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 2024-11-06.

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	13% 6% 2 6% 4 5% 5% 4 6% 8% 41%	100862
	Night	14% 6% 2 5% 4 4 4 4 6% 8% 44%	98140
Transparency	Day	38% 7% 12% 23% 16% 4	104512
	Night	37% 6% 11% 24% 18% 4	102629
Seeing	Day	19% 10% 22% 24% 17% 7%	15156
	Night	17% 8% 20% 34% 18% 2	25402

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% 5% 2 4 3 4 4 3 6% 7% 47%	2836
	11th to 20th	Day	11% 8% 3 6% 5% 5% 4 4 5% 6% 43%	2840
	21st to end	Day	16% 9% 3 7% 4 4 4 3 4 5% 40%	3124
February	1st to 10th	Day	10% 8% 2 7% 5% 5% 6% 5% 5% 6% 40%	1710
	11th to 20th	Day	16% 6% 2 7% 5% 5% 4 4 5% 8% 39%	1698
	21st to end	Day	18% 6% 3 6% 4 4 4 3 6% 7% 38%	1650
March	1st to 10th	Day	21% 9% 3 5% 3 3 4 3 5% 8% 36%	3435
	11th to 20th	Day	10% 5% 2 5% 4 4 3 4 4 7% 52%	3480
	21st to end	Day	17% 9% 3 6% 4 4 5% 3 5% 6% 38%	3815
April	1st to 10th	Day	10% 5% 2 5% 4 4 5% 4 6% 7% 48%	2164
	11th to 20th	Day	12% 5% 2 6% 4 5% 4 4 6% 8% 44%	2254
	21st to end	Day	16% 6% 2 6% 4 4 4 4 6% 6% 41%	2399
May	1st to 10th	Day	15% 7% 3 8% 5% 6% 6% 5% 7% 8% 31%	4082
	11th to 20th	Day	20% 7% 3 6% 5% 4 5% 4 6% 8% 32%	4081
	21st to end	Day	9% 4 2 5% 4 5% 6% 5% 8% 10% 43%	4476
June	1st to 10th	Day	10% 4 1 6% 5% 5% 5% 5% 8% 9% 41%	2470
	11th to 20th	Day	8% 5% 1 6% 5% 4 7% 5% 8% 11% 39%	2470
	21st to end	Day	8% 4 2 5% 5% 6% 6% 6% 8% 11% 39%	2470
July	1st to 10th	Day	6% 4 1 6% 5% 5% 5% 5% 10% 12% 40%	4120
	11th to 20th	Day	8% 5% 2 6% 5% 6% 5% 6% 9% 12% 36%	4120
	21st to end	Day	12% 7% 3 8% 7% 6% 7% 6% 8% 9% 25%	4466
August	1st to 10th	Day	13% 7% 2 7% 5% 6% 6% 5% 8% 10% 30%	4104
	11th to 20th	Day	9% 3 2 6% 5% 5% 6% 5% 9% 11% 38%	4044
	21st to end	Day	8% 4 2 5% 5% 4 4 4 6% 10% 48%	4212
September	1st to 10th	Day	13% 4 2 5% 4 4 3 4 6% 7% 46%	2254
	11th to 20th	Day	16% 6% 2 6% 4 5% 4 3 5% 7% 40%	2090
	21st to end	Day	24% 6% 2 6% 4 3 4 4 5% 6% 37%	2090
October	1st to 10th	Day	16% 5% 2 5% 3 3 4 2 4 7% 50%	1908
	11th to 20th	Day	14% 5% 1 5% 3 3 4 3 5% 6% 52%	1900
	21st to end	Day	12% 6% 2 4 4 4 4 3 4 7% 51%	2090
November	1st to 10th	Day	11% 5% 2 6% 3 3 4 3 4 6% 55%	1908
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 4 7% 51%	1756
	21st to end	Day	13% 4 2 5% 4 4 3 4 5% 7% 49%	1641
December	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 6% 6% 7% 38%	1520
	11th to 20th	Day	14% 4 1 4 3 3 4 3 5% 7% 51%	2061
	21st to end	Day	18% 6% 2 6% 4 4 5% 5% 7% 9% 34%	3124
January	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 5% 7% 48%	4109
	11th to 20th	Night	14% 8% 2 5% 3 3 3 3 4 6% 47%	4120
	21st to end	Night	18% 9% 3 6% 4 3 4 3 5% 5% 41%	4532
February	1st to 10th	Night	13% 8% 2 6% 4 3 4 4 6% 6% 44%	2470
	11th to 20th	Night	19% 8% 2 6% 4 3 4 4 6% 5% 40%	2365
	21st to end	Night	14% 7% 2 6% 5% 4 5% 4 4 7% 41%	1803
March	1st to 10th	Night	22% 8% 3 6% 4 4 5% 4 5% 6% 34%	3418
	11th to 20th	Night	11% 5% 2 4 3 4 4 4 5% 8% 51%	3480
	21st to end	Night	16% 6% 2 5% 3 4 4 3 5% 6% 48%	3817
April	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 5% 7% 45%	2014
	11th to 20th	Night	13% 4 2 6% 4 3 3 3 5% 7% 50%	1875
	21st to end	Night	14% 5% 1 4 4 4 4 4 6% 6% 48%	1796
May	1st to 10th	Night	15% 7% 3 7% 5% 4 4 4 5% 8% 37%	2813
	11th to 20th	Night	24% 8% 2 6% 3 4 3 3 4 6% 37%	2812
	21st to end	Night	10% 3 1 5% 4 3 5% 4 7% 11% 47%	3079
June	1st to 10th	Night	9% 4 2 5% 5% 4 6% 5% 7% 11% 42%	1710
	11th to 20th	Night	9% 5% 2 5% 5% 3 4 3 7% 10% 46%	1710
	21st to end	Night	7% 4 1 4 4 5% 5% 4 8% 11% 46%	1710
July	1st to 10th	Night	6% 4 2 5% 5% 5% 6% 5% 8% 11% 44%	2840
	11th to 20th	Night	8% 6% 2 5% 4 4 5% 5% 7% 12% 42%	2840
	21st to end	Night	9% 6% 3 9% 6% 6% 6% 5% 8% 9% 32%	3087
August	1st to 10th	Night	17% 5% 2 6% 4 4 5% 4 7% 11% 35%	2825
	11th to 20th	Night	8% 4 2 6% 4 5% 6% 5% 8% 9% 44%	2785
	21st to end	Night	8% 3 2 5% 4 4 4 4 6% 8% 51%	3410
September	1st to 10th	Night	11% 4 3 7% 4 4 4 4 6% 8% 47%	1922
	11th to 20th	Night	17% 5% 2 5% 5% 4 4 3 5% 7% 43%	2090
	21st to end	Night	22% 5% 2 7% 4 4 4 3 4 5% 39%	2090
October	1st to 10th	Night	18% 5% 2 4 3 3 4 2 5% 6% 48%	2270
	11th to 20th	Night	15% 5% 1 5% 3 3 4 3 4 6% 51%	2280
	21st to end	Night	13% 4 2 4 3 3 3 3 5% 6% 56%	2508
November	1st to 10th	Night	13% 6% 2 4 4 3 3 3 4 6% 53%	2292
	11th to 20th	Night	12% 5% 2 4 4 4 3 4 5% 5% 52%	2421
	21st to end	Night	12% 5% 2 4 3 4 4 4 5% 7% 50%	2531
December	1st to 10th	Night	16% 7% 3 6% 5% 4 4 3 5% 6% 42%	2660
	11th to 20th	Night	14% 6% 2 5% 4 4 4 3 5% 7% 45%	3124
	21st to end	Night	17% 5% 2 6% 4 4 5% 5% 7% 8% 37%	4532

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	35% 5% 7% 30% 19% 4	2837
	11th to 20th	Day	39% 7% 7% 21% 22% 4	2840
	21st to end	Day	46% 5% 7% 23% 16% 3	3124
February	1st to 10th	Day	43% 6% 11% 21% 17% 2	1710
	11th to 20th	Day	43% 6% 8% 22% 17% 4	1707
	21st to end	Day	45% 5% 11% 16% 18% 5%	1650
March	1st to 10th	Day	46% 5% 8% 22% 13% 6%	3435
	11th to 20th	Day	31% 7% 8% 26% 19% 10%	3480
	21st to end	Day	46% 6% 8% 22% 15% 3	3815
April	1st to 10th	Day	34% 6% 11% 23% 18% 8%	2164
	11th to 20th	Day	35% 7% 11% 23% 19% 5%	2254
	21st to end	Day	42% 6% 11% 23% 14% 4	2399
May	1st to 10th	Day	46% 11% 12% 19% 11% 2	4120
	11th to 20th	Day	47% 8% 10% 24% 9% 1	4085
	21st to end	Day	31% 11% 16% 22% 16% 4	4489
June	1st to 10th	Day	34% 10% 16% 25% 14% 1	2470
	11th to 20th	Day	35% 10% 17% 20% 16% 2	2470
	21st to end	Day	35% 11% 15% 23% 15% 2	2470
July	1st to 10th	Day	33% 10% 23% 18% 10% 5%	4055
	11th to 20th	Day	38% 7% 20% 24% 10% 1	4051
	21st to end	Day	51% 10% 12% 18% 9% 1	4324
August	1st to 10th	Day	47% 7% 18% 22% 6%	4091
	11th to 20th	Day	35% 9% 16% 28% 12% 1	4083
	21st to end	Day	30% 7% 19% 29% 14% 1	4212
September	1st to 10th	Day	35% 8% 12% 21% 20% 4	2254
	11th to 20th	Day	41% 7% 9% 24% 18% 2	2090
	21st to end	Day	48% 5% 10% 18% 17% 2	2090
October	1st to 10th	Day	32% 4 9% 20% 28% 8%	3168
	11th to 20th	Day	25% 4 7% 24% 26% 14%	3140
	21st to end	Day	24% 3 11% 27% 23% 11%	3428
November	1st to 10th	Day	32% 5% 9% 22% 27% 5%	1908
	11th to 20th	Day	34% 4 10% 23% 23% 7%	1756
	21st to end	Day	34% 5% 10% 23% 24% 3	1641
December	1st to 10th	Day	43% 8% 11% 17% 17% 3	1520
	11th to 20th	Day	33% 4 8% 25% 22% 7%	2058
	21st to end	Day	44% 7% 9% 26% 13% 2	3124
January	1st to 10th	Night	34% 6% 8% 26% 22% 4	4109
	11th to 20th	Night	38% 5% 8% 21% 23% 5%	4120
	21st to end	Night	45% 5% 7% 24% 14% 5%	4532
February	1st to 10th	Night	39% 6% 11% 23% 19% 2	2470
	11th to 20th	Night	44% 5% 9% 20% 17% 5%	2367
	21st to end	Night	42% 6% 10% 23% 14% 5%	1803
March	1st to 10th	Night	49% 7% 8% 19% 13% 3	3418
	11th to 20th	Night	30% 7% 7% 23% 25% 9%	3480
	21st to end	Night	38% 4 10% 24% 19% 4	3817
April	1st to 10th	Night	38% 5% 11% 22% 20% 5%	2014
	11th to 20th	Night	35% 4 11% 23% 21% 6%	1875
	21st to end	Night	35% 6% 11% 26% 18% 3	1796
May	1st to 10th	Night	44% 9% 13% 20% 11% 1	2840
	11th to 20th	Night	47% 7% 10% 24% 11% 1	2813
	21st to end	Night	29% 10% 18% 27% 11% 4	3087
June	1st to 10th	Night	33% 9% 16% 26% 14%	1710
	11th to 20th	Night	33% 9% 17% 24% 15% 2	1710
	21st to end	Night	29% 10% 17% 23% 17% 3	1710
July	1st to 10th	Night	34% 11% 21% 19% 12% 3	2795
	11th to 20th	Night	37% 8% 21% 25% 8% 2	2786
	21st to end	Night	46% 9% 16% 20% 8%	2988
August	1st to 10th	Night	45% 7% 19% 22% 7%	2814
	11th to 20th	Night	36% 7% 16% 29% 11% 1	2812
	21st to end	Night	31% 6% 15% 35% 10% 1	3410
September	1st to 10th	Night	36% 10% 13% 20% 17% 6%	1922
	11th to 20th	Night	41% 6% 9% 23% 19% 2	2090
	21st to end	Night	48% 5% 9% 20% 15% 3	2090
October	1st to 10th	Night	38% 4 8% 18% 26% 6%	3790
	11th to 20th	Night	25% 3 7% 23% 32% 9%	3779
	21st to end	Night	20% 3 9% 30% 28% 11%	4120
November	1st to 10th	Night	34% 4 10% 22% 25% 7%	2292
	11th to 20th	Night	33% 4 9% 23% 24% 7%	2421
	21st to end	Night	35% 4 9% 27% 20% 5%	2531
December	1st to 10th	Night	43% 5% 8% 25% 16% 3	2660
	11th to 20th	Night	39% 5% 8% 23% 19% 6%	3126
	21st to end	Night	41% 7% 10% 22% 18% 2	4532

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	29% 23% 28% 16% 4 1	270
	11th to 20th	Day	31% 19% 29% 18% 3	270
	21st to end	Day	28% 22% 28% 15% 7%	297
February	1st to 10th	Day	25% 17% 37% 16% 6%	270
	11th to 20th	Day	36% 21% 31% 8% 3	270
	21st to end	Day	30% 18% 30% 17% 3	305
March	1st to 10th	Day	24% 18% 28% 22% 7%	460
	11th to 20th	Day	28% 20% 25% 22% 5% 1	460
	21st to end	Day	25% 15% 30% 21% 9%	506
April	1st to 10th	Day	22% 17% 27% 23% 9% 1	460
	11th to 20th	Day	20% 14% 27% 23% 13% 4	452
	21st to end	Day	24% 11% 29% 19% 13% 4	460
May	1st to 10th	Day	22% 14% 29% 19% 12% 4	463
	11th to 20th	Day	20% 7% 23% 25% 17% 8%	487
	21st to end	Day	16% 5% 19% 24% 25% 10%	529
June	1st to 10th	Day	12% 5% 24% 29% 22% 7%	490
	11th to 20th	Day	10% 7% 18% 31% 21% 12%	490
	21st to end	Day	11% 4 23% 31% 22% 9%	490
July	1st to 10th	Day	12% 1 14% 28% 29% 16%	485
	11th to 20th	Day	6% 3 13% 31% 34% 13%	484
	21st to end	Day	13% 3 13% 31% 26% 15%	532
August	1st to 10th	Day	14% 4 19% 24% 24% 16%	486
	11th to 20th	Day	12% 3 13% 27% 26% 19%	479
	21st to end	Day	12% 1 10% 29% 25% 23%	539
September	1st to 10th	Day	14% 3 12% 27% 28% 16%	490
	11th to 20th	Day	20% 2 12% 22% 29% 16%	490
	21st to end	Day	29% 6% 16% 23% 21% 4	490
October	1st to 10th	Day	22% 5% 15% 29% 23% 7%	490
	11th to 20th	Day	14% 13% 20% 27% 20% 6%	490
	21st to end	Day	19% 8% 28% 27% 16% 3	539
November	1st to 10th	Day	17% 7% 25% 33% 14% 4	476
	11th to 20th	Day	23% 15% 22% 27% 12% 2	316
	21st to end	Day	18% 19% 28% 23% 11% 2	237
December	1st to 10th	Day	23% 17% 35% 17% 7%	180
	11th to 20th	Day	24% 16% 27% 25% 6% 2	227
	21st to end	Day	26% 12% 32% 19% 10% 1	297
January	1st to 10th	Night	20% 15% 31% 30% 4	760
	11th to 20th	Night	20% 14% 29% 33% 4	760
	21st to end	Night	21% 13% 33% 31% 2	836
February	1st to 10th	Night	20% 13% 29% 35% 3	760
	11th to 20th	Night	28% 16% 29% 25% 2	743
	21st to end	Night	21% 13% 34% 29% 2	628
March	1st to 10th	Night	18% 13% 31% 34% 4	760
	11th to 20th	Night	25% 12% 30% 26% 7% 1	760
	21st to end	Night	20% 12% 28% 32% 8%	836
April	1st to 10th	Night	21% 15% 26% 29% 8%	760
	11th to 20th	Night	18% 11% 26% 32% 12% 1	751
	21st to end	Night	17% 8% 23% 38% 13% 1	656
May	1st to 10th	Night	18% 7% 24% 35% 16% 1	570
	11th to 20th	Night	20% 6% 18% 34% 20% 2	561
	21st to end	Night	15% 3 10% 32% 35% 5%	617
June	1st to 10th	Night	10% 2 14% 40% 33% 2	570
	11th to 20th	Night	10% 2 11% 34% 39% 4	570
	21st to end	Night	9% 2 12% 42% 30% 6%	570
July	1st to 10th	Night	12% 3 6% 32% 41% 6%	555
	11th to 20th	Night	8% 2 7% 30% 49% 5%	552
	21st to end	Night	7% 2 6% 38% 40% 6%	624
August	1st to 10th	Night	15% 2 8% 32% 39% 5%	567
	11th to 20th	Night	12% 1 5% 38% 39% 5%	564
	21st to end	Night	12% 1 6% 33% 40% 9%	796
September	1st to 10th	Night	12% 3 8% 34% 36% 7%	760
	11th to 20th	Night	18% 3 8% 31% 34% 6%	760
	21st to end	Night	27% 4 13% 29% 26% 1	760
October	1st to 10th	Night	18% 6% 13% 30% 28% 5%	760
	11th to 20th	Night	17% 8% 22% 33% 18% 2	760
	21st to end	Night	16% 7% 18% 43% 14% 2	836
November	1st to 10th	Night	17% 7% 19% 40% 15% 2	764
	11th to 20th	Night	17% 11% 24% 35% 12% 1	760
	21st to end	Night	16% 10% 25% 36% 12% 1	760
December	1st to 10th	Night	22% 12% 27% 34% 6%	760
	11th to 20th	Night	18% 9% 30% 38% 5%	760
	21st to end	Night	21% 12% 26% 36% 5%	836

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