Greenbrier State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Greenbrier State Park 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	14% 7% 2 6% 5% 4 5% 4 6% 8% 39%	120229
	Night	14% 7% 2 6% 4 4 4 4 6% 7% 40%	118898
Transparency	Day	40% 7% 13% 23% 15% 3	123866
	Night	39% 7% 13% 23% 16% 3	123398
Seeing	Day	19% 13% 26% 23% 14% 5%	15792
	Night	17% 11% 29% 31% 12% 1	25298

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	14% 5% 2 8% 6% 6% 5% 4 4 6% 39%	3712
	11th to 20th	Day	13% 10% 3 8% 5% 5% 5% 3 5% 7% 38%	4060
	21st to end	Day	14% 8% 2 8% 5% 5% 5% 3 6% 6% 38%	4466
February	1st to 10th	Day	14% 10% 3 7% 6% 4 4 4 5% 6% 37%	1800
	11th to 20th	Day	16% 9% 3 7% 6% 4 4 3 5% 6% 38%	1773
	21st to end	Day	17% 9% 3 8% 4 4 4 3 5% 5% 38%	1650
March	1st to 10th	Day	18% 6% 2 6% 5% 4 4 3 5% 6% 40%	4905
	11th to 20th	Day	11% 6% 2 6% 5% 4 4 4 5% 8% 45%	4980
	21st to end	Day	15% 7% 2 7% 5% 5% 4 4 5% 7% 38%	5510
April	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 4 6% 7% 36%	2400
	11th to 20th	Day	13% 7% 2 6% 5% 4 5% 4 4 8% 42%	2374
	21st to end	Day	17% 7% 2 6% 4 4 4 4 6% 7% 40%	2410
May	1st to 10th	Day	20% 7% 3 7% 4 5% 6% 4 7% 8% 29%	5862
	11th to 20th	Day	20% 8% 3 7% 4 4 5% 4 7% 9% 29%	6131
	21st to end	Day	15% 6% 2 6% 4 5% 5% 5% 7% 9% 35%	6934
June	1st to 10th	Day	12% 5% 2 5% 4 4 5% 5% 7% 10% 41%	2800
	11th to 20th	Day	12% 6% 2 5% 4 5% 6% 5% 6% 9% 39%	2800
	21st to end	Day	9% 5% 2 5% 5% 5% 5% 4 7% 9% 45%	2800
July	1st to 10th	Day	5% 4 2 6% 5% 5% 7% 6% 8% 10% 42%	4580
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 5% 8% 11% 41%	4428
	21st to end	Day	10% 8% 2 8% 6% 6% 5% 4 7% 8% 36%	4466
August	1st to 10th	Day	14% 6% 3 7% 5% 6% 6% 5% 7% 10% 33%	4104
	11th to 20th	Day	10% 6% 3 6% 5% 5% 5% 5% 8% 9% 40%	3929
	21st to end	Day	9% 7% 2 6% 5% 4 5% 3 6% 8% 46%	4180
September	1st to 10th	Day	15% 6% 2 5% 4 4 4 3 5% 7% 45%	2280
	11th to 20th	Day	11% 4 2 5% 4 4 4 4 6% 8% 47%	2280
	21st to end	Day	20% 8% 2 7% 4 3 3 4 5% 6% 38%	1984
October	1st to 10th	Day	16% 6% 1 6% 4 4 3 3 5% 8% 44%	1900
	11th to 20th	Day	16% 5% 2 5% 3 4 4 3 5% 6% 47%	1900
	21st to end	Day	13% 7% 3 6% 3 4 4 3 5% 6% 47%	2090
November	1st to 10th	Day	11% 7% 2 5% 3 3 3 4 5% 7% 50%	1866
	11th to 20th	Day	16% 5% 2 4 4 3 4 4 5% 7% 46%	1725
	21st to end	Day	15% 6% 2 7% 5% 4 4 4 5% 7% 40%	1600
December	1st to 10th	Day	13% 7% 2 7% 5% 4 5% 4 7% 7% 40%	2600
	11th to 20th	Day	20% 9% 2 6% 4 4 3 3 5% 6% 37%	2990
	21st to end	Day	16% 9% 3 7% 5% 4 5% 4 6% 9% 31%	3960
January	1st to 10th	Night	11% 8% 2 7% 5% 5% 5% 4 6% 7% 40%	6230
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 5% 7% 42%	5900
	21st to end	Night	14% 8% 3 6% 4 4 4 4 5% 7% 42%	6490
February	1st to 10th	Night	14% 9% 2 6% 4 4 5% 4 6% 7% 39%	2600
	11th to 20th	Night	18% 8% 2 7% 4 4 4 4 5% 6% 38%	2513
	21st to end	Night	16% 9% 3 7% 5% 5% 5% 3 5% 7% 35%	1980
March	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 4 6% 44%	4946
	11th to 20th	Night	10% 5% 2 6% 5% 4 4 3 5% 7% 48%	4980
	21st to end	Night	11% 8% 2 7% 4 4 4 3 6% 7% 44%	5422
April	1st to 10th	Night	15% 9% 3 7% 6% 5% 5% 4 6% 5% 35%	2000
	11th to 20th	Night	13% 6% 3 7% 4 3 4 4 6% 8% 42%	1975
	21st to end	Night	16% 5% 2 5% 4 4 5% 4 6% 8% 42%	2000
May	1st to 10th	Night	23% 9% 3 8% 5% 5% 4 3 6% 8% 27%	4039
	11th to 20th	Night	21% 10% 2 7% 4 4 4 4 6% 8% 30%	3759
	21st to end	Night	16% 7% 3 7% 4 5% 6% 5% 7% 7% 33%	3920
June	1st to 10th	Night	11% 5% 2 5% 4 5% 4 4 7% 8% 46%	1600
	11th to 20th	Night	11% 6% 2 9% 5% 5% 5% 4 7% 8% 40%	1600
	21st to end	Night	7% 6% 2 6% 6% 4 5% 5% 7% 9% 44%	1600
July	1st to 10th	Night	7% 5% 1 7% 6% 6% 6% 4 7% 11% 40%	2600
	11th to 20th	Night	6% 6% 2 8% 6% 6% 6% 6% 8% 9% 37%	2748
	21st to end	Night	11% 7% 2 6% 4 5% 5% 5% 8% 8% 38%	3087
August	1st to 10th	Night	14% 7% 3 8% 6% 5% 4 5% 6% 9% 33%	2825
	11th to 20th	Night	9% 7% 2 7% 5% 5% 5% 5% 6% 8% 40%	2867
	21st to end	Night	10% 6% 2 6% 5% 4 3 4 6% 8% 46%	3476
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 5% 8% 46%	1900
	11th to 20th	Night	10% 6% 2 5% 3 4 3 3 4 7% 52%	1900
	21st to end	Night	18% 7% 3 5% 3 5% 4 3 5% 6% 40%	2190
October	1st to 10th	Night	18% 6% 2 5% 4 4 4 3 4 6% 44%	2280
	11th to 20th	Night	13% 5% 2 5% 3 5% 3 3 5% 6% 50%	2280
	21st to end	Night	15% 6% 2 5% 3 3 4 3 6% 7% 48%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 6% 53%	2431
	11th to 20th	Night	15% 7% 2 4 4 3 4 4 6% 6% 44%	2671
	21st to end	Night	14% 5% 2 6% 5% 4 5% 4 6% 7% 42%	2800
December	1st to 10th	Night	11% 7% 3 6% 4 4 4 5% 5% 8% 45%	4580
	11th to 20th	Night	20% 6% 2 6% 5% 5% 5% 4 5% 6% 36%	5205
	21st to end	Night	16% 7% 3 8% 5% 5% 6% 5% 7% 8% 29%	6996

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	39% 6% 11% 25% 16% 4	3713
	11th to 20th	Day	43% 5% 10% 19% 18% 5%	4060
	21st to end	Day	42% 7% 11% 24% 13% 2	4466
February	1st to 10th	Day	46% 7% 13% 22% 10% 2	1800
	11th to 20th	Day	47% 5% 10% 23% 13% 2	1782
	21st to end	Day	47% 5% 11% 20% 13% 4	1650
March	1st to 10th	Day	40% 7% 11% 21% 15% 6%	4905
	11th to 20th	Day	34% 6% 13% 25% 16% 5%	4980
	21st to end	Day	44% 5% 10% 18% 18% 4	5510
April	1st to 10th	Day	43% 8% 12% 17% 16% 4	2400
	11th to 20th	Day	39% 6% 12% 21% 17% 4	2374
	21st to end	Day	42% 7% 10% 22% 17% 3	2410
May	1st to 10th	Day	48% 9% 13% 19% 10% 2	5897
	11th to 20th	Day	47% 9% 13% 22% 9% 1	6135
	21st to end	Day	40% 9% 14% 21% 15% 1	6949
June	1st to 10th	Day	35% 9% 15% 24% 16% 1	2800
	11th to 20th	Day	38% 11% 13% 22% 13% 2	2800
	21st to end	Day	32% 8% 16% 26% 16% 2	2800
July	1st to 10th	Day	34% 10% 17% 22% 12% 5%	4510
	11th to 20th	Day	35% 8% 16% 28% 12% 1	4353
	21st to end	Day	43% 7% 10% 21% 17% 1	4324
August	1st to 10th	Day	44% 9% 14% 24% 8%	4091
	11th to 20th	Day	37% 8% 13% 27% 13% 1	3968
	21st to end	Day	35% 6% 14% 26% 16% 2	4180
September	1st to 10th	Day	39% 7% 11% 20% 20% 4	2280
	11th to 20th	Day	33% 6% 12% 25% 21% 3	2280
	21st to end	Day	46% 6% 9% 19% 18% 2	1984
October	1st to 10th	Day	38% 5% 8% 19% 25% 5%	3160
	11th to 20th	Day	28% 4 11% 26% 21% 9%	3140
	21st to end	Day	25% 4 12% 30% 26% 3	3428
November	1st to 10th	Day	33% 5% 13% 25% 19% 4	1866
	11th to 20th	Day	36% 5% 14% 23% 19% 3	1725
	21st to end	Day	40% 6% 13% 27% 12% 1	1600
December	1st to 10th	Day	39% 7% 16% 28% 7% 3	2600
	11th to 20th	Day	46% 6% 14% 25% 8% 2	2986
	21st to end	Day	45% 9% 18% 15% 12% 1	3960
January	1st to 10th	Night	34% 7% 13% 24% 19% 3	6230
	11th to 20th	Night	37% 5% 12% 24% 16% 6%	5900
	21st to end	Night	38% 6% 11% 27% 15% 2	6490
February	1st to 10th	Night	42% 7% 14% 24% 12% 1	2600
	11th to 20th	Night	44% 5% 11% 22% 15% 3	2515
	21st to end	Night	46% 6% 12% 20% 13% 5%	1980
March	1st to 10th	Night	38% 6% 9% 26% 12% 8%	4946
	11th to 20th	Night	34% 6% 12% 30% 13% 5%	4980
	21st to end	Night	38% 5% 11% 21% 21% 4	5422
April	1st to 10th	Night	46% 7% 11% 18% 15% 3	2000
	11th to 20th	Night	39% 7% 11% 21% 17% 5%	1975
	21st to end	Night	39% 7% 12% 24% 15% 4	2000
May	1st to 10th	Night	55% 6% 10% 15% 10% 4	4066
	11th to 20th	Night	47% 8% 13% 21% 10% 1	3760
	21st to end	Night	43% 10% 14% 24% 8% 1	3927
June	1st to 10th	Night	34% 9% 11% 26% 17% 3	1600
	11th to 20th	Night	39% 10% 11% 24% 14% 2	1600
	21st to end	Night	34% 10% 14% 24% 18% 2	1600
July	1st to 10th	Night	37% 9% 19% 17% 13% 5%	2560
	11th to 20th	Night	38% 9% 16% 21% 13% 1	2700
	21st to end	Night	39% 9% 15% 19% 17% 1	2988
August	1st to 10th	Night	45% 8% 17% 22% 7% 1	2814
	11th to 20th	Night	39% 6% 12% 27% 14% 1	2894
	21st to end	Night	35% 6% 14% 28% 16% 2	3476
September	1st to 10th	Night	37% 7% 12% 20% 20% 5%	1900
	11th to 20th	Night	33% 5% 10% 28% 22% 2	1900
	21st to end	Night	45% 6% 10% 20% 17% 3	2190
October	1st to 10th	Night	40% 5% 10% 22% 21% 3	3800
	11th to 20th	Night	24% 4 12% 24% 28% 7%	3779
	21st to end	Night	25% 4 11% 31% 24% 4	4120
November	1st to 10th	Night	32% 5% 11% 26% 22% 4	2431
	11th to 20th	Night	37% 6% 11% 23% 19% 5%	2671
	21st to end	Night	38% 7% 13% 23% 16% 2	2800
December	1st to 10th	Night	36% 6% 15% 30% 12% 1	4580
	11th to 20th	Night	46% 5% 13% 19% 14% 2	5208
	21st to end	Night	45% 9% 16% 17% 12%	6996

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% 25% 27% 15% 3	225
	11th to 20th	Day	31% 27% 28% 10% 3	300
	21st to end	Day	22% 28% 33% 13% 3 1	330
February	1st to 10th	Day	25% 25% 35% 12% 3	300
	11th to 20th	Day	36% 26% 30% 8% 1	320
	21st to end	Day	28% 21% 30% 18% 2	414
March	1st to 10th	Day	22% 23% 30% 20% 4	500
	11th to 20th	Day	24% 22% 30% 18% 5% 1	500
	21st to end	Day	22% 20% 31% 20% 5% 1	550
April	1st to 10th	Day	23% 22% 27% 21% 6% 2	500
	11th to 20th	Day	20% 16% 31% 16% 13% 4	492
	21st to end	Day	24% 16% 28% 20% 10% 2	500
May	1st to 10th	Day	23% 15% 30% 19% 10% 3	503
	11th to 20th	Day	21% 15% 25% 22% 13% 4	527
	21st to end	Day	17% 8% 24% 25% 20% 7%	573
June	1st to 10th	Day	15% 8% 28% 23% 18% 7%	530
	11th to 20th	Day	16% 7% 26% 27% 18% 5%	530
	21st to end	Day	10% 6% 23% 32% 23% 6%	530
July	1st to 10th	Day	9% 3 19% 31% 26% 11%	525
	11th to 20th	Day	7% 3 20% 30% 29% 11%	524
	21st to end	Day	12% 3 19% 34% 24% 8%	532
August	1st to 10th	Day	11% 2 19% 27% 28% 12%	486
	11th to 20th	Day	11% 4 20% 32% 22% 12%	479
	21st to end	Day	11% 3 17% 32% 23% 14%	539
September	1st to 10th	Day	17% 7% 18% 24% 23% 12%	490
	11th to 20th	Day	12% 4 19% 29% 25% 11%	490
	21st to end	Day	26% 8% 22% 23% 16% 5%	490
October	1st to 10th	Day	20% 9% 20% 24% 21% 6%	490
	11th to 20th	Day	17% 12% 27% 28% 14% 2	490
	21st to end	Day	14% 14% 34% 26% 11% 2	539
November	1st to 10th	Day	15% 13% 30% 27% 12% 3	449
	11th to 20th	Day	18% 17% 32% 20% 12% 1	325
	21st to end	Day	22% 20% 34% 18% 6% 1	200
December	1st to 10th	Day	22% 24% 33% 16% 4 2	200
	11th to 20th	Day	24% 23% 27% 20% 7%	200
	21st to end	Day	23% 18% 32% 20% 7%	220
January	1st to 10th	Night	18% 19% 40% 20% 3	800
	11th to 20th	Night	22% 17% 33% 26% 3	800
	21st to end	Night	18% 17% 41% 24% 1	880
February	1st to 10th	Night	19% 17% 34% 28% 2	800
	11th to 20th	Night	26% 20% 35% 18% 1	783
	21st to end	Night	23% 18% 38% 18% 3	660
March	1st to 10th	Night	20% 16% 38% 22% 4	800
	11th to 20th	Night	22% 18% 31% 24% 5%	800
	21st to end	Night	18% 16% 39% 23% 4	880
April	1st to 10th	Night	19% 18% 35% 24% 4	800
	11th to 20th	Night	15% 13% 34% 29% 8%	791
	21st to end	Night	20% 12% 32% 30% 7%	800
May	1st to 10th	Night	22% 8% 27% 33% 11%	606
	11th to 20th	Night	20% 6% 26% 36% 11%	474
	21st to end	Night	16% 3 23% 39% 18% 1	433
June	1st to 10th	Night	13% 3 25% 36% 23% 1	400
	11th to 20th	Night	12% 5% 20% 43% 19% 2	400
	21st to end	Night	9% 4 21% 48% 19%	400
July	1st to 10th	Night	11% 3 17% 39% 27% 3	390
	11th to 20th	Night	7% 2 15% 41% 32% 3	471
	21st to end	Night	11% 2 13% 43% 29% 3	624
August	1st to 10th	Night	12% 1 9% 45% 30% 2	567
	11th to 20th	Night	12% 3 14% 39% 29% 3	607
	21st to end	Night	12% 3 14% 40% 28% 3	836
September	1st to 10th	Night	16% 1 18% 36% 25% 3	760
	11th to 20th	Night	12% 3 18% 36% 28% 3	760
	21st to end	Night	23% 4 18% 34% 19% 2	760
October	1st to 10th	Night	20% 6% 21% 30% 19% 3	760
	11th to 20th	Night	14% 9% 30% 33% 12% 1	760
	21st to end	Night	18% 9% 31% 30% 11%	836
November	1st to 10th	Night	12% 8% 32% 37% 9% 1	780
	11th to 20th	Night	18% 14% 32% 29% 7% 1	800
	21st to end	Night	16% 15% 33% 29% 6%	800
December	1st to 10th	Night	21% 13% 36% 23% 6%	800
	11th to 20th	Night	19% 11% 39% 27% 4	800
	21st to end	Night	20% 11% 35% 32% 3	880

Page generated at 2025-07-18 01:10:04. Version ac07

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