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 2024-01-31.

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	20% 11% 4 7% 6% 3 5% 4 5% 6% 30%	74922
	Night	20% 12% 4 8% 6% 3 4 4 4 6% 29%	75209
Transparency	Day	55% 7% 12% 18% 8% 1	74627
	Night	57% 6% 11% 17% 8% 1	75038
Seeing	Day	25% 17% 30% 19% 8% 2	13561
	Night	25% 14% 29% 23% 8% 1	22605

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	21% 21% 7% 11% 7% 3 5% 5% 3 3 14%	1517
	11th to 20th	Day	19% 17% 7% 12% 7% 4 4 4 3 4 19%	1520
	21st to end	Day	11% 16% 6% 12% 8% 3 5% 6% 5% 6% 23%	1664
February	1st to 10th	Day	17% 18% 6% 9% 8% 5% 6% 4 4 4 19%	1508
	11th to 20th	Day	16% 18% 6% 11% 7% 3 5% 4 4 5% 20%	1723
	21st to end	Day	16% 18% 6% 10% 6% 4 4 4 4 5% 23%	1480
March	1st to 10th	Day	17% 15% 5% 9% 8% 4 5% 4 3 5% 25%	1797
	11th to 20th	Day	19% 13% 4 8% 6% 3 4 4 4 5% 29%	1849
	21st to end	Day	19% 14% 5% 7% 5% 3 4 3 4 4 31%	2365
April	1st to 10th	Day	24% 13% 4 8% 7% 3 4 3 3 4 27%	2160
	11th to 20th	Day	22% 11% 3 5% 4 3 4 4 4 5% 35%	2160
	21st to end	Day	25% 12% 3 6% 5% 3 4 4 4 6% 29%	2340
May	1st to 10th	Day	24% 8% 3 6% 5% 3 4 4 4 5% 33%	2412
	11th to 20th	Day	19% 7% 3 5% 5% 3 5% 4 5% 6% 36%	2478
	21st to end	Day	19% 7% 3 7% 6% 4 5% 5% 6% 7% 32%	2725
June	1st to 10th	Day	25% 7% 3 5% 5% 3 5% 5% 6% 8% 28%	2520
	11th to 20th	Day	19% 6% 3 6% 4 3 4 5% 5% 8% 36%	2520
	21st to end	Day	22% 7% 3 6% 7% 3 5% 5% 5% 7% 29%	2520
July	1st to 10th	Day	16% 5% 3 5% 6% 4 5% 5% 6% 10% 35%	2520
	11th to 20th	Day	13% 6% 4 6% 6% 4 5% 5% 7% 9% 34%	2520
	21st to end	Day	17% 6% 3 5% 6% 3 6% 5% 7% 9% 33%	2744
August	1st to 10th	Day	18% 6% 3 6% 6% 4 5% 6% 7% 8% 31%	2506
	11th to 20th	Day	15% 6% 3 5% 5% 4 5% 5% 6% 9% 37%	2313
	21st to end	Day	17% 6% 2 5% 6% 3 5% 4 6% 9% 38%	2378
September	1st to 10th	Day	15% 7% 2 5% 4 4 5% 5% 6% 8% 40%	2160
	11th to 20th	Day	20% 5% 2 4 3 2 4 4 5% 7% 43%	2034
	21st to end	Day	20% 7% 3 4 4 2 4 5% 5% 7% 39%	1980
October	1st to 10th	Day	22% 8% 2 6% 6% 4 4 4 5% 7% 30%	1934
	11th to 20th	Day	24% 11% 2 7% 5% 3 5% 4 4 6% 27%	1702
	21st to end	Day	28% 14% 3 7% 5% 3 3 3 3 5% 24%	1782
November	1st to 10th	Day	21% 12% 4 8% 5% 3 4 4 4 5% 31%	1671
	11th to 20th	Day	20% 14% 6% 9% 5% 3 4 4 4 5% 25%	1702
	21st to end	Day	22% 16% 5% 9% 6% 3 3 5% 4 5% 22%	1520
December	1st to 10th	Day	25% 20% 6% 9% 7% 3 4 5% 3 4 15%	1520
	11th to 20th	Day	22% 16% 6% 9% 5% 3 4 4 3 4 24%	1906
	21st to end	Day	26% 19% 6% 10% 6% 2 3 2 3 4 18%	2772
January	1st to 10th	Night	20% 19% 7% 10% 7% 3 4 4 3 4 17%	2649
	11th to 20th	Night	16% 18% 7% 13% 8% 3 5% 3 3 4 20%	2660
	21st to end	Night	13% 16% 6% 10% 9% 4 5% 4 4 5% 24%	2919
February	1st to 10th	Night	16% 15% 7% 12% 9% 4 5% 4 4 4 20%	2449
	11th to 20th	Night	14% 15% 7% 12% 8% 4 5% 4 4 5% 23%	2122
	21st to end	Night	15% 16% 6% 9% 7% 4 4 4 4 4 29%	1776
March	1st to 10th	Night	16% 15% 6% 10% 7% 3 4 4 4 4 27%	2150
	11th to 20th	Night	20% 12% 5% 7% 5% 4 5% 3 4 4 29%	2110
	21st to end	Night	19% 13% 4 6% 5% 3 4 3 4 4 34%	1990
April	1st to 10th	Night	24% 12% 4 8% 6% 3 3 4 4 6% 27%	1800
	11th to 20th	Night	23% 8% 3 6% 5% 3 4 4 4 6% 34%	1749
	21st to end	Night	24% 11% 3 7% 4 2 4 3 4 5% 32%	1620
May	1st to 10th	Night	24% 10% 4 5% 4 3 4 4 5% 6% 30%	1564
	11th to 20th	Night	20% 8% 3 6% 5% 3 5% 4 4 7% 35%	1415
	21st to end	Night	21% 5% 3 5% 5% 3 5% 5% 5% 7% 36%	1550
June	1st to 10th	Night	25% 6% 3 5% 6% 3 5% 5% 6% 8% 30%	1440
	11th to 20th	Night	21% 6% 3 5% 4 3 4 5% 5% 8% 37%	1440
	21st to end	Night	19% 6% 3 6% 6% 3 6% 6% 5% 8% 32%	1440
July	1st to 10th	Night	14% 6% 3 5% 7% 3 5% 5% 7% 9% 37%	1440
	11th to 20th	Night	11% 6% 2 7% 6% 2 7% 6% 6% 10% 37%	1440
	21st to end	Night	14% 6% 3 7% 4 3 5% 6% 7% 9% 37%	1574
August	1st to 10th	Night	17% 6% 1 4 4 4 5% 5% 6% 9% 39%	1432
	11th to 20th	Night	16% 6% 4 6% 6% 3 5% 5% 5% 7% 37%	1570
	21st to end	Night	16% 6% 3 5% 6% 3 4 5% 5% 8% 41%	1975
September	1st to 10th	Night	16% 8% 2 6% 5% 3 4 4 5% 7% 41%	1800
	11th to 20th	Night	18% 5% 2 5% 4 2 4 3 4 7% 46%	1926
	21st to end	Night	18% 6% 3 5% 5% 3 4 3 4 7% 41%	1980
October	1st to 10th	Night	19% 7% 3 6% 6% 3 4 4 5% 6% 37%	2024
	11th to 20th	Night	24% 9% 3 8% 6% 3 4 4 4 6% 30%	2258
	21st to end	Night	26% 11% 4 6% 6% 3 4 4 4 5% 27%	2574
November	1st to 10th	Night	21% 13% 4 7% 5% 3 4 4 4 5% 29%	2407
	11th to 20th	Night	21% 13% 5% 8% 5% 3 4 3 5% 5% 29%	2474
	21st to end	Night	22% 15% 5% 9% 5% 3 4 4 3 5% 25%	2660
December	1st to 10th	Night	25% 20% 7% 9% 7% 3 3 3 3 5% 16%	2660
	11th to 20th	Night	21% 18% 7% 11% 7% 3 4 3 3 3 21%	3288
	21st to end	Night	26% 19% 6% 9% 5% 3 3 3 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	75% 6% 8% 9% 2	1518
	11th to 20th	Day	70% 5% 7% 11% 5% 1	1520
	21st to end	Day	61% 8% 11% 14% 6% 1	1664
February	1st to 10th	Day	70% 4 8% 13% 4 1	1508
	11th to 20th	Day	67% 5% 11% 10% 6% 1	1732
	21st to end	Day	64% 5% 9% 14% 6% 2	1480
March	1st to 10th	Day	62% 6% 9% 14% 7% 2	1800
	11th to 20th	Day	57% 6% 12% 13% 11% 1	1849
	21st to end	Day	58% 4 9% 19% 9% 1	2365
April	1st to 10th	Day	63% 5% 8% 15% 8% 2	2160
	11th to 20th	Day	51% 5% 9% 20% 13% 2	2160
	21st to end	Day	57% 5% 9% 16% 10% 2	2340
May	1st to 10th	Day	53% 5% 9% 19% 12% 2	2412
	11th to 20th	Day	47% 7% 12% 20% 12% 1	2482
	21st to end	Day	50% 9% 10% 18% 12% 1	2725
June	1st to 10th	Day	53% 7% 12% 18% 8% 1	2520
	11th to 20th	Day	46% 7% 14% 21% 12% 1	2520
	21st to end	Day	55% 7% 11% 18% 9%	2520
July	1st to 10th	Day	43% 9% 16% 24% 8%	2450
	11th to 20th	Day	44% 9% 16% 24% 7%	2445
	21st to end	Day	47% 9% 17% 20% 6%	2591
August	1st to 10th	Day	47% 9% 19% 21% 4	2492
	11th to 20th	Day	42% 10% 18% 21% 9%	2313
	21st to end	Day	43% 8% 16% 23% 12%	2378
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	71% 6% 9% 10% 3	2649
	11th to 20th	Night	70% 5% 9% 10% 5% 1	2660
	21st to end	Night	64% 5% 10% 13% 8% 1	2919
February	1st to 10th	Night	68% 4 9% 13% 6% 1	2449
	11th to 20th	Night	64% 5% 10% 13% 7%	2124
	21st to end	Night	59% 6% 9% 14% 11% 1	1776
March	1st to 10th	Night	61% 4 9% 16% 8% 1	2160
	11th to 20th	Night	59% 6% 9% 14% 11% 2	2111
	21st to end	Night	55% 4 8% 20% 11% 3	1990
April	1st to 10th	Night	60% 5% 8% 16% 10% 1	1800
	11th to 20th	Night	52% 5% 9% 19% 13% 2	1749
	21st to end	Night	54% 6% 9% 17% 12% 2	1620
May	1st to 10th	Night	54% 6% 7% 16% 14% 3	1564
	11th to 20th	Night	49% 6% 10% 18% 15% 3	1416
	21st to end	Night	46% 9% 10% 19% 14% 2	1551
June	1st to 10th	Night	51% 7% 12% 20% 9% 1	1440
	11th to 20th	Night	45% 7% 10% 23% 12% 3	1440
	21st to end	Night	49% 9% 13% 18% 11%	1440
July	1st to 10th	Night	42% 9% 14% 23% 12%	1400
	11th to 20th	Night	40% 9% 17% 24% 11%	1392
	21st to end	Night	41% 10% 18% 24% 8% 1	1486
August	1st to 10th	Night	40% 9% 19% 24% 7%	1422
	11th to 20th	Night	44% 9% 16% 23% 8%	1570
	21st to end	Night	41% 8% 15% 25% 10% 1	1975
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% 33% 15% 6% 1	196
February	1st to 10th	Day	40% 28% 26% 5% 1 1	190
	11th to 20th	Day	28% 23% 38% 8% 2 1	391
	21st to end	Day	27% 25% 30% 16% 2	346
March	1st to 10th	Day	26% 22% 31% 17% 4	420
	11th to 20th	Day	28% 23% 29% 16% 4 1	420
	21st to end	Day	30% 22% 25% 15% 7% 1	462
April	1st to 10th	Day	33% 16% 28% 15% 6% 1	420
	11th to 20th	Day	33% 19% 25% 15% 6% 3	412
	21st to end	Day	32% 19% 31% 10% 6% 2	420
May	1st to 10th	Day	33% 16% 30% 14% 6% 1	423
	11th to 20th	Day	22% 15% 34% 20% 7% 1	447
	21st to end	Day	20% 22% 31% 19% 7% 1	485
June	1st to 10th	Day	26% 17% 32% 18% 6% 2	450
	11th to 20th	Day	20% 16% 33% 20% 11% 1	450
	21st to end	Day	24% 15% 31% 20% 10% 1	450
July	1st to 10th	Day	18% 10% 31% 27% 13% 2	445
	11th to 20th	Day	14% 10% 36% 30% 8% 2	444
	21st to end	Day	18% 8% 35% 28% 9% 1	488
August	1st to 10th	Day	20% 11% 25% 28% 14% 2	446
	11th to 20th	Day	19% 6% 30% 26% 17% 3	439
	21st to end	Day	18% 13% 28% 21% 14% 6%	495
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%	835
February	1st to 10th	Night	25% 20% 34% 19% 3	720
	11th to 20th	Night	23% 20% 35% 17% 3	703
	21st to end	Night	24% 17% 34% 22% 3	592
March	1st to 10th	Night	26% 17% 34% 20% 3	720
	11th to 20th	Night	27% 18% 31% 18% 5% 1	720
	21st to end	Night	26% 15% 28% 24% 6% 1	792
April	1st to 10th	Night	32% 14% 27% 22% 4	720
	11th to 20th	Night	27% 16% 28% 23% 6%	685
	21st to end	Night	32% 14% 28% 19% 7%	540
May	1st to 10th	Night	28% 12% 29% 24% 7%	484
	11th to 20th	Night	23% 11% 31% 28% 7% 1	354
	21st to end	Night	22% 10% 36% 25% 7%	389
June	1st to 10th	Night	29% 8% 30% 24% 9%	360
	11th to 20th	Night	21% 7% 28% 33% 11%	360
	21st to end	Night	18% 6% 33% 31% 12% 1	360
July	1st to 10th	Night	16% 5% 33% 33% 13% 1	350
	11th to 20th	Night	12% 4 28% 39% 15% 1	348
	21st to end	Night	14% 5% 28% 42% 10%	394
August	1st to 10th	Night	19% 6% 20% 35% 20%	358
	11th to 20th	Night	19% 4 22% 35% 18% 1	506
	21st to end	Night	17% 7% 24% 35% 15% 2	787
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

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