Richmond
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Richmond 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	20% 12% 4 9% 5% 5% 5% 4 5% 6% 25%	131633
	Night	22% 12% 4 9% 5% 5% 4 4 5% 6% 24%	130458
Transparency	Day	57% 7% 12% 17% 7% 1	135094
	Night	58% 6% 12% 17% 7% 1	135095
Seeing	Day	27% 12% 28% 23% 9% 1	19538
	Night	28% 10% 28% 25% 8% 1	27025

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	12% 15% 6% 16% 8% 6% 6% 5% 5% 6% 14%	4690
	11th to 20th	Day	18% 12% 6% 12% 7% 7% 6% 4 5% 5% 19%	4681
	21st to end	Day	12% 10% 5% 11% 9% 10% 6% 4 5% 6% 22%	5249
February	1st to 10th	Day	20% 10% 4 13% 6% 6% 6% 6% 7% 7% 16%	1890
	11th to 20th	Day	15% 12% 3 10% 7% 7% 6% 5% 6% 8% 21%	1869
	21st to end	Day	13% 11% 4 9% 7% 6% 5% 4 6% 6% 30%	1730
March	1st to 10th	Day	21% 11% 4 8% 7% 6% 5% 3 5% 5% 26%	4574
	11th to 20th	Day	14% 17% 6% 8% 5% 5% 5% 4 5% 7% 24%	4996
	21st to end	Day	29% 11% 4 8% 5% 5% 5% 3 5% 6% 19%	5957
April	1st to 10th	Day	24% 10% 3 8% 5% 4 5% 4 4 6% 25%	2412
	11th to 20th	Day	23% 11% 4 7% 3 4 3 3 4 6% 31%	2374
	21st to end	Day	25% 12% 3 6% 5% 4 3 3 4 6% 30%	2550
May	1st to 10th	Day	28% 15% 4 8% 4 3 3 3 4 4 23%	6345
	11th to 20th	Day	22% 12% 3 7% 5% 4 4 3 4 6% 28%	6318
	21st to end	Day	22% 13% 4 7% 5% 3 3 3 4 6% 29%	6948
June	1st to 10th	Day	30% 10% 4 8% 5% 5% 4 3 5% 6% 20%	2800
	11th to 20th	Day	22% 11% 4 6% 5% 4 5% 4 6% 7% 28%	2800
	21st to end	Day	32% 15% 4 7% 5% 4 4 3 4 5% 18%	2800
July	1st to 10th	Day	23% 14% 5% 11% 6% 5% 5% 3 4 5% 19%	6360
	11th to 20th	Day	20% 14% 4 9% 5% 5% 5% 4 5% 6% 25%	6360
	21st to end	Day	16% 9% 3 7% 4 4 5% 4 5% 8% 34%	6927
August	1st to 10th	Day	21% 11% 3 9% 5% 4 5% 4 6% 7% 26%	4564
	11th to 20th	Day	19% 8% 3 7% 5% 5% 5% 4 7% 9% 28%	4099
	21st to end	Day	13% 8% 3 6% 4 4 4 4 5% 8% 41%	4312
September	1st to 10th	Day	18% 8% 3 6% 4 5% 4 4 6% 7% 35%	2400
	11th to 20th	Day	19% 8% 2 6% 5% 4 5% 4 6% 8% 33%	2379
	21st to end	Day	21% 7% 2 6% 4 3 4 3 6% 7% 36%	2170
October	1st to 10th	Day	18% 8% 2 8% 4 4 4 5% 6% 9% 34%	2000
	11th to 20th	Day	20% 8% 3 9% 5% 5% 5% 4 6% 9% 27%	2000
	21st to end	Day	21% 13% 3 9% 5% 6% 4 4 5% 7% 23%	2005
November	1st to 10th	Day	23% 10% 4 6% 5% 5% 5% 5% 5% 8% 25%	1850
	11th to 20th	Day	23% 13% 3 10% 6% 6% 6% 4 5% 6% 19%	1708
	21st to end	Day	18% 11% 5% 10% 7% 6% 6% 5% 7% 7% 19%	1680
December	1st to 10th	Day	15% 13% 6% 13% 7% 6% 6% 4 5% 6% 19%	2690
	11th to 20th	Day	25% 13% 4 11% 5% 5% 5% 4 7% 6% 16%	3076
	21st to end	Day	21% 10% 5% 12% 8% 6% 6% 4 5% 6% 18%	4070
January	1st to 10th	Night	15% 16% 6% 13% 7% 5% 5% 4 5% 6% 17%	8234
	11th to 20th	Night	22% 12% 7% 13% 6% 5% 5% 4 5% 5% 14%	8234
	21st to end	Night	11% 9% 4 13% 11% 10% 6% 4 6% 7% 17%	8955
February	1st to 10th	Night	20% 11% 4 12% 6% 6% 7% 5% 6% 6% 17%	2730
	11th to 20th	Night	16% 13% 5% 10% 8% 6% 6% 5% 5% 7% 20%	2635
	21st to end	Night	12% 12% 5% 11% 7% 5% 4 4 6% 6% 28%	2076
March	1st to 10th	Night	22% 13% 4 8% 5% 5% 4 3 4 6% 25%	5486
	11th to 20th	Night	19% 16% 5% 7% 4 4 3 3 5% 6% 28%	5167
	21st to end	Night	31% 12% 5% 10% 4 5% 4 2 3 3 20%	5217
April	1st to 10th	Night	25% 11% 3 6% 4 4 4 4 5% 6% 27%	2011
	11th to 20th	Night	21% 10% 4 8% 5% 4 3 3 5% 6% 32%	1976
	21st to end	Night	26% 12% 2 7% 4 3 3 3 4 5% 31%	1843
May	1st to 10th	Night	31% 15% 3 5% 3 2 3 3 4 5% 25%	3615
	11th to 20th	Night	23% 14% 3 6% 3 4 3 3 4 6% 30%	3596
	21st to end	Night	18% 17% 5% 8% 4 4 3 3 4 6% 28%	3948
June	1st to 10th	Night	33% 12% 4 8% 4 3 3 2 4 5% 21%	1600
	11th to 20th	Night	26% 11% 2 6% 4 3 3 3 5% 5% 32%	1600
	21st to end	Night	35% 17% 3 7% 4 2 3 2 4 5% 19%	1600
July	1st to 10th	Night	29% 17% 5% 8% 4 3 4 3 5% 5% 17%	3619
	11th to 20th	Night	27% 13% 4 6% 4 3 4 3 4 6% 27%	3620
	21st to end	Night	23% 13% 3 7% 4 4 4 3 5% 7% 30%	3948
August	1st to 10th	Night	23% 13% 4 7% 4 4 4 3 5% 6% 28%	2596
	11th to 20th	Night	20% 10% 3 7% 5% 5% 4 4 7% 8% 28%	2943
	21st to end	Night	17% 7% 2 4 3 3 4 4 5% 8% 42%	3586
September	1st to 10th	Night	18% 7% 4 5% 4 4 5% 4 5% 7% 38%	2000
	11th to 20th	Night	21% 8% 2 5% 4 4 3 4 5% 7% 38%	2020
	21st to end	Night	20% 10% 3 6% 3 5% 3 3 4 6% 38%	2228
October	1st to 10th	Night	19% 7% 2 6% 4 3 5% 3 5% 7% 39%	2400
	11th to 20th	Night	21% 8% 2 7% 4 4 4 3 5% 7% 35%	2400
	21st to end	Night	24% 10% 3 8% 4 5% 4 3 5% 6% 28%	2835
November	1st to 10th	Night	20% 11% 3 7% 4 5% 4 4 6% 8% 27%	2668
	11th to 20th	Night	24% 13% 3 8% 5% 6% 4 4 5% 6% 22%	2909
	21st to end	Night	22% 11% 4 8% 6% 6% 5% 4 7% 7% 20%	2940
December	1st to 10th	Night	18% 10% 4 9% 6% 5% 6% 4 6% 7% 24%	4720
	11th to 20th	Night	25% 10% 4 12% 6% 5% 4 3 5% 5% 21%	5353
	21st to end	Night	23% 12% 5% 10% 5% 5% 5% 3 4 5% 21%	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	66% 8% 12% 12% 2	4691
	11th to 20th	Day	63% 8% 11% 14% 4	4710
	21st to end	Day	57% 7% 10% 16% 8% 1	5249
February	1st to 10th	Day	63% 6% 12% 13% 6%	1890
	11th to 20th	Day	57% 8% 13% 15% 6% 1	1877
	21st to end	Day	52% 7% 11% 17% 11% 2	1730
March	1st to 10th	Day	58% 6% 8% 16% 11% 2	4578
	11th to 20th	Day	60% 10% 11% 13% 6% 1	4996
	21st to end	Day	63% 7% 9% 12% 7% 1	5957
April	1st to 10th	Day	58% 7% 8% 15% 9% 3	2412
	11th to 20th	Day	53% 6% 9% 19% 11% 1	2374
	21st to end	Day	57% 7% 9% 15% 11% 1	2550
May	1st to 10th	Day	66% 5% 8% 14% 7% 1	6357
	11th to 20th	Day	55% 7% 14% 16% 8%	6322
	21st to end	Day	55% 7% 10% 22% 6%	6948
June	1st to 10th	Day	64% 6% 8% 15% 6%	2800
	11th to 20th	Day	54% 6% 12% 18% 8% 1	2800
	21st to end	Day	68% 6% 9% 11% 6%	2800
July	1st to 10th	Day	65% 7% 10% 10% 7%	6290
	11th to 20th	Day	56% 9% 13% 16% 6%	6284
	21st to end	Day	46% 7% 19% 21% 7%	6730
August	1st to 10th	Day	56% 9% 13% 16% 6%	4550
	11th to 20th	Day	49% 8% 17% 22% 4	4138
	21st to end	Day	41% 6% 14% 28% 11%	4312
September	1st to 10th	Day	46% 8% 12% 23% 10% 1	2400
	11th to 20th	Day	47% 8% 12% 23% 9% 1	2379
	21st to end	Day	46% 6% 10% 27% 11% 1	2170
October	1st to 10th	Day	52% 7% 14% 20% 7%	3260
	11th to 20th	Day	42% 8% 12% 26% 11% 1	3240
	21st to end	Day	52% 7% 14% 19% 8%	3227
November	1st to 10th	Day	55% 8% 13% 17% 6%	1850
	11th to 20th	Day	64% 7% 11% 13% 4	1708
	21st to end	Day	60% 8% 14% 13% 5%	1680
December	1st to 10th	Day	60% 8% 12% 14% 6%	2690
	11th to 20th	Day	64% 6% 13% 12% 4	3075
	21st to end	Day	64% 7% 13% 10% 5%	4070
January	1st to 10th	Night	63% 7% 13% 13% 3	8234
	11th to 20th	Night	69% 8% 10% 10% 2	8280
	21st to end	Night	58% 9% 13% 14% 6%	8955
February	1st to 10th	Night	65% 6% 13% 11% 6% 1	2730
	11th to 20th	Night	62% 5% 12% 15% 5% 1	2638
	21st to end	Night	55% 5% 11% 16% 11% 1	2076
March	1st to 10th	Night	59% 4 7% 16% 11% 2	5495
	11th to 20th	Night	57% 6% 13% 15% 8% 1	5168
	21st to end	Night	69% 4 5% 14% 6% 1	5217
April	1st to 10th	Night	57% 6% 10% 17% 10% 1	2011
	11th to 20th	Night	53% 5% 9% 19% 12% 1	1976
	21st to end	Night	56% 5% 8% 19% 11% 1	1843
May	1st to 10th	Night	61% 4 11% 15% 8%	3620
	11th to 20th	Night	55% 5% 12% 21% 7%	3597
	21st to end	Night	57% 6% 10% 20% 7%	3949
June	1st to 10th	Night	66% 4 8% 14% 7% 1	1600
	11th to 20th	Night	54% 5% 10% 20% 11% 1	1600
	21st to end	Night	69% 4 8% 13% 6% 1	1600
July	1st to 10th	Night	68% 5% 8% 13% 6%	3581
	11th to 20th	Night	58% 5% 9% 18% 10% 1	3572
	21st to end	Night	54% 4 14% 23% 5%	3831
August	1st to 10th	Night	57% 4 14% 20% 5%	2585
	11th to 20th	Night	51% 7% 19% 19% 4	2970
	21st to end	Night	39% 7% 17% 30% 7%	3585
September	1st to 10th	Night	44% 8% 13% 23% 12%	2000
	11th to 20th	Night	46% 6% 14% 23% 11% 1	2020
	21st to end	Night	49% 4 11% 25% 11% 1	2228
October	1st to 10th	Night	47% 7% 12% 21% 12%	3920
	11th to 20th	Night	40% 6% 14% 31% 9%	3899
	21st to end	Night	49% 6% 17% 19% 8%	4573
November	1st to 10th	Night	53% 8% 15% 16% 7%	2668
	11th to 20th	Night	61% 6% 14% 14% 6%	2909
	21st to end	Night	61% 7% 14% 13% 4	2940
December	1st to 10th	Night	53% 8% 14% 19% 6%	4720
	11th to 20th	Night	63% 6% 12% 13% 6%	5355
	21st to end	Night	63% 5% 14% 13% 4	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	28% 21% 30% 17% 4	430
	11th to 20th	Day	29% 15% 31% 15% 8% 1	430
	21st to end	Day	21% 15% 39% 20% 4	473
February	1st to 10th	Day	28% 18% 31% 20% 3	430
	11th to 20th	Day	23% 24% 30% 17% 4 1	426
	21st to end	Day	19% 17% 37% 22% 5%	355
March	1st to 10th	Day	22% 18% 36% 17% 6%	430
	11th to 20th	Day	19% 18% 33% 21% 7% 1	518
	21st to end	Day	28% 19% 30% 17% 5% 1	594
April	1st to 10th	Day	30% 21% 31% 13% 5%	504
	11th to 20th	Day	28% 15% 31% 19% 6%	492
	21st to end	Day	30% 12% 29% 24% 5% 1	590
May	1st to 10th	Day	32% 12% 30% 18% 6% 2	703
	11th to 20th	Day	25% 9% 27% 26% 11% 3	724
	21st to end	Day	27% 8% 30% 25% 9% 1	790
June	1st to 10th	Day	34% 8% 22% 25% 9% 1	730
	11th to 20th	Day	28% 6% 28% 28% 9% 2	730
	21st to end	Day	40% 5% 19% 25% 9% 2	730
July	1st to 10th	Day	29% 5% 22% 29% 13% 2	720
	11th to 20th	Day	29% 4 22% 29% 15% 1	718
	21st to end	Day	30% 6% 20% 29% 13% 2	795
August	1st to 10th	Day	21% 4 22% 33% 17% 3	725
	11th to 20th	Day	25% 7% 20% 27% 18% 2	530
	21st to end	Day	21% 7% 25% 31% 14% 3	583
September	1st to 10th	Day	18% 9% 27% 32% 12% 2	530
	11th to 20th	Day	23% 11% 25% 24% 14% 3	518
	21st to end	Day	26% 6% 24% 27% 13% 3	420
October	1st to 10th	Day	19% 9% 29% 27% 13% 3	420
	11th to 20th	Day	23% 9% 29% 23% 12% 3	420
	21st to end	Day	27% 13% 31% 19% 10%	462
November	1st to 10th	Day	25% 16% 31% 20% 7% 2	425
	11th to 20th	Day	30% 18% 29% 18% 4 1	430
	21st to end	Day	28% 19% 30% 19% 4 1	430
December	1st to 10th	Day	26% 16% 33% 20% 5%	430
	11th to 20th	Day	29% 21% 33% 14% 3	430
	21st to end	Day	22% 19% 37% 15% 6% 1	473
January	1st to 10th	Night	27% 17% 32% 20% 4	950
	11th to 20th	Night	28% 16% 34% 17% 5% 1	950
	21st to end	Night	21% 16% 37% 21% 5%	1026
February	1st to 10th	Night	27% 12% 33% 25% 3	840
	11th to 20th	Night	23% 18% 36% 18% 3 1	823
	21st to end	Night	18% 13% 34% 26% 8%	692
March	1st to 10th	Night	26% 13% 35% 22% 4	840
	11th to 20th	Night	23% 11% 31% 25% 9%	840
	21st to end	Night	29% 11% 29% 25% 5% 1	761
April	1st to 10th	Night	31% 10% 30% 24% 5%	604
	11th to 20th	Night	26% 7% 31% 27% 8%	594
	21st to end	Night	33% 8% 27% 23% 9% 1	600
May	1st to 10th	Night	37% 5% 27% 21% 9% 3	600
	11th to 20th	Night	30% 5% 25% 27% 11% 1	592
	21st to end	Night	27% 5% 29% 29% 9% 1	649
June	1st to 10th	Night	40% 5% 20% 26% 9%	600
	11th to 20th	Night	32% 5% 19% 32% 11% 1	600
	21st to end	Night	42% 5% 16% 30% 7% 1	600
July	1st to 10th	Night	32% 3 21% 30% 13% 1	586
	11th to 20th	Night	32% 5% 15% 34% 14%	582
	21st to end	Night	37% 4 17% 30% 11% 1	656
August	1st to 10th	Night	28% 3 15% 36% 16% 2	598
	11th to 20th	Night	25% 4 16% 35% 17% 3	592
	21st to end	Night	24% 4 17% 33% 21% 1	660
September	1st to 10th	Night	22% 4 20% 37% 14% 3	600
	11th to 20th	Night	25% 5% 25% 28% 14% 3	600
	21st to end	Night	25% 6% 28% 28% 11% 1	628
October	1st to 10th	Night	22% 7% 27% 31% 12% 1	800
	11th to 20th	Night	24% 7% 29% 28% 11% 2	800
	21st to end	Night	27% 11% 31% 24% 7% 1	987
November	1st to 10th	Night	27% 12% 31% 23% 6% 1	930
	11th to 20th	Night	31% 15% 27% 20% 7% 1	950
	21st to end	Night	30% 14% 33% 19% 4 1	950
December	1st to 10th	Night	30% 13% 33% 19% 5%	950
	11th to 20th	Night	31% 15% 34% 16% 3 1	950
	21st to end	Night	27% 16% 33% 19% 4 1	1045

Page generated at 2026-03-30 03:18:39. Version ac07

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