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 2025-03-27.

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% 7% 42%	108949
	Night	14% 6% 2 5% 4 4 4 3 5% 7% 46%	110371
Transparency	Day	38% 8% 12% 22% 16% 4	112599
	Night	36% 7% 12% 23% 18% 4	114860
Seeing	Day	19% 14% 26% 23% 13% 5%	15427
	Night	17% 10% 26% 32% 14% 2	25293

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% 7% 2 5% 4 4 4 3 5% 5% 50%	4056
	11th to 20th	Day	12% 8% 3 6% 4 4 3 3 3 4 50%	4060
	21st to end	Day	15% 7% 3 5% 4 4 4 3 4 6% 46%	4466
February	1st to 10th	Day	12% 8% 3 7% 5% 5% 4 4 5% 6% 41%	1800
	11th to 20th	Day	17% 7% 2 6% 5% 3 5% 4 5% 7% 40%	1753
	21st to end	Day	17% 8% 3 6% 5% 4 3 3 6% 6% 40%	1711
March	1st to 10th	Day	19% 8% 3 7% 4 4 5% 4 4 6% 37%	3545
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 6% 7% 50%	3590
	21st to end	Day	18% 7% 3 7% 5% 4 5% 4 5% 5% 38%	3881
April	1st to 10th	Day	10% 7% 2 5% 5% 4 5% 4 6% 7% 46%	2242
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 5% 6% 46%	2254
	21st to end	Day	16% 7% 2 7% 5% 4 5% 3 4 6% 40%	2323
May	1st to 10th	Day	18% 7% 2 7% 6% 6% 6% 4 6% 7% 30%	4082
	11th to 20th	Day	23% 7% 2 6% 4 4 4 4 5% 8% 32%	4081
	21st to end	Day	11% 5% 2 5% 5% 5% 5% 4 7% 10% 42%	4719
June	1st to 10th	Day	11% 5% 2 5% 6% 5% 5% 4 7% 8% 42%	2660
	11th to 20th	Day	11% 6% 2 7% 5% 6% 6% 5% 7% 9% 37%	2660
	21st to end	Day	8% 5% 2 5% 5% 5% 6% 5% 8% 10% 43%	2660
July	1st to 10th	Day	7% 4 2 5% 5% 5% 6% 5% 8% 10% 42%	4252
	11th to 20th	Day	10% 5% 2 5% 5% 5% 6% 6% 8% 13% 35%	4120
	21st to end	Day	13% 6% 3 8% 6% 6% 6% 5% 7% 9% 31%	4466
August	1st to 10th	Day	15% 5% 2 6% 4 6% 5% 5% 8% 10% 33%	4104
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 5% 8% 10% 39%	3983
	21st to end	Day	11% 4 2 5% 4 4 5% 4 6% 8% 48%	4180
September	1st to 10th	Day	13% 6% 2 6% 5% 4 4 4 5% 6% 46%	2280
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 4 5% 8% 44%	2195
	21st to end	Day	24% 7% 2 6% 4 4 4 3 5% 6% 35%	2028
October	1st to 10th	Day	15% 6% 3 5% 4 3 4 3 5% 6% 47%	1900
	11th to 20th	Day	16% 4 2 4 3 3 3 3 4 6% 51%	1900
	21st to end	Day	14% 6% 2 4 3 3 4 3 4 5% 52%	2090
November	1st to 10th	Day	12% 6% 2 4 3 3 3 3 4 6% 54%	1957
	11th to 20th	Day	15% 4 2 4 4 4 4 3 6% 5% 50%	1755
	21st to end	Day	14% 4 2 4 4 3 5% 3 5% 6% 50%	1600
December	1st to 10th	Day	12% 5% 2 5% 3 4 4 3 5% 7% 51%	2600
	11th to 20th	Day	18% 6% 2 5% 3 3 3 3 4 7% 47%	2990
	21st to end	Day	20% 7% 3 6% 4 4 3 4 5% 7% 38%	4006
January	1st to 10th	Night	11% 7% 2 4 3 3 3 3 4 6% 55%	5889
	11th to 20th	Night	13% 7% 2 4 2 2 2 2 4 5% 56%	5900
	21st to end	Night	15% 6% 2 5% 4 3 3 2 4 5% 51%	6490
February	1st to 10th	Night	13% 8% 2 6% 4 4 5% 5% 5% 6% 43%	2600
	11th to 20th	Night	19% 7% 2 5% 4 4 4 4 4 6% 41%	2533
	21st to end	Night	14% 7% 2 6% 4 3 5% 4 5% 6% 44%	1914
March	1st to 10th	Night	21% 7% 2 7% 4 4 4 3 5% 6% 38%	3528
	11th to 20th	Night	11% 5% 1 5% 3 3 4 4 6% 7% 53%	3590
	21st to end	Night	14% 8% 2 5% 3 3 3 3 5% 6% 48%	3890
April	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 5% 7% 45%	1938
	11th to 20th	Night	14% 4 2 5% 4 3 4 4 6% 8% 47%	1875
	21st to end	Night	16% 4 1 5% 4 5% 4 4 5% 7% 45%	1872
May	1st to 10th	Night	21% 7% 3 7% 4 4 4 3 5% 9% 32%	2813
	11th to 20th	Night	25% 6% 2 5% 4 4 4 3 5% 6% 36%	2812
	21st to end	Night	12% 4 2 6% 4 4 3 4 7% 9% 44%	2831
June	1st to 10th	Night	10% 5% 2 6% 5% 4 4 4 7% 10% 44%	1520
	11th to 20th	Night	12% 5% 2 5% 4 4 5% 5% 6% 9% 43%	1520
	21st to end	Night	10% 4 2 4 5% 4 6% 5% 6% 8% 45%	1520
July	1st to 10th	Night	7% 4 2 5% 5% 5% 5% 6% 8% 11% 44%	2701
	11th to 20th	Night	10% 6% 2 6% 5% 5% 5% 5% 8% 9% 37%	2840
	21st to end	Night	10% 7% 3 8% 6% 4 6% 5% 7% 9% 34%	3087
August	1st to 10th	Night	17% 5% 2 7% 5% 5% 5% 4 5% 10% 35%	2825
	11th to 20th	Night	11% 7% 3 8% 5% 6% 5% 4 7% 8% 37%	2812
	21st to end	Night	13% 5% 2 6% 4 4 4 4 6% 8% 46%	3476
September	1st to 10th	Night	14% 6% 2 5% 3 3 5% 4 5% 8% 45%	1900
	11th to 20th	Night	14% 5% 2 6% 4 3 3 3 5% 7% 46%	1981
	21st to end	Night	25% 7% 2 5% 3 3 3 3 4 6% 40%	2152
October	1st to 10th	Night	19% 6% 2 5% 3 4 3 3 4 6% 45%	2280
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 4 6% 51%	2280
	21st to end	Night	15% 4 2 4 3 3 3 3 5% 5% 53%	2508
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 2 5% 5% 52%	2341
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 5% 5% 50%	2636
	21st to end	Night	14% 4 1 4 4 4 3 3 5% 7% 50%	2800
December	1st to 10th	Night	10% 6% 2 4 3 4 4 3 5% 7% 53%	4580
	11th to 20th	Night	18% 7% 2 5% 3 3 4 3 5% 6% 44%	5205
	21st to end	Night	16% 5% 2 5% 4 4 4 4 6% 8% 42%	6932

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 6% 22% 23% 8%	4057
	11th to 20th	Day	37% 5% 6% 19% 21% 12%	4060
	21st to end	Day	39% 5% 7% 25% 19% 5%	4466
February	1st to 10th	Day	43% 7% 10% 23% 14% 3	1800
	11th to 20th	Day	43% 6% 9% 20% 18% 4	1762
	21st to end	Day	44% 6% 10% 17% 16% 6%	1711
March	1st to 10th	Day	46% 7% 9% 18% 14% 5%	3545
	11th to 20th	Day	31% 7% 9% 27% 21% 6%	3590
	21st to end	Day	47% 7% 10% 17% 15% 4	3881
April	1st to 10th	Day	36% 8% 12% 20% 18% 7%	2242
	11th to 20th	Day	36% 7% 11% 20% 20% 6%	2254
	21st to end	Day	44% 7% 11% 22% 13% 4	2323
May	1st to 10th	Day	49% 10% 11% 18% 10% 1	4120
	11th to 20th	Day	48% 9% 13% 20% 9% 1	4085
	21st to end	Day	34% 11% 16% 21% 16% 3	4733
June	1st to 10th	Day	37% 9% 15% 23% 15% 1	2660
	11th to 20th	Day	39% 11% 15% 20% 13% 2	2660
	21st to end	Day	33% 11% 16% 25% 14% 1	2660
July	1st to 10th	Day	33% 12% 21% 19% 9% 6%	4187
	11th to 20th	Day	37% 11% 20% 24% 8% 1	4051
	21st to end	Day	46% 10% 12% 22% 9% 1	4324
August	1st to 10th	Day	42% 11% 19% 22% 6%	4091
	11th to 20th	Day	36% 11% 14% 26% 12% 1	4022
	21st to end	Day	33% 9% 18% 28% 11% 1	4180
September	1st to 10th	Day	39% 9% 12% 17% 20% 5%	2280
	11th to 20th	Day	37% 7% 9% 25% 19% 3	2195
	21st to end	Day	49% 6% 10% 17% 16% 2	2028
October	1st to 10th	Day	34% 5% 8% 19% 28% 6%	3160
	11th to 20th	Day	23% 4 9% 26% 26% 12%	3140
	21st to end	Day	21% 4 11% 31% 21% 12%	3428
November	1st to 10th	Day	32% 5% 9% 25% 24% 4	1957
	11th to 20th	Day	35% 5% 10% 20% 23% 7%	1755
	21st to end	Day	35% 6% 9% 25% 23% 3	1600
December	1st to 10th	Day	33% 6% 11% 24% 19% 7%	2600
	11th to 20th	Day	38% 5% 14% 24% 14% 4	2986
	21st to end	Day	45% 7% 12% 20% 15% 1	4006
January	1st to 10th	Night	30% 5% 8% 23% 25% 9%	5889
	11th to 20th	Night	32% 4 6% 24% 24% 10%	5900
	21st to end	Night	37% 4 7% 26% 22% 4	6490
February	1st to 10th	Night	40% 7% 12% 23% 16% 2	2600
	11th to 20th	Night	43% 5% 10% 19% 17% 6%	2535
	21st to end	Night	40% 6% 12% 20% 15% 6%	1914
March	1st to 10th	Night	44% 7% 10% 22% 13% 3	3528
	11th to 20th	Night	30% 6% 10% 24% 26% 4	3590
	21st to end	Night	37% 7% 10% 22% 20% 4	3890
April	1st to 10th	Night	37% 8% 12% 18% 20% 5%	1938
	11th to 20th	Night	35% 6% 14% 20% 20% 5%	1875
	21st to end	Night	38% 7% 12% 25% 14% 4	1872
May	1st to 10th	Night	49% 8% 12% 19% 9% 2	2840
	11th to 20th	Night	48% 9% 13% 22% 8% 1	2813
	21st to end	Night	33% 12% 17% 25% 11% 3	2838
June	1st to 10th	Night	33% 11% 14% 27% 14% 1	1520
	11th to 20th	Night	35% 13% 15% 24% 12% 1	1520
	21st to end	Night	33% 11% 17% 19% 18% 3	1520
July	1st to 10th	Night	31% 13% 24% 16% 13% 2	2656
	11th to 20th	Night	39% 11% 19% 22% 8% 1	2786
	21st to end	Night	42% 11% 18% 19% 9%	2988
August	1st to 10th	Night	44% 8% 18% 23% 6% 1	2814
	11th to 20th	Night	40% 11% 15% 21% 13%	2839
	21st to end	Night	33% 9% 17% 29% 10% 1	3476
September	1st to 10th	Night	36% 9% 14% 19% 16% 5%	1900
	11th to 20th	Night	37% 8% 11% 23% 19% 2	1981
	21st to end	Night	47% 6% 10% 19% 16% 2	2152
October	1st to 10th	Night	40% 6% 10% 18% 21% 6%	3800
	11th to 20th	Night	24% 5% 10% 22% 28% 12%	3779
	21st to end	Night	20% 4 10% 32% 24% 11%	4120
November	1st to 10th	Night	34% 5% 10% 22% 23% 6%	2341
	11th to 20th	Night	36% 5% 8% 22% 23% 6%	2636
	21st to end	Night	34% 5% 11% 24% 21% 5%	2800
December	1st to 10th	Night	31% 6% 10% 27% 23% 3	4580
	11th to 20th	Night	42% 5% 12% 18% 17% 6%	5208
	21st to end	Night	39% 8% 14% 22% 16% 1	6932

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% 25% 30% 12% 3	300
	11th to 20th	Day	31% 31% 25% 11% 3	300
	21st to end	Day	23% 30% 33% 11% 2 1	330
February	1st to 10th	Day	24% 34% 27% 12% 2	300
	11th to 20th	Day	42% 26% 25% 6% 1	300
	21st to end	Day	31% 22% 29% 15% 3	397
March	1st to 10th	Day	24% 24% 29% 18% 5%	500
	11th to 20th	Day	23% 23% 30% 18% 5% 1	500
	21st to end	Day	26% 20% 29% 17% 6% 1	530
April	1st to 10th	Day	22% 25% 25% 18% 7% 2	460
	11th to 20th	Day	20% 18% 29% 19% 10% 4	452
	21st to end	Day	24% 15% 30% 19% 9% 2	460
May	1st to 10th	Day	22% 19% 28% 18% 10% 3	463
	11th to 20th	Day	21% 14% 27% 23% 11% 4	487
	21st to end	Day	17% 6% 26% 28% 17% 6%	529
June	1st to 10th	Day	13% 9% 27% 28% 19% 5%	490
	11th to 20th	Day	12% 10% 26% 33% 14% 6%	490
	21st to end	Day	8% 8% 29% 32% 18% 5%	490
July	1st to 10th	Day	12% 4 20% 31% 24% 9%	485
	11th to 20th	Day	6% 2 19% 38% 24% 10%	484
	21st to end	Day	11% 3 25% 32% 21% 8%	532
August	1st to 10th	Day	13% 6% 20% 28% 25% 9%	486
	11th to 20th	Day	11% 4 22% 29% 23% 11%	479
	21st to end	Day	9% 4 18% 32% 22% 16%	539
September	1st to 10th	Day	14% 5% 20% 27% 20% 15%	490
	11th to 20th	Day	16% 3 19% 30% 23% 10%	490
	21st to end	Day	32% 8% 19% 22% 14% 4	490
October	1st to 10th	Day	19% 6% 23% 29% 17% 6%	490
	11th to 20th	Day	15% 18% 25% 24% 16% 2	490
	21st to end	Day	17% 13% 33% 23% 11% 3	539
November	1st to 10th	Day	17% 12% 30% 28% 10% 3	495
	11th to 20th	Day	21% 14% 30% 22% 11% 1	330
	21st to end	Day	23% 19% 34% 20% 5% 1	200
December	1st to 10th	Day	20% 25% 35% 16% 4 1	200
	11th to 20th	Day	24% 18% 32% 18% 8% 1	200
	21st to end	Day	28% 19% 30% 18% 5%	230
January	1st to 10th	Night	20% 17% 35% 25% 3	800
	11th to 20th	Night	20% 15% 33% 29% 3	800
	21st to end	Night	18% 14% 40% 25% 3	880
February	1st to 10th	Night	17% 14% 36% 31% 3	800
	11th to 20th	Night	24% 19% 37% 19% 1	783
	21st to end	Night	19% 18% 38% 23% 2	660
March	1st to 10th	Night	18% 18% 36% 25% 4	800
	11th to 20th	Night	22% 18% 31% 23% 6% 1	800
	21st to end	Night	19% 16% 34% 26% 5% 1	863
April	1st to 10th	Night	17% 18% 34% 25% 6%	760
	11th to 20th	Night	16% 14% 32% 28% 9% 1	751
	21st to end	Night	18% 10% 31% 34% 7% 1	732
May	1st to 10th	Night	20% 9% 23% 30% 17% 1	570
	11th to 20th	Night	20% 5% 21% 37% 16% 1	561
	21st to end	Night	15% 2 17% 38% 24% 3	472
June	1st to 10th	Night	10% 2 18% 45% 23% 1	380
	11th to 20th	Night	11% 3 16% 41% 27% 2	380
	21st to end	Night	9% 2 17% 48% 22% 2	380
July	1st to 10th	Night	11% 2 12% 36% 34% 5%	468
	11th to 20th	Night	7% 2 13% 32% 42% 5%	552
	21st to end	Night	7% 3 13% 39% 33% 5%	624
August	1st to 10th	Night	13% 3 8% 41% 31% 3	567
	11th to 20th	Night	12% 3 13% 37% 31% 4	578
	21st to end	Night	12% 2 12% 37% 33% 4	836
September	1st to 10th	Night	16% 3 14% 35% 28% 5%	760
	11th to 20th	Night	14% 3 13% 36% 31% 3	760
	21st to end	Night	26% 4 17% 32% 20% 1	760
October	1st to 10th	Night	20% 6% 17% 33% 21% 4	760
	11th to 20th	Night	15% 11% 26% 31% 17% 1	760
	21st to end	Night	16% 10% 28% 34% 12% 1	836
November	1st to 10th	Night	15% 8% 27% 36% 12% 2	780
	11th to 20th	Night	16% 11% 31% 33% 8% 1	800
	21st to end	Night	17% 11% 31% 31% 9% 1	800
December	1st to 10th	Night	19% 12% 34% 28% 7%	800
	11th to 20th	Night	18% 10% 31% 35% 6%	800
	21st to end	Night	18% 12% 29% 37% 4	880

Page generated at 2025-03-24 09:04:31. Version ac07

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