James River State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the James River State Park clear sky chart from 2005-11-06 to 2026-07-16.

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% 5% 2 6% 4 4 4 4 6% 8% 43%	137503
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 45%	135162
Transparency	Day	37% 7% 12% 22% 17% 4	141109
	Night	38% 6% 11% 22% 18% 5%	139628
Seeing	Day	19% 11% 25% 24% 15% 6%	17278
	Night	17% 8% 27% 32% 15% 2	27151

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	15% 6% 2 5% 3 3 4 3 4 6% 48%	5258
	11th to 20th	Day	11% 6% 2 6% 3 3 3 3 4 4 55%	5280
	21st to end	Day	15% 6% 3 7% 4 3 4 3 4 5% 47%	5780
February	1st to 10th	Day	12% 7% 2 6% 4 5% 5% 3 6% 8% 42%	1890
	11th to 20th	Day	20% 7% 1 5% 4 4 4 3 5% 6% 42%	1843
	21st to end	Day	18% 8% 3 6% 5% 4 4 3 5% 6% 39%	1824
March	1st to 10th	Day	17% 5% 2 6% 4 4 5% 4 5% 8% 40%	5045
	11th to 20th	Day	11% 5% 2 5% 4 3 3 3 5% 7% 51%	5090
	21st to end	Day	15% 6% 3 7% 5% 4 4 4 5% 6% 41%	5586
April	1st to 10th	Day	11% 6% 2 5% 3 4 5% 4 5% 7% 49%	2461
	11th to 20th	Day	12% 5% 2 5% 4 3 4 4 6% 7% 48%	2494
	21st to end	Day	15% 5% 2 6% 5% 5% 4 3 5% 7% 42%	2560
May	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 4 7% 10% 37%	7616
	11th to 20th	Day	18% 5% 2 6% 4 4 4 4 6% 8% 40%	7626
	21st to end	Day	19% 6% 3 5% 4 4 4 4 5% 8% 38%	8562
June	1st to 10th	Day	9% 4 2 5% 5% 5% 5% 4 7% 10% 45%	2940
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 5% 8% 12% 40%	2940
	21st to end	Day	8% 4 2 6% 5% 5% 5% 5% 7% 12% 42%	2907
July	1st to 10th	Day	7% 4 2 6% 5% 5% 5% 5% 8% 12% 41%	6030
	11th to 20th	Day	8% 5% 2 8% 6% 6% 6% 6% 9% 13% 30%	5965
	21st to end	Day	9% 5% 3 7% 5% 6% 5% 5% 8% 11% 36%	6424
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 5% 5% 8% 11% 31%	4234
	11th to 20th	Day	8% 4 2 7% 5% 6% 6% 5% 8% 11% 39%	4108
	21st to end	Day	8% 4 2 4 4 5% 4 4 7% 10% 48%	4312
September	1st to 10th	Day	10% 6% 2 6% 4 4 4 4 5% 8% 48%	2400
	11th to 20th	Day	13% 5% 3 5% 4 4 5% 4 6% 7% 44%	2287
	21st to end	Day	23% 7% 3 6% 4 3 4 4 5% 6% 36%	2147
October	1st to 10th	Day	14% 5% 2 6% 3 3 4 3 5% 6% 50%	2000
	11th to 20th	Day	15% 4 2 4 3 3 4 3 4 5% 53%	2000
	21st to end	Day	13% 5% 2 5% 3 3 3 3 4 6% 53%	2200
November	1st to 10th	Day	12% 4 2 4 3 4 3 3 5% 6% 55%	2057
	11th to 20th	Day	13% 6% 2 5% 3 3 4 2 5% 6% 51%	1924
	21st to end	Day	14% 5% 2 4 4 3 4 3 5% 6% 50%	1680
December	1st to 10th	Day	13% 6% 2 5% 3 3 4 3 5% 6% 49%	2680
	11th to 20th	Day	20% 5% 2 4 2 3 3 3 4 6% 47%	3070
	21st to end	Day	20% 6% 2 6% 4 4 3 4 5% 6% 39%	4283
January	1st to 10th	Night	12% 5% 2 5% 3 3 4 4 5% 6% 51%	7635
	11th to 20th	Night	12% 6% 2 5% 3 3 3 2 4 4 57%	7680
	21st to end	Night	14% 6% 2 6% 3 3 3 3 4 5% 51%	8401
February	1st to 10th	Night	14% 7% 2 6% 4 4 4 4 5% 7% 44%	2730
	11th to 20th	Night	20% 8% 2 5% 4 4 3 3 4 5% 43%	2663
	21st to end	Night	14% 6% 2 5% 4 4 5% 3 5% 7% 45%	1977
March	1st to 10th	Night	16% 6% 2 5% 4 3 4 4 5% 6% 46%	5028
	11th to 20th	Night	10% 3 2 4 3 3 4 3 5% 8% 54%	5090
	21st to end	Night	12% 7% 2 6% 4 4 5% 4 5% 7% 44%	5588
April	1st to 10th	Night	13% 7% 3 5% 4 4 4 3 6% 8% 45%	2158
	11th to 20th	Night	11% 5% 2 5% 4 3 3 3 6% 8% 49%	2075
	21st to end	Night	14% 5% 2 5% 5% 4 5% 4 5% 7% 45%	2077
May	1st to 10th	Night	13% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	5237
	11th to 20th	Night	17% 6% 2 6% 4 4 5% 4 6% 7% 40%	5234
	21st to end	Night	21% 7% 2 6% 4 4 4 4 6% 8% 35%	5582
June	1st to 10th	Night	8% 6% 2 5% 4 4 5% 5% 7% 8% 45%	1680
	11th to 20th	Night	10% 7% 2 6% 5% 4 5% 4 6% 10% 42%	1680
	21st to end	Night	7% 4 2 5% 4 4 5% 5% 8% 11% 45%	1705
July	1st to 10th	Night	10% 5% 2 7% 6% 6% 6% 5% 7% 9% 38%	4148
	11th to 20th	Night	12% 8% 3 9% 7% 6% 6% 5% 8% 10% 27%	4102
	21st to end	Night	9% 7% 3 8% 6% 6% 6% 6% 8% 9% 31%	4427
August	1st to 10th	Night	16% 5% 3 8% 5% 5% 5% 5% 6% 10% 31%	2915
	11th to 20th	Night	10% 6% 3 7% 5% 6% 7% 4 7% 9% 36%	2908
	21st to end	Night	10% 6% 2 7% 5% 4 5% 4 6% 9% 43%	3586
September	1st to 10th	Night	12% 6% 3 5% 4 4 4 4 5% 7% 48%	2000
	11th to 20th	Night	12% 5% 3 5% 5% 4 5% 4 5% 7% 44%	2110
	21st to end	Night	24% 8% 3 5% 3 4 4 2 5% 6% 37%	2252
October	1st to 10th	Night	17% 5% 2 5% 3 3 4 3 4 6% 48%	2400
	11th to 20th	Night	14% 6% 2 4 3 3 4 3 5% 6% 50%	2400
	21st to end	Night	14% 5% 1 3 4 3 3 3 5% 5% 55%	2640
November	1st to 10th	Night	12% 7% 2 6% 4 3 3 2 3 4 53%	2461
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 5% 6% 51%	2691
	21st to end	Night	13% 4 1 5% 4 4 3 4 5% 7% 51%	2940
December	1st to 10th	Night	11% 5% 2 5% 4 4 3 3 4 6% 53%	4720
	11th to 20th	Night	20% 8% 2 5% 4 4 3 3 4 6% 42%	5345
	21st to end	Night	17% 5% 2 5% 4 4 4 4 5% 7% 42%	6897

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	37% 4 10% 22% 20% 7%	5259
	11th to 20th	Day	33% 3 5% 17% 27% 15%	5280
	21st to end	Day	41% 4 6% 27% 19% 4	5780
February	1st to 10th	Day	40% 6% 10% 24% 16% 4	1890
	11th to 20th	Day	42% 6% 8% 20% 20% 4	1852
	21st to end	Day	47% 4 11% 17% 16% 5%	1824
March	1st to 10th	Day	41% 6% 9% 17% 17% 10%	5045
	11th to 20th	Day	31% 5% 9% 22% 22% 10%	5090
	21st to end	Day	43% 6% 9% 18% 17% 6%	5586
April	1st to 10th	Day	33% 6% 11% 23% 19% 8%	2461
	11th to 20th	Day	34% 5% 12% 24% 19% 6%	2494
	21st to end	Day	41% 6% 11% 23% 15% 4	2560
May	1st to 10th	Day	40% 8% 13% 24% 14% 1	7651
	11th to 20th	Day	42% 7% 12% 24% 15% 1	7630
	21st to end	Day	43% 7% 12% 18% 17% 3	8577
June	1st to 10th	Day	33% 9% 16% 25% 16% 1	2940
	11th to 20th	Day	34% 10% 17% 22% 15% 2	2940
	21st to end	Day	32% 10% 17% 24% 14% 2	2907
July	1st to 10th	Day	33% 10% 21% 20% 12% 4	5965
	11th to 20th	Day	40% 12% 21% 19% 7% 1	5896
	21st to end	Day	38% 10% 15% 23% 14%	6240
August	1st to 10th	Day	44% 9% 17% 24% 6%	4221
	11th to 20th	Day	37% 9% 16% 28% 10%	4147
	21st to end	Day	29% 8% 20% 26% 15% 1	4312
September	1st to 10th	Day	34% 8% 14% 17% 22% 4	2400
	11th to 20th	Day	37% 6% 10% 24% 19% 3	2287
	21st to end	Day	47% 6% 11% 17% 16% 2	2147
October	1st to 10th	Day	33% 4 8% 19% 31% 5%	3260
	11th to 20th	Day	25% 3 6% 23% 29% 14%	3240
	21st to end	Day	22% 3 11% 28% 24% 12%	3538
November	1st to 10th	Day	30% 4 11% 25% 25% 5%	2057
	11th to 20th	Day	34% 5% 9% 23% 21% 7%	1924
	21st to end	Day	34% 4 10% 23% 25% 4	1680
December	1st to 10th	Day	36% 5% 9% 23% 19% 8%	2680
	11th to 20th	Day	39% 4 13% 22% 17% 6%	3066
	21st to end	Day	44% 6% 11% 24% 15% 1	4283
January	1st to 10th	Night	33% 4 10% 23% 21% 9%	7636
	11th to 20th	Night	33% 3 5% 19% 29% 11%	7680
	21st to end	Night	37% 3 6% 27% 20% 7%	8401
February	1st to 10th	Night	39% 5% 11% 24% 19% 3	2730
	11th to 20th	Night	44% 4 8% 18% 19% 7%	2665
	21st to end	Night	38% 5% 11% 22% 17% 7%	1977
March	1st to 10th	Night	38% 6% 8% 20% 20% 9%	5028
	11th to 20th	Night	27% 6% 8% 20% 29% 10%	5090
	21st to end	Night	39% 5% 10% 21% 20% 6%	5588
April	1st to 10th	Night	36% 6% 13% 17% 22% 6%	2158
	11th to 20th	Night	33% 5% 13% 23% 21% 5%	2075
	21st to end	Night	38% 6% 12% 24% 17% 4	2077
May	1st to 10th	Night	44% 7% 14% 20% 11% 3	5264
	11th to 20th	Night	41% 7% 12% 25% 13% 1	5235
	21st to end	Night	45% 8% 13% 20% 12% 3	5591
June	1st to 10th	Night	34% 9% 13% 26% 17% 1	1680
	11th to 20th	Night	36% 8% 19% 20% 16% 2	1680
	21st to end	Night	31% 10% 17% 21% 18% 3	1705
July	1st to 10th	Night	40% 10% 19% 17% 13% 2	4105
	11th to 20th	Night	49% 11% 18% 16% 5% 1	4051
	21st to end	Night	45% 9% 14% 19% 12%	4297
August	1st to 10th	Night	46% 9% 16% 22% 6% 1	2904
	11th to 20th	Night	42% 8% 15% 24% 10%	2935
	21st to end	Night	37% 8% 15% 30% 10% 2	3586
September	1st to 10th	Night	36% 8% 13% 20% 18% 6%	2000
	11th to 20th	Night	38% 8% 11% 22% 19% 2	2110
	21st to end	Night	49% 6% 9% 18% 15% 3	2252
October	1st to 10th	Night	40% 4 10% 18% 22% 5%	3920
	11th to 20th	Night	27% 4 7% 21% 30% 12%	3899
	21st to end	Night	21% 3 7% 29% 27% 12%	4252
November	1st to 10th	Night	36% 4 7% 21% 25% 7%	2461
	11th to 20th	Night	34% 5% 8% 20% 24% 9%	2691
	21st to end	Night	33% 5% 9% 24% 22% 6%	2940
December	1st to 10th	Night	33% 4 7% 28% 23% 5%	4720
	11th to 20th	Night	44% 4 10% 19% 15% 7%	5348
	21st to end	Night	39% 6% 11% 26% 15% 2	6897

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	23% 27% 27% 19% 4 1	330
	11th to 20th	Day	28% 28% 32% 10% 1	330
	21st to end	Day	25% 26% 32% 13% 4	363
February	1st to 10th	Day	23% 22% 39% 12% 4	330
	11th to 20th	Day	39% 25% 26% 7% 3	330
	21st to end	Day	31% 19% 29% 18% 4	428
March	1st to 10th	Day	26% 15% 33% 21% 5%	540
	11th to 20th	Day	23% 20% 34% 18% 5%	540
	21st to end	Day	24% 17% 33% 20% 4 1	594
April	1st to 10th	Day	21% 20% 28% 21% 8% 1	540
	11th to 20th	Day	20% 12% 33% 23% 10% 3	532
	21st to end	Day	20% 12% 32% 24% 10% 2	540
May	1st to 10th	Day	20% 16% 33% 20% 8% 3	543
	11th to 20th	Day	20% 11% 26% 26% 13% 4	567
	21st to end	Day	20% 6% 21% 30% 17% 6%	617
June	1st to 10th	Day	10% 7% 28% 26% 22% 7%	570
	11th to 20th	Day	12% 8% 21% 32% 18% 9%	570
	21st to end	Day	10% 5% 23% 32% 22% 8%	570
July	1st to 10th	Day	11% 2 15% 28% 29% 14%	565
	11th to 20th	Day	7% 3 15% 33% 29% 13%	544
	21st to end	Day	12% 3 19% 30% 23% 13%	576
August	1st to 10th	Day	17% 4 17% 27% 23% 12%	526
	11th to 20th	Day	11% 3 18% 30% 25% 14%	519
	21st to end	Day	12% 2 15% 28% 27% 17%	583
September	1st to 10th	Day	13% 3 17% 28% 22% 16%	530
	11th to 20th	Day	17% 4 13% 28% 27% 10%	530
	21st to end	Day	30% 8% 18% 25% 15% 4	530
October	1st to 10th	Day	20% 6% 19% 29% 18% 9%	530
	11th to 20th	Day	15% 12% 24% 27% 20% 3	530
	21st to end	Day	17% 11% 30% 28% 10% 3	583
November	1st to 10th	Day	16% 11% 29% 28% 13% 3	535
	11th to 20th	Day	19% 13% 32% 22% 12% 1	406
	21st to end	Day	21% 17% 30% 20% 10% 2	220
December	1st to 10th	Day	25% 14% 37% 18% 5% 1	220
	11th to 20th	Day	26% 13% 33% 22% 5%	220
	21st to end	Day	28% 15% 29% 19% 7% 1	297
January	1st to 10th	Night	18% 13% 38% 27% 3	840
	11th to 20th	Night	20% 14% 37% 26% 3	840
	21st to end	Night	17% 11% 44% 25% 3	924
February	1st to 10th	Night	17% 12% 40% 27% 3	840
	11th to 20th	Night	27% 14% 38% 20% 1	823
	21st to end	Night	19% 15% 37% 27% 2	692
March	1st to 10th	Night	18% 13% 36% 29% 4	840
	11th to 20th	Night	21% 12% 35% 26% 6% 1	840
	21st to end	Night	18% 12% 37% 28% 5%	924
April	1st to 10th	Night	16% 14% 39% 24% 6%	840
	11th to 20th	Night	14% 10% 34% 32% 9% 1	831
	21st to end	Night	16% 6% 31% 37% 8% 1	817
May	1st to 10th	Night	16% 8% 31% 33% 12% 1	630
	11th to 20th	Night	20% 5% 21% 35% 16% 1	621
	21st to end	Night	18% 3 15% 35% 27% 2	620
June	1st to 10th	Night	9% 3 15% 46% 25% 2	420
	11th to 20th	Night	12% 3 17% 39% 26% 2	420
	21st to end	Night	7% 2 15% 50% 24% 3	445
July	1st to 10th	Night	10% 2 11% 37% 35% 5%	615
	11th to 20th	Night	7% 1 9% 35% 41% 7%	597
	21st to end	Night	9% 2 10% 42% 33% 4	657
August	1st to 10th	Night	14% 3 8% 39% 31% 5%	597
	11th to 20th	Night	16% 1 7% 37% 35% 4	614
	21st to end	Night	12% 2 11% 35% 35% 5%	880
September	1st to 10th	Night	15% 2 10% 35% 31% 7%	800
	11th to 20th	Night	15% 4 11% 34% 31% 5%	800
	21st to end	Night	28% 4 15% 31% 20% 2	800
October	1st to 10th	Night	19% 5% 17% 31% 24% 5%	800
	11th to 20th	Night	17% 7% 23% 35% 17% 1	800
	21st to end	Night	16% 7% 25% 36% 15% 1	880
November	1st to 10th	Night	16% 6% 27% 37% 12% 2	820
	11th to 20th	Night	17% 10% 32% 31% 10% 1	840
	21st to end	Night	15% 9% 32% 35% 9% 1	840
December	1st to 10th	Night	20% 8% 35% 29% 7%	840
	11th to 20th	Night	18% 8% 35% 33% 6%	840
	21st to end	Night	21% 10% 31% 35% 3	924

Page generated at 2026-07-13 03:14:52. Version ac07

Copyright © 2026 Attilla Danko.