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 2024-11-30.

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% 7% 2 7% 5% 5% 5% 4 6% 8% 37%	101459
	Night	14% 7% 2 6% 4 4 4 4 6% 7% 41%	98095
Transparency	Day	41% 8% 12% 20% 14% 5%	105162
	Night	38% 8% 13% 21% 15% 5%	102531
Seeing	Day	21% 16% 24% 20% 13% 6%	15266
	Night	18% 15% 22% 26% 17% 2	25347

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% 6% 3 5% 3 3 3 3 5% 8% 45%	2836
	11th to 20th	Day	12% 8% 3 8% 4 4 4 4 5% 5% 42%	2840
	21st to end	Day	19% 9% 3 7% 3 4 3 3 4 5% 40%	3124
February	1st to 10th	Day	13% 9% 3 7% 5% 3 5% 4 5% 6% 39%	1710
	11th to 20th	Day	17% 10% 3 7% 4 4 4 4 5% 6% 38%	1806
	21st to end	Day	17% 8% 3 7% 4 4 5% 3 6% 6% 38%	1584
March	1st to 10th	Day	22% 7% 2 5% 3 4 4 4 5% 6% 38%	3435
	11th to 20th	Day	11% 7% 2 6% 4 4 4 3 4 6% 49%	3480
	21st to end	Day	18% 8% 3 6% 4 3 4 3 6% 6% 39%	3815
April	1st to 10th	Day	11% 6% 2 6% 4 5% 4 4 5% 7% 45%	2090
	11th to 20th	Day	16% 7% 2 5% 4 4 4 4 6% 6% 42%	2268
	21st to end	Day	15% 7% 3 6% 5% 4 4 5% 5% 6% 39%	2470
May	1st to 10th	Day	15% 9% 3 8% 5% 5% 6% 5% 7% 8% 30%	4082
	11th to 20th	Day	18% 8% 3 8% 6% 5% 5% 5% 7% 8% 28%	4081
	21st to end	Day	10% 6% 2 6% 5% 5% 6% 5% 9% 9% 37%	4476
June	1st to 10th	Day	12% 7% 3 6% 6% 6% 6% 4 8% 9% 33%	2470
	11th to 20th	Day	9% 6% 2 8% 7% 5% 6% 5% 7% 10% 34%	2470
	21st to end	Day	10% 6% 2 7% 6% 5% 6% 6% 9% 10% 33%	2470
July	1st to 10th	Day	7% 5% 2 8% 6% 7% 8% 6% 8% 10% 33%	4120
	11th to 20th	Day	9% 7% 3 8% 7% 7% 7% 6% 8% 11% 29%	4120
	21st to end	Day	16% 10% 4 11% 9% 7% 7% 5% 7% 7% 18%	4466
August	1st to 10th	Day	14% 7% 3 9% 6% 7% 7% 5% 8% 10% 23%	4104
	11th to 20th	Day	10% 5% 2 8% 6% 7% 7% 6% 9% 10% 28%	4044
	21st to end	Day	8% 4 2 6% 5% 5% 6% 4 7% 10% 43%	4312
September	1st to 10th	Day	13% 7% 3 6% 5% 5% 5% 4 6% 8% 39%	2118
	11th to 20th	Day	18% 7% 2 7% 5% 5% 4 4 6% 8% 33%	2090
	21st to end	Day	22% 6% 3 7% 4 4 5% 3 5% 6% 35%	2090
October	1st to 10th	Day	16% 7% 2 5% 4 3 4 3 4 6% 46%	1979
	11th to 20th	Day	14% 6% 2 4 3 4 3 4 4 6% 49%	1900
	21st to end	Day	15% 6% 2 5% 3 3 3 2 4 8% 50%	2090
November	1st to 10th	Day	12% 5% 2 5% 3 4 3 2 5% 6% 53%	1935
	11th to 20th	Day	12% 5% 2 4 4 5% 4 4 5% 5% 50%	1800
	21st to end	Day	15% 6% 3 6% 3 4 3 3 5% 6% 47%	1797
December	1st to 10th	Day	15% 11% 3 7% 5% 5% 4 4 5% 6% 35%	1710
	11th to 20th	Day	14% 5% 3 5% 3 3 3 3 5% 8% 47%	2153
	21st to end	Day	18% 8% 4 6% 3 4 5% 4 6% 6% 37%	3124
January	1st to 10th	Night	14% 8% 3 5% 4 3 4 4 5% 7% 42%	4109
	11th to 20th	Night	11% 8% 3 7% 5% 4 4 4 5% 5% 43%	4120
	21st to end	Night	20% 10% 2 6% 4 3 4 3 4 5% 39%	4532
February	1st to 10th	Night	13% 7% 3 6% 4 4 5% 4 7% 7% 42%	2470
	11th to 20th	Night	17% 10% 3 5% 4 4 4 3 5% 6% 39%	2256
	21st to end	Night	13% 9% 3 8% 5% 4 4 4 6% 7% 37%	1868
March	1st to 10th	Night	22% 12% 3 6% 4 4 4 3 5% 5% 32%	3418
	11th to 20th	Night	9% 5% 2 5% 5% 4 4 5% 5% 7% 49%	3480
	21st to end	Night	16% 6% 2 5% 4 3 4 4 6% 7% 44%	3817
April	1st to 10th	Night	11% 7% 2 5% 5% 4 4 3 6% 8% 44%	2090
	11th to 20th	Night	14% 7% 2 5% 4 4 3 3 7% 7% 45%	1874
	21st to end	Night	14% 6% 3 4 4 3 4 4 5% 7% 46%	1710
May	1st to 10th	Night	15% 8% 3 7% 4 4 5% 3 6% 8% 36%	2813
	11th to 20th	Night	18% 9% 3 4 4 4 4 4 6% 8% 38%	2812
	21st to end	Night	10% 5% 2 5% 4 4 5% 5% 7% 11% 41%	3079
June	1st to 10th	Night	10% 5% 2 5% 4 4 6% 5% 8% 9% 41%	1710
	11th to 20th	Night	10% 6% 1 5% 3 4 5% 4 8% 10% 43%	1710
	21st to end	Night	7% 5% 2 6% 5% 4 4 4 8% 12% 42%	1710
July	1st to 10th	Night	8% 6% 2 7% 5% 5% 6% 5% 9% 10% 37%	2840
	11th to 20th	Night	8% 5% 2 6% 5% 4 5% 5% 8% 11% 40%	2840
	21st to end	Night	11% 10% 4 11% 8% 6% 5% 5% 7% 9% 25%	3087
August	1st to 10th	Night	14% 8% 3 8% 5% 4 5% 4 8% 11% 29%	2825
	11th to 20th	Night	11% 6% 3 8% 5% 6% 6% 5% 7% 8% 36%	2785
	21st to end	Night	7% 4 2 5% 4 4 5% 3 6% 9% 52%	3311
September	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 6% 7% 45%	2058
	11th to 20th	Night	18% 7% 2 6% 3 3 4 3 6% 7% 41%	2090
	21st to end	Night	19% 8% 2 7% 4 4 4 3 5% 6% 40%	2090
October	1st to 10th	Night	16% 7% 2 5% 3 3 4 3 5% 6% 47%	2199
	11th to 20th	Night	14% 6% 1 5% 3 3 4 3 4 5% 51%	2280
	21st to end	Night	14% 5% 2 4 3 2 3 3 4 6% 54%	2508
November	1st to 10th	Night	13% 7% 2 5% 4 3 3 3 4 5% 50%	2361
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 4 7% 51%	2600
	21st to end	Night	13% 6% 2 5% 4 4 4 4 5% 6% 48%	2599
December	1st to 10th	Night	16% 9% 2 6% 4 3 5% 3 5% 7% 39%	2470
	11th to 20th	Night	14% 6% 3 8% 5% 4 4 3 5% 6% 42%	3042
	21st to end	Night	19% 9% 4 7% 4 4 4 4 6% 7% 33%	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	34% 6% 8% 24% 23% 5%	2837
	11th to 20th	Day	39% 8% 8% 18% 24% 4	2840
	21st to end	Day	43% 5% 7% 20% 19% 5%	3124
February	1st to 10th	Day	43% 6% 11% 18% 18% 4	1710
	11th to 20th	Day	46% 5% 9% 17% 18% 5%	1816
	21st to end	Day	44% 7% 12% 15% 16% 6%	1584
March	1st to 10th	Day	43% 7% 10% 18% 14% 8%	3435
	11th to 20th	Day	33% 7% 9% 24% 18% 9%	3480
	21st to end	Day	43% 6% 9% 24% 14% 4	3815
April	1st to 10th	Day	34% 8% 12% 23% 19% 5%	2090
	11th to 20th	Day	38% 7% 12% 19% 18% 5%	2268
	21st to end	Day	42% 8% 12% 21% 13% 2	2470
May	1st to 10th	Day	46% 12% 13% 18% 9% 1	4120
	11th to 20th	Day	48% 11% 13% 20% 7% 1	4085
	21st to end	Day	35% 13% 16% 20% 14% 2	4489
June	1st to 10th	Day	44% 12% 15% 19% 10% 1	2470
	11th to 20th	Day	41% 12% 15% 18% 12% 2	2470
	21st to end	Day	41% 12% 15% 18% 13% 1	2470
July	1st to 10th	Day	43% 12% 18% 17% 7% 3	4055
	11th to 20th	Day	47% 8% 22% 17% 5% 1	4051
	21st to end	Day	61% 10% 11% 13% 4 1	4324
August	1st to 10th	Day	53% 9% 17% 18% 4	4091
	11th to 20th	Day	45% 9% 20% 16% 7% 3	4083
	21st to end	Day	36% 7% 18% 29% 8% 2	4312
September	1st to 10th	Day	41% 10% 13% 18% 15% 4	2118
	11th to 20th	Day	46% 8% 10% 19% 15% 1	2090
	21st to end	Day	49% 6% 11% 17% 14% 3	2090
October	1st to 10th	Day	32% 5% 7% 20% 26% 11%	3291
	11th to 20th	Day	25% 4 7% 24% 19% 22%	3140
	21st to end	Day	28% 3 8% 28% 17% 17%	3428
November	1st to 10th	Day	32% 4 10% 22% 24% 7%	1935
	11th to 20th	Day	33% 5% 9% 20% 27% 7%	1800
	21st to end	Day	38% 4 8% 21% 25% 4	1797
December	1st to 10th	Day	47% 7% 8% 18% 15% 4	1710
	11th to 20th	Day	33% 5% 10% 20% 23% 8%	2150
	21st to end	Day	41% 8% 10% 21% 17% 3	3124
January	1st to 10th	Night	35% 8% 11% 20% 18% 7%	4109
	11th to 20th	Night	37% 9% 8% 17% 21% 8%	4120
	21st to end	Night	45% 5% 9% 17% 19% 6%	4532
February	1st to 10th	Night	39% 6% 11% 23% 18% 3	2470
	11th to 20th	Night	44% 6% 9% 19% 16% 6%	2257
	21st to end	Night	43% 7% 12% 20% 13% 5%	1868
March	1st to 10th	Night	49% 8% 10% 16% 13% 4	3418
	11th to 20th	Night	30% 8% 9% 24% 19% 10%	3480
	21st to end	Night	37% 7% 12% 25% 13% 6%	3817
April	1st to 10th	Night	35% 8% 13% 23% 17% 4	2090
	11th to 20th	Night	35% 7% 12% 21% 18% 6%	1874
	21st to end	Night	35% 8% 15% 26% 14% 2	1710
May	1st to 10th	Night	43% 10% 15% 23% 8% 1	2840
	11th to 20th	Night	42% 12% 14% 24% 8%	2813
	21st to end	Night	30% 13% 19% 26% 10% 2	3087
June	1st to 10th	Night	35% 13% 18% 21% 11% 1	1710
	11th to 20th	Night	33% 11% 21% 24% 11% 1	1710
	21st to end	Night	32% 12% 19% 23% 11% 3	1710
July	1st to 10th	Night	39% 12% 18% 21% 8% 1	2795
	11th to 20th	Night	36% 10% 26% 22% 5% 1	2786
	21st to end	Night	53% 10% 16% 16% 5%	2988
August	1st to 10th	Night	47% 7% 23% 16% 6%	2814
	11th to 20th	Night	42% 8% 21% 20% 9% 1	2812
	21st to end	Night	28% 7% 19% 36% 8% 2	3311
September	1st to 10th	Night	37% 9% 17% 18% 16% 4	2058
	11th to 20th	Night	42% 7% 14% 20% 17% 1	2090
	21st to end	Night	45% 7% 10% 20% 15% 2	2090
October	1st to 10th	Night	35% 4 7% 20% 22% 12%	3667
	11th to 20th	Night	25% 4 9% 22% 23% 19%	3779
	21st to end	Night	27% 3 7% 26% 21% 17%	4120
November	1st to 10th	Night	35% 5% 11% 19% 23% 7%	2361
	11th to 20th	Night	32% 5% 9% 20% 25% 9%	2600
	21st to end	Night	34% 5% 8% 23% 24% 5%	2599
December	1st to 10th	Night	42% 7% 10% 20% 17% 5%	2470
	11th to 20th	Night	37% 7% 11% 16% 23% 5%	3044
	21st to end	Night	43% 9% 11% 21% 15% 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	32% 30% 27% 9% 2	270
	11th to 20th	Day	36% 26% 29% 8% 1	270
	21st to end	Day	30% 35% 25% 8% 2	297
February	1st to 10th	Day	29% 32% 26% 12% 1	270
	11th to 20th	Day	45% 29% 22% 4	270
	21st to end	Day	36% 31% 27% 5%	239
March	1st to 10th	Day	26% 25% 27% 16% 5%	460
	11th to 20th	Day	29% 28% 26% 12% 3 1	460
	21st to end	Day	27% 29% 23% 15% 5% 1	506
April	1st to 10th	Day	26% 23% 29% 14% 7% 1	460
	11th to 20th	Day	23% 24% 25% 15% 11% 2	452
	21st to end	Day	20% 22% 28% 17% 10% 3	460
May	1st to 10th	Day	20% 21% 33% 13% 8% 4	463
	11th to 20th	Day	19% 15% 26% 24% 11% 6%	487
	21st to end	Day	19% 9% 24% 24% 15% 9%	529
June	1st to 10th	Day	14% 9% 27% 26% 16% 8%	490
	11th to 20th	Day	11% 9% 23% 29% 18% 9%	490
	21st to end	Day	13% 8% 30% 28% 13% 9%	490
July	1st to 10th	Day	11% 4 19% 29% 25% 12%	485
	11th to 20th	Day	8% 3 17% 33% 27% 12%	484
	21st to end	Day	13% 3 18% 29% 25% 12%	532
August	1st to 10th	Day	16% 6% 23% 23% 17% 14%	486
	11th to 20th	Day	14% 4 19% 26% 24% 13%	479
	21st to end	Day	13% 3 15% 29% 24% 16%	539
September	1st to 10th	Day	14% 3 19% 26% 25% 12%	490
	11th to 20th	Day	19% 4 17% 23% 26% 11%	490
	21st to end	Day	24% 10% 20% 26% 16% 5%	490
October	1st to 10th	Day	23% 8% 20% 24% 19% 6%	490
	11th to 20th	Day	17% 19% 23% 21% 17% 2	490
	21st to end	Day	20% 15% 29% 24% 9% 3	539
November	1st to 10th	Day	21% 16% 27% 23% 10% 3	473
	11th to 20th	Day	23% 26% 29% 17% 5% 1	300
	21st to end	Day	26% 23% 29% 18% 3 1	299
December	1st to 10th	Day	26% 31% 32% 9% 1	270
	11th to 20th	Day	23% 26% 33% 15% 4	270
	21st to end	Day	30% 21% 35% 8% 5% 1	297
January	1st to 10th	Night	20% 28% 29% 18% 4	760
	11th to 20th	Night	22% 27% 27% 20% 4	760
	21st to end	Night	19% 28% 32% 18% 3	836
February	1st to 10th	Night	17% 24% 28% 28% 3	760
	11th to 20th	Night	27% 31% 27% 14% 1	743
	21st to end	Night	20% 27% 34% 16% 2	628
March	1st to 10th	Night	20% 24% 31% 22% 3	760
	11th to 20th	Night	25% 21% 28% 18% 6% 1	760
	21st to end	Night	19% 20% 32% 22% 6%	836
April	1st to 10th	Night	19% 22% 28% 24% 6%	760
	11th to 20th	Night	19% 20% 26% 24% 10% 1	746
	21st to end	Night	18% 14% 24% 35% 9%	570
May	1st to 10th	Night	18% 11% 27% 29% 14% 1	570
	11th to 20th	Night	17% 9% 22% 33% 16% 3	561
	21st to end	Night	18% 5% 16% 27% 29% 6%	617
June	1st to 10th	Night	12% 3 18% 35% 27% 5%	570
	11th to 20th	Night	13% 3 17% 28% 34% 5%	570
	21st to end	Night	8% 3 17% 35% 34% 3	570
July	1st to 10th	Night	13% 1 10% 29% 41% 7%	555
	11th to 20th	Night	7% 2 9% 30% 45% 7%	552
	21st to end	Night	7% 2 11% 34% 40% 7%	624
August	1st to 10th	Night	15% 1 13% 30% 36% 5%	567
	11th to 20th	Night	15% 2 11% 31% 34% 8%	564
	21st to end	Night	13% 1 10% 29% 40% 8%	736
September	1st to 10th	Night	14% 4 13% 27% 36% 7%	760
	11th to 20th	Night	20% 4 13% 25% 32% 6%	760
	21st to end	Night	23% 6% 16% 31% 24% 1	760
October	1st to 10th	Night	19% 11% 17% 25% 22% 6%	760
	11th to 20th	Night	19% 13% 22% 29% 16% 2	760
	21st to end	Night	19% 14% 21% 31% 13% 2	836
November	1st to 10th	Night	17% 14% 22% 31% 13% 3	780
	11th to 20th	Night	19% 18% 22% 31% 9% 1	800
	21st to end	Night	19% 19% 25% 27% 10% 1	800
December	1st to 10th	Night	23% 22% 25% 24% 6%	760
	11th to 20th	Night	19% 21% 29% 25% 6% 1	760
	21st to end	Night	25% 21% 24% 25% 5%	836

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