Black Hills State University
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black Hills State University clear sky chart from 2005-11-06 to 2026-02-01.

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	7% 7% 2 5% 4 4 4 4 6% 8% 50%	136510
	Night	7% 8% 3 6% 4 4 4 4 6% 7% 49%	132940
Transparency	Day	32% 6% 14% 27% 18% 4	131261
	Night	33% 6% 13% 26% 18% 4	129195
Seeing	Day	14% 16% 33% 25% 10% 2	17591
	Night	12% 15% 33% 29% 9% 1	27705

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	3 9% 5% 11% 5% 4 5% 4 7% 8% 39%	3718
	11th to 20th	Day	2 13% 7% 8% 6% 6% 4 3 5% 6% 38%	3720
	21st to end	Day	2 7% 3 9% 4 4 4 3 5% 6% 52%	4106
February	1st to 10th	Day	5% 16% 4 9% 7% 7% 5% 3 4 6% 34%	1800
	11th to 20th	Day	6% 13% 6% 9% 5% 4 4 3 6% 7% 38%	1877
	21st to end	Day	4 11% 4 9% 6% 4 5% 4 5% 8% 40%	1650
March	1st to 10th	Day	3 10% 3 5% 4 4 3 3 5% 7% 53%	4500
	11th to 20th	Day	8% 9% 2 5% 3 3 3 3 6% 9% 51%	4696
	21st to end	Day	13% 13% 4 8% 4 4 4 4 6% 8% 33%	5961
April	1st to 10th	Day	11% 11% 3 6% 4 4 4 3 6% 7% 41%	2400
	11th to 20th	Day	13% 12% 4 8% 5% 4 4 4 6% 7% 33%	2371
	21st to end	Day	15% 11% 3 5% 4 5% 5% 4 5% 8% 35%	2531
May	1st to 10th	Day	12% 10% 3 6% 4 4 4 3 5% 7% 42%	6154
	11th to 20th	Day	14% 9% 2 6% 4 4 4 4 6% 8% 39%	6298
	21st to end	Day	12% 6% 2 5% 4 3 4 4 6% 9% 45%	6925
June	1st to 10th	Day	8% 5% 2 5% 4 4 5% 4 7% 9% 47%	2800
	11th to 20th	Day	8% 4 2 4 3 4 5% 5% 8% 9% 49%	2800
	21st to end	Day	5% 4 1 3 3 3 3 4 6% 8% 60%	2800
July	1st to 10th	Day	5% 3 1 4 4 4 4 4 7% 9% 54%	6339
	11th to 20th	Day	7% 2 1 3 2 3 4 3 6% 8% 62%	6340
	21st to end	Day	4 3 1 3 3 3 4 4 7% 10% 59%	6903
August	1st to 10th	Day	6% 2 1 4 4 4 5% 4 7% 10% 54%	6232
	11th to 20th	Day	5% 3 1 4 3 4 4 4 6% 8% 59%	5619
	21st to end	Day	7% 4 2 5% 3 3 3 3 5% 7% 59%	5984
September	1st to 10th	Day	7% 4 1 4 2 3 3 3 4 7% 64%	2400
	11th to 20th	Day	9% 3 1 3 2 3 3 4 5% 6% 60%	2335
	21st to end	Day	10% 4 1 3 3 3 3 3 4 7% 59%	2200
October	1st to 10th	Day	6% 5% 1 3 2 2 2 2 4 5% 68%	3369
	11th to 20th	Day	9% 5% 2 4 2 2 2 2 4 6% 61%	3218
	21st to end	Day	9% 8% 2 4 3 3 4 3 5% 9% 49%	3190
November	1st to 10th	Day	7% 8% 3 5% 4 4 4 4 7% 7% 48%	1853
	11th to 20th	Day	5% 8% 3 7% 4 4 3 4 5% 8% 49%	1843
	21st to end	Day	4 8% 2 6% 4 4 4 5% 6% 10% 49%	1680
December	1st to 10th	Day	3 12% 6% 7% 4 4 4 3 5% 6% 45%	2700
	11th to 20th	Day	3 9% 3 6% 4 4 4 4 6% 10% 48%	3106
	21st to end	Day	5% 10% 3 7% 4 4 5% 4 5% 7% 45%	4092
January	1st to 10th	Night	4 13% 5% 10% 4 5% 4 3 5% 5% 41%	6467
	11th to 20th	Night	3 12% 4 10% 7% 5% 4 4 5% 6% 40%	6480
	21st to end	Night	2 8% 4 8% 3 3 3 3 5% 6% 55%	7128
February	1st to 10th	Night	6% 15% 5% 9% 6% 5% 5% 4 5% 6% 35%	2616
	11th to 20th	Night	5% 13% 5% 10% 6% 5% 4 4 6% 6% 39%	2410
	21st to end	Night	4 10% 4 9% 5% 5% 4 3 7% 6% 43%	1980
March	1st to 10th	Night	5% 9% 3 6% 3 3 4 3 4 7% 51%	5374
	11th to 20th	Night	6% 8% 2 5% 4 3 4 3 6% 10% 48%	5276
	21st to end	Night	9% 12% 4 9% 5% 5% 6% 4 7% 8% 30%	4991
April	1st to 10th	Night	12% 11% 3 5% 4 4 3 4 6% 7% 40%	2000
	11th to 20th	Night	12% 9% 4 7% 4 4 4 4 5% 6% 42%	1977
	21st to end	Night	13% 9% 3 5% 4 4 4 4 5% 8% 41%	1868
May	1st to 10th	Night	15% 9% 2 5% 3 3 3 3 5% 6% 46%	3823
	11th to 20th	Night	14% 8% 2 6% 3 4 4 3 6% 8% 42%	3617
	21st to end	Night	13% 7% 2 6% 4 4 4 3 6% 7% 44%	3972
June	1st to 10th	Night	9% 4 1 5% 4 4 4 4 6% 11% 49%	1600
	11th to 20th	Night	8% 4 2 5% 4 5% 5% 4 8% 10% 45%	1600
	21st to end	Night	6% 3 2 5% 4 3 3 4 6% 7% 57%	1600
July	1st to 10th	Night	5% 3 2 4 4 3 4 3 6% 11% 56%	3640
	11th to 20th	Night	6% 2 1 4 3 4 4 4 7% 9% 57%	3640
	21st to end	Night	6% 4 2 4 3 4 4 4 7% 9% 53%	3970
August	1st to 10th	Night	5% 3 1 4 3 3 5% 5% 7% 10% 52%	3580
	11th to 20th	Night	4 2 1 4 3 3 4 4 7% 10% 56%	4212
	21st to end	Night	6% 4 2 5% 4 4 4 4 5% 7% 56%	4993
September	1st to 10th	Night	6% 3 1 4 3 3 3 3 4 8% 62%	2000
	11th to 20th	Night	8% 4 1 4 3 3 3 3 5% 7% 58%	2064
	21st to end	Night	10% 3 1 4 2 3 3 3 4 5% 62%	2200
October	1st to 10th	Night	8% 4 1 2 2 2 2 2 3 4 70%	3815
	11th to 20th	Night	9% 5% 2 3 3 3 4 4 5% 7% 55%	3898
	21st to end	Night	8% 6% 2 4 3 3 4 3 5% 8% 53%	4610
November	1st to 10th	Night	7% 8% 3 6% 4 4 4 4 6% 8% 46%	2665
	11th to 20th	Night	6% 9% 3 7% 4 4 4 3 6% 6% 48%	2775
	21st to end	Night	5% 8% 2 6% 4 4 4 3 6% 8% 50%	2940
December	1st to 10th	Night	3 10% 5% 9% 5% 5% 4 3 5% 6% 46%	4710
	11th to 20th	Night	3 9% 5% 8% 5% 4 4 4 6% 8% 45%	5321
	21st to end	Night	6% 11% 3 6% 4 4 5% 4 6% 7% 44%	7128

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	39% 8% 17% 25% 10% 2	3719
	11th to 20th	Day	42% 8% 13% 20% 12% 4	3720
	21st to end	Day	30% 6% 14% 31% 15% 3	4106
February	1st to 10th	Day	51% 6% 12% 19% 9% 3	1800
	11th to 20th	Day	44% 7% 14% 19% 13% 4	1886
	21st to end	Day	42% 6% 12% 23% 13% 4	1650
March	1st to 10th	Day	29% 6% 10% 27% 25% 3	4500
	11th to 20th	Day	29% 6% 16% 31% 18% 1	4696
	21st to end	Day	47% 8% 16% 22% 7%	5961
April	1st to 10th	Day	42% 6% 16% 25% 10% 2	2400
	11th to 20th	Day	48% 7% 12% 19% 12% 2	2374
	21st to end	Day	46% 7% 15% 23% 8%	2531
May	1st to 10th	Day	40% 7% 11% 24% 17% 2	6154
	11th to 20th	Day	40% 8% 14% 22% 13% 3	6302
	21st to end	Day	33% 9% 15% 28% 15% 1	6926
June	1st to 10th	Day	31% 9% 21% 25% 12% 2	2800
	11th to 20th	Day	28% 9% 20% 26% 15% 3	2800
	21st to end	Day	21% 7% 15% 30% 22% 5%	2800
July	1st to 10th	Day	24% 6% 12% 33% 21% 3	6271
	11th to 20th	Day	20% 5% 12% 33% 23% 6%	6265
	21st to end	Day	19% 7% 13% 26% 24% 11%	6705
August	1st to 10th	Day	26% 7% 15% 24% 18% 10%	4536
	11th to 20th	Day	25% 6% 15% 29% 21% 4	4064
	21st to end	Day	17% 5% 10% 26% 31% 10%	4311
September	1st to 10th	Day	21% 4 10% 29% 26% 9%	2400
	11th to 20th	Day	24% 5% 12% 28% 25% 7%	2335
	21st to end	Day	26% 4 9% 32% 25% 3	2200
October	1st to 10th	Day	20% 3 9% 21% 34% 14%	3369
	11th to 20th	Day	27% 3 12% 31% 23% 4	3218
	21st to end	Day	33% 4 14% 29% 15% 4	3190
November	1st to 10th	Day	32% 8% 12% 25% 20% 3	1853
	11th to 20th	Day	32% 5% 14% 26% 19% 4	1843
	21st to end	Day	31% 6% 16% 28% 16% 3	1680
December	1st to 10th	Day	38% 5% 15% 26% 15% 1	2700
	11th to 20th	Day	29% 6% 21% 33% 9% 1	3104
	21st to end	Day	37% 5% 9% 30% 17% 2	4092
January	1st to 10th	Night	42% 6% 11% 28% 11% 2	6467
	11th to 20th	Night	41% 8% 13% 21% 14% 3	6480
	21st to end	Night	30% 6% 14% 28% 18% 5%	7128
February	1st to 10th	Night	46% 7% 15% 18% 12% 2	2616
	11th to 20th	Night	44% 6% 13% 21% 14% 3	2411
	21st to end	Night	38% 7% 13% 22% 14% 5%	1980
March	1st to 10th	Night	30% 7% 9% 26% 23% 5%	5374
	11th to 20th	Night	29% 6% 18% 24% 20% 2	5276
	21st to end	Night	45% 12% 15% 18% 9%	4991
April	1st to 10th	Night	41% 7% 14% 23% 12% 3	2000
	11th to 20th	Night	42% 6% 12% 24% 14% 2	1977
	21st to end	Night	41% 6% 13% 28% 10% 1	1868
May	1st to 10th	Night	38% 6% 10% 24% 22% 1	3823
	11th to 20th	Night	37% 8% 15% 26% 13% 2	3618
	21st to end	Night	37% 8% 11% 28% 16%	3972
June	1st to 10th	Night	29% 8% 21% 28% 13% 1	1600
	11th to 20th	Night	31% 8% 20% 23% 15% 3	1600
	21st to end	Night	24% 7% 15% 26% 23% 5%	1600
July	1st to 10th	Night	22% 5% 14% 29% 26% 4	3600
	11th to 20th	Night	22% 6% 11% 30% 26% 5%	3592
	21st to end	Night	26% 7% 12% 26% 22% 8%	3856
August	1st to 10th	Night	28% 7% 14% 23% 20% 9%	2600
	11th to 20th	Night	23% 6% 17% 28% 21% 5%	3042
	21st to end	Night	20% 5% 11% 23% 31% 11%	3597
September	1st to 10th	Night	22% 4 7% 28% 26% 12%	2000
	11th to 20th	Night	24% 6% 11% 26% 24% 8%	2064
	21st to end	Night	26% 4 9% 29% 25% 8%	2200
October	1st to 10th	Night	21% 3 9% 23% 30% 15%	3815
	11th to 20th	Night	28% 5% 10% 27% 26% 5%	3898
	21st to end	Night	29% 5% 13% 30% 16% 7%	4610
November	1st to 10th	Night	34% 7% 14% 21% 19% 5%	2665
	11th to 20th	Night	36% 5% 13% 25% 18% 4	2775
	21st to end	Night	31% 6% 16% 26% 17% 4	2940
December	1st to 10th	Night	39% 5% 13% 26% 15% 2	4710
	11th to 20th	Night	36% 6% 15% 31% 11% 1	5322
	21st to end	Night	36% 7% 9% 31% 15% 2	7128

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	13% 20% 32% 25% 7% 3	330
	11th to 20th	Day	21% 23% 27% 19% 8% 2	330
	21st to end	Day	21% 33% 26% 13% 6% 1	363
February	1st to 10th	Day	17% 25% 41% 15% 2	300
	11th to 20th	Day	15% 26% 39% 17% 3	300
	21st to end	Day	18% 24% 42% 14% 2	249
March	1st to 10th	Day	14% 28% 33% 16% 8% 1	300
	11th to 20th	Day	14% 22% 37% 19% 6% 2	365
	21st to end	Day	23% 18% 31% 20% 6% 1	550
April	1st to 10th	Day	21% 24% 36% 14% 3	500
	11th to 20th	Day	25% 26% 36% 10% 3	492
	21st to end	Day	23% 14% 31% 24% 7% 1	631
May	1st to 10th	Day	19% 18% 29% 21% 11% 2	703
	11th to 20th	Day	22% 13% 26% 24% 12% 3	725
	21st to end	Day	18% 15% 32% 20% 12% 3	789
June	1st to 10th	Day	11% 11% 30% 30% 15% 3	730
	11th to 20th	Day	12% 17% 37% 26% 8% 1	730
	21st to end	Day	9% 11% 37% 28% 13% 2	730
July	1st to 10th	Day	5% 7% 32% 35% 18% 2	721
	11th to 20th	Day	5% 9% 37% 35% 14% 1	718
	21st to end	Day	4 8% 32% 35% 18% 2	794
August	1st to 10th	Day	6% 7% 34% 37% 15% 1	726
	11th to 20th	Day	7% 7% 36% 34% 13% 3	571
	21st to end	Day	5% 13% 33% 35% 10% 4	583
September	1st to 10th	Day	8% 9% 37% 31% 12% 3	530
	11th to 20th	Day	10% 9% 33% 31% 13% 3	465
	21st to end	Day	13% 15% 33% 22% 12% 5%	330
October	1st to 10th	Day	15% 17% 27% 24% 14% 3	330
	11th to 20th	Day	21% 20% 34% 15% 6% 3	330
	21st to end	Day	16% 27% 28% 19% 8% 1	363
November	1st to 10th	Day	18% 22% 35% 18% 5% 2	330
	11th to 20th	Day	17% 27% 32% 18% 5% 1	330
	21st to end	Day	12% 21% 35% 25% 6% 1	330
December	1st to 10th	Day	18% 27% 34% 17% 4 1	330
	11th to 20th	Day	17% 26% 32% 22% 3	330
	21st to end	Day	16% 25% 37% 16% 4 1	363
January	1st to 10th	Night	11% 23% 37% 18% 10% 1	1050
	11th to 20th	Night	16% 27% 29% 18% 7% 1	1050
	21st to end	Night	14% 26% 34% 20% 6%	1155
February	1st to 10th	Night	15% 25% 34% 20% 6% 1	816
	11th to 20th	Night	11% 21% 36% 26% 6% 1	784
	21st to end	Night	12% 17% 38% 25% 6% 1	660
March	1st to 10th	Night	11% 19% 36% 25% 9% 1	800
	11th to 20th	Night	10% 16% 39% 27% 7% 2	800
	21st to end	Night	16% 13% 28% 33% 8% 1	667
April	1st to 10th	Night	18% 11% 35% 30% 6%	600
	11th to 20th	Night	20% 14% 32% 27% 6% 1	593
	21st to end	Night	18% 11% 30% 32% 8% 1	600
May	1st to 10th	Night	16% 8% 28% 35% 11% 2	600
	11th to 20th	Night	18% 10% 25% 33% 13% 1	591
	21st to end	Night	16% 11% 31% 30% 10%	650
June	1st to 10th	Night	10% 11% 28% 37% 14% 1	600
	11th to 20th	Night	10% 10% 41% 32% 6%	600
	21st to end	Night	8% 6% 35% 41% 10% 1	600
July	1st to 10th	Night	6% 5% 29% 41% 18% 1	585
	11th to 20th	Night	6% 5% 33% 43% 13% 1	582
	21st to end	Night	4 3 28% 51% 13%	657
August	1st to 10th	Night	6% 3 30% 46% 15%	597
	11th to 20th	Night	6% 4 26% 44% 18% 2	594
	21st to end	Night	4 5% 28% 43% 18% 1	660
September	1st to 10th	Night	7% 5% 25% 45% 17% 2	600
	11th to 20th	Night	11% 8% 26% 37% 17% 2	600
	21st to end	Night	12% 10% 29% 34% 14% 3	600
October	1st to 10th	Night	17% 10% 26% 34% 12% 1	734
	11th to 20th	Night	15% 14% 30% 29% 11% 1	800
	21st to end	Night	16% 16% 33% 25% 7% 2	1100
November	1st to 10th	Night	12% 19% 35% 26% 7% 2	1025
	11th to 20th	Night	13% 21% 37% 22% 6% 1	1050
	21st to end	Night	11% 21% 37% 24% 6%	1050
December	1st to 10th	Night	12% 25% 36% 20% 6%	1050
	11th to 20th	Night	12% 22% 35% 24% 6%	1050
	21st to end	Night	16% 22% 34% 22% 6%	1155

Page generated at 2026-01-28 23:21:47. Version ac07

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