Graceland University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Graceland University Observatory 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	12% 7% 3 7% 4 4 4 4 5% 7% 41%	127555
	Night	11% 8% 3 7% 4 4 4 3 5% 6% 44%	124705
Transparency	Day	27% 36% 8% 17% 11% 1	131159
	Night	27% 36% 7% 16% 12% 2	129177
Seeing	Day	15% 9% 29% 28% 14% 4	17427
	Night	15% 9% 33% 31% 11% 1	27389

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	12% 10% 4 12% 5% 6% 5% 4 5% 7% 30%	3911
	11th to 20th	Day	7% 10% 4 10% 6% 5% 8% 3 4 6% 38%	4170
	21st to end	Day	11% 12% 6% 13% 5% 5% 5% 3 5% 6% 29%	4587
February	1st to 10th	Day	12% 12% 3 10% 8% 5% 5% 3 5% 7% 30%	1800
	11th to 20th	Day	12% 11% 4 9% 5% 5% 4 3 5% 7% 35%	1754
	21st to end	Day	13% 8% 2 7% 5% 4 4 3 6% 7% 41%	1636
March	1st to 10th	Day	17% 10% 3 6% 3 3 3 2 3 4 46%	4936
	11th to 20th	Day	10% 7% 2 6% 4 4 4 3 5% 8% 46%	4980
	21st to end	Day	16% 13% 5% 8% 4 5% 4 4 5% 7% 28%	5462
April	1st to 10th	Day	16% 9% 3 5% 5% 3 3 4 5% 7% 39%	2363
	11th to 20th	Day	15% 9% 2 7% 5% 4 4 3 6% 6% 38%	2374
	21st to end	Day	21% 8% 3 8% 4 5% 3 3 4 6% 35%	2531
May	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 6% 8% 40%	5862
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 6% 46%	5999
	21st to end	Day	15% 8% 3 6% 5% 4 4 4 6% 7% 38%	6935
June	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 5% 6% 8% 46%	2800
	11th to 20th	Day	8% 5% 2 6% 4 4 6% 4 7% 10% 46%	2800
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 9% 44%	2800
July	1st to 10th	Day	9% 6% 2 6% 4 4 5% 5% 7% 9% 43%	6360
	11th to 20th	Day	8% 4 2 6% 5% 4 5% 5% 6% 8% 45%	6020
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 45%	6422
August	1st to 10th	Day	6% 4 2 5% 4 5% 6% 5% 7% 11% 46%	4234
	11th to 20th	Day	12% 6% 2 6% 4 5% 4 4 6% 7% 44%	4171
	21st to end	Day	7% 4 2 6% 4 4 5% 3 6% 9% 50%	4388
September	1st to 10th	Day	13% 4 2 4 3 3 4 3 4 6% 55%	2400
	11th to 20th	Day	10% 4 2 5% 4 4 4 4 5% 9% 50%	2299
	21st to end	Day	11% 4 1 5% 3 3 3 4 4 8% 52%	2143
October	1st to 10th	Day	14% 6% 2 5% 4 3 3 3 4 6% 49%	2000
	11th to 20th	Day	13% 7% 2 6% 3 2 3 3 4 5% 51%	2000
	21st to end	Day	13% 8% 3 7% 5% 4 4 3 5% 5% 44%	2200
November	1st to 10th	Day	10% 6% 2 5% 3 3 5% 3 4 7% 52%	2010
	11th to 20th	Day	12% 9% 3 7% 4 2 4 3 5% 7% 44%	1680
	21st to end	Day	17% 11% 3 6% 4 3 2 2 5% 7% 39%	1680
December	1st to 10th	Day	7% 6% 3 7% 4 4 3 4 5% 6% 52%	2690
	11th to 20th	Day	11% 8% 4 10% 5% 5% 4 3 5% 7% 38%	3088
	21st to end	Day	20% 15% 6% 13% 5% 4 4 3 4 5% 20%	4070
January	1st to 10th	Night	10% 10% 5% 19% 7% 5% 5% 4 4 5% 26%	6271
	11th to 20th	Night	8% 9% 4 10% 6% 5% 6% 4 7% 6% 35%	6030
	21st to end	Night	12% 12% 5% 11% 4 4 4 3 4 4 36%	6633
February	1st to 10th	Night	13% 11% 4 9% 5% 4 4 3 5% 6% 36%	2601
	11th to 20th	Night	11% 10% 4 7% 4 4 4 3 5% 6% 41%	2533
	21st to end	Night	12% 9% 3 7% 3 4 4 3 4 5% 46%	2010
March	1st to 10th	Night	17% 8% 2 6% 3 3 3 2 4 4 48%	4919
	11th to 20th	Night	10% 7% 2 5% 3 3 3 4 4 6% 53%	4980
	21st to end	Night	15% 11% 4 11% 5% 4 4 4 5% 6% 31%	5467
April	1st to 10th	Night	12% 6% 2 6% 4 3 4 4 5% 6% 47%	2036
	11th to 20th	Night	15% 7% 3 6% 4 3 5% 3 5% 5% 45%	1975
	21st to end	Night	17% 8% 3 6% 4 4 4 3 5% 6% 41%	1865
May	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 5% 7% 45%	4053
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 4 6% 51%	3898
	21st to end	Night	15% 6% 3 7% 4 4 4 3 5% 6% 43%	3942
June	1st to 10th	Night	6% 4 1 5% 3 3 3 4 7% 9% 54%	1600
	11th to 20th	Night	6% 5% 2 4 3 4 4 4 7% 12% 50%	1600
	21st to end	Night	5% 4 2 4 4 4 5% 4 6% 9% 52%	1600
July	1st to 10th	Night	9% 4 2 4 4 4 4 4 6% 9% 50%	3618
	11th to 20th	Night	5% 3 2 6% 4 4 4 3 6% 8% 54%	3939
	21st to end	Night	8% 4 2 6% 4 4 5% 4 6% 9% 48%	4449
August	1st to 10th	Night	3 3 2 4 3 4 4 4 6% 9% 58%	2926
	11th to 20th	Night	11% 6% 1 4 3 3 3 3 5% 7% 52%	2888
	21st to end	Night	6% 4 2 4 3 3 4 3 5% 8% 58%	3512
September	1st to 10th	Night	10% 4 1 5% 3 2 3 3 5% 6% 57%	2000
	11th to 20th	Night	10% 5% 2 5% 4 3 4 3 5% 7% 53%	2097
	21st to end	Night	8% 4 2 5% 3 3 4 3 5% 7% 57%	2235
October	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 5% 54%	2400
	11th to 20th	Night	12% 6% 2 6% 4 3 4 2 3 5% 52%	2400
	21st to end	Night	15% 8% 2 5% 4 3 3 3 3 5% 48%	2640
November	1st to 10th	Night	8% 4 2 6% 3 3 3 3 5% 7% 55%	2499
	11th to 20th	Night	11% 6% 2 6% 4 3 4 3 5% 7% 49%	2940
	21st to end	Night	15% 10% 3 6% 4 3 3 3 5% 6% 43%	2940
December	1st to 10th	Night	7% 8% 2 7% 4 4 4 3 4 6% 50%	4720
	11th to 20th	Night	11% 8% 3 9% 5% 4 4 3 4 7% 42%	5339
	21st to end	Night	20% 16% 6% 11% 5% 3 3 3 4 4 25%	7150

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	42% 25% 9% 15% 8% 1	3912
	11th to 20th	Day	37% 25% 7% 16% 13% 3	4170
	21st to end	Day	42% 27% 7% 17% 6% 1	4587
February	1st to 10th	Day	24% 61% 3 7% 4	1800
	11th to 20th	Day	21% 60% 5% 6% 7% 1	1763
	21st to end	Day	15% 61% 3 10% 9% 1	1636
March	1st to 10th	Day	31% 28% 4 18% 16% 2	4936
	11th to 20th	Day	24% 28% 5% 20% 20% 3	4980
	21st to end	Day	40% 29% 14% 13% 3 1	5464
April	1st to 10th	Day	20% 61% 4 10% 5%	2363
	11th to 20th	Day	17% 62% 6% 9% 5%	2374
	21st to end	Day	21% 61% 3 9% 5%	2531
May	1st to 10th	Day	29% 29% 9% 20% 13% 1	5900
	11th to 20th	Day	28% 28% 6% 23% 13% 2	6003
	21st to end	Day	32% 28% 7% 19% 13%	6949
June	1st to 10th	Day	11% 62% 6% 13% 7% 2	2800
	11th to 20th	Day	14% 62% 6% 12% 6% 1	2800
	21st to end	Day	14% 60% 9% 11% 5%	2800
July	1st to 10th	Day	31% 25% 10% 24% 10% 1	6291
	11th to 20th	Day	30% 25% 11% 21% 13%	5948
	21st to end	Day	27% 25% 14% 26% 7% 1	6240
August	1st to 10th	Day	21% 35% 10% 23% 11%	4220
	11th to 20th	Day	29% 35% 8% 18% 10%	4210
	21st to end	Day	23% 36% 14% 19% 7% 1	4387
September	1st to 10th	Day	13% 56% 8% 13% 8% 1	2400
	11th to 20th	Day	14% 56% 7% 15% 8%	2299
	21st to end	Day	16% 56% 4 14% 10%	2143
October	1st to 10th	Day	13% 34% 2 11% 38% 2	3260
	11th to 20th	Day	10% 35% 4 20% 25% 7%	3240
	21st to end	Day	23% 34% 6% 21% 15% 2	3538
November	1st to 10th	Day	19% 53% 7% 11% 9% 1	2010
	11th to 20th	Day	20% 53% 4 13% 10%	1680
	21st to end	Day	24% 53% 4 12% 6%	1680
December	1st to 10th	Day	20% 33% 14% 23% 10% 1	2690
	11th to 20th	Day	32% 29% 10% 20% 9%	3085
	21st to end	Day	56% 25% 8% 8% 3	4070
January	1st to 10th	Night	51% 26% 7% 11% 5% 1	6271
	11th to 20th	Night	38% 24% 8% 14% 14% 2	6030
	21st to end	Night	40% 27% 6% 14% 12% 2	6633
February	1st to 10th	Night	20% 61% 6% 8% 4 1	2601
	11th to 20th	Night	19% 60% 4 10% 6% 1	2534
	21st to end	Night	16% 61% 3 8% 10% 2	2010
March	1st to 10th	Night	32% 28% 4 16% 18% 2	4919
	11th to 20th	Night	22% 28% 3 24% 18% 5%	4980
	21st to end	Night	38% 28% 12% 15% 5% 2	5467
April	1st to 10th	Night	16% 61% 5% 11% 6% 1	2036
	11th to 20th	Night	15% 62% 6% 11% 5% 1	1975
	21st to end	Night	18% 61% 4 9% 6% 2	1865
May	1st to 10th	Night	28% 28% 7% 21% 14% 1	4080
	11th to 20th	Night	24% 28% 10% 20% 14% 4	3899
	21st to end	Night	33% 28% 6% 17% 15% 1	3950
June	1st to 10th	Night	10% 61% 6% 13% 8% 2	1600
	11th to 20th	Night	12% 61% 4 14% 7% 2	1600
	21st to end	Night	12% 60% 8% 13% 7% 1	1600
July	1st to 10th	Night	27% 24% 9% 23% 13% 3	3580
	11th to 20th	Night	24% 25% 14% 21% 16%	3891
	21st to end	Night	27% 25% 16% 22% 9%	4320
August	1st to 10th	Night	15% 35% 11% 25% 14%	2916
	11th to 20th	Night	24% 35% 9% 19% 13% 1	2915
	21st to end	Night	18% 36% 13% 21% 10% 2	3512
September	1st to 10th	Night	13% 57% 6% 12% 9% 3	2000
	11th to 20th	Night	13% 57% 5% 16% 8% 1	2097
	21st to end	Night	14% 56% 4 12% 12% 1	2235
October	1st to 10th	Night	14% 34% 2 10% 38% 1	3920
	11th to 20th	Night	13% 34% 4 18% 25% 6%	3899
	21st to end	Night	26% 34% 4 18% 16% 2	4252
November	1st to 10th	Night	16% 52% 6% 15% 10% 1	2499
	11th to 20th	Night	16% 53% 4 15% 10% 2	2940
	21st to end	Night	23% 53% 4 13% 6%	2940
December	1st to 10th	Night	23% 33% 5% 29% 9%	4720
	11th to 20th	Night	29% 31% 9% 19% 11%	5341
	21st to end	Night	53% 25% 7% 12% 3 1	7150

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	23% 11% 34% 24% 7% 1	330
	11th to 20th	Day	19% 16% 39% 22% 4	330
	21st to end	Day	17% 13% 42% 25% 3	363
February	1st to 10th	Day	20% 17% 38% 20% 4 1	300
	11th to 20th	Day	19% 16% 41% 18% 7% 1	300
	21st to end	Day	18% 16% 43% 21% 3	249
March	1st to 10th	Day	20% 16% 45% 15% 4	300
	11th to 20th	Day	23% 13% 36% 24% 4	300
	21st to end	Day	38% 18% 26% 14% 3	330
April	1st to 10th	Day	25% 17% 34% 19% 5% 1	463
	11th to 20th	Day	20% 16% 36% 22% 6% 1	492
	21st to end	Day	23% 17% 34% 19% 6% 1	631
May	1st to 10th	Day	21% 12% 31% 26% 8% 2	703
	11th to 20th	Day	21% 12% 29% 23% 12% 3	725
	21st to end	Day	18% 7% 29% 28% 15% 3	789
June	1st to 10th	Day	7% 6% 23% 33% 25% 6%	730
	11th to 20th	Day	10% 4 27% 36% 20% 4	730
	21st to end	Day	7% 7% 36% 32% 15% 3	730
July	1st to 10th	Day	9% 3 17% 33% 27% 11%	721
	11th to 20th	Day	7% 3 24% 37% 21% 8%	718
	21st to end	Day	8% 3 19% 40% 25% 5%	794
August	1st to 10th	Day	9% 3 21% 39% 24% 6%	726
	11th to 20th	Day	12% 3 17% 37% 26% 5%	715
	21st to end	Day	10% 3 18% 34% 28% 7%	633
September	1st to 10th	Day	13% 4 20% 29% 27% 8%	530
	11th to 20th	Day	10% 2 19% 37% 27% 6%	429
	21st to end	Day	16% 5% 28% 29% 15% 7%	330
October	1st to 10th	Day	18% 7% 29% 26% 15% 5%	330
	11th to 20th	Day	12% 21% 38% 20% 8% 1	330
	21st to end	Day	17% 18% 36% 20% 7% 2	363
November	1st to 10th	Day	18% 9% 43% 22% 7% 2	330
	11th to 20th	Day	19% 14% 38% 20% 7% 1	330
	21st to end	Day	25% 13% 34% 22% 6% 1	330
December	1st to 10th	Day	11% 16% 41% 23% 7% 2	330
	11th to 20th	Day	17% 16% 35% 25% 5% 1	330
	21st to end	Day	26% 16% 36% 20% 2	363
January	1st to 10th	Night	13% 12% 41% 26% 8%	1050
	11th to 20th	Night	15% 15% 42% 22% 6%	1050
	21st to end	Night	16% 12% 39% 28% 5%	1155
February	1st to 10th	Night	19% 12% 40% 25% 5%	1001
	11th to 20th	Night	16% 14% 39% 25% 5%	979
	21st to end	Night	17% 15% 41% 22% 4 1	690
March	1st to 10th	Night	16% 15% 37% 27% 6%	600
	11th to 20th	Night	20% 10% 39% 25% 5%	600
	21st to end	Night	25% 13% 38% 20% 4	660
April	1st to 10th	Night	15% 15% 41% 26% 4	600
	11th to 20th	Night	19% 14% 36% 26% 6%	593
	21st to end	Night	22% 9% 36% 27% 7% 1	600
May	1st to 10th	Night	16% 9% 33% 35% 7% 1	600
	11th to 20th	Night	16% 9% 29% 33% 13% 1	591
	21st to end	Night	15% 6% 29% 36% 14%	650
June	1st to 10th	Night	7% 4 26% 40% 22% 1	600
	11th to 20th	Night	8% 5% 26% 44% 17% 1	600
	21st to end	Night	8% 3 31% 42% 16%	600
July	1st to 10th	Night	6% 2 14% 42% 32% 3	585
	11th to 20th	Night	6% 1 17% 48% 26% 2	582
	21st to end	Night	7% 2 16% 47% 27% 1	657
August	1st to 10th	Night	6% 2 18% 48% 25% 2	597
	11th to 20th	Night	10% 3 14% 43% 27% 3	594
	21st to end	Night	9% 2 14% 42% 31% 2	660
September	1st to 10th	Night	12% 3 21% 35% 26% 3	600
	11th to 20th	Night	13% 2 21% 39% 23% 2	600
	21st to end	Night	9% 5% 28% 35% 22% 1	635
October	1st to 10th	Night	16% 6% 27% 34% 15% 2	800
	11th to 20th	Night	16% 10% 38% 28% 8%	800
	21st to end	Night	21% 11% 35% 26% 6% 1	880
November	1st to 10th	Night	10% 11% 40% 31% 8%	825
	11th to 20th	Night	14% 15% 40% 24% 7%	1050
	21st to end	Night	20% 12% 36% 26% 6%	1050
December	1st to 10th	Night	16% 13% 38% 27% 6%	1050
	11th to 20th	Night	18% 10% 38% 28% 5%	1050
	21st to end	Night	22% 14% 35% 25% 4	1155

Page generated at 2026-01-29 00:21:45. Version ac07

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