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 2025-04-26.

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	16% 11% 4 9% 6% 5% 5% 4 5% 6% 28%	113087
	Night	14% 11% 4 10% 6% 6% 5% 4 6% 6% 28%	115578
Transparency	Day	55% 7% 13% 17% 8% 1	116578
	Night	54% 8% 13% 17% 8% 1	120252
Seeing	Day	26% 20% 30% 16% 6% 1	16316
	Night	24% 18% 29% 21% 7% 1	24572

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	12% 18% 8% 14% 9% 9% 6% 4 6% 5% 10%	3201
	11th to 20th	Day	9% 13% 6% 19% 13% 6% 6% 5% 6% 5% 12%	3570
	21st to end	Day	5% 6% 4 15% 7% 8% 7% 6% 7% 7% 28%	3960
February	1st to 10th	Day	7% 14% 5% 10% 10% 9% 7% 5% 7% 7% 19%	1692
	11th to 20th	Day	12% 16% 5% 11% 7% 6% 6% 5% 6% 7% 19%	1802
	21st to end	Day	12% 12% 4 10% 8% 8% 6% 5% 5% 6% 25%	1650
March	1st to 10th	Day	18% 17% 5% 10% 6% 5% 5% 3 4 6% 22%	4476
	11th to 20th	Day	14% 15% 5% 10% 8% 6% 5% 4 5% 6% 21%	4952
	21st to end	Day	17% 18% 5% 10% 6% 3 4 3 4 5% 26%	5528
April	1st to 10th	Day	20% 14% 3 10% 6% 5% 4 3 5% 6% 25%	2408
	11th to 20th	Day	17% 12% 3 9% 5% 4 5% 3 5% 6% 31%	2572
	21st to end	Day	21% 14% 3 7% 4 4 4 3 5% 5% 31%	2535
May	1st to 10th	Day	15% 12% 4 9% 7% 5% 5% 3 5% 6% 29%	4106
	11th to 20th	Day	17% 11% 3 8% 6% 5% 5% 4 6% 6% 29%	4449
	21st to end	Day	18% 8% 3 7% 5% 5% 6% 5% 6% 8% 30%	5185
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 4 6% 6% 29%	2850
	11th to 20th	Day	20% 7% 2 7% 4 4 4 4 6% 8% 33%	2850
	21st to end	Day	24% 10% 3 8% 6% 5% 4 3 6% 7% 25%	2850
July	1st to 10th	Day	14% 8% 3 8% 6% 6% 5% 4 7% 8% 31%	4760
	11th to 20th	Day	13% 6% 3 7% 6% 5% 6% 5% 7% 9% 34%	4644
	21st to end	Day	11% 6% 2 7% 5% 5% 7% 5% 6% 8% 38%	4814
August	1st to 10th	Day	14% 7% 3 8% 5% 5% 6% 5% 7% 8% 33%	4265
	11th to 20th	Day	17% 8% 3 9% 5% 5% 6% 5% 7% 7% 29%	4045
	21st to end	Day	17% 7% 3 7% 4 4 4 4 6% 7% 36%	4532
September	1st to 10th	Day	15% 8% 2 6% 5% 4 4 3 6% 8% 40%	2257
	11th to 20th	Day	15% 7% 2 6% 4 3 4 2 5% 6% 46%	2090
	21st to end	Day	18% 7% 3 8% 4 4 4 3 4 7% 38%	2090
October	1st to 10th	Day	17% 11% 3 7% 5% 5% 4 3 5% 6% 35%	2021
	11th to 20th	Day	20% 13% 4 10% 4 5% 5% 4 5% 6% 26%	1815
	21st to end	Day	23% 13% 4 9% 5% 4 4 4 4 5% 25%	1881
November	1st to 10th	Day	17% 15% 5% 9% 5% 5% 4 4 4 5% 27%	1761
	11th to 20th	Day	17% 14% 5% 12% 5% 6% 5% 4 5% 6% 21%	1612
	21st to end	Day	18% 16% 4 13% 7% 6% 5% 4 4 4 20%	1490
December	1st to 10th	Day	15% 16% 6% 14% 6% 6% 5% 4 5% 5% 19%	2280
	11th to 20th	Day	15% 14% 4 12% 7% 5% 5% 4 5% 4 24%	2618
	21st to end	Day	16% 14% 4 11% 8% 6% 6% 4 5% 5% 21%	3476
January	1st to 10th	Night	10% 16% 7% 14% 10% 10% 7% 5% 6% 5% 10%	6745
	11th to 20th	Night	9% 15% 6% 13% 10% 7% 6% 5% 6% 7% 16%	6315
	21st to end	Night	4 9% 4 11% 6% 6% 7% 5% 7% 9% 32%	6996
February	1st to 10th	Night	8% 13% 4 10% 9% 7% 6% 5% 6% 6% 26%	2706
	11th to 20th	Night	10% 14% 5% 13% 7% 6% 6% 5% 7% 6% 21%	2483
	21st to end	Night	10% 15% 4 9% 7% 6% 4 4 4 7% 30%	1980
March	1st to 10th	Night	17% 14% 4 9% 5% 5% 6% 4 4 6% 25%	5364
	11th to 20th	Night	12% 16% 6% 12% 6% 5% 5% 4 5% 7% 23%	5006
	21st to end	Night	19% 16% 5% 9% 5% 4 3 2 3 5% 29%	5390
April	1st to 10th	Night	21% 13% 4 8% 5% 3 3 4 5% 4 29%	1991
	11th to 20th	Night	20% 9% 3 8% 4 5% 5% 4 6% 6% 31%	1778
	21st to end	Night	22% 12% 2 6% 4 4 5% 4 4 5% 31%	1762
May	1st to 10th	Night	19% 10% 3 7% 4 3 4 4 5% 7% 34%	2834
	11th to 20th	Night	18% 10% 3 8% 5% 5% 4 4 5% 6% 32%	2462
	21st to end	Night	15% 7% 2 6% 5% 4 5% 5% 8% 8% 36%	2401
June	1st to 10th	Night	20% 9% 3 7% 5% 4 4 4 6% 6% 32%	1330
	11th to 20th	Night	15% 7% 2 5% 5% 6% 5% 4 6% 8% 38%	1330
	21st to end	Night	23% 9% 3 7% 5% 5% 5% 4 6% 6% 27%	1330
July	1st to 10th	Night	13% 6% 2 7% 5% 5% 5% 4 6% 8% 38%	2210
	11th to 20th	Night	16% 6% 2 6% 5% 5% 5% 5% 7% 8% 36%	2319
	21st to end	Night	11% 6% 2 6% 4 4 5% 5% 6% 8% 44%	2739
August	1st to 10th	Night	17% 6% 3 7% 6% 4 5% 4 6% 8% 34%	2634
	11th to 20th	Night	13% 7% 3 9% 6% 5% 6% 5% 7% 9% 31%	2782
	21st to end	Night	16% 7% 3 7% 4 4 4 3 4 6% 43%	3124
September	1st to 10th	Night	16% 5% 2 6% 4 4 4 4 7% 6% 42%	1919
	11th to 20th	Night	17% 7% 2 6% 3 3 4 4 5% 6% 43%	2090
	21st to end	Night	15% 7% 3 6% 4 4 4 4 5% 7% 42%	2090
October	1st to 10th	Night	17% 8% 3 7% 4 3 4 2 5% 7% 39%	2158
	11th to 20th	Night	21% 10% 2 6% 4 5% 5% 3 6% 6% 31%	2363
	21st to end	Night	21% 11% 3 8% 4 4 4 4 5% 7% 29%	2717
November	1st to 10th	Night	17% 10% 4 9% 6% 4 5% 4 6% 6% 28%	2537
	11th to 20th	Night	16% 13% 3 8% 5% 5% 5% 4 6% 6% 27%	2785
	21st to end	Night	16% 14% 4 12% 7% 6% 5% 3 6% 6% 20%	2905
December	1st to 10th	Night	13% 13% 6% 13% 8% 7% 6% 4 6% 7% 18%	4910
	11th to 20th	Night	18% 12% 4 12% 8% 6% 5% 4 5% 6% 20%	5591
	21st to end	Night	15% 12% 5% 12% 8% 8% 5% 4 4 5% 23%	7502

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	72% 9% 9% 8% 1	3202
	11th to 20th	Day	70% 8% 9% 10% 3 1	3600
	21st to end	Day	52% 8% 9% 12% 19%	3960
February	1st to 10th	Day	61% 7% 10% 12% 8% 1	1692
	11th to 20th	Day	63% 8% 12% 12% 5%	1810
	21st to end	Day	58% 6% 11% 17% 8% 1	1650
March	1st to 10th	Day	64% 5% 9% 10% 9% 3	4479
	11th to 20th	Day	62% 7% 12% 10% 7% 1	4952
	21st to end	Day	62% 4 5% 17% 8% 3	5528
April	1st to 10th	Day	60% 5% 10% 14% 8% 3	2408
	11th to 20th	Day	53% 6% 10% 19% 11% 2	2572
	21st to end	Day	56% 5% 8% 17% 12% 2	2535
May	1st to 10th	Day	55% 7% 11% 18% 9% 1	4120
	11th to 20th	Day	52% 8% 14% 17% 9%	4453
	21st to end	Day	49% 9% 12% 18% 12%	5185
June	1st to 10th	Day	53% 8% 10% 18% 10% 1	2850
	11th to 20th	Day	48% 7% 13% 19% 11% 1	2850
	21st to end	Day	58% 7% 12% 15% 8%	2850
July	1st to 10th	Day	48% 7% 14% 22% 9%	4685
	11th to 20th	Day	44% 7% 19% 25% 4	4563
	21st to end	Day	42% 9% 18% 24% 8%	4661
August	1st to 10th	Day	46% 9% 18% 19% 7%	4251
	11th to 20th	Day	51% 8% 17% 18% 5%	4084
	21st to end	Day	45% 7% 18% 19% 11%	4532
September	1st to 10th	Day	42% 7% 14% 26% 11%	2257
	11th to 20th	Day	40% 6% 15% 26% 12%	2090
	21st to end	Day	46% 7% 13% 24% 10%	2090
October	1st to 10th	Day	56% 7% 11% 21% 6%	3359
	11th to 20th	Day	57% 6% 11% 23% 3	2992
	21st to end	Day	49% 6% 17% 19% 8%	3081
November	1st to 10th	Day	58% 7% 12% 17% 4	1761
	11th to 20th	Day	63% 8% 11% 13% 5%	1612
	21st to end	Day	66% 8% 11% 12% 3	1490
December	1st to 10th	Day	66% 5% 14% 9% 7%	2280
	11th to 20th	Day	59% 5% 12% 16% 7%	2618
	21st to end	Day	65% 6% 10% 11% 7% 1	3476
January	1st to 10th	Night	71% 9% 10% 9% 1	6745
	11th to 20th	Night	63% 9% 13% 9% 4	6360
	21st to end	Night	46% 8% 10% 20% 15% 1	6996
February	1st to 10th	Night	56% 7% 11% 14% 11% 1	2706
	11th to 20th	Night	60% 10% 11% 12% 7%	2486
	21st to end	Night	54% 6% 10% 19% 10% 1	1980
March	1st to 10th	Night	60% 6% 7% 14% 12% 1	5374
	11th to 20th	Night	60% 6% 14% 11% 8% 1	5007
	21st to end	Night	58% 3 7% 19% 11% 1	5390
April	1st to 10th	Night	57% 5% 9% 14% 13% 3	1991
	11th to 20th	Night	52% 6% 9% 18% 14% 1	1778
	21st to end	Night	54% 5% 7% 18% 13% 2	1762
May	1st to 10th	Night	48% 7% 15% 17% 11% 2	2840
	11th to 20th	Night	49% 9% 14% 20% 7% 1	2463
	21st to end	Night	41% 10% 13% 24% 11%	2402
June	1st to 10th	Night	50% 7% 12% 21% 10% 1	1330
	11th to 20th	Night	43% 7% 15% 19% 14% 2	1330
	21st to end	Night	55% 8% 12% 16% 8%	1330
July	1st to 10th	Night	41% 8% 21% 25% 5%	2175
	11th to 20th	Night	43% 8% 22% 23% 4 1	2277
	21st to end	Night	36% 9% 26% 21% 8%	2651
August	1st to 10th	Night	48% 8% 17% 22% 5%	2624
	11th to 20th	Night	47% 11% 22% 13% 7%	2809
	21st to end	Night	43% 7% 15% 23% 12%	3124
September	1st to 10th	Night	39% 10% 16% 25% 10% 1	1919
	11th to 20th	Night	41% 9% 17% 22% 10%	2090
	21st to end	Night	41% 7% 12% 27% 12% 1	2090
October	1st to 10th	Night	48% 5% 11% 28% 7% 1	3600
	11th to 20th	Night	47% 8% 14% 25% 5%	3925
	21st to end	Night	47% 7% 18% 19% 9%	4467
November	1st to 10th	Night	54% 9% 14% 18% 5% 1	2537
	11th to 20th	Night	55% 9% 15% 14% 7%	2785
	21st to end	Night	64% 8% 13% 11% 3	2905
December	1st to 10th	Night	61% 8% 12% 16% 4	4910
	11th to 20th	Night	61% 8% 9% 17% 5%	5592
	21st to end	Night	63% 6% 9% 15% 6%	7502

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% 24% 35% 12% 2 1	200
	11th to 20th	Day	31% 22% 31% 10% 7% 1	200
	21st to end	Day	21% 28% 29% 14% 6% 2	220
February	1st to 10th	Day	22% 27% 34% 10% 7% 1	292
	11th to 20th	Day	27% 27% 30% 13% 3	421
	21st to end	Day	26% 28% 28% 12% 6%	414
March	1st to 10th	Day	29% 22% 31% 14% 3	500
	11th to 20th	Day	27% 24% 29% 12% 6% 1	500
	21st to end	Day	35% 23% 23% 13% 5% 1	550
April	1st to 10th	Day	34% 24% 26% 11% 4 1	500
	11th to 20th	Day	30% 20% 33% 11% 4 1	492
	21st to end	Day	32% 23% 32% 11% 3 1	480
May	1st to 10th	Day	29% 21% 31% 14% 5%	463
	11th to 20th	Day	26% 20% 32% 16% 6% 1	552
	21st to end	Day	30% 22% 27% 15% 6% 1	735
June	1st to 10th	Day	27% 18% 30% 19% 6% 1	680
	11th to 20th	Day	25% 20% 25% 20% 8% 1	680
	21st to end	Day	26% 17% 29% 21% 7%	680
July	1st to 10th	Day	22% 15% 33% 23% 7% 1	670
	11th to 20th	Day	17% 14% 37% 22% 8% 2	604
	21st to end	Day	16% 14% 40% 22% 7% 1	532
August	1st to 10th	Day	19% 17% 34% 20% 8% 1	486
	11th to 20th	Day	18% 12% 34% 28% 6% 2	479
	21st to end	Day	22% 17% 28% 19% 10% 3	539
September	1st to 10th	Day	19% 21% 30% 17% 10% 3	490
	11th to 20th	Day	20% 18% 33% 18% 9% 3	490
	21st to end	Day	24% 18% 29% 16% 7% 5%	490
October	1st to 10th	Day	22% 21% 27% 17% 9% 2	453
	11th to 20th	Day	35% 21% 21% 13% 9% 1	390
	21st to end	Day	35% 19% 29% 13% 4	429
November	1st to 10th	Day	35% 19% 26% 16% 3 1	395
	11th to 20th	Day	29% 23% 29% 14% 5% 2	400
	21st to end	Day	35% 25% 27% 10% 3	290
December	1st to 10th	Day	29% 25% 28% 15% 3 2	200
	11th to 20th	Day	30% 28% 31% 9% 3	200
	21st to end	Day	35% 21% 26% 13% 3 2	220
January	1st to 10th	Night	23% 22% 35% 16% 3	887
	11th to 20th	Night	25% 25% 27% 17% 6%	800
	21st to end	Night	15% 21% 38% 22% 4	880
February	1st to 10th	Night	18% 26% 31% 19% 5% 1	800
	11th to 20th	Night	24% 23% 33% 16% 4	783
	21st to end	Night	23% 19% 35% 19% 4	660
March	1st to 10th	Night	23% 17% 34% 21% 4	800
	11th to 20th	Night	24% 22% 30% 17% 6% 1	800
	21st to end	Night	25% 22% 27% 20% 6% 1	880
April	1st to 10th	Night	31% 20% 25% 18% 5% 1	791
	11th to 20th	Night	25% 18% 28% 22% 7%	594
	21st to end	Night	34% 15% 25% 20% 6% 1	588
May	1st to 10th	Night	28% 12% 30% 23% 6% 1	570
	11th to 20th	Night	23% 13% 30% 25% 8% 1	430
	21st to end	Night	23% 17% 28% 24% 8%	411
June	1st to 10th	Night	24% 9% 26% 34% 7% 1	380
	11th to 20th	Night	19% 10% 27% 33% 11% 1	380
	21st to end	Night	27% 9% 28% 27% 7% 1	380
July	1st to 10th	Night	16% 11% 33% 28% 11% 1	370
	11th to 20th	Night	14% 10% 32% 31% 12%	368
	21st to end	Night	17% 7% 29% 34% 12% 1	416
August	1st to 10th	Night	17% 7% 31% 32% 13%	454
	11th to 20th	Night	18% 8% 31% 31% 11% 1	562
	21st to end	Night	13% 14% 31% 28% 12% 2	627
September	1st to 10th	Night	18% 15% 30% 25% 11% 1	722
	11th to 20th	Night	22% 10% 28% 26% 12% 2	760
	21st to end	Night	19% 14% 30% 23% 13% 1	760
October	1st to 10th	Night	23% 17% 27% 24% 9% 1	760
	11th to 20th	Night	27% 17% 26% 19% 10%	760
	21st to end	Night	28% 21% 25% 21% 5%	836
November	1st to 10th	Night	27% 20% 27% 19% 6%	780
	11th to 20th	Night	28% 22% 26% 18% 5% 1	893
	21st to end	Night	30% 21% 27% 17% 5%	900
December	1st to 10th	Night	27% 20% 32% 14% 5% 1	900
	11th to 20th	Night	30% 22% 26% 17% 4	900
	21st to end	Night	29% 22% 27% 17% 5%	990

Page generated at 2025-04-23 09:25:07. Version ac07

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