Lyndon B. Johnson National Grasslands
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lyndon B. Johnson National Grasslands clear sky chart from 2005-11-06 to 2025-08-05.

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 5% 3 3 3 3 5% 7% 54%	125732
	Night	9% 5% 2 4 3 3 3 3 4 6% 58%	122613
Transparency	Day	30% 4 9% 24% 24% 9%	129462
	Night	27% 4 8% 23% 26% 12%	126946
Seeing	Day	11% 8% 24% 29% 21% 6%	16229
	Night	11% 8% 31% 35% 14% 1	26066

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	12% 7% 2 5% 3 3 3 2 3 5% 54%	4077
	11th to 20th	Day	6% 4 1 4 3 2 2 2 4 6% 67%	4080
	21st to end	Day	18% 7% 2 5% 3 2 2 2 3 4 51%	4488
February	1st to 10th	Day	14% 7% 2 5% 3 3 3 2 4 6% 51%	1965
	11th to 20th	Day	14% 6% 2 5% 3 5% 3 3 4 5% 50%	1949
	21st to end	Day	17% 7% 3 4 2 2 3 2 3 4 54%	1650
March	1st to 10th	Day	14% 5% 1 4 3 3 3 3 4 6% 54%	4884
	11th to 20th	Day	10% 4 2 5% 3 3 3 3 4 5% 59%	4980
	21st to end	Day	10% 6% 2 5% 3 3 4 4 6% 7% 50%	5462
April	1st to 10th	Day	11% 5% 2 5% 4 3 3 3 4 5% 54%	2200
	11th to 20th	Day	12% 5% 2 7% 3 4 3 3 5% 6% 50%	2276
	21st to end	Day	12% 5% 2 5% 4 4 4 4 5% 6% 50%	2600
May	1st to 10th	Day	15% 7% 2 6% 4 3 4 4 5% 7% 43%	5862
	11th to 20th	Day	12% 6% 2 6% 3 4 4 3 5% 7% 48%	5862
	21st to end	Day	13% 7% 3 7% 5% 5% 4 4 6% 7% 40%	6434
June	1st to 10th	Day	6% 4 2 5% 4 4 4 5% 7% 8% 51%	2600
	11th to 20th	Day	5% 3 1 4 4 4 5% 4 7% 8% 54%	2600
	21st to end	Day	5% 4 2 3 4 4 4 3 6% 9% 57%	2600
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 5% 8% 44%	5900
	11th to 20th	Day	4 3 1 4 3 4 4 3 6% 9% 60%	5900
	21st to end	Day	3 3 1 4 3 4 4 4 6% 9% 59%	6422
August	1st to 10th	Day	2 2 1 3 3 4 4 3 5% 8% 65%	4157
	11th to 20th	Day	4 2 1 3 3 4 4 4 6% 9% 59%	4041
	21st to end	Day	3 2 1 4 3 4 4 5% 6% 9% 60%	4040
September	1st to 10th	Day	9% 4 2 5% 4 4 4 3 4 7% 55%	2090
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 4 6% 6% 48%	2090
	21st to end	Day	7% 3 1 5% 3 3 3 3 5% 7% 59%	2090
October	1st to 10th	Day	5% 3 1 4 3 3 3 3 5% 6% 63%	2080
	11th to 20th	Day	10% 3 2 4 2 2 2 2 4 5% 65%	1900
	21st to end	Day	13% 5% 1 3 2 2 3 2 2 3 63%	2090
November	1st to 10th	Day	12% 5% 2 3 3 3 3 2 3 5% 59%	1944
	11th to 20th	Day	6% 4 1 3 2 3 3 2 4 6% 66%	1800
	21st to end	Day	13% 5% 2 4 2 3 3 3 4 4 58%	1800
December	1st to 10th	Day	13% 8% 2 5% 3 3 2 3 4 5% 53%	2940
	11th to 20th	Day	13% 7% 2 6% 5% 3 3 3 3 4 51%	3391
	21st to end	Day	18% 7% 2 5% 2 3 3 2 3 5% 52%	4488
January	1st to 10th	Night	13% 4 2 4 3 3 3 2 4 5% 56%	5888
	11th to 20th	Night	6% 4 1 3 2 2 2 2 3 5% 72%	5900
	21st to end	Night	18% 8% 3 5% 3 2 2 2 3 4 50%	6490
February	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 5% 55%	2450
	11th to 20th	Night	14% 6% 2 4 3 3 4 3 4 6% 52%	2338
	21st to end	Night	14% 6% 2 4 3 3 2 3 3 4 57%	1980
March	1st to 10th	Night	12% 6% 2 5% 3 3 4 3 5% 6% 51%	4971
	11th to 20th	Night	8% 4 1 4 3 3 3 2 4 7% 61%	4980
	21st to end	Night	10% 6% 2 6% 4 3 4 3 4 5% 53%	5467
April	1st to 10th	Night	9% 5% 2 4 3 3 4 4 6% 7% 54%	2200
	11th to 20th	Night	10% 5% 3 5% 3 3 3 3 4 7% 54%	2067
	21st to end	Night	11% 4 1 5% 3 3 3 3 5% 8% 54%	1800
May	1st to 10th	Night	15% 8% 2 5% 4 3 4 3 4 6% 46%	4053
	11th to 20th	Night	8% 5% 2 6% 3 4 4 4 5% 7% 52%	4053
	21st to end	Night	9% 5% 2 5% 4 3 4 4 5% 8% 50%	4443
June	1st to 10th	Night	3 3 1 4 3 3 4 4 7% 10% 57%	1800
	11th to 20th	Night	3 2 1 3 2 3 4 4 5% 10% 63%	1800
	21st to end	Night	2 2 1 3 2 3 2 3 6% 8% 67%	1800
July	1st to 10th	Night	7% 5% 2 6% 4 4 5% 4 6% 10% 48%	4078
	11th to 20th	Night	2 1 1 3 2 2 3 2 5% 9% 68%	4077
	21st to end	Night	2 2 1 3 3 3 3 3 5% 8% 67%	4449
August	1st to 10th	Night	2 1 2 2 2 3 2 5% 8% 73%	2872
	11th to 20th	Night	3 2 1 3 2 2 2 3 5% 10% 69%	2798
	21st to end	Night	2 2 1 3 2 3 3 3 5% 9% 67%	3615
September	1st to 10th	Night	6% 4 2 6% 4 3 3 3 4 7% 58%	2090
	11th to 20th	Night	9% 6% 2 6% 4 5% 3 3 5% 7% 52%	2090
	21st to end	Night	6% 2 1 2 2 3 3 4 5% 6% 64%	2090
October	1st to 10th	Night	5% 3 1 3 3 3 3 3 4 8% 65%	2090
	11th to 20th	Night	9% 3 2 3 2 2 2 2 3 4 66%	2270
	21st to end	Night	13% 4 1 3 2 2 2 2 3 4 64%	2508
November	1st to 10th	Night	10% 3 1 4 3 2 3 2 3 5% 63%	2350
	11th to 20th	Night	5% 4 1 4 2 3 3 3 5% 5% 65%	2600
	21st to end	Night	14% 5% 2 4 3 3 3 3 4 5% 55%	2600
December	1st to 10th	Night	11% 6% 2 4 2 3 3 3 4 5% 57%	4250
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 3 4 54%	4816
	21st to end	Night	14% 7% 2 5% 2 2 3 2 3 5% 55%	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	35% 3 6% 26% 26% 5%	4078
	11th to 20th	Day	22% 1 5% 21% 40% 10%	4080
	21st to end	Day	40% 2 3 18% 23% 14%	4488
February	1st to 10th	Day	36% 4 5% 20% 24% 11%	1965
	11th to 20th	Day	38% 3 7% 21% 22% 9%	1959
	21st to end	Day	37% 2 3 14% 28% 16%	1650
March	1st to 10th	Day	31% 3 5% 15% 24% 21%	4884
	11th to 20th	Day	29% 4 8% 18% 19% 22%	4980
	21st to end	Day	33% 5% 10% 23% 22% 7%	5464
April	1st to 10th	Day	34% 4 8% 23% 20% 12%	2200
	11th to 20th	Day	35% 5% 9% 20% 20% 12%	2276
	21st to end	Day	35% 4 9% 24% 24% 5%	2600
May	1st to 10th	Day	40% 5% 10% 23% 16% 7%	5900
	11th to 20th	Day	36% 4 9% 21% 25% 5%	5866
	21st to end	Day	41% 6% 11% 22% 14% 6%	6447
June	1st to 10th	Day	28% 6% 17% 29% 17% 3	2600
	11th to 20th	Day	25% 8% 12% 29% 22% 5%	2600
	21st to end	Day	24% 6% 12% 27% 26% 5%	2600
July	1st to 10th	Day	39% 5% 11% 31% 13% 2	5836
	11th to 20th	Day	21% 4 14% 34% 25% 3	5831
	21st to end	Day	21% 5% 13% 34% 26% 2	6240
August	1st to 10th	Day	17% 5% 10% 26% 34% 8%	4143
	11th to 20th	Day	20% 4 13% 37% 23% 3	4080
	21st to end	Day	20% 5% 15% 38% 18% 3	4039
September	1st to 10th	Day	30% 5% 9% 21% 25% 11%	2090
	11th to 20th	Day	35% 7% 9% 19% 23% 7%	2090
	21st to end	Day	25% 5% 7% 23% 28% 12%	2090
October	1st to 10th	Day	14% 3 6% 14% 36% 27%	3457
	11th to 20th	Day	17% 2 5% 21% 40% 16%	3140
	21st to end	Day	22% 2 4 30% 30% 12%	3428
November	1st to 10th	Day	30% 3 7% 21% 27% 12%	1944
	11th to 20th	Day	21% 4 9% 22% 31% 14%	1800
	21st to end	Day	31% 3 5% 22% 28% 12%	1800
December	1st to 10th	Day	36% 3 4 29% 23% 5%	2940
	11th to 20th	Day	39% 3 4 12% 29% 13%	3389
	21st to end	Day	38% 3 3 25% 22% 9%	4488
January	1st to 10th	Night	31% 3 6% 27% 27% 7%	5888
	11th to 20th	Night	19% 2 6% 19% 39% 16%	5900
	21st to end	Night	41% 2 4 14% 25% 14%	6490
February	1st to 10th	Night	33% 4 6% 20% 24% 14%	2450
	11th to 20th	Night	35% 4 7% 18% 25% 11%	2338
	21st to end	Night	33% 3 5% 17% 23% 19%	1980
March	1st to 10th	Night	34% 4 4 14% 24% 19%	4971
	11th to 20th	Night	25% 4 6% 20% 19% 27%	4980
	21st to end	Night	34% 4 7% 18% 31% 6%	5467
April	1st to 10th	Night	28% 5% 11% 19% 21% 15%	2200
	11th to 20th	Night	30% 5% 9% 20% 22% 14%	2067
	21st to end	Night	29% 5% 12% 24% 25% 6%	1800
May	1st to 10th	Night	40% 5% 9% 23% 17% 6%	4080
	11th to 20th	Night	32% 4 11% 20% 24% 9%	4054
	21st to end	Night	32% 6% 13% 26% 13% 9%	4452
June	1st to 10th	Night	21% 7% 15% 33% 19% 5%	1800
	11th to 20th	Night	18% 6% 13% 32% 24% 7%	1800
	21st to end	Night	15% 5% 14% 28% 30% 7%	1800
July	1st to 10th	Night	32% 5% 13% 33% 15% 2	4035
	11th to 20th	Night	15% 4 12% 38% 28% 5%	4026
	21st to end	Night	16% 4 14% 37% 27% 2	4320
August	1st to 10th	Night	11% 3 8% 29% 41% 7%	2862
	11th to 20th	Night	13% 3 13% 36% 28% 7%	2825
	21st to end	Night	16% 4 19% 36% 21% 4	3615
September	1st to 10th	Night	26% 5% 8% 22% 27% 12%	2090
	11th to 20th	Night	32% 5% 10% 19% 22% 11%	2090
	21st to end	Night	20% 4 8% 22% 29% 17%	2090
October	1st to 10th	Night	12% 3 6% 14% 33% 33%	3480
	11th to 20th	Night	16% 2 6% 18% 34% 24%	3769
	21st to end	Night	23% 2 4 26% 31% 14%	4120
November	1st to 10th	Night	26% 4 7% 20% 27% 17%	2350
	11th to 20th	Night	21% 4 7% 22% 30% 16%	2600
	21st to end	Night	32% 3 5% 17% 28% 13%	2600
December	1st to 10th	Night	30% 3 5% 26% 27% 9%	4250
	11th to 20th	Night	35% 2 5% 14% 28% 16%	4817
	21st to end	Night	35% 2 5% 21% 27% 10%	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	17% 13% 38% 25% 6%	300
	11th to 20th	Day	15% 17% 35% 25% 6% 2	300
	21st to end	Day	20% 13% 38% 21% 8% 1	330
February	1st to 10th	Day	20% 14% 38% 23% 5%	300
	11th to 20th	Day	21% 16% 42% 16% 4 1	300
	21st to end	Day	24% 16% 41% 16% 4	249
March	1st to 10th	Day	23% 17% 33% 21% 6%	300
	11th to 20th	Day	19% 20% 41% 15% 5%	300
	21st to end	Day	15% 18% 45% 17% 4 1	330
April	1st to 10th	Day	17% 17% 36% 23% 7% 1	300
	11th to 20th	Day	14% 17% 44% 20% 6%	394
	21st to end	Day	15% 16% 38% 22% 7% 1	700
May	1st to 10th	Day	13% 11% 37% 30% 8% 1	703
	11th to 20th	Day	16% 9% 31% 30% 13% 2	725
	21st to end	Day	13% 9% 31% 30% 14% 4	789
June	1st to 10th	Day	6% 4 19% 41% 25% 6%	730
	11th to 20th	Day	7% 2 18% 42% 25% 5%	730
	21st to end	Day	4 2 12% 39% 34% 8%	730
July	1st to 10th	Day	6% 2 8% 37% 35% 13%	721
	11th to 20th	Day	3 1 7% 33% 44% 12%	718
	21st to end	Day	2 1 8% 31% 43% 15%	794
August	1st to 10th	Day	3 2 6% 35% 41% 13%	697
	11th to 20th	Day	7% 2 6% 34% 40% 11%	665
	21st to end	Day	4 1 5% 34% 36% 20%	464
September	1st to 10th	Day	9% 1 6% 23% 43% 19%	300
	11th to 20th	Day	4 2 9% 29% 38% 19%	300
	21st to end	Day	12% 2 10% 30% 33% 13%	300
October	1st to 10th	Day	3 2 21% 32% 31% 11%	300
	11th to 20th	Day	9% 8% 31% 32% 18% 1	300
	21st to end	Day	22% 9% 25% 27% 15% 3	330
November	1st to 10th	Day	17% 7% 34% 25% 13% 3	300
	11th to 20th	Day	12% 10% 30% 33% 10% 5%	300
	21st to end	Day	17% 13% 38% 21% 10% 2	300
December	1st to 10th	Day	14% 11% 37% 26% 11% 2	300
	11th to 20th	Day	15% 11% 41% 26% 7%	300
	21st to end	Day	20% 12% 37% 23% 8%	330
January	1st to 10th	Night	13% 10% 38% 32% 8%	1000
	11th to 20th	Night	13% 14% 35% 32% 6%	1000
	21st to end	Night	14% 11% 36% 33% 7%	1100
February	1st to 10th	Night	14% 12% 41% 28% 4	850
	11th to 20th	Night	18% 13% 40% 27% 3	784
	21st to end	Night	16% 17% 42% 23% 2	660
March	1st to 10th	Night	15% 15% 37% 29% 4	628
	11th to 20th	Night	15% 12% 42% 28% 4	600
	21st to end	Night	12% 13% 43% 28% 4	660
April	1st to 10th	Night	12% 13% 45% 26% 4	600
	11th to 20th	Night	11% 15% 43% 26% 5%	593
	21st to end	Night	12% 12% 42% 28% 6% 1	600
May	1st to 10th	Night	10% 11% 42% 31% 6% 1	600
	11th to 20th	Night	9% 7% 40% 31% 11% 1	591
	21st to end	Night	8% 4 35% 38% 13% 2	650
June	1st to 10th	Night	4 3 23% 48% 20% 3	600
	11th to 20th	Night	3 2 28% 53% 15%	600
	21st to end	Night	2 1 19% 49% 27% 3	600
July	1st to 10th	Night	4 2 11% 48% 31% 4	585
	11th to 20th	Night	2 1 11% 48% 35% 3	582
	21st to end	Night	2 9% 47% 40% 3	657
August	1st to 10th	Night	2 1 8% 50% 35% 3	579
	11th to 20th	Night	7% 1 7% 46% 35% 4	564
	21st to end	Night	3 6% 46% 39% 5%	627
September	1st to 10th	Night	6% 1 11% 40% 39% 3	570
	11th to 20th	Night	10% 1 12% 43% 31% 3	570
	21st to end	Night	7% 1 16% 45% 29% 2	570
October	1st to 10th	Night	7% 2 22% 40% 26% 2	570
	11th to 20th	Night	11% 6% 29% 39% 14% 1	750
	21st to end	Night	15% 11% 29% 32% 12% 1	836
November	1st to 10th	Night	11% 4 34% 36% 14% 1	790
	11th to 20th	Night	8% 10% 35% 35% 11% 1	1000
	21st to end	Night	16% 10% 37% 27% 9%	1000
December	1st to 10th	Night	13% 10% 36% 36% 6%	1000
	11th to 20th	Night	17% 10% 36% 31% 6%	1000
	21st to end	Night	16% 11% 39% 28% 6%	1100

Page generated at 2025-08-02 00:32:29. Version ac07

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