Observatoire de L'Avenir
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire de L'Avenir clear sky chart from 2005-11-06 to 2024-08-30.

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	19% 11% 4 8% 5% 4 4 4 5% 6% 29%	95748
	Night	19% 13% 4 9% 5% 4 4 3 4 5% 29%	93306
Transparency	Day	54% 7% 13% 18% 8% 1	95467
	Night	57% 7% 11% 17% 8% 1	93141
Seeing	Day	25% 17% 30% 19% 8% 2	14389
	Night	25% 14% 29% 23% 8% 1	23233

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	19% 20% 7% 15% 6% 5% 4 3 2 3 16%	2517
	11th to 20th	Day	14% 16% 6% 14% 9% 6% 4 3 3 4 21%	2520
	21st to end	Day	12% 18% 6% 14% 7% 5% 5% 4 5% 5% 19%	2772
February	1st to 10th	Day	17% 19% 6% 10% 7% 6% 5% 3 4 4 20%	1591
	11th to 20th	Day	15% 19% 6% 13% 7% 5% 5% 3 4 5% 20%	1821
	21st to end	Day	15% 19% 5% 9% 6% 6% 4 4 4 5% 24%	1570
March	1st to 10th	Day	20% 14% 4 10% 6% 5% 4 3 4 5% 24%	3116
	11th to 20th	Day	16% 21% 6% 11% 4 3 4 3 4 4 24%	3251
	21st to end	Day	16% 14% 5% 9% 4 4 3 2 3 4 34%	4155
April	1st to 10th	Day	24% 15% 3 9% 5% 4 4 2 3 4 27%	2280
	11th to 20th	Day	21% 11% 3 6% 3 4 4 4 4 5% 35%	2282
	21st to end	Day	25% 12% 3 7% 4 3 4 3 4 5% 30%	2470
May	1st to 10th	Day	24% 9% 3 7% 4 4 5% 4 5% 6% 28%	4234
	11th to 20th	Day	19% 8% 3 7% 5% 6% 5% 4 6% 7% 30%	4398
	21st to end	Day	15% 7% 3 8% 5% 5% 6% 4 6% 7% 33%	4837
June	1st to 10th	Day	25% 7% 3 7% 4 4 5% 4 6% 8% 28%	2660
	11th to 20th	Day	18% 7% 2 7% 4 4 5% 4 6% 8% 35%	2660
	21st to end	Day	23% 8% 3 7% 5% 4 5% 4 5% 6% 28%	2660
July	1st to 10th	Day	14% 7% 2 7% 6% 6% 6% 5% 6% 9% 32%	4440
	11th to 20th	Day	14% 7% 3 8% 6% 5% 6% 4 7% 8% 33%	4440
	21st to end	Day	15% 6% 2 6% 4 4 5% 3 6% 8% 42%	4814
August	1st to 10th	Day	18% 6% 3 7% 5% 6% 5% 5% 7% 8% 31%	2646
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 4 6% 9% 36%	2443
	21st to end	Day	17% 6% 2 6% 5% 5% 4 3 6% 9% 38%	2488
September	1st to 10th	Day	15% 7% 2 5% 4 5% 5% 4 6% 8% 39%	2160
	11th to 20th	Day	19% 6% 2 5% 3 3 3 3 5% 7% 43%	2034
	21st to end	Day	20% 7% 3 5% 3 3 5% 3 5% 7% 39%	1980
October	1st to 10th	Day	22% 8% 2 7% 5% 4 5% 3 6% 8% 30%	1934
	11th to 20th	Day	24% 11% 2 8% 5% 4 4 4 4 7% 27%	1702
	21st to end	Day	27% 15% 3 8% 4 4 4 3 3 5% 24%	1782
November	1st to 10th	Day	20% 14% 3 8% 4 4 3 3 5% 5% 31%	1671
	11th to 20th	Day	19% 16% 5% 10% 5% 5% 3 4 4 5% 25%	1702
	21st to end	Day	22% 18% 4 10% 5% 4 4 3 4 4 22%	1520
December	1st to 10th	Day	24% 21% 6% 10% 6% 4 3 3 3 4 15%	1520
	11th to 20th	Day	22% 17% 5% 9% 4 4 4 3 3 4 24%	1906
	21st to end	Day	25% 20% 6% 11% 5% 3 3 2 3 3 18%	2772
January	1st to 10th	Night	17% 20% 7% 12% 6% 4 4 3 4 4 20%	4429
	11th to 20th	Night	13% 18% 6% 16% 8% 6% 4 3 4 5% 18%	4440
	21st to end	Night	15% 19% 5% 13% 8% 4 4 3 3 4 22%	4884
February	1st to 10th	Night	15% 17% 6% 14% 7% 5% 4 3 4 4 21%	2585
	11th to 20th	Night	13% 16% 6% 14% 6% 5% 5% 3 4 5% 22%	2244
	21st to end	Night	15% 17% 5% 9% 6% 5% 4 3 5% 4 29%	1884
March	1st to 10th	Night	21% 13% 4 8% 5% 4 3 2 4 4 31%	3724
	11th to 20th	Night	19% 21% 5% 9% 4 4 4 2 3 3 24%	3708
	21st to end	Night	15% 15% 5% 10% 5% 4 3 2 3 4 32%	3476
April	1st to 10th	Night	23% 13% 3 8% 5% 3 3 3 5% 5% 28%	1900
	11th to 20th	Night	23% 9% 3 7% 4 4 4 3 4 6% 33%	1847
	21st to end	Night	23% 11% 2 7% 4 4 3 3 5% 5% 32%	1710
May	1st to 10th	Night	24% 10% 4 8% 4 5% 4 4 5% 6% 27%	2722
	11th to 20th	Night	22% 8% 3 6% 4 3 4 3 5% 7% 34%	2495
	21st to end	Night	18% 6% 2 7% 5% 5% 5% 4 5% 8% 35%	2738
June	1st to 10th	Night	24% 7% 2 6% 5% 3 5% 4 6% 8% 30%	1520
	11th to 20th	Night	21% 7% 2 6% 3 4 5% 3 6% 7% 38%	1520
	21st to end	Night	20% 7% 3 7% 5% 5% 5% 5% 5% 7% 31%	1520
July	1st to 10th	Night	14% 8% 3 8% 5% 5% 5% 4 7% 8% 33%	2520
	11th to 20th	Night	15% 7% 2 7% 4 4 5% 4 6% 8% 37%	2520
	21st to end	Night	14% 6% 3 6% 3 4 5% 4 6% 8% 40%	2739
August	1st to 10th	Night	17% 7% 1 5% 4 5% 5% 4 6% 9% 37%	1512
	11th to 20th	Night	16% 6% 3 7% 5% 4 5% 4 6% 7% 37%	1659
	21st to end	Night	16% 6% 3 6% 5% 4 4 3 5% 8% 40%	2075
September	1st to 10th	Night	16% 8% 2 7% 4 4 3 3 5% 7% 41%	1800
	11th to 20th	Night	18% 5% 2 6% 3 3 4 2 5% 7% 46%	1926
	21st to end	Night	18% 7% 2 5% 4 4 4 3 4 7% 41%	1980
October	1st to 10th	Night	19% 8% 3 8% 4 4 4 3 5% 6% 37%	2024
	11th to 20th	Night	23% 9% 3 9% 5% 4 4 3 4 6% 30%	2258
	21st to end	Night	26% 11% 4 7% 5% 5% 4 3 4 5% 27%	2574
November	1st to 10th	Night	20% 13% 4 8% 4 4 4 4 4 6% 29%	2407
	11th to 20th	Night	20% 14% 4 9% 5% 3 3 3 5% 5% 29%	2474
	21st to end	Night	21% 17% 5% 10% 4 3 4 3 4 5% 25%	2660
December	1st to 10th	Night	24% 22% 6% 11% 6% 4 3 2 3 4 16%	2660
	11th to 20th	Night	21% 19% 6% 12% 6% 4 3 3 3 3 21%	3288
	21st to end	Night	25% 19% 5% 10% 4 4 3 2 3 3 20%	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	73% 5% 11% 9% 1	2518
	11th to 20th	Day	64% 8% 10% 14% 4	2520
	21st to end	Day	65% 9% 12% 10% 4	2772
February	1st to 10th	Day	69% 4 8% 14% 4 1	1591
	11th to 20th	Day	67% 5% 11% 10% 6% 1	1830
	21st to end	Day	63% 6% 10% 14% 6% 2	1570
March	1st to 10th	Day	61% 7% 8% 16% 7% 1	3119
	11th to 20th	Day	63% 6% 11% 10% 9% 1	3251
	21st to end	Day	53% 5% 9% 19% 13% 1	4155
April	1st to 10th	Day	62% 5% 8% 16% 8% 2	2280
	11th to 20th	Day	52% 5% 9% 20% 13% 2	2282
	21st to end	Day	57% 5% 9% 17% 10% 2	2470
May	1st to 10th	Day	54% 6% 11% 20% 8% 1	4248
	11th to 20th	Day	49% 10% 18% 15% 7% 1	4402
	21st to end	Day	45% 10% 11% 21% 12% 1	4837
June	1st to 10th	Day	53% 7% 12% 19% 8% 1	2660
	11th to 20th	Day	45% 7% 15% 21% 11% 1	2660
	21st to end	Day	56% 7% 11% 18% 9%	2660
July	1st to 10th	Day	46% 7% 15% 25% 7%	4370
	11th to 20th	Day	46% 9% 16% 25% 4	4365
	21st to end	Day	40% 7% 15% 30% 7%	4661
August	1st to 10th	Day	47% 9% 19% 21% 4	2632
	11th to 20th	Day	42% 10% 18% 21% 9%	2443
	21st to end	Day	43% 7% 16% 22% 12%	2488
September	1st to 10th	Day	42% 7% 16% 24% 10% 1	2160
	11th to 20th	Day	40% 6% 13% 30% 11%	2034
	21st to end	Day	44% 7% 14% 25% 9%	1980
October	1st to 10th	Day	52% 7% 14% 19% 7%	1934
	11th to 20th	Day	56% 8% 15% 17% 4	1702
	21st to end	Day	64% 5% 11% 14% 6% 1	1782
November	1st to 10th	Day	55% 6% 14% 17% 8%	1671
	11th to 20th	Day	61% 7% 11% 13% 7% 1	1702
	21st to end	Day	65% 7% 13% 12% 3	1520
December	1st to 10th	Day	74% 5% 10% 9% 2	1520
	11th to 20th	Day	66% 4 9% 16% 5%	1906
	21st to end	Day	72% 5% 10% 9% 5%	2772
January	1st to 10th	Night	67% 8% 13% 9% 3	4429
	11th to 20th	Night	68% 8% 11% 9% 3	4440
	21st to end	Night	67% 5% 11% 11% 5%	4884
February	1st to 10th	Night	66% 5% 9% 13% 6% 1	2585
	11th to 20th	Night	64% 6% 10% 13% 7%	2246
	21st to end	Night	58% 6% 9% 14% 11% 1	1884
March	1st to 10th	Night	56% 5% 10% 17% 8% 3	3734
	11th to 20th	Night	64% 6% 8% 12% 9% 1	3709
	21st to end	Night	56% 5% 9% 19% 9% 2	3476
April	1st to 10th	Night	58% 6% 8% 17% 10% 1	1900
	11th to 20th	Night	52% 5% 9% 19% 12% 2	1847
	21st to end	Night	53% 5% 9% 17% 12% 2	1710
May	1st to 10th	Night	55% 9% 10% 16% 8% 2	2728
	11th to 20th	Night	47% 9% 14% 19% 9% 1	2496
	21st to end	Night	44% 9% 11% 19% 16% 1	2739
June	1st to 10th	Night	51% 7% 12% 20% 10% 1	1520
	11th to 20th	Night	45% 7% 11% 22% 12% 3	1520
	21st to end	Night	50% 9% 12% 17% 11% 1	1520
July	1st to 10th	Night	47% 7% 16% 20% 10%	2480
	11th to 20th	Night	43% 7% 19% 23% 8%	2472
	21st to end	Night	40% 8% 19% 24% 9%	2651
August	1st to 10th	Night	42% 9% 18% 24% 7%	1502
	11th to 20th	Night	44% 9% 16% 23% 8%	1659
	21st to end	Night	41% 8% 15% 25% 10%	2075
September	1st to 10th	Night	43% 7% 14% 23% 13%	1800
	11th to 20th	Night	38% 6% 16% 29% 11% 1	1926
	21st to end	Night	43% 6% 11% 26% 12% 1	1980
October	1st to 10th	Night	48% 6% 12% 22% 11%	2024
	11th to 20th	Night	55% 8% 11% 21% 5%	2258
	21st to end	Night	60% 6% 10% 15% 8% 1	2574
November	1st to 10th	Night	57% 6% 12% 18% 6% 1	2407
	11th to 20th	Night	58% 7% 12% 14% 8% 2	2474
	21st to end	Night	63% 5% 12% 15% 4	2660
December	1st to 10th	Night	74% 5% 10% 10% 2	2660
	11th to 20th	Night	69% 5% 8% 13% 5%	3288
	21st to end	Night	69% 6% 9% 13% 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	31% 18% 33% 15% 2 1	180
	11th to 20th	Day	32% 21% 33% 11% 2	180
	21st to end	Day	23% 23% 35% 13% 6% 1	198
February	1st to 10th	Day	37% 26% 27% 8% 1	213
	11th to 20th	Day	27% 24% 38% 9% 2	429
	21st to end	Day	27% 26% 29% 16% 2	382
March	1st to 10th	Day	27% 22% 30% 18% 3	460
	11th to 20th	Day	28% 23% 30% 15% 3 1	460
	21st to end	Day	29% 24% 25% 14% 6% 1	506
April	1st to 10th	Day	33% 16% 28% 16% 6% 1	460
	11th to 20th	Day	33% 20% 26% 14% 6% 2	452
	21st to end	Day	32% 20% 31% 10% 6% 2	460
May	1st to 10th	Day	32% 17% 29% 14% 7% 1	463
	11th to 20th	Day	21% 16% 33% 21% 8% 1	487
	21st to end	Day	21% 21% 31% 19% 6% 1	529
June	1st to 10th	Day	24% 18% 31% 18% 7% 2	490
	11th to 20th	Day	19% 15% 34% 20% 10% 1	490
	21st to end	Day	27% 15% 30% 19% 9% 1	490
July	1st to 10th	Day	17% 12% 31% 26% 12% 2	485
	11th to 20th	Day	15% 11% 36% 29% 7% 1	484
	21st to end	Day	18% 8% 35% 29% 9% 1	532
August	1st to 10th	Day	20% 10% 26% 29% 13% 2	486
	11th to 20th	Day	19% 6% 30% 26% 17% 2	479
	21st to end	Day	18% 13% 27% 21% 14% 6%	532
September	1st to 10th	Day	17% 11% 32% 23% 11% 5%	450
	11th to 20th	Day	21% 11% 25% 21% 15% 7%	450
	21st to end	Day	18% 15% 27% 22% 12% 5%	450
October	1st to 10th	Day	24% 13% 28% 20% 11% 4	450
	11th to 20th	Day	28% 19% 24% 19% 9% 1	401
	21st to end	Day	36% 17% 27% 16% 5%	396
November	1st to 10th	Day	32% 16% 29% 19% 4 1	365
	11th to 20th	Day	31% 21% 26% 15% 5% 1	362
	21st to end	Day	31% 22% 33% 12% 3	180
December	1st to 10th	Day	38% 14% 25% 16% 4 2	180
	11th to 20th	Day	32% 28% 26% 12% 2 1	180
	21st to end	Day	35% 26% 24% 10% 5% 1	198
January	1st to 10th	Night	32% 18% 31% 15% 4	760
	11th to 20th	Night	27% 19% 31% 18% 4	760
	21st to end	Night	21% 16% 35% 22% 5%	836
February	1st to 10th	Night	24% 19% 34% 19% 3	760
	11th to 20th	Night	23% 21% 36% 17% 3	743
	21st to end	Night	24% 18% 35% 21% 2	628
March	1st to 10th	Night	26% 16% 34% 21% 3	760
	11th to 20th	Night	28% 18% 31% 19% 5% 1	760
	21st to end	Night	26% 16% 29% 23% 5% 1	836
April	1st to 10th	Night	31% 15% 28% 22% 4	760
	11th to 20th	Night	28% 16% 28% 22% 6%	723
	21st to end	Night	32% 15% 28% 18% 7%	570
May	1st to 10th	Night	28% 12% 30% 24% 7%	510
	11th to 20th	Night	22% 11% 31% 28% 7% 1	374
	21st to end	Night	21% 10% 37% 24% 7%	411
June	1st to 10th	Night	28% 8% 29% 25% 9%	380
	11th to 20th	Night	21% 8% 27% 33% 11%	380
	21st to end	Night	19% 6% 32% 30% 11% 1	380
July	1st to 10th	Night	16% 5% 32% 34% 12% 1	370
	11th to 20th	Night	13% 5% 28% 39% 14% 1	368
	21st to end	Night	15% 5% 27% 42% 11%	416
August	1st to 10th	Night	20% 6% 20% 35% 19%	378
	11th to 20th	Night	19% 4 22% 34% 19% 1	535
	21st to end	Night	17% 7% 24% 35% 15% 2	827
September	1st to 10th	Night	18% 8% 28% 33% 12% 1	720
	11th to 20th	Night	19% 7% 25% 29% 18% 2	720
	21st to end	Night	20% 9% 30% 25% 15% 2	720
October	1st to 10th	Night	23% 13% 27% 26% 10% 1	720
	11th to 20th	Night	28% 14% 26% 23% 8% 1	720
	21st to end	Night	32% 14% 31% 20% 4	792
November	1st to 10th	Night	29% 15% 31% 19% 5%	740
	11th to 20th	Night	27% 20% 28% 16% 8% 1	760
	21st to end	Night	31% 21% 25% 17% 6% 1	760
December	1st to 10th	Night	36% 17% 26% 17% 4	760
	11th to 20th	Night	30% 18% 31% 16% 5% 1	760
	21st to end	Night	35% 20% 25% 16% 4	836

Page generated at 2024-08-27 09:49:38. Version ac07

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