Blue Ridge
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Ridge clear sky chart from 2005-11-06 to 2025-07-21.

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	7% 3 1 3 2 2 3 3 4 6% 65%	124119
	Night	8% 4 1 4 3 3 3 3 4 6% 62%	123864
Transparency	Day	21% 4 8% 19% 25% 23%	123876
	Night	24% 4 9% 19% 23% 22%	123694
Seeing	Day	8% 8% 21% 31% 24% 8%	16204
	Night	9% 4 18% 38% 27% 4	25490

By Month

Percentage Cloud Cover SW (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% 7% 3 5% 3 3 3 4 5% 7% 47%	3759
	11th to 20th	Day	13% 7% 3 6% 4 3 3 2 3 4 54%	4100
	21st to end	Day	17% 6% 2 5% 4 3 4 3 4 6% 45%	4510
February	1st to 10th	Day	16% 6% 2 6% 4 4 4 3 6% 5% 45%	1800
	11th to 20th	Day	12% 5% 2 6% 5% 5% 4 4 5% 8% 43%	1833
	21st to end	Day	12% 6% 2 6% 4 3 4 3 5% 6% 49%	1650
March	1st to 10th	Day	11% 9% 3 8% 5% 5% 5% 4 6% 7% 38%	4839
	11th to 20th	Day	12% 5% 2 5% 4 3 4 4 5% 7% 50%	5000
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 5% 8% 8% 39%	5586
April	1st to 10th	Day	11% 8% 2 4 4 4 5% 4 6% 7% 45%	2400
	11th to 20th	Day	9% 3 2 4 3 3 4 4 5% 7% 57%	2369
	21st to end	Day	3 4 1 3 3 3 3 3 5% 7% 62%	2400
May	1st to 10th	Day	3 2 1 2 2 1 1 2 3 5% 79%	5674
	11th to 20th	Day	2 1 1 2 1 1 2 2 4 6% 79%	6074
	21st to end	Day	1 1 1 1 1 2 2 4 6% 82%	6902
June	1st to 10th	Day	4 3 1 3 2 3 3 3 5% 7% 68%	2800
	11th to 20th	Day	1 1 1 2 2 2 4 6% 81%	2800
	21st to end	Day	1 1 1 1 1 2 2 3 5% 83%	2800
July	1st to 10th	Day	1 1 1 1 3 92%	4560
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 3 87%	4369
	21st to end	Day	1 1 1 1 1 2 5% 88%	4456
August	1st to 10th	Day	1 1 1 1 1 2 2 3 5% 6% 76%	4091
	11th to 20th	Day	1 1 1 1 1 1 2 3 88%	3851
	21st to end	Day	1 1 1 1 1 1 1 2 5% 86%	4180
September	1st to 10th	Day	1 1 1 2 1 2 4 86%	2280
	11th to 20th	Day	2 1 1 3 2 2 2 3 4 6% 74%	2280
	21st to end	Day	2 2 1 2 2 2 1 2 3 4 79%	2016
October	1st to 10th	Day	1 2 1 3 2 2 2 2 4 5% 76%	3180
	11th to 20th	Day	3 3 1 3 2 2 2 2 4 6% 73%	3161
	21st to end	Day	4 3 2 3 3 3 3 3 5% 7% 64%	3448
November	1st to 10th	Day	7% 5% 1 3 2 3 4 3 5% 7% 59%	1889
	11th to 20th	Day	9% 7% 2 6% 4 3 3 4 6% 6% 50%	1800
	21st to end	Day	11% 4 1 4 3 4 4 3 6% 6% 55%	1610
December	1st to 10th	Day	8% 4 2 3 3 2 3 3 6% 10% 55%	2620
	11th to 20th	Day	24% 8% 3 5% 4 3 4 3 6% 7% 33%	3028
	21st to end	Day	23% 10% 3 7% 3 3 3 4 5% 7% 31%	4004
January	1st to 10th	Night	9% 5% 2 5% 3 3 4 3 6% 9% 51%	6174
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 4 54%	5880
	21st to end	Night	14% 6% 2 6% 4 3 3 3 4 6% 48%	6468
February	1st to 10th	Night	16% 5% 2 5% 3 3 4 3 4 5% 50%	2600
	11th to 20th	Night	11% 5% 2 6% 4 3 4 4 5% 7% 48%	2452
	21st to end	Night	11% 4 2 4 3 3 2 4 5% 6% 55%	1980
March	1st to 10th	Night	10% 7% 3 7% 4 4 4 4 6% 8% 43%	5040
	11th to 20th	Night	13% 5% 1 4 3 3 4 3 5% 7% 54%	5000
	21st to end	Night	12% 7% 2 6% 4 4 4 3 5% 8% 45%	5387
April	1st to 10th	Night	7% 5% 1 5% 5% 4 3 4 4 8% 54%	2000
	11th to 20th	Night	7% 4 1 3 3 3 3 4 5% 7% 60%	1979
	21st to end	Night	5% 3 1 2 2 2 3 3 5% 9% 65%	2000
May	1st to 10th	Night	2 2 1 2 2 1 2 1 3 6% 78%	4298
	11th to 20th	Night	1 1 1 2 2 2 2 2 4 6% 78%	3812
	21st to end	Night	1 1 1 1 1 1 2 3 5% 85%	3972
June	1st to 10th	Night	3 2 1 3 3 3 2 3 4 8% 70%	1600
	11th to 20th	Night	1 1 1 1 1 2 1 3 6% 82%	1600
	21st to end	Night	1 1 1 1 1 3 2 3 5% 82%	1600
July	1st to 10th	Night	1 1 1 3 92%	2620
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 85%	2812
	21st to end	Night	1 1 1 1 1 2 2 4 8% 81%	3109
August	1st to 10th	Night	3 1 2 1 2 2 3 5% 8% 72%	2848
	11th to 20th	Night	1 1 1 1 2 3 5% 86%	2994
	21st to end	Night	2 1 2 1 1 1 1 3 6% 83%	3498
September	1st to 10th	Night	1 1 1 1 1 3 7% 85%	1900
	11th to 20th	Night	2 2 1 2 1 2 3 2 4 6% 75%	1900
	21st to end	Night	1 1 1 2 2 1 2 2 4 6% 80%	2161
October	1st to 10th	Night	2 1 1 2 2 2 2 2 4 5% 77%	3800
	11th to 20th	Night	2 1 1 2 2 2 2 2 4 7% 73%	3777
	21st to end	Night	4 2 1 3 3 3 4 3 5% 6% 66%	4121
November	1st to 10th	Night	6% 3 1 5% 3 3 3 3 6% 8% 58%	2409
	11th to 20th	Night	8% 5% 2 4 3 3 3 3 5% 8% 56%	2600
	21st to end	Night	9% 4 1 4 3 3 3 2 5% 7% 58%	2785
December	1st to 10th	Night	9% 3 2 4 3 3 3 3 5% 7% 59%	4560
	11th to 20th	Night	22% 7% 2 5% 4 4 5% 4 7% 8% 31%	5176
	21st to end	Night	22% 9% 3 8% 5% 4 4 3 4 5% 33%	6952

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% 9% 15% 21% 11% 8%	3759
	11th to 20th	Day	36% 5% 7% 16% 22% 15%	4100
	21st to end	Day	40% 5% 8% 15% 12% 20%	4510
February	1st to 10th	Day	40% 5% 8% 20% 19% 9%	1800
	11th to 20th	Day	37% 6% 10% 19% 20% 7%	1842
	21st to end	Day	35% 4 11% 24% 18% 9%	1650
March	1st to 10th	Day	42% 6% 9% 18% 21% 4	4839
	11th to 20th	Day	31% 6% 13% 26% 16% 9%	5000
	21st to end	Day	36% 9% 15% 26% 12% 2	5586
April	1st to 10th	Day	35% 7% 12% 21% 19% 6%	2400
	11th to 20th	Day	26% 7% 13% 23% 22% 9%	2372
	21st to end	Day	20% 5% 15% 27% 27% 6%	2400
May	1st to 10th	Day	10% 2 7% 24% 31% 26%	5674
	11th to 20th	Day	8% 3 7% 30% 38% 14%	6077
	21st to end	Day	7% 3 9% 29% 41% 11%	6904
June	1st to 10th	Day	16% 5% 12% 27% 30% 10%	2800
	11th to 20th	Day	7% 3 10% 21% 35% 24%	2800
	21st to end	Day	6% 2 6% 16% 34% 36%	2800
July	1st to 10th	Day	3 1 3 8% 15% 71%	4491
	11th to 20th	Day	6% 1 4 8% 15% 66%	4295
	21st to end	Day	4 1 3 7% 24% 61%	4316
August	1st to 10th	Day	9% 2 4 11% 26% 48%	4076
	11th to 20th	Day	4 1 4 10% 41% 41%	3890
	21st to end	Day	6% 1 5% 12% 36% 40%	4179
September	1st to 10th	Day	5% 1 4 9% 30% 50%	2280
	11th to 20th	Day	12% 2 9% 22% 38% 16%	2280
	21st to end	Day	11% 2 6% 17% 34% 30%	2016
October	1st to 10th	Day	12% 3 5% 11% 27% 43%	3180
	11th to 20th	Day	14% 3 5% 25% 30% 22%	3161
	21st to end	Day	20% 3 8% 23% 32% 15%	3448
November	1st to 10th	Day	25% 5% 9% 27% 22% 12%	1889
	11th to 20th	Day	33% 6% 11% 23% 20% 7%	1800
	21st to end	Day	30% 4 10% 20% 24% 11%	1610
December	1st to 10th	Day	25% 4 7% 31% 25% 8%	2620
	11th to 20th	Day	51% 4 9% 18% 15% 3	3028
	21st to end	Day	51% 6% 13% 17% 11% 2	4004
January	1st to 10th	Night	29% 9% 23% 20% 15% 5%	6174
	11th to 20th	Night	34% 6% 9% 15% 23% 14%	5880
	21st to end	Night	37% 5% 9% 12% 19% 19%	6468
February	1st to 10th	Night	37% 4 8% 19% 21% 11%	2600
	11th to 20th	Night	35% 5% 11% 21% 19% 10%	2454
	21st to end	Night	30% 4 10% 25% 23% 9%	1980
March	1st to 10th	Night	36% 8% 11% 17% 22% 6%	5040
	11th to 20th	Night	30% 4 14% 25% 18% 10%	5000
	21st to end	Night	35% 6% 15% 24% 16% 3	5387
April	1st to 10th	Night	29% 6% 14% 21% 22% 8%	2000
	11th to 20th	Night	23% 6% 13% 27% 18% 14%	1979
	21st to end	Night	17% 5% 12% 31% 29% 7%	2000
May	1st to 10th	Night	11% 4 6% 23% 31% 26%	4295
	11th to 20th	Night	10% 2 10% 26% 33% 20%	3812
	21st to end	Night	5% 2 9% 26% 36% 22%	3972
June	1st to 10th	Night	14% 5% 10% 25% 32% 13%	1600
	11th to 20th	Night	7% 2 8% 20% 35% 29%	1600
	21st to end	Night	6% 3 5% 14% 31% 41%	1600
July	1st to 10th	Night	3 1 2 9% 14% 72%	2580
	11th to 20th	Night	7% 1 3 9% 12% 68%	2763
	21st to end	Night	4 1 3 10% 20% 62%	3011
August	1st to 10th	Night	11% 2 4 11% 18% 53%	2839
	11th to 20th	Night	4 1 4 9% 27% 56%	3021
	21st to end	Night	7% 1 3 12% 31% 45%	3498
September	1st to 10th	Night	4 1 4 9% 28% 53%	1900
	11th to 20th	Night	11% 3 8% 20% 35% 23%	1900
	21st to end	Night	8% 3 7% 17% 29% 37%	2161
October	1st to 10th	Night	11% 2 6% 10% 18% 53%	3800
	11th to 20th	Night	12% 3 8% 25% 28% 24%	3777
	21st to end	Night	19% 3 7% 21% 29% 22%	4121
November	1st to 10th	Night	24% 5% 10% 24% 21% 16%	2409
	11th to 20th	Night	27% 4 12% 27% 21% 9%	2600
	21st to end	Night	28% 5% 9% 20% 25% 14%	2785
December	1st to 10th	Night	27% 4 6% 21% 29% 14%	4560
	11th to 20th	Night	49% 5% 10% 17% 17% 3	5176
	21st to end	Night	51% 7% 11% 17% 10% 3	6952

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	30% 11% 23% 20% 15% 2	255
	11th to 20th	Day	16% 11% 19% 33% 16% 5%	400
	21st to end	Day	16% 8% 20% 27% 25% 5%	440
February	1st to 10th	Day	20% 13% 17% 25% 21% 4	400
	11th to 20th	Day	19% 14% 24% 24% 14% 5%	435
	21st to end	Day	14% 11% 27% 30% 15% 3	414
March	1st to 10th	Day	19% 13% 26% 28% 10% 4	500
	11th to 20th	Day	21% 11% 23% 23% 16% 5%	500
	21st to end	Day	15% 14% 26% 26% 16% 2	550
April	1st to 10th	Day	14% 11% 27% 25% 17% 5%	500
	11th to 20th	Day	9% 12% 30% 24% 19% 6%	492
	21st to end	Day	4 12% 26% 26% 23% 8%	500
May	1st to 10th	Day	6% 12% 30% 27% 21% 5%	503
	11th to 20th	Day	6% 9% 26% 31% 21% 8%	528
	21st to end	Day	2 7% 24% 34% 24% 9%	573
June	1st to 10th	Day	4 7% 25% 35% 23% 6%	530
	11th to 20th	Day	2 8% 23% 38% 27% 2	530
	21st to end	Day	1 3 22% 38% 29% 6%	530
July	1st to 10th	Day	1 2 16% 43% 31% 7%	526
	11th to 20th	Day	1 1 14% 38% 37% 9%	524
	21st to end	Day	1 1 13% 41% 34% 11%	532
August	1st to 10th	Day	2 2 14% 41% 30% 11%	487
	11th to 20th	Day	2 2 17% 33% 32% 14%	477
	21st to end	Day	1 2 12% 36% 37% 14%	539
September	1st to 10th	Day	1 2 12% 37% 36% 13%	490
	11th to 20th	Day	2 7% 16% 34% 30% 10%	490
	21st to end	Day	2 4 18% 33% 30% 12%	490
October	1st to 10th	Day	3 8% 20% 28% 27% 14%	490
	11th to 20th	Day	7% 6% 19% 27% 29% 13%	490
	21st to end	Day	9% 7% 19% 29% 25% 11%	539
November	1st to 10th	Day	8% 10% 21% 23% 28% 9%	425
	11th to 20th	Day	12% 9% 19% 25% 27% 7%	300
	21st to end	Day	15% 6% 22% 31% 23% 2	205
December	1st to 10th	Day	16% 9% 24% 25% 20% 7%	200
	11th to 20th	Day	20% 7% 25% 27% 16% 6%	200
	21st to end	Day	21% 12% 21% 24% 18% 4	220
January	1st to 10th	Night	19% 5% 19% 33% 20% 5%	800
	11th to 20th	Night	16% 6% 18% 37% 19% 4	800
	21st to end	Night	16% 5% 18% 34% 24% 3	880
February	1st to 10th	Night	18% 7% 18% 34% 21% 3	800
	11th to 20th	Night	13% 8% 23% 31% 21% 4	784
	21st to end	Night	14% 7% 21% 38% 18% 2	660
March	1st to 10th	Night	16% 7% 23% 31% 22% 2	800
	11th to 20th	Night	17% 7% 20% 31% 24% 2	800
	21st to end	Night	13% 7% 24% 37% 18% 1	880
April	1st to 10th	Night	9% 6% 26% 38% 19% 2	800
	11th to 20th	Night	8% 7% 27% 34% 20% 3	791
	21st to end	Night	6% 5% 25% 39% 23% 3	800
May	1st to 10th	Night	5% 4 26% 36% 26% 3	694
	11th to 20th	Night	4 2 20% 43% 29% 2	486
	21st to end	Night	4 3 25% 43% 25% 1	433
June	1st to 10th	Night	3 2 22% 47% 25% 2	400
	11th to 20th	Night	2 2 24% 48% 24% 1	400
	21st to end	Night	1 1 19% 51% 28% 1	400
July	1st to 10th	Night	1 1 13% 56% 30%	390
	11th to 20th	Night	1 10% 50% 35% 3	501
	21st to end	Night	1 1 9% 49% 37% 4	624
August	1st to 10th	Night	1 1 13% 44% 37% 4	567
	11th to 20th	Night	1 14% 43% 37% 5%	668
	21st to end	Night	1 1 9% 43% 42% 4	836
September	1st to 10th	Night	1 1 9% 43% 42% 4	760
	11th to 20th	Night	2 2 15% 41% 35% 5%	760
	21st to end	Night	2 2 13% 40% 37% 7%	760
October	1st to 10th	Night	4 3 14% 39% 33% 6%	760
	11th to 20th	Night	5% 4 16% 35% 35% 6%	760
	21st to end	Night	7% 5% 17% 33% 30% 9%	836
November	1st to 10th	Night	7% 5% 18% 31% 33% 6%	780
	11th to 20th	Night	10% 4 18% 34% 29% 5%	800
	21st to end	Night	11% 7% 17% 36% 24% 6%	800
December	1st to 10th	Night	15% 6% 20% 31% 23% 5%	800
	11th to 20th	Night	22% 5% 17% 32% 22% 3	800
	21st to end	Night	20% 8% 22% 31% 19%	880

Page generated at 2025-07-17 22:10:00. Version ac07

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