Shepherd University Observatory
Clear Sky Chart
History

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

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	13% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	127227
	Night	13% 7% 2 6% 4 4 5% 4 6% 7% 41%	124135
Transparency	Day	27% 38% 7% 16% 10% 2	130823
	Night	27% 38% 7% 16% 11% 2	128608
Seeing	Day	19% 13% 25% 23% 15% 5%	16349
	Night	18% 11% 27% 31% 12% 1	25959

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	14% 5% 2 7% 5% 5% 5% 4 5% 6% 41%	3766
	11th to 20th	Day	12% 9% 3 7% 5% 4 4 4 5% 6% 40%	4060
	21st to end	Day	14% 7% 3 8% 5% 5% 5% 4 5% 6% 39%	4466
February	1st to 10th	Day	14% 9% 3 8% 5% 5% 4 4 5% 6% 37%	1800
	11th to 20th	Day	16% 8% 3 8% 5% 5% 4 3 5% 6% 38%	1763
	21st to end	Day	17% 8% 3 7% 4 4 4 3 5% 6% 39%	1650
March	1st to 10th	Day	17% 7% 2 6% 5% 4 5% 3 5% 6% 40%	4933
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 46%	4980
	21st to end	Day	15% 7% 2 7% 5% 5% 5% 4 5% 6% 39%	5500
April	1st to 10th	Day	15% 7% 3 6% 6% 5% 5% 3 6% 7% 37%	2400
	11th to 20th	Day	13% 7% 2 5% 5% 4 4 3 7% 6% 44%	2374
	21st to end	Day	17% 6% 2 6% 5% 4 4 4 6% 5% 41%	2421
May	1st to 10th	Day	19% 6% 3 7% 4 5% 5% 5% 7% 10% 30%	5862
	11th to 20th	Day	22% 7% 3 6% 4 4 5% 4 6% 10% 30%	6084
	21st to end	Day	15% 6% 2 6% 4 5% 5% 4 7% 9% 36%	6934
June	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 7% 9% 43%	2800
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	2800
	21st to end	Day	9% 4 1 5% 4 5% 5% 4 7% 11% 45%	2800
July	1st to 10th	Day	6% 4 2 6% 4 5% 5% 5% 7% 11% 44%	6360
	11th to 20th	Day	8% 6% 3 7% 6% 6% 7% 6% 9% 11% 33%	6089
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 4 7% 10% 40%	6424
August	1st to 10th	Day	14% 6% 2 6% 5% 5% 6% 5% 7% 8% 34%	4234
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 4 7% 9% 42%	4076
	21st to end	Day	9% 5% 2 5% 5% 4 5% 4 6% 8% 48%	4312
September	1st to 10th	Day	15% 5% 2 5% 3 4 4 3 5% 7% 46%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 6% 8% 48%	2389
	21st to end	Day	21% 8% 3 5% 4 4 4 3 5% 7% 37%	2091
October	1st to 10th	Day	16% 4 2 5% 3 4 3 3 5% 7% 47%	2000
	11th to 20th	Day	15% 4 2 5% 4 4 5% 3 4 6% 47%	2000
	21st to end	Day	13% 5% 2 6% 4 4 4 3 5% 6% 49%	2200
November	1st to 10th	Day	11% 5% 1 5% 3 4 4 3 5% 7% 51%	1980
	11th to 20th	Day	14% 7% 2 4 4 4 4 3 5% 7% 46%	1801
	21st to end	Day	14% 7% 3 6% 4 3 4 4 4 7% 43%	1680
December	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 7% 7% 42%	2680
	11th to 20th	Day	19% 8% 3 5% 4 4 4 3 5% 7% 38%	3070
	21st to end	Day	15% 9% 3 7% 6% 5% 6% 4 7% 9% 30%	4048
January	1st to 10th	Night	11% 8% 2 8% 5% 5% 4 4 6% 7% 43%	6198
	11th to 20th	Night	14% 7% 2 6% 4 4 3 4 5% 6% 45%	5900
	21st to end	Night	15% 7% 3 6% 4 4 4 3 5% 6% 44%	6490
February	1st to 10th	Night	14% 8% 2 6% 5% 4 4 4 6% 6% 40%	2600
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 3 5% 6% 39%	2522
	21st to end	Night	16% 9% 3 7% 5% 4 4 4 5% 7% 37%	1979
March	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 5% 45%	4918
	11th to 20th	Night	10% 5% 2 6% 4 5% 5% 3 5% 8% 48%	4980
	21st to end	Night	11% 7% 2 7% 4 5% 5% 4 5% 7% 44%	5432
April	1st to 10th	Night	16% 8% 3 7% 5% 5% 5% 5% 5% 5% 37%	2000
	11th to 20th	Night	14% 6% 2 6% 5% 3 4 4 5% 9% 42%	1975
	21st to end	Night	17% 4 1 5% 4 4 5% 4 5% 8% 43%	1995
May	1st to 10th	Night	22% 8% 3 8% 5% 5% 4 4 5% 7% 28%	4033
	11th to 20th	Night	23% 8% 2 7% 4 3 4 4 6% 9% 30%	3807
	21st to end	Night	17% 7% 2 7% 5% 5% 5% 5% 7% 8% 33%	3920
June	1st to 10th	Night	10% 5% 2 5% 5% 4 5% 4 7% 9% 45%	1600
	11th to 20th	Night	11% 6% 2 6% 4 5% 5% 5% 7% 9% 40%	1600
	21st to end	Night	8% 6% 1 5% 5% 5% 4 5% 7% 10% 44%	1600
July	1st to 10th	Night	7% 6% 2 7% 6% 6% 5% 5% 7% 9% 40%	3598
	11th to 20th	Night	7% 7% 3 9% 7% 7% 7% 5% 8% 9% 29%	3863
	21st to end	Night	8% 6% 2 6% 5% 4 6% 4 8% 10% 41%	4427
August	1st to 10th	Night	15% 5% 2 8% 5% 5% 5% 4 7% 9% 34%	2915
	11th to 20th	Night	10% 7% 2 7% 5% 6% 5% 4 6% 9% 40%	2938
	21st to end	Night	10% 6% 2 5% 4 5% 5% 4 6% 8% 46%	3586
September	1st to 10th	Night	13% 6% 2 6% 3 3 5% 4 5% 6% 46%	2000
	11th to 20th	Night	11% 6% 2 6% 4 4 4 2 4 7% 49%	2008
	21st to end	Night	19% 8% 2 6% 4 4 5% 3 4 6% 39%	2307
October	1st to 10th	Night	19% 4 2 5% 4 4 4 2 4 5% 46%	2400
	11th to 20th	Night	13% 5% 2 6% 3 3 4 3 5% 6% 51%	2400
	21st to end	Night	16% 5% 2 4 3 3 3 3 5% 6% 49%	2640
November	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 6% 6% 53%	2535
	11th to 20th	Night	14% 7% 2 5% 4 4 4 3 5% 7% 46%	2814
	21st to end	Night	14% 5% 2 6% 4 5% 5% 4 5% 8% 43%	2940
December	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 8% 47%	4720
	11th to 20th	Night	18% 7% 2 7% 5% 5% 5% 4 5% 6% 37%	5345
	21st to end	Night	15% 8% 3 7% 5% 5% 6% 5% 7% 8% 30%	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	33% 27% 4 18% 13% 4	3767
	11th to 20th	Day	33% 27% 5% 14% 17% 5%	4060
	21st to end	Day	32% 29% 7% 18% 11% 2	4466
February	1st to 10th	Day	20% 62% 6% 8% 4 1	1800
	11th to 20th	Day	20% 61% 3 9% 6% 1	1772
	21st to end	Day	18% 62% 4 9% 6% 1	1650
March	1st to 10th	Day	34% 30% 5% 14% 10% 6%	4933
	11th to 20th	Day	24% 29% 8% 22% 13% 4	4980
	21st to end	Day	33% 29% 6% 14% 14% 4	5500
April	1st to 10th	Day	20% 63% 3 6% 6% 1	2400
	11th to 20th	Day	15% 63% 5% 10% 7% 1	2374
	21st to end	Day	17% 62% 3 10% 7%	2421
May	1st to 10th	Day	38% 31% 10% 15% 5% 1	5897
	11th to 20th	Day	36% 31% 8% 21% 5%	6088
	21st to end	Day	32% 31% 9% 15% 12% 1	6949
June	1st to 10th	Day	16% 62% 5% 10% 6%	2800
	11th to 20th	Day	14% 64% 5% 10% 7%	2800
	21st to end	Day	13% 61% 6% 11% 8% 1	2800
July	1st to 10th	Day	27% 29% 12% 18% 12% 3	6290
	11th to 20th	Day	35% 27% 14% 19% 5%	6015
	21st to end	Day	30% 26% 7% 19% 18% 1	6240
August	1st to 10th	Day	31% 37% 10% 18% 4	4221
	11th to 20th	Day	25% 37% 8% 22% 9%	4115
	21st to end	Day	22% 36% 8% 20% 14% 1	4312
September	1st to 10th	Day	16% 57% 6% 11% 8% 1	2400
	11th to 20th	Day	15% 57% 5% 13% 9% 1	2389
	21st to end	Day	24% 57% 4 8% 6% 1	2091
October	1st to 10th	Day	24% 35% 4 13% 20% 4	3260
	11th to 20th	Day	16% 35% 5% 21% 17% 6%	3240
	21st to end	Day	12% 35% 9% 21% 22% 2	3538
November	1st to 10th	Day	18% 53% 7% 10% 9% 3	1980
	11th to 20th	Day	15% 54% 5% 12% 13% 1	1801
	21st to end	Day	19% 54% 7% 14% 6% 1	1680
December	1st to 10th	Day	28% 35% 11% 19% 5% 3	2680
	11th to 20th	Day	33% 31% 10% 18% 6% 1	3066
	21st to end	Day	36% 29% 13% 11% 11% 1	4048
January	1st to 10th	Night	29% 27% 6% 19% 16% 2	6198
	11th to 20th	Night	28% 26% 7% 17% 15% 6%	5900
	21st to end	Night	29% 28% 8% 20% 13% 2	6490
February	1st to 10th	Night	17% 63% 7% 10% 3 1	2600
	11th to 20th	Night	19% 60% 3 9% 8% 1	2524
	21st to end	Night	17% 62% 5% 8% 6% 2	1979
March	1st to 10th	Night	32% 30% 5% 18% 8% 7%	4918
	11th to 20th	Night	23% 29% 9% 24% 10% 4	4980
	21st to end	Night	28% 29% 7% 16% 17% 2	5432
April	1st to 10th	Night	18% 63% 4 9% 5% 1	2000
	11th to 20th	Night	16% 63% 4 10% 6% 1	1975
	21st to end	Night	15% 63% 4 10% 7% 1	1995
May	1st to 10th	Night	44% 30% 7% 11% 6% 2	4060
	11th to 20th	Night	38% 31% 7% 19% 5%	3808
	21st to end	Night	36% 31% 9% 18% 4 1	3927
June	1st to 10th	Night	15% 63% 4 10% 8%	1600
	11th to 20th	Night	16% 63% 4 11% 6%	1600
	21st to end	Night	14% 62% 6% 9% 8%	1600
July	1st to 10th	Night	30% 28% 12% 14% 12% 3	3560
	11th to 20th	Night	42% 27% 13% 11% 7%	3817
	21st to end	Night	30% 26% 8% 18% 17% 1	4297
August	1st to 10th	Night	34% 37% 11% 14% 4 1	2904
	11th to 20th	Night	26% 37% 7% 21% 9%	2965
	21st to end	Night	23% 36% 8% 23% 11%	3586
September	1st to 10th	Night	16% 57% 7% 11% 7% 1	2000
	11th to 20th	Night	15% 57% 5% 13% 10% 1	2008
	21st to end	Night	24% 58% 5% 8% 6%	2307
October	1st to 10th	Night	26% 35% 6% 14% 16% 2	3920
	11th to 20th	Night	14% 35% 7% 16% 23% 5%	3899
	21st to end	Night	13% 35% 7% 23% 19% 3	4252
November	1st to 10th	Night	16% 54% 8% 11% 9% 2	2535
	11th to 20th	Night	17% 54% 4 11% 12% 2	2814
	21st to end	Night	18% 55% 7% 13% 7% 1	2940
December	1st to 10th	Night	23% 34% 12% 22% 9% 1	4720
	11th to 20th	Night	34% 31% 8% 14% 11% 2	5348
	21st to end	Night	38% 29% 10% 13% 10%	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	31% 26% 25% 14% 3 1	237
	11th to 20th	Day	31% 26% 27% 12% 4	300
	21st to end	Day	23% 30% 29% 13% 4 1	330
February	1st to 10th	Day	28% 22% 34% 12% 3	300
	11th to 20th	Day	37% 25% 28% 8% 1	310
	21st to end	Day	29% 19% 30% 19% 3	414
March	1st to 10th	Day	23% 23% 29% 21% 4	500
	11th to 20th	Day	25% 22% 29% 17% 7% 1	500
	21st to end	Day	23% 20% 31% 18% 6% 1	550
April	1st to 10th	Day	25% 21% 26% 19% 6% 2	500
	11th to 20th	Day	20% 16% 32% 15% 13% 4	492
	21st to end	Day	26% 15% 28% 19% 11% 2	500
May	1st to 10th	Day	24% 17% 26% 18% 12% 3	503
	11th to 20th	Day	20% 14% 26% 21% 14% 4	527
	21st to end	Day	17% 8% 22% 25% 20% 8%	573
June	1st to 10th	Day	15% 8% 27% 24% 19% 7%	530
	11th to 20th	Day	15% 7% 25% 28% 19% 5%	530
	21st to end	Day	11% 6% 23% 29% 24% 6%	530
July	1st to 10th	Day	9% 3 18% 30% 27% 12%	525
	11th to 20th	Day	7% 4 19% 28% 31% 11%	524
	21st to end	Day	11% 3 19% 31% 26% 9%	576
August	1st to 10th	Day	12% 3 18% 29% 25% 13%	526
	11th to 20th	Day	14% 5% 18% 30% 21% 12%	519
	21st to end	Day	10% 3 20% 30% 23% 14%	583
September	1st to 10th	Day	17% 6% 17% 27% 22% 12%	530
	11th to 20th	Day	12% 4 19% 29% 25% 11%	530
	21st to end	Day	27% 8% 21% 23% 16% 4	530
October	1st to 10th	Day	19% 7% 18% 27% 22% 6%	530
	11th to 20th	Day	17% 13% 26% 28% 13% 3	530
	21st to end	Day	16% 15% 32% 24% 11% 2	583
November	1st to 10th	Day	17% 13% 30% 24% 14% 3	494
	11th to 20th	Day	23% 14% 31% 20% 10% 2	341
	21st to end	Day	25% 17% 32% 19% 5% 1	220
December	1st to 10th	Day	23% 21% 36% 15% 4 1	220
	11th to 20th	Day	27% 21% 26% 18% 8%	220
	21st to end	Day	24% 21% 29% 18% 7% 1	242
January	1st to 10th	Night	18% 20% 37% 22% 3	805
	11th to 20th	Night	22% 17% 32% 27% 3	800
	21st to end	Night	18% 18% 37% 25% 2	880
February	1st to 10th	Night	18% 18% 32% 29% 2	800
	11th to 20th	Night	25% 22% 32% 20% 1	783
	21st to end	Night	24% 19% 35% 20% 3	660
March	1st to 10th	Night	20% 17% 37% 23% 4	800
	11th to 20th	Night	23% 20% 29% 23% 6%	800
	21st to end	Night	20% 15% 37% 24% 4 1	880
April	1st to 10th	Night	20% 18% 34% 23% 5%	800
	11th to 20th	Night	17% 13% 34% 28% 9%	791
	21st to end	Night	19% 12% 30% 30% 7% 1	795
May	1st to 10th	Night	22% 7% 26% 33% 12%	600
	11th to 20th	Night	19% 7% 23% 39% 11% 1	496
	21st to end	Night	16% 4 22% 39% 19%	433
June	1st to 10th	Night	13% 2 26% 36% 23% 1	400
	11th to 20th	Night	11% 6% 19% 43% 19% 2	400
	21st to end	Night	9% 3 20% 50% 18% 1	400
July	1st to 10th	Night	12% 2 16% 40% 26% 4	390
	11th to 20th	Night	9% 2 11% 40% 34% 4	497
	21st to end	Night	10% 2 13% 41% 31% 3	657
August	1st to 10th	Night	12% 2 10% 43% 31% 3	597
	11th to 20th	Night	12% 3 13% 40% 29% 3	631
	21st to end	Night	11% 3 14% 42% 26% 4	880
September	1st to 10th	Night	15% 2 16% 37% 27% 2	800
	11th to 20th	Night	12% 4 17% 37% 27% 3	800
	21st to end	Night	22% 5% 19% 34% 19% 2	800
October	1st to 10th	Night	20% 6% 19% 31% 20% 3	800
	11th to 20th	Night	15% 11% 27% 33% 12% 1	800
	21st to end	Night	20% 8% 31% 30% 11%	880
November	1st to 10th	Night	13% 10% 31% 35% 10% 1	820
	11th to 20th	Night	19% 14% 28% 30% 7% 1	840
	21st to end	Night	17% 16% 32% 29% 7%	840
December	1st to 10th	Night	21% 14% 34% 24% 6%	840
	11th to 20th	Night	20% 11% 36% 29% 4	840
	21st to end	Night	21% 13% 33% 31% 3	924

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