Behlen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Behlen Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	11% 7% 3 7% 4 4 4 3 5% 7% 45%	126474
	Night	10% 7% 2 6% 4 4 4 3 5% 7% 48%	124174
Transparency	Day	38% 3 13% 26% 17% 2	130125
	Night	36% 3 12% 27% 19% 3	128586
Seeing	Day	15% 7% 29% 30% 16% 3	17230
	Night	14% 7% 32% 36% 11% 1	27072

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	10% 12% 6% 14% 8% 5% 4 3 5% 6% 27%	4077
	11th to 20th	Day	7% 9% 3 10% 5% 4 6% 4 5% 6% 41%	4080
	21st to end	Day	6% 10% 4 11% 5% 5% 5% 3 4 7% 40%	4488
February	1st to 10th	Day	10% 12% 4 10% 6% 5% 4 4 6% 7% 33%	1800
	11th to 20th	Day	9% 11% 3 8% 5% 4 4 4 5% 7% 39%	1806
	21st to end	Day	10% 8% 3 6% 4 4 5% 4 5% 7% 45%	1650
March	1st to 10th	Day	14% 8% 3 5% 3 2 2 2 3 4 53%	4842
	11th to 20th	Day	12% 7% 2 5% 3 3 3 3 4 7% 51%	4980
	21st to end	Day	19% 10% 3 8% 4 4 4 4 6% 8% 30%	5462
April	1st to 10th	Day	15% 8% 3 5% 5% 4 3 3 5% 7% 44%	2245
	11th to 20th	Day	15% 8% 3 6% 4 4 4 4 6% 6% 41%	2511
	21st to end	Day	20% 7% 3 6% 4 4 3 3 5% 7% 38%	2600
May	1st to 10th	Day	12% 8% 3 6% 4 4 4 3 4 6% 45%	5862
	11th to 20th	Day	14% 6% 2 5% 3 3 3 2 4 5% 51%	5862
	21st to end	Day	17% 7% 3 7% 4 4 4 4 6% 7% 38%	6520
June	1st to 10th	Day	7% 4 2 5% 5% 4 5% 4 6% 9% 50%	2800
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 48%	2800
	21st to end	Day	7% 5% 2 5% 4 5% 4 4 6% 8% 50%	2788
July	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 6% 9% 47%	5900
	11th to 20th	Day	7% 5% 2 7% 4 4 4 4 6% 8% 48%	5900
	21st to end	Day	6% 4 2 5% 3 4 4 4 7% 10% 50%	6422
August	1st to 10th	Day	6% 4 2 6% 5% 4 5% 4 7% 10% 46%	4234
	11th to 20th	Day	12% 6% 3 6% 4 4 5% 3 6% 9% 42%	4171
	21st to end	Day	6% 4 2 6% 4 4 5% 4 6% 8% 51%	4618
September	1st to 10th	Day	12% 5% 2 5% 4 3 4 3 4 6% 53%	2300
	11th to 20th	Day	10% 5% 2 4 4 3 4 3 6% 7% 53%	2200
	21st to end	Day	10% 4 2 4 4 2 4 3 5% 6% 58%	2200
October	1st to 10th	Day	13% 6% 2 4 3 3 3 3 4 6% 52%	2060
	11th to 20th	Day	11% 5% 2 5% 4 3 3 3 3 5% 56%	2000
	21st to end	Day	13% 8% 3 6% 3 4 4 3 4 6% 47%	2200
November	1st to 10th	Day	9% 6% 2 5% 3 3 3 3 5% 5% 55%	1858
	11th to 20th	Day	10% 6% 2 6% 6% 4 3 3 4 6% 49%	1831
	21st to end	Day	13% 8% 2 5% 5% 3 3 3 4 8% 44%	1600
December	1st to 10th	Day	5% 5% 2 6% 4 4 4 4 5% 6% 54%	2610
	11th to 20th	Day	9% 9% 4 10% 5% 5% 4 4 5% 7% 41%	3008
	21st to end	Day	17% 10% 4 13% 5% 4 3 3 4 6% 32%	4189
January	1st to 10th	Night	9% 11% 4 18% 7% 5% 5% 3 5% 5% 26%	5888
	11th to 20th	Night	6% 8% 3 8% 5% 4 7% 4 7% 6% 42%	5900
	21st to end	Night	8% 10% 5% 9% 4 4 4 3 4 6% 42%	6490
February	1st to 10th	Night	11% 10% 3 9% 5% 4 4 3 5% 7% 38%	2600
	11th to 20th	Night	9% 9% 3 7% 5% 5% 5% 3 6% 7% 41%	2498
	21st to end	Night	9% 7% 2 5% 3 4 4 4 5% 5% 51%	1980
March	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 5% 52%	5014
	11th to 20th	Night	10% 6% 2 4 2 3 3 3 4 6% 56%	4980
	21st to end	Night	15% 10% 2 7% 4 4 4 3 7% 7% 36%	5467
April	1st to 10th	Night	12% 8% 3 5% 3 3 4 4 5% 6% 47%	2155
	11th to 20th	Night	12% 7% 3 5% 4 4 3 4 5% 7% 47%	1836
	21st to end	Night	16% 7% 2 6% 3 4 3 3 4 6% 45%	1800
May	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 4 6% 50%	4053
	11th to 20th	Night	13% 4 2 4 3 3 3 4 4 6% 55%	4053
	21st to end	Night	17% 6% 3 5% 4 4 4 3 4 6% 44%	4352
June	1st to 10th	Night	5% 4 2 5% 5% 4 4 4 6% 8% 54%	1600
	11th to 20th	Night	7% 4 1 4 4 4 5% 5% 7% 10% 50%	1600
	21st to end	Night	4 3 2 4 4 4 4 3 7% 8% 57%	1606
July	1st to 10th	Night	7% 5% 2 5% 4 4 4 3 5% 8% 54%	4078
	11th to 20th	Night	6% 4 2 5% 3 3 4 4 7% 9% 54%	4077
	21st to end	Night	4 3 2 4 4 4 4 4 6% 10% 54%	4449
August	1st to 10th	Night	4 3 1 4 3 3 4 4 6% 10% 58%	2926
	11th to 20th	Night	10% 5% 1 6% 4 3 3 3 6% 9% 50%	2888
	21st to end	Night	7% 4 2 5% 4 3 4 3 6% 7% 55%	3288
September	1st to 10th	Night	9% 4 2 3 2 3 3 2 5% 6% 61%	2096
	11th to 20th	Night	9% 3 1 3 2 3 4 3 6% 8% 58%	2200
	21st to end	Night	9% 5% 1 4 3 3 4 3 4 5% 59%	2200
October	1st to 10th	Night	14% 4 2 4 3 4 4 4 5% 6% 51%	2318
	11th to 20th	Night	13% 5% 2 4 4 3 2 3 4 6% 56%	2400
	21st to end	Night	14% 5% 2 4 3 3 3 3 4 5% 53%	2640
November	1st to 10th	Night	8% 4 2 4 3 3 4 3 5% 7% 56%	2658
	11th to 20th	Night	8% 7% 2 6% 3 3 3 3 5% 6% 54%	2721
	21st to end	Night	11% 7% 2 5% 3 3 3 3 4 6% 52%	2800
December	1st to 10th	Night	5% 6% 2 5% 4 4 4 4 5% 7% 55%	4580
	11th to 20th	Night	10% 7% 3 8% 4 4 4 3 5% 8% 46%	5199
	21st to end	Night	16% 13% 5% 10% 4 3 3 3 4 5% 34%	6784

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% 3 12% 19% 7%	4078
	11th to 20th	Day	44% 2 14% 21% 17% 2	4080
	21st to end	Day	45% 3 14% 25% 13% 1	4488
February	1st to 10th	Day	51% 3 15% 20% 11% 1	1800
	11th to 20th	Day	45% 3 12% 22% 15% 3	1815
	21st to end	Day	38% 4 12% 27% 16% 3	1650
March	1st to 10th	Day	38% 2 8% 27% 19% 6%	4842
	11th to 20th	Day	34% 2 8% 27% 24% 4	4980
	21st to end	Day	53% 3 21% 16% 5% 1	5464
April	1st to 10th	Day	41% 3 13% 26% 15% 2	2245
	11th to 20th	Day	43% 3 14% 23% 14% 3	2511
	21st to end	Day	47% 3 10% 26% 13% 1	2600
May	1st to 10th	Day	38% 5% 10% 27% 18% 2	5900
	11th to 20th	Day	37% 2 9% 23% 24% 5%	5866
	21st to end	Day	45% 5% 11% 23% 15% 1	6534
June	1st to 10th	Day	31% 4 17% 31% 14% 2	2800
	11th to 20th	Day	33% 4 13% 32% 16% 2	2800
	21st to end	Day	31% 4 16% 29% 15% 4	2788
July	1st to 10th	Day	33% 4 14% 33% 16% 1	5836
	11th to 20th	Day	33% 5% 13% 29% 17% 2	5831
	21st to end	Day	28% 5% 19% 29% 17% 2	6240
August	1st to 10th	Day	31% 4 17% 28% 17% 3	4220
	11th to 20th	Day	39% 4 14% 26% 16% 1	4210
	21st to end	Day	31% 5% 15% 26% 17% 6%	4617
September	1st to 10th	Day	33% 3 10% 31% 18% 5%	2300
	11th to 20th	Day	31% 3 12% 33% 19% 1	2200
	21st to end	Day	29% 2 9% 33% 25% 2	2200
October	1st to 10th	Day	22% 2 6% 21% 45% 4	3359
	11th to 20th	Day	23% 2 13% 30% 25% 8%	3240
	21st to end	Day	36% 2 11% 27% 22% 3	3538
November	1st to 10th	Day	32% 2 14% 29% 21% 2	1858
	11th to 20th	Day	37% 3 12% 27% 20% 1	1831
	21st to end	Day	41% 2 12% 30% 14% 2	1600
December	1st to 10th	Day	31% 2 18% 30% 18% 1	2610
	11th to 20th	Day	43% 3 19% 25% 10% 1	3005
	21st to end	Day	56% 2 12% 23% 7% 1	4189
January	1st to 10th	Night	59% 4 10% 17% 11%	5888
	11th to 20th	Night	40% 3 15% 24% 17% 2	5900
	21st to end	Night	44% 4 12% 21% 18% 2	6490
February	1st to 10th	Night	47% 3 13% 21% 13% 2	2600
	11th to 20th	Night	42% 3 13% 23% 15% 3	2499
	21st to end	Night	35% 3 10% 27% 21% 5%	1980
March	1st to 10th	Night	35% 2 7% 29% 23% 3	5014
	11th to 20th	Night	30% 2 7% 26% 28% 7%	4980
	21st to end	Night	46% 4 19% 24% 6% 1	5467
April	1st to 10th	Night	37% 4 10% 28% 18% 3	2155
	11th to 20th	Night	38% 3 11% 26% 18% 3	1836
	21st to end	Night	41% 4 13% 24% 17% 1	1800
May	1st to 10th	Night	36% 3 10% 27% 22% 2	4080
	11th to 20th	Night	31% 3 13% 19% 28% 6%	4054
	21st to end	Night	42% 4 13% 24% 16% 2	4360
June	1st to 10th	Night	28% 5% 14% 34% 17% 3	1600
	11th to 20th	Night	28% 5% 15% 28% 20% 5%	1600
	21st to end	Night	23% 4 15% 33% 21% 3	1606
July	1st to 10th	Night	30% 4 13% 34% 17% 3	4035
	11th to 20th	Night	28% 5% 15% 31% 20% 2	4026
	21st to end	Night	25% 6% 17% 28% 22% 2	4320
August	1st to 10th	Night	22% 4 17% 36% 19% 2	2916
	11th to 20th	Night	33% 4 16% 28% 19% 1	2915
	21st to end	Night	28% 4 15% 28% 17% 8%	3288
September	1st to 10th	Night	25% 3 11% 31% 23% 7%	2096
	11th to 20th	Night	25% 4 12% 36% 20% 4	2200
	21st to end	Night	28% 2 8% 29% 28% 5%	2200
October	1st to 10th	Night	21% 2 7% 17% 47% 6%	3786
	11th to 20th	Night	22% 2 7% 30% 30% 8%	3899
	21st to end	Night	33% 2 9% 31% 22% 2	4252
November	1st to 10th	Night	28% 2 15% 27% 24% 4	2658
	11th to 20th	Night	33% 2 12% 31% 20% 3	2721
	21st to end	Night	35% 2 13% 29% 19% 2	2800
December	1st to 10th	Night	29% 3 15% 34% 17% 1	4580
	11th to 20th	Night	37% 3 16% 30% 13%	5201
	21st to end	Night	54% 2 11% 24% 9% 1	6784

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	20% 10% 35% 25% 9% 2	300
	11th to 20th	Day	18% 14% 34% 27% 6% 1	300
	21st to end	Day	16% 8% 42% 23% 9% 2	330
February	1st to 10th	Day	18% 18% 38% 23% 3	300
	11th to 20th	Day	16% 12% 39% 24% 8% 2	300
	21st to end	Day	15% 17% 40% 21% 6% 1	249
March	1st to 10th	Day	16% 11% 41% 27% 4 1	300
	11th to 20th	Day	27% 7% 34% 22% 9% 1	300
	21st to end	Day	29% 12% 38% 16% 3 1	330
April	1st to 10th	Day	21% 14% 37% 23% 5%	345
	11th to 20th	Day	21% 13% 37% 21% 8%	629
	21st to end	Day	26% 11% 31% 22% 9% 2	700
May	1st to 10th	Day	21% 8% 32% 26% 10% 3	703
	11th to 20th	Day	20% 7% 29% 26% 15% 4	725
	21st to end	Day	19% 6% 27% 29% 16% 2	789
June	1st to 10th	Day	8% 4 23% 34% 25% 6%	730
	11th to 20th	Day	9% 4 27% 37% 18% 5%	730
	21st to end	Day	9% 2 34% 35% 17% 3	730
July	1st to 10th	Day	11% 2 15% 34% 29% 10%	721
	11th to 20th	Day	6% 2 20% 39% 25% 7%	718
	21st to end	Day	7% 1 17% 41% 29% 5%	794
August	1st to 10th	Day	7% 2 19% 42% 25% 5%	726
	11th to 20th	Day	8% 2 16% 42% 27% 3	715
	21st to end	Day	11% 2 19% 39% 25% 5%	772
September	1st to 10th	Day	16% 3 22% 36% 19% 4	430
	11th to 20th	Day	8% 2 30% 35% 19% 5%	330
	21st to end	Day	13% 6% 29% 31% 15% 5%	330
October	1st to 10th	Day	20% 4 24% 31% 15% 6%	330
	11th to 20th	Day	12% 14% 40% 23% 9% 1	330
	21st to end	Day	20% 13% 35% 20% 10% 2	363
November	1st to 10th	Day	18% 9% 38% 26% 8% 2	330
	11th to 20th	Day	14% 13% 40% 26% 7% 1	321
	21st to end	Day	21% 10% 31% 26% 10% 2	300
December	1st to 10th	Day	9% 10% 38% 33% 8% 1	300
	11th to 20th	Day	19% 8% 37% 27% 9% 1	300
	21st to end	Day	21% 11% 38% 22% 6% 1	330
January	1st to 10th	Night	13% 11% 33% 35% 8% 1	1000
	11th to 20th	Night	15% 11% 36% 32% 7% 1	1000
	21st to end	Night	16% 6% 39% 32% 6%	1100
February	1st to 10th	Night	15% 10% 39% 32% 5%	1000
	11th to 20th	Night	14% 10% 40% 31% 6%	944
	21st to end	Night	14% 10% 41% 30% 5% 1	660
March	1st to 10th	Night	16% 12% 35% 29% 8%	647
	11th to 20th	Night	19% 7% 37% 30% 7%	600
	21st to end	Night	23% 8% 38% 26% 5%	660
April	1st to 10th	Night	16% 14% 39% 26% 6%	600
	11th to 20th	Night	16% 12% 38% 30% 3 1	593
	21st to end	Night	21% 9% 33% 30% 7%	600
May	1st to 10th	Night	17% 8% 32% 33% 10%	600
	11th to 20th	Night	16% 7% 29% 36% 12% 1	591
	21st to end	Night	17% 5% 29% 35% 13% 1	650
June	1st to 10th	Night	9% 3 22% 46% 20% 1	600
	11th to 20th	Night	10% 4 28% 42% 16% 1	600
	21st to end	Night	6% 3 31% 45% 15%	600
July	1st to 10th	Night	8% 1 16% 44% 29% 1	585
	11th to 20th	Night	4 2 17% 51% 24% 2	582
	21st to end	Night	4 1 12% 56% 26% 1	657
August	1st to 10th	Night	6% 2 17% 50% 23% 2	597
	11th to 20th	Night	7% 2 15% 50% 24% 1	594
	21st to end	Night	9% 1 19% 44% 24% 3	660
September	1st to 10th	Night	11% 2 24% 40% 21% 2	600
	11th to 20th	Night	9% 2 28% 42% 17% 2	600
	21st to end	Night	11% 4 29% 35% 19% 1	600
October	1st to 10th	Night	17% 6% 28% 34% 13% 2	718
	11th to 20th	Night	16% 9% 36% 31% 8%	800
	21st to end	Night	19% 8% 29% 34% 9%	880
November	1st to 10th	Night	11% 9% 34% 36% 10% 1	1018
	11th to 20th	Night	13% 10% 39% 32% 6% 1	1036
	21st to end	Night	15% 10% 33% 32% 9% 1	1000
December	1st to 10th	Night	13% 10% 38% 31% 7%	1000
	11th to 20th	Night	16% 8% 33% 36% 7%	1000
	21st to end	Night	20% 9% 32% 33% 5%	1100

Page generated at 2025-11-14 23:55:58. Version ac07

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