Green Bank Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Green Bank Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	13% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	125094
	Night	13% 9% 3 8% 5% 5% 5% 4 5% 7% 35%	122923
Transparency	Day	48% 5% 10% 20% 14% 3	128699
	Night	47% 5% 11% 20% 13% 3	127387
Seeing	Day	27% 14% 25% 19% 11% 5%	15913
	Night	24% 13% 21% 26% 14% 2	25889

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	11% 13% 5% 9% 5% 4 4 3 4 5% 36%	4056
	11th to 20th	Day	10% 12% 6% 14% 5% 4 4 3 3 4 33%	4060
	21st to end	Day	15% 10% 4 7% 4 4 3 3 4 6% 40%	4466
February	1st to 10th	Day	13% 13% 4 8% 5% 4 5% 3 6% 6% 33%	1800
	11th to 20th	Day	15% 10% 4 9% 6% 4 4 4 5% 7% 31%	1756
	21st to end	Day	16% 11% 4 8% 4 4 5% 3 4 6% 35%	1722
March	1st to 10th	Day	15% 8% 6% 8% 4 4 4 4 5% 7% 34%	4935
	11th to 20th	Day	11% 8% 3 7% 3 3 4 3 5% 7% 45%	4980
	21st to end	Day	12% 11% 4 9% 5% 5% 5% 4 4 6% 34%	5465
April	1st to 10th	Day	13% 11% 4 8% 6% 4 4 3 5% 7% 35%	2295
	11th to 20th	Day	14% 9% 3 6% 5% 3 5% 3 5% 7% 39%	2374
	21st to end	Day	18% 9% 3 7% 5% 5% 5% 4 5% 5% 34%	2555
May	1st to 10th	Day	16% 8% 3 9% 6% 6% 7% 6% 7% 8% 25%	5862
	11th to 20th	Day	21% 9% 3 8% 5% 5% 5% 4 6% 8% 27%	5861
	21st to end	Day	17% 7% 3 8% 6% 6% 6% 5% 6% 8% 28%	6433
June	1st to 10th	Day	14% 7% 3 6% 5% 5% 5% 5% 7% 7% 37%	2755
	11th to 20th	Day	12% 7% 3 7% 6% 6% 6% 5% 8% 8% 32%	2800
	21st to end	Day	10% 6% 3 8% 6% 5% 6% 5% 7% 9% 36%	2672
July	1st to 10th	Day	7% 6% 3 8% 6% 6% 6% 5% 7% 9% 37%	5900
	11th to 20th	Day	8% 6% 3 9% 8% 7% 8% 6% 9% 10% 27%	5900
	21st to end	Day	11% 8% 4 9% 7% 7% 7% 5% 8% 9% 25%	6424
August	1st to 10th	Day	13% 9% 3 9% 6% 7% 7% 6% 7% 10% 25%	4156
	11th to 20th	Day	10% 6% 3 10% 7% 7% 7% 5% 7% 9% 27%	4044
	21st to end	Day	7% 5% 2 7% 6% 6% 6% 4 7% 9% 41%	4228
September	1st to 10th	Day	14% 7% 2 7% 5% 5% 4 3 6% 6% 39%	2278
	11th to 20th	Day	13% 8% 2 7% 5% 4 4 4 5% 6% 40%	2090
	21st to end	Day	23% 8% 3 7% 4 4 4 3 5% 6% 33%	2087
October	1st to 10th	Day	15% 7% 3 6% 4 5% 4 3 5% 7% 41%	1900
	11th to 20th	Day	16% 9% 3 6% 4 3 3 3 4 4 44%	1900
	21st to end	Day	16% 9% 2 6% 4 3 4 3 5% 6% 43%	2090
November	1st to 10th	Day	10% 8% 3 5% 4 4 4 4 5% 5% 47%	1957
	11th to 20th	Day	15% 8% 3 7% 3 4 4 4 5% 7% 41%	1793
	21st to end	Day	14% 10% 4 6% 5% 4 5% 3 5% 5% 39%	1600
December	1st to 10th	Day	12% 10% 6% 9% 4 4 4 3 4 6% 38%	2600
	11th to 20th	Day	19% 10% 5% 6% 4 4 4 3 5% 6% 35%	2990
	21st to end	Day	18% 11% 4 10% 5% 4 5% 3 5% 7% 27%	4310
January	1st to 10th	Night	10% 12% 6% 12% 8% 6% 5% 3 4 5% 29%	5889
	11th to 20th	Night	9% 10% 3 12% 8% 5% 4 3 4 5% 36%	5900
	21st to end	Night	15% 9% 3 7% 4 4 3 3 5% 5% 41%	6490
February	1st to 10th	Night	12% 10% 4 8% 6% 4 5% 4 6% 8% 33%	2600
	11th to 20th	Night	15% 9% 3 10% 6% 5% 5% 5% 6% 6% 30%	2528
	21st to end	Night	14% 10% 4 10% 5% 4 5% 3 5% 7% 33%	1906
March	1st to 10th	Night	17% 13% 4 7% 4 4 3 3 4 5% 35%	4918
	11th to 20th	Night	10% 7% 3 8% 4 4 4 3 5% 8% 45%	4980
	21st to end	Night	12% 9% 3 7% 5% 5% 4 4 6% 7% 37%	5467
April	1st to 10th	Night	14% 12% 4 7% 5% 4 5% 3 5% 7% 34%	2104
	11th to 20th	Night	11% 8% 2 7% 6% 5% 5% 3 6% 9% 39%	1975
	21st to end	Night	14% 8% 3 7% 4 5% 5% 4 6% 7% 38%	1860
May	1st to 10th	Night	18% 10% 3 8% 6% 5% 4 4 5% 8% 29%	4033
	11th to 20th	Night	21% 11% 3 8% 5% 4 4 4 6% 7% 25%	4032
	21st to end	Night	17% 11% 4 9% 5% 5% 5% 4 6% 7% 26%	4421
June	1st to 10th	Night	10% 6% 3 5% 5% 4 5% 4 7% 10% 41%	1635
	11th to 20th	Night	12% 6% 2 7% 4 5% 4 4 7% 9% 40%	1600
	21st to end	Night	9% 5% 2 5% 5% 5% 4 5% 8% 11% 42%	1717
July	1st to 10th	Night	8% 7% 3 8% 5% 5% 5% 5% 7% 10% 39%	4058
	11th to 20th	Night	10% 9% 4 10% 6% 5% 5% 4 7% 9% 31%	4057
	21st to end	Night	10% 9% 4 10% 7% 7% 6% 5% 7% 9% 26%	4427
August	1st to 10th	Night	13% 9% 3 9% 5% 5% 5% 5% 6% 9% 31%	2861
	11th to 20th	Night	13% 8% 3 7% 5% 5% 6% 4 7% 7% 35%	2785
	21st to end	Night	7% 6% 2 6% 4 4 4 5% 5% 9% 47%	3380
September	1st to 10th	Night	13% 7% 2 6% 4 4 5% 3 5% 7% 44%	1900
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 2 5% 8% 45%	2088
	21st to end	Night	20% 8% 2 6% 4 5% 5% 3 5% 6% 35%	2092
October	1st to 10th	Night	16% 9% 2 4 3 4 4 4 5% 6% 43%	2280
	11th to 20th	Night	13% 10% 3 6% 3 3 4 3 4 7% 42%	2280
	21st to end	Night	14% 8% 3 6% 4 4 5% 3 5% 6% 42%	2508
November	1st to 10th	Night	9% 9% 3 7% 4 4 4 4 5% 6% 44%	2341
	11th to 20th	Night	13% 8% 4 7% 4 5% 5% 5% 5% 7% 38%	2598
	21st to end	Night	14% 8% 2 7% 6% 5% 5% 4 6% 7% 36%	2800
December	1st to 10th	Night	9% 10% 4 10% 6% 5% 4 4 6% 7% 35%	4580
	11th to 20th	Night	16% 12% 3 10% 5% 5% 4 3 4 6% 32%	5205
	21st to end	Night	16% 8% 6% 9% 5% 5% 5% 5% 6% 7% 27%	6628

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	51% 4 4 17% 21% 4	4057
	11th to 20th	Day	55% 6% 5% 11% 16% 7%	4060
	21st to end	Day	48% 2 6% 21% 17% 5%	4466
February	1st to 10th	Day	52% 4 8% 20% 13% 3	1800
	11th to 20th	Day	53% 4 7% 17% 15% 3	1765
	21st to end	Day	53% 4 7% 18% 14% 5%	1722
March	1st to 10th	Day	48% 5% 7% 18% 16% 6%	4935
	11th to 20th	Day	39% 5% 6% 25% 19% 5%	4980
	21st to end	Day	51% 4 8% 17% 16% 4	5465
April	1st to 10th	Day	50% 4 10% 17% 15% 4	2295
	11th to 20th	Day	46% 3 10% 21% 17% 3	2374
	21st to end	Day	52% 4 10% 21% 12% 2	2555
May	1st to 10th	Day	54% 6% 10% 19% 8% 2	5897
	11th to 20th	Day	54% 5% 11% 22% 7% 1	5865
	21st to end	Day	52% 5% 13% 18% 9% 3	6447
June	1st to 10th	Day	45% 6% 13% 23% 13%	2755
	11th to 20th	Day	48% 6% 14% 19% 12% 1	2800
	21st to end	Day	43% 6% 14% 24% 11% 1	2672
July	1st to 10th	Day	42% 8% 12% 24% 11% 3	5835
	11th to 20th	Day	47% 10% 15% 22% 6%	5831
	21st to end	Day	51% 7% 11% 20% 9% 1	6240
August	1st to 10th	Day	51% 7% 15% 20% 7%	4143
	11th to 20th	Day	51% 6% 10% 22% 10%	4083
	21st to end	Day	38% 7% 14% 27% 13% 1	4228
September	1st to 10th	Day	45% 4 10% 18% 17% 5%	2278
	11th to 20th	Day	44% 3 6% 24% 19% 3	2090
	21st to end	Day	52% 3 7% 20% 15% 3	2087
October	1st to 10th	Day	31% 4 8% 23% 26% 8%	3160
	11th to 20th	Day	35% 3 9% 24% 14% 15%	3140
	21st to end	Day	32% 2 8% 27% 19% 12%	3428
November	1st to 10th	Day	39% 3 8% 23% 22% 5%	1957
	11th to 20th	Day	44% 4 7% 17% 22% 6%	1793
	21st to end	Day	48% 3 8% 19% 19% 2	1600
December	1st to 10th	Day	48% 4 7% 20% 18% 2	2600
	11th to 20th	Day	52% 4 8% 17% 16% 3	2986
	21st to end	Day	57% 5% 9% 15% 13% 2	4310
January	1st to 10th	Night	54% 7% 5% 14% 15% 5%	5889
	11th to 20th	Night	52% 3 6% 16% 16% 7%	5900
	21st to end	Night	47% 3 7% 21% 18% 3	6490
February	1st to 10th	Night	50% 5% 12% 19% 13% 2	2600
	11th to 20th	Night	56% 5% 8% 16% 12% 4	2530
	21st to end	Night	52% 4 10% 19% 9% 6%	1906
March	1st to 10th	Night	51% 4 6% 19% 14% 5%	4918
	11th to 20th	Night	38% 5% 9% 27% 18% 4	4980
	21st to end	Night	46% 5% 12% 18% 17% 3	5467
April	1st to 10th	Night	50% 5% 10% 18% 14% 2	2104
	11th to 20th	Night	44% 5% 15% 21% 13% 3	1975
	21st to end	Night	45% 5% 13% 24% 12% 1	1860
May	1st to 10th	Night	54% 8% 14% 16% 8% 1	4060
	11th to 20th	Night	56% 6% 12% 20% 5% 1	4033
	21st to end	Night	55% 8% 16% 16% 3 3	4429
June	1st to 10th	Night	38% 8% 15% 26% 13%	1635
	11th to 20th	Night	39% 7% 18% 23% 10% 1	1600
	21st to end	Night	34% 8% 19% 26% 12% 1	1717
July	1st to 10th	Night	40% 9% 17% 24% 9% 2	4015
	11th to 20th	Night	48% 9% 17% 20% 6% 1	4006
	21st to end	Night	48% 9% 16% 20% 7%	4297
August	1st to 10th	Night	49% 8% 17% 21% 6%	2850
	11th to 20th	Night	45% 6% 15% 23% 10% 1	2812
	21st to end	Night	33% 6% 17% 32% 12% 1	3380
September	1st to 10th	Night	41% 3 16% 19% 18% 3	1900
	11th to 20th	Night	39% 4 10% 26% 18% 2	2088
	21st to end	Night	49% 5% 9% 21% 14% 1	2092
October	1st to 10th	Night	39% 4 8% 26% 19% 5%	3800
	11th to 20th	Night	38% 3 9% 17% 18% 14%	3779
	21st to end	Night	31% 4 9% 26% 19% 12%	4120
November	1st to 10th	Night	41% 4 11% 19% 20% 6%	2341
	11th to 20th	Night	46% 5% 6% 17% 20% 5%	2598
	21st to end	Night	48% 4 9% 20% 16% 3	2800
December	1st to 10th	Night	49% 4 8% 23% 15% 2	4580
	11th to 20th	Night	55% 4 10% 13% 16% 2	5208
	21st to end	Night	55% 6% 10% 17% 11% 1	6628

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	47% 20% 22% 7% 3 1	300
	11th to 20th	Day	43% 23% 25% 7% 1	300
	21st to end	Day	42% 25% 25% 7% 2	330
February	1st to 10th	Day	44% 22% 24% 7% 3	300
	11th to 20th	Day	53% 21% 21% 4	300
	21st to end	Day	45% 21% 24% 8% 2	321
March	1st to 10th	Day	36% 22% 24% 14% 3	500
	11th to 20th	Day	35% 24% 24% 13% 5% 1	500
	21st to end	Day	34% 22% 26% 13% 5%	550
April	1st to 10th	Day	35% 23% 26% 11% 5%	500
	11th to 20th	Day	29% 19% 27% 14% 10% 2	492
	21st to end	Day	32% 18% 28% 12% 6% 3	500
May	1st to 10th	Day	34% 15% 26% 13% 8% 3	503
	11th to 20th	Day	28% 15% 27% 18% 9% 4	527
	21st to end	Day	21% 12% 24% 21% 15% 7%	573
June	1st to 10th	Day	22% 12% 25% 22% 15% 5%	530
	11th to 20th	Day	20% 10% 28% 20% 15% 7%	530
	21st to end	Day	13% 13% 28% 26% 15% 5%	530
July	1st to 10th	Day	14% 6% 26% 25% 19% 10%	525
	11th to 20th	Day	7% 7% 29% 31% 15% 10%	524
	21st to end	Day	15% 5% 24% 28% 20% 9%	576
August	1st to 10th	Day	17% 8% 24% 24% 19% 10%	502
	11th to 20th	Day	17% 5% 23% 26% 17% 12%	479
	21st to end	Day	12% 4 23% 28% 20% 13%	539
September	1st to 10th	Day	18% 5% 19% 27% 19% 12%	490
	11th to 20th	Day	14% 6% 20% 29% 21% 11%	490
	21st to end	Day	30% 11% 21% 21% 14% 3	490
October	1st to 10th	Day	20% 10% 23% 24% 17% 5%	490
	11th to 20th	Day	26% 16% 21% 23% 12% 3	490
	21st to end	Day	27% 15% 30% 18% 10% 1	539
November	1st to 10th	Day	22% 17% 28% 20% 10% 3	495
	11th to 20th	Day	36% 14% 29% 16% 6%	301
	21st to end	Day	35% 15% 32% 16% 2 1	200
December	1st to 10th	Day	35% 24% 26% 14% 2 1	200
	11th to 20th	Day	39% 17% 27% 14% 4 1	200
	21st to end	Day	35% 23% 24% 12% 5%	297
January	1st to 10th	Night	36% 22% 25% 15% 2	800
	11th to 20th	Night	35% 17% 24% 22% 2	800
	21st to end	Night	32% 21% 30% 16% 2	880
February	1st to 10th	Night	28% 20% 27% 22% 3	800
	11th to 20th	Night	37% 25% 24% 13% 1	783
	21st to end	Night	34% 20% 30% 15% 1	660
March	1st to 10th	Night	28% 23% 25% 22% 3	800
	11th to 20th	Night	34% 21% 20% 19% 5% 1	800
	21st to end	Night	26% 23% 26% 20% 5% 1	880
April	1st to 10th	Night	29% 20% 28% 20% 3	800
	11th to 20th	Night	22% 17% 25% 24% 10% 1	791
	21st to end	Night	23% 13% 25% 30% 8% 1	660
May	1st to 10th	Night	25% 9% 22% 29% 15% 1	600
	11th to 20th	Night	26% 9% 19% 31% 14% 1	591
	21st to end	Night	18% 6% 17% 32% 22% 5%	650
June	1st to 10th	Night	15% 4 22% 31% 26% 3	435
	11th to 20th	Night	15% 5% 18% 41% 20% 2	400
	21st to end	Night	12% 4 19% 39% 22% 4	517
July	1st to 10th	Night	13% 3 14% 35% 30% 6%	585
	11th to 20th	Night	9% 3 13% 37% 31% 7%	582
	21st to end	Night	12% 3 12% 35% 34% 4	657
August	1st to 10th	Night	12% 2 12% 39% 33% 3	579
	11th to 20th	Night	15% 3 10% 34% 32% 5%	564
	21st to end	Night	14% 2 12% 33% 33% 6%	779
September	1st to 10th	Night	17% 3 13% 32% 30% 6%	760
	11th to 20th	Night	17% 4 13% 31% 31% 4	760
	21st to end	Night	24% 7% 18% 26% 22% 2	760
October	1st to 10th	Night	24% 8% 15% 27% 21% 5%	760
	11th to 20th	Night	23% 14% 19% 28% 15% 1	760
	21st to end	Night	25% 14% 20% 28% 13% 1	836
November	1st to 10th	Night	18% 12% 25% 30% 12% 2	780
	11th to 20th	Night	27% 16% 21% 26% 9% 2	800
	21st to end	Night	29% 17% 22% 26% 6% 1	800
December	1st to 10th	Night	30% 23% 22% 21% 4	800
	11th to 20th	Night	28% 19% 25% 25% 4	800
	21st to end	Night	28% 15% 26% 27% 4	880

Page generated at 2025-08-02 01:32:25. Version ac07

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