Greenbrier Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Greenbrier Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	12% 9% 3 8% 5% 5% 5% 4 6% 8% 34%	126385
	Night	12% 9% 4 8% 5% 5% 5% 4 6% 7% 37%	123983
Transparency	Day	26% 41% 7% 13% 9% 2	130004
	Night	26% 41% 7% 14% 10% 2	128433
Seeing	Day	23% 15% 24% 20% 12% 5%	16461
	Night	20% 13% 21% 28% 16% 2	26704

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% 16% 4 8% 5% 4 4 3 4 6% 33%	4056
	11th to 20th	Day	9% 13% 6% 14% 7% 5% 3 3 4 4 32%	4060
	21st to end	Day	15% 12% 3 8% 4 3 3 3 4 6% 39%	4466
February	1st to 10th	Day	13% 10% 4 10% 5% 6% 5% 3 5% 6% 34%	1800
	11th to 20th	Day	17% 8% 4 10% 6% 5% 4 4 6% 6% 31%	1808
	21st to end	Day	16% 10% 4 9% 5% 4 4 3 5% 6% 33%	1703
March	1st to 10th	Day	15% 10% 5% 8% 5% 4 5% 3 5% 6% 32%	4935
	11th to 20th	Day	11% 7% 3 8% 4 3 4 3 4 7% 46%	4980
	21st to end	Day	12% 10% 4 9% 5% 5% 5% 4 5% 7% 35%	5465
April	1st to 10th	Day	11% 11% 3 7% 5% 5% 5% 3 5% 7% 39%	2263
	11th to 20th	Day	12% 8% 3 7% 6% 4 4 4 5% 7% 40%	2374
	21st to end	Day	15% 9% 3 7% 5% 4 5% 4 5% 6% 37%	2577
May	1st to 10th	Day	14% 8% 3 9% 6% 6% 6% 5% 8% 9% 25%	5862
	11th to 20th	Day	19% 9% 4 9% 5% 5% 5% 5% 6% 8% 25%	5861
	21st to end	Day	13% 7% 3 8% 6% 5% 7% 5% 7% 9% 31%	6433
June	1st to 10th	Day	12% 5% 3 7% 5% 6% 6% 5% 6% 8% 38%	2600
	11th to 20th	Day	9% 6% 3 8% 6% 6% 6% 5% 8% 11% 34%	2600
	21st to end	Day	9% 6% 2 7% 7% 5% 6% 5% 8% 10% 37%	2600
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 6% 5% 8% 10% 38%	5900
	11th to 20th	Day	7% 6% 3 9% 6% 6% 7% 7% 9% 12% 28%	5900
	21st to end	Day	10% 8% 4 10% 6% 7% 7% 5% 8% 10% 25%	6424
August	1st to 10th	Day	12% 7% 3 8% 6% 6% 6% 6% 8% 11% 27%	4234
	11th to 20th	Day	9% 6% 3 9% 6% 6% 7% 6% 7% 9% 32%	4174
	21st to end	Day	6% 4 2 7% 5% 5% 5% 5% 7% 10% 44%	4391
September	1st to 10th	Day	10% 6% 3 6% 5% 5% 5% 4 6% 7% 43%	2344
	11th to 20th	Day	13% 8% 3 8% 5% 5% 5% 4 5% 7% 38%	2200
	21st to end	Day	20% 8% 3 7% 5% 4 4 4 6% 6% 33%	2200
October	1st to 10th	Day	15% 6% 3 6% 4 3 4 3 4 8% 45%	2025
	11th to 20th	Day	14% 8% 3 5% 4 4 4 3 5% 6% 45%	2000
	21st to end	Day	14% 9% 2 7% 4 4 4 3 4 6% 44%	2200
November	1st to 10th	Day	9% 8% 3 5% 4 4 4 3 6% 6% 48%	2057
	11th to 20th	Day	12% 9% 3 6% 5% 5% 5% 3 5% 6% 41%	1863
	21st to end	Day	12% 10% 3 6% 4 5% 4 3 5% 8% 41%	1739
December	1st to 10th	Day	12% 11% 4 8% 4 4 4 4 5% 6% 39%	2600
	11th to 20th	Day	20% 13% 4 7% 4 4 4 3 4 6% 32%	3225
	21st to end	Day	16% 12% 3 10% 7% 5% 5% 4 5% 6% 26%	4466
January	1st to 10th	Night	11% 13% 7% 12% 7% 5% 4 3 4 5% 28%	5889
	11th to 20th	Night	11% 10% 4 12% 8% 4 4 3 4 4 36%	5900
	21st to end	Night	15% 10% 3 7% 5% 4 4 3 4 5% 40%	6490
February	1st to 10th	Night	13% 10% 4 8% 5% 5% 4 4 7% 8% 33%	2600
	11th to 20th	Night	18% 9% 4 10% 7% 4 4 4 6% 5% 30%	2476
	21st to end	Night	13% 9% 4 9% 6% 4 5% 4 6% 5% 33%	1925
March	1st to 10th	Night	16% 10% 3 7% 4 4 4 3 5% 6% 37%	4918
	11th to 20th	Night	12% 7% 4 6% 4 3 4 3 5% 6% 46%	4980
	21st to end	Night	12% 9% 3 8% 5% 5% 5% 4 6% 7% 38%	5467
April	1st to 10th	Night	10% 11% 4 8% 6% 4 5% 3 5% 7% 38%	2135
	11th to 20th	Night	11% 7% 3 7% 4 5% 4 4 7% 7% 41%	1975
	21st to end	Night	13% 7% 2 5% 4 4 6% 4 6% 7% 41%	1840
May	1st to 10th	Night	15% 9% 3 8% 5% 4 5% 4 6% 8% 32%	4033
	11th to 20th	Night	18% 10% 4 9% 5% 5% 4 4 6% 7% 28%	4032
	21st to end	Night	16% 9% 4 9% 5% 5% 5% 4 6% 8% 30%	4421
June	1st to 10th	Night	8% 6% 2 5% 6% 5% 5% 4 8% 10% 42%	1800
	11th to 20th	Night	9% 7% 2 7% 5% 4 4 5% 6% 9% 43%	1800
	21st to end	Night	8% 5% 2 6% 4 4 6% 4 7% 10% 45%	1800
July	1st to 10th	Night	6% 5% 3 8% 6% 5% 6% 4 8% 10% 39%	4058
	11th to 20th	Night	6% 8% 4 10% 6% 6% 5% 5% 7% 10% 34%	4057
	21st to end	Night	8% 8% 4 10% 7% 7% 6% 5% 7% 10% 29%	4427
August	1st to 10th	Night	13% 7% 3 8% 6% 5% 6% 4 7% 8% 34%	2915
	11th to 20th	Night	9% 7% 3 9% 5% 5% 6% 5% 7% 8% 37%	2875
	21st to end	Night	6% 5% 3 6% 4 3 4 3 6% 9% 51%	3472
September	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 6% 6% 47%	2052
	11th to 20th	Night	11% 7% 3 7% 4 4 3 3 5% 7% 45%	2200
	21st to end	Night	15% 10% 3 7% 4 4 5% 4 5% 7% 36%	2200
October	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 5% 6% 47%	2374
	11th to 20th	Night	12% 8% 3 7% 4 3 4 3 5% 6% 45%	2400
	21st to end	Night	13% 9% 2 5% 4 3 4 3 5% 6% 45%	2640
November	1st to 10th	Night	10% 7% 3 7% 4 4 3 3 5% 5% 48%	2461
	11th to 20th	Night	12% 10% 4 6% 4 3 4 4 6% 6% 41%	2689
	21st to end	Night	12% 9% 3 7% 5% 5% 4 4 6% 6% 40%	2652
December	1st to 10th	Night	10% 8% 4 8% 5% 6% 5% 5% 6% 7% 37%	4580
	11th to 20th	Night	16% 12% 7% 10% 5% 5% 4 3 4 6% 29%	4960
	21st to end	Night	15% 11% 6% 9% 6% 5% 6% 4 5% 7% 26%	6490

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	33% 31% 3 13% 17% 2	4057
	11th to 20th	Day	33% 33% 4 10% 13% 6%	4060
	21st to end	Day	31% 31% 5% 19% 13% 2	4466
February	1st to 10th	Day	19% 64% 3 7% 6% 1	1800
	11th to 20th	Day	21% 62% 3 6% 6% 1	1817
	21st to end	Day	19% 62% 3 6% 8% 2	1703
March	1st to 10th	Day	36% 35% 6% 11% 8% 5%	4935
	11th to 20th	Day	22% 31% 6% 16% 20% 5%	4980
	21st to end	Day	31% 33% 6% 14% 13% 3	5465
April	1st to 10th	Day	18% 64% 4 8% 5% 1	2263
	11th to 20th	Day	16% 64% 5% 10% 4 1	2374
	21st to end	Day	17% 63% 5% 10% 4 1	2577
May	1st to 10th	Day	35% 35% 8% 15% 6% 1	5897
	11th to 20th	Day	38% 33% 8% 14% 6%	5865
	21st to end	Day	30% 34% 10% 14% 9% 3	6447
June	1st to 10th	Day	15% 64% 6% 9% 6%	2600
	11th to 20th	Day	15% 65% 6% 8% 6%	2600
	21st to end	Day	17% 63% 6% 9% 4	2600
July	1st to 10th	Day	26% 33% 15% 17% 8% 2	5835
	11th to 20th	Day	31% 34% 14% 17% 3	5831
	21st to end	Day	36% 31% 11% 16% 6%	6240
August	1st to 10th	Day	30% 40% 11% 15% 4	4221
	11th to 20th	Day	24% 42% 9% 16% 8% 1	4213
	21st to end	Day	17% 40% 11% 22% 10%	4391
September	1st to 10th	Day	16% 60% 6% 8% 8% 2	2344
	11th to 20th	Day	20% 58% 4 9% 8%	2200
	21st to end	Day	24% 59% 5% 8% 4	2200
October	1st to 10th	Day	17% 38% 5% 11% 22% 6%	3298
	11th to 20th	Day	15% 37% 5% 17% 10% 16%	3240
	21st to end	Day	14% 36% 6% 21% 10% 12%	3538
November	1st to 10th	Day	19% 55% 6% 11% 9% 1	2057
	11th to 20th	Day	17% 55% 3 9% 12% 3	1863
	21st to end	Day	17% 59% 6% 11% 8%	1739
December	1st to 10th	Day	26% 37% 5% 13% 16% 2	2600
	11th to 20th	Day	37% 34% 5% 11% 12% 1	3222
	21st to end	Day	40% 32% 8% 12% 8%	4466
January	1st to 10th	Night	34% 35% 4 11% 13% 3	5889
	11th to 20th	Night	32% 29% 4 15% 14% 6%	5900
	21st to end	Night	30% 31% 6% 18% 13% 2	6490
February	1st to 10th	Night	16% 64% 7% 7% 5% 1	2600
	11th to 20th	Night	21% 62% 4 6% 5% 1	2478
	21st to end	Night	17% 63% 4 9% 5% 3	1925
March	1st to 10th	Night	31% 33% 6% 14% 12% 5%	4918
	11th to 20th	Night	20% 32% 5% 17% 22% 3	4980
	21st to end	Night	29% 32% 8% 15% 14% 2	5467
April	1st to 10th	Night	18% 65% 4 7% 6%	2135
	11th to 20th	Night	16% 64% 6% 9% 5% 1	1975
	21st to end	Night	14% 63% 6% 11% 5%	1840
May	1st to 10th	Night	33% 35% 11% 14% 5%	4060
	11th to 20th	Night	39% 34% 10% 13% 5%	4033
	21st to end	Night	33% 37% 11% 12% 3 3	4429
June	1st to 10th	Night	13% 66% 6% 8% 7%	1800
	11th to 20th	Night	13% 64% 7% 10% 6% 1	1800
	21st to end	Night	13% 64% 7% 10% 5%	1800
July	1st to 10th	Night	26% 34% 14% 17% 7% 1	4015
	11th to 20th	Night	32% 33% 15% 18% 2	4006
	21st to end	Night	37% 33% 11% 13% 7%	4297
August	1st to 10th	Night	31% 40% 12% 15% 1	2904
	11th to 20th	Night	24% 40% 12% 17% 6%	2902
	21st to end	Night	18% 37% 9% 25% 10%	3472
September	1st to 10th	Night	15% 59% 8% 10% 7% 2	2052
	11th to 20th	Night	17% 59% 4 11% 9%	2200
	21st to end	Night	24% 60% 5% 8% 3	2200
October	1st to 10th	Night	21% 37% 4 15% 20% 3	3881
	11th to 20th	Night	14% 39% 6% 14% 16% 12%	3899
	21st to end	Night	10% 36% 8% 21% 15% 10%	4252
November	1st to 10th	Night	19% 54% 6% 11% 9% 1	2461
	11th to 20th	Night	15% 56% 5% 9% 13% 3	2689
	21st to end	Night	17% 59% 6% 10% 7% 1	2652
December	1st to 10th	Night	22% 38% 7% 19% 13% 1	4580
	11th to 20th	Night	37% 35% 7% 12% 7% 1	4962
	21st to end	Night	37% 34% 8% 14% 8%	6490

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	43% 25% 19% 12% 1 1	300
	11th to 20th	Day	43% 23% 26% 8% 1	300
	21st to end	Day	41% 25% 24% 8% 2	330
February	1st to 10th	Day	38% 23% 26% 10% 2	300
	11th to 20th	Day	49% 25% 19% 6%	300
	21st to end	Day	40% 23% 26% 9% 1	302
March	1st to 10th	Day	33% 23% 23% 17% 5%	500
	11th to 20th	Day	32% 26% 23% 15% 3 1	500
	21st to end	Day	30% 24% 25% 17% 5%	550
April	1st to 10th	Day	28% 24% 24% 14% 9%	500
	11th to 20th	Day	25% 18% 28% 18% 8% 3	492
	21st to end	Day	24% 21% 28% 15% 11% 2	500
May	1st to 10th	Day	26% 19% 30% 12% 10% 2	503
	11th to 20th	Day	24% 13% 30% 16% 12% 5%	527
	21st to end	Day	20% 13% 22% 22% 17% 7%	573
June	1st to 10th	Day	14% 13% 29% 23% 14% 7%	530
	11th to 20th	Day	12% 10% 28% 24% 17% 7%	530
	21st to end	Day	9% 11% 28% 28% 15% 8%	530
July	1st to 10th	Day	12% 6% 24% 29% 20% 10%	525
	11th to 20th	Day	6% 5% 23% 35% 21% 10%	524
	21st to end	Day	13% 6% 24% 27% 19% 10%	576
August	1st to 10th	Day	17% 8% 25% 20% 21% 10%	526
	11th to 20th	Day	13% 6% 24% 27% 18% 11%	519
	21st to end	Day	11% 5% 21% 26% 21% 15%	583
September	1st to 10th	Day	14% 7% 20% 26% 22% 11%	530
	11th to 20th	Day	14% 5% 19% 30% 23% 8%	530
	21st to end	Day	28% 11% 20% 24% 15% 2	530
October	1st to 10th	Day	20% 10% 20% 27% 18% 5%	530
	11th to 20th	Day	19% 18% 25% 22% 15% 2	530
	21st to end	Day	25% 17% 25% 21% 11% 2	583
November	1st to 10th	Day	20% 18% 27% 22% 10% 2	535
	11th to 20th	Day	33% 17% 27% 17% 5%	321
	21st to end	Day	28% 21% 29% 16% 5% 1	269
December	1st to 10th	Day	35% 30% 20% 12% 4 1	200
	11th to 20th	Day	37% 21% 23% 13% 6%	253
	21st to end	Day	32% 22% 26% 13% 6% 1	330
January	1st to 10th	Night	31% 25% 23% 18% 3	800
	11th to 20th	Night	29% 19% 29% 20% 3	800
	21st to end	Night	29% 21% 30% 19% 1	880
February	1st to 10th	Night	26% 21% 25% 26% 3	800
	11th to 20th	Night	33% 26% 24% 16% 1	783
	21st to end	Night	26% 23% 30% 18% 2	660
March	1st to 10th	Night	25% 19% 28% 25% 3	800
	11th to 20th	Night	29% 20% 24% 21% 7% 1	800
	21st to end	Night	22% 19% 28% 23% 7%	880
April	1st to 10th	Night	22% 22% 27% 25% 5%	800
	11th to 20th	Night	19% 18% 27% 26% 10% 1	791
	21st to end	Night	17% 14% 26% 32% 10%	640
May	1st to 10th	Night	18% 12% 26% 30% 14% 1	600
	11th to 20th	Night	20% 10% 20% 30% 18% 1	591
	21st to end	Night	16% 5% 16% 31% 26% 6%	650
June	1st to 10th	Night	9% 5% 18% 39% 27% 3	600
	11th to 20th	Night	11% 3 17% 34% 34% 2	600
	21st to end	Night	9% 4 16% 39% 29% 4	600
July	1st to 10th	Night	11% 1 13% 39% 31% 6%	585
	11th to 20th	Night	6% 3 11% 33% 42% 5%	582
	21st to end	Night	6% 4 13% 37% 37% 4	657
August	1st to 10th	Night	15% 2 12% 34% 33% 4	597
	11th to 20th	Night	13% 2 12% 34% 35% 5%	594
	21st to end	Night	10% 3 11% 33% 37% 6%	805
September	1st to 10th	Night	13% 4 13% 31% 33% 6%	800
	11th to 20th	Night	17% 4 11% 30% 33% 4	800
	21st to end	Night	22% 10% 18% 26% 23% 2	800
October	1st to 10th	Night	16% 10% 18% 27% 27% 3	800
	11th to 20th	Night	18% 14% 21% 29% 17% 1	800
	21st to end	Night	21% 13% 20% 33% 12% 1	880
November	1st to 10th	Night	18% 11% 24% 33% 13% 2	820
	11th to 20th	Night	24% 16% 21% 30% 9% 1	829
	21st to end	Night	21% 17% 23% 30% 8% 1	800
December	1st to 10th	Night	29% 20% 24% 21% 6%	800
	11th to 20th	Night	23% 19% 27% 26% 5%	800
	21st to end	Night	25% 17% 23% 31% 4	880

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