York River State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the York River State Park clear sky chart from 2005-11-06 to 2026-04-02.

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 5% 4 6% 8% 42%	131211
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	130583
Transparency	Day	25% 38% 8% 15% 12% 3	134804
	Night	25% 36% 7% 16% 12% 3	135060
Seeing	Day	19% 14% 30% 23% 11% 3	16967
	Night	18% 11% 29% 28% 11% 1	26659

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	13% 4 2 5% 3 3 3 3 5% 5% 54%	4961
	11th to 20th	Day	12% 6% 2 5% 3 3 3 3 4 5% 55%	5280
	21st to end	Day	14% 8% 3 6% 4 4 4 3 5% 5% 45%	5780
February	1st to 10th	Day	12% 8% 3 6% 4 4 5% 4 5% 8% 41%	1890
	11th to 20th	Day	20% 7% 3 5% 4 3 3 3 4 5% 44%	1971
	21st to end	Day	16% 8% 2 8% 4 5% 4 4 5% 5% 39%	1730
March	1st to 10th	Day	18% 7% 3 7% 4 4 4 4 5% 6% 39%	4925
	11th to 20th	Day	10% 6% 2 6% 4 3 4 3 5% 7% 52%	5090
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 6% 45%	5720
April	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 4 8% 49%	2412
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 4 7% 48%	2374
	21st to end	Day	13% 6% 2 6% 5% 4 5% 3 6% 7% 44%	2400
May	1st to 10th	Day	14% 7% 3 7% 5% 5% 6% 4 6% 10% 33%	5512
	11th to 20th	Day	18% 7% 2 8% 5% 4 5% 4 7% 9% 32%	6061
	21st to end	Day	14% 6% 2 5% 4 4 4 4 6% 9% 42%	6934
June	1st to 10th	Day	10% 5% 2 6% 5% 4 6% 5% 7% 10% 42%	2800
	11th to 20th	Day	11% 6% 1 6% 5% 5% 5% 4 7% 9% 42%	2800
	21st to end	Day	7% 5% 2 6% 6% 5% 6% 5% 8% 11% 41%	2800
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	6360
	11th to 20th	Day	11% 7% 3 8% 6% 6% 6% 6% 8% 12% 27%	6003
	21st to end	Day	7% 5% 2 7% 5% 6% 6% 6% 9% 12% 36%	6424
August	1st to 10th	Day	14% 9% 3 7% 5% 6% 6% 5% 8% 10% 26%	4234
	11th to 20th	Day	10% 5% 3 7% 6% 6% 6% 5% 7% 10% 34%	3881
	21st to end	Day	9% 4 2 5% 4 3 4 4 7% 11% 48%	4312
September	1st to 10th	Day	13% 5% 2 5% 4 4 5% 4 6% 8% 43%	2400
	11th to 20th	Day	13% 7% 2 7% 4 4 5% 4 6% 7% 40%	2400
	21st to end	Day	20% 8% 2 6% 4 5% 4 3 5% 6% 35%	2124
October	1st to 10th	Day	15% 6% 2 6% 4 4 3 3 5% 6% 45%	2000
	11th to 20th	Day	15% 5% 2 5% 3 2 2 3 4 5% 51%	2000
	21st to end	Day	12% 5% 2 5% 4 4 3 3 4 5% 53%	2200
November	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 5% 6% 52%	1971
	11th to 20th	Day	13% 7% 2 5% 3 3 3 3 5% 6% 51%	1890
	21st to end	Day	14% 5% 1 5% 4 3 3 3 5% 6% 51%	1774
December	1st to 10th	Day	11% 6% 2 5% 3 3 4 2 5% 6% 53%	2680
	11th to 20th	Day	18% 8% 2 5% 3 2 3 2 3 5% 47%	3070
	21st to end	Day	12% 6% 2 5% 4 3 4 3 5% 7% 48%	4048
January	1st to 10th	Night	11% 6% 2 5% 4 3 4 4 4 6% 49%	7915
	11th to 20th	Night	12% 6% 2 6% 3 3 3 2 4 4 55%	7680
	21st to end	Night	16% 6% 2 5% 3 3 3 2 4 5% 49%	8401
February	1st to 10th	Night	16% 8% 2 5% 3 4 4 4 5% 6% 42%	2730
	11th to 20th	Night	21% 7% 2 4 4 3 4 3 4 6% 41%	2534
	21st to end	Night	16% 8% 2 6% 4 3 3 3 5% 5% 46%	2076
March	1st to 10th	Night	18% 7% 2 6% 3 3 3 3 4 6% 44%	5132
	11th to 20th	Night	13% 6% 3 5% 4 4 4 3 4 6% 49%	5090
	21st to end	Night	14% 7% 2 5% 4 4 3 3 5% 6% 47%	5453
April	1st to 10th	Night	14% 7% 3 5% 5% 4 4 4 6% 8% 42%	2011
	11th to 20th	Night	12% 7% 2 5% 4 3 4 2 5% 9% 48%	1975
	21st to end	Night	14% 6% 2 5% 4 3 4 4 6% 8% 44%	2000
May	1st to 10th	Night	15% 8% 3 7% 5% 6% 5% 4 6% 9% 32%	4412
	11th to 20th	Night	21% 8% 3 7% 4 5% 4 3 5% 7% 34%	3817
	21st to end	Night	17% 6% 3 7% 4 4 4 3 5% 8% 39%	3920
June	1st to 10th	Night	11% 5% 2 6% 5% 6% 5% 5% 8% 9% 39%	1600
	11th to 20th	Night	10% 6% 2 6% 4 5% 6% 5% 6% 10% 39%	1600
	21st to end	Night	7% 6% 2 7% 5% 5% 5% 5% 5% 11% 43%	1600
July	1st to 10th	Night	9% 5% 2 7% 5% 6% 6% 5% 8% 10% 37%	3598
	11th to 20th	Night	11% 7% 3 7% 6% 5% 6% 5% 9% 12% 29%	3936
	21st to end	Night	10% 7% 3 7% 6% 5% 6% 5% 7% 9% 36%	4427
August	1st to 10th	Night	17% 9% 3 7% 5% 5% 4 4 7% 9% 30%	2915
	11th to 20th	Night	8% 7% 3 7% 5% 4 5% 4 6% 10% 40%	3133
	21st to end	Night	9% 5% 2 5% 4 4 4 4 6% 9% 48%	3586
September	1st to 10th	Night	12% 5% 2 6% 4 5% 4 3 6% 8% 45%	2000
	11th to 20th	Night	12% 6% 2 7% 3 4 5% 5% 6% 7% 44%	2000
	21st to end	Night	21% 8% 3 7% 4 3 4 3 6% 6% 35%	2270
October	1st to 10th	Night	14% 5% 2 6% 3 4 4 3 5% 7% 47%	2400
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 4 6% 50%	2400
	21st to end	Night	15% 5% 2 4 4 3 3 2 4 6% 53%	2640
November	1st to 10th	Night	13% 6% 2 5% 3 4 4 4 4 5% 50%	2545
	11th to 20th	Night	15% 6% 2 5% 3 4 4 3 5% 6% 48%	2730
	21st to end	Night	12% 5% 2 5% 4 4 4 3 5% 6% 51%	2842
December	1st to 10th	Night	11% 5% 2 4 3 4 4 3 4 6% 54%	4720
	11th to 20th	Night	18% 8% 3 6% 4 4 4 4 5% 6% 40%	5345
	21st to end	Night	12% 6% 2 6% 3 3 4 3 5% 6% 48%	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	26% 21% 8% 21% 16% 8%	4962
	11th to 20th	Day	26% 21% 3 15% 25% 10%	5280
	21st to end	Day	34% 23% 6% 23% 13% 2	5780
February	1st to 10th	Day	18% 59% 4 12% 6% 1	1890
	11th to 20th	Day	21% 58% 3 8% 8% 2	1980
	21st to end	Day	21% 58% 4 8% 6% 2	1730
March	1st to 10th	Day	37% 29% 6% 10% 13% 5%	4925
	11th to 20th	Day	22% 29% 6% 18% 18% 7%	5090
	21st to end	Day	28% 30% 9% 12% 16% 5%	5720
April	1st to 10th	Day	16% 62% 4 9% 7% 2	2412
	11th to 20th	Day	14% 62% 5% 10% 8% 2	2374
	21st to end	Day	16% 62% 5% 10% 5% 2	2400
May	1st to 10th	Day	33% 34% 12% 14% 6% 1	5544
	11th to 20th	Day	35% 32% 11% 15% 6%	6065
	21st to end	Day	28% 31% 10% 17% 12% 2	6949
June	1st to 10th	Day	16% 63% 6% 9% 6% 1	2800
	11th to 20th	Day	13% 62% 7% 10% 7%	2800
	21st to end	Day	12% 61% 7% 13% 7%	2800
July	1st to 10th	Day	27% 31% 16% 15% 9% 2	6290
	11th to 20th	Day	39% 31% 18% 9% 2	5931
	21st to end	Day	29% 29% 10% 16% 15%	6240
August	1st to 10th	Day	36% 37% 12% 12% 3	4221
	11th to 20th	Day	27% 39% 9% 17% 8%	3918
	21st to end	Day	16% 37% 10% 21% 15% 1	4312
September	1st to 10th	Day	18% 58% 7% 11% 6% 1	2400
	11th to 20th	Day	20% 57% 5% 9% 7% 2	2400
	21st to end	Day	26% 56% 4 10% 3 1	2124
October	1st to 10th	Day	22% 37% 5% 14% 19% 3	3260
	11th to 20th	Day	13% 36% 3 13% 29% 7%	3240
	21st to end	Day	10% 35% 6% 20% 20% 10%	3538
November	1st to 10th	Day	18% 52% 5% 13% 9% 2	1971
	11th to 20th	Day	17% 53% 4 10% 12% 3	1890
	21st to end	Day	17% 53% 3 14% 10% 2	1774
December	1st to 10th	Day	22% 35% 4 21% 13% 5%	2680
	11th to 20th	Day	30% 30% 9% 14% 13% 4	3066
	21st to end	Day	26% 27% 10% 23% 13% 1	4048
January	1st to 10th	Night	30% 21% 8% 17% 18% 6%	7916
	11th to 20th	Night	26% 20% 2 19% 24% 8%	7680
	21st to end	Night	31% 22% 4 23% 16% 4	8401
February	1st to 10th	Night	18% 58% 6% 10% 7% 1	2730
	11th to 20th	Night	20% 58% 3 9% 8% 2	2536
	21st to end	Night	17% 58% 3 10% 7% 3	2076
March	1st to 10th	Night	33% 29% 5% 13% 13% 7%	5132
	11th to 20th	Night	24% 28% 5% 15% 19% 9%	5090
	21st to end	Night	29% 28% 5% 18% 14% 5%	5453
April	1st to 10th	Night	18% 62% 4 6% 9% 1	2011
	11th to 20th	Night	15% 62% 4 10% 7% 1	1975
	21st to end	Night	16% 62% 5% 9% 7% 1	2000
May	1st to 10th	Night	36% 33% 11% 14% 6%	4442
	11th to 20th	Night	36% 31% 9% 18% 5%	3818
	21st to end	Night	33% 31% 10% 17% 10%	3927
June	1st to 10th	Night	17% 62% 6% 7% 7% 1	1600
	11th to 20th	Night	14% 62% 7% 9% 8% 1	1600
	21st to end	Night	12% 61% 6% 12% 7% 2	1600
July	1st to 10th	Night	32% 33% 12% 13% 9% 1	3560
	11th to 20th	Night	38% 33% 17% 11% 1	3888
	21st to end	Night	33% 29% 8% 17% 12%	4297
August	1st to 10th	Night	37% 37% 10% 13% 2	2904
	11th to 20th	Night	23% 39% 13% 18% 8%	3162
	21st to end	Night	18% 36% 5% 20% 20% 1	3586
September	1st to 10th	Night	16% 59% 9% 10% 5% 2	2000
	11th to 20th	Night	18% 57% 4 11% 8% 1	2000
	21st to end	Night	26% 58% 5% 8% 3	2270
October	1st to 10th	Night	22% 36% 7% 15% 16% 3	3920
	11th to 20th	Night	13% 35% 4 15% 25% 9%	3899
	21st to end	Night	10% 34% 3 24% 18% 11%	4252
November	1st to 10th	Night	21% 53% 4 11% 10% 2	2545
	11th to 20th	Night	17% 54% 3 10% 10% 6%	2730
	21st to end	Night	15% 54% 4 13% 12% 2	2842
December	1st to 10th	Night	20% 34% 5% 20% 18% 4	4720
	11th to 20th	Night	35% 31% 8% 12% 9% 5%	5348
	21st to end	Night	27% 27% 10% 23% 12% 1	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	29% 28% 31% 10% 2	275
	11th to 20th	Day	29% 31% 30% 9% 1	330
	21st to end	Day	26% 27% 39% 7% 1	363
February	1st to 10th	Day	25% 28% 35% 8% 3	330
	11th to 20th	Day	35% 25% 30% 9% 1	459
	21st to end	Day	32% 19% 34% 12% 4	446
March	1st to 10th	Day	25% 26% 32% 13% 4	540
	11th to 20th	Day	27% 24% 31% 13% 4 1	540
	21st to end	Day	23% 24% 37% 13% 4	594
April	1st to 10th	Day	24% 27% 31% 13% 5% 1	504
	11th to 20th	Day	18% 18% 36% 20% 5% 3	492
	21st to end	Day	19% 16% 39% 20% 5% 1	500
May	1st to 10th	Day	18% 17% 40% 17% 6% 1	503
	11th to 20th	Day	21% 13% 37% 20% 8% 2	527
	21st to end	Day	21% 8% 28% 26% 13% 3	573
June	1st to 10th	Day	14% 8% 33% 29% 14% 3	530
	11th to 20th	Day	15% 7% 26% 33% 15% 5%	530
	21st to end	Day	9% 5% 28% 37% 17% 5%	530
July	1st to 10th	Day	10% 4 21% 38% 23% 5%	525
	11th to 20th	Day	8% 2 20% 40% 24% 6%	524
	21st to end	Day	9% 3 27% 34% 20% 7%	576
August	1st to 10th	Day	13% 6% 25% 33% 18% 6%	526
	11th to 20th	Day	12% 6% 21% 34% 21% 6%	519
	21st to end	Day	10% 3 25% 30% 23% 10%	583
September	1st to 10th	Day	15% 4 26% 27% 17% 11%	530
	11th to 20th	Day	16% 7% 25% 28% 16% 9%	530
	21st to end	Day	30% 9% 26% 22% 9% 4	530
October	1st to 10th	Day	19% 7% 25% 25% 15% 9%	530
	11th to 20th	Day	21% 14% 25% 27% 12% 3	530
	21st to end	Day	18% 16% 32% 20% 10% 4	583
November	1st to 10th	Day	18% 12% 33% 23% 10% 3	489
	11th to 20th	Day	21% 19% 31% 20% 9% 1	430
	21st to end	Day	23% 18% 30% 18% 10% 1	314
December	1st to 10th	Day	27% 20% 36% 13% 4	220
	11th to 20th	Day	26% 21% 30% 20% 4	220
	21st to end	Day	24% 23% 31% 15% 7%	242
January	1st to 10th	Night	20% 22% 35% 21% 3	840
	11th to 20th	Night	22% 20% 33% 20% 5%	840
	21st to end	Night	23% 17% 36% 22% 2	924
February	1st to 10th	Night	22% 16% 40% 19% 3	840
	11th to 20th	Night	29% 21% 34% 14% 2	823
	21st to end	Night	25% 19% 36% 17% 3	692
March	1st to 10th	Night	22% 15% 35% 23% 4	840
	11th to 20th	Night	23% 18% 33% 20% 4 1	840
	21st to end	Night	20% 16% 41% 19% 4	924
April	1st to 10th	Night	20% 20% 39% 18% 3	805
	11th to 20th	Night	18% 12% 40% 23% 6% 1	791
	21st to end	Night	18% 12% 35% 29% 6%	800
May	1st to 10th	Night	18% 13% 37% 25% 6% 1	774
	11th to 20th	Night	17% 8% 29% 36% 9% 1	506
	21st to end	Night	16% 3 22% 40% 17% 2	433
June	1st to 10th	Night	11% 2 26% 43% 15% 2	400
	11th to 20th	Night	11% 4 23% 42% 18% 2	400
	21st to end	Night	7% 4 23% 42% 21% 3	400
July	1st to 10th	Night	11% 3 19% 42% 24% 1	390
	11th to 20th	Night	7% 2 16% 44% 28% 4	529
	21st to end	Night	7% 2 22% 38% 28% 3	657
August	1st to 10th	Night	11% 3 15% 41% 26% 4	597
	11th to 20th	Night	12% 3 15% 42% 25% 3	750
	21st to end	Night	10% 4 16% 36% 29% 6%	880
September	1st to 10th	Night	14% 4 17% 38% 23% 4	800
	11th to 20th	Night	15% 3 17% 36% 24% 4	800
	21st to end	Night	25% 5% 21% 30% 16% 2	800
October	1st to 10th	Night	17% 6% 21% 30% 21% 5%	800
	11th to 20th	Night	19% 11% 27% 28% 13% 2	800
	21st to end	Night	18% 12% 28% 26% 13% 2	880
November	1st to 10th	Night	17% 10% 29% 29% 13% 2	820
	11th to 20th	Night	19% 14% 32% 28% 7%	840
	21st to end	Night	15% 15% 30% 30% 9% 1	840
December	1st to 10th	Night	23% 13% 34% 21% 8% 1	840
	11th to 20th	Night	20% 12% 38% 25% 5%	840
	21st to end	Night	21% 15% 31% 28% 6%	924

Page generated at 2026-03-30 02:18:46. Version ac07

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