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 2024-01-12.

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% 8% 3 7% 6% 3 5% 5% 6% 8% 35%	74592
	Night	13% 8% 4 6% 6% 3 5% 5% 6% 7% 38%	75127
Transparency	Day	30% 39% 8% 13% 9% 1	74314
	Night	28% 40% 8% 13% 9% 1	74924
Seeing	Day	23% 15% 24% 21% 13% 5%	13937
	Night	20% 13% 21% 28% 16% 2	24250

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	13% 12% 4 7% 7% 4 5% 5% 5% 6% 32%	1706
	11th to 20th	Day	15% 14% 5% 10% 6% 4 5% 5% 4 6% 28%	1635
	21st to end	Day	15% 13% 5% 8% 5% 3 4 4 4 6% 35%	1782
February	1st to 10th	Day	13% 9% 5% 9% 6% 4 6% 4 4 7% 33%	1620
	11th to 20th	Day	16% 7% 4 9% 7% 4 4 5% 5% 7% 32%	1623
	21st to end	Day	17% 10% 4 9% 6% 3 4 4 4 6% 32%	1527
March	1st to 10th	Day	14% 6% 3 6% 4 3 4 4 5% 7% 43%	1980
	11th to 20th	Day	18% 9% 4 8% 5% 3 5% 4 5% 6% 33%	1980
	21st to end	Day	15% 11% 5% 7% 6% 3 5% 4 4 7% 33%	2178
April	1st to 10th	Day	11% 8% 4 6% 6% 3 5% 5% 5% 7% 41%	2036
	11th to 20th	Day	13% 8% 3 6% 6% 3 4 4 5% 7% 40%	2134
	21st to end	Day	16% 8% 3 6% 6% 3 5% 5% 5% 6% 37%	2319
May	1st to 10th	Day	17% 7% 3 7% 7% 3 5% 5% 6% 7% 31%	2340
	11th to 20th	Day	19% 8% 4 6% 5% 3 5% 7% 6% 8% 30%	2301
	21st to end	Day	11% 7% 3 7% 7% 4 7% 6% 7% 9% 32%	2531
June	1st to 10th	Day	13% 5% 3 6% 5% 4 6% 6% 6% 8% 38%	2340
	11th to 20th	Day	10% 6% 3 7% 7% 4 7% 6% 7% 11% 33%	2340
	21st to end	Day	10% 5% 2 6% 7% 4 6% 6% 8% 10% 36%	2340
July	1st to 10th	Day	11% 6% 3 7% 7% 4 6% 6% 6% 9% 34%	2340
	11th to 20th	Day	8% 5% 3 6% 6% 4 6% 7% 8% 11% 34%	2340
	21st to end	Day	11% 6% 4 8% 7% 4 8% 7% 8% 9% 29%	2547
August	1st to 10th	Day	12% 7% 3 7% 7% 5% 6% 7% 8% 10% 28%	2327
	11th to 20th	Day	15% 7% 3 7% 7% 4 7% 7% 7% 8% 28%	2303
	21st to end	Day	10% 6% 3 7% 7% 3 7% 8% 7% 10% 33%	2425
September	1st to 10th	Day	11% 6% 3 5% 6% 4 5% 5% 6% 7% 41%	2110
	11th to 20th	Day	13% 7% 4 6% 6% 3 6% 5% 5% 7% 39%	1980
	21st to end	Day	20% 7% 3 6% 5% 3 4 4 6% 6% 35%	1980
October	1st to 10th	Day	16% 6% 3 5% 4 2 4 4 5% 7% 44%	1823
	11th to 20th	Day	16% 8% 3 5% 5% 3 4 4 5% 6% 43%	1800
	21st to end	Day	15% 9% 3 6% 5% 3 5% 3 4 7% 43%	1980
November	1st to 10th	Day	10% 8% 3 4 4 3 4 4 5% 5% 50%	1857
	11th to 20th	Day	12% 8% 3 5% 5% 4 5% 5% 5% 6% 41%	1710
	21st to end	Day	13% 8% 3 5% 4 3 5% 4 5% 8% 42%	1653
December	1st to 10th	Day	18% 12% 4 8% 6% 3 4 5% 6% 7% 27%	1520
	11th to 20th	Day	16% 12% 3 6% 4 2 3 4 4 8% 38%	2061
	21st to end	Day	17% 12% 4 8% 8% 4 4 6% 6% 7% 24%	3124
January	1st to 10th	Night	12% 10% 5% 9% 7% 4 6% 5% 5% 6% 31%	2459
	11th to 20th	Night	16% 10% 5% 9% 6% 4 5% 4 5% 6% 29%	2365
	21st to end	Night	13% 10% 6% 7% 6% 4 5% 5% 5% 6% 33%	2574
February	1st to 10th	Night	13% 9% 5% 7% 6% 4 5% 5% 6% 8% 33%	2340
	11th to 20th	Night	18% 8% 4 8% 8% 3 5% 5% 5% 6% 30%	2222
	21st to end	Night	15% 9% 4 8% 6% 4 5% 5% 6% 5% 32%	1727
March	1st to 10th	Night	13% 7% 3 5% 5% 3 5% 5% 5% 7% 41%	1980
	11th to 20th	Night	17% 8% 3 6% 7% 3 5% 5% 6% 7% 32%	1980
	21st to end	Night	13% 9% 3 6% 5% 3 5% 5% 6% 7% 37%	2178
April	1st to 10th	Night	10% 8% 4 7% 7% 3 5% 4 5% 8% 40%	1922
	11th to 20th	Night	12% 6% 3 6% 5% 4 5% 5% 7% 8% 42%	1775
	21st to end	Night	15% 7% 3 4 5% 2 6% 5% 6% 7% 41%	1656
May	1st to 10th	Night	14% 8% 2 7% 7% 3 4 5% 5% 6% 38%	1620
	11th to 20th	Night	15% 8% 3 6% 5% 3 4 6% 5% 9% 35%	1592
	21st to end	Night	11% 7% 3 6% 6% 3 5% 5% 6% 10% 38%	1744
June	1st to 10th	Night	9% 5% 2 4 5% 4 5% 5% 7% 11% 43%	1620
	11th to 20th	Night	9% 6% 2 6% 5% 3 4 5% 6% 8% 44%	1620
	21st to end	Night	8% 4 2 5% 5% 3 6% 5% 7% 10% 45%	1620
July	1st to 10th	Night	9% 5% 3 5% 7% 4 5% 6% 8% 8% 41%	1620
	11th to 20th	Night	6% 4 2 5% 5% 3 4 5% 7% 12% 46%	1620
	21st to end	Night	6% 4 2 6% 7% 4 7% 5% 7% 11% 41%	1771
August	1st to 10th	Night	11% 6% 3 5% 7% 3 6% 6% 7% 8% 38%	1611
	11th to 20th	Night	12% 7% 3 6% 5% 2 6% 6% 6% 8% 41%	1598
	21st to end	Night	8% 5% 3 5% 6% 3 3 6% 7% 11% 42%	1912
September	1st to 10th	Night	11% 6% 2 5% 5% 3 4 5% 6% 7% 46%	1846
	11th to 20th	Night	11% 6% 3 6% 5% 3 4 4 5% 8% 46%	1980
	21st to end	Night	14% 9% 3 5% 4 3 5% 6% 5% 7% 39%	1980
October	1st to 10th	Night	14% 7% 2 4 5% 3 5% 4 5% 6% 46%	2136
	11th to 20th	Night	13% 8% 3 7% 5% 2 3 4 5% 6% 44%	2160
	21st to end	Night	14% 9% 2 5% 5% 3 4 4 5% 7% 44%	2376
November	1st to 10th	Night	11% 6% 3 6% 5% 3 3 4 5% 5% 49%	2221
	11th to 20th	Night	12% 9% 4 5% 4 3 4 5% 6% 7% 42%	2467
	21st to end	Night	12% 8% 3 5% 6% 4 4 5% 5% 7% 41%	2519
December	1st to 10th	Night	16% 11% 5% 8% 5% 4 4 5% 5% 6% 30%	2660
	11th to 20th	Night	14% 9% 5% 9% 5% 4 6% 4 5% 6% 33%	3124
	21st to end	Night	15% 12% 7% 8% 7% 4 6% 5% 5% 6% 25%	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	33% 43% 5% 14% 5%	1707
	11th to 20th	Day	35% 43% 6% 10% 6% 1	1635
	21st to end	Day	35% 38% 7% 12% 8% 1	1782
February	1st to 10th	Day	33% 37% 6% 14% 9% 1	1620
	11th to 20th	Day	32% 39% 6% 13% 7% 2	1632
	21st to end	Day	35% 37% 6% 11% 8% 2	1527
March	1st to 10th	Day	24% 37% 7% 17% 12% 1	1980
	11th to 20th	Day	35% 38% 6% 11% 7% 3	1980
	21st to end	Day	36% 38% 6% 12% 7% 1	2178
April	1st to 10th	Day	27% 38% 8% 12% 12% 4	2036
	11th to 20th	Day	28% 37% 7% 14% 12% 3	2134
	21st to end	Day	31% 39% 7% 13% 9% 2	2319
May	1st to 10th	Day	33% 40% 7% 10% 8% 1	2340
	11th to 20th	Day	35% 40% 7% 12% 4 1	2305
	21st to end	Day	29% 42% 10% 12% 8%	2531
June	1st to 10th	Day	27% 41% 9% 14% 9%	2340
	11th to 20th	Day	29% 39% 10% 11% 10% 1	2340
	21st to end	Day	25% 41% 8% 17% 8% 1	2340
July	1st to 10th	Day	28% 42% 9% 11% 7% 2	2275
	11th to 20th	Day	27% 42% 10% 11% 9% 1	2271
	21st to end	Day	30% 42% 10% 12% 5% 1	2405
August	1st to 10th	Day	30% 40% 11% 14% 5%	2314
	11th to 20th	Day	32% 40% 8% 13% 6%	2303
	21st to end	Day	29% 41% 11% 12% 6% 2	2425
September	1st to 10th	Day	26% 39% 6% 13% 13% 2	2110
	11th to 20th	Day	28% 37% 6% 15% 12% 1	1980
	21st to end	Day	33% 37% 6% 11% 12% 1	1980
October	1st to 10th	Day	25% 37% 7% 17% 13% 2	1823
	11th to 20th	Day	28% 39% 8% 14% 11% 1	1800
	21st to end	Day	27% 38% 6% 14% 11% 3	1980
November	1st to 10th	Day	18% 41% 6% 14% 17% 3	1857
	11th to 20th	Day	26% 42% 6% 12% 12% 2	1710
	21st to end	Day	26% 41% 7% 13% 11% 2	1653
December	1st to 10th	Day	33% 41% 8% 11% 7%	1520
	11th to 20th	Day	30% 36% 6% 12% 14% 1	2058
	21st to end	Day	39% 31% 9% 15% 5%	3124
January	1st to 10th	Night	32% 44% 8% 10% 6%	2459
	11th to 20th	Night	34% 44% 5% 8% 7% 1	2365
	21st to end	Night	34% 39% 6% 14% 6% 1	2574
February	1st to 10th	Night	32% 38% 7% 13% 9% 1	2340
	11th to 20th	Night	35% 39% 5% 10% 8% 2	2224
	21st to end	Night	35% 39% 7% 11% 6% 3	1727
March	1st to 10th	Night	27% 38% 7% 14% 12% 2	1980
	11th to 20th	Night	32% 40% 7% 13% 7% 1	1980
	21st to end	Night	30% 39% 8% 13% 8% 2	2178
April	1st to 10th	Night	28% 38% 9% 13% 9% 3	1922
	11th to 20th	Night	24% 38% 8% 13% 13% 3	1775
	21st to end	Night	28% 39% 8% 16% 8% 1	1656
May	1st to 10th	Night	31% 40% 9% 11% 9% 1	1620
	11th to 20th	Night	31% 41% 10% 11% 5% 1	1593
	21st to end	Night	26% 40% 12% 15% 5%	1745
June	1st to 10th	Night	23% 41% 11% 17% 9%	1620
	11th to 20th	Night	24% 41% 12% 15% 8% 1	1620
	21st to end	Night	19% 42% 11% 16% 10% 2	1620
July	1st to 10th	Night	25% 42% 10% 11% 9% 2	1575
	11th to 20th	Night	22% 43% 11% 12% 10% 2	1566
	21st to end	Night	20% 43% 13% 17% 7%	1672
August	1st to 10th	Night	24% 42% 11% 16% 6%	1600
	11th to 20th	Night	27% 38% 10% 15% 9% 1	1598
	21st to end	Night	24% 41% 11% 15% 7% 2	1912
September	1st to 10th	Night	25% 40% 8% 12% 12% 3	1846
	11th to 20th	Night	24% 38% 7% 17% 12% 2	1980
	21st to end	Night	27% 39% 7% 13% 12% 1	1980
October	1st to 10th	Night	25% 37% 8% 16% 12% 2	2136
	11th to 20th	Night	26% 38% 7% 16% 10% 2	2160
	21st to end	Night	25% 39% 6% 15% 11% 3	2376
November	1st to 10th	Night	19% 41% 6% 14% 16% 4	2221
	11th to 20th	Night	25% 42% 7% 13% 11% 2	2467
	21st to end	Night	27% 41% 6% 14% 10% 3	2519
December	1st to 10th	Night	31% 41% 7% 14% 7%	2660
	11th to 20th	Night	35% 36% 6% 12% 11%	3126
	21st to end	Night	39% 34% 8% 11% 7%	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	39% 26% 20% 13% 1 1	270
	11th to 20th	Day	34% 26% 31% 8% 1	245
	21st to end	Day	42% 25% 22% 8% 3	264
February	1st to 10th	Day	40% 22% 28% 8% 2	240
	11th to 20th	Day	45% 29% 20% 6%	240
	21st to end	Day	41% 24% 24% 9% 1	245
March	1st to 10th	Day	31% 24% 24% 17% 5%	420
	11th to 20th	Day	32% 25% 23% 15% 4 1	420
	21st to end	Day	27% 23% 24% 19% 6%	462
April	1st to 10th	Day	25% 24% 25% 16% 10%	420
	11th to 20th	Day	23% 18% 27% 19% 9% 3	412
	21st to end	Day	27% 20% 25% 14% 11% 2	420
May	1st to 10th	Day	26% 18% 30% 13% 10% 3	423
	11th to 20th	Day	22% 12% 31% 17% 12% 5%	447
	21st to end	Day	17% 12% 22% 22% 19% 8%	485
June	1st to 10th	Day	16% 11% 26% 24% 15% 8%	450
	11th to 20th	Day	14% 10% 29% 23% 17% 8%	450
	21st to end	Day	10% 11% 29% 30% 13% 7%	450
July	1st to 10th	Day	13% 6% 22% 27% 21% 11%	445
	11th to 20th	Day	5% 5% 20% 38% 21% 11%	444
	21st to end	Day	14% 5% 24% 27% 19% 11%	488
August	1st to 10th	Day	17% 6% 23% 21% 22% 11%	446
	11th to 20th	Day	14% 6% 21% 27% 19% 12%	439
	21st to end	Day	11% 4 19% 26% 22% 17%	495
September	1st to 10th	Day	16% 6% 19% 24% 23% 11%	450
	11th to 20th	Day	14% 6% 18% 30% 25% 8%	450
	21st to end	Day	27% 11% 18% 26% 16% 2	450
October	1st to 10th	Day	22% 10% 19% 26% 19% 4	450
	11th to 20th	Day	19% 17% 24% 23% 15% 2	450
	21st to end	Day	23% 18% 25% 22% 11% 2	495
November	1st to 10th	Day	19% 16% 27% 23% 12% 2	455
	11th to 20th	Day	33% 15% 26% 19% 6%	270
	21st to end	Day	25% 21% 30% 17% 6% 1	243
December	1st to 10th	Day	34% 28% 19% 13% 4 1	180
	11th to 20th	Day	37% 21% 23% 13% 6%	227
	21st to end	Day	33% 21% 26% 13% 6% 1	297
January	1st to 10th	Night	29% 24% 24% 19% 4	760
	11th to 20th	Night	27% 19% 29% 22% 3	728
	21st to end	Night	28% 20% 30% 20% 1	792
February	1st to 10th	Night	26% 20% 24% 26% 3	720
	11th to 20th	Night	31% 27% 24% 17% 1	703
	21st to end	Night	26% 24% 30% 18% 2	592
March	1st to 10th	Night	24% 20% 28% 25% 3	720
	11th to 20th	Night	28% 20% 23% 21% 7% 1	720
	21st to end	Night	21% 18% 27% 25% 8%	792
April	1st to 10th	Night	20% 21% 27% 26% 6%	720
	11th to 20th	Night	20% 16% 26% 27% 11% 1	711
	21st to end	Night	19% 13% 26% 33% 10%	576
May	1st to 10th	Night	19% 12% 25% 29% 14% 1	540
	11th to 20th	Night	19% 10% 20% 31% 19% 1	531
	21st to end	Night	15% 4 15% 32% 28% 6%	584
June	1st to 10th	Night	10% 4 16% 40% 28% 3	540
	11th to 20th	Night	11% 3 17% 32% 35% 2	540
	21st to end	Night	9% 4 16% 40% 28% 4	540
July	1st to 10th	Night	11% 1 13% 38% 31% 6%	525
	11th to 20th	Night	7% 2 10% 33% 44% 5%	522
	21st to end	Night	5% 3 14% 37% 38% 4	591
August	1st to 10th	Night	14% 2 13% 32% 34% 4	537
	11th to 20th	Night	13% 2 11% 34% 36% 5%	534
	21st to end	Night	11% 2 10% 32% 38% 7%	724
September	1st to 10th	Night	14% 4 12% 29% 35% 7%	720
	11th to 20th	Night	16% 4 12% 30% 34% 4	720
	21st to end	Night	20% 9% 17% 26% 25% 2	720
October	1st to 10th	Night	16% 10% 18% 26% 26% 3	720
	11th to 20th	Night	19% 14% 21% 28% 17% 1	720
	21st to end	Night	20% 13% 21% 32% 12% 1	792
November	1st to 10th	Night	17% 10% 25% 32% 14% 2	740
	11th to 20th	Night	24% 15% 20% 31% 10% 1	760
	21st to end	Night	20% 16% 23% 31% 8% 1	760
December	1st to 10th	Night	29% 20% 23% 22% 6%	760
	11th to 20th	Night	22% 19% 28% 26% 5%	760
	21st to end	Night	25% 18% 23% 31% 3	836

Page generated at 2024-01-09 16:24:22. Version ac07

Copyright 2024 Attilla Danko