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 2024-01-27.

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	15% 9% 4 7% 6% 3 5% 5% 5% 7% 35%	75072
	Night	15% 9% 4 7% 6% 3 4 4 5% 7% 37%	74983
Transparency	Day	48% 5% 11% 21% 13% 2	74785
	Night	47% 5% 11% 22% 14% 2	74791
Seeing	Day	17% 9% 26% 24% 16% 7%	14362
	Night	18% 9% 28% 29% 15% 2	24287

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% 13% 6% 8% 7% 4 6% 4 5% 6% 23%	1517
	11th to 20th	Day	18% 14% 8% 11% 6% 4 5% 3 4 4 22%	1524
	21st to end	Day	16% 17% 6% 11% 8% 3 5% 4 4 6% 19%	1836
February	1st to 10th	Day	18% 16% 6% 9% 7% 3 5% 4 3 4 24%	1724
	11th to 20th	Day	16% 10% 5% 11% 9% 4 5% 5% 4 6% 26%	1748
	21st to end	Day	19% 12% 7% 10% 5% 4 3 3 3 6% 29%	1480
March	1st to 10th	Day	18% 12% 5% 6% 6% 4 5% 5% 4 6% 31%	1800
	11th to 20th	Day	18% 11% 5% 8% 7% 2 4 4 4 8% 29%	1968
	21st to end	Day	22% 12% 5% 7% 5% 4 4 4 4 6% 26%	2180
April	1st to 10th	Day	15% 11% 4 7% 7% 3 4 4 4 7% 34%	2160
	11th to 20th	Day	18% 9% 4 6% 5% 2 5% 3 4 6% 38%	2171
	21st to end	Day	20% 8% 3 6% 6% 3 4 4 5% 7% 33%	2340
May	1st to 10th	Day	19% 9% 3 6% 7% 3 5% 4 5% 7% 32%	2340
	11th to 20th	Day	20% 8% 3 7% 5% 3 5% 4 5% 5% 34%	2301
	21st to end	Day	10% 6% 3 7% 7% 4 7% 6% 7% 10% 34%	2531
June	1st to 10th	Day	13% 5% 3 5% 5% 3 5% 5% 6% 8% 41%	2446
	11th to 20th	Day	10% 5% 3 5% 5% 3 5% 6% 7% 9% 42%	2520
	21st to end	Day	10% 5% 3 6% 6% 4 6% 6% 6% 9% 39%	2520
July	1st to 10th	Day	10% 5% 2 6% 7% 5% 6% 7% 6% 9% 37%	2520
	11th to 20th	Day	8% 4 2 5% 5% 5% 5% 6% 7% 9% 43%	2520
	21st to end	Day	9% 5% 2 6% 6% 3 6% 6% 7% 8% 42%	2557
August	1st to 10th	Day	7% 4 2 6% 6% 5% 5% 6% 7% 9% 43%	2327
	11th to 20th	Day	9% 6% 3 5% 6% 4 6% 7% 7% 9% 39%	2303
	21st to end	Day	8% 5% 2 5% 5% 4 5% 4 7% 9% 45%	2377
September	1st to 10th	Day	12% 5% 3 5% 5% 3 5% 4 4 8% 46%	2075
	11th to 20th	Day	11% 5% 2 6% 6% 3 4 4 6% 8% 47%	1980
	21st to end	Day	14% 5% 3 4 5% 3 3 3 4 7% 49%	1980
October	1st to 10th	Day	13% 5% 2 5% 6% 3 4 5% 5% 6% 46%	1980
	11th to 20th	Day	15% 7% 2 4 4 2 5% 6% 5% 7% 44%	1844
	21st to end	Day	20% 9% 3 8% 5% 3 4 4 5% 6% 33%	1800
November	1st to 10th	Day	10% 6% 3 5% 4 3 4 4 5% 6% 51%	1672
	11th to 20th	Day	16% 11% 4 7% 6% 2 4 4 4 6% 35%	1710
	21st to end	Day	20% 11% 5% 7% 5% 2 4 3 4 6% 32%	1710
December	1st to 10th	Day	20% 14% 7% 11% 6% 2 4 4 5% 5% 23%	1710
	11th to 20th	Day	16% 14% 7% 10% 5% 3 5% 4 4 6% 26%	2118
	21st to end	Day	21% 23% 9% 10% 6% 3 3 4 4 5% 14%	2783
January	1st to 10th	Night	17% 12% 7% 10% 8% 4 4 4 4 5% 24%	2648
	11th to 20th	Night	18% 15% 7% 11% 6% 4 5% 3 4 5% 22%	2652
	21st to end	Night	17% 13% 7% 10% 7% 4 4 4 4 5% 24%	2664
February	1st to 10th	Night	21% 13% 6% 7% 8% 3 4 4 4 5% 26%	2232
	11th to 20th	Night	15% 11% 5% 8% 7% 3 5% 4 4 7% 30%	2098
	21st to end	Night	19% 13% 6% 8% 6% 3 3 4 5% 6% 28%	1776
March	1st to 10th	Night	17% 9% 4 7% 6% 4 4 4 4 5% 37%	2160
	11th to 20th	Night	15% 10% 3 6% 6% 3 4 4 5% 7% 38%	1992
	21st to end	Night	20% 10% 3 7% 5% 4 4 3 4 6% 33%	2178
April	1st to 10th	Night	14% 8% 4 6% 5% 3 4 6% 5% 8% 37%	1814
	11th to 20th	Night	15% 9% 2 4 5% 3 4 5% 6% 7% 39%	1736
	21st to end	Night	17% 8% 2 7% 5% 4 5% 4 5% 7% 37%	1620
May	1st to 10th	Night	18% 10% 4 7% 5% 4 5% 4 4 6% 32%	1620
	11th to 20th	Night	17% 7% 3 6% 4 3 5% 4 4 6% 39%	1592
	21st to end	Night	8% 4 3 6% 5% 3 5% 5% 8% 11% 42%	1744
June	1st to 10th	Night	9% 5% 2 6% 5% 3 5% 4 5% 8% 47%	1509
	11th to 20th	Night	10% 4 2 4 5% 3 5% 4 5% 11% 48%	1440
	21st to end	Night	6% 4 3 5% 4 3 6% 6% 6% 10% 47%	1440
July	1st to 10th	Night	10% 5% 3 5% 6% 3 5% 5% 6% 7% 46%	1440
	11th to 20th	Night	8% 3 2 4 4 2 4 5% 6% 8% 52%	1440
	21st to end	Night	9% 4 2 4 5% 3 4 5% 5% 8% 50%	1758
August	1st to 10th	Night	6% 3 2 6% 5% 3 5% 6% 7% 10% 48%	1611
	11th to 20th	Night	6% 5% 2 5% 6% 3 4 5% 6% 9% 49%	1598
	21st to end	Night	5% 4 2 5% 5% 3 5% 5% 6% 9% 51%	1975
September	1st to 10th	Night	12% 5% 1 4 5% 3 4 4 5% 7% 48%	1867
	11th to 20th	Night	9% 4 2 4 3 2 3 4 5% 8% 54%	1980
	21st to end	Night	13% 4 2 5% 4 3 4 4 4 6% 51%	1980
October	1st to 10th	Night	10% 5% 2 5% 4 3 3 4 4 7% 52%	1980
	11th to 20th	Night	15% 7% 3 6% 5% 2 3 4 5% 6% 42%	2114
	21st to end	Night	19% 8% 3 5% 5% 2 4 4 4 7% 39%	2551
November	1st to 10th	Night	12% 7% 3 5% 5% 2 4 4 5% 7% 46%	2406
	11th to 20th	Night	16% 8% 4 5% 5% 3 4 4 5% 7% 38%	2470
	21st to end	Night	18% 12% 5% 7% 6% 3 5% 4 5% 6% 30%	2470
December	1st to 10th	Night	19% 13% 4 8% 6% 3 5% 4 5% 6% 26%	2470
	11th to 20th	Night	19% 13% 7% 9% 5% 3 4 4 4 6% 27%	3074
	21st to end	Night	22% 18% 8% 10% 6% 4 4 4 4 5% 17%	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	61% 5% 8% 16% 10%	1518
	11th to 20th	Day	65% 4 10% 15% 6%	1524
	21st to end	Day	65% 4 9% 16% 5% 1	1836
February	1st to 10th	Day	63% 3 10% 14% 8% 1	1724
	11th to 20th	Day	61% 3 9% 19% 8% 1	1758
	21st to end	Day	58% 4 9% 20% 8% 1	1480
March	1st to 10th	Day	55% 4 9% 22% 10%	1800
	11th to 20th	Day	55% 4 12% 18% 10% 1	1968
	21st to end	Day	59% 5% 11% 18% 5% 1	2180
April	1st to 10th	Day	49% 4 11% 21% 13% 1	2160
	11th to 20th	Day	48% 4 11% 20% 13% 4	2171
	21st to end	Day	51% 5% 10% 21% 11% 2	2340
May	1st to 10th	Day	52% 5% 12% 21% 10% 1	2340
	11th to 20th	Day	51% 6% 12% 18% 12% 1	2305
	21st to end	Day	43% 8% 15% 21% 12% 1	2531
June	1st to 10th	Day	39% 6% 11% 26% 16% 3	2446
	11th to 20th	Day	36% 6% 14% 23% 19% 2	2520
	21st to end	Day	39% 8% 14% 21% 15% 4	2520
July	1st to 10th	Day	41% 7% 13% 21% 15% 3	2451
	11th to 20th	Day	34% 7% 15% 25% 17% 2	2445
	21st to end	Day	37% 7% 15% 24% 14% 4	2415
August	1st to 10th	Day	35% 7% 14% 29% 14% 2	2314
	11th to 20th	Day	38% 6% 13% 26% 15% 1	2303
	21st to end	Day	33% 7% 13% 25% 16% 6%	2377
September	1st to 10th	Day	37% 5% 12% 23% 18% 5%	2075
	11th to 20th	Day	35% 5% 10% 27% 21% 2	1980
	21st to end	Day	37% 3 10% 25% 23% 2	1980
October	1st to 10th	Day	38% 5% 10% 24% 21% 2	1980
	11th to 20th	Day	39% 4 9% 27% 19% 3	1844
	21st to end	Day	51% 3 10% 21% 12% 2	1800
November	1st to 10th	Day	33% 5% 11% 29% 19% 3	1672
	11th to 20th	Day	50% 4 10% 20% 13% 3	1710
	21st to end	Day	54% 3 10% 21% 11% 1	1710
December	1st to 10th	Day	64% 3 10% 16% 6% 1	1710
	11th to 20th	Day	60% 4 8% 19% 8%	2115
	21st to end	Day	74% 3 7% 10% 6%	2783
January	1st to 10th	Night	63% 4 6% 17% 9%	2648
	11th to 20th	Night	65% 4 9% 16% 6%	2652
	21st to end	Night	62% 4 9% 17% 8% 1	2664
February	1st to 10th	Night	62% 3 10% 17% 7% 1	2232
	11th to 20th	Night	53% 5% 11% 19% 11% 1	2099
	21st to end	Night	58% 3 11% 17% 10% 2	1776
March	1st to 10th	Night	50% 4 8% 23% 13% 2	2160
	11th to 20th	Night	47% 4 12% 23% 12% 3	1992
	21st to end	Night	52% 4 10% 23% 9% 2	2178
April	1st to 10th	Night	45% 4 11% 26% 13% 2	1814
	11th to 20th	Night	43% 4 10% 20% 18% 5%	1736
	21st to end	Night	46% 5% 12% 21% 13% 3	1620
May	1st to 10th	Night	53% 6% 10% 20% 9% 3	1620
	11th to 20th	Night	45% 5% 13% 21% 14% 2	1593
	21st to end	Night	34% 7% 18% 23% 14% 4	1745
June	1st to 10th	Night	35% 6% 9% 26% 21% 3	1509
	11th to 20th	Night	31% 6% 16% 23% 18% 4	1440
	21st to end	Night	30% 8% 16% 24% 18% 4	1440
July	1st to 10th	Night	35% 8% 14% 21% 16% 6%	1400
	11th to 20th	Night	30% 5% 15% 29% 18% 3	1392
	21st to end	Night	30% 5% 13% 31% 17% 3	1660
August	1st to 10th	Night	31% 6% 16% 29% 16% 3	1600
	11th to 20th	Night	31% 6% 15% 28% 19% 1	1598
	21st to end	Night	28% 7% 14% 24% 22% 5%	1975
September	1st to 10th	Night	33% 5% 13% 24% 17% 7%	1867
	11th to 20th	Night	29% 4 10% 28% 26% 3	1980
	21st to end	Night	34% 4 10% 25% 25% 3	1980
October	1st to 10th	Night	31% 5% 10% 27% 23% 3	1980
	11th to 20th	Night	42% 4 8% 25% 20% 2	2114
	21st to end	Night	47% 3 8% 24% 15% 2	2551
November	1st to 10th	Night	38% 4 8% 24% 22% 3	2406
	11th to 20th	Night	45% 4 8% 23% 17% 3	2470
	21st to end	Night	54% 4 10% 21% 10% 1	2470
December	1st to 10th	Night	58% 4 10% 19% 9% 1	2470
	11th to 20th	Night	57% 4 8% 21% 9% 1	3076
	21st to end	Night	71% 4 8% 12% 5%	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	24% 15% 37% 18% 5% 1	270
	11th to 20th	Day	26% 17% 37% 15% 4	270
	21st to end	Day	32% 15% 32% 16% 5%	282
February	1st to 10th	Day	30% 15% 35% 16% 4	240
	11th to 20th	Day	25% 18% 36% 18% 3	240
	21st to end	Day	23% 16% 44% 14% 3	198
March	1st to 10th	Day	22% 20% 36% 21% 2	240
	11th to 20th	Day	25% 13% 36% 20% 6% 1	408
	21st to end	Day	29% 12% 34% 18% 7% 1	462
April	1st to 10th	Day	18% 15% 33% 22% 10% 3	420
	11th to 20th	Day	20% 14% 29% 21% 11% 4	412
	21st to end	Day	24% 11% 35% 19% 10% 2	420
May	1st to 10th	Day	25% 9% 29% 24% 8% 4	423
	11th to 20th	Day	23% 6% 29% 22% 12% 8%	447
	21st to end	Day	11% 5% 25% 27% 18% 14%	485
June	1st to 10th	Day	12% 5% 22% 34% 21% 6%	556
	11th to 20th	Day	11% 4 25% 30% 22% 7%	630
	21st to end	Day	9% 6% 26% 30% 19% 9%	630
July	1st to 10th	Day	12% 2 15% 27% 25% 19%	621
	11th to 20th	Day	8% 3 20% 30% 25% 15%	618
	21st to end	Day	10% 3 14% 28% 28% 16%	498
August	1st to 10th	Day	8% 4 11% 27% 31% 19%	446
	11th to 20th	Day	11% 5% 14% 29% 27% 14%	439
	21st to end	Day	10% 4 11% 27% 34% 15%	495
September	1st to 10th	Day	12% 4 16% 25% 30% 14%	450
	11th to 20th	Day	11% 3 14% 24% 32% 16%	450
	21st to end	Day	16% 9% 19% 24% 23% 10%	450
October	1st to 10th	Day	12% 6% 26% 28% 19% 10%	450
	11th to 20th	Day	16% 14% 29% 26% 13% 1	450
	21st to end	Day	29% 12% 35% 21% 4	315
November	1st to 10th	Day	14% 13% 33% 24% 13% 4	270
	11th to 20th	Day	23% 19% 26% 18% 11% 3	270
	21st to end	Day	26% 11% 36% 21% 6%	270
December	1st to 10th	Day	19% 21% 34% 19% 6% 1	270
	11th to 20th	Day	24% 13% 37% 19% 6% 1	270
	21st to end	Day	27% 19% 33% 16% 5%	297
January	1st to 10th	Night	22% 16% 36% 22% 5%	950
	11th to 20th	Night	25% 14% 37% 20% 4	942
	21st to end	Night	23% 12% 39% 22% 4	820
February	1st to 10th	Night	27% 11% 36% 23% 2	720
	11th to 20th	Night	21% 16% 36% 24% 2	703
	21st to end	Night	27% 10% 39% 20% 4	592
March	1st to 10th	Night	22% 13% 33% 26% 6% 1	720
	11th to 20th	Night	18% 12% 36% 26% 7% 1	720
	21st to end	Night	25% 10% 30% 27% 6% 1	792
April	1st to 10th	Night	17% 11% 36% 27% 8% 1	554
	11th to 20th	Night	20% 9% 31% 29% 10% 2	533
	21st to end	Night	20% 8% 38% 22% 11% 1	540
May	1st to 10th	Night	23% 6% 31% 29% 10% 1	540
	11th to 20th	Night	21% 5% 25% 30% 17% 2	531
	21st to end	Night	11% 3 22% 35% 26% 3	584
June	1st to 10th	Night	10% 3 22% 41% 23% 1	540
	11th to 20th	Night	11% 3 20% 36% 28% 2	540
	21st to end	Night	8% 3 20% 42% 25% 3	540
July	1st to 10th	Night	11% 2 13% 33% 33% 8%	525
	11th to 20th	Night	8% 2 14% 37% 33% 6%	522
	21st to end	Night	9% 3 12% 39% 35% 3	591
August	1st to 10th	Night	8% 2 10% 39% 36% 5%	537
	11th to 20th	Night	7% 2 10% 43% 31% 7%	534
	21st to end	Night	6% 3 12% 35% 37% 7%	594
September	1st to 10th	Night	13% 3 12% 34% 31% 6%	607
	11th to 20th	Night	10% 4 12% 33% 35% 6%	720
	21st to end	Night	14% 4 22% 33% 24% 4	720
October	1st to 10th	Night	12% 6% 25% 31% 21% 5%	720
	11th to 20th	Night	19% 10% 28% 29% 13% 1	720
	21st to end	Night	23% 10% 33% 25% 8%	792
November	1st to 10th	Night	14% 10% 33% 27% 15% 1	740
	11th to 20th	Night	19% 16% 30% 24% 9% 1	760
	21st to end	Night	23% 13% 30% 28% 6% 1	760
December	1st to 10th	Night	24% 11% 36% 23% 5% 1	760
	11th to 20th	Night	26% 9% 33% 25% 5% 1	779
	21st to end	Night	28% 13% 34% 21% 4	1045

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