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 2024-01-12.

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% 8% 3 6% 5% 3 4 4 5% 6% 32%	74741
	Night	27% 9% 3 6% 5% 3 4 4 4 6% 30%	74975
Transparency	Day	54% 8% 14% 16% 7% 1	74430
	Night	57% 7% 12% 15% 7% 1	74804
Seeing	Day	22% 5% 16% 26% 24% 7%	14215
	Night	30% 5% 13% 24% 22% 6%	22564

By Month

Percentage Cloud Cover NW (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	45% 9% 4 8% 4 3 4 3 4 4 12%	1329
	11th to 20th	Day	35% 11% 5% 7% 6% 3 6% 4 3 6% 15%	1267
	21st to end	Day	38% 11% 3 7% 6% 3 5% 3 5% 4 15%	1594
February	1st to 10th	Day	34% 10% 5% 6% 6% 4 4 5% 6% 6% 15%	1620
	11th to 20th	Day	31% 9% 3 7% 6% 5% 5% 3 4 6% 22%	1575
	21st to end	Day	24% 12% 4 8% 6% 4 5% 5% 4 5% 24%	1348
March	1st to 10th	Day	28% 11% 5% 7% 7% 3 5% 4 4 6% 20%	1858
	11th to 20th	Day	33% 11% 5% 7% 6% 3 3 3 3 4 22%	1980
	21st to end	Day	26% 10% 4 8% 6% 3 5% 5% 4 6% 22%	2233
April	1st to 10th	Day	27% 9% 4 7% 5% 4 6% 5% 6% 6% 21%	2160
	11th to 20th	Day	21% 9% 4 8% 6% 4 5% 6% 5% 6% 27%	2219
	21st to end	Day	21% 10% 4 7% 6% 3 5% 5% 5% 6% 28%	2392
May	1st to 10th	Day	15% 8% 3 6% 5% 3 6% 6% 6% 6% 36%	2520
	11th to 20th	Day	15% 7% 3 6% 5% 3 5% 6% 5% 8% 36%	2477
	21st to end	Day	17% 8% 3 7% 6% 3 5% 5% 6% 7% 34%	2767
June	1st to 10th	Day	21% 6% 3 6% 6% 4 5% 5% 5% 7% 32%	2700
	11th to 20th	Day	19% 7% 3 7% 7% 3 5% 5% 6% 8% 29%	2700
	21st to end	Day	12% 6% 3 5% 5% 4 5% 5% 6% 8% 41%	2700
July	1st to 10th	Day	11% 5% 2 5% 4 2 4 4 5% 7% 51%	2700
	11th to 20th	Day	9% 4 2 4 4 3 3 4 5% 6% 55%	2700
	21st to end	Day	7% 3 1 3 2 2 3 4 5% 6% 64%	2788
August	1st to 10th	Day	9% 5% 2 4 4 2 3 3 5% 7% 57%	2488
	11th to 20th	Day	8% 5% 2 3 3 3 4 4 5% 6% 57%	2298
	21st to end	Day	11% 5% 2 5% 5% 3 4 4 6% 8% 47%	2516
September	1st to 10th	Day	13% 6% 2 5% 5% 3 4 5% 5% 6% 47%	2160
	11th to 20th	Day	17% 8% 3 7% 4 2 4 4 5% 6% 40%	2160
	21st to end	Day	24% 5% 3 5% 5% 3 5% 4 4 6% 36%	1894
October	1st to 10th	Day	23% 8% 3 5% 5% 3 4 5% 5% 7% 32%	1800
	11th to 20th	Day	31% 8% 3 7% 5% 4 4 5% 5% 7% 22%	1800
	21st to end	Day	34% 8% 4 6% 4 3 3 4 4 6% 23%	1837
November	1st to 10th	Day	39% 8% 3 7% 5% 4 5% 4 4 5% 18%	1488
	11th to 20th	Day	36% 9% 4 6% 6% 3 4 4 4 4 21%	1520
	21st to end	Day	39% 9% 3 6% 6% 3 4 3 5% 4 18%	1520
December	1st to 10th	Day	31% 13% 5% 5% 6% 3 4 5% 4 4 22%	1506
	11th to 20th	Day	35% 14% 5% 8% 6% 4 4 4 4 4 12%	1685
	21st to end	Day	39% 13% 5% 9% 6% 3 4 3 3 3 11%	2442
January	1st to 10th	Night	42% 11% 3 6% 5% 3 4 3 3 4 16%	2836
	11th to 20th	Night	40% 10% 3 5% 5% 3 4 4 5% 5% 18%	2715
	21st to end	Night	33% 11% 3 7% 5% 3 5% 3 4 7% 19%	2757
February	1st to 10th	Night	35% 10% 3 7% 5% 3 4 4 4 6% 20%	2340
	11th to 20th	Night	30% 11% 3 7% 5% 3 3 4 4 5% 26%	2272
	21st to end	Night	29% 10% 3 6% 5% 4 4 3 5% 5% 27%	1907
March	1st to 10th	Night	30% 10% 3 7% 5% 3 4 4 4 5% 24%	2099
	11th to 20th	Night	33% 9% 3 6% 5% 3 4 4 5% 5% 23%	1979
	21st to end	Night	30% 11% 5% 8% 6% 3 4 4 5% 4 20%	2122
April	1st to 10th	Night	27% 9% 4 7% 6% 4 5% 4 5% 7% 23%	1800
	11th to 20th	Night	22% 9% 3 6% 7% 3 5% 5% 5% 7% 28%	1690
	21st to end	Night	22% 8% 4 8% 6% 3 5% 5% 6% 6% 27%	1566
May	1st to 10th	Night	16% 6% 3 5% 5% 3 3 5% 6% 6% 40%	1440
	11th to 20th	Night	15% 6% 2 6% 5% 3 5% 6% 7% 8% 37%	1416
	21st to end	Night	16% 7% 3 6% 5% 4 5% 6% 5% 7% 37%	1503
June	1st to 10th	Night	20% 8% 3 6% 6% 2 5% 4 5% 7% 34%	1260
	11th to 20th	Night	15% 9% 4 7% 7% 3 6% 6% 5% 8% 30%	1260
	21st to end	Night	10% 5% 3 5% 6% 3 5% 6% 6% 8% 43%	1260
July	1st to 10th	Night	10% 5% 2 4 5% 2 4 4 4 6% 55%	1260
	11th to 20th	Night	7% 4 2 3 4 2 3 4 4 7% 60%	1260
	21st to end	Night	4 2 1 3 3 2 3 3 5% 5% 68%	1527
August	1st to 10th	Night	7% 4 1 3 3 2 3 3 4 6% 63%	1449
	11th to 20th	Night	6% 3 1 3 3 2 2 4 5% 8% 63%	1602
	21st to end	Night	9% 5% 2 5% 5% 3 5% 5% 7% 8% 46%	1840
September	1st to 10th	Night	9% 3 1 4 4 2 4 4 5% 7% 56%	1800
	11th to 20th	Night	18% 6% 3 4 6% 2 5% 4 4 6% 43%	1800
	21st to end	Night	20% 6% 3 6% 6% 3 4 4 4 5% 41%	2064
October	1st to 10th	Night	20% 6% 3 5% 5% 3 5% 5% 4 5% 38%	2160
	11th to 20th	Night	30% 8% 3 4 5% 2 4 4 4 5% 30%	2160
	21st to end	Night	33% 8% 3 5% 4 2 3 4 4 5% 28%	2518
November	1st to 10th	Night	37% 9% 3 6% 5% 3 5% 4 4 4 18%	2590
	11th to 20th	Night	36% 9% 3 5% 5% 3 4 4 4 5% 23%	2660
	21st to end	Night	36% 9% 4 7% 7% 4 4 3 3 4 20%	2660
December	1st to 10th	Night	32% 9% 4 6% 6% 3 4 4 3 5% 24%	2669
	11th to 20th	Night	43% 13% 4 8% 5% 2 3 3 3 4 12%	3520
	21st to end	Night	40% 13% 4 7% 5% 3 3 2 3 4 16%	5214

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	77% 6% 7% 6% 4 1	1329
	11th to 20th	Day	72% 9% 8% 8% 3 1	1267
	21st to end	Day	71% 7% 7% 11% 4	1594
February	1st to 10th	Day	70% 6% 10% 7% 4 2	1620
	11th to 20th	Day	65% 8% 9% 12% 6%	1584
	21st to end	Day	63% 8% 11% 12% 4 2	1348
March	1st to 10th	Day	66% 7% 12% 11% 3 1	1858
	11th to 20th	Day	68% 6% 9% 12% 4 1	1980
	21st to end	Day	65% 9% 11% 12% 2	2233
April	1st to 10th	Day	63% 9% 14% 10% 3	2160
	11th to 20th	Day	59% 8% 11% 14% 8% 1	2219
	21st to end	Day	58% 8% 13% 13% 7% 1	2392
May	1st to 10th	Day	50% 9% 15% 18% 7% 1	2520
	11th to 20th	Day	48% 9% 18% 18% 6%	2480
	21st to end	Day	51% 10% 17% 19% 3	2768
June	1st to 10th	Day	54% 7% 17% 16% 5% 2	2700
	11th to 20th	Day	54% 12% 18% 12% 4	2700
	21st to end	Day	44% 9% 20% 18% 8% 2	2700
July	1st to 10th	Day	35% 9% 16% 25% 11% 3	2626
	11th to 20th	Day	31% 9% 19% 29% 11% 2	2619
	21st to end	Day	21% 8% 18% 34% 16% 3	2633
August	1st to 10th	Day	30% 9% 18% 29% 12% 2	2471
	11th to 20th	Day	28% 9% 21% 26% 15% 2	2298
	21st to end	Day	37% 10% 18% 28% 7% 1	2516
September	1st to 10th	Day	39% 9% 15% 22% 11% 4	2160
	11th to 20th	Day	46% 7% 13% 18% 14% 2	2160
	21st to end	Day	50% 8% 14% 17% 9% 1	1894
October	1st to 10th	Day	51% 8% 15% 17% 9%	1800
	11th to 20th	Day	61% 9% 13% 13% 3 1	1800
	21st to end	Day	64% 8% 10% 11% 8% 1	1837
November	1st to 10th	Day	69% 8% 9% 9% 4 1	1488
	11th to 20th	Day	69% 6% 9% 10% 4 2	1520
	21st to end	Day	69% 7% 7% 9% 7% 1	1520
December	1st to 10th	Day	67% 7% 6% 11% 8% 1	1506
	11th to 20th	Day	73% 11% 7% 6% 3 1	1688
	21st to end	Day	77% 8% 7% 6% 2 1	2442
January	1st to 10th	Night	74% 6% 6% 11% 3 1	2836
	11th to 20th	Night	67% 7% 11% 10% 3 1	2715
	21st to end	Night	66% 8% 8% 14% 4 1	2757
February	1st to 10th	Night	67% 7% 10% 9% 4 3	2340
	11th to 20th	Night	64% 7% 9% 12% 7% 1	2274
	21st to end	Night	60% 6% 10% 15% 6% 2	1907
March	1st to 10th	Night	63% 6% 13% 13% 5%	2099
	11th to 20th	Night	64% 7% 10% 12% 6% 1	1980
	21st to end	Night	68% 7% 12% 10% 2	2122
April	1st to 10th	Night	62% 7% 13% 16% 3	1800
	11th to 20th	Night	57% 7% 12% 15% 7% 2	1690
	21st to end	Night	58% 8% 13% 12% 8%	1566
May	1st to 10th	Night	44% 8% 19% 19% 9% 1	1440
	11th to 20th	Night	44% 9% 20% 19% 8%	1416
	21st to end	Night	46% 8% 20% 19% 6%	1503
June	1st to 10th	Night	51% 8% 17% 16% 5% 3	1260
	11th to 20th	Night	53% 9% 19% 15% 3	1260
	21st to end	Night	39% 8% 22% 18% 9% 3	1260
July	1st to 10th	Night	34% 6% 15% 28% 14% 4	1225
	11th to 20th	Night	27% 7% 19% 31% 14% 2	1218
	21st to end	Night	19% 5% 15% 35% 22% 5%	1442
August	1st to 10th	Night	24% 6% 18% 30% 18% 3	1441
	11th to 20th	Night	21% 6% 23% 30% 14% 6%	1602
	21st to end	Night	33% 8% 18% 30% 10% 1	1840
September	1st to 10th	Night	29% 9% 17% 27% 15% 4	1800
	11th to 20th	Night	44% 6% 12% 17% 16% 4	1800
	21st to end	Night	47% 7% 14% 22% 8% 2	2064
October	1st to 10th	Night	48% 7% 13% 21% 10% 1	2160
	11th to 20th	Night	57% 7% 14% 16% 6% 1	2160
	21st to end	Night	59% 7% 11% 14% 9%	2518
November	1st to 10th	Night	70% 6% 10% 10% 4 1	2590
	11th to 20th	Night	65% 8% 10% 10% 5% 2	2660
	21st to end	Night	68% 7% 8% 8% 7% 1	2660
December	1st to 10th	Night	63% 6% 11% 11% 8% 1	2669
	11th to 20th	Night	77% 7% 7% 5% 4 1	3516
	21st to end	Night	74% 5% 7% 9% 3 1	5214

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	39% 8% 18% 14% 12% 8%	180
	11th to 20th	Day	43% 12% 12% 14% 12% 6%	162
	21st to end	Day	46% 5% 12% 17% 16% 5%	198
February	1st to 10th	Day	41% 6% 16% 21% 15% 1	340
	11th to 20th	Day	37% 6% 18% 22% 15% 3	335
	21st to end	Day	27% 10% 19% 28% 12% 4	288
March	1st to 10th	Day	31% 5% 18% 25% 15% 5%	420
	11th to 20th	Day	31% 7% 18% 18% 20% 5%	420
	21st to end	Day	31% 7% 21% 21% 16% 3	462
April	1st to 10th	Day	29% 10% 16% 23% 19% 3	420
	11th to 20th	Day	24% 5% 21% 25% 18% 6%	412
	21st to end	Day	21% 6% 24% 26% 19% 5%	420
May	1st to 10th	Day	15% 4 19% 31% 26% 5%	423
	11th to 20th	Day	17% 4 13% 27% 30% 10%	448
	21st to end	Day	15% 4 17% 31% 24% 9%	529
June	1st to 10th	Day	19% 4 18% 32% 22% 6%	630
	11th to 20th	Day	17% 5% 20% 32% 21% 4	630
	21st to end	Day	12% 4 18% 28% 31% 7%	630
July	1st to 10th	Day	12% 2 13% 31% 33% 9%	621
	11th to 20th	Day	6% 3 16% 33% 34% 8%	618
	21st to end	Day	5% 2 9% 28% 43% 13%	534
August	1st to 10th	Day	10% 2 12% 22% 39% 15%	447
	11th to 20th	Day	7% 2 8% 32% 38% 13%	437
	21st to end	Day	11% 4 15% 29% 30% 10%	495
September	1st to 10th	Day	15% 3 16% 32% 28% 7%	450
	11th to 20th	Day	18% 4 15% 26% 29% 9%	450
	21st to end	Day	25% 4 15% 21% 26% 8%	450
October	1st to 10th	Day	26% 4 15% 23% 21% 10%	450
	11th to 20th	Day	33% 5% 14% 20% 18% 10%	450
	21st to end	Day	34% 9% 17% 20% 14% 6%	368
November	1st to 10th	Day	37% 7% 18% 21% 14% 3	180
	11th to 20th	Day	36% 9% 13% 12% 19% 10%	180
	21st to end	Day	37% 9% 17% 17% 11% 9%	180
December	1st to 10th	Day	42% 7% 17% 11% 11% 12%	180
	11th to 20th	Day	41% 11% 12% 19% 16% 2	180
	21st to end	Day	41% 6% 21% 20% 10% 3	198
January	1st to 10th	Night	45% 9% 16% 15% 11% 3	850
	11th to 20th	Night	44% 6% 11% 18% 18% 3	805
	21st to end	Night	38% 6% 14% 24% 14% 3	796
February	1st to 10th	Night	39% 6% 18% 22% 11% 3	720
	11th to 20th	Night	34% 8% 18% 24% 15% 1	704
	21st to end	Night	32% 8% 18% 24% 15% 3	592
March	1st to 10th	Night	33% 7% 17% 22% 18% 3	720
	11th to 20th	Night	35% 8% 16% 23% 15% 3	720
	21st to end	Night	34% 6% 15% 22% 17% 5%	792
April	1st to 10th	Night	31% 6% 13% 26% 20% 4	720
	11th to 20th	Night	23% 6% 16% 30% 17% 7%	622
	21st to end	Night	26% 6% 15% 29% 19% 5%	486
May	1st to 10th	Night	19% 3 13% 26% 34% 5%	360
	11th to 20th	Night	19% 2 6% 26% 39% 8%	354
	21st to end	Night	19% 4 7% 29% 35% 6%	389
June	1st to 10th	Night	21% 6% 11% 28% 30% 5%	360
	11th to 20th	Night	17% 5% 14% 33% 27% 4	360
	21st to end	Night	13% 2 10% 31% 36% 8%	360
July	1st to 10th	Night	13% 1 7% 26% 45% 8%	350
	11th to 20th	Night	7% 3 8% 28% 43% 10%	348
	21st to end	Night	4 1 6% 26% 49% 14%	394
August	1st to 10th	Night	9% 2 3 21% 43% 21%	375
	11th to 20th	Night	7% 1 4 22% 49% 17%	534
	21st to end	Night	11% 2 9% 23% 42% 12%	652
September	1st to 10th	Night	11% 2 5% 34% 37% 12%	720
	11th to 20th	Night	20% 3 12% 18% 32% 15%	720
	21st to end	Night	22% 3 12% 26% 29% 7%	720
October	1st to 10th	Night	23% 3 13% 23% 27% 11%	720
	11th to 20th	Night	33% 4 13% 23% 20% 8%	720
	21st to end	Night	37% 6% 14% 24% 15% 4	792
November	1st to 10th	Night	41% 7% 15% 23% 13% 1	740
	11th to 20th	Night	39% 8% 16% 21% 13% 4	760
	21st to end	Night	37% 8% 16% 21% 14% 4	760
December	1st to 10th	Night	34% 6% 15% 27% 15% 4	764
	11th to 20th	Night	48% 8% 14% 21% 8% 1	850
	21st to end	Night	45% 8% 20% 18% 8% 1	935

Page generated at 2024-01-08 19:31:25. Version ac07

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