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 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% 7% 3 6% 6% 4 5% 5% 6% 7% 38%	74598
	Night	13% 6% 3 5% 5% 3 5% 5% 6% 7% 42%	75107
Transparency	Day	42% 7% 12% 21% 15% 3	74319
	Night	38% 8% 14% 23% 15% 3	74905
Seeing	Day	19% 14% 26% 23% 14% 5%	13799
	Night	16% 11% 27% 31% 13% 1	23670

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% 8% 3 8% 7% 3 5% 5% 5% 5% 40%	1706
	11th to 20th	Day	14% 9% 4 8% 5% 4 5% 4 5% 5% 37%	1629
	21st to end	Day	13% 10% 4 6% 5% 3 4 4 5% 5% 41%	1782
February	1st to 10th	Day	12% 10% 4 8% 5% 4 4 4 7% 7% 35%	1620
	11th to 20th	Day	14% 8% 3 7% 6% 4 5% 4 5% 6% 38%	1573
	21st to end	Day	16% 8% 3 6% 5% 3 4 4 5% 6% 40%	1528
March	1st to 10th	Day	12% 7% 3 5% 4 3 4 4 5% 7% 46%	1980
	11th to 20th	Day	15% 9% 3 6% 6% 3 4 5% 5% 6% 38%	1980
	21st to end	Day	16% 9% 4 6% 5% 4 5% 5% 5% 6% 35%	2178
April	1st to 10th	Day	10% 8% 4 6% 5% 4 5% 5% 5% 7% 42%	2103
	11th to 20th	Day	14% 6% 2 6% 4 3 4 3 5% 7% 45%	2134
	21st to end	Day	18% 7% 3 6% 6% 3 4 5% 4 6% 38%	2249
May	1st to 10th	Day	17% 7% 3 6% 7% 4 6% 4 5% 6% 34%	2340
	11th to 20th	Day	20% 8% 3 6% 6% 3 5% 5% 5% 7% 32%	2301
	21st to end	Day	15% 5% 3 7% 6% 4 6% 6% 5% 9% 33%	2633
June	1st to 10th	Day	14% 4 3 4 5% 4 6% 6% 7% 9% 39%	2520
	11th to 20th	Day	12% 5% 3 6% 6% 4 6% 6% 6% 10% 34%	2520
	21st to end	Day	9% 5% 2 6% 7% 4 6% 7% 7% 9% 37%	2520
July	1st to 10th	Day	11% 5% 4 8% 6% 5% 7% 7% 6% 8% 34%	2386
	11th to 20th	Day	8% 5% 3 6% 7% 5% 7% 8% 8% 10% 33%	2340
	21st to end	Day	13% 6% 3 7% 7% 5% 7% 6% 7% 8% 30%	2547
August	1st to 10th	Day	10% 7% 3 7% 8% 5% 6% 7% 7% 10% 30%	2327
	11th to 20th	Day	14% 6% 3 7% 8% 4 6% 6% 6% 8% 33%	2303
	21st to end	Day	12% 6% 3 7% 7% 5% 6% 6% 6% 9% 34%	2376
September	1st to 10th	Day	15% 6% 3 6% 5% 4 5% 5% 5% 7% 41%	2160
	11th to 20th	Day	14% 6% 2 6% 6% 3 4 4 5% 7% 42%	2044
	21st to end	Day	22% 5% 3 5% 5% 3 4 5% 5% 6% 37%	1943
October	1st to 10th	Day	17% 6% 2 5% 5% 3 5% 4 5% 6% 42%	1800
	11th to 20th	Day	16% 6% 4 5% 5% 3 4 4 4 5% 44%	1800
	21st to end	Day	15% 6% 2 4 4 3 4 4 5% 6% 47%	1980
November	1st to 10th	Day	10% 6% 2 4 5% 3 4 3 4 6% 52%	1857
	11th to 20th	Day	13% 7% 2 5% 4 3 4 3 5% 7% 47%	1649
	21st to end	Day	14% 6% 2 6% 4 3 4 5% 4 6% 46%	1520
December	1st to 10th	Day	15% 9% 3 6% 6% 3 5% 5% 6% 7% 35%	1520
	11th to 20th	Day	14% 6% 3 5% 4 3 4 3 4 7% 47%	1910
	21st to end	Day	16% 7% 2 6% 6% 4 6% 6% 7% 8% 31%	2840
January	1st to 10th	Night	10% 8% 3 6% 4 3 5% 4 5% 7% 44%	2459
	11th to 20th	Night	14% 7% 3 6% 5% 4 5% 5% 5% 7% 39%	2360
	21st to end	Night	12% 8% 4 5% 6% 3 5% 5% 5% 6% 41%	2574
February	1st to 10th	Night	11% 8% 3 5% 6% 3 6% 6% 6% 7% 39%	2340
	11th to 20th	Night	15% 8% 3 6% 5% 4 5% 6% 6% 7% 36%	2273
	21st to end	Night	14% 8% 3 6% 5% 3 5% 5% 5% 6% 38%	1723
March	1st to 10th	Night	10% 6% 2 4 4 3 5% 5% 6% 7% 48%	1980
	11th to 20th	Night	15% 8% 2 5% 4 3 5% 5% 5% 7% 40%	1980
	21st to end	Night	15% 7% 3 5% 5% 3 4 5% 6% 7% 39%	2178
April	1st to 10th	Night	12% 7% 3 6% 6% 3 4 5% 5% 7% 42%	1855
	11th to 20th	Night	12% 5% 2 5% 6% 3 4 4 6% 9% 44%	1775
	21st to end	Night	17% 5% 2 5% 4 3 5% 4 6% 7% 42%	1726
May	1st to 10th	Night	16% 7% 2 6% 5% 3 4 5% 5% 8% 40%	1620
	11th to 20th	Night	17% 8% 3 6% 5% 2 5% 4 4 7% 39%	1592
	21st to end	Night	14% 6% 2 4 5% 2 4 5% 7% 10% 40%	1637
June	1st to 10th	Night	11% 4 2 4 5% 3 5% 5% 7% 9% 46%	1440
	11th to 20th	Night	11% 4 2 6% 5% 4 6% 5% 7% 9% 42%	1440
	21st to end	Night	8% 3 3 4 4 4 4 6% 7% 10% 46%	1440
July	1st to 10th	Night	10% 6% 2 5% 5% 3 5% 6% 6% 10% 41%	1568
	11th to 20th	Night	6% 4 2 4 4 4 5% 6% 7% 10% 47%	1620
	21st to end	Night	8% 4 2 6% 5% 5% 6% 6% 7% 11% 40%	1771
August	1st to 10th	Night	8% 6% 2 6% 6% 3 5% 6% 7% 9% 41%	1611
	11th to 20th	Night	12% 6% 3 5% 6% 3 5% 6% 5% 8% 42%	1598
	21st to end	Night	11% 6% 2 5% 7% 5% 4 5% 7% 9% 39%	1962
September	1st to 10th	Night	14% 5% 2 5% 4 3 4 3 5% 7% 48%	1800
	11th to 20th	Night	13% 6% 2 4 4 3 4 5% 5% 7% 46%	1912
	21st to end	Night	21% 6% 2 5% 4 2 4 4 4 5% 42%	2016
October	1st to 10th	Night	18% 5% 2 4 5% 3 4 4 5% 6% 44%	2160
	11th to 20th	Night	14% 6% 3 5% 4 2 3 4 4 6% 49%	2160
	21st to end	Night	14% 6% 2 5% 5% 2 4 4 5% 6% 46%	2376
November	1st to 10th	Night	11% 7% 3 5% 4 3 4 5% 4 6% 50%	2221
	11th to 20th	Night	13% 6% 2 4 5% 2 4 5% 5% 7% 47%	2524
	21st to end	Night	13% 5% 2 5% 5% 4 4 5% 5% 6% 45%	2660
December	1st to 10th	Night	13% 9% 3 7% 5% 3 5% 5% 5% 7% 38%	2660
	11th to 20th	Night	14% 7% 3 6% 5% 4 4 4 5% 7% 42%	3285
	21st to end	Night	15% 6% 2 6% 6% 4 5% 6% 6% 8% 34%	4811

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	44% 5% 9% 21% 17% 4	1707
	11th to 20th	Day	48% 5% 11% 21% 15% 1	1629
	21st to end	Day	44% 4 12% 20% 17% 4	1782
February	1st to 10th	Day	45% 6% 12% 21% 13% 3	1620
	11th to 20th	Day	46% 5% 9% 22% 16% 2	1582
	21st to end	Day	43% 5% 13% 18% 16% 5%	1528
March	1st to 10th	Day	37% 5% 11% 24% 20% 3	1980
	11th to 20th	Day	45% 5% 10% 19% 16% 5%	1980
	21st to end	Day	46% 6% 12% 18% 12% 4	2178
April	1st to 10th	Day	38% 7% 11% 21% 17% 6%	2103
	11th to 20th	Day	39% 5% 10% 20% 21% 5%	2134
	21st to end	Day	45% 6% 11% 21% 13% 4	2249
May	1st to 10th	Day	46% 9% 13% 18% 12% 2	2340
	11th to 20th	Day	49% 9% 13% 18% 9% 2	2305
	21st to end	Day	45% 9% 15% 21% 9% 1	2633
June	1st to 10th	Day	38% 9% 14% 23% 15% 1	2520
	11th to 20th	Day	41% 11% 15% 19% 12% 2	2520
	21st to end	Day	38% 10% 15% 24% 13% 1	2520
July	1st to 10th	Day	42% 11% 12% 19% 13% 2	2321
	11th to 20th	Day	38% 12% 16% 21% 11% 2	2271
	21st to end	Day	45% 10% 13% 18% 14% 1	2405
August	1st to 10th	Day	45% 10% 16% 19% 10%	2314
	11th to 20th	Day	46% 9% 14% 19% 10% 1	2303
	21st to end	Day	44% 8% 15% 20% 11% 2	2376
September	1st to 10th	Day	41% 8% 14% 17% 17% 3	2160
	11th to 20th	Day	39% 5% 9% 24% 18% 4	2044
	21st to end	Day	45% 7% 9% 20% 16% 2	1943
October	1st to 10th	Day	41% 5% 12% 22% 15% 5%	1800
	11th to 20th	Day	40% 5% 9% 23% 20% 3	1800
	21st to end	Day	36% 6% 9% 24% 20% 5%	1980
November	1st to 10th	Day	32% 5% 9% 26% 24% 5%	1857
	11th to 20th	Day	36% 4 12% 20% 22% 6%	1649
	21st to end	Day	37% 4 11% 25% 20% 2	1520
December	1st to 10th	Day	46% 6% 12% 22% 12% 3	1520
	11th to 20th	Day	36% 4 11% 29% 16% 5%	1906
	21st to end	Day	43% 10% 15% 21% 10% 1	2840
January	1st to 10th	Night	38% 6% 12% 22% 19% 4	2459
	11th to 20th	Night	43% 7% 12% 21% 15% 3	2360
	21st to end	Night	40% 5% 11% 25% 16% 3	2574
February	1st to 10th	Night	40% 7% 15% 23% 13% 2	2340
	11th to 20th	Night	44% 6% 11% 21% 14% 4	2275
	21st to end	Night	43% 8% 12% 20% 13% 5%	1723
March	1st to 10th	Night	33% 6% 12% 28% 17% 4	1980
	11th to 20th	Night	40% 8% 11% 24% 15% 2	1980
	21st to end	Night	41% 6% 15% 19% 14% 4	2178
April	1st to 10th	Night	39% 7% 12% 22% 16% 4	1855
	11th to 20th	Night	34% 6% 15% 21% 18% 5%	1775
	21st to end	Night	38% 7% 14% 25% 13% 3	1726
May	1st to 10th	Night	41% 8% 16% 21% 11% 3	1620
	11th to 20th	Night	45% 8% 14% 21% 12% 1	1593
	21st to end	Night	38% 9% 19% 24% 9% 1	1638
June	1st to 10th	Night	33% 11% 15% 27% 14%	1440
	11th to 20th	Night	35% 11% 17% 23% 12% 1	1440
	21st to end	Night	30% 12% 19% 21% 17% 2	1440
July	1st to 10th	Night	37% 11% 17% 21% 12% 3	1523
	11th to 20th	Night	28% 13% 19% 26% 13% 2	1566
	21st to end	Night	32% 10% 20% 24% 13%	1672
August	1st to 10th	Night	36% 12% 18% 22% 12% 1	1600
	11th to 20th	Night	39% 9% 16% 24% 12%	1598
	21st to end	Night	39% 10% 16% 24% 10% 2	1962
September	1st to 10th	Night	36% 8% 16% 19% 17% 4	1800
	11th to 20th	Night	36% 6% 11% 28% 18% 2	1912
	21st to end	Night	43% 6% 11% 22% 17% 2	2016
October	1st to 10th	Night	40% 6% 11% 24% 16% 2	2160
	11th to 20th	Night	35% 5% 12% 27% 20% 2	2160
	21st to end	Night	35% 5% 11% 23% 22% 3	2376
November	1st to 10th	Night	34% 5% 11% 23% 22% 5%	2221
	11th to 20th	Night	34% 6% 9% 25% 21% 5%	2524
	21st to end	Night	37% 5% 12% 23% 18% 4	2660
December	1st to 10th	Night	44% 6% 13% 22% 13% 3	2660
	11th to 20th	Night	38% 6% 11% 23% 21% 2	3288
	21st to end	Night	39% 11% 14% 23% 12% 1	4811

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	24% 29% 32% 11% 3	270
	11th to 20th	Day	23% 27% 32% 16% 2	243
	21st to end	Day	27% 30% 29% 11% 2	264
February	1st to 10th	Day	28% 30% 32% 7% 2	240
	11th to 20th	Day	34% 31% 30% 5%	240
	21st to end	Day	30% 24% 29% 13% 3	299
March	1st to 10th	Day	20% 28% 29% 21% 3	420
	11th to 20th	Day	23% 24% 29% 18% 5% 1	420
	21st to end	Day	21% 21% 32% 17% 7% 1	462
April	1st to 10th	Day	21% 25% 26% 20% 9%	420
	11th to 20th	Day	18% 20% 25% 23% 11% 3	412
	21st to end	Day	27% 17% 28% 19% 9% 2	420
May	1st to 10th	Day	21% 20% 27% 19% 9% 4	423
	11th to 20th	Day	22% 12% 29% 23% 11% 3	447
	21st to end	Day	19% 6% 24% 26% 18% 8%	485
June	1st to 10th	Day	13% 9% 24% 31% 16% 6%	450
	11th to 20th	Day	16% 8% 28% 24% 17% 7%	450
	21st to end	Day	10% 6% 29% 35% 16% 4	450
July	1st to 10th	Day	12% 4 18% 29% 25% 11%	445
	11th to 20th	Day	5% 3 19% 34% 27% 11%	444
	21st to end	Day	14% 4 18% 30% 25% 9%	488
August	1st to 10th	Day	13% 7% 19% 26% 21% 14%	446
	11th to 20th	Day	15% 3 19% 28% 24% 11%	439
	21st to end	Day	11% 4 18% 28% 24% 16%	495
September	1st to 10th	Day	18% 6% 19% 23% 21% 13%	450
	11th to 20th	Day	16% 4 16% 29% 25% 10%	450
	21st to end	Day	25% 9% 22% 23% 17% 4	450
October	1st to 10th	Day	20% 6% 22% 27% 20% 5%	450
	11th to 20th	Day	16% 16% 25% 27% 14% 2	450
	21st to end	Day	19% 16% 33% 23% 7% 2	495
November	1st to 10th	Day	15% 13% 31% 27% 12% 2	455
	11th to 20th	Day	24% 18% 26% 23% 7% 1	269
	21st to end	Day	20% 20% 36% 19% 4 1	180
December	1st to 10th	Day	22% 22% 38% 13% 4 1	180
	11th to 20th	Day	26% 18% 35% 14% 7% 1	180
	21st to end	Day	20% 23% 36% 16% 5%	218
January	1st to 10th	Night	17% 21% 36% 22% 3	760
	11th to 20th	Night	19% 16% 34% 27% 4	727
	21st to end	Night	15% 16% 44% 23% 2	792
February	1st to 10th	Night	15% 19% 38% 24% 3	720
	11th to 20th	Night	23% 20% 36% 20% 1	703
	21st to end	Night	22% 17% 39% 20% 2	592
March	1st to 10th	Night	15% 18% 39% 25% 3	720
	11th to 20th	Night	22% 17% 32% 23% 5% 1	720
	21st to end	Night	20% 16% 34% 23% 6% 1	792
April	1st to 10th	Night	17% 22% 31% 25% 6%	720
	11th to 20th	Night	14% 16% 31% 28% 11% 1	711
	21st to end	Night	19% 11% 29% 33% 8%	646
May	1st to 10th	Night	18% 8% 27% 31% 14% 1	540
	11th to 20th	Night	19% 6% 24% 36% 14% 2	531
	21st to end	Night	15% 2 18% 42% 21% 2	477
June	1st to 10th	Night	11% 3 21% 41% 23% 1	360
	11th to 20th	Night	10% 5% 16% 46% 21% 2	360
	21st to end	Night	8% 3 17% 52% 19% 1	360
July	1st to 10th	Night	13% 2 11% 36% 32% 5%	473
	11th to 20th	Night	6% 2 11% 40% 35% 6%	522
	21st to end	Night	7% 2 12% 42% 33% 4	591
August	1st to 10th	Night	10% 2 12% 41% 32% 3	537
	11th to 20th	Night	11% 3 10% 39% 34% 3	534
	21st to end	Night	13% 3 14% 37% 30% 3	774
September	1st to 10th	Night	17% 3 15% 32% 28% 5%	720
	11th to 20th	Night	14% 4 14% 36% 27% 5%	720
	21st to end	Night	22% 4 19% 32% 20% 2	720
October	1st to 10th	Night	18% 9% 18% 31% 21% 3	720
	11th to 20th	Night	16% 9% 30% 30% 13% 1	720
	21st to end	Night	16% 13% 27% 32% 10% 1	792
November	1st to 10th	Night	14% 9% 28% 36% 11% 2	740
	11th to 20th	Night	16% 15% 28% 31% 10% 1	760
	21st to end	Night	16% 15% 30% 30% 8% 1	760
December	1st to 10th	Night	17% 16% 35% 27% 4	760
	11th to 20th	Night	15% 13% 37% 31% 4	760
	21st to end	Night	16% 16% 33% 33% 3	836

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