Union Grove State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Union Grove State Park clear sky chart from 2005-11-06 to 2025-05-11.

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% 7% 3 7% 5% 4 4 3 5% 7% 43%	114006
	Night	10% 7% 3 7% 4 4 4 3 5% 7% 45%	115093
Transparency	Day	39% 6% 14% 25% 14% 2	117639
	Night	37% 6% 13% 25% 16% 2	119593
Seeing	Day	15% 10% 31% 29% 13% 3	16535
	Night	13% 9% 34% 34% 10% 1	26556

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% 13% 5% 16% 9% 8% 6% 4 5% 4 22%	3592
	11th to 20th	Day	6% 11% 4 9% 7% 5% 7% 3 7% 8% 32%	3974
	21st to end	Day	5% 10% 5% 13% 6% 6% 5% 4 5% 6% 36%	4488
February	1st to 10th	Day	10% 11% 4 10% 8% 8% 6% 4 4 6% 29%	1800
	11th to 20th	Day	9% 12% 4 8% 6% 4 5% 3 5% 7% 38%	1788
	21st to end	Day	12% 10% 3 8% 6% 5% 5% 3 6% 6% 36%	1650
March	1st to 10th	Day	12% 9% 3 7% 3 3 3 3 4 6% 48%	4763
	11th to 20th	Day	8% 8% 3 5% 4 3 4 3 5% 7% 50%	4980
	21st to end	Day	23% 11% 4 10% 5% 5% 5% 3 4 6% 25%	5462
April	1st to 10th	Day	15% 9% 3 7% 4 4 4 4 5% 6% 39%	2249
	11th to 20th	Day	17% 10% 3 6% 4 4 4 3 5% 6% 39%	2572
	21st to end	Day	22% 7% 3 6% 3 4 4 4 4 6% 38%	2600
May	1st to 10th	Day	16% 7% 2 6% 5% 5% 5% 4 6% 8% 36%	4212
	11th to 20th	Day	11% 6% 2 6% 4 4 3 3 4 6% 50%	4083
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 7% 41%	4730
June	1st to 10th	Day	11% 5% 2 5% 4 3 4 4 6% 8% 49%	2700
	11th to 20th	Day	13% 5% 2 5% 5% 4 4 4 6% 7% 45%	2850
	21st to end	Day	9% 4 2 5% 4 4 4 4 7% 8% 49%	2850
July	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 7% 10% 44%	4656
	11th to 20th	Day	7% 3 2 4 4 3 3 3 5% 7% 58%	4285
	21st to end	Day	4 3 1 5% 3 4 4 4 7% 10% 55%	4465
August	1st to 10th	Day	8% 6% 2 5% 4 4 5% 4 6% 8% 48%	4104
	11th to 20th	Day	17% 7% 3 6% 4 4 4 4 5% 7% 39%	4041
	21st to end	Day	9% 5% 1 5% 4 3 4 3 5% 7% 54%	4532
September	1st to 10th	Day	14% 5% 2 5% 3 4 3 3 4 5% 52%	2197
	11th to 20th	Day	9% 5% 2 5% 3 3 3 3 5% 6% 55%	2090
	21st to end	Day	11% 4 2 5% 3 3 3 4 5% 6% 56%	2090
October	1st to 10th	Day	15% 5% 2 5% 3 4 3 2 4 6% 51%	1962
	11th to 20th	Day	13% 6% 1 4 3 3 3 2 5% 6% 53%	1900
	21st to end	Day	15% 7% 2 6% 3 3 3 3 4 5% 47%	2024
November	1st to 10th	Day	10% 7% 3 5% 3 4 4 4 5% 7% 47%	1763
	11th to 20th	Day	9% 8% 3 9% 5% 4 3 3 5% 6% 44%	1689
	21st to end	Day	12% 8% 2 8% 4 4 5% 4 6% 7% 41%	1600
December	1st to 10th	Day	5% 6% 4 9% 6% 5% 5% 4 6% 7% 44%	2610
	11th to 20th	Day	9% 11% 4 11% 6% 6% 5% 4 5% 6% 33%	3008
	21st to end	Day	14% 10% 4 14% 5% 4 4 3 5% 7% 29%	3647
January	1st to 10th	Night	9% 11% 4 13% 10% 7% 6% 4 6% 6% 24%	6356
	11th to 20th	Night	4 9% 4 9% 7% 5% 7% 4 8% 6% 37%	5975
	21st to end	Night	4 9% 6% 10% 4 4 4 4 6% 7% 41%	6490
February	1st to 10th	Night	10% 11% 3 8% 6% 5% 4 3 5% 6% 38%	2600
	11th to 20th	Night	9% 11% 3 8% 5% 4 4 3 5% 7% 41%	2517
	21st to end	Night	9% 8% 3 6% 4 4 5% 4 6% 6% 45%	1980
March	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 4 6% 53%	5098
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 8% 50%	4980
	21st to end	Night	17% 10% 2 8% 5% 4 4 4 5% 6% 35%	5467
April	1st to 10th	Night	14% 8% 2 7% 4 5% 4 3 6% 6% 42%	2147
	11th to 20th	Night	14% 9% 2 6% 4 4 3 3 6% 6% 43%	1779
	21st to end	Night	19% 7% 2 5% 4 3 4 3 5% 6% 41%	1800
May	1st to 10th	Night	17% 6% 2 4 4 3 4 3 5% 7% 46%	2913
	11th to 20th	Night	9% 4 2 6% 4 4 4 3 5% 7% 51%	2828
	21st to end	Night	14% 6% 2 6% 4 3 4 4 5% 7% 44%	2819
June	1st to 10th	Night	7% 4 2 3 3 3 5% 3 6% 9% 54%	1475
	11th to 20th	Night	8% 4 1 4 3 2 4 4 8% 9% 52%	1330
	21st to end	Night	7% 3 1 4 4 4 4 4 5% 10% 56%	1330
July	1st to 10th	Night	7% 5% 2 6% 4 4 4 3 6% 8% 53%	2309
	11th to 20th	Night	6% 2 1 3 2 3 3 3 6% 7% 65%	2681
	21st to end	Night	2 3 1 4 3 3 3 3 7% 10% 61%	3098
August	1st to 10th	Night	6% 6% 2 5% 4 3 5% 4 6% 9% 50%	2836
	11th to 20th	Night	10% 5% 2 5% 3 3 3 4 6% 8% 50%	2798
	21st to end	Night	7% 4 2 4 4 3 4 3 6% 9% 54%	3135
September	1st to 10th	Night	9% 4 2 4 3 4 3 3 4 6% 59%	1980
	11th to 20th	Night	9% 4 1 4 3 3 4 3 5% 7% 58%	2090
	21st to end	Night	8% 5% 1 3 3 2 3 2 5% 6% 61%	2090
October	1st to 10th	Night	16% 5% 1 4 2 3 4 3 4 5% 54%	2198
	11th to 20th	Night	12% 5% 2 5% 4 3 3 2 4 6% 54%	2280
	21st to end	Night	14% 7% 2 5% 3 3 3 3 5% 5% 51%	2570
November	1st to 10th	Night	9% 5% 1 6% 4 3 3 3 5% 6% 53%	2535
	11th to 20th	Night	10% 6% 2 7% 4 4 3 3 5% 7% 49%	2709
	21st to end	Night	9% 8% 2 6% 4 3 3 4 6% 7% 49%	2800
December	1st to 10th	Night	5% 8% 4 8% 5% 5% 4 4 5% 6% 47%	4580
	11th to 20th	Night	10% 9% 3 9% 7% 6% 5% 4 6% 7% 35%	5199
	21st to end	Night	15% 12% 4 13% 5% 4 4 3 4 6% 28%	7321

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	63% 6% 13% 14% 4	3593
	11th to 20th	Day	49% 4 13% 18% 15% 1	3974
	21st to end	Day	47% 7% 14% 21% 8% 3	4488
February	1st to 10th	Day	55% 6% 14% 15% 9% 1	1800
	11th to 20th	Day	47% 5% 13% 20% 12% 1	1797
	21st to end	Day	48% 6% 12% 21% 11% 2	1650
March	1st to 10th	Day	39% 5% 10% 31% 13% 2	4763
	11th to 20th	Day	33% 5% 9% 31% 19% 2	4980
	21st to end	Day	59% 7% 13% 16% 3 2	5464
April	1st to 10th	Day	45% 6% 13% 24% 11% 2	2249
	11th to 20th	Day	45% 6% 12% 21% 14% 2	2572
	21st to end	Day	47% 7% 11% 23% 12% 1	2600
May	1st to 10th	Day	41% 9% 13% 22% 14%	4250
	11th to 20th	Day	33% 7% 13% 27% 17% 3	4087
	21st to end	Day	41% 9% 12% 24% 14% 1	4743
June	1st to 10th	Day	31% 8% 16% 29% 13% 2	2700
	11th to 20th	Day	37% 7% 14% 24% 15% 3	2850
	21st to end	Day	30% 7% 17% 30% 13% 3	2850
July	1st to 10th	Day	35% 7% 19% 24% 13% 1	4584
	11th to 20th	Day	26% 6% 13% 33% 18% 4	4210
	21st to end	Day	23% 8% 18% 29% 21% 3	4325
August	1st to 10th	Day	33% 7% 15% 30% 14% 2	4090
	11th to 20th	Day	44% 7% 13% 27% 9%	4080
	21st to end	Day	30% 7% 15% 27% 17% 3	4531
September	1st to 10th	Day	34% 6% 11% 29% 19% 2	2197
	11th to 20th	Day	29% 6% 14% 32% 17% 1	2090
	21st to end	Day	28% 7% 12% 29% 22% 2	2090
October	1st to 10th	Day	23% 4 7% 26% 36% 4	3261
	11th to 20th	Day	26% 5% 16% 27% 21% 6%	3140
	21st to end	Day	36% 4 13% 26% 17% 3	3317
November	1st to 10th	Day	35% 6% 17% 25% 15% 2	1763
	11th to 20th	Day	39% 7% 15% 24% 14% 1	1689
	21st to end	Day	41% 5% 16% 23% 14% 1	1600
December	1st to 10th	Day	37% 6% 18% 28% 10% 1	2610
	11th to 20th	Day	51% 6% 17% 19% 7%	3005
	21st to end	Day	54% 6% 13% 21% 6% 1	3647
January	1st to 10th	Night	58% 7% 12% 15% 8%	6356
	11th to 20th	Night	43% 6% 15% 18% 16% 2	5975
	21st to end	Night	41% 7% 13% 25% 14%	6490
February	1st to 10th	Night	46% 6% 14% 22% 11% 1	2600
	11th to 20th	Night	44% 5% 13% 23% 12% 3	2518
	21st to end	Night	39% 6% 14% 24% 14% 4	1980
March	1st to 10th	Night	33% 4 9% 32% 21% 1	5098
	11th to 20th	Night	31% 5% 9% 29% 22% 4	4980
	21st to end	Night	48% 9% 14% 21% 6% 1	5467
April	1st to 10th	Night	41% 7% 13% 23% 15% 1	2147
	11th to 20th	Night	39% 6% 11% 25% 18% 2	1779
	21st to end	Night	43% 5% 13% 23% 14% 2	1800
May	1st to 10th	Night	37% 7% 14% 21% 20% 1	2940
	11th to 20th	Night	29% 9% 13% 25% 21% 4	2829
	21st to end	Night	36% 8% 12% 26% 15% 2	2827
June	1st to 10th	Night	24% 9% 16% 28% 18% 4	1475
	11th to 20th	Night	26% 6% 17% 26% 20% 5%	1330
	21st to end	Night	24% 7% 16% 30% 21% 2	1330
July	1st to 10th	Night	30% 6% 21% 28% 12% 3	2272
	11th to 20th	Night	19% 5% 14% 38% 20% 4	2633
	21st to end	Night	18% 7% 19% 30% 22% 4	2999
August	1st to 10th	Night	29% 8% 14% 31% 16% 3	2826
	11th to 20th	Night	31% 8% 17% 29% 14% 2	2825
	21st to end	Night	27% 7% 18% 23% 22% 4	3135
September	1st to 10th	Night	26% 6% 11% 29% 23% 5%	1980
	11th to 20th	Night	25% 6% 14% 32% 20% 3	2090
	21st to end	Night	24% 6% 10% 33% 24% 3	2090
October	1st to 10th	Night	20% 3 7% 20% 46% 4	3666
	11th to 20th	Night	25% 3 12% 31% 26% 4	3779
	21st to end	Night	38% 4 11% 30% 16% 1	4231
November	1st to 10th	Night	30% 5% 16% 24% 21% 4	2535
	11th to 20th	Night	34% 5% 15% 24% 18% 3	2709
	21st to end	Night	34% 6% 15% 28% 16% 1	2800
December	1st to 10th	Night	38% 4 15% 32% 10%	4580
	11th to 20th	Night	47% 6% 17% 23% 7%	5201
	21st to end	Night	57% 5% 11% 20% 6%	7321

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	16% 13% 32% 25% 12% 2	300
	11th to 20th	Day	18% 12% 33% 28% 7% 1	300
	21st to end	Day	15% 9% 38% 27% 10% 1	330
February	1st to 10th	Day	14% 20% 37% 27% 3	300
	11th to 20th	Day	15% 13% 41% 25% 5% 1	300
	21st to end	Day	20% 12% 40% 22% 5%	249
March	1st to 10th	Day	15% 16% 39% 26% 3 1	300
	11th to 20th	Day	20% 12% 37% 22% 8% 1	300
	21st to end	Day	29% 13% 36% 17% 4 1	330
April	1st to 10th	Day	22% 16% 38% 20% 4	349
	11th to 20th	Day	24% 16% 32% 22% 6%	690
	21st to end	Day	25% 13% 31% 19% 9% 2	700
May	1st to 10th	Day	18% 13% 29% 28% 10% 2	703
	11th to 20th	Day	18% 12% 29% 26% 13% 3	676
	21st to end	Day	20% 8% 31% 27% 11% 2	734
June	1st to 10th	Day	8% 7% 23% 36% 21% 4	680
	11th to 20th	Day	13% 6% 34% 30% 14% 3	680
	21st to end	Day	9% 8% 31% 31% 18% 2	680
July	1st to 10th	Day	8% 3 18% 39% 25% 7%	671
	11th to 20th	Day	8% 4 28% 35% 21% 4	668
	21st to end	Day	5% 3 25% 41% 22% 4	739
August	1st to 10th	Day	10% 4 22% 39% 22% 4	676
	11th to 20th	Day	13% 3 24% 34% 21% 5%	665
	21st to end	Day	8% 4 27% 35% 22% 4	748
September	1st to 10th	Day	16% 5% 27% 32% 16% 4	407
	11th to 20th	Day	7% 5% 37% 31% 16% 4	300
	21st to end	Day	14% 11% 34% 25% 12% 4	300
October	1st to 10th	Day	16% 9% 27% 30% 13% 4	300
	11th to 20th	Day	15% 17% 34% 23% 9% 1	300
	21st to end	Day	21% 14% 37% 22% 6%	330
November	1st to 10th	Day	20% 13% 30% 24% 9% 3	300
	11th to 20th	Day	14% 18% 40% 21% 7%	300
	21st to end	Day	19% 16% 29% 26% 9% 1	300
December	1st to 10th	Day	12% 11% 40% 26% 10% 1	300
	11th to 20th	Day	15% 15% 41% 21% 7%	300
	21st to end	Day	24% 11% 37% 25% 3	330
January	1st to 10th	Night	12% 10% 39% 31% 8% 1	1000
	11th to 20th	Night	12% 14% 35% 32% 7% 1	1000
	21st to end	Night	14% 10% 40% 29% 7%	1100
February	1st to 10th	Night	15% 11% 40% 29% 5%	1000
	11th to 20th	Night	14% 12% 39% 28% 6%	963
	21st to end	Night	13% 10% 38% 30% 8%	660
March	1st to 10th	Night	13% 10% 38% 31% 6%	680
	11th to 20th	Night	17% 12% 37% 30% 5%	600
	21st to end	Night	20% 13% 37% 23% 6%	660
April	1st to 10th	Night	19% 13% 40% 26% 3	600
	11th to 20th	Night	19% 15% 36% 24% 5% 1	593
	21st to end	Night	23% 10% 32% 27% 8% 1	600
May	1st to 10th	Night	16% 9% 33% 32% 9%	600
	11th to 20th	Night	16% 8% 31% 33% 12%	563
	21st to end	Night	17% 6% 31% 36% 9%	617
June	1st to 10th	Night	8% 6% 26% 45% 15% 1	570
	11th to 20th	Night	10% 3 36% 38% 12%	570
	21st to end	Night	7% 5% 26% 45% 16%	570
July	1st to 10th	Night	6% 2 21% 45% 27%	555
	11th to 20th	Night	4 1 23% 53% 18% 1	552
	21st to end	Night	3 2 22% 55% 18%	624
August	1st to 10th	Night	8% 3 22% 45% 21% 1	567
	11th to 20th	Night	7% 3 25% 42% 22% 1	564
	21st to end	Night	6% 3 27% 44% 18% 2	627
September	1st to 10th	Night	9% 5% 25% 43% 16% 1	570
	11th to 20th	Night	9% 5% 32% 39% 13% 2	570
	21st to end	Night	10% 5% 37% 34% 13% 1	570
October	1st to 10th	Night	18% 7% 33% 31% 10% 1	678
	11th to 20th	Night	15% 12% 35% 31% 7% 1	760
	21st to end	Night	17% 13% 31% 30% 9%	898
November	1st to 10th	Night	12% 10% 37% 31% 9% 1	975
	11th to 20th	Night	13% 11% 41% 29% 6%	1000
	21st to end	Night	13% 10% 37% 33% 8%	1000
December	1st to 10th	Night	13% 12% 38% 32% 6%	1000
	11th to 20th	Night	17% 8% 35% 32% 8% 1	1000
	21st to end	Night	21% 10% 35% 27% 6%	1100

Page generated at 2025-05-08 08:33:55. Version ac07

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