Washington and Old Dominion Trail
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Washington and Old Dominion Trail clear sky chart from 2005-11-06 to 2026-09-29.

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% 4 4 5% 4 6% 8% 41%	149176
	Night	14% 6% 2 6% 4 4 4 4 6% 7% 42%	142234
Transparency	Day	27% 35% 8% 17% 11% 2	152773
	Night	27% 35% 8% 16% 12% 2	146708
Seeing	Day	19% 13% 27% 23% 13% 4	17436
	Night	18% 12% 29% 29% 11% 1	26927

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% 4 2 6% 4 4 4 4 5% 6% 46%	4878
	11th to 20th	Day	13% 9% 3 6% 4 3 3 3 4 4 49%	5280
	21st to end	Day	13% 7% 2 6% 4 4 4 4 6% 7% 42%	5780
February	1st to 10th	Day	13% 8% 2 6% 6% 4 5% 4 5% 7% 40%	1890
	11th to 20th	Day	18% 8% 2 5% 5% 4 4 3 5% 6% 40%	1880
	21st to end	Day	17% 8% 3 6% 4 4 5% 3 5% 5% 40%	1730
March	1st to 10th	Day	18% 6% 2 5% 4 5% 5% 3 4 7% 41%	5013
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 7% 49%	5090
	21st to end	Day	15% 6% 3 6% 5% 4 5% 4 5% 7% 40%	5646
April	1st to 10th	Day	14% 7% 2 6% 5% 4 4 4 6% 7% 42%	2520
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 5% 6% 46%	2494
	21st to end	Day	18% 6% 2 6% 4 4 4 4 5% 6% 42%	2520
May	1st to 10th	Day	17% 7% 2 6% 4 4 5% 4 6% 9% 34%	7572
	11th to 20th	Day	18% 6% 2 6% 4 4 5% 4 7% 10% 36%	7923
	21st to end	Day	22% 6% 2 6% 4 4 4 3 5% 7% 38%	9045
June	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 7% 9% 44%	2940
	11th to 20th	Day	10% 6% 2 6% 4 4 6% 4 7% 9% 41%	2940
	21st to end	Day	9% 4 2 5% 5% 5% 5% 4 7% 10% 45%	2940
July	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 5% 7% 10% 41%	8280
	11th to 20th	Day	8% 6% 3 7% 5% 6% 6% 5% 9% 12% 32%	7929
	21st to end	Day	8% 5% 2 7% 5% 6% 6% 5% 7% 10% 39%	8340
August	1st to 10th	Day	13% 6% 3 8% 7% 7% 6% 6% 8% 10% 29%	6014
	11th to 20th	Day	10% 5% 2 7% 5% 5% 6% 5% 7% 10% 38%	5746
	21st to end	Day	9% 4 2 5% 4 3 6% 4 6% 9% 48%	6080
September	1st to 10th	Day	14% 5% 2 4 4 4 4 3 5% 8% 47%	2520
	11th to 20th	Day	12% 6% 2 6% 3 4 4 3 5% 8% 47%	2520
	21st to end	Day	21% 7% 3 6% 4 3 4 3 5% 6% 36%	2176
October	1st to 10th	Day	16% 6% 1 4 4 4 3 3 5% 6% 47%	2000
	11th to 20th	Day	16% 4 2 6% 3 3 3 3 5% 6% 49%	2000
	21st to end	Day	13% 5% 3 5% 3 4 4 3 5% 6% 50%	2200
November	1st to 10th	Day	11% 5% 2 5% 4 4 3 3 6% 7% 51%	1971
	11th to 20th	Day	14% 6% 2 5% 3 3 3 4 6% 6% 50%	1841
	21st to end	Day	14% 6% 2 5% 3 4 5% 4 5% 7% 45%	1680
December	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 48%	2680
	11th to 20th	Day	18% 8% 2 5% 3 4 4 4 5% 6% 42%	3070
	21st to end	Day	17% 8% 3 7% 5% 4 4 4 6% 8% 35%	4048
January	1st to 10th	Night	11% 7% 2 6% 5% 5% 4 4 6% 6% 43%	8013
	11th to 20th	Night	13% 6% 2 6% 3 3 3 2 4 5% 50%	7680
	21st to end	Night	13% 7% 2 5% 3 3 4 3 4 6% 48%	8401
February	1st to 10th	Night	14% 8% 2 6% 5% 4 5% 4 5% 5% 42%	2730
	11th to 20th	Night	19% 7% 2 6% 4 4 4 3 5% 6% 40%	2625
	21st to end	Night	16% 9% 2 6% 4 4 4 3 5% 6% 42%	2076
March	1st to 10th	Night	16% 6% 2 6% 4 3 4 3 4 5% 47%	5057
	11th to 20th	Night	11% 6% 2 4 3 4 4 4 5% 7% 50%	5090
	21st to end	Night	12% 7% 2 6% 4 4 5% 4 6% 6% 43%	5526
April	1st to 10th	Night	14% 8% 2 5% 5% 4 5% 4 6% 6% 41%	2100
	11th to 20th	Night	13% 6% 2 6% 3 4 4 3 6% 7% 44%	2075
	21st to end	Night	15% 5% 2 5% 4 4 5% 4 6% 7% 45%	2100
May	1st to 10th	Night	19% 7% 2 7% 5% 5% 5% 4 6% 7% 33%	5325
	11th to 20th	Night	20% 7% 2 6% 4 4 5% 4 6% 8% 35%	4919
	21st to end	Night	23% 7% 3 6% 4 4 4 4 5% 6% 33%	5108
June	1st to 10th	Night	10% 5% 2 4 4 5% 5% 4 6% 9% 45%	1680
	11th to 20th	Night	12% 5% 2 5% 5% 4 6% 4 6% 8% 41%	1680
	21st to end	Night	8% 5% 3 6% 5% 5% 5% 4 7% 10% 43%	1680
July	1st to 10th	Night	10% 6% 3 8% 5% 5% 5% 4 7% 9% 39%	4678
	11th to 20th	Night	9% 8% 3 9% 7% 6% 6% 5% 7% 9% 31%	5010
	21st to end	Night	8% 6% 3 7% 5% 5% 6% 5% 7% 9% 38%	5736
August	1st to 10th	Night	14% 6% 3 9% 6% 6% 6% 5% 9% 10% 27%	4135
	11th to 20th	Night	10% 6% 3 9% 6% 6% 6% 5% 7% 9% 34%	4253
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 6% 9% 45%	5045
September	1st to 10th	Night	13% 5% 2 6% 4 4 4 3 5% 7% 46%	2100
	11th to 20th	Night	10% 6% 2 5% 4 4 3 3 5% 7% 50%	2100
	21st to end	Night	21% 7% 3 7% 4 4 4 3 4 6% 37%	2395
October	1st to 10th	Night	20% 5% 2 5% 3 3 3 3 4 6% 47%	2400
	11th to 20th	Night	15% 5% 2 4 3 3 3 2 4 5% 53%	2400
	21st to end	Night	14% 5% 2 5% 3 4 4 2 4 6% 51%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 6% 54%	2545
	11th to 20th	Night	14% 6% 2 4 4 4 3 3 4 6% 50%	2777
	21st to end	Night	13% 4 2 6% 4 4 4 4 6% 7% 47%	2940
December	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 51%	4720
	11th to 20th	Night	20% 6% 2 5% 4 4 4 4 5% 6% 40%	5345
	21st to end	Night	16% 7% 2 6% 5% 5% 5% 4 7% 8% 35%	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	31% 22% 7% 21% 14% 5%	4879
	11th to 20th	Day	31% 21% 4 18% 16% 9%	5280
	21st to end	Day	31% 24% 7% 25% 10% 2	5780
February	1st to 10th	Day	18% 60% 6% 8% 6% 1	1890
	11th to 20th	Day	20% 58% 3 9% 7% 1	1889
	21st to end	Day	20% 59% 4 10% 6% 1	1730
March	1st to 10th	Day	32% 31% 6% 14% 11% 7%	5013
	11th to 20th	Day	23% 29% 8% 21% 15% 4	5090
	21st to end	Day	30% 30% 8% 13% 14% 5%	5646
April	1st to 10th	Day	19% 60% 4 8% 8% 1	2520
	11th to 20th	Day	15% 60% 5% 10% 8% 1	2494
	21st to end	Day	18% 60% 4 11% 7%	2520
May	1st to 10th	Day	36% 26% 10% 17% 10% 1	7607
	11th to 20th	Day	32% 26% 10% 24% 8%	7927
	21st to end	Day	37% 25% 8% 13% 14% 2	9061
June	1st to 10th	Day	16% 61% 5% 10% 8% 1	2940
	11th to 20th	Day	14% 61% 6% 11% 7% 1	2940
	21st to end	Day	14% 59% 7% 12% 8%	2940
July	1st to 10th	Day	31% 26% 13% 17% 11% 3	8210
	11th to 20th	Day	34% 25% 11% 22% 7%	7855
	21st to end	Day	31% 23% 8% 20% 18%	8156
August	1st to 10th	Day	38% 31% 12% 15% 4	6001
	11th to 20th	Day	28% 30% 12% 21% 8%	5785
	21st to end	Day	22% 29% 12% 22% 14%	6080
September	1st to 10th	Day	16% 56% 8% 11% 8% 1	2520
	11th to 20th	Day	17% 55% 4 13% 9% 2	2520
	21st to end	Day	26% 55% 4 9% 5% 1	2176
October	1st to 10th	Day	23% 36% 4 10% 22% 4	3260
	11th to 20th	Day	13% 36% 5% 20% 18% 8%	3240
	21st to end	Day	9% 35% 8% 21% 22% 5%	3538
November	1st to 10th	Day	17% 54% 7% 10% 9% 3	1971
	11th to 20th	Day	14% 55% 4 12% 13% 2	1841
	21st to end	Day	18% 54% 6% 15% 7% 1	1680
December	1st to 10th	Day	23% 36% 10% 19% 9% 4	2680
	11th to 20th	Day	31% 31% 10% 19% 8% 2	3066
	21st to end	Day	34% 29% 12% 14% 10% 1	4048
January	1st to 10th	Night	31% 23% 7% 21% 16% 3	8014
	11th to 20th	Night	29% 21% 5% 18% 20% 7%	7680
	21st to end	Night	28% 23% 6% 25% 16% 2	8401
February	1st to 10th	Night	17% 60% 6% 10% 5% 1	2730
	11th to 20th	Night	19% 58% 3 9% 8% 2	2627
	21st to end	Night	18% 59% 5% 9% 6% 3	2076
March	1st to 10th	Night	31% 30% 5% 17% 9% 8%	5057
	11th to 20th	Night	21% 30% 9% 21% 14% 5%	5090
	21st to end	Night	28% 30% 7% 15% 17% 3	5526
April	1st to 10th	Night	18% 60% 4 9% 8% 1	2100
	11th to 20th	Night	16% 60% 6% 10% 6% 1	2075
	21st to end	Night	16% 60% 5% 11% 7% 1	2100
May	1st to 10th	Night	40% 25% 9% 15% 9% 2	5352
	11th to 20th	Night	36% 26% 10% 21% 7%	4920
	21st to end	Night	43% 25% 7% 14% 10% 1	5115
June	1st to 10th	Night	16% 60% 5% 10% 9% 1	1680
	11th to 20th	Night	16% 61% 6% 11% 6%	1680
	21st to end	Night	15% 59% 6% 11% 8% 1	1680
July	1st to 10th	Night	35% 25% 12% 15% 12% 2	4640
	11th to 20th	Night	40% 25% 12% 16% 8%	4964
	21st to end	Night	33% 24% 11% 17% 14% 1	5606
August	1st to 10th	Night	40% 31% 12% 12% 4	4124
	11th to 20th	Night	34% 30% 12% 16% 8%	4280
	21st to end	Night	26% 28% 10% 23% 12%	5045
September	1st to 10th	Night	15% 56% 8% 13% 7% 2	2100
	11th to 20th	Night	17% 54% 5% 12% 9% 1	2100
	21st to end	Night	26% 56% 4 8% 5%	2395
October	1st to 10th	Night	26% 36% 6% 13% 14% 4	3920
	11th to 20th	Night	12% 36% 7% 14% 24% 6%	3899
	21st to end	Night	11% 35% 6% 24% 18% 5%	4252
November	1st to 10th	Night	16% 54% 7% 11% 10% 3	2545
	11th to 20th	Night	15% 54% 4 10% 13% 3	2777
	21st to end	Night	15% 54% 6% 14% 9% 2	2940
December	1st to 10th	Night	21% 35% 10% 20% 13% 1	4720
	11th to 20th	Night	32% 32% 8% 13% 12% 3	5348
	21st to end	Night	34% 30% 11% 13% 11% 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	32% 24% 27% 15% 1 1	259
	11th to 20th	Day	35% 25% 28% 11% 2	330
	21st to end	Day	23% 29% 34% 10% 3 1	363
February	1st to 10th	Day	25% 25% 34% 12% 4	330
	11th to 20th	Day	36% 24% 32% 5% 2	367
	21st to end	Day	27% 21% 33% 17% 2	446
March	1st to 10th	Day	25% 23% 31% 18% 4	540
	11th to 20th	Day	28% 21% 29% 16% 4 1	540
	21st to end	Day	23% 20% 33% 18% 5% 1	594
April	1st to 10th	Day	26% 22% 28% 17% 5% 2	540
	11th to 20th	Day	19% 20% 30% 17% 12% 3	532
	21st to end	Day	26% 13% 32% 19% 9% 1	540
May	1st to 10th	Day	22% 19% 30% 18% 8% 3	543
	11th to 20th	Day	21% 12% 28% 23% 12% 3	567
	21st to end	Day	20% 8% 25% 25% 18% 4	617
June	1st to 10th	Day	13% 9% 29% 24% 19% 5%	570
	11th to 20th	Day	14% 9% 25% 32% 16% 4	570
	21st to end	Day	10% 6% 28% 32% 19% 6%	570
July	1st to 10th	Day	12% 4 17% 31% 26% 10%	565
	11th to 20th	Day	6% 4 20% 32% 30% 8%	564
	21st to end	Day	11% 4 22% 32% 24% 7%	620
August	1st to 10th	Day	13% 5% 20% 28% 24% 11%	566
	11th to 20th	Day	11% 5% 21% 32% 22% 9%	559
	21st to end	Day	9% 5% 22% 31% 21% 12%	627
September	1st to 10th	Day	16% 6% 22% 25% 20% 10%	570
	11th to 20th	Day	13% 5% 20% 31% 21% 10%	570
	21st to end	Day	28% 9% 25% 24% 10% 4	562
October	1st to 10th	Day	21% 7% 19% 27% 18% 8%	530
	11th to 20th	Day	17% 15% 27% 24% 14% 2	530
	21st to end	Day	17% 14% 33% 22% 11% 3	583
November	1st to 10th	Day	15% 14% 34% 24% 11% 2	489
	11th to 20th	Day	22% 15% 34% 20% 9% 1	381
	21st to end	Day	25% 20% 32% 14% 8%	220
December	1st to 10th	Day	22% 23% 37% 15% 3	220
	11th to 20th	Day	26% 20% 29% 20% 5%	220
	21st to end	Day	27% 21% 29% 17% 5%	242
January	1st to 10th	Night	21% 19% 40% 17% 3	840
	11th to 20th	Night	23% 18% 33% 24% 3	840
	21st to end	Night	20% 18% 41% 21% 1	924
February	1st to 10th	Night	19% 17% 37% 25% 2	840
	11th to 20th	Night	26% 22% 35% 16% 1	823
	21st to end	Night	24% 18% 37% 18% 2	692
March	1st to 10th	Night	21% 18% 38% 21% 3	840
	11th to 20th	Night	23% 20% 33% 19% 5%	840
	21st to end	Night	19% 20% 38% 19% 4 1	924
April	1st to 10th	Night	20% 21% 34% 21% 4	840
	11th to 20th	Night	15% 16% 35% 26% 7% 1	831
	21st to end	Night	18% 13% 33% 28% 7%	840
May	1st to 10th	Night	20% 10% 29% 32% 9%	666
	11th to 20th	Night	20% 7% 28% 34% 11%	514
	21st to end	Night	16% 6% 25% 37% 16% 1	455
June	1st to 10th	Night	10% 5% 26% 36% 22%	420
	11th to 20th	Night	12% 7% 24% 40% 16% 1	420
	21st to end	Night	9% 4 25% 45% 17% 1	420
July	1st to 10th	Night	12% 2 17% 41% 25% 3	410
	11th to 20th	Night	6% 1 14% 41% 36% 2	519
	21st to end	Night	10% 2 17% 43% 26% 3	690
August	1st to 10th	Night	13% 2 12% 42% 29% 3	627
	11th to 20th	Night	12% 5% 14% 42% 25% 3	691
	21st to end	Night	11% 3 17% 40% 25% 4	924
September	1st to 10th	Night	14% 3 21% 33% 26% 3	840
	11th to 20th	Night	11% 5% 17% 36% 28% 3	840
	21st to end	Night	24% 4 22% 31% 19%	833
October	1st to 10th	Night	21% 6% 21% 29% 20% 4	800
	11th to 20th	Night	17% 12% 28% 29% 14% 1	800
	21st to end	Night	18% 11% 32% 27% 12%	880
November	1st to 10th	Night	14% 11% 32% 33% 9% 1	820
	11th to 20th	Night	18% 16% 32% 27% 5% 1	840
	21st to end	Night	17% 16% 33% 28% 7%	840
December	1st to 10th	Night	21% 16% 33% 23% 6%	840
	11th to 20th	Night	20% 14% 37% 25% 4	840
	21st to end	Night	20% 15% 36% 28% 2	924

Page generated at 2026-09-26 04:18:39. Version ac07

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