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 2025-09-04.

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 4 4 6% 8% 43%	125758
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	122964
Transparency	Day	37% 6% 12% 22% 18% 4	129357
	Night	38% 6% 10% 22% 19% 5%	127434
Seeing	Day	18% 10% 27% 25% 15% 5%	16211
	Night	17% 10% 28% 30% 13% 1	25840

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% 4 1 4 3 3 3 3 4 5% 54%	3964
	11th to 20th	Day	13% 7% 2 5% 3 3 3 3 5% 4 51%	4060
	21st to end	Day	14% 7% 3 6% 5% 4 4 3 5% 5% 45%	4466
February	1st to 10th	Day	13% 7% 3 7% 4 4 4 5% 6% 7% 40%	1800
	11th to 20th	Day	22% 7% 2 3 4 2 4 3 6% 6% 42%	1823
	21st to end	Day	17% 6% 2 7% 6% 4 4 3 5% 6% 39%	1652
March	1st to 10th	Day	17% 7% 2 7% 4 4 4 3 5% 7% 38%	4935
	11th to 20th	Day	11% 5% 2 5% 3 3 4 4 5% 6% 52%	4980
	21st to end	Day	16% 6% 2 6% 5% 4 4 3 5% 6% 43%	5505
April	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 5% 7% 50%	2400
	11th to 20th	Day	11% 6% 2 5% 4 3 4 3 5% 7% 48%	2374
	21st to end	Day	15% 6% 2 6% 4 4 4 4 5% 7% 43%	2400
May	1st to 10th	Day	13% 8% 2 7% 5% 5% 5% 5% 8% 9% 33%	5676
	11th to 20th	Day	18% 7% 2 6% 5% 4 4 4 7% 10% 32%	5901
	21st to end	Day	14% 5% 2 6% 4 4 5% 4 6% 9% 42%	6934
June	1st to 10th	Day	11% 4 2 5% 4 5% 5% 4 7% 10% 42%	2800
	11th to 20th	Day	11% 4 2 5% 5% 4 5% 5% 7% 10% 42%	2800
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 11% 42%	2800
July	1st to 10th	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 12% 40%	6278
	11th to 20th	Day	11% 6% 3 8% 6% 6% 7% 5% 9% 13% 28%	5900
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 11% 41%	6424
August	1st to 10th	Day	15% 8% 3 8% 5% 5% 6% 4 7% 11% 28%	4234
	11th to 20th	Day	9% 6% 2 8% 5% 5% 5% 5% 7% 10% 37%	3958
	21st to end	Day	8% 4 2 5% 3 3 4 4 7% 11% 49%	4312
September	1st to 10th	Day	13% 5% 2 5% 3 4 4 4 6% 7% 46%	2316
	11th to 20th	Day	11% 8% 3 6% 5% 3 3 3 5% 8% 46%	2280
	21st to end	Day	20% 7% 2 8% 4 4 4 3 4 7% 37%	1986
October	1st to 10th	Day	16% 6% 2 5% 4 4 4 3 3 6% 48%	1900
	11th to 20th	Day	15% 5% 2 4 2 3 3 3 5% 5% 53%	1900
	21st to end	Day	12% 5% 1 5% 3 3 3 2 4 6% 54%	2090
November	1st to 10th	Day	12% 5% 1 5% 3 3 4 3 4 5% 54%	1935
	11th to 20th	Day	13% 7% 2 4 3 4 3 3 4 6% 49%	1800
	21st to end	Day	14% 5% 2 5% 3 3 4 3 4 5% 53%	1625
December	1st to 10th	Day	11% 6% 2 5% 4 3 3 3 5% 6% 53%	2600
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 3 6% 47%	2990
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 8% 45%	3960
January	1st to 10th	Night	14% 6% 2 5% 3 2 3 2 4 5% 54%	5963
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 4 5% 55%	5900
	21st to end	Night	16% 7% 2 5% 3 3 3 3 4 5% 50%	6490
February	1st to 10th	Night	16% 6% 3 5% 4 3 4 4 5% 6% 43%	2600
	11th to 20th	Night	20% 7% 2 6% 4 3 4 3 4 6% 42%	2462
	21st to end	Night	16% 6% 2 6% 5% 3 3 2 4 6% 46%	1975
March	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 5% 6% 46%	4918
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 6% 51%	4980
	21st to end	Night	13% 7% 2 5% 4 4 4 3 4 6% 47%	5427
April	1st to 10th	Night	13% 7% 3 7% 5% 4 4 2 4 7% 44%	2000
	11th to 20th	Night	11% 6% 2 5% 3 3 4 3 5% 7% 49%	1975
	21st to end	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	2000
May	1st to 10th	Night	14% 8% 3 7% 5% 5% 5% 4 6% 9% 33%	4260
	11th to 20th	Night	21% 8% 3 7% 5% 4 4 4 5% 7% 32%	3991
	21st to end	Night	15% 7% 2 6% 4 5% 4 3 6% 8% 40%	3920
June	1st to 10th	Night	10% 6% 2 6% 5% 5% 6% 4 7% 8% 41%	1600
	11th to 20th	Night	10% 5% 2 6% 6% 5% 5% 4 7% 8% 41%	1600
	21st to end	Night	8% 5% 2 5% 4 4 5% 5% 8% 10% 44%	1600
July	1st to 10th	Night	9% 6% 2 8% 4 5% 5% 5% 8% 10% 39%	3662
	11th to 20th	Night	12% 7% 3 8% 6% 6% 6% 6% 8% 11% 28%	4057
	21st to end	Night	8% 6% 3 9% 6% 6% 7% 5% 8% 8% 34%	4427
August	1st to 10th	Night	16% 7% 3 8% 5% 5% 4 4 7% 9% 31%	2915
	11th to 20th	Night	10% 7% 2 7% 5% 4 5% 5% 7% 9% 38%	3057
	21st to end	Night	9% 6% 2 5% 4 4 4 4 6% 8% 48%	3586
September	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 6% 7% 45%	1931
	11th to 20th	Night	12% 5% 2 5% 5% 4 4 3 6% 6% 46%	1900
	21st to end	Night	21% 8% 3 6% 3 4 3 3 5% 6% 36%	2187
October	1st to 10th	Night	14% 5% 3 6% 3 4 4 3 4 6% 48%	2280
	11th to 20th	Night	15% 5% 2 5% 4 3 3 2 4 7% 49%	2280
	21st to end	Night	14% 5% 2 5% 3 3 4 3 4 6% 53%	2508
November	1st to 10th	Night	12% 7% 2 5% 4 4 4 3 4 5% 52%	2361
	11th to 20th	Night	15% 6% 2 5% 3 3 3 4 4 5% 49%	2600
	21st to end	Night	12% 5% 2 4 4 4 4 3 5% 7% 51%	2771
December	1st to 10th	Night	10% 5% 2 4 3 3 3 3 5% 6% 55%	4580
	11th to 20th	Night	20% 6% 3 7% 4 4 4 3 4 6% 40%	5205
	21st to end	Night	14% 7% 2 5% 3 3 4 3 5% 7% 46%	6996

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	33% 3 5% 27% 24% 9%	3965
	11th to 20th	Day	35% 4 6% 20% 26% 9%	4060
	21st to end	Day	42% 4 8% 26% 17% 3	4466
February	1st to 10th	Day	41% 5% 11% 25% 15% 3	1800
	11th to 20th	Day	42% 4 8% 22% 20% 5%	1832
	21st to end	Day	47% 4 9% 17% 18% 5%	1652
March	1st to 10th	Day	45% 5% 7% 18% 18% 8%	4935
	11th to 20th	Day	31% 5% 9% 24% 23% 9%	4980
	21st to end	Day	41% 5% 10% 19% 18% 6%	5505
April	1st to 10th	Day	33% 5% 12% 23% 20% 8%	2400
	11th to 20th	Day	34% 5% 11% 22% 22% 6%	2374
	21st to end	Day	40% 6% 11% 23% 15% 5%	2400
May	1st to 10th	Day	44% 9% 16% 19% 11% 1	5708
	11th to 20th	Day	46% 8% 13% 21% 10% 1	5905
	21st to end	Day	37% 7% 14% 23% 17% 2	6949
June	1st to 10th	Day	34% 8% 16% 26% 15% 2	2800
	11th to 20th	Day	35% 9% 16% 23% 16% 1	2800
	21st to end	Day	33% 8% 17% 24% 16% 2	2800
July	1st to 10th	Day	34% 11% 20% 18% 13% 4	6209
	11th to 20th	Day	44% 12% 22% 16% 5% 1	5831
	21st to end	Day	35% 9% 14% 23% 18% 1	6240
August	1st to 10th	Day	47% 8% 17% 22% 6%	4221
	11th to 20th	Day	39% 9% 15% 24% 13%	3997
	21st to end	Day	29% 6% 16% 29% 19% 1	4312
September	1st to 10th	Day	36% 8% 14% 16% 21% 4	2316
	11th to 20th	Day	37% 6% 9% 24% 20% 3	2280
	21st to end	Day	48% 5% 9% 20% 15% 3	1986
October	1st to 10th	Day	36% 5% 7% 19% 27% 5%	3160
	11th to 20th	Day	24% 3 5% 20% 34% 13%	3140
	21st to end	Day	21% 3 8% 29% 26% 13%	3428
November	1st to 10th	Day	33% 4 9% 26% 25% 4	1935
	11th to 20th	Day	36% 4 9% 22% 22% 7%	1800
	21st to end	Day	34% 3 7% 26% 24% 5%	1625
December	1st to 10th	Day	32% 4 8% 29% 20% 8%	2600
	11th to 20th	Day	41% 4 11% 24% 17% 4	2986
	21st to end	Day	38% 5% 11% 28% 16% 1	3960
January	1st to 10th	Night	33% 3 6% 21% 26% 10%	5963
	11th to 20th	Night	34% 3 5% 22% 28% 7%	5900
	21st to end	Night	38% 3 6% 27% 21% 5%	6490
February	1st to 10th	Night	41% 4 12% 23% 17% 3	2600
	11th to 20th	Night	45% 3 7% 20% 19% 6%	2464
	21st to end	Night	40% 4 8% 22% 18% 8%	1975
March	1st to 10th	Night	39% 4 6% 21% 23% 7%	4918
	11th to 20th	Night	33% 5% 7% 20% 24% 11%	4980
	21st to end	Night	38% 5% 7% 22% 21% 6%	5427
April	1st to 10th	Night	40% 5% 9% 18% 22% 6%	2000
	11th to 20th	Night	34% 5% 11% 23% 21% 6%	1975
	21st to end	Night	38% 6% 12% 23% 17% 5%	2000
May	1st to 10th	Night	45% 9% 17% 18% 10% 1	4290
	11th to 20th	Night	49% 7% 11% 23% 9% 1	3992
	21st to end	Night	41% 8% 14% 24% 13% 1	3927
June	1st to 10th	Night	38% 8% 14% 23% 16% 1	1600
	11th to 20th	Night	37% 10% 17% 19% 16% 1	1600
	21st to end	Night	32% 9% 14% 24% 18% 4	1600
July	1st to 10th	Night	39% 12% 16% 18% 13% 2	3623
	11th to 20th	Night	46% 12% 22% 15% 4 1	4006
	21st to end	Night	44% 9% 12% 20% 15%	4297
August	1st to 10th	Night	48% 8% 17% 22% 6%	2904
	11th to 20th	Night	37% 9% 18% 23% 12%	3084
	21st to end	Night	32% 8% 11% 28% 20% 1	3586
September	1st to 10th	Night	37% 8% 14% 19% 16% 6%	1931
	11th to 20th	Night	36% 7% 10% 24% 21% 2	1900
	21st to end	Night	48% 6% 9% 18% 16% 4	2187
October	1st to 10th	Night	38% 6% 11% 18% 22% 5%	3800
	11th to 20th	Night	24% 3 6% 22% 31% 13%	3779
	21st to end	Night	21% 3 5% 31% 26% 13%	4120
November	1st to 10th	Night	35% 4 9% 22% 24% 5%	2361
	11th to 20th	Night	37% 5% 8% 19% 22% 10%	2600
	21st to end	Night	33% 4 9% 26% 23% 5%	2771
December	1st to 10th	Night	30% 3 8% 28% 25% 6%	4580
	11th to 20th	Night	46% 4 10% 19% 15% 6%	5208
	21st to end	Night	38% 5% 11% 29% 17% 1	6996

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	26% 24% 33% 14% 4	280
	11th to 20th	Day	27% 31% 28% 11% 3	300
	21st to end	Day	23% 23% 40% 13% 1	330
February	1st to 10th	Day	23% 26% 37% 11% 3	300
	11th to 20th	Day	35% 21% 32% 10% 2	370
	21st to end	Day	28% 15% 33% 19% 4	414
March	1st to 10th	Day	23% 19% 34% 19% 5%	500
	11th to 20th	Day	23% 20% 34% 17% 5% 1	500
	21st to end	Day	24% 15% 39% 16% 5% 1	550
April	1st to 10th	Day	23% 17% 34% 18% 7% 1	500
	11th to 20th	Day	17% 13% 33% 25% 7% 4	492
	21st to end	Day	22% 13% 32% 23% 8% 1	500
May	1st to 10th	Day	17% 15% 33% 23% 10% 2	503
	11th to 20th	Day	21% 9% 29% 27% 11% 3	527
	21st to end	Day	20% 5% 23% 28% 18% 5%	573
June	1st to 10th	Day	12% 5% 27% 29% 23% 4	530
	11th to 20th	Day	12% 5% 22% 35% 18% 8%	530
	21st to end	Day	11% 5% 22% 34% 23% 6%	530
July	1st to 10th	Day	10% 2 16% 35% 26% 10%	525
	11th to 20th	Day	6% 2 15% 37% 29% 11%	524
	21st to end	Day	9% 2 19% 33% 26% 11%	576
August	1st to 10th	Day	15% 3 17% 31% 26% 7%	526
	11th to 20th	Day	11% 4 16% 30% 26% 12%	519
	21st to end	Day	9% 4 18% 29% 25% 16%	583
September	1st to 10th	Day	14% 3 17% 28% 22% 15%	502
	11th to 20th	Day	16% 4 17% 29% 21% 13%	490
	21st to end	Day	28% 7% 23% 26% 12% 3	490
October	1st to 10th	Day	21% 5% 22% 25% 19% 8%	490
	11th to 20th	Day	16% 12% 21% 29% 19% 3	490
	21st to end	Day	15% 10% 33% 26% 11% 5%	539
November	1st to 10th	Day	17% 8% 32% 25% 13% 4	483
	11th to 20th	Day	18% 14% 32% 24% 11% 1	400
	21st to end	Day	20% 15% 34% 18% 11% 1	225
December	1st to 10th	Day	24% 16% 40% 14% 7% 1	200
	11th to 20th	Day	22% 16% 33% 21% 9%	200
	21st to end	Day	20% 13% 35% 19% 11% 1	220
January	1st to 10th	Night	21% 18% 38% 20% 4	800
	11th to 20th	Night	20% 18% 34% 24% 4	800
	21st to end	Night	21% 15% 39% 24% 1	880
February	1st to 10th	Night	21% 15% 37% 25% 3	800
	11th to 20th	Night	25% 20% 36% 18% 2	783
	21st to end	Night	22% 19% 36% 21% 3	660
March	1st to 10th	Night	20% 17% 36% 24% 4	800
	11th to 20th	Night	23% 16% 33% 22% 5% 1	800
	21st to end	Night	18% 16% 38% 23% 4	880
April	1st to 10th	Night	17% 20% 38% 22% 2	800
	11th to 20th	Night	17% 13% 37% 25% 8%	791
	21st to end	Night	17% 10% 38% 29% 7%	800
May	1st to 10th	Night	16% 12% 35% 30% 7%	700
	11th to 20th	Night	18% 5% 30% 35% 11% 1	576
	21st to end	Night	17% 4 18% 40% 19% 2	433
June	1st to 10th	Night	11% 3 21% 43% 21% 1	400
	11th to 20th	Night	10% 4 21% 42% 21% 3	400
	21st to end	Night	9% 5% 21% 45% 19% 1	400
July	1st to 10th	Night	10% 2 17% 39% 29% 2	414
	11th to 20th	Night	5% 3 12% 40% 35% 5%	582
	21st to end	Night	8% 3 16% 42% 26% 5%	657
August	1st to 10th	Night	12% 3 13% 42% 26% 4	597
	11th to 20th	Night	13% 3 13% 42% 27% 3	698
	21st to end	Night	11% 4 13% 36% 30% 5%	880
September	1st to 10th	Night	16% 2 13% 36% 28% 5%	773
	11th to 20th	Night	14% 3 14% 36% 29% 4	760
	21st to end	Night	25% 4 21% 30% 19% 1	760
October	1st to 10th	Night	17% 4 21% 30% 24% 3	760
	11th to 20th	Night	18% 9% 24% 31% 16% 1	760
	21st to end	Night	16% 10% 28% 29% 15% 1	836
November	1st to 10th	Night	16% 7% 30% 31% 13% 2	780
	11th to 20th	Night	19% 11% 30% 31% 9%	800
	21st to end	Night	14% 13% 32% 31% 9% 1	800
December	1st to 10th	Night	20% 13% 35% 23% 8%	800
	11th to 20th	Night	19% 11% 36% 30% 6%	800
	21st to end	Night	20% 14% 29% 32% 5%	880

Page generated at 2025-09-01 01:51:09. Version ac07

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