Astraeus Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astraeus Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	13% 8% 3 7% 5% 5% 5% 4 6% 8% 37%	149545
	Night	14% 9% 3 8% 5% 4 4 3 5% 7% 38%	141879
Transparency	Day	45% 4 11% 24% 15% 2	153213
	Night	45% 4 10% 23% 16% 2	146283
Seeing	Day	17% 9% 23% 24% 18% 9%	17718
	Night	18% 7% 23% 32% 18% 2	27152

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	16% 14% 7% 9% 5% 4 4 3 4 6% 28%	5258
	11th to 20th	Day	12% 11% 5% 15% 4 4 3 3 3 4 36%	5280
	21st to end	Day	21% 11% 5% 7% 5% 4 4 3 5% 5% 28%	5780
February	1st to 10th	Day	17% 13% 4 9% 4 4 4 3 4 6% 32%	2079
	11th to 20th	Day	19% 9% 4 9% 5% 5% 4 3 4 5% 33%	2048
	21st to end	Day	18% 12% 4 8% 4 4 5% 3 6% 5% 31%	1730
March	1st to 10th	Day	19% 12% 4 7% 4 4 4 3 4 5% 36%	4580
	11th to 20th	Day	12% 7% 3 7% 4 4 4 4 6% 7% 42%	5086
	21st to end	Day	14% 8% 3 8% 5% 4 4 3 5% 7% 37%	5586
April	1st to 10th	Day	14% 7% 3 6% 5% 4 5% 4 5% 7% 40%	2499
	11th to 20th	Day	15% 8% 3 6% 4 4 4 2 6% 6% 41%	2499
	21st to end	Day	16% 7% 3 6% 5% 4 5% 4 5% 7% 37%	2730
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 6% 4 7% 8% 29%	7616
	11th to 20th	Day	14% 8% 2 8% 5% 5% 5% 4 5% 7% 38%	7626
	21st to end	Day	17% 8% 3 8% 5% 5% 6% 4 7% 9% 26%	8391
June	1st to 10th	Day	11% 6% 2 6% 5% 6% 5% 5% 6% 7% 42%	2730
	11th to 20th	Day	7% 5% 2 6% 5% 5% 7% 5% 7% 9% 41%	2836
	21st to end	Day	8% 6% 3 5% 4 5% 6% 5% 7% 11% 42%	2940
July	1st to 10th	Day	9% 7% 3 8% 5% 5% 5% 4 6% 10% 37%	8280
	11th to 20th	Day	9% 6% 2 7% 5% 6% 6% 5% 8% 11% 34%	7747
	21st to end	Day	10% 8% 3 9% 6% 6% 7% 5% 7% 10% 29%	8340
August	1st to 10th	Day	9% 5% 2 7% 5% 5% 5% 5% 8% 11% 38%	6014
	11th to 20th	Day	7% 5% 2 7% 5% 5% 6% 5% 9% 12% 36%	5793
	21st to end	Day	4 3 1 5% 4 3 4 3 5% 9% 58%	6080
September	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 6% 8% 44%	2401
	11th to 20th	Day	11% 5% 2 5% 3 3 4 4 6% 8% 48%	2310
	21st to end	Day	15% 6% 2 5% 4 4 4 4 4 7% 46%	2288
October	1st to 10th	Day	10% 4 2 4 3 4 5% 3 5% 6% 54%	2200
	11th to 20th	Day	12% 6% 2 5% 3 3 4 3 5% 6% 51%	2072
	21st to end	Day	19% 7% 2 6% 2 4 3 3 4 5% 45%	2046
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 2 4 6% 52%	1852
	11th to 20th	Day	16% 8% 3 6% 5% 3 3 3 4 6% 43%	1890
	21st to end	Day	18% 10% 3 7% 4 3 4 4 4 5% 38%	1890
December	1st to 10th	Day	17% 11% 3 8% 4 3 3 3 4 5% 38%	3020
	11th to 20th	Day	19% 12% 4 10% 5% 4 4 3 4 4 29%	3463
	21st to end	Day	20% 16% 5% 10% 5% 5% 4 3 4 5% 22%	4565
January	1st to 10th	Night	14% 14% 6% 12% 5% 4 4 3 4 5% 29%	7635
	11th to 20th	Night	15% 12% 5% 10% 5% 4 4 3 4 5% 32%	7680
	21st to end	Night	18% 11% 4 9% 5% 4 3 3 4 5% 35%	8401
February	1st to 10th	Night	21% 10% 5% 8% 4 4 4 4 5% 6% 28%	2537
	11th to 20th	Night	21% 9% 4 9% 5% 3 4 3 4 4 33%	2458
	21st to end	Night	20% 12% 3 10% 5% 3 4 3 4 5% 31%	2076
March	1st to 10th	Night	19% 14% 2 6% 4 4 3 3 4 5% 35%	5493
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 5% 6% 46%	5094
	21st to end	Night	15% 9% 3 8% 5% 5% 5% 4 5% 6% 34%	5588
April	1st to 10th	Night	13% 6% 2 5% 5% 5% 5% 4 6% 7% 41%	2137
	11th to 20th	Night	15% 7% 3 6% 4 4 4 3 6% 6% 41%	2069
	21st to end	Night	15% 8% 3 7% 5% 4 5% 4 5% 7% 39%	1890
May	1st to 10th	Night	17% 9% 3 8% 5% 5% 6% 4 6% 8% 29%	5237
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 5% 6% 8% 36%	5234
	21st to end	Night	15% 8% 3 9% 5% 5% 5% 4 7% 8% 30%	5761
June	1st to 10th	Night	8% 6% 2 5% 4 4 4 4 6% 11% 47%	1890
	11th to 20th	Night	5% 5% 2 6% 5% 4 5% 4 7% 10% 48%	1775
	21st to end	Night	6% 4 2 5% 5% 5% 4 5% 8% 12% 45%	1680
July	1st to 10th	Night	7% 6% 3 7% 5% 5% 5% 4 6% 9% 45%	4708
	11th to 20th	Night	8% 6% 3 7% 5% 5% 5% 4 7% 10% 40%	5180
	21st to end	Night	9% 7% 3 9% 6% 6% 6% 5% 7% 10% 32%	5736
August	1st to 10th	Night	7% 7% 3 7% 4 6% 5% 3 7% 9% 42%	4135
	11th to 20th	Night	5% 6% 2 7% 5% 5% 6% 4 7% 11% 42%	4236
	21st to end	Night	3 3 1 3 3 3 3 3 6% 10% 62%	5046
September	1st to 10th	Night	11% 5% 2 4 4 4 4 3 5% 8% 50%	2197
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 6% 8% 51%	2310
	21st to end	Night	13% 6% 2 5% 4 3 3 2 4 6% 51%	2289
October	1st to 10th	Night	10% 4 1 4 4 3 3 3 4 6% 58%	2200
	11th to 20th	Night	13% 6% 2 5% 4 3 4 2 5% 6% 50%	2326
	21st to end	Night	17% 7% 2 5% 4 4 3 3 4 6% 45%	2790
November	1st to 10th	Night	12% 6% 2 5% 4 3 4 3 5% 5% 49%	2666
	11th to 20th	Night	14% 8% 3 6% 4 3 3 3 4 6% 45%	2730
	21st to end	Night	18% 11% 3 6% 4 4 3 3 4 6% 39%	2730
December	1st to 10th	Night	18% 12% 4 9% 4 4 3 3 3 4 37%	4380
	11th to 20th	Night	21% 11% 4 7% 4 3 3 2 5% 6% 34%	4952
	21st to end	Night	20% 16% 5% 10% 6% 5% 5% 3 4 5% 21%	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	59% 2 8% 21% 9% 1	5259
	11th to 20th	Day	53% 2 6% 21% 15% 2	5280
	21st to end	Day	58% 2 8% 21% 9% 3	5780
February	1st to 10th	Day	54% 3 7% 22% 11% 1	2079
	11th to 20th	Day	55% 3 8% 20% 11% 3	2058
	21st to end	Day	54% 3 8% 21% 11% 3	1730
March	1st to 10th	Day	53% 2 6% 17% 17% 6%	4580
	11th to 20th	Day	41% 3 9% 25% 20% 2	5086
	21st to end	Day	47% 3 11% 27% 12% 1	5586
April	1st to 10th	Day	42% 4 13% 24% 14% 2	2499
	11th to 20th	Day	45% 3 9% 23% 17% 3	2499
	21st to end	Day	46% 4 10% 25% 12% 2	2730
May	1st to 10th	Day	50% 4 13% 22% 10%	7651
	11th to 20th	Day	46% 4 12% 23% 15% 1	7630
	21st to end	Day	52% 5% 13% 15% 13% 1	8405
June	1st to 10th	Day	40% 5% 9% 26% 17% 2	2730
	11th to 20th	Day	37% 7% 15% 24% 16% 2	2836
	21st to end	Day	35% 7% 15% 22% 17% 4	2940
July	1st to 10th	Day	41% 7% 12% 24% 15% 1	8211
	11th to 20th	Day	41% 8% 14% 26% 10% 1	7676
	21st to end	Day	47% 7% 13% 21% 11% 1	8156
August	1st to 10th	Day	37% 7% 13% 30% 12% 1	6001
	11th to 20th	Day	36% 7% 16% 27% 12% 1	5832
	21st to end	Day	23% 5% 13% 39% 18% 2	6079
September	1st to 10th	Day	37% 4 12% 22% 20% 5%	2401
	11th to 20th	Day	33% 5% 11% 25% 23% 3	2310
	21st to end	Day	40% 4 7% 23% 23% 4	2288
October	1st to 10th	Day	22% 3 5% 27% 41% 3	3590
	11th to 20th	Day	25% 2 9% 29% 32% 3	3351
	21st to end	Day	31% 3 8% 30% 21% 8%	3283
November	1st to 10th	Day	35% 2 7% 29% 23% 4	1852
	11th to 20th	Day	44% 2 7% 24% 18% 5%	1890
	21st to end	Day	48% 3 8% 22% 18% 1	1890
December	1st to 10th	Day	49% 3 6% 24% 16%	3020
	11th to 20th	Day	60% 2 7% 18% 12% 1	3460
	21st to end	Day	65% 3 10% 16% 6% 1	4565
January	1st to 10th	Night	59% 2 9% 21% 8% 1	7636
	11th to 20th	Night	55% 2 6% 19% 15% 3	7680
	21st to end	Night	53% 2 8% 21% 13% 2	8401
February	1st to 10th	Night	56% 3 10% 19% 10% 1	2537
	11th to 20th	Night	55% 2 7% 22% 13% 2	2459
	21st to end	Night	57% 2 9% 16% 13% 3	2076
March	1st to 10th	Night	53% 2 5% 22% 14% 4	5493
	11th to 20th	Night	40% 2 8% 26% 20% 3	5094
	21st to end	Night	51% 3 9% 24% 13% 1	5588
April	1st to 10th	Night	42% 4 11% 23% 17% 3	2137
	11th to 20th	Night	44% 4 9% 20% 18% 6%	2069
	21st to end	Night	44% 5% 10% 24% 14% 2	1890
May	1st to 10th	Night	52% 5% 10% 21% 11% 1	5264
	11th to 20th	Night	45% 6% 13% 23% 13% 1	5235
	21st to end	Night	50% 6% 12% 20% 11% 1	5771
June	1st to 10th	Night	33% 4 13% 27% 19% 4	1890
	11th to 20th	Night	30% 6% 15% 24% 22% 4	1775
	21st to end	Night	30% 7% 18% 23% 16% 6%	1680
July	1st to 10th	Night	36% 7% 11% 27% 17% 1	4670
	11th to 20th	Night	37% 8% 15% 26% 12% 1	5131
	21st to end	Night	44% 8% 15% 20% 12% 1	5606
August	1st to 10th	Night	37% 6% 14% 28% 13% 1	4124
	11th to 20th	Night	34% 7% 18% 28% 12% 1	4263
	21st to end	Night	18% 5% 14% 39% 21% 3	5046
September	1st to 10th	Night	32% 5% 13% 22% 23% 6%	2197
	11th to 20th	Night	32% 4 11% 26% 24% 3	2310
	21st to end	Night	35% 4 8% 24% 25% 5%	2289
October	1st to 10th	Night	21% 2 7% 23% 44% 3	3590
	11th to 20th	Night	25% 2 6% 26% 35% 6%	3786
	21st to end	Night	31% 2 7% 21% 30% 9%	4503
November	1st to 10th	Night	36% 3 7% 26% 23% 4	2666
	11th to 20th	Night	41% 3 7% 22% 21% 5%	2730
	21st to end	Night	47% 3 8% 24% 16% 2	2730
December	1st to 10th	Night	52% 3 9% 22% 14%	4380
	11th to 20th	Night	52% 3 9% 22% 13% 1	4954
	21st to end	Night	67% 3 8% 15% 7%	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	22% 18% 34% 19% 6% 1	330
	11th to 20th	Day	26% 25% 32% 14% 4	330
	21st to end	Day	24% 18% 39% 16% 3	363
February	1st to 10th	Day	22% 17% 42% 16% 4	330
	11th to 20th	Day	30% 21% 35% 11% 3	330
	21st to end	Day	27% 18% 35% 16% 3	273
March	1st to 10th	Day	25% 16% 41% 13% 5%	330
	11th to 20th	Day	21% 15% 39% 18% 7%	536
	21st to end	Day	25% 14% 34% 19% 7% 1	594
April	1st to 10th	Day	20% 15% 29% 25% 10% 1	540
	11th to 20th	Day	19% 14% 28% 24% 11% 4	532
	21st to end	Day	20% 11% 30% 25% 10% 4	540
May	1st to 10th	Day	19% 11% 31% 22% 11% 5%	543
	11th to 20th	Day	17% 7% 25% 26% 16% 8%	567
	21st to end	Day	14% 6% 19% 27% 19% 13%	617
June	1st to 10th	Day	13% 4 18% 26% 26% 13%	570
	11th to 20th	Day	9% 3 19% 31% 26% 13%	676
	21st to end	Day	10% 4 18% 31% 24% 14%	780
July	1st to 10th	Day	12% 4 13% 23% 27% 22%	771
	11th to 20th	Day	9% 2 13% 28% 29% 19%	590
	21st to end	Day	11% 3 10% 28% 30% 17%	620
August	1st to 10th	Day	12% 1 11% 27% 29% 20%	566
	11th to 20th	Day	12% 3 11% 28% 32% 15%	559
	21st to end	Day	7% 2 7% 28% 34% 23%	627
September	1st to 10th	Day	13% 2 11% 26% 29% 19%	570
	11th to 20th	Day	10% 3 10% 23% 36% 18%	570
	21st to end	Day	16% 7% 15% 24% 25% 11%	562
October	1st to 10th	Day	13% 2 15% 32% 27% 10%	530
	11th to 20th	Day	13% 12% 23% 32% 14% 6%	530
	21st to end	Day	21% 10% 32% 23% 13% 1	429
November	1st to 10th	Day	17% 7% 30% 30% 12% 4	330
	11th to 20th	Day	22% 16% 28% 26% 8% 1	330
	21st to end	Day	25% 12% 31% 24% 7% 2	330
December	1st to 10th	Day	24% 17% 30% 22% 7% 1	330
	11th to 20th	Day	19% 15% 38% 18% 9% 1	330
	21st to end	Day	29% 18% 29% 18% 6%	363
January	1st to 10th	Night	22% 13% 32% 28% 4	840
	11th to 20th	Night	27% 15% 33% 22% 3	840
	21st to end	Night	23% 11% 37% 27% 2	924
February	1st to 10th	Night	27% 10% 33% 28% 2	840
	11th to 20th	Night	29% 11% 34% 24% 2	823
	21st to end	Night	26% 11% 39% 22% 2	692
March	1st to 10th	Night	23% 12% 33% 26% 6%	840
	11th to 20th	Night	24% 9% 34% 27% 5% 1	840
	21st to end	Night	21% 12% 31% 29% 7%	924
April	1st to 10th	Night	16% 10% 32% 33% 8%	667
	11th to 20th	Night	17% 8% 32% 31% 11% 1	623
	21st to end	Night	18% 6% 31% 32% 11% 1	630
May	1st to 10th	Night	19% 5% 26% 34% 14% 2	630
	11th to 20th	Night	15% 6% 15% 40% 19% 3	621
	21st to end	Night	10% 3 15% 38% 28% 6%	683
June	1st to 10th	Night	10% 2 14% 43% 29% 3	630
	11th to 20th	Night	7% 3 13% 43% 31% 4	630
	21st to end	Night	7% 2 13% 42% 32% 4	630
July	1st to 10th	Night	11% 1 8% 30% 41% 9%	615
	11th to 20th	Night	7% 3 8% 35% 41% 7%	612
	21st to end	Night	10% 3 7% 36% 39% 6%	690
August	1st to 10th	Night	9% 3 6% 35% 42% 5%	627
	11th to 20th	Night	7% 2 8% 39% 39% 5%	624
	21st to end	Night	5% 1 5% 35% 48% 6%	693
September	1st to 10th	Night	13% 2 8% 33% 38% 6%	727
	11th to 20th	Night	11% 3 11% 31% 38% 7%	840
	21st to end	Night	14% 4 16% 34% 30% 3	833
October	1st to 10th	Night	11% 3 16% 34% 31% 5%	800
	11th to 20th	Night	16% 6% 22% 36% 19% 1	800
	21st to end	Night	20% 6% 28% 32% 13% 1	880
November	1st to 10th	Night	15% 6% 25% 37% 16% 2	820
	11th to 20th	Night	18% 9% 32% 31% 10%	840
	21st to end	Night	23% 10% 23% 33% 11% 1	840
December	1st to 10th	Night	27% 9% 28% 30% 6%	840
	11th to 20th	Night	25% 9% 33% 25% 7%	840
	21st to end	Night	29% 11% 28% 27% 5%	924

Page generated at 2026-09-26 04:18:39. Version ac07

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