Chambersburg
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chambersburg clear sky chart from 2005-11-06 to 2026-08-30.

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% 8% 3 7% 5% 5% 5% 4 6% 8% 37%	144094
	Night	13% 8% 3 7% 5% 5% 4 4 5% 7% 38%	138104
Transparency	Day	42% 7% 13% 22% 14% 2	147690
	Night	42% 7% 12% 23% 14% 2	142579
Seeing	Day	20% 14% 27% 22% 13% 4	17204
	Night	18% 12% 27% 30% 12% 1	26645

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% 10% 4 13% 7% 4 3 3 4 5% 32%	4758
	11th to 20th	Day	10% 11% 5% 10% 7% 6% 5% 3 5% 6% 32%	5280
	21st to end	Day	13% 9% 3 9% 6% 5% 5% 4 6% 6% 34%	5780
February	1st to 10th	Day	14% 11% 3 7% 6% 5% 4 4 5% 7% 34%	1890
	11th to 20th	Day	14% 11% 4 8% 6% 4 4 3 6% 6% 34%	1853
	21st to end	Day	17% 12% 3 9% 4 4 3 3 5% 5% 35%	1730
March	1st to 10th	Day	18% 9% 3 6% 5% 4 4 3 5% 5% 38%	5000
	11th to 20th	Day	10% 7% 2 6% 4 5% 4 4 6% 8% 43%	5090
	21st to end	Day	14% 9% 3 7% 5% 4 5% 4 5% 6% 38%	5632
April	1st to 10th	Day	16% 10% 2 6% 5% 4 5% 4 6% 6% 35%	2520
	11th to 20th	Day	13% 7% 3 7% 4 5% 4 4 5% 6% 42%	2494
	21st to end	Day	18% 7% 2 7% 4 4 4 4 6% 6% 38%	2550
May	1st to 10th	Day	19% 7% 2 7% 5% 4 5% 4 7% 9% 31%	7616
	11th to 20th	Day	17% 7% 3 7% 5% 5% 4 4 6% 9% 33%	8038
	21st to end	Day	23% 7% 2 6% 4 4 5% 4 6% 6% 34%	9045
June	1st to 10th	Day	11% 6% 2 5% 5% 5% 5% 4 7% 10% 40%	2940
	11th to 20th	Day	13% 6% 2 6% 5% 6% 5% 4 7% 10% 38%	2940
	21st to end	Day	11% 5% 1 6% 4 4 4 4 7% 10% 43%	2940
July	1st to 10th	Day	8% 6% 3 7% 5% 5% 6% 4 7% 9% 40%	8280
	11th to 20th	Day	6% 6% 3 7% 6% 6% 7% 6% 8% 11% 34%	8084
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 7% 10% 39%	8340
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 9% 33%	4364
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 4 8% 9% 39%	4207
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 8% 46%	4420
September	1st to 10th	Day	15% 5% 2 6% 4 4 4 3 4 6% 46%	2400
	11th to 20th	Day	11% 5% 2 5% 3 4 4 4 6% 8% 47%	2400
	21st to end	Day	20% 9% 2 6% 4 4 4 3 5% 7% 36%	2087
October	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 4 6% 45%	2000
	11th to 20th	Day	14% 5% 3 6% 4 4 4 4 5% 7% 44%	2000
	21st to end	Day	14% 7% 2 5% 4 4 4 4 5% 7% 44%	2200
November	1st to 10th	Day	11% 6% 3 5% 4 3 4 3 5% 8% 48%	1948
	11th to 20th	Day	13% 8% 3 6% 4 5% 4 4 5% 6% 42%	1790
	21st to end	Day	15% 8% 3 6% 6% 5% 5% 4 4 7% 38%	1680
December	1st to 10th	Day	14% 7% 5% 8% 5% 5% 5% 5% 7% 7% 34%	2680
	11th to 20th	Day	20% 13% 3 6% 4 4 4 4 5% 6% 33%	3070
	21st to end	Day	16% 12% 4 9% 5% 5% 5% 4 6% 7% 26%	4048
January	1st to 10th	Night	10% 12% 5% 13% 6% 5% 5% 4 5% 7% 31%	8132
	11th to 20th	Night	11% 9% 5% 10% 6% 5% 5% 3 5% 6% 36%	7680
	21st to end	Night	14% 8% 2 7% 5% 5% 4 4 6% 8% 38%	8401
February	1st to 10th	Night	15% 9% 3 7% 5% 4 5% 3 6% 6% 36%	2730
	11th to 20th	Night	16% 10% 4 8% 5% 5% 4 4 5% 6% 33%	2652
	21st to end	Night	16% 10% 3 9% 4 4 4 3 5% 7% 35%	2076
March	1st to 10th	Night	17% 9% 3 6% 4 4 3 3 4 6% 42%	5058
	11th to 20th	Night	10% 7% 3 7% 4 4 4 3 5% 6% 47%	5090
	21st to end	Night	11% 7% 3 7% 5% 5% 5% 4 5% 6% 43%	5542
April	1st to 10th	Night	17% 10% 3 8% 5% 4 5% 4 5% 5% 34%	2100
	11th to 20th	Night	13% 7% 2 6% 5% 5% 4 3 6% 9% 40%	2075
	21st to end	Night	16% 5% 2 5% 5% 4 5% 4 6% 7% 41%	2087
May	1st to 10th	Night	20% 8% 3 8% 5% 4 5% 4 6% 8% 31%	5237
	11th to 20th	Night	17% 8% 3 6% 4 3 4 4 6% 9% 34%	4821
	21st to end	Night	23% 8% 3 6% 4 4 4 4 5% 7% 33%	5108
June	1st to 10th	Night	11% 6% 2 5% 4 4 4 4 6% 7% 47%	1680
	11th to 20th	Night	12% 5% 2 7% 5% 5% 5% 5% 5% 9% 39%	1680
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 7% 9% 44%	1680
July	1st to 10th	Night	9% 7% 3 7% 5% 5% 5% 4 6% 9% 41%	4678
	11th to 20th	Night	8% 8% 3 9% 6% 5% 5% 5% 7% 9% 35%	4855
	21st to end	Night	8% 6% 2 7% 5% 5% 5% 4 7% 8% 41%	5736
August	1st to 10th	Night	14% 8% 2 8% 5% 4 5% 5% 7% 9% 34%	3005
	11th to 20th	Night	11% 7% 2 7% 4 5% 4 4 6% 8% 42%	3028
	21st to end	Night	11% 5% 2 6% 4 5% 4 4 5% 9% 46%	3677
September	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 7% 47%	2000
	11th to 20th	Night	9% 6% 2 5% 4 3 4 3 4 7% 52%	2000
	21st to end	Night	17% 8% 3 6% 4 4 5% 4 5% 6% 38%	2309
October	1st to 10th	Night	16% 5% 2 5% 4 3 4 3 5% 6% 45%	2400
	11th to 20th	Night	14% 5% 2 5% 4 5% 4 3 5% 6% 48%	2400
	21st to end	Night	15% 6% 2 5% 4 4 4 3 5% 7% 45%	2640
November	1st to 10th	Night	9% 7% 2 7% 4 4 4 3 4 6% 51%	2567
	11th to 20th	Night	14% 8% 3 6% 4 4 4 3 6% 6% 42%	2825
	21st to end	Night	14% 7% 3 7% 6% 4 5% 4 5% 8% 38%	2940
December	1st to 10th	Night	10% 7% 3 8% 5% 5% 5% 4 6% 7% 39%	4720
	11th to 20th	Night	19% 9% 3 7% 5% 5% 4 4 5% 6% 32%	5345
	21st to end	Night	17% 10% 4 10% 6% 6% 5% 4 5% 6% 27%	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	52% 5% 11% 20% 10% 2	4759
	11th to 20th	Day	46% 7% 11% 17% 14% 5%	5280
	21st to end	Day	47% 6% 12% 23% 10% 1	5780
February	1st to 10th	Day	48% 6% 12% 21% 11% 1	1890
	11th to 20th	Day	49% 6% 11% 21% 11% 1	1862
	21st to end	Day	50% 5% 12% 20% 10% 3	1730
March	1st to 10th	Day	44% 7% 10% 21% 13% 5%	5000
	11th to 20th	Day	36% 7% 15% 26% 12% 5%	5090
	21st to end	Day	45% 6% 10% 18% 16% 3	5632
April	1st to 10th	Day	46% 6% 13% 17% 14% 3	2520
	11th to 20th	Day	41% 7% 11% 21% 16% 3	2494
	21st to end	Day	44% 7% 9% 22% 15% 2	2550
May	1st to 10th	Day	48% 8% 11% 21% 11% 1	7651
	11th to 20th	Day	47% 8% 12% 24% 9% 1	8042
	21st to end	Day	47% 7% 11% 17% 16% 1	9061
June	1st to 10th	Day	38% 8% 14% 22% 16% 1	2940
	11th to 20th	Day	40% 10% 15% 20% 14% 1	2940
	21st to end	Day	35% 8% 16% 27% 14% 1	2940
July	1st to 10th	Day	40% 9% 13% 23% 14% 2	8210
	11th to 20th	Day	39% 8% 17% 25% 10% 1	8009
	21st to end	Day	39% 7% 11% 24% 18% 1	8156
August	1st to 10th	Day	45% 8% 15% 23% 8%	4351
	11th to 20th	Day	39% 7% 13% 28% 12% 1	4246
	21st to end	Day	35% 6% 13% 28% 16% 2	4420
September	1st to 10th	Day	38% 7% 11% 21% 20% 3	2400
	11th to 20th	Day	33% 8% 11% 26% 19% 3	2400
	21st to end	Day	48% 5% 10% 17% 18% 2	2087
October	1st to 10th	Day	36% 4 9% 23% 24% 4	3260
	11th to 20th	Day	30% 6% 13% 27% 16% 8%	3240
	21st to end	Day	27% 4 13% 28% 26% 2	3538
November	1st to 10th	Day	34% 5% 14% 23% 20% 4	1948
	11th to 20th	Day	41% 6% 11% 21% 17% 3	1790
	21st to end	Day	45% 6% 12% 26% 10% 1	1680
December	1st to 10th	Day	45% 7% 18% 23% 5% 2	2680
	11th to 20th	Day	50% 5% 16% 22% 7% 1	3066
	21st to end	Day	51% 9% 15% 15% 9% 1	4048
January	1st to 10th	Night	48% 7% 12% 21% 10% 1	8133
	11th to 20th	Night	45% 6% 11% 18% 15% 5%	7680
	21st to end	Night	40% 6% 12% 28% 12% 1	8401
February	1st to 10th	Night	47% 6% 12% 22% 12% 1	2730
	11th to 20th	Night	49% 6% 10% 18% 15% 2	2654
	21st to end	Night	47% 6% 12% 19% 13% 3	2076
March	1st to 10th	Night	43% 6% 10% 25% 11% 6%	5058
	11th to 20th	Night	34% 6% 14% 29% 11% 5%	5090
	21st to end	Night	40% 6% 11% 19% 20% 3	5542
April	1st to 10th	Night	49% 7% 9% 18% 15% 3	2100
	11th to 20th	Night	40% 7% 11% 21% 17% 4	2075
	21st to end	Night	39% 8% 12% 23% 15% 4	2087
May	1st to 10th	Night	52% 6% 11% 18% 12% 2	5264
	11th to 20th	Night	45% 8% 13% 24% 9% 1	4822
	21st to end	Night	50% 7% 10% 22% 10% 1	5115
June	1st to 10th	Night	34% 9% 10% 27% 17% 3	1680
	11th to 20th	Night	40% 9% 13% 23% 14% 2	1680
	21st to end	Night	36% 8% 14% 25% 16% 1	1680
July	1st to 10th	Night	41% 7% 15% 20% 15% 3	4640
	11th to 20th	Night	43% 7% 14% 22% 12% 1	4810
	21st to end	Night	37% 7% 14% 22% 19% 1	5606
August	1st to 10th	Night	45% 8% 17% 21% 9% 1	2994
	11th to 20th	Night	37% 9% 13% 26% 14% 1	3055
	21st to end	Night	34% 7% 14% 29% 15% 2	3677
September	1st to 10th	Night	36% 8% 12% 21% 20% 4	2000
	11th to 20th	Night	32% 5% 12% 27% 22% 2	2000
	21st to end	Night	44% 7% 12% 19% 16% 2	2309
October	1st to 10th	Night	37% 5% 11% 23% 22% 1	3920
	11th to 20th	Night	29% 5% 9% 26% 23% 7%	3899
	21st to end	Night	27% 4 12% 30% 23% 3	4252
November	1st to 10th	Night	34% 6% 10% 26% 21% 4	2567
	11th to 20th	Night	42% 5% 10% 23% 16% 4	2825
	21st to end	Night	42% 6% 13% 24% 14% 2	2940
December	1st to 10th	Night	42% 6% 17% 27% 8%	4720
	11th to 20th	Night	49% 6% 15% 19% 10% 1	5348
	21st to end	Night	51% 9% 13% 16% 11%	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% 24% 30% 13% 3 1	237
	11th to 20th	Day	32% 25% 32% 9% 2	330
	21st to end	Day	24% 28% 32% 13% 3	363
February	1st to 10th	Day	29% 25% 31% 11% 4	330
	11th to 20th	Day	39% 24% 32% 4 1	340
	21st to end	Day	30% 19% 34% 16% 1	446
March	1st to 10th	Day	29% 21% 27% 20% 3	540
	11th to 20th	Day	27% 24% 28% 14% 6% 1	540
	21st to end	Day	27% 21% 30% 17% 4 1	594
April	1st to 10th	Day	28% 20% 27% 19% 5% 2	540
	11th to 20th	Day	19% 20% 30% 17% 10% 3	532
	21st to end	Day	26% 16% 31% 17% 8% 2	540
May	1st to 10th	Day	27% 17% 28% 17% 8% 3	543
	11th to 20th	Day	22% 15% 25% 24% 11% 3	567
	21st to end	Day	21% 11% 22% 25% 14% 6%	617
June	1st to 10th	Day	15% 11% 27% 23% 19% 5%	570
	11th to 20th	Day	15% 10% 29% 27% 14% 4	570
	21st to end	Day	12% 8% 29% 26% 19% 5%	570
July	1st to 10th	Day	10% 4 20% 30% 25% 11%	565
	11th to 20th	Day	7% 3 24% 30% 26% 10%	564
	21st to end	Day	13% 5% 21% 30% 24% 7%	620
August	1st to 10th	Day	11% 4 21% 30% 25% 10%	566
	11th to 20th	Day	13% 5% 19% 32% 21% 9%	559
	21st to end	Day	11% 4 21% 31% 21% 12%	619
September	1st to 10th	Day	17% 8% 19% 25% 21% 10%	530
	11th to 20th	Day	12% 6% 18% 31% 23% 11%	530
	21st to end	Day	28% 10% 22% 22% 13% 5%	530
October	1st to 10th	Day	19% 9% 22% 25% 20% 5%	530
	11th to 20th	Day	18% 16% 26% 25% 12% 3	530
	21st to end	Day	17% 16% 33% 24% 9% 2	583
November	1st to 10th	Day	14% 14% 33% 25% 12% 2	477
	11th to 20th	Day	24% 18% 28% 22% 7% 1	330
	21st to end	Day	21% 24% 31% 20% 4	220
December	1st to 10th	Day	28% 20% 34% 13% 4 1	220
	11th to 20th	Day	28% 20% 33% 13% 6%	220
	21st to end	Day	26% 17% 33% 19% 5%	242
January	1st to 10th	Night	18% 21% 38% 20% 3	840
	11th to 20th	Night	22% 18% 34% 24% 3	840
	21st to end	Night	18% 20% 39% 23% 1	924
February	1st to 10th	Night	19% 19% 31% 28% 2	840
	11th to 20th	Night	24% 24% 33% 19% 1	823
	21st to end	Night	22% 20% 36% 20% 2	692
March	1st to 10th	Night	22% 17% 35% 21% 4	840
	11th to 20th	Night	25% 18% 28% 23% 5%	840
	21st to end	Night	20% 16% 36% 22% 5% 1	924
April	1st to 10th	Night	23% 18% 31% 23% 5%	840
	11th to 20th	Night	17% 15% 30% 30% 8%	831
	21st to end	Night	20% 13% 29% 30% 9%	827
May	1st to 10th	Night	21% 9% 27% 28% 14% 1	630
	11th to 20th	Night	19% 6% 27% 38% 10%	481
	21st to end	Night	18% 4 22% 36% 19% 2	455
June	1st to 10th	Night	13% 3 24% 37% 22%	420
	11th to 20th	Night	14% 5% 22% 42% 17%	420
	21st to end	Night	12% 2 20% 45% 21%	420
July	1st to 10th	Night	11% 2 18% 37% 29% 3	410
	11th to 20th	Night	9% 1 13% 43% 31% 2	469
	21st to end	Night	9% 2 15% 42% 29% 3	690
August	1st to 10th	Night	11% 2 10% 44% 30% 2	627
	11th to 20th	Night	11% 3 11% 40% 30% 3	661
	21st to end	Night	12% 3 15% 39% 27% 4	917
September	1st to 10th	Night	16% 3 18% 34% 26% 3	800
	11th to 20th	Night	12% 4 15% 36% 28% 5%	800
	21st to end	Night	21% 5% 21% 30% 21% 3	800
October	1st to 10th	Night	21% 5% 20% 28% 22% 4	800
	11th to 20th	Night	17% 11% 26% 34% 11% 1	800
	21st to end	Night	19% 9% 30% 32% 10%	880
November	1st to 10th	Night	13% 11% 30% 36% 10% 1	820
	11th to 20th	Night	19% 19% 25% 29% 7% 1	840
	21st to end	Night	18% 18% 30% 28% 5%	840
December	1st to 10th	Night	22% 16% 34% 23% 6%	840
	11th to 20th	Night	22% 15% 32% 28% 3	840
	21st to end	Night	20% 16% 32% 29% 3	924

Page generated at 2026-08-27 04:07:12. Version ac07

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