Rehoboth Beach
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rehoboth Beach clear sky chart from 2005-11-06 to 2025-11-18.

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% 6% 2 6% 4 4 4 4 6% 8% 42%	126745
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 43%	123612
Transparency	Day	38% 7% 13% 24% 16% 3	130304
	Night	39% 5% 12% 23% 17% 3	128122
Seeing	Day	15% 7% 22% 30% 20% 5%	16495
	Night	17% 8% 26% 31% 16% 2	25811

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% 5% 2 4 2 3 3 3 4 6% 53%	3597
	11th to 20th	Day	13% 7% 3 7% 4 3 4 3 4 5% 47%	3830
	21st to end	Day	13% 8% 3 8% 4 5% 4 3 5% 6% 42%	4466
February	1st to 10th	Day	14% 8% 2 7% 5% 4 5% 4 7% 8% 38%	1800
	11th to 20th	Day	18% 8% 2 5% 3 3 4 4 5% 7% 42%	1946
	21st to end	Day	16% 10% 3 6% 4 4 4 3 5% 6% 40%	1650
March	1st to 10th	Day	18% 5% 3 5% 3 3 4 3 5% 7% 45%	4479
	11th to 20th	Day	10% 8% 3 5% 3 3 3 3 5% 6% 48%	4980
	21st to end	Day	14% 8% 2 6% 4 4 4 3 5% 6% 44%	5800
April	1st to 10th	Day	14% 7% 2 6% 4 3 3 3 6% 7% 44%	2400
	11th to 20th	Day	11% 6% 2 5% 3 4 4 3 5% 8% 48%	2374
	21st to end	Day	14% 6% 3 5% 4 4 4 3 6% 8% 44%	2400
May	1st to 10th	Day	17% 9% 3 7% 5% 4 5% 4 7% 8% 31%	5603
	11th to 20th	Day	21% 9% 3 7% 4 4 4 4 5% 7% 33%	6244
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 10% 38%	6934
June	1st to 10th	Day	11% 6% 2 6% 4 4 5% 5% 7% 9% 42%	2800
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 4 7% 9% 44%	2800
	21st to end	Day	8% 5% 2 4 5% 4 5% 5% 7% 11% 45%	2800
July	1st to 10th	Day	7% 5% 2 7% 4 5% 5% 5% 7% 10% 43%	6360
	11th to 20th	Day	9% 6% 3 8% 5% 6% 7% 6% 9% 11% 30%	6360
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	6662
August	1st to 10th	Day	14% 6% 2 6% 4 4 5% 4 8% 11% 35%	4152
	11th to 20th	Day	8% 5% 3 7% 4 5% 7% 4 7% 8% 40%	3850
	21st to end	Day	7% 4 3 5% 5% 5% 5% 4 6% 9% 48%	4312
September	1st to 10th	Day	13% 5% 2 4 3 4 5% 4 6% 7% 46%	2400
	11th to 20th	Day	11% 6% 3 6% 4 3 4 3 5% 7% 48%	2360
	21st to end	Day	18% 7% 3 7% 3 4 4 4 5% 7% 38%	2200
October	1st to 10th	Day	15% 5% 2 5% 3 3 5% 3 5% 7% 46%	2021
	11th to 20th	Day	17% 5% 2 4 2 3 3 3 5% 6% 50%	2000
	21st to end	Day	14% 6% 2 5% 4 4 4 3 5% 7% 47%	2156
November	1st to 10th	Day	13% 5% 2 4 3 4 3 3 5% 7% 51%	1851
	11th to 20th	Day	12% 6% 3 5% 5% 3 4 3 4 8% 46%	1863
	21st to end	Day	14% 4 2 5% 4 4 4 3 4 7% 48%	1745
December	1st to 10th	Day	10% 5% 2 6% 3 3 4 3 5% 8% 50%	2600
	11th to 20th	Day	18% 9% 3 6% 3 3 3 2 3 5% 45%	2990
	21st to end	Day	12% 6% 3 5% 4 4 4 4 5% 7% 46%	3960
January	1st to 10th	Night	12% 7% 2 5% 3 3 4 3 5% 7% 49%	6349
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 4 5% 52%	6113
	21st to end	Night	15% 7% 2 6% 4 4 4 3 4 6% 43%	6490
February	1st to 10th	Night	18% 7% 2 7% 3 4 3 4 5% 7% 41%	2600
	11th to 20th	Night	19% 7% 2 6% 4 3 4 3 5% 6% 41%	2338
	21st to end	Night	18% 6% 3 6% 3 4 3 3 4 6% 44%	1980
March	1st to 10th	Night	16% 8% 3 5% 3 3 4 2 4 6% 47%	5356
	11th to 20th	Night	12% 10% 2 5% 4 3 3 3 5% 7% 45%	4980
	21st to end	Night	15% 8% 3 6% 4 4 3 3 4 5% 44%	5130
April	1st to 10th	Night	17% 7% 2 6% 5% 4 3 3 5% 6% 40%	2000
	11th to 20th	Night	11% 7% 2 7% 4 4 4 3 4 7% 46%	1975
	21st to end	Night	16% 6% 2 6% 4 4 5% 4 5% 7% 42%	2000
May	1st to 10th	Night	14% 10% 3 8% 4 4 5% 4 6% 8% 32%	4288
	11th to 20th	Night	21% 10% 3 7% 4 4 5% 4 5% 7% 31%	3677
	21st to end	Night	18% 8% 3 8% 4 4 5% 3 5% 7% 35%	3920
June	1st to 10th	Night	14% 7% 2 5% 5% 5% 4 4 6% 8% 41%	1600
	11th to 20th	Night	12% 5% 2 5% 4 6% 5% 6% 6% 10% 40%	1600
	21st to end	Night	8% 6% 1 6% 5% 5% 6% 5% 7% 12% 40%	1600
July	1st to 10th	Night	8% 6% 3 7% 6% 6% 6% 4 8% 11% 36%	3598
	11th to 20th	Night	10% 8% 3 8% 6% 5% 6% 4 7% 10% 32%	3597
	21st to end	Night	9% 6% 3 7% 6% 5% 6% 4 7% 11% 36%	4186
August	1st to 10th	Night	15% 8% 3 8% 5% 5% 5% 4 6% 9% 33%	2966
	11th to 20th	Night	10% 6% 2 7% 4 4 6% 4 6% 9% 41%	3197
	21st to end	Night	7% 5% 2 6% 4 3 4 3 6% 9% 52%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 5% 8% 44%	2000
	11th to 20th	Night	11% 5% 2 5% 4 4 4 4 5% 9% 47%	2040
	21st to end	Night	20% 7% 2 7% 4 4 5% 3 5% 7% 35%	2200
October	1st to 10th	Night	17% 5% 2 6% 4 3 4 3 4 6% 46%	2375
	11th to 20th	Night	17% 4 2 4 3 3 3 3 3 6% 51%	2400
	21st to end	Night	14% 7% 2 5% 3 3 4 3 5% 6% 48%	2681
November	1st to 10th	Night	12% 5% 2 6% 4 3 4 3 4 5% 51%	2667
	11th to 20th	Night	15% 7% 3 5% 3 4 4 3 4 6% 47%	2692
	21st to end	Night	12% 5% 2 5% 4 4 4 3 6% 7% 49%	2650
December	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 4 7% 51%	4580
	11th to 20th	Night	19% 8% 3 7% 4 4 4 3 5% 6% 38%	5205
	21st to end	Night	13% 8% 3 6% 4 3 4 4 5% 7% 43%	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 7% 29% 26% 3	3598
	11th to 20th	Day	41% 4 8% 22% 20% 6%	3830
	21st to end	Day	43% 4 11% 28% 12% 2	4466
February	1st to 10th	Day	42% 7% 14% 24% 12% 2	1800
	11th to 20th	Day	42% 5% 10% 24% 16% 3	1955
	21st to end	Day	45% 5% 11% 20% 15% 5%	1650
March	1st to 10th	Day	39% 4 8% 23% 18% 7%	4479
	11th to 20th	Day	35% 6% 9% 26% 19% 6%	4980
	21st to end	Day	41% 6% 10% 18% 19% 7%	5800
April	1st to 10th	Day	38% 6% 13% 21% 17% 5%	2400
	11th to 20th	Day	35% 6% 12% 23% 19% 6%	2374
	21st to end	Day	37% 6% 14% 20% 19% 4	2400
May	1st to 10th	Day	48% 9% 14% 18% 8% 1	5635
	11th to 20th	Day	49% 8% 11% 22% 9% 1	6248
	21st to end	Day	40% 8% 13% 23% 15% 1	6949
June	1st to 10th	Day	36% 10% 15% 22% 15% 2	2800
	11th to 20th	Day	34% 10% 16% 24% 14% 3	2800
	21st to end	Day	30% 10% 17% 24% 18% 1	2800
July	1st to 10th	Day	34% 10% 20% 20% 14% 2	6290
	11th to 20th	Day	43% 11% 20% 19% 5% 1	6285
	21st to end	Day	36% 9% 13% 22% 19% 1	6471
August	1st to 10th	Day	41% 8% 18% 25% 7%	4139
	11th to 20th	Day	38% 8% 12% 31% 9% 1	3886
	21st to end	Day	31% 7% 13% 30% 17% 2	4312
September	1st to 10th	Day	35% 8% 16% 22% 17% 3	2400
	11th to 20th	Day	36% 5% 11% 25% 21% 3	2360
	21st to end	Day	46% 7% 9% 19% 15% 3	2200
October	1st to 10th	Day	32% 5% 10% 24% 24% 6%	3294
	11th to 20th	Day	27% 3 6% 28% 27% 8%	3240
	21st to end	Day	25% 4 10% 29% 24% 7%	3458
November	1st to 10th	Day	34% 4 11% 30% 17% 4	1851
	11th to 20th	Day	38% 5% 9% 23% 21% 4	1863
	21st to end	Day	36% 5% 10% 30% 17% 2	1745
December	1st to 10th	Day	33% 4 19% 27% 13% 4	2600
	11th to 20th	Day	43% 3 13% 26% 12% 3	2986
	21st to end	Day	36% 6% 17% 23% 18%	3960
January	1st to 10th	Night	35% 3 8% 24% 25% 5%	6349
	11th to 20th	Night	35% 4 7% 28% 20% 6%	6113
	21st to end	Night	41% 4 8% 30% 14% 2	6490
February	1st to 10th	Night	43% 4 13% 25% 14% 1	2600
	11th to 20th	Night	44% 4 9% 21% 17% 4	2340
	21st to end	Night	42% 4 9% 23% 18% 5%	1980
March	1st to 10th	Night	40% 4 9% 23% 15% 9%	5356
	11th to 20th	Night	38% 6% 9% 23% 18% 6%	4980
	21st to end	Night	43% 5% 8% 18% 22% 4	5130
April	1st to 10th	Night	43% 5% 10% 19% 18% 5%	2000
	11th to 20th	Night	38% 5% 12% 19% 20% 6%	1975
	21st to end	Night	41% 5% 10% 20% 19% 4	2000
May	1st to 10th	Night	48% 7% 14% 21% 9% 2	4318
	11th to 20th	Night	52% 6% 10% 19% 11% 1	3678
	21st to end	Night	47% 8% 11% 20% 14% 1	3927
June	1st to 10th	Night	40% 8% 11% 21% 18% 1	1600
	11th to 20th	Night	38% 10% 13% 24% 15% 2	1600
	21st to end	Night	36% 9% 15% 23% 15% 3	1600
July	1st to 10th	Night	41% 10% 17% 19% 12% 1	3560
	11th to 20th	Night	45% 9% 19% 20% 5% 1	3552
	21st to end	Night	40% 8% 16% 20% 16%	4063
August	1st to 10th	Night	47% 8% 15% 22% 8%	2955
	11th to 20th	Night	38% 7% 15% 27% 12%	3227
	21st to end	Night	29% 6% 13% 27% 23% 2	3586
September	1st to 10th	Night	39% 6% 13% 23% 16% 4	2000
	11th to 20th	Night	33% 7% 12% 26% 20% 2	2040
	21st to end	Night	48% 7% 9% 18% 15% 2	2200
October	1st to 10th	Night	36% 4 10% 24% 23% 2	3882
	11th to 20th	Night	26% 3 12% 21% 32% 6%	3899
	21st to end	Night	24% 3 12% 29% 25% 6%	4329
November	1st to 10th	Night	35% 5% 9% 25% 20% 5%	2667
	11th to 20th	Night	40% 4 9% 21% 21% 5%	2692
	21st to end	Night	34% 5% 12% 26% 20% 3	2650
December	1st to 10th	Night	34% 4 13% 27% 19% 3	4580
	11th to 20th	Night	46% 4 12% 20% 13% 4	5208
	21st to end	Night	40% 4 16% 22% 17% 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	15% 16% 38% 27% 6%	200
	11th to 20th	Day	17% 18% 40% 18% 7%	250
	21st to end	Day	18% 12% 35% 27% 8%	330
February	1st to 10th	Day	19% 10% 37% 25% 9%	300
	11th to 20th	Day	26% 14% 39% 18% 4	494
	21st to end	Day	22% 10% 37% 24% 7%	414
March	1st to 10th	Day	16% 13% 38% 26% 7%	500
	11th to 20th	Day	18% 10% 34% 28% 7% 2	500
	21st to end	Day	19% 10% 34% 30% 7%	550
April	1st to 10th	Day	18% 14% 31% 29% 6% 2	500
	11th to 20th	Day	14% 9% 26% 38% 12% 1	492
	21st to end	Day	17% 8% 25% 33% 15% 2	500
May	1st to 10th	Day	20% 8% 26% 31% 13% 2	503
	11th to 20th	Day	20% 4 24% 36% 15% 2	527
	21st to end	Day	16% 3 18% 31% 23% 9%	573
June	1st to 10th	Day	14% 3 15% 36% 27% 6%	530
	11th to 20th	Day	12% 2 13% 33% 32% 8%	530
	21st to end	Day	10% 1 12% 36% 32% 8%	530
July	1st to 10th	Day	10% 2 9% 26% 41% 11%	525
	11th to 20th	Day	3 2 6% 31% 44% 14%	524
	21st to end	Day	9% 2 8% 34% 36% 11%	576
August	1st to 10th	Day	11% 3 7% 37% 33% 9%	526
	11th to 20th	Day	11% 2 6% 32% 36% 13%	519
	21st to end	Day	10% 2 9% 32% 34% 12%	583
September	1st to 10th	Day	15% 2 14% 30% 24% 15%	530
	11th to 20th	Day	14% 3 12% 31% 32% 9%	530
	21st to end	Day	23% 6% 15% 33% 19% 4	530
October	1st to 10th	Day	18% 3 17% 29% 22% 11%	530
	11th to 20th	Day	17% 6% 23% 28% 21% 4	530
	21st to end	Day	16% 8% 28% 28% 18% 3	558
November	1st to 10th	Day	15% 9% 25% 28% 20% 4	425
	11th to 20th	Day	12% 15% 34% 27% 11% 1	421
	21st to end	Day	14% 14% 29% 28% 14% 2	345
December	1st to 10th	Day	18% 11% 37% 26% 9% 1	200
	11th to 20th	Day	21% 12% 33% 26% 9%	200
	21st to end	Day	18% 11% 26% 32% 12%	220
January	1st to 10th	Night	18% 18% 37% 23% 4	800
	11th to 20th	Night	19% 14% 36% 25% 6%	800
	21st to end	Night	18% 14% 39% 27% 2	880
February	1st to 10th	Night	21% 10% 37% 28% 5%	800
	11th to 20th	Night	23% 15% 38% 22% 2	783
	21st to end	Night	20% 14% 35% 26% 5%	660
March	1st to 10th	Night	16% 13% 38% 25% 8%	800
	11th to 20th	Night	21% 10% 34% 26% 7% 2	800
	21st to end	Night	20% 9% 33% 31% 6% 1	880
April	1st to 10th	Night	19% 11% 35% 28% 7% 1	800
	11th to 20th	Night	15% 6% 31% 37% 11% 1	791
	21st to end	Night	19% 6% 27% 35% 12% 1	800
May	1st to 10th	Night	17% 6% 26% 38% 11% 2	715
	11th to 20th	Night	19% 6% 23% 36% 15% 1	444
	21st to end	Night	15% 4 13% 37% 26% 5%	433
June	1st to 10th	Night	15% 4 14% 41% 23% 2	400
	11th to 20th	Night	14% 1 14% 38% 31% 3	400
	21st to end	Night	10% 2 12% 44% 30% 4	400
July	1st to 10th	Night	8% 1 10% 38% 38% 4	390
	11th to 20th	Night	6% 1 10% 32% 46% 6%	388
	21st to end	Night	6% 2 10% 38% 39% 5%	546
August	1st to 10th	Night	12% 1 7% 42% 34% 4	622
	11th to 20th	Night	11% 3 9% 35% 35% 7%	790
	21st to end	Night	10% 2 9% 36% 34% 9%	880
September	1st to 10th	Night	16% 3 14% 29% 31% 7%	800
	11th to 20th	Night	14% 2 12% 31% 35% 6%	800
	21st to end	Night	24% 4 18% 29% 24% 3	800
October	1st to 10th	Night	20% 4 17% 29% 23% 7%	800
	11th to 20th	Night	19% 8% 23% 32% 17% 2	800
	21st to end	Night	16% 9% 29% 29% 15% 2	880
November	1st to 10th	Night	14% 9% 28% 31% 15% 3	820
	11th to 20th	Night	19% 11% 32% 28% 10% 1	829
	21st to end	Night	13% 15% 31% 28% 12% 1	800
December	1st to 10th	Night	21% 12% 34% 21% 9% 2	800
	11th to 20th	Night	19% 12% 38% 25% 6%	800
	21st to end	Night	17% 10% 36% 32% 5%	880

Page generated at 2025-11-15 00:56:01. Version ac07

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