Von Braun Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Von Braun Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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% 7% 3 6% 4 4 5% 4 6% 8% 39%	126486
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	123413
Transparency	Day	41% 6% 11% 21% 17% 4	130223
	Night	40% 5% 10% 21% 19% 5%	127746
Seeing	Day	18% 12% 23% 22% 17% 8%	15868
	Night	17% 10% 21% 28% 21% 4	25809

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	18% 11% 5% 5% 3 3 3 3 4 5% 39%	4056
	11th to 20th	Day	17% 6% 6% 10% 3 3 2 2 3 5% 45%	4078
	21st to end	Day	22% 11% 4 5% 3 2 3 3 4 7% 36%	4972
February	1st to 10th	Day	20% 10% 3 6% 3 4 3 3 4 7% 37%	2000
	11th to 20th	Day	22% 9% 3 7% 3 4 4 3 5% 5% 35%	1948
	21st to end	Day	22% 8% 3 6% 4 4 4 3 4 5% 38%	1650
March	1st to 10th	Day	20% 9% 3 7% 5% 5% 4 4 5% 5% 33%	4480
	11th to 20th	Day	13% 8% 2 6% 3 4 4 3 4 6% 46%	4796
	21st to end	Day	14% 7% 3 7% 5% 4 4 3 5% 6% 42%	5691
April	1st to 10th	Day	14% 7% 2 5% 4 5% 4 3 6% 7% 43%	2400
	11th to 20th	Day	14% 7% 3 6% 4 3 3 3 5% 7% 46%	2374
	21st to end	Day	11% 7% 2 6% 6% 4 4 4 6% 8% 42%	2418
May	1st to 10th	Day	19% 7% 2 6% 4 5% 5% 4 7% 8% 32%	5862
	11th to 20th	Day	17% 8% 3 8% 5% 6% 6% 4 5% 8% 29%	5861
	21st to end	Day	17% 7% 2 7% 4 4 4 3 5% 7% 39%	6433
June	1st to 10th	Day	10% 5% 3 6% 5% 6% 6% 5% 8% 10% 36%	2600
	11th to 20th	Day	6% 4 2 5% 4 6% 7% 5% 8% 10% 43%	2600
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 5% 8% 12% 38%	2617
July	1st to 10th	Day	7% 4 2 7% 5% 6% 7% 6% 9% 11% 36%	5901
	11th to 20th	Day	8% 5% 2 6% 5% 6% 7% 6% 10% 13% 32%	5900
	21st to end	Day	12% 7% 3 8% 7% 6% 6% 6% 8% 11% 25%	6424
August	1st to 10th	Day	5% 3 2 5% 4 6% 5% 5% 8% 10% 47%	3951
	11th to 20th	Day	5% 5% 2 7% 6% 6% 6% 5% 7% 9% 43%	3849
	21st to end	Day	7% 4 2 5% 4 4 5% 4 5% 7% 52%	4312
September	1st to 10th	Day	12% 5% 2 5% 5% 5% 5% 5% 6% 9% 41%	2345
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 8% 43%	2200
	21st to end	Day	12% 4 2 6% 4 4 4 4 5% 7% 48%	2200
October	1st to 10th	Day	9% 4 2 6% 4 5% 3 3 4 5% 55%	2200
	11th to 20th	Day	10% 4 1 5% 3 4 4 3 4 7% 56%	2200
	21st to end	Day	15% 6% 1 5% 4 3 4 2 4 6% 51%	2019
November	1st to 10th	Day	12% 6% 2 5% 3 4 4 3 5% 5% 52%	1780
	11th to 20th	Day	13% 5% 2 6% 5% 3 3 2 5% 7% 49%	1800
	21st to end	Day	16% 7% 2 5% 4 3 5% 3 4 7% 44%	1800
December	1st to 10th	Day	18% 7% 2 6% 5% 3 3 3 4 4 44%	2930
	11th to 20th	Day	21% 11% 4 8% 4 4 4 3 4 5% 34%	3373
	21st to end	Day	20% 10% 4 7% 4 4 4 3 5% 7% 32%	4466
January	1st to 10th	Night	17% 12% 3 7% 3 3 3 3 4 5% 41%	5889
	11th to 20th	Night	19% 9% 4 7% 3 2 2 2 3 4 44%	5877
	21st to end	Night	20% 10% 2 5% 3 3 4 3 4 5% 41%	5984
February	1st to 10th	Night	20% 10% 2 6% 3 3 3 3 4 6% 40%	2400
	11th to 20th	Night	23% 9% 3 6% 4 3 3 3 5% 3 38%	2338
	21st to end	Night	23% 7% 2 6% 4 4 4 4 4 6% 37%	1980
March	1st to 10th	Night	25% 9% 3 7% 4 4 4 3 4 6% 31%	5373
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 5% 7% 48%	5164
	21st to end	Night	16% 7% 3 7% 5% 4 4 3 4 5% 42%	5266
April	1st to 10th	Night	13% 4 2 6% 4 4 4 4 5% 9% 46%	2000
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 5% 8% 49%	1975
	21st to end	Night	12% 6% 2 5% 4 4 4 3 5% 7% 49%	1980
May	1st to 10th	Night	16% 7% 3 7% 5% 5% 5% 4 6% 7% 34%	4033
	11th to 20th	Night	19% 9% 4 9% 5% 5% 5% 4 4 6% 30%	4032
	21st to end	Night	14% 6% 2 6% 4 4 5% 4 6% 8% 40%	4421
June	1st to 10th	Night	8% 6% 2 8% 5% 4 6% 4 7% 10% 41%	1800
	11th to 20th	Night	5% 3 1 4 5% 4 5% 6% 7% 11% 51%	1800
	21st to end	Night	6% 4 1 5% 4 5% 4 4 7% 12% 47%	1717
July	1st to 10th	Night	7% 5% 3 7% 6% 6% 5% 5% 7% 9% 40%	3947
	11th to 20th	Night	8% 5% 2 6% 5% 5% 5% 6% 9% 12% 37%	4057
	21st to end	Night	9% 7% 3 8% 6% 5% 6% 6% 8% 12% 30%	4427
August	1st to 10th	Night	3 2 1 4 3 4 4 4 7% 11% 57%	3193
	11th to 20th	Night	6% 4 1 5% 4 4 4 4 8% 10% 51%	3200
	21st to end	Night	6% 3 2 4 3 3 4 3 5% 7% 59%	3586
September	1st to 10th	Night	8% 5% 1 6% 4 5% 4 4 5% 10% 48%	2033
	11th to 20th	Night	9% 6% 3 7% 5% 4 4 4 5% 9% 45%	2200
	21st to end	Night	9% 5% 2 6% 4 4 3 3 5% 7% 52%	2200
October	1st to 10th	Night	10% 4 2 5% 3 3 3 3 4 5% 60%	2200
	11th to 20th	Night	12% 6% 2 6% 4 3 3 3 4 4 56%	2200
	21st to end	Night	15% 6% 2 4 3 3 4 2 4 5% 52%	2816
November	1st to 10th	Night	12% 6% 2 4 3 3 4 4 4 6% 52%	2563
	11th to 20th	Night	12% 7% 2 4 4 3 4 3 4 6% 51%	2600
	21st to end	Night	16% 8% 2 5% 4 3 3 3 4 5% 47%	2600
December	1st to 10th	Night	21% 7% 2 5% 3 3 3 3 4 5% 45%	4250
	11th to 20th	Night	23% 10% 3 5% 3 3 3 2 3 4 41%	4822
	21st to end	Night	20% 10% 4 7% 4 4 4 3 5% 5% 34%	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	47% 4 7% 20% 21% 2	4057
	11th to 20th	Day	45% 3 5% 22% 25% 1	4078
	21st to end	Day	49% 4 7% 19% 18% 3	4972
February	1st to 10th	Day	48% 4 6% 20% 18% 3	2000
	11th to 20th	Day	52% 4 6% 18% 15% 5%	1958
	21st to end	Day	50% 4 6% 14% 20% 6%	1650
March	1st to 10th	Day	52% 5% 7% 12% 17% 8%	4480
	11th to 20th	Day	39% 4 6% 20% 20% 12%	4796
	21st to end	Day	43% 4 10% 24% 17% 2	5693
April	1st to 10th	Day	40% 5% 9% 24% 16% 5%	2400
	11th to 20th	Day	38% 5% 7% 22% 22% 6%	2374
	21st to end	Day	39% 5% 10% 27% 16% 3	2418
May	1st to 10th	Day	46% 9% 13% 20% 10% 2	5897
	11th to 20th	Day	51% 8% 14% 19% 9%	5865
	21st to end	Day	44% 6% 9% 23% 16% 2	6447
June	1st to 10th	Day	41% 9% 13% 20% 15% 1	2600
	11th to 20th	Day	32% 8% 17% 24% 17% 2	2600
	21st to end	Day	36% 9% 17% 20% 14% 3	2617
July	1st to 10th	Day	37% 10% 18% 25% 10% 1	5836
	11th to 20th	Day	36% 12% 26% 20% 4 1	5831
	21st to end	Day	48% 10% 21% 16% 4	6240
August	1st to 10th	Day	28% 9% 13% 34% 15% 1	3938
	11th to 20th	Day	34% 8% 15% 27% 13% 3	3885
	21st to end	Day	30% 7% 12% 25% 24% 2	4311
September	1st to 10th	Day	38% 8% 14% 18% 17% 5%	2345
	11th to 20th	Day	38% 7% 11% 19% 22% 3	2200
	21st to end	Day	35% 6% 11% 21% 20% 7%	2200
October	1st to 10th	Day	22% 4 5% 15% 37% 16%	3590
	11th to 20th	Day	20% 3 5% 18% 31% 22%	3568
	21st to end	Day	28% 3 6% 23% 30% 9%	3231
November	1st to 10th	Day	35% 3 8% 18% 29% 7%	1780
	11th to 20th	Day	37% 4 5% 21% 26% 8%	1800
	21st to end	Day	42% 4 5% 20% 24% 5%	1800
December	1st to 10th	Day	44% 3 4 20% 24% 4	2930
	11th to 20th	Day	53% 4 6% 21% 13% 4	3370
	21st to end	Day	52% 4 9% 22% 11% 2	4466
January	1st to 10th	Night	47% 3 7% 19% 20% 3	5889
	11th to 20th	Night	46% 3 5% 18% 23% 5%	5877
	21st to end	Night	46% 3 4 19% 21% 6%	5984
February	1st to 10th	Night	46% 3 8% 19% 20% 4	2400
	11th to 20th	Night	51% 3 7% 16% 16% 7%	2339
	21st to end	Night	50% 4 6% 16% 17% 7%	1980
March	1st to 10th	Night	55% 4 6% 14% 17% 5%	5373
	11th to 20th	Night	36% 4 7% 17% 22% 14%	5164
	21st to end	Night	46% 4 7% 21% 19% 4	5266
April	1st to 10th	Night	36% 5% 12% 22% 20% 5%	2000
	11th to 20th	Night	35% 4 7% 22% 22% 10%	1975
	21st to end	Night	36% 5% 11% 27% 18% 3	1980
May	1st to 10th	Night	45% 10% 12% 19% 13% 1	4060
	11th to 20th	Night	54% 7% 12% 18% 9%	4033
	21st to end	Night	40% 8% 13% 23% 16% 1	4429
June	1st to 10th	Night	37% 8% 14% 22% 18% 2	1800
	11th to 20th	Night	26% 8% 17% 28% 19% 3	1800
	21st to end	Night	30% 7% 18% 24% 17% 4	1717
July	1st to 10th	Night	38% 10% 18% 24% 9% 2	3904
	11th to 20th	Night	34% 12% 27% 20% 5% 2	4006
	21st to end	Night	44% 11% 23% 17% 5%	4297
August	1st to 10th	Night	19% 8% 14% 36% 21% 1	3182
	11th to 20th	Night	27% 7% 21% 27% 16% 2	3230
	21st to end	Night	25% 6% 13% 26% 27% 3	3586
September	1st to 10th	Night	33% 7% 17% 21% 17% 6%	2033
	11th to 20th	Night	35% 7% 13% 18% 24% 3	2200
	21st to end	Night	32% 6% 10% 22% 22% 9%	2200
October	1st to 10th	Night	20% 4 6% 18% 34% 18%	3590
	11th to 20th	Night	22% 2 4 18% 33% 22%	3571
	21st to end	Night	25% 2 4 26% 29% 14%	4554
November	1st to 10th	Night	34% 4 8% 20% 24% 10%	2563
	11th to 20th	Night	35% 3 6% 18% 28% 10%	2600
	21st to end	Night	41% 2 5% 21% 24% 6%	2600
December	1st to 10th	Night	43% 4 5% 20% 26% 3	4250
	11th to 20th	Night	49% 3 4 19% 22% 3	4824
	21st to end	Night	52% 3 7% 24% 12% 1	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	31% 27% 27% 13% 3	300
	11th to 20th	Day	30% 28% 27% 10% 4 1	300
	21st to end	Day	23% 31% 31% 12% 2	330
February	1st to 10th	Day	27% 26% 32% 12% 2	300
	11th to 20th	Day	41% 23% 26% 9% 1	300
	21st to end	Day	36% 26% 29% 8%	249
March	1st to 10th	Day	31% 24% 32% 10% 3	300
	11th to 20th	Day	26% 23% 30% 15% 5% 1	420
	21st to end	Day	26% 22% 30% 16% 5% 1	550
April	1st to 10th	Day	23% 18% 27% 20% 10% 1	500
	11th to 20th	Day	22% 19% 28% 22% 8% 2	492
	21st to end	Day	19% 15% 30% 25% 9% 1	500
May	1st to 10th	Day	18% 14% 30% 22% 11% 5%	503
	11th to 20th	Day	13% 11% 28% 26% 13% 8%	527
	21st to end	Day	15% 6% 23% 29% 15% 12%	573
June	1st to 10th	Day	13% 6% 21% 25% 22% 13%	530
	11th to 20th	Day	8% 5% 17% 26% 26% 18%	530
	21st to end	Day	11% 4 16% 26% 26% 17%	547
July	1st to 10th	Day	9% 3 14% 26% 29% 19%	526
	11th to 20th	Day	11% 2 12% 22% 28% 25%	524
	21st to end	Day	10% 3 11% 25% 31% 20%	576
August	1st to 10th	Day	11% 2 13% 24% 32% 17%	526
	11th to 20th	Day	10% 3 15% 28% 32% 12%	519
	21st to end	Day	14% 3 13% 23% 34% 14%	583
September	1st to 10th	Day	12% 4 13% 30% 28% 13%	530
	11th to 20th	Day	11% 4 18% 24% 29% 14%	530
	21st to end	Day	15% 5% 16% 30% 26% 7%	530
October	1st to 10th	Day	11% 7% 27% 26% 21% 8%	530
	11th to 20th	Day	13% 10% 31% 27% 17% 3	530
	21st to end	Day	22% 14% 27% 25% 11% 2	377
November	1st to 10th	Day	21% 13% 31% 23% 11% 2	306
	11th to 20th	Day	16% 19% 40% 20% 5%	300
	21st to end	Day	21% 22% 30% 17% 10% 1	300
December	1st to 10th	Day	30% 21% 29% 14% 5%	300
	11th to 20th	Day	25% 24% 33% 11% 7%	300
	21st to end	Day	29% 21% 32% 14% 4	330
January	1st to 10th	Night	26% 19% 29% 21% 5% 1	800
	11th to 20th	Night	27% 19% 27% 21% 7%	800
	21st to end	Night	21% 16% 32% 27% 4	880
February	1st to 10th	Night	25% 14% 28% 28% 5%	800
	11th to 20th	Night	29% 18% 26% 25% 2	783
	21st to end	Night	28% 18% 27% 23% 3	660
March	1st to 10th	Night	22% 18% 26% 30% 5%	800
	11th to 20th	Night	21% 16% 30% 26% 7% 1	800
	21st to end	Night	20% 13% 30% 26% 10%	795
April	1st to 10th	Night	16% 14% 30% 28% 12% 1	600
	11th to 20th	Night	16% 11% 33% 25% 14% 1	593
	21st to end	Night	16% 8% 26% 35% 14% 1	600
May	1st to 10th	Night	15% 6% 25% 32% 20% 3	600
	11th to 20th	Night	15% 6% 20% 32% 24% 3	591
	21st to end	Night	7% 3 15% 38% 31% 6%	650
June	1st to 10th	Night	9% 3 15% 31% 35% 6%	600
	11th to 20th	Night	6% 2 10% 30% 41% 11%	600
	21st to end	Night	8% 1 9% 29% 43% 10%	600
July	1st to 10th	Night	11% 1 8% 27% 40% 14%	585
	11th to 20th	Night	9% 1 6% 23% 43% 18%	582
	21st to end	Night	9% 1 8% 23% 45% 13%	657
August	1st to 10th	Night	5% 1 6% 26% 51% 11%	597
	11th to 20th	Night	11% 1 8% 29% 43% 8%	594
	21st to end	Night	10% 2 10% 27% 43% 9%	660
September	1st to 10th	Night	10% 2 13% 26% 42% 8%	633
	11th to 20th	Night	11% 3 16% 28% 37% 6%	800
	21st to end	Night	12% 3 13% 31% 38% 3	800
October	1st to 10th	Night	11% 5% 20% 30% 28% 6%	800
	11th to 20th	Night	15% 8% 23% 32% 21% 2	800
	21st to end	Night	18% 10% 20% 33% 18% 2	880
November	1st to 10th	Night	15% 9% 22% 33% 18% 3	789
	11th to 20th	Night	15% 14% 26% 31% 13% 1	800
	21st to end	Night	21% 13% 24% 29% 13% 1	800
December	1st to 10th	Night	26% 16% 24% 27% 8%	800
	11th to 20th	Night	25% 14% 24% 29% 9%	800
	21st to end	Night	27% 15% 25% 28% 5%	880

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