Observatoire Le Valhalla
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Le Valhalla clear sky chart from 2005-11-06 to 2025-07-21.

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 5% 6% 31%	120314
	Night	18% 11% 4 9% 5% 5% 4 3 5% 6% 30%	118799
Transparency	Day	32% 42% 8% 13% 6%	123802
	Night	34% 41% 7% 11% 6% 1	123472
Seeing	Day	21% 16% 30% 21% 9% 2	15402
	Night	22% 13% 30% 26% 9% 1	24403

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% 19% 6% 13% 6% 5% 4 3 4 4 21%	3597
	11th to 20th	Day	13% 16% 6% 12% 6% 5% 5% 4 5% 4 24%	3570
	21st to end	Day	12% 17% 6% 12% 6% 5% 5% 4 6% 6% 23%	3960
February	1st to 10th	Day	15% 15% 4 12% 6% 5% 5% 3 5% 6% 24%	1800
	11th to 20th	Day	14% 13% 4 11% 7% 6% 5% 3 5% 6% 26%	1827
	21st to end	Day	20% 13% 4 10% 5% 4 5% 3 5% 4 26%	1650
March	1st to 10th	Day	19% 12% 5% 12% 7% 5% 4 3 4 5% 25%	4476
	11th to 20th	Day	12% 13% 4 10% 6% 5% 4 4 6% 6% 31%	4790
	21st to end	Day	19% 9% 3 8% 6% 5% 4 3 4 4 35%	5913
April	1st to 10th	Day	21% 12% 3 8% 5% 4 3 3 4 5% 32%	2400
	11th to 20th	Day	21% 9% 2 7% 4 4 3 3 5% 5% 35%	2374
	21st to end	Day	23% 8% 2 8% 5% 4 5% 3 4 6% 31%	2525
May	1st to 10th	Day	26% 10% 3 9% 5% 5% 5% 4 5% 6% 23%	6344
	11th to 20th	Day	20% 9% 3 9% 5% 4 5% 4 4 7% 31%	6318
	21st to end	Day	21% 8% 3 7% 4 4 5% 4 6% 7% 32%	6949
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 4 5% 8% 30%	2800
	11th to 20th	Day	14% 5% 3 7% 5% 5% 5% 5% 6% 7% 38%	2800
	21st to end	Day	20% 6% 3 6% 5% 5% 6% 4 7% 7% 31%	2800
July	1st to 10th	Day	12% 6% 3 7% 6% 5% 6% 5% 6% 9% 36%	4580
	11th to 20th	Day	14% 5% 2 6% 5% 5% 5% 5% 7% 8% 38%	4580
	21st to end	Day	12% 6% 2 5% 4 4 5% 4 7% 9% 44%	4814
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 5% 4 6% 10% 34%	4423
	11th to 20th	Day	14% 6% 3 7% 4 5% 5% 4 6% 9% 39%	3962
	21st to end	Day	16% 4 2 6% 4 4 6% 4 6% 9% 39%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 7% 8% 43%	2280
	11th to 20th	Day	12% 5% 2 5% 3 4 5% 3 4 7% 50%	2227
	21st to end	Day	17% 6% 2 6% 3 4 5% 3 5% 7% 42%	2090
October	1st to 10th	Day	19% 8% 3 6% 5% 4 4 4 5% 6% 37%	1921
	11th to 20th	Day	19% 10% 3 8% 6% 5% 5% 4 5% 5% 31%	1891
	21st to end	Day	24% 10% 3 8% 4 4 5% 4 5% 6% 28%	1881
November	1st to 10th	Day	17% 9% 3 7% 4 5% 5% 4 5% 7% 33%	1761
	11th to 20th	Day	16% 11% 3 8% 5% 5% 6% 3 5% 6% 30%	1685
	21st to end	Day	21% 13% 4 8% 5% 6% 5% 5% 6% 6% 22%	1600
December	1st to 10th	Day	20% 20% 5% 10% 5% 4 4 4 6% 7% 16%	2600
	11th to 20th	Day	19% 15% 5% 10% 5% 5% 5% 3 5% 5% 25%	2986
	21st to end	Day	24% 15% 4 9% 5% 4 5% 3 4 4 23%	3960
January	1st to 10th	Night	14% 22% 8% 12% 6% 4 4 3 4 5% 20%	6349
	11th to 20th	Night	11% 16% 6% 17% 8% 6% 5% 4 5% 5% 18%	6315
	21st to end	Night	14% 16% 5% 11% 7% 6% 4 3 4 5% 24%	6982
February	1st to 10th	Night	14% 13% 5% 11% 7% 5% 6% 4 5% 6% 23%	2600
	11th to 20th	Night	14% 12% 4 10% 6% 5% 5% 4 6% 5% 27%	2458
	21st to end	Night	15% 15% 4 10% 5% 4 4 4 4 4 30%	1980
March	1st to 10th	Night	21% 13% 4 8% 5% 4 4 3 4 5% 30%	5364
	11th to 20th	Night	15% 13% 4 8% 5% 5% 5% 4 6% 7% 29%	5168
	21st to end	Night	17% 10% 3 9% 6% 5% 5% 4 4 5% 32%	5018
April	1st to 10th	Night	23% 10% 3 6% 5% 5% 4 3 4 7% 31%	2000
	11th to 20th	Night	22% 8% 3 7% 5% 3 4 3 5% 5% 34%	1975
	21st to end	Night	21% 8% 3 6% 5% 4 5% 3 5% 6% 34%	1875
May	1st to 10th	Night	25% 9% 3 8% 6% 5% 5% 3 5% 6% 24%	3626
	11th to 20th	Night	19% 9% 4 8% 4 4 4 3 5% 7% 34%	3575
	21st to end	Night	18% 9% 4 8% 5% 5% 4 3 5% 7% 31%	3926
June	1st to 10th	Night	19% 8% 2 6% 6% 5% 5% 4 7% 9% 30%	1600
	11th to 20th	Night	16% 7% 2 5% 4 5% 4 4 6% 7% 40%	1600
	21st to end	Night	18% 7% 2 7% 4 4 5% 4 6% 7% 35%	1600
July	1st to 10th	Night	13% 6% 3 8% 6% 5% 6% 4 7% 7% 36%	2600
	11th to 20th	Night	14% 5% 2 6% 4 5% 4 4 6% 8% 42%	2600
	21st to end	Night	12% 7% 2 5% 4 4 4 4 6% 7% 44%	2739
August	1st to 10th	Night	15% 7% 3 7% 4 4 4 4 6% 7% 40%	2506
	11th to 20th	Night	16% 6% 2 6% 4 5% 4 4 5% 7% 41%	2833
	21st to end	Night	14% 5% 3 6% 5% 4 5% 4 6% 7% 41%	3476
September	1st to 10th	Night	11% 6% 2 7% 5% 5% 5% 4 5% 7% 44%	1900
	11th to 20th	Night	14% 7% 2 4 4 3 3 3 4 7% 50%	1952
	21st to end	Night	17% 5% 2 5% 5% 4 3 4 6% 6% 44%	2090
October	1st to 10th	Night	19% 6% 2 7% 4 4 4 4 5% 6% 38%	2256
	11th to 20th	Night	19% 8% 3 8% 5% 4 5% 4 5% 6% 34%	2288
	21st to end	Night	22% 10% 3 7% 5% 5% 5% 4 4 5% 30%	2717
November	1st to 10th	Night	19% 9% 3 8% 4 4 4 4 5% 7% 33%	2537
	11th to 20th	Night	18% 9% 4 8% 5% 5% 5% 4 5% 6% 32%	2710
	21st to end	Night	21% 12% 4 9% 5% 5% 4 3 5% 6% 26%	2800
December	1st to 10th	Night	18% 16% 6% 12% 7% 5% 5% 4 5% 4 18%	4580
	11th to 20th	Night	23% 15% 5% 9% 5% 4 4 3 4 4 25%	5208
	21st to end	Night	26% 16% 4 9% 4 3 3 3 4 6% 22%	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	49% 31% 6% 11% 2	3598
	11th to 20th	Day	44% 31% 7% 12% 5% 1	3600
	21st to end	Day	45% 32% 8% 8% 6% 1	3960
February	1st to 10th	Day	24% 62% 3 7% 2 1	1800
	11th to 20th	Day	23% 61% 4 8% 5%	1836
	21st to end	Day	22% 63% 3 5% 5% 1	1650
March	1st to 10th	Day	44% 32% 5% 9% 8% 2	4479
	11th to 20th	Day	36% 32% 13% 10% 8% 1	4790
	21st to end	Day	39% 30% 4 12% 14% 1	5913
April	1st to 10th	Day	21% 63% 3 7% 6% 1	2400
	11th to 20th	Day	20% 62% 2 9% 6% 1	2374
	21st to end	Day	21% 62% 4 7% 6%	2525
May	1st to 10th	Day	47% 33% 6% 12% 3	6355
	11th to 20th	Day	40% 33% 10% 12% 5%	6322
	21st to end	Day	38% 32% 8% 14% 7%	6949
June	1st to 10th	Day	20% 63% 3 8% 6% 1	2800
	11th to 20th	Day	16% 63% 5% 9% 6% 1	2800
	21st to end	Day	19% 61% 5% 10% 5% 1	2800
July	1st to 10th	Day	30% 38% 9% 16% 6%	4510
	11th to 20th	Day	26% 39% 10% 22% 4	4505
	21st to end	Day	21% 38% 10% 25% 6%	4661
August	1st to 10th	Day	29% 40% 11% 14% 6%	4409
	11th to 20th	Day	29% 40% 10% 18% 2	4003
	21st to end	Day	25% 40% 12% 14% 8%	4180
September	1st to 10th	Day	14% 62% 7% 12% 6%	2280
	11th to 20th	Day	13% 61% 6% 13% 7% 1	2227
	21st to end	Day	14% 61% 6% 12% 7% 1	2090
October	1st to 10th	Day	28% 39% 10% 20% 5%	3194
	11th to 20th	Day	23% 39% 13% 21% 4	3119
	21st to end	Day	25% 39% 15% 15% 4	3081
November	1st to 10th	Day	25% 57% 6% 8% 3 1	1761
	11th to 20th	Day	20% 59% 5% 11% 5% 1	1685
	21st to end	Day	26% 61% 6% 5% 2	1600
December	1st to 10th	Day	45% 38% 7% 9% 1	2600
	11th to 20th	Day	43% 34% 6% 10% 6%	2986
	21st to end	Day	47% 29% 6% 13% 5%	3960
January	1st to 10th	Night	50% 30% 8% 9% 3	6349
	11th to 20th	Night	49% 31% 9% 9% 3	6360
	21st to end	Night	46% 30% 7% 7% 8%	6982
February	1st to 10th	Night	24% 62% 4 7% 2	2600
	11th to 20th	Night	22% 62% 4 9% 4	2460
	21st to end	Night	20% 62% 3 8% 6%	1980
March	1st to 10th	Night	41% 30% 5% 10% 9% 4	5374
	11th to 20th	Night	37% 32% 11% 11% 7% 3	5169
	21st to end	Night	39% 31% 5% 15% 8% 1	5018
April	1st to 10th	Night	22% 62% 3 8% 6%	2000
	11th to 20th	Night	22% 63% 2 7% 6%	1975
	21st to end	Night	21% 62% 4 6% 6% 1	1875
May	1st to 10th	Night	44% 33% 8% 12% 3	3632
	11th to 20th	Night	38% 33% 8% 14% 7%	3576
	21st to end	Night	41% 32% 7% 12% 9%	3927
June	1st to 10th	Night	19% 62% 4 8% 6% 1	1600
	11th to 20th	Night	16% 62% 5% 9% 8% 1	1600
	21st to end	Night	19% 61% 4 8% 6% 1	1600
July	1st to 10th	Night	32% 38% 10% 11% 8%	2560
	11th to 20th	Night	26% 38% 11% 22% 3	2552
	21st to end	Night	24% 38% 12% 17% 10%	2651
August	1st to 10th	Night	29% 40% 9% 18% 5%	2496
	11th to 20th	Night	31% 39% 12% 15% 2	2858
	21st to end	Night	26% 42% 9% 14% 9% 1	3476
September	1st to 10th	Night	14% 60% 6% 12% 8%	1900
	11th to 20th	Night	13% 61% 6% 15% 5% 1	1952
	21st to end	Night	14% 60% 6% 13% 7%	2090
October	1st to 10th	Night	24% 40% 11% 19% 5%	3763
	11th to 20th	Night	24% 39% 10% 22% 5%	3799
	21st to end	Night	27% 39% 13% 13% 8%	4467
November	1st to 10th	Night	25% 57% 7% 7% 3	2537
	11th to 20th	Night	23% 58% 5% 9% 5%	2710
	21st to end	Night	26% 58% 6% 7% 2	2800
December	1st to 10th	Night	44% 38% 9% 7% 2	4580
	11th to 20th	Night	46% 33% 3 10% 8%	5208
	21st to end	Night	48% 27% 5% 11% 9%	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	26% 20% 29% 22% 4 1	200
	11th to 20th	Day	26% 24% 34% 12% 5% 1	200
	21st to end	Day	19% 19% 38% 18% 6%	220
February	1st to 10th	Day	30% 27% 28% 13% 2	300
	11th to 20th	Day	25% 22% 39% 13% 1	374
	21st to end	Day	29% 20% 30% 19% 2	414
March	1st to 10th	Day	26% 19% 32% 19% 4	500
	11th to 20th	Day	21% 23% 34% 17% 4 1	500
	21st to end	Day	27% 20% 29% 15% 8% 1	550
April	1st to 10th	Day	26% 22% 29% 15% 6% 1	500
	11th to 20th	Day	27% 22% 26% 16% 8% 2	492
	21st to end	Day	28% 18% 31% 14% 7% 1	500
May	1st to 10th	Day	22% 17% 34% 20% 6% 1	503
	11th to 20th	Day	18% 16% 33% 21% 10% 2	527
	21st to end	Day	20% 16% 30% 23% 9% 2	573
June	1st to 10th	Day	21% 16% 29% 25% 8% 2	530
	11th to 20th	Day	15% 13% 33% 24% 13% 1	530
	21st to end	Day	21% 14% 32% 22% 9% 3	530
July	1st to 10th	Day	11% 12% 31% 28% 14% 3	525
	11th to 20th	Day	15% 10% 31% 29% 13% 2	524
	21st to end	Day	15% 8% 30% 30% 14% 3	532
August	1st to 10th	Day	15% 8% 26% 32% 14% 4	486
	11th to 20th	Day	13% 7% 26% 30% 16% 7%	479
	21st to end	Day	15% 9% 28% 25% 17% 6%	539
September	1st to 10th	Day	14% 9% 32% 23% 16% 6%	490
	11th to 20th	Day	12% 10% 23% 23% 19% 13%	490
	21st to end	Day	17% 12% 29% 20% 17% 4	490
October	1st to 10th	Day	21% 13% 27% 21% 14% 3	490
	11th to 20th	Day	22% 21% 27% 20% 9% 1	485
	21st to end	Day	29% 15% 28% 21% 6%	429
November	1st to 10th	Day	28% 17% 25% 22% 7% 1	395
	11th to 20th	Day	25% 21% 33% 17% 4	285
	21st to end	Day	29% 18% 33% 16% 5%	200
December	1st to 10th	Day	29% 18% 27% 20% 5% 1	200
	11th to 20th	Day	30% 21% 33% 14% 2 1	200
	21st to end	Day	32% 20% 27% 17% 4 1	220
January	1st to 10th	Night	25% 18% 34% 19% 3	800
	11th to 20th	Night	27% 18% 29% 21% 5% 1	800
	21st to end	Night	19% 15% 36% 25% 5%	880
February	1st to 10th	Night	20% 17% 36% 24% 3	800
	11th to 20th	Night	21% 20% 35% 21% 4	783
	21st to end	Night	23% 16% 32% 26% 3	660
March	1st to 10th	Night	25% 16% 32% 23% 4	800
	11th to 20th	Night	25% 17% 32% 22% 4 1	800
	21st to end	Night	23% 15% 30% 25% 7%	880
April	1st to 10th	Night	27% 15% 29% 24% 5%	800
	11th to 20th	Night	25% 15% 31% 23% 5% 1	791
	21st to end	Night	27% 12% 30% 23% 8%	675
May	1st to 10th	Night	24% 11% 31% 24% 7% 2	419
	11th to 20th	Night	21% 9% 32% 28% 9% 1	394
	21st to end	Night	18% 7% 36% 29% 10%	433
June	1st to 10th	Night	22% 9% 29% 31% 9%	400
	11th to 20th	Night	18% 9% 28% 34% 11% 1	400
	21st to end	Night	20% 8% 28% 30% 14% 1	400
July	1st to 10th	Night	12% 6% 28% 37% 16% 1	390
	11th to 20th	Night	13% 5% 25% 40% 16%	388
	21st to end	Night	13% 4 27% 39% 15% 1	416
August	1st to 10th	Night	15% 4 21% 43% 17% 1	378
	11th to 20th	Night	14% 4 23% 32% 24% 2	584
	21st to end	Night	13% 7% 28% 33% 18% 2	836
September	1st to 10th	Night	13% 6% 28% 36% 15% 1	760
	11th to 20th	Night	18% 6% 19% 32% 22% 3	760
	21st to end	Night	18% 9% 26% 27% 18% 2	760
October	1st to 10th	Night	21% 11% 28% 27% 13%	760
	11th to 20th	Night	23% 13% 29% 26% 9%	760
	21st to end	Night	27% 14% 30% 25% 4	836
November	1st to 10th	Night	25% 16% 27% 25% 7%	780
	11th to 20th	Night	22% 19% 30% 21% 8%	800
	21st to end	Night	28% 18% 28% 19% 7%	800
December	1st to 10th	Night	29% 17% 31% 20% 4	800
	11th to 20th	Night	30% 20% 28% 17% 4 1	800
	21st to end	Night	35% 15% 29% 15% 5%	880

Page generated at 2025-07-18 01:10:00. Version ac07

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