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 2024-12-06.

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%	101556
	Night	15% 7% 2 6% 4 4 4 4 6% 7% 41%	98113
Transparency	Day	23% 47% 7% 14% 8% 1	105195
	Night	23% 47% 6% 14% 9% 1	102613
Seeing	Day	19% 15% 27% 22% 13% 4	14981
	Night	18% 13% 29% 28% 11% 1	24468

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	6% 4 2 6% 5% 6% 6% 5% 8% 10% 43%	4440
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 10% 41%	4388
	21st to end	Day	10% 7% 3 7% 6% 6% 5% 4 6% 8% 38%	4466
August	1st to 10th	Day	16% 5% 2 6% 5% 5% 6% 4 7% 10% 34%	4104
	11th to 20th	Day	11% 5% 2 5% 4 4 6% 4 8% 8% 42%	3901
	21st to end	Day	13% 5% 2 5% 3 4 4 4 5% 8% 47%	4180
September	1st to 10th	Day	16% 5% 3 5% 3 4 4 3 5% 6% 46%	2280
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 5% 7% 49%	2280
	21st to end	Day	21% 7% 2 5% 4 4 4 3 5% 7% 38%	1997
October	1st to 10th	Day	16% 6% 2 5% 4 4 4 4 5% 6% 44%	1900
	11th to 20th	Day	17% 4 2 6% 3 3 4 4 5% 6% 47%	1900
	21st to end	Day	14% 6% 3 6% 4 3 4 3 5% 6% 46%	2090
November	1st to 10th	Day	12% 7% 2 4 3 3 4 4 5% 7% 52%	1828
	11th to 20th	Day	15% 7% 1 4 4 4 3 4 4 7% 47%	1739
	21st to end	Day	14% 7% 2 6% 5% 4 3 4 5% 7% 43%	1600
December	1st to 10th	Day	17% 7% 3 6% 5% 4 4 4 6% 7% 36%	1560
	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	6% 4 2 6% 5% 5% 6% 5% 8% 11% 41%	2520
	11th to 20th	Night	6% 5% 2 7% 5% 5% 6% 5% 7% 11% 41%	2568
	21st to end	Night	11% 7% 2 6% 5% 4 5% 3 6% 9% 42%	3087
August	1st to 10th	Night	18% 5% 2 7% 5% 4 5% 5% 7% 9% 35%	2825
	11th to 20th	Night	11% 6% 3 7% 5% 5% 5% 4 6% 7% 42%	2881
	21st to end	Night	14% 5% 2 5% 3 4 4 3 5% 8% 48%	3476
September	1st to 10th	Night	13% 5% 2 6% 4 4 4 3 5% 7% 46%	1900
	11th to 20th	Night	11% 6% 2 5% 3 3 4 3 5% 6% 53%	1900
	21st to end	Night	21% 6% 2 5% 3 3 4 3 5% 5% 42%	2179
October	1st to 10th	Night	19% 6% 3 4 4 3 4 3 5% 5% 44%	2280
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 5% 6% 50%	2280
	21st to end	Night	15% 5% 2 4 3 4 3 3 6% 7% 47%	2508
November	1st to 10th	Night	10% 6% 2 6% 3 4 3 3 5% 5% 53%	2470
	11th to 20th	Night	15% 7% 2 5% 4 3 4 3 5% 6% 46%	2659
	21st to end	Night	13% 5% 2 6% 5% 4 5% 4 6% 7% 44%	2800
December	1st to 10th	Night	15% 8% 2 6% 5% 5% 5% 4 6% 6% 38%	2737
	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	23% 40% 12% 16% 7% 3	4370
	11th to 20th	Day	25% 38% 11% 19% 7% 1	4313
	21st to end	Day	29% 37% 7% 15% 12% 1	4324
August	1st to 10th	Day	31% 39% 9% 17% 4	4091
	11th to 20th	Day	23% 39% 8% 21% 10%	3940
	21st to end	Day	20% 38% 8% 20% 14% 1	4180
September	1st to 10th	Day	16% 60% 5% 10% 7% 2	2280
	11th to 20th	Day	16% 59% 4 11% 8% 2	2280
	21st to end	Day	21% 60% 4 9% 7% 1	1997
October	1st to 10th	Day	24% 36% 3 13% 20% 4	3160
	11th to 20th	Day	14% 37% 6% 20% 16% 7%	3140
	21st to end	Day	11% 36% 8% 22% 22% 2	3428
November	1st to 10th	Day	17% 56% 7% 10% 8% 3	1828
	11th to 20th	Day	13% 57% 4 13% 12% 1	1739
	21st to end	Day	17% 57% 6% 14% 5% 1	1600
December	1st to 10th	Day	22% 58% 5% 9% 5% 1	1560
	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	21% 39% 14% 12% 10% 4	2480
	11th to 20th	Night	26% 38% 10% 16% 9%	2520
	21st to end	Night	30% 37% 7% 12% 13% 1	2988
August	1st to 10th	Night	33% 38% 12% 13% 3 1	2814
	11th to 20th	Night	26% 39% 5% 21% 9%	2908
	21st to end	Night	21% 37% 6% 24% 11%	3476
September	1st to 10th	Night	14% 61% 6% 10% 7% 1	1900
	11th to 20th	Night	14% 60% 4 11% 9% 2	1900
	21st to end	Night	20% 60% 4 9% 7%	2179
October	1st to 10th	Night	27% 37% 7% 12% 16% 2	3800
	11th to 20th	Night	13% 37% 6% 18% 21% 5%	3779
	21st to end	Night	12% 37% 7% 23% 18% 2	4120
November	1st to 10th	Night	15% 57% 6% 12% 8% 2	2470
	11th to 20th	Night	17% 57% 3 10% 11% 2	2659
	21st to end	Night	16% 57% 6% 13% 7% 1	2800
December	1st to 10th	Night	21% 59% 6% 9% 5%	2737
	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	12% 4 22% 29% 21% 11%	485
	11th to 20th	Day	6% 5% 22% 32% 27% 8%	484
	21st to end	Day	11% 6% 21% 35% 20% 6%	532
August	1st to 10th	Day	12% 3 21% 30% 25% 9%	486
	11th to 20th	Day	13% 4 22% 30% 22% 10%	479
	21st to end	Day	12% 4 21% 28% 23% 11%	539
September	1st to 10th	Day	17% 9% 19% 23% 22% 10%	490
	11th to 20th	Day	14% 4 19% 29% 23% 10%	490
	21st to end	Day	29% 10% 23% 20% 13% 5%	490
October	1st to 10th	Day	19% 9% 26% 23% 18% 5%	490
	11th to 20th	Day	18% 16% 27% 23% 13% 3	490
	21st to end	Day	16% 16% 36% 22% 9% 2	539
November	1st to 10th	Day	17% 14% 29% 26% 12% 2	430
	11th to 20th	Day	19% 21% 30% 22% 8%	339
	21st to end	Day	24% 26% 30% 17% 5%	200
December	1st to 10th	Day	25% 26% 29% 14% 4 1	190
	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	370
	11th to 20th	Night	6% 2 19% 44% 27% 2	390
	21st to end	Night	9% 3 16% 45% 25% 3	624
August	1st to 10th	Night	12% 1 12% 43% 30% 2	567
	11th to 20th	Night	11% 4 15% 39% 28% 3	621
	21st to end	Night	14% 3 14% 39% 26% 4	836
September	1st to 10th	Night	15% 3 20% 35% 24% 3	760
	11th to 20th	Night	12% 4 19% 33% 28% 3	760
	21st to end	Night	25% 7% 18% 30% 18% 2	760
October	1st to 10th	Night	21% 7% 22% 27% 20% 3	760
	11th to 20th	Night	16% 12% 29% 30% 11% 1	760
	21st to end	Night	19% 12% 31% 28% 10%	836
November	1st to 10th	Night	13% 11% 33% 32% 10% 1	780
	11th to 20th	Night	18% 19% 31% 24% 7% 1	800
	21st to end	Night	16% 20% 32% 26% 6% 1	800
December	1st to 10th	Night	21% 18% 36% 19% 5% 1	783
	11th to 20th	Night	18% 18% 36% 24% 4	760
	21st to end	Night	20% 17% 34% 26% 2	836

Page generated at 2024-12-03 06:45:12. Version ac07

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