Gettysburg College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gettysburg College Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 40%	131360
	Night	14% 7% 2 6% 4 4 4 4 6% 7% 42%	130134
Transparency	Day	27% 37% 8% 16% 10% 2	134955
	Night	27% 36% 7% 16% 11% 2	134609
Seeing	Day	19% 16% 27% 21% 13% 4	16578
	Night	18% 14% 29% 28% 11% 1	26157

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% 7% 3 8% 5% 5% 5% 3 5% 4 42%	4676
	11th to 20th	Day	11% 9% 4 8% 5% 5% 4 3 5% 6% 40%	5280
	21st to end	Day	13% 8% 2 8% 5% 5% 5% 4 6% 6% 39%	5780
February	1st to 10th	Day	14% 8% 3 6% 5% 4 5% 5% 5% 6% 38%	1890
	11th to 20th	Day	15% 9% 3 7% 5% 5% 5% 4 4 6% 38%	1880
	21st to end	Day	17% 9% 3 7% 5% 4 4 3 4 6% 38%	1730
March	1st to 10th	Day	18% 7% 2 4 3 4 4 3 5% 6% 43%	4943
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 7% 45%	5057
	21st to end	Day	15% 7% 2 6% 4 5% 5% 3 5% 6% 41%	5556
April	1st to 10th	Day	15% 8% 2 7% 5% 5% 5% 4 6% 7% 37%	2400
	11th to 20th	Day	14% 7% 2 5% 5% 4 5% 4 5% 7% 43%	2374
	21st to end	Day	18% 7% 1 5% 4 5% 4 4 6% 7% 40%	2400
May	1st to 10th	Day	24% 6% 2 5% 4 4 4 4 7% 9% 30%	5852
	11th to 20th	Day	24% 7% 2 6% 4 4 4 3 6% 8% 31%	6244
	21st to end	Day	16% 5% 2 6% 5% 5% 5% 4 6% 9% 37%	6934
June	1st to 10th	Day	12% 5% 2 5% 5% 5% 5% 4 7% 10% 40%	2800
	11th to 20th	Day	12% 4 3 7% 5% 5% 5% 4 7% 8% 40%	2800
	21st to end	Day	9% 4 2 5% 5% 5% 6% 4 7% 11% 44%	2800
July	1st to 10th	Day	6% 4 2 6% 5% 5% 5% 5% 7% 11% 44%	6360
	11th to 20th	Day	8% 5% 3 7% 6% 6% 6% 6% 8% 12% 33%	6280
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 4 6% 9% 42%	6424
August	1st to 10th	Day	16% 5% 3 6% 5% 5% 6% 4 7% 10% 34%	4234
	11th to 20th	Day	11% 5% 2 6% 4 5% 6% 4 8% 8% 42%	4027
	21st to end	Day	13% 5% 2 5% 3 4 4 4 5% 8% 48%	4312
September	1st to 10th	Day	16% 5% 3 5% 3 4 4 2 5% 6% 47%	2400
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 5% 7% 49%	2400
	21st to end	Day	21% 8% 2 6% 4 4 4 3 4 7% 37%	2102
October	1st to 10th	Day	15% 6% 2 5% 4 4 4 4 5% 6% 46%	2000
	11th to 20th	Day	16% 5% 2 6% 3 3 4 4 5% 6% 47%	2000
	21st to end	Day	14% 6% 3 6% 4 3 3 3 6% 6% 46%	2200
November	1st to 10th	Day	11% 6% 2 4 3 3 4 4 5% 7% 52%	1921
	11th to 20th	Day	14% 7% 1 4 4 4 3 4 4 7% 47%	1826
	21st to end	Day	14% 7% 2 6% 4 4 3 4 5% 7% 43%	1680
December	1st to 10th	Day	14% 6% 4 7% 5% 4 4 4 6% 7% 40%	2680
	11th to 20th	Day	19% 8% 2 6% 4 4 4 3 5% 6% 37%	3070
	21st to end	Day	16% 9% 3 8% 5% 4 6% 4 6% 7% 32%	4048
January	1st to 10th	Night	10% 8% 3 7% 4 4 5% 4 6% 7% 42%	8215
	11th to 20th	Night	13% 9% 2 6% 4 3 4 3 5% 6% 46%	7680
	21st to end	Night	14% 6% 2 5% 4 4 3 3 6% 7% 45%	8401
February	1st to 10th	Night	15% 8% 3 6% 5% 4 4 4 6% 7% 40%	2730
	11th to 20th	Night	16% 9% 3 7% 4 5% 4 3 6% 6% 39%	2624
	21st to end	Night	15% 8% 3 7% 4 4 4 3 6% 6% 39%	2076
March	1st to 10th	Night	17% 8% 2 5% 4 3 3 3 4 5% 46%	5124
	11th to 20th	Night	10% 6% 2 5% 4 4 4 4 5% 6% 50%	5058
	21st to end	Night	11% 8% 2 6% 4 4 4 3 5% 6% 45%	5376
April	1st to 10th	Night	16% 9% 3 7% 6% 5% 3 4 6% 6% 36%	2000
	11th to 20th	Night	14% 7% 2 5% 4 4 4 3 5% 9% 43%	1975
	21st to end	Night	17% 6% 2 5% 3 3 3 4 6% 7% 42%	2000
May	1st to 10th	Night	25% 8% 3 7% 4 4 4 4 5% 7% 29%	4084
	11th to 20th	Night	23% 8% 3 6% 4 4 4 4 6% 8% 31%	3634
	21st to end	Night	19% 6% 3 6% 4 4 4 3 6% 7% 36%	3920
June	1st to 10th	Night	12% 6% 2 5% 4 3 4 4 6% 8% 47%	1600
	11th to 20th	Night	13% 5% 2 5% 5% 4 5% 5% 8% 10% 39%	1600
	21st to end	Night	9% 5% 2 5% 5% 5% 5% 4 7% 9% 44%	1600
July	1st to 10th	Night	7% 6% 2 7% 5% 5% 6% 5% 7% 11% 40%	3598
	11th to 20th	Night	9% 6% 2 9% 7% 6% 7% 5% 7% 10% 31%	3659
	21st to end	Night	9% 6% 2 5% 4 4 5% 4 6% 9% 45%	4427
August	1st to 10th	Night	17% 5% 2 7% 5% 4 5% 5% 7% 9% 35%	2915
	11th to 20th	Night	11% 6% 3 7% 5% 4 5% 4 6% 7% 42%	2974
	21st to end	Night	14% 5% 2 5% 3 4 4 3 5% 8% 48%	3586
September	1st to 10th	Night	13% 5% 2 6% 4 4 5% 3 5% 7% 46%	2000
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 5% 6% 52%	2000
	21st to end	Night	21% 7% 2 5% 4 4 4 3 5% 6% 40%	2294
October	1st to 10th	Night	18% 6% 2 4 3 3 4 3 5% 5% 46%	2400
	11th to 20th	Night	14% 5% 2 5% 4 2 4 3 5% 6% 50%	2400
	21st to end	Night	15% 5% 2 5% 3 4 3 3 6% 6% 47%	2640
November	1st to 10th	Night	10% 6% 2 6% 3 4 3 3 5% 5% 53%	2597
	11th to 20th	Night	14% 7% 2 5% 4 4 4 4 5% 6% 45%	2792
	21st to end	Night	13% 5% 2 6% 5% 4 5% 4 6% 7% 43%	2940
December	1st to 10th	Night	10% 6% 2 6% 4 5% 5% 4 6% 7% 44%	4720
	11th to 20th	Night	21% 7% 2 5% 4 4 4 3 6% 6% 38%	5345
	21st to end	Night	17% 8% 3 7% 5% 5% 5% 5% 6% 8% 31%	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	38% 21% 8% 19% 13% 2	4677
	11th to 20th	Day	36% 21% 5% 15% 15% 7%	5280
	21st to end	Day	35% 23% 8% 22% 10% 2	5780
February	1st to 10th	Day	19% 59% 6% 9% 5% 1	1890
	11th to 20th	Day	21% 57% 3 10% 7% 1	1889
	21st to end	Day	19% 59% 5% 10% 5% 2	1730
March	1st to 10th	Day	33% 29% 6% 15% 11% 5%	4943
	11th to 20th	Day	26% 29% 8% 22% 11% 4	5057
	21st to end	Day	32% 30% 7% 13% 15% 4	5556
April	1st to 10th	Day	21% 62% 3 6% 6% 1	2400
	11th to 20th	Day	15% 63% 6% 9% 6% 1	2374
	21st to end	Day	17% 62% 3 10% 7%	2400
May	1st to 10th	Day	38% 30% 10% 14% 6% 1	5887
	11th to 20th	Day	37% 31% 8% 18% 5%	6248
	21st to end	Day	32% 31% 9% 16% 11% 1	6949
June	1st to 10th	Day	17% 63% 4 10% 6%	2800
	11th to 20th	Day	16% 63% 4 10% 7% 1	2800
	21st to end	Day	14% 61% 5% 12% 8%	2800
July	1st to 10th	Day	26% 31% 12% 17% 13% 2	6290
	11th to 20th	Day	34% 29% 14% 17% 5%	6205
	21st to end	Day	28% 27% 8% 19% 18% 1	6240
August	1st to 10th	Day	31% 38% 9% 17% 5%	4221
	11th to 20th	Day	24% 38% 8% 21% 9%	4066
	21st to end	Day	20% 36% 7% 21% 14% 1	4312
September	1st to 10th	Day	16% 58% 6% 11% 8% 2	2400
	11th to 20th	Day	17% 57% 4 12% 9% 2	2400
	21st to end	Day	24% 57% 4 9% 7% 1	2102
October	1st to 10th	Day	24% 35% 4 13% 20% 4	3260
	11th to 20th	Day	14% 36% 7% 21% 16% 7%	3240
	21st to end	Day	12% 35% 8% 22% 22% 1	3538
November	1st to 10th	Day	18% 53% 7% 10% 9% 3	1921
	11th to 20th	Day	15% 54% 4 13% 13% 1	1826
	21st to end	Day	18% 55% 6% 15% 5% 1	1680
December	1st to 10th	Day	29% 35% 10% 19% 5% 3	2680
	11th to 20th	Day	35% 31% 9% 19% 6% 1	3066
	21st to end	Day	39% 28% 12% 11% 9% 1	4048
January	1st to 10th	Night	35% 23% 9% 19% 12% 2	8216
	11th to 20th	Night	33% 21% 6% 18% 17% 6%	7680
	21st to end	Night	30% 22% 7% 24% 15% 2	8401
February	1st to 10th	Night	18% 59% 7% 11% 5% 1	2730
	11th to 20th	Night	20% 58% 3 10% 8% 2	2626
	21st to end	Night	18% 59% 4 9% 7% 2	2076
March	1st to 10th	Night	33% 29% 5% 18% 7% 8%	5124
	11th to 20th	Night	23% 29% 9% 24% 9% 4	5058
	21st to end	Night	29% 29% 6% 14% 19% 2	5376
April	1st to 10th	Night	19% 62% 3 9% 6% 1	2000
	11th to 20th	Night	15% 62% 5% 10% 6% 1	1975
	21st to end	Night	16% 63% 4 10% 7% 1	2000
May	1st to 10th	Night	44% 30% 7% 10% 7% 2	4111
	11th to 20th	Night	38% 31% 8% 17% 6%	3635
	21st to end	Night	36% 31% 8% 18% 6% 1	3927
June	1st to 10th	Night	16% 62% 3 11% 8%	1600
	11th to 20th	Night	15% 63% 4 10% 7%	1600
	21st to end	Night	14% 61% 6% 10% 8% 1	1600
July	1st to 10th	Night	29% 29% 12% 14% 12% 3	3560
	11th to 20th	Night	40% 29% 11% 13% 7%	3614
	21st to end	Night	29% 28% 7% 19% 17% 1	4297
August	1st to 10th	Night	33% 37% 12% 13% 4 1	2904
	11th to 20th	Night	26% 38% 6% 21% 9%	3001
	21st to end	Night	21% 36% 6% 24% 12%	3586
September	1st to 10th	Night	15% 58% 6% 11% 8% 1	2000
	11th to 20th	Night	16% 57% 4 12% 9% 2	2000
	21st to end	Night	23% 58% 4 8% 7%	2294
October	1st to 10th	Night	27% 36% 7% 12% 16% 2	3920
	11th to 20th	Night	14% 36% 6% 18% 21% 5%	3899
	21st to end	Night	13% 36% 7% 24% 19% 2	4252
November	1st to 10th	Night	17% 54% 6% 12% 9% 2	2597
	11th to 20th	Night	18% 54% 3 11% 12% 2	2792
	21st to end	Night	17% 54% 6% 14% 7% 1	2940
December	1st to 10th	Night	24% 35% 10% 22% 8% 1	4720
	11th to 20th	Night	34% 31% 8% 15% 10% 2	5348
	21st to end	Night	40% 28% 8% 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	25% 32% 26% 14% 2 1	222
	11th to 20th	Day	26% 33% 31% 8% 2	330
	21st to end	Day	20% 35% 31% 10% 3	363
February	1st to 10th	Day	23% 29% 33% 12% 2	330
	11th to 20th	Day	34% 30% 31% 5% 1	367
	21st to end	Day	28% 23% 32% 16% 2	446
March	1st to 10th	Day	26% 24% 27% 19% 3	540
	11th to 20th	Day	23% 30% 26% 13% 5% 1	528
	21st to end	Day	24% 24% 30% 16% 4 1	550
April	1st to 10th	Day	26% 22% 30% 15% 5% 1	500
	11th to 20th	Day	19% 21% 30% 16% 12% 2	492
	21st to end	Day	25% 18% 31% 17% 7% 2	500
May	1st to 10th	Day	27% 16% 29% 18% 7% 3	503
	11th to 20th	Day	22% 16% 27% 20% 11% 3	527
	21st to end	Day	19% 9% 23% 24% 21% 4	573
June	1st to 10th	Day	15% 12% 28% 21% 18% 5%	530
	11th to 20th	Day	17% 9% 28% 26% 17% 4	530
	21st to end	Day	11% 9% 26% 30% 18% 6%	530
July	1st to 10th	Day	12% 4 21% 30% 22% 11%	525
	11th to 20th	Day	6% 4 21% 32% 27% 9%	524
	21st to end	Day	11% 6% 21% 34% 21% 7%	576
August	1st to 10th	Day	12% 3 21% 30% 25% 9%	526
	11th to 20th	Day	13% 4 21% 29% 23% 10%	519
	21st to end	Day	12% 4 22% 28% 23% 12%	583
September	1st to 10th	Day	16% 9% 19% 23% 22% 10%	530
	11th to 20th	Day	14% 4 19% 28% 24% 10%	530
	21st to end	Day	29% 10% 24% 21% 12% 5%	530
October	1st to 10th	Day	18% 8% 26% 23% 19% 6%	530
	11th to 20th	Day	18% 16% 28% 23% 12% 3	530
	21st to end	Day	16% 17% 36% 21% 9% 2	583
November	1st to 10th	Day	16% 15% 30% 25% 11% 2	463
	11th to 20th	Day	19% 22% 29% 22% 7%	366
	21st to end	Day	23% 25% 30% 17% 4	220
December	1st to 10th	Day	24% 26% 32% 14% 3 1	220
	11th to 20th	Day	23% 24% 31% 17% 4	220
	21st to end	Day	21% 24% 30% 19% 5%	242
January	1st to 10th	Night	17% 25% 37% 20% 2	840
	11th to 20th	Night	21% 20% 34% 22% 3	840
	21st to end	Night	17% 22% 39% 20% 2	924
February	1st to 10th	Night	19% 21% 33% 25% 2	840
	11th to 20th	Night	24% 26% 36% 13% 1	823
	21st to end	Night	22% 22% 35% 18% 3	692
March	1st to 10th	Night	20% 21% 36% 20% 3	840
	11th to 20th	Night	22% 21% 31% 20% 5%	829
	21st to end	Night	20% 17% 41% 18% 5%	880
April	1st to 10th	Night	22% 23% 31% 20% 4	800
	11th to 20th	Night	17% 17% 33% 26% 7% 1	791
	21st to end	Night	21% 14% 31% 27% 8% 1	800
May	1st to 10th	Night	24% 9% 28% 29% 10%	625
	11th to 20th	Night	20% 8% 28% 33% 10%	427
	21st to end	Night	17% 7% 23% 33% 18% 1	433
June	1st to 10th	Night	14% 2 30% 33% 21% 1	400
	11th to 20th	Night	16% 4 23% 41% 16%	400
	21st to end	Night	9% 3 22% 47% 18% 1	400
July	1st to 10th	Night	12% 4 16% 38% 27% 3	390
	11th to 20th	Night	7% 3 18% 43% 27% 2	411
	21st to end	Night	9% 3 15% 45% 26% 2	657
August	1st to 10th	Night	11% 1 12% 43% 30% 2	597
	11th to 20th	Night	11% 4 15% 39% 28% 3	654
	21st to end	Night	13% 3 15% 40% 26% 4	880
September	1st to 10th	Night	15% 3 20% 36% 24% 3	800
	11th to 20th	Night	12% 4 19% 34% 28% 3	800
	21st to end	Night	24% 7% 18% 32% 17% 2	800
October	1st to 10th	Night	21% 7% 22% 27% 21% 3	800
	11th to 20th	Night	17% 12% 29% 31% 11% 1	800
	21st to end	Night	19% 12% 31% 28% 10%	880
November	1st to 10th	Night	13% 12% 33% 32% 10% 1	820
	11th to 20th	Night	18% 20% 31% 24% 7% 1	840
	21st to end	Night	16% 20% 32% 26% 5%	840
December	1st to 10th	Night	20% 17% 37% 21% 5%	840
	11th to 20th	Night	19% 18% 35% 25% 3	840
	21st to end	Night	20% 17% 34% 26% 3	924

Page generated at 2026-03-15 02:23:57. Version ac07

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