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 2025-03-12.

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	14% 5% 2 6% 4 4 4 4 6% 8% 43%	108723
	Night	14% 6% 2 5% 4 4 4 3 5% 7% 45%	110271
Transparency	Day	37% 7% 12% 22% 17% 4	112373
	Night	37% 6% 11% 22% 19% 5%	114760
Seeing	Day	19% 11% 25% 25% 15% 6%	15452
	Night	17% 7% 26% 33% 15% 2	25443

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 5% 3 3 4 3 4 5% 51%	4056
	11th to 20th	Day	13% 6% 2 6% 3 3 3 3 4 4 53%	4060
	21st to end	Day	16% 6% 2 6% 3 3 4 3 4 5% 47%	4466
February	1st to 10th	Day	12% 7% 2 6% 4 5% 5% 4 7% 8% 41%	1800
	11th to 20th	Day	20% 7% 1 5% 4 4 4 3 5% 6% 41%	1753
	21st to end	Day	18% 7% 3 6% 5% 4 4 3 5% 6% 40%	1740
March	1st to 10th	Day	20% 7% 2 6% 4 4 4 3 5% 8% 38%	3545
	11th to 20th	Day	10% 5% 2 5% 4 4 3 3 6% 7% 52%	3491
	21st to end	Day	19% 7% 2 7% 4 4 4 3 5% 6% 38%	3815
April	1st to 10th	Day	10% 5% 2 5% 3 4 5% 4 5% 7% 50%	2227
	11th to 20th	Day	12% 5% 2 5% 4 3 4 4 6% 7% 48%	2254
	21st to end	Day	16% 5% 2 6% 5% 5% 4 3 6% 7% 42%	2316
May	1st to 10th	Day	16% 7% 2 8% 6% 5% 5% 4 6% 9% 33%	4082
	11th to 20th	Day	22% 5% 2 6% 4 4 4 4 6% 8% 34%	4081
	21st to end	Day	9% 4 2 4 4 4 5% 4 7% 11% 46%	4563
June	1st to 10th	Day	10% 4 2 5% 5% 5% 5% 4 6% 10% 44%	2660
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 5% 8% 11% 40%	2660
	21st to end	Day	8% 4 2 6% 5% 5% 5% 5% 7% 12% 42%	2631
July	1st to 10th	Day	6% 4 1 6% 5% 4 6% 5% 9% 13% 41%	4120
	11th to 20th	Day	10% 5% 2 7% 5% 5% 5% 5% 8% 12% 36%	4120
	21st to end	Day	12% 6% 3 9% 6% 6% 6% 5% 7% 9% 31%	4466
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 5% 5% 8% 11% 31%	4104
	11th to 20th	Day	8% 4 2 6% 5% 6% 6% 5% 8% 11% 39%	3980
	21st to end	Day	8% 4 2 4 4 5% 4 4 7% 10% 48%	4180
September	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 5% 7% 47%	2280
	11th to 20th	Day	13% 6% 3 5% 4 4 5% 3 6% 7% 44%	2173
	21st to end	Day	22% 6% 3 6% 4 3 4 4 5% 5% 38%	2040
October	1st to 10th	Day	15% 6% 2 6% 3 3 4 3 5% 6% 50%	1900
	11th to 20th	Day	15% 4 2 4 3 3 4 3 4 6% 53%	1900
	21st to end	Day	13% 4 2 5% 3 3 3 3 4 6% 53%	2090
November	1st to 10th	Day	12% 4 1 4 3 4 3 3 4 6% 55%	1957
	11th to 20th	Day	14% 6% 1 5% 3 3 4 3 5% 6% 50%	1833
	21st to end	Day	14% 4 1 4 4 3 4 3 5% 7% 51%	1600
December	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 5% 6% 49%	2600
	11th to 20th	Day	21% 5% 2 4 2 3 3 3 4 6% 47%	2990
	21st to end	Day	20% 6% 2 6% 4 4 3 4 5% 6% 39%	4190
January	1st to 10th	Night	12% 5% 2 4 3 3 3 3 5% 5% 54%	5889
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 4 4 56%	5900
	21st to end	Night	15% 7% 2 5% 3 3 3 2 4 5% 50%	6490
February	1st to 10th	Night	14% 7% 2 6% 4 4 4 4 5% 7% 43%	2600
	11th to 20th	Night	20% 8% 2 5% 4 4 3 3 4 5% 43%	2533
	21st to end	Night	14% 7% 2 5% 4 4 4 3 5% 7% 45%	1885
March	1st to 10th	Night	20% 7% 2 7% 5% 4 4 4 5% 6% 37%	3528
	11th to 20th	Night	11% 4 2 4 4 3 3 4 5% 8% 51%	3498
	21st to end	Night	15% 7% 2 5% 3 3 4 3 4 6% 47%	3817
April	1st to 10th	Night	12% 7% 3 5% 4 4 4 3 6% 7% 45%	1952
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 6% 8% 50%	1875
	21st to end	Night	15% 5% 2 5% 5% 4 5% 4 4 7% 45%	1879
May	1st to 10th	Night	17% 8% 3 8% 5% 4 5% 4 6% 7% 34%	2813
	11th to 20th	Night	24% 7% 2 5% 4 4 4 4 4 6% 35%	2812
	21st to end	Night	10% 5% 2 5% 4 4 5% 5% 7% 9% 44%	2984
June	1st to 10th	Night	9% 6% 2 5% 4 5% 5% 5% 7% 9% 44%	1520
	11th to 20th	Night	9% 6% 2 6% 5% 4 5% 4 6% 10% 43%	1520
	21st to end	Night	8% 5% 2 5% 3 4 5% 4 7% 11% 45%	1543
July	1st to 10th	Night	7% 4 2 7% 5% 5% 6% 5% 8% 10% 41%	2840
	11th to 20th	Night	9% 6% 2 7% 6% 5% 5% 5% 8% 10% 36%	2840
	21st to end	Night	11% 9% 3 9% 6% 6% 6% 5% 7% 8% 29%	3087
August	1st to 10th	Night	16% 5% 3 8% 5% 5% 5% 5% 6% 10% 31%	2825
	11th to 20th	Night	11% 6% 2 7% 5% 5% 7% 4 7% 9% 36%	2817
	21st to end	Night	10% 6% 2 7% 4 4 5% 4 6% 9% 43%	3476
September	1st to 10th	Night	12% 6% 3 5% 4 4 4 4 5% 7% 46%	1900
	11th to 20th	Night	12% 6% 3 5% 5% 4 5% 4 5% 7% 44%	2004
	21st to end	Night	24% 8% 2 4 3 4 4 2 5% 6% 38%	2139
October	1st to 10th	Night	17% 5% 2 5% 3 4 4 3 4 6% 47%	2280
	11th to 20th	Night	14% 6% 2 4 3 3 4 3 4 6% 50%	2280
	21st to end	Night	13% 4 1 3 4 3 3 3 5% 5% 55%	2508
November	1st to 10th	Night	12% 7% 2 6% 4 3 3 2 3 4 53%	2341
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 5% 6% 51%	2563
	21st to end	Night	13% 4 1 5% 4 4 3 4 5% 7% 51%	2800
December	1st to 10th	Night	11% 5% 2 5% 4 4 3 3 4 6% 54%	4580
	11th to 20th	Night	20% 8% 2 5% 4 4 3 3 4 6% 42%	5205
	21st to end	Night	17% 5% 2 5% 4 4 5% 4 5% 7% 42%	6748

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	34% 5% 7% 23% 23% 9%	4057
	11th to 20th	Day	34% 4 6% 18% 25% 12%	4060
	21st to end	Day	40% 5% 7% 25% 20% 4	4466
February	1st to 10th	Day	40% 6% 11% 24% 15% 3	1800
	11th to 20th	Day	43% 6% 8% 20% 20% 5%	1762
	21st to end	Day	46% 4 11% 16% 17% 6%	1740
March	1st to 10th	Day	44% 7% 9% 19% 14% 7%	3545
	11th to 20th	Day	29% 7% 9% 24% 22% 9%	3491
	21st to end	Day	47% 7% 9% 18% 16% 3	3815
April	1st to 10th	Day	33% 7% 11% 23% 19% 8%	2227
	11th to 20th	Day	35% 6% 11% 23% 20% 6%	2254
	21st to end	Day	41% 7% 12% 22% 15% 4	2316
May	1st to 10th	Day	46% 9% 14% 19% 11% 2	4120
	11th to 20th	Day	46% 8% 12% 22% 11% 1	4085
	21st to end	Day	29% 10% 17% 21% 20% 2	4577
June	1st to 10th	Day	33% 10% 16% 26% 14% 1	2660
	11th to 20th	Day	35% 10% 17% 22% 15% 2	2660
	21st to end	Day	33% 10% 17% 24% 14% 2	2631
July	1st to 10th	Day	32% 11% 23% 19% 10% 5%	4055
	11th to 20th	Day	37% 9% 20% 24% 9% 1	4051
	21st to end	Day	46% 10% 13% 19% 11%	4324
August	1st to 10th	Day	44% 9% 17% 23% 6%	4091
	11th to 20th	Day	36% 9% 16% 28% 10%	4019
	21st to end	Day	29% 8% 20% 26% 15% 1	4180
September	1st to 10th	Day	35% 8% 13% 17% 22% 4	2280
	11th to 20th	Day	38% 6% 10% 24% 18% 3	2173
	21st to end	Day	47% 6% 11% 17% 17% 2	2040
October	1st to 10th	Day	34% 4 8% 19% 30% 5%	3160
	11th to 20th	Day	25% 3 6% 23% 29% 14%	3140
	21st to end	Day	21% 3 11% 28% 24% 12%	3428
November	1st to 10th	Day	31% 4 10% 24% 26% 5%	1957
	11th to 20th	Day	35% 5% 10% 22% 21% 8%	1833
	21st to end	Day	33% 5% 11% 23% 25% 4	1600
December	1st to 10th	Day	36% 5% 9% 23% 20% 8%	2600
	11th to 20th	Day	39% 4 13% 21% 17% 6%	2986
	21st to end	Day	44% 6% 11% 24% 15% 1	4190
January	1st to 10th	Night	31% 4 7% 21% 25% 12%	5889
	11th to 20th	Night	33% 3 6% 20% 29% 10%	5900
	21st to end	Night	38% 3 6% 27% 20% 7%	6490
February	1st to 10th	Night	39% 6% 11% 24% 18% 3	2600
	11th to 20th	Night	44% 4 8% 19% 19% 7%	2535
	21st to end	Night	38% 5% 11% 23% 17% 7%	1885
March	1st to 10th	Night	45% 7% 9% 19% 16% 4	3528
	11th to 20th	Night	29% 6% 8% 20% 27% 9%	3498
	21st to end	Night	38% 5% 11% 22% 19% 5%	3817
April	1st to 10th	Night	37% 6% 12% 17% 22% 6%	1952
	11th to 20th	Night	32% 6% 13% 22% 22% 6%	1875
	21st to end	Night	38% 6% 12% 22% 17% 5%	1879
May	1st to 10th	Night	47% 8% 14% 18% 11% 3	2840
	11th to 20th	Night	47% 9% 11% 22% 10% 1	2813
	21st to end	Night	31% 11% 17% 25% 13% 3	2991
June	1st to 10th	Night	33% 10% 13% 27% 16% 1	1520
	11th to 20th	Night	36% 8% 18% 20% 15% 2	1520
	21st to end	Night	31% 10% 17% 21% 18% 4	1543
July	1st to 10th	Night	36% 11% 22% 16% 13% 3	2795
	11th to 20th	Night	40% 10% 20% 22% 7% 1	2786
	21st to end	Night	49% 9% 14% 18% 9%	2988
August	1st to 10th	Night	46% 9% 17% 22% 6% 1	2814
	11th to 20th	Night	41% 8% 15% 25% 10%	2844
	21st to end	Night	37% 8% 15% 30% 9% 2	3476
September	1st to 10th	Night	37% 8% 13% 19% 17% 6%	1900
	11th to 20th	Night	38% 8% 11% 21% 19% 2	2004
	21st to end	Night	47% 6% 9% 19% 16% 3	2139
October	1st to 10th	Night	41% 4 10% 18% 21% 6%	3800
	11th to 20th	Night	26% 4 7% 21% 30% 12%	3779
	21st to end	Night	20% 4 7% 30% 27% 12%	4120
November	1st to 10th	Night	36% 4 7% 21% 25% 7%	2341
	11th to 20th	Night	34% 6% 8% 20% 24% 9%	2563
	21st to end	Night	33% 5% 9% 24% 23% 6%	2800
December	1st to 10th	Night	33% 4 7% 28% 24% 5%	4580
	11th to 20th	Night	45% 4 10% 19% 15% 7%	5208
	21st to end	Night	39% 7% 11% 26% 16% 2	6748

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% 26% 27% 18% 4 1	300
	11th to 20th	Day	27% 27% 34% 11% 1	300
	21st to end	Day	24% 25% 33% 14% 5%	330
February	1st to 10th	Day	23% 24% 39% 11% 4	300
	11th to 20th	Day	40% 24% 26% 7% 3	300
	21st to end	Day	30% 20% 28% 18% 4	397
March	1st to 10th	Day	26% 16% 32% 21% 5%	500
	11th to 20th	Day	20% 21% 34% 18% 5% 1	464
	21st to end	Day	25% 16% 32% 22% 4 1	506
April	1st to 10th	Day	21% 21% 27% 21% 8% 1	460
	11th to 20th	Day	20% 13% 32% 22% 10% 3	452
	21st to end	Day	21% 13% 32% 22% 10% 2	460
May	1st to 10th	Day	21% 13% 33% 21% 10% 3	463
	11th to 20th	Day	20% 9% 26% 25% 14% 5%	487
	21st to end	Day	17% 4 21% 33% 18% 7%	529
June	1st to 10th	Day	11% 8% 25% 27% 23% 7%	490
	11th to 20th	Day	12% 7% 20% 32% 19% 10%	490
	21st to end	Day	10% 6% 23% 32% 22% 7%	490
July	1st to 10th	Day	12% 2 16% 29% 28% 13%	485
	11th to 20th	Day	6% 2 15% 34% 30% 12%	484
	21st to end	Day	12% 4 20% 30% 23% 12%	532
August	1st to 10th	Day	16% 5% 16% 27% 24% 12%	486
	11th to 20th	Day	11% 3 18% 29% 25% 14%	479
	21st to end	Day	12% 2 13% 27% 28% 18%	539
September	1st to 10th	Day	13% 3 16% 27% 24% 17%	490
	11th to 20th	Day	17% 4 13% 28% 27% 10%	490
	21st to end	Day	30% 8% 17% 26% 14% 4	490
October	1st to 10th	Day	21% 6% 19% 29% 18% 7%	490
	11th to 20th	Day	14% 11% 23% 28% 21% 3	490
	21st to end	Day	15% 10% 31% 30% 11% 4	539
November	1st to 10th	Day	16% 9% 27% 29% 15% 3	495
	11th to 20th	Day	18% 12% 31% 23% 13% 2	375
	21st to end	Day	19% 17% 32% 22% 10% 2	200
December	1st to 10th	Day	25% 15% 37% 19% 5% 1	200
	11th to 20th	Day	26% 12% 34% 23% 5% 1	200
	21st to end	Day	27% 14% 30% 20% 8% 1	270
January	1st to 10th	Night	19% 13% 38% 27% 3	800
	11th to 20th	Night	20% 13% 36% 28% 3	800
	21st to end	Night	17% 10% 44% 26% 3	880
February	1st to 10th	Night	18% 12% 41% 27% 3	800
	11th to 20th	Night	27% 14% 38% 21% 1	783
	21st to end	Night	20% 14% 37% 27% 2	660
March	1st to 10th	Night	18% 13% 36% 28% 4 1	800
	11th to 20th	Night	21% 12% 35% 25% 6% 1	767
	21st to end	Night	19% 12% 35% 29% 5%	836
April	1st to 10th	Night	16% 14% 40% 24% 6%	760
	11th to 20th	Night	14% 11% 32% 32% 10% 1	751
	21st to end	Night	17% 7% 31% 37% 8% 1	739
May	1st to 10th	Night	17% 8% 29% 34% 12% 1	570
	11th to 20th	Night	21% 5% 20% 35% 17% 1	561
	21st to end	Night	15% 2 14% 37% 29% 3	560
June	1st to 10th	Night	9% 3 15% 46% 25% 2	380
	11th to 20th	Night	12% 3 16% 39% 28% 3	380
	21st to end	Night	7% 2 15% 50% 23% 3	403
July	1st to 10th	Night	10% 2 11% 38% 35% 4	555
	11th to 20th	Night	7% 1 7% 37% 42% 7%	552
	21st to end	Night	9% 2 10% 43% 32% 3	624
August	1st to 10th	Night	13% 3 7% 40% 31% 5%	567
	11th to 20th	Night	16% 2 6% 38% 34% 4	583
	21st to end	Night	12% 2 11% 34% 35% 5%	836
September	1st to 10th	Night	16% 2 10% 34% 31% 7%	760
	11th to 20th	Night	15% 4 11% 34% 31% 5%	760
	21st to end	Night	28% 4 16% 30% 21% 2	760
October	1st to 10th	Night	19% 4 17% 31% 23% 5%	760
	11th to 20th	Night	16% 7% 23% 35% 18% 1	760
	21st to end	Night	15% 6% 25% 37% 15% 1	836
November	1st to 10th	Night	16% 6% 26% 37% 13% 2	780
	11th to 20th	Night	17% 9% 32% 31% 10% 1	800
	21st to end	Night	15% 9% 32% 35% 9% 1	800
December	1st to 10th	Night	21% 8% 35% 29% 7%	800
	11th to 20th	Night	18% 8% 34% 34% 6%	800
	21st to end	Night	21% 9% 30% 36% 3	880

Page generated at 2025-03-09 08:08:47. Version ac07

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