Wilmington
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wilmington clear sky chart from 2005-11-06 to 2025-05-26.

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	11% 6% 2 6% 4 4 5% 4 6% 8% 43%	113810
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	115445
Transparency	Day	36% 7% 14% 20% 17% 5%	117454
	Night	37% 7% 12% 19% 19% 6%	119940
Seeing	Day	13% 4 17% 28% 28% 11%	16327
	Night	13% 5% 20% 30% 25% 7%	26698

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 3 3 4 5% 56%	4056
	11th to 20th	Day	11% 5% 2 5% 4 3 4 2 4 6% 53%	4060
	21st to end	Day	13% 8% 3 7% 4 4 4 3 5% 5% 43%	4466
February	1st to 10th	Day	15% 6% 3 7% 6% 4 4 3 6% 7% 41%	1800
	11th to 20th	Day	19% 6% 3 5% 4 3 3 3 4 5% 45%	1870
	21st to end	Day	13% 7% 2 5% 4 4 4 3 5% 8% 44%	1755
March	1st to 10th	Day	20% 9% 3 6% 3 3 4 3 4 6% 38%	4935
	11th to 20th	Day	8% 4 1 4 3 4 4 4 6% 7% 54%	4980
	21st to end	Day	12% 6% 2 6% 4 4 4 3 5% 5% 51%	5465
April	1st to 10th	Day	9% 4 2 5% 3 4 4 5% 6% 7% 52%	2365
	11th to 20th	Day	9% 4 2 5% 4 3 4 4 6% 8% 52%	2374
	21st to end	Day	8% 4 1 4 4 4 4 4 5% 9% 53%	2400
May	1st to 10th	Day	6% 5% 2 6% 5% 6% 5% 5% 7% 10% 41%	3885
	11th to 20th	Day	16% 7% 3 8% 5% 5% 4 4 6% 8% 35%	4210
	21st to end	Day	6% 3 2 6% 5% 5% 6% 5% 8% 11% 44%	4541
June	1st to 10th	Day	10% 7% 3 7% 6% 5% 6% 5% 8% 9% 34%	2470
	11th to 20th	Day	8% 4 2 6% 4 4 5% 6% 9% 11% 40%	2470
	21st to end	Day	7% 5% 3 6% 6% 6% 7% 5% 9% 11% 35%	2470
July	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 5% 9% 14% 31%	4120
	11th to 20th	Day	8% 6% 3 8% 7% 6% 7% 6% 8% 12% 28%	4120
	21st to end	Day	8% 7% 3 8% 6% 6% 7% 6% 10% 13% 26%	4466
August	1st to 10th	Day	20% 12% 3 8% 5% 5% 5% 4 6% 10% 21%	4104
	11th to 20th	Day	7% 6% 2 7% 5% 6% 7% 6% 8% 12% 35%	3738
	21st to end	Day	7% 6% 2 7% 5% 4 5% 4 7% 10% 42%	4180
September	1st to 10th	Day	10% 6% 3 6% 5% 4 6% 5% 7% 9% 40%	2280
	11th to 20th	Day	16% 7% 2 7% 5% 4 5% 4 5% 8% 36%	2239
	21st to end	Day	17% 7% 3 7% 5% 5% 4 4 6% 6% 37%	2000
October	1st to 10th	Day	14% 6% 3 7% 4 4 4 3 5% 5% 44%	1900
	11th to 20th	Day	10% 4 2 5% 3 4 4 3 4 7% 55%	1900
	21st to end	Day	10% 5% 2 6% 3 3 3 3 4 6% 56%	2090
November	1st to 10th	Day	12% 5% 1 4 3 4 4 3 5% 7% 52%	1957
	11th to 20th	Day	13% 6% 2 6% 4 3 4 3 4 6% 50%	2000
	21st to end	Day	9% 6% 2 5% 4 5% 3 3 5% 8% 50%	1837
December	1st to 10th	Day	9% 6% 2 5% 4 4 4 4 5% 6% 53%	2600
	11th to 20th	Day	15% 7% 3 6% 4 3 4 3 5% 6% 45%	3241
	21st to end	Day	13% 7% 2 5% 4 3 4 3 5% 7% 46%	4466
January	1st to 10th	Night	10% 6% 2 5% 3 3 3 2 4 6% 55%	5889
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 5% 7% 48%	5900
	21st to end	Night	12% 7% 3 7% 5% 4 4 3 4 5% 46%	6490
February	1st to 10th	Night	17% 6% 2 6% 4 4 4 4 4 6% 43%	2600
	11th to 20th	Night	18% 9% 3 6% 3 3 4 3 4 7% 41%	2415
	21st to end	Night	16% 6% 2 7% 4 3 4 3 5% 6% 44%	1871
March	1st to 10th	Night	17% 10% 3 7% 4 4 4 2 4 5% 40%	4918
	11th to 20th	Night	12% 5% 1 5% 3 3 3 4 6% 8% 49%	4980
	21st to end	Night	13% 5% 2 5% 3 3 3 3 4 6% 53%	5467
April	1st to 10th	Night	10% 6% 2 6% 3 3 4 4 6% 7% 49%	2033
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 6% 6% 50%	1975
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 7% 55%	2000
May	1st to 10th	Night	6% 5% 3 7% 5% 5% 5% 5% 6% 10% 44%	3230
	11th to 20th	Night	12% 7% 2 8% 6% 4 5% 4 5% 7% 39%	2918
	21st to end	Night	8% 5% 2 6% 4 4 4 6% 6% 10% 46%	3130
June	1st to 10th	Night	9% 7% 2 8% 5% 6% 5% 4 8% 11% 35%	1710
	11th to 20th	Night	9% 5% 1 6% 4 4 5% 5% 9% 12% 41%	1710
	21st to end	Night	8% 6% 1 7% 5% 6% 6% 6% 9% 11% 34%	1710
July	1st to 10th	Night	7% 5% 3 7% 6% 7% 6% 6% 8% 12% 34%	2840
	11th to 20th	Night	7% 6% 3 8% 7% 6% 6% 5% 10% 12% 31%	2840
	21st to end	Night	8% 5% 2 7% 6% 6% 7% 6% 9% 13% 30%	3087
August	1st to 10th	Night	19% 10% 3 8% 4 4 4 4 7% 11% 26%	2825
	11th to 20th	Night	7% 6% 2 7% 4 5% 6% 5% 8% 11% 39%	3058
	21st to end	Night	6% 4 2 7% 5% 5% 5% 5% 7% 11% 44%	3476
September	1st to 10th	Night	11% 5% 2 6% 4 5% 5% 5% 6% 10% 41%	1900
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 5% 7% 9% 36%	1935
	21st to end	Night	17% 7% 3 6% 4 5% 5% 3 5% 6% 39%	2178
October	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 4 6% 46%	2280
	11th to 20th	Night	9% 4 2 5% 4 3 4 3 5% 6% 55%	2280
	21st to end	Night	9% 4 2 4 3 4 4 3 5% 7% 55%	2508
November	1st to 10th	Night	11% 6% 2 4 4 4 4 3 5% 5% 53%	2341
	11th to 20th	Night	13% 7% 2 6% 4 4 3 3 5% 7% 46%	2400
	21st to end	Night	10% 6% 2 5% 3 3 4 4 6% 7% 50%	2550
December	1st to 10th	Night	10% 5% 2 5% 3 4 3 3 5% 6% 53%	4580
	11th to 20th	Night	13% 8% 2 7% 5% 3 4 4 5% 6% 44%	4931
	21st to end	Night	13% 5% 3 6% 4 3 4 3 5% 7% 47%	6490

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	30% 4 7% 18% 34% 7%	4057
	11th to 20th	Day	33% 4 5% 18% 31% 9%	4060
	21st to end	Day	45% 4 6% 17% 20% 8%	4466
February	1st to 10th	Day	42% 5% 9% 21% 17% 5%	1800
	11th to 20th	Day	43% 5% 8% 16% 21% 7%	1879
	21st to end	Day	39% 6% 11% 17% 18% 9%	1755
March	1st to 10th	Day	47% 5% 8% 14% 19% 7%	4935
	11th to 20th	Day	27% 5% 10% 16% 25% 16%	4980
	21st to end	Day	35% 5% 10% 23% 19% 8%	5465
April	1st to 10th	Day	29% 5% 15% 23% 22% 6%	2365
	11th to 20th	Day	28% 5% 13% 26% 19% 9%	2374
	21st to end	Day	26% 7% 16% 26% 23% 3	2400
May	1st to 10th	Day	32% 11% 23% 23% 10% 2	3920
	11th to 20th	Day	42% 7% 15% 24% 12%	4214
	21st to end	Day	27% 11% 20% 28% 14% 1	4554
June	1st to 10th	Day	39% 10% 17% 22% 11%	2470
	11th to 20th	Day	31% 12% 20% 25% 11% 1	2470
	21st to end	Day	36% 13% 21% 23% 8% 1	2470
July	1st to 10th	Day	37% 12% 26% 21% 4 1	4055
	11th to 20th	Day	42% 14% 28% 12% 4	4051
	21st to end	Day	40% 14% 24% 15% 7%	4324
August	1st to 10th	Day	55% 10% 21% 12% 2	4091
	11th to 20th	Day	35% 12% 21% 23% 9%	3774
	21st to end	Day	34% 8% 21% 32% 5% 1	4180
September	1st to 10th	Day	37% 11% 19% 17% 13% 3	2280
	11th to 20th	Day	44% 8% 14% 17% 14% 3	2239
	21st to end	Day	44% 8% 12% 18% 15% 3	2000
October	1st to 10th	Day	41% 4 7% 22% 22% 4	3160
	11th to 20th	Day	20% 4 6% 17% 28% 24%	3140
	21st to end	Day	29% 4 6% 26% 26% 9%	3428
November	1st to 10th	Day	33% 4 10% 20% 25% 9%	1957
	11th to 20th	Day	37% 4 8% 20% 23% 8%	2000
	21st to end	Day	33% 5% 10% 22% 23% 7%	1837
December	1st to 10th	Day	32% 5% 8% 17% 26% 11%	2600
	11th to 20th	Day	41% 5% 10% 19% 19% 6%	3238
	21st to end	Day	37% 6% 14% 26% 16% 2	4466
January	1st to 10th	Night	32% 4 6% 17% 35% 6%	5889
	11th to 20th	Night	37% 4 6% 15% 32% 6%	5900
	21st to end	Night	39% 4 6% 15% 27% 8%	6490
February	1st to 10th	Night	42% 5% 9% 19% 19% 6%	2600
	11th to 20th	Night	44% 5% 8% 15% 20% 7%	2417
	21st to end	Night	41% 4 11% 17% 18% 9%	1871
March	1st to 10th	Night	47% 5% 7% 15% 20% 6%	4918
	11th to 20th	Night	33% 5% 9% 13% 23% 16%	4980
	21st to end	Night	34% 4 11% 24% 20% 7%	5467
April	1st to 10th	Night	33% 6% 14% 18% 22% 7%	2033
	11th to 20th	Night	34% 5% 12% 22% 18% 9%	1975
	21st to end	Night	28% 7% 15% 28% 20% 4	2000
May	1st to 10th	Night	33% 9% 20% 23% 12% 2	3260
	11th to 20th	Night	42% 8% 13% 23% 13%	2919
	21st to end	Night	30% 10% 19% 26% 15%	3138
June	1st to 10th	Night	41% 9% 19% 21% 10%	1710
	11th to 20th	Night	32% 12% 20% 26% 9% 1	1710
	21st to end	Night	38% 13% 19% 22% 7%	1710
July	1st to 10th	Night	39% 11% 23% 22% 5% 1	2795
	11th to 20th	Night	39% 16% 30% 11% 3	2786
	21st to end	Night	41% 16% 20% 18% 6%	2988
August	1st to 10th	Night	51% 10% 24% 13% 2	2814
	11th to 20th	Night	35% 12% 22% 23% 6%	3088
	21st to end	Night	34% 9% 18% 33% 6% 1	3476
September	1st to 10th	Night	38% 12% 20% 16% 11% 3	1900
	11th to 20th	Night	45% 8% 14% 18% 13% 2	1935
	21st to end	Night	46% 7% 12% 20% 13% 2	2178
October	1st to 10th	Night	39% 5% 9% 18% 23% 6%	3800
	11th to 20th	Night	20% 4 6% 16% 22% 32%	3779
	21st to end	Night	24% 4 7% 27% 27% 11%	4120
November	1st to 10th	Night	34% 5% 9% 19% 24% 9%	2341
	11th to 20th	Night	38% 5% 9% 16% 24% 8%	2400
	21st to end	Night	33% 5% 10% 22% 23% 6%	2550
December	1st to 10th	Night	32% 4 8% 15% 29% 12%	4580
	11th to 20th	Night	42% 5% 12% 17% 18% 6%	4933
	21st to end	Night	37% 5% 12% 25% 18% 2	6490

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	14% 7% 38% 31% 10%	300
	11th to 20th	Day	10% 5% 30% 38% 15% 1	300
	21st to end	Day	17% 6% 32% 34% 10% 1	330
February	1st to 10th	Day	16% 6% 24% 41% 13%	300
	11th to 20th	Day	25% 6% 29% 29% 10% 1	418
	21st to end	Day	15% 7% 28% 38% 11% 2	414
March	1st to 10th	Day	13% 6% 30% 31% 18% 1	500
	11th to 20th	Day	14% 8% 27% 37% 12% 2	500
	21st to end	Day	13% 5% 29% 37% 15% 2	550
April	1st to 10th	Day	12% 6% 26% 35% 19% 3	500
	11th to 20th	Day	9% 4 24% 31% 26% 7%	492
	21st to end	Day	9% 3 17% 34% 27% 10%	500
May	1st to 10th	Day	6% 3 19% 41% 21% 9%	503
	11th to 20th	Day	14% 3 14% 27% 32% 10%	527
	21st to end	Day	11% 1 8% 27% 33% 20%	549
June	1st to 10th	Day	13% 1 8% 29% 36% 13%	490
	11th to 20th	Day	10% 1 10% 22% 34% 22%	490
	21st to end	Day	8% 1 9% 22% 44% 16%	490
July	1st to 10th	Day	10% 1 5% 14% 46% 24%	485
	11th to 20th	Day	10% 1 4 15% 48% 23%	484
	21st to end	Day	8% 1 6% 19% 48% 19%	532
August	1st to 10th	Day	17% 2 7% 17% 35% 22%	486
	11th to 20th	Day	9% 2 6% 18% 38% 27%	479
	21st to end	Day	11% 2 7% 15% 41% 24%	539
September	1st to 10th	Day	11% 1 8% 19% 37% 23%	490
	11th to 20th	Day	22% 1 9% 21% 37% 11%	490
	21st to end	Day	20% 2 9% 25% 30% 14%	490
October	1st to 10th	Day	17% 3 13% 23% 30% 13%	490
	11th to 20th	Day	11% 17% 31% 31% 9%	490
	21st to end	Day	9% 5% 17% 32% 29% 8%	539
November	1st to 10th	Day	15% 5% 15% 30% 25% 8%	495
	11th to 20th	Day	16% 6% 24% 35% 18% 2	500
	21st to end	Day	11% 6% 28% 32% 20% 4	395
December	1st to 10th	Day	18% 3 26% 34% 19% 1	200
	11th to 20th	Day	16% 5% 25% 32% 17% 4	260
	21st to end	Day	18% 7% 25% 31% 15% 4	330
January	1st to 10th	Night	14% 13% 35% 29% 9% 1	800
	11th to 20th	Night	14% 8% 29% 37% 12% 1	800
	21st to end	Night	15% 9% 35% 35% 6%	880
February	1st to 10th	Night	20% 6% 28% 36% 10%	800
	11th to 20th	Night	21% 9% 32% 32% 6%	783
	21st to end	Night	18% 7% 28% 35% 10% 1	660
March	1st to 10th	Night	15% 6% 31% 34% 13% 1	800
	11th to 20th	Night	15% 6% 26% 37% 14% 2	800
	21st to end	Night	14% 5% 29% 34% 16% 2	880
April	1st to 10th	Night	13% 5% 28% 34% 18% 2	800
	11th to 20th	Night	13% 4 22% 33% 23% 5%	791
	21st to end	Night	7% 4 21% 37% 27% 5%	800
May	1st to 10th	Night	7% 5% 19% 39% 25% 5%	800
	11th to 20th	Night	15% 2 17% 34% 26% 6%	607
	21st to end	Night	11% 1 9% 25% 36% 18%	634
June	1st to 10th	Night	11% 1 10% 32% 36% 10%	570
	11th to 20th	Night	11% 2 7% 25% 41% 14%	570
	21st to end	Night	8% 4 7% 26% 44% 11%	570
July	1st to 10th	Night	8% 1 5% 20% 50% 15%	555
	11th to 20th	Night	7% 1 5% 19% 53% 15%	552
	21st to end	Night	6% 1 6% 25% 49% 13%	624
August	1st to 10th	Night	13% 2 5% 23% 42% 17%	567
	11th to 20th	Night	11% 1 6% 20% 45% 17%	723
	21st to end	Night	12% 1 7% 19% 43% 18%	836
September	1st to 10th	Night	12% 2 6% 22% 44% 14%	760
	11th to 20th	Night	17% 1 8% 26% 38% 10%	760
	21st to end	Night	20% 2 11% 24% 32% 10%	760
October	1st to 10th	Night	17% 3 12% 26% 32% 10%	760
	11th to 20th	Night	12% 2 22% 32% 25% 7%	760
	21st to end	Night	11% 5% 18% 29% 28% 8%	836
November	1st to 10th	Night	13% 4 22% 33% 24% 5%	780
	11th to 20th	Night	16% 7% 26% 29% 19% 3	800
	21st to end	Night	11% 9% 26% 31% 20% 4	800
December	1st to 10th	Night	18% 9% 25% 33% 13% 2	800
	11th to 20th	Night	15% 8% 27% 32% 16% 2	800
	21st to end	Night	16% 7% 27% 33% 15% 1	880

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