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 2024-07-05.

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	15% 7% 2 6% 5% 4 5% 4 6% 8% 40%	89676
	Night	15% 7% 2 6% 4 4 4 4 6% 7% 41%	89544
Transparency	Day	21% 54% 6% 11% 6% 1	89438
	Night	21% 53% 6% 12% 7% 1	89386
Seeing	Day	19% 15% 27% 22% 13% 4	14416
	Night	18% 14% 29% 28% 11% 1	23909

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	15% 7% 2 7% 5% 5% 5% 4 4 5% 42%	2518
	11th to 20th	Day	13% 11% 4 9% 5% 5% 5% 4 5% 6% 32%	2840
	21st to end	Day	16% 11% 2 9% 5% 5% 5% 3 5% 5% 34%	3124
February	1st to 10th	Day	15% 8% 3 6% 6% 4 5% 5% 5% 5% 37%	1710
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 4 6% 39%	1696
	21st to end	Day	18% 9% 3 7% 5% 4 3 3 4 6% 37%	1570
March	1st to 10th	Day	22% 7% 2 5% 3 4 5% 3 5% 7% 37%	3364
	11th to 20th	Day	9% 6% 3 6% 4 5% 5% 4 6% 7% 45%	3480
	21st to end	Day	19% 8% 3 5% 4 4 3 3 4 6% 42%	3878
April	1st to 10th	Day	14% 8% 2 7% 5% 5% 5% 5% 6% 7% 38%	2280
	11th to 20th	Day	14% 7% 2 5% 5% 4 5% 4 5% 7% 43%	2254
	21st to end	Day	18% 7% 1 5% 4 5% 4 4 6% 6% 39%	2280
May	1st to 10th	Day	27% 6% 2 5% 4 4 4 4 6% 8% 30%	4072
	11th to 20th	Day	23% 8% 3 7% 5% 5% 4 4 6% 7% 29%	4352
	21st to end	Day	10% 5% 2 6% 5% 4 5% 4 6% 9% 44%	4823
June	1st to 10th	Day	12% 5% 2 5% 5% 4 5% 4 7% 11% 40%	2660
	11th to 20th	Day	11% 4 3 7% 5% 5% 5% 4 7% 9% 40%	2660
	21st to end	Day	9% 4 2 5% 5% 4 6% 4 7% 11% 44%	2660
July	1st to 10th	Day	9% 4 2 6% 5% 5% 6% 4 7% 10% 42%	2576
	11th to 20th	Day	6% 4 2 5% 5% 5% 5% 5% 8% 10% 44%	2496
	21st to end	Day	8% 7% 3 6% 6% 5% 6% 5% 7% 10% 37%	2547
August	1st to 10th	Day	9% 5% 2 6% 6% 6% 7% 4 7% 10% 38%	2327
	11th to 20th	Day	11% 7% 2 6% 5% 4 5% 4 8% 8% 41%	2230
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 41%	2376
September	1st to 10th	Day	17% 5% 3 5% 4 4 4 3 5% 7% 44%	2160
	11th to 20th	Day	13% 5% 2 4 4 4 4 4 5% 7% 49%	2160
	21st to end	Day	18% 7% 2 5% 4 4 4 3 5% 7% 40%	1893
October	1st to 10th	Day	15% 6% 2 5% 4 4 4 4 5% 6% 44%	1800
	11th to 20th	Day	18% 5% 2 6% 3 3 4 4 5% 6% 45%	1800
	21st to end	Day	15% 7% 3 6% 4 3 3 3 6% 6% 44%	1980
November	1st to 10th	Day	12% 7% 2 4 3 3 4 4 4 7% 51%	1735
	11th to 20th	Day	15% 7% 2 4 4 4 3 4 4 7% 46%	1653
	21st to end	Day	15% 7% 2 6% 4 4 3 4 5% 7% 43%	1520
December	1st to 10th	Day	18% 7% 3 6% 5% 4 4 4 6% 7% 36%	1520
	11th to 20th	Day	16% 5% 3 6% 5% 5% 5% 4 7% 7% 37%	1910
	21st to end	Day	18% 8% 2 8% 6% 5% 6% 4 7% 7% 31%	2772
January	1st to 10th	Night	13% 8% 2 7% 5% 4 5% 4 5% 7% 40%	4425
	11th to 20th	Night	16% 8% 3 6% 4 3 4 3 6% 6% 40%	4120
	21st to end	Night	20% 9% 3 7% 4 4 3 3 5% 6% 35%	4532
February	1st to 10th	Night	15% 9% 3 6% 5% 3 4 4 5% 7% 40%	2470
	11th to 20th	Night	16% 9% 3 7% 4 4 3 3 6% 6% 39%	2368
	21st to end	Night	17% 8% 3 7% 4 4 5% 3 6% 6% 37%	1884
March	1st to 10th	Night	20% 9% 2 5% 5% 4 3 3 5% 6% 38%	3484
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 5% 6% 49%	3480
	21st to end	Night	13% 9% 3 6% 4 3 4 3 4 5% 47%	3754
April	1st to 10th	Night	15% 9% 2 7% 6% 5% 3 4 6% 7% 37%	1900
	11th to 20th	Night	13% 7% 2 6% 4 4 3 3 5% 9% 44%	1875
	21st to end	Night	18% 6% 2 5% 3 3 3 4 6% 7% 42%	1900
May	1st to 10th	Night	25% 8% 3 7% 4 3 4 4 5% 6% 31%	2850
	11th to 20th	Night	22% 9% 4 7% 4 4 4 4 6% 6% 29%	2540
	21st to end	Night	11% 7% 2 6% 4 5% 5% 4 8% 9% 40%	2732
June	1st to 10th	Night	12% 5% 2 5% 4 3 4 4 6% 8% 47%	1520
	11th to 20th	Night	11% 5% 2 5% 6% 4 5% 5% 8% 10% 39%	1520
	21st to end	Night	9% 5% 2 5% 5% 5% 5% 4 7% 9% 45%	1520
July	1st to 10th	Night	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 39%	1478
	11th to 20th	Night	5% 4 2 6% 4 4 5% 5% 7% 10% 48%	1460
	21st to end	Night	8% 5% 2 4 4 4 5% 5% 8% 11% 45%	1771
August	1st to 10th	Night	10% 4 2 7% 5% 4 6% 6% 8% 10% 41%	1611
	11th to 20th	Night	9% 6% 3 8% 6% 5% 5% 4 6% 7% 42%	1652
	21st to end	Night	12% 5% 2 5% 4 4 5% 4 6% 9% 45%	1980
September	1st to 10th	Night	14% 5% 2 7% 4 4 4 3 5% 8% 45%	1800
	11th to 20th	Night	11% 6% 2 5% 3 3 4 3 5% 6% 53%	1800
	21st to end	Night	18% 6% 2 5% 4 4 4 3 5% 6% 44%	2064
October	1st to 10th	Night	18% 6% 3 4 4 3 4 3 5% 6% 44%	2160
	11th to 20th	Night	14% 5% 2 6% 4 3 4 3 5% 6% 49%	2160
	21st to end	Night	16% 5% 2 5% 3 4 3 3 6% 6% 46%	2376
November	1st to 10th	Night	10% 6% 2 6% 3 3 3 3 5% 5% 52%	2343
	11th to 20th	Night	14% 7% 2 5% 4 3 4 3 5% 6% 46%	2526
	21st to end	Night	13% 5% 2 6% 5% 4 5% 4 6% 7% 44%	2660
December	1st to 10th	Night	15% 8% 2 6% 5% 5% 5% 4 6% 6% 37%	2660
	11th to 20th	Night	18% 6% 2 5% 5% 4 4 4 6% 6% 40%	3285
	21st to end	Night	17% 7% 3 6% 5% 6% 6% 5% 6% 8% 32%	4884

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	29% 37% 7% 17% 7% 3	2519
	11th to 20th	Day	36% 38% 6% 10% 8% 3	2840
	21st to end	Day	36% 40% 5% 14% 4 1	3124
February	1st to 10th	Day	18% 66% 6% 6% 3 1	1710
	11th to 20th	Day	17% 63% 3 9% 6% 2	1705
	21st to end	Day	17% 65% 4 7% 5% 2	1570
March	1st to 10th	Day	33% 42% 6% 12% 6% 1	3364
	11th to 20th	Day	20% 42% 6% 20% 10% 2	3480
	21st to end	Day	27% 41% 7% 13% 11% 2	3878
April	1st to 10th	Day	18% 65% 4 6% 6% 1	2280
	11th to 20th	Day	14% 66% 5% 9% 5% 1	2254
	21st to end	Day	17% 65% 3 8% 6%	2280
May	1st to 10th	Day	36% 41% 6% 13% 4	4110
	11th to 20th	Day	34% 42% 10% 12% 2	4356
	21st to end	Day	19% 42% 10% 16% 12% 1	4837
June	1st to 10th	Day	15% 66% 4 10% 5%	2660
	11th to 20th	Day	14% 66% 4 9% 6% 1	2660
	21st to end	Day	12% 64% 5% 11% 7%	2660
July	1st to 10th	Day	15% 63% 6% 8% 6% 1	2506
	11th to 20th	Day	12% 64% 6% 11% 7% 1	2421
	21st to end	Day	14% 63% 5% 11% 6% 1	2405
August	1st to 10th	Day	14% 65% 6% 10% 5%	2314
	11th to 20th	Day	13% 65% 7% 10% 5%	2230
	21st to end	Day	12% 64% 6% 11% 6% 1	2376
September	1st to 10th	Day	16% 64% 6% 9% 5% 1	2160
	11th to 20th	Day	14% 63% 4 10% 7% 2	2160
	21st to end	Day	17% 63% 4 9% 7% 1	1893
October	1st to 10th	Day	16% 63% 5% 9% 7% 1	1800
	11th to 20th	Day	14% 63% 4 9% 10% 1	1800
	21st to end	Day	14% 62% 5% 11% 8% 1	1980
November	1st to 10th	Day	17% 59% 6% 9% 6% 3	1735
	11th to 20th	Day	12% 60% 4 12% 11% 1	1653
	21st to end	Day	15% 60% 6% 14% 5% 1	1520
December	1st to 10th	Day	22% 60% 5% 8% 5%	1520
	11th to 20th	Day	26% 48% 4 17% 4 1	1906
	21st to end	Day	34% 40% 11% 10% 5%	2772
January	1st to 10th	Night	29% 39% 7% 15% 7% 2	4425
	11th to 20th	Night	29% 37% 7% 17% 8% 2	4120
	21st to end	Night	36% 39% 5% 15% 4 1	4532
February	1st to 10th	Night	15% 65% 7% 10% 3 1	2470
	11th to 20th	Night	16% 64% 2 8% 8% 2	2370
	21st to end	Night	16% 64% 4 7% 6% 2	1884
March	1st to 10th	Night	35% 41% 4 15% 4 1	3484
	11th to 20th	Night	19% 42% 5% 24% 9% 1	3480
	21st to end	Night	24% 41% 4 13% 15% 3	3754
April	1st to 10th	Night	16% 65% 3 9% 5% 1	1900
	11th to 20th	Night	13% 66% 5% 9% 6% 1	1875
	21st to end	Night	15% 66% 4 9% 6% 1	1900
May	1st to 10th	Night	38% 41% 4 10% 6% 1	2877
	11th to 20th	Night	36% 43% 7% 10% 3	2541
	21st to end	Night	23% 43% 10% 17% 5% 2	2739
June	1st to 10th	Night	14% 66% 3 10% 7%	1520
	11th to 20th	Night	13% 66% 4 10% 7%	1520
	21st to end	Night	13% 64% 6% 10% 7% 1	1520
July	1st to 10th	Night	14% 63% 7% 7% 7% 2	1438
	11th to 20th	Night	10% 63% 7% 11% 7% 1	1412
	21st to end	Night	13% 63% 7% 10% 6% 1	1672
August	1st to 10th	Night	14% 65% 7% 9% 5% 1	1600
	11th to 20th	Night	13% 65% 6% 9% 6%	1652
	21st to end	Night	11% 63% 6% 12% 7%	1980
September	1st to 10th	Night	14% 64% 6% 10% 5% 1	1800
	11th to 20th	Night	13% 63% 4 11% 8% 2	1800
	21st to end	Night	16% 64% 4 9% 7%	2064
October	1st to 10th	Night	16% 63% 6% 8% 6% 1	2160
	11th to 20th	Night	13% 63% 3 12% 8% 1	2160
	21st to end	Night	14% 62% 4 11% 8% 1	2376
November	1st to 10th	Night	15% 60% 6% 11% 6% 2	2343
	11th to 20th	Night	14% 60% 3 10% 11% 2	2526
	21st to end	Night	14% 60% 6% 13% 6% 1	2660
December	1st to 10th	Night	21% 60% 6% 8% 5%	2660
	11th to 20th	Night	23% 50% 4 12% 9% 2	3288
	21st to end	Night	33% 40% 10% 12% 5%	4884

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	27% 30% 27% 14% 2 1	181
	11th to 20th	Day	27% 31% 31% 9% 2	270
	21st to end	Day	21% 37% 28% 11% 3	297
February	1st to 10th	Day	25% 29% 33% 11% 1	270
	11th to 20th	Day	32% 31% 31% 4 1	303
	21st to end	Day	31% 23% 28% 16% 2	382
March	1st to 10th	Day	25% 24% 28% 19% 3	460
	11th to 20th	Day	23% 31% 26% 14% 5% 1	460
	21st to end	Day	25% 23% 30% 16% 5% 1	506
April	1st to 10th	Day	24% 22% 31% 15% 6% 2	460
	11th to 20th	Day	18% 20% 30% 16% 13% 2	452
	21st to end	Day	26% 17% 32% 17% 6% 2	460
May	1st to 10th	Day	29% 16% 28% 17% 7% 3	463
	11th to 20th	Day	22% 16% 27% 21% 11% 3	487
	21st to end	Day	17% 8% 23% 25% 22% 5%	529
June	1st to 10th	Day	16% 12% 29% 21% 17% 5%	490
	11th to 20th	Day	15% 9% 28% 26% 18% 4	490
	21st to end	Day	11% 10% 28% 30% 17% 4	490
July	1st to 10th	Day	13% 5% 21% 31% 20% 11%	461
	11th to 20th	Day	6% 5% 21% 32% 27% 9%	444
	21st to end	Day	11% 6% 22% 34% 21% 7%	488
August	1st to 10th	Day	11% 3 21% 30% 25% 10%	446
	11th to 20th	Day	13% 4 21% 30% 22% 11%	439
	21st to end	Day	12% 4 22% 29% 23% 11%	495
September	1st to 10th	Day	18% 8% 18% 22% 22% 11%	450
	11th to 20th	Day	15% 4 20% 29% 22% 10%	450
	21st to end	Day	25% 10% 24% 22% 14% 5%	450
October	1st to 10th	Day	20% 9% 25% 23% 19% 5%	450
	11th to 20th	Day	19% 16% 27% 24% 12% 2	450
	21st to end	Day	17% 17% 36% 20% 8% 1	495
November	1st to 10th	Day	18% 14% 28% 25% 12% 2	397
	11th to 20th	Day	20% 22% 28% 21% 8%	313
	21st to end	Day	23% 24% 32% 17% 4	180
December	1st to 10th	Day	26% 27% 30% 13% 4 1	180
	11th to 20th	Day	24% 26% 29% 17% 4	180
	21st to end	Day	22% 25% 28% 20% 5%	198
January	1st to 10th	Night	18% 24% 36% 20% 2	760
	11th to 20th	Night	22% 19% 33% 23% 3	760
	21st to end	Night	18% 22% 38% 20% 2	836
February	1st to 10th	Night	19% 21% 33% 25% 2	760
	11th to 20th	Night	23% 27% 36% 13% 1	743
	21st to end	Night	24% 23% 33% 17% 3	628
March	1st to 10th	Night	19% 21% 36% 20% 3	760
	11th to 20th	Night	23% 21% 31% 20% 5%	760
	21st to end	Night	20% 17% 40% 18% 5%	836
April	1st to 10th	Night	21% 23% 32% 20% 4	760
	11th to 20th	Night	16% 16% 33% 26% 7% 1	751
	21st to end	Night	21% 13% 31% 26% 8% 1	760
May	1st to 10th	Night	23% 9% 28% 28% 11% 1	594
	11th to 20th	Night	20% 8% 28% 33% 10%	406
	21st to end	Night	16% 7% 23% 34% 19% 1	411
June	1st to 10th	Night	13% 2 30% 33% 21% 1	380
	11th to 20th	Night	14% 3 24% 41% 17%	380
	21st to end	Night	8% 3 23% 48% 17%	380
July	1st to 10th	Night	12% 4 16% 38% 27% 3	360
	11th to 20th	Night	7% 2 19% 44% 27% 2	368
	21st to end	Night	9% 3 17% 45% 25% 3	591
August	1st to 10th	Night	11% 1 13% 44% 29% 2	537
	11th to 20th	Night	12% 4 15% 38% 28% 3	588
	21st to end	Night	14% 3 15% 39% 26% 4	792
September	1st to 10th	Night	16% 3 20% 34% 24% 3	720
	11th to 20th	Night	13% 4 20% 32% 28% 3	720
	21st to end	Night	22% 7% 19% 31% 19% 2	720
October	1st to 10th	Night	21% 7% 22% 28% 20% 3	720
	11th to 20th	Night	17% 13% 28% 30% 11% 1	720
	21st to end	Night	20% 12% 31% 27% 9%	792
November	1st to 10th	Night	13% 12% 33% 31% 10% 1	740
	11th to 20th	Night	18% 20% 31% 23% 7% 1	760
	21st to end	Night	16% 20% 32% 26% 6% 1	760
December	1st to 10th	Night	21% 18% 36% 19% 6% 1	760
	11th to 20th	Night	18% 18% 36% 24% 4	760
	21st to end	Night	20% 17% 34% 26% 2	836

Page generated at 2024-07-02 06:41:30. Version ac07

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