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 2024-10-25.

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% 3 9% 6% 5% 5% 4 6% 7% 29%	101557
	Night	15% 11% 4 9% 6% 5% 5% 4 6% 6% 29%	97302
Transparency	Day	54% 7% 13% 17% 8% 1	101303
	Night	54% 8% 13% 17% 7% 1	97177
Seeing	Day	26% 20% 30% 16% 6% 1	15776
	Night	24% 18% 29% 21% 7% 1	23798

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% 15% 6% 12% 10% 9% 6% 4 6% 6% 14%	2238
	11th to 20th	Day	11% 11% 4 16% 12% 7% 5% 5% 7% 6% 15%	2520
	21st to end	Day	7% 8% 4 13% 7% 9% 7% 6% 6% 7% 25%	2772
February	1st to 10th	Day	8% 15% 5% 10% 10% 9% 7% 5% 6% 6% 19%	1607
	11th to 20th	Day	12% 16% 5% 11% 7% 6% 6% 5% 6% 7% 19%	1709
	21st to end	Day	12% 12% 4 10% 8% 8% 6% 5% 5% 6% 24%	1570
March	1st to 10th	Day	15% 16% 4 9% 6% 6% 5% 3 4 6% 25%	3116
	11th to 20th	Day	16% 19% 6% 11% 8% 5% 4 4 4 5% 18%	3466
	21st to end	Day	17% 17% 4 9% 6% 3 4 3 4 5% 29%	3864
April	1st to 10th	Day	20% 14% 3 10% 6% 5% 4 3 5% 6% 24%	2287
	11th to 20th	Day	17% 12% 3 9% 5% 4 5% 3 5% 6% 31%	2442
	21st to end	Day	21% 14% 3 7% 4 4 4 3 5% 5% 32%	2470
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% 14% 4 9% 5% 4 4 4 4 5% 25%	1825
November	1st to 10th	Day	17% 15% 5% 10% 5% 5% 4 4 4 5% 27%	1671
	11th to 20th	Day	18% 12% 5% 12% 6% 6% 6% 4 6% 6% 21%	1532
	21st to end	Day	17% 16% 4 13% 7% 6% 5% 4 4 5% 20%	1416
December	1st to 10th	Day	19% 16% 5% 13% 7% 7% 5% 4 4 5% 14%	1330
	11th to 20th	Day	17% 17% 4 11% 6% 6% 5% 4 4 4 21%	1668
	21st to end	Day	19% 13% 4 11% 9% 5% 5% 3 5% 4 23%	2431
January	1st to 10th	Night	9% 15% 6% 14% 10% 11% 7% 5% 6% 6% 11%	4713
	11th to 20th	Night	11% 14% 4 10% 9% 7% 6% 5% 7% 7% 19%	4440
	21st to end	Night	6% 11% 5% 12% 7% 7% 6% 5% 6% 7% 28%	4884
February	1st to 10th	Night	9% 13% 4 10% 9% 7% 6% 5% 5% 6% 25%	2571
	11th to 20th	Night	10% 14% 5% 13% 8% 6% 5% 6% 7% 5% 21%	2356
	21st to end	Night	10% 15% 4 9% 7% 6% 4 4 5% 7% 30%	1884
March	1st to 10th	Night	15% 14% 4 9% 5% 6% 7% 4 4 5% 27%	3724
	11th to 20th	Night	12% 18% 7% 12% 7% 5% 4 3 4 6% 20%	3492
	21st to end	Night	19% 16% 4 9% 5% 4 3 2 4 5% 28%	3768
April	1st to 10th	Night	21% 13% 4 7% 5% 3 3 4 5% 4 29%	1892
	11th to 20th	Night	20% 8% 3 7% 5% 5% 5% 4 6% 6% 32%	1688
	21st to end	Night	22% 12% 3 6% 4 4 5% 4 4 5% 32%	1710
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%	2637
November	1st to 10th	Night	17% 10% 4 9% 6% 4 5% 4 6% 6% 28%	2407
	11th to 20th	Night	16% 12% 3 8% 5% 5% 5% 4 7% 7% 27%	2645
	21st to end	Night	16% 13% 4 12% 7% 6% 5% 3 6% 6% 21%	2759
December	1st to 10th	Night	19% 16% 5% 12% 7% 6% 5% 4 5% 5% 18%	2850
	11th to 20th	Night	18% 14% 4 11% 10% 6% 6% 5% 6% 5% 15%	3531
	21st to end	Night	16% 13% 5% 11% 9% 7% 5% 4 4 5% 20%	5236

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	66% 10% 13% 9% 2	2239
	11th to 20th	Day	65% 10% 10% 11% 3 1	2520
	21st to end	Day	56% 9% 11% 11% 12%	2772
February	1st to 10th	Day	62% 7% 11% 11% 8% 1	1607
	11th to 20th	Day	63% 8% 12% 12% 5%	1717
	21st to end	Day	58% 6% 11% 16% 8% 1	1570
March	1st to 10th	Day	59% 6% 12% 13% 7% 3	3119
	11th to 20th	Day	67% 8% 9% 9% 5% 1	3466
	21st to end	Day	59% 5% 6% 18% 9% 3	3864
April	1st to 10th	Day	60% 6% 10% 14% 7% 3	2287
	11th to 20th	Day	53% 6% 10% 19% 11% 2	2442
	21st to end	Day	56% 5% 7% 17% 13% 2	2470
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	50% 7% 12% 22% 9%	2021
	11th to 20th	Day	60% 7% 12% 17% 3	1815
	21st to end	Day	62% 6% 13% 13% 6% 1	1825
November	1st to 10th	Day	58% 8% 13% 17% 4	1671
	11th to 20th	Day	63% 8% 12% 13% 5%	1532
	21st to end	Day	65% 8% 11% 13% 3	1416
December	1st to 10th	Day	72% 5% 11% 8% 3	1330
	11th to 20th	Day	66% 6% 9% 12% 8%	1668
	21st to end	Day	65% 7% 12% 11% 4 1	2431
January	1st to 10th	Night	69% 9% 12% 8% 2	4713
	11th to 20th	Night	60% 10% 12% 11% 6%	4440
	21st to end	Night	52% 9% 10% 17% 11% 1	4884
February	1st to 10th	Night	56% 7% 11% 14% 10% 1	2571
	11th to 20th	Night	60% 10% 11% 12% 7%	2359
	21st to end	Night	54% 6% 10% 19% 10% 1	1884
March	1st to 10th	Night	59% 7% 8% 17% 8% 1	3734
	11th to 20th	Night	64% 6% 12% 11% 6% 1	3493
	21st to end	Night	59% 4 9% 19% 7% 1	3768
April	1st to 10th	Night	57% 5% 9% 14% 12% 3	1892
	11th to 20th	Night	51% 7% 9% 18% 14% 1	1688
	21st to end	Night	54% 5% 7% 19% 13% 2	1710
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	45% 7% 12% 26% 10% 1	2158
	11th to 20th	Night	52% 8% 14% 21% 5%	2363
	21st to end	Night	54% 8% 15% 15% 7% 1	2637
November	1st to 10th	Night	54% 9% 14% 17% 5% 1	2407
	11th to 20th	Night	55% 9% 15% 13% 7%	2645
	21st to end	Night	63% 9% 14% 11% 3 1	2759
December	1st to 10th	Night	67% 7% 11% 10% 4	2850
	11th to 20th	Night	66% 9% 11% 11% 3	3532
	21st to end	Night	66% 7% 11% 11% 5%	5236

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% 32% 13% 2 1	180
	11th to 20th	Day	33% 22% 31% 8% 4 1	180
	21st to end	Day	21% 29% 28% 14% 7% 2	198
February	1st to 10th	Day	22% 28% 34% 10% 6% 1	267
	11th to 20th	Day	26% 26% 31% 13% 4	388
	21st to end	Day	25% 29% 28% 12% 6%	382
March	1st to 10th	Day	28% 22% 31% 15% 3	460
	11th to 20th	Day	28% 23% 29% 13% 7% 1	460
	21st to end	Day	34% 23% 24% 12% 6% 1	506
April	1st to 10th	Day	34% 23% 26% 11% 5% 1	460
	11th to 20th	Day	29% 21% 33% 12% 4 2	452
	21st to end	Day	32% 23% 32% 11% 3 1	460
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	36% 19% 28% 13% 5%	411
November	1st to 10th	Day	34% 20% 26% 16% 3 1	365
	11th to 20th	Day	28% 22% 28% 14% 5% 2	370
	21st to end	Day	33% 26% 27% 11% 3	266
December	1st to 10th	Day	29% 24% 28% 14% 3 2	180
	11th to 20th	Day	29% 29% 32% 8% 2	180
	21st to end	Day	36% 22% 26% 13% 2 1	198
January	1st to 10th	Night	22% 22% 36% 17% 3	839
	11th to 20th	Night	25% 26% 28% 17% 5%	760
	21st to end	Night	16% 21% 36% 22% 5%	836
February	1st to 10th	Night	18% 26% 30% 19% 5% 1	760
	11th to 20th	Night	23% 23% 33% 16% 4	743
	21st to end	Night	22% 19% 35% 18% 4	628
March	1st to 10th	Night	23% 17% 34% 22% 4	760
	11th to 20th	Night	23% 21% 31% 17% 7% 1	760
	21st to end	Night	25% 22% 27% 19% 6% 1	836
April	1st to 10th	Night	31% 19% 26% 19% 5% 1	752
	11th to 20th	Night	25% 18% 28% 22% 7%	564
	21st to end	Night	33% 15% 25% 20% 6% 1	570
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% 24% 21% 5%	812
November	1st to 10th	Night	27% 21% 26% 20% 6%	740
	11th to 20th	Night	27% 23% 26% 18% 6% 1	843
	21st to end	Night	28% 21% 28% 17% 5%	850
December	1st to 10th	Night	28% 20% 32% 13% 5% 1	850
	11th to 20th	Night	29% 22% 27% 17% 4	850
	21st to end	Night	30% 22% 26% 17% 5%	935

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