NASA Wallops Visitor Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the NASA Wallops Visitor Center clear sky chart from 2005-11-06 to 2026-07-31.

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% 6% 2 6% 4 4 4 4 6% 8% 44%	138003
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 43%	135043
Transparency	Day	37% 6% 13% 24% 16% 3	141591
	Night	39% 5% 11% 24% 17% 3	139526
Seeing	Day	15% 8% 24% 30% 19% 5%	17462
	Night	16% 8% 26% 31% 16% 3	26835

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% 5% 2 4 3 3 4 3 5% 8% 51%	4661
	11th to 20th	Day	12% 6% 2 7% 4 3 3 2 4 5% 52%	5178
	21st to end	Day	12% 7% 3 7% 5% 4 4 3 5% 5% 45%	5780
February	1st to 10th	Day	12% 7% 3 7% 5% 4 5% 5% 6% 8% 39%	1890
	11th to 20th	Day	19% 8% 3 4 3 4 4 3 4 5% 42%	2047
	21st to end	Day	17% 9% 3 6% 3 3 4 3 5% 7% 40%	1730
March	1st to 10th	Day	18% 6% 2 6% 4 4 4 3 5% 7% 42%	4668
	11th to 20th	Day	10% 5% 2 6% 4 4 3 3 5% 7% 51%	5090
	21st to end	Day	13% 7% 2 6% 4 4 4 3 5% 7% 46%	5883
April	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 6% 7% 47%	2520
	11th to 20th	Day	10% 6% 1 5% 4 4 4 4 6% 7% 50%	2494
	21st to end	Day	13% 6% 3 5% 4 4 4 4 5% 7% 47%	2520
May	1st to 10th	Day	15% 7% 3 6% 4 4 4 4 6% 9% 39%	7098
	11th to 20th	Day	15% 7% 3 7% 4 4 4 3 5% 7% 42%	8095
	21st to end	Day	18% 6% 3 6% 4 4 4 3 5% 8% 37%	9045
June	1st to 10th	Day	10% 5% 2 5% 4 4 5% 5% 8% 10% 43%	2940
	11th to 20th	Day	11% 4 2 5% 4 4 4 4 7% 11% 45%	2940
	21st to end	Day	7% 4 1 5% 5% 4 4 5% 7% 11% 46%	2940
July	1st to 10th	Day	7% 4 2 6% 4 4 5% 4 7% 12% 46%	6500
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 11% 33%	6500
	21st to end	Day	7% 5% 2 6% 5% 5% 7% 5% 9% 12% 39%	6608
August	1st to 10th	Day	13% 8% 2 6% 4 4 5% 5% 8% 10% 34%	4147
	11th to 20th	Day	8% 5% 2 7% 5% 6% 7% 5% 7% 10% 39%	3850
	21st to end	Day	8% 4 2 5% 4 4 5% 5% 8% 10% 46%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 5% 8% 45%	2400
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 3 5% 8% 45%	2375
	21st to end	Day	19% 7% 3 7% 5% 3 3 3 5% 7% 38%	2200
October	1st to 10th	Day	13% 5% 2 6% 4 4 4 3 5% 6% 47%	2000
	11th to 20th	Day	16% 5% 2 4 2 4 3 3 5% 5% 50%	2000
	21st to end	Day	12% 6% 2 6% 4 4 4 3 4 6% 49%	2200
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 3 5% 7% 51%	1853
	11th to 20th	Day	11% 7% 2 5% 5% 4 5% 3 5% 7% 47%	1890
	21st to end	Day	14% 5% 2 5% 4 4 3 3 6% 6% 48%	1851
December	1st to 10th	Day	11% 6% 2 5% 4 4 3 3 5% 6% 52%	2680
	11th to 20th	Day	17% 10% 2 7% 3 3 2 2 3 5% 45%	3070
	21st to end	Day	11% 6% 2 6% 4 3 5% 4 5% 7% 47%	4048
January	1st to 10th	Night	11% 7% 2 6% 3 4 4 4 6% 7% 47%	8233
	11th to 20th	Night	13% 6% 3 6% 3 3 3 3 3 4 52%	7764
	21st to end	Night	13% 7% 3 6% 4 4 4 3 4 5% 47%	8401
February	1st to 10th	Night	16% 7% 2 6% 4 4 3 3 5% 6% 42%	2730
	11th to 20th	Night	20% 8% 1 4 4 4 3 3 5% 5% 41%	2458
	21st to end	Night	17% 7% 2 6% 4 3 4 3 5% 5% 44%	2076
March	1st to 10th	Night	18% 7% 2 5% 3 3 3 3 4 5% 45%	5401
	11th to 20th	Night	12% 8% 3 6% 4 3 4 3 5% 7% 46%	5090
	21st to end	Night	14% 9% 2 5% 4 3 4 4 5% 6% 45%	5290
April	1st to 10th	Night	15% 8% 2 5% 5% 4 4 3 6% 8% 40%	2100
	11th to 20th	Night	11% 6% 2 4 4 3 5% 3 5% 7% 50%	2075
	21st to end	Night	14% 6% 2 6% 4 4 5% 3 6% 8% 42%	2100
May	1st to 10th	Night	13% 7% 3 8% 4 4 5% 4 7% 9% 36%	5756
	11th to 20th	Night	15% 8% 3 5% 3 3 5% 3 5% 8% 41%	4820
	21st to end	Night	21% 9% 3 8% 4 4 4 3 5% 6% 33%	5108
June	1st to 10th	Night	12% 8% 3 7% 4 3 5% 4 6% 8% 41%	1680
	11th to 20th	Night	9% 5% 3 6% 6% 5% 5% 4 7% 11% 40%	1680
	21st to end	Night	7% 6% 3 5% 4 5% 5% 4 7% 11% 43%	1680
July	1st to 10th	Night	8% 6% 2 7% 5% 6% 6% 5% 7% 11% 37%	3678
	11th to 20th	Night	9% 7% 3 8% 6% 6% 5% 5% 8% 12% 31%	3677
	21st to end	Night	10% 7% 3 7% 5% 5% 5% 4 8% 10% 38%	4458
August	1st to 10th	Night	14% 9% 3 7% 5% 4 5% 4 6% 9% 32%	2970
	11th to 20th	Night	9% 7% 3 8% 5% 6% 5% 4 7% 9% 40%	3197
	21st to end	Night	8% 4 2 5% 4 4 4 3 5% 9% 51%	3586
September	1st to 10th	Night	12% 6% 3 5% 5% 4 4 3 5% 7% 46%	2000
	11th to 20th	Night	10% 6% 3 5% 5% 4 4 3 5% 8% 46%	2025
	21st to end	Night	19% 8% 2 7% 5% 4 5% 4 6% 6% 34%	2200
October	1st to 10th	Night	14% 6% 2 6% 4 3 3 2 5% 7% 46%	2396
	11th to 20th	Night	16% 6% 2 5% 4 3 4 2 4 5% 50%	2400
	21st to end	Night	13% 7% 2 5% 4 3 3 3 5% 6% 49%	2640
November	1st to 10th	Night	13% 7% 2 5% 3 4 4 3 4 6% 50%	2662
	11th to 20th	Night	14% 6% 3 5% 4 3 4 3 5% 6% 47%	2730
	21st to end	Night	11% 6% 2 5% 4 3 4 4 5% 7% 50%	2767
December	1st to 10th	Night	10% 6% 2 5% 3 4 4 4 5% 6% 51%	4720
	11th to 20th	Night	17% 7% 3 8% 4 4 4 3 4 7% 38%	5345
	21st to end	Night	12% 7% 2 6% 4 4 4 3 5% 8% 44%	7150

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% 3 10% 28% 22% 4	4662
	11th to 20th	Day	36% 3 6% 22% 27% 6%	5178
	21st to end	Day	42% 4 8% 31% 14% 2	5780
February	1st to 10th	Day	41% 6% 13% 24% 13% 3	1890
	11th to 20th	Day	44% 4 9% 23% 16% 4	2056
	21st to end	Day	44% 4 11% 21% 15% 4	1730
March	1st to 10th	Day	42% 4 9% 18% 20% 6%	4668
	11th to 20th	Day	34% 4 10% 26% 20% 6%	5090
	21st to end	Day	39% 5% 12% 19% 18% 7%	5883
April	1st to 10th	Day	36% 6% 13% 21% 20% 5%	2520
	11th to 20th	Day	34% 6% 12% 26% 18% 5%	2494
	21st to end	Day	37% 5% 13% 24% 18% 3	2520
May	1st to 10th	Day	44% 7% 14% 21% 13% 2	7130
	11th to 20th	Day	40% 5% 13% 26% 14% 1	8099
	21st to end	Day	46% 6% 12% 21% 13% 2	9061
June	1st to 10th	Day	33% 8% 16% 25% 16% 1	2940
	11th to 20th	Day	32% 8% 17% 27% 14% 2	2940
	21st to end	Day	29% 8% 18% 28% 15% 1	2940
July	1st to 10th	Day	30% 8% 25% 23% 13% 2	6430
	11th to 20th	Day	41% 11% 24% 20% 4	6425
	21st to end	Day	34% 8% 18% 23% 17% 1	6422
August	1st to 10th	Day	42% 7% 21% 24% 7%	4134
	11th to 20th	Day	36% 7% 17% 30% 10%	3886
	21st to end	Day	29% 8% 13% 31% 18% 2	4312
September	1st to 10th	Day	36% 7% 16% 21% 17% 3	2400
	11th to 20th	Day	37% 5% 11% 26% 20% 2	2375
	21st to end	Day	46% 6% 10% 20% 16% 2	2200
October	1st to 10th	Day	30% 5% 9% 29% 23% 5%	3260
	11th to 20th	Day	27% 3 8% 24% 30% 7%	3240
	21st to end	Day	26% 4 10% 28% 23% 9%	3538
November	1st to 10th	Day	36% 4 10% 30% 16% 4	1853
	11th to 20th	Day	37% 5% 12% 22% 19% 5%	1890
	21st to end	Day	37% 4 9% 29% 16% 3	1851
December	1st to 10th	Day	36% 4 13% 27% 18% 3	2680
	11th to 20th	Day	43% 3 13% 22% 16% 3	3066
	21st to end	Day	34% 6% 16% 29% 14% 1	4048
January	1st to 10th	Night	36% 3 12% 24% 22% 4	8234
	11th to 20th	Night	37% 3 5% 25% 24% 6%	7764
	21st to end	Night	40% 3 8% 30% 16% 3	8401
February	1st to 10th	Night	43% 3 11% 25% 15% 2	2730
	11th to 20th	Night	44% 3 8% 23% 17% 4	2460
	21st to end	Night	42% 4 8% 22% 18% 5%	2076
March	1st to 10th	Night	41% 4 9% 21% 17% 8%	5401
	11th to 20th	Night	37% 4 8% 23% 21% 6%	5090
	21st to end	Night	42% 4 9% 21% 17% 7%	5290
April	1st to 10th	Night	41% 6% 11% 19% 18% 5%	2100
	11th to 20th	Night	34% 6% 11% 22% 21% 5%	2075
	21st to end	Night	39% 6% 11% 22% 18% 4	2100
May	1st to 10th	Night	44% 6% 15% 25% 10% 1	5786
	11th to 20th	Night	42% 6% 14% 25% 13% 1	4821
	21st to end	Night	53% 5% 9% 18% 14% 1	5115
June	1st to 10th	Night	40% 8% 12% 21% 18% 1	1680
	11th to 20th	Night	37% 8% 15% 23% 15% 1	1680
	21st to end	Night	34% 9% 16% 24% 16% 2	1680
July	1st to 10th	Night	39% 9% 19% 19% 12% 2	3640
	11th to 20th	Night	44% 9% 22% 20% 5%	3632
	21st to end	Night	39% 7% 15% 23% 15%	4330
August	1st to 10th	Night	48% 6% 16% 23% 7%	2959
	11th to 20th	Night	39% 7% 14% 27% 12%	3227
	21st to end	Night	31% 6% 11% 26% 25% 1	3586
September	1st to 10th	Night	37% 7% 16% 19% 17% 4	2000
	11th to 20th	Night	37% 5% 11% 26% 19% 2	2025
	21st to end	Night	48% 7% 10% 18% 14% 2	2200
October	1st to 10th	Night	34% 4 10% 24% 24% 3	3916
	11th to 20th	Night	28% 4 7% 26% 29% 7%	3899
	21st to end	Night	24% 3 9% 31% 23% 10%	4252
November	1st to 10th	Night	37% 5% 9% 26% 18% 5%	2662
	11th to 20th	Night	40% 4 11% 21% 19% 5%	2730
	21st to end	Night	34% 5% 10% 27% 21% 3	2767
December	1st to 10th	Night	34% 4 12% 25% 21% 3	4720
	11th to 20th	Night	47% 5% 12% 20% 12% 3	5348
	21st to end	Night	39% 5% 14% 25% 15% 1	7150

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	17% 19% 31% 28% 5%	220
	11th to 20th	Day	17% 19% 41% 17% 6%	311
	21st to end	Day	15% 14% 38% 27% 6%	363
February	1st to 10th	Day	15% 14% 36% 27% 8%	330
	11th to 20th	Day	27% 16% 34% 20% 2	535
	21st to end	Day	22% 14% 36% 23% 6%	446
March	1st to 10th	Day	17% 15% 30% 31% 7%	540
	11th to 20th	Day	18% 16% 32% 29% 4 2	540
	21st to end	Day	15% 11% 37% 29% 6% 1	594
April	1st to 10th	Day	12% 14% 35% 30% 7% 2	540
	11th to 20th	Day	12% 9% 28% 38% 11% 2	532
	21st to end	Day	17% 8% 26% 33% 14% 1	540
May	1st to 10th	Day	15% 8% 31% 31% 12% 2	543
	11th to 20th	Day	16% 4 26% 35% 16% 2	567
	21st to end	Day	19% 3 18% 30% 22% 7%	617
June	1st to 10th	Day	12% 3 17% 35% 26% 7%	570
	11th to 20th	Day	11% 2 19% 33% 28% 7%	570
	21st to end	Day	8% 1 15% 39% 30% 7%	570
July	1st to 10th	Day	8% 3 8% 35% 34% 12%	565
	11th to 20th	Day	7% 2 7% 33% 42% 10%	564
	21st to end	Day	10% 2 11% 34% 36% 8%	616
August	1st to 10th	Day	10% 3 12% 31% 34% 9%	526
	11th to 20th	Day	11% 2 9% 30% 37% 11%	519
	21st to end	Day	9% 2 13% 30% 34% 13%	583
September	1st to 10th	Day	15% 2 14% 30% 25% 14%	530
	11th to 20th	Day	14% 2 15% 28% 31% 9%	530
	21st to end	Day	23% 5% 21% 27% 20% 3	530
October	1st to 10th	Day	16% 5% 16% 30% 22% 10%	530
	11th to 20th	Day	15% 8% 24% 30% 19% 4	530
	21st to end	Day	14% 9% 28% 28% 16% 5%	583
November	1st to 10th	Day	17% 8% 27% 27% 17% 4	425
	11th to 20th	Day	14% 15% 35% 24% 11% 1	430
	21st to end	Day	18% 16% 24% 26% 14% 3	391
December	1st to 10th	Day	21% 11% 33% 25% 9%	220
	11th to 20th	Day	19% 13% 29% 32% 7%	220
	21st to end	Day	18% 12% 32% 27% 10% 1	242
January	1st to 10th	Night	17% 18% 34% 26% 5%	840
	11th to 20th	Night	18% 17% 33% 26% 7%	840
	21st to end	Night	17% 16% 35% 29% 3	924
February	1st to 10th	Night	20% 12% 36% 28% 5%	840
	11th to 20th	Night	25% 15% 39% 19% 2	823
	21st to end	Night	20% 13% 31% 31% 4 1	692
March	1st to 10th	Night	19% 11% 33% 29% 8%	840
	11th to 20th	Night	18% 12% 33% 29% 6% 2	840
	21st to end	Night	19% 8% 34% 32% 6%	924
April	1st to 10th	Night	18% 9% 37% 29% 7% 1	840
	11th to 20th	Night	13% 8% 31% 38% 10% 1	831
	21st to end	Night	15% 5% 29% 37% 13% 1	840
May	1st to 10th	Night	18% 7% 28% 36% 10% 1	814
	11th to 20th	Night	18% 3 26% 36% 16% 1	480
	21st to end	Night	19% 2 15% 35% 24% 6%	455
June	1st to 10th	Night	13% 4 16% 40% 25% 2	420
	11th to 20th	Night	9% 1 13% 44% 29% 4	420
	21st to end	Night	10% 2 14% 40% 30% 4	420
July	1st to 10th	Night	9% 1 8% 44% 33% 5%	410
	11th to 20th	Night	9% 1 7% 33% 42% 8%	408
	21st to end	Night	10% 2 9% 37% 36% 6%	654
August	1st to 10th	Night	10% 1 10% 36% 36% 6%	626
	11th to 20th	Night	13% 2 9% 35% 35% 5%	790
	21st to end	Night	9% 2 10% 34% 35% 9%	880
September	1st to 10th	Night	13% 2 12% 33% 31% 9%	800
	11th to 20th	Night	13% 2 12% 33% 34% 7%	800
	21st to end	Night	22% 5% 15% 32% 24% 3	800
October	1st to 10th	Night	17% 3 19% 31% 23% 7%	800
	11th to 20th	Night	19% 8% 23% 30% 19% 3	800
	21st to end	Night	17% 10% 25% 30% 15% 3	880
November	1st to 10th	Night	16% 8% 27% 29% 17% 4	820
	11th to 20th	Night	17% 12% 32% 30% 9% 1	840
	21st to end	Night	13% 15% 29% 30% 13% 1	840
December	1st to 10th	Night	20% 11% 34% 23% 9% 2	840
	11th to 20th	Night	18% 12% 38% 25% 7%	840
	21st to end	Night	17% 13% 33% 28% 8%	924

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