%	5364
	11th to 20th	Night	10% 14% 5% 10% 5% 5% 5% 4 6% 8% 28%	5194
	21st to end	Night	14% 12% 5% 9% 6% 5% 5% 3 5% 6% 30%	5010
April	1st to 10th	Night	20% 12% 3 8% 5% 4 3 3 5% 7% 30%	2000
	11th to 20th	Night	17% 8% 4 7% 5% 4 4 4 6% 5% 36%	1975
	21st to end	Night	19% 10% 3 8% 4 4 4 3 5% 5% 34%	1850
May	1st to 10th	Night	21% 10% 4 9% 5% 5% 5% 4 5% 6% 26%	3601
	11th to 20th	Night	20% 8% 3 7% 5% 4 4 3 5% 6% 34%	3575
	21st to end	Night	21% 10% 3 8% 5% 5% 5% 4 6% 6% 28%	3926
June	1st to 10th	Night	18% 9% 3 7% 5% 5% 4 4 7% 8% 32%	1600
	11th to 20th	Night	18% 7% 3 6% 5% 4 5% 4 5% 8% 37%	1600
	21st to end	Night	20% 8% 2 6% 5% 4 5% 4 6% 6% 35%	1600
July	1st to 10th	Night	15% 6% 3 7% 6% 5% 5% 5% 6% 8% 33%	3600
	11th to 20th	Night	13% 7% 2 7% 4 4 5% 4 6% 7% 42%	3600
	21st to end	Night	9% 5% 2 6% 4 5% 5% 4 6% 8% 48%	3926
August	1st to 10th	Night	11% 6% 3 8% 5% 4 5% 4 6% 8% 40%	2586
	11th to 20th	Night	15% 7% 3 7% 4 3 4 4 5% 6% 42%	2855
	21st to end	Night	17% 6% 2 6% 4 4 5% 4 5% 6% 41%	3586
September	1st to 10th	Night	12% 7% 2 5% 3 4 4 4 5% 7% 47%	2000
	11th to 20th	Night	14% 4 2 5% 3 3 3 3 5% 8% 49%	2062
	21st to end	Night	18% 6% 2 5% 4 3 3 4 5% 7% 43%	2200
October	1st to 10th	Night	16% 8% 3 6% 5% 4 4 3 5% 6% 39%	2368
	11th to 20th	Night	19% 10% 3 7% 5% 4 5% 4 4 6% 33%	2430
	21st to end	Night	22% 10% 3 9% 5% 4 4 3 5% 7% 27%	2860
November	1st to 10th	Night	17% 11% 5% 9% 5% 5% 5% 4 6% 6% 28%	2667
	11th to 20th	Night	16% 12% 5% 10% 6% 5% 5% 4 5% 7% 26%	2822
	21st to end	Night	17% 18% 5% 11% 6% 4 4 3 4 5% 23%	2800
December	1st to 10th	Night	15% 14% 6% 13% 7% 6% 6% 4 5% 6% 17%	4580
	11th to 20th	Night	21% 14% 5% 12% 7% 5% 4 3 3 3 24%	5208
	21st to end	Night	19% 20% 5% 9% 5% 3 3 2 3 4 26%	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	67% 6% 8% 14% 4	3598
	11th to 20th	Day	64% 9% 4 13% 8% 2	3600
	21st to end	Day	59% 7% 7% 13% 12% 2	3960
February	1st to 10th	Day	61% 5% 8% 16% 8% 2	1765
	11th to 20th	Day	59% 6% 10% 14% 10% 1	1831
	21st to end	Day	61% 5% 8% 14% 9% 3	1650
March	1st to 10th	Day	64% 6% 5% 10% 11% 3	4479
	11th to 20th	Day	52% 8% 15% 12% 11% 3	4750
	21st to end	Day	52% 6% 8% 15% 17% 2	5921
April	1st to 10th	Day	55% 6% 9% 16% 12% 3	2400
	11th to 20th	Day	51% 7% 9% 20% 12% 2	2374
	21st to end	Day	55% 7% 8% 16% 12% 3	2560
May	1st to 10th	Day	60% 7% 8% 16% 8% 1	6357
	11th to 20th	Day	51% 9% 16% 15% 8% 2	6322
	21st to end	Day	53% 8% 10% 15% 12% 1	6949
June	1st to 10th	Day	54% 8% 11% 16% 10% 1	2800
	11th to 20th	Day	47% 8% 13% 19% 10% 3	2800
	21st to end	Day	56% 8% 11% 16% 9% 1	2800
July	1st to 10th	Day	50% 7% 13% 22% 7%	6290
	11th to 20th	Day	39% 7% 16% 30% 7%	6285
	21st to end	Day	35% 7% 17% 31% 9%	6728
August	1st to 10th	Day	45% 9% 18% 21% 6%	4549
	11th to 20th	Day	44% 9% 16% 25% 6%	4198
	21st to end	Day	44% 7% 15% 21% 12% 1	4312
September	1st to 10th	Day	40% 8% 14% 26% 12%	2400
	11th to 20th	Day	36% 7% 15% 29% 12% 1	2337
	21st to end	Day	45% 8% 12% 24% 12%	2200
October	1st to 10th	Day	45% 7% 13% 27% 9%	3303
	11th to 20th	Day	47% 8% 13% 24% 8% 1	3186
	21st to end	Day	48% 6% 21% 16% 9% 1	3180
November	1st to 10th	Day	52% 8% 13% 18% 9% 1	1851
	11th to 20th	Day	57% 9% 11% 13% 10% 1	1729
	21st to end	Day	62% 10% 11% 13% 4 1	1600
December	1st to 10th	Day	69% 6% 9% 12% 4	2600
	11th to 20th	Day	61% 6% 8% 15% 9%	2986
	21st to end	Day	61% 6% 8% 14% 10%	3960
January	1st to 10th	Night	65% 8% 10% 13% 4	6349
	11th to 20th	Night	61% 10% 10% 12% 5% 1	6360
	21st to end	Night	59% 8% 8% 10% 14% 1	6996
February	1st to 10th	Night	57% 6% 10% 17% 8% 1	2633
	11th to 20th	Night	54% 8% 11% 18% 8% 1	2465
	21st to end	Night	54% 7% 10% 17% 11% 2	1980
March	1st to 10th	Night	60% 6% 7% 11% 12% 3	5374
	11th to 20th	Night	49% 9% 13% 15% 11% 3	5195
	21st to end	Night	51% 6% 9% 20% 11% 3	5010
April	1st to 10th	Night	54% 5% 9% 17% 13% 2	2000
	11th to 20th	Night	46% 7% 11% 20% 14% 2	1975
	21st to end	Night	49% 8% 10% 19% 13% 2	1850
May	1st to 10th	Night	55% 9% 11% 17% 7% 2	3607
	11th to 20th	Night	47% 9% 16% 16% 10% 2	3576
	21st to end	Night	53% 8% 11% 16% 11% 1	3927
June	1st to 10th	Night	47% 9% 13% 20% 12%	1600
	11th to 20th	Night	45% 8% 11% 22% 13% 2	1600
	21st to end	Night	48% 8% 12% 19% 12% 1	1600
July	1st to 10th	Night	45% 7% 20% 20% 7%	3560
	11th to 20th	Night	38% 8% 19% 30% 6%	3552
	21st to end	Night	33% 7% 18% 31% 11%	3809
August	1st to 10th	Night	40% 8% 17% 27% 8%	2576
	11th to 20th	Night	41% 7% 19% 25% 8%	2879
	21st to end	Night	42% 7% 16% 23% 11% 2	3586
September	1st to 10th	Night	34% 9% 16% 27% 14% 1	2000
	11th to 20th	Night	33% 7% 17% 32% 11% 1	2062
	21st to end	Night	39% 8% 13% 28% 13%	2200
October	1st to 10th	Night	43% 6% 13% 25% 12% 1	3875
	11th to 20th	Night	43% 7% 12% 27% 10%	3953
	21st to end	Night	46% 7% 18% 17% 11% 2	4610
November	1st to 10th	Night	52% 8% 14% 19% 6% 1	2667
	11th to 20th	Night	52% 9% 12% 17% 9% 2	2822
	21st to end	Night	58% 9% 12% 15% 6% 1	2800
December	1st to 10th	Night	62% 7% 13% 14% 4	4580
	11th to 20th	Night	62% 6% 6% 14% 10% 1	5208
	21st to end	Night	61% 6% 6% 12% 14% 1	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	17% 27% 40% 15% 2	200
	11th to 20th	Day	27% 22% 34% 13% 5% 1	200
	21st to end	Day	19% 23% 40% 13% 4 1	220
February	1st to 10th	Day	32% 32% 29% 6% 1	283
	11th to 20th	Day	25% 28% 38% 8% 1	369
	21st to end	Day	28% 25% 31% 14% 2	414
March	1st to 10th	Day	27% 22% 33% 14% 3	500
	11th to 20th	Day	25% 26% 33% 13% 3	500
	21st to end	Day	31% 24% 26% 14% 5% 1	550
April	1st to 10th	Day	32% 20% 31% 12% 4 1	500
	11th to 20th	Day	35% 21% 24% 12% 7% 1	492
	21st to end	Day	30% 20% 34% 11% 3 1	500
May	1st to 10th	Day	28% 22% 32% 14% 3 1	503
	11th to 20th	Day	25% 20% 31% 18% 6% 1	527
	21st to end	Day	26% 20% 30% 19% 5%	573
June	1st to 10th	Day	25% 17% 36% 15% 6% 1	530
	11th to 20th	Day	18% 21% 34% 16% 9% 1	530
	21st to end	Day	25% 17% 33% 18% 6% 1	530
July	1st to 10th	Day	15% 13% 39% 23% 8% 2	525
	11th to 20th	Day	16% 14% 38% 27% 6% 1	524
	21st to end	Day	14% 13% 37% 26% 9% 1	576
August	1st to 10th	Day	16% 10% 30% 32% 10% 2	526
	11th to 20th	Day	14% 11% 30% 27% 13% 5%	519
	21st to end	Day	17% 17% 29% 21% 11% 5%	583
September	1st to 10th	Day	18% 13% 34% 20% 10% 5%	530
	11th to 20th	Day	12% 14% 27% 25% 16% 7%	530
	21st to end	Day	22% 15% 25% 21% 12% 4	530
October	1st to 10th	Day	25% 17% 27% 19% 10% 2	530
	11th to 20th	Day	26% 22% 28% 17% 6% 1	513
	21st to end	Day	31% 17% 32% 16% 3	462
November	1st to 10th	Day	31% 20% 28% 16% 3 1	425
	11th to 20th	Day	26% 24% 31% 13% 5%	287
	21st to end	Day	30% 28% 30% 11% 2	200
December	1st to 10th	Day	30% 21% 29% 16% 6%	200
	11th to 20th	Day	26% 29% 35% 9% 2	200
	21st to end	Day	33% 24% 26% 11% 5% 1	220
January	1st to 10th	Night	23% 21% 36% 17% 4	800
	11th to 20th	Night	21% 23% 31% 21% 4	800
	21st to end	Night	15% 16% 41% 23% 5% 1	880
February	1st to 10th	Night	18% 23% 37% 19% 4	800
	11th to 20th	Night	21% 21% 40% 15% 3	783
	21st to end	Night	21% 19% 36% 22% 2	660
March	1st to 10th	Night	25% 19% 34% 19% 3	800
	11th to 20th	Night	22% 21% 34% 18% 4 1	800
	21st to end	Night	22% 18% 32% 22% 5% 1	880
April	1st to 10th	Night	27% 19% 30% 20% 4	800
	11th to 20th	Night	24% 18% 30% 22% 5% 1	791
	21st to end	Night	26% 16% 32% 19% 7%	650
May	1st to 10th	Night	23% 14% 35% 22% 5% 1	407
	11th to 20th	Night	21% 15% 28% 29% 7% 1	394
	21st to end	Night	20% 12% 39% 22% 7%	433
June	1st to 10th	Night	22% 8% 37% 25% 7%	400
	11th to 20th	Night	21% 9% 27% 32% 11% 1	400
	21st to end	Night	21% 8% 28% 31% 13% 1	400
July	1st to 10th	Night	14% 8% 31% 35% 12% 1	390
	11th to 20th	Night	12% 7% 26% 41% 13% 1	388
	21st to end	Night	12% 7% 27% 41% 12% 1	438
August	1st to 10th	Night	13% 5% 22% 44% 16% 1	398
	11th to 20th	Night	12% 6% 25% 33% 22% 1	571
	21st to end	Night	16% 9% 29% 29% 15% 2	880
September	1st to 10th	Night	14% 7% 32% 33% 12% 1	800
	11th to 20th	Night	14% 10% 25% 31% 19% 2	800
	21st to end	Night	20% 11% 26% 26% 15% 2	800
October	1st to 10th	Night	20% 14% 28% 25% 12% 1	800
	11th to 20th	Night	25% 15% 30% 22% 8% 1	800
	21st to end	Night	27% 14% 32% 22% 5%	880
November	1st to 10th	Night	24% 19% 30% 21% 5% 1	820
	11th to 20th	Night	24% 18% 31% 18% 8% 1	829
	21st to end	Night	27% 24% 27% 17% 4 1	800
December	1st to 10th	Night	26% 19% 31% 17% 5% 1	800
	11th to 20th	Night	27% 21% 29% 18% 4 1	800
	21st to end	Night	30% 20% 30% 16% 4	880

Page generated at 2025-11-15 00:55:55. Version ac07

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