Lyndon B. Johnson State Park & Historic Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lyndon B. Johnson State Park & Historic Site 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	10% 5% 2 6% 4 4 4 4 5% 7% 49%	132314
	Night	10% 5% 2 5% 3 3 4 3 5% 7% 52%	130844
Transparency	Day	34% 5% 9% 21% 20% 11%	136119
	Night	31% 5% 10% 20% 20% 13%	135103
Seeing	Day	12% 9% 25% 31% 19% 5%	16805
	Night	12% 8% 29% 36% 14% 1	26822

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	10% 6% 3 5% 3 4 3 3 4 5% 54%	5289
	11th to 20th	Day	12% 6% 2 4 3 2 2 3 4 5% 56%	5310
	21st to end	Day	20% 7% 2 7% 5% 3 2 2 3 6% 43%	6348
February	1st to 10th	Day	14% 8% 2 5% 3 4 3 2 4 6% 48%	2100
	11th to 20th	Day	14% 7% 2 5% 3 3 4 2 5% 6% 48%	2049
	21st to end	Day	15% 6% 1 4 3 2 3 3 5% 5% 52%	1730
March	1st to 10th	Day	10% 6% 2 7% 3 3 3 2 5% 6% 54%	4910
	11th to 20th	Day	12% 7% 2 6% 4 3 3 3 3 4 53%	5090
	21st to end	Day	11% 6% 2 6% 4 4 4 3 5% 6% 49%	5583
April	1st to 10th	Day	12% 7% 3 7% 4 4 3 3 5% 6% 45%	2310
	11th to 20th	Day	17% 7% 2 6% 4 4 4 3 5% 5% 43%	2351
	21st to end	Day	13% 6% 2 6% 4 5% 5% 3 4 6% 46%	2644
May	1st to 10th	Day	16% 6% 3 6% 5% 5% 5% 5% 5% 8% 36%	5992
	11th to 20th	Day	10% 5% 2 6% 4 4 4 3 6% 9% 47%	5941
	21st to end	Day	13% 6% 2 7% 5% 5% 6% 5% 6% 9% 36%	6434
June	1st to 10th	Day	5% 4 2 5% 5% 4 5% 6% 7% 8% 49%	2600
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 9% 50%	2600
	21st to end	Day	6% 4 2 6% 5% 5% 5% 5% 9% 9% 43%	2600
July	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 6% 10% 38%	5900
	11th to 20th	Day	4 3 1 4 4 5% 6% 5% 8% 12% 49%	5900
	21st to end	Day	6% 4 1 5% 4 5% 6% 5% 7% 10% 47%	6422
August	1st to 10th	Day	1 1 2 2 3 3 2 5% 8% 71%	4122
	11th to 20th	Day	4 2 1 4 3 3 4 4 6% 9% 60%	3846
	21st to end	Day	3 3 1 6% 5% 5% 6% 5% 7% 11% 48%	4190
September	1st to 10th	Day	11% 6% 3 6% 5% 4 4 4 5% 6% 47%	2200
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 4 6% 8% 42%	2200
	21st to end	Day	8% 4 1 5% 4 4 4 4 6% 7% 51%	2200
October	1st to 10th	Day	6% 3 1 4 3 4 4 3 5% 7% 58%	2200
	11th to 20th	Day	11% 5% 2 4 3 4 3 4 4 5% 56%	2100
	21st to end	Day	10% 4 2 4 4 3 3 3 4 5% 59%	2200
November	1st to 10th	Day	13% 5% 2 6% 4 3 3 3 5% 6% 49%	2058
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 4 7% 53%	1907
	21st to end	Day	15% 7% 2 6% 4 3 3 3 3 6% 48%	1890
December	1st to 10th	Day	21% 8% 3 7% 3 3 2 2 4 5% 42%	3030
	11th to 20th	Day	10% 7% 3 7% 5% 4 4 4 5% 6% 45%	3481
	21st to end	Day	10% 5% 2 4 3 3 3 2 3 4 60%	4587
January	1st to 10th	Night	11% 5% 3 5% 3 3 3 2 4 5% 57%	7634
	11th to 20th	Night	9% 3 2 3 2 2 2 2 5% 5% 65%	7680
	21st to end	Night	22% 8% 3 6% 4 3 3 2 4 5% 41%	7910
February	1st to 10th	Night	13% 5% 3 6% 3 2 3 3 5% 5% 51%	2520
	11th to 20th	Night	15% 6% 2 4 3 3 3 3 5% 6% 50%	2458
	21st to end	Night	14% 7% 2 5% 5% 3 3 3 4 5% 50%	2076
March	1st to 10th	Night	9% 6% 2 7% 4 3 4 3 6% 8% 47%	5170
	11th to 20th	Night	11% 7% 2 5% 3 3 3 3 5% 5% 53%	5090
	21st to end	Night	11% 8% 2 6% 4 4 4 4 5% 6% 48%	5588
April	1st to 10th	Night	9% 7% 2 5% 5% 4 4 3 6% 7% 48%	2310
	11th to 20th	Night	13% 7% 3 6% 4 5% 4 3 4 6% 46%	2213
	21st to end	Night	14% 8% 2 5% 5% 3 4 3 4 6% 47%	1974
May	1st to 10th	Night	19% 10% 3 5% 3 4 3 3 5% 6% 40%	4143
	11th to 20th	Night	7% 5% 1 6% 4 4 5% 4 6% 8% 50%	4107
	21st to end	Night	12% 6% 2 6% 4 5% 5% 4 6% 10% 40%	4443
June	1st to 10th	Night	3 3 2 4 4 3 4 3 5% 9% 61%	1800
	11th to 20th	Night	4 3 1 4 3 3 4 3 6% 9% 61%	1800
	21st to end	Night	6% 4 1 4 3 3 4 5% 6% 9% 56%	1800
July	1st to 10th	Night	8% 6% 2 7% 4 4 4 4 7% 10% 45%	4078
	11th to 20th	Night	2 2 1 3 3 4 5% 5% 7% 11% 57%	4077
	21st to end	Night	3 4 2 5% 4 5% 5% 5% 7% 11% 49%	4449
August	1st to 10th	Night	1 1 1 2 1 2 3 2 4 7% 75%	3018
	11th to 20th	Night	3 2 1 3 2 2 4 3 7% 10% 63%	3213
	21st to end	Night	4 3 1 4 3 4 4 4 6% 11% 56%	3715
September	1st to 10th	Night	8% 5% 2 5% 4 4 5% 4 5% 7% 52%	2200
	11th to 20th	Night	8% 6% 2 6% 5% 5% 4 4 6% 8% 47%	2200
	21st to end	Night	7% 4 1 5% 4 4 4 4 6% 8% 54%	2200
October	1st to 10th	Night	5% 3 1 3 3 2 3 3 5% 7% 65%	2200
	11th to 20th	Night	10% 4 2 4 3 3 4 3 5% 6% 57%	2280
	21st to end	Night	12% 4 2 4 3 3 2 2 3 4 62%	2640
November	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 4 6% 56%	2460
	11th to 20th	Night	8% 6% 2 3 3 3 3 2 5% 6% 58%	2709
	21st to end	Night	14% 5% 2 5% 4 4 3 3 5% 6% 49%	2730
December	1st to 10th	Night	18% 8% 3 6% 3 3 3 3 4 5% 43%	4380
	11th to 20th	Night	11% 8% 3 7% 4 4 4 3 4 5% 48%	4946
	21st to end	Night	11% 6% 2 4 3 2 3 2 3 5% 59%	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	34% 2 5% 22% 26% 11%	5290
	11th to 20th	Day	32% 2 4 15% 26% 21%	5310
	21st to end	Day	45% 3 4 13% 24% 12%	6348
February	1st to 10th	Day	39% 4 5% 12% 21% 19%	2100
	11th to 20th	Day	38% 4 7% 19% 19% 13%	2059
	21st to end	Day	35% 3 5% 17% 23% 16%	1730
March	1st to 10th	Day	33% 4 5% 17% 18% 25%	4910
	11th to 20th	Day	34% 6% 4 8% 14% 33%	5090
	21st to end	Day	35% 5% 8% 19% 20% 12%	5585
April	1st to 10th	Day	40% 5% 8% 17% 17% 13%	2310
	11th to 20th	Day	43% 5% 7% 16% 18% 11%	2351
	21st to end	Day	40% 5% 7% 20% 17% 10%	2644
May	1st to 10th	Day	44% 7% 8% 19% 17% 5%	6030
	11th to 20th	Day	35% 6% 11% 16% 22% 10%	5945
	21st to end	Day	42% 7% 9% 25% 13% 3	6447
June	1st to 10th	Day	29% 8% 13% 27% 20% 3	2600
	11th to 20th	Day	30% 7% 11% 27% 20% 5%	2600
	21st to end	Day	34% 9% 13% 24% 17% 4	2600
July	1st to 10th	Day	41% 6% 19% 29% 6% 1	5836
	11th to 20th	Day	26% 6% 18% 33% 15% 2	5831
	21st to end	Day	29% 8% 17% 27% 18% 1	6240
August	1st to 10th	Day	13% 4 11% 40% 31% 1	4108
	11th to 20th	Day	20% 5% 22% 39% 11% 2	3882
	21st to end	Day	28% 8% 19% 24% 18% 2	4189
September	1st to 10th	Day	37% 7% 9% 19% 18% 9%	2200
	11th to 20th	Day	37% 8% 11% 20% 18% 5%	2200
	21st to end	Day	31% 6% 9% 22% 18% 13%	2200
October	1st to 10th	Day	16% 4 7% 12% 26% 36%	3590
	11th to 20th	Day	22% 3 8% 20% 25% 22%	3405
	21st to end	Day	24% 3 5% 17% 36% 14%	3538
November	1st to 10th	Day	36% 5% 7% 17% 21% 12%	2058
	11th to 20th	Day	33% 4 7% 18% 21% 17%	1907
	21st to end	Day	39% 4 5% 17% 21% 14%	1890
December	1st to 10th	Day	48% 3 5% 16% 18% 10%	3030
	11th to 20th	Day	39% 5% 7% 12% 22% 15%	3479
	21st to end	Day	29% 3 4 16% 28% 20%	4587
January	1st to 10th	Night	32% 3 5% 18% 27% 15%	7635
	11th to 20th	Night	23% 1 4 18% 30% 24%	7680
	21st to end	Night	46% 3 3 10% 22% 15%	7910
February	1st to 10th	Night	35% 4 6% 13% 23% 19%	2520
	11th to 20th	Night	36% 4 7% 18% 19% 16%	2458
	21st to end	Night	38% 4 6% 13% 22% 18%	2076
March	1st to 10th	Night	35% 5% 6% 19% 20% 16%	5170
	11th to 20th	Night	34% 6% 6% 10% 15% 28%	5090
	21st to end	Night	35% 6% 7% 19% 20% 13%	5588
April	1st to 10th	Night	35% 5% 10% 18% 16% 15%	2310
	11th to 20th	Night	40% 5% 8% 14% 17% 15%	2213
	21st to end	Night	40% 5% 9% 19% 17% 9%	1974
May	1st to 10th	Night	45% 6% 8% 17% 15% 9%	4170
	11th to 20th	Night	31% 7% 12% 19% 18% 13%	4108
	21st to end	Night	38% 7% 13% 28% 10% 3	4452
June	1st to 10th	Night	20% 8% 15% 33% 20% 3	1800
	11th to 20th	Night	20% 6% 15% 30% 23% 6%	1800
	21st to end	Night	24% 8% 18% 29% 18% 4	1800
July	1st to 10th	Night	35% 8% 25% 28% 4 1	4035
	11th to 20th	Night	21% 7% 24% 35% 13% 1	4026
	21st to end	Night	27% 8% 22% 27% 13% 3	4320
August	1st to 10th	Night	10% 4 14% 43% 28% 1	3008
	11th to 20th	Night	17% 6% 31% 33% 11% 2	3243
	21st to end	Night	22% 10% 22% 25% 18% 2	3715
September	1st to 10th	Night	31% 7% 15% 21% 18% 8%	2200
	11th to 20th	Night	33% 8% 13% 24% 16% 5%	2200
	21st to end	Night	28% 7% 12% 22% 19% 13%	2200
October	1st to 10th	Night	13% 4 8% 15% 23% 37%	3590
	11th to 20th	Night	20% 4 9% 18% 29% 20%	3701
	21st to end	Night	22% 3 4 20% 34% 16%	4252
November	1st to 10th	Night	31% 5% 8% 19% 24% 14%	2460
	11th to 20th	Night	28% 4 8% 21% 21% 18%	2709
	21st to end	Night	36% 4 8% 17% 19% 15%	2730
December	1st to 10th	Night	45% 3 7% 16% 17% 12%	4380
	11th to 20th	Night	40% 3 5% 12% 21% 18%	4947
	21st to end	Night	30% 3 4 17% 28% 18%	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	20% 14% 40% 21% 5% 1	330
	11th to 20th	Day	19% 16% 35% 21% 7% 1	330
	21st to end	Day	18% 17% 40% 19% 4 1	363
February	1st to 10th	Day	25% 13% 33% 25% 4	330
	11th to 20th	Day	21% 21% 38% 18% 1	330
	21st to end	Day	22% 21% 39% 16% 2	273
March	1st to 10th	Day	19% 20% 37% 22% 2	330
	11th to 20th	Day	17% 19% 38% 20% 5%	330
	21st to end	Day	15% 17% 42% 19% 6% 2	363
April	1st to 10th	Day	18% 19% 40% 18% 4	330
	11th to 20th	Day	19% 22% 35% 20% 3	389
	21st to end	Day	16% 15% 42% 22% 5%	664
May	1st to 10th	Day	14% 15% 41% 24% 6%	753
	11th to 20th	Day	17% 11% 32% 29% 9% 2	756
	21st to end	Day	15% 6% 29% 36% 13% 1	789
June	1st to 10th	Day	6% 3 22% 43% 22% 4	730
	11th to 20th	Day	7% 2 19% 49% 19% 4	730
	21st to end	Day	6% 2 15% 39% 31% 7%	730
July	1st to 10th	Day	10% 2 13% 36% 32% 9%	721
	11th to 20th	Day	4 1 7% 41% 40% 7%	718
	21st to end	Day	3 1 8% 38% 39% 10%	794
August	1st to 10th	Day	2 4 40% 41% 12%	666
	11th to 20th	Day	6% 7% 36% 39% 13%	517
	21st to end	Day	4 1 6% 29% 42% 19%	513
September	1st to 10th	Day	11% 1 10% 27% 35% 16%	330
	11th to 20th	Day	5% 2 5% 32% 43% 14%	330
	21st to end	Day	9% 1 8% 34% 35% 13%	330
October	1st to 10th	Day	6% 12% 39% 32% 11%	330
	11th to 20th	Day	9% 4 26% 37% 19% 4	330
	21st to end	Day	15% 15% 25% 28% 15% 2	363
November	1st to 10th	Day	17% 8% 28% 33% 11% 4	330
	11th to 20th	Day	16% 10% 31% 28% 12% 2	330
	21st to end	Day	20% 18% 33% 18% 9% 2	330
December	1st to 10th	Day	18% 11% 35% 27% 7% 2	330
	11th to 20th	Day	15% 18% 34% 23% 8% 2	330
	21st to end	Day	15% 13% 37% 29% 5% 1	363
January	1st to 10th	Night	16% 11% 35% 31% 7%	1050
	11th to 20th	Night	16% 12% 36% 31% 5%	1050
	21st to end	Night	19% 12% 34% 29% 6%	984
February	1st to 10th	Night	18% 12% 38% 27% 5%	840
	11th to 20th	Night	19% 15% 40% 24% 3	824
	21st to end	Night	19% 16% 45% 20% 1	692
March	1st to 10th	Night	17% 15% 39% 27% 2	698
	11th to 20th	Night	17% 12% 38% 30% 4	630
	21st to end	Night	13% 15% 43% 26% 4	693
April	1st to 10th	Night	10% 15% 45% 27% 3	630
	11th to 20th	Night	15% 15% 41% 25% 2	623
	21st to end	Night	16% 12% 44% 22% 6%	630
May	1st to 10th	Night	13% 14% 42% 28% 4	630
	11th to 20th	Night	11% 9% 39% 31% 10% 1	609
	21st to end	Night	12% 7% 35% 35% 10% 1	650
June	1st to 10th	Night	3 3 28% 46% 20% 1	600
	11th to 20th	Night	5% 2 34% 45% 13% 1	600
	21st to end	Night	5% 2 18% 52% 20% 3	600
July	1st to 10th	Night	9% 3 11% 50% 25% 2	585
	11th to 20th	Night	4 1 13% 53% 29% 2	582
	21st to end	Night	3 1 12% 53% 29% 3	657
August	1st to 10th	Night	1 1 8% 56% 31% 3	597
	11th to 20th	Night	6% 1 7% 53% 31% 3	594
	21st to end	Night	5% 2 7% 44% 37% 5%	660
September	1st to 10th	Night	9% 1 9% 44% 32% 6%	600
	11th to 20th	Night	9% 2 10% 41% 35% 4	600
	21st to end	Night	8% 1 15% 43% 31% 2	600
October	1st to 10th	Night	6% 1 14% 47% 29% 3	600
	11th to 20th	Night	11% 5% 22% 43% 17% 2	680
	21st to end	Night	13% 9% 29% 29% 19% 1	880
November	1st to 10th	Night	11% 6% 25% 40% 16% 1	820
	11th to 20th	Night	11% 8% 29% 37% 14% 1	1029
	21st to end	Night	16% 11% 33% 30% 10%	1050
December	1st to 10th	Night	17% 9% 31% 35% 8% 1	1050
	11th to 20th	Night	18% 11% 32% 33% 6%	1050
	21st to end	Night	15% 11% 34% 31% 8%	1155

Page generated at 2026-05-14 00:58:54. Version ac07

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