Brenham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brenham clear sky chart from 2005-11-06 to 2026-07-31.

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	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	138194
	Night	14% 6% 2 5% 4 3 4 3 5% 7% 47%	135125
Transparency	Day	38% 6% 11% 20% 17% 8%	141898
	Night	36% 6% 11% 20% 18% 10%	139488
Seeing	Day	13% 7% 20% 27% 24% 10%	17194
	Night	14% 7% 25% 34% 18% 2	27010

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	16% 8% 3 6% 3 3 3 3 4 5% 45%	5289
	11th to 20th	Day	13% 6% 2 5% 3 4 3 3 4 5% 51%	5310
	21st to end	Day	27% 9% 3 7% 4 3 2 2 3 6% 33%	5812
February	1st to 10th	Day	21% 9% 3 6% 3 3 3 3 3 5% 41%	2078
	11th to 20th	Day	20% 8% 2 6% 4 4 4 2 3 6% 40%	2049
	21st to end	Day	17% 7% 3 6% 4 3 3 3 5% 4 44%	1835
March	1st to 10th	Day	13% 6% 3 8% 5% 4 5% 3 5% 7% 41%	5046
	11th to 20th	Day	13% 6% 2 6% 4 4 3 3 4 6% 49%	5090
	21st to end	Day	17% 7% 3 7% 4 4 4 3 4 6% 41%	5583
April	1st to 10th	Day	15% 9% 3 7% 6% 4 5% 4 4 7% 37%	2310
	11th to 20th	Day	16% 7% 3 7% 4 5% 4 4 5% 6% 38%	2376
	21st to end	Day	14% 8% 3 6% 4 5% 5% 3 5% 7% 41%	2520
May	1st to 10th	Day	19% 8% 2 7% 4 5% 5% 4 6% 7% 34%	7557
	11th to 20th	Day	10% 6% 3 7% 4 5% 5% 4 6% 8% 42%	7627
	21st to end	Day	11% 7% 3 7% 6% 6% 6% 5% 8% 9% 33%	8392
June	1st to 10th	Day	5% 4 2 6% 5% 6% 6% 5% 7% 10% 43%	2730
	11th to 20th	Day	7% 3 2 5% 5% 5% 6% 6% 7% 10% 44%	2730
	21st to end	Day	5% 5% 2 7% 7% 7% 7% 6% 8% 11% 36%	2730
July	1st to 10th	Day	8% 5% 3 7% 5% 5% 5% 5% 8% 11% 38%	6030
	11th to 20th	Day	6% 5% 2 6% 5% 6% 6% 6% 9% 13% 37%	6030
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 12% 38%	6553
August	1st to 10th	Day	2 1 1 3 3 3 4 3 6% 11% 62%	4234
	11th to 20th	Day	3 2 1 4 3 4 5% 6% 10% 14% 49%	4143
	21st to end	Day	6% 4 2 6% 6% 6% 6% 5% 7% 10% 42%	4016
September	1st to 10th	Day	6% 5% 2 6% 6% 6% 6% 5% 7% 9% 42%	2200
	11th to 20th	Day	10% 7% 3 7% 5% 6% 6% 5% 6% 7% 40%	2200
	21st to end	Day	6% 4 2 6% 5% 6% 5% 5% 6% 7% 49%	2200
October	1st to 10th	Day	5% 4 1 4 4 4 4 3 3 8% 59%	2144
	11th to 20th	Day	11% 6% 2 5% 4 3 4 3 4 5% 54%	2000
	21st to end	Day	12% 6% 1 5% 3 3 3 3 4 6% 55%	2200
November	1st to 10th	Day	12% 6% 1 5% 4 4 5% 3 5% 6% 48%	2058
	11th to 20th	Day	11% 5% 2 5% 4 3 3 4 5% 6% 53%	2100
	21st to end	Day	16% 6% 3 7% 4 4 4 4 5% 5% 43%	1924
December	1st to 10th	Day	24% 7% 2 6% 4 3 3 3 4 6% 38%	3030
	11th to 20th	Day	18% 7% 2 6% 4 3 3 3 4 6% 43%	3481
	21st to end	Day	15% 7% 2 5% 3 3 4 3 4 5% 48%	4587
January	1st to 10th	Night	18% 7% 2 5% 3 3 3 2 3 5% 49%	7634
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 4 5% 57%	7680
	21st to end	Night	29% 8% 2 6% 3 3 3 2 4 5% 35%	8402
February	1st to 10th	Night	19% 9% 3 6% 3 3 3 3 3 7% 42%	2559
	11th to 20th	Night	21% 8% 2 5% 4 2 2 2 3 5% 45%	2458
	21st to end	Night	21% 6% 2 7% 3 4 3 3 5% 5% 43%	1969
March	1st to 10th	Night	18% 9% 2 6% 4 3 3 3 5% 6% 40%	5029
	11th to 20th	Night	16% 7% 2 6% 4 3 3 3 5% 5% 47%	5090
	21st to end	Night	18% 10% 3 6% 4 3 3 3 3 5% 42%	5588
April	1st to 10th	Night	12% 7% 2 5% 4 5% 4 4 5% 7% 45%	2310
	11th to 20th	Night	15% 7% 2 6% 4 3 4 3 6% 6% 43%	2191
	21st to end	Night	15% 8% 2 5% 5% 4 4 3 4 6% 44%	2100
May	1st to 10th	Night	24% 10% 3 7% 4 4 4 3 5% 7% 30%	5293
	11th to 20th	Night	12% 7% 3 7% 4 4 5% 4 6% 7% 41%	5265
	21st to end	Night	12% 8% 3 7% 4 5% 5% 4 6% 10% 35%	5793
June	1st to 10th	Night	5% 4 2 5% 3 4 4 3 7% 10% 54%	1890
	11th to 20th	Night	5% 4 1 4 3 4 4 4 6% 11% 55%	1890
	21st to end	Night	4 3 2 5% 4 4 5% 5% 7% 11% 48%	1890
July	1st to 10th	Night	7% 5% 2 6% 4 4 6% 5% 7% 11% 43%	4168
	11th to 20th	Night	4 4 2 5% 4 4 4 4 7% 11% 51%	4167
	21st to end	Night	7% 5% 3 6% 4 4 5% 4 6% 10% 46%	4539
August	1st to 10th	Night	1 1 2 2 2 3 3 5% 11% 71%	2926
	11th to 20th	Night	2 2 1 3 2 3 4 4 7% 12% 60%	2912
	21st to end	Night	3 3 1 5% 4 5% 5% 4 8% 11% 51%	3876
September	1st to 10th	Night	5% 4 2 6% 3 4 5% 5% 6% 10% 50%	2200
	11th to 20th	Night	9% 6% 2 6% 5% 4 4 4 6% 7% 47%	2200
	21st to end	Night	7% 4 2 4 3 3 4 3 6% 8% 55%	2200
October	1st to 10th	Night	4 3 1 4 3 3 3 3 5% 7% 64%	2235
	11th to 20th	Night	10% 4 1 4 3 3 3 3 4 5% 61%	2400
	21st to end	Night	11% 5% 1 4 3 3 3 2 3 5% 60%	2640
November	1st to 10th	Night	11% 6% 2 5% 4 3 3 2 4 5% 54%	2460
	11th to 20th	Night	11% 5% 2 5% 4 4 3 3 4 7% 53%	2520
	21st to end	Night	16% 6% 2 6% 4 3 3 2 4 7% 47%	2692
December	1st to 10th	Night	22% 10% 3 7% 4 3 3 3 4 5% 37%	4380
	11th to 20th	Night	17% 9% 2 6% 3 3 3 3 4 5% 45%	4946
	21st to end	Night	16% 6% 2 4 3 3 3 2 4 5% 53%	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	42% 4 7% 22% 18% 7%	5290
	11th to 20th	Day	36% 3 4 18% 24% 14%	5310
	21st to end	Day	55% 2 4 11% 17% 11%	5812
February	1st to 10th	Day	48% 3 5% 12% 19% 13%	2078
	11th to 20th	Day	47% 5% 8% 16% 16% 9%	2059
	21st to end	Day	43% 4 8% 15% 18% 11%	1835
March	1st to 10th	Day	42% 6% 7% 16% 17% 13%	5046
	11th to 20th	Day	38% 4 6% 10% 18% 23%	5090
	21st to end	Day	44% 6% 7% 19% 18% 6%	5585
April	1st to 10th	Day	46% 6% 7% 16% 14% 10%	2310
	11th to 20th	Day	44% 7% 8% 14% 18% 9%	2376
	21st to end	Day	42% 6% 7% 19% 19% 6%	2520
May	1st to 10th	Day	48% 7% 10% 22% 11% 3	7595
	11th to 20th	Day	38% 7% 12% 20% 20% 3	7631
	21st to end	Day	43% 8% 13% 25% 10% 1	8405
June	1st to 10th	Day	33% 8% 14% 26% 16% 2	2730
	11th to 20th	Day	32% 9% 14% 25% 18% 3	2730
	21st to end	Day	39% 9% 14% 25% 11% 1	2730
July	1st to 10th	Day	37% 9% 26% 25% 3	5966
	11th to 20th	Day	34% 11% 22% 25% 8%	5961
	21st to end	Day	37% 9% 17% 25% 10% 1	6371
August	1st to 10th	Day	16% 7% 13% 40% 23%	4220
	11th to 20th	Day	22% 8% 31% 31% 8% 1	4182
	21st to end	Day	33% 12% 18% 23% 13% 1	4015
September	1st to 10th	Day	34% 12% 13% 20% 15% 6%	2200
	11th to 20th	Day	39% 9% 12% 18% 18% 4	2200
	21st to end	Day	33% 7% 9% 21% 17% 13%	2200
October	1st to 10th	Day	17% 3 6% 14% 22% 39%	3495
	11th to 20th	Day	21% 3 7% 16% 35% 17%	3240
	21st to end	Day	26% 4 4 20% 31% 15%	3538
November	1st to 10th	Day	35% 6% 9% 18% 22% 11%	2058
	11th to 20th	Day	33% 6% 9% 17% 22% 14%	2100
	21st to end	Day	43% 5% 8% 15% 18% 11%	1924
December	1st to 10th	Day	49% 4 6% 15% 20% 6%	3030
	11th to 20th	Day	42% 5% 9% 11% 23% 10%	3479
	21st to end	Day	39% 4 5% 15% 26% 12%	4587
January	1st to 10th	Night	39% 3 8% 19% 19% 11%	7635
	11th to 20th	Night	31% 4 4 19% 27% 16%	7680
	21st to end	Night	53% 3 3 12% 18% 11%	8402
February	1st to 10th	Night	45% 4 6% 11% 19% 15%	2559
	11th to 20th	Night	44% 4 6% 17% 17% 12%	2458
	21st to end	Night	45% 5% 6% 14% 16% 14%	1969
March	1st to 10th	Night	45% 5% 7% 15% 18% 11%	5029
	11th to 20th	Night	40% 5% 6% 14% 14% 21%	5090
	21st to end	Night	45% 5% 6% 16% 19% 9%	5588
April	1st to 10th	Night	39% 6% 9% 19% 16% 11%	2310
	11th to 20th	Night	40% 5% 10% 16% 15% 14%	2191
	21st to end	Night	41% 5% 10% 19% 17% 7%	2100
May	1st to 10th	Night	54% 7% 8% 18% 10% 3	5320
	11th to 20th	Night	40% 6% 11% 21% 17% 5%	5266
	21st to end	Night	43% 9% 15% 26% 7% 1	5805
June	1st to 10th	Night	24% 8% 16% 32% 18% 2	1890
	11th to 20th	Night	23% 8% 18% 29% 17% 4	1890
	21st to end	Night	28% 9% 20% 28% 14% 1	1890
July	1st to 10th	Night	34% 10% 30% 24% 3	4125
	11th to 20th	Night	27% 8% 25% 32% 9%	4116
	21st to end	Night	32% 9% 20% 27% 11% 1	4410
August	1st to 10th	Night	9% 5% 16% 45% 24% 1	2916
	11th to 20th	Night	16% 7% 34% 34% 8% 1	2939
	21st to end	Night	25% 12% 21% 29% 12% 1	3876
September	1st to 10th	Night	29% 10% 19% 21% 14% 7%	2200
	11th to 20th	Night	35% 8% 13% 23% 15% 6%	2200
	21st to end	Night	26% 6% 15% 22% 18% 13%	2200
October	1st to 10th	Night	14% 4 7% 17% 19% 40%	3651
	11th to 20th	Night	16% 3 6% 21% 33% 21%	3899
	21st to end	Night	22% 3 4 22% 31% 18%	4252
November	1st to 10th	Night	32% 6% 9% 19% 22% 12%	2460
	11th to 20th	Night	33% 4 9% 18% 21% 15%	2520
	21st to end	Night	39% 4 8% 17% 19% 12%	2692
December	1st to 10th	Night	51% 4 5% 15% 19% 8%	4380
	11th to 20th	Night	42% 4 9% 9% 21% 15%	4947
	21st to end	Night	36% 4 4 16% 29% 11%	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	24% 8% 38% 23% 5% 2	330
	11th to 20th	Day	25% 11% 31% 25% 7% 1	330
	21st to end	Day	25% 18% 30% 20% 7%	363
February	1st to 10th	Day	28% 14% 28% 22% 7%	330
	11th to 20th	Day	29% 17% 29% 18% 7%	330
	21st to end	Day	26% 18% 33% 20% 4	273
March	1st to 10th	Day	21% 16% 35% 22% 6%	330
	11th to 20th	Day	19% 16% 39% 19% 6% 1	330
	21st to end	Day	18% 13% 33% 26% 8% 2	363
April	1st to 10th	Day	16% 19% 37% 22% 6% 1	330
	11th to 20th	Day	22% 15% 31% 22% 9% 1	414
	21st to end	Day	19% 11% 31% 27% 11% 1	540
May	1st to 10th	Day	14% 11% 39% 26% 10% 1	732
	11th to 20th	Day	15% 9% 28% 29% 17% 3	775
	21st to end	Day	12% 5% 25% 35% 20% 3	844
June	1st to 10th	Day	6% 2 14% 37% 31% 9%	780
	11th to 20th	Day	9% 1 12% 40% 31% 8%	780
	21st to end	Day	7% 1 10% 32% 36% 14%	780
July	1st to 10th	Day	8% 2 7% 27% 39% 16%	771
	11th to 20th	Day	5% 1 4 30% 43% 16%	768
	21st to end	Day	5% 1 5% 25% 46% 17%	845
August	1st to 10th	Day	3 2 21% 50% 23%	726
	11th to 20th	Day	4 1 4 20% 47% 24%	700
	21st to end	Day	7% 1 4 17% 31% 40%	404
September	1st to 10th	Day	6% 2 7% 25% 33% 27%	330
	11th to 20th	Day	8% 8% 19% 38% 27%	330
	21st to end	Day	6% 2 8% 24% 38% 22%	330
October	1st to 10th	Day	4 2 9% 33% 38% 15%	330
	11th to 20th	Day	9% 4 17% 35% 23% 12%	330
	21st to end	Day	20% 9% 20% 24% 21% 6%	363
November	1st to 10th	Day	16% 5% 19% 32% 20% 7%	330
	11th to 20th	Day	15% 7% 23% 34% 18% 4	330
	21st to end	Day	21% 15% 33% 18% 11% 3	330
December	1st to 10th	Day	21% 8% 31% 23% 12% 5%	330
	11th to 20th	Day	23% 15% 27% 22% 10% 2	330
	21st to end	Day	20% 9% 33% 27% 9% 2	363
January	1st to 10th	Night	21% 10% 33% 28% 8%	1050
	11th to 20th	Night	21% 12% 31% 29% 7%	1050
	21st to end	Night	23% 11% 30% 30% 6%	1155
February	1st to 10th	Night	23% 13% 34% 26% 4	879
	11th to 20th	Night	27% 10% 33% 26% 4	824
	21st to end	Night	25% 13% 39% 19% 3	585
March	1st to 10th	Night	24% 10% 38% 25% 3	630
	11th to 20th	Night	18% 11% 36% 28% 6%	630
	21st to end	Night	16% 10% 40% 28% 5%	693
April	1st to 10th	Night	14% 10% 43% 29% 4	630
	11th to 20th	Night	17% 13% 35% 30% 6%	623
	21st to end	Night	18% 11% 37% 26% 8% 1	630
May	1st to 10th	Night	17% 10% 39% 29% 5%	630
	11th to 20th	Night	11% 7% 33% 35% 13% 1	621
	21st to end	Night	8% 5% 30% 41% 16% 1	683
June	1st to 10th	Night	5% 2 16% 50% 25% 2	630
	11th to 20th	Night	5% 2 21% 50% 21% 1	630
	21st to end	Night	5% 1 12% 46% 32% 4	630
July	1st to 10th	Night	7% 1 7% 44% 36% 5%	615
	11th to 20th	Night	4 1 7% 46% 38% 3	612
	21st to end	Night	4 1 7% 46% 39% 4	687
August	1st to 10th	Night	1 4 49% 41% 4	597
	11th to 20th	Night	4 4 46% 40% 6%	594
	21st to end	Night	4 6% 33% 50% 7%	660
September	1st to 10th	Night	6% 1 8% 33% 45% 7%	600
	11th to 20th	Night	10% 1 7% 34% 43% 4	600
	21st to end	Night	9% 1 12% 33% 40% 6%	600
October	1st to 10th	Night	5% 1 9% 44% 34% 6%	635
	11th to 20th	Night	11% 3 22% 41% 21% 3	800
	21st to end	Night	13% 8% 26% 32% 19% 2	880
November	1st to 10th	Night	12% 5% 22% 39% 20% 2	820
	11th to 20th	Night	12% 8% 26% 38% 15% 1	840
	21st to end	Night	18% 9% 28% 32% 14% 1	1012
December	1st to 10th	Night	20% 10% 27% 31% 12% 1	1050
	11th to 20th	Night	24% 10% 28% 31% 7%	1050
	21st to end	Night	21% 9% 30% 31% 9% 1	1155

Page generated at 2026-07-28 02:25:20. Version ac07

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