South Llano River State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the South Llano River State Park clear sky chart from 2005-11-06 to 2026-05-17.

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	9% 5% 2 5% 4 4 4 3 5% 7% 54%	132334
	Night	9% 5% 2 4 3 3 3 3 5% 7% 57%	130832
Transparency	Day	29% 5% 9% 21% 22% 13%	136191
	Night	26% 5% 10% 21% 22% 15%	135040
Seeing	Day	10% 8% 25% 32% 20% 5%	16810
	Night	10% 7% 31% 38% 13% 1	26850

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	8% 4 3 4 2 3 3 2 3 6% 62%	5289
	11th to 20th	Day	8% 6% 1 4 2 2 3 3 4 4 61%	5472
	21st to end	Day	17% 5% 2 5% 4 4 3 2 4 6% 48%	6468
February	1st to 10th	Day	9% 7% 2 5% 3 3 3 4 4 5% 54%	2100
	11th to 20th	Day	10% 6% 2 4 4 4 4 3 5% 6% 53%	2049
	21st to end	Day	11% 5% 2 3 2 3 4 3 5% 6% 58%	1730
March	1st to 10th	Day	8% 5% 2 6% 3 3 3 3 4 5% 57%	4724
	11th to 20th	Day	11% 6% 2 5% 3 3 3 2 3 5% 57%	5090
	21st to end	Day	8% 6% 2 5% 4 3 4 3 5% 6% 55%	5583
April	1st to 10th	Day	9% 5% 2 5% 4 3 3 3 5% 6% 53%	2331
	11th to 20th	Day	12% 6% 2 6% 5% 4 3 3 5% 6% 49%	2494
	21st to end	Day	9% 6% 2 5% 4 4 3 4 4 5% 54%	2546
May	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 5% 6% 45%	5992
	11th to 20th	Day	9% 6% 2 6% 4 4 4 3 5% 7% 50%	5941
	21st to end	Day	11% 6% 2 6% 4 4 5% 4 6% 9% 44%	6434
June	1st to 10th	Day	4 3 1 4 3 4 5% 3 6% 8% 57%	2600
	11th to 20th	Day	4 3 1 4 4 4 4 4 6% 9% 56%	2600
	21st to end	Day	5% 4 1 4 5% 5% 6% 5% 8% 8% 49%	2600
July	1st to 10th	Day	11% 5% 3 7% 6% 4 5% 4 6% 8% 42%	5900
	11th to 20th	Day	3 4 2 5% 4 5% 5% 4 7% 9% 53%	5900
	21st to end	Day	5% 3 1 5% 4 4 4 4 6% 9% 54%	6405
August	1st to 10th	Day	1 2 1 3 2 3 3 3 4 8% 71%	3905
	11th to 20th	Day	3 3 1 3 3 3 3 3 5% 8% 64%	3846
	21st to end	Day	4 4 2 5% 5% 5% 6% 5% 6% 9% 49%	4312
September	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 3 4 7% 47%	2213
	11th to 20th	Day	10% 4 2 7% 4 4 4 5% 6% 8% 45%	2200
	21st to end	Day	9% 4 2 4 4 4 4 4 5% 6% 54%	2200
October	1st to 10th	Day	5% 4 1 4 3 4 4 3 5% 7% 60%	2200
	11th to 20th	Day	9% 5% 2 5% 3 3 3 2 4 5% 59%	2185
	21st to end	Day	10% 4 2 4 2 2 3 2 3 5% 63%	2200
November	1st to 10th	Day	11% 6% 2 4 3 2 3 3 5% 5% 57%	1947
	11th to 20th	Day	8% 5% 2 4 3 2 3 3 4 6% 60%	1890
	21st to end	Day	12% 7% 2 5% 3 3 2 2 4 6% 54%	1890
December	1st to 10th	Day	19% 8% 3 5% 3 2 3 2 4 5% 46%	3030
	11th to 20th	Day	10% 7% 2 6% 4 4 4 3 5% 5% 49%	3481
	21st to end	Day	8% 5% 2 4 3 2 2 2 3 4 65%	4587
January	1st to 10th	Night	9% 4 2 4 2 2 3 2 3 5% 62%	7634
	11th to 20th	Night	6% 4 1 3 2 2 2 2 3 5% 70%	7560
	21st to end	Night	22% 6% 2 5% 3 2 3 2 3 5% 46%	7746
February	1st to 10th	Night	9% 6% 2 4 3 2 3 3 4 7% 58%	2520
	11th to 20th	Night	12% 6% 1 5% 3 3 2 2 5% 6% 54%	2458
	21st to end	Night	10% 5% 1 4 3 3 4 3 5% 7% 55%	2076
March	1st to 10th	Night	6% 5% 2 4 3 3 4 3 5% 8% 57%	5361
	11th to 20th	Night	11% 5% 1 4 3 3 3 2 4 5% 58%	5090
	21st to end	Night	7% 6% 2 5% 3 3 3 3 5% 6% 56%	5588
April	1st to 10th	Night	7% 5% 2 4 3 4 4 4 5% 7% 56%	2283
	11th to 20th	Night	11% 5% 2 5% 3 4 3 3 4 6% 53%	2077
	21st to end	Night	10% 5% 1 4 3 3 3 3 5% 6% 56%	2072
May	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 4 7% 48%	4143
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 5% 8% 55%	4107
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 7% 10% 46%	4443
June	1st to 10th	Night	2 2 1 3 3 3 3 4 6% 11% 62%	1800
	11th to 20th	Night	3 2 1 2 3 3 4 4 6% 9% 65%	1800
	21st to end	Night	4 3 1 4 4 3 3 4 5% 9% 61%	1800
July	1st to 10th	Night	7% 4 2 6% 4 5% 6% 4 7% 10% 46%	4078
	11th to 20th	Night	2 2 1 2 3 3 3 3 6% 10% 66%	4077
	21st to end	Night	3 3 1 4 4 4 4 4 6% 9% 58%	4449
August	1st to 10th	Night	1 1 1 3 2 2 3 2 4 6% 76%	3254
	11th to 20th	Night	2 3 1 2 2 3 3 3 7% 10% 64%	3213
	21st to end	Night	4 4 1 4 4 3 4 4 7% 11% 54%	3597
September	1st to 10th	Night	7% 5% 2 4 4 4 3 3 6% 9% 53%	2185
	11th to 20th	Night	7% 5% 2 4 4 4 5% 5% 6% 7% 51%	2200
	21st to end	Night	8% 4 2 4 3 3 4 3 7% 7% 57%	2200
October	1st to 10th	Night	4 2 1 3 2 2 3 3 5% 7% 67%	2200
	11th to 20th	Night	9% 4 1 4 3 3 3 2 4 5% 63%	2200
	21st to end	Night	10% 4 1 4 3 2 2 2 3 4 64%	2635
November	1st to 10th	Night	10% 4 2 3 3 2 3 2 4 5% 62%	2567
	11th to 20th	Night	7% 3 1 3 3 3 4 3 4 6% 62%	2730
	21st to end	Night	12% 5% 2 4 3 4 4 3 4 6% 54%	2730
December	1st to 10th	Night	19% 7% 2 4 3 3 3 3 4 5% 45%	4380
	11th to 20th	Night	12% 7% 2 6% 4 3 3 3 4 5% 53%	4946
	21st to end	Night	9% 5% 2 4 3 2 2 2 3 5% 63%	6633

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	26% 2 6% 24% 25% 16%	5290
	11th to 20th	Day	27% 2 4 17% 28% 22%	5472
	21st to end	Day	39% 3 5% 13% 26% 13%	6468
February	1st to 10th	Day	31% 4 7% 14% 22% 23%	2100
	11th to 20th	Day	32% 4 7% 20% 21% 16%	2059
	21st to end	Day	29% 4 6% 14% 28% 20%	1730
March	1st to 10th	Day	29% 3 5% 15% 18% 30%	4724
	11th to 20th	Day	31% 4 5% 10% 16% 34%	5090
	21st to end	Day	29% 5% 6% 18% 22% 20%	5585
April	1st to 10th	Day	31% 5% 9% 19% 19% 17%	2331
	11th to 20th	Day	37% 5% 6% 16% 21% 15%	2494
	21st to end	Day	32% 5% 8% 21% 20% 14%	2546
May	1st to 10th	Day	37% 7% 8% 16% 20% 11%	6030
	11th to 20th	Day	33% 5% 8% 15% 24% 15%	5945
	21st to end	Day	34% 7% 11% 26% 17% 5%	6447
June	1st to 10th	Day	24% 6% 12% 31% 23% 4	2600
	11th to 20th	Day	22% 7% 12% 27% 24% 7%	2600
	21st to end	Day	28% 9% 13% 25% 19% 6%	2600
July	1st to 10th	Day	39% 7% 16% 30% 8% 1	5836
	11th to 20th	Day	26% 8% 15% 33% 16% 2	5831
	21st to end	Day	24% 7% 18% 28% 20% 1	6223
August	1st to 10th	Day	13% 5% 10% 39% 32% 1	3892
	11th to 20th	Day	19% 6% 17% 42% 14% 2	3882
	21st to end	Day	28% 10% 16% 23% 20% 2	4311
September	1st to 10th	Day	36% 8% 9% 19% 18% 9%	2213
	11th to 20th	Day	36% 8% 10% 21% 20% 6%	2200
	21st to end	Day	29% 6% 10% 23% 20% 12%	2200
October	1st to 10th	Day	14% 4 6% 14% 27% 34%	3590
	11th to 20th	Day	24% 4 6% 19% 22% 25%	3541
	21st to end	Day	20% 3 5% 18% 41% 13%	3538
November	1st to 10th	Day	30% 4 8% 21% 24% 14%	1947
	11th to 20th	Day	26% 5% 7% 19% 23% 20%	1890
	21st to end	Day	33% 3 5% 19% 24% 15%	1890
December	1st to 10th	Day	43% 3 5% 17% 22% 11%	3030
	11th to 20th	Day	35% 5% 7% 12% 25% 15%	3479
	21st to end	Day	25% 3 4 18% 27% 23%	4587
January	1st to 10th	Night	26% 3 6% 20% 30% 16%	7635
	11th to 20th	Night	19% 2 4 18% 30% 28%	7560
	21st to end	Night	42% 3 4 11% 23% 17%	7746
February	1st to 10th	Night	29% 3 7% 15% 24% 22%	2520
	11th to 20th	Night	32% 4 6% 19% 22% 18%	2458
	21st to end	Night	29% 4 6% 15% 25% 22%	2076
March	1st to 10th	Night	24% 5% 8% 19% 24% 20%	5361
	11th to 20th	Night	29% 4 6% 12% 18% 32%	5090
	21st to end	Night	27% 5% 7% 18% 21% 21%	5588
April	1st to 10th	Night	27% 6% 10% 20% 17% 20%	2283
	11th to 20th	Night	32% 5% 10% 17% 18% 17%	2077
	21st to end	Night	28% 5% 11% 23% 20% 12%	2072
May	1st to 10th	Night	35% 5% 9% 21% 17% 13%	4170
	11th to 20th	Night	27% 7% 11% 19% 20% 17%	4108
	21st to end	Night	31% 8% 14% 26% 14% 7%	4452
June	1st to 10th	Night	14% 7% 18% 36% 23% 3	1800
	11th to 20th	Night	15% 6% 15% 31% 24% 7%	1800
	21st to end	Night	20% 6% 18% 31% 20% 6%	1800
July	1st to 10th	Night	30% 10% 21% 31% 8% 1	4035
	11th to 20th	Night	14% 7% 19% 39% 18% 2	4026
	21st to end	Night	19% 8% 20% 31% 20% 2	4320
August	1st to 10th	Night	11% 5% 13% 37% 33% 1	3244
	11th to 20th	Night	15% 6% 21% 42% 14% 2	3243
	21st to end	Night	22% 10% 21% 25% 20% 2	3597
September	1st to 10th	Night	27% 7% 11% 25% 20% 9%	2185
	11th to 20th	Night	28% 9% 16% 23% 17% 7%	2200
	21st to end	Night	25% 6% 12% 23% 21% 13%	2200
October	1st to 10th	Night	12% 4 8% 17% 23% 36%	3590
	11th to 20th	Night	18% 4 9% 17% 28% 23%	3570
	21st to end	Night	19% 3 6% 22% 34% 17%	4247
November	1st to 10th	Night	25% 3 8% 19% 27% 17%	2567
	11th to 20th	Night	22% 5% 9% 21% 23% 20%	2730
	21st to end	Night	32% 4 8% 17% 24% 16%	2730
December	1st to 10th	Night	41% 4 7% 17% 17% 14%	4380
	11th to 20th	Night	34% 4 6% 15% 23% 18%	4947
	21st to end	Night	25% 3 5% 19% 29% 19%	6633

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	12% 15% 42% 22% 8% 1	330
	11th to 20th	Day	15% 16% 31% 28% 10% 1	330
	21st to end	Day	13% 15% 41% 23% 8%	363
February	1st to 10th	Day	18% 13% 36% 22% 9% 2	330
	11th to 20th	Day	17% 15% 40% 23% 4	330
	21st to end	Day	15% 15% 44% 23% 3	273
March	1st to 10th	Day	16% 19% 39% 22% 5%	330
	11th to 20th	Day	16% 16% 41% 20% 8%	330
	21st to end	Day	14% 16% 42% 20% 8% 1	363
April	1st to 10th	Day	16% 18% 39% 22% 4	351
	11th to 20th	Day	15% 21% 41% 20% 2	532
	21st to end	Day	11% 18% 42% 24% 5%	566
May	1st to 10th	Day	12% 13% 41% 29% 5%	753
	11th to 20th	Day	14% 10% 36% 28% 10% 2	756
	21st to end	Day	11% 8% 31% 37% 11% 1	789
June	1st to 10th	Day	4 4 21% 48% 20% 3	730
	11th to 20th	Day	4 1 25% 49% 17% 3	730
	21st to end	Day	4 1 17% 41% 33% 5%	730
July	1st to 10th	Day	8% 2 9% 35% 37% 8%	721
	11th to 20th	Day	3 1 5% 44% 40% 7%	718
	21st to end	Day	4 1 6% 38% 39% 12%	789
August	1st to 10th	Day	2 6% 35% 43% 14%	527
	11th to 20th	Day	6% 2 7% 34% 38% 14%	517
	21st to end	Day	5% 1 6% 30% 39% 19%	583
September	1st to 10th	Day	13% 1 4 26% 36% 19%	343
	11th to 20th	Day	6% 1 8% 26% 42% 18%	330
	21st to end	Day	12% 2 4 35% 33% 14%	330
October	1st to 10th	Day	5% 2 10% 35% 36% 11%	330
	11th to 20th	Day	9% 4 21% 39% 22% 4	330
	21st to end	Day	14% 13% 25% 28% 17% 4	363
November	1st to 10th	Day	16% 9% 25% 30% 18% 2	330
	11th to 20th	Day	14% 11% 30% 24% 15% 7%	330
	21st to end	Day	17% 18% 32% 20% 11% 2	330
December	1st to 10th	Day	16% 9% 31% 31% 10% 3	330
	11th to 20th	Day	12% 17% 34% 25% 10% 2	330
	21st to end	Day	12% 14% 34% 29% 10%	363
January	1st to 10th	Night	14% 8% 37% 32% 8%	1050
	11th to 20th	Night	14% 9% 36% 35% 6%	1008
	21st to end	Night	17% 9% 38% 30% 6%	924
February	1st to 10th	Night	14% 10% 38% 33% 4	840
	11th to 20th	Night	15% 14% 41% 26% 3	824
	21st to end	Night	13% 16% 46% 25% 1	692
March	1st to 10th	Night	14% 15% 44% 24% 3	785
	11th to 20th	Night	13% 11% 43% 29% 4	630
	21st to end	Night	8% 13% 52% 26% 2	693
April	1st to 10th	Night	8% 13% 51% 25% 3	630
	11th to 20th	Night	11% 15% 50% 23% 1	623
	21st to end	Night	10% 12% 52% 21% 5%	630
May	1st to 10th	Night	9% 11% 49% 28% 3	630
	11th to 20th	Night	10% 7% 41% 34% 8%	609
	21st to end	Night	8% 7% 42% 35% 8%	650
June	1st to 10th	Night	2 2 34% 50% 13%	600
	11th to 20th	Night	3 1 38% 49% 9%	600
	21st to end	Night	3 2 22% 53% 19% 1	600
July	1st to 10th	Night	5% 2 10% 56% 25% 3	585
	11th to 20th	Night	3 1 13% 58% 25% 1	582
	21st to end	Night	2 1 9% 59% 29% 2	657
August	1st to 10th	Night	1 1 5% 63% 28% 2	597
	11th to 20th	Night	5% 1 7% 53% 32% 3	594
	21st to end	Night	4 1 8% 46% 37% 5%	660
September	1st to 10th	Night	7% 2 9% 46% 35% 3	600
	11th to 20th	Night	8% 1 8% 45% 36% 2	600
	21st to end	Night	9% 1 13% 48% 28% 1	600
October	1st to 10th	Night	5% 1 14% 51% 29% 1	600
	11th to 20th	Night	10% 5% 23% 45% 16% 1	600
	21st to end	Night	10% 9% 30% 34% 16% 1	875
November	1st to 10th	Night	12% 5% 28% 40% 15% 1	927
	11th to 20th	Night	10% 6% 33% 36% 14% 1	1050
	21st to end	Night	14% 9% 34% 32% 11% 1	1050
December	1st to 10th	Night	14% 6% 32% 39% 9%	1050
	11th to 20th	Night	16% 10% 32% 35% 7%	1050
	21st to end	Night	11% 10% 36% 35% 7%	1155

Page generated at 2026-05-14 00:58:57. Version ac07

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