Williamsburg
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Williamsburg clear sky chart from 2005-11-06 to 2026-04-17.

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	12% 8% 3 8% 5% 5% 5% 4 5% 7% 38%	131984
	Night	12% 9% 4 8% 4 4 4 3 5% 6% 41%	130498
Transparency	Day	44% 3 12% 25% 14% 2	135566
	Night	44% 3 11% 25% 15% 2	134993
Seeing	Day	16% 9% 29% 27% 15% 4	17735
	Night	17% 9% 31% 31% 12% 1	27730

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	8% 12% 4 19% 7% 7% 6% 4 5% 6% 23%	4691
	11th to 20th	Day	6% 9% 4 11% 7% 7% 6% 5% 6% 7% 31%	4987
	21st to end	Day	9% 13% 5% 11% 7% 7% 5% 3 4 5% 30%	5812
February	1st to 10th	Day	11% 13% 4 10% 7% 5% 4 3 5% 8% 29%	1890
	11th to 20th	Day	13% 11% 4 8% 5% 5% 5% 4 4 7% 34%	1859
	21st to end	Day	13% 9% 4 10% 5% 3 4 3 5% 7% 36%	1730
March	1st to 10th	Day	18% 11% 3 6% 3 3 2 2 3 5% 45%	4852
	11th to 20th	Day	10% 6% 3 5% 4 4 4 4 5% 8% 47%	5090
	21st to end	Day	20% 12% 4 9% 4 5% 4 4 5% 6% 26%	5738
April	1st to 10th	Day	17% 12% 4 8% 5% 4 4 3 5% 5% 33%	2520
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 4 6% 6% 33%	2446
	21st to end	Day	22% 9% 2 6% 4 4 4 4 5% 7% 33%	2422
May	1st to 10th	Day	15% 8% 2 7% 5% 5% 5% 4 6% 7% 37%	6015
	11th to 20th	Day	15% 6% 2 5% 4 4 4 3 5% 7% 45%	6319
	21st to end	Day	15% 7% 2 7% 5% 4 5% 4 6% 8% 38%	6935
June	1st to 10th	Day	10% 5% 2 6% 5% 4 4 5% 6% 7% 45%	2800
	11th to 20th	Day	10% 5% 2 5% 5% 5% 5% 4 7% 7% 45%	2800
	21st to end	Day	11% 7% 2 6% 5% 4 5% 4 7% 9% 41%	2800
July	1st to 10th	Day	9% 7% 3 7% 5% 5% 5% 4 7% 9% 40%	6360
	11th to 20th	Day	9% 6% 2 6% 4 4 4 4 6% 8% 46%	6360
	21st to end	Day	10% 7% 2 6% 5% 5% 5% 5% 7% 9% 39%	6658
August	1st to 10th	Day	8% 5% 2 5% 5% 4 5% 5% 7% 10% 45%	4234
	11th to 20th	Day	10% 5% 3 7% 5% 5% 5% 5% 6% 9% 41%	4113
	21st to end	Day	7% 5% 2 6% 4 4 4 3 6% 8% 50%	4312
September	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 5% 7% 52%	2400
	11th to 20th	Day	10% 5% 2 5% 4 3 5% 3 6% 8% 48%	2400
	21st to end	Day	11% 5% 2 5% 4 4 4 3 6% 8% 48%	2105
October	1st to 10th	Day	14% 6% 2 5% 3 3 4 3 4 6% 50%	2000
	11th to 20th	Day	16% 7% 2 7% 4 4 4 3 4 6% 44%	2000
	21st to end	Day	15% 10% 3 7% 4 4 4 3 5% 5% 42%	2195
November	1st to 10th	Day	11% 8% 3 5% 4 4 5% 4 6% 6% 45%	1852
	11th to 20th	Day	16% 11% 3 8% 5% 3 3 3 5% 7% 37%	1761
	21st to end	Day	16% 10% 4 6% 3 4 4 4 4 7% 38%	1680
December	1st to 10th	Day	8% 9% 3 8% 4 4 4 3 5% 7% 47%	2690
	11th to 20th	Day	14% 11% 4 15% 6% 6% 4 4 5% 5% 26%	3088
	21st to end	Day	23% 21% 9% 12% 5% 3 3 2 3 4 16%	4070
January	1st to 10th	Night	11% 12% 7% 14% 5% 4 4 3 4 5% 31%	8232
	11th to 20th	Night	8% 11% 4 10% 7% 6% 7% 4 6% 6% 30%	7998
	21st to end	Night	11% 12% 5% 10% 5% 4 4 3 4 6% 35%	8402
February	1st to 10th	Night	13% 12% 4 9% 5% 4 4 3 5% 6% 35%	2730
	11th to 20th	Night	12% 9% 3 8% 5% 4 4 4 5% 6% 40%	2647
	21st to end	Night	13% 9% 3 8% 5% 4 3 4 4 5% 41%	2076
March	1st to 10th	Night	17% 8% 2 5% 3 3 3 2 4 5% 46%	5263
	11th to 20th	Night	10% 7% 3 5% 4 4 3 3 5% 7% 50%	5090
	21st to end	Night	18% 12% 5% 11% 5% 5% 4 3 5% 5% 28%	5432
April	1st to 10th	Night	16% 9% 3 7% 4 4 4 3 5% 6% 40%	2100
	11th to 20th	Night	15% 10% 3 6% 4 4 4 4 5% 6% 40%	2035
	21st to end	Night	18% 7% 2 6% 4 4 5% 3 5% 7% 39%	1975
May	1st to 10th	Night	16% 7% 2 6% 4 3 4 3 5% 7% 43%	3897
	11th to 20th	Night	12% 5% 2 5% 5% 4 5% 4 5% 7% 47%	3596
	21st to end	Night	15% 7% 3 6% 4 4 5% 4 4 7% 42%	3942
June	1st to 10th	Night	7% 5% 3 6% 4 4 4 4 6% 9% 49%	1600
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 8% 11% 46%	1600
	21st to end	Night	10% 5% 2 5% 4 5% 5% 3 6% 9% 47%	1600
July	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 7% 10% 43%	3618
	11th to 20th	Night	8% 4 2 5% 3 3 4 4 5% 8% 53%	3617
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 8% 46%	4210
August	1st to 10th	Night	4 3 2 5% 3 5% 5% 4 6% 11% 53%	2926
	11th to 20th	Night	8% 4 2 5% 3 3 4 3 4 7% 56%	2942
	21st to end	Night	8% 3 2 5% 4 4 4 3 6% 7% 54%	3586
September	1st to 10th	Night	9% 4 2 5% 4 2 3 3 5% 7% 56%	2000
	11th to 20th	Night	11% 5% 1 6% 5% 4 4 3 4 7% 51%	2000
	21st to end	Night	8% 5% 2 4 4 3 3 3 4 7% 55%	2269
October	1st to 10th	Night	12% 6% 2 5% 3 3 3 2 4 6% 53%	2400
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 4 6% 48%	2400
	21st to end	Night	16% 7% 3 7% 4 4 4 3 4 6% 42%	2644
November	1st to 10th	Night	11% 6% 2 4 3 3 3 3 5% 6% 53%	2666
	11th to 20th	Night	14% 9% 3 7% 4 4 4 3 4 6% 41%	2856
	21st to end	Night	16% 10% 3 8% 4 4 4 3 4 6% 39%	2940
December	1st to 10th	Night	7% 9% 3 7% 5% 4 4 4 4 6% 47%	4720
	11th to 20th	Night	14% 11% 5% 11% 6% 4 4 3 4 5% 33%	5339
	21st to end	Night	23% 17% 8% 11% 5% 4 4 3 4 4 17%	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% 2 12% 14% 8%	4692
	11th to 20th	Day	51% 3 9% 21% 14% 2	4987
	21st to end	Day	58% 2 12% 21% 7%	5812
February	1st to 10th	Day	54% 2 15% 21% 8% 1	1890
	11th to 20th	Day	51% 3 12% 22% 10% 2	1868
	21st to end	Day	49% 2 13% 24% 9% 3	1730
March	1st to 10th	Day	45% 2 9% 26% 15% 2	4852
	11th to 20th	Day	37% 2 10% 32% 17% 2	5090
	21st to end	Day	60% 2 15% 17% 6% 1	5740
April	1st to 10th	Day	54% 3 10% 23% 10%	2520
	11th to 20th	Day	50% 3 11% 23% 12% 2	2446
	21st to end	Day	51% 3 10% 21% 13% 1	2422
May	1st to 10th	Day	47% 3 11% 21% 18% 1	6053
	11th to 20th	Day	40% 3 11% 29% 15% 3	6323
	21st to end	Day	45% 4 10% 26% 14% 1	6949
June	1st to 10th	Day	36% 4 15% 27% 16% 2	2800
	11th to 20th	Day	35% 5% 12% 24% 21% 3	2800
	21st to end	Day	39% 5% 13% 24% 16% 2	2800
July	1st to 10th	Day	39% 6% 12% 29% 12% 1	6291
	11th to 20th	Day	36% 5% 13% 29% 16% 1	6285
	21st to end	Day	40% 7% 18% 22% 14%	6469
August	1st to 10th	Day	33% 5% 18% 30% 14% 1	4220
	11th to 20th	Day	40% 5% 13% 26% 15% 1	4152
	21st to end	Day	32% 4 14% 31% 17% 2	4311
September	1st to 10th	Day	33% 4 10% 27% 23% 4	2400
	11th to 20th	Day	34% 3 12% 31% 18% 2	2400
	21st to end	Day	36% 3 9% 32% 19% 2	2105
October	1st to 10th	Day	24% 1 6% 27% 40% 2	3260
	11th to 20th	Day	29% 2 10% 25% 27% 7%	3240
	21st to end	Day	41% 2 9% 29% 16% 3	3521
November	1st to 10th	Day	39% 3 14% 26% 14% 3	1852
	11th to 20th	Day	48% 2 12% 22% 14% 1	1761
	21st to end	Day	47% 3 15% 24% 10%	1680
December	1st to 10th	Day	39% 2 13% 37% 8% 1	2690
	11th to 20th	Day	61% 2 11% 15% 11% 1	3085
	21st to end	Day	76% 2 8% 11% 3	4070
January	1st to 10th	Night	57% 2 15% 19% 8% 1	8233
	11th to 20th	Night	53% 3 12% 16% 15% 1	7998
	21st to end	Night	52% 2 13% 21% 12% 1	8402
February	1st to 10th	Night	51% 2 11% 24% 11% 1	2730
	11th to 20th	Night	45% 3 10% 28% 12% 2	2648
	21st to end	Night	46% 3 10% 24% 16% 2	2076
March	1st to 10th	Night	43% 1 7% 28% 19% 2	5263
	11th to 20th	Night	35% 2 8% 33% 18% 4	5090
	21st to end	Night	60% 3 14% 15% 7% 1	5432
April	1st to 10th	Night	46% 2 9% 28% 13% 2	2100
	11th to 20th	Night	45% 3 10% 23% 16% 3	2035
	21st to end	Night	45% 3 12% 23% 14% 3	1975
May	1st to 10th	Night	43% 3 10% 23% 19% 3	3924
	11th to 20th	Night	37% 3 13% 25% 18% 3	3597
	21st to end	Night	43% 3 9% 27% 15% 2	3950
June	1st to 10th	Night	33% 4 15% 29% 16% 4	1600
	11th to 20th	Night	32% 4 13% 26% 21% 4	1600
	21st to end	Night	34% 4 15% 25% 18% 5%	1600
July	1st to 10th	Night	35% 5% 13% 30% 14% 2	3580
	11th to 20th	Night	29% 4 15% 30% 21% 1	3572
	21st to end	Night	36% 6% 18% 25% 15% 1	4088
August	1st to 10th	Night	27% 4 17% 34% 16% 2	2916
	11th to 20th	Night	29% 5% 15% 34% 16% 1	2969
	21st to end	Night	30% 5% 11% 30% 18% 6%	3586
September	1st to 10th	Night	29% 3 9% 30% 26% 3	2000
	11th to 20th	Night	33% 3 9% 32% 19% 3	2000
	21st to end	Night	29% 3 9% 31% 25% 3	2269
October	1st to 10th	Night	24% 2 6% 27% 38% 3	3920
	11th to 20th	Night	28% 2 9% 23% 30% 8%	3899
	21st to end	Night	46% 2 8% 23% 18% 4	4268
November	1st to 10th	Night	33% 2 14% 28% 18% 4	2666
	11th to 20th	Night	45% 2 11% 26% 14% 2	2856
	21st to end	Night	49% 2 11% 26% 12% 1	2940
December	1st to 10th	Night	39% 2 12% 36% 11% 1	4720
	11th to 20th	Night	54% 2 12% 24% 6% 1	5341
	21st to end	Night	72% 3 8% 12% 5%	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	23% 9% 35% 23% 9% 2	330
	11th to 20th	Day	18% 16% 39% 22% 4	330
	21st to end	Day	18% 14% 37% 26% 3 1	363
February	1st to 10th	Day	17% 17% 38% 22% 5% 1	330
	11th to 20th	Day	20% 15% 36% 21% 7%	330
	21st to end	Day	19% 15% 47% 15% 4	273
March	1st to 10th	Day	17% 20% 44% 15% 4	330
	11th to 20th	Day	23% 15% 34% 20% 7% 1	330
	21st to end	Day	33% 14% 31% 16% 5%	428
April	1st to 10th	Day	24% 15% 36% 19% 6% 1	540
	11th to 20th	Day	22% 18% 31% 21% 7% 1	516
	21st to end	Day	24% 11% 36% 20% 8% 1	522
May	1st to 10th	Day	19% 11% 34% 26% 7% 3	703
	11th to 20th	Day	19% 12% 27% 26% 13% 2	725
	21st to end	Day	19% 6% 26% 29% 15% 5%	789
June	1st to 10th	Day	10% 4 27% 33% 21% 5%	730
	11th to 20th	Day	11% 5% 28% 34% 18% 3	730
	21st to end	Day	11% 7% 31% 32% 15% 4	730
July	1st to 10th	Day	8% 5% 16% 32% 28% 10%	721
	11th to 20th	Day	8% 4 24% 34% 22% 7%	718
	21st to end	Day	8% 3 22% 35% 27% 5%	794
August	1st to 10th	Day	8% 3 23% 33% 26% 7%	726
	11th to 20th	Day	10% 4 18% 34% 26% 7%	683
	21st to end	Day	9% 5% 17% 36% 27% 7%	583
September	1st to 10th	Day	13% 6% 20% 29% 26% 6%	530
	11th to 20th	Day	10% 3 18% 38% 24% 8%	530
	21st to end	Day	13% 7% 28% 23% 23% 6%	385
October	1st to 10th	Day	19% 6% 28% 25% 17% 5%	330
	11th to 20th	Day	18% 19% 34% 18% 9% 1	330
	21st to end	Day	23% 15% 34% 20% 6% 2	363
November	1st to 10th	Day	18% 11% 40% 18% 11% 2	330
	11th to 20th	Day	18% 18% 34% 23% 5% 1	330
	21st to end	Day	25% 12% 37% 22% 5% 1	330
December	1st to 10th	Day	14% 13% 46% 18% 8% 2	330
	11th to 20th	Day	18% 14% 35% 25% 7% 1	330
	21st to end	Day	31% 10% 38% 17% 3	363
January	1st to 10th	Night	16% 12% 38% 27% 7%	1050
	11th to 20th	Night	19% 13% 37% 25% 6%	1050
	21st to end	Night	19% 11% 38% 26% 5%	1155
February	1st to 10th	Night	19% 12% 38% 26% 5%	1050
	11th to 20th	Night	17% 13% 37% 29% 4	1013
	21st to end	Night	19% 12% 41% 23% 4 1	692
March	1st to 10th	Night	20% 11% 41% 22% 6% 1	740
	11th to 20th	Night	19% 9% 35% 28% 8%	630
	21st to end	Night	24% 11% 38% 22% 5% 1	693
April	1st to 10th	Night	22% 10% 41% 23% 3	630
	11th to 20th	Night	20% 14% 33% 27% 6%	611
	21st to end	Night	21% 7% 37% 26% 8% 1	600
May	1st to 10th	Night	18% 5% 38% 33% 6% 1	600
	11th to 20th	Night	19% 5% 31% 34% 11%	591
	21st to end	Night	14% 3 30% 35% 16% 1	650
June	1st to 10th	Night	9% 4 24% 43% 18% 2	600
	11th to 20th	Night	9% 6% 24% 47% 14%	600
	21st to end	Night	11% 4 27% 40% 17% 1	600
July	1st to 10th	Night	6% 2 14% 44% 32% 2	585
	11th to 20th	Night	6% 3 19% 41% 27% 4	582
	21st to end	Night	8% 3 15% 47% 25% 2	657
August	1st to 10th	Night	6% 3 14% 47% 29% 3	597
	11th to 20th	Night	8% 3 14% 43% 28% 4	594
	21st to end	Night	11% 2 13% 43% 29% 2	660
September	1st to 10th	Night	10% 4 19% 39% 26% 3	600
	11th to 20th	Night	12% 2 19% 37% 27% 3	600
	21st to end	Night	10% 6% 26% 37% 20% 2	729
October	1st to 10th	Night	15% 6% 26% 33% 19% 2	800
	11th to 20th	Night	18% 10% 34% 30% 9%	800
	21st to end	Night	21% 11% 32% 28% 8%	880
November	1st to 10th	Night	15% 9% 39% 29% 9%	820
	11th to 20th	Night	20% 13% 39% 23% 6%	966
	21st to end	Night	21% 12% 34% 26% 6%	1050
December	1st to 10th	Night	19% 11% 37% 28% 5% 1	1050
	11th to 20th	Night	21% 11% 34% 27% 7% 1	1050
	21st to end	Night	27% 12% 34% 22% 5%	1155

Page generated at 2026-04-14 01:44:25. Version ac07

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