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 2024-12-24.

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	13% 8% 3 7% 5% 4 4 4 5% 7% 38%	101892
	Night	13% 8% 3 7% 4 4 4 3 5% 6% 41%	98320
Transparency	Day	44% 3 12% 24% 15% 2	105514
	Night	44% 3 11% 25% 16% 2	102838
Seeing	Day	16% 9% 29% 27% 15% 4	15989
	Night	17% 8% 31% 31% 12% 1	25906

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	9% 11% 5% 21% 8% 7% 5% 4 4 6% 20%	2527
	11th to 20th	Day	11% 12% 5% 11% 8% 7% 6% 5% 7% 6% 24%	2667
	21st to end	Day	14% 18% 7% 15% 7% 5% 4 3 3 5% 18%	3135
February	1st to 10th	Day	12% 13% 4 9% 7% 5% 4 4 5% 8% 30%	1710
	11th to 20th	Day	13% 11% 4 7% 5% 6% 5% 4 5% 7% 34%	1677
	21st to end	Day	14% 10% 4 11% 5% 3 4 3 5% 7% 35%	1570
March	1st to 10th	Day	17% 12% 3 7% 3 3 3 3 3 5% 41%	3302
	11th to 20th	Day	10% 6% 3 6% 5% 4 4 3 5% 7% 46%	3480
	21st to end	Day	23% 14% 5% 10% 4 4 4 3 5% 5% 22%	3924
April	1st to 10th	Day	17% 12% 4 7% 5% 4 4 3 5% 5% 33%	2280
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 4 6% 6% 34%	2254
	21st to end	Day	23% 8% 2 6% 4 4 4 4 5% 7% 34%	2301
May	1st to 10th	Day	18% 8% 3 7% 5% 4 4 4 6% 7% 34%	4193
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 5% 6% 41%	4399
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 5% 8% 43%	4824
June	1st to 10th	Day	10% 5% 2 6% 5% 4 4 5% 6% 8% 45%	2660
	11th to 20th	Day	10% 4 2 5% 5% 5% 5% 4 7% 7% 46%	2660
	21st to end	Day	11% 7% 2 6% 5% 4 5% 4 7% 9% 40%	2660
July	1st to 10th	Day	10% 8% 3 7% 5% 6% 6% 4 7% 8% 36%	4440
	11th to 20th	Day	6% 4 1 5% 3 4 4 4 6% 9% 54%	4440
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 6% 8% 10% 41%	4631
August	1st to 10th	Day	8% 4 2 5% 5% 4 5% 5% 7% 10% 45%	4104
	11th to 20th	Day	11% 5% 3 7% 5% 5% 5% 4 6% 9% 40%	3985
	21st to end	Day	8% 4 2 6% 4 4 4 3 6% 8% 50%	4180
September	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 4 6% 52%	2280
	11th to 20th	Day	10% 5% 2 4 4 3 5% 4 5% 8% 50%	2280
	21st to end	Day	11% 5% 2 5% 4 4 4 3 5% 8% 48%	2000
October	1st to 10th	Day	15% 6% 2 6% 3 3 4 3 4 6% 49%	1900
	11th to 20th	Day	16% 7% 2 6% 4 4 3 3 4 6% 44%	1900
	21st to end	Day	15% 10% 3 7% 4 4 4 2 4 5% 43%	2085
November	1st to 10th	Day	12% 8% 3 5% 4 4 5% 4 6% 6% 44%	1762
	11th to 20th	Day	16% 11% 3 8% 5% 3 3 3 5% 7% 36%	1677
	21st to end	Day	16% 10% 4 6% 3 4 4 4 4 7% 38%	1600
December	1st to 10th	Day	13% 13% 4 10% 5% 4 4 3 4 7% 34%	1600
	11th to 20th	Day	14% 10% 4 10% 5% 6% 4 4 5% 5% 33%	1998
	21st to end	Day	22% 19% 8% 12% 5% 3 4 3 3 4 17%	2807
January	1st to 10th	Night	11% 12% 9% 16% 5% 4 4 3 4 5% 27%	4428
	11th to 20th	Night	12% 15% 6% 12% 5% 4 5% 3 7% 6% 24%	4298
	21st to end	Night	17% 16% 7% 12% 5% 4 4 3 4 6% 23%	4532
February	1st to 10th	Night	13% 12% 4 8% 5% 4 4 3 5% 6% 35%	2470
	11th to 20th	Night	12% 9% 3 7% 5% 4 4 4 5% 6% 40%	2389
	21st to end	Night	14% 9% 3 8% 5% 4 3 4 4 5% 40%	1884
March	1st to 10th	Night	17% 9% 2 5% 3 4 4 3 4 5% 44%	3578
	11th to 20th	Night	11% 6% 3 5% 4 4 3 3 4 6% 51%	3480
	21st to end	Night	22% 12% 5% 12% 5% 4 4 3 4 4 23%	3706
April	1st to 10th	Night	16% 9% 3 7% 4 3 4 3 5% 6% 41%	1900
	11th to 20th	Night	16% 10% 2 6% 4 4 4 4 5% 6% 40%	1875
	21st to end	Night	18% 7% 2 5% 4 4 5% 3 5% 6% 40%	1876
May	1st to 10th	Night	19% 7% 2 6% 4 3 4 3 5% 7% 39%	2709
	11th to 20th	Night	13% 6% 2 6% 5% 4 5% 4 5% 6% 45%	2506
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 5% 8% 48%	2743
June	1st to 10th	Night	7% 5% 3 6% 4 4 4 4 6% 9% 48%	1520
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 8% 10% 48%	1520
	21st to end	Night	10% 5% 2 5% 4 5% 5% 3 6% 9% 46%	1520
July	1st to 10th	Night	10% 5% 2 6% 5% 5% 5% 5% 7% 9% 41%	2530
	11th to 20th	Night	5% 3 1 4 3 3 3 4 6% 9% 60%	2530
	21st to end	Night	6% 5% 2 5% 4 4 4 4 6% 8% 51%	2929
August	1st to 10th	Night	4 3 2 5% 3 4 5% 4 6% 11% 53%	2836
	11th to 20th	Night	8% 4 2 5% 3 3 4 3 4 7% 56%	2850
	21st to end	Night	8% 4 2 5% 4 4 4 3 6% 7% 53%	3476
September	1st to 10th	Night	10% 4 2 4 4 2 3 3 5% 7% 56%	1900
	11th to 20th	Night	11% 5% 1 5% 4 3 4 3 4 7% 53%	1900
	21st to end	Night	8% 5% 2 4 3 3 3 3 5% 7% 55%	2155
October	1st to 10th	Night	12% 6% 2 5% 3 3 3 2 4 6% 53%	2280
	11th to 20th	Night	14% 6% 2 6% 4 3 4 3 4 6% 48%	2280
	21st to end	Night	16% 7% 3 7% 4 4 4 3 4 6% 42%	2512
November	1st to 10th	Night	12% 6% 2 4 3 3 3 3 5% 6% 53%	2536
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 4 6% 41%	2720
	21st to end	Night	16% 9% 3 8% 4 4 4 3 4 6% 40%	2800
December	1st to 10th	Night	12% 11% 4 9% 5% 4 4 4 4 5% 37%	2800
	11th to 20th	Night	12% 11% 5% 9% 5% 4 4 4 4 5% 36%	3419
	21st to end	Night	21% 16% 7% 11% 5% 5% 4 3 4 5% 21%	4933

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	67% 1 11% 14% 6%	2528
	11th to 20th	Day	61% 1 11% 20% 7%	2667
	21st to end	Day	71% 2 10% 12% 5%	3135
February	1st to 10th	Day	54% 2 14% 21% 9% 1	1710
	11th to 20th	Day	50% 3 12% 23% 10% 1	1686
	21st to end	Day	51% 2 12% 23% 9% 3	1570
March	1st to 10th	Day	48% 2 11% 25% 14% 1	3302
	11th to 20th	Day	39% 2 12% 28% 17% 2	3480
	21st to end	Day	65% 2 13% 16% 4 1	3924
April	1st to 10th	Day	54% 3 10% 24% 9%	2280
	11th to 20th	Day	49% 3 10% 23% 12% 2	2254
	21st to end	Day	51% 3 11% 21% 14% 1	2301
May	1st to 10th	Day	51% 2 11% 21% 14% 1	4231
	11th to 20th	Day	45% 2 11% 29% 13%	4403
	21st to end	Day	41% 3 9% 27% 17% 2	4837
June	1st to 10th	Day	36% 4 15% 27% 16% 2	2660
	11th to 20th	Day	35% 4 12% 23% 22% 3	2660
	21st to end	Day	40% 5% 14% 24% 16% 2	2660
July	1st to 10th	Day	44% 6% 11% 28% 9% 2	4371
	11th to 20th	Day	27% 5% 15% 33% 19% 2	4365
	21st to end	Day	34% 8% 18% 21% 18% 1	4485
August	1st to 10th	Day	33% 5% 17% 30% 14% 1	4090
	11th to 20th	Day	40% 4 13% 26% 15% 1	4024
	21st to end	Day	32% 4 14% 31% 17% 2	4179
September	1st to 10th	Day	34% 3 10% 25% 23% 4	2280
	11th to 20th	Day	33% 3 11% 32% 19% 2	2280
	21st to end	Day	36% 3 9% 32% 19% 2	2000
October	1st to 10th	Day	23% 1 6% 27% 40% 2	3160
	11th to 20th	Day	29% 2 10% 26% 28% 7%	3140
	21st to end	Day	41% 2 9% 29% 16% 3	3411
November	1st to 10th	Day	40% 3 14% 26% 14% 3	1762
	11th to 20th	Day	49% 2 12% 21% 15% 1	1677
	21st to end	Day	47% 3 15% 25% 10% 1	1600
December	1st to 10th	Day	52% 2 13% 24% 8% 1	1600
	11th to 20th	Day	54% 2 14% 19% 10% 1	1995
	21st to end	Day	73% 2 8% 12% 4	2807
January	1st to 10th	Night	61% 2 10% 17% 9%	4428
	11th to 20th	Night	61% 1 12% 14% 11% 1	4298
	21st to end	Night	65% 2 10% 16% 6% 1	4532
February	1st to 10th	Night	52% 2 10% 24% 11% 1	2470
	11th to 20th	Night	44% 3 11% 27% 13% 2	2390
	21st to end	Night	47% 2 10% 23% 15% 3	1884
March	1st to 10th	Night	46% 1 8% 27% 17% 1	3578
	11th to 20th	Night	36% 2 10% 34% 15% 4	3480
	21st to end	Night	66% 2 15% 12% 5% 1	3706
April	1st to 10th	Night	45% 2 9% 29% 14% 2	1900
	11th to 20th	Night	45% 2 9% 23% 17% 3	1875
	21st to end	Night	45% 3 11% 24% 15% 3	1876
May	1st to 10th	Night	47% 2 10% 19% 18% 3	2736
	11th to 20th	Night	41% 2 12% 28% 14% 2	2507
	21st to end	Night	37% 3 10% 28% 20% 3	2751
June	1st to 10th	Night	33% 3 14% 29% 16% 5%	1520
	11th to 20th	Night	30% 4 14% 26% 21% 4	1520
	21st to end	Night	34% 3 15% 25% 17% 5%	1520
July	1st to 10th	Night	39% 5% 13% 28% 12% 3	2490
	11th to 20th	Night	21% 4 17% 34% 22% 2	2482
	21st to end	Night	30% 6% 18% 25% 19% 2	2836
August	1st to 10th	Night	27% 4 16% 35% 16% 2	2826
	11th to 20th	Night	29% 5% 15% 34% 16% 1	2877
	21st to end	Night	30% 5% 11% 30% 18% 6%	3476
September	1st to 10th	Night	29% 3 8% 29% 27% 3	1900
	11th to 20th	Night	33% 3 9% 33% 19% 4	1900
	21st to end	Night	29% 3 9% 32% 24% 3	2155
October	1st to 10th	Night	24% 2 6% 27% 38% 3	3800
	11th to 20th	Night	27% 2 9% 23% 31% 8%	3779
	21st to end	Night	46% 2 8% 23% 17% 4	4136
November	1st to 10th	Night	34% 2 15% 27% 18% 4	2536
	11th to 20th	Night	45% 2 11% 26% 14% 2	2720
	21st to end	Night	48% 2 11% 27% 12% 1	2800
December	1st to 10th	Night	50% 3 12% 25% 10% 1	2800
	11th to 20th	Night	51% 2 12% 26% 8% 2	3421
	21st to end	Night	68% 2 8% 15% 6% 1	4933

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% 9% 32% 23% 10% 1	270
	11th to 20th	Day	21% 14% 37% 24% 4	270
	21st to end	Day	21% 11% 34% 29% 4 1	297
February	1st to 10th	Day	20% 16% 37% 21% 6% 1	270
	11th to 20th	Day	20% 14% 35% 22% 9%	270
	21st to end	Day	22% 14% 45% 14% 4	225
March	1st to 10th	Day	17% 20% 42% 16% 5%	270
	11th to 20th	Day	22% 14% 35% 19% 9% 1	270
	21st to end	Day	36% 13% 27% 18% 6% 1	356
April	1st to 10th	Day	24% 13% 37% 20% 6% 1	460
	11th to 20th	Day	21% 16% 31% 23% 8% 1	452
	21st to end	Day	25% 10% 36% 19% 9% 1	481
May	1st to 10th	Day	20% 12% 33% 26% 6% 2	653
	11th to 20th	Day	19% 11% 28% 26% 13% 2	675
	21st to end	Day	19% 6% 26% 29% 16% 5%	734
June	1st to 10th	Day	10% 3 27% 33% 22% 6%	680
	11th to 20th	Day	11% 5% 28% 34% 19% 3	680
	21st to end	Day	11% 8% 30% 31% 15% 4	680
July	1st to 10th	Day	8% 5% 16% 32% 28% 11%	671
	11th to 20th	Day	8% 4 24% 34% 23% 7%	668
	21st to end	Day	7% 3 22% 35% 28% 5%	739
August	1st to 10th	Day	8% 3 23% 33% 27% 7%	676
	11th to 20th	Day	11% 4 19% 34% 26% 6%	635
	21st to end	Day	9% 5% 15% 36% 27% 7%	539
September	1st to 10th	Day	14% 5% 20% 29% 26% 6%	490
	11th to 20th	Day	10% 3 18% 38% 23% 8%	490
	21st to end	Day	14% 7% 30% 23% 22% 4	352
October	1st to 10th	Day	20% 7% 26% 27% 16% 4	300
	11th to 20th	Day	19% 18% 34% 18% 10% 1	300
	21st to end	Day	25% 14% 34% 19% 6% 3	330
November	1st to 10th	Day	18% 9% 39% 19% 12% 3	300
	11th to 20th	Day	18% 19% 33% 23% 6% 1	300
	21st to end	Day	24% 11% 38% 22% 4 1	300
December	1st to 10th	Day	13% 12% 47% 18% 7% 2	300
	11th to 20th	Day	17% 14% 35% 25% 7% 1	300
	21st to end	Day	29% 10% 40% 18% 3	306
January	1st to 10th	Night	15% 12% 39% 26% 8%	950
	11th to 20th	Night	19% 12% 36% 26% 7%	950
	21st to end	Night	20% 11% 36% 27% 6%	1045
February	1st to 10th	Night	20% 11% 38% 26% 5%	950
	11th to 20th	Night	17% 12% 37% 30% 5%	915
	21st to end	Night	20% 12% 40% 23% 5% 1	628
March	1st to 10th	Night	19% 11% 41% 22% 7% 1	670
	11th to 20th	Night	19% 8% 34% 29% 9%	570
	21st to end	Night	25% 9% 37% 23% 5% 1	627
April	1st to 10th	Night	22% 10% 41% 24% 4	570
	11th to 20th	Night	21% 13% 32% 28% 6%	563
	21st to end	Night	21% 7% 37% 26% 8% 1	570
May	1st to 10th	Night	18% 5% 38% 32% 6% 1	570
	11th to 20th	Night	19% 5% 31% 34% 11%	561
	21st to end	Night	13% 3 29% 36% 17% 1	617
June	1st to 10th	Night	9% 4 25% 44% 17% 2	570
	11th to 20th	Night	9% 5% 24% 47% 14%	570
	21st to end	Night	11% 4 26% 40% 17% 1	570
July	1st to 10th	Night	6% 2 14% 43% 33% 3	555
	11th to 20th	Night	6% 2 20% 41% 28% 4	552
	21st to end	Night	8% 3 15% 47% 25% 2	624
August	1st to 10th	Night	6% 3 14% 46% 29% 3	567
	11th to 20th	Night	8% 2 15% 43% 27% 5%	564
	21st to end	Night	11% 2 13% 42% 29% 3	627
September	1st to 10th	Night	10% 4 19% 39% 25% 3	570
	11th to 20th	Night	12% 2 19% 36% 26% 4	570
	21st to end	Night	10% 6% 27% 37% 18% 2	692
October	1st to 10th	Night	15% 6% 26% 32% 18% 2	760
	11th to 20th	Night	17% 10% 33% 30% 10%	760
	21st to end	Night	21% 11% 32% 28% 8%	836
November	1st to 10th	Night	15% 8% 38% 29% 9% 1	780
	11th to 20th	Night	19% 12% 39% 23% 6%	920
	21st to end	Night	21% 11% 35% 27% 7%	1000
December	1st to 10th	Night	19% 11% 37% 28% 6% 1	1000
	11th to 20th	Night	21% 10% 34% 28% 7% 1	1000
	21st to end	Night	26% 12% 34% 23% 5%	1063

Page generated at 2024-12-21 03:13:49. Version ac07

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