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 2025-04-26.

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% 7% 38%	113720
	Night	14% 8% 3 7% 5% 4 4 4 5% 7% 39%	114879
Transparency	Day	42% 7% 13% 22% 14% 2	117359
	Night	41% 7% 12% 22% 14% 3	119379
Seeing	Day	20% 14% 26% 22% 13% 4	15400
	Night	18% 12% 27% 30% 12% 1	25035

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% 11% 4 13% 6% 5% 3 3 4 5% 34%	3666
	11th to 20th	Day	12% 11% 5% 11% 6% 6% 4 3 5% 6% 30%	4060
	21st to end	Day	15% 12% 4 9% 5% 4 4 3 5% 6% 34%	4466
February	1st to 10th	Day	14% 12% 3 8% 6% 5% 4 4 5% 7% 33%	1800
	11th to 20th	Day	14% 11% 3 8% 6% 5% 4 3 6% 6% 34%	1762
	21st to end	Day	17% 11% 3 8% 4 4 4 3 5% 5% 35%	1650
March	1st to 10th	Day	17% 9% 3 6% 5% 4 4 3 5% 5% 39%	4891
	11th to 20th	Day	10% 7% 2 6% 4 5% 4 4 6% 8% 43%	4980
	21st to end	Day	14% 9% 3 7% 5% 4 5% 4 5% 6% 38%	5510
April	1st to 10th	Day	16% 10% 2 6% 6% 4 5% 4 6% 6% 35%	2400
	11th to 20th	Day	13% 8% 3 8% 4 5% 4 3 5% 6% 41%	2374
	21st to end	Day	18% 7% 2 7% 4 4 4 4 6% 6% 38%	2367
May	1st to 10th	Day	26% 7% 2 6% 4 4 4 4 5% 7% 29%	4082
	11th to 20th	Day	22% 8% 3 6% 5% 5% 4 4 6% 7% 29%	4297
	21st to end	Day	11% 5% 2 5% 4 5% 5% 4 7% 8% 42%	4823
June	1st to 10th	Day	12% 6% 2 5% 5% 5% 5% 4 7% 10% 39%	2660
	11th to 20th	Day	12% 6% 2 6% 5% 6% 5% 4 7% 9% 38%	2660
	21st to end	Day	10% 5% 1 6% 4 5% 4 4 7% 10% 43%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 6% 7% 5% 7% 11% 41%	4440
	11th to 20th	Day	7% 6% 2 6% 5% 5% 5% 5% 8% 10% 41%	4328
	21st to end	Day	11% 8% 3 7% 5% 5% 5% 4 6% 8% 38%	4466
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 9% 33%	4104
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 4 7% 9% 40%	3953
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 8% 46%	4180
September	1st to 10th	Day	15% 5% 2 6% 4 4 4 4 5% 6% 45%	2280
	11th to 20th	Day	11% 5% 2 5% 3 4 4 4 6% 8% 48%	2280
	21st to end	Day	19% 8% 2 6% 4 4 4 3 5% 7% 38%	1983
October	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 4 7% 44%	1900
	11th to 20th	Day	15% 5% 3 6% 4 4 4 4 5% 7% 44%	1900
	21st to end	Day	15% 7% 2 5% 4 3 4 4 5% 7% 44%	2090
November	1st to 10th	Day	11% 7% 3 5% 4 3 4 3 4 8% 48%	1853
	11th to 20th	Day	14% 8% 3 6% 4 4 4 4 5% 6% 43%	1705
	21st to end	Day	15% 8% 3 6% 6% 4 5% 4 4 7% 38%	1600
December	1st to 10th	Day	14% 7% 5% 7% 5% 5% 5% 5% 7% 7% 34%	2600
	11th to 20th	Day	20% 13% 3 5% 4 4 4 4 5% 5% 33%	2990
	21st to end	Day	16% 12% 4 9% 5% 5% 5% 4 6% 7% 26%	3960
January	1st to 10th	Night	10% 10% 4 13% 6% 5% 5% 3 5% 6% 33%	6276
	11th to 20th	Night	12% 9% 5% 10% 6% 5% 4 3 4 6% 36%	5900
	21st to end	Night	16% 9% 3 8% 5% 4 4 3 5% 7% 38%	6490
February	1st to 10th	Night	16% 9% 3 7% 5% 4 5% 3 6% 6% 36%	2600
	11th to 20th	Night	16% 10% 3 7% 5% 4 4 4 6% 6% 34%	2523
	21st to end	Night	16% 10% 3 9% 4 4 4 3 5% 7% 34%	1980
March	1st to 10th	Night	17% 9% 3 6% 4 4 3 3 4 6% 42%	4947
	11th to 20th	Night	10% 7% 3 7% 4 4 4 3 5% 6% 47%	4980
	21st to end	Night	11% 7% 3 7% 5% 5% 5% 4 5% 6% 43%	5422
April	1st to 10th	Night	17% 10% 3 8% 5% 4 5% 4 6% 5% 33%	2000
	11th to 20th	Night	13% 7% 2 6% 5% 4 4 3 6% 9% 40%	1975
	21st to end	Night	17% 5% 2 5% 5% 4 5% 4 6% 7% 41%	1946
May	1st to 10th	Night	26% 8% 3 7% 4 4 4 3 5% 7% 30%	2813
	11th to 20th	Night	22% 8% 4 7% 5% 4 5% 3 5% 7% 31%	2595
	21st to end	Night	11% 6% 2 6% 4 5% 5% 5% 7% 10% 39%	2732
June	1st to 10th	Night	11% 6% 2 5% 4 4 4 4 6% 7% 46%	1520
	11th to 20th	Night	11% 5% 2 7% 4 5% 5% 5% 5% 9% 40%	1520
	21st to end	Night	9% 5% 2 6% 4 4 5% 3 7% 9% 45%	1520
July	1st to 10th	Night	7% 5% 2 5% 5% 5% 6% 5% 6% 11% 42%	2520
	11th to 20th	Night	6% 6% 2 7% 5% 5% 6% 5% 8% 11% 40%	2614
	21st to end	Night	11% 8% 2 7% 5% 5% 4 4 6% 8% 39%	3087
August	1st to 10th	Night	14% 8% 3 7% 5% 4 5% 5% 7% 9% 34%	2825
	11th to 20th	Night	11% 7% 2 6% 4 5% 4 4 6% 7% 42%	2844
	21st to end	Night	11% 5% 2 6% 4 5% 4 4 5% 9% 46%	3476
September	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 7% 47%	1900
	11th to 20th	Night	9% 6% 2 5% 4 3 3 3 4 7% 53%	1900
	21st to end	Night	17% 7% 3 5% 4 4 5% 4 5% 6% 40%	2193
October	1st to 10th	Night	16% 6% 3 6% 4 4 4 3 5% 6% 44%	2280
	11th to 20th	Night	13% 5% 2 5% 4 5% 4 3 5% 6% 47%	2280
	21st to end	Night	15% 6% 2 5% 4 4 4 4 6% 7% 44%	2508
November	1st to 10th	Night	9% 7% 2 7% 3 4 4 3 4 5% 52%	2442
	11th to 20th	Night	14% 8% 3 6% 4 4 4 3 6% 6% 42%	2690
	21st to end	Night	14% 7% 3 7% 6% 4 5% 4 6% 8% 38%	2800
December	1st to 10th	Night	10% 7% 3 8% 5% 5% 5% 4 6% 6% 39%	4580
	11th to 20th	Night	20% 9% 3 7% 5% 5% 4 4 5% 6% 32%	5205
	21st to end	Night	17% 10% 4 9% 6% 6% 5% 4 6% 6% 27%	6996

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	50% 6% 10% 21% 11% 2	3667
	11th to 20th	Day	48% 8% 10% 16% 15% 3	4060
	21st to end	Day	49% 7% 10% 20% 12% 1	4466
February	1st to 10th	Day	50% 6% 12% 21% 10% 1	1800
	11th to 20th	Day	48% 7% 11% 22% 11% 1	1771
	21st to end	Day	50% 5% 12% 19% 10% 3	1650
March	1st to 10th	Day	44% 7% 10% 21% 13% 5%	4891
	11th to 20th	Day	35% 7% 15% 26% 12% 5%	4980
	21st to end	Day	45% 6% 10% 18% 17% 3	5510
April	1st to 10th	Day	47% 6% 12% 17% 14% 3	2400
	11th to 20th	Day	42% 7% 10% 22% 16% 3	2374
	21st to end	Day	45% 7% 9% 22% 15% 2	2367
May	1st to 10th	Day	52% 9% 9% 19% 10% 1	4120
	11th to 20th	Day	52% 9% 13% 17% 7% 2	4301
	21st to end	Day	34% 10% 15% 23% 16% 2	4837
June	1st to 10th	Day	38% 8% 15% 23% 15% 1	2660
	11th to 20th	Day	39% 10% 15% 20% 13% 1	2660
	21st to end	Day	34% 8% 15% 28% 13% 1	2660
July	1st to 10th	Day	36% 9% 16% 25% 12% 3	4370
	11th to 20th	Day	35% 8% 19% 25% 13% 1	4253
	21st to end	Day	43% 7% 10% 21% 17% 1	4324
August	1st to 10th	Day	45% 8% 15% 23% 7%	4091
	11th to 20th	Day	38% 7% 13% 29% 13% 1	3992
	21st to end	Day	35% 6% 13% 27% 17% 2	4180
September	1st to 10th	Day	39% 7% 11% 20% 20% 3	2280
	11th to 20th	Day	33% 8% 11% 25% 20% 3	2280
	21st to end	Day	46% 5% 11% 17% 19% 2	1983
October	1st to 10th	Day	36% 4 9% 22% 23% 4	3160
	11th to 20th	Day	30% 6% 14% 27% 16% 8%	3140
	21st to end	Day	27% 4 13% 29% 26% 2	3428
November	1st to 10th	Day	35% 5% 14% 23% 20% 4	1853
	11th to 20th	Day	41% 6% 11% 22% 17% 3	1705
	21st to end	Day	44% 6% 12% 26% 10% 1	1600
December	1st to 10th	Day	45% 7% 18% 23% 5% 2	2600
	11th to 20th	Day	50% 5% 16% 22% 7% 1	2986
	21st to end	Day	51% 9% 15% 15% 9% 1	3960
January	1st to 10th	Night	46% 8% 11% 21% 13% 2	6276
	11th to 20th	Night	45% 7% 12% 19% 12% 5%	5900
	21st to end	Night	43% 6% 12% 24% 13% 2	6490
February	1st to 10th	Night	47% 6% 13% 22% 11% 1	2600
	11th to 20th	Night	48% 6% 10% 18% 15% 2	2525
	21st to end	Night	48% 6% 12% 18% 13% 4	1980
March	1st to 10th	Night	42% 6% 10% 25% 11% 6%	4947
	11th to 20th	Night	34% 6% 14% 29% 11% 5%	4980
	21st to end	Night	40% 6% 11% 19% 21% 3	5422
April	1st to 10th	Night	49% 7% 9% 17% 15% 3	2000
	11th to 20th	Night	40% 7% 11% 21% 17% 5%	1975
	21st to end	Night	39% 8% 12% 23% 15% 4	1946
May	1st to 10th	Night	54% 7% 8% 17% 12% 3	2840
	11th to 20th	Night	52% 10% 12% 16% 9% 1	2596
	21st to end	Night	35% 12% 16% 26% 9% 2	2739
June	1st to 10th	Night	34% 9% 11% 27% 16% 3	1520
	11th to 20th	Night	38% 9% 13% 25% 13% 2	1520
	21st to end	Night	34% 9% 15% 25% 16% 2	1520
July	1st to 10th	Night	36% 8% 19% 18% 14% 5%	2480
	11th to 20th	Night	35% 9% 17% 23% 15% 2	2566
	21st to end	Night	40% 8% 14% 18% 18% 1	2988
August	1st to 10th	Night	45% 8% 17% 21% 8%	2814
	11th to 20th	Night	37% 9% 12% 27% 15% 1	2871
	21st to end	Night	34% 7% 14% 28% 15% 2	3476
September	1st to 10th	Night	36% 8% 12% 21% 19% 4	1900
	11th to 20th	Night	31% 5% 11% 28% 22% 3	1900
	21st to end	Night	43% 7% 12% 20% 17% 2	2193
October	1st to 10th	Night	38% 5% 11% 23% 22% 1	3800
	11th to 20th	Night	29% 5% 9% 26% 23% 7%	3779
	21st to end	Night	27% 4 12% 30% 24% 3	4120
November	1st to 10th	Night	33% 6% 10% 26% 21% 4	2442
	11th to 20th	Night	42% 5% 10% 23% 16% 4	2690
	21st to end	Night	42% 6% 13% 23% 14% 2	2800
December	1st to 10th	Night	42% 6% 16% 27% 8%	4580
	11th to 20th	Night	49% 6% 15% 19% 10% 1	5208
	21st to end	Night	51% 9% 13% 16% 11%	6996

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	30% 26% 28% 12% 3 1	215
	11th to 20th	Day	34% 23% 31% 10% 2	300
	21st to end	Day	25% 28% 31% 14% 3	330
February	1st to 10th	Day	30% 25% 31% 10% 3	300
	11th to 20th	Day	39% 26% 30% 4 1	309
	21st to end	Day	31% 20% 32% 16% 1	414
March	1st to 10th	Day	28% 22% 27% 20% 3	500
	11th to 20th	Day	27% 24% 29% 14% 6% 1	500
	21st to end	Day	28% 21% 29% 17% 4 1	550
April	1st to 10th	Day	28% 21% 27% 18% 4 2	500
	11th to 20th	Day	21% 21% 29% 16% 10% 3	492
	21st to end	Day	26% 16% 31% 16% 8% 2	480
May	1st to 10th	Day	28% 18% 26% 16% 8% 3	463
	11th to 20th	Day	24% 15% 25% 22% 10% 3	487
	21st to end	Day	17% 11% 23% 26% 16% 7%	529
June	1st to 10th	Day	16% 12% 27% 22% 18% 5%	490
	11th to 20th	Day	14% 10% 29% 27% 16% 4	490
	21st to end	Day	10% 10% 31% 26% 18% 4	490
July	1st to 10th	Day	9% 5% 22% 31% 23% 10%	485
	11th to 20th	Day	6% 3 26% 31% 25% 9%	484
	21st to end	Day	14% 5% 22% 30% 23% 6%	532
August	1st to 10th	Day	10% 4 21% 31% 25% 9%	486
	11th to 20th	Day	11% 5% 20% 32% 22% 10%	479
	21st to end	Day	12% 5% 20% 30% 22% 12%	539
September	1st to 10th	Day	18% 7% 19% 24% 21% 10%	490
	11th to 20th	Day	12% 6% 19% 31% 22% 11%	490
	21st to end	Day	27% 11% 23% 22% 14% 5%	490
October	1st to 10th	Day	20% 9% 23% 24% 18% 5%	490
	11th to 20th	Day	18% 17% 25% 25% 12% 3	490
	21st to end	Day	17% 16% 32% 24% 9% 2	539
November	1st to 10th	Day	15% 13% 31% 26% 13% 2	442
	11th to 20th	Day	24% 17% 28% 22% 7% 1	305
	21st to end	Day	22% 25% 30% 19% 5%	200
December	1st to 10th	Day	28% 22% 32% 13% 5% 1	200
	11th to 20th	Day	30% 20% 32% 14% 6%	200
	21st to end	Day	25% 17% 32% 20% 6%	220
January	1st to 10th	Night	18% 22% 38% 20% 3	800
	11th to 20th	Night	22% 17% 33% 25% 3	800
	21st to end	Night	18% 20% 38% 23% 1	880
February	1st to 10th	Night	20% 20% 32% 27% 2	800
	11th to 20th	Night	23% 25% 32% 19% 1	783
	21st to end	Night	22% 21% 35% 19% 3	660
March	1st to 10th	Night	21% 18% 36% 21% 4	800
	11th to 20th	Night	26% 18% 28% 23% 6%	800
	21st to end	Night	21% 16% 36% 21% 5% 1	880
April	1st to 10th	Night	22% 19% 31% 23% 5%	800
	11th to 20th	Night	17% 15% 30% 30% 8%	791
	21st to end	Night	20% 14% 29% 28% 9%	772
May	1st to 10th	Night	22% 10% 25% 28% 15% 1	570
	11th to 20th	Night	19% 7% 27% 37% 10%	435
	21st to end	Night	14% 4 22% 36% 21% 2	411
June	1st to 10th	Night	13% 3 24% 37% 22% 1	380
	11th to 20th	Night	13% 5% 22% 41% 19%	380
	21st to end	Night	11% 3 21% 46% 20%	380
July	1st to 10th	Night	11% 2 19% 37% 28% 3	370
	11th to 20th	Night	8% 1 15% 43% 31% 2	423
	21st to end	Night	9% 2 15% 42% 28% 3	624
August	1st to 10th	Night	11% 2 11% 44% 29% 3	567
	11th to 20th	Night	11% 4 12% 40% 30% 4	597
	21st to end	Night	12% 3 15% 39% 28% 4	836
September	1st to 10th	Night	16% 3 18% 34% 26% 3	760
	11th to 20th	Night	12% 4 16% 35% 28% 5%	760
	21st to end	Night	21% 5% 21% 28% 21% 3	760
October	1st to 10th	Night	21% 6% 21% 28% 21% 3	760
	11th to 20th	Night	17% 11% 25% 34% 11% 1	760
	21st to end	Night	19% 10% 29% 31% 10%	836
November	1st to 10th	Night	13% 10% 29% 36% 11% 1	780
	11th to 20th	Night	19% 19% 25% 29% 7% 1	800
	21st to end	Night	18% 18% 30% 28% 6% 1	800
December	1st to 10th	Night	22% 16% 33% 23% 6%	800
	11th to 20th	Night	22% 15% 31% 28% 3	800
	21st to end	Night	19% 16% 32% 30% 3	880

Page generated at 2025-04-23 09:25:07. Version ac07

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