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 2025-10-04.

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	21% 8% 3 7% 4 4 4 3 5% 6% 35%	130593
	Night	25% 9% 3 7% 4 4 4 3 5% 6% 30%	127470
Transparency	Day	50% 8% 15% 18% 8% 1	130300
	Night	55% 7% 13% 16% 7% 1	127340
Seeing	Day	22% 6% 17% 26% 23% 7%	16565
	Night	29% 6% 14% 24% 22% 6%	24662

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	39% 11% 4 9% 6% 4 5% 3 5% 5% 9%	3179
	11th to 20th	Day	21% 9% 3 7% 4 5% 5% 3 4 9% 29%	3159
	21st to end	Day	44% 10% 2 6% 3 3 3 2 3 3 21%	4042
February	1st to 10th	Day	32% 12% 4 7% 5% 5% 4 4 6% 6% 16%	1800
	11th to 20th	Day	30% 10% 3 8% 6% 6% 4 2 4 5% 22%	1755
	21st to end	Day	26% 12% 4 8% 6% 4 5% 4 5% 5% 22%	1503
March	1st to 10th	Day	25% 14% 4 10% 6% 5% 5% 4 5% 6% 16%	4667
	11th to 20th	Day	24% 11% 4 8% 5% 4 4 3 5% 6% 25%	5000
	21st to end	Day	29% 12% 4 9% 5% 5% 5% 4 5% 5% 17%	5631
April	1st to 10th	Day	26% 10% 3 8% 5% 5% 6% 4 6% 7% 20%	2400
	11th to 20th	Day	19% 10% 3 8% 5% 5% 5% 4 5% 6% 29%	2469
	21st to end	Day	21% 10% 3 8% 5% 4 4 4 5% 7% 29%	2658
May	1st to 10th	Day	13% 8% 2 6% 4 4 4 3 5% 6% 44%	6320
	11th to 20th	Day	17% 7% 3 8% 4 4 5% 4 6% 9% 32%	6277
	21st to end	Day	20% 7% 3 7% 4 4 5% 5% 7% 8% 29%	7031
June	1st to 10th	Day	19% 6% 2 7% 5% 5% 4 4 5% 7% 33%	3000
	11th to 20th	Day	18% 8% 3 8% 6% 4 5% 4 6% 8% 29%	3000
	21st to end	Day	13% 7% 3 6% 5% 5% 5% 4 7% 7% 39%	3000
July	1st to 10th	Day	8% 4 1 4 3 3 3 3 4 6% 60%	6780
	11th to 20th	Day	4 3 1 3 2 2 2 2 3 6% 71%	6780
	21st to end	Day	8% 3 1 4 2 2 3 3 5% 7% 61%	6977
August	1st to 10th	Day	5% 3 1 3 3 3 2 2 4 6% 68%	4495
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 3 5% 7% 48%	4156
	21st to end	Day	14% 6% 3 6% 4 4 4 3 5% 7% 43%	4551
September	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 47%	2400
	11th to 20th	Day	17% 8% 3 7% 4 4 4 4 5% 6% 39%	2400
	21st to end	Day	24% 5% 2 6% 4 5% 4 3 5% 6% 35%	2104
October	1st to 10th	Day	22% 7% 3 8% 6% 5% 5% 4 6% 7% 29%	3211
	11th to 20th	Day	35% 7% 2 6% 4 4 4 3 4 7% 23%	3161
	21st to end	Day	30% 12% 4 9% 5% 4 4 4 5% 6% 17%	3184
November	1st to 10th	Day	38% 9% 3 8% 4 5% 4 3 4 5% 17%	1568
	11th to 20th	Day	36% 10% 3 7% 5% 4 4 3 4 4 20%	1600
	21st to end	Day	38% 10% 3 7% 5% 4 4 3 5% 4 17%	1600
December	1st to 10th	Day	24% 11% 3 9% 8% 5% 5% 4 6% 7% 18%	2592
	11th to 20th	Day	40% 13% 4 8% 4 4 4 3 4 4 10%	2645
	21st to end	Day	39% 14% 4 9% 6% 5% 4 3 3 3 10%	3498
January	1st to 10th	Night	38% 13% 4 9% 5% 4 5% 3 4 4 13%	6766
	11th to 20th	Night	23% 7% 2 6% 4 4 5% 4 7% 9% 29%	6739
	21st to end	Night	38% 10% 3 6% 3 3 3 2 4 6% 23%	6912
February	1st to 10th	Night	31% 12% 3 8% 5% 4 4 3 4 6% 20%	2600
	11th to 20th	Night	29% 12% 2 8% 4 4 3 3 4 5% 25%	2532
	21st to end	Night	30% 11% 3 6% 4 5% 3 3 5% 5% 25%	2126
March	1st to 10th	Night	26% 13% 4 8% 5% 4 4 3 5% 6% 22%	5211
	11th to 20th	Night	26% 10% 3 6% 4 4 4 4 6% 7% 25%	4999
	21st to end	Night	36% 13% 4 9% 5% 4 3 3 4 4 17%	5343
April	1st to 10th	Night	27% 10% 3 9% 6% 5% 4 3 5% 6% 23%	2000
	11th to 20th	Night	20% 10% 3 7% 6% 4 4 4 5% 7% 30%	1879
	21st to end	Night	21% 8% 4 8% 5% 4 4 4 6% 6% 29%	1740
May	1st to 10th	Night	16% 8% 3 6% 3 3 2 3 4 5% 48%	3640
	11th to 20th	Night	16% 7% 3 7% 4 4 4 4 6% 8% 37%	3616
	21st to end	Night	17% 9% 4 9% 6% 6% 5% 4 4 7% 29%	3839
June	1st to 10th	Night	19% 9% 3 7% 5% 4 4 3 5% 6% 35%	1400
	11th to 20th	Night	14% 10% 4 8% 6% 4 6% 5% 5% 8% 31%	1400
	21st to end	Night	11% 6% 3 7% 6% 5% 4 5% 6% 7% 42%	1400
July	1st to 10th	Night	9% 4 2 3 3 2 3 3 4 6% 62%	3180
	11th to 20th	Night	3 2 1 2 2 2 2 2 3 5% 77%	3180
	21st to end	Night	6% 4 2 4 3 3 3 3 5% 6% 63%	3850
August	1st to 10th	Night	4 3 1 2 2 2 2 2 4 7% 72%	2641
	11th to 20th	Night	12% 5% 2 5% 4 3 3 3 4 7% 51%	2902
	21st to end	Night	12% 5% 2 6% 4 4 4 4 6% 8% 44%	3362
September	1st to 10th	Night	9% 3 1 4 4 3 4 4 6% 7% 56%	2000
	11th to 20th	Night	17% 6% 2 6% 5% 3 5% 3 4 6% 44%	2000
	21st to end	Night	21% 6% 3 7% 5% 4 4 3 4 5% 39%	2294
October	1st to 10th	Night	20% 7% 3 8% 5% 5% 5% 4 6% 6% 31%	3836
	11th to 20th	Night	30% 9% 3 6% 3 4 4 3 5% 6% 28%	3777
	21st to end	Night	31% 10% 3 7% 4 4 3 3 5% 5% 25%	4375
November	1st to 10th	Night	37% 10% 3 7% 5% 4 4 3 4 4 17%	2730
	11th to 20th	Night	37% 10% 3 6% 4 4 4 4 4 5% 22%	2800
	21st to end	Night	35% 11% 3 8% 6% 4 4 2 3 4 19%	2800
December	1st to 10th	Night	27% 9% 3 7% 6% 6% 6% 5% 6% 6% 20%	4583
	11th to 20th	Night	43% 13% 4 8% 4 3 4 3 4 4 10%	5560
	21st to end	Night	39% 14% 4 8% 4 4 3 2 3 4 15%	7458

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	76% 6% 7% 8% 2	3179
	11th to 20th	Day	53% 8% 10% 19% 7% 2	3180
	21st to end	Day	68% 4 6% 16% 5%	4042
February	1st to 10th	Day	69% 6% 10% 8% 4 3	1800
	11th to 20th	Day	65% 7% 9% 12% 6% 1	1764
	21st to end	Day	65% 7% 11% 11% 4 2	1503
March	1st to 10th	Day	66% 7% 12% 12% 3	4667
	11th to 20th	Day	60% 8% 10% 19% 3	5000
	21st to end	Day	68% 9% 12% 9% 2	5631
April	1st to 10th	Day	63% 10% 15% 10% 3	2400
	11th to 20th	Day	56% 8% 11% 16% 8% 2	2469
	21st to end	Day	56% 8% 14% 14% 7%	2658
May	1st to 10th	Day	41% 6% 18% 23% 11%	6320
	11th to 20th	Day	49% 10% 20% 18% 4	6280
	21st to end	Day	50% 11% 20% 17% 3	7032
June	1st to 10th	Day	51% 7% 18% 16% 6% 2	3000
	11th to 20th	Day	53% 11% 18% 13% 4	3000
	21st to end	Day	45% 9% 20% 17% 7% 2	3000
July	1st to 10th	Day	26% 7% 18% 23% 18% 7%	6706
	11th to 20th	Day	17% 5% 21% 33% 21% 2	6699
	21st to end	Day	25% 6% 29% 27% 11% 2	6781
August	1st to 10th	Day	20% 6% 15% 41% 17% 1	4478
	11th to 20th	Day	37% 9% 26% 18% 9% 1	4195
	21st to end	Day	42% 8% 18% 21% 11% 1	4550
September	1st to 10th	Day	37% 9% 17% 22% 11% 4	2400
	11th to 20th	Day	46% 8% 13% 18% 13% 2	2400
	21st to end	Day	51% 9% 14% 17% 9% 1	2104
October	1st to 10th	Day	54% 8% 17% 16% 5%	3211
	11th to 20th	Day	61% 8% 13% 12% 6%	3161
	21st to end	Day	68% 9% 9% 10% 4	3184
November	1st to 10th	Day	70% 8% 9% 9% 4 1	1568
	11th to 20th	Day	69% 6% 9% 10% 4 2	1600
	21st to end	Day	69% 8% 7% 9% 6% 1	1600
December	1st to 10th	Day	60% 8% 9% 16% 6%	2592
	11th to 20th	Day	76% 9% 7% 5% 2 1	2648
	21st to end	Day	79% 7% 7% 5% 2	3498
January	1st to 10th	Night	75% 6% 6% 10% 2	6766
	11th to 20th	Night	50% 8% 13% 20% 7% 2	6780
	21st to end	Night	64% 5% 10% 16% 4 1	6912
February	1st to 10th	Night	66% 7% 10% 9% 5% 3	2600
	11th to 20th	Night	63% 7% 9% 12% 7% 2	2534
	21st to end	Night	61% 6% 10% 15% 6% 2	2126
March	1st to 10th	Night	62% 9% 11% 12% 6%	5211
	11th to 20th	Night	56% 9% 13% 18% 3	5000
	21st to end	Night	71% 8% 12% 8% 1	5343
April	1st to 10th	Night	62% 7% 13% 15% 3	2000
	11th to 20th	Night	55% 7% 12% 16% 8% 2	1879
	21st to end	Night	57% 8% 14% 13% 8%	1740
May	1st to 10th	Night	40% 5% 12% 30% 12% 1	3637
	11th to 20th	Night	44% 9% 23% 18% 6%	3616
	21st to end	Night	54% 8% 18% 17% 2	3839
June	1st to 10th	Night	50% 8% 18% 16% 6% 2	1400
	11th to 20th	Night	52% 9% 18% 18% 3	1400
	21st to end	Night	42% 8% 21% 18% 9% 3	1400
July	1st to 10th	Night	26% 4 18% 28% 17% 7%	3145
	11th to 20th	Night	13% 4 20% 36% 23% 3	3138
	21st to end	Night	23% 5% 23% 32% 15% 2	3741
August	1st to 10th	Night	16% 5% 15% 37% 25% 2	2633
	11th to 20th	Night	35% 7% 27% 20% 8% 3	2929
	21st to end	Night	38% 7% 20% 23% 11% 1	3362
September	1st to 10th	Night	28% 9% 18% 26% 15% 4	2000
	11th to 20th	Night	43% 6% 14% 18% 15% 4	2000
	21st to end	Night	48% 7% 14% 22% 7% 2	2294
October	1st to 10th	Night	53% 9% 17% 15% 6%	3836
	11th to 20th	Night	58% 6% 12% 17% 7%	3777
	21st to end	Night	62% 8% 12% 12% 5%	4375
November	1st to 10th	Night	70% 6% 10% 10% 3 1	2730
	11th to 20th	Night	66% 7% 10% 9% 5% 2	2800
	21st to end	Night	69% 7% 9% 8% 7% 1	2800
December	1st to 10th	Night	61% 9% 14% 10% 5% 1	4583
	11th to 20th	Night	77% 6% 7% 6% 3 1	5556
	21st to end	Night	75% 5% 8% 9% 2 1	7458

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	40% 8% 18% 17% 11% 7%	200
	11th to 20th	Day	38% 13% 11% 20% 14% 6%	200
	21st to end	Day	47% 5% 11% 16% 14% 6%	245
February	1st to 10th	Day	38% 8% 17% 21% 16% 2	400
	11th to 20th	Day	35% 7% 17% 23% 15% 3	395
	21st to end	Day	30% 10% 20% 26% 11% 4	341
March	1st to 10th	Day	31% 6% 19% 24% 15% 5%	500
	11th to 20th	Day	31% 8% 17% 19% 20% 6%	500
	21st to end	Day	32% 7% 22% 20% 15% 3	550
April	1st to 10th	Day	29% 9% 17% 23% 19% 4	500
	11th to 20th	Day	21% 7% 22% 27% 17% 5%	492
	21st to end	Day	22% 6% 23% 26% 17% 6%	500
May	1st to 10th	Day	15% 5% 19% 30% 26% 5%	503
	11th to 20th	Day	18% 5% 15% 26% 28% 9%	528
	21st to end	Day	17% 5% 19% 29% 22% 9%	623
June	1st to 10th	Day	17% 5% 18% 33% 21% 5%	730
	11th to 20th	Day	17% 5% 20% 33% 21% 4	730
	21st to end	Day	13% 5% 18% 28% 28% 7%	730
July	1st to 10th	Day	11% 3 15% 32% 31% 9%	721
	11th to 20th	Day	5% 4 17% 35% 32% 8%	718
	21st to end	Day	7% 3 11% 29% 40% 12%	626
August	1st to 10th	Day	10% 3 11% 26% 37% 13%	527
	11th to 20th	Day	8% 3 11% 31% 36% 11%	517
	21st to end	Day	11% 5% 15% 30% 28% 10%	583
September	1st to 10th	Day	13% 2 15% 34% 27% 9%	530
	11th to 20th	Day	18% 5% 16% 26% 27% 8%	530
	21st to end	Day	25% 6% 16% 21% 24% 7%	530
October	1st to 10th	Day	27% 5% 16% 23% 20% 9%	503
	11th to 20th	Day	34% 5% 14% 20% 18% 10%	490
	21st to end	Day	33% 9% 17% 21% 13% 6%	403
November	1st to 10th	Day	37% 7% 18% 22% 14% 3	200
	11th to 20th	Day	36% 11% 15% 12% 18% 9%	200
	21st to end	Day	38% 9% 17% 16% 11% 10%	200
December	1st to 10th	Day	39% 7% 17% 12% 12% 14%	200
	11th to 20th	Day	43% 11% 13% 19% 14% 2	200
	21st to end	Day	43% 6% 20% 19% 9% 2	220
January	1st to 10th	Night	44% 9% 16% 16% 11% 3	900
	11th to 20th	Night	41% 6% 13% 20% 17% 3	900
	21st to end	Night	39% 6% 14% 24% 14% 4	885
February	1st to 10th	Night	37% 8% 19% 23% 11% 3	800
	11th to 20th	Night	34% 8% 18% 24% 15% 1	784
	21st to end	Night	34% 8% 19% 23% 13% 2	660
March	1st to 10th	Night	33% 8% 17% 22% 18% 3	800
	11th to 20th	Night	34% 9% 16% 22% 16% 4	800
	21st to end	Night	35% 7% 15% 22% 17% 4	880
April	1st to 10th	Night	31% 7% 13% 27% 19% 4	800
	11th to 20th	Night	22% 7% 16% 31% 17% 6%	691
	21st to end	Night	25% 5% 16% 29% 19% 5%	540
May	1st to 10th	Night	19% 3 13% 27% 34% 5%	400
	11th to 20th	Night	20% 3 6% 27% 37% 8%	394
	21st to end	Night	18% 4 9% 29% 34% 6%	433
June	1st to 10th	Night	21% 5% 12% 28% 30% 5%	400
	11th to 20th	Night	16% 6% 15% 33% 27% 4	400
	21st to end	Night	14% 2 10% 30% 36% 8%	400
July	1st to 10th	Night	13% 1 7% 28% 44% 7%	390
	11th to 20th	Night	6% 3 10% 29% 43% 9%	388
	21st to end	Night	3 3 6% 27% 48% 13%	438
August	1st to 10th	Night	9% 2 4 24% 42% 19%	417
	11th to 20th	Night	9% 1 6% 23% 46% 15%	594
	21st to end	Night	11% 3 10% 23% 41% 13%	725
September	1st to 10th	Night	10% 2 6% 33% 37% 13%	800
	11th to 20th	Night	19% 3 12% 19% 32% 14%	800
	21st to end	Night	23% 4 12% 27% 28% 7%	800
October	1st to 10th	Night	24% 4 13% 24% 26% 10%	772
	11th to 20th	Night	33% 4 13% 23% 20% 8%	760
	21st to end	Night	36% 6% 15% 24% 14% 4	836
November	1st to 10th	Night	42% 7% 15% 23% 13% 1	780
	11th to 20th	Night	41% 8% 16% 20% 12% 4	800
	21st to end	Night	37% 8% 16% 21% 15% 4	800
December	1st to 10th	Night	33% 6% 14% 27% 16% 4	805
	11th to 20th	Night	48% 8% 14% 20% 8% 1	900
	21st to end	Night	45% 9% 20% 17% 8% 1	990

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