Sam Rayburn
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sam Rayburn clear sky chart from 2005-11-06 to 2026-07-16.

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	13% 7% 2 6% 4 4 5% 4 5% 8% 42%	137664
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 45%	135294
Transparency	Day	39% 6% 11% 19% 18% 6%	141271
	Night	37% 6% 11% 19% 19% 8%	139767
Seeing	Day	13% 7% 21% 25% 23% 11%	17253
	Night	15% 8% 24% 32% 19% 3	27024

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	18% 9% 3 6% 4 3 4 2 4 5% 42%	5289
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 6% 52%	5310
	21st to end	Day	26% 10% 3 6% 3 3 3 2 3 5% 37%	5812
February	1st to 10th	Day	19% 9% 3 5% 5% 4 4 3 4 5% 39%	1890
	11th to 20th	Day	22% 11% 3 6% 3 3 3 2 4 4 40%	2085
	21st to end	Day	18% 9% 3 7% 4 3 4 3 4 6% 39%	1903
March	1st to 10th	Day	19% 11% 3 8% 4 4 4 2 4 5% 36%	5046
	11th to 20th	Day	13% 7% 2 6% 4 3 4 3 4 6% 48%	5090
	21st to end	Day	15% 10% 3 7% 4 4 4 3 5% 5% 41%	5583
April	1st to 10th	Day	12% 9% 3 8% 6% 5% 5% 4 5% 6% 38%	2369
	11th to 20th	Day	14% 7% 2 7% 5% 4 5% 4 6% 7% 39%	2494
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	2520
May	1st to 10th	Day	23% 8% 3 7% 4 5% 5% 3 5% 6% 32%	7021
	11th to 20th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	7506
	21st to end	Day	12% 7% 3 8% 5% 5% 6% 5% 7% 9% 32%	8392
June	1st to 10th	Day	7% 4 2 6% 6% 6% 7% 6% 8% 10% 37%	2730
	11th to 20th	Day	6% 5% 1 6% 6% 6% 7% 5% 8% 11% 39%	2730
	21st to end	Day	7% 6% 2 6% 6% 6% 6% 5% 8% 10% 37%	2730
July	1st to 10th	Day	8% 5% 2 7% 5% 6% 6% 6% 8% 12% 34%	6030
	11th to 20th	Day	6% 5% 2 7% 5% 5% 6% 5% 8% 11% 37%	5966
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 5% 6% 9% 36%	6422
August	1st to 10th	Day	2 2 1 3 3 4 4 3 6% 9% 63%	4234
	11th to 20th	Day	3 2 1 4 3 4 4 4 6% 12% 55%	3892
	21st to end	Day	9% 4 2 5% 5% 6% 6% 6% 7% 10% 40%	4312
September	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 9% 43%	2362
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	2200
	21st to end	Day	9% 4 1 5% 3 5% 4 4 6% 7% 52%	2190
October	1st to 10th	Day	7% 3 1 4 3 4 3 4 4 6% 61%	2000
	11th to 20th	Day	10% 5% 2 4 3 3 4 3 5% 6% 55%	2000
	21st to end	Day	14% 5% 2 5% 3 3 3 3 3 6% 53%	2200
November	1st to 10th	Day	11% 6% 2 5% 4 4 5% 3 4 8% 49%	2058
	11th to 20th	Day	11% 6% 2 6% 4 4 3 3 4 6% 52%	2100
	21st to end	Day	15% 8% 2 7% 4 3 3 3 4 5% 46%	2100
December	1st to 10th	Day	22% 7% 2 5% 4 3 3 3 3 5% 44%	3030
	11th to 20th	Day	15% 11% 3 7% 4 3 4 3 4 5% 40%	3481
	21st to end	Day	16% 8% 3 7% 4 3 3 2 4 5% 45%	4587
January	1st to 10th	Night	18% 7% 2 5% 3 3 3 2 3 5% 49%	7634
	11th to 20th	Night	13% 8% 2 5% 3 3 3 3 4 5% 52%	7680
	21st to end	Night	28% 10% 3 7% 3 3 3 2 4 4 33%	8402
February	1st to 10th	Night	17% 9% 3 5% 4 3 4 2 5% 6% 43%	2730
	11th to 20th	Night	21% 9% 3 4 3 3 2 3 4 5% 42%	2437
	21st to end	Night	21% 7% 2 6% 4 4 4 3 4 4 42%	1903
March	1st to 10th	Night	21% 10% 2 6% 3 3 3 2 4 6% 39%	5029
	11th to 20th	Night	15% 8% 2 5% 4 4 3 2 5% 5% 46%	5090
	21st to end	Night	16% 9% 3 7% 4 4 3 3 3 4 43%	5588
April	1st to 10th	Night	14% 6% 3 5% 5% 4 4 4 5% 7% 42%	2248
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 5% 7% 44%	2075
	21st to end	Night	13% 6% 3 6% 4 5% 4 3 5% 6% 46%	2100
May	1st to 10th	Night	22% 10% 3 8% 4 4 4 3 5% 7% 30%	5832
	11th to 20th	Night	14% 7% 3 7% 5% 5% 5% 4 6% 7% 38%	5362
	21st to end	Night	11% 8% 2 8% 5% 5% 5% 5% 7% 10% 33%	5793
June	1st to 10th	Night	6% 4 2 6% 4 4 5% 4 7% 9% 48%	1890
	11th to 20th	Night	4 3 1 4 4 4 4 4 6% 10% 56%	1890
	21st to end	Night	5% 5% 1 5% 4 4 5% 5% 8% 11% 47%	1890
July	1st to 10th	Night	5% 5% 2 7% 5% 6% 6% 5% 9% 13% 37%	4168
	11th to 20th	Night	5% 4 2 6% 5% 4 5% 5% 9% 12% 44%	4122
	21st to end	Night	9% 7% 3 7% 4 4 4 4 6% 10% 41%	4449
August	1st to 10th	Night	1 2 1 2 2 2 3 2 5% 10% 71%	2926
	11th to 20th	Night	2 2 1 3 2 3 4 4 8% 12% 58%	3156
	21st to end	Night	7% 4 2 6% 5% 5% 6% 5% 7% 9% 43%	3586
September	1st to 10th	Night	6% 5% 2 6% 5% 4 4 4 6% 10% 48%	2032
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 4 6% 9% 46%	2200
	21st to end	Night	8% 4 2 4 3 3 4 4 5% 7% 56%	2200
October	1st to 10th	Night	5% 3 2 4 3 4 4 3 5% 7% 61%	2388
	11th to 20th	Night	10% 4 1 5% 3 3 3 3 4 5% 60%	2400
	21st to end	Night	12% 5% 1 4 3 2 2 3 4 5% 59%	2640
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 6% 54%	2460
	11th to 20th	Night	11% 5% 2 5% 4 3 3 3 4 5% 54%	2520
	21st to end	Night	15% 7% 2 5% 3 4 4 3 5% 7% 46%	2520
December	1st to 10th	Night	21% 9% 3 6% 4 3 3 3 3 5% 40%	4375
	11th to 20th	Night	15% 10% 2 7% 3 3 4 3 4 4 43%	4946
	21st to end	Night	15% 9% 2 5% 3 3 3 3 4 5% 49%	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	44% 4 6% 23% 20% 3	5290
	11th to 20th	Day	35% 3 5% 17% 30% 10%	5310
	21st to end	Day	53% 3 3 13% 17% 12%	5812
February	1st to 10th	Day	48% 4 5% 14% 18% 10%	1890
	11th to 20th	Day	49% 4 7% 15% 16% 9%	2095
	21st to end	Day	47% 5% 7% 13% 17% 11%	1903
March	1st to 10th	Day	50% 4 7% 10% 18% 11%	5046
	11th to 20th	Day	37% 6% 6% 14% 21% 16%	5090
	21st to end	Day	44% 5% 9% 20% 17% 5%	5585
April	1st to 10th	Day	46% 7% 7% 17% 16% 8%	2369
	11th to 20th	Day	42% 8% 10% 14% 17% 9%	2494
	21st to end	Day	40% 7% 10% 17% 21% 5%	2520
May	1st to 10th	Day	51% 7% 9% 20% 11% 3	7056
	11th to 20th	Day	41% 7% 14% 17% 19% 2	7510
	21st to end	Day	44% 8% 15% 20% 11% 2	8405
June	1st to 10th	Day	38% 10% 17% 20% 15% 1	2730
	11th to 20th	Day	36% 10% 15% 23% 14% 2	2730
	21st to end	Day	38% 11% 17% 23% 10% 1	2730
July	1st to 10th	Day	38% 11% 27% 20% 4	5966
	11th to 20th	Day	37% 9% 21% 27% 6%	5897
	21st to end	Day	42% 9% 15% 25% 8%	6240
August	1st to 10th	Day	18% 6% 16% 34% 25% 1	4220
	11th to 20th	Day	21% 8% 25% 38% 7% 1	3928
	21st to end	Day	38% 10% 16% 18% 16% 1	4311
September	1st to 10th	Day	36% 9% 14% 18% 15% 7%	2362
	11th to 20th	Day	41% 7% 12% 18% 16% 6%	2200
	21st to end	Day	31% 6% 11% 21% 19% 11%	2190
October	1st to 10th	Day	17% 3 7% 13% 25% 35%	3260
	11th to 20th	Day	19% 3 5% 20% 31% 21%	3240
	21st to end	Day	27% 4 4 12% 40% 13%	3538
November	1st to 10th	Day	34% 6% 9% 18% 22% 11%	2058
	11th to 20th	Day	34% 4 7% 17% 25% 12%	2100
	21st to end	Day	42% 4 7% 17% 21% 8%	2100
December	1st to 10th	Day	44% 3 6% 17% 25% 5%	3030
	11th to 20th	Day	45% 5% 7% 11% 23% 9%	3479
	21st to end	Day	42% 3 6% 15% 28% 6%	4587
January	1st to 10th	Night	39% 3 7% 23% 23% 6%	7635
	11th to 20th	Night	36% 3 4 17% 29% 11%	7680
	21st to end	Night	56% 3 4 14% 14% 9%	8402
February	1st to 10th	Night	44% 3 6% 15% 18% 12%	2730
	11th to 20th	Night	46% 4 7% 15% 19% 9%	2437
	21st to end	Night	46% 5% 7% 12% 16% 14%	1903
March	1st to 10th	Night	47% 4 6% 12% 19% 11%	5029
	11th to 20th	Night	40% 5% 7% 14% 16% 17%	5090
	21st to end	Night	45% 5% 8% 15% 21% 7%	5588
April	1st to 10th	Night	41% 6% 9% 17% 16% 10%	2248
	11th to 20th	Night	37% 7% 11% 18% 16% 11%	2075
	21st to end	Night	39% 5% 11% 17% 23% 5%	2100
May	1st to 10th	Night	52% 8% 10% 16% 12% 2	5862
	11th to 20th	Night	44% 7% 11% 18% 18% 3	5363
	21st to end	Night	42% 9% 19% 20% 9% 1	5805
June	1st to 10th	Night	30% 9% 18% 26% 17% 1	1890
	11th to 20th	Night	24% 8% 18% 32% 16% 3	1890
	21st to end	Night	28% 10% 20% 29% 12% 1	1890
July	1st to 10th	Night	34% 11% 32% 19% 3	4125
	11th to 20th	Night	28% 11% 29% 26% 5%	4071
	21st to end	Night	39% 8% 18% 27% 8%	4320
August	1st to 10th	Night	11% 5% 19% 34% 30% 1	2916
	11th to 20th	Night	16% 7% 29% 39% 8% 1	3186
	21st to end	Night	34% 12% 17% 19% 16% 2	3586
September	1st to 10th	Night	31% 10% 17% 19% 15% 7%	2032
	11th to 20th	Night	35% 8% 14% 20% 16% 7%	2200
	21st to end	Night	26% 6% 13% 23% 20% 13%	2200
October	1st to 10th	Night	15% 4 7% 17% 20% 38%	3908
	11th to 20th	Night	16% 3 4 22% 30% 25%	3899
	21st to end	Night	23% 3 5% 13% 38% 17%	4252
November	1st to 10th	Night	32% 4 11% 18% 23% 12%	2460
	11th to 20th	Night	32% 5% 9% 18% 24% 13%	2520
	21st to end	Night	39% 4 8% 17% 22% 10%	2520
December	1st to 10th	Night	48% 3 5% 16% 23% 5%	4375
	11th to 20th	Night	44% 4 7% 12% 23% 11%	4947
	21st to end	Night	38% 4 7% 15% 28% 7%	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	25% 11% 38% 21% 5% 1	330
	11th to 20th	Day	22% 15% 37% 21% 5% 1	330
	21st to end	Day	25% 20% 34% 18% 2	363
February	1st to 10th	Day	22% 18% 35% 21% 4 1	330
	11th to 20th	Day	31% 15% 32% 18% 3	330
	21st to end	Day	23% 21% 44% 10% 3	273
March	1st to 10th	Day	26% 17% 28% 22% 7%	330
	11th to 20th	Day	16% 21% 38% 20% 4 1	330
	21st to end	Day	17% 14% 37% 24% 7% 1	363
April	1st to 10th	Day	13% 19% 35% 25% 7%	389
	11th to 20th	Day	21% 15% 29% 25% 8% 1	532
	21st to end	Day	18% 11% 30% 28% 10% 2	540
May	1st to 10th	Day	20% 9% 31% 27% 13%	543
	11th to 20th	Day	12% 8% 27% 33% 17% 4	730
	21st to end	Day	9% 5% 26% 35% 21% 5%	844
June	1st to 10th	Day	8% 3 13% 33% 30% 12%	780
	11th to 20th	Day	8% 2 14% 36% 31% 9%	780
	21st to end	Day	8% 3 12% 25% 34% 18%	780
July	1st to 10th	Day	8% 2 8% 23% 38% 22%	771
	11th to 20th	Day	6% 1 5% 25% 40% 23%	744
	21st to end	Day	8% 1 4 21% 43% 22%	794
August	1st to 10th	Day	4 4 21% 44% 27%	726
	11th to 20th	Day	5% 1 2 13% 45% 34%	550
	21st to end	Day	12% 2 5% 20% 33% 28%	583
September	1st to 10th	Day	7% 2 6% 22% 39% 24%	492
	11th to 20th	Day	10% 1 6% 25% 38% 20%	330
	21st to end	Day	12% 2 10% 29% 29% 17%	330
October	1st to 10th	Day	5% 1 17% 38% 32% 7%	330
	11th to 20th	Day	9% 5% 21% 34% 22% 9%	330
	21st to end	Day	20% 11% 21% 24% 18% 6%	363
November	1st to 10th	Day	15% 6% 28% 26% 20% 5%	330
	11th to 20th	Day	13% 9% 32% 32% 13% 1	330
	21st to end	Day	17% 16% 34% 23% 8% 2	330
December	1st to 10th	Day	22% 9% 34% 24% 7% 4	330
	11th to 20th	Day	21% 12% 37% 24% 7% 1	330
	21st to end	Day	20% 13% 37% 23% 6% 1	363
January	1st to 10th	Night	22% 13% 33% 27% 5%	1050
	11th to 20th	Night	22% 14% 31% 25% 8%	1050
	21st to end	Night	25% 12% 31% 29% 3	1155
February	1st to 10th	Night	21% 14% 35% 26% 4	1050
	11th to 20th	Night	26% 13% 34% 23% 4	802
	21st to end	Night	23% 17% 37% 21% 2	519
March	1st to 10th	Night	25% 13% 32% 25% 6%	630
	11th to 20th	Night	19% 12% 36% 27% 5% 1	630
	21st to end	Night	18% 9% 37% 29% 7%	693
April	1st to 10th	Night	16% 10% 40% 28% 7%	630
	11th to 20th	Night	17% 11% 34% 28% 10%	623
	21st to end	Night	16% 10% 35% 27% 11%	630
May	1st to 10th	Night	15% 11% 34% 31% 9%	630
	11th to 20th	Night	14% 6% 27% 36% 15% 1	621
	21st to end	Night	7% 4 25% 38% 22% 3	683
June	1st to 10th	Night	5% 3 15% 38% 34% 4	630
	11th to 20th	Night	5% 2 17% 44% 29% 3	630
	21st to end	Night	6% 2 10% 38% 37% 7%	630
July	1st to 10th	Night	4 1 5% 38% 42% 9%	615
	11th to 20th	Night	4 1 7% 37% 44% 8%	597
	21st to end	Night	5% 1 6% 38% 43% 7%	657
August	1st to 10th	Night	3 1 4 39% 46% 9%	597
	11th to 20th	Night	6% 1 3 35% 49% 7%	594
	21st to end	Night	8% 1 6% 30% 43% 11%	660
September	1st to 10th	Night	6% 1 8% 32% 44% 9%	600
	11th to 20th	Night	12% 1 9% 28% 41% 9%	600
	21st to end	Night	10% 2 11% 32% 37% 9%	600
October	1st to 10th	Night	6% 2 12% 38% 38% 5%	788
	11th to 20th	Night	11% 5% 22% 39% 20% 4	800
	21st to end	Night	15% 6% 25% 33% 19% 2	880
November	1st to 10th	Night	13% 5% 24% 35% 20% 2	820
	11th to 20th	Night	12% 11% 25% 39% 13%	840
	21st to end	Night	18% 10% 31% 28% 11% 1	840
December	1st to 10th	Night	25% 10% 26% 29% 9%	1045
	11th to 20th	Night	25% 9% 30% 30% 6%	1050
	21st to end	Night	23% 12% 29% 30% 6%	1155

Page generated at 2026-07-13 02:14:49. Version ac07

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