Strohbehn Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Strohbehn Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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	12% 8% 3 8% 5% 4 4 4 5% 7% 38%	113887
	Night	11% 8% 3 8% 5% 4 4 3 5% 6% 41%	114902
Transparency	Day	43% 6% 13% 24% 13% 1	117529
	Night	42% 5% 13% 24% 15% 2	119403
Seeing	Day	15% 10% 31% 28% 13% 3	16438
	Night	15% 10% 34% 31% 10% 1	26470

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	5% 14% 6% 22% 9% 6% 4 3 5% 5% 21%	3617
	11th to 20th	Day	5% 11% 4 12% 6% 7% 8% 6% 7% 8% 26%	4016
	21st to end	Day	10% 12% 7% 13% 6% 4 4 3 4 5% 31%	4488
February	1st to 10th	Day	10% 12% 5% 12% 7% 6% 5% 4 6% 7% 26%	1800
	11th to 20th	Day	11% 11% 5% 10% 5% 5% 6% 4 5% 8% 32%	1754
	21st to end	Day	14% 9% 4 10% 6% 4 4 4 5% 6% 35%	1575
March	1st to 10th	Day	14% 9% 3 7% 3 3 3 2 4 5% 47%	4936
	11th to 20th	Day	12% 6% 2 6% 4 3 3 3 5% 8% 46%	4980
	21st to end	Day	21% 13% 7% 8% 5% 4 4 3 4 6% 25%	5462
April	1st to 10th	Day	17% 11% 4 7% 4 4 4 3 5% 6% 33%	2382
	11th to 20th	Day	18% 10% 3 8% 5% 5% 3 3 5% 6% 35%	2374
	21st to end	Day	21% 9% 3 7% 4 4 4 3 5% 5% 35%	2516
May	1st to 10th	Day	20% 8% 3 6% 5% 4 4 4 7% 8% 31%	4082
	11th to 20th	Day	15% 7% 3 5% 4 4 4 4 5% 7% 42%	4356
	21st to end	Day	16% 7% 2 6% 4 4 5% 3 5% 7% 40%	4824
June	1st to 10th	Day	10% 7% 2 6% 3 3 4 4 7% 8% 45%	2660
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 4 6% 9% 43%	2660
	21st to end	Day	10% 6% 3 6% 5% 5% 5% 5% 6% 7% 41%	2660
July	1st to 10th	Day	10% 6% 2 7% 4 5% 6% 4 7% 10% 39%	4440
	11th to 20th	Day	6% 3 2 5% 4 3 4 3 5% 8% 58%	4426
	21st to end	Day	5% 3 2 6% 5% 5% 5% 5% 8% 9% 47%	4465
August	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 7% 8% 44%	4104
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 5% 6% 8% 39%	4041
	21st to end	Day	7% 5% 2 7% 5% 4 5% 4 6% 8% 48%	4294
September	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 4 7% 52%	2280
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 3 6% 7% 49%	2204
	21st to end	Day	10% 5% 2 5% 4 4 4 3 6% 6% 50%	2027
October	1st to 10th	Day	15% 6% 2 5% 4 4 3 3 5% 7% 46%	1900
	11th to 20th	Day	15% 6% 3 7% 4 3 3 3 5% 5% 47%	1900
	21st to end	Day	16% 9% 4 6% 4 3 3 3 5% 6% 41%	2090
November	1st to 10th	Day	11% 7% 2 7% 3 3 4 4 6% 8% 45%	1774
	11th to 20th	Day	13% 11% 4 9% 5% 4 4 3 6% 6% 36%	1600
	21st to end	Day	13% 10% 4 7% 4 4 4 3 5% 7% 39%	1600
December	1st to 10th	Day	7% 8% 3 7% 6% 5% 4 4 5% 9% 41%	2610
	11th to 20th	Day	14% 12% 4 13% 9% 6% 4 4 5% 6% 23%	3008
	21st to end	Day	20% 16% 8% 16% 5% 3 3 2 3 4 17%	3982
January	1st to 10th	Night	7% 12% 5% 21% 7% 5% 4 4 5% 6% 24%	6348
	11th to 20th	Night	7% 11% 4 10% 7% 6% 7% 5% 8% 7% 28%	5946
	21st to end	Night	10% 11% 5% 13% 5% 4 4 3 5% 6% 33%	6490
February	1st to 10th	Night	9% 13% 5% 12% 7% 4 5% 4 5% 5% 32%	2600
	11th to 20th	Night	10% 10% 4 8% 5% 5% 5% 4 5% 6% 38%	2533
	21st to end	Night	12% 9% 4 9% 5% 4 4 3 5% 5% 41%	2070
March	1st to 10th	Night	15% 7% 3 5% 3 3 3 2 4 5% 51%	4919
	11th to 20th	Night	9% 6% 3 5% 3 4 4 3 6% 9% 49%	4980
	21st to end	Night	21% 11% 4 11% 5% 4 4 3 4 5% 29%	5467
April	1st to 10th	Night	14% 8% 3 7% 5% 4 4 3 5% 6% 41%	2017
	11th to 20th	Night	15% 9% 3 6% 5% 5% 4 4 5% 6% 39%	1975
	21st to end	Night	18% 9% 2 5% 4 3 3 3 5% 6% 41%	1776
May	1st to 10th	Night	19% 7% 2 6% 4 3 4 3 5% 6% 41%	2823
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 4 6% 50%	2548
	21st to end	Night	13% 6% 2 6% 3 3 4 4 5% 7% 46%	2743
June	1st to 10th	Night	7% 4 2 5% 4 5% 5% 4 7% 9% 48%	1520
	11th to 20th	Night	9% 5% 2 5% 4 4 4 3 6% 9% 48%	1520
	21st to end	Night	9% 5% 2 5% 4 4 4 3 7% 9% 48%	1520
July	1st to 10th	Night	8% 5% 1 5% 5% 5% 4 4 6% 9% 49%	2530
	11th to 20th	Night	4 3 1 3 3 3 4 4 6% 8% 61%	2530
	21st to end	Night	5% 3 1 4 4 5% 5% 5% 7% 9% 52%	3095
August	1st to 10th	Night	6% 5% 2 5% 4 4 4 4 6% 8% 52%	2836
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 4 8% 54%	2798
	21st to end	Night	9% 5% 2 5% 3 3 3 3 4 6% 56%	3352
September	1st to 10th	Night	8% 4 2 5% 3 3 4 3 5% 6% 57%	1900
	11th to 20th	Night	10% 4 2 4 3 3 4 2 5% 7% 55%	1973
	21st to end	Night	9% 5% 1 4 3 3 3 3 5% 6% 57%	2133
October	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 6% 53%	2280
	11th to 20th	Night	15% 7% 2 5% 4 4 4 3 4 6% 47%	2280
	21st to end	Night	15% 8% 2 6% 4 4 3 3 5% 6% 44%	2508
November	1st to 10th	Night	11% 7% 2 5% 4 3 4 4 5% 7% 49%	2517
	11th to 20th	Night	12% 10% 3 8% 4 4 4 3 5% 6% 42%	2800
	21st to end	Night	13% 10% 4 7% 3 4 3 3 4 6% 43%	2800
December	1st to 10th	Night	6% 7% 3 8% 6% 5% 4 4 6% 8% 42%	4580
	11th to 20th	Night	12% 11% 4 12% 6% 5% 4 4 4 5% 33%	5199
	21st to end	Night	19% 17% 8% 16% 5% 3 3 2 3 4 19%	6996

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	64% 6% 11% 15% 5%	3618
	11th to 20th	Day	54% 4 11% 16% 14% 1	4016
	21st to end	Day	55% 4 14% 19% 7% 1	4488
February	1st to 10th	Day	56% 6% 13% 17% 7% 1	1800
	11th to 20th	Day	50% 6% 11% 21% 11% 2	1763
	21st to end	Day	50% 5% 13% 19% 11% 2	1575
March	1st to 10th	Day	42% 4 10% 29% 14% 2	4936
	11th to 20th	Day	36% 4 10% 32% 16% 2	4980
	21st to end	Day	60% 6% 14% 15% 4 1	5464
April	1st to 10th	Day	50% 6% 12% 22% 9% 1	2382
	11th to 20th	Day	51% 5% 11% 20% 13% 1	2374
	21st to end	Day	50% 6% 11% 21% 12% 1	2516
May	1st to 10th	Day	48% 7% 14% 18% 12% 1	4120
	11th to 20th	Day	40% 8% 11% 28% 13% 1	4360
	21st to end	Day	41% 8% 12% 23% 15% 1	4837
June	1st to 10th	Day	35% 7% 16% 25% 16% 2	2660
	11th to 20th	Day	36% 7% 14% 23% 18% 2	2660
	21st to end	Day	38% 9% 15% 22% 14% 2	2660
July	1st to 10th	Day	39% 7% 17% 28% 8% 1	4371
	11th to 20th	Day	25% 6% 14% 33% 20% 2	4351
	21st to end	Day	29% 10% 18% 25% 17% 2	4325
August	1st to 10th	Day	36% 7% 16% 26% 14% 1	4090
	11th to 20th	Day	43% 6% 14% 24% 12% 1	4080
	21st to end	Day	33% 6% 15% 26% 17% 2	4293
September	1st to 10th	Day	32% 6% 12% 27% 21% 2	2280
	11th to 20th	Day	33% 6% 14% 31% 15%	2204
	21st to end	Day	34% 6% 11% 29% 20% 1	2027
October	1st to 10th	Day	23% 3 7% 30% 35% 2	3160
	11th to 20th	Day	29% 4 13% 27% 25% 2	3140
	21st to end	Day	41% 4 9% 30% 14% 3	3428
November	1st to 10th	Day	37% 7% 15% 23% 17% 2	1774
	11th to 20th	Day	47% 6% 13% 22% 13% 1	1600
	21st to end	Day	45% 5% 16% 23% 11% 1	1600
December	1st to 10th	Day	39% 6% 15% 32% 7% 1	2610
	11th to 20th	Day	61% 5% 14% 13% 6%	3005
	21st to end	Day	72% 4 8% 12% 4	3982
January	1st to 10th	Night	60% 5% 12% 15% 8% 1	6348
	11th to 20th	Night	51% 4 14% 14% 15% 2	5946
	21st to end	Night	52% 4 12% 19% 12%	6490
February	1st to 10th	Night	53% 5% 12% 20% 10% 1	2600
	11th to 20th	Night	46% 5% 11% 23% 13% 2	2534
	21st to end	Night	44% 4 13% 20% 15% 3	2070
March	1st to 10th	Night	36% 4 9% 32% 18% 2	4919
	11th to 20th	Night	33% 4 10% 32% 17% 4	4980
	21st to end	Night	58% 6% 13% 15% 6% 1	5467
April	1st to 10th	Night	42% 6% 12% 25% 14% 1	2017
	11th to 20th	Night	45% 5% 12% 19% 17% 2	1975
	21st to end	Night	43% 5% 12% 23% 14% 2	1776
May	1st to 10th	Night	43% 6% 11% 20% 19% 1	2850
	11th to 20th	Night	36% 7% 11% 26% 15% 6%	2549
	21st to end	Night	37% 7% 13% 25% 16% 3	2751
June	1st to 10th	Night	30% 9% 15% 27% 16% 3	1520
	11th to 20th	Night	31% 7% 14% 22% 20% 6%	1520
	21st to end	Night	31% 7% 16% 27% 16% 3	1520
July	1st to 10th	Night	31% 6% 17% 30% 13% 2	2490
	11th to 20th	Night	21% 6% 19% 34% 19% 2	2482
	21st to end	Night	25% 9% 18% 27% 19% 2	2997
August	1st to 10th	Night	29% 7% 15% 33% 14% 3	2826
	11th to 20th	Night	29% 6% 18% 32% 14% 2	2825
	21st to end	Night	30% 5% 16% 28% 18% 3	3352
September	1st to 10th	Night	28% 5% 12% 26% 26% 3	1900
	11th to 20th	Night	29% 5% 13% 32% 21% 1	1973
	21st to end	Night	28% 5% 11% 29% 24% 2	2133
October	1st to 10th	Night	21% 3 7% 27% 39% 3	3800
	11th to 20th	Night	27% 3 10% 28% 25% 6%	3779
	21st to end	Night	42% 4 10% 26% 16% 2	4120
November	1st to 10th	Night	34% 5% 15% 25% 17% 3	2517
	11th to 20th	Night	42% 6% 13% 25% 13% 2	2800
	21st to end	Night	45% 3 13% 26% 12% 2	2800
December	1st to 10th	Night	39% 4 14% 34% 8%	4580
	11th to 20th	Night	52% 4 14% 24% 5%	5201
	21st to end	Night	70% 4 10% 12% 4	6996

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	14% 14% 36% 25% 8% 1	300
	11th to 20th	Day	17% 14% 46% 21% 2 1	300
	21st to end	Day	20% 12% 40% 22% 4 2	330
February	1st to 10th	Day	14% 21% 41% 21% 2 1	300
	11th to 20th	Day	14% 22% 37% 22% 5% 1	300
	21st to end	Day	21% 16% 41% 20% 2	249
March	1st to 10th	Day	17% 14% 47% 16% 6% 1	300
	11th to 20th	Day	22% 16% 34% 21% 7%	300
	21st to end	Day	33% 18% 28% 17% 3	330
April	1st to 10th	Day	27% 13% 35% 21% 4	482
	11th to 20th	Day	23% 17% 30% 22% 7% 1	492
	21st to end	Day	24% 16% 30% 22% 7% 2	656
May	1st to 10th	Day	22% 14% 33% 21% 9% 1	653
	11th to 20th	Day	23% 12% 29% 23% 11% 3	675
	21st to end	Day	18% 9% 28% 28% 14% 2	734
June	1st to 10th	Day	7% 8% 28% 33% 21% 4	680
	11th to 20th	Day	12% 6% 32% 33% 15% 3	680
	21st to end	Day	10% 7% 36% 30% 14% 3	680
July	1st to 10th	Day	7% 4 20% 35% 25% 9%	671
	11th to 20th	Day	6% 3 26% 37% 22% 6%	668
	21st to end	Day	5% 3 24% 40% 24% 5%	739
August	1st to 10th	Day	9% 4 21% 39% 23% 5%	676
	11th to 20th	Day	12% 4 21% 34% 23% 6%	665
	21st to end	Day	9% 4 22% 39% 20% 6%	614
September	1st to 10th	Day	13% 5% 24% 35% 18% 5%	490
	11th to 20th	Day	9% 4 26% 38% 18% 6%	414
	21st to end	Day	10% 12% 31% 32% 13% 3	300
October	1st to 10th	Day	17% 9% 28% 25% 18% 4	300
	11th to 20th	Day	18% 19% 36% 20% 7% 1	300
	21st to end	Day	21% 16% 35% 19% 8% 2	330
November	1st to 10th	Day	16% 11% 41% 22% 7% 2	300
	11th to 20th	Day	18% 23% 35% 18% 6% 1	300
	21st to end	Day	19% 14% 42% 20% 4 1	300
December	1st to 10th	Day	10% 15% 45% 22% 8% 1	300
	11th to 20th	Day	16% 14% 38% 24% 8%	300
	21st to end	Day	24% 18% 37% 18% 2 1	330
January	1st to 10th	Night	12% 13% 40% 28% 7%	1000
	11th to 20th	Night	16% 12% 38% 28% 6%	1000
	21st to end	Night	18% 12% 38% 26% 6%	1100
February	1st to 10th	Night	16% 11% 41% 27% 5%	1000
	11th to 20th	Night	16% 13% 40% 26% 5% 1	979
	21st to end	Night	16% 13% 41% 23% 6% 1	750
March	1st to 10th	Night	18% 12% 38% 24% 7% 1	600
	11th to 20th	Night	18% 11% 40% 24% 7%	600
	21st to end	Night	23% 13% 38% 20% 6% 1	660
April	1st to 10th	Night	17% 16% 41% 23% 3	600
	11th to 20th	Night	19% 14% 38% 23% 6% 1	593
	21st to end	Night	21% 11% 35% 25% 7% 1	587
May	1st to 10th	Night	19% 10% 33% 29% 9%	570
	11th to 20th	Night	16% 8% 33% 32% 10% 1	561
	21st to end	Night	15% 6% 30% 35% 13%	617
June	1st to 10th	Night	8% 6% 29% 40% 16% 1	570
	11th to 20th	Night	11% 5% 28% 45% 11%	570
	21st to end	Night	9% 4 31% 42% 14% 1	570
July	1st to 10th	Night	6% 2 17% 46% 27% 2	555
	11th to 20th	Night	5% 3 25% 43% 23% 3	552
	21st to end	Night	4 2 16% 54% 23% 2	624
August	1st to 10th	Night	7% 2 20% 47% 23% 1	567
	11th to 20th	Night	9% 3 20% 41% 24% 3	564
	21st to end	Night	9% 3 22% 41% 21% 3	627
September	1st to 10th	Night	9% 5% 24% 38% 22% 2	570
	11th to 20th	Night	12% 2 27% 41% 17% 1	570
	21st to end	Night	10% 7% 33% 35% 14% 1	613
October	1st to 10th	Night	14% 8% 33% 32% 12% 1	760
	11th to 20th	Night	17% 11% 37% 28% 6%	760
	21st to end	Night	19% 12% 35% 26% 8%	836
November	1st to 10th	Night	14% 10% 38% 30% 8%	845
	11th to 20th	Night	16% 14% 42% 22% 6%	1000
	21st to end	Night	17% 14% 36% 26% 8%	1000
December	1st to 10th	Night	14% 12% 41% 27% 6%	1000
	11th to 20th	Night	20% 10% 35% 30% 6%	1000
	21st to end	Night	23% 12% 36% 23% 5%	1100

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