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 2025-05-26.

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	14% 8% 3 7% 4 4 5% 4 5% 7% 38%	114867
	Night	14% 9% 3 7% 5% 4 4 3 5% 7% 39%	114420
Transparency	Day	44% 4 10% 24% 16% 2	118579
	Night	45% 3 9% 23% 17% 3	118847
Seeing	Day	17% 9% 23% 24% 18% 9%	15840
	Night	18% 7% 23% 32% 18% 2	25448

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	15% 15% 8% 9% 4 4 4 4 5% 7% 27%	4056
	11th to 20th	Day	13% 12% 5% 16% 4 5% 4 3 3 4 32%	4060
	21st to end	Day	21% 11% 5% 7% 5% 4 4 3 4 5% 31%	4466
February	1st to 10th	Day	17% 13% 4 9% 4 4 4 3 4 6% 32%	1980
	11th to 20th	Day	19% 10% 4 9% 4 5% 4 3 4 5% 33%	1948
	21st to end	Day	18% 11% 4 7% 4 4 5% 3 6% 5% 31%	1650
March	1st to 10th	Day	19% 12% 4 7% 4 4 4 3 4 5% 36%	4480
	11th to 20th	Day	12% 7% 3 7% 4 4 4 4 6% 7% 43%	4976
	21st to end	Day	14% 8% 3 8% 5% 4 4 3 5% 7% 37%	5465
April	1st to 10th	Day	15% 7% 3 6% 4 4 5% 4 5% 7% 39%	2380
	11th to 20th	Day	16% 8% 3 7% 5% 4 4 2 5% 5% 41%	2379
	21st to end	Day	17% 7% 3 6% 5% 5% 5% 4 5% 7% 37%	2600
May	1st to 10th	Day	19% 9% 3 8% 5% 5% 5% 4 6% 7% 29%	4212
	11th to 20th	Day	17% 8% 2 8% 5% 4 4 3 4 7% 37%	4211
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 5% 7% 10% 37%	4541
June	1st to 10th	Day	11% 6% 2 6% 5% 6% 5% 5% 6% 6% 42%	2470
	11th to 20th	Day	7% 4 2 6% 5% 5% 6% 5% 7% 9% 42%	2566
	21st to end	Day	8% 5% 3 5% 4 5% 6% 5% 7% 10% 42%	2660
July	1st to 10th	Day	10% 6% 3 8% 5% 5% 6% 4 6% 10% 37%	4440
	11th to 20th	Day	7% 5% 2 5% 4 5% 5% 5% 8% 11% 43%	4159
	21st to end	Day	13% 11% 4 9% 6% 6% 7% 5% 6% 7% 25%	4466
August	1st to 10th	Day	7% 4 2 5% 4 4 4 4 8% 12% 46%	4104
	11th to 20th	Day	7% 5% 2 6% 5% 5% 6% 5% 9% 11% 38%	3931
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 9% 54%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 6% 8% 43%	2173
	11th to 20th	Day	11% 5% 2 6% 3 3 4 4 6% 8% 47%	2090
	21st to end	Day	15% 6% 2 5% 5% 4 4 4 4 7% 46%	2090
October	1st to 10th	Day	10% 4 2 4 3 4 5% 3 5% 6% 54%	2090
	11th to 20th	Day	13% 6% 2 6% 3 3 4 3 5% 6% 49%	1969
	21st to end	Day	19% 7% 2 6% 2 4 3 3 5% 5% 45%	1944
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 2 4 6% 53%	1762
	11th to 20th	Day	16% 8% 3 6% 5% 3 3 3 4 6% 43%	1800
	21st to end	Day	18% 10% 3 6% 4 3 4 4 4 5% 39%	1800
December	1st to 10th	Day	18% 11% 3 7% 4 3 3 3 4 5% 39%	2930
	11th to 20th	Day	19% 12% 4 10% 6% 4 4 3 4 4 29%	3373
	21st to end	Day	20% 16% 5% 10% 5% 5% 4 3 4 5% 22%	4466
January	1st to 10th	Night	15% 15% 6% 14% 5% 4 4 3 4 5% 24%	5889
	11th to 20th	Night	17% 14% 5% 11% 6% 4 4 3 3 4 29%	5900
	21st to end	Night	20% 10% 3 7% 5% 4 3 3 4 5% 36%	6490
February	1st to 10th	Night	22% 11% 5% 9% 4 4 4 4 5% 6% 28%	2416
	11th to 20th	Night	21% 10% 4 9% 5% 3 3 3 4 4 33%	2338
	21st to end	Night	20% 13% 3 9% 5% 3 4 3 4 5% 31%	1980
March	1st to 10th	Night	19% 14% 2 6% 4 4 3 3 4 5% 36%	5373
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 5% 6% 46%	4984
	21st to end	Night	15% 9% 3 8% 5% 6% 4 4 5% 6% 34%	5467
April	1st to 10th	Night	14% 6% 3 5% 5% 5% 5% 4 6% 7% 40%	2035
	11th to 20th	Night	15% 7% 2 7% 4 4 4 3 5% 6% 42%	1969
	21st to end	Night	15% 8% 3 7% 5% 4 5% 4 5% 7% 38%	1800
May	1st to 10th	Night	19% 9% 3 8% 6% 5% 6% 4 5% 7% 28%	2903
	11th to 20th	Night	16% 7% 3 5% 4 4 4 4 6% 7% 38%	2902
	21st to end	Night	8% 5% 2 8% 4 5% 5% 4 8% 10% 41%	3130
June	1st to 10th	Night	8% 6% 2 5% 4 4 4 4 6% 11% 47%	1710
	11th to 20th	Night	5% 5% 2 5% 4 4 4 4 8% 10% 49%	1606
	21st to end	Night	6% 3 2 4 5% 5% 4 4 8% 13% 45%	1520
July	1st to 10th	Night	7% 5% 2 6% 5% 4 5% 4 6% 10% 46%	2530
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 9% 51%	2795
	21st to end	Night	11% 8% 3 10% 6% 6% 6% 5% 6% 9% 29%	3087
August	1st to 10th	Night	5% 5% 2 6% 3 5% 4 3 7% 10% 50%	2825
	11th to 20th	Night	6% 6% 2 7% 5% 5% 5% 4 6% 10% 43%	2881
	21st to end	Night	3 3 1 3 3 3 4 3 6% 9% 62%	3476
September	1st to 10th	Night	12% 6% 2 4 4 4 4 3 5% 8% 49%	1987
	11th to 20th	Night	10% 5% 2 6% 4 4 3 3 5% 8% 51%	2090
	21st to end	Night	12% 6% 1 5% 4 4 3 2 4 6% 52%	2090
October	1st to 10th	Night	9% 4 1 4 4 3 3 3 5% 6% 58%	2090
	11th to 20th	Night	14% 6% 3 5% 4 3 4 2 5% 6% 49%	2209
	21st to end	Night	16% 7% 2 5% 4 4 3 3 4 6% 45%	2650
November	1st to 10th	Night	12% 6% 2 5% 5% 3 3 4 5% 5% 49%	2536
	11th to 20th	Night	15% 8% 3 6% 4 3 4 3 4 6% 44%	2600
	21st to end	Night	17% 10% 3 6% 4 3 3 3 4 6% 40%	2600
December	1st to 10th	Night	18% 12% 4 8% 3 4 3 3 3 4 38%	4250
	11th to 20th	Night	21% 11% 4 7% 4 3 3 2 5% 6% 34%	4822
	21st to end	Night	19% 16% 5% 10% 6% 6% 5% 3 4 5% 21%	6490

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	58% 2 8% 22% 9% 1	4057
	11th to 20th	Day	58% 2 6% 18% 14% 2	4060
	21st to end	Day	56% 2 8% 21% 11% 2	4466
February	1st to 10th	Day	55% 3 7% 22% 12% 2	1980
	11th to 20th	Day	55% 3 8% 20% 11% 3	1958
	21st to end	Day	53% 3 8% 20% 12% 3	1650
March	1st to 10th	Day	53% 2 5% 18% 17% 6%	4480
	11th to 20th	Day	41% 3 9% 26% 20% 2	4976
	21st to end	Day	47% 3 11% 27% 12% 1	5465
April	1st to 10th	Day	43% 4 12% 24% 15% 2	2380
	11th to 20th	Day	46% 3 8% 23% 17% 3	2379
	21st to end	Day	47% 4 10% 25% 12% 2	2600
May	1st to 10th	Day	54% 3 12% 20% 9%	4250
	11th to 20th	Day	48% 3 12% 23% 13% 1	4215
	21st to end	Day	42% 5% 13% 19% 20% 1	4554
June	1st to 10th	Day	40% 5% 9% 27% 16% 2	2470
	11th to 20th	Day	35% 7% 15% 24% 17% 2	2566
	21st to end	Day	35% 6% 15% 21% 18% 4	2660
July	1st to 10th	Day	42% 6% 13% 22% 15% 1	4371
	11th to 20th	Day	33% 6% 12% 34% 13% 2	4088
	21st to end	Day	56% 6% 11% 16% 9% 1	4324
August	1st to 10th	Day	30% 6% 12% 38% 13% 1	4091
	11th to 20th	Day	36% 7% 12% 29% 15% 2	3970
	21st to end	Day	27% 6% 10% 37% 17% 3	4179
September	1st to 10th	Day	39% 4 12% 20% 19% 5%	2173
	11th to 20th	Day	35% 5% 10% 24% 23% 3	2090
	21st to end	Day	39% 4 7% 24% 22% 3	2090
October	1st to 10th	Day	22% 3 5% 27% 41% 3	3480
	11th to 20th	Day	26% 2 9% 28% 32% 3	3248
	21st to end	Day	30% 3 8% 30% 21% 8%	3181
November	1st to 10th	Day	34% 2 7% 29% 23% 4	1762
	11th to 20th	Day	44% 2 7% 23% 18% 5%	1800
	21st to end	Day	47% 3 8% 22% 19% 2	1800
December	1st to 10th	Day	49% 3 6% 24% 17% 1	2930
	11th to 20th	Day	60% 2 7% 17% 12% 1	3370
	21st to end	Day	65% 3 10% 16% 6% 1	4466
January	1st to 10th	Night	64% 2 7% 18% 9% 1	5889
	11th to 20th	Night	60% 2 6% 18% 11% 3	5900
	21st to end	Night	51% 2 7% 23% 15% 2	6490
February	1st to 10th	Night	57% 3 10% 18% 10% 1	2416
	11th to 20th	Night	55% 2 7% 20% 13% 2	2339
	21st to end	Night	57% 2 9% 16% 13% 3	1980
March	1st to 10th	Night	53% 2 5% 22% 14% 4	5373
	11th to 20th	Night	39% 2 8% 26% 21% 3	4984
	21st to end	Night	51% 3 9% 24% 13% 1	5467
April	1st to 10th	Night	42% 4 10% 23% 18% 3	2035
	11th to 20th	Night	44% 4 7% 21% 19% 6%	1969
	21st to end	Night	45% 5% 11% 24% 14% 2	1800
May	1st to 10th	Night	55% 4 10% 20% 8% 2	2930
	11th to 20th	Night	43% 5% 13% 27% 11% 1	2903
	21st to end	Night	36% 6% 13% 25% 17% 3	3138
June	1st to 10th	Night	32% 4 14% 29% 17% 4	1710
	11th to 20th	Night	27% 6% 17% 25% 21% 4	1606
	21st to end	Night	29% 7% 19% 23% 16% 6%	1520
July	1st to 10th	Night	34% 7% 11% 27% 18% 3	2490
	11th to 20th	Night	29% 5% 13% 34% 16% 2	2743
	21st to end	Night	50% 7% 13% 19% 11% 1	2988
August	1st to 10th	Night	29% 5% 15% 36% 14% 1	2814
	11th to 20th	Night	35% 6% 12% 29% 16% 2	2908
	21st to end	Night	19% 5% 14% 36% 21% 4	3476
September	1st to 10th	Night	34% 5% 13% 21% 21% 6%	1987
	11th to 20th	Night	32% 4 10% 25% 25% 3	2090
	21st to end	Night	34% 4 7% 25% 24% 5%	2090
October	1st to 10th	Night	21% 2 7% 22% 44% 3	3480
	11th to 20th	Night	26% 2 6% 26% 35% 6%	3669
	21st to end	Night	31% 2 7% 22% 30% 9%	4363
November	1st to 10th	Night	37% 3 7% 26% 23% 4	2536
	11th to 20th	Night	42% 3 7% 21% 21% 6%	2600
	21st to end	Night	47% 3 8% 24% 16% 2	2600
December	1st to 10th	Night	52% 3 8% 22% 14%	4250
	11th to 20th	Night	52% 3 9% 21% 13% 1	4824
	21st to end	Night	67% 3 8% 15% 7%	6490

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	23% 18% 34% 19% 6% 1	300
	11th to 20th	Day	26% 24% 31% 15% 4	300
	21st to end	Day	23% 17% 40% 16% 3	330
February	1st to 10th	Day	24% 17% 41% 16% 3	300
	11th to 20th	Day	30% 21% 35% 9% 4	300
	21st to end	Day	29% 17% 36% 15% 3	249
March	1st to 10th	Day	24% 16% 40% 14% 6%	300
	11th to 20th	Day	21% 14% 38% 19% 7%	496
	21st to end	Day	26% 15% 32% 20% 7% 1	550
April	1st to 10th	Day	21% 14% 28% 25% 10% 1	500
	11th to 20th	Day	20% 14% 26% 24% 12% 4	492
	21st to end	Day	21% 11% 29% 25% 10% 4	500
May	1st to 10th	Day	20% 10% 29% 23% 12% 6%	503
	11th to 20th	Day	17% 7% 24% 26% 17% 9%	527
	21st to end	Day	12% 6% 19% 28% 21% 15%	549
June	1st to 10th	Day	13% 4 17% 25% 27% 14%	490
	11th to 20th	Day	8% 3 18% 31% 27% 14%	586
	21st to end	Day	10% 4 19% 31% 23% 13%	680
July	1st to 10th	Day	13% 3 13% 23% 26% 22%	671
	11th to 20th	Day	9% 1 14% 26% 30% 19%	508
	21st to end	Day	11% 2 10% 29% 31% 17%	532
August	1st to 10th	Day	13% 1 11% 26% 29% 20%	486
	11th to 20th	Day	13% 3 11% 28% 32% 14%	479
	21st to end	Day	8% 2 7% 27% 31% 25%	539
September	1st to 10th	Day	13% 2 11% 27% 28% 18%	490
	11th to 20th	Day	11% 3 11% 24% 36% 16%	490
	21st to end	Day	16% 7% 16% 23% 25% 12%	490
October	1st to 10th	Day	13% 2 16% 32% 27% 9%	490
	11th to 20th	Day	13% 13% 25% 30% 13% 7%	490
	21st to end	Day	20% 9% 32% 24% 15% 1	393
November	1st to 10th	Day	17% 6% 30% 29% 14% 4	300
	11th to 20th	Day	22% 15% 27% 27% 8% 1	300
	21st to end	Day	24% 11% 31% 25% 7% 2	300
December	1st to 10th	Day	24% 17% 28% 23% 7% 1	300
	11th to 20th	Day	20% 14% 37% 18% 10% 1	300
	21st to end	Day	29% 17% 29% 18% 6%	330
January	1st to 10th	Night	23% 13% 32% 28% 4	800
	11th to 20th	Night	28% 14% 32% 22% 4	800
	21st to end	Night	23% 10% 38% 27% 2	880
February	1st to 10th	Night	28% 10% 34% 27% 2	800
	11th to 20th	Night	29% 11% 34% 24% 2	783
	21st to end	Night	26% 12% 39% 21% 2	660
March	1st to 10th	Night	23% 12% 32% 26% 6%	800
	11th to 20th	Night	24% 9% 34% 28% 6% 1	800
	21st to end	Night	22% 11% 30% 30% 7%	880
April	1st to 10th	Night	16% 9% 32% 33% 8%	635
	11th to 20th	Night	17% 8% 31% 32% 12% 1	593
	21st to end	Night	18% 6% 32% 32% 11% 2	600
May	1st to 10th	Night	20% 6% 25% 33% 15% 2	600
	11th to 20th	Night	16% 6% 15% 40% 20% 4	591
	21st to end	Night	8% 3 15% 38% 30% 6%	634
June	1st to 10th	Night	10% 2 13% 42% 29% 3	570
	11th to 20th	Night	5% 2 12% 44% 32% 4	570
	21st to end	Night	6% 2 14% 44% 31% 4	570
July	1st to 10th	Night	12% 1 9% 30% 39% 9%	555
	11th to 20th	Night	7% 2 8% 35% 42% 6%	552
	21st to end	Night	9% 3 6% 36% 39% 6%	624
August	1st to 10th	Night	9% 2 6% 34% 43% 5%	567
	11th to 20th	Night	7% 2 8% 40% 37% 5%	564
	21st to end	Night	6% 1 6% 32% 50% 6%	627
September	1st to 10th	Night	14% 2 7% 33% 39% 5%	657
	11th to 20th	Night	11% 3 11% 32% 37% 6%	760
	21st to end	Night	14% 3 16% 34% 31% 3	760
October	1st to 10th	Night	11% 3 16% 34% 30% 5%	760
	11th to 20th	Night	17% 6% 22% 34% 19% 1	760
	21st to end	Night	19% 6% 27% 33% 13% 1	836
November	1st to 10th	Night	15% 6% 24% 37% 16% 2	780
	11th to 20th	Night	18% 9% 31% 31% 11%	800
	21st to end	Night	23% 10% 23% 33% 11% 1	800
December	1st to 10th	Night	27% 9% 28% 30% 6%	800
	11th to 20th	Night	25% 9% 33% 26% 8% 1	800
	21st to end	Night	28% 11% 28% 28% 5%	880

Page generated at 2025-05-23 09:21:00. Version ac07

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