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 2026-09-14.

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	11% 7% 3 7% 4 4 4 3 5% 7% 43%	149303
	Night	10% 7% 3 7% 4 4 4 3 5% 7% 46%	142106
Transparency	Day	39% 6% 14% 25% 14% 2	152895
	Night	36% 6% 14% 26% 16% 2	146585
Seeing	Day	15% 10% 31% 29% 13% 2	18531
	Night	13% 9% 34% 34% 10% 1	28293

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% 15% 8% 7% 5% 4 4 4 25%	4663
	11th to 20th	Day	5% 10% 4 10% 7% 7% 7% 4 6% 8% 32%	5178
	21st to end	Day	4 10% 5% 12% 6% 6% 6% 4 6% 6% 35%	5812
February	1st to 10th	Day	9% 12% 4 11% 7% 8% 6% 4 4 6% 29%	1890
	11th to 20th	Day	9% 11% 4 8% 6% 4 5% 2 5% 7% 39%	1880
	21st to end	Day	12% 10% 3 8% 6% 5% 5% 3 6% 6% 37%	1730
March	1st to 10th	Day	12% 9% 3 7% 3 3 3 3 4 6% 47%	4869
	11th to 20th	Day	8% 8% 3 5% 4 4 3 3 5% 7% 50%	5090
	21st to end	Day	22% 11% 4 10% 5% 5% 5% 3 4 6% 25%	5583
April	1st to 10th	Day	15% 10% 3 7% 4 4 4 3 5% 6% 38%	2361
	11th to 20th	Day	17% 9% 3 6% 4 4 4 3 5% 6% 39%	2702
	21st to end	Day	22% 7% 3 6% 4 4 4 4 4 6% 38%	2730
May	1st to 10th	Day	9% 5% 2 5% 4 4 5% 4 6% 8% 48%	7617
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 4 6% 48%	7627
	21st to end	Day	19% 7% 2 6% 4 4 4 4 5% 7% 37%	8869
June	1st to 10th	Day	11% 5% 2 5% 4 3 4 4 6% 8% 48%	2984
	11th to 20th	Day	13% 5% 2 5% 4 4 4 4 6% 8% 45%	3150
	21st to end	Day	10% 4 2 6% 4 4 4 4 7% 8% 48%	3150
July	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 4 6% 10% 44%	8672
	11th to 20th	Day	7% 3 2 4 4 3 3 3 4 6% 61%	7988
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 46%	8337
August	1st to 10th	Day	8% 5% 2 5% 4 4 4 3 5% 8% 51%	6014
	11th to 20th	Day	18% 7% 3 8% 4 4 5% 4 6% 8% 34%	5948
	21st to end	Day	9% 4 1 5% 4 4 4 3 5% 8% 53%	6593
September	1st to 10th	Day	13% 5% 2 5% 3 4 4 3 5% 5% 51%	2427
	11th to 20th	Day	10% 5% 2 5% 3 3 3 3 5% 7% 54%	2233
	21st to end	Day	10% 4 2 5% 3 3 3 4 5% 6% 56%	2200
October	1st to 10th	Day	15% 5% 2 5% 3 4 3 2 4 6% 51%	2065
	11th to 20th	Day	14% 6% 1 5% 3 3 3 2 5% 6% 52%	2000
	21st to end	Day	15% 8% 2 6% 3 4 3 3 4 5% 46%	2130
November	1st to 10th	Day	10% 7% 3 5% 3 4 4 4 5% 7% 47%	1853
	11th to 20th	Day	9% 8% 3 9% 5% 4 3 3 5% 6% 44%	1773
	21st to end	Day	12% 8% 3 8% 5% 4 4 4 6% 7% 41%	1680
December	1st to 10th	Day	5% 6% 4 9% 6% 5% 5% 4 6% 7% 43%	2690
	11th to 20th	Day	9% 11% 4 11% 6% 6% 5% 4 5% 6% 33%	3088
	21st to end	Day	14% 10% 4 14% 5% 4 4 3 5% 7% 29%	3727
January	1st to 10th	Night	8% 11% 4 12% 9% 6% 5% 4 6% 6% 29%	8243
	11th to 20th	Night	3 8% 3 9% 7% 6% 7% 5% 8% 7% 38%	7781
	21st to end	Night	4 8% 5% 10% 4 4 4 4 7% 7% 42%	8402
February	1st to 10th	Night	10% 11% 3 8% 6% 5% 4 4 5% 6% 38%	2730
	11th to 20th	Night	9% 11% 3 8% 5% 4 4 3 5% 7% 42%	2646
	21st to end	Night	9% 8% 3 6% 4 4 5% 4 6% 6% 46%	2076
March	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 4 6% 53%	5212
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 8% 50%	5090
	21st to end	Night	17% 10% 2 8% 5% 4 4 4 5% 6% 35%	5588
April	1st to 10th	Night	14% 8% 2 7% 4 5% 4 3 5% 6% 42%	2255
	11th to 20th	Night	14% 9% 2 6% 4 4 3 3 6% 6% 44%	1869
	21st to end	Night	19% 7% 2 5% 4 4 4 3 5% 6% 41%	1890
May	1st to 10th	Night	11% 5% 2 4 3 3 4 3 5% 8% 52%	5266
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 6% 53%	5265
	21st to end	Night	16% 7% 3 7% 4 4 5% 4 5% 7% 40%	5298
June	1st to 10th	Night	7% 4 2 4 3 4 4 3 6% 9% 53%	1630
	11th to 20th	Night	8% 4 1 4 3 2 4 4 8% 9% 53%	1470
	21st to end	Night	9% 3 1 4 4 3 4 4 4 10% 54%	1470
July	1st to 10th	Night	7% 4 2 7% 4 4 4 4 7% 9% 47%	4311
	11th to 20th	Night	7% 3 1 3 2 3 3 3 5% 6% 65%	4982
	21st to end	Night	7% 4 1 5% 3 4 4 4 6% 9% 54%	5770
August	1st to 10th	Night	6% 5% 2 5% 4 3 4 4 5% 9% 54%	4156
	11th to 20th	Night	11% 5% 2 6% 3 4 4 4 6% 9% 45%	4118
	21st to end	Night	6% 4 2 4 3 3 4 3 6% 10% 54%	4555
September	1st to 10th	Night	9% 4 1 4 3 4 3 3 4 6% 59%	2189
	11th to 20th	Night	8% 4 2 5% 3 3 4 3 6% 7% 57%	2233
	21st to end	Night	8% 5% 1 3 3 3 4 3 5% 6% 60%	2200
October	1st to 10th	Night	15% 5% 1 4 2 3 4 3 4 5% 54%	2314
	11th to 20th	Night	12% 5% 2 5% 4 3 3 2 4 6% 53%	2400
	21st to end	Night	14% 7% 2 5% 3 3 3 2 5% 5% 50%	2706
November	1st to 10th	Night	9% 5% 1 6% 4 3 4 3 5% 6% 53%	2665
	11th to 20th	Night	10% 6% 2 7% 4 4 4 3 5% 7% 49%	2845
	21st to end	Night	9% 8% 2 6% 4 3 4 4 6% 7% 48%	2940
December	1st to 10th	Night	5% 8% 4 9% 5% 5% 4 4 5% 6% 46%	4720
	11th to 20th	Night	10% 9% 3 9% 7% 6% 5% 4 6% 7% 35%	5339
	21st to end	Night	15% 12% 4 13% 5% 4 4 3 4 6% 28%	7482

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	61% 5% 14% 14% 6% 1	4664
	11th to 20th	Day	49% 4 12% 20% 15% 1	5178
	21st to end	Day	47% 6% 15% 21% 9% 2	5812
February	1st to 10th	Day	56% 6% 15% 15% 8%	1890
	11th to 20th	Day	46% 5% 13% 22% 13% 2	1889
	21st to end	Day	47% 6% 12% 22% 11% 2	1730
March	1st to 10th	Day	39% 5% 10% 31% 12% 2	4869
	11th to 20th	Day	34% 5% 9% 31% 19% 2	5090
	21st to end	Day	59% 7% 13% 16% 4 2	5585
April	1st to 10th	Day	46% 6% 12% 24% 10% 1	2361
	11th to 20th	Day	44% 6% 12% 22% 14% 2	2702
	21st to end	Day	47% 6% 11% 22% 13% 1	2730
May	1st to 10th	Day	31% 6% 10% 28% 23% 1	7655
	11th to 20th	Day	36% 5% 10% 23% 18% 8%	7631
	21st to end	Day	46% 7% 10% 23% 13% 1	8883
June	1st to 10th	Day	32% 8% 16% 29% 13% 2	2984
	11th to 20th	Day	36% 7% 15% 24% 16% 3	3150
	21st to end	Day	31% 7% 16% 29% 14% 3	3150
July	1st to 10th	Day	35% 6% 18% 28% 12% 1	8600
	11th to 20th	Day	24% 5% 13% 29% 23% 6%	7913
	21st to end	Day	33% 7% 18% 28% 13% 1	8155
August	1st to 10th	Day	31% 6% 14% 35% 13% 2	6000
	11th to 20th	Day	47% 8% 16% 21% 7%	5987
	21st to end	Day	30% 7% 19% 27% 15% 2	6592
September	1st to 10th	Day	34% 5% 12% 29% 18% 2	2427
	11th to 20th	Day	30% 6% 15% 32% 17% 1	2233
	21st to end	Day	27% 7% 12% 29% 22% 2	2200
October	1st to 10th	Day	23% 4 7% 27% 36% 4	3364
	11th to 20th	Day	27% 5% 16% 26% 20% 6%	3240
	21st to end	Day	37% 4 13% 26% 17% 3	3423
November	1st to 10th	Day	35% 5% 17% 26% 15% 2	1853
	11th to 20th	Day	39% 6% 15% 24% 14% 2	1773
	21st to end	Day	42% 5% 15% 23% 13% 1	1680
December	1st to 10th	Day	38% 6% 18% 27% 9% 1	2690
	11th to 20th	Day	51% 6% 17% 19% 7%	3085
	21st to end	Day	53% 6% 14% 21% 6% 1	3727
January	1st to 10th	Night	54% 6% 14% 18% 8% 1	8244
	11th to 20th	Night	41% 5% 14% 21% 16% 2	7781
	21st to end	Night	40% 6% 15% 25% 15%	8402
February	1st to 10th	Night	46% 6% 14% 22% 11% 1	2730
	11th to 20th	Night	43% 5% 13% 24% 12% 3	2647
	21st to end	Night	38% 6% 14% 25% 14% 3	2076
March	1st to 10th	Night	33% 4 10% 32% 20% 1	5212
	11th to 20th	Night	32% 5% 9% 29% 22% 4	5090
	21st to end	Night	47% 9% 14% 22% 7% 1	5588
April	1st to 10th	Night	42% 7% 13% 23% 15% 1	2255
	11th to 20th	Night	38% 6% 11% 25% 18% 2	1869
	21st to end	Night	43% 5% 13% 22% 14% 2	1890
May	1st to 10th	Night	30% 5% 10% 28% 25% 1	5293
	11th to 20th	Night	32% 6% 10% 21% 24% 8%	5266
	21st to end	Night	42% 6% 11% 26% 13% 1	5309
June	1st to 10th	Night	25% 9% 16% 28% 17% 4	1630
	11th to 20th	Night	26% 6% 17% 25% 22% 4	1470
	21st to end	Night	27% 7% 16% 29% 19% 2	1470
July	1st to 10th	Night	32% 6% 20% 28% 12% 2	4276
	11th to 20th	Night	21% 5% 12% 33% 25% 4	4937
	21st to end	Night	27% 7% 23% 28% 13% 2	5641
August	1st to 10th	Night	27% 7% 15% 31% 18% 3	4146
	11th to 20th	Night	34% 9% 21% 24% 10% 1	4145
	21st to end	Night	26% 6% 20% 27% 18% 3	4555
September	1st to 10th	Night	26% 6% 13% 28% 22% 5%	2189
	11th to 20th	Night	26% 6% 15% 32% 19% 2	2233
	21st to end	Night	24% 6% 10% 33% 25% 3	2200
October	1st to 10th	Night	20% 3 7% 21% 46% 3	3782
	11th to 20th	Night	25% 3 11% 31% 26% 4	3899
	21st to end	Night	38% 4 12% 30% 16% 1	4367
November	1st to 10th	Night	30% 5% 16% 24% 21% 4	2665
	11th to 20th	Night	34% 5% 15% 25% 18% 2	2845
	21st to end	Night	35% 5% 15% 28% 15% 1	2940
December	1st to 10th	Night	38% 4 15% 32% 10%	4720
	11th to 20th	Night	47% 6% 16% 23% 7%	5341
	21st to end	Night	57% 5% 11% 20% 6%	7482

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% 11% 2	330
	11th to 20th	Day	18% 13% 34% 27% 7% 1	330
	21st to end	Day	14% 10% 40% 26% 10% 1	363
February	1st to 10th	Day	13% 19% 39% 25% 3 1	330
	11th to 20th	Day	15% 13% 41% 25% 5% 1	330
	21st to end	Day	19% 15% 40% 21% 5%	273
March	1st to 10th	Day	16% 16% 39% 25% 3 1	330
	11th to 20th	Day	20% 13% 36% 22% 8% 1	330
	21st to end	Day	28% 15% 37% 16% 4 1	363
April	1st to 10th	Day	24% 16% 37% 19% 4	381
	11th to 20th	Day	24% 17% 32% 22% 5%	740
	21st to end	Day	25% 13% 32% 19% 9% 2	750
May	1st to 10th	Day	17% 13% 30% 28% 10% 2	753
	11th to 20th	Day	19% 12% 28% 25% 13% 2	775
	21st to end	Day	20% 8% 32% 26% 11% 2	844
June	1st to 10th	Day	8% 8% 24% 35% 20% 4	780
	11th to 20th	Day	13% 8% 34% 30% 13% 2	780
	21st to end	Day	10% 9% 33% 30% 16% 2	780
July	1st to 10th	Day	7% 3 18% 40% 25% 6%	771
	11th to 20th	Day	8% 4 27% 35% 21% 5%	768
	21st to end	Day	6% 3 26% 41% 20% 4	849
August	1st to 10th	Day	10% 4 23% 39% 20% 4	776
	11th to 20th	Day	12% 4 25% 33% 21% 5%	765
	21st to end	Day	7% 4 27% 36% 22% 3	858
September	1st to 10th	Day	15% 6% 28% 31% 14% 5%	477
	11th to 20th	Day	9% 5% 38% 30% 15% 4	339
	21st to end	Day	14% 10% 32% 26% 13% 5%	330
October	1st to 10th	Day	15% 11% 28% 30% 13% 3	330
	11th to 20th	Day	17% 16% 36% 22% 8% 1	330
	21st to end	Day	22% 13% 37% 21% 7%	363
November	1st to 10th	Day	19% 13% 32% 24% 8% 3	330
	11th to 20th	Day	13% 18% 42% 21% 6%	330
	21st to end	Day	20% 17% 29% 24% 8% 2	330
December	1st to 10th	Day	12% 12% 41% 25% 9% 1	330
	11th to 20th	Day	16% 17% 41% 20% 6%	330
	21st to end	Day	23% 10% 39% 25% 3	363
January	1st to 10th	Night	12% 10% 39% 31% 8% 1	1050
	11th to 20th	Night	12% 15% 35% 31% 6% 1	1050
	21st to end	Night	14% 10% 40% 29% 6%	1155
February	1st to 10th	Night	14% 12% 40% 29% 5%	1050
	11th to 20th	Night	14% 12% 40% 28% 6%	1012
	21st to end	Night	13% 11% 38% 30% 8%	692
March	1st to 10th	Night	14% 11% 39% 31% 6%	714
	11th to 20th	Night	18% 11% 37% 29% 4	630
	21st to end	Night	20% 13% 38% 23% 6%	693
April	1st to 10th	Night	19% 13% 40% 25% 3	630
	11th to 20th	Night	19% 14% 38% 24% 5% 1	623
	21st to end	Night	23% 10% 33% 27% 8%	630
May	1st to 10th	Night	16% 9% 34% 33% 8%	630
	11th to 20th	Night	16% 9% 31% 33% 10%	621
	21st to end	Night	18% 6% 30% 35% 10%	683
June	1st to 10th	Night	8% 6% 26% 45% 14%	630
	11th to 20th	Night	10% 4 37% 38% 11%	630
	21st to end	Night	9% 5% 27% 44% 15%	630
July	1st to 10th	Night	6% 2 21% 45% 26%	615
	11th to 20th	Night	6% 1 22% 52% 19% 1	612
	21st to end	Night	5% 2 23% 53% 17%	690
August	1st to 10th	Night	8% 3 22% 46% 20% 1	627
	11th to 20th	Night	7% 4 25% 42% 22% 1	624
	21st to end	Night	6% 3 27% 45% 18% 2	693
September	1st to 10th	Night	9% 6% 24% 44% 16% 1	630
	11th to 20th	Night	9% 5% 31% 41% 12% 2	609
	21st to end	Night	10% 5% 37% 34% 14% 1	600
October	1st to 10th	Night	17% 8% 32% 31% 10% 1	714
	11th to 20th	Night	16% 13% 35% 30% 7% 1	800
	21st to end	Night	17% 13% 32% 29% 9%	946
November	1st to 10th	Night	11% 11% 38% 30% 9% 1	1025
	11th to 20th	Night	13% 11% 41% 29% 6%	1050
	21st to end	Night	13% 10% 36% 33% 8%	1050
December	1st to 10th	Night	13% 11% 38% 32% 6%	1050
	11th to 20th	Night	17% 9% 34% 32% 7% 1	1050
	21st to end	Night	21% 10% 37% 27% 5%	1155

Page generated at 2026-09-11 03:11:52. Version ac07

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