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 2025-03-27.

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%	108871
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	110489
Transparency	Day	37% 6% 14% 25% 16% 3	112502
	Night	39% 5% 11% 24% 18% 3	114997
Seeing	Day	15% 7% 23% 30% 20% 5%	15645
	Night	16% 8% 25% 31% 16% 3	25186

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	14% 4 2 4 3 3 3 3 4 7% 54%	3597
	11th to 20th	Day	12% 7% 2 7% 4 4 3 2 4 5% 50%	3986
	21st to end	Day	12% 7% 3 8% 5% 4 4 3 5% 5% 43%	4466
February	1st to 10th	Day	12% 7% 3 7% 5% 4 5% 5% 6% 8% 38%	1800
	11th to 20th	Day	19% 8% 3 4 3 4 4 3 4 5% 43%	1947
	21st to end	Day	17% 9% 3 6% 3 3 4 3 5% 7% 40%	1650
March	1st to 10th	Day	23% 7% 2 6% 4 4 3 3 4 6% 38%	3280
	11th to 20th	Day	10% 4 2 4 3 3 3 3 5% 7% 55%	3590
	21st to end	Day	18% 8% 2 5% 4 3 4 3 4 6% 43%	4089
April	1st to 10th	Day	13% 5% 2 5% 4 5% 4 3 6% 6% 47%	2280
	11th to 20th	Day	11% 6% 1 5% 4 4 4 4 5% 7% 50%	2254
	21st to end	Day	13% 6% 3 5% 4 4 4 4 5% 7% 47%	2280
May	1st to 10th	Day	15% 9% 3 8% 5% 5% 4 4 6% 10% 31%	3815
	11th to 20th	Day	19% 9% 3 8% 5% 4 4 4 5% 6% 32%	4327
	21st to end	Day	9% 4 2 5% 4 4 5% 4 7% 10% 45%	4823
June	1st to 10th	Day	10% 4 2 5% 4 4 5% 5% 8% 10% 43%	2660
	11th to 20th	Day	10% 4 2 5% 4 4 4 4 7% 11% 45%	2660
	21st to end	Day	7% 5% 2 5% 5% 5% 4 5% 7% 12% 45%	2660
July	1st to 10th	Day	6% 4 2 5% 4 5% 6% 5% 8% 12% 44%	4440
	11th to 20th	Day	11% 6% 2 6% 4 5% 4 5% 8% 10% 39%	4440
	21st to end	Day	8% 5% 2 7% 5% 6% 7% 5% 9% 10% 34%	4505
August	1st to 10th	Day	13% 8% 2 6% 4 4 5% 5% 8% 11% 34%	4020
	11th to 20th	Day	8% 5% 2 7% 5% 6% 7% 5% 7% 10% 40%	3730
	21st to end	Day	8% 4 2 5% 4 4 5% 5% 8% 10% 46%	4180
September	1st to 10th	Day	14% 5% 2 5% 5% 4 4 4 5% 8% 45%	2280
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 5% 8% 46%	2256
	21st to end	Day	19% 7% 3 7% 5% 3 3 3 5% 7% 38%	2090
October	1st to 10th	Day	13% 5% 2 7% 4 4 4 3 4 6% 46%	1900
	11th to 20th	Day	16% 4 2 4 2 4 3 3 5% 5% 51%	1900
	21st to end	Day	12% 6% 2 6% 4 4 4 3 4 6% 49%	2090
November	1st to 10th	Day	12% 6% 2 5% 3 3 4 3 5% 7% 50%	1763
	11th to 20th	Day	12% 7% 2 5% 5% 4 5% 3 4 7% 47%	1800
	21st to end	Day	14% 5% 2 5% 4 3 3 3 6% 6% 48%	1763
December	1st to 10th	Day	10% 6% 2 5% 4 4 4 3 5% 6% 52%	2600
	11th to 20th	Day	17% 10% 2 7% 3 3 2 2 3 5% 45%	2990
	21st to end	Day	11% 6% 2 5% 4 3 5% 4 5% 7% 48%	3960
January	1st to 10th	Night	11% 7% 2 6% 3 3 3 3 4 6% 52%	6349
	11th to 20th	Night	13% 6% 3 6% 4 4 4 3 3 5% 50%	5970
	21st to end	Night	13% 8% 3 7% 4 4 4 3 4 5% 47%	6490
February	1st to 10th	Night	17% 7% 2 6% 4 3 3 3 5% 6% 43%	2600
	11th to 20th	Night	20% 8% 1 5% 3 3 4 4 5% 5% 41%	2338
	21st to end	Night	17% 7% 2 6% 4 3 4 3 4 5% 45%	1980
March	1st to 10th	Night	23% 9% 3 7% 3 3 3 3 4 6% 37%	3789
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 5% 7% 47%	3590
	21st to end	Night	17% 10% 2 5% 4 3 3 3 3 4 45%	3681
April	1st to 10th	Night	15% 8% 2 5% 5% 4 4 3 6% 7% 39%	1900
	11th to 20th	Night	11% 6% 2 4 4 4 4 3 5% 7% 49%	1875
	21st to end	Night	14% 6% 2 7% 4 4 6% 3 6% 7% 41%	1900
May	1st to 10th	Night	17% 9% 4 9% 5% 4 5% 3 6% 6% 31%	3080
	11th to 20th	Night	22% 10% 3 5% 4 3 4 3 5% 7% 33%	2594
	21st to end	Night	10% 8% 3 8% 5% 4 5% 4 6% 8% 39%	2732
June	1st to 10th	Night	12% 8% 3 7% 5% 3 5% 4 7% 8% 40%	1520
	11th to 20th	Night	8% 5% 3 5% 6% 5% 5% 4 7% 11% 41%	1520
	21st to end	Night	7% 6% 3 5% 4 5% 5% 4 7% 11% 42%	1520
July	1st to 10th	Night	8% 6% 2 6% 6% 6% 6% 6% 7% 10% 37%	2520
	11th to 20th	Night	8% 7% 2 7% 6% 5% 5% 5% 8% 11% 37%	2520
	21st to end	Night	12% 9% 3 8% 6% 4 5% 4 7% 10% 32%	3044
August	1st to 10th	Night	14% 9% 3 7% 5% 5% 5% 4 6% 9% 32%	2878
	11th to 20th	Night	8% 6% 3 7% 5% 5% 5% 4 7% 9% 40%	3097
	21st to end	Night	9% 4 2 5% 4 4 4 3 5% 9% 51%	3476
September	1st to 10th	Night	12% 6% 2 5% 6% 4 4 3 6% 7% 45%	1900
	11th to 20th	Night	9% 6% 3 6% 5% 4 4 3 5% 8% 46%	1924
	21st to end	Night	19% 7% 2 7% 4 4 5% 4 5% 6% 35%	2090
October	1st to 10th	Night	14% 7% 2 6% 4 3 3 3 6% 7% 45%	2276
	11th to 20th	Night	15% 6% 2 5% 4 3 4 2 4 5% 50%	2280
	21st to end	Night	13% 7% 2 5% 4 4 4 3 5% 6% 49%	2508
November	1st to 10th	Night	13% 7% 2 5% 3 4 4 3 4 6% 49%	2532
	11th to 20th	Night	14% 7% 3 5% 4 3 3 3 5% 6% 47%	2600
	21st to end	Night	11% 6% 2 5% 4 3 4 4 5% 7% 50%	2635
December	1st to 10th	Night	9% 5% 2 5% 3 4 4 4 5% 6% 51%	4580
	11th to 20th	Night	18% 8% 3 8% 4 4 4 3 4 6% 37%	5205
	21st to end	Night	12% 7% 2 7% 4 4 4 3 5% 8% 44%	6996

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	32% 3 5% 28% 27% 5%	3598
	11th to 20th	Day	38% 3 7% 22% 24% 5%	3986
	21st to end	Day	44% 4 9% 29% 12% 2	4466
February	1st to 10th	Day	42% 6% 13% 24% 12% 2	1800
	11th to 20th	Day	44% 4 9% 23% 16% 4	1956
	21st to end	Day	43% 5% 12% 21% 15% 4	1650
March	1st to 10th	Day	47% 5% 9% 19% 15% 4	3280
	11th to 20th	Day	28% 5% 10% 30% 23% 5%	3590
	21st to end	Day	43% 6% 10% 19% 17% 4	4089
April	1st to 10th	Day	37% 6% 12% 21% 20% 5%	2280
	11th to 20th	Day	34% 6% 11% 26% 19% 5%	2254
	21st to end	Day	37% 5% 13% 24% 18% 4	2280
May	1st to 10th	Day	51% 9% 12% 19% 8% 2	3850
	11th to 20th	Day	48% 7% 14% 21% 7% 2	4331
	21st to end	Day	35% 9% 16% 23% 14% 3	4837
June	1st to 10th	Day	33% 9% 16% 26% 14% 1	2660
	11th to 20th	Day	31% 8% 17% 27% 14% 2	2660
	21st to end	Day	29% 9% 18% 27% 15% 2	2660
July	1st to 10th	Day	30% 9% 27% 21% 11% 2	4370
	11th to 20th	Day	37% 8% 24% 25% 5%	4365
	21st to end	Day	39% 10% 18% 20% 13% 1	4361
August	1st to 10th	Day	42% 7% 22% 23% 6%	4007
	11th to 20th	Day	35% 7% 17% 31% 10%	3766
	21st to end	Day	29% 8% 13% 31% 18% 1	4180
September	1st to 10th	Day	36% 7% 16% 20% 18% 3	2280
	11th to 20th	Day	36% 5% 11% 27% 20% 2	2256
	21st to end	Day	45% 6% 10% 20% 16% 3	2090
October	1st to 10th	Day	29% 5% 9% 29% 23% 5%	3160
	11th to 20th	Day	27% 3 8% 25% 31% 7%	3140
	21st to end	Day	25% 4 10% 29% 22% 9%	3428
November	1st to 10th	Day	37% 4 10% 29% 16% 4	1763
	11th to 20th	Day	38% 5% 12% 21% 19% 5%	1800
	21st to end	Day	37% 4 9% 29% 16% 3	1763
December	1st to 10th	Day	35% 4 13% 27% 18% 3	2600
	11th to 20th	Day	43% 3 13% 22% 16% 3	2986
	21st to end	Day	34% 6% 16% 29% 14% 1	3960
January	1st to 10th	Night	34% 3 7% 23% 27% 6%	6349
	11th to 20th	Night	38% 4 6% 25% 24% 4	5970
	21st to end	Night	40% 4 8% 30% 15% 2	6490
February	1st to 10th	Night	43% 3 12% 26% 14% 2	2600
	11th to 20th	Night	45% 3 8% 23% 17% 4	2340
	21st to end	Night	41% 4 8% 22% 18% 5%	1980
March	1st to 10th	Night	48% 4 10% 21% 14% 3	3789
	11th to 20th	Night	35% 5% 9% 25% 21% 5%	3590
	21st to end	Night	44% 4 7% 22% 17% 6%	3681
April	1st to 10th	Night	42% 7% 10% 18% 19% 5%	1900
	11th to 20th	Night	35% 6% 11% 22% 20% 6%	1875
	21st to end	Night	40% 6% 11% 22% 18% 3	1900
May	1st to 10th	Night	51% 8% 12% 20% 8% 1	3110
	11th to 20th	Night	49% 7% 13% 22% 8% 1	2595
	21st to end	Night	42% 8% 14% 21% 14% 1	2739
June	1st to 10th	Night	41% 8% 13% 21% 16% 1	1520
	11th to 20th	Night	35% 9% 15% 24% 15% 2	1520
	21st to end	Night	35% 9% 16% 23% 15% 2	1520
July	1st to 10th	Night	39% 11% 19% 19% 10% 2	2480
	11th to 20th	Night	37% 8% 23% 25% 7% 1	2472
	21st to end	Night	46% 8% 16% 17% 13%	2947
August	1st to 10th	Night	48% 6% 17% 22% 7%	2867
	11th to 20th	Night	39% 7% 14% 28% 12%	3127
	21st to end	Night	30% 6% 11% 26% 25% 1	3476
September	1st to 10th	Night	37% 8% 16% 18% 17% 4	1900
	11th to 20th	Night	37% 5% 11% 26% 19% 2	1924
	21st to end	Night	47% 7% 9% 18% 15% 3	2090
October	1st to 10th	Night	35% 5% 10% 24% 24% 3	3796
	11th to 20th	Night	27% 4 7% 26% 29% 7%	3779
	21st to end	Night	24% 3 9% 32% 23% 10%	4120
November	1st to 10th	Night	38% 5% 9% 26% 17% 5%	2532
	11th to 20th	Night	40% 4 11% 21% 19% 5%	2600
	21st to end	Night	34% 5% 10% 27% 21% 2	2635
December	1st to 10th	Night	34% 4 12% 25% 21% 3	4580
	11th to 20th	Night	48% 5% 12% 20% 12% 3	5208
	21st to end	Night	39% 5% 14% 24% 15% 1	6996

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	18% 20% 31% 27% 5%	200
	11th to 20th	Day	16% 19% 41% 17% 7%	283
	21st to end	Day	15% 13% 36% 29% 7%	330
February	1st to 10th	Day	16% 13% 35% 28% 8%	300
	11th to 20th	Day	27% 16% 34% 20% 3	495
	21st to end	Day	21% 14% 37% 23% 6%	414
March	1st to 10th	Day	17% 16% 30% 30% 7%	500
	11th to 20th	Day	17% 16% 31% 30% 4 2	500
	21st to end	Day	17% 11% 34% 31% 6% 1	530
April	1st to 10th	Day	12% 15% 34% 31% 6% 2	460
	11th to 20th	Day	12% 8% 26% 40% 12% 2	452
	21st to end	Day	17% 8% 27% 34% 14% 1	460
May	1st to 10th	Day	14% 8% 29% 33% 13% 3	463
	11th to 20th	Day	16% 4 24% 36% 16% 2	487
	21st to end	Day	17% 2 17% 31% 25% 8%	529
June	1st to 10th	Day	12% 2 16% 35% 28% 7%	490
	11th to 20th	Day	9% 2 17% 34% 30% 8%	490
	21st to end	Day	8% 1 14% 40% 30% 7%	490
July	1st to 10th	Day	8% 2 9% 34% 34% 12%	485
	11th to 20th	Day	6% 2 7% 32% 43% 11%	484
	21st to end	Day	10% 1 10% 36% 36% 7%	532
August	1st to 10th	Day	10% 2 11% 31% 36% 10%	486
	11th to 20th	Day	11% 1 9% 31% 37% 12%	479
	21st to end	Day	9% 2 13% 29% 34% 13%	539
September	1st to 10th	Day	15% 2 12% 30% 25% 15%	490
	11th to 20th	Day	13% 1 16% 29% 31% 10%	490
	21st to end	Day	23% 6% 21% 27% 21% 3	490
October	1st to 10th	Day	17% 5% 16% 30% 22% 9%	490
	11th to 20th	Day	15% 6% 24% 31% 20% 4	490
	21st to end	Day	14% 8% 27% 29% 17% 5%	539
November	1st to 10th	Day	16% 8% 25% 28% 18% 5%	395
	11th to 20th	Day	15% 13% 35% 24% 12% 2	400
	21st to end	Day	18% 17% 22% 25% 14% 3	363
December	1st to 10th	Day	20% 12% 35% 24% 10% 1	200
	11th to 20th	Day	20% 12% 28% 33% 8%	200
	21st to end	Day	19% 10% 31% 28% 11% 1	220
January	1st to 10th	Night	18% 18% 34% 25% 5%	800
	11th to 20th	Night	19% 16% 32% 27% 7%	800
	21st to end	Night	18% 15% 35% 30% 3	880
February	1st to 10th	Night	21% 11% 37% 27% 5%	800
	11th to 20th	Night	25% 16% 39% 18% 2	783
	21st to end	Night	20% 13% 31% 32% 4 1	660
March	1st to 10th	Night	18% 12% 33% 28% 8%	800
	11th to 20th	Night	18% 12% 34% 28% 7% 2	800
	21st to end	Night	19% 9% 33% 33% 6%	863
April	1st to 10th	Night	19% 9% 36% 29% 6%	760
	11th to 20th	Night	14% 8% 30% 38% 11% 1	751
	21st to end	Night	16% 4 30% 37% 12% 1	760
May	1st to 10th	Night	19% 7% 27% 36% 10% 1	736
	11th to 20th	Night	19% 3 25% 36% 16% 1	434
	21st to end	Night	16% 1 14% 36% 26% 6%	411
June	1st to 10th	Night	14% 3 16% 40% 25% 2	380
	11th to 20th	Night	8% 1 13% 44% 31% 4	380
	21st to end	Night	10% 2 13% 42% 30% 3	380
July	1st to 10th	Night	8% 9% 45% 32% 5%	370
	11th to 20th	Night	8% 2 6% 33% 43% 8%	368
	21st to end	Night	10% 1 9% 38% 36% 5%	594
August	1st to 10th	Night	9% 2 10% 35% 37% 7%	594
	11th to 20th	Night	12% 2 10% 36% 35% 5%	750
	21st to end	Night	9% 2 10% 33% 36% 10%	836
September	1st to 10th	Night	13% 2 11% 33% 31% 9%	760
	11th to 20th	Night	12% 2 12% 33% 34% 7%	760
	21st to end	Night	22% 5% 15% 32% 23% 3	760
October	1st to 10th	Night	18% 3 18% 31% 23% 7%	760
	11th to 20th	Night	18% 7% 23% 30% 20% 3	760
	21st to end	Night	17% 9% 24% 30% 16% 3	836
November	1st to 10th	Night	16% 8% 26% 29% 17% 4	780
	11th to 20th	Night	18% 11% 31% 30% 9% 1	800
	21st to end	Night	13% 15% 29% 29% 13% 1	800
December	1st to 10th	Night	20% 11% 35% 23% 10% 2	800
	11th to 20th	Night	19% 11% 38% 25% 7%	800
	21st to end	Night	17% 13% 33% 29% 9%	880

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