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 2024-11-04.

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	17% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	101545
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 32%	97480
Transparency	Day	26% 48% 7% 12% 5%	105006
	Night	28% 48% 7% 11% 5%	102108
Seeing	Day	21% 16% 30% 22% 10% 2	14549
	Night	22% 13% 29% 26% 9% 1	23537

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	17% 16% 5% 12% 6% 5% 4 3 4 5% 23%	2517
	11th to 20th	Day	15% 14% 6% 10% 7% 5% 6% 4 5% 5% 24%	2520
	21st to end	Day	15% 17% 5% 11% 5% 5% 5% 4 6% 6% 20%	2772
February	1st to 10th	Day	16% 15% 4 11% 5% 5% 5% 4 4 6% 24%	1710
	11th to 20th	Day	14% 13% 4 10% 7% 6% 5% 3 5% 6% 26%	1733
	21st to end	Day	20% 14% 3 10% 6% 4 5% 3 5% 5% 26%	1570
March	1st to 10th	Day	21% 12% 4 10% 7% 4 4 3 4 5% 27%	3116
	11th to 20th	Day	13% 17% 5% 11% 6% 5% 4 4 4 5% 27%	3346
	21st to end	Day	16% 8% 3 7% 6% 4 4 4 5% 5% 39%	4123
April	1st to 10th	Day	21% 12% 3 8% 5% 4 3 3 4 5% 32%	2280
	11th to 20th	Day	22% 8% 2 7% 4 4 3 3 5% 6% 35%	2254
	21st to end	Day	24% 8% 2 8% 5% 4 5% 3 4 6% 32%	2399
May	1st to 10th	Day	21% 9% 3 8% 4 5% 5% 4 6% 6% 29%	4424
	11th to 20th	Day	19% 8% 3 8% 5% 4 5% 4 4 7% 33%	4398
	21st to end	Day	16% 6% 2 7% 4 4 5% 4 6% 8% 37%	4837
June	1st to 10th	Day	22% 8% 2 7% 5% 5% 5% 4 5% 8% 30%	2660
	11th to 20th	Day	15% 5% 2 7% 5% 5% 5% 5% 6% 7% 38%	2660
	21st to end	Day	20% 7% 3 6% 5% 5% 6% 4 6% 7% 31%	2660
July	1st to 10th	Day	12% 6% 3 7% 6% 5% 5% 4 6% 9% 36%	4440
	11th to 20th	Day	14% 5% 2 6% 5% 5% 5% 5% 7% 8% 37%	4440
	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% 34%	1698
	11th to 20th	Day	17% 11% 3 8% 5% 5% 6% 3 5% 6% 29%	1601
	21st to end	Day	20% 12% 4 8% 5% 6% 5% 5% 5% 6% 23%	1520
December	1st to 10th	Day	24% 15% 4 9% 6% 5% 5% 3 5% 6% 18%	1520
	11th to 20th	Day	22% 14% 5% 9% 5% 4 4 3 5% 6% 24%	1906
	21st to end	Day	29% 17% 4 9% 5% 3 4 3 3 3 20%	2772
January	1st to 10th	Night	17% 17% 6% 10% 6% 5% 4 3 4 5% 22%	4429
	11th to 20th	Night	13% 14% 5% 14% 7% 6% 5% 4 5% 6% 21%	4440
	21st to end	Night	17% 18% 6% 10% 6% 5% 3 3 4 5% 23%	4884
February	1st to 10th	Night	15% 13% 5% 11% 7% 5% 6% 5% 5% 6% 23%	2470
	11th to 20th	Night	14% 12% 4 10% 6% 5% 5% 4 6% 5% 28%	2332
	21st to end	Night	16% 15% 4 10% 5% 4 4 4 4 4 30%	1884
March	1st to 10th	Night	20% 11% 4 8% 5% 4 4 3 4 5% 33%	3724
	11th to 20th	Night	19% 17% 5% 9% 6% 4 4 3 4 4 26%	3612
	21st to end	Night	14% 10% 3 8% 5% 4 4 4 4 5% 38%	3508
April	1st to 10th	Night	23% 10% 3 6% 5% 5% 4 3 4 6% 31%	1900
	11th to 20th	Night	22% 8% 3 7% 5% 3 4 3 5% 5% 35%	1875
	21st to end	Night	21% 9% 3 6% 5% 4 5% 3 5% 6% 34%	1781
May	1st to 10th	Night	22% 9% 3 7% 5% 4 5% 3 6% 6% 30%	2532
	11th to 20th	Night	21% 8% 3 6% 4 4 3 2 5% 6% 37%	2495
	21st to end	Night	15% 6% 3 6% 5% 4 4 3 5% 9% 38%	2738
June	1st to 10th	Night	20% 8% 2 6% 6% 5% 5% 4 7% 9% 31%	1520
	11th to 20th	Night	17% 7% 2 5% 4 4 4 4 6% 7% 40%	1520
	21st to end	Night	18% 7% 3 8% 4 4 5% 4 6% 7% 35%	1520
July	1st to 10th	Night	14% 6% 3 8% 6% 5% 5% 4 7% 7% 35%	2520
	11th to 20th	Night	14% 5% 2 6% 4 4 4 4 6% 8% 42%	2520
	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 3 5% 7% 33%	2453
	11th to 20th	Night	19% 9% 4 8% 5% 5% 5% 4 5% 6% 32%	2574
	21st to end	Night	22% 12% 4 9% 5% 5% 4 3 5% 6% 26%	2660
December	1st to 10th	Night	20% 15% 6% 10% 7% 4 5% 4 5% 4 20%	2660
	11th to 20th	Night	22% 16% 4 9% 5% 4 4 3 5% 4 25%	3288
	21st to end	Night	31% 17% 4 9% 4 3 3 2 3 4 21%	4884

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	39% 43% 8% 6% 3 1	2518
	11th to 20th	Day	35% 42% 9% 10% 4	2520
	21st to end	Day	41% 44% 9% 5% 2	2772
February	1st to 10th	Day	23% 65% 3 7% 2 1	1710
	11th to 20th	Day	20% 64% 3 8% 5%	1742
	21st to end	Day	21% 65% 3 5% 5% 1	1570
March	1st to 10th	Day	36% 43% 4 11% 5%	3119
	11th to 20th	Day	36% 43% 6% 7% 7% 1	3346
	21st to end	Day	26% 42% 5% 14% 12% 1	4123
April	1st to 10th	Day	19% 66% 3 6% 5% 1	2280
	11th to 20th	Day	18% 65% 2 8% 5% 1	2254
	21st to end	Day	20% 65% 3 6% 5%	2399
May	1st to 10th	Day	32% 45% 6% 14% 3	4438
	11th to 20th	Day	31% 45% 10% 11% 4	4402
	21st to end	Day	26% 43% 8% 16% 7%	4837
June	1st to 10th	Day	18% 65% 3 8% 5%	2660
	11th to 20th	Day	16% 66% 5% 7% 5% 1	2660
	21st to end	Day	18% 64% 5% 9% 4 1	2660
July	1st to 10th	Day	29% 39% 9% 16% 7%	4370
	11th to 20th	Day	25% 40% 10% 21% 4	4365
	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	24% 59% 6% 7% 3 1	1698
	11th to 20th	Day	20% 62% 5% 9% 4	1601
	21st to end	Day	24% 63% 5% 5% 2	1520
December	1st to 10th	Day	27% 61% 4 6% 1	1520
	11th to 20th	Day	34% 51% 8% 5% 2	1906
	21st to end	Day	44% 39% 4 9% 3	2772
January	1st to 10th	Night	39% 42% 11% 6% 2	4429
	11th to 20th	Night	36% 43% 10% 7% 2	4440
	21st to end	Night	42% 42% 8% 6% 2	4884
February	1st to 10th	Night	23% 65% 4 6% 1	2470
	11th to 20th	Night	19% 65% 3 9% 4	2334
	21st to end	Night	18% 65% 3 7% 7%	1884
March	1st to 10th	Night	30% 42% 6% 12% 6% 3	3734
	11th to 20th	Night	37% 43% 5% 7% 6% 1	3613
	21st to end	Night	29% 41% 4 15% 10% 1	3508
April	1st to 10th	Night	20% 65% 3 7% 6%	1900
	11th to 20th	Night	20% 66% 2 7% 5%	1875
	21st to end	Night	19% 65% 3 5% 6% 1	1781
May	1st to 10th	Night	31% 46% 8% 12% 3	2538
	11th to 20th	Night	29% 44% 8% 12% 6%	2496
	21st to end	Night	28% 43% 6% 12% 10%	2739
June	1st to 10th	Night	17% 66% 3 8% 5% 1	1520
	11th to 20th	Night	14% 65% 5% 8% 8%	1520
	21st to end	Night	18% 64% 4 8% 6%	1520
July	1st to 10th	Night	32% 39% 10% 10% 8%	2480
	11th to 20th	Night	25% 39% 11% 22% 3	2472
	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	24% 58% 7% 7% 3	2453
	11th to 20th	Night	22% 61% 5% 8% 4	2574
	21st to end	Night	24% 61% 6% 7% 2	2660
December	1st to 10th	Night	26% 61% 5% 6% 2	2660
	11th to 20th	Night	35% 50% 4 8% 3	3288
	21st to end	Night	45% 38% 4 10% 3	4884

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% 19% 26% 24% 4 1	180
	11th to 20th	Day	26% 24% 32% 12% 5% 1	180
	21st to end	Day	19% 18% 37% 19% 7%	198
February	1st to 10th	Day	30% 26% 29% 13% 2	270
	11th to 20th	Day	24% 22% 40% 13% 1	340
	21st to end	Day	29% 20% 29% 20% 2	382
March	1st to 10th	Day	25% 18% 32% 20% 4	460
	11th to 20th	Day	21% 24% 33% 17% 4 1	460
	21st to end	Day	26% 20% 29% 15% 8% 1	506
April	1st to 10th	Day	26% 20% 31% 16% 7% 1	460
	11th to 20th	Day	26% 22% 27% 16% 8% 2	452
	21st to end	Day	28% 18% 31% 14% 8% 2	460
May	1st to 10th	Day	21% 17% 33% 21% 6% 2	463
	11th to 20th	Day	18% 15% 32% 22% 11% 1	487
	21st to end	Day	19% 16% 31% 24% 8% 2	529
June	1st to 10th	Day	22% 15% 28% 25% 8% 2	490
	11th to 20th	Day	16% 13% 33% 24% 13% 1	490
	21st to end	Day	22% 13% 30% 22% 9% 3	490
July	1st to 10th	Day	11% 12% 31% 28% 14% 4	485
	11th to 20th	Day	15% 10% 30% 29% 13% 2	484
	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	27% 18% 25% 22% 7% 1	374
	11th to 20th	Day	26% 22% 33% 16% 4	261
	21st to end	Day	26% 18% 33% 18% 6%	180
December	1st to 10th	Day	29% 17% 28% 20% 5% 1	180
	11th to 20th	Day	30% 20% 33% 14% 2 1	180
	21st to end	Day	35% 19% 28% 15% 3 1	198
January	1st to 10th	Night	25% 18% 34% 20% 3	760
	11th to 20th	Night	27% 18% 30% 21% 4 1	760
	21st to end	Night	19% 15% 36% 24% 6%	836
February	1st to 10th	Night	21% 17% 36% 24% 3	760
	11th to 20th	Night	20% 20% 35% 21% 4	743
	21st to end	Night	23% 17% 32% 25% 3	628
March	1st to 10th	Night	24% 16% 32% 24% 4	760
	11th to 20th	Night	25% 18% 31% 21% 4 1	760
	21st to end	Night	22% 14% 31% 25% 7%	836
April	1st to 10th	Night	27% 15% 29% 24% 5%	760
	11th to 20th	Night	25% 15% 31% 23% 5% 1	751
	21st to end	Night	26% 12% 30% 23% 8%	641
May	1st to 10th	Night	23% 11% 32% 24% 8% 3	398
	11th to 20th	Night	21% 8% 32% 28% 9% 1	374
	21st to end	Night	17% 7% 37% 29% 10%	411
June	1st to 10th	Night	22% 9% 28% 31% 10%	380
	11th to 20th	Night	17% 8% 28% 34% 11% 1	380
	21st to end	Night	19% 8% 28% 29% 14% 1	380
July	1st to 10th	Night	12% 5% 29% 36% 16% 1	370
	11th to 20th	Night	13% 5% 26% 40% 16%	368
	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	24% 16% 27% 26% 7%	755
	11th to 20th	Night	23% 20% 29% 20% 8%	760
	21st to end	Night	29% 17% 28% 20% 7%	760
December	1st to 10th	Night	29% 17% 30% 19% 4	760
	11th to 20th	Night	29% 20% 29% 17% 4 1	760
	21st to end	Night	36% 16% 29% 15% 5%	836

Page generated at 2024-11-01 22:47:33. Version ac07

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