Lake Somerville State Park - Birch Creek Unit
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Somerville State Park - Birch Creek Unit clear sky chart from 2005-11-06 to 2025-12-18.

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% 6% 2 6% 4 4 4 4 6% 8% 44%	127096
	Night	12% 6% 2 5% 3 3 4 3 5% 7% 49%	124193
Transparency	Day	37% 5% 11% 21% 18% 8%	130800
	Night	34% 5% 11% 21% 19% 11%	128539
Seeing	Day	12% 7% 20% 27% 25% 10%	16338
	Night	13% 7% 25% 35% 17% 2	26239

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% 11% 3 6% 4 3 3 3 4 5% 43%	4077
	11th to 20th	Day	14% 6% 2 5% 3 3 4 2 5% 7% 50%	4080
	21st to end	Day	28% 10% 3 6% 3 2 2 2 3 5% 37%	4488
February	1st to 10th	Day	20% 9% 3 6% 4 3 3 2 4 5% 41%	1995
	11th to 20th	Day	20% 9% 2 7% 3 4 3 2 3 5% 41%	1949
	21st to end	Day	17% 8% 2 7% 3 5% 3 4 5% 5% 42%	1730
March	1st to 10th	Day	13% 7% 3 7% 4 4 4 3 5% 7% 43%	4936
	11th to 20th	Day	13% 7% 2 5% 4 4 4 2 4 6% 49%	4980
	21st to end	Day	17% 8% 3 7% 4 4 4 3 4 6% 41%	5462
April	1st to 10th	Day	13% 8% 3 7% 5% 4 4 4 5% 8% 39%	2200
	11th to 20th	Day	14% 7% 3 8% 5% 4 4 4 5% 5% 41%	2248
	21st to end	Day	12% 7% 3 6% 4 5% 4 4 5% 7% 44%	2415
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 7% 8% 33%	5862
	11th to 20th	Day	10% 5% 2 6% 5% 4 5% 4 6% 8% 45%	5862
	21st to end	Day	10% 6% 2 7% 6% 5% 6% 5% 8% 10% 35%	6434
June	1st to 10th	Day	4 4 2 5% 5% 6% 6% 6% 8% 10% 46%	2600
	11th to 20th	Day	6% 4 2 6% 4 5% 6% 5% 7% 10% 46%	2600
	21st to end	Day	5% 6% 2 6% 6% 6% 8% 5% 9% 10% 36%	2600
July	1st to 10th	Day	8% 6% 2 6% 6% 5% 6% 5% 8% 12% 37%	5900
	11th to 20th	Day	5% 5% 2 6% 5% 5% 6% 5% 8% 13% 40%	5900
	21st to end	Day	7% 6% 2 6% 5% 5% 5% 5% 7% 12% 39%	6422
August	1st to 10th	Day	2 1 1 3 2 3 3 3 5% 9% 69%	4234
	11th to 20th	Day	3 2 1 4 3 3 4 5% 8% 13% 53%	4171
	21st to end	Day	4 4 2 6% 5% 7% 5% 5% 9% 10% 44%	3969
September	1st to 10th	Day	5% 6% 2 6% 6% 6% 5% 5% 6% 10% 43%	2200
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 5% 7% 43%	2200
	21st to end	Day	6% 3 2 5% 5% 5% 5% 4 6% 7% 50%	2200
October	1st to 10th	Day	5% 3 2 4 3 4 3 4 5% 7% 60%	2157
	11th to 20th	Day	12% 5% 2 4 3 2 4 3 4 5% 56%	2000
	21st to end	Day	12% 5% 1 4 3 3 3 2 4 6% 56%	2200
November	1st to 10th	Day	12% 6% 2 6% 3 4 4 4 5% 6% 49%	2058
	11th to 20th	Day	10% 7% 2 5% 4 3 4 3 5% 6% 53%	2100
	21st to end	Day	17% 7% 3 6% 4 4 4 4 4 5% 43%	1895
December	1st to 10th	Day	23% 8% 2 6% 4 2 3 3 4 5% 39%	3030
	11th to 20th	Day	18% 7% 2 7% 4 4 4 3 5% 6% 42%	3454
	21st to end	Day	14% 7% 2 6% 4 4 3 2 4 5% 49%	4488
January	1st to 10th	Night	18% 7% 2 4 3 3 3 2 4 5% 49%	5888
	11th to 20th	Night	13% 6% 1 3 2 2 3 2 3 5% 58%	5900
	21st to end	Night	27% 8% 2 5% 3 2 3 2 3 5% 39%	6490
February	1st to 10th	Night	17% 8% 3 7% 3 3 3 3 4 5% 44%	2421
	11th to 20th	Night	20% 8% 2 5% 3 2 2 3 3 4 46%	2338
	21st to end	Night	19% 7% 3 5% 4 3 3 3 4 5% 43%	1898
March	1st to 10th	Night	15% 8% 3 7% 3 4 4 3 5% 8% 41%	4919
	11th to 20th	Night	16% 7% 2 5% 4 3 3 3 4 5% 48%	4980
	21st to end	Night	17% 10% 2 6% 4 3 3 3 3 5% 43%	5467
April	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 6% 7% 46%	2200
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 6% 7% 45%	2100
	21st to end	Night	13% 6% 2 6% 4 4 3 3 6% 6% 47%	1979
May	1st to 10th	Night	18% 8% 3 8% 5% 4 4 3 6% 8% 32%	4053
	11th to 20th	Night	9% 5% 2 7% 4 4 5% 4 7% 8% 46%	4053
	21st to end	Night	9% 7% 2 6% 6% 5% 5% 5% 7% 10% 38%	4443
June	1st to 10th	Night	4 3 2 5% 3 3 4 4 7% 10% 58%	1800
	11th to 20th	Night	4 3 1 3 2 3 3 5% 7% 9% 60%	1800
	21st to end	Night	3 4 2 5% 5% 4 4 5% 8% 11% 50%	1800
July	1st to 10th	Night	6% 5% 2 6% 5% 4 5% 5% 8% 13% 41%	4078
	11th to 20th	Night	3 3 1 5% 4 4 5% 4 6% 12% 53%	4077
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 7% 10% 46%	4449
August	1st to 10th	Night	1 1 1 1 2 2 2 2 4 9% 74%	2926
	11th to 20th	Night	2 2 1 3 3 3 4 4 6% 14% 58%	2888
	21st to end	Night	3 3 1 4 3 4 5% 5% 8% 12% 52%	3907
September	1st to 10th	Night	6% 4 2 5% 3 4 5% 4 6% 9% 53%	2200
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 48%	2200
	21st to end	Night	7% 4 2 4 3 3 4 4 6% 8% 55%	2200
October	1st to 10th	Night	4 3 1 4 4 3 3 3 4 7% 65%	2220
	11th to 20th	Night	10% 4 1 4 3 2 3 3 4 6% 61%	2400
	21st to end	Night	11% 5% 2 4 3 2 2 3 4 4 61%	2640
November	1st to 10th	Night	11% 5% 1 5% 3 3 4 3 4 5% 55%	2460
	11th to 20th	Night	10% 6% 2 4 4 4 4 3 4 6% 54%	2520
	21st to end	Night	15% 6% 3 6% 3 3 4 3 5% 7% 46%	2721
December	1st to 10th	Night	22% 9% 3 6% 4 3 3 3 4 5% 38%	4380
	11th to 20th	Night	15% 9% 3 6% 4 3 3 3 4 5% 46%	4908
	21st to end	Night	14% 6% 2 5% 3 3 2 2 3 4 56%	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	44% 3 5% 23% 17% 8%	4078
	11th to 20th	Day	36% 3 4 20% 25% 12%	4080
	21st to end	Day	52% 1 3 10% 19% 15%	4488
February	1st to 10th	Day	47% 4 5% 12% 19% 13%	1995
	11th to 20th	Day	47% 3 7% 16% 18% 9%	1959
	21st to end	Day	44% 4 7% 16% 16% 13%	1730
March	1st to 10th	Day	42% 4 6% 17% 17% 13%	4936
	11th to 20th	Day	39% 3 6% 10% 17% 24%	4980
	21st to end	Day	45% 4 7% 19% 18% 6%	5464
April	1st to 10th	Day	45% 6% 6% 18% 14% 12%	2200
	11th to 20th	Day	45% 4 8% 14% 19% 10%	2248
	21st to end	Day	40% 4 8% 20% 21% 7%	2415
May	1st to 10th	Day	48% 5% 10% 21% 12% 4	5900
	11th to 20th	Day	37% 5% 13% 19% 22% 4	5866
	21st to end	Day	41% 7% 11% 27% 12% 2	6447
June	1st to 10th	Day	31% 7% 14% 27% 17% 3	2600
	11th to 20th	Day	30% 8% 15% 26% 18% 4	2600
	21st to end	Day	37% 10% 14% 25% 12% 2	2600
July	1st to 10th	Day	38% 8% 24% 26% 4	5836
	11th to 20th	Day	32% 7% 23% 27% 10%	5831
	21st to end	Day	36% 8% 18% 26% 11% 1	6240
August	1st to 10th	Day	14% 5% 13% 41% 26% 1	4220
	11th to 20th	Day	21% 7% 30% 33% 8% 2	4210
	21st to end	Day	33% 10% 18% 25% 13% 1	3968
September	1st to 10th	Day	36% 9% 11% 20% 16% 7%	2200
	11th to 20th	Day	39% 8% 13% 19% 18% 4	2200
	21st to end	Day	32% 6% 9% 23% 18% 14%	2200
October	1st to 10th	Day	16% 3 6% 14% 22% 39%	3508
	11th to 20th	Day	21% 3 7% 17% 35% 18%	3240
	21st to end	Day	26% 3 5% 21% 30% 15%	3538
November	1st to 10th	Day	35% 6% 8% 18% 22% 11%	2058
	11th to 20th	Day	32% 4 9% 18% 22% 14%	2100
	21st to end	Day	42% 4 8% 16% 18% 11%	1895
December	1st to 10th	Day	49% 3 7% 16% 18% 8%	3030
	11th to 20th	Day	43% 4 8% 12% 23% 11%	3452
	21st to end	Day	38% 4 4 15% 27% 12%	4488
January	1st to 10th	Night	39% 4 4 21% 21% 11%	5888
	11th to 20th	Night	31% 3 3 20% 29% 14%	5900
	21st to end	Night	51% 2 3 12% 18% 14%	6490
February	1st to 10th	Night	45% 3 5% 11% 20% 16%	2421
	11th to 20th	Night	44% 4 5% 16% 18% 13%	2338
	21st to end	Night	45% 4 6% 14% 17% 14%	1898
March	1st to 10th	Night	43% 5% 7% 15% 19% 12%	4919
	11th to 20th	Night	39% 4 6% 14% 14% 22%	4980
	21st to end	Night	45% 4 7% 16% 19% 9%	5467
April	1st to 10th	Night	36% 5% 10% 19% 17% 13%	2200
	11th to 20th	Night	39% 4 10% 16% 16% 14%	2100
	21st to end	Night	38% 5% 10% 22% 19% 7%	1979
May	1st to 10th	Night	50% 5% 8% 22% 10% 4	4080
	11th to 20th	Night	36% 5% 12% 23% 18% 6%	4054
	21st to end	Night	40% 6% 13% 30% 10% 1	4452
June	1st to 10th	Night	23% 6% 17% 31% 21% 2	1800
	11th to 20th	Night	20% 8% 16% 33% 18% 5%	1800
	21st to end	Night	27% 9% 19% 29% 15% 2	1800
July	1st to 10th	Night	34% 9% 28% 26% 4	4035
	11th to 20th	Night	24% 7% 23% 36% 10%	4026
	21st to end	Night	33% 8% 20% 26% 11% 2	4320
August	1st to 10th	Night	9% 4 16% 42% 27% 1	2916
	11th to 20th	Night	17% 5% 32% 34% 10% 1	2915
	21st to end	Night	23% 11% 23% 29% 12% 1	3907
September	1st to 10th	Night	28% 8% 17% 23% 15% 8%	2200
	11th to 20th	Night	35% 6% 15% 24% 14% 6%	2200
	21st to end	Night	26% 7% 15% 21% 18% 14%	2200
October	1st to 10th	Night	13% 3 8% 16% 19% 40%	3623
	11th to 20th	Night	16% 2 6% 20% 34% 21%	3899
	21st to end	Night	21% 3 5% 24% 31% 18%	4252
November	1st to 10th	Night	32% 5% 9% 19% 23% 13%	2460
	11th to 20th	Night	32% 4 9% 19% 21% 15%	2520
	21st to end	Night	38% 4 9% 18% 19% 13%	2721
December	1st to 10th	Night	50% 3 5% 16% 19% 8%	4380
	11th to 20th	Night	41% 4 9% 9% 22% 15%	4909
	21st to end	Night	34% 3 4 16% 30% 12%	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	23% 9% 37% 23% 6% 2	300
	11th to 20th	Day	20% 16% 28% 27% 6% 2	300
	21st to end	Day	22% 16% 34% 21% 7% 1	330
February	1st to 10th	Day	29% 12% 28% 23% 8%	300
	11th to 20th	Day	29% 16% 29% 20% 6%	300
	21st to end	Day	27% 17% 35% 16% 4	249
March	1st to 10th	Day	18% 19% 34% 22% 7%	300
	11th to 20th	Day	16% 18% 43% 17% 5% 1	300
	21st to end	Day	18% 13% 38% 23% 7% 1	330
April	1st to 10th	Day	17% 17% 40% 20% 6% 1	300
	11th to 20th	Day	18% 17% 32% 24% 8% 1	366
	21st to end	Day	15% 13% 34% 27% 10% 1	515
May	1st to 10th	Day	12% 11% 36% 30% 10% 1	703
	11th to 20th	Day	14% 7% 27% 33% 16% 3	725
	21st to end	Day	10% 6% 26% 35% 19% 3	789
June	1st to 10th	Day	5% 2 14% 40% 31% 8%	730
	11th to 20th	Day	6% 2 14% 40% 32% 7%	730
	21st to end	Day	5% 2 11% 31% 37% 15%	730
July	1st to 10th	Day	7% 2 8% 28% 38% 17%	721
	11th to 20th	Day	4 1 3 29% 45% 18%	718
	21st to end	Day	5% 2 4 25% 46% 18%	794
August	1st to 10th	Day	3 3 22% 51% 22%	726
	11th to 20th	Day	5% 3 23% 49% 21%	715
	21st to end	Day	7% 1 3 18% 33% 37%	383
September	1st to 10th	Day	6% 2 6% 24% 36% 27%	330
	11th to 20th	Day	7% 2 5% 18% 39% 29%	330
	21st to end	Day	6% 1 7% 26% 39% 20%	330
October	1st to 10th	Day	2 2 11% 31% 38% 16%	330
	11th to 20th	Day	10% 4 17% 33% 25% 11%	330
	21st to end	Day	18% 9% 22% 22% 22% 7%	363
November	1st to 10th	Day	15% 7% 19% 31% 20% 7%	330
	11th to 20th	Day	16% 8% 23% 31% 18% 5%	330
	21st to end	Day	19% 15% 33% 20% 10% 3	330
December	1st to 10th	Day	22% 8% 30% 23% 12% 5%	330
	11th to 20th	Day	22% 14% 26% 25% 9% 3	321
	21st to end	Day	22% 7% 37% 24% 7% 2	330
January	1st to 10th	Night	20% 10% 32% 31% 7%	1000
	11th to 20th	Night	20% 12% 31% 30% 7%	1000
	21st to end	Night	20% 12% 31% 31% 5%	1100
February	1st to 10th	Night	23% 12% 34% 27% 4	821
	11th to 20th	Night	24% 12% 35% 25% 4	784
	21st to end	Night	24% 13% 41% 19% 3	578
March	1st to 10th	Night	18% 14% 40% 25% 3	600
	11th to 20th	Night	15% 14% 38% 29% 5%	600
	21st to end	Night	15% 10% 42% 29% 4	660
April	1st to 10th	Night	13% 11% 43% 30% 3	600
	11th to 20th	Night	14% 15% 37% 29% 5%	593
	21st to end	Night	13% 12% 39% 27% 8% 1	600
May	1st to 10th	Night	13% 11% 42% 30% 4	600
	11th to 20th	Night	9% 7% 35% 36% 12% 1	591
	21st to end	Night	5% 6% 33% 41% 15% 1	650
June	1st to 10th	Night	4 2 17% 50% 25% 2	600
	11th to 20th	Night	4 1 23% 51% 21% 1	600
	21st to end	Night	4 1 10% 48% 34% 3	600
July	1st to 10th	Night	6% 2 9% 45% 36% 3	585
	11th to 20th	Night	3 1 7% 50% 35% 3	582
	21st to end	Night	5% 6% 51% 35% 3	657
August	1st to 10th	Night	1 4 54% 37% 3	597
	11th to 20th	Night	4 4 51% 35% 5%	594
	21st to end	Night	4 1 5% 36% 47% 6%	660
September	1st to 10th	Night	6% 1 9% 35% 44% 6%	600
	11th to 20th	Night	9% 1 7% 36% 43% 4	600
	21st to end	Night	8% 1 13% 35% 40% 4	600
October	1st to 10th	Night	4 1 11% 44% 36% 5%	620
	11th to 20th	Night	10% 3 21% 41% 22% 2	800
	21st to end	Night	12% 8% 27% 33% 18% 2	880
November	1st to 10th	Night	12% 5% 22% 41% 19% 1	820
	11th to 20th	Night	11% 9% 25% 39% 15% 1	840
	21st to end	Night	16% 11% 30% 29% 14% 1	1041
December	1st to 10th	Night	19% 9% 28% 32% 11% 1	1050
	11th to 20th	Night	22% 11% 27% 31% 8% 1	1036
	21st to end	Night	19% 9% 31% 32% 9%	1100

Page generated at 2025-12-15 00:13:52. Version ac07

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