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 2026-02-16.

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% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	131204
	Night	13% 7% 2 6% 4 4 4 4 6% 7% 42%	129652
Transparency	Day	42% 7% 12% 19% 15% 5%	134863
	Night	39% 7% 12% 21% 16% 5%	134063
Seeing	Day	22% 16% 24% 20% 13% 5%	16848
	Night	19% 15% 22% 26% 16% 2	26989

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% 7% 2 5% 3 3 4 4 4 6% 46%	5258
	11th to 20th	Day	8% 7% 3 7% 4 4 3 3 4 5% 53%	5280
	21st to end	Day	14% 8% 3 7% 3 3 3 3 4 5% 45%	5780
February	1st to 10th	Day	12% 9% 3 8% 6% 3 5% 4 5% 6% 39%	1890
	11th to 20th	Day	17% 10% 3 6% 4 4 4 4 5% 6% 38%	1952
	21st to end	Day	16% 8% 3 7% 4 3 5% 3 6% 6% 39%	1665
March	1st to 10th	Day	17% 7% 2 7% 5% 5% 5% 4 5% 5% 39%	4935
	11th to 20th	Day	11% 7% 2 6% 4 4 4 3 4 7% 50%	4980
	21st to end	Day	15% 8% 3 6% 4 4 4 3 5% 7% 42%	5465
April	1st to 10th	Day	12% 7% 2 6% 4 5% 4 4 5% 8% 44%	2200
	11th to 20th	Day	15% 7% 2 5% 4 4 4 4 6% 7% 42%	2389
	21st to end	Day	15% 7% 3 6% 5% 5% 4 5% 6% 7% 39%	2600
May	1st to 10th	Day	12% 8% 3 9% 6% 6% 7% 6% 8% 9% 27%	5862
	11th to 20th	Day	20% 8% 3 8% 6% 5% 5% 5% 7% 8% 25%	5861
	21st to end	Day	14% 6% 2 7% 5% 5% 6% 5% 8% 9% 33%	6433
June	1st to 10th	Day	12% 7% 2 6% 6% 6% 6% 5% 8% 9% 33%	2600
	11th to 20th	Day	9% 6% 2 8% 7% 6% 6% 5% 7% 10% 34%	2600
	21st to end	Day	10% 6% 2 7% 6% 5% 6% 6% 8% 10% 33%	2600
July	1st to 10th	Day	7% 5% 2 8% 6% 7% 7% 6% 7% 9% 35%	5900
	11th to 20th	Day	8% 6% 3 9% 8% 8% 8% 6% 9% 11% 25%	5900
	21st to end	Day	13% 9% 3 10% 8% 7% 7% 6% 8% 9% 20%	6424
August	1st to 10th	Day	15% 7% 3 10% 6% 7% 7% 5% 8% 9% 23%	4234
	11th to 20th	Day	10% 6% 2 8% 6% 7% 7% 6% 9% 10% 28%	4174
	21st to end	Day	8% 4 2 6% 5% 5% 6% 4 7% 10% 43%	4448
September	1st to 10th	Day	12% 7% 3 6% 5% 5% 5% 4 6% 8% 39%	2229
	11th to 20th	Day	17% 7% 2 7% 5% 5% 5% 4 6% 9% 34%	2200
	21st to end	Day	21% 7% 3 7% 5% 4 5% 3 5% 6% 34%	2200
October	1st to 10th	Day	15% 6% 2 5% 4 3 4 3 4 6% 47%	2083
	11th to 20th	Day	13% 6% 2 5% 3 4 3 4 4 6% 50%	2000
	21st to end	Day	16% 7% 2 5% 3 3 3 2 4 8% 50%	2200
November	1st to 10th	Day	12% 5% 2 5% 3 3 3 2 5% 6% 54%	2034
	11th to 20th	Day	11% 5% 2 4 4 5% 4 4 5% 5% 51%	1890
	21st to end	Day	15% 6% 3 6% 3 4 3 3 5% 6% 47%	1890
December	1st to 10th	Day	9% 9% 2 7% 5% 4 4 4 5% 5% 46%	3020
	11th to 20th	Day	15% 7% 3 7% 5% 4 4 3 5% 7% 41%	3463
	21st to end	Day	16% 8% 4 7% 4 4 4 4 6% 7% 36%	4565
January	1st to 10th	Night	11% 9% 3 6% 4 3 4 3 5% 6% 46%	7635
	11th to 20th	Night	9% 7% 2 7% 5% 3 3 3 5% 5% 51%	7680
	21st to end	Night	14% 9% 3 6% 3 3 3 3 4 5% 46%	8401
February	1st to 10th	Night	12% 7% 3 6% 4 4 5% 4 7% 6% 41%	2730
	11th to 20th	Night	17% 10% 3 5% 4 4 4 3 5% 6% 39%	2441
	21st to end	Night	12% 8% 3 8% 5% 4 4 4 6% 7% 39%	1963
March	1st to 10th	Night	18% 9% 3 6% 4 4 4 3 5% 4 39%	4918
	11th to 20th	Night	10% 5% 2 4 4 4 4 4 4 7% 52%	4980
	21st to end	Night	13% 7% 3 7% 4 4 4 4 5% 7% 42%	5467
April	1st to 10th	Night	11% 8% 2 6% 5% 4 4 3 6% 8% 43%	2200
	11th to 20th	Night	13% 7% 2 5% 4 4 3 3 7% 7% 46%	1974
	21st to end	Night	14% 6% 3 5% 4 3 4 4 5% 7% 45%	1800
May	1st to 10th	Night	15% 8% 3 7% 4 4 5% 4 6% 11% 34%	4033
	11th to 20th	Night	20% 8% 3 6% 4 5% 4 4 6% 9% 32%	4032
	21st to end	Night	17% 7% 2 6% 5% 4 4 4 6% 9% 35%	4421
June	1st to 10th	Night	10% 6% 2 5% 4 4 6% 5% 8% 9% 41%	1800
	11th to 20th	Night	10% 6% 2 5% 3 4 5% 4 9% 10% 42%	1800
	21st to end	Night	7% 5% 2 6% 6% 4 4 4 8% 12% 42%	1800
July	1st to 10th	Night	8% 6% 2 7% 5% 5% 6% 5% 8% 10% 38%	4058
	11th to 20th	Night	7% 6% 3 9% 5% 5% 7% 5% 9% 11% 33%	4057
	21st to end	Night	9% 9% 3 10% 7% 6% 6% 5% 8% 10% 26%	4427
August	1st to 10th	Night	15% 8% 3 8% 5% 4 5% 4 7% 11% 29%	2915
	11th to 20th	Night	11% 6% 3 8% 5% 6% 6% 5% 7% 8% 36%	2875
	21st to end	Night	7% 4 2 5% 3 4 5% 3 6% 9% 52%	3416
September	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 6% 7% 45%	2167
	11th to 20th	Night	17% 7% 2 6% 3 3 4 3 6% 7% 41%	2200
	21st to end	Night	19% 8% 2 7% 4 4 4 3 5% 6% 39%	2200
October	1st to 10th	Night	16% 7% 2 5% 3 3 4 3 5% 6% 47%	2315
	11th to 20th	Night	14% 6% 1 5% 3 3 4 2 4 5% 51%	2400
	21st to end	Night	15% 6% 2 4 3 2 2 3 4 6% 53%	2640
November	1st to 10th	Night	13% 7% 2 5% 4 3 3 3 4 5% 50%	2482
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 7% 51%	2730
	21st to end	Night	13% 6% 2 5% 4 3 4 4 5% 6% 48%	2730
December	1st to 10th	Night	11% 8% 2 7% 4 3 5% 3 4 5% 47%	4380
	11th to 20th	Night	15% 9% 5% 9% 5% 4 4 3 5% 5% 36%	4952
	21st to end	Night	17% 9% 3 6% 4 4 4 4 6% 8% 37%	6633

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	36% 4 11% 18% 27% 4	5259
	11th to 20th	Day	32% 5% 4 15% 32% 13%	5280
	21st to end	Day	41% 3 6% 23% 20% 7%	5780
February	1st to 10th	Day	43% 6% 10% 19% 18% 4	1890
	11th to 20th	Day	46% 4 9% 18% 18% 5%	1962
	21st to end	Day	43% 7% 11% 16% 17% 6%	1665
March	1st to 10th	Day	43% 6% 7% 16% 16% 12%	4935
	11th to 20th	Day	33% 5% 6% 21% 21% 14%	4980
	21st to end	Day	41% 4 9% 25% 16% 4	5465
April	1st to 10th	Day	35% 8% 12% 23% 18% 5%	2200
	11th to 20th	Day	38% 7% 13% 19% 18% 5%	2389
	21st to end	Day	43% 8% 13% 22% 13% 2	2600
May	1st to 10th	Day	48% 9% 13% 19% 9% 1	5897
	11th to 20th	Day	53% 9% 14% 18% 6% 1	5865
	21st to end	Day	42% 10% 13% 18% 14% 3	6447
June	1st to 10th	Day	43% 12% 15% 20% 10% 1	2600
	11th to 20th	Day	41% 12% 15% 18% 12% 2	2600
	21st to end	Day	40% 12% 16% 19% 12% 1	2600
July	1st to 10th	Day	44% 9% 17% 20% 8% 2	5835
	11th to 20th	Day	50% 9% 23% 13% 3	5831
	21st to end	Day	57% 8% 14% 14% 5%	6240
August	1st to 10th	Day	54% 9% 16% 18% 3	4221
	11th to 20th	Day	46% 9% 19% 16% 7% 3	4213
	21st to end	Day	36% 7% 17% 29% 9% 2	4448
September	1st to 10th	Day	40% 9% 13% 18% 16% 3	2229
	11th to 20th	Day	46% 8% 11% 20% 15% 1	2200
	21st to end	Day	50% 6% 11% 17% 13% 3	2200
October	1st to 10th	Day	31% 4 6% 20% 27% 10%	3395
	11th to 20th	Day	25% 3 7% 24% 20% 21%	3240
	21st to end	Day	29% 3 7% 27% 18% 16%	3538
November	1st to 10th	Day	32% 4 11% 22% 25% 7%	2034
	11th to 20th	Day	32% 5% 9% 21% 26% 6%	1890
	21st to end	Day	38% 4 8% 22% 24% 4	1890
December	1st to 10th	Day	40% 4 6% 19% 27% 5%	3020
	11th to 20th	Day	42% 4 10% 17% 21% 6%	3460
	21st to end	Day	44% 6% 12% 22% 14% 3	4565
January	1st to 10th	Night	35% 5% 11% 22% 18% 8%	7636
	11th to 20th	Night	34% 5% 5% 18% 27% 11%	7680
	21st to end	Night	39% 3 8% 20% 24% 7%	8401
February	1st to 10th	Night	40% 6% 12% 22% 18% 3	2730
	11th to 20th	Night	44% 5% 9% 19% 16% 6%	2442
	21st to end	Night	42% 7% 12% 20% 14% 6%	1963
March	1st to 10th	Night	44% 7% 7% 19% 16% 8%	4918
	11th to 20th	Night	29% 6% 7% 21% 23% 13%	4980
	21st to end	Night	41% 6% 11% 25% 13% 5%	5467
April	1st to 10th	Night	36% 8% 13% 23% 17% 4	2200
	11th to 20th	Night	36% 7% 12% 21% 18% 6%	1974
	21st to end	Night	37% 8% 15% 26% 13% 2	1800
May	1st to 10th	Night	44% 11% 15% 21% 9% 1	4060
	11th to 20th	Night	49% 10% 15% 20% 6%	4033
	21st to end	Night	42% 9% 16% 22% 9% 2	4429
June	1st to 10th	Night	35% 12% 18% 22% 11% 1	1800
	11th to 20th	Night	33% 11% 20% 23% 11% 1	1800
	21st to end	Night	32% 11% 19% 24% 11% 3	1800
July	1st to 10th	Night	41% 9% 16% 25% 8% 1	4015
	11th to 20th	Night	43% 11% 26% 16% 4 1	4006
	21st to end	Night	51% 9% 18% 15% 7%	4297
August	1st to 10th	Night	48% 7% 23% 16% 5%	2904
	11th to 20th	Night	43% 8% 21% 19% 8% 1	2902
	21st to end	Night	29% 7% 18% 36% 8% 2	3416
September	1st to 10th	Night	36% 9% 17% 17% 16% 4	2167
	11th to 20th	Night	41% 7% 14% 21% 17% 1	2200
	21st to end	Night	47% 7% 11% 20% 14% 2	2200
October	1st to 10th	Night	35% 4 7% 20% 22% 11%	3783
	11th to 20th	Night	25% 4 8% 22% 23% 19%	3899
	21st to end	Night	28% 3 7% 25% 21% 16%	4252
November	1st to 10th	Night	35% 5% 10% 20% 23% 7%	2482
	11th to 20th	Night	32% 5% 9% 21% 25% 9%	2730
	21st to end	Night	35% 5% 8% 23% 24% 5%	2730
December	1st to 10th	Night	38% 5% 9% 22% 24% 3	4380
	11th to 20th	Night	46% 5% 10% 16% 18% 5%	4954
	21st to end	Night	42% 7% 14% 22% 13% 3	6633

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	38% 28% 24% 8% 2	330
	11th to 20th	Day	41% 25% 27% 7% 1	330
	21st to end	Day	34% 34% 24% 7% 1	363
February	1st to 10th	Day	29% 34% 26% 10% 1	330
	11th to 20th	Day	49% 27% 20% 3	315
	21st to end	Day	36% 30% 28% 5%	264
March	1st to 10th	Day	28% 25% 27% 15% 5%	500
	11th to 20th	Day	30% 29% 25% 12% 3 1	500
	21st to end	Day	29% 29% 23% 14% 5% 1	550
April	1st to 10th	Day	27% 24% 28% 13% 6% 1	500
	11th to 20th	Day	24% 24% 26% 14% 10% 2	492
	21st to end	Day	20% 21% 30% 17% 9% 3	500
May	1st to 10th	Day	21% 20% 34% 13% 8% 4	503
	11th to 20th	Day	21% 16% 24% 23% 11% 5%	527
	21st to end	Day	21% 9% 24% 23% 14% 8%	573
June	1st to 10th	Day	14% 10% 28% 24% 15% 8%	530
	11th to 20th	Day	11% 8% 24% 31% 17% 9%	530
	21st to end	Day	12% 7% 28% 29% 16% 8%	530
July	1st to 10th	Day	11% 3 19% 31% 25% 12%	525
	11th to 20th	Day	8% 3 17% 33% 28% 12%	524
	21st to end	Day	13% 3 18% 29% 25% 13%	576
August	1st to 10th	Day	19% 5% 22% 24% 17% 13%	526
	11th to 20th	Day	14% 4 18% 26% 25% 13%	519
	21st to end	Day	12% 4 17% 29% 23% 15%	583
September	1st to 10th	Day	14% 4 20% 27% 25% 11%	530
	11th to 20th	Day	18% 4 17% 24% 25% 12%	530
	21st to end	Day	24% 10% 21% 26% 15% 5%	530
October	1st to 10th	Day	22% 8% 20% 25% 19% 6%	530
	11th to 20th	Day	17% 20% 25% 20% 16% 2	530
	21st to end	Day	23% 16% 28% 22% 9% 3	583
November	1st to 10th	Day	22% 16% 27% 23% 10% 3	512
	11th to 20th	Day	25% 26% 28% 15% 5% 1	330
	21st to end	Day	29% 22% 28% 17% 3 1	330
December	1st to 10th	Day	28% 32% 30% 8% 1	330
	11th to 20th	Day	26% 25% 32% 13% 4	330
	21st to end	Day	33% 21% 34% 7% 5% 1	363
January	1st to 10th	Night	24% 27% 28% 18% 4	840
	11th to 20th	Night	25% 26% 27% 19% 3	840
	21st to end	Night	21% 28% 32% 17% 2	924
February	1st to 10th	Night	19% 23% 28% 27% 3	840
	11th to 20th	Night	29% 31% 27% 13% 1	804
	21st to end	Night	20% 26% 36% 15% 2	660
March	1st to 10th	Night	21% 24% 31% 21% 3	800
	11th to 20th	Night	26% 21% 28% 19% 6% 1	800
	21st to end	Night	21% 20% 32% 21% 6%	880
April	1st to 10th	Night	21% 22% 28% 24% 6%	800
	11th to 20th	Night	19% 20% 27% 24% 10% 1	786
	21st to end	Night	18% 14% 24% 35% 9%	600
May	1st to 10th	Night	18% 11% 28% 29% 14% 1	600
	11th to 20th	Night	18% 8% 22% 33% 16% 3	591
	21st to end	Night	20% 5% 16% 27% 27% 6%	650
June	1st to 10th	Night	13% 4 19% 34% 27% 5%	600
	11th to 20th	Night	13% 3 17% 29% 34% 5%	600
	21st to end	Night	8% 3 16% 34% 35% 4	600
July	1st to 10th	Night	13% 1 9% 28% 42% 6%	585
	11th to 20th	Night	7% 2 9% 29% 46% 7%	582
	21st to end	Night	7% 2 11% 33% 40% 7%	657
August	1st to 10th	Night	16% 1 12% 29% 37% 5%	597
	11th to 20th	Night	16% 2 11% 30% 34% 7%	594
	21st to end	Night	13% 1 11% 29% 38% 7%	775
September	1st to 10th	Night	14% 3 13% 28% 36% 6%	800
	11th to 20th	Night	20% 4 13% 26% 33% 6%	800
	21st to end	Night	23% 6% 16% 31% 24% 1	800
October	1st to 10th	Night	19% 11% 17% 25% 23% 6%	800
	11th to 20th	Night	20% 13% 22% 29% 15% 2	800
	21st to end	Night	20% 14% 21% 30% 12% 2	880
November	1st to 10th	Night	18% 14% 22% 31% 12% 3	820
	11th to 20th	Night	20% 18% 23% 30% 9% 1	840
	21st to end	Night	20% 19% 25% 26% 9% 1	840
December	1st to 10th	Night	23% 23% 27% 23% 5%	840
	11th to 20th	Night	21% 20% 29% 25% 5% 1	840
	21st to end	Night	26% 21% 24% 24% 5%	924

Page generated at 2026-02-13 01:42:16. Version ac07

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