Gettysburg Observatory
Clear Sky Chart
History

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

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	16% 10% 4 9% 5% 5% 5% 4 6% 7% 31%	130996
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	130175
Transparency	Day	51% 6% 11% 21% 10% 1	134600
	Night	52% 5% 11% 20% 11% 2	134641
Seeing	Day	22% 12% 26% 22% 13% 5%	16338
	Night	22% 10% 26% 29% 13% 1	26155

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	17% 17% 7% 20% 5% 4 4 2 4 5% 14%	5235
	11th to 20th	Day	12% 19% 7% 18% 10% 7% 4 3 3 3 15%	5280
	21st to end	Day	18% 15% 5% 12% 9% 7% 5% 3 5% 5% 16%	5780
February	1st to 10th	Day	20% 16% 5% 9% 7% 5% 4 3 4 5% 22%	1890
	11th to 20th	Day	16% 17% 5% 12% 5% 5% 4 3 5% 5% 23%	1843
	21st to end	Day	19% 14% 4 12% 5% 4 4 4 4 5% 25%	1735
March	1st to 10th	Day	17% 12% 4 9% 5% 5% 4 3 5% 5% 30%	4957
	11th to 20th	Day	13% 9% 4 8% 6% 5% 5% 4 5% 6% 36%	4980
	21st to end	Day	17% 10% 3 9% 6% 5% 5% 4 5% 6% 31%	5465
April	1st to 10th	Day	17% 12% 3 8% 5% 4 5% 4 6% 7% 29%	2310
	11th to 20th	Day	16% 10% 3 7% 5% 5% 5% 4 5% 7% 34%	2397
	21st to end	Day	20% 9% 3 7% 5% 4 4 4 5% 7% 33%	2600
May	1st to 10th	Day	21% 9% 3 8% 5% 5% 5% 4 7% 7% 27%	5862
	11th to 20th	Day	18% 7% 3 7% 5% 5% 5% 4 7% 7% 31%	5861
	21st to end	Day	18% 7% 2 6% 4 5% 5% 4 7% 9% 32%	6832
June	1st to 10th	Day	13% 7% 2 6% 5% 5% 6% 4 7% 9% 35%	2800
	11th to 20th	Day	14% 6% 3 6% 5% 6% 5% 4 7% 9% 35%	2800
	21st to end	Day	12% 7% 2 6% 5% 5% 5% 4 7% 9% 38%	2800
July	1st to 10th	Day	9% 7% 3 7% 5% 6% 5% 4 6% 8% 42%	6183
	11th to 20th	Day	11% 6% 3 9% 6% 6% 6% 5% 7% 11% 32%	5900
	21st to end	Day	11% 7% 3 8% 5% 5% 5% 4 6% 8% 38%	6424
August	1st to 10th	Day	14% 8% 3 8% 5% 6% 7% 4 7% 8% 31%	4234
	11th to 20th	Day	12% 6% 3 7% 5% 6% 6% 5% 7% 8% 37%	4174
	21st to end	Day	8% 4 2 7% 5% 5% 6% 4 8% 10% 41%	4444
September	1st to 10th	Day	15% 6% 2 5% 3 4 5% 4 6% 7% 45%	2400
	11th to 20th	Day	12% 5% 2 5% 4 5% 5% 3 5% 7% 47%	2215
	21st to end	Day	21% 9% 3 6% 5% 5% 4 3 5% 6% 35%	2191
October	1st to 10th	Day	16% 6% 2 7% 3 4 4 3 6% 8% 42%	2000
	11th to 20th	Day	18% 7% 3 7% 5% 4 5% 4 5% 6% 36%	2000
	21st to end	Day	22% 10% 2 7% 4 4 4 3 5% 7% 33%	2200
November	1st to 10th	Day	12% 9% 3 7% 5% 4 3 4 5% 6% 42%	1983
	11th to 20th	Day	17% 13% 3 7% 5% 5% 4 4 5% 6% 31%	1743
	21st to end	Day	18% 15% 5% 9% 5% 5% 4 3 5% 6% 25%	1680
December	1st to 10th	Day	18% 14% 7% 15% 7% 4 4 3 4 5% 20%	2680
	11th to 20th	Day	23% 17% 6% 11% 4 4 3 2 4 5% 22%	3070
	21st to end	Day	22% 19% 6% 12% 6% 4 4 3 5% 5% 15%	4048
January	1st to 10th	Night	15% 18% 6% 19% 6% 5% 5% 3 4 4 15%	7653
	11th to 20th	Night	14% 16% 8% 15% 9% 6% 4 3 4 4 16%	7680
	21st to end	Night	19% 13% 5% 11% 8% 6% 4 3 4 5% 21%	8401
February	1st to 10th	Night	20% 13% 4 10% 6% 5% 4 3 5% 5% 25%	2730
	11th to 20th	Night	20% 12% 5% 12% 6% 4 4 4 4 5% 25%	2663
	21st to end	Night	20% 13% 4 12% 6% 4 4 4 4 4 26%	2067
March	1st to 10th	Night	21% 11% 4 9% 5% 3 3 3 4 5% 33%	4941
	11th to 20th	Night	13% 10% 4 7% 4 4 3 2 4 7% 42%	4980
	21st to end	Night	18% 9% 4 8% 5% 5% 4 3 5% 5% 35%	5467
April	1st to 10th	Night	18% 13% 3 8% 5% 3 4 3 4 6% 34%	2088
	11th to 20th	Night	16% 8% 3 7% 4 4 5% 4 5% 8% 36%	1969
	21st to end	Night	16% 9% 2 7% 4 4 4 4 5% 7% 38%	1800
May	1st to 10th	Night	25% 11% 3 8% 5% 4 4 3 6% 6% 25%	4033
	11th to 20th	Night	17% 9% 3 8% 4 4 4 3 5% 7% 36%	4032
	21st to end	Night	19% 7% 3 7% 4 4 4 4 5% 8% 35%	4020
June	1st to 10th	Night	10% 7% 2 6% 5% 4 4 4 6% 9% 44%	1600
	11th to 20th	Night	12% 6% 3 7% 5% 5% 5% 4 6% 8% 40%	1600
	21st to end	Night	11% 4 2 4 4 5% 5% 5% 8% 10% 41%	1600
July	1st to 10th	Night	11% 7% 3 8% 5% 5% 4 3 6% 8% 41%	3770
	11th to 20th	Night	12% 6% 3 7% 5% 5% 6% 5% 7% 10% 35%	4057
	21st to end	Night	9% 7% 3 7% 5% 5% 5% 4 7% 9% 41%	4427
August	1st to 10th	Night	11% 9% 3 8% 5% 6% 4 4 7% 7% 34%	2915
	11th to 20th	Night	13% 7% 2 7% 4 5% 4 4 5% 7% 43%	2875
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 4 6% 9% 44%	3421
September	1st to 10th	Night	13% 4 1 4 4 4 5% 3 6% 7% 49%	2000
	11th to 20th	Night	10% 4 2 5% 4 3 4 3 5% 7% 53%	2180
	21st to end	Night	16% 8% 4 7% 4 4 4 3 5% 6% 40%	2207
October	1st to 10th	Night	14% 6% 2 7% 4 4 4 4 5% 6% 45%	2400
	11th to 20th	Night	17% 7% 2 6% 5% 5% 5% 3 5% 6% 40%	2400
	21st to end	Night	22% 8% 3 6% 3 3 4 3 4 6% 38%	2640
November	1st to 10th	Night	13% 8% 3 7% 4 4 4 3 5% 5% 44%	2529
	11th to 20th	Night	18% 12% 4 7% 5% 4 4 4 5% 6% 32%	2875
	21st to end	Night	21% 14% 4 7% 4 3 4 4 5% 5% 30%	2940
December	1st to 10th	Night	19% 15% 10% 13% 5% 3 4 3 5% 5% 18%	4720
	11th to 20th	Night	22% 17% 6% 10% 4 4 3 2 4 4 24%	5345
	21st to end	Night	23% 17% 5% 11% 5% 5% 4 3 5% 5% 17%	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	74% 5% 9% 9% 3	5236
	11th to 20th	Day	75% 4 7% 9% 4 1	5280
	21st to end	Day	69% 5% 9% 14% 4	5780
February	1st to 10th	Day	65% 3 9% 15% 6%	1890
	11th to 20th	Day	63% 4 10% 16% 6%	1852
	21st to end	Day	61% 3 13% 15% 7% 1	1735
March	1st to 10th	Day	54% 5% 10% 23% 7% 1	4957
	11th to 20th	Day	48% 5% 12% 26% 9% 2	4980
	21st to end	Day	53% 5% 9% 20% 12% 1	5465
April	1st to 10th	Day	53% 5% 11% 19% 11% 2	2310
	11th to 20th	Day	49% 5% 12% 18% 13% 2	2397
	21st to end	Day	50% 5% 10% 24% 10% 2	2600
May	1st to 10th	Day	53% 7% 12% 18% 8% 2	5897
	11th to 20th	Day	49% 7% 12% 22% 9% 1	5865
	21st to end	Day	45% 7% 15% 22% 9% 2	6847
June	1st to 10th	Day	43% 7% 14% 23% 11% 1	2800
	11th to 20th	Day	44% 7% 14% 22% 12% 1	2800
	21st to end	Day	41% 6% 14% 26% 13% 1	2800
July	1st to 10th	Day	39% 7% 10% 26% 16% 2	6116
	11th to 20th	Day	43% 9% 14% 23% 11%	5831
	21st to end	Day	41% 7% 10% 22% 19% 1	6240
August	1st to 10th	Day	48% 8% 14% 22% 9%	4221
	11th to 20th	Day	40% 8% 12% 27% 12% 1	4213
	21st to end	Day	35% 8% 13% 28% 14% 2	4444
September	1st to 10th	Day	37% 7% 11% 22% 20% 3	2400
	11th to 20th	Day	36% 5% 13% 29% 16% 1	2215
	21st to end	Day	50% 4 10% 22% 14% 1	2191
October	1st to 10th	Day	32% 5% 11% 31% 20% 1	3260
	11th to 20th	Day	42% 7% 10% 22% 14% 5%	3240
	21st to end	Day	36% 4 12% 28% 18% 1	3538
November	1st to 10th	Day	42% 5% 12% 22% 16% 3	1983
	11th to 20th	Day	54% 4 10% 19% 11% 2	1743
	21st to end	Day	60% 4 13% 17% 6% 1	1680
December	1st to 10th	Day	68% 4 13% 11% 4	2680
	11th to 20th	Day	66% 3 13% 14% 4	3066
	21st to end	Day	71% 4 9% 11% 4	4048
January	1st to 10th	Night	72% 5% 9% 11% 3	7654
	11th to 20th	Night	71% 5% 8% 11% 5% 1	7680
	21st to end	Night	65% 5% 9% 15% 6% 1	8401
February	1st to 10th	Night	61% 4 9% 19% 7%	2730
	11th to 20th	Night	62% 4 9% 18% 7%	2665
	21st to end	Night	61% 4 9% 16% 9% 1	2067
March	1st to 10th	Night	54% 4 9% 23% 9% 1	4941
	11th to 20th	Night	43% 4 13% 28% 9% 3	4980
	21st to end	Night	50% 5% 10% 19% 15% 1	5467
April	1st to 10th	Night	52% 4 9% 20% 13% 2	2088
	11th to 20th	Night	46% 5% 11% 20% 15% 3	1969
	21st to end	Night	46% 5% 9% 25% 12% 3	1800
May	1st to 10th	Night	59% 6% 9% 14% 9% 3	4060
	11th to 20th	Night	47% 6% 10% 25% 11% 1	4033
	21st to end	Night	46% 6% 13% 22% 10% 3	4027
June	1st to 10th	Night	37% 7% 12% 25% 16% 2	1600
	11th to 20th	Night	41% 6% 13% 25% 14% 2	1600
	21st to end	Night	35% 9% 14% 24% 17% 2	1600
July	1st to 10th	Night	41% 6% 9% 24% 18% 2	3729
	11th to 20th	Night	40% 10% 17% 20% 11% 1	4006
	21st to end	Night	38% 7% 11% 19% 22% 2	4297
August	1st to 10th	Night	45% 8% 13% 22% 10% 1	2904
	11th to 20th	Night	40% 5% 11% 27% 16% 1	2902
	21st to end	Night	35% 8% 13% 30% 13% 2	3421
September	1st to 10th	Night	33% 6% 12% 23% 21% 5%	2000
	11th to 20th	Night	30% 6% 12% 28% 22% 1	2180
	21st to end	Night	46% 4 10% 22% 17% 1	2207
October	1st to 10th	Night	34% 5% 9% 27% 23% 2	3920
	11th to 20th	Night	37% 6% 10% 25% 15% 7%	3899
	21st to end	Night	35% 3 9% 29% 22% 2	4252
November	1st to 10th	Night	41% 4 11% 22% 19% 3	2529
	11th to 20th	Night	53% 4 9% 21% 11% 2	2875
	21st to end	Night	57% 4 10% 22% 7% 1	2940
December	1st to 10th	Night	67% 4 11% 14% 3	4720
	11th to 20th	Night	65% 4 14% 15% 3	5348
	21st to end	Night	69% 5% 10% 13% 3	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	28% 25% 25% 15% 5% 2	325
	11th to 20th	Day	31% 27% 28% 11% 2 1	330
	21st to end	Day	28% 21% 33% 14% 3 1	363
February	1st to 10th	Day	30% 18% 33% 15% 3	330
	11th to 20th	Day	32% 25% 28% 12% 2	330
	21st to end	Day	31% 19% 37% 12% 2	315
March	1st to 10th	Day	26% 19% 32% 20% 4	508
	11th to 20th	Day	29% 17% 33% 14% 5% 1	500
	21st to end	Day	30% 17% 29% 18% 5% 1	550
April	1st to 10th	Day	29% 17% 26% 20% 7% 2	500
	11th to 20th	Day	22% 18% 31% 16% 9% 4	492
	21st to end	Day	27% 14% 28% 20% 9% 2	500
May	1st to 10th	Day	27% 13% 27% 20% 10% 3	503
	11th to 20th	Day	21% 12% 28% 20% 13% 6%	527
	21st to end	Day	17% 7% 25% 25% 20% 7%	573
June	1st to 10th	Day	17% 8% 28% 26% 17% 5%	530
	11th to 20th	Day	18% 7% 27% 24% 19% 5%	530
	21st to end	Day	14% 8% 27% 27% 19% 5%	530
July	1st to 10th	Day	13% 4 19% 30% 22% 13%	525
	11th to 20th	Day	11% 7% 23% 28% 22% 9%	524
	21st to end	Day	13% 6% 21% 31% 22% 8%	576
August	1st to 10th	Day	14% 4 21% 28% 24% 9%	526
	11th to 20th	Day	15% 6% 18% 27% 23% 11%	519
	21st to end	Day	11% 4 22% 28% 23% 12%	583
September	1st to 10th	Day	16% 7% 18% 28% 20% 10%	530
	11th to 20th	Day	12% 3 15% 31% 26% 13%	530
	21st to end	Day	24% 11% 21% 21% 15% 7%	530
October	1st to 10th	Day	17% 7% 25% 24% 21% 5%	530
	11th to 20th	Day	20% 15% 28% 23% 11% 2	530
	21st to end	Day	27% 15% 29% 20% 7% 1	583
November	1st to 10th	Day	20% 17% 30% 21% 10% 2	461
	11th to 20th	Day	32% 17% 29% 14% 6% 2	253
	21st to end	Day	27% 18% 41% 11% 3	220
December	1st to 10th	Day	31% 24% 28% 12% 4 1	220
	11th to 20th	Day	32% 16% 36% 12% 4	220
	21st to end	Day	33% 18% 30% 14% 5%	242
January	1st to 10th	Night	28% 17% 33% 18% 4	840
	11th to 20th	Night	31% 14% 32% 20% 3	840
	21st to end	Night	28% 15% 31% 23% 3	924
February	1st to 10th	Night	27% 15% 28% 28% 3	840
	11th to 20th	Night	28% 15% 35% 20% 2	823
	21st to end	Night	28% 11% 37% 21% 3	692
March	1st to 10th	Night	24% 15% 30% 27% 4	809
	11th to 20th	Night	25% 15% 29% 24% 6% 1	800
	21st to end	Night	25% 12% 33% 24% 5% 1	880
April	1st to 10th	Night	26% 13% 30% 24% 6%	800
	11th to 20th	Night	21% 10% 32% 27% 10%	785
	21st to end	Night	19% 8% 32% 35% 6%	600
May	1st to 10th	Night	24% 8% 21% 35% 11% 1	600
	11th to 20th	Night	21% 7% 25% 34% 13%	591
	21st to end	Night	12% 3 21% 38% 23% 3	482
June	1st to 10th	Night	12% 5% 19% 46% 18% 1	400
	11th to 20th	Night	14% 5% 22% 40% 20% 1	400
	21st to end	Night	12% 3 25% 38% 21% 2	400
July	1st to 10th	Night	9% 2 18% 43% 22% 5%	455
	11th to 20th	Night	10% 4 19% 33% 30% 4	582
	21st to end	Night	10% 3 15% 40% 30% 2	657
August	1st to 10th	Night	10% 2 15% 41% 29% 3	597
	11th to 20th	Night	12% 3 13% 38% 30% 4	594
	21st to end	Night	13% 3 16% 35% 28% 4	780
September	1st to 10th	Night	13% 5% 17% 34% 28% 3	800
	11th to 20th	Night	11% 5% 13% 36% 32% 4	800
	21st to end	Night	20% 7% 22% 28% 21% 3	800
October	1st to 10th	Night	17% 6% 19% 31% 24% 2	800
	11th to 20th	Night	21% 8% 28% 31% 12% 1	800
	21st to end	Night	26% 10% 24% 31% 8% 1	880
November	1st to 10th	Night	17% 10% 26% 34% 12% 1	820
	11th to 20th	Night	26% 13% 28% 25% 8% 1	840
	21st to end	Night	29% 13% 28% 25% 5%	840
December	1st to 10th	Night	30% 15% 29% 21% 6%	840
	11th to 20th	Night	28% 15% 29% 24% 4	840
	21st to end	Night	29% 13% 30% 26% 2	924

Page generated at 2026-02-28 01:40:25. Version ac07

Copyright © 2026 Attilla Danko.