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 2026-06-01.

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% 12% 4 9% 5% 5% 4 4 5% 6% 28%	132095
	Night	18% 14% 5% 10% 6% 4 4 3 4 5% 27%	131068
Transparency	Day	55% 6% 12% 18% 8% 1	135550
	Night	58% 6% 11% 16% 8% 1	135700
Seeing	Day	26% 17% 30% 18% 7% 2	16550
	Night	26% 14% 29% 23% 7% 1	25502

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% 21% 7% 15% 6% 6% 4 3 3 4 16%	4661
	11th to 20th	Day	11% 18% 8% 18% 10% 6% 5% 3 3 4 15%	4650
	21st to end	Day	9% 15% 6% 14% 9% 9% 6% 4 4 4 18%	5124
February	1st to 10th	Day	15% 17% 6% 11% 8% 6% 5% 3 4 5% 21%	1759
	11th to 20th	Day	14% 19% 6% 13% 7% 5% 4 3 4 5% 20%	2019
	21st to end	Day	14% 19% 5% 10% 7% 5% 4 3 4 5% 23%	1730
March	1st to 10th	Day	19% 15% 6% 13% 6% 5% 4 3 4 4 21%	4576
	11th to 20th	Day	15% 17% 5% 10% 4 4 4 3 5% 6% 29%	4756
	21st to end	Day	20% 14% 5% 9% 4 4 3 2 4 4 29%	6090
April	1st to 10th	Day	23% 15% 4 9% 5% 4 4 2 3 4 27%	2520
	11th to 20th	Day	22% 11% 3 6% 3 4 4 3 4 4 34%	2525
	21st to end	Day	24% 12% 3 7% 4 3 4 3 4 5% 30%	2730
May	1st to 10th	Day	28% 11% 3 8% 5% 4 5% 4 5% 6% 22%	6204
	11th to 20th	Day	21% 8% 3 7% 5% 5% 5% 4 6% 6% 30%	6458
	21st to end	Day	19% 10% 3 7% 4 5% 5% 4 6% 7% 30%	7103
June	1st to 10th	Day	24% 7% 3 7% 4 4 5% 4 6% 8% 28%	2800
	11th to 20th	Day	18% 7% 2 7% 4 4 5% 4 6% 8% 35%	2800
	21st to end	Day	23% 8% 3 7% 5% 4 5% 4 6% 6% 29%	2800
July	1st to 10th	Day	16% 8% 3 8% 6% 6% 6% 5% 7% 9% 27%	6360
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 7% 9% 36%	6360
	21st to end	Day	13% 6% 2 5% 4 3 4 4 6% 9% 43%	6926
August	1st to 10th	Day	17% 7% 3 7% 5% 5% 6% 5% 8% 9% 29%	4563
	11th to 20th	Day	18% 6% 3 7% 5% 5% 5% 4 7% 8% 33%	4184
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 7% 38%	4314
September	1st to 10th	Day	14% 6% 2 5% 4 5% 5% 3 6% 8% 42%	2400
	11th to 20th	Day	18% 5% 2 5% 3 4 4 3 5% 7% 45%	2260
	21st to end	Day	20% 7% 3 5% 3 3 5% 4 5% 7% 38%	2200
October	1st to 10th	Day	20% 8% 2 7% 5% 4 5% 3 6% 7% 33%	2148
	11th to 20th	Day	22% 12% 3 8% 5% 4 4 4 4 7% 28%	1890
	21st to end	Day	26% 15% 3 8% 4 4 3 3 3 5% 25%	1980
November	1st to 10th	Day	20% 15% 4 8% 4 4 3 3 4 4 30%	1851
	11th to 20th	Day	18% 18% 5% 10% 5% 4 3 3 4 5% 25%	1880
	21st to end	Day	23% 19% 5% 10% 5% 4 4 3 4 4 20%	1680
December	1st to 10th	Day	19% 21% 6% 13% 6% 5% 4 3 3 4 16%	2680
	11th to 20th	Day	21% 16% 6% 11% 5% 4 4 3 4 4 23%	3066
	21st to end	Day	20% 18% 5% 11% 6% 4 4 3 5% 5% 19%	4048
January	1st to 10th	Night	14% 21% 7% 14% 7% 4 4 3 3 4 18%	8234
	11th to 20th	Night	12% 22% 8% 18% 8% 6% 4 3 3 4 12%	8235
	21st to end	Night	9% 15% 6% 15% 11% 6% 4 3 4 5% 22%	9057
February	1st to 10th	Night	14% 16% 6% 14% 7% 5% 4 3 4 4 22%	2857
	11th to 20th	Night	13% 16% 6% 15% 6% 5% 4 3 4 5% 23%	2486
	21st to end	Night	14% 18% 5% 9% 6% 4 4 3 5% 4 28%	2076
March	1st to 10th	Night	19% 15% 5% 10% 6% 4 4 3 4 4 27%	5484
	11th to 20th	Night	16% 17% 5% 9% 5% 5% 5% 3 5% 6% 25%	5423
	21st to end	Night	17% 17% 5% 11% 5% 4 4 3 3 4 27%	5082
April	1st to 10th	Night	24% 13% 4 8% 5% 3 3 3 5% 5% 28%	2100
	11th to 20th	Night	23% 11% 3 7% 4 4 4 3 4 5% 32%	2043
	21st to end	Night	23% 11% 2 8% 4 4 3 3 4 5% 33%	1890
May	1st to 10th	Night	27% 11% 4 8% 5% 4 4 3 4 5% 23%	3987
	11th to 20th	Night	25% 7% 3 7% 4 4 5% 4 5% 7% 30%	3655
	21st to end	Night	19% 8% 3 8% 5% 5% 5% 4 5% 7% 30%	4014
June	1st to 10th	Night	24% 7% 2 6% 5% 3 5% 4 6% 8% 30%	1600
	11th to 20th	Night	20% 7% 2 6% 4 4 5% 3 6% 7% 37%	1600
	21st to end	Night	20% 7% 3 7% 5% 5% 5% 5% 5% 7% 32%	1600
July	1st to 10th	Night	16% 8% 3 8% 5% 5% 5% 5% 7% 9% 29%	3600
	11th to 20th	Night	17% 7% 2 7% 4 4 5% 4 5% 8% 37%	3600
	21st to end	Night	12% 6% 2 7% 4 4 5% 4 7% 8% 39%	3926
August	1st to 10th	Night	21% 8% 2 6% 4 4 4 3 5% 7% 34%	2586
	11th to 20th	Night	17% 7% 3 8% 5% 5% 5% 4 6% 7% 34%	2832
	21st to end	Night	19% 6% 3 6% 4 4 4 3 5% 7% 40%	3581
September	1st to 10th	Night	15% 8% 2 7% 4 4 3 3 5% 6% 43%	2000
	11th to 20th	Night	16% 5% 2 6% 3 3 4 2 4 7% 48%	2140
	21st to end	Night	18% 7% 2 5% 5% 4 4 3 4 7% 40%	2200
October	1st to 10th	Night	19% 7% 3 7% 4 4 4 3 5% 6% 39%	2250
	11th to 20th	Night	22% 10% 2 9% 5% 4 4 3 5% 6% 31%	2510
	21st to end	Night	25% 11% 3 7% 5% 5% 4 3 4 5% 28%	2860
November	1st to 10th	Night	21% 14% 4 9% 4 4 4 3 4 5% 28%	2667
	11th to 20th	Night	20% 15% 4 9% 4 4 3 3 5% 5% 28%	2735
	21st to end	Night	22% 17% 5% 10% 4 3 4 3 4 5% 23%	2940
December	1st to 10th	Night	18% 23% 9% 13% 6% 5% 3 3 3 4 13%	4720
	11th to 20th	Night	21% 19% 5% 12% 5% 4 3 3 3 3 21%	5348
	21st to end	Night	21% 19% 5% 10% 4 4 3 2 4 5% 22%	7150

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% 4 9% 11% 3	4662
	11th to 20th	Day	72% 5% 8% 10% 5%	4680
	21st to end	Day	68% 6% 8% 9% 9%	5124
February	1st to 10th	Day	67% 4 8% 14% 5% 1	1759
	11th to 20th	Day	67% 4 11% 12% 6% 1	2028
	21st to end	Day	64% 5% 9% 14% 6% 2	1730
March	1st to 10th	Day	67% 5% 7% 11% 7% 2	4579
	11th to 20th	Day	56% 6% 14% 12% 11% 1	4756
	21st to end	Day	58% 4 7% 16% 13% 1	6090
April	1st to 10th	Day	62% 5% 8% 16% 8% 2	2520
	11th to 20th	Day	53% 5% 9% 19% 13% 1	2525
	21st to end	Day	56% 5% 9% 17% 11% 2	2730
May	1st to 10th	Day	62% 6% 9% 17% 6% 1	6215
	11th to 20th	Day	51% 9% 15% 16% 8% 1	6462
	21st to end	Day	51% 8% 11% 18% 11% 1	7103
June	1st to 10th	Day	52% 7% 12% 19% 9% 1	2800
	11th to 20th	Day	45% 7% 14% 21% 11% 1	2800
	21st to end	Day	55% 7% 11% 19% 9%	2800
July	1st to 10th	Day	50% 7% 15% 22% 6%	6290
	11th to 20th	Day	42% 8% 15% 28% 7%	6285
	21st to end	Day	38% 6% 16% 33% 7%	6728
August	1st to 10th	Day	48% 8% 18% 20% 5%	4549
	11th to 20th	Day	46% 9% 20% 19% 6%	4223
	21st to end	Day	45% 6% 15% 19% 13% 1	4314
September	1st to 10th	Day	40% 7% 16% 28% 9% 1	2400
	11th to 20th	Day	39% 5% 13% 31% 11%	2260
	21st to end	Day	44% 7% 14% 25% 10%	2200
October	1st to 10th	Day	46% 7% 15% 26% 6%	3499
	11th to 20th	Day	50% 6% 16% 25% 4	3054
	21st to end	Day	52% 5% 18% 19% 6%	3180
November	1st to 10th	Day	57% 5% 13% 17% 7%	1851
	11th to 20th	Day	61% 6% 11% 13% 8% 1	1880
	21st to end	Day	68% 7% 11% 11% 3	1680
December	1st to 10th	Day	74% 5% 9% 10% 3	2680
	11th to 20th	Day	66% 4 7% 16% 6%	3066
	21st to end	Day	68% 4 9% 11% 8%	4048
January	1st to 10th	Night	70% 6% 10% 10% 4	8234
	11th to 20th	Night	76% 5% 8% 7% 3	8280
	21st to end	Night	66% 4 8% 11% 10%	9057
February	1st to 10th	Night	65% 5% 8% 15% 6% 1	2857
	11th to 20th	Night	64% 5% 10% 14% 7%	2488
	21st to end	Night	58% 6% 9% 16% 10% 1	2076
March	1st to 10th	Night	61% 4 9% 13% 10% 3	5494
	11th to 20th	Night	59% 6% 12% 14% 8% 1	5424
	21st to end	Night	60% 4 8% 18% 8% 1	5082
April	1st to 10th	Night	59% 5% 7% 17% 10% 1	2100
	11th to 20th	Night	54% 5% 8% 18% 12% 2	2043
	21st to end	Night	53% 5% 9% 18% 13% 2	1890
May	1st to 10th	Night	61% 7% 9% 14% 7% 1	3993
	11th to 20th	Night	52% 8% 12% 19% 8% 1	3656
	21st to end	Night	51% 8% 11% 16% 14% 1	4015
June	1st to 10th	Night	51% 7% 12% 20% 11% 1	1600
	11th to 20th	Night	46% 7% 11% 21% 12% 3	1600
	21st to end	Night	50% 9% 12% 18% 12% 1	1600
July	1st to 10th	Night	49% 7% 18% 19% 7%	3560
	11th to 20th	Night	44% 7% 17% 25% 8%	3552
	21st to end	Night	39% 7% 18% 26% 9%	3809
August	1st to 10th	Night	49% 7% 14% 24% 6%	2576
	11th to 20th	Night	47% 9% 16% 21% 6%	2859
	21st to end	Night	44% 6% 12% 24% 14%	3581
September	1st to 10th	Night	41% 7% 14% 25% 13%	2000
	11th to 20th	Night	36% 5% 16% 31% 12% 1	2140
	21st to end	Night	44% 6% 10% 26% 13%	2200
October	1st to 10th	Night	49% 5% 13% 24% 9%	3679
	11th to 20th	Night	49% 6% 12% 27% 6%	4085
	21st to end	Night	52% 4 14% 19% 9% 2	4610
November	1st to 10th	Night	58% 6% 12% 18% 5% 1	2667
	11th to 20th	Night	58% 7% 11% 14% 9% 1	2735
	21st to end	Night	65% 5% 12% 14% 4	2940
December	1st to 10th	Night	77% 4 9% 8% 2	4720
	11th to 20th	Night	69% 4 7% 13% 7%	5348
	21st to end	Night	65% 5% 9% 13% 8%	7150

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% 19% 33% 14% 2	220
	11th to 20th	Day	32% 22% 34% 10% 2	220
	21st to end	Day	26% 21% 35% 12% 5%	242
February	1st to 10th	Day	33% 25% 31% 9% 2	261
	11th to 20th	Day	27% 23% 36% 11% 2	507
	21st to end	Day	26% 26% 31% 15% 2	446
March	1st to 10th	Day	28% 24% 29% 16% 3	540
	11th to 20th	Day	30% 21% 30% 14% 3 1	540
	21st to end	Day	32% 24% 24% 13% 5% 1	594
April	1st to 10th	Day	35% 18% 28% 14% 5% 1	540
	11th to 20th	Day	34% 19% 26% 14% 5% 2	532
	21st to end	Day	31% 20% 31% 9% 6% 2	540
May	1st to 10th	Day	33% 17% 29% 14% 6% 1	543
	11th to 20th	Day	23% 17% 31% 20% 8% 1	567
	21st to end	Day	23% 22% 30% 18% 6% 1	617
June	1st to 10th	Day	24% 17% 33% 18% 6% 2	530
	11th to 20th	Day	20% 15% 35% 20% 10% 1	530
	21st to end	Day	26% 14% 33% 18% 8% 1	530
July	1st to 10th	Day	18% 14% 31% 25% 12% 2	525
	11th to 20th	Day	15% 12% 36% 29% 7% 2	524
	21st to end	Day	17% 9% 36% 28% 9% 1	576
August	1st to 10th	Day	19% 10% 27% 30% 13% 2	526
	11th to 20th	Day	18% 6% 30% 26% 18% 2	519
	21st to end	Day	18% 13% 29% 22% 13% 6%	583
September	1st to 10th	Day	16% 12% 32% 23% 11% 5%	530
	11th to 20th	Day	18% 9% 24% 23% 18% 8%	530
	21st to end	Day	19% 15% 27% 22% 12% 5%	530
October	1st to 10th	Day	22% 13% 29% 22% 11% 4	530
	11th to 20th	Day	28% 18% 24% 20% 8% 1	469
	21st to end	Day	35% 18% 27% 16% 5%	462
November	1st to 10th	Day	34% 16% 29% 17% 3	425
	11th to 20th	Day	32% 20% 27% 16% 5% 1	420
	21st to end	Day	34% 21% 32% 10% 2	220
December	1st to 10th	Day	35% 15% 28% 16% 4 1	220
	11th to 20th	Day	30% 29% 28% 11% 1	220
	21st to end	Day	32% 24% 26% 12% 5% 1	242
January	1st to 10th	Night	31% 19% 32% 15% 3	840
	11th to 20th	Night	28% 20% 30% 18% 4	840
	21st to end	Night	21% 17% 36% 21% 5%	924
February	1st to 10th	Night	23% 19% 34% 20% 4	840
	11th to 20th	Night	23% 21% 35% 17% 3	823
	21st to end	Night	24% 17% 35% 21% 2	692
March	1st to 10th	Night	27% 17% 33% 19% 3	840
	11th to 20th	Night	28% 19% 30% 18% 4	840
	21st to end	Night	27% 16% 29% 22% 5% 1	924
April	1st to 10th	Night	33% 15% 28% 21% 4	840
	11th to 20th	Night	29% 16% 28% 21% 6%	799
	21st to end	Night	31% 15% 28% 18% 7%	630
May	1st to 10th	Night	29% 12% 30% 23% 6%	564
	11th to 20th	Night	23% 11% 33% 27% 7% 1	414
	21st to end	Night	22% 10% 36% 25% 6%	455
June	1st to 10th	Night	28% 9% 30% 25% 8%	400
	11th to 20th	Night	21% 8% 28% 34% 10%	400
	21st to end	Night	20% 7% 33% 30% 11% 1	400
July	1st to 10th	Night	16% 6% 32% 33% 12% 1	390
	11th to 20th	Night	13% 5% 28% 39% 14% 1	388
	21st to end	Night	14% 5% 27% 43% 10%	438
August	1st to 10th	Night	19% 6% 20% 35% 20%	398
	11th to 20th	Night	19% 4 22% 34% 20% 1	564
	21st to end	Night	17% 7% 25% 36% 14% 2	875
September	1st to 10th	Night	18% 8% 27% 33% 12% 1	800
	11th to 20th	Night	18% 7% 23% 29% 21% 2	800
	21st to end	Night	20% 9% 29% 26% 14% 1	800
October	1st to 10th	Night	22% 12% 28% 27% 10% 1	800
	11th to 20th	Night	27% 13% 27% 24% 8% 1	800
	21st to end	Night	31% 14% 31% 20% 4	880
November	1st to 10th	Night	31% 15% 30% 18% 5%	820
	11th to 20th	Night	27% 18% 28% 17% 8% 1	840
	21st to end	Night	32% 21% 26% 16% 5% 1	840
December	1st to 10th	Night	34% 18% 27% 17% 4	840
	11th to 20th	Night	31% 19% 30% 16% 4 1	840
	21st to end	Night	35% 19% 25% 17% 4	924

Page generated at 2026-05-29 01:53:32. Version ac07

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