Asteria Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Asteria Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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% 10% 4 8% 5% 5% 5% 4 6% 7% 33%	143489
	Night	15% 10% 4 8% 5% 4 4 3 5% 7% 35%	138413
Transparency	Day	48% 5% 10% 22% 13% 2	147150
	Night	48% 5% 10% 21% 13% 2	142812
Seeing	Day	22% 14% 27% 20% 12% 5%	17240
	Night	21% 12% 26% 27% 14% 2	27274

By Month

Percentage Cloud Cover SE (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	17% 19% 7% 12% 6% 4 4 2 4 5% 20%	5258
	11th to 20th	Day	12% 18% 7% 15% 6% 5% 4 2 3 4 24%	5280
	21st to end	Day	20% 13% 6% 10% 9% 6% 5% 3 4 4 19%	5780
February	1st to 10th	Day	21% 13% 4 9% 5% 4 3 3 5% 6% 27%	1890
	11th to 20th	Day	18% 13% 5% 12% 5% 4 4 3 5% 5% 27%	2022
	21st to end	Day	18% 14% 5% 8% 4 3 3 3 4 5% 32%	1730
March	1st to 10th	Day	18% 14% 5% 8% 4 4 4 3 4 6% 30%	4850
	11th to 20th	Day	13% 8% 3 7% 4 4 5% 3 6% 7% 40%	5090
	21st to end	Day	18% 9% 3 8% 5% 5% 4 3 5% 7% 34%	5586
April	1st to 10th	Day	15% 12% 3 8% 5% 4 5% 4 5% 6% 34%	2322
	11th to 20th	Day	16% 9% 3 7% 4 4 4 4 5% 6% 38%	2660
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 3 6% 7% 35%	2730
May	1st to 10th	Day	18% 9% 3 7% 5% 5% 5% 4 6% 7% 29%	7616
	11th to 20th	Day	17% 8% 3 8% 4 4 5% 4 7% 8% 32%	7626
	21st to end	Day	19% 8% 3 7% 5% 4 5% 4 6% 8% 31%	8391
June	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 4 7% 8% 38%	2730
	11th to 20th	Day	9% 7% 3 7% 6% 6% 5% 5% 6% 9% 38%	2730
	21st to end	Day	11% 6% 2 6% 6% 5% 6% 5% 8% 10% 35%	2730
July	1st to 10th	Day	9% 6% 3 8% 5% 5% 5% 4 6% 8% 41%	7680
	11th to 20th	Day	11% 7% 2 7% 5% 5% 5% 5% 8% 10% 36%	7680
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 5% 7% 10% 33%	8340
August	1st to 10th	Day	7% 6% 2 7% 6% 6% 6% 5% 7% 10% 38%	4364
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 5% 7% 11% 38%	4226
	21st to end	Day	5% 4 2 6% 5% 4 5% 4 6% 9% 50%	4587
September	1st to 10th	Day	15% 5% 2 5% 3 3 4 4 5% 8% 47%	2200
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 5% 7% 51%	2200
	21st to end	Day	18% 7% 3 6% 4 4 3 3 5% 6% 42%	2200
October	1st to 10th	Day	11% 6% 2 6% 4 3 4 3 7% 8% 45%	2133
	11th to 20th	Day	14% 8% 2 5% 4 4 4 3 5% 6% 45%	2000
	21st to end	Day	20% 10% 2 6% 4 3 4 2 4 6% 38%	2178
November	1st to 10th	Day	11% 8% 2 5% 4 4 3 3 5% 7% 49%	1852
	11th to 20th	Day	16% 12% 4 7% 5% 3 3 4 5% 6% 35%	1890
	21st to end	Day	18% 14% 4 7% 4 4 4 3 4 5% 32%	1890
December	1st to 10th	Day	19% 13% 5% 12% 6% 5% 4 3 4 4 26%	3020
	11th to 20th	Day	23% 14% 6% 10% 4 4 4 3 4 6% 22%	3463
	21st to end	Day	19% 21% 5% 11% 5% 5% 4 3 5% 5% 17%	4565
January	1st to 10th	Night	14% 17% 7% 16% 7% 5% 5% 3 4 4 18%	7635
	11th to 20th	Night	14% 16% 7% 15% 8% 5% 4 3 4 4 21%	7680
	21st to end	Night	18% 12% 5% 9% 7% 5% 4 3 4 5% 29%	8401
February	1st to 10th	Night	20% 13% 4 9% 5% 4 4 3 4 5% 29%	2730
	11th to 20th	Night	18% 14% 5% 9% 5% 4 4 3 4 5% 28%	2479
	21st to end	Night	19% 15% 6% 8% 4 4 3 3 3 5% 30%	2076
March	1st to 10th	Night	19% 13% 3 7% 3 4 4 3 4 6% 35%	5222
	11th to 20th	Night	13% 9% 3 6% 3 4 4 3 6% 8% 42%	5090
	21st to end	Night	17% 10% 3 8% 5% 5% 4 4 5% 5% 34%	5588
April	1st to 10th	Night	16% 11% 3 7% 4 3 4 4 6% 6% 37%	2310
	11th to 20th	Night	13% 8% 2 6% 5% 4 5% 5% 7% 8% 38%	1914
	21st to end	Night	14% 8% 3 7% 5% 5% 4 3 6% 7% 37%	1890
May	1st to 10th	Night	22% 8% 3 7% 4 5% 4 4 5% 7% 30%	5237
	11th to 20th	Night	14% 7% 3 6% 4 4 4 4 6% 10% 37%	5234
	21st to end	Night	18% 8% 3 7% 4 4 4 3 6% 9% 33%	5761
June	1st to 10th	Night	10% 6% 2 6% 3 4 5% 4 7% 9% 45%	1890
	11th to 20th	Night	8% 5% 2 5% 4 5% 5% 4 7% 10% 44%	1890
	21st to end	Night	10% 5% 2 6% 4 3 5% 5% 8% 9% 43%	1890
July	1st to 10th	Night	8% 7% 3 7% 5% 4 4 4 6% 8% 45%	5278
	11th to 20th	Night	11% 7% 2 7% 5% 5% 5% 4 6% 9% 38%	5277
	21st to end	Night	8% 6% 2 8% 5% 5% 6% 4 7% 9% 40%	5736
August	1st to 10th	Night	7% 7% 3 8% 5% 5% 5% 4 7% 10% 39%	3005
	11th to 20th	Night	9% 8% 3 7% 5% 4 4 4 5% 7% 43%	2911
	21st to end	Night	5% 3 2 5% 5% 4 5% 5% 7% 10% 51%	3279
September	1st to 10th	Night	11% 5% 2 5% 3 4 3 3 5% 7% 52%	2195
	11th to 20th	Night	8% 4 2 5% 4 4 4 4 5% 7% 54%	2200
	21st to end	Night	16% 7% 3 6% 3 4 3 3 4 5% 47%	2200
October	1st to 10th	Night	10% 6% 2 6% 2 3 4 3 5% 7% 52%	2265
	11th to 20th	Night	14% 6% 2 6% 3 4 4 3 5% 7% 46%	2400
	21st to end	Night	18% 8% 3 5% 4 4 4 4 5% 6% 40%	2659
November	1st to 10th	Night	14% 7% 2 5% 4 3 3 3 4 6% 49%	2666
	11th to 20th	Night	15% 11% 3 8% 4 3 4 3 5% 6% 37%	2730
	21st to end	Night	19% 13% 4 7% 4 4 4 3 4 6% 31%	2730
December	1st to 10th	Night	18% 12% 5% 10% 5% 4 4 3 4 5% 30%	4380
	11th to 20th	Night	19% 17% 7% 10% 4 3 4 3 4 5% 24%	4952
	21st to end	Night	21% 20% 6% 10% 6% 5% 4 3 4 5% 17%	6633

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	68% 3 13% 13% 3	5259
	11th to 20th	Day	66% 2 9% 11% 11% 1	5280
	21st to end	Day	69% 3 9% 13% 6% 1	5780
February	1st to 10th	Day	58% 4 12% 17% 8% 1	1890
	11th to 20th	Day	60% 4 9% 16% 10%	2032
	21st to end	Day	56% 3 11% 20% 9% 1	1730
March	1st to 10th	Day	56% 4 6% 22% 10% 2	4850
	11th to 20th	Day	43% 4 10% 27% 14% 2	5090
	21st to end	Day	51% 4 9% 21% 12% 2	5586
April	1st to 10th	Day	50% 5% 10% 21% 11% 2	2322
	11th to 20th	Day	46% 6% 10% 21% 15% 4	2660
	21st to end	Day	48% 5% 11% 21% 12% 2	2730
May	1st to 10th	Day	52% 6% 10% 21% 10% 1	7651
	11th to 20th	Day	48% 6% 11% 23% 12% 1	7630
	21st to end	Day	48% 7% 10% 16% 14% 4	8405
June	1st to 10th	Day	40% 8% 11% 22% 17% 2	2730
	11th to 20th	Day	41% 8% 13% 20% 16% 1	2730
	21st to end	Day	40% 9% 12% 22% 14% 2	2730
July	1st to 10th	Day	41% 6% 11% 26% 15% 1	7615
	11th to 20th	Day	41% 7% 10% 30% 11%	7611
	21st to end	Day	44% 6% 11% 20% 17% 2	8156
August	1st to 10th	Day	38% 8% 14% 29% 11% 1	4351
	11th to 20th	Day	38% 7% 10% 30% 15% 1	4265
	21st to end	Day	30% 6% 14% 30% 18% 3	4587
September	1st to 10th	Day	35% 7% 10% 22% 22% 5%	2200
	11th to 20th	Day	32% 7% 11% 25% 24% 2	2200
	21st to end	Day	44% 4 9% 22% 19% 1	2200
October	1st to 10th	Day	25% 4 9% 30% 31% 2	3471
	11th to 20th	Day	33% 4 10% 29% 22% 3	3240
	21st to end	Day	36% 3 11% 27% 20% 2	3492
November	1st to 10th	Day	36% 6% 9% 29% 18% 3	1852
	11th to 20th	Day	50% 4 9% 21% 14% 2	1890
	21st to end	Day	55% 4 10% 20% 11% 1	1890
December	1st to 10th	Day	62% 3 10% 20% 5%	3020
	11th to 20th	Day	65% 3 9% 18% 5%	3460
	21st to end	Day	70% 4 10% 12% 5%	4565
January	1st to 10th	Night	70% 3 9% 13% 4	7636
	11th to 20th	Night	68% 3 7% 12% 8% 1	7680
	21st to end	Night	60% 3 10% 19% 7% 1	8401
February	1st to 10th	Night	58% 4 11% 18% 8% 1	2730
	11th to 20th	Night	58% 4 9% 17% 11% 1	2480
	21st to end	Night	59% 4 10% 17% 9% 2	2076
March	1st to 10th	Night	51% 4 7% 26% 11% 2	5222
	11th to 20th	Night	40% 4 10% 30% 15% 1	5090
	21st to end	Night	52% 4 8% 23% 12% 2	5588
April	1st to 10th	Night	47% 5% 12% 20% 14% 2	2310
	11th to 20th	Night	41% 6% 12% 19% 18% 5%	1914
	21st to end	Night	45% 6% 10% 23% 15% 2	1890
May	1st to 10th	Night	51% 6% 11% 19% 9% 3	5264
	11th to 20th	Night	43% 7% 12% 24% 14% 1	5235
	21st to end	Night	47% 6% 12% 19% 11% 4	5771
June	1st to 10th	Night	34% 8% 11% 23% 21% 3	1890
	11th to 20th	Night	33% 9% 15% 22% 19% 2	1890
	21st to end	Night	34% 9% 14% 22% 18% 3	1890
July	1st to 10th	Night	38% 6% 10% 26% 18% 3	5235
	11th to 20th	Night	41% 7% 12% 28% 11% 1	5226
	21st to end	Night	38% 6% 12% 23% 17% 3	5606
August	1st to 10th	Night	39% 8% 15% 27% 11% 1	2994
	11th to 20th	Night	40% 7% 9% 24% 21% 1	2938
	21st to end	Night	28% 6% 14% 29% 20% 3	3279
September	1st to 10th	Night	31% 7% 12% 23% 22% 5%	2195
	11th to 20th	Night	29% 6% 10% 25% 27% 3	2200
	21st to end	Night	40% 5% 9% 21% 25% 1	2200
October	1st to 10th	Night	27% 4 7% 27% 32% 3	3707
	11th to 20th	Night	30% 4 9% 26% 24% 7%	3899
	21st to end	Night	33% 4 8% 24% 26% 5%	4283
November	1st to 10th	Night	37% 3 11% 25% 20% 3	2666
	11th to 20th	Night	48% 4 8% 21% 16% 3	2730
	21st to end	Night	55% 4 10% 20% 10% 1	2730
December	1st to 10th	Night	57% 4 12% 22% 5%	4380
	11th to 20th	Night	63% 3 11% 17% 5%	4954
	21st to end	Night	69% 4 9% 14% 4	6633

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% 22% 33% 9% 4	330
	11th to 20th	Day	35% 28% 29% 7% 1	330
	21st to end	Day	37% 23% 30% 8% 1	363
February	1st to 10th	Day	34% 22% 28% 13% 2	330
	11th to 20th	Day	39% 25% 30% 4 2	330
	21st to end	Day	35% 24% 35% 5% 1	273
March	1st to 10th	Day	32% 26% 26% 12% 3	444
	11th to 20th	Day	32% 21% 31% 11% 5%	540
	21st to end	Day	34% 19% 31% 13% 3	594
April	1st to 10th	Day	28% 20% 31% 14% 7% 1	540
	11th to 20th	Day	27% 18% 29% 18% 6% 2	532
	21st to end	Day	27% 17% 31% 17% 6% 2	540
May	1st to 10th	Day	28% 20% 29% 15% 7% 1	543
	11th to 20th	Day	22% 15% 28% 21% 9% 4	567
	21st to end	Day	19% 9% 26% 25% 16% 5%	617
June	1st to 10th	Day	14% 10% 27% 28% 15% 5%	570
	11th to 20th	Day	17% 9% 25% 31% 13% 4	570
	21st to end	Day	11% 11% 27% 30% 15% 7%	570
July	1st to 10th	Day	12% 4 20% 27% 20% 16%	565
	11th to 20th	Day	11% 6% 22% 29% 22% 11%	564
	21st to end	Day	11% 4 25% 28% 20% 11%	620
August	1st to 10th	Day	11% 4 23% 30% 23% 9%	566
	11th to 20th	Day	14% 5% 22% 31% 17% 11%	535
	21st to end	Day	10% 4 21% 31% 23% 11%	583
September	1st to 10th	Day	14% 5% 19% 24% 25% 12%	530
	11th to 20th	Day	10% 3 14% 29% 31% 13%	530
	21st to end	Day	23% 9% 21% 25% 17% 4	530
October	1st to 10th	Day	12% 7% 28% 27% 20% 6%	530
	11th to 20th	Day	20% 12% 32% 21% 12% 3	530
	21st to end	Day	25% 15% 33% 19% 7%	561
November	1st to 10th	Day	19% 16% 36% 21% 6% 1	330
	11th to 20th	Day	33% 17% 29% 15% 5% 1	330
	21st to end	Day	32% 20% 32% 12% 4	330
December	1st to 10th	Day	30% 21% 35% 11% 2 1	330
	11th to 20th	Day	28% 26% 34% 7% 5%	330
	21st to end	Day	34% 22% 29% 12% 3	363
January	1st to 10th	Night	30% 19% 30% 17% 4	840
	11th to 20th	Night	31% 22% 28% 16% 3	840
	21st to end	Night	27% 20% 32% 19% 2	924
February	1st to 10th	Night	30% 15% 31% 23% 1	840
	11th to 20th	Night	31% 22% 28% 18% 1	823
	21st to end	Night	30% 17% 32% 19% 2	692
March	1st to 10th	Night	26% 17% 29% 23% 4	840
	11th to 20th	Night	27% 18% 28% 21% 6% 1	840
	21st to end	Night	26% 16% 31% 20% 7%	924
April	1st to 10th	Night	23% 17% 32% 22% 6%	840
	11th to 20th	Night	17% 13% 36% 24% 9% 1	628
	21st to end	Night	19% 11% 36% 26% 6% 1	630
May	1st to 10th	Night	20% 11% 31% 27% 10% 2	630
	11th to 20th	Night	19% 8% 29% 28% 15% 1	621
	21st to end	Night	13% 5% 24% 32% 24% 2	683
June	1st to 10th	Night	12% 4 23% 39% 21% 1	630
	11th to 20th	Night	11% 4 22% 38% 23% 2	630
	21st to end	Night	11% 4 22% 36% 24% 3	630
July	1st to 10th	Night	11% 3 15% 33% 31% 7%	615
	11th to 20th	Night	7% 2 17% 37% 31% 6%	612
	21st to end	Night	10% 3 17% 33% 33% 4	690
August	1st to 10th	Night	7% 3 15% 40% 32% 3	627
	11th to 20th	Night	9% 3 15% 38% 30% 5%	606
	21st to end	Night	9% 2 17% 32% 33% 7%	660
September	1st to 10th	Night	14% 4 16% 33% 28% 5%	795
	11th to 20th	Night	9% 4 14% 36% 33% 5%	800
	21st to end	Night	18% 7% 22% 32% 19% 3	800
October	1st to 10th	Night	14% 6% 22% 33% 22% 4	800
	11th to 20th	Night	18% 10% 27% 31% 13% 1	800
	21st to end	Night	22% 13% 28% 27% 10% 1	880
November	1st to 10th	Night	20% 10% 26% 28% 16%	820
	11th to 20th	Night	25% 16% 27% 24% 8% 1	840
	21st to end	Night	28% 14% 25% 27% 6% 1	840
December	1st to 10th	Night	31% 17% 27% 21% 4	840
	11th to 20th	Night	27% 16% 30% 21% 5%	840
	21st to end	Night	30% 17% 27% 23% 2	924

Page generated at 2026-08-12 03:37:20. Version ac07

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