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 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	14% 5% 2 5% 5% 3 5% 5% 6% 8% 43%	74462
	Night	15% 5% 2 5% 4 3 4 5% 5% 7% 45%	75243
Transparency	Day	38% 8% 12% 22% 16% 4	74183
	Night	37% 7% 11% 22% 18% 4	75040
Seeing	Day	17% 10% 24% 24% 18% 8%	14010
	Night	17% 8% 26% 30% 17% 2	24383

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	16% 6% 2 4 4 2 4 4 5% 7% 46%	1706
	11th to 20th	Day	17% 7% 2 4 5% 3 4 4 5% 6% 43%	1629
	21st to end	Day	16% 6% 3 4 4 3 4 4 5% 5% 46%	1782
February	1st to 10th	Day	14% 6% 3 5% 5% 3 6% 5% 7% 7% 39%	1620
	11th to 20th	Day	19% 5% 2 5% 4 2 4 5% 6% 7% 42%	1587
	21st to end	Day	20% 6% 3 5% 5% 3 4 4 5% 7% 39%	1577
March	1st to 10th	Day	14% 4 2 4 3 2 3 5% 5% 7% 50%	1980
	11th to 20th	Day	18% 5% 2 5% 5% 4 4 5% 5% 7% 42%	1980
	21st to end	Day	18% 6% 3 5% 5% 3 4 5% 4 6% 40%	2178
April	1st to 10th	Day	11% 4 2 3 4 3 4 5% 6% 7% 51%	2065
	11th to 20th	Day	14% 5% 2 4 4 2 4 4 4 7% 49%	2134
	21st to end	Day	16% 5% 2 5% 5% 3 5% 4 4 7% 43%	2238
May	1st to 10th	Day	14% 5% 2 5% 5% 3 5% 6% 6% 8% 40%	2340
	11th to 20th	Day	17% 5% 3 5% 5% 3 5% 5% 6% 8% 37%	2301
	21st to end	Day	13% 5% 2 4 6% 4 6% 5% 6% 9% 41%	2531
June	1st to 10th	Day	11% 4 2 4 5% 3 5% 5% 7% 8% 46%	2340
	11th to 20th	Day	10% 4 2 5% 6% 3 6% 7% 7% 11% 41%	2340
	21st to end	Day	9% 4 2 5% 6% 4 6% 6% 8% 9% 42%	2340
July	1st to 10th	Day	9% 4 2 5% 7% 5% 5% 6% 8% 10% 38%	2340
	11th to 20th	Day	5% 4 2 5% 6% 4 6% 7% 8% 12% 40%	2340
	21st to end	Day	8% 5% 3 7% 6% 5% 7% 7% 8% 9% 35%	2547
August	1st to 10th	Day	11% 6% 3 5% 6% 4 6% 7% 8% 9% 35%	2327
	11th to 20th	Day	13% 5% 2 6% 5% 4 7% 6% 6% 10% 35%	2299
	21st to end	Day	11% 5% 2 5% 5% 4 4 5% 7% 11% 40%	2376
September	1st to 10th	Day	13% 4 2 5% 4 2 4 5% 5% 7% 48%	2160
	11th to 20th	Day	13% 5% 2 6% 4 3 4 5% 5% 8% 45%	1985
	21st to end	Day	23% 5% 2 4 4 3 4 4 5% 7% 38%	1972
October	1st to 10th	Day	18% 5% 2 3 3 3 3 3 4 6% 52%	1800
	11th to 20th	Day	16% 4 2 3 3 3 3 3 4 7% 52%	1800
	21st to end	Day	14% 5% 2 4 4 2 3 3 4 6% 53%	1980
November	1st to 10th	Day	13% 4 1 3 3 2 4 4 4 6% 57%	1857
	11th to 20th	Day	14% 5% 2 4 3 3 4 4 4 6% 51%	1788
	21st to end	Day	14% 4 1 4 4 2 4 3 5% 9% 50%	1591
December	1st to 10th	Day	16% 6% 2 5% 5% 4 5% 6% 5% 7% 39%	1520
	11th to 20th	Day	15% 4 2 3 3 2 3 3 4 6% 54%	1988
	21st to end	Day	20% 5% 2 5% 4 3 5% 6% 6% 8% 37%	3124
January	1st to 10th	Night	16% 5% 2 4 3 2 4 4 4 6% 48%	2459
	11th to 20th	Night	15% 6% 3 6% 5% 3 4 4 4 6% 43%	2360
	21st to end	Night	16% 5% 2 4 4 3 4 5% 4 6% 47%	2574
February	1st to 10th	Night	16% 6% 2 5% 4 2 5% 4 5% 7% 44%	2340
	11th to 20th	Night	20% 5% 3 4 5% 2 4 4 4 5% 44%	2257
	21st to end	Night	16% 6% 2 5% 4 3 5% 4 5% 6% 43%	1678
March	1st to 10th	Night	13% 5% 2 3 3 3 4 5% 6% 6% 51%	1980
	11th to 20th	Night	18% 5% 2 4 5% 3 5% 4 6% 8% 40%	1980
	21st to end	Night	16% 6% 2 5% 5% 3 4 4 4 7% 44%	2178
April	1st to 10th	Night	15% 6% 3 5% 4 3 4 4 5% 6% 45%	1894
	11th to 20th	Night	14% 4 2 5% 4 2 3 4 6% 6% 50%	1775
	21st to end	Night	17% 4 2 4 5% 2 4 4 5% 9% 45%	1737
May	1st to 10th	Night	14% 5% 2 5% 5% 3 5% 5% 6% 7% 44%	1620
	11th to 20th	Night	16% 7% 3 6% 5% 3 4 5% 5% 7% 39%	1592
	21st to end	Night	15% 5% 2 5% 5% 3 4 4 5% 10% 43%	1744
June	1st to 10th	Night	10% 4 2 5% 4 3 6% 5% 7% 12% 42%	1620
	11th to 20th	Night	11% 4 2 5% 4 3 5% 5% 6% 9% 47%	1620
	21st to end	Night	8% 2 3 4 5% 4 5% 6% 8% 11% 45%	1620
July	1st to 10th	Night	9% 4 2 5% 5% 3 6% 5% 8% 10% 43%	1620
	11th to 20th	Night	6% 4 1 4 5% 3 7% 6% 6% 12% 46%	1620
	21st to end	Night	6% 5% 2 5% 6% 4 6% 6% 8% 11% 41%	1771
August	1st to 10th	Night	13% 5% 2 5% 5% 4 5% 6% 7% 10% 37%	1611
	11th to 20th	Night	11% 4 3 5% 6% 4 5% 6% 7% 9% 40%	1598
	21st to end	Night	10% 5% 3 6% 5% 4 6% 6% 6% 10% 39%	1965
September	1st to 10th	Night	11% 5% 3 6% 4 3 5% 5% 6% 7% 46%	1800
	11th to 20th	Night	15% 6% 2 5% 4 3 5% 4 6% 7% 42%	1970
	21st to end	Night	23% 6% 3 6% 4 3 4 3 5% 6% 38%	1988
October	1st to 10th	Night	19% 4 1 4 4 2 3 4 4 7% 48%	2160
	11th to 20th	Night	15% 4 2 4 4 3 4 4 4 6% 50%	2160
	21st to end	Night	15% 3 2 3 4 2 4 3 5% 6% 54%	2376
November	1st to 10th	Night	13% 6% 2 4 4 2 3 3 4 5% 53%	2221
	11th to 20th	Night	14% 4 2 3 3 2 4 5% 4 6% 53%	2389
	21st to end	Night	14% 3 2 4 4 2 4 5% 5% 7% 50%	2584
December	1st to 10th	Night	18% 6% 3 5% 4 3 4 4 5% 6% 43%	2660
	11th to 20th	Night	16% 6% 3 4 5% 3 4 4 4 6% 47%	3190
	21st to end	Night	19% 5% 2 5% 5% 3 5% 6% 6% 9% 35%	4532

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	38% 6% 11% 22% 18% 5%	1707
	11th to 20th	Day	43% 5% 8% 22% 18% 3	1629
	21st to end	Day	39% 5% 8% 24% 20% 4	1782
February	1st to 10th	Day	41% 7% 12% 21% 17% 3	1620
	11th to 20th	Day	41% 5% 10% 23% 17% 4	1596
	21st to end	Day	45% 6% 13% 14% 17% 6%	1577
March	1st to 10th	Day	34% 6% 9% 24% 22% 5%	1980
	11th to 20th	Day	42% 7% 10% 18% 17% 6%	1980
	21st to end	Day	45% 6% 9% 20% 16% 4	2178
April	1st to 10th	Day	31% 7% 12% 23% 19% 8%	2065
	11th to 20th	Day	35% 6% 12% 22% 19% 7%	2134
	21st to end	Day	41% 7% 12% 21% 15% 4	2238
May	1st to 10th	Day	40% 10% 16% 19% 13% 3	2340
	11th to 20th	Day	45% 9% 13% 22% 10% 2	2305
	21st to end	Day	39% 10% 15% 25% 11%	2531
June	1st to 10th	Day	34% 9% 16% 26% 14% 1	2340
	11th to 20th	Day	36% 11% 15% 20% 15% 2	2340
	21st to end	Day	35% 12% 15% 21% 14% 2	2340
July	1st to 10th	Day	39% 12% 16% 17% 14% 3	2275
	11th to 20th	Day	32% 13% 19% 20% 15% 1	2271
	21st to end	Day	40% 11% 15% 21% 12% 1	2405
August	1st to 10th	Day	41% 11% 15% 23% 8% 1	2314
	11th to 20th	Day	42% 13% 15% 20% 10% 1	2299
	21st to end	Day	38% 9% 18% 20% 12% 3	2376
September	1st to 10th	Day	35% 9% 13% 21% 19% 4	2160
	11th to 20th	Day	39% 6% 10% 22% 20% 2	1985
	21st to end	Day	46% 8% 9% 17% 18% 2	1972
October	1st to 10th	Day	34% 5% 9% 25% 22% 4	1800
	11th to 20th	Day	34% 4 10% 26% 21% 6%	1800
	21st to end	Day	34% 5% 8% 26% 21% 7%	1980
November	1st to 10th	Day	30% 6% 9% 22% 27% 6%	1857
	11th to 20th	Day	35% 5% 8% 22% 21% 8%	1788
	21st to end	Day	32% 5% 10% 24% 25% 4	1591
December	1st to 10th	Day	44% 8% 9% 18% 17% 4	1520
	11th to 20th	Day	33% 4 8% 25% 23% 7%	1984
	21st to end	Day	40% 8% 12% 25% 13% 2	3124
January	1st to 10th	Night	37% 5% 9% 21% 20% 7%	2459
	11th to 20th	Night	42% 6% 9% 20% 19% 4	2360
	21st to end	Night	38% 5% 9% 24% 20% 4	2574
February	1st to 10th	Night	39% 6% 10% 24% 18% 3	2340
	11th to 20th	Night	44% 4 9% 19% 18% 7%	2259
	21st to end	Night	41% 6% 10% 22% 15% 6%	1678
March	1st to 10th	Night	33% 6% 9% 25% 21% 6%	1980
	11th to 20th	Night	41% 6% 11% 19% 18% 3	1980
	21st to end	Night	41% 6% 10% 21% 18% 5%	2178
April	1st to 10th	Night	39% 6% 10% 19% 21% 5%	1894
	11th to 20th	Night	34% 6% 10% 22% 21% 7%	1775
	21st to end	Night	36% 6% 12% 25% 17% 4	1737
May	1st to 10th	Night	38% 9% 13% 23% 15% 3	1620
	11th to 20th	Night	44% 8% 12% 22% 12% 2	1593
	21st to end	Night	38% 9% 16% 26% 11%	1745
June	1st to 10th	Night	33% 11% 16% 26% 14%	1620
	11th to 20th	Night	33% 10% 18% 21% 16% 3	1620
	21st to end	Night	31% 12% 16% 21% 16% 4	1620
July	1st to 10th	Night	34% 13% 16% 19% 12% 5%	1575
	11th to 20th	Night	28% 12% 21% 23% 13% 2	1566
	21st to end	Night	32% 13% 18% 24% 12%	1672
August	1st to 10th	Night	39% 13% 16% 20% 12% 1	1600
	11th to 20th	Night	38% 10% 16% 22% 12% 1	1598
	21st to end	Night	37% 11% 14% 25% 10% 3	1965
September	1st to 10th	Night	36% 10% 12% 20% 16% 5%	1800
	11th to 20th	Night	40% 8% 10% 20% 20% 2	1970
	21st to end	Night	48% 6% 8% 19% 16% 3	1988
October	1st to 10th	Night	37% 6% 9% 24% 22% 3	2160
	11th to 20th	Night	35% 5% 8% 26% 21% 5%	2160
	21st to end	Night	31% 4 8% 24% 26% 7%	2376
November	1st to 10th	Night	34% 4 9% 20% 25% 7%	2221
	11th to 20th	Night	32% 5% 9% 21% 24% 8%	2389
	21st to end	Night	33% 4 9% 25% 22% 6%	2584
December	1st to 10th	Night	43% 5% 8% 24% 17% 3	2660
	11th to 20th	Night	38% 5% 8% 22% 21% 6%	3192
	21st to end	Night	41% 7% 12% 21% 18% 2	4532

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% 26% 29% 16% 5% 1	270
	11th to 20th	Day	25% 20% 40% 14% 2	243
	21st to end	Day	27% 20% 37% 14% 3	264
February	1st to 10th	Day	20% 24% 36% 15% 5%	240
	11th to 20th	Day	33% 25% 32% 9% 1	240
	21st to end	Day	27% 18% 35% 17% 4	295
March	1st to 10th	Day	23% 15% 33% 21% 7%	420
	11th to 20th	Day	21% 20% 32% 20% 6% 1	420
	21st to end	Day	24% 11% 35% 23% 5%	462
April	1st to 10th	Day	18% 18% 32% 21% 10% 2	420
	11th to 20th	Day	19% 15% 25% 23% 13% 5%	412
	21st to end	Day	20% 12% 33% 21% 11% 3	420
May	1st to 10th	Day	17% 14% 32% 24% 10% 4	423
	11th to 20th	Day	18% 6% 25% 27% 16% 8%	447
	21st to end	Day	18% 3 18% 27% 26% 9%	485
June	1st to 10th	Day	12% 4 22% 32% 21% 9%	450
	11th to 20th	Day	11% 5% 18% 28% 25% 13%	450
	21st to end	Day	9% 4 18% 33% 26% 10%	450
July	1st to 10th	Day	12% 2 12% 25% 32% 17%	445
	11th to 20th	Day	4 2 10% 26% 41% 16%	444
	21st to end	Day	9% 2 15% 30% 28% 16%	488
August	1st to 10th	Day	13% 3 21% 20% 30% 15%	446
	11th to 20th	Day	11% 2 13% 25% 28% 20%	439
	21st to end	Day	12% 2 8% 27% 30% 21%	495
September	1st to 10th	Day	15% 3 12% 23% 28% 20%	450
	11th to 20th	Day	16% 4 13% 22% 29% 16%	450
	21st to end	Day	28% 6% 17% 27% 17% 5%	450
October	1st to 10th	Day	22% 5% 16% 26% 24% 7%	450
	11th to 20th	Day	16% 12% 20% 25% 24% 4	450
	21st to end	Day	15% 11% 27% 30% 14% 3	495
November	1st to 10th	Day	18% 9% 26% 28% 14% 4	455
	11th to 20th	Day	18% 16% 29% 22% 13% 1	348
	21st to end	Day	15% 20% 27% 25% 10% 2	213
December	1st to 10th	Day	23% 18% 34% 18% 7% 1	180
	11th to 20th	Day	22% 13% 33% 23% 9% 1	204
	21st to end	Day	24% 15% 32% 18% 9% 2	297
January	1st to 10th	Night	21% 16% 34% 25% 3	760
	11th to 20th	Night	18% 14% 34% 31% 3	727
	21st to end	Night	19% 13% 42% 24% 3	792
February	1st to 10th	Night	19% 13% 39% 25% 3	720
	11th to 20th	Night	24% 16% 41% 19% 1	703
	21st to end	Night	19% 13% 41% 24% 3	592
March	1st to 10th	Night	17% 13% 37% 29% 4	720
	11th to 20th	Night	21% 13% 35% 23% 8% 1	720
	21st to end	Night	18% 11% 35% 31% 5%	792
April	1st to 10th	Night	18% 16% 33% 27% 6%	720
	11th to 20th	Night	16% 10% 34% 27% 13%	711
	21st to end	Night	18% 6% 32% 35% 10%	657
May	1st to 10th	Night	15% 9% 35% 27% 14%	540
	11th to 20th	Night	19% 5% 25% 32% 18% 2	531
	21st to end	Night	16% 15% 32% 32% 4	584
June	1st to 10th	Night	11% 2 15% 39% 30% 3	540
	11th to 20th	Night	12% 1 14% 32% 36% 4	540
	21st to end	Night	8% 2 14% 43% 30% 3	540
July	1st to 10th	Night	10% 1 10% 33% 38% 8%	525
	11th to 20th	Night	6% 1 6% 37% 44% 6%	522
	21st to end	Night	8% 1 10% 42% 35% 5%	591
August	1st to 10th	Night	13% 1 10% 35% 36% 4	537
	11th to 20th	Night	12% 2 8% 33% 40% 4	534
	21st to end	Night	11% 1 11% 30% 40% 7%	777
September	1st to 10th	Night	14% 2 10% 28% 38% 8%	720
	11th to 20th	Night	17% 2 12% 30% 35% 5%	720
	21st to end	Night	26% 4 17% 28% 23% 2	720
October	1st to 10th	Night	20% 5% 18% 27% 25% 5%	720
	11th to 20th	Night	17% 7% 25% 32% 17% 2	720
	21st to end	Night	16% 7% 27% 37% 12% 1	792
November	1st to 10th	Night	16% 7% 28% 35% 13% 2	740
	11th to 20th	Night	15% 11% 32% 31% 11% 1	760
	21st to end	Night	16% 11% 31% 32% 11% 1	760
December	1st to 10th	Night	22% 12% 35% 26% 6%	760
	11th to 20th	Night	17% 10% 38% 30% 5%	760
	21st to end	Night	21% 12% 33% 29% 4	836

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