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 2026-06-01.

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% 9% 4 8% 5% 5% 5% 4 5% 7% 34%	131874
	Night	14% 10% 4 8% 5% 4 4 3 5% 7% 35%	131409
Transparency	Day	47% 5% 11% 22% 13% 2	135500
	Night	48% 4 10% 22% 14% 2	135854
Seeing	Day	17% 10% 27% 23% 16% 7%	17553
	Night	18% 9% 28% 28% 14% 2	27391

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	17% 14% 8% 12% 6% 4 4 3 5% 5% 21%	4691
	11th to 20th	Day	11% 13% 6% 16% 8% 9% 5% 4 3 3 21%	4740
	21st to end	Day	18% 14% 7% 13% 6% 6% 5% 3 4 5% 19%	5780
February	1st to 10th	Day	17% 17% 6% 10% 6% 4 4 4 3 5% 25%	2011
	11th to 20th	Day	15% 11% 5% 13% 7% 4 5% 3 4 6% 26%	2048
	21st to end	Day	17% 13% 5% 11% 4 4 4 3 4 6% 30%	1730
March	1st to 10th	Day	19% 14% 8% 10% 4 4 3 2 3 4 28%	4580
	11th to 20th	Day	11% 11% 3 7% 4 4 4 3 5% 7% 40%	5077
	21st to end	Day	18% 11% 4 8% 5% 5% 4 3 5% 6% 31%	5601
April	1st to 10th	Day	16% 12% 3 8% 6% 4 4 3 5% 6% 33%	2520
	11th to 20th	Day	16% 10% 4 7% 5% 3 5% 3 5% 7% 37%	2538
	21st to end	Day	19% 8% 3 7% 5% 4 4 3 6% 7% 34%	2730
May	1st to 10th	Day	21% 9% 3 8% 5% 5% 4 3 5% 7% 28%	5992
	11th to 20th	Day	19% 8% 3 7% 4 4 4 3 5% 6% 36%	5991
	21st to end	Day	15% 7% 3 8% 5% 5% 5% 4 7% 10% 31%	6576
June	1st to 10th	Day	13% 5% 3 6% 5% 4 5% 4 7% 7% 41%	2718
	11th to 20th	Day	10% 6% 3 6% 4 5% 5% 4 7% 8% 42%	2800
	21st to end	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 8% 40%	2800
July	1st to 10th	Day	10% 6% 2 7% 6% 5% 5% 5% 7% 9% 38%	6360
	11th to 20th	Day	9% 5% 2 7% 5% 6% 5% 5% 7% 11% 38%	6360
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 5% 7% 9% 35%	6434
August	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 4 7% 10% 45%	4234
	11th to 20th	Day	9% 6% 2 6% 5% 5% 6% 5% 7% 10% 40%	4174
	21st to end	Day	6% 5% 2 5% 4 4 4 3 6% 9% 52%	4313
September	1st to 10th	Day	11% 5% 2 5% 4 5% 4 3 5% 7% 49%	2305
	11th to 20th	Day	10% 5% 2 6% 5% 4 4 3 7% 8% 49%	2200
	21st to end	Day	15% 7% 2 6% 4 3 3 3 4 6% 47%	2200
October	1st to 10th	Day	12% 5% 2 6% 4 3 4 3 5% 6% 49%	2200
	11th to 20th	Day	14% 7% 2 4 3 4 5% 4 5% 6% 46%	2048
	21st to end	Day	19% 9% 3 8% 4 4 4 3 5% 6% 35%	2000
November	1st to 10th	Day	10% 7% 2 5% 4 4 3 3 5% 6% 49%	1852
	11th to 20th	Day	16% 12% 4 7% 5% 4 4 4 5% 6% 34%	1890
	21st to end	Day	19% 13% 5% 9% 4 3 3 3 4 6% 31%	1890
December	1st to 10th	Day	16% 13% 5% 9% 3 3 3 3 4 5% 37%	3020
	11th to 20th	Day	18% 14% 7% 11% 4 5% 5% 3 4 5% 23%	3401
	21st to end	Day	23% 21% 7% 10% 5% 4 3 3 4 5% 15%	4070
January	1st to 10th	Night	14% 15% 7% 14% 7% 6% 5% 3 4 5% 20%	8232
	11th to 20th	Night	10% 16% 8% 18% 6% 7% 5% 3 4 4 20%	8244
	21st to end	Night	17% 13% 5% 8% 5% 5% 4 4 5% 7% 28%	8401
February	1st to 10th	Night	18% 13% 5% 9% 7% 5% 3 3 4 5% 27%	2604
	11th to 20th	Night	15% 11% 4 9% 6% 4 4 4 4 7% 31%	2458
	21st to end	Night	17% 14% 4 10% 5% 4 3 3 4 6% 30%	2076
March	1st to 10th	Night	19% 14% 3 6% 3 4 3 3 3 4 38%	5493
	11th to 20th	Night	12% 7% 3 7% 4 4 4 3 5% 7% 43%	5103
	21st to end	Night	18% 9% 3 8% 4 5% 4 3 5% 7% 35%	5588
April	1st to 10th	Night	15% 10% 3 6% 4 3 4 4 5% 8% 36%	2115
	11th to 20th	Night	15% 9% 2 6% 4 4 4 4 6% 7% 38%	2029
	21st to end	Night	16% 8% 2 7% 5% 5% 4 3 5% 7% 37%	1890
May	1st to 10th	Night	21% 11% 3 7% 4 5% 4 3 4 7% 28%	4123
	11th to 20th	Night	15% 8% 3 7% 4 4 5% 4 6% 8% 37%	4122
	21st to end	Night	14% 7% 3 7% 4 4 4 3 6% 8% 39%	4520
June	1st to 10th	Night	10% 5% 2 6% 4 5% 4 4 5% 8% 47%	1676
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 5% 10% 47%	1600
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 7% 10% 47%	1600
July	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 47%	3618
	11th to 20th	Night	9% 5% 2 5% 4 4 5% 4 6% 10% 45%	3617
	21st to end	Night	9% 6% 2 6% 5% 4 5% 4 7% 11% 40%	4414
August	1st to 10th	Night	9% 6% 2 7% 4 4 4 3 6% 9% 46%	2915
	11th to 20th	Night	7% 7% 3 6% 4 4 4 4 5% 8% 48%	2875
	21st to end	Night	4 3 2 6% 4 4 5% 3 7% 9% 53%	3581
September	1st to 10th	Night	11% 5% 1 5% 4 4 4 3 5% 7% 51%	2075
	11th to 20th	Night	8% 5% 2 5% 3 3 3 4 5% 8% 55%	2200
	21st to end	Night	14% 5% 2 5% 3 3 5% 3 4 6% 49%	2200
October	1st to 10th	Night	9% 5% 2 6% 3 4 3 3 5% 7% 54%	2200
	11th to 20th	Night	14% 7% 3 6% 4 3 3 3 5% 6% 45%	2350
	21st to end	Night	18% 8% 2 6% 4 4 4 3 4 7% 40%	2835
November	1st to 10th	Night	12% 8% 3 5% 4 3 4 4 5% 6% 46%	2666
	11th to 20th	Night	16% 9% 4 6% 5% 3 4 3 5% 7% 38%	2730
	21st to end	Night	18% 14% 5% 8% 5% 4 4 3 5% 5% 28%	2730
December	1st to 10th	Night	17% 11% 3 9% 5% 4 4 3 4 5% 35%	4380
	11th to 20th	Night	19% 16% 7% 11% 4 3 4 3 4 5% 23%	4999
	21st to end	Night	25% 18% 7% 10% 5% 4 4 3 4 4 16%	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	64% 3 10% 17% 5%	4692
	11th to 20th	Day	68% 3 8% 11% 9% 1	4740
	21st to end	Day	68% 2 8% 13% 6% 2	5780
February	1st to 10th	Day	61% 4 9% 16% 9% 1	2011
	11th to 20th	Day	60% 3 9% 19% 9% 1	2058
	21st to end	Day	56% 4 10% 21% 8% 1	1730
March	1st to 10th	Day	61% 2 6% 18% 11% 1	4580
	11th to 20th	Day	44% 3 9% 30% 13% 1	5077
	21st to end	Day	53% 5% 12% 19% 9% 2	5601
April	1st to 10th	Day	51% 5% 11% 21% 13% 1	2520
	11th to 20th	Day	48% 4 11% 22% 12% 3	2538
	21st to end	Day	49% 5% 10% 23% 11% 2	2730
May	1st to 10th	Day	54% 5% 12% 18% 11%	6027
	11th to 20th	Day	49% 5% 11% 24% 11%	5995
	21st to end	Day	46% 6% 12% 23% 12% 1	6590
June	1st to 10th	Day	39% 6% 10% 26% 17% 3	2718
	11th to 20th	Day	37% 6% 14% 23% 19% 2	2800
	21st to end	Day	39% 7% 14% 20% 16% 4	2800
July	1st to 10th	Day	40% 5% 10% 26% 18% 1	6291
	11th to 20th	Day	38% 7% 15% 24% 15% 1	6285
	21st to end	Day	44% 7% 13% 20% 14% 2	6250
August	1st to 10th	Day	32% 7% 15% 32% 14% 1	4221
	11th to 20th	Day	36% 6% 13% 31% 15% 1	4213
	21st to end	Day	29% 6% 13% 31% 17% 3	4312
September	1st to 10th	Day	35% 5% 11% 25% 19% 5%	2305
	11th to 20th	Day	33% 4 10% 28% 23% 3	2200
	21st to end	Day	39% 3 9% 23% 23% 2	2200
October	1st to 10th	Day	25% 3 9% 30% 32% 2	3590
	11th to 20th	Day	31% 3 9% 30% 25% 2	3314
	21st to end	Day	40% 2 10% 24% 22% 2	3212
November	1st to 10th	Day	35% 5% 11% 29% 18% 3	1852
	11th to 20th	Day	51% 3 9% 20% 13% 3	1890
	21st to end	Day	55% 3 9% 21% 10% 1	1890
December	1st to 10th	Day	51% 2 10% 26% 10%	3020
	11th to 20th	Day	63% 4 9% 18% 6%	3398
	21st to end	Day	73% 3 8% 11% 5%	4070
January	1st to 10th	Night	66% 3 9% 15% 7%	8233
	11th to 20th	Night	68% 3 7% 11% 9% 2	8244
	21st to end	Night	57% 2 11% 20% 8% 1	8401
February	1st to 10th	Night	59% 4 10% 18% 8% 1	2604
	11th to 20th	Night	53% 4 11% 19% 12% 1	2459
	21st to end	Night	56% 3 12% 17% 10% 2	2076
March	1st to 10th	Night	52% 3 6% 27% 11% 2	5493
	11th to 20th	Night	41% 4 11% 27% 16% 2	5103
	21st to end	Night	50% 4 9% 26% 10% 2	5588
April	1st to 10th	Night	47% 4 11% 24% 13% 1	2115
	11th to 20th	Night	43% 5% 11% 20% 17% 4	2029
	21st to end	Night	45% 5% 12% 23% 13% 3	1890
May	1st to 10th	Night	56% 5% 9% 19% 8% 3	4150
	11th to 20th	Night	44% 6% 13% 25% 12% 1	4123
	21st to end	Night	41% 6% 16% 23% 11% 3	4528
June	1st to 10th	Night	35% 5% 9% 26% 22% 2	1676
	11th to 20th	Night	33% 6% 15% 23% 19% 4	1600
	21st to end	Night	30% 7% 17% 24% 18% 4	1600
July	1st to 10th	Night	34% 6% 12% 25% 18% 5%	3580
	11th to 20th	Night	34% 6% 16% 28% 15% 1	3572
	21st to end	Night	37% 6% 14% 25% 15% 3	4285
August	1st to 10th	Night	35% 6% 14% 31% 12% 1	2904
	11th to 20th	Night	34% 5% 13% 30% 18%	2902
	21st to end	Night	26% 6% 14% 29% 22% 3	3581
September	1st to 10th	Night	32% 5% 12% 25% 20% 6%	2075
	11th to 20th	Night	28% 4 10% 27% 28% 4	2200
	21st to end	Night	37% 4 9% 23% 25% 2	2200
October	1st to 10th	Night	21% 3 7% 30% 37% 3	3590
	11th to 20th	Night	29% 3 10% 24% 33% 2	3823
	21st to end	Night	35% 3 7% 26% 25% 4	4573
November	1st to 10th	Night	38% 4 8% 25% 22% 3	2666
	11th to 20th	Night	45% 4 8% 24% 17% 3	2730
	21st to end	Night	56% 4 9% 20% 9% 1	2730
December	1st to 10th	Night	51% 3 11% 24% 10%	4380
	11th to 20th	Night	63% 4 9% 17% 7% 1	5001
	21st to end	Night	72% 4 8% 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	24% 16% 35% 19% 5% 1	330
	11th to 20th	Day	24% 23% 36% 14% 3	330
	21st to end	Day	29% 18% 31% 17% 4	363
February	1st to 10th	Day	26% 14% 37% 16% 6%	330
	11th to 20th	Day	24% 19% 36% 16% 4	330
	21st to end	Day	22% 18% 43% 15% 2	273
March	1st to 10th	Day	24% 19% 34% 20% 3	330
	11th to 20th	Day	24% 15% 36% 18% 6% 1	527
	21st to end	Day	26% 16% 35% 17% 6% 1	594
April	1st to 10th	Day	20% 17% 32% 20% 8% 2	540
	11th to 20th	Day	20% 16% 31% 20% 10% 3	532
	21st to end	Day	21% 12% 34% 21% 10% 1	540
May	1st to 10th	Day	24% 11% 32% 22% 8% 4	543
	11th to 20th	Day	21% 8% 28% 23% 12% 8%	567
	21st to end	Day	13% 7% 25% 27% 16% 11%	617
June	1st to 10th	Day	12% 6% 25% 32% 19% 6%	648
	11th to 20th	Day	12% 5% 25% 31% 22% 6%	730
	21st to end	Day	9% 6% 26% 31% 19% 9%	730
July	1st to 10th	Day	12% 3 16% 26% 25% 17%	721
	11th to 20th	Day	8% 4 21% 29% 25% 13%	718
	21st to end	Day	10% 4 16% 28% 27% 15%	586
August	1st to 10th	Day	8% 4 13% 26% 31% 17%	526
	11th to 20th	Day	10% 5% 15% 29% 26% 14%	519
	21st to end	Day	8% 4 11% 28% 34% 14%	583
September	1st to 10th	Day	10% 5% 15% 25% 29% 14%	530
	11th to 20th	Day	10% 2 13% 24% 34% 17%	530
	21st to end	Day	18% 10% 17% 23% 22% 9%	530
October	1st to 10th	Day	11% 6% 24% 28% 21% 11%	530
	11th to 20th	Day	15% 13% 29% 26% 15% 3	530
	21st to end	Day	27% 11% 36% 20% 5%	383
November	1st to 10th	Day	14% 15% 35% 23% 11% 3	330
	11th to 20th	Day	23% 20% 27% 18% 9% 2	330
	21st to end	Day	28% 12% 37% 18% 5%	330
December	1st to 10th	Day	18% 21% 38% 17% 5% 1	330
	11th to 20th	Day	25% 15% 37% 16% 6% 1	330
	21st to end	Day	29% 18% 34% 15% 5%	363
January	1st to 10th	Night	21% 16% 35% 22% 5%	1050
	11th to 20th	Night	24% 16% 38% 19% 3	1040
	21st to end	Night	22% 13% 40% 21% 4	924
February	1st to 10th	Night	24% 13% 37% 24% 3	840
	11th to 20th	Night	21% 16% 37% 24% 2	823
	21st to end	Night	25% 12% 38% 22% 3	692
March	1st to 10th	Night	23% 13% 33% 25% 5% 1	840
	11th to 20th	Night	18% 14% 37% 25% 6%	840
	21st to end	Night	24% 12% 31% 26% 6% 1	924
April	1st to 10th	Night	19% 11% 36% 26% 7% 1	645
	11th to 20th	Night	19% 11% 32% 28% 9% 2	623
	21st to end	Night	19% 8% 39% 22% 11% 1	630
May	1st to 10th	Night	23% 7% 32% 28% 10% 1	630
	11th to 20th	Night	19% 6% 25% 31% 16% 2	621
	21st to end	Night	12% 3 24% 35% 23% 2	683
June	1st to 10th	Night	12% 3 23% 40% 22% 1	600
	11th to 20th	Night	11% 3 20% 39% 26% 2	600
	21st to end	Night	7% 3 21% 41% 25% 3	600
July	1st to 10th	Night	10% 3 13% 34% 33% 7%	585
	11th to 20th	Night	9% 2 14% 38% 32% 5%	582
	21st to end	Night	9% 3 11% 40% 34% 3	657
August	1st to 10th	Night	8% 2 12% 38% 36% 5%	597
	11th to 20th	Night	7% 2 11% 42% 31% 6%	594
	21st to end	Night	6% 3 12% 36% 37% 7%	660
September	1st to 10th	Night	12% 3 13% 35% 31% 5%	675
	11th to 20th	Night	9% 4 11% 35% 36% 6%	800
	21st to end	Night	16% 4 20% 33% 23% 4	800
October	1st to 10th	Night	11% 6% 24% 32% 23% 5%	800
	11th to 20th	Night	18% 10% 27% 30% 14% 2	800
	21st to end	Night	21% 10% 35% 25% 9%	880
November	1st to 10th	Night	14% 10% 34% 27% 14% 1	820
	11th to 20th	Night	19% 16% 31% 24% 8% 1	840
	21st to end	Night	23% 15% 30% 26% 5% 1	840
December	1st to 10th	Night	24% 12% 36% 23% 5% 1	840
	11th to 20th	Night	26% 10% 34% 25% 4 1	861
	21st to end	Night	29% 14% 33% 20% 4	1155

Page generated at 2026-05-29 01:53:30. Version ac07

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