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 2025-02-06.

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 3 5% 6% 29%	108438
	Night	18% 14% 4 10% 5% 4 4 3 4 5% 28%	109810
Transparency	Day	55% 7% 13% 18% 7% 1	111941
	Night	58% 6% 11% 17% 8% 1	114471
Seeing	Day	25% 17% 30% 19% 8% 2	14836
	Night	25% 14% 29% 23% 8% 1	23870

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	18% 24% 7% 15% 6% 6% 4 3 2 2 13%	3597
	11th to 20th	Day	11% 17% 7% 20% 10% 6% 5% 3 3 3 15%	3570
	21st to end	Day	10% 18% 7% 13% 6% 6% 5% 4 4 5% 21%	3960
February	1st to 10th	Day	16% 19% 6% 10% 7% 6% 5% 3 4 4 20%	1631
	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% 7% 3 7% 5% 5% 6% 5% 8% 9% 28%	4423
	11th to 20th	Day	18% 7% 2 6% 5% 5% 5% 4 7% 9% 32%	4054
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 7% 38%	4182
September	1st to 10th	Day	15% 6% 2 5% 4 5% 5% 4 6% 8% 40%	2280
	11th to 20th	Day	18% 5% 2 5% 3 4 4 3 5% 6% 44%	2147
	21st to end	Day	20% 7% 3 5% 3 3 5% 3 5% 7% 38%	2090
October	1st to 10th	Day	21% 8% 2 8% 5% 4 5% 3 6% 7% 31%	2041
	11th to 20th	Day	23% 12% 2 8% 5% 4 4 4 4 6% 28%	1796
	21st to end	Day	26% 14% 3 8% 4 4 3 3 3 5% 26%	1881
November	1st to 10th	Day	20% 14% 3 8% 4 4 3 3 5% 5% 31%	1761
	11th to 20th	Day	19% 17% 5% 10% 5% 5% 3 3 4 5% 26%	1791
	21st to end	Day	23% 18% 4 10% 5% 4 4 3 4 4 21%	1600
December	1st to 10th	Day	19% 21% 6% 13% 6% 5% 3 3 3 4 15%	2600
	11th to 20th	Day	21% 17% 6% 10% 5% 4 4 3 3 4 23%	2986
	21st to end	Day	20% 18% 5% 11% 6% 4 4 3 4 5% 19%	3960
January	1st to 10th	Night	15% 23% 8% 15% 7% 4 3 3 3 3 17%	6349
	11th to 20th	Night	11% 21% 8% 18% 8% 6% 4 3 3 4 13%	6315
	21st to end	Night	11% 17% 6% 14% 8% 5% 4 3 4 5% 23%	6996
February	1st to 10th	Night	15% 17% 5% 13% 7% 5% 4 3 4 4 21%	2656
	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	22% 9% 2 7% 5% 4 4 3 5% 7% 33%	2506
	11th to 20th	Night	17% 7% 3 8% 5% 5% 5% 4 6% 7% 33%	2743
	21st to end	Night	19% 6% 3 6% 4 4 4 3 5% 7% 40%	3471
September	1st to 10th	Night	16% 8% 2 7% 4 4 3 3 5% 6% 42%	1900
	11th to 20th	Night	17% 5% 2 6% 3 3 3 2 5% 7% 48%	2033
	21st to end	Night	18% 7% 3 5% 5% 5% 4 3 4 7% 40%	2090
October	1st to 10th	Night	19% 8% 3 7% 4 4 4 3 6% 6% 37%	2137
	11th to 20th	Night	23% 10% 3 9% 5% 4 4 3 4 5% 30%	2384
	21st to end	Night	25% 11% 4 7% 5% 5% 4 3 4 5% 28%	2717
November	1st to 10th	Night	20% 14% 4 9% 4 4 4 4 4 5% 28%	2537
	11th to 20th	Night	20% 14% 4 9% 4 3 3 3 5% 5% 29%	2605
	21st to end	Night	22% 17% 5% 10% 4 3 4 3 4 5% 23%	2800
December	1st to 10th	Night	19% 24% 9% 13% 6% 5% 3 3 3 4 12%	4580
	11th to 20th	Night	21% 19% 5% 12% 5% 4 3 2 3 3 21%	5208
	21st to end	Night	21% 19% 5% 9% 4 4 3 2 4 5% 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	77% 4 8% 9% 1	3598
	11th to 20th	Day	72% 6% 8% 10% 3	3600
	21st to end	Day	64% 7% 10% 10% 8%	3960
February	1st to 10th	Day	68% 4 8% 14% 4 1	1631
	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	50% 8% 18% 20% 5%	4409
	11th to 20th	Day	46% 9% 21% 19% 5%	4093
	21st to end	Day	45% 6% 16% 19% 13% 1	4182
September	1st to 10th	Day	41% 7% 16% 25% 10% 1	2280
	11th to 20th	Day	40% 6% 13% 30% 11%	2147
	21st to end	Day	44% 7% 14% 25% 9%	2090
October	1st to 10th	Day	47% 7% 16% 25% 5%	3392
	11th to 20th	Day	50% 6% 16% 24% 3	2960
	21st to end	Day	52% 5% 18% 19% 6%	3081
November	1st to 10th	Day	55% 6% 13% 18% 7%	1761
	11th to 20th	Day	61% 6% 11% 14% 8% 1	1791
	21st to end	Day	66% 7% 12% 12% 3	1600
December	1st to 10th	Day	74% 5% 9% 10% 3	2600
	11th to 20th	Day	66% 4 7% 17% 6%	2986
	21st to end	Day	68% 4 9% 11% 8%	3960
January	1st to 10th	Night	73% 6% 9% 8% 3	6349
	11th to 20th	Night	74% 6% 9% 8% 2	6360
	21st to end	Night	65% 4 9% 11% 11%	6996
February	1st to 10th	Night	66% 5% 9% 14% 6% 1	2656
	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	51% 7% 14% 23% 5%	2496
	11th to 20th	Night	48% 10% 16% 21% 5%	2770
	21st to end	Night	44% 6% 12% 24% 14%	3471
September	1st to 10th	Night	42% 7% 14% 24% 13%	1900
	11th to 20th	Night	37% 6% 16% 30% 11% 1	2033
	21st to end	Night	44% 6% 11% 26% 12%	2090
October	1st to 10th	Night	50% 5% 13% 23% 9%	3566
	11th to 20th	Night	50% 6% 12% 27% 5%	3959
	21st to end	Night	52% 4 14% 19% 9% 2	4467
November	1st to 10th	Night	58% 6% 12% 18% 6% 1	2537
	11th to 20th	Night	57% 7% 11% 15% 9% 2	2605
	21st to end	Night	65% 5% 12% 14% 4	2800
December	1st to 10th	Night	78% 4 9% 7% 2	4580
	11th to 20th	Night	69% 4 7% 13% 7%	5208
	21st to end	Night	65% 5% 9% 13% 8%	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	32% 19% 33% 14% 2 1	200
	11th to 20th	Day	31% 22% 35% 12% 2	200
	21st to end	Day	25% 22% 35% 12% 5%	220
February	1st to 10th	Day	37% 26% 27% 8% 1	223
	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% 28% 22% 14% 6%	539
September	1st to 10th	Day	17% 12% 32% 23% 10% 5%	490
	11th to 20th	Day	19% 10% 23% 23% 17% 7%	490
	21st to end	Day	19% 15% 27% 22% 12% 5%	490
October	1st to 10th	Day	23% 13% 28% 21% 11% 4	490
	11th to 20th	Day	29% 19% 23% 19% 9% 1	435
	21st to end	Day	34% 19% 27% 16% 5%	429
November	1st to 10th	Day	33% 16% 29% 18% 3 1	395
	11th to 20th	Day	32% 20% 27% 15% 5% 1	391
	21st to end	Day	32% 23% 32% 11% 3	200
December	1st to 10th	Day	37% 15% 28% 16% 5% 2	200
	11th to 20th	Day	32% 28% 27% 12% 2 1	200
	21st to end	Day	32% 25% 26% 11% 5% 1	220
January	1st to 10th	Night	31% 19% 32% 15% 4	800
	11th to 20th	Night	27% 20% 31% 19% 4	800
	21st to end	Night	21% 16% 37% 21% 5%	880
February	1st to 10th	Night	24% 19% 34% 19% 3	781
	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% 25% 35% 14% 2	831
September	1st to 10th	Night	19% 8% 28% 33% 12% 1	760
	11th to 20th	Night	18% 6% 24% 29% 20% 2	760
	21st to end	Night	20% 9% 29% 26% 14% 1	760
October	1st to 10th	Night	23% 13% 27% 26% 10% 1	760
	11th to 20th	Night	28% 14% 26% 23% 8% 1	760
	21st to end	Night	31% 14% 31% 20% 4	836
November	1st to 10th	Night	30% 15% 31% 18% 5%	780
	11th to 20th	Night	27% 19% 28% 17% 8% 1	800
	21st to end	Night	32% 21% 25% 16% 5% 1	800
December	1st to 10th	Night	35% 18% 26% 17% 4	800
	11th to 20th	Night	31% 18% 30% 16% 5% 1	800
	21st to end	Night	35% 19% 25% 17% 4	880

Page generated at 2025-02-03 03:05:50. Version ac07

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