Roanoke
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Roanoke clear sky chart from 2005-11-06 to 2025-11-03.

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	12% 6% 2 6% 4 5% 5% 4 6% 8% 41%	126220
	Night	13% 6% 2 5% 4 4 4 4 6% 7% 45%	123816
Transparency	Day	38% 7% 12% 22% 17% 4	129826
	Night	37% 6% 11% 23% 19% 4	128279
Seeing	Day	19% 10% 22% 24% 17% 7%	16481
	Night	17% 8% 21% 33% 18% 2	26727

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	12% 6% 2 4 3 4 4 3 5% 6% 50%	4056
	11th to 20th	Day	9% 7% 2 6% 4 4 4 3 5% 5% 50%	4060
	21st to end	Day	13% 8% 2 6% 4 4 4 3 4 5% 47%	4466
February	1st to 10th	Day	10% 8% 2 7% 6% 5% 6% 5% 5% 6% 40%	1800
	11th to 20th	Day	17% 6% 2 7% 5% 5% 4 4 5% 7% 38%	1790
	21st to end	Day	17% 6% 3 6% 4 4 4 3 6% 7% 39%	1734
March	1st to 10th	Day	17% 7% 2 5% 5% 5% 6% 4 6% 8% 38%	4935
	11th to 20th	Day	10% 5% 2 5% 4 4 3 4 5% 8% 52%	4980
	21st to end	Day	14% 7% 3 7% 4 4 5% 4 5% 6% 40%	5465
April	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 6% 7% 47%	2278
	11th to 20th	Day	12% 5% 2 6% 4 5% 4 4 6% 8% 45%	2374
	21st to end	Day	16% 6% 2 6% 5% 4 4 4 6% 7% 41%	2526
May	1st to 10th	Day	11% 5% 2 7% 5% 6% 7% 5% 8% 10% 33%	5862
	11th to 20th	Day	20% 7% 2 7% 5% 4 5% 5% 7% 8% 29%	5861
	21st to end	Day	13% 5% 2 5% 4 5% 5% 4 7% 9% 40%	6433
June	1st to 10th	Day	10% 4 1 5% 5% 5% 5% 5% 8% 9% 42%	2600
	11th to 20th	Day	8% 5% 2 6% 6% 4 7% 5% 8% 11% 39%	2600
	21st to end	Day	8% 4 2 5% 5% 6% 6% 6% 8% 11% 40%	2600
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 11% 40%	5900
	11th to 20th	Day	7% 5% 2 7% 6% 7% 7% 6% 10% 13% 31%	5900
	21st to end	Day	9% 6% 3 7% 6% 6% 8% 6% 9% 11% 29%	6424
August	1st to 10th	Day	14% 7% 2 7% 5% 6% 6% 5% 8% 10% 30%	4234
	11th to 20th	Day	9% 3 2 6% 5% 5% 6% 5% 9% 11% 38%	4174
	21st to end	Day	8% 4 2 5% 5% 4 4 4 6% 10% 48%	4345
September	1st to 10th	Day	12% 4 2 5% 4 4 3 4 6% 8% 47%	2372
	11th to 20th	Day	16% 6% 2 6% 4 5% 4 3 5% 7% 42%	2200
	21st to end	Day	24% 6% 3 6% 4 3 4 4 5% 6% 36%	2200
October	1st to 10th	Day	16% 5% 2 5% 3 3 4 2 4 7% 51%	2008
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 5% 6% 53%	2000
	21st to end	Day	13% 6% 2 5% 4 4 4 3 4 6% 51%	2200
November	1st to 10th	Day	11% 5% 2 6% 3 3 4 3 4 6% 54%	1977
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 4 7% 51%	1848
	21st to end	Day	12% 4 2 5% 4 4 3 4 5% 7% 49%	1727
December	1st to 10th	Day	9% 5% 2 5% 4 4 5% 5% 6% 7% 47%	2600
	11th to 20th	Day	17% 5% 2 5% 3 4 4 4 6% 8% 43%	3225
	21st to end	Day	17% 7% 2 7% 4 4 5% 5% 7% 8% 34%	4466
January	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 7% 54%	5889
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 5% 6% 51%	5900
	21st to end	Night	15% 7% 2 5% 3 3 3 2 4 5% 50%	6490
February	1st to 10th	Night	13% 8% 2 6% 4 3 4 4 6% 6% 44%	2600
	11th to 20th	Night	19% 8% 2 6% 4 4 4 4 5% 5% 40%	2493
	21st to end	Night	14% 6% 2 6% 5% 4 5% 5% 5% 7% 42%	1895
March	1st to 10th	Night	17% 6% 2 5% 4 4 4 4 6% 6% 44%	4918
	11th to 20th	Night	10% 4 2 4 3 3 3 4 5% 8% 54%	4980
	21st to end	Night	13% 6% 2 7% 4 5% 5% 3 5% 6% 44%	5467
April	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 8% 44%	2120
	11th to 20th	Night	13% 4 2 6% 4 3 4 3 5% 7% 49%	1975
	21st to end	Night	13% 5% 1 4 4 5% 4 4 6% 6% 47%	1890
May	1st to 10th	Night	14% 7% 3 6% 5% 4 4 3 5% 9% 40%	4033
	11th to 20th	Night	22% 8% 3 7% 4 4 4 4 5% 7% 33%	4032
	21st to end	Night	15% 5% 2 5% 4 4 5% 4 7% 10% 40%	4421
June	1st to 10th	Night	9% 4 2 5% 5% 4 6% 5% 7% 11% 43%	1800
	11th to 20th	Night	9% 5% 2 5% 5% 3 4 4 7% 10% 46%	1800
	21st to end	Night	7% 4 1 4 4 5% 5% 4 8% 11% 47%	1800
July	1st to 10th	Night	7% 5% 2 6% 5% 5% 6% 5% 8% 10% 41%	4058
	11th to 20th	Night	8% 6% 2 6% 5% 5% 7% 6% 9% 12% 34%	4057
	21st to end	Night	8% 6% 3 8% 6% 6% 6% 6% 8% 9% 34%	4427
August	1st to 10th	Night	18% 5% 2 6% 4 4 5% 4 7% 11% 35%	2915
	11th to 20th	Night	8% 4 2 5% 4 5% 6% 5% 8% 9% 44%	2875
	21st to end	Night	8% 3 2 5% 4 4 4 4 6% 8% 50%	3518
September	1st to 10th	Night	11% 4 3 6% 4 4 4 4 5% 8% 47%	2023
	11th to 20th	Night	16% 5% 2 5% 5% 4 4 3 5% 7% 44%	2200
	21st to end	Night	23% 6% 2 7% 4 4 4 3 4 6% 38%	2200
October	1st to 10th	Night	17% 5% 2 4 3 3 4 2 4 6% 49%	2390
	11th to 20th	Night	15% 5% 1 5% 3 3 3 3 5% 6% 51%	2400
	21st to end	Night	14% 4 2 4 3 3 3 3 4 6% 55%	2640
November	1st to 10th	Night	13% 6% 2 4 4 3 3 3 4 6% 51%	2366
	11th to 20th	Night	11% 6% 2 4 4 4 3 4 5% 5% 52%	2549
	21st to end	Night	12% 5% 2 4 4 4 5% 4 5% 7% 50%	2665
December	1st to 10th	Night	10% 5% 2 5% 4 3 4 4 6% 6% 52%	4580
	11th to 20th	Night	16% 7% 2 7% 5% 5% 4 3 5% 6% 40%	4960
	21st to end	Night	15% 6% 2 5% 4 4 5% 4 6% 8% 41%	6490

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	34% 4 5% 24% 28% 4	4057
	11th to 20th	Day	33% 6% 6% 20% 28% 8%	4060
	21st to end	Day	41% 4 6% 26% 20% 3	4466
February	1st to 10th	Day	43% 6% 11% 21% 16% 3	1800
	11th to 20th	Day	44% 6% 8% 22% 17% 4	1799
	21st to end	Day	44% 5% 10% 18% 18% 5%	1734
March	1st to 10th	Day	43% 6% 7% 20% 15% 9%	4935
	11th to 20th	Day	31% 5% 7% 24% 22% 10%	4980
	21st to end	Day	44% 5% 9% 20% 18% 5%	5465
April	1st to 10th	Day	36% 5% 11% 23% 18% 8%	2278
	11th to 20th	Day	35% 6% 11% 24% 19% 5%	2374
	21st to end	Day	41% 6% 11% 23% 14% 4	2526
May	1st to 10th	Day	43% 9% 14% 21% 12% 2	5897
	11th to 20th	Day	50% 7% 11% 20% 10% 1	5865
	21st to end	Day	38% 8% 14% 20% 16% 4	6447
June	1st to 10th	Day	34% 10% 16% 25% 15% 1	2600
	11th to 20th	Day	36% 10% 17% 19% 16% 2	2600
	21st to end	Day	34% 11% 15% 24% 14% 2	2600
July	1st to 10th	Day	36% 9% 20% 20% 11% 4	5835
	11th to 20th	Day	41% 9% 23% 19% 7%	5831
	21st to end	Day	45% 9% 16% 20% 10%	6240
August	1st to 10th	Day	47% 6% 18% 22% 6%	4221
	11th to 20th	Day	36% 9% 16% 28% 11% 1	4213
	21st to end	Day	30% 7% 19% 29% 14% 1	4345
September	1st to 10th	Day	34% 8% 12% 21% 20% 4	2372
	11th to 20th	Day	40% 7% 9% 24% 18% 2	2200
	21st to end	Day	49% 5% 11% 17% 17% 2	2200
October	1st to 10th	Day	32% 4 9% 21% 28% 8%	3268
	11th to 20th	Day	25% 4 7% 24% 26% 14%	3240
	21st to end	Day	25% 3 10% 27% 24% 11%	3538
November	1st to 10th	Day	32% 5% 9% 22% 27% 5%	1977
	11th to 20th	Day	34% 4 10% 23% 23% 7%	1848
	21st to end	Day	33% 6% 10% 23% 25% 3	1727
December	1st to 10th	Day	34% 6% 10% 21% 23% 6%	2600
	11th to 20th	Day	40% 3 12% 21% 19% 5%	3222
	21st to end	Day	47% 6% 10% 23% 13% 1	4466
January	1st to 10th	Night	30% 5% 6% 24% 27% 8%	5889
	11th to 20th	Night	33% 4 6% 21% 29% 7%	5900
	21st to end	Night	38% 3 7% 28% 19% 5%	6490
February	1st to 10th	Night	40% 6% 11% 22% 19% 2	2600
	11th to 20th	Night	44% 5% 9% 19% 17% 5%	2495
	21st to end	Night	40% 6% 10% 23% 15% 6%	1895
March	1st to 10th	Night	40% 6% 6% 22% 18% 8%	4918
	11th to 20th	Night	28% 5% 7% 22% 28% 10%	4980
	21st to end	Night	40% 5% 8% 22% 20% 4	5467
April	1st to 10th	Night	39% 5% 11% 21% 19% 5%	2120
	11th to 20th	Night	35% 4 11% 23% 21% 6%	1975
	21st to end	Night	36% 6% 11% 26% 18% 3	1890
May	1st to 10th	Night	42% 8% 16% 20% 12% 2	4060
	11th to 20th	Night	52% 6% 11% 20% 11% 1	4033
	21st to end	Night	38% 8% 16% 25% 10% 3	4429
June	1st to 10th	Night	34% 9% 16% 26% 15% 1	1800
	11th to 20th	Night	34% 9% 17% 23% 16% 2	1800
	21st to end	Night	30% 10% 17% 23% 17% 3	1800
July	1st to 10th	Night	38% 10% 19% 19% 12% 2	4015
	11th to 20th	Night	44% 9% 22% 18% 6% 1	4006
	21st to end	Night	45% 8% 16% 20% 10%	4297
August	1st to 10th	Night	45% 7% 18% 22% 7%	2904
	11th to 20th	Night	36% 7% 17% 29% 11% 1	2902
	21st to end	Night	32% 6% 15% 35% 11% 1	3518
September	1st to 10th	Night	35% 9% 13% 20% 17% 6%	2023
	11th to 20th	Night	41% 6% 9% 23% 20% 2	2200
	21st to end	Night	50% 5% 9% 19% 15% 3	2200
October	1st to 10th	Night	37% 4 8% 18% 26% 6%	3910
	11th to 20th	Night	25% 3 7% 23% 32% 9%	3899
	21st to end	Night	21% 3 9% 29% 28% 11%	4252
November	1st to 10th	Night	35% 4 9% 21% 24% 6%	2366
	11th to 20th	Night	34% 4 8% 23% 24% 7%	2549
	21st to end	Night	35% 4 9% 26% 21% 5%	2665
December	1st to 10th	Night	33% 4 8% 28% 24% 2	4580
	11th to 20th	Night	45% 3 11% 19% 14% 7%	4962
	21st to end	Night	39% 6% 12% 25% 17% 1	6490

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	31% 23% 28% 14% 4 1	300
	11th to 20th	Day	30% 22% 28% 18% 3	300
	21st to end	Day	27% 23% 28% 15% 7%	330
February	1st to 10th	Day	23% 20% 37% 15% 5%	300
	11th to 20th	Day	38% 22% 29% 7% 3	300
	21st to end	Day	29% 18% 32% 18% 3	333
March	1st to 10th	Day	25% 18% 29% 21% 7%	500
	11th to 20th	Day	28% 19% 24% 23% 5% 1	500
	21st to end	Day	26% 15% 30% 20% 8%	550
April	1st to 10th	Day	22% 18% 28% 22% 8% 1	500
	11th to 20th	Day	20% 14% 27% 23% 12% 4	492
	21st to end	Day	22% 11% 29% 20% 13% 4	500
May	1st to 10th	Day	20% 15% 30% 19% 12% 3	503
	11th to 20th	Day	21% 8% 24% 24% 16% 7%	527
	21st to end	Day	19% 6% 20% 23% 23% 9%	573
June	1st to 10th	Day	12% 5% 26% 30% 21% 7%	530
	11th to 20th	Day	10% 7% 19% 31% 22% 12%	530
	21st to end	Day	10% 4 22% 29% 25% 10%	530
July	1st to 10th	Day	11% 1 15% 27% 30% 16%	525
	11th to 20th	Day	6% 2 13% 31% 34% 15%	524
	21st to end	Day	12% 3 12% 30% 27% 15%	576
August	1st to 10th	Day	16% 4 18% 24% 23% 16%	526
	11th to 20th	Day	12% 3 12% 28% 27% 18%	519
	21st to end	Day	11% 2 11% 30% 24% 21%	583
September	1st to 10th	Day	13% 3 14% 27% 28% 15%	530
	11th to 20th	Day	19% 2 12% 23% 29% 15%	530
	21st to end	Day	29% 6% 16% 24% 20% 4	530
October	1st to 10th	Day	21% 5% 15% 29% 23% 8%	530
	11th to 20th	Day	14% 12% 22% 27% 20% 5%	530
	21st to end	Day	20% 9% 26% 26% 15% 3	583
November	1st to 10th	Day	17% 7% 26% 33% 13% 4	503
	11th to 20th	Day	22% 14% 24% 27% 11% 2	348
	21st to end	Day	19% 21% 28% 21% 10% 2	263
December	1st to 10th	Day	22% 17% 38% 17% 7%	200
	11th to 20th	Day	25% 15% 27% 26% 6% 2	253
	21st to end	Day	25% 14% 31% 20% 9% 1	330
January	1st to 10th	Night	21% 16% 31% 28% 4	800
	11th to 20th	Night	20% 14% 30% 32% 4	800
	21st to end	Night	21% 13% 35% 30% 2	880
February	1st to 10th	Night	20% 13% 30% 34% 3	800
	11th to 20th	Night	29% 16% 29% 24% 2	783
	21st to end	Night	20% 13% 35% 29% 2	660
March	1st to 10th	Night	18% 14% 32% 33% 4	800
	11th to 20th	Night	24% 12% 30% 26% 7% 1	800
	21st to end	Night	19% 13% 29% 31% 7%	880
April	1st to 10th	Night	22% 16% 26% 29% 8%	800
	11th to 20th	Night	17% 12% 27% 32% 12% 1	791
	21st to end	Night	17% 8% 22% 39% 14% 1	690
May	1st to 10th	Night	17% 7% 24% 36% 16% 1	600
	11th to 20th	Night	20% 6% 18% 34% 20% 2	591
	21st to end	Night	16% 3 12% 31% 33% 5%	650
June	1st to 10th	Night	10% 2 14% 40% 32% 2	600
	11th to 20th	Night	10% 3 11% 34% 38% 4	600
	21st to end	Night	9% 2 11% 41% 32% 6%	600
July	1st to 10th	Night	12% 3 7% 32% 41% 6%	585
	11th to 20th	Night	8% 2 7% 30% 48% 6%	582
	21st to end	Night	7% 3 6% 37% 41% 6%	657
August	1st to 10th	Night	16% 2 7% 32% 38% 5%	597
	11th to 20th	Night	12% 1 5% 37% 39% 5%	594
	21st to end	Night	11% 2 7% 34% 38% 8%	838
September	1st to 10th	Night	12% 3 8% 35% 36% 7%	800
	11th to 20th	Night	18% 3 8% 32% 34% 6%	800
	21st to end	Night	27% 5% 13% 30% 25% 1	800
October	1st to 10th	Night	18% 6% 13% 30% 29% 5%	800
	11th to 20th	Night	17% 8% 22% 34% 18% 2	800
	21st to end	Night	17% 8% 19% 42% 13% 2	880
November	1st to 10th	Night	17% 7% 20% 40% 14% 2	789
	11th to 20th	Night	17% 10% 25% 35% 12% 1	800
	21st to end	Night	16% 11% 26% 35% 12% 1	800
December	1st to 10th	Night	21% 12% 28% 33% 6%	800
	11th to 20th	Night	19% 9% 30% 38% 5%	800
	21st to end	Night	21% 12% 26% 37% 5%	880

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