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 2024-01-16.

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	22% 10% 4 7% 6% 4 5% 5% 5% 7% 25%	74776
	Night	23% 11% 4 7% 6% 4 5% 5% 5% 6% 25%	75040
Transparency	Day	58% 7% 11% 16% 7% 1	74479
	Night	59% 5% 12% 16% 7% 1	74872
Seeing	Day	27% 12% 28% 22% 9% 2	16503
	Night	28% 10% 28% 25% 8% 1	23972

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	19% 12% 6% 10% 7% 4 6% 6% 6% 7% 17%	1516
	11th to 20th	Day	21% 13% 5% 8% 8% 6% 6% 5% 5% 6% 16%	1481
	21st to end	Day	15% 11% 6% 10% 9% 5% 8% 6% 6% 8% 17%	1611
February	1st to 10th	Day	20% 8% 4 12% 6% 5% 6% 7% 7% 8% 16%	1620
	11th to 20th	Day	17% 11% 3 8% 8% 5% 8% 6% 6% 9% 20%	1595
	21st to end	Day	12% 10% 4 9% 8% 5% 6% 5% 5% 7% 29%	1480
March	1st to 10th	Day	16% 11% 5% 8% 6% 4 5% 5% 6% 7% 27%	1796
	11th to 20th	Day	16% 9% 5% 7% 8% 5% 6% 5% 5% 7% 27%	1944
	21st to end	Day	18% 11% 4 7% 7% 4 6% 5% 5% 6% 26%	2317
April	1st to 10th	Day	23% 9% 4 7% 6% 3 5% 5% 4 6% 26%	2160
	11th to 20th	Day	23% 10% 4 7% 4 3 4 4 5% 6% 31%	2134
	21st to end	Day	27% 11% 4 5% 5% 3 3 3 4 6% 29%	2294
May	1st to 10th	Day	28% 9% 4 6% 5% 3 5% 4 5% 6% 25%	2520
	11th to 20th	Day	23% 11% 3 7% 6% 3 4 4 4 6% 29%	2478
	21st to end	Day	25% 10% 4 6% 5% 3 4 5% 5% 7% 27%	2724
June	1st to 10th	Day	32% 9% 4 7% 5% 3 5% 4 5% 6% 20%	2520
	11th to 20th	Day	23% 10% 4 6% 6% 3 5% 5% 6% 7% 27%	2520
	21st to end	Day	33% 14% 5% 6% 5% 3 5% 4 3 5% 17%	2520
July	1st to 10th	Day	21% 10% 4 8% 6% 3 5% 5% 5% 8% 26%	2520
	11th to 20th	Day	23% 12% 4 6% 4 4 5% 5% 6% 7% 23%	2520
	21st to end	Day	25% 11% 4 7% 6% 3 6% 4 5% 7% 22%	2745
August	1st to 10th	Day	19% 9% 3 8% 6% 3 4 5% 6% 8% 29%	2506
	11th to 20th	Day	20% 9% 3 7% 7% 3 4 5% 6% 8% 29%	2273
	21st to end	Day	19% 8% 3 6% 5% 3 4 5% 5% 9% 34%	2376
September	1st to 10th	Day	19% 7% 4 5% 4 3 5% 5% 6% 8% 35%	2160
	11th to 20th	Day	20% 8% 2 5% 6% 3 4 5% 5% 8% 33%	2142
	21st to end	Day	22% 8% 3 5% 4 2 3 4 6% 7% 37%	1954
October	1st to 10th	Day	19% 7% 3 6% 5% 3 5% 6% 5% 9% 33%	1800
	11th to 20th	Day	21% 8% 3 8% 6% 4 5% 5% 6% 9% 26%	1800
	21st to end	Day	22% 12% 4 8% 6% 4 5% 4 5% 7% 23%	1805
November	1st to 10th	Day	24% 9% 4 5% 6% 3 5% 6% 4 8% 26%	1670
	11th to 20th	Day	22% 11% 4 8% 7% 5% 6% 5% 6% 6% 20%	1546
	21st to end	Day	18% 9% 5% 8% 8% 5% 5% 7% 7% 8% 20%	1520
December	1st to 10th	Day	23% 13% 4 10% 8% 4 6% 6% 5% 7% 15%	1520
	11th to 20th	Day	27% 14% 6% 11% 7% 3 6% 5% 6% 6% 10%	1906
	21st to end	Day	21% 10% 5% 10% 9% 5% 7% 5% 5% 5% 17%	2783
January	1st to 10th	Night	20% 13% 6% 9% 6% 5% 6% 6% 6% 8% 16%	2649
	11th to 20th	Night	21% 12% 5% 8% 8% 4 6% 6% 6% 7% 16%	2601
	21st to end	Night	17% 10% 5% 9% 8% 5% 7% 6% 7% 9% 18%	2741
February	1st to 10th	Night	21% 9% 4 9% 7% 5% 6% 7% 7% 7% 18%	2340
	11th to 20th	Night	17% 12% 5% 9% 8% 5% 7% 6% 6% 7% 19%	2250
	21st to end	Night	11% 11% 6% 9% 8% 4 5% 5% 5% 6% 30%	1776
March	1st to 10th	Night	16% 13% 5% 9% 6% 4 6% 5% 5% 7% 26%	2151
	11th to 20th	Night	18% 10% 4 5% 6% 4 5% 5% 6% 7% 31%	2013
	21st to end	Night	22% 11% 4 7% 6% 4 5% 4 4 5% 28%	2039
April	1st to 10th	Night	25% 9% 4 6% 4 3 4 5% 5% 6% 28%	1800
	11th to 20th	Night	22% 9% 4 7% 5% 2 3 4 5% 6% 32%	1776
	21st to end	Night	26% 12% 3 6% 4 3 3 4 4 5% 30%	1660
May	1st to 10th	Night	32% 9% 3 4 4 2 4 5% 3 5% 29%	1440
	11th to 20th	Night	24% 10% 2 5% 3 2 3 4 4 6% 36%	1416
	21st to end	Night	22% 12% 4 7% 5% 3 5% 4 5% 7% 26%	1550
June	1st to 10th	Night	34% 12% 4 7% 5% 3 3 3 3 5% 20%	1440
	11th to 20th	Night	28% 10% 3 5% 4 2 4 3 4 5% 30%	1440
	21st to end	Night	36% 16% 3 6% 4 2 3 2 4 5% 18%	1440
July	1st to 10th	Night	26% 14% 4 7% 6% 3 4 4 4 6% 22%	1440
	11th to 20th	Night	30% 12% 4 6% 4 3 5% 4 5% 6% 22%	1440
	21st to end	Night	29% 15% 3 6% 5% 2 5% 5% 4 5% 20%	1573
August	1st to 10th	Night	22% 10% 3 6% 5% 3 4 4 6% 7% 29%	1433
	11th to 20th	Night	22% 11% 3 6% 5% 3 4 5% 6% 7% 28%	1623
	21st to end	Night	20% 9% 3 4 5% 3 4 4 5% 7% 37%	1980
September	1st to 10th	Night	17% 6% 4 5% 5% 3 5% 4 6% 7% 38%	1800
	11th to 20th	Night	23% 8% 2 4 5% 3 3 5% 5% 7% 36%	1818
	21st to end	Night	21% 10% 3 5% 4 3 4 3 3 5% 38%	2004
October	1st to 10th	Night	19% 6% 2 5% 5% 3 4 5% 5% 7% 38%	2160
	11th to 20th	Night	22% 8% 2 6% 5% 3 4 4 5% 7% 34%	2160
	21st to end	Night	24% 9% 4 7% 6% 3 4 4 5% 6% 28%	2551
November	1st to 10th	Night	21% 10% 4 5% 5% 4 4 5% 5% 8% 28%	2408
	11th to 20th	Night	24% 11% 3 6% 6% 5% 5% 5% 5% 7% 23%	2631
	21st to end	Night	21% 10% 4 7% 6% 5% 6% 6% 6% 8% 21%	2660
December	1st to 10th	Night	24% 12% 5% 7% 7% 4 6% 6% 6% 6% 17%	2660
	11th to 20th	Night	29% 10% 4 10% 8% 5% 5% 5% 5% 5% 15%	3293
	21st to end	Night	28% 12% 5% 8% 7% 4 5% 5% 4 5% 19%	4884

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	65% 6% 11% 13% 3	1517
	11th to 20th	Day	69% 6% 12% 10% 3	1481
	21st to end	Day	64% 5% 10% 15% 6%	1611
February	1st to 10th	Day	62% 6% 13% 13% 6%	1620
	11th to 20th	Day	58% 8% 13% 14% 6% 1	1603
	21st to end	Day	53% 7% 11% 16% 11% 2	1480
March	1st to 10th	Day	55% 7% 8% 19% 9% 2	1800
	11th to 20th	Day	56% 7% 8% 18% 9% 1	1944
	21st to end	Day	58% 7% 8% 16% 10% 2	2317
April	1st to 10th	Day	57% 7% 8% 15% 9% 3	2160
	11th to 20th	Day	54% 6% 8% 19% 12% 2	2134
	21st to end	Day	58% 7% 10% 15% 10% 1	2294
May	1st to 10th	Day	60% 6% 10% 14% 9% 2	2520
	11th to 20th	Day	55% 7% 8% 21% 8% 1	2482
	21st to end	Day	57% 6% 10% 19% 8% 1	2724
June	1st to 10th	Day	64% 6% 9% 16% 5%	2520
	11th to 20th	Day	55% 7% 11% 18% 9% 1	2520
	21st to end	Day	69% 6% 8% 11% 6%	2520
July	1st to 10th	Day	55% 7% 13% 16% 7%	2450
	11th to 20th	Day	58% 6% 15% 15% 6%	2444
	21st to end	Day	61% 6% 13% 13% 6%	2592
August	1st to 10th	Day	51% 8% 14% 20% 7%	2492
	11th to 20th	Day	52% 8% 13% 20% 6%	2273
	21st to end	Day	48% 7% 13% 22% 8% 1	2376
September	1st to 10th	Day	46% 8% 13% 22% 9% 1	2160
	11th to 20th	Day	49% 8% 11% 22% 10% 1	2142
	21st to end	Day	46% 6% 10% 27% 10% 1	1954
October	1st to 10th	Day	46% 8% 14% 23% 10%	1800
	11th to 20th	Day	54% 7% 13% 20% 5%	1800
	21st to end	Day	60% 6% 13% 14% 7%	1805
November	1st to 10th	Day	55% 8% 12% 18% 7%	1670
	11th to 20th	Day	62% 7% 12% 14% 4 1	1546
	21st to end	Day	59% 8% 14% 13% 5%	1520
December	1st to 10th	Day	68% 6% 13% 10% 3	1520
	11th to 20th	Day	74% 5% 8% 10% 3	1905
	21st to end	Day	67% 7% 15% 9% 2 1	2783
January	1st to 10th	Night	65% 4 14% 13% 3	2649
	11th to 20th	Night	66% 6% 14% 11% 3	2601
	21st to end	Night	62% 6% 14% 14% 4	2741
February	1st to 10th	Night	64% 5% 13% 12% 6% 1	2340
	11th to 20th	Night	63% 4 13% 14% 4 1	2253
	21st to end	Night	55% 5% 11% 15% 11% 2	1776
March	1st to 10th	Night	59% 5% 10% 17% 9%	2160
	11th to 20th	Night	52% 4 13% 20% 9% 1	2014
	21st to end	Night	60% 4 8% 17% 9% 3	2039
April	1st to 10th	Night	56% 6% 10% 17% 10% 1	1800
	11th to 20th	Night	53% 5% 9% 19% 13% 2	1776
	21st to end	Night	57% 4 8% 19% 11% 1	1660
May	1st to 10th	Night	58% 4 10% 18% 10% 1	1440
	11th to 20th	Night	51% 3 9% 23% 14% 1	1417
	21st to end	Night	58% 5% 10% 19% 8%	1551
June	1st to 10th	Night	68% 4 8% 14% 6% 1	1440
	11th to 20th	Night	56% 5% 10% 18% 10%	1440
	21st to end	Night	70% 4 7% 12% 6% 1	1440
July	1st to 10th	Night	64% 5% 11% 13% 7%	1401
	11th to 20th	Night	62% 6% 7% 16% 9%	1392
	21st to end	Night	65% 5% 14% 11% 4 1	1486
August	1st to 10th	Night	55% 5% 14% 21% 5%	1422
	11th to 20th	Night	54% 7% 15% 18% 6%	1623
	21st to end	Night	46% 6% 15% 24% 8%	1980
September	1st to 10th	Night	44% 8% 14% 23% 12%	1800
	11th to 20th	Night	48% 5% 12% 22% 11% 1	1818
	21st to end	Night	50% 3 10% 25% 11% 1	2004
October	1st to 10th	Night	44% 6% 15% 23% 11% 1	2160
	11th to 20th	Night	51% 5% 15% 25% 5%	2160
	21st to end	Night	57% 5% 13% 17% 7%	2551
November	1st to 10th	Night	54% 8% 14% 16% 7%	2408
	11th to 20th	Night	59% 6% 14% 14% 6%	2631
	21st to end	Night	60% 7% 15% 14% 4	2660
December	1st to 10th	Night	64% 7% 13% 12% 3	2660
	11th to 20th	Night	70% 4 12% 9% 4	3295
	21st to end	Night	69% 4 14% 11% 2	4884

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	29% 20% 32% 15% 4 1	370
	11th to 20th	Day	28% 15% 34% 15% 7% 1	356
	21st to end	Day	23% 14% 38% 22% 4	374
February	1st to 10th	Day	29% 19% 32% 18% 1	340
	11th to 20th	Day	26% 23% 30% 16% 4 1	336
	21st to end	Day	16% 19% 39% 20% 6%	280
March	1st to 10th	Day	21% 19% 35% 17% 6%	340
	11th to 20th	Day	19% 17% 34% 20% 8% 1	404
	21st to end	Day	25% 19% 31% 18% 5% 2	462
April	1st to 10th	Day	28% 21% 33% 12% 5%	420
	11th to 20th	Day	28% 14% 32% 18% 6%	412
	21st to end	Day	32% 10% 28% 24% 6% 1	500
May	1st to 10th	Day	32% 11% 32% 17% 6% 2	603
	11th to 20th	Day	25% 8% 27% 26% 10% 3	624
	21st to end	Day	30% 7% 31% 24% 8% 1	680
June	1st to 10th	Day	36% 7% 21% 25% 9% 1	630
	11th to 20th	Day	29% 6% 27% 27% 9% 2	630
	21st to end	Day	43% 6% 17% 22% 10% 2	630
July	1st to 10th	Day	29% 5% 23% 29% 13% 2	620
	11th to 20th	Day	30% 4 24% 28% 14% 1	618
	21st to end	Day	33% 6% 20% 26% 12% 2	685
August	1st to 10th	Day	22% 5% 23% 31% 16% 3	625
	11th to 20th	Day	28% 7% 20% 28% 14% 3	450
	21st to end	Day	23% 6% 22% 31% 14% 3	495
September	1st to 10th	Day	18% 9% 30% 32% 10% 3	450
	11th to 20th	Day	25% 12% 26% 23% 11% 3	441
	21st to end	Day	28% 7% 25% 24% 12% 3	360
October	1st to 10th	Day	21% 7% 31% 27% 11% 3	360
	11th to 20th	Day	25% 9% 29% 22% 13% 3	360
	21st to end	Day	28% 12% 33% 17% 9%	396
November	1st to 10th	Day	25% 16% 29% 20% 8% 2	365
	11th to 20th	Day	28% 18% 30% 19% 4 1	370
	21st to end	Day	27% 19% 29% 19% 5% 1	370
December	1st to 10th	Day	28% 15% 33% 19% 5%	370
	11th to 20th	Day	29% 22% 33% 13% 3	370
	21st to end	Day	21% 20% 37% 14% 7% 1	407
January	1st to 10th	Night	27% 16% 32% 20% 4	850
	11th to 20th	Night	27% 16% 36% 17% 3	829
	21st to end	Night	23% 14% 36% 23% 5%	866
February	1st to 10th	Night	27% 13% 34% 24% 2	720
	11th to 20th	Night	24% 19% 37% 17% 3 1	703
	21st to end	Night	17% 14% 34% 26% 8%	592
March	1st to 10th	Night	25% 12% 36% 23% 5%	720
	11th to 20th	Night	23% 12% 30% 25% 9% 1	720
	21st to end	Night	27% 11% 28% 27% 6% 1	653
April	1st to 10th	Night	30% 10% 29% 25% 6%	540
	11th to 20th	Night	27% 6% 31% 27% 9%	534
	21st to end	Night	34% 8% 25% 23% 10% 1	540
May	1st to 10th	Night	36% 4 27% 22% 8% 3	540
	11th to 20th	Night	30% 5% 26% 27% 11% 2	532
	21st to end	Night	28% 5% 30% 27% 8% 1	583
June	1st to 10th	Night	41% 5% 20% 26% 8%	540
	11th to 20th	Night	33% 5% 19% 30% 11% 1	540
	21st to end	Night	43% 5% 16% 28% 7% 1	540
July	1st to 10th	Night	32% 3 22% 31% 12% 1	526
	11th to 20th	Night	33% 5% 16% 34% 12%	522
	21st to end	Night	39% 4 16% 29% 11% 1	590
August	1st to 10th	Night	29% 3 16% 36% 15% 1	538
	11th to 20th	Night	27% 4 16% 36% 15% 2	532
	21st to end	Night	24% 4 16% 33% 21% 1	594
September	1st to 10th	Night	20% 4 21% 38% 14% 3	540
	11th to 20th	Night	27% 5% 26% 27% 12% 2	540
	21st to end	Night	27% 6% 29% 26% 11% 2	564
October	1st to 10th	Night	22% 7% 28% 33% 11% 1	720
	11th to 20th	Night	25% 6% 29% 27% 11% 2	720
	21st to end	Night	28% 10% 31% 23% 7% 1	879
November	1st to 10th	Night	27% 11% 31% 24% 7% 1	830
	11th to 20th	Night	30% 15% 28% 20% 7% 1	850
	21st to end	Night	30% 14% 33% 18% 4 1	850
December	1st to 10th	Night	31% 12% 32% 19% 5%	850
	11th to 20th	Night	31% 15% 34% 16% 3	850
	21st to end	Night	27% 16% 33% 19% 4 1	935

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