Blue Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Ridge Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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% 42%	137943
	Night	14% 6% 2 5% 4 4 4 4 5% 7% 45%	135036
Transparency	Day	38% 8% 12% 22% 16% 4	141549
	Night	37% 7% 12% 22% 18% 5%	139500
Seeing	Day	19% 14% 27% 23% 12% 4	17245
	Night	17% 10% 27% 31% 14% 1	26964

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% 6% 2 6% 4 4 4 3 5% 6% 48%	5258
	11th to 20th	Day	11% 8% 3 6% 4 4 3 2 3 4 52%	5280
	21st to end	Day	14% 6% 2 6% 5% 4 4 3 4 5% 46%	5780
February	1st to 10th	Day	12% 8% 3 7% 5% 5% 4 4 5% 6% 42%	1890
	11th to 20th	Day	17% 7% 2 6% 4 3 5% 4 5% 7% 40%	1843
	21st to end	Day	17% 9% 3 6% 5% 4 3 3 6% 6% 39%	1794
March	1st to 10th	Day	17% 7% 2 6% 4 4 5% 4 5% 6% 42%	5045
	11th to 20th	Day	11% 5% 2 4 4 4 4 3 5% 6% 51%	5090
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 4 5% 6% 40%	5586
April	1st to 10th	Day	11% 7% 2 5% 5% 4 5% 4 6% 7% 46%	2478
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 6% 6% 46%	2494
	21st to end	Day	16% 7% 2 7% 5% 4 5% 3 5% 6% 41%	2567
May	1st to 10th	Day	16% 6% 2 6% 5% 5% 6% 5% 6% 8% 35%	7616
	11th to 20th	Day	19% 5% 2 5% 4 5% 4 4 6% 8% 38%	7626
	21st to end	Day	22% 5% 2 5% 4 4 4 4 6% 8% 37%	8844
June	1st to 10th	Day	11% 5% 2 5% 6% 5% 5% 4 7% 8% 42%	2940
	11th to 20th	Day	11% 5% 2 6% 5% 6% 6% 5% 7% 9% 37%	2940
	21st to end	Day	9% 5% 2 5% 5% 5% 6% 5% 7% 10% 43%	2940
July	1st to 10th	Day	8% 4 2 6% 5% 5% 5% 5% 8% 10% 43%	6222
	11th to 20th	Day	10% 5% 2 6% 5% 6% 7% 6% 9% 13% 29%	6030
	21st to end	Day	10% 5% 2 6% 6% 5% 6% 5% 7% 10% 36%	6554
August	1st to 10th	Day	16% 5% 2 6% 4 6% 5% 5% 7% 10% 33%	4234
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 5% 8% 10% 38%	4111
	21st to end	Day	11% 4 2 5% 4 4 5% 4 6% 8% 47%	4312
September	1st to 10th	Day	12% 5% 2 6% 5% 4 5% 4 5% 6% 47%	2400
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 4 5% 8% 45%	2310
	21st to end	Day	24% 8% 2 6% 4 3 4 3 5% 6% 34%	2135
October	1st to 10th	Day	15% 5% 3 5% 4 3 4 3 5% 6% 48%	2000
	11th to 20th	Day	16% 4 2 4 3 3 3 3 4 6% 51%	2000
	21st to end	Day	14% 6% 2 4 3 3 4 3 4 5% 51%	2200
November	1st to 10th	Day	12% 6% 2 4 3 3 3 3 4 6% 54%	2057
	11th to 20th	Day	14% 4 2 4 4 4 3 3 5% 5% 51%	1842
	21st to end	Day	14% 5% 2 5% 4 3 4 3 5% 6% 49%	1680
December	1st to 10th	Day	12% 6% 2 5% 3 4 4 3 5% 7% 51%	2680
	11th to 20th	Day	18% 6% 2 5% 3 3 3 3 4 7% 47%	3070
	21st to end	Day	20% 7% 3 6% 4 4 3 4 5% 7% 38%	4095
January	1st to 10th	Night	10% 6% 2 4 3 4 4 4 5% 6% 51%	7635
	11th to 20th	Night	12% 7% 2 4 2 3 2 2 4 5% 56%	7680
	21st to end	Night	14% 6% 2 5% 4 3 3 2 4 5% 52%	8401
February	1st to 10th	Night	13% 8% 2 6% 4 4 5% 5% 5% 6% 44%	2730
	11th to 20th	Night	19% 7% 2 5% 4 4 4 4 4 6% 41%	2663
	21st to end	Night	14% 7% 2 6% 4 3 5% 4 5% 6% 44%	2007
March	1st to 10th	Night	17% 5% 2 6% 3 3 3 3 4 6% 47%	5028
	11th to 20th	Night	11% 4 1 5% 3 3 3 3 6% 6% 55%	5090
	21st to end	Night	12% 7% 2 5% 4 4 4 3 5% 6% 47%	5588
April	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 5% 7% 45%	2142
	11th to 20th	Night	13% 5% 2 5% 4 4 4 4 6% 8% 46%	2075
	21st to end	Night	15% 4 1 5% 4 4 4 4 5% 7% 45%	2069
May	1st to 10th	Night	18% 6% 2 6% 4 4 4 4 5% 8% 37%	5237
	11th to 20th	Night	19% 4 2 5% 4 4 5% 4 6% 8% 39%	5234
	21st to end	Night	22% 5% 2 6% 4 3 3 3 5% 8% 36%	5291
June	1st to 10th	Night	10% 5% 2 6% 5% 4 4 4 7% 9% 45%	1680
	11th to 20th	Night	12% 5% 2 5% 4 4 5% 4 6% 9% 43%	1680
	21st to end	Night	10% 4 2 4 5% 4 6% 5% 6% 8% 45%	1680
July	1st to 10th	Night	9% 5% 2 5% 5% 5% 5% 5% 7% 9% 43%	3948
	11th to 20th	Night	13% 7% 3 8% 6% 6% 6% 5% 8% 9% 29%	4147
	21st to end	Night	8% 6% 2 7% 5% 5% 6% 5% 8% 10% 37%	4517
August	1st to 10th	Night	17% 5% 2 7% 5% 5% 5% 4 5% 10% 36%	2915
	11th to 20th	Night	11% 7% 2 7% 5% 6% 5% 4 7% 9% 37%	2903
	21st to end	Night	13% 5% 2 6% 4 4 4 4 6% 8% 45%	3586
September	1st to 10th	Night	14% 6% 2 5% 3 3 5% 3 5% 8% 47%	2000
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 5% 8% 47%	2085
	21st to end	Night	25% 7% 2 5% 3 3 3 3 4 5% 39%	2265
October	1st to 10th	Night	19% 6% 2 5% 3 4 3 3 4 6% 47%	2400
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 4 6% 51%	2400
	21st to end	Night	14% 4 2 5% 3 3 3 3 5% 5% 53%	2640
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 5% 5% 52%	2461
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 5% 5% 50%	2769
	21st to end	Night	14% 4 1 5% 4 4 3 3 5% 7% 49%	2940
December	1st to 10th	Night	10% 6% 2 4 3 4 4 3 5% 7% 53%	4720
	11th to 20th	Night	18% 7% 2 5% 3 3 4 3 5% 6% 44%	5345
	21st to end	Night	16% 5% 2 5% 4 4 4 4 6% 7% 42%	7085

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	38% 4 10% 21% 21% 6%	5259
	11th to 20th	Day	36% 5% 5% 18% 23% 14%	5280
	21st to end	Day	40% 4 6% 27% 18% 5%	5780
February	1st to 10th	Day	42% 6% 10% 22% 15% 3	1890
	11th to 20th	Day	43% 6% 9% 20% 18% 4	1852
	21st to end	Day	45% 6% 10% 18% 16% 5%	1794
March	1st to 10th	Day	41% 7% 9% 19% 16% 9%	5045
	11th to 20th	Day	32% 6% 8% 27% 19% 7%	5090
	21st to end	Day	43% 7% 10% 16% 17% 7%	5586
April	1st to 10th	Day	36% 8% 12% 20% 18% 7%	2478
	11th to 20th	Day	35% 7% 11% 21% 20% 6%	2494
	21st to end	Day	43% 7% 11% 23% 13% 4	2567
May	1st to 10th	Day	44% 9% 12% 21% 13% 1	7651
	11th to 20th	Day	42% 8% 14% 23% 12% 1	7630
	21st to end	Day	43% 9% 12% 18% 15% 3	8859
June	1st to 10th	Day	36% 9% 15% 23% 17% 1	2940
	11th to 20th	Day	39% 11% 15% 20% 13% 1	2940
	21st to end	Day	33% 11% 16% 25% 14% 1	2940
July	1st to 10th	Day	34% 12% 19% 20% 11% 4	6157
	11th to 20th	Day	41% 14% 20% 20% 6% 1	5961
	21st to end	Day	39% 10% 14% 24% 12% 1	6370
August	1st to 10th	Day	42% 11% 19% 22% 6%	4221
	11th to 20th	Day	36% 11% 14% 26% 11% 1	4150
	21st to end	Day	33% 9% 18% 28% 11% 1	4312
September	1st to 10th	Day	37% 9% 13% 17% 20% 5%	2400
	11th to 20th	Day	37% 7% 9% 25% 19% 3	2310
	21st to end	Day	50% 6% 10% 16% 15% 2	2135
October	1st to 10th	Day	33% 5% 8% 19% 28% 6%	3260
	11th to 20th	Day	23% 5% 9% 26% 26% 12%	3240
	21st to end	Day	21% 4 11% 30% 21% 12%	3538
November	1st to 10th	Day	31% 5% 10% 25% 25% 4	2057
	11th to 20th	Day	34% 5% 10% 21% 23% 6%	1842
	21st to end	Day	35% 6% 9% 25% 22% 3	1680
December	1st to 10th	Day	34% 6% 11% 24% 19% 7%	2680
	11th to 20th	Day	38% 5% 14% 24% 14% 4	3066
	21st to end	Day	44% 7% 12% 20% 15% 1	4095
January	1st to 10th	Night	33% 6% 10% 24% 21% 7%	7636
	11th to 20th	Night	32% 4 6% 22% 25% 11%	7680
	21st to end	Night	35% 4 7% 27% 23% 5%	8401
February	1st to 10th	Night	39% 7% 12% 23% 16% 2	2730
	11th to 20th	Night	43% 5% 10% 19% 17% 6%	2665
	21st to end	Night	41% 6% 12% 20% 15% 6%	2007
March	1st to 10th	Night	37% 6% 8% 22% 18% 9%	5028
	11th to 20th	Night	28% 5% 10% 24% 26% 6%	5090
	21st to end	Night	37% 7% 11% 20% 21% 5%	5588
April	1st to 10th	Night	37% 8% 13% 18% 20% 4	2142
	11th to 20th	Night	35% 6% 14% 21% 20% 5%	2075
	21st to end	Night	37% 7% 12% 26% 14% 4	2069
May	1st to 10th	Night	45% 8% 13% 20% 11% 3	5264
	11th to 20th	Night	41% 8% 14% 25% 11% 1	5235
	21st to end	Night	46% 9% 13% 18% 10% 3	5298
June	1st to 10th	Night	33% 10% 14% 26% 15% 1	1680
	11th to 20th	Night	36% 12% 16% 23% 13% 1	1680
	21st to end	Night	34% 11% 16% 19% 18% 2	1680
July	1st to 10th	Night	35% 13% 19% 18% 14% 2	3905
	11th to 20th	Night	48% 13% 17% 16% 5% 1	4096
	21st to end	Night	37% 12% 18% 23% 10%	4387
August	1st to 10th	Night	44% 8% 18% 23% 6% 1	2904
	11th to 20th	Night	40% 11% 15% 21% 12%	2930
	21st to end	Night	34% 9% 16% 29% 11% 1	3586
September	1st to 10th	Night	35% 9% 14% 20% 17% 5%	2000
	11th to 20th	Night	36% 8% 11% 24% 19% 2	2085
	21st to end	Night	48% 6% 10% 18% 15% 2	2265
October	1st to 10th	Night	40% 6% 9% 18% 21% 6%	3920
	11th to 20th	Night	24% 5% 10% 22% 27% 11%	3899
	21st to end	Night	20% 4 10% 31% 24% 10%	4252
November	1st to 10th	Night	34% 5% 10% 22% 24% 6%	2461
	11th to 20th	Night	36% 5% 8% 22% 23% 6%	2769
	21st to end	Night	35% 5% 11% 24% 21% 5%	2940
December	1st to 10th	Night	31% 6% 10% 27% 23% 3	4720
	11th to 20th	Night	42% 5% 12% 18% 17% 6%	5348
	21st to end	Night	39% 8% 14% 22% 16% 1	7085

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	28% 26% 32% 12% 3	330
	11th to 20th	Day	32% 32% 24% 11% 2	330
	21st to end	Day	24% 31% 33% 10% 2 1	363
February	1st to 10th	Day	24% 33% 29% 12% 2	330
	11th to 20th	Day	42% 26% 25% 6% 1	330
	21st to end	Day	31% 23% 29% 14% 3	428
March	1st to 10th	Day	23% 24% 29% 19% 5%	540
	11th to 20th	Day	25% 23% 29% 17% 5% 1	540
	21st to end	Day	25% 21% 30% 17% 5% 1	594
April	1st to 10th	Day	23% 25% 25% 18% 7% 1	540
	11th to 20th	Day	21% 18% 30% 17% 9% 3	532
	21st to end	Day	23% 16% 31% 19% 9% 2	540
May	1st to 10th	Day	21% 21% 29% 17% 9% 3	543
	11th to 20th	Day	21% 15% 29% 22% 10% 3	567
	21st to end	Day	20% 10% 26% 24% 15% 5%	617
June	1st to 10th	Day	12% 9% 30% 26% 18% 5%	570
	11th to 20th	Day	13% 10% 27% 31% 14% 5%	570
	21st to end	Day	9% 8% 29% 31% 19% 5%	570
July	1st to 10th	Day	12% 4 19% 33% 23% 9%	565
	11th to 20th	Day	6% 3 19% 38% 24% 9%	564
	21st to end	Day	9% 5% 25% 32% 21% 8%	616
August	1st to 10th	Day	13% 6% 20% 28% 23% 9%	526
	11th to 20th	Day	11% 4 22% 30% 22% 11%	519
	21st to end	Day	9% 5% 20% 31% 21% 15%	583
September	1st to 10th	Day	13% 5% 21% 27% 19% 14%	530
	11th to 20th	Day	16% 3 20% 29% 23% 9%	530
	21st to end	Day	32% 8% 20% 22% 14% 4	530
October	1st to 10th	Day	18% 6% 23% 30% 17% 6%	530
	11th to 20th	Day	16% 18% 26% 23% 15% 2	530
	21st to end	Day	20% 13% 32% 22% 10% 2	583
November	1st to 10th	Day	17% 14% 31% 27% 9% 3	535
	11th to 20th	Day	22% 15% 31% 21% 10% 1	357
	21st to end	Day	23% 20% 32% 20% 5%	220
December	1st to 10th	Day	20% 25% 36% 15% 4	220
	11th to 20th	Day	25% 19% 32% 16% 7% 1	220
	21st to end	Day	29% 20% 29% 17% 5%	253
January	1st to 10th	Night	19% 17% 36% 25% 3	840
	11th to 20th	Night	21% 16% 33% 28% 3	840
	21st to end	Night	18% 15% 40% 24% 3	924
February	1st to 10th	Night	17% 15% 35% 30% 2	840
	11th to 20th	Night	24% 19% 38% 18% 1	823
	21st to end	Night	19% 18% 38% 22% 2	692
March	1st to 10th	Night	17% 18% 36% 25% 4	840
	11th to 20th	Night	23% 18% 31% 23% 5% 1	840
	21st to end	Night	18% 16% 35% 25% 5% 1	924
April	1st to 10th	Night	18% 18% 34% 24% 6%	840
	11th to 20th	Night	16% 15% 33% 27% 8% 1	831
	21st to end	Night	17% 10% 31% 34% 7%	809
May	1st to 10th	Night	20% 10% 25% 29% 16% 1	630
	11th to 20th	Night	20% 6% 23% 36% 15% 1	621
	21st to end	Night	18% 3 18% 36% 23% 3	522
June	1st to 10th	Night	10% 2 20% 44% 23% 1	420
	11th to 20th	Night	12% 3 18% 40% 25% 2	420
	21st to end	Night	9% 2 17% 47% 22% 2	420
July	1st to 10th	Night	11% 3 12% 35% 35% 5%	519
	11th to 20th	Night	7% 3 13% 32% 41% 5%	612
	21st to end	Night	7% 3 15% 38% 32% 5%	687
August	1st to 10th	Night	13% 4 9% 41% 30% 4	597
	11th to 20th	Night	12% 3 13% 36% 31% 4	609
	21st to end	Night	12% 3 13% 36% 32% 4	880
September	1st to 10th	Night	16% 2 14% 36% 26% 5%	800
	11th to 20th	Night	14% 3 13% 37% 30% 3	800
	21st to end	Night	27% 4 17% 31% 19% 1	800
October	1st to 10th	Night	20% 6% 17% 32% 21% 4	800
	11th to 20th	Night	15% 11% 26% 30% 16% 1	800
	21st to end	Night	16% 10% 29% 33% 11% 1	880
November	1st to 10th	Night	16% 9% 27% 35% 11% 2	820
	11th to 20th	Night	17% 11% 31% 32% 8% 1	840
	21st to end	Night	17% 11% 32% 30% 9% 1	840
December	1st to 10th	Night	19% 12% 35% 27% 6%	840
	11th to 20th	Night	19% 9% 32% 35% 5%	840
	21st to end	Night	18% 13% 30% 36% 4	924

Page generated at 2026-07-28 03:25:24. Version ac07

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