Sheldon Lake State Park & Environmental Learning Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sheldon Lake State Park & Environmental Learning Center 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	13% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	126960
	Night	15% 7% 2 5% 4 4 4 3 5% 7% 45%	124318
Transparency	Day	40% 5% 12% 20% 16% 7%	130609
	Night	38% 4 12% 20% 17% 8%	128732
Seeing	Day	14% 5% 18% 27% 24% 13%	16302
	Night	16% 4 19% 33% 25% 3	26160

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	21% 11% 2 6% 3 3 3 3 3 4 40%	4077
	11th to 20th	Day	16% 6% 2 5% 3 3 3 2 4 6% 51%	4080
	21st to end	Day	29% 8% 3 7% 3 3 3 2 4 6% 33%	4488
February	1st to 10th	Day	22% 9% 3 7% 4 4 3 4 5% 6% 35%	1874
	11th to 20th	Day	22% 10% 3 6% 4 3 4 2 4 6% 36%	1989
	21st to end	Day	20% 9% 3 7% 4 3 3 3 5% 5% 39%	1815
March	1st to 10th	Day	17% 8% 3 8% 5% 4 4 4 5% 7% 35%	4936
	11th to 20th	Day	14% 6% 2 5% 5% 4 5% 3 5% 5% 45%	4980
	21st to end	Day	19% 7% 2 7% 5% 4 4 3 5% 5% 38%	5462
April	1st to 10th	Day	15% 8% 3 9% 4 5% 5% 4 5% 5% 37%	2200
	11th to 20th	Day	17% 7% 3 7% 5% 4 4 3 5% 6% 39%	2345
	21st to end	Day	10% 7% 3 7% 5% 4 5% 4 6% 7% 41%	2400
May	1st to 10th	Day	18% 8% 3 8% 5% 6% 5% 4 6% 7% 30%	5443
	11th to 20th	Day	11% 5% 2 7% 5% 5% 4 4 5% 7% 44%	5862
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 11% 35%	6434
June	1st to 10th	Day	6% 5% 2 6% 6% 6% 7% 6% 8% 9% 40%	2600
	11th to 20th	Day	5% 4 2 7% 6% 7% 7% 7% 8% 10% 38%	2600
	21st to end	Day	7% 7% 2 7% 7% 8% 7% 7% 9% 9% 29%	2600
July	1st to 10th	Day	7% 5% 2 8% 6% 7% 7% 6% 9% 12% 29%	5900
	11th to 20th	Day	6% 6% 2 9% 7% 7% 8% 6% 9% 12% 28%	5900
	21st to end	Day	11% 7% 2 7% 6% 5% 6% 5% 7% 10% 34%	6422
August	1st to 10th	Day	2 2 1 3 4 5% 5% 5% 7% 10% 56%	4234
	11th to 20th	Day	2 3 1 4 4 5% 7% 7% 9% 13% 45%	3974
	21st to end	Day	9% 5% 2 7% 6% 7% 7% 6% 8% 9% 35%	4288
September	1st to 10th	Day	7% 5% 3 8% 6% 6% 7% 5% 7% 8% 38%	2200
	11th to 20th	Day	14% 6% 2 7% 5% 5% 6% 4 6% 7% 37%	2200
	21st to end	Day	8% 6% 2 6% 4 5% 4 4 6% 7% 48%	2200
October	1st to 10th	Day	6% 4 1 4 4 4 4 3 5% 7% 57%	2063
	11th to 20th	Day	9% 4 2 6% 4 4 4 4 5% 7% 51%	2000
	21st to end	Day	13% 6% 2 4 3 3 3 3 4 5% 54%	2200
November	1st to 10th	Day	10% 5% 2 6% 4 4 6% 4 5% 6% 47%	2058
	11th to 20th	Day	10% 7% 2 6% 4 4 4 4 5% 6% 47%	2100
	21st to end	Day	16% 7% 2 8% 5% 4 4 3 4 7% 41%	2064
December	1st to 10th	Day	25% 8% 3 5% 4 3 3 2 4 7% 36%	3030
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 5% 7% 42%	3454
	21st to end	Day	16% 7% 2 5% 4 4 3 3 4 6% 46%	4488
January	1st to 10th	Night	20% 7% 2 6% 3 4 3 3 4 5% 44%	5888
	11th to 20th	Night	16% 8% 2 5% 3 3 3 2 4 5% 49%	5900
	21st to end	Night	30% 8% 2 5% 3 3 3 2 4 6% 33%	6490
February	1st to 10th	Night	21% 9% 2 7% 4 3 3 3 5% 6% 38%	2542
	11th to 20th	Night	22% 9% 2 6% 3 3 3 2 3 5% 42%	2295
	21st to end	Night	23% 9% 2 6% 4 3 3 3 4 5% 39%	1815
March	1st to 10th	Night	24% 10% 3 6% 3 3 3 3 5% 5% 37%	4919
	11th to 20th	Night	19% 8% 2 6% 3 3 3 2 4 5% 44%	4980
	21st to end	Night	20% 9% 3 5% 4 4 3 3 3 5% 41%	5467
April	1st to 10th	Night	16% 7% 2 7% 4 4 4 4 6% 6% 39%	2200
	11th to 20th	Night	19% 6% 2 5% 4 3 3 2 5% 7% 44%	2001
	21st to end	Night	15% 5% 2 6% 4 4 5% 3 4 7% 44%	2000
May	1st to 10th	Night	27% 9% 2 7% 4 4 4 4 5% 7% 28%	4448
	11th to 20th	Night	15% 7% 2 5% 4 4 4 4 6% 7% 41%	4053
	21st to end	Night	16% 7% 3 7% 5% 5% 6% 4 7% 10% 29%	4443
June	1st to 10th	Night	5% 3 2 3 3 3 4 3 6% 12% 57%	1800
	11th to 20th	Night	5% 3 2 3 3 4 4 3 7% 12% 55%	1800
	21st to end	Night	6% 4 2 6% 4 6% 5% 4 9% 11% 45%	1800
July	1st to 10th	Night	5% 5% 2 5% 4 5% 6% 5% 8% 13% 43%	4078
	11th to 20th	Night	3 4 2 6% 5% 5% 5% 4 6% 12% 49%	4077
	21st to end	Night	7% 5% 2 7% 4 4 4 5% 7% 9% 46%	4449
August	1st to 10th	Night	1 1 1 3 2 3 3 2 5% 10% 69%	2926
	11th to 20th	Night	2 2 1 2 3 2 4 3 7% 13% 61%	3078
	21st to end	Night	5% 4 1 5% 3 5% 5% 5% 8% 12% 47%	3604
September	1st to 10th	Night	5% 5% 3 6% 4 4 5% 4 7% 10% 46%	2200
	11th to 20th	Night	11% 7% 2 5% 3 5% 4 3 6% 8% 46%	2200
	21st to end	Night	9% 4 2 5% 3 3 3 4 5% 7% 55%	2200
October	1st to 10th	Night	5% 3 1 4 4 3 4 4 5% 7% 61%	2315
	11th to 20th	Night	9% 3 1 4 3 3 3 3 5% 6% 61%	2400
	21st to end	Night	11% 5% 2 5% 3 2 2 3 3 4 60%	2640
November	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 4 5% 54%	2460
	11th to 20th	Night	11% 5% 2 4 4 3 3 3 5% 6% 53%	2520
	21st to end	Night	16% 7% 2 5% 3 4 4 3 4 6% 45%	2552
December	1st to 10th	Night	22% 10% 4 8% 4 3 3 2 4 6% 35%	4380
	11th to 20th	Night	16% 9% 2 7% 4 3 4 2 4 5% 45%	4908
	21st to end	Night	16% 7% 2 6% 4 3 3 2 4 5% 47%	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	50% 2 3 20% 18% 7%	4078
	11th to 20th	Day	37% 2 4 20% 23% 13%	4080
	21st to end	Day	54% 2 3 13% 16% 13%	4488
February	1st to 10th	Day	52% 2 5% 13% 16% 11%	1874
	11th to 20th	Day	52% 2 6% 14% 17% 9%	1999
	21st to end	Day	50% 3 8% 16% 14% 10%	1815
March	1st to 10th	Day	48% 3 10% 14% 15% 11%	4936
	11th to 20th	Day	41% 3 7% 10% 21% 18%	4980
	21st to end	Day	47% 3 8% 19% 19% 4	5464
April	1st to 10th	Day	47% 4 8% 17% 15% 9%	2200
	11th to 20th	Day	46% 3 9% 17% 16% 9%	2345
	21st to end	Day	40% 4 11% 20% 20% 6%	2400
May	1st to 10th	Day	51% 5% 11% 19% 11% 3	5478
	11th to 20th	Day	39% 3 12% 23% 20% 3	5866
	21st to end	Day	40% 5% 15% 30% 9% 1	6447
June	1st to 10th	Day	35% 7% 16% 26% 16% 1	2600
	11th to 20th	Day	35% 7% 15% 27% 14% 2	2600
	21st to end	Day	45% 7% 19% 21% 8% 1	2600
July	1st to 10th	Day	41% 9% 30% 19% 2	5836
	11th to 20th	Day	43% 10% 25% 18% 4	5831
	21st to end	Day	42% 8% 19% 25% 6%	6240
August	1st to 10th	Day	20% 5% 17% 40% 17%	4220
	11th to 20th	Day	25% 9% 31% 30% 4 1	4013
	21st to end	Day	41% 10% 16% 19% 13% 1	4287
September	1st to 10th	Day	40% 8% 15% 17% 15% 4	2200
	11th to 20th	Day	45% 6% 12% 19% 16% 3	2200
	21st to end	Day	34% 4 13% 22% 17% 11%	2200
October	1st to 10th	Day	20% 3 6% 15% 17% 39%	3362
	11th to 20th	Day	22% 4 5% 17% 37% 15%	3240
	21st to end	Day	28% 3 6% 17% 35% 12%	3538
November	1st to 10th	Day	36% 3 9% 20% 22% 10%	2058
	11th to 20th	Day	37% 3 10% 15% 22% 13%	2100
	21st to end	Day	44% 4 7% 17% 18% 11%	2064
December	1st to 10th	Day	50% 2 6% 16% 20% 5%	3030
	11th to 20th	Day	41% 4 9% 15% 21% 9%	3452
	21st to end	Day	41% 3 6% 17% 23% 11%	4488
January	1st to 10th	Night	43% 3 6% 21% 18% 9%	5888
	11th to 20th	Night	40% 2 4 18% 25% 11%	5900
	21st to end	Night	54% 2 3 14% 16% 11%	6490
February	1st to 10th	Night	49% 2 7% 12% 17% 14%	2542
	11th to 20th	Night	47% 2 6% 14% 19% 11%	2295
	21st to end	Night	49% 3 5% 14% 17% 12%	1815
March	1st to 10th	Night	51% 3 7% 14% 17% 10%	4919
	11th to 20th	Night	45% 2 8% 13% 17% 16%	4980
	21st to end	Night	47% 2 8% 15% 20% 8%	5467
April	1st to 10th	Night	45% 4 9% 17% 16% 9%	2200
	11th to 20th	Night	41% 3 10% 18% 16% 13%	2001
	21st to end	Night	41% 4 10% 21% 18% 6%	2000
May	1st to 10th	Night	55% 5% 10% 18% 9% 3	4478
	11th to 20th	Night	42% 4 11% 21% 18% 3	4054
	21st to end	Night	47% 6% 19% 22% 6% 1	4452
June	1st to 10th	Night	22% 5% 22% 33% 18%	1800
	11th to 20th	Night	24% 4 20% 31% 18% 3	1800
	21st to end	Night	31% 7% 24% 28% 10% 1	1800
July	1st to 10th	Night	31% 7% 36% 24% 2	4035
	11th to 20th	Night	29% 8% 31% 28% 4	4026
	21st to end	Night	34% 9% 21% 30% 6%	4320
August	1st to 10th	Night	13% 4 20% 48% 15%	2916
	11th to 20th	Night	15% 6% 38% 36% 5% 1	3105
	21st to end	Night	29% 12% 22% 27% 9% 1	3604
September	1st to 10th	Night	32% 8% 22% 21% 13% 4	2200
	11th to 20th	Night	36% 6% 15% 23% 16% 4	2200
	21st to end	Night	29% 4 14% 24% 18% 11%	2200
October	1st to 10th	Night	18% 3 8% 18% 17% 36%	3783
	11th to 20th	Night	16% 3 5% 24% 33% 19%	3899
	21st to end	Night	22% 2 5% 16% 38% 16%	4252
November	1st to 10th	Night	32% 3 11% 22% 22% 10%	2460
	11th to 20th	Night	32% 3 9% 21% 22% 13%	2520
	21st to end	Night	41% 2 8% 20% 18% 10%	2552
December	1st to 10th	Night	52% 2 6% 16% 17% 6%	4380
	11th to 20th	Night	43% 4 9% 11% 20% 13%	4909
	21st to end	Night	40% 3 6% 14% 28% 8%	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	29% 6% 35% 21% 8% 1	300
	11th to 20th	Day	23% 11% 30% 28% 6% 2	300
	21st to end	Day	24% 11% 30% 26% 9%	330
February	1st to 10th	Day	24% 9% 33% 25% 8%	300
	11th to 20th	Day	28% 15% 29% 19% 7% 1	300
	21st to end	Day	26% 10% 42% 18% 4	249
March	1st to 10th	Day	29% 13% 31% 21% 7%	300
	11th to 20th	Day	18% 14% 44% 17% 6% 1	300
	21st to end	Day	22% 13% 32% 23% 9% 1	330
April	1st to 10th	Day	20% 14% 34% 23% 9% 1	300
	11th to 20th	Day	24% 10% 29% 22% 14% 1	463
	21st to end	Day	16% 6% 29% 30% 16% 2	500
May	1st to 10th	Day	17% 6% 30% 30% 16% 1	515
	11th to 20th	Day	13% 4 28% 33% 19% 3	725
	21st to end	Day	12% 3 24% 37% 20% 4	789
June	1st to 10th	Day	8% 1 12% 34% 31% 14%	730
	11th to 20th	Day	7% 2 10% 37% 30% 14%	730
	21st to end	Day	8% 8% 29% 33% 21%	730
July	1st to 10th	Day	9% 2 4 25% 37% 24%	721
	11th to 20th	Day	6% 2 4 20% 40% 29%	718
	21st to end	Day	6% 1 3 23% 38% 28%	794
August	1st to 10th	Day	2 1 2 21% 43% 31%	726
	11th to 20th	Day	5% 1 1 13% 41% 39%	596
	21st to end	Day	10% 1 5% 16% 34% 34%	572
September	1st to 10th	Day	9% 2 5% 23% 33% 27%	330
	11th to 20th	Day	12% 2 4 26% 34% 22%	330
	21st to end	Day	11% 1 4 28% 35% 20%	330
October	1st to 10th	Day	6% 9% 39% 33% 12%	330
	11th to 20th	Day	7% 3 15% 39% 25% 12%	330
	21st to end	Day	17% 6% 23% 31% 17% 6%	363
November	1st to 10th	Day	15% 5% 20% 32% 22% 6%	330
	11th to 20th	Day	15% 3 21% 39% 18% 3	330
	21st to end	Day	18% 12% 30% 20% 16% 3	330
December	1st to 10th	Day	22% 5% 32% 24% 13% 5%	330
	11th to 20th	Day	17% 9% 31% 29% 12% 2	321
	21st to end	Day	20% 7% 35% 26% 10% 2	330
January	1st to 10th	Night	23% 6% 30% 31% 10%	1000
	11th to 20th	Night	24% 7% 26% 35% 8% 1	1000
	21st to end	Night	24% 6% 27% 36% 7%	1100
February	1st to 10th	Night	27% 7% 27% 34% 6%	942
	11th to 20th	Night	27% 7% 28% 30% 7%	741
	21st to end	Night	26% 8% 33% 27% 6%	495
March	1st to 10th	Night	25% 6% 30% 33% 7%	600
	11th to 20th	Night	21% 7% 28% 35% 9%	600
	21st to end	Night	19% 5% 30% 35% 10% 1	660
April	1st to 10th	Night	21% 5% 31% 35% 9%	600
	11th to 20th	Night	23% 6% 25% 37% 9% 1	593
	21st to end	Night	19% 5% 26% 37% 13% 1	600
May	1st to 10th	Night	20% 4 25% 41% 10%	600
	11th to 20th	Night	13% 2 22% 42% 18% 2	591
	21st to end	Night	10% 2 19% 40% 26% 2	650
June	1st to 10th	Night	6% 1 7% 45% 38% 4	600
	11th to 20th	Night	6% 1 9% 41% 41% 3	600
	21st to end	Night	7% 1 7% 28% 49% 9%	600
July	1st to 10th	Night	6% 1 2 26% 54% 11%	585
	11th to 20th	Night	5% 1 23% 59% 11%	582
	21st to end	Night	4 1 2 25% 58% 10%	657
August	1st to 10th	Night	3 2 20% 65% 10%	597
	11th to 20th	Night	6% 1 18% 63% 12%	594
	21st to end	Night	7% 1 1 21% 56% 14%	660
September	1st to 10th	Night	8% 1 5% 23% 52% 12%	600
	11th to 20th	Night	13% 2 25% 50% 9%	600
	21st to end	Night	10% 1 5% 28% 50% 7%	600
October	1st to 10th	Night	6% 1 8% 36% 42% 6%	715
	11th to 20th	Night	9% 1 16% 41% 29% 3	800
	21st to end	Night	13% 4 19% 37% 24% 3	880
November	1st to 10th	Night	12% 3 16% 40% 27% 3	820
	11th to 20th	Night	13% 4 22% 41% 19% 1	840
	21st to end	Night	18% 6% 26% 35% 14% 1	872
December	1st to 10th	Night	23% 5% 24% 32% 16% 1	1050
	11th to 20th	Night	23% 5% 24% 37% 11%	1036
	21st to end	Night	22% 5% 27% 36% 9%	1100

Page generated at 2025-12-15 00:13:56. Version ac07

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