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 2026-10-14.

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	17% 12% 4 9% 5% 5% 5% 4 5% 7% 29%	149973
	Night	16% 13% 5% 10% 5% 4 4 3 5% 6% 29%	141743
Transparency	Day	54% 6% 11% 19% 9% 1	153406
	Night	56% 6% 11% 18% 9% 1	146399
Seeing	Day	26% 18% 30% 18% 6% 2	16937
	Night	25% 14% 28% 24% 8% 1	25859

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	13% 19% 6% 18% 8% 6% 4 2 4 4 16%	4661
	11th to 20th	Day	11% 23% 8% 17% 9% 4 3 2 3 5% 16%	4650
	21st to end	Day	7% 15% 9% 16% 7% 7% 6% 3 5% 6% 19%	5124
February	1st to 10th	Day	11% 18% 5% 14% 8% 5% 4 3 3 6% 23%	1880
	11th to 20th	Day	12% 19% 5% 12% 6% 5% 4 3 4 5% 25%	1896
	21st to end	Day	16% 17% 5% 10% 8% 6% 3 3 4 5% 23%	1730
March	1st to 10th	Day	18% 16% 5% 12% 6% 5% 4 3 4 5% 22%	4576
	11th to 20th	Day	12% 16% 6% 11% 6% 4 4 3 4 6% 28%	4918
	21st to end	Day	18% 13% 3 9% 5% 4 4 3 4 5% 32%	6008
April	1st to 10th	Day	21% 15% 3 8% 4 4 3 3 4 5% 30%	2520
	11th to 20th	Day	23% 11% 4 6% 4 3 3 3 4 5% 32%	2494
	21st to end	Day	22% 12% 3 7% 5% 4 4 3 5% 4 30%	2688
May	1st to 10th	Day	22% 12% 3 8% 5% 5% 5% 4 6% 6% 22%	8237
	11th to 20th	Day	19% 9% 3 8% 5% 5% 5% 4 6% 8% 30%	8221
	21st to end	Day	21% 10% 3 7% 4 4 5% 3 5% 7% 32%	9060
June	1st to 10th	Day	23% 9% 3 7% 5% 4 5% 4 6% 7% 27%	2940
	11th to 20th	Day	17% 7% 3 6% 5% 5% 6% 4 6% 7% 33%	2940
	21st to end	Day	22% 8% 2 7% 6% 5% 6% 4 6% 7% 28%	2940
July	1st to 10th	Day	14% 7% 3 7% 5% 5% 6% 4 6% 9% 34%	8280
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 7% 9% 41%	8280
	21st to end	Day	12% 7% 2 6% 4 4 5% 4 7% 10% 39%	8993
August	1st to 10th	Day	18% 7% 3 8% 6% 5% 5% 4 6% 8% 30%	6483
	11th to 20th	Day	16% 7% 2 7% 4 4 5% 4 7% 10% 33%	5911
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 4 6% 9% 35%	6080
September	1st to 10th	Day	18% 6% 1 5% 4 4 4 4 5% 7% 41%	2520
	11th to 20th	Day	15% 6% 2 5% 4 3 5% 3 5% 8% 46%	2471
	21st to end	Day	20% 8% 2 5% 4 3 4 3 5% 7% 39%	2310
October	1st to 10th	Day	21% 9% 3 6% 4 4 4 4 5% 7% 34%	2105
	11th to 20th	Day	24% 14% 3 8% 4 4 4 3 5% 5% 26%	2020
	21st to end	Day	25% 14% 3 8% 4 3 3 3 5% 5% 25%	1980
November	1st to 10th	Day	17% 15% 3 9% 4 4 4 3 5% 6% 29%	1851
	11th to 20th	Day	20% 17% 5% 10% 6% 4 4 3 5% 5% 22%	1732
	21st to end	Day	23% 17% 8% 10% 6% 4 4 3 4 4 19%	1680
December	1st to 10th	Day	19% 23% 9% 10% 6% 4 4 3 4 4 13%	2680
	11th to 20th	Day	18% 20% 6% 13% 5% 4 4 3 3 3 21%	3066
	21st to end	Day	19% 19% 5% 10% 5% 5% 4 3 4 4 22%	4048
January	1st to 10th	Night	12% 22% 9% 14% 6% 5% 4 3 4 4 18%	8234
	11th to 20th	Night	11% 21% 9% 19% 10% 5% 3 2 3 4 14%	8235
	21st to end	Night	8% 15% 7% 16% 8% 8% 6% 4 5% 5% 18%	9057
February	1st to 10th	Night	11% 16% 7% 13% 7% 5% 4 4 5% 4 24%	2739
	11th to 20th	Night	12% 17% 6% 13% 6% 5% 4 3 4 5% 25%	2608
	21st to end	Night	14% 16% 6% 11% 5% 5% 4 3 5% 5% 26%	2076
March	1st to 10th	Night	20% 15% 5% 11% 5% 4 4 2 3 4 27%	5484
	11th to 20th	Night	14% 15% 5% 9% 5% 5% 4 3 5% 7% 28%	5259
	21st to end	Night	17% 14% 5% 10% 6% 4 4 3 5% 5% 28%	5152
April	1st to 10th	Night	23% 13% 4 6% 5% 3 3 3 4 6% 30%	2100
	11th to 20th	Night	20% 10% 4 7% 5% 3 4 3 5% 5% 35%	2075
	21st to end	Night	20% 11% 3 7% 4 4 4 3 5% 5% 34%	1948
May	1st to 10th	Night	18% 10% 4 9% 6% 5% 5% 4 5% 7% 26%	4660
	11th to 20th	Night	19% 8% 3 8% 4 4 5% 4 5% 7% 33%	4639
	21st to end	Night	21% 10% 3 7% 4 4 5% 4 5% 6% 31%	5114
June	1st to 10th	Night	18% 9% 2 6% 5% 4 5% 4 7% 7% 32%	1680
	11th to 20th	Night	18% 8% 3 6% 5% 3 5% 3 5% 8% 37%	1680
	21st to end	Night	20% 8% 2 6% 4 5% 4 4 5% 6% 36%	1680
July	1st to 10th	Night	14% 7% 3 7% 5% 5% 5% 4 7% 9% 34%	4680
	11th to 20th	Night	14% 7% 3 6% 3 3 4 3 5% 8% 45%	4680
	21st to end	Night	13% 5% 2 7% 5% 5% 5% 4 6% 8% 40%	5084
August	1st to 10th	Night	17% 8% 3 7% 4 4 5% 4 6% 7% 35%	3666
	11th to 20th	Night	16% 7% 2 6% 4 3 4 3 6% 7% 41%	4085
	21st to end	Night	17% 8% 3 6% 4 4 4 3 5% 7% 40%	5045
September	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 5% 7% 45%	2100
	11th to 20th	Night	15% 6% 2 5% 3 4 3 3 4 7% 48%	2148
	21st to end	Night	18% 7% 2 4 4 4 4 3 4 7% 44%	2310
October	1st to 10th	Night	18% 8% 3 7% 4 4 3 3 5% 5% 39%	2511
	11th to 20th	Night	23% 11% 3 7% 5% 4 4 3 4 5% 31%	2447
	21st to end	Night	24% 13% 4 8% 4 3 4 3 4 6% 28%	2860
November	1st to 10th	Night	21% 13% 4 7% 5% 4 4 3 6% 6% 28%	2667
	11th to 20th	Night	18% 14% 6% 10% 5% 4 5% 3 4 5% 26%	2882
	21st to end	Night	20% 19% 6% 10% 5% 4 4 3 4 4 20%	2940
December	1st to 10th	Night	18% 19% 7% 15% 7% 4 4 3 4 5% 14%	4720
	11th to 20th	Night	23% 17% 6% 12% 6% 4 3 2 3 3 23%	5348
	21st to end	Night	21% 21% 4 10% 5% 3 3 2 3 4 23%	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	70% 6% 7% 13% 4	4662
	11th to 20th	Day	72% 4 4 10% 9% 1	4680
	21st to end	Day	65% 5% 8% 10% 11% 1	5124
February	1st to 10th	Day	64% 3 7% 17% 7% 2	1880
	11th to 20th	Day	62% 5% 9% 14% 9% 1	1905
	21st to end	Day	64% 4 7% 14% 10% 2	1730
March	1st to 10th	Day	65% 5% 7% 9% 12% 2	4579
	11th to 20th	Day	57% 5% 14% 12% 11% 2	4918
	21st to end	Day	54% 4 7% 16% 17% 2	6008
April	1st to 10th	Day	57% 4 8% 17% 11% 3	2520
	11th to 20th	Day	54% 5% 7% 19% 12% 2	2494
	21st to end	Day	57% 5% 7% 16% 12% 3	2688
May	1st to 10th	Day	59% 7% 8% 18% 8% 1	8249
	11th to 20th	Day	51% 7% 14% 19% 8% 1	8225
	21st to end	Day	52% 6% 10% 19% 12% 1	9061
June	1st to 10th	Day	55% 6% 9% 19% 11% 1	2940
	11th to 20th	Day	48% 7% 11% 21% 10% 3	2940
	21st to end	Day	55% 7% 10% 19% 8% 1	2940
July	1st to 10th	Day	46% 7% 14% 23% 10%	8210
	11th to 20th	Day	38% 6% 16% 30% 10%	8205
	21st to end	Day	40% 7% 17% 29% 7%	8795
August	1st to 10th	Day	51% 6% 15% 19% 7% 1	6469
	11th to 20th	Day	44% 8% 17% 25% 6%	5953
	21st to end	Day	44% 7% 17% 22% 10% 1	6080
September	1st to 10th	Day	42% 6% 14% 27% 12%	2520
	11th to 20th	Day	37% 6% 13% 32% 11% 1	2471
	21st to end	Day	45% 6% 13% 24% 12%	2310
October	1st to 10th	Day	49% 5% 11% 27% 7%	3365
	11th to 20th	Day	52% 7% 12% 22% 6% 1	3248
	21st to end	Day	53% 6% 19% 15% 7% 1	3180
November	1st to 10th	Day	55% 6% 13% 19% 7% 1	1851
	11th to 20th	Day	63% 5% 11% 11% 9% 1	1732
	21st to end	Day	69% 6% 10% 10% 4 1	1680
December	1st to 10th	Day	74% 6% 7% 10% 3	2680
	11th to 20th	Day	67% 5% 7% 15% 6%	3066
	21st to end	Day	65% 5% 8% 15% 8%	4048
January	1st to 10th	Night	69% 6% 9% 13% 4	8234
	11th to 20th	Night	74% 6% 6% 9% 4 1	8280
	21st to end	Night	66% 5% 7% 10% 11% 1	9057
February	1st to 10th	Night	62% 5% 8% 17% 7% 1	2739
	11th to 20th	Night	61% 5% 9% 17% 8% 1	2610
	21st to end	Night	58% 6% 8% 17% 10% 2	2076
March	1st to 10th	Night	62% 5% 8% 11% 12% 3	5494
	11th to 20th	Night	53% 8% 12% 15% 9% 3	5260
	21st to end	Night	57% 5% 7% 19% 10% 3	5152
April	1st to 10th	Night	55% 4 8% 18% 13% 2	2100
	11th to 20th	Night	50% 5% 9% 20% 14% 1	2075
	21st to end	Night	52% 5% 8% 19% 13% 3	1948
May	1st to 10th	Night	55% 7% 12% 18% 7% 1	4666
	11th to 20th	Night	48% 8% 14% 20% 9% 1	4640
	21st to end	Night	53% 6% 9% 18% 13% 1	5115
June	1st to 10th	Night	48% 8% 11% 22% 10% 1	1680
	11th to 20th	Night	45% 7% 11% 22% 13% 2	1680
	21st to end	Night	48% 7% 12% 21% 12% 1	1680
July	1st to 10th	Night	45% 7% 17% 20% 10%	4640
	11th to 20th	Night	38% 6% 17% 31% 7%	4632
	21st to end	Night	40% 7% 16% 28% 9%	4967
August	1st to 10th	Night	47% 7% 15% 22% 8% 1	3656
	11th to 20th	Night	41% 7% 18% 27% 7%	4109
	21st to end	Night	43% 7% 16% 23% 10% 1	5045
September	1st to 10th	Night	38% 6% 15% 29% 12% 1	2100
	11th to 20th	Night	36% 5% 15% 33% 11% 1	2148
	21st to end	Night	41% 5% 12% 27% 14%	2310
October	1st to 10th	Night	45% 5% 14% 24% 11% 1	4031
	11th to 20th	Night	51% 5% 13% 22% 9%	3958
	21st to end	Night	52% 5% 16% 17% 9% 1	4610
November	1st to 10th	Night	55% 6% 13% 19% 6%	2667
	11th to 20th	Night	59% 6% 11% 15% 8% 1	2882
	21st to end	Night	68% 5% 10% 13% 4	2940
December	1st to 10th	Night	72% 6% 10% 10% 3	4720
	11th to 20th	Night	68% 4 5% 14% 9% 1	5348
	21st to end	Night	66% 4 5% 12% 13%	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	24% 21% 38% 13% 4	220
	11th to 20th	Day	29% 24% 32% 10% 5%	220
	21st to end	Day	22% 24% 35% 15% 4	242
February	1st to 10th	Day	33% 28% 29% 9% 1	325
	11th to 20th	Day	28% 29% 34% 8% 1	383
	21st to end	Day	32% 22% 32% 13% 2	446
March	1st to 10th	Day	29% 22% 34% 12% 2 1	540
	11th to 20th	Day	31% 22% 31% 13% 2	540
	21st to end	Day	33% 22% 25% 14% 5% 1	594
April	1st to 10th	Day	34% 21% 28% 13% 3 1	540
	11th to 20th	Day	36% 20% 22% 13% 7% 2	532
	21st to end	Day	34% 18% 31% 11% 4 1	540
May	1st to 10th	Day	30% 22% 29% 14% 5% 1	543
	11th to 20th	Day	26% 18% 32% 18% 5% 1	567
	21st to end	Day	27% 19% 32% 17% 5%	617
June	1st to 10th	Day	27% 16% 33% 17% 6% 1	570
	11th to 20th	Day	22% 17% 32% 20% 8% 1	570
	21st to end	Day	26% 14% 34% 16% 7% 2	570
July	1st to 10th	Day	18% 13% 33% 26% 8% 2	565
	11th to 20th	Day	17% 14% 35% 26% 7% 1	564
	21st to end	Day	16% 11% 36% 26% 8% 2	620
August	1st to 10th	Day	18% 12% 28% 31% 10% 2	566
	11th to 20th	Day	18% 10% 27% 28% 13% 5%	559
	21st to end	Day	19% 16% 26% 21% 12% 6%	627
September	1st to 10th	Day	23% 11% 31% 19% 11% 5%	570
	11th to 20th	Day	15% 12% 26% 22% 17% 8%	570
	21st to end	Day	20% 16% 24% 23% 11% 6%	570
October	1st to 10th	Day	28% 17% 22% 20% 11% 2	570
	11th to 20th	Day	30% 21% 27% 15% 6% 1	536
	21st to end	Day	35% 13% 31% 16% 5%	462
November	1st to 10th	Day	37% 16% 25% 17% 4 1	425
	11th to 20th	Day	30% 22% 29% 14% 5%	272
	21st to end	Day	36% 25% 26% 12% 1	220
December	1st to 10th	Day	34% 19% 28% 15% 4	220
	11th to 20th	Day	33% 26% 30% 9% 1 1	220
	21st to end	Day	33% 22% 29% 12% 3 1	242
January	1st to 10th	Night	28% 16% 36% 17% 3	840
	11th to 20th	Night	30% 20% 28% 16% 6%	840
	21st to end	Night	18% 19% 35% 21% 6% 1	924
February	1st to 10th	Night	21% 21% 32% 20% 5% 1	840
	11th to 20th	Night	23% 22% 34% 18% 3	823
	21st to end	Night	25% 17% 33% 22% 3	692
March	1st to 10th	Night	26% 17% 34% 20% 3	840
	11th to 20th	Night	26% 21% 32% 18% 3 1	840
	21st to end	Night	27% 16% 30% 21% 5% 1	924
April	1st to 10th	Night	31% 17% 28% 21% 3	840
	11th to 20th	Night	28% 15% 30% 21% 6%	831
	21st to end	Night	27% 17% 28% 19% 8%	688
May	1st to 10th	Night	25% 12% 30% 23% 8% 1	420
	11th to 20th	Night	22% 11% 30% 28% 8% 1	414
	21st to end	Night	19% 13% 36% 24% 8%	455
June	1st to 10th	Night	21% 9% 31% 30% 9%	420
	11th to 20th	Night	22% 8% 26% 32% 11% 1	420
	21st to end	Night	22% 7% 21% 36% 14%	420
July	1st to 10th	Night	14% 8% 24% 40% 14%	410
	11th to 20th	Night	15% 6% 24% 38% 16% 1	408
	21st to end	Night	13% 7% 24% 40% 15% 1	460
August	1st to 10th	Night	14% 4 18% 46% 17% 1	418
	11th to 20th	Night	14% 4 21% 36% 24% 2	611
	21st to end	Night	19% 7% 22% 34% 16% 2	924
September	1st to 10th	Night	18% 6% 27% 34% 14% 1	840
	11th to 20th	Night	19% 6% 20% 32% 21% 2	840
	21st to end	Night	21% 9% 24% 26% 18% 2	840
October	1st to 10th	Night	23% 13% 24% 27% 12% 1	840
	11th to 20th	Night	29% 12% 26% 24% 7% 1	813
	21st to end	Night	31% 12% 28% 24% 5%	880
November	1st to 10th	Night	30% 15% 26% 23% 6% 1	820
	11th to 20th	Night	28% 18% 31% 15% 8% 1	840
	21st to end	Night	33% 19% 26% 17% 5%	840
December	1st to 10th	Night	30% 20% 27% 18% 5%	840
	11th to 20th	Night	31% 21% 26% 16% 4 1	840
	21st to end	Night	36% 17% 27% 18% 3	924

Page generated at 2026-10-11 05:06:29. Version ac07

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