C&O Canal National Historical Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the C&O Canal National Historical Park clear sky chart from 2005-11-06 to 2026-04-17.

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% 5% 5% 5% 4 6% 8% 39%	131626
	Night	14% 7% 2 6% 4 4 5% 4 6% 8% 40%	130501
Transparency	Day	41% 5% 11% 24% 16% 3	135223
	Night	41% 5% 11% 24% 16% 3	134975
Seeing	Day	21% 14% 24% 21% 14% 5%	16685
	Night	19% 12% 25% 30% 13% 1	26355

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	13% 6% 3 9% 6% 5% 5% 4 6% 6% 37%	4883
	11th to 20th	Day	11% 9% 3 8% 4 5% 5% 5% 6% 8% 36%	5280
	21st to end	Day	13% 6% 2 7% 5% 5% 6% 5% 7% 8% 36%	5780
February	1st to 10th	Day	13% 9% 3 8% 5% 4 5% 3 5% 6% 37%	1890
	11th to 20th	Day	14% 9% 2 8% 6% 5% 4 4 5% 7% 36%	1843
	21st to end	Day	16% 8% 3 8% 5% 5% 4 3 4 5% 38%	1735
March	1st to 10th	Day	18% 7% 2 6% 4 5% 5% 4 4 6% 39%	5045
	11th to 20th	Day	11% 7% 2 5% 4 4 5% 4 7% 8% 44%	5090
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 5% 6% 6% 38%	5586
April	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 4 7% 8% 36%	2520
	11th to 20th	Day	14% 5% 2 6% 4 4 4 3 6% 7% 43%	2446
	21st to end	Day	18% 7% 2 6% 4 4 4 4 5% 6% 40%	2444
May	1st to 10th	Day	21% 7% 2 6% 4 5% 5% 4 7% 9% 30%	5862
	11th to 20th	Day	23% 6% 2 6% 4 4 5% 4 7% 9% 30%	6078
	21st to end	Day	15% 5% 2 6% 4 5% 6% 4 7% 10% 35%	6934
June	1st to 10th	Day	12% 5% 2 5% 4 4 5% 5% 7% 11% 41%	2800
	11th to 20th	Day	11% 5% 3 6% 5% 4 6% 5% 7% 11% 38%	2800
	21st to end	Day	9% 4 2 6% 4 4 5% 5% 6% 10% 45%	2800
July	1st to 10th	Day	5% 4 2 6% 5% 5% 5% 5% 7% 11% 45%	6360
	11th to 20th	Day	7% 6% 2 7% 5% 6% 6% 6% 9% 11% 34%	6070
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 4 7% 10% 41%	6424
August	1st to 10th	Day	16% 5% 2 6% 5% 5% 5% 5% 7% 10% 34%	4234
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 4 7% 9% 42%	4091
	21st to end	Day	11% 4 2 5% 4 4 4 4 6% 9% 47%	4312
September	1st to 10th	Day	15% 6% 2 4 4 4 4 4 5% 7% 46%	2400
	11th to 20th	Day	12% 4 2 6% 4 3 4 4 6% 7% 48%	2372
	21st to end	Day	21% 7% 3 6% 4 4 5% 2 6% 6% 37%	2100
October	1st to 10th	Day	16% 5% 2 5% 4 4 4 3 6% 6% 47%	2000
	11th to 20th	Day	16% 4 2 5% 4 3 4 3 6% 7% 45%	2000
	21st to end	Day	14% 5% 1 5% 4 4 4 4 5% 7% 46%	2200
November	1st to 10th	Day	11% 5% 2 4 4 3 4 3 6% 7% 51%	1989
	11th to 20th	Day	16% 6% 1 5% 4 3 4 4 5% 7% 44%	1780
	21st to end	Day	15% 7% 3 6% 4 5% 4 4 5% 7% 41%	1680
December	1st to 10th	Day	13% 7% 2 7% 6% 5% 5% 4 6% 7% 38%	2680
	11th to 20th	Day	20% 9% 2 6% 4 4 4 3 5% 6% 36%	3070
	21st to end	Day	17% 9% 3 8% 6% 5% 6% 5% 7% 8% 27%	4048
January	1st to 10th	Night	9% 9% 3 8% 5% 5% 5% 4 6% 8% 38%	7993
	11th to 20th	Night	11% 8% 3 7% 5% 5% 4 4 5% 6% 43%	7680
	21st to end	Night	13% 7% 2 5% 4 4 5% 4 6% 8% 42%	8401
February	1st to 10th	Night	15% 7% 2 6% 5% 4 5% 4 6% 7% 39%	2730
	11th to 20th	Night	16% 9% 2 7% 5% 5% 4 3 5% 6% 37%	2663
	21st to end	Night	14% 10% 3 6% 6% 4 4 4 6% 7% 36%	2067
March	1st to 10th	Night	17% 7% 2 6% 4 4 3 3 5% 7% 43%	5028
	11th to 20th	Night	10% 5% 3 6% 4 4 4 3 5% 7% 48%	5090
	21st to end	Night	11% 7% 2 6% 4 5% 5% 4 6% 7% 44%	5574
April	1st to 10th	Night	16% 9% 2 6% 5% 5% 4 4 6% 6% 37%	2100
	11th to 20th	Night	13% 5% 3 6% 4 3 4 5% 6% 8% 43%	2036
	21st to end	Night	17% 5% 2 5% 4 4 5% 4 5% 7% 43%	1973
May	1st to 10th	Night	23% 9% 3 7% 5% 4 5% 4 6% 8% 27%	4033
	11th to 20th	Night	26% 7% 3 5% 4 3 4 3 6% 9% 30%	3814
	21st to end	Night	18% 7% 2 7% 5% 5% 5% 4 7% 8% 34%	3920
June	1st to 10th	Night	10% 6% 2 5% 5% 4 4 4 6% 8% 47%	1600
	11th to 20th	Night	12% 4 2 7% 6% 5% 4 4 6% 9% 40%	1600
	21st to end	Night	9% 4 2 5% 4 5% 4 5% 9% 10% 43%	1600
July	1st to 10th	Night	7% 5% 2 7% 4 5% 6% 6% 7% 9% 41%	3598
	11th to 20th	Night	7% 7% 3 9% 6% 7% 6% 5% 8% 10% 30%	3869
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 5% 8% 10% 41%	4427
August	1st to 10th	Night	15% 5% 3 8% 6% 5% 5% 5% 6% 9% 33%	2915
	11th to 20th	Night	10% 6% 2 7% 5% 5% 5% 4 5% 8% 42%	2923
	21st to end	Night	13% 5% 1 5% 4 4 4 4 6% 7% 46%	3586
September	1st to 10th	Night	14% 4 2 5% 4 4 5% 3 6% 7% 46%	2000
	11th to 20th	Night	11% 6% 3 6% 4 3 3 3 5% 7% 50%	2024
	21st to end	Night	20% 7% 2 5% 3 5% 5% 4 4 6% 39%	2298
October	1st to 10th	Night	19% 5% 2 4 3 3 4 3 4 6% 46%	2400
	11th to 20th	Night	15% 4 2 5% 3 3 3 3 4 7% 50%	2400
	21st to end	Night	15% 5% 2 4 4 3 3 4 5% 6% 48%	2640
November	1st to 10th	Night	9% 6% 2 5% 4 3 4 3 5% 6% 52%	2528
	11th to 20th	Night	15% 7% 2 5% 4 4 4 3 5% 7% 44%	2836
	21st to end	Night	14% 6% 2 7% 4 4 5% 4 6% 7% 41%	2940
December	1st to 10th	Night	10% 6% 2 7% 5% 4 5% 4 5% 8% 44%	4720
	11th to 20th	Night	19% 9% 2 6% 4 5% 5% 4 5% 7% 36%	5345
	21st to end	Night	17% 8% 3 7% 5% 6% 6% 5% 7% 8% 28%	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	46% 3 10% 23% 14% 4	4884
	11th to 20th	Day	45% 4 7% 18% 19% 7%	5280
	21st to end	Day	44% 4 10% 27% 12% 3	5780
February	1st to 10th	Day	46% 4 12% 23% 12% 2	1890
	11th to 20th	Day	48% 5% 10% 22% 13% 2	1852
	21st to end	Day	49% 3 10% 21% 13% 3	1735
March	1st to 10th	Day	45% 4 8% 21% 14% 7%	5045
	11th to 20th	Day	36% 4 11% 27% 17% 5%	5090
	21st to end	Day	46% 4 9% 19% 18% 4	5586
April	1st to 10th	Day	44% 6% 12% 18% 17% 4	2520
	11th to 20th	Day	39% 5% 12% 22% 18% 4	2446
	21st to end	Day	44% 5% 9% 23% 17% 3	2444
May	1st to 10th	Day	50% 5% 13% 19% 11% 2	5897
	11th to 20th	Day	50% 6% 9% 25% 9% 1	6082
	21st to end	Day	42% 7% 12% 22% 16% 2	6949
June	1st to 10th	Day	35% 8% 14% 24% 17% 1	2800
	11th to 20th	Day	39% 10% 14% 22% 14% 2	2800
	21st to end	Day	33% 9% 14% 26% 17% 1	2800
July	1st to 10th	Day	34% 7% 14% 25% 16% 3	6290
	11th to 20th	Day	42% 6% 17% 24% 10% 1	5997
	21st to end	Day	38% 5% 10% 24% 22% 1	6240
August	1st to 10th	Day	45% 6% 14% 26% 9%	4221
	11th to 20th	Day	38% 6% 11% 29% 15% 1	4130
	21st to end	Day	35% 5% 13% 28% 17% 2	4312
September	1st to 10th	Day	37% 7% 11% 20% 22% 4	2400
	11th to 20th	Day	34% 6% 9% 28% 21% 2	2372
	21st to end	Day	48% 4 9% 18% 18% 2	2100
October	1st to 10th	Day	37% 3 7% 23% 26% 4	3260
	11th to 20th	Day	29% 4 10% 30% 22% 6%	3240
	21st to end	Day	26% 3 11% 31% 26% 3	3538
November	1st to 10th	Day	32% 5% 12% 25% 21% 4	1989
	11th to 20th	Day	39% 4 11% 22% 21% 3	1780
	21st to end	Day	41% 6% 12% 27% 12% 1	1680
December	1st to 10th	Day	43% 5% 14% 26% 8% 4	2680
	11th to 20th	Day	48% 4 14% 24% 9% 2	3066
	21st to end	Day	50% 7% 15% 14% 13% 1	4048
January	1st to 10th	Night	43% 4 10% 26% 16% 2	7994
	11th to 20th	Night	41% 4 8% 21% 20% 6%	7680
	21st to end	Night	40% 3 9% 30% 16% 2	8401
February	1st to 10th	Night	42% 6% 13% 25% 13% 2	2730
	11th to 20th	Night	47% 4 10% 20% 15% 3	2665
	21st to end	Night	46% 5% 11% 21% 13% 5%	2067
March	1st to 10th	Night	41% 4 8% 26% 13% 7%	5028
	11th to 20th	Night	35% 5% 11% 30% 14% 5%	5090
	21st to end	Night	39% 4 11% 22% 20% 4	5574
April	1st to 10th	Night	46% 5% 10% 20% 16% 3	2100
	11th to 20th	Night	39% 6% 10% 23% 17% 5%	2036
	21st to end	Night	39% 6% 10% 26% 15% 4	1973
May	1st to 10th	Night	55% 6% 10% 16% 10% 4	4060
	11th to 20th	Night	50% 5% 10% 23% 11% 1	3815
	21st to end	Night	45% 7% 13% 26% 8% 2	3927
June	1st to 10th	Night	35% 7% 11% 27% 18% 3	1600
	11th to 20th	Night	38% 8% 12% 26% 13% 2	1600
	21st to end	Night	32% 8% 15% 25% 17% 2	1600
July	1st to 10th	Night	39% 7% 16% 19% 15% 4	3560
	11th to 20th	Night	48% 6% 16% 18% 11% 1	3823
	21st to end	Night	37% 5% 13% 24% 21% 1	4297
August	1st to 10th	Night	46% 6% 16% 22% 9% 1	2904
	11th to 20th	Night	39% 5% 11% 27% 17% 1	2950
	21st to end	Night	36% 4 13% 30% 15% 2	3586
September	1st to 10th	Night	36% 7% 11% 22% 21% 4	2000
	11th to 20th	Night	34% 4 10% 27% 23% 2	2024
	21st to end	Night	44% 6% 12% 19% 17% 2	2298
October	1st to 10th	Night	39% 3 9% 25% 22% 3	3920
	11th to 20th	Night	26% 3 11% 25% 29% 6%	3899
	21st to end	Night	26% 3 10% 32% 25% 4	4252
November	1st to 10th	Night	32% 4 12% 27% 22% 4	2528
	11th to 20th	Night	39% 4 8% 24% 20% 5%	2836
	21st to end	Night	40% 5% 12% 24% 16% 2	2940
December	1st to 10th	Night	38% 4 13% 31% 13% 1	4720
	11th to 20th	Night	48% 3 12% 21% 13% 2	5348
	21st to end	Night	49% 7% 13% 18% 12% 1	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	35% 27% 24% 10% 4	261
	11th to 20th	Day	38% 25% 24% 11% 2	330
	21st to end	Day	28% 33% 26% 10% 3 1	363
February	1st to 10th	Day	29% 26% 29% 12% 3	330
	11th to 20th	Day	44% 23% 25% 7% 1	330
	21st to end	Day	30% 20% 28% 19% 3	441
March	1st to 10th	Day	31% 21% 24% 20% 4	540
	11th to 20th	Day	31% 22% 25% 16% 6% 1	540
	21st to end	Day	26% 22% 32% 15% 4 2	594
April	1st to 10th	Day	26% 23% 26% 18% 6% 1	540
	11th to 20th	Day	20% 20% 30% 16% 10% 4	516
	21st to end	Day	30% 16% 27% 16% 9% 2	500
May	1st to 10th	Day	24% 18% 28% 16% 11% 3	503
	11th to 20th	Day	25% 17% 24% 18% 14% 4	527
	21st to end	Day	18% 10% 23% 22% 18% 8%	573
June	1st to 10th	Day	15% 11% 28% 20% 19% 6%	530
	11th to 20th	Day	16% 10% 25% 27% 18% 5%	530
	21st to end	Day	12% 8% 26% 29% 17% 8%	530
July	1st to 10th	Day	12% 4 20% 29% 23% 13%	525
	11th to 20th	Day	7% 4 22% 28% 27% 11%	524
	21st to end	Day	13% 6% 20% 31% 22% 9%	576
August	1st to 10th	Day	16% 4 20% 26% 22% 13%	526
	11th to 20th	Day	14% 3 21% 27% 24% 10%	519
	21st to end	Day	10% 5% 21% 28% 24% 13%	583
September	1st to 10th	Day	18% 7% 18% 23% 23% 11%	530
	11th to 20th	Day	15% 5% 15% 30% 25% 10%	530
	21st to end	Day	28% 8% 22% 20% 17% 4	530
October	1st to 10th	Day	20% 7% 19% 27% 21% 6%	530
	11th to 20th	Day	18% 16% 25% 25% 13% 3	530
	21st to end	Day	19% 16% 30% 21% 11% 2	583
November	1st to 10th	Day	17% 17% 27% 23% 13% 3	499
	11th to 20th	Day	26% 15% 25% 24% 8% 1	320
	21st to end	Day	27% 17% 30% 19% 5% 1	220
December	1st to 10th	Day	28% 23% 29% 15% 4 1	220
	11th to 20th	Day	27% 21% 25% 21% 5%	220
	21st to end	Day	26% 19% 29% 18% 8%	242
January	1st to 10th	Night	19% 22% 33% 22% 3	840
	11th to 20th	Night	25% 17% 30% 26% 3	840
	21st to end	Night	21% 20% 34% 23% 2	924
February	1st to 10th	Night	20% 19% 31% 28% 3	840
	11th to 20th	Night	27% 23% 30% 19% 1	823
	21st to end	Night	26% 19% 33% 21% 2	692
March	1st to 10th	Night	23% 17% 34% 23% 4	840
	11th to 20th	Night	25% 19% 28% 22% 5%	840
	21st to end	Night	19% 17% 35% 23% 5% 1	924
April	1st to 10th	Night	25% 18% 31% 22% 5%	840
	11th to 20th	Night	19% 14% 31% 28% 8%	816
	21st to end	Night	21% 13% 26% 34% 7%	773
May	1st to 10th	Night	22% 7% 24% 33% 13% 1	600
	11th to 20th	Night	23% 7% 24% 32% 13%	503
	21st to end	Night	14% 6% 22% 36% 20% 2	433
June	1st to 10th	Night	12% 4 24% 36% 24% 1	400
	11th to 20th	Night	12% 6% 19% 41% 21% 1	400
	21st to end	Night	9% 3 20% 45% 22% 2	400
July	1st to 10th	Night	12% 4 15% 36% 31% 3	390
	11th to 20th	Night	7% 2 12% 41% 33% 5%	503
	21st to end	Night	9% 3 14% 40% 32% 3	657
August	1st to 10th	Night	11% 2 12% 41% 31% 3	597
	11th to 20th	Night	11% 4 11% 40% 30% 4	616
	21st to end	Night	13% 3 14% 39% 28% 4	880
September	1st to 10th	Night	16% 3 16% 35% 27% 4	800
	11th to 20th	Night	13% 5% 15% 34% 30% 3	800
	21st to end	Night	24% 5% 17% 32% 19% 2	800
October	1st to 10th	Night	21% 7% 17% 31% 22% 3	800
	11th to 20th	Night	17% 10% 26% 34% 13% 1	800
	21st to end	Night	18% 12% 27% 31% 10%	880
November	1st to 10th	Night	14% 10% 28% 36% 10% 1	820
	11th to 20th	Night	20% 17% 25% 29% 8%	840
	21st to end	Night	19% 15% 29% 29% 7%	840
December	1st to 10th	Night	21% 16% 31% 25% 6%	840
	11th to 20th	Night	20% 15% 31% 30% 3	840
	21st to end	Night	20% 15% 29% 33% 3	924

Page generated at 2026-04-14 02:44:27. Version ac07

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