Arundel
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arundel clear sky chart from 2005-11-06 to 2025-06-10.

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	18% 12% 4 9% 5% 4 4 3 5% 6% 28%	119744
	Night	17% 14% 5% 10% 5% 4 4 3 4 5% 28%	118454
Transparency	Day	55% 6% 11% 18% 9% 1	123220
	Night	56% 6% 11% 17% 9% 1	123139
Seeing	Day	27% 18% 30% 17% 6% 2	15174
	Night	25% 14% 28% 24% 8% 1	24251

By Month

Percentage Cloud Cover NE (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% 22% 6% 18% 8% 6% 4 2 3 3 14%	3597
	11th to 20th	Day	12% 18% 7% 17% 11% 4 3 2 3 5% 18%	3570
	21st to end	Day	9% 17% 10% 15% 5% 5% 5% 3 5% 6% 20%	3960
February	1st to 10th	Day	12% 18% 5% 14% 8% 5% 4 3 3 6% 22%	1790
	11th to 20th	Day	12% 20% 6% 13% 6% 5% 4 3 4 4 24%	1803
	21st to end	Day	17% 17% 5% 10% 8% 5% 3 3 4 4 23%	1650
March	1st to 10th	Day	18% 16% 5% 12% 6% 5% 4 3 4 5% 21%	4476
	11th to 20th	Day	12% 16% 6% 11% 6% 4 4 3 5% 6% 28%	4812
	21st to end	Day	18% 13% 3 8% 5% 4 4 3 4 5% 32%	5878
April	1st to 10th	Day	21% 15% 3 8% 4 4 3 3 4 5% 31%	2400
	11th to 20th	Day	22% 12% 4 6% 4 3 3 3 4 5% 33%	2374
	21st to end	Day	22% 12% 3 7% 5% 4 4 3 5% 4 30%	2560
May	1st to 10th	Day	24% 12% 4 8% 5% 5% 5% 4 5% 6% 23%	6346
	11th to 20th	Day	19% 9% 3 9% 5% 5% 4 3 5% 7% 30%	6318
	21st to end	Day	22% 10% 3 7% 5% 4 5% 3 5% 6% 30%	6949
June	1st to 10th	Day	23% 9% 3 7% 5% 5% 5% 4 6% 7% 27%	2786
	11th to 20th	Day	17% 7% 3 6% 5% 5% 6% 4 6% 7% 34%	2660
	21st to end	Day	23% 9% 2 7% 6% 5% 6% 4 5% 7% 27%	2660
July	1st to 10th	Day	14% 8% 3 8% 6% 5% 6% 4 6% 9% 32%	4440
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 4 7% 8% 33%	4440
	21st to end	Day	13% 7% 2 6% 4 4 4 4 6% 7% 43%	4814
August	1st to 10th	Day	16% 7% 3 8% 6% 6% 5% 4 6% 9% 29%	4423
	11th to 20th	Day	17% 7% 2 7% 4 4 5% 4 6% 9% 34%	4018
	21st to end	Day	17% 6% 2 7% 5% 5% 5% 3 6% 8% 37%	4180
September	1st to 10th	Day	18% 6% 2 5% 4 4 5% 3 5% 6% 41%	2280
	11th to 20th	Day	16% 6% 2 5% 4 3 5% 3 5% 8% 44%	2236
	21st to end	Day	21% 7% 2 5% 4 3 4 3 6% 7% 38%	2090
October	1st to 10th	Day	22% 9% 3 6% 4 4 4 4 5% 7% 32%	1905
	11th to 20th	Day	24% 14% 3 8% 4 4 4 3 5% 5% 25%	1891
	21st to end	Day	25% 14% 3 8% 4 3 3 4 5% 6% 24%	1881
November	1st to 10th	Day	17% 14% 3 8% 4 4 4 4 5% 6% 31%	1761
	11th to 20th	Day	20% 16% 5% 9% 6% 4 4 3 5% 5% 22%	1650
	21st to end	Day	23% 16% 7% 10% 6% 4 4 3 4 4 20%	1600
December	1st to 10th	Day	20% 23% 9% 10% 6% 4 4 3 4 4 13%	2600
	11th to 20th	Day	18% 20% 6% 12% 5% 4 4 3 3 3 21%	2986
	21st to end	Day	19% 19% 5% 10% 4 5% 4 3 4 4 22%	3960
January	1st to 10th	Night	13% 25% 9% 13% 6% 4 3 3 3 3 17%	6349
	11th to 20th	Night	10% 18% 8% 18% 10% 5% 4 3 4 4 16%	6315
	21st to end	Night	10% 17% 8% 16% 7% 7% 5% 3 4 5% 19%	6996
February	1st to 10th	Night	12% 16% 7% 12% 7% 5% 4 4 5% 4 24%	2609
	11th to 20th	Night	12% 17% 6% 13% 7% 5% 3 3 4 5% 25%	2481
	21st to end	Night	14% 16% 6% 10% 5% 5% 4 3 5% 5% 26%	1980
March	1st to 10th	Night	20% 16% 5% 11% 5% 4 4 2 3 4 27%	5364
	11th to 20th	Night	13% 15% 5% 9% 5% 5% 5% 3 5% 7% 29%	5145
	21st to end	Night	17% 14% 5% 9% 6% 4 4 3 5% 5% 29%	5040
April	1st to 10th	Night	22% 13% 4 6% 5% 3 3 3 4 6% 31%	2000
	11th to 20th	Night	20% 10% 4 7% 5% 3 3 3 5% 5% 35%	1975
	21st to end	Night	20% 11% 3 7% 4 4 4 3 5% 5% 34%	1855
May	1st to 10th	Night	21% 10% 4 9% 5% 4 5% 4 5% 7% 26%	3594
	11th to 20th	Night	19% 8% 3 8% 4 4 5% 3 5% 6% 35%	3575
	21st to end	Night	21% 10% 3 8% 5% 4 5% 4 5% 6% 29%	3926
June	1st to 10th	Night	19% 9% 2 7% 5% 5% 5% 4 7% 7% 31%	1598
	11th to 20th	Night	17% 8% 3 6% 5% 3 5% 4 5% 8% 37%	1520
	21st to end	Night	21% 8% 2 6% 4 4 4 4 5% 5% 36%	1520
July	1st to 10th	Night	14% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	2520
	11th to 20th	Night	17% 7% 3 5% 4 3 4 3 6% 7% 40%	2520
	21st to end	Night	12% 5% 2 5% 4 4 4 4 7% 9% 45%	2739
August	1st to 10th	Night	14% 8% 3 7% 5% 4 4 4 6% 7% 39%	2506
	11th to 20th	Night	17% 7% 2 7% 4 4 4 3 6% 6% 41%	2775
	21st to end	Night	16% 7% 3 7% 4 4 3 4 5% 6% 41%	3476
September	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 4 6% 46%	1900
	11th to 20th	Night	16% 6% 2 4 3 3 3 3 4 7% 48%	1943
	21st to end	Night	19% 7% 2 4 3 3 4 3 4 7% 43%	2090
October	1st to 10th	Night	18% 8% 3 7% 4 5% 3 3 5% 5% 37%	2271
	11th to 20th	Night	23% 11% 3 7% 5% 4 4 3 4 5% 30%	2289
	21st to end	Night	24% 13% 4 8% 4 3 4 3 4 6% 27%	2717
November	1st to 10th	Night	20% 12% 4 7% 5% 4 4 3 6% 6% 29%	2537
	11th to 20th	Night	18% 13% 5% 10% 6% 4 5% 3 4 5% 26%	2745
	21st to end	Night	21% 19% 6% 10% 5% 4 4 3 4 4 20%	2800
December	1st to 10th	Night	18% 19% 7% 15% 7% 4 4 3 4 5% 14%	4580
	11th to 20th	Night	23% 17% 6% 11% 6% 3 3 2 3 3 23%	5208
	21st to end	Night	22% 21% 4 10% 5% 3 3 2 3 4 24%	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	73% 6% 5% 12% 3	3598
	11th to 20th	Day	70% 5% 4 12% 8% 1	3600
	21st to end	Day	64% 5% 8% 11% 10% 1	3960
February	1st to 10th	Day	65% 3 7% 16% 7% 2	1790
	11th to 20th	Day	63% 5% 9% 12% 9% 1	1812
	21st to end	Day	64% 4 7% 14% 10% 2	1650
March	1st to 10th	Day	65% 5% 6% 9% 12% 2	4479
	11th to 20th	Day	57% 5% 14% 12% 11% 2	4812
	21st to end	Day	54% 4 7% 16% 17% 2	5878
April	1st to 10th	Day	56% 4 8% 17% 12% 3	2400
	11th to 20th	Day	54% 5% 7% 20% 12% 2	2374
	21st to end	Day	57% 5% 8% 15% 12% 3	2560
May	1st to 10th	Day	61% 7% 7% 17% 7% 1	6357
	11th to 20th	Day	52% 7% 15% 16% 9% 1	6322
	21st to end	Day	54% 6% 10% 16% 12% 1	6949
June	1st to 10th	Day	55% 6% 9% 17% 11% 1	2786
	11th to 20th	Day	47% 8% 12% 20% 11% 3	2660
	21st to end	Day	57% 7% 10% 17% 8%	2660
July	1st to 10th	Day	47% 7% 15% 24% 7%	4370
	11th to 20th	Day	46% 8% 16% 24% 6%	4365
	21st to end	Day	39% 7% 15% 31% 8%	4661
August	1st to 10th	Day	50% 7% 16% 21% 6%	4409
	11th to 20th	Day	45% 7% 16% 27% 5%	4060
	21st to end	Day	44% 7% 15% 22% 12% 1	4180
September	1st to 10th	Day	43% 6% 13% 26% 12%	2280
	11th to 20th	Day	39% 6% 13% 30% 11% 1	2236
	21st to end	Day	45% 6% 13% 25% 10%	2090
October	1st to 10th	Day	50% 5% 12% 27% 6%	3165
	11th to 20th	Day	52% 7% 12% 22% 6%	3119
	21st to end	Day	52% 6% 20% 14% 7% 1	3081
November	1st to 10th	Day	53% 6% 13% 19% 7% 1	1761
	11th to 20th	Day	63% 5% 11% 12% 9% 1	1650
	21st to end	Day	67% 7% 11% 11% 4 1	1600
December	1st to 10th	Day	74% 6% 7% 10% 3	2600
	11th to 20th	Day	67% 5% 7% 15% 6%	2986
	21st to end	Day	65% 5% 8% 15% 8%	3960
January	1st to 10th	Night	71% 6% 9% 11% 3	6349
	11th to 20th	Night	70% 7% 8% 11% 4 1	6360
	21st to end	Night	66% 5% 8% 8% 11% 1	6996
February	1st to 10th	Night	62% 5% 8% 17% 7% 1	2609
	11th to 20th	Night	62% 5% 9% 16% 8% 1	2483
	21st to end	Night	58% 6% 8% 17% 10% 2	1980
March	1st to 10th	Night	62% 4 8% 11% 12% 3	5374
	11th to 20th	Night	53% 8% 12% 15% 9% 3	5146
	21st to end	Night	56% 5% 7% 19% 10% 3	5040
April	1st to 10th	Night	55% 4 8% 17% 13% 2	2000
	11th to 20th	Night	50% 5% 9% 20% 14% 2	1975
	21st to end	Night	52% 5% 9% 19% 12% 3	1855
May	1st to 10th	Night	56% 7% 11% 18% 7% 2	3600
	11th to 20th	Night	48% 8% 15% 17% 10% 2	3576
	21st to end	Night	54% 7% 9% 17% 12% 1	3927
June	1st to 10th	Night	49% 8% 11% 21% 10%	1598
	11th to 20th	Night	44% 7% 11% 23% 13% 2	1520
	21st to end	Night	49% 7% 11% 20% 12% 1	1520
July	1st to 10th	Night	48% 5% 18% 19% 9%	2480
	11th to 20th	Night	42% 7% 18% 27% 6%	2472
	21st to end	Night	35% 7% 19% 28% 11%	2651
August	1st to 10th	Night	44% 7% 15% 26% 7%	2496
	11th to 20th	Night	43% 7% 19% 26% 6%	2799
	21st to end	Night	42% 7% 15% 23% 11% 2	3476
September	1st to 10th	Night	38% 6% 14% 28% 13% 1	1900
	11th to 20th	Night	37% 5% 15% 32% 10% 1	1943
	21st to end	Night	42% 5% 13% 29% 11%	2090
October	1st to 10th	Night	45% 6% 15% 24% 10% 1	3791
	11th to 20th	Night	52% 5% 13% 22% 8%	3800
	21st to end	Night	53% 5% 17% 16% 9% 1	4467
November	1st to 10th	Night	55% 7% 13% 19% 6%	2537
	11th to 20th	Night	58% 6% 11% 15% 8% 2	2745
	21st to end	Night	67% 5% 10% 13% 5%	2800
December	1st to 10th	Night	72% 6% 10% 9% 3	4580
	11th to 20th	Night	67% 4 5% 14% 9% 1	5208
	21st to end	Night	65% 4 5% 12% 13%	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	25% 22% 38% 12% 4	200
	11th to 20th	Day	28% 24% 33% 11% 5% 1	200
	21st to end	Day	24% 23% 36% 12% 5%	220
February	1st to 10th	Day	36% 30% 27% 6% 1	295
	11th to 20th	Day	29% 29% 34% 7% 1	350
	21st to end	Day	33% 23% 30% 12% 2	414
March	1st to 10th	Day	29% 21% 34% 13% 2 1	500
	11th to 20th	Day	29% 23% 31% 13% 3	500
	21st to end	Day	33% 22% 24% 15% 5% 1	550
April	1st to 10th	Day	34% 21% 28% 13% 3 1	500
	11th to 20th	Day	36% 20% 23% 13% 8% 2	492
	21st to end	Day	34% 19% 31% 12% 4 1	500
May	1st to 10th	Day	30% 20% 30% 14% 5% 1	503
	11th to 20th	Day	25% 17% 33% 18% 5% 1	527
	21st to end	Day	26% 17% 33% 18% 5%	573
June	1st to 10th	Day	27% 16% 35% 16% 6%	526
	11th to 20th	Day	20% 18% 32% 20% 9% 1	490
	21st to end	Day	27% 14% 32% 17% 7% 2	490
July	1st to 10th	Day	18% 11% 33% 26% 9% 2	485
	11th to 20th	Day	17% 13% 35% 26% 8% 1	484
	21st to end	Day	16% 10% 37% 27% 8% 2	532
August	1st to 10th	Day	19% 13% 27% 30% 10% 2	486
	11th to 20th	Day	18% 9% 26% 28% 13% 6%	479
	21st to end	Day	19% 16% 27% 22% 11% 6%	539
September	1st to 10th	Day	22% 10% 31% 20% 12% 5%	490
	11th to 20th	Day	16% 12% 26% 22% 16% 8%	490
	21st to end	Day	21% 17% 23% 23% 10% 6%	490
October	1st to 10th	Day	28% 17% 22% 19% 12% 2	490
	11th to 20th	Day	31% 21% 26% 14% 6% 1	485
	21st to end	Day	36% 14% 31% 15% 4	429
November	1st to 10th	Day	36% 16% 25% 18% 4 1	395
	11th to 20th	Day	30% 24% 28% 14% 4	250
	21st to end	Day	35% 25% 28% 12% 1	200
December	1st to 10th	Day	36% 18% 28% 15% 4	200
	11th to 20th	Day	34% 27% 28% 9% 1 1	200
	21st to end	Day	34% 22% 28% 12% 3 1	220
January	1st to 10th	Night	29% 17% 36% 16% 3	800
	11th to 20th	Night	29% 20% 29% 17% 6%	800
	21st to end	Night	18% 19% 36% 20% 5% 1	880
February	1st to 10th	Night	22% 22% 33% 19% 4 1	800
	11th to 20th	Night	23% 22% 34% 17% 3	783
	21st to end	Night	25% 17% 33% 22% 3	660
March	1st to 10th	Night	26% 18% 33% 20% 3	800
	11th to 20th	Night	25% 21% 32% 18% 3 1	800
	21st to end	Night	27% 16% 30% 21% 6% 1	880
April	1st to 10th	Night	30% 17% 27% 22% 4	800
	11th to 20th	Night	28% 15% 30% 21% 6%	791
	21st to end	Night	27% 17% 28% 19% 8%	655
May	1st to 10th	Night	26% 12% 30% 23% 8% 1	400
	11th to 20th	Night	22% 11% 30% 27% 9% 1	394
	21st to end	Night	19% 12% 36% 24% 8%	433
June	1st to 10th	Night	22% 10% 32% 28% 8%	400
	11th to 20th	Night	21% 7% 26% 33% 12% 1	380
	21st to end	Night	22% 7% 21% 35% 14% 1	380
July	1st to 10th	Night	14% 8% 24% 38% 15% 1	370
	11th to 20th	Night	14% 6% 24% 38% 17% 1	368
	21st to end	Night	13% 7% 25% 39% 15% 1	416
August	1st to 10th	Night	14% 4 19% 46% 17% 1	378
	11th to 20th	Night	14% 5% 21% 34% 24% 2	551
	21st to end	Night	18% 7% 23% 33% 16% 2	836
September	1st to 10th	Night	18% 6% 27% 33% 14% 1	760
	11th to 20th	Night	20% 6% 20% 31% 21% 1	760
	21st to end	Night	22% 9% 25% 25% 17% 2	760
October	1st to 10th	Night	23% 13% 25% 26% 12% 1	760
	11th to 20th	Night	30% 12% 26% 23% 7% 1	760
	21st to end	Night	31% 13% 28% 23% 5%	836
November	1st to 10th	Night	29% 15% 26% 23% 6% 1	780
	11th to 20th	Night	27% 18% 31% 15% 8% 1	800
	21st to end	Night	33% 19% 25% 17% 5%	800
December	1st to 10th	Night	30% 20% 27% 18% 5% 1	800
	11th to 20th	Night	32% 21% 26% 17% 4 1	800
	21st to end	Night	36% 16% 27% 18% 3	880

Page generated at 2025-06-07 10:28:18. Version ac07

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