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 2024-12-24.

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% 4 5% 4 6% 8% 39%	101616
	Night	15% 7% 2 6% 4 4 4 4 6% 7% 40%	98409
Transparency	Day	41% 6% 12% 24% 15% 2	105257
	Night	40% 6% 12% 24% 15% 3	102908
Seeing	Day	21% 14% 24% 22% 14% 5%	14999
	Night	18% 12% 25% 31% 13% 1	24614

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% 6% 3 7% 5% 4 5% 3 5% 6% 39%	2627
	11th to 20th	Day	13% 11% 4 9% 5% 6% 5% 5% 6% 7% 31%	2840
	21st to end	Day	17% 9% 3 8% 5% 4 6% 3 6% 6% 31%	3124
February	1st to 10th	Day	14% 9% 3 8% 6% 4 5% 3 5% 6% 36%	1710
	11th to 20th	Day	14% 9% 2 7% 6% 6% 4 4 5% 7% 37%	1663
	21st to end	Day	17% 9% 3 8% 6% 5% 4 2 4 5% 38%	1574
March	1st to 10th	Day	21% 9% 2 6% 5% 5% 5% 4 4 7% 33%	3435
	11th to 20th	Day	10% 6% 2 6% 5% 4 5% 4 7% 8% 44%	3480
	21st to end	Day	17% 9% 3 6% 4 4 4 4 5% 6% 39%	3815
April	1st to 10th	Day	13% 8% 2 7% 6% 5% 5% 4 7% 8% 36%	2280
	11th to 20th	Day	14% 5% 2 6% 4 5% 4 4 5% 6% 43%	2254
	21st to end	Day	19% 7% 2 6% 4 4 4 4 5% 6% 40%	2322
May	1st to 10th	Day	24% 7% 3 6% 5% 4 4 4 6% 8% 29%	4082
	11th to 20th	Day	24% 7% 2 6% 4 4 5% 4 6% 8% 31%	4228
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 7% 10% 42%	4823
June	1st to 10th	Day	12% 5% 2 5% 4 4 5% 5% 7% 11% 41%	2660
	11th to 20th	Day	10% 5% 3 6% 6% 4 6% 5% 7% 11% 38%	2660
	21st to end	Day	9% 4 2 6% 4 4 5% 5% 7% 10% 44%	2660
July	1st to 10th	Day	5% 4 2 6% 5% 5% 5% 5% 8% 10% 45%	4440
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 5% 8% 10% 43%	4234
	21st to end	Day	10% 7% 3 8% 5% 5% 5% 4 6% 8% 38%	4466
August	1st to 10th	Day	16% 5% 2 6% 5% 5% 6% 5% 7% 10% 34%	4104
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 4 7% 9% 43%	3963
	21st to end	Day	12% 4 2 5% 4 4 4 4 6% 8% 47%	4180
September	1st to 10th	Day	16% 6% 2 4 4 4 4 4 5% 7% 46%	2280
	11th to 20th	Day	12% 5% 2 6% 4 3 4 4 6% 8% 48%	2254
	21st to end	Day	21% 6% 3 6% 4 4 5% 2 6% 6% 38%	1995
October	1st to 10th	Day	16% 6% 2 5% 4 4 4 3 5% 6% 45%	1900
	11th to 20th	Day	16% 4 2 5% 4 3 4 3 6% 7% 45%	1900
	21st to end	Day	15% 5% 1 6% 4 4 4 3 5% 7% 47%	2090
November	1st to 10th	Day	11% 5% 2 4 4 3 4 3 5% 6% 51%	1892
	11th to 20th	Day	16% 6% 1 5% 4 3 4 4 5% 7% 45%	1695
	21st to end	Day	15% 6% 2 6% 4 5% 4 4 5% 7% 41%	1600
December	1st to 10th	Day	17% 9% 3 6% 5% 4 4 5% 6% 8% 32%	1600
	11th to 20th	Day	17% 7% 2 6% 4 5% 4 4 6% 6% 37%	1990
	21st to end	Day	17% 7% 3 8% 7% 5% 6% 5% 7% 7% 28%	2796
January	1st to 10th	Night	11% 8% 3 8% 5% 5% 5% 4 6% 7% 38%	4301
	11th to 20th	Night	16% 9% 3 7% 5% 4 4 4 5% 6% 38%	4120
	21st to end	Night	19% 10% 4 8% 5% 4 4 3 5% 6% 32%	4532
February	1st to 10th	Night	15% 7% 3 6% 5% 4 5% 3 6% 7% 39%	2470
	11th to 20th	Night	16% 9% 2 7% 4 5% 4 3 5% 6% 38%	2403
	21st to end	Night	15% 11% 3 6% 6% 4 4 4 6% 7% 35%	1877
March	1st to 10th	Night	20% 8% 2 6% 5% 4 4 3 5% 6% 36%	3418
	11th to 20th	Night	11% 5% 2 7% 4 5% 4 4 5% 7% 45%	3480
	21st to end	Night	13% 8% 2 6% 4 4 4 3 4 6% 46%	3803
April	1st to 10th	Night	16% 9% 2 6% 5% 5% 5% 4 6% 6% 37%	1900
	11th to 20th	Night	13% 5% 3 6% 4 3 4 5% 6% 7% 44%	1875
	21st to end	Night	17% 5% 2 5% 4 4 5% 4 5% 7% 42%	1875
May	1st to 10th	Night	24% 8% 2 7% 4 4 4 4 5% 8% 29%	2813
	11th to 20th	Night	25% 8% 3 6% 4 4 4 3 6% 7% 30%	2664
	21st to end	Night	11% 5% 2 6% 5% 5% 5% 4 8% 9% 40%	2732
June	1st to 10th	Night	10% 5% 2 4 5% 3 4 5% 6% 8% 47%	1520
	11th to 20th	Night	11% 4 2 7% 6% 5% 4 5% 6% 10% 40%	1520
	21st to end	Night	9% 4 2 5% 4 5% 4 5% 9% 10% 43%	1520
July	1st to 10th	Night	7% 4 2 6% 4 5% 6% 6% 7% 10% 42%	2520
	11th to 20th	Night	5% 5% 2 7% 6% 6% 6% 5% 8% 10% 39%	2708
	21st to end	Night	10% 6% 3 7% 6% 5% 5% 5% 7% 8% 38%	3087
August	1st to 10th	Night	15% 5% 3 8% 5% 5% 5% 5% 6% 10% 33%	2825
	11th to 20th	Night	10% 6% 2 7% 4 5% 5% 4 5% 8% 42%	2832
	21st to end	Night	13% 5% 1 5% 4 4 4 4 6% 7% 46%	3476
September	1st to 10th	Night	14% 4 2 5% 4 4 5% 3 6% 7% 46%	1900
	11th to 20th	Night	11% 6% 2 5% 4 3 3 2 5% 7% 50%	1923
	21st to end	Night	20% 6% 2 4 3 4 5% 4 4 6% 41%	2183
October	1st to 10th	Night	19% 5% 2 5% 4 4 4 3 4 6% 44%	2280
	11th to 20th	Night	14% 4 2 5% 3 3 3 3 5% 7% 50%	2280
	21st to end	Night	15% 5% 2 4 4 3 3 4 5% 6% 48%	2508
November	1st to 10th	Night	10% 6% 2 5% 4 3 4 2 5% 6% 53%	2405
	11th to 20th	Night	15% 7% 2 4 4 4 4 3 5% 7% 44%	2701
	21st to end	Night	14% 6% 2 6% 4 4 5% 4 6% 7% 42%	2800
December	1st to 10th	Night	15% 8% 2 7% 5% 4 4 4 6% 7% 38%	2800
	11th to 20th	Night	17% 8% 2 6% 4 5% 4 4 6% 7% 38%	3425
	21st to end	Night	17% 7% 2 6% 5% 6% 6% 5% 8% 8% 30%	4933

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	44% 6% 10% 27% 10% 3	2628
	11th to 20th	Day	50% 7% 9% 17% 14% 3	2840
	21st to end	Day	52% 7% 8% 22% 10% 1	3124
February	1st to 10th	Day	48% 5% 13% 22% 11% 2	1710
	11th to 20th	Day	46% 5% 10% 23% 13% 2	1672
	21st to end	Day	50% 4 11% 20% 13% 3	1574
March	1st to 10th	Day	49% 6% 10% 21% 12% 2	3435
	11th to 20th	Day	35% 5% 10% 28% 19% 3	3480
	21st to end	Day	46% 5% 11% 21% 14% 3	3815
April	1st to 10th	Day	43% 7% 12% 17% 17% 4	2280
	11th to 20th	Day	39% 6% 11% 21% 18% 5%	2254
	21st to end	Day	45% 5% 9% 22% 16% 3	2322
May	1st to 10th	Day	52% 7% 10% 19% 12% 1	4120
	11th to 20th	Day	50% 8% 10% 22% 8% 2	4232
	21st to end	Day	34% 9% 14% 25% 16% 2	4837
June	1st to 10th	Day	35% 8% 14% 24% 17% 1	2660
	11th to 20th	Day	38% 10% 15% 22% 14% 2	2660
	21st to end	Day	33% 9% 14% 27% 16% 2	2660
July	1st to 10th	Day	33% 7% 16% 27% 12% 5%	4370
	11th to 20th	Day	35% 6% 15% 29% 14% 1	4161
	21st to end	Day	43% 6% 9% 22% 19% 2	4324
August	1st to 10th	Day	46% 6% 14% 25% 9%	4091
	11th to 20th	Day	38% 6% 11% 29% 16% 1	4002
	21st to end	Day	35% 5% 14% 28% 17% 2	4180
September	1st to 10th	Day	38% 7% 11% 20% 21% 4	2280
	11th to 20th	Day	34% 7% 9% 27% 21% 3	2254
	21st to end	Day	47% 4 10% 19% 18% 2	1995
October	1st to 10th	Day	37% 3 7% 22% 26% 4	3160
	11th to 20th	Day	29% 4 10% 29% 22% 6%	3140
	21st to end	Day	26% 3 11% 31% 26% 3	3428
November	1st to 10th	Day	33% 5% 12% 25% 21% 4	1892
	11th to 20th	Day	38% 5% 12% 22% 21% 3	1695
	21st to end	Day	41% 7% 12% 27% 13% 1	1600
December	1st to 10th	Day	47% 7% 14% 22% 8% 1	1600
	11th to 20th	Day	44% 7% 12% 28% 8% 2	1986
	21st to end	Day	48% 10% 15% 17% 10%	2796
January	1st to 10th	Night	42% 7% 10% 26% 13% 2	4301
	11th to 20th	Night	45% 6% 12% 23% 12% 2	4120
	21st to end	Night	51% 6% 9% 24% 9% 1	4532
February	1st to 10th	Night	42% 6% 13% 25% 12% 2	2470
	11th to 20th	Night	46% 5% 10% 20% 16% 3	2405
	21st to end	Night	48% 6% 11% 19% 12% 5%	1877
March	1st to 10th	Night	48% 6% 9% 26% 10% 1	3418
	11th to 20th	Night	36% 7% 9% 32% 14% 2	3480
	21st to end	Night	39% 5% 9% 24% 19% 4	3803
April	1st to 10th	Night	45% 5% 10% 19% 16% 4	1900
	11th to 20th	Night	38% 7% 10% 22% 17% 5%	1875
	21st to end	Night	39% 6% 11% 26% 14% 4	1875
May	1st to 10th	Night	53% 7% 8% 17% 12% 3	2840
	11th to 20th	Night	51% 8% 10% 20% 10% 1	2665
	21st to end	Night	35% 10% 16% 27% 10% 3	2739
June	1st to 10th	Night	34% 8% 11% 27% 18% 3	1520
	11th to 20th	Night	37% 8% 12% 27% 13% 2	1520
	21st to end	Night	32% 8% 15% 26% 17% 2	1520
July	1st to 10th	Night	37% 8% 19% 19% 12% 6%	2480
	11th to 20th	Night	38% 6% 16% 23% 16% 2	2659
	21st to end	Night	40% 7% 14% 20% 18% 2	2988
August	1st to 10th	Night	46% 6% 16% 22% 9% 1	2814
	11th to 20th	Night	39% 5% 11% 27% 17% 1	2859
	21st to end	Night	36% 5% 13% 29% 15% 2	3476
September	1st to 10th	Night	37% 7% 11% 21% 20% 4	1900
	11th to 20th	Night	34% 4 10% 28% 23% 2	1923
	21st to end	Night	43% 6% 11% 20% 17% 2	2183
October	1st to 10th	Night	40% 3 9% 24% 22% 2	3800
	11th to 20th	Night	25% 3 10% 25% 30% 6%	3779
	21st to end	Night	26% 3 11% 32% 25% 4	4120
November	1st to 10th	Night	31% 4 12% 27% 21% 4	2405
	11th to 20th	Night	39% 5% 8% 25% 19% 5%	2701
	21st to end	Night	40% 5% 12% 24% 17% 2	2800
December	1st to 10th	Night	44% 6% 12% 26% 11% 1	2800
	11th to 20th	Night	44% 5% 12% 23% 15% 2	3428
	21st to end	Night	45% 9% 16% 19% 11%	4933

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	32% 28% 26% 10% 4	213
	11th to 20th	Day	36% 24% 25% 13% 3	270
	21st to end	Day	26% 34% 24% 12% 3 1	297
February	1st to 10th	Day	28% 26% 30% 12% 4	270
	11th to 20th	Day	40% 26% 27% 7%	270
	21st to end	Day	32% 20% 25% 19% 3	377
March	1st to 10th	Day	30% 22% 24% 21% 3	460
	11th to 20th	Day	29% 23% 25% 17% 5% 1	460
	21st to end	Day	26% 22% 30% 15% 5% 2	506
April	1st to 10th	Day	24% 24% 26% 19% 7% 1	460
	11th to 20th	Day	19% 20% 29% 16% 11% 5%	452
	21st to end	Day	31% 16% 26% 17% 9% 2	460
May	1st to 10th	Day	24% 18% 27% 16% 11% 4	463
	11th to 20th	Day	24% 16% 23% 18% 14% 4	487
	21st to end	Day	16% 10% 23% 23% 19% 8%	529
June	1st to 10th	Day	15% 12% 28% 20% 19% 6%	490
	11th to 20th	Day	14% 9% 26% 27% 18% 6%	490
	21st to end	Day	12% 9% 27% 29% 16% 7%	490
July	1st to 10th	Day	12% 5% 21% 28% 22% 13%	485
	11th to 20th	Day	7% 4 22% 29% 27% 11%	484
	21st to end	Day	14% 6% 20% 30% 22% 8%	532
August	1st to 10th	Day	16% 4 20% 25% 23% 13%	486
	11th to 20th	Day	13% 4 22% 28% 23% 11%	479
	21st to end	Day	9% 5% 21% 28% 25% 13%	539
September	1st to 10th	Day	19% 7% 18% 22% 23% 11%	490
	11th to 20th	Day	15% 6% 16% 31% 23% 10%	490
	21st to end	Day	27% 9% 22% 20% 18% 4	490
October	1st to 10th	Day	22% 7% 19% 26% 21% 5%	490
	11th to 20th	Day	18% 16% 23% 26% 14% 3	490
	21st to end	Day	18% 16% 29% 22% 12% 2	539
November	1st to 10th	Day	17% 15% 27% 24% 14% 3	462
	11th to 20th	Day	24% 15% 26% 25% 9% 1	295
	21st to end	Day	26% 18% 31% 19% 5% 2	200
December	1st to 10th	Day	28% 23% 28% 17% 5% 1	200
	11th to 20th	Day	29% 21% 25% 21% 6%	200
	21st to end	Day	25% 20% 28% 19% 9%	204
January	1st to 10th	Night	18% 21% 34% 23% 3	760
	11th to 20th	Night	24% 17% 29% 27% 3	760
	21st to end	Night	21% 21% 32% 24% 2	836
February	1st to 10th	Night	19% 20% 30% 28% 3	760
	11th to 20th	Night	25% 24% 31% 19% 2	743
	21st to end	Night	27% 19% 32% 20% 3	628
March	1st to 10th	Night	21% 18% 33% 24% 4	760
	11th to 20th	Night	25% 20% 28% 21% 6%	760
	21st to end	Night	19% 17% 36% 22% 5% 1	836
April	1st to 10th	Night	23% 19% 31% 22% 5%	760
	11th to 20th	Night	18% 13% 31% 28% 9%	751
	21st to end	Night	21% 13% 26% 33% 7%	735
May	1st to 10th	Night	22% 8% 24% 33% 14% 1	570
	11th to 20th	Night	23% 7% 24% 33% 13%	478
	21st to end	Night	13% 6% 21% 37% 21% 2	411
June	1st to 10th	Night	11% 4 24% 36% 24% 1	380
	11th to 20th	Night	10% 6% 20% 42% 22% 1	380
	21st to end	Night	9% 3 21% 46% 21% 1	380
July	1st to 10th	Night	12% 4 15% 36% 29% 4	370
	11th to 20th	Night	7% 2 12% 40% 34% 4	477
	21st to end	Night	9% 3 14% 40% 32% 3	624
August	1st to 10th	Night	11% 2 13% 41% 31% 3	567
	11th to 20th	Night	10% 4 12% 40% 30% 4	585
	21st to end	Night	14% 3 13% 39% 28% 4	836
September	1st to 10th	Night	16% 3 16% 35% 27% 4	760
	11th to 20th	Night	13% 5% 15% 34% 29% 3	760
	21st to end	Night	24% 6% 18% 31% 20% 2	760
October	1st to 10th	Night	22% 7% 17% 31% 21% 3	760
	11th to 20th	Night	16% 10% 26% 33% 13% 1	760
	21st to end	Night	18% 12% 27% 31% 11%	836
November	1st to 10th	Night	13% 10% 29% 36% 10% 1	780
	11th to 20th	Night	19% 17% 25% 29% 9%	800
	21st to end	Night	19% 15% 29% 30% 7%	800
December	1st to 10th	Night	21% 16% 31% 25% 7%	800
	11th to 20th	Night	20% 15% 32% 30% 3	800
	21st to end	Night	19% 16% 29% 33% 4	851

Page generated at 2024-12-21 04:13:47. Version ac07

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