Observatoire Aster
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Aster clear sky chart from 2005-11-06 to 2026-08-30.

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	15% 11% 4 9% 6% 5% 5% 4 6% 7% 28%	144355
	Night	14% 11% 4 10% 6% 5% 5% 4 6% 7% 28%	137987
Transparency	Day	54% 7% 12% 18% 8% 1	147800
	Night	53% 7% 13% 18% 8% 1	142632
Seeing	Day	26% 20% 30% 16% 6% 1	18195
	Night	23% 18% 29% 22% 7% 1	26153

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	11% 16% 7% 15% 8% 8% 6% 5% 6% 5% 14%	4150
	11th to 20th	Day	9% 16% 6% 18% 12% 6% 6% 5% 6% 5% 12%	4650
	21st to end	Day	4 6% 4 15% 8% 10% 8% 6% 8% 8% 24%	5124
February	1st to 10th	Day	7% 14% 5% 10% 10% 9% 7% 5% 7% 7% 20%	1777
	11th to 20th	Day	11% 16% 5% 11% 7% 6% 6% 5% 6% 7% 19%	1895
	21st to end	Day	11% 12% 4 11% 8% 8% 6% 5% 5% 6% 25%	1730
March	1st to 10th	Day	17% 17% 5% 10% 6% 5% 5% 3 4 6% 22%	4576
	11th to 20th	Day	14% 15% 5% 10% 8% 5% 5% 4 5% 6% 21%	5061
	21st to end	Day	16% 18% 6% 10% 6% 3 4 3 4 5% 26%	5650
April	1st to 10th	Day	20% 14% 4 10% 6% 5% 4 3 5% 6% 24%	2528
	11th to 20th	Day	18% 12% 3 9% 5% 4 4 3 5% 6% 30%	2702
	21st to end	Day	21% 14% 3 7% 4 3 4 3 5% 5% 31%	2730
May	1st to 10th	Day	21% 13% 4 9% 6% 5% 4 3 4 5% 25%	7639
	11th to 20th	Day	20% 10% 3 8% 5% 5% 5% 4 5% 6% 28%	8315
	21st to end	Day	18% 10% 3 8% 5% 5% 5% 4 6% 7% 30%	9716
June	1st to 10th	Day	20% 9% 3 6% 5% 5% 5% 4 6% 7% 30%	3150
	11th to 20th	Day	20% 7% 2 7% 5% 4 5% 4 6% 8% 33%	3150
	21st to end	Day	23% 10% 3 8% 6% 5% 5% 3 6% 7% 25%	3150
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 4 7% 9% 35%	8880
	11th to 20th	Day	13% 6% 3 7% 6% 6% 6% 5% 7% 8% 35%	8666
	21st to end	Day	10% 6% 2 7% 6% 6% 7% 5% 7% 8% 36%	8993
August	1st to 10th	Day	13% 7% 3 7% 5% 5% 6% 5% 7% 8% 34%	4535
	11th to 20th	Day	16% 7% 3 8% 5% 5% 6% 5% 7% 7% 30%	4305
	21st to end	Day	17% 7% 3 7% 4 4 4 4 6% 8% 36%	4792
September	1st to 10th	Day	15% 7% 2 6% 5% 4 4 3 6% 8% 40%	2376
	11th to 20th	Day	14% 7% 2 6% 4 4 4 2 5% 6% 47%	2200
	21st to end	Day	17% 7% 3 8% 4 4 4 3 4 7% 38%	2200
October	1st to 10th	Day	17% 11% 3 7% 5% 5% 4 3 5% 6% 35%	2127
	11th to 20th	Day	20% 12% 4 10% 4 5% 5% 3 5% 6% 26%	1910
	21st to end	Day	23% 13% 4 9% 5% 4 5% 3 4 5% 25%	1980
November	1st to 10th	Day	16% 15% 5% 10% 6% 5% 4 4 4 5% 26%	1851
	11th to 20th	Day	17% 14% 5% 12% 6% 5% 5% 4 5% 6% 21%	1692
	21st to end	Day	18% 16% 4 13% 6% 6% 5% 4 4 5% 20%	1564
December	1st to 10th	Day	15% 15% 6% 14% 7% 6% 5% 4 5% 6% 19%	2350
	11th to 20th	Day	15% 13% 4 12% 7% 6% 5% 4 5% 4 24%	2688
	21st to end	Day	16% 14% 4 11% 8% 6% 7% 4 5% 5% 21%	3553
January	1st to 10th	Night	9% 15% 6% 14% 9% 9% 7% 5% 7% 7% 13%	8754
	11th to 20th	Night	9% 16% 6% 14% 10% 7% 6% 4 6% 7% 17%	8235
	21st to end	Night	3 8% 5% 12% 6% 7% 8% 6% 7% 8% 30%	9057
February	1st to 10th	Night	8% 12% 4 10% 9% 7% 6% 5% 6% 7% 27%	2841
	11th to 20th	Night	10% 14% 5% 13% 7% 6% 5% 5% 7% 6% 22%	2610
	21st to end	Night	9% 15% 4 9% 7% 5% 4 4 4 7% 31%	2076
March	1st to 10th	Night	17% 14% 4 9% 5% 5% 6% 4 4 5% 26%	5484
	11th to 20th	Night	12% 15% 6% 12% 6% 5% 5% 4 5% 7% 23%	5117
	21st to end	Night	19% 16% 5% 9% 5% 4 3 3 3 5% 29%	5510
April	1st to 10th	Night	22% 13% 4 7% 5% 3 3 4 5% 4 29%	2091
	11th to 20th	Night	20% 9% 3 7% 4 5% 5% 4 6% 6% 31%	1868
	21st to end	Night	21% 12% 2 6% 4 4 5% 4 4 6% 31%	1890
May	1st to 10th	Night	22% 11% 3 7% 4 3 4 3 5% 6% 30%	5259
	11th to 20th	Night	20% 9% 3 8% 5% 4 4 4 5% 6% 32%	4584
	21st to end	Night	15% 9% 2 6% 4 4 5% 4 7% 8% 37%	4491
June	1st to 10th	Night	20% 8% 3 7% 5% 4 4 4 6% 7% 32%	1470
	11th to 20th	Night	16% 7% 2 5% 5% 6% 5% 4 5% 9% 37%	1470
	21st to end	Night	22% 9% 3 7% 5% 5% 5% 4 6% 6% 27%	1470
July	1st to 10th	Night	12% 7% 2 6% 4 5% 6% 4 7% 8% 39%	4110
	11th to 20th	Night	14% 5% 2 6% 5% 5% 5% 4 7% 8% 38%	4297
	21st to end	Night	9% 6% 2 7% 5% 4 5% 5% 6% 8% 43%	5084
August	1st to 10th	Night	16% 6% 3 7% 6% 4 5% 4 5% 8% 35%	2802
	11th to 20th	Night	13% 7% 3 8% 6% 5% 6% 5% 7% 9% 33%	2962
	21st to end	Night	16% 7% 3 7% 4 4 4 3 4 6% 43%	3305
September	1st to 10th	Night	15% 5% 2 6% 4 4 4 4 7% 6% 43%	2020
	11th to 20th	Night	16% 7% 2 6% 3 4 5% 4 4 6% 43%	2200
	21st to end	Night	14% 7% 2 6% 4 4 4 4 5% 6% 44%	2200
October	1st to 10th	Night	17% 7% 3 7% 4 3 4 2 5% 7% 40%	2272
	11th to 20th	Night	20% 10% 2 7% 4 5% 5% 4 6% 6% 31%	2488
	21st to end	Night	20% 11% 3 8% 4 5% 4 4 5% 7% 29%	2860
November	1st to 10th	Night	17% 11% 4 9% 6% 4 5% 4 6% 6% 28%	2667
	11th to 20th	Night	16% 13% 4 9% 5% 5% 5% 4 6% 7% 27%	2925
	21st to end	Night	16% 14% 4 12% 7% 6% 5% 3 6% 6% 20%	3050
December	1st to 10th	Night	12% 13% 5% 13% 8% 7% 6% 4 6% 7% 18%	5060
	11th to 20th	Night	17% 12% 4 12% 8% 6% 5% 4 5% 6% 20%	5741
	21st to end	Night	15% 12% 5% 12% 8% 8% 5% 4 4 5% 23%	7667

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	67% 8% 9% 12% 3	4151
	11th to 20th	Day	70% 7% 9% 9% 4	4680
	21st to end	Day	54% 8% 9% 13% 16%	5124
February	1st to 10th	Day	60% 7% 11% 13% 8% 1	1777
	11th to 20th	Day	62% 8% 12% 13% 5%	1903
	21st to end	Day	58% 6% 10% 17% 8% 1	1730
March	1st to 10th	Day	63% 5% 10% 10% 9% 3	4579
	11th to 20th	Day	62% 7% 12% 11% 7% 1	5061
	21st to end	Day	62% 4 5% 17% 8% 3	5650
April	1st to 10th	Day	60% 5% 10% 14% 8% 2	2528
	11th to 20th	Day	54% 6% 10% 19% 10% 2	2702
	21st to end	Day	56% 5% 7% 17% 13% 2	2730
May	1st to 10th	Day	60% 6% 8% 16% 10%	7651
	11th to 20th	Day	54% 7% 11% 18% 8% 1	8319
	21st to end	Day	52% 6% 11% 17% 14% 1	9717
June	1st to 10th	Day	52% 8% 10% 19% 10% 1	3150
	11th to 20th	Day	49% 7% 13% 19% 10% 1	3150
	21st to end	Day	57% 7% 13% 15% 8%	3150
July	1st to 10th	Day	44% 6% 15% 24% 12%	8805
	11th to 20th	Day	45% 7% 16% 26% 6%	8585
	21st to end	Day	42% 8% 18% 26% 6%	8795
August	1st to 10th	Day	45% 9% 18% 20% 8%	4521
	11th to 20th	Day	50% 8% 17% 20% 5%	4344
	21st to end	Day	45% 7% 18% 20% 11%	4792
September	1st to 10th	Day	41% 7% 15% 26% 10%	2376
	11th to 20th	Day	39% 6% 16% 27% 12%	2200
	21st to end	Day	45% 7% 12% 25% 10%	2200
October	1st to 10th	Day	55% 7% 11% 22% 6%	3465
	11th to 20th	Day	56% 6% 12% 23% 3	3087
	21st to end	Day	50% 5% 17% 19% 8%	3180
November	1st to 10th	Day	58% 7% 12% 17% 4	1851
	11th to 20th	Day	63% 8% 11% 14% 5%	1692
	21st to end	Day	66% 8% 11% 12% 4	1564
December	1st to 10th	Day	65% 5% 14% 9% 7%	2350
	11th to 20th	Day	60% 5% 12% 16% 7%	2688
	21st to end	Day	66% 6% 10% 11% 7% 1	3553
January	1st to 10th	Night	66% 8% 11% 12% 2	8754
	11th to 20th	Night	64% 8% 12% 10% 5%	8280
	21st to end	Night	48% 7% 10% 20% 15% 1	9057
February	1st to 10th	Night	55% 7% 12% 16% 10% 1	2841
	11th to 20th	Night	59% 9% 11% 12% 8%	2613
	21st to end	Night	53% 6% 10% 20% 11% 1	2076
March	1st to 10th	Night	60% 6% 7% 14% 12% 1	5494
	11th to 20th	Night	61% 6% 14% 11% 8% 1	5118
	21st to end	Night	58% 4 7% 19% 11% 1	5510
April	1st to 10th	Night	57% 5% 9% 15% 12% 3	2091
	11th to 20th	Night	52% 7% 9% 19% 13% 1	1868
	21st to end	Night	53% 5% 8% 19% 12% 2	1890
May	1st to 10th	Night	54% 5% 10% 18% 11% 1	5265
	11th to 20th	Night	51% 7% 11% 22% 9% 1	4585
	21st to end	Night	42% 8% 11% 22% 16% 1	4492
June	1st to 10th	Night	49% 7% 12% 22% 10% 1	1470
	11th to 20th	Night	44% 7% 15% 20% 13% 2	1470
	21st to end	Night	55% 8% 12% 17% 8%	1470
July	1st to 10th	Night	40% 7% 20% 23% 9%	4075
	11th to 20th	Night	41% 7% 22% 26% 4	4255
	21st to end	Night	38% 7% 23% 26% 6%	4967
August	1st to 10th	Night	47% 8% 17% 23% 6%	2792
	11th to 20th	Night	46% 10% 22% 15% 7%	2989
	21st to end	Night	43% 7% 15% 23% 12%	3305
September	1st to 10th	Night	38% 10% 16% 26% 10% 1	2020
	11th to 20th	Night	40% 9% 17% 23% 10%	2200
	21st to end	Night	41% 7% 12% 27% 13% 1	2200
October	1st to 10th	Night	47% 5% 11% 29% 8% 1	3714
	11th to 20th	Night	47% 8% 14% 25% 6%	4050
	21st to end	Night	47% 7% 17% 20% 9%	4610
November	1st to 10th	Night	54% 9% 13% 18% 5% 1	2667
	11th to 20th	Night	55% 9% 15% 14% 7%	2925
	21st to end	Night	64% 9% 13% 11% 3	3050
December	1st to 10th	Night	61% 8% 12% 16% 4	5060
	11th to 20th	Night	61% 8% 9% 17% 5%	5742
	21st to end	Night	63% 6% 9% 15% 6%	7667

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	29% 23% 34% 12% 2 1	220
	11th to 20th	Day	33% 22% 29% 9% 6% 1	220
	21st to end	Day	20% 28% 31% 14% 5% 2	242
February	1st to 10th	Day	20% 26% 34% 10% 7% 3	317
	11th to 20th	Day	26% 27% 30% 13% 4	454
	21st to end	Day	26% 27% 28% 13% 6%	446
March	1st to 10th	Day	31% 21% 31% 14% 3	540
	11th to 20th	Day	29% 24% 29% 12% 6% 1	540
	21st to end	Day	35% 23% 23% 13% 5% 1	594
April	1st to 10th	Day	34% 24% 27% 10% 4 1	540
	11th to 20th	Day	31% 20% 33% 11% 4 1	532
	21st to end	Day	33% 20% 32% 11% 3 1	540
May	1st to 10th	Day	31% 21% 31% 13% 4	543
	11th to 20th	Day	28% 18% 32% 16% 6% 1	639
	21st to end	Day	30% 20% 27% 16% 6% 1	845
June	1st to 10th	Day	26% 17% 30% 19% 6% 1	780
	11th to 20th	Day	26% 20% 26% 19% 8% 1	780
	21st to end	Day	27% 16% 29% 20% 7% 1	780
July	1st to 10th	Day	21% 16% 33% 21% 7% 1	770
	11th to 20th	Day	18% 15% 38% 21% 8% 2	697
	21st to end	Day	16% 15% 39% 21% 7% 1	620
August	1st to 10th	Day	17% 16% 35% 22% 8% 2	566
	11th to 20th	Day	17% 12% 35% 27% 7% 2	559
	21st to end	Day	22% 17% 30% 19% 10% 3	619
September	1st to 10th	Day	18% 21% 30% 18% 11% 3	530
	11th to 20th	Day	19% 18% 32% 18% 10% 3	530
	21st to end	Day	24% 18% 29% 17% 8% 5%	530
October	1st to 10th	Day	22% 21% 29% 17% 10% 2	489
	11th to 20th	Day	34% 20% 21% 13% 10% 1	420
	21st to end	Day	35% 18% 29% 13% 4	462
November	1st to 10th	Day	35% 20% 27% 15% 3 1	425
	11th to 20th	Day	28% 23% 29% 14% 5% 2	430
	21st to end	Day	35% 25% 26% 11% 3	314
December	1st to 10th	Day	28% 25% 30% 14% 3 1	220
	11th to 20th	Day	30% 27% 31% 9% 2	220
	21st to end	Day	34% 22% 26% 14% 3 2	242
January	1st to 10th	Night	22% 22% 35% 17% 3	936
	11th to 20th	Night	24% 25% 27% 17% 6%	840
	21st to end	Night	15% 20% 38% 22% 4	924
February	1st to 10th	Night	17% 25% 31% 19% 7% 1	840
	11th to 20th	Night	23% 23% 32% 17% 4	823
	21st to end	Night	22% 18% 36% 19% 4	692
March	1st to 10th	Night	24% 17% 34% 21% 4	840
	11th to 20th	Night	25% 22% 29% 17% 6% 1	840
	21st to end	Night	24% 22% 27% 20% 6% 1	924
April	1st to 10th	Night	31% 20% 25% 18% 5% 1	831
	11th to 20th	Night	26% 18% 28% 22% 7%	624
	21st to end	Night	32% 15% 25% 20% 6% 1	630
May	1st to 10th	Night	28% 12% 30% 23% 6%	630
	11th to 20th	Night	24% 15% 29% 23% 8% 1	476
	21st to end	Night	23% 16% 28% 25% 8%	455
June	1st to 10th	Night	23% 9% 27% 34% 7%	420
	11th to 20th	Night	20% 10% 25% 32% 11% 1	420
	21st to end	Night	27% 9% 28% 28% 7%	420
July	1st to 10th	Night	16% 11% 33% 28% 10% 1	410
	11th to 20th	Night	13% 11% 34% 30% 12%	408
	21st to end	Night	16% 7% 30% 34% 12% 1	460
August	1st to 10th	Night	17% 6% 30% 33% 14%	502
	11th to 20th	Night	17% 7% 31% 33% 11% 1	622
	21st to end	Night	13% 13% 31% 29% 12% 1	688
September	1st to 10th	Night	18% 15% 30% 25% 11% 1	760
	11th to 20th	Night	21% 11% 27% 26% 13% 2	800
	21st to end	Night	19% 14% 31% 23% 13% 1	800
October	1st to 10th	Night	22% 17% 28% 24% 8% 1	800
	11th to 20th	Night	26% 17% 26% 20% 10%	800
	21st to end	Night	28% 20% 25% 21% 5%	880
November	1st to 10th	Night	27% 21% 27% 19% 5%	820
	11th to 20th	Night	27% 22% 26% 18% 6% 1	943
	21st to end	Night	29% 21% 27% 17% 5%	950
December	1st to 10th	Night	27% 20% 33% 15% 5% 1	950
	11th to 20th	Night	30% 22% 27% 17% 4	950
	21st to end	Night	29% 21% 27% 17% 5%	1045

Page generated at 2026-08-27 04:07:10. Version ac07

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