Smith Mountain Lake State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Smith Mountain Lake State Park 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	13% 5% 2 6% 4 4 5% 4 6% 8% 43%	125732
	Night	14% 5% 2 5% 4 4 4 4 5% 7% 45%	123644
Transparency	Day	38% 7% 12% 22% 17% 4	129337
	Night	37% 6% 11% 22% 19% 5%	128107
Seeing	Day	17% 10% 24% 24% 17% 7%	16367
	Night	17% 8% 26% 30% 17% 2	26672

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	14% 6% 1 4 3 4 3 3 4 6% 51%	4056
	11th to 20th	Day	13% 6% 2 4 4 3 3 3 4 5% 53%	4060
	21st to end	Day	17% 6% 2 5% 3 4 3 3 4 5% 47%	4466
February	1st to 10th	Day	13% 7% 2 6% 5% 4 6% 4 6% 7% 40%	1800
	11th to 20th	Day	20% 5% 1 5% 4 4 3 4 6% 7% 41%	1768
	21st to end	Day	19% 6% 3 5% 4 4 4 3 5% 7% 40%	1759
March	1st to 10th	Day	18% 6% 3 6% 4 4 4 4 6% 7% 39%	4935
	11th to 20th	Day	12% 4 2 4 3 3 3 4 5% 7% 53%	4980
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 6% 42%	5465
April	1st to 10th	Day	12% 5% 2 4 4 4 4 4 5% 7% 49%	2295
	11th to 20th	Day	13% 6% 2 5% 3 4 4 3 5% 7% 49%	2374
	21st to end	Day	15% 5% 2 6% 5% 5% 4 3 5% 7% 43%	2488
May	1st to 10th	Day	12% 5% 2 7% 5% 5% 6% 5% 8% 10% 34%	5862
	11th to 20th	Day	21% 7% 3 7% 4 4 5% 4 6% 8% 31%	5861
	21st to end	Day	14% 4 2 5% 4 5% 5% 4 6% 9% 43%	6433
June	1st to 10th	Day	10% 4 2 5% 5% 4 5% 5% 7% 9% 45%	2600
	11th to 20th	Day	10% 4 2 6% 5% 4 5% 5% 7% 11% 40%	2600
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 10% 44%	2600
July	1st to 10th	Day	7% 5% 2 7% 5% 5% 5% 5% 8% 11% 39%	5900
	11th to 20th	Day	7% 6% 3 7% 6% 5% 7% 6% 9% 13% 32%	5900
	21st to end	Day	9% 6% 2 8% 6% 6% 7% 5% 9% 10% 33%	6424
August	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 5% 8% 11% 31%	4234
	11th to 20th	Day	9% 4 2 6% 4 5% 7% 5% 7% 10% 41%	4156
	21st to end	Day	8% 4 1 5% 4 4 4 3 6% 11% 50%	4312
September	1st to 10th	Day	12% 4 2 5% 3 3 4 4 5% 7% 50%	2400
	11th to 20th	Day	14% 6% 2 6% 4 4 3 4 5% 8% 44%	2205
	21st to end	Day	24% 6% 2 5% 4 4 3 4 5% 7% 36%	2190
October	1st to 10th	Day	18% 5% 2 3 2 4 2 2 4 6% 52%	1930
	11th to 20th	Day	16% 4 1 4 2 3 3 3 4 7% 53%	1900
	21st to end	Day	13% 5% 2 5% 3 3 3 3 4 6% 54%	2090
November	1st to 10th	Day	13% 4 1 4 3 3 3 3 4 6% 55%	1957
	11th to 20th	Day	14% 6% 2 4 3 4 4 3 4 6% 51%	1882
	21st to end	Day	13% 4 1 5% 3 4 4 3 5% 8% 50%	1674
December	1st to 10th	Day	13% 6% 1 5% 3 4 4 4 5% 6% 49%	2600
	11th to 20th	Day	20% 5% 2 4 3 3 3 3 6% 6% 46%	3110
	21st to end	Day	21% 5% 2 5% 4 4 4 4 6% 7% 37%	4466
January	1st to 10th	Night	13% 4 2 4 3 3 3 3 4 6% 56%	5889
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 4 6% 55%	5900
	21st to end	Night	19% 6% 2 4 2 3 3 3 4 6% 50%	6490
February	1st to 10th	Night	15% 6% 2 6% 4 3 4 4 6% 7% 44%	2600
	11th to 20th	Night	20% 6% 2 5% 4 3 4 3 5% 5% 43%	2515
	21st to end	Night	15% 6% 2 6% 4 4 5% 3 5% 6% 43%	1870
March	1st to 10th	Night	19% 5% 2 5% 4 4 4 3 5% 5% 44%	4918
	11th to 20th	Night	10% 4 1 4 3 3 3 3 6% 7% 56%	4980
	21st to end	Night	14% 7% 2 5% 4 4 5% 3 5% 7% 45%	5467
April	1st to 10th	Night	16% 6% 3 6% 4 3 4 3 5% 6% 45%	2104
	11th to 20th	Night	13% 5% 2 6% 3 3 3 3 6% 6% 50%	1975
	21st to end	Night	15% 4 2 5% 4 3 4 3 6% 8% 45%	1929
May	1st to 10th	Night	15% 7% 2 7% 4 5% 5% 4 6% 8% 38%	4033
	11th to 20th	Night	22% 8% 3 7% 5% 4 4 3 5% 7% 33%	4032
	21st to end	Night	16% 5% 2 6% 4 4 3 4 6% 9% 42%	4421
June	1st to 10th	Night	9% 5% 1 6% 4 4 5% 5% 7% 12% 42%	1800
	11th to 20th	Night	11% 4 2 6% 4 3 5% 4 7% 9% 45%	1800
	21st to end	Night	8% 3 2 4 5% 4 6% 4 8% 11% 45%	1800
July	1st to 10th	Night	10% 5% 2 8% 5% 5% 6% 5% 8% 10% 37%	4058
	11th to 20th	Night	9% 7% 3 7% 6% 6% 6% 6% 8% 12% 30%	4057
	21st to end	Night	9% 7% 3 8% 6% 6% 7% 5% 7% 9% 33%	4427
August	1st to 10th	Night	17% 7% 2 6% 4 5% 5% 4 8% 10% 30%	2915
	11th to 20th	Night	8% 4 2 7% 5% 5% 6% 5% 8% 10% 41%	2875
	21st to end	Night	8% 4 2 6% 4 4 5% 4 6% 9% 48%	3570
September	1st to 10th	Night	10% 6% 2 6% 4 4 5% 3 6% 6% 47%	2000
	11th to 20th	Night	16% 6% 2 6% 3 4 4 3 6% 7% 42%	2190
	21st to end	Night	24% 7% 3 7% 4 4 3 3 5% 5% 36%	2210
October	1st to 10th	Night	19% 5% 2 4 4 3 3 3 4 7% 47%	2317
	11th to 20th	Night	14% 5% 2 5% 4 4 3 3 4 6% 51%	2280
	21st to end	Night	14% 3 1 3 3 3 3 2 4 6% 56%	2508
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 5% 52%	2341
	11th to 20th	Night	14% 5% 2 4 3 3 4 4 5% 5% 52%	2515
	21st to end	Night	13% 4 2 5% 3 4 4 4 5% 7% 50%	2720
December	1st to 10th	Night	11% 4 2 4 3 4 4 4 5% 6% 53%	4580
	11th to 20th	Night	21% 7% 3 6% 4 4 3 3 4 5% 40%	5068
	21st to end	Night	20% 4 2 5% 3 4 4 4 6% 8% 39%	6490

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	33% 4 7% 23% 27% 6%	4057
	11th to 20th	Day	33% 3 5% 20% 29% 9%	4060
	21st to end	Day	41% 4 7% 25% 21% 3	4466
February	1st to 10th	Day	40% 7% 12% 22% 17% 3	1800
	11th to 20th	Day	42% 5% 9% 23% 17% 5%	1777
	21st to end	Day	43% 5% 13% 15% 18% 6%	1759
March	1st to 10th	Day	44% 5% 7% 20% 15% 10%	4935
	11th to 20th	Day	30% 6% 8% 22% 24% 11%	4980
	21st to end	Day	42% 5% 9% 20% 18% 6%	5465
April	1st to 10th	Day	33% 7% 11% 23% 19% 8%	2295
	11th to 20th	Day	35% 6% 12% 24% 18% 6%	2374
	21st to end	Day	41% 7% 12% 22% 15% 4	2488
May	1st to 10th	Day	41% 10% 15% 21% 11% 2	5897
	11th to 20th	Day	51% 8% 10% 20% 10% 2	5865
	21st to end	Day	36% 8% 15% 21% 16% 4	6447
June	1st to 10th	Day	33% 10% 16% 26% 14% 1	2600
	11th to 20th	Day	36% 10% 16% 21% 15% 2	2600
	21st to end	Day	33% 12% 15% 23% 15% 2	2600
July	1st to 10th	Day	37% 11% 18% 18% 12% 3	5835
	11th to 20th	Day	39% 13% 23% 19% 6%	5831
	21st to end	Day	42% 11% 15% 20% 11%	6240
August	1st to 10th	Day	45% 9% 16% 24% 5% 1	4221
	11th to 20th	Day	36% 11% 17% 26% 10% 1	4195
	21st to end	Day	29% 7% 18% 29% 16% 1	4312
September	1st to 10th	Day	34% 9% 13% 20% 20% 4	2400
	11th to 20th	Day	40% 6% 11% 22% 20% 2	2205
	21st to end	Day	48% 8% 10% 16% 17% 2	2190
October	1st to 10th	Day	33% 4 8% 19% 29% 8%	3190
	11th to 20th	Day	24% 3 6% 24% 28% 15%	3140
	21st to end	Day	24% 4 10% 25% 24% 12%	3428
November	1st to 10th	Day	31% 5% 9% 23% 26% 6%	1957
	11th to 20th	Day	35% 5% 8% 22% 22% 8%	1882
	21st to end	Day	32% 5% 10% 24% 25% 4	1674
December	1st to 10th	Day	35% 6% 7% 24% 22% 7%	2600
	11th to 20th	Day	39% 4 11% 20% 21% 5%	3106
	21st to end	Day	43% 7% 11% 24% 14% 1	4466
January	1st to 10th	Night	30% 4 6% 23% 28% 9%	5889
	11th to 20th	Night	32% 4 6% 20% 32% 7%	5900
	21st to end	Night	38% 4 6% 29% 19% 5%	6490
February	1st to 10th	Night	39% 6% 10% 24% 18% 3	2600
	11th to 20th	Night	43% 4 9% 19% 19% 6%	2517
	21st to end	Night	41% 6% 9% 22% 15% 7%	1870
March	1st to 10th	Night	41% 5% 6% 21% 19% 8%	4918
	11th to 20th	Night	27% 5% 8% 20% 28% 12%	4980
	21st to end	Night	39% 4 8% 23% 21% 5%	5467
April	1st to 10th	Night	39% 6% 11% 19% 20% 5%	2104
	11th to 20th	Night	34% 6% 10% 23% 21% 6%	1975
	21st to end	Night	35% 7% 12% 24% 18% 4	1929
May	1st to 10th	Night	43% 9% 16% 19% 12% 2	4060
	11th to 20th	Night	51% 6% 11% 19% 12% 1	4033
	21st to end	Night	36% 8% 15% 26% 12% 3	4429
June	1st to 10th	Night	33% 11% 16% 24% 15% 1	1800
	11th to 20th	Night	33% 11% 18% 21% 16% 2	1800
	21st to end	Night	30% 12% 16% 22% 17% 4	1800
July	1st to 10th	Night	40% 12% 17% 18% 11% 2	4015
	11th to 20th	Night	43% 13% 21% 16% 5% 1	4006
	21st to end	Night	45% 10% 14% 21% 10%	4297
August	1st to 10th	Night	46% 10% 16% 21% 7% 1	2904
	11th to 20th	Night	34% 10% 18% 27% 10% 1	2902
	21st to end	Night	31% 8% 12% 35% 12% 2	3570
September	1st to 10th	Night	36% 9% 13% 20% 17% 6%	2000
	11th to 20th	Night	41% 8% 10% 21% 20% 2	2190
	21st to end	Night	50% 7% 9% 17% 15% 3	2210
October	1st to 10th	Night	38% 5% 8% 19% 24% 6%	3837
	11th to 20th	Night	25% 3 7% 22% 30% 13%	3779
	21st to end	Night	21% 3 7% 29% 27% 12%	4120
November	1st to 10th	Night	35% 4 9% 20% 25% 7%	2341
	11th to 20th	Night	33% 5% 9% 21% 25% 8%	2515
	21st to end	Night	33% 4 9% 25% 23% 6%	2720
December	1st to 10th	Night	32% 4 7% 28% 25% 3	4580
	11th to 20th	Night	47% 4 9% 19% 14% 7%	5070
	21st to end	Night	39% 6% 13% 24% 17% 2	6490

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	27% 26% 27% 14% 4 1	300
	11th to 20th	Day	28% 23% 35% 12% 2	300
	21st to end	Day	24% 21% 39% 13% 3	330
February	1st to 10th	Day	17% 23% 38% 16% 5%	300
	11th to 20th	Day	35% 25% 28% 9% 2 1	300
	21st to end	Day	27% 17% 34% 18% 4	358
March	1st to 10th	Day	24% 16% 32% 20% 8% 1	500
	11th to 20th	Day	21% 18% 32% 21% 6% 1	500
	21st to end	Day	25% 13% 35% 21% 5%	550
April	1st to 10th	Day	20% 20% 31% 19% 9% 1	500
	11th to 20th	Day	19% 15% 28% 23% 11% 4	492
	21st to end	Day	18% 12% 33% 24% 10% 2	500
May	1st to 10th	Day	16% 14% 33% 24% 9% 3	503
	11th to 20th	Day	20% 7% 25% 25% 16% 7%	527
	21st to end	Day	18% 5% 20% 26% 23% 8%	573
June	1st to 10th	Day	10% 5% 24% 33% 19% 9%	530
	11th to 20th	Day	10% 5% 19% 29% 25% 12%	530
	21st to end	Day	9% 4 17% 32% 27% 10%	530
July	1st to 10th	Day	11% 2 12% 27% 31% 16%	525
	11th to 20th	Day	5% 2 11% 27% 38% 17%	524
	21st to end	Day	10% 2 15% 30% 27% 16%	576
August	1st to 10th	Day	15% 4 20% 20% 28% 13%	526
	11th to 20th	Day	10% 3 14% 28% 28% 17%	519
	21st to end	Day	11% 2 10% 27% 30% 19%	583
September	1st to 10th	Day	13% 3 14% 25% 27% 17%	530
	11th to 20th	Day	17% 3 14% 22% 29% 15%	530
	21st to end	Day	28% 7% 19% 26% 15% 5%	530
October	1st to 10th	Day	21% 5% 16% 28% 23% 7%	502
	11th to 20th	Day	14% 13% 20% 25% 23% 4	490
	21st to end	Day	14% 10% 27% 30% 15% 4	539
November	1st to 10th	Day	19% 9% 26% 29% 14% 4	495
	11th to 20th	Day	18% 16% 31% 22% 12% 1	382
	21st to end	Day	18% 20% 26% 24% 10% 2	236
December	1st to 10th	Day	24% 19% 35% 16% 7% 1	200
	11th to 20th	Day	23% 11% 33% 23% 8% 1	227
	21st to end	Day	24% 15% 31% 19% 9% 2	330
January	1st to 10th	Night	22% 17% 33% 24% 3	800
	11th to 20th	Night	19% 16% 35% 29% 3	800
	21st to end	Night	20% 13% 41% 24% 2	880
February	1st to 10th	Night	19% 14% 39% 26% 3	800
	11th to 20th	Night	24% 16% 40% 18% 1	783
	21st to end	Night	19% 13% 40% 25% 3	660
March	1st to 10th	Night	17% 14% 36% 28% 4	800
	11th to 20th	Night	20% 14% 35% 24% 7% 1	800
	21st to end	Night	18% 12% 36% 30% 5%	880
April	1st to 10th	Night	19% 17% 32% 27% 6%	800
	11th to 20th	Night	15% 11% 34% 28% 12%	791
	21st to end	Night	16% 7% 32% 35% 9%	729
May	1st to 10th	Night	15% 8% 35% 27% 15%	600
	11th to 20th	Night	19% 5% 25% 32% 17% 2	591
	21st to end	Night	17% 1 16% 33% 30% 4	650
June	1st to 10th	Night	11% 2 16% 40% 30% 2	600
	11th to 20th	Night	12% 1 14% 33% 36% 4	600
	21st to end	Night	8% 2 14% 42% 31% 4	600
July	1st to 10th	Night	10% 1 10% 34% 38% 7%	585
	11th to 20th	Night	6% 2 7% 35% 44% 6%	582
	21st to end	Night	8% 1 10% 40% 35% 5%	657
August	1st to 10th	Night	15% 2 10% 35% 35% 4	597
	11th to 20th	Night	12% 2 9% 34% 39% 5%	594
	21st to end	Night	11% 2 11% 31% 39% 6%	864
September	1st to 10th	Night	13% 2 11% 30% 36% 7%	800
	11th to 20th	Night	17% 2 12% 29% 34% 5%	800
	21st to end	Night	27% 4 17% 29% 22% 2	800
October	1st to 10th	Night	20% 5% 18% 28% 25% 5%	773
	11th to 20th	Night	16% 8% 24% 32% 17% 2	760
	21st to end	Night	16% 7% 26% 37% 13% 2	836
November	1st to 10th	Night	16% 7% 28% 35% 12% 2	780
	11th to 20th	Night	15% 11% 32% 31% 10% 1	800
	21st to end	Night	16% 12% 31% 31% 10% 1	800
December	1st to 10th	Night	21% 13% 35% 25% 6%	800
	11th to 20th	Night	18% 10% 37% 29% 5%	800
	21st to end	Night	21% 12% 31% 30% 5%	880

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