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 2024-01-17.

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% 3 5% 5% 3 5% 5% 6% 8% 40%	74696
	Night	14% 6% 3 5% 5% 3 5% 5% 6% 8% 41%	75131
Transparency	Day	41% 7% 12% 22% 15% 2	74408
	Night	40% 7% 12% 23% 16% 3	74939
Seeing	Day	21% 14% 24% 22% 14% 5%	13753
	Night	18% 12% 25% 30% 13% 1	23418

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% 7% 3 5% 8% 3 5% 5% 6% 7% 37%	1588
	11th to 20th	Day	15% 8% 3 7% 5% 4 6% 6% 6% 8% 34%	1674
	21st to end	Day	14% 8% 3 8% 6% 4 5% 4 5% 6% 36%	1782
February	1st to 10th	Day	15% 9% 4 8% 6% 4 5% 4 5% 7% 34%	1620
	11th to 20th	Day	14% 8% 3 6% 6% 4 5% 5% 5% 7% 36%	1573
	21st to end	Day	18% 8% 3 8% 6% 3 5% 3 3 5% 38%	1484
March	1st to 10th	Day	13% 6% 2 5% 5% 3 5% 4 4 7% 46%	1980
	11th to 20th	Day	16% 7% 2 6% 6% 3 5% 5% 6% 7% 37%	1980
	21st to end	Day	17% 7% 3 6% 6% 3 5% 5% 5% 6% 36%	2178
April	1st to 10th	Day	12% 7% 2 6% 6% 4 5% 5% 6% 9% 37%	2160
	11th to 20th	Day	15% 4 3 5% 5% 3 5% 4 5% 6% 43%	2134
	21st to end	Day	20% 6% 2 5% 4 3 4 5% 5% 6% 40%	2199
May	1st to 10th	Day	17% 7% 3 5% 6% 3 5% 5% 6% 9% 34%	2340
	11th to 20th	Day	21% 5% 2 5% 4 3 6% 5% 6% 7% 36%	2378
	21st to end	Day	14% 6% 3 6% 5% 3 5% 5% 6% 9% 39%	2725
June	1st to 10th	Day	12% 5% 2 5% 4 3 4 6% 7% 11% 41%	2520
	11th to 20th	Day	11% 5% 3 5% 6% 3 6% 7% 7% 11% 37%	2520
	21st to end	Day	9% 3 2 5% 5% 3 6% 6% 6% 10% 44%	2520
July	1st to 10th	Day	8% 4 2 6% 6% 4 5% 6% 7% 9% 42%	2520
	11th to 20th	Day	6% 4 2 5% 5% 4 6% 6% 8% 10% 44%	2412
	21st to end	Day	10% 6% 3 6% 6% 4 6% 6% 7% 9% 38%	2547
August	1st to 10th	Day	9% 5% 2 6% 7% 4 6% 8% 8% 10% 37%	2327
	11th to 20th	Day	11% 6% 3 5% 5% 3 6% 5% 7% 9% 42%	2264
	21st to end	Day	11% 4 2 6% 4 3 5% 5% 6% 9% 44%	2376
September	1st to 10th	Day	16% 6% 2 4 4 3 4 5% 5% 7% 44%	2160
	11th to 20th	Day	12% 4 2 5% 4 2 4 5% 6% 8% 47%	2136
	21st to end	Day	18% 6% 3 5% 4 3 5% 4 6% 6% 40%	1890
October	1st to 10th	Day	16% 5% 2 4 5% 3 3 4 6% 6% 45%	1800
	11th to 20th	Day	17% 4 2 4 5% 2 4 4 6% 7% 44%	1800
	21st to end	Day	16% 5% 2 5% 5% 2 4 4 5% 7% 45%	1980
November	1st to 10th	Day	12% 5% 2 3 4 3 4 3 5% 7% 51%	1796
	11th to 20th	Day	16% 6% 1 4 4 3 4 4 5% 7% 45%	1611
	21st to end	Day	16% 5% 3 5% 4 3 5% 5% 5% 7% 41%	1520
December	1st to 10th	Day	18% 9% 3 6% 6% 3 5% 6% 6% 8% 32%	1520
	11th to 20th	Day	17% 6% 3 5% 5% 4 5% 5% 6% 7% 37%	1910
	21st to end	Day	18% 6% 3 7% 8% 4 6% 7% 7% 7% 27%	2772
January	1st to 10th	Night	13% 8% 3 6% 5% 4 5% 4 6% 7% 40%	2573
	11th to 20th	Night	17% 8% 4 6% 5% 3 5% 6% 5% 7% 35%	2425
	21st to end	Night	14% 7% 3 6% 6% 3 4 5% 6% 7% 38%	2574
February	1st to 10th	Night	16% 6% 3 6% 5% 3 5% 5% 6% 7% 38%	2340
	11th to 20th	Night	16% 8% 3 6% 5% 3 5% 4 5% 6% 38%	2273
	21st to end	Night	16% 10% 4 5% 7% 3 4 5% 5% 7% 35%	1770
March	1st to 10th	Night	13% 6% 3 4 5% 3 5% 4 5% 7% 45%	1980
	11th to 20th	Night	17% 6% 3 5% 4 4 4 5% 6% 7% 41%	1980
	21st to end	Night	15% 6% 3 6% 5% 3 5% 4 5% 7% 40%	2177
April	1st to 10th	Night	16% 7% 3 6% 6% 3 5% 5% 5% 6% 37%	1800
	11th to 20th	Night	13% 5% 3 5% 5% 2 3 6% 6% 8% 44%	1775
	21st to end	Night	18% 5% 2 4 5% 2 5% 5% 5% 7% 42%	1777
May	1st to 10th	Night	16% 6% 3 6% 6% 3 4 5% 5% 8% 38%	1620
	11th to 20th	Night	17% 7% 3 4 5% 3 5% 5% 6% 8% 37%	1514
	21st to end	Night	13% 5% 2 5% 5% 3 5% 4 6% 9% 42%	1550
June	1st to 10th	Night	10% 5% 2 4 5% 3 4 5% 6% 9% 48%	1440
	11th to 20th	Night	12% 4 3 6% 7% 4 4 6% 6% 10% 40%	1440
	21st to end	Night	9% 3 2 4 5% 3 4 5% 9% 11% 44%	1440
July	1st to 10th	Night	10% 4 2 6% 5% 3 5% 7% 7% 8% 42%	1440
	11th to 20th	Night	5% 4 2 5% 5% 3 6% 5% 8% 9% 47%	1544
	21st to end	Night	8% 3 2 5% 5% 3 6% 7% 8% 11% 41%	1771
August	1st to 10th	Night	8% 5% 3 6% 7% 3 5% 6% 6% 10% 40%	1611
	11th to 20th	Night	9% 6% 2 7% 6% 4 6% 5% 5% 8% 43%	1617
	21st to end	Night	12% 4 1 5% 5% 3 5% 5% 6% 8% 45%	1980
September	1st to 10th	Night	15% 4 3 4 5% 3 5% 5% 6% 7% 45%	1800
	11th to 20th	Night	11% 6% 3 4 5% 2 4 3 5% 7% 51%	1821
	21st to end	Night	18% 6% 2 4 3 3 5% 6% 4 6% 43%	2068
October	1st to 10th	Night	19% 5% 3 4 4 3 4 4 4 6% 44%	2160
	11th to 20th	Night	15% 4 2 5% 4 2 4 4 5% 7% 49%	2160
	21st to end	Night	16% 5% 2 4 4 2 3 5% 5% 6% 47%	2376
November	1st to 10th	Night	10% 5% 3 4 4 2 4 3 5% 7% 52%	2281
	11th to 20th	Night	15% 6% 3 4 4 2 4 4 5% 8% 45%	2565
	21st to end	Night	14% 5% 3 5% 5% 3 5% 5% 6% 7% 42%	2660
December	1st to 10th	Night	16% 8% 3 6% 6% 3 4 5% 5% 7% 37%	2660
	11th to 20th	Night	17% 6% 2 5% 5% 4 4 5% 6% 8% 38%	3285
	21st to end	Night	17% 7% 3 5% 5% 4 7% 6% 7% 9% 30%	4884

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% 23% 14% 2	1589
	11th to 20th	Day	47% 7% 12% 21% 12% 1	1674
	21st to end	Day	48% 5% 10% 21% 13% 2	1782
February	1st to 10th	Day	49% 5% 13% 21% 10% 2	1620
	11th to 20th	Day	47% 6% 10% 23% 12% 2	1582
	21st to end	Day	50% 4 10% 20% 13% 3	1484
March	1st to 10th	Day	39% 6% 11% 25% 18% 2	1980
	11th to 20th	Day	45% 6% 11% 18% 17% 5%	1980
	21st to end	Day	48% 5% 10% 19% 15% 3	2178
April	1st to 10th	Day	42% 7% 12% 18% 17% 4	2160
	11th to 20th	Day	39% 6% 11% 21% 18% 5%	2134
	21st to end	Day	45% 5% 9% 21% 16% 3	2199
May	1st to 10th	Day	45% 8% 12% 19% 15% 2	2340
	11th to 20th	Day	47% 7% 11% 19% 13% 3	2382
	21st to end	Day	41% 7% 15% 23% 12% 1	2725
June	1st to 10th	Day	35% 8% 15% 24% 17% 1	2520
	11th to 20th	Day	39% 10% 15% 21% 14% 2	2520
	21st to end	Day	33% 9% 14% 26% 16% 1	2520
July	1st to 10th	Day	37% 9% 13% 22% 16% 3	2450
	11th to 20th	Day	33% 9% 14% 24% 18% 3	2339
	21st to end	Day	39% 9% 12% 21% 17% 2	2405
August	1st to 10th	Day	39% 10% 14% 26% 11%	2314
	11th to 20th	Day	38% 9% 13% 24% 15% 1	2264
	21st to end	Day	35% 8% 15% 25% 14% 3	2376
September	1st to 10th	Day	39% 7% 12% 19% 20% 4	2160
	11th to 20th	Day	33% 7% 9% 27% 20% 3	2136
	21st to end	Day	45% 5% 10% 20% 19% 2	1890
October	1st to 10th	Day	40% 5% 12% 24% 16% 3	1800
	11th to 20th	Day	38% 6% 11% 25% 19% 1	1800
	21st to end	Day	38% 6% 10% 25% 18% 2	1980
November	1st to 10th	Day	33% 5% 12% 25% 20% 4	1796
	11th to 20th	Day	38% 5% 11% 22% 20% 3	1611
	21st to end	Day	40% 7% 12% 28% 12% 1	1520
December	1st to 10th	Day	48% 8% 14% 22% 8% 1	1520
	11th to 20th	Day	43% 7% 11% 28% 8% 2	1906
	21st to end	Day	49% 10% 15% 17% 10%	2772
January	1st to 10th	Night	42% 6% 10% 23% 15% 3	2573
	11th to 20th	Night	47% 6% 14% 21% 11% 1	2425
	21st to end	Night	44% 5% 10% 26% 13% 2	2574
February	1st to 10th	Night	43% 6% 13% 24% 12% 2	2340
	11th to 20th	Night	46% 5% 11% 20% 16% 3	2275
	21st to end	Night	49% 5% 10% 19% 12% 4	1770
March	1st to 10th	Night	39% 5% 11% 27% 17% 2	1980
	11th to 20th	Night	42% 6% 11% 22% 17% 3	1980
	21st to end	Night	44% 6% 11% 20% 15% 4	2177
April	1st to 10th	Night	46% 5% 10% 19% 16% 4	1800
	11th to 20th	Night	38% 7% 10% 22% 18% 5%	1775
	21st to end	Night	40% 6% 11% 25% 14% 4	1777
May	1st to 10th	Night	43% 7% 10% 23% 13% 4	1620
	11th to 20th	Night	45% 7% 10% 18% 17% 2	1515
	21st to end	Night	37% 8% 16% 26% 12% 2	1551
June	1st to 10th	Night	34% 8% 11% 27% 18% 3	1440
	11th to 20th	Night	38% 9% 12% 26% 13% 2	1440
	21st to end	Night	31% 9% 16% 26% 17% 2	1440
July	1st to 10th	Night	36% 12% 15% 20% 13% 4	1400
	11th to 20th	Night	30% 9% 17% 25% 17% 3	1495
	21st to end	Night	30% 10% 19% 22% 16% 2	1672
August	1st to 10th	Night	36% 9% 16% 23% 15% 1	1600
	11th to 20th	Night	38% 8% 13% 22% 17% 1	1617
	21st to end	Night	34% 8% 14% 24% 16% 3	1980
September	1st to 10th	Night	38% 8% 11% 21% 19% 4	1800
	11th to 20th	Night	34% 4 10% 27% 23% 2	1821
	21st to end	Night	41% 6% 12% 21% 18% 2	2068
October	1st to 10th	Night	41% 5% 11% 24% 17% 2	2160
	11th to 20th	Night	34% 6% 10% 29% 20% 2	2160
	21st to end	Night	37% 5% 10% 27% 20% 2	2376
November	1st to 10th	Night	32% 5% 12% 27% 20% 5%	2281
	11th to 20th	Night	38% 5% 8% 25% 20% 5%	2565
	21st to end	Night	39% 5% 12% 24% 17% 2	2660
December	1st to 10th	Night	45% 6% 11% 25% 11% 1	2660
	11th to 20th	Night	43% 5% 12% 24% 14% 2	3288
	21st to end	Night	45% 9% 16% 19% 10%	4884

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	34% 24% 26% 13% 3	258
	21st to end	Day	28% 36% 23% 9% 3 1	264
February	1st to 10th	Day	30% 29% 28% 9% 3	240
	11th to 20th	Day	38% 27% 28% 7%	240
	21st to end	Day	32% 20% 26% 19% 3	342
March	1st to 10th	Day	28% 23% 25% 21% 3	420
	11th to 20th	Day	28% 24% 26% 17% 5% 1	420
	21st to end	Day	26% 22% 30% 16% 5% 2	462
April	1st to 10th	Day	23% 24% 25% 19% 7% 1	420
	11th to 20th	Day	18% 19% 30% 17% 11% 5%	412
	21st to end	Day	33% 15% 25% 16% 9% 2	420
May	1st to 10th	Day	24% 18% 28% 16% 11% 4	423
	11th to 20th	Day	23% 17% 24% 18% 14% 3	447
	21st to end	Day	18% 9% 22% 23% 19% 9%	485
June	1st to 10th	Day	15% 12% 28% 21% 20% 5%	450
	11th to 20th	Day	15% 10% 28% 24% 18% 6%	450
	21st to end	Day	12% 8% 27% 30% 16% 6%	450
July	1st to 10th	Day	13% 5% 20% 27% 22% 14%	445
	11th to 20th	Day	7% 3 22% 30% 27% 12%	444
	21st to end	Day	13% 7% 20% 31% 21% 9%	488
August	1st to 10th	Day	14% 4 19% 26% 24% 14%	446
	11th to 20th	Day	13% 4 20% 27% 24% 12%	439
	21st to end	Day	9% 5% 21% 28% 24% 13%	495
September	1st to 10th	Day	19% 7% 17% 22% 23% 12%	450
	11th to 20th	Day	14% 6% 16% 31% 23% 10%	450
	21st to end	Day	22% 10% 22% 22% 19% 5%	450
October	1st to 10th	Day	22% 6% 19% 26% 22% 5%	450
	11th to 20th	Day	18% 16% 24% 26% 14% 2	450
	21st to end	Day	20% 17% 29% 22% 9% 2	495
November	1st to 10th	Day	17% 15% 26% 24% 14% 4	426
	11th to 20th	Day	25% 15% 25% 24% 9% 1	271
	21st to end	Day	24% 18% 31% 20% 6% 2	180
December	1st to 10th	Day	28% 23% 27% 16% 5% 1	180
	11th to 20th	Day	26% 23% 25% 20% 6%	180
	21st to end	Day	24% 20% 28% 19% 9%	198
January	1st to 10th	Night	18% 21% 34% 23% 3	760
	11th to 20th	Night	24% 16% 29% 28% 3	747
	21st to end	Night	20% 21% 33% 24% 2	792
February	1st to 10th	Night	20% 21% 29% 27% 2	720
	11th to 20th	Night	24% 24% 30% 19% 2	703
	21st to end	Night	27% 19% 32% 19% 3	592
March	1st to 10th	Night	21% 18% 34% 24% 3	720
	11th to 20th	Night	25% 19% 29% 21% 6%	720
	21st to end	Night	19% 16% 36% 23% 6% 1	792
April	1st to 10th	Night	23% 19% 31% 22% 6%	720
	11th to 20th	Night	18% 13% 31% 28% 9%	711
	21st to end	Night	22% 13% 25% 32% 7%	697
May	1st to 10th	Night	21% 8% 24% 32% 14% 1	540
	11th to 20th	Night	22% 7% 24% 34% 13%	453
	21st to end	Night	14% 6% 20% 38% 21% 2	389
June	1st to 10th	Night	11% 3 24% 37% 24% 1	360
	11th to 20th	Night	11% 6% 20% 40% 22% 1	360
	21st to end	Night	9% 3 20% 46% 21% 1	360
July	1st to 10th	Night	12% 3 15% 35% 31% 4	350
	11th to 20th	Night	7% 2 11% 40% 35% 5%	451
	21st to end	Night	8% 3 14% 40% 32% 3	591
August	1st to 10th	Night	10% 2 13% 40% 31% 3	537
	11th to 20th	Night	10% 4 11% 39% 31% 4	553
	21st to end	Night	13% 3 13% 39% 28% 5%	792
September	1st to 10th	Night	17% 3 16% 34% 27% 4	720
	11th to 20th	Night	13% 5% 16% 34% 29% 3	720
	21st to end	Night	21% 6% 18% 32% 21% 2	720
October	1st to 10th	Night	21% 7% 18% 31% 21% 3	720
	11th to 20th	Night	17% 10% 25% 33% 13% 1	720
	21st to end	Night	19% 13% 27% 30% 10%	792
November	1st to 10th	Night	14% 10% 29% 36% 10% 1	740
	11th to 20th	Night	19% 18% 24% 29% 9%	760
	21st to end	Night	18% 16% 28% 31% 7%	760
December	1st to 10th	Night	22% 16% 31% 23% 7%	760
	11th to 20th	Night	19% 15% 33% 30% 4	760
	21st to end	Night	19% 16% 29% 33% 3	836

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