George Washington National Forest Helicopter Landing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the George Washington National Forest Helicopter Landing Site clear sky chart from 2005-11-06 to 2025-10-04.

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% 7% 3 7% 5% 5% 5% 4 6% 7% 38%	125987
	Night	12% 7% 2 6% 4 4 4 4 6% 7% 42%	123361
Transparency	Day	39% 8% 12% 22% 15% 4	129593
	Night	36% 8% 13% 23% 16% 4	127824
Seeing	Day	19% 15% 26% 22% 13% 5%	16134
	Night	16% 12% 27% 31% 13% 1	25880

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	10% 9% 3 8% 5% 4 4 3 4 5% 44%	4056
	11th to 20th	Day	9% 11% 4 9% 4 4 4 3 4 3 45%	4060
	21st to end	Day	13% 9% 3 6% 4 4 5% 4 5% 5% 44%	4466
February	1st to 10th	Day	11% 10% 4 8% 4 5% 4 4 7% 6% 37%	1800
	11th to 20th	Day	14% 8% 3 7% 5% 4 5% 3 5% 6% 38%	1753
	21st to end	Day	15% 8% 3 7% 4 4 4 3 6% 6% 41%	1705
March	1st to 10th	Day	14% 9% 3 6% 4 4 4 4 6% 7% 40%	4935
	11th to 20th	Day	10% 6% 2 6% 4 3 4 3 5% 6% 50%	4980
	21st to end	Day	13% 9% 3 8% 6% 5% 5% 4 5% 5% 37%	5465
April	1st to 10th	Day	11% 9% 3 7% 5% 5% 4 4 6% 6% 40%	2337
	11th to 20th	Day	13% 7% 2 6% 4 4 4 4 5% 7% 44%	2374
	21st to end	Day	17% 7% 2 7% 5% 5% 4 4 5% 6% 38%	2499
May	1st to 10th	Day	15% 7% 3 8% 7% 7% 6% 5% 7% 8% 27%	5862
	11th to 20th	Day	22% 8% 2 7% 5% 4 5% 4 6% 8% 30%	5861
	21st to end	Day	17% 6% 2 7% 5% 5% 6% 4 7% 8% 32%	6702
June	1st to 10th	Day	13% 5% 2 5% 5% 5% 5% 5% 7% 9% 39%	2800
	11th to 20th	Day	12% 5% 3 7% 6% 6% 6% 5% 6% 10% 34%	2800
	21st to end	Day	9% 6% 2 7% 6% 5% 6% 5% 7% 9% 38%	2800
July	1st to 10th	Day	7% 5% 2 8% 6% 6% 6% 5% 7% 9% 39%	6002
	11th to 20th	Day	9% 6% 3 8% 7% 8% 8% 6% 8% 10% 27%	5900
	21st to end	Day	11% 6% 3 9% 6% 6% 7% 5% 7% 9% 30%	6424
August	1st to 10th	Day	14% 6% 2 8% 7% 6% 7% 5% 7% 9% 28%	4234
	11th to 20th	Day	9% 5% 3 8% 7% 6% 7% 6% 7% 9% 33%	4160
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 4 6% 8% 43%	4312
September	1st to 10th	Day	14% 6% 2 7% 4 4 5% 3 5% 7% 43%	2400
	11th to 20th	Day	14% 7% 2 6% 4 4 4 3 5% 8% 42%	2270
	21st to end	Day	23% 7% 3 6% 5% 4 4 4 5% 5% 34%	2158
October	1st to 10th	Day	16% 6% 2 6% 4 4 5% 3 5% 6% 43%	1930
	11th to 20th	Day	15% 7% 3 6% 4 4 4 3 4 5% 45%	1900
	21st to end	Day	14% 6% 2 5% 3 3 5% 3 5% 5% 48%	2090
November	1st to 10th	Day	10% 7% 2 5% 4 3 4 3 4 6% 52%	1957
	11th to 20th	Day	13% 8% 2 5% 4 3 4 3 5% 7% 47%	1735
	21st to end	Day	13% 7% 2 6% 4 5% 3 4 4 6% 45%	1600
December	1st to 10th	Day	10% 7% 3 6% 4 5% 4 3 6% 6% 47%	2600
	11th to 20th	Day	16% 7% 3 7% 4 5% 4 3 5% 6% 41%	2990
	21st to end	Day	15% 8% 2 7% 5% 5% 5% 5% 7% 8% 33%	4070
January	1st to 10th	Night	9% 9% 3 6% 4 4 4 4 6% 7% 43%	5889
	11th to 20th	Night	10% 8% 2 7% 5% 4 3 3 5% 6% 46%	5900
	21st to end	Night	15% 7% 2 5% 3 3 4 3 5% 6% 48%	6490
February	1st to 10th	Night	10% 9% 3 6% 5% 5% 5% 5% 6% 7% 40%	2600
	11th to 20th	Night	15% 8% 3 7% 4 5% 5% 5% 6% 6% 36%	2533
	21st to end	Night	13% 8% 3 7% 5% 4 5% 4 6% 7% 39%	1920
March	1st to 10th	Night	15% 7% 2 5% 3 4 4 3 5% 6% 45%	4918
	11th to 20th	Night	9% 6% 2 5% 3 3 4 4 6% 7% 50%	4980
	21st to end	Night	11% 7% 3 7% 5% 4 5% 4 5% 6% 43%	5467
April	1st to 10th	Night	13% 9% 2 7% 5% 4 5% 4 5% 7% 40%	2061
	11th to 20th	Night	11% 6% 2 6% 5% 5% 4 3 6% 8% 43%	1975
	21st to end	Night	16% 6% 2 6% 3 4 4 3 6% 7% 42%	1917
May	1st to 10th	Night	18% 8% 3 7% 5% 4 5% 4 6% 8% 32%	4033
	11th to 20th	Night	23% 8% 3 6% 5% 4 5% 4 5% 7% 31%	4032
	21st to end	Night	14% 6% 2 7% 5% 5% 5% 5% 7% 9% 34%	4147
June	1st to 10th	Night	11% 5% 2 5% 4 5% 4 4 7% 9% 45%	1600
	11th to 20th	Night	11% 5% 2 7% 4 5% 5% 5% 7% 9% 42%	1600
	21st to end	Night	8% 5% 2 5% 5% 4 4 5% 7% 10% 46%	1600
July	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	3936
	11th to 20th	Night	8% 7% 3 9% 5% 7% 6% 6% 7% 10% 31%	4057
	21st to end	Night	7% 6% 2 8% 5% 7% 6% 5% 8% 10% 35%	4427
August	1st to 10th	Night	14% 5% 2 7% 5% 5% 5% 5% 7% 9% 36%	2915
	11th to 20th	Night	9% 7% 3 7% 6% 5% 5% 5% 5% 7% 38%	2875
	21st to end	Night	9% 6% 2 5% 4 4 4 4 5% 9% 47%	3553
September	1st to 10th	Night	13% 5% 2 6% 3 4 4 2 5% 6% 49%	2000
	11th to 20th	Night	14% 6% 2 5% 4 3 4 4 6% 8% 45%	2126
	21st to end	Night	23% 7% 2 5% 4 3 4 3 5% 5% 39%	2241
October	1st to 10th	Night	18% 5% 2 5% 4 4 4 3 5% 6% 45%	2317
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 4 6% 50%	2280
	21st to end	Night	14% 6% 2 6% 4 4 3 4 5% 6% 48%	2508
November	1st to 10th	Night	11% 7% 3 6% 3 4 4 4 4 6% 49%	2341
	11th to 20th	Night	12% 7% 2 5% 4 4 4 4 5% 7% 46%	2657
	21st to end	Night	13% 5% 2 6% 4 5% 4 4 5% 6% 44%	2800
December	1st to 10th	Night	8% 6% 3 6% 4 4 4 4 6% 9% 48%	4580
	11th to 20th	Night	13% 7% 3 8% 5% 5% 4 4 5% 7% 39%	5205
	21st to end	Night	13% 7% 3 7% 5% 6% 6% 5% 7% 8% 33%	6881

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	38% 6% 7% 21% 22% 7%	4057
	11th to 20th	Day	40% 4 8% 17% 20% 11%	4060
	21st to end	Day	39% 4 9% 24% 19% 5%	4466
February	1st to 10th	Day	43% 6% 12% 21% 15% 3	1800
	11th to 20th	Day	45% 5% 9% 21% 17% 3	1762
	21st to end	Day	42% 5% 13% 19% 16% 5%	1705
March	1st to 10th	Day	38% 7% 10% 19% 18% 8%	4935
	11th to 20th	Day	31% 6% 9% 26% 22% 6%	4980
	21st to end	Day	43% 7% 10% 17% 18% 5%	5465
April	1st to 10th	Day	40% 7% 10% 20% 16% 5%	2337
	11th to 20th	Day	38% 6% 11% 21% 20% 5%	2374
	21st to end	Day	44% 6% 10% 22% 14% 4	2499
May	1st to 10th	Day	46% 12% 11% 19% 9% 2	5897
	11th to 20th	Day	48% 9% 10% 22% 9% 1	5865
	21st to end	Day	41% 11% 14% 19% 13% 3	6717
June	1st to 10th	Day	38% 10% 14% 22% 16% 1	2800
	11th to 20th	Day	40% 11% 16% 20% 12% 2	2800
	21st to end	Day	36% 10% 16% 24% 13% 2	2800
July	1st to 10th	Day	37% 13% 14% 22% 11% 4	5937
	11th to 20th	Day	43% 15% 16% 20% 6% 1	5831
	21st to end	Day	42% 10% 12% 24% 12% 1	6240
August	1st to 10th	Day	45% 11% 15% 21% 7%	4221
	11th to 20th	Day	38% 11% 13% 26% 10% 1	4199
	21st to end	Day	35% 10% 15% 27% 13% 1	4312
September	1st to 10th	Day	39% 7% 13% 17% 20% 4	2400
	11th to 20th	Day	39% 5% 9% 25% 19% 3	2270
	21st to end	Day	48% 7% 9% 19% 15% 2	2158
October	1st to 10th	Day	32% 6% 9% 20% 26% 7%	3190
	11th to 20th	Day	27% 5% 9% 29% 17% 14%	3140
	21st to end	Day	24% 5% 9% 28% 23% 11%	3428
November	1st to 10th	Day	32% 5% 9% 26% 24% 4	1957
	11th to 20th	Day	35% 4 12% 19% 23% 6%	1735
	21st to end	Day	37% 5% 11% 25% 20% 2	1600
December	1st to 10th	Day	34% 6% 11% 24% 20% 5%	2600
	11th to 20th	Day	41% 6% 12% 22% 15% 3	2986
	21st to end	Day	42% 9% 15% 19% 14% 1	4070
January	1st to 10th	Night	32% 6% 12% 23% 23% 5%	5889
	11th to 20th	Night	34% 5% 11% 24% 17% 10%	5900
	21st to end	Night	36% 5% 9% 28% 18% 4	6490
February	1st to 10th	Night	39% 7% 14% 24% 14% 2	2600
	11th to 20th	Night	44% 6% 11% 20% 15% 4	2535
	21st to end	Night	40% 8% 13% 20% 13% 6%	1920
March	1st to 10th	Night	36% 6% 10% 24% 16% 8%	4918
	11th to 20th	Night	27% 6% 11% 28% 23% 5%	4980
	21st to end	Night	36% 7% 14% 21% 18% 4	5467
April	1st to 10th	Night	40% 7% 12% 21% 16% 3	2061
	11th to 20th	Night	35% 7% 14% 22% 18% 4	1975
	21st to end	Night	37% 7% 14% 25% 14% 3	1917
May	1st to 10th	Night	45% 11% 15% 18% 9% 2	4060
	11th to 20th	Night	47% 10% 12% 23% 8%	4033
	21st to end	Night	40% 11% 18% 23% 6% 2	4154
June	1st to 10th	Night	33% 10% 15% 26% 15% 1	1600
	11th to 20th	Night	35% 11% 17% 23% 12% 1	1600
	21st to end	Night	30% 12% 18% 21% 17% 2	1600
July	1st to 10th	Night	34% 12% 20% 22% 11% 2	3893
	11th to 20th	Night	43% 14% 17% 19% 6% 1	4006
	21st to end	Night	36% 12% 18% 24% 10%	4297
August	1st to 10th	Night	41% 10% 18% 24% 7%	2904
	11th to 20th	Night	37% 12% 16% 22% 13%	2902
	21st to end	Night	32% 8% 16% 31% 11% 1	3553
September	1st to 10th	Night	35% 8% 16% 19% 19% 4	2000
	11th to 20th	Night	36% 6% 11% 28% 18% 2	2126
	21st to end	Night	45% 7% 11% 20% 15% 2	2241
October	1st to 10th	Night	38% 5% 9% 21% 23% 3	3837
	11th to 20th	Night	24% 6% 12% 21% 26% 12%	3779
	21st to end	Night	21% 4 12% 30% 23% 9%	4120
November	1st to 10th	Night	33% 5% 12% 23% 21% 5%	2341
	11th to 20th	Night	35% 6% 9% 25% 21% 5%	2657
	21st to end	Night	37% 6% 13% 23% 18% 4	2800
December	1st to 10th	Night	30% 6% 13% 28% 21% 2	4580
	11th to 20th	Night	41% 6% 14% 18% 18% 3	5208
	21st to end	Night	39% 11% 14% 22% 13% 1	6881

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	28% 30% 29% 11% 2	300
	11th to 20th	Day	30% 27% 27% 14% 2	300
	21st to end	Day	28% 29% 31% 11% 2	330
February	1st to 10th	Day	29% 29% 31% 9% 2	300
	11th to 20th	Day	39% 29% 26% 5% 1	300
	21st to end	Day	30% 25% 30% 12% 3	362
March	1st to 10th	Day	22% 26% 29% 20% 3	500
	11th to 20th	Day	23% 24% 29% 18% 5% 1	500
	21st to end	Day	23% 23% 31% 17% 6% 1	550
April	1st to 10th	Day	24% 25% 26% 17% 7%	500
	11th to 20th	Day	20% 21% 26% 21% 9% 3	492
	21st to end	Day	26% 16% 29% 18% 9% 1	500
May	1st to 10th	Day	21% 20% 27% 18% 9% 4	503
	11th to 20th	Day	24% 14% 27% 21% 11% 3	527
	21st to end	Day	20% 10% 24% 24% 16% 6%	573
June	1st to 10th	Day	12% 11% 25% 30% 15% 6%	530
	11th to 20th	Day	15% 8% 29% 25% 17% 6%	530
	21st to end	Day	9% 8% 28% 34% 17% 4	530
July	1st to 10th	Day	11% 5% 21% 29% 24% 10%	525
	11th to 20th	Day	6% 4 20% 34% 25% 11%	524
	21st to end	Day	13% 5% 19% 30% 24% 9%	576
August	1st to 10th	Day	14% 7% 22% 26% 19% 13%	526
	11th to 20th	Day	13% 4 22% 29% 22% 9%	519
	21st to end	Day	10% 5% 20% 29% 22% 14%	583
September	1st to 10th	Day	16% 7% 22% 24% 20% 12%	530
	11th to 20th	Day	17% 5% 18% 28% 23% 9%	530
	21st to end	Day	28% 10% 21% 22% 15% 3	530
October	1st to 10th	Day	20% 6% 24% 27% 19% 5%	502
	11th to 20th	Day	16% 16% 24% 27% 14% 2	490
	21st to end	Day	17% 15% 34% 23% 8% 2	539
November	1st to 10th	Day	15% 15% 32% 25% 12% 2	495
	11th to 20th	Day	23% 19% 28% 22% 6% 1	295
	21st to end	Day	24% 21% 34% 17% 4 1	200
December	1st to 10th	Day	23% 24% 37% 12% 4 1	200
	11th to 20th	Day	27% 18% 34% 15% 6% 1	200
	21st to end	Day	23% 22% 36% 14% 5%	243
January	1st to 10th	Night	18% 23% 34% 21% 3	800
	11th to 20th	Night	19% 18% 34% 25% 4	800
	21st to end	Night	15% 18% 44% 22% 1	880
February	1st to 10th	Night	15% 19% 38% 25% 3	800
	11th to 20th	Night	24% 22% 36% 19% 1	783
	21st to end	Night	21% 18% 40% 19% 2	660
March	1st to 10th	Night	16% 19% 37% 24% 3	800
	11th to 20th	Night	21% 18% 32% 23% 5% 1	800
	21st to end	Night	20% 18% 34% 22% 5% 1	880
April	1st to 10th	Night	18% 22% 31% 24% 6%	800
	11th to 20th	Night	15% 17% 31% 26% 10% 1	791
	21st to end	Night	19% 11% 29% 33% 7%	717
May	1st to 10th	Night	19% 8% 28% 32% 13% 1	600
	11th to 20th	Night	19% 7% 24% 35% 14% 2	591
	21st to end	Night	15% 3 20% 40% 20% 2	531
June	1st to 10th	Night	10% 5% 22% 42% 21% 1	400
	11th to 20th	Night	10% 6% 17% 47% 19% 2	400
	21st to end	Night	8% 3 18% 49% 20% 1	400
July	1st to 10th	Night	12% 2 12% 38% 31% 5%	528
	11th to 20th	Night	6% 3 13% 40% 33% 6%	582
	21st to end	Night	7% 2 13% 43% 31% 3	657
August	1st to 10th	Night	11% 2 12% 42% 30% 3	597
	11th to 20th	Night	11% 3 12% 38% 32% 3	594
	21st to end	Night	13% 3 16% 37% 28% 3	860
September	1st to 10th	Night	16% 3 16% 33% 27% 5%	800
	11th to 20th	Night	15% 4 14% 37% 26% 4	800
	21st to end	Night	25% 5% 19% 32% 19% 2	800
October	1st to 10th	Night	18% 9% 19% 30% 21% 3	773
	11th to 20th	Night	16% 10% 30% 30% 14% 1	760
	21st to end	Night	16% 12% 27% 33% 11% 1	836
November	1st to 10th	Night	14% 10% 29% 36% 10% 2	780
	11th to 20th	Night	16% 15% 29% 30% 9% 1	800
	21st to end	Night	17% 16% 30% 29% 7% 1	800
December	1st to 10th	Night	17% 16% 36% 26% 4	800
	11th to 20th	Night	15% 14% 36% 31% 4	800
	21st to end	Night	16% 16% 33% 33% 3	880

Page generated at 2025-10-01 01:39:02. Version ac07

Copyright © 2025 Attilla Danko.