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 2025-03-12.

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% 5% 4 5% 4 5% 7% 42%	109280
	Night	14% 7% 2 5% 4 4 4 3 5% 7% 46%	109908
Transparency	Day	40% 6% 11% 19% 18% 7%	112926
	Night	37% 6% 11% 19% 20% 8%	114402
Seeing	Day	13% 7% 20% 25% 23% 11%	15371
	Night	15% 8% 24% 32% 19% 3	25339

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% 10% 3 6% 4 3 3 2 4 5% 42%	4077
	11th to 20th	Day	16% 6% 2 5% 4 3 3 2 4 5% 51%	4080
	21st to end	Day	25% 10% 2 5% 3 2 2 2 3 5% 39%	4488
February	1st to 10th	Day	19% 9% 3 5% 5% 4 4 3 4 5% 38%	1800
	11th to 20th	Day	22% 10% 2 5% 3 3 3 2 3 4 40%	1983
	21st to end	Day	19% 9% 3 7% 4 4 4 3 4 6% 38%	1815
March	1st to 10th	Day	20% 12% 4 9% 5% 4 4 3 4 5% 31%	3546
	11th to 20th	Day	18% 9% 3 7% 4 4 5% 4 5% 8% 33%	3491
	21st to end	Day	16% 11% 3 7% 5% 3 3 3 5% 5% 40%	3814
April	1st to 10th	Day	12% 9% 3 7% 5% 5% 4 4 5% 6% 40%	2143
	11th to 20th	Day	15% 7% 2 6% 5% 4 5% 4 6% 7% 39%	2254
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 6% 7% 44%	2280
May	1st to 10th	Day	19% 8% 3 7% 5% 5% 5% 3 6% 7% 30%	3765
	11th to 20th	Day	13% 6% 2 7% 4 4 4 3 5% 7% 43%	4015
	21st to end	Day	10% 7% 2 7% 6% 6% 7% 5% 7% 9% 34%	4477
June	1st to 10th	Day	7% 4 2 6% 6% 6% 6% 6% 8% 10% 39%	2470
	11th to 20th	Day	5% 5% 1 5% 6% 6% 6% 5% 9% 11% 41%	2470
	21st to end	Day	7% 6% 2 7% 6% 7% 6% 5% 9% 10% 35%	2470
July	1st to 10th	Day	9% 5% 2 7% 6% 6% 6% 6% 8% 12% 33%	4120
	11th to 20th	Day	7% 7% 3 8% 6% 6% 7% 6% 9% 10% 32%	4120
	21st to end	Day	16% 9% 3 7% 6% 5% 5% 5% 6% 9% 29%	4465
August	1st to 10th	Day	2 2 1 3 3 3 4 3 6% 9% 63%	4104
	11th to 20th	Day	3 2 1 4 3 4 4 4 6% 12% 55%	3771
	21st to end	Day	8% 4 2 5% 5% 6% 6% 6% 7% 10% 40%	4180
September	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 8% 43%	2244
	11th to 20th	Day	13% 6% 2 6% 5% 5% 6% 4 6% 8% 40%	2090
	21st to end	Day	10% 4 1 5% 3 5% 4 4 6% 7% 52%	2081
October	1st to 10th	Day	7% 3 1 4 3 4 3 4 4 6% 60%	1900
	11th to 20th	Day	11% 5% 2 4 3 3 4 3 5% 6% 54%	1900
	21st to end	Day	15% 5% 2 5% 3 3 3 3 3 6% 52%	2090
November	1st to 10th	Day	11% 7% 2 5% 4 4 5% 3 4 8% 48%	1958
	11th to 20th	Day	11% 6% 2 6% 4 3 3 3 4 6% 52%	2000
	21st to end	Day	15% 7% 3 7% 4 3 3 3 4 5% 46%	2000
December	1st to 10th	Day	22% 7% 2 5% 4 3 3 3 3 5% 45%	2940
	11th to 20th	Day	15% 11% 3 7% 4 3 4 3 4 5% 40%	3391
	21st to end	Day	16% 8% 3 7% 4 3 3 2 4 5% 46%	4488
January	1st to 10th	Night	17% 7% 2 5% 3 2 3 2 3 5% 51%	5888
	11th to 20th	Night	16% 9% 2 5% 2 2 3 2 3 4 51%	5900
	21st to end	Night	27% 11% 3 6% 3 3 2 2 3 4 36%	6490
February	1st to 10th	Night	18% 9% 3 5% 4 3 4 3 5% 6% 41%	2600
	11th to 20th	Night	21% 10% 3 4 3 3 2 3 4 4 42%	2318
	21st to end	Night	21% 8% 2 6% 4 4 4 3 4 4 41%	1815
March	1st to 10th	Night	23% 12% 3 6% 4 4 3 3 5% 6% 32%	3529
	11th to 20th	Night	20% 9% 3 6% 5% 5% 4 3 5% 6% 33%	3497
	21st to end	Night	13% 8% 3 6% 4 4 4 4 4 6% 43%	3817
April	1st to 10th	Night	13% 6% 2 5% 5% 4 4 4 5% 8% 44%	2034
	11th to 20th	Night	15% 7% 2 6% 4 4 3 3 5% 7% 44%	1875
	21st to end	Night	12% 5% 3 6% 3 5% 4 3 4 6% 49%	1900
May	1st to 10th	Night	19% 9% 3 8% 4 4 5% 4 5% 8% 33%	3130
	11th to 20th	Night	15% 7% 2 7% 4 4 4 3 4 7% 43%	2870
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 5% 8% 10% 38%	3090
June	1st to 10th	Night	5% 4 2 6% 4 4 5% 4 7% 9% 50%	1710
	11th to 20th	Night	3 3 1 3 4 4 4 4 6% 10% 58%	1710
	21st to end	Night	6% 5% 1 5% 4 4 5% 5% 8% 11% 46%	1710
July	1st to 10th	Night	7% 5% 2 6% 5% 5% 6% 5% 8% 12% 39%	2850
	11th to 20th	Night	4 5% 2 6% 5% 5% 6% 5% 10% 12% 40%	2850
	21st to end	Night	12% 8% 3 8% 5% 4 4 4 6% 10% 34%	3098
August	1st to 10th	Night	1 2 1 2 2 2 3 2 5% 10% 70%	2836
	11th to 20th	Night	2 2 1 3 2 3 4 4 8% 12% 58%	3058
	21st to end	Night	7% 4 2 6% 5% 5% 6% 5% 7% 9% 44%	3476
September	1st to 10th	Night	6% 5% 2 6% 5% 4 4 4 7% 10% 48%	1930
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 4 5% 8% 46%	2090
	21st to end	Night	8% 4 2 4 3 3 4 4 5% 7% 56%	2090
October	1st to 10th	Night	5% 3 2 4 3 4 4 4 5% 7% 60%	2268
	11th to 20th	Night	10% 4 1 5% 3 3 3 3 4 5% 58%	2280
	21st to end	Night	13% 5% 1 4 3 2 2 3 4 4 59%	2508
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 6% 53%	2340
	11th to 20th	Night	11% 5% 2 5% 5% 3 3 3 4 5% 54%	2400
	21st to end	Night	15% 7% 2 5% 3 4 4 3 5% 7% 46%	2400
December	1st to 10th	Night	20% 9% 3 6% 4 4 3 3 4 5% 41%	4245
	11th to 20th	Night	15% 10% 2 6% 4 3 4 3 4 5% 43%	4816
	21st to end	Night	15% 9% 2 5% 3 3 3 3 4 5% 49%	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	44% 4 4 24% 20% 4	4078
	11th to 20th	Day	37% 3 4 17% 28% 11%	4080
	21st to end	Day	50% 3 3 14% 17% 13%	4488
February	1st to 10th	Day	50% 4 6% 13% 18% 10%	1800
	11th to 20th	Day	49% 4 6% 16% 16% 9%	1993
	21st to end	Day	49% 5% 7% 13% 16% 11%	1815
March	1st to 10th	Day	55% 5% 8% 11% 16% 5%	3546
	11th to 20th	Day	48% 7% 8% 17% 14% 5%	3491
	21st to end	Day	46% 5% 8% 16% 22% 3	3814
April	1st to 10th	Day	45% 7% 7% 17% 17% 8%	2143
	11th to 20th	Day	43% 7% 10% 14% 16% 10%	2254
	21st to end	Day	38% 7% 9% 18% 23% 5%	2280
May	1st to 10th	Day	50% 9% 11% 17% 9% 4	3800
	11th to 20th	Day	39% 7% 10% 17% 24% 2	4019
	21st to end	Day	42% 9% 14% 24% 10% 2	4489
June	1st to 10th	Day	37% 9% 16% 21% 15% 1	2470
	11th to 20th	Day	34% 11% 15% 24% 15% 2	2470
	21st to end	Day	40% 11% 17% 20% 11% 1	2470
July	1st to 10th	Day	40% 12% 24% 20% 4	4056
	11th to 20th	Day	42% 10% 17% 23% 8%	4051
	21st to end	Day	50% 9% 15% 19% 6%	4325
August	1st to 10th	Day	18% 6% 15% 34% 26% 1	4090
	11th to 20th	Day	22% 8% 25% 38% 8% 1	3807
	21st to end	Day	38% 11% 16% 18% 16% 1	4179
September	1st to 10th	Day	37% 9% 14% 18% 14% 7%	2244
	11th to 20th	Day	42% 7% 12% 16% 16% 6%	2090
	21st to end	Day	31% 6% 11% 21% 19% 12%	2081
October	1st to 10th	Day	18% 4 7% 13% 24% 36%	3160
	11th to 20th	Day	19% 4 5% 20% 31% 21%	3140
	21st to end	Day	27% 4 4 12% 40% 13%	3428
November	1st to 10th	Day	36% 6% 9% 17% 21% 10%	1958
	11th to 20th	Day	35% 4 7% 16% 26% 13%	2000
	21st to end	Day	42% 4 7% 17% 21% 9%	2000
December	1st to 10th	Day	44% 3 7% 16% 25% 5%	2940
	11th to 20th	Day	45% 5% 7% 11% 22% 9%	3389
	21st to end	Day	42% 3 6% 15% 28% 6%	4488
January	1st to 10th	Night	37% 3 6% 24% 24% 5%	5888
	11th to 20th	Night	39% 3 4 17% 30% 8%	5900
	21st to end	Night	53% 3 3 13% 15% 11%	6490
February	1st to 10th	Night	45% 3 6% 15% 17% 13%	2600
	11th to 20th	Night	46% 4 6% 15% 19% 10%	2318
	21st to end	Night	47% 5% 7% 12% 15% 14%	1815
March	1st to 10th	Night	53% 6% 8% 12% 15% 7%	3529
	11th to 20th	Night	50% 7% 9% 13% 15% 6%	3497
	21st to end	Night	42% 6% 10% 14% 24% 5%	3817
April	1st to 10th	Night	39% 6% 9% 18% 16% 10%	2034
	11th to 20th	Night	38% 7% 10% 17% 16% 12%	1875
	21st to end	Night	37% 5% 10% 18% 25% 6%	1900
May	1st to 10th	Night	48% 10% 11% 16% 12% 4	3160
	11th to 20th	Night	41% 6% 10% 18% 23% 3	2871
	21st to end	Night	35% 11% 19% 23% 11% 1	3099
June	1st to 10th	Night	28% 9% 18% 27% 17% 1	1710
	11th to 20th	Night	22% 8% 17% 32% 17% 4	1710
	21st to end	Night	29% 10% 20% 28% 13% 1	1710
July	1st to 10th	Night	34% 12% 31% 18% 4	2805
	11th to 20th	Night	30% 13% 25% 24% 7%	2796
	21st to end	Night	44% 9% 21% 19% 6%	2999
August	1st to 10th	Night	11% 5% 19% 34% 30% 1	2826
	11th to 20th	Night	16% 7% 29% 38% 8% 1	3088
	21st to end	Night	34% 12% 17% 20% 16% 2	3476
September	1st to 10th	Night	31% 10% 17% 20% 15% 7%	1930
	11th to 20th	Night	36% 8% 14% 19% 16% 7%	2090
	21st to end	Night	27% 6% 12% 22% 20% 13%	2090
October	1st to 10th	Night	15% 4 7% 16% 19% 38%	3788
	11th to 20th	Night	17% 3 4 21% 30% 25%	3779
	21st to end	Night	24% 3 5% 13% 38% 17%	4120
November	1st to 10th	Night	33% 4 11% 18% 22% 11%	2340
	11th to 20th	Night	32% 5% 9% 17% 25% 13%	2400
	21st to end	Night	38% 4 8% 17% 22% 10%	2400
December	1st to 10th	Night	47% 3 5% 16% 23% 5%	4245
	11th to 20th	Night	44% 4 7% 12% 22% 11%	4817
	21st to end	Night	37% 4 7% 15% 29% 7%	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	25% 11% 39% 19% 5% 1	300
	11th to 20th	Day	23% 14% 37% 21% 5% 1	300
	21st to end	Day	24% 19% 35% 18% 3	330
February	1st to 10th	Day	23% 18% 33% 21% 4 1	300
	11th to 20th	Day	32% 14% 33% 18% 2	300
	21st to end	Day	25% 20% 43% 10% 3	249
March	1st to 10th	Day	27% 17% 28% 20% 8%	300
	11th to 20th	Day	15% 21% 38% 21% 4 1	273
	21st to end	Day	19% 16% 38% 21% 6%	297
April	1st to 10th	Day	12% 19% 37% 25% 7%	323
	11th to 20th	Day	23% 14% 26% 26% 10% 2	452
	21st to end	Day	15% 12% 30% 29% 11% 3	460
May	1st to 10th	Day	18% 9% 30% 28% 14%	463
	11th to 20th	Day	11% 6% 25% 35% 19% 4	634
	21st to end	Day	8% 5% 26% 34% 21% 6%	734
June	1st to 10th	Day	8% 3 12% 32% 32% 13%	680
	11th to 20th	Day	5% 2 14% 36% 33% 10%	680
	21st to end	Day	9% 3 12% 25% 34% 18%	680
July	1st to 10th	Day	8% 2 8% 24% 38% 21%	671
	11th to 20th	Day	5% 1 5% 25% 40% 25%	668
	21st to end	Day	8% 1 5% 22% 42% 22%	739
August	1st to 10th	Day	4 4 20% 44% 28%	676
	11th to 20th	Day	6% 1 3 13% 46% 32%	509
	21st to end	Day	11% 2 5% 19% 33% 29%	539
September	1st to 10th	Day	8% 2 5% 20% 39% 25%	454
	11th to 20th	Day	11% 1 7% 27% 36% 18%	300
	21st to end	Day	13% 2 9% 27% 31% 18%	300
October	1st to 10th	Day	6% 1 18% 37% 32% 7%	300
	11th to 20th	Day	9% 5% 22% 34% 22% 8%	300
	21st to end	Day	21% 10% 21% 23% 17% 7%	330
November	1st to 10th	Day	15% 6% 28% 27% 19% 5%	300
	11th to 20th	Day	14% 9% 33% 30% 13% 1	300
	21st to end	Day	18% 15% 34% 23% 9% 2	300
December	1st to 10th	Day	21% 8% 34% 25% 8% 4	300
	11th to 20th	Day	21% 12% 37% 25% 5%	300
	21st to end	Day	20% 13% 38% 22% 6% 1	330
January	1st to 10th	Night	22% 13% 33% 27% 5%	1000
	11th to 20th	Night	23% 13% 31% 25% 8%	1000
	21st to end	Night	24% 12% 32% 29% 3	1100
February	1st to 10th	Night	22% 14% 35% 25% 4	1000
	11th to 20th	Night	26% 14% 33% 23% 4	763
	21st to end	Night	23% 16% 37% 21% 2	495
March	1st to 10th	Night	24% 12% 31% 26% 6%	600
	11th to 20th	Night	18% 11% 37% 28% 5% 1	575
	21st to end	Night	18% 9% 38% 28% 7%	627
April	1st to 10th	Night	14% 10% 41% 28% 7%	570
	11th to 20th	Night	17% 11% 32% 29% 11%	563
	21st to end	Night	14% 9% 35% 29% 12% 1	570
May	1st to 10th	Night	14% 11% 34% 32% 9% 1	570
	11th to 20th	Night	14% 4 27% 38% 15% 1	561
	21st to end	Night	7% 4 25% 37% 23% 4	617
June	1st to 10th	Night	5% 2 13% 39% 36% 4	570
	11th to 20th	Night	4 1 17% 45% 29% 4	570
	21st to end	Night	6% 2 10% 37% 38% 7%	570
July	1st to 10th	Night	5% 2 5% 39% 41% 9%	555
	11th to 20th	Night	3 1 8% 38% 42% 8%	552
	21st to end	Night	5% 1 6% 38% 44% 7%	624
August	1st to 10th	Night	3 1 4 40% 45% 8%	567
	11th to 20th	Night	6% 1 3 37% 48% 6%	564
	21st to end	Night	8% 1 6% 31% 44% 11%	627
September	1st to 10th	Night	6% 1 7% 32% 45% 9%	570
	11th to 20th	Night	13% 1 9% 27% 40% 9%	570
	21st to end	Night	10% 2 11% 31% 37% 9%	570
October	1st to 10th	Night	6% 2 12% 38% 38% 5%	748
	11th to 20th	Night	11% 5% 22% 39% 19% 3	760
	21st to end	Night	14% 6% 25% 33% 20% 2	836
November	1st to 10th	Night	13% 6% 25% 34% 20% 2	780
	11th to 20th	Night	12% 11% 25% 38% 13%	800
	21st to end	Night	18% 11% 32% 28% 11% 1	800
December	1st to 10th	Night	24% 10% 26% 30% 10%	995
	11th to 20th	Night	26% 9% 30% 29% 6%	1000
	21st to end	Night	22% 11% 30% 30% 6%	1100

Page generated at 2025-03-09 07:08:45. Version ac07

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