Ruhenstroth Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ruhenstroth Observatory clear sky chart from 2005-11-06 to 2024-11-12.

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	2 3 1 4 3 3 3 3 5% 6% 66%	105166
	Night	2 4 2 4 3 3 3 3 5% 7% 65%	102727
Transparency	Day	19% 5% 9% 19% 26% 23%	104911
	Night	19% 6% 11% 23% 25% 17%	102572
Seeing	Day	9% 16% 26% 25% 17% 7%	15097
	Night	5% 7% 21% 37% 24% 5%	24467

By Month

Percentage Cloud Cover SW (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	1 4 3 5% 4 4 5% 5% 8% 10% 52%	2539
	11th to 20th	Day	2 5% 2 8% 5% 5% 5% 4 8% 8% 48%	2604
	21st to end	Day	4 8% 3 7% 5% 5% 5% 5% 8% 9% 41%	3146
February	1st to 10th	Day	2 5% 2 5% 3 4 3 4 7% 8% 57%	1737
	11th to 20th	Day	1 5% 2 5% 4 4 4 5% 8% 10% 52%	1850
	21st to end	Day	1 3 2 6% 3 4 4 4 7% 9% 57%	1570
March	1st to 10th	Day	1 5% 3 8% 5% 6% 6% 6% 9% 10% 41%	3138
	11th to 20th	Day	4 1 3 3 3 3 4 5% 8% 66%	3414
	21st to end	Day	1 8% 3 6% 3 4 5% 4 6% 8% 52%	4092
April	1st to 10th	Day	2 5% 2 6% 4 5% 4 4 6% 9% 55%	2280
	11th to 20th	Day	2 5% 1 4 4 4 3 4 6% 8% 60%	2249
	21st to end	Day	2 4 1 4 3 3 3 3 6% 7% 63%	2280
May	1st to 10th	Day	2 3 1 4 3 3 3 2 4 6% 69%	4002
	11th to 20th	Day	2 2 1 3 2 2 3 3 4 5% 72%	4248
	21st to end	Day	3 4 1 4 3 4 3 3 4 6% 64%	4813
June	1st to 10th	Day	2 3 2 4 3 3 4 4 5% 7% 64%	2660
	11th to 20th	Day	2 2 2 2 2 2 2 4 5% 77%	2660
	21st to end	Day	2 2 1 2 3 2 3 3 4 4 74%	2660
July	1st to 10th	Day	1 1 1 2 2 1 2 3 86%	4420
	11th to 20th	Day	2 2 1 4 4 3 4 3 5% 5% 68%	4420
	21st to end	Day	1 2 1 2 2 2 3 3 4 4 75%	4686
August	1st to 10th	Day	2 2 1 2 3 2 3 2 4 4 75%	4004
	11th to 20th	Day	1 1 2 1 2 2 2 3 3 84%	3724
	21st to end	Day	1 1 1 2 1 1 2 1 2 2 85%	4180
September	1st to 10th	Day	1 1 1 1 1 2 2 2 4 5% 79%	2280
	11th to 20th	Day	1 2 1 3 2 2 3 2 4 5% 74%	2237
	21st to end	Day	2 2 1 2 2 2 2 2 3 3 79%	2090
October	1st to 10th	Day	2 2 1 2 2 3 3 3 4 6% 72%	3258
	11th to 20th	Day	2 3 1 2 1 2 2 1 3 4 80%	3161
	21st to end	Day	1 2 1 3 2 3 3 3 5% 7% 70%	3355
November	1st to 10th	Day	2 3 1 4 3 3 3 4 6% 8% 64%	1764
	11th to 20th	Day	1 3 1 3 2 3 3 4 6% 9% 64%	1719
	21st to end	Day	1 3 1 3 3 4 4 5% 6% 9% 61%	1684
December	1st to 10th	Day	2 5% 2 4 3 3 4 5% 7% 10% 57%	1520
	11th to 20th	Day	3 6% 2 8% 6% 5% 7% 5% 9% 9% 41%	1928
	21st to end	Day	1 5% 3 7% 5% 5% 5% 5% 8% 12% 43%	2794
January	1st to 10th	Night	1 5% 3 7% 3 4 4 4 7% 9% 53%	4406
	11th to 20th	Night	1 7% 3 7% 4 5% 5% 5% 7% 9% 46%	4354
	21st to end	Night	4 7% 3 7% 5% 5% 6% 4 7% 8% 44%	4521
February	1st to 10th	Night	2 5% 2 6% 4 3 4 4 6% 7% 57%	2442
	11th to 20th	Night	1 5% 1 5% 4 4 4 3 7% 11% 55%	2218
	21st to end	Night	4 2 5% 4 4 3 4 5% 9% 60%	1884
March	1st to 10th	Night	1 5% 3 6% 4 4 4 5% 8% 11% 48%	3736
	11th to 20th	Night	1 4 1 4 3 3 4 4 6% 8% 63%	3562
	21st to end	Night	1 8% 2 7% 4 4 5% 4 6% 9% 48%	3560
April	1st to 10th	Night	2 3 2 4 3 4 4 4 7% 9% 58%	1900
	11th to 20th	Night	1 4 1 3 3 3 4 3 6% 7% 64%	1879
	21st to end	Night	3 4 1 3 2 2 3 3 5% 7% 66%	1900
May	1st to 10th	Night	2 3 1 3 2 2 3 2 4 6% 73%	2970
	11th to 20th	Night	1 2 1 3 2 2 2 2 4 5% 75%	2645
	21st to end	Night	3 3 1 3 3 3 3 3 4 6% 71%	2762
June	1st to 10th	Night	4 3 1 3 3 3 3 2 4 6% 69%	1520
	11th to 20th	Night	1 1 1 2 1 2 2 3 4 82%	1520
	21st to end	Night	2 2 1 2 1 2 3 2 2 3 81%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 2 91%	2540
	11th to 20th	Night	1 2 1 3 2 2 3 3 4 5% 73%	2540
	21st to end	Night	2 1 1 2 1 2 2 2 4 4 80%	2852
August	1st to 10th	Night	1 2 1 2 2 2 3 3 3 5% 77%	2936
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 87%	3126
	21st to end	Night	1 1 2 1 1 1 1 2 2 88%	3498
September	1st to 10th	Night	1 1 1 1 1 2 1 2 4 86%	1900
	11th to 20th	Night	1 2 1 2 1 2 2 2 3 5% 77%	1943
	21st to end	Night	1 2 1 3 1 2 2 2 3 4 79%	2090
October	1st to 10th	Night	2 2 1 2 1 2 2 2 3 4 78%	3707
	11th to 20th	Night	1 2 1 2 1 1 2 2 3 4 81%	3777
	21st to end	Night	2 3 1 4 2 2 3 3 4 6% 71%	4210
November	1st to 10th	Night	2 2 1 4 2 2 3 3 6% 9% 66%	2535
	11th to 20th	Night	1 3 1 3 3 3 3 3 6% 10% 65%	2483
	21st to end	Night	2 3 2 3 3 3 3 4 7% 7% 64%	2493
December	1st to 10th	Night	2 5% 2 5% 3 4 4 4 6% 8% 57%	2660
	11th to 20th	Night	3 6% 2 6% 4 4 5% 4 7% 11% 46%	3276
	21st to end	Night	2 6% 3 7% 5% 5% 6% 6% 9% 10% 42%	4862

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	25% 10% 18% 30% 15% 1	2539
	11th to 20th	Day	30% 10% 13% 26% 16% 4	2604
	21st to end	Day	36% 9% 13% 24% 16% 3	3146
February	1st to 10th	Day	25% 8% 12% 27% 21% 7%	1737
	11th to 20th	Day	26% 7% 11% 26% 24% 6%	1860
	21st to end	Day	22% 8% 15% 30% 19% 5%	1570
March	1st to 10th	Day	31% 9% 15% 26% 15% 4	3138
	11th to 20th	Day	17% 6% 15% 33% 26% 4	3414
	21st to end	Day	29% 6% 18% 24% 21% 2	4092
April	1st to 10th	Day	28% 6% 14% 25% 20% 7%	2280
	11th to 20th	Day	22% 7% 11% 27% 26% 8%	2252
	21st to end	Day	21% 7% 12% 29% 26% 5%	2280
May	1st to 10th	Day	17% 5% 8% 27% 36% 8%	4002
	11th to 20th	Day	16% 4 7% 18% 41% 15%	4251
	21st to end	Day	21% 4 9% 23% 37% 7%	4814
June	1st to 10th	Day	20% 6% 9% 21% 32% 12%	2660
	11th to 20th	Day	11% 4 6% 19% 37% 23%	2660
	21st to end	Day	14% 3 5% 9% 28% 41%	2660
July	1st to 10th	Day	7% 2 2 5% 12% 72%	4351
	11th to 20th	Day	19% 2 4 9% 14% 52%	4345
	21st to end	Day	13% 3 5% 9% 20% 49%	4538
August	1st to 10th	Day	14% 3 4 8% 22% 49%	3989
	11th to 20th	Day	9% 1 3 5% 17% 66%	3760
	21st to end	Day	9% 2 2 8% 33% 47%	4179
September	1st to 10th	Day	10% 3 4 7% 30% 47%	2280
	11th to 20th	Day	14% 3 5% 16% 35% 26%	2237
	21st to end	Day	12% 3 4 16% 36% 28%	2090
October	1st to 10th	Day	16% 3 6% 11% 31% 33%	3258
	11th to 20th	Day	12% 2 4 17% 48% 17%	3161
	21st to end	Day	14% 5% 7% 26% 34% 15%	3355
November	1st to 10th	Day	20% 5% 12% 24% 29% 11%	1764
	11th to 20th	Day	16% 6% 16% 22% 33% 7%	1719
	21st to end	Day	20% 7% 12% 26% 26% 8%	1684
December	1st to 10th	Day	23% 8% 14% 26% 23% 6%	1520
	11th to 20th	Day	35% 13% 13% 19% 15% 6%	1928
	21st to end	Day	31% 9% 17% 21% 18% 3	2794
January	1st to 10th	Night	27% 7% 20% 30% 16% 1	4406
	11th to 20th	Night	31% 9% 16% 25% 14% 4	4354
	21st to end	Night	34% 10% 14% 26% 13% 2	4521
February	1st to 10th	Night	27% 7% 13% 26% 20% 7%	2442
	11th to 20th	Night	23% 7% 11% 32% 22% 4	2219
	21st to end	Night	22% 7% 16% 30% 20% 4	1884
March	1st to 10th	Night	25% 9% 15% 27% 20% 3	3736
	11th to 20th	Night	18% 8% 16% 40% 15% 3	3562
	21st to end	Night	29% 7% 20% 27% 14% 2	3560
April	1st to 10th	Night	22% 7% 17% 29% 18% 5%	1900
	11th to 20th	Night	19% 7% 16% 31% 20% 7%	1879
	21st to end	Night	19% 7% 15% 34% 22% 3	1900
May	1st to 10th	Night	14% 4 9% 33% 33% 7%	2970
	11th to 20th	Night	13% 3 8% 22% 41% 12%	2645
	21st to end	Night	16% 3 10% 31% 36% 3	2762
June	1st to 10th	Night	18% 6% 11% 24% 30% 11%	1520
	11th to 20th	Night	9% 4 6% 22% 37% 22%	1520
	21st to end	Night	11% 2 4 13% 33% 36%	1520
July	1st to 10th	Night	4 1 1 5% 18% 70%	2500
	11th to 20th	Night	15% 1 4 8% 17% 54%	2492
	21st to end	Night	11% 2 4 10% 21% 51%	2763
August	1st to 10th	Night	12% 2 5% 10% 28% 43%	2927
	11th to 20th	Night	7% 1 2 6% 22% 62%	3156
	21st to end	Night	7% 1 2 9% 40% 40%	3498
September	1st to 10th	Night	6% 2 3 9% 35% 45%	1900
	11th to 20th	Night	11% 4 6% 19% 35% 25%	1943
	21st to end	Night	12% 3 6% 17% 36% 27%	2090
October	1st to 10th	Night	13% 2 5% 15% 32% 33%	3707
	11th to 20th	Night	10% 2 5% 19% 48% 15%	3777
	21st to end	Night	15% 4 7% 23% 38% 13%	4210
November	1st to 10th	Night	15% 7% 11% 28% 26% 12%	2535
	11th to 20th	Night	16% 8% 12% 28% 30% 7%	2483
	21st to end	Night	20% 6% 11% 29% 26% 8%	2493
December	1st to 10th	Night	24% 8% 15% 25% 23% 6%	2660
	11th to 20th	Night	30% 12% 16% 21% 17% 4	3276
	21st to end	Night	34% 10% 16% 22% 15% 4	4862

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	18% 13% 32% 21% 14% 2	180
	11th to 20th	Day	12% 19% 25% 22% 17% 6%	218
	21st to end	Day	10% 14% 26% 29% 17% 5%	407
February	1st to 10th	Day	16% 16% 27% 23% 15% 3	383
	11th to 20th	Day	13% 18% 26% 27% 14% 2	455
	21st to end	Day	10% 19% 31% 27% 12% 1	382
March	1st to 10th	Day	14% 23% 29% 22% 11% 1	460
	11th to 20th	Day	13% 23% 24% 23% 13% 4	460
	21st to end	Day	14% 25% 28% 21% 11% 1	506
April	1st to 10th	Day	14% 22% 29% 19% 13% 4	460
	11th to 20th	Day	17% 23% 27% 20% 8% 4	452
	21st to end	Day	11% 22% 27% 22% 12% 6%	460
May	1st to 10th	Day	13% 24% 29% 21% 10% 4	463
	11th to 20th	Day	13% 20% 26% 20% 14% 8%	488
	21st to end	Day	11% 17% 23% 23% 18% 9%	529
June	1st to 10th	Day	10% 17% 26% 20% 18% 8%	490
	11th to 20th	Day	10% 18% 30% 24% 12% 7%	490
	21st to end	Day	4 12% 31% 28% 18% 8%	490
July	1st to 10th	Day	2 13% 29% 28% 20% 8%	486
	11th to 20th	Day	3 12% 24% 29% 20% 12%	484
	21st to end	Day	3 9% 26% 28% 21% 12%	531
August	1st to 10th	Day	6% 12% 26% 22% 23% 11%	487
	11th to 20th	Day	1 12% 29% 26% 21% 12%	477
	21st to end	Day	3 10% 27% 29% 22% 9%	539
September	1st to 10th	Day	3 10% 28% 29% 21% 10%	490
	11th to 20th	Day	4 15% 24% 30% 20% 7%	490
	21st to end	Day	6% 12% 24% 32% 21% 5%	490
October	1st to 10th	Day	6% 14% 24% 26% 20% 10%	490
	11th to 20th	Day	6% 11% 23% 29% 22% 8%	490
	21st to end	Day	6% 13% 23% 29% 22% 7%	482
November	1st to 10th	Day	8% 12% 18% 29% 21% 12%	300
	11th to 20th	Day	7% 8% 21% 31% 22% 11%	273
	21st to end	Day	5% 12% 28% 26% 20% 8%	257
December	1st to 10th	Day	12% 12% 21% 27% 19% 9%	180
	11th to 20th	Day	7% 14% 31% 25% 14% 9%	180
	21st to end	Day	17% 14% 21% 24% 18% 6%	198
January	1st to 10th	Night	7% 12% 22% 30% 20% 8%	760
	11th to 20th	Night	4 9% 24% 39% 17% 6%	760
	21st to end	Night	6% 6% 27% 36% 18% 6%	836
February	1st to 10th	Night	7% 9% 24% 36% 21% 4	760
	11th to 20th	Night	4 11% 27% 33% 20% 6%	744
	21st to end	Night	6% 14% 24% 38% 15% 3	628
March	1st to 10th	Night	7% 13% 30% 31% 16% 4	760
	11th to 20th	Night	5% 12% 26% 30% 23% 5%	760
	21st to end	Night	6% 12% 30% 34% 15% 3	836
April	1st to 10th	Night	7% 9% 25% 36% 21% 2	760
	11th to 20th	Night	8% 11% 25% 33% 18% 5%	751
	21st to end	Night	6% 11% 22% 35% 21% 5%	760
May	1st to 10th	Night	6% 10% 24% 36% 20% 4	641
	11th to 20th	Night	4 7% 21% 40% 25% 3	451
	21st to end	Night	6% 4 19% 36% 33% 2	411
June	1st to 10th	Night	6% 3 19% 42% 27% 4	380
	11th to 20th	Night	1 4 21% 44% 29% 1	380
	21st to end	Night	2 1 17% 48% 29% 3	380
July	1st to 10th	Night	1 1 10% 54% 33% 2	370
	11th to 20th	Night	2 1 12% 49% 34% 1	368
	21st to end	Night	3 1 10% 40% 39% 6%	471
August	1st to 10th	Night	3 1 11% 41% 39% 5%	616
	11th to 20th	Night	2 2 15% 45% 32% 5%	752
	21st to end	Night	2 2 14% 46% 32% 5%	836
September	1st to 10th	Night	1 1 11% 43% 37% 6%	760
	11th to 20th	Night	3 4 15% 35% 35% 8%	760
	21st to end	Night	3 4 15% 36% 33% 8%	760
October	1st to 10th	Night	5% 3 18% 38% 27% 9%	760
	11th to 20th	Night	4 7% 12% 39% 31% 8%	760
	21st to end	Night	4 5% 18% 35% 25% 13%	836
November	1st to 10th	Night	4 8% 19% 34% 27% 8%	780
	11th to 20th	Night	5% 5% 23% 41% 19% 7%	764
	21st to end	Night	7% 8% 21% 33% 24% 6%	760
December	1st to 10th	Night	7% 8% 24% 36% 19% 6%	760
	11th to 20th	Night	7% 10% 23% 36% 17% 6%	760
	21st to end	Night	7% 11% 27% 36% 15% 3	836

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