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 2024-10-20.

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 5% 5% 4 6% 8% 42%	100891
	Night	12% 6% 2 5% 4 3 4 3 5% 7% 48%	97876
Transparency	Day	38% 7% 12% 20% 17% 6%	100705
	Night	35% 6% 12% 21% 17% 8%	97733
Seeing	Day	13% 6% 19% 26% 26% 10%	14812
	Night	14% 6% 24% 35% 18% 2	24657

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% 9% 2 5% 3 4 4 3 5% 5% 45%	2847
	11th to 20th	Day	16% 8% 2 4 2 3 2 2 4 7% 51%	2850
	21st to end	Day	23% 9% 3 6% 3 3 2 2 4 8% 36%	3135
February	1st to 10th	Day	22% 9% 3 6% 3 3 3 3 3 5% 40%	1880
	11th to 20th	Day	20% 8% 2 6% 4 4 4 2 3 6% 41%	1849
	21st to end	Day	18% 7% 3 6% 4 3 3 3 5% 5% 43%	1665
March	1st to 10th	Day	15% 8% 3 9% 6% 5% 5% 4 5% 8% 34%	3436
	11th to 20th	Day	19% 9% 3 9% 5% 5% 5% 4 5% 6% 31%	3480
	21st to end	Day	16% 7% 2 6% 4 4 4 3 4 6% 42%	3814
April	1st to 10th	Day	15% 9% 3 6% 5% 4 5% 4 4 7% 38%	2090
	11th to 20th	Day	16% 7% 3 7% 4 5% 3 4 5% 6% 39%	2147
	21st to end	Day	12% 7% 3 5% 4 4 5% 3 5% 7% 44%	2280
May	1st to 10th	Day	17% 8% 3 8% 5% 6% 5% 4 7% 7% 31%	4050
	11th to 20th	Day	13% 7% 2 7% 4 4 4 4 5% 7% 42%	4082
	21st to end	Day	9% 5% 2 7% 6% 6% 6% 5% 9% 10% 35%	4477
June	1st to 10th	Day	5% 4 2 6% 5% 6% 6% 5% 7% 10% 45%	2470
	11th to 20th	Day	6% 3 1 5% 5% 5% 6% 6% 7% 10% 46%	2470
	21st to end	Day	5% 6% 2 7% 7% 7% 8% 6% 8% 11% 34%	2470
July	1st to 10th	Day	8% 4 2 5% 5% 4 5% 5% 8% 10% 44%	4120
	11th to 20th	Day	6% 5% 2 7% 5% 6% 6% 6% 8% 11% 38%	4120
	21st to end	Day	12% 7% 3 7% 5% 6% 6% 5% 7% 11% 30%	4465
August	1st to 10th	Day	2 2 1 3 3 3 4 3 6% 11% 63%	4104
	11th to 20th	Day	3 2 1 4 3 4 5% 6% 9% 14% 49%	4014
	21st to end	Day	5% 4 2 6% 6% 6% 6% 5% 7% 10% 43%	3893
September	1st to 10th	Day	6% 5% 2 6% 6% 6% 6% 6% 8% 9% 41%	2090
	11th to 20th	Day	10% 7% 3 7% 5% 6% 6% 5% 6% 7% 38%	2090
	21st to end	Day	6% 4 2 6% 5% 6% 5% 4 6% 7% 49%	2090
October	1st to 10th	Day	5% 4 1 4 5% 5% 4 3 3 8% 58%	2037
	11th to 20th	Day	11% 6% 2 5% 4 4 3 3 4 6% 52%	1890
	21st to end	Day	13% 6% 1 5% 3 3 2 3 4 5% 55%	1980
November	1st to 10th	Day	11% 7% 1 5% 4 4 5% 4 6% 7% 47%	1858
	11th to 20th	Day	11% 6% 2 5% 4 3 3 4 5% 6% 52%	1900
	21st to end	Day	16% 6% 3 8% 4 4 3 4 4 5% 42%	1742
December	1st to 10th	Day	16% 7% 2 7% 5% 4 4 3 4 7% 43%	1710
	11th to 20th	Day	17% 7% 3 7% 4 4 4 3 4 5% 43%	2161
	21st to end	Day	16% 7% 2 6% 3 3 4 2 4 4 48%	3135
January	1st to 10th	Night	18% 8% 2 4 3 3 3 2 4 5% 48%	4108
	11th to 20th	Night	16% 7% 2 4 2 2 2 2 3 4 56%	4120
	21st to end	Night	22% 9% 2 6% 3 3 2 2 4 6% 41%	4532
February	1st to 10th	Night	19% 9% 3 7% 4 3 3 3 3 6% 41%	2316
	11th to 20th	Night	21% 8% 2 5% 4 2 2 2 3 5% 46%	2218
	21st to end	Night	21% 7% 2 7% 3 4 3 3 5% 4 41%	1787
March	1st to 10th	Night	20% 10% 3 7% 4 3 4 4 5% 6% 34%	3419
	11th to 20th	Night	22% 9% 3 8% 5% 4 4 3 5% 5% 31%	3480
	21st to end	Night	14% 8% 3 7% 5% 4 3 4 4 5% 44%	3817
April	1st to 10th	Night	12% 7% 2 5% 4 4 4 4 6% 8% 45%	2090
	11th to 20th	Night	14% 7% 2 6% 4 3 4 3 6% 7% 43%	1980
	21st to end	Night	13% 8% 2 5% 5% 4 4 3 4 6% 47%	1900
May	1st to 10th	Night	20% 9% 3 7% 4 4 4 3 5% 7% 33%	2837
	11th to 20th	Night	11% 6% 2 6% 4 4 5% 4 6% 6% 46%	2823
	21st to end	Night	9% 8% 2 6% 4 4 5% 4 7% 10% 40%	3090
June	1st to 10th	Night	4 4 2 4 3 4 4 3 7% 10% 56%	1710
	11th to 20th	Night	4 3 1 3 3 4 4 4 6% 11% 57%	1710
	21st to end	Night	4 3 2 5% 5% 4 5% 5% 8% 11% 48%	1710
July	1st to 10th	Night	6% 3 1 4 4 4 5% 4 6% 11% 52%	2850
	11th to 20th	Night	3 4 2 6% 4 5% 4 4 7% 10% 50%	2850
	21st to end	Night	9% 7% 3 7% 5% 4 5% 4 6% 11% 39%	3098
August	1st to 10th	Night	1 1 2 2 2 3 3 5% 11% 71%	2836
	11th to 20th	Night	2 2 1 3 2 3 5% 4 7% 12% 60%	2821
	21st to end	Night	3 3 1 4 4 5% 5% 4 8% 11% 51%	3757
September	1st to 10th	Night	5% 4 2 6% 3 4 5% 5% 6% 10% 50%	2090
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 6% 7% 46%	2090
	21st to end	Night	8% 4 2 4 3 3 4 3 6% 9% 54%	2090
October	1st to 10th	Night	4 3 1 4 3 3 3 3 6% 7% 63%	2123
	11th to 20th	Night	10% 4 1 4 3 3 3 3 4 5% 60%	2275
	21st to end	Night	12% 5% 1 4 3 3 3 2 3 5% 59%	2376
November	1st to 10th	Night	10% 6% 2 5% 4 3 3 3 4 6% 54%	2220
	11th to 20th	Night	11% 5% 2 5% 4 4 3 3 5% 7% 51%	2280
	21st to end	Night	16% 6% 2 5% 4 4 3 3 5% 7% 45%	2435
December	1st to 10th	Night	17% 8% 2 4 4 3 3 3 4 6% 45%	2470
	11th to 20th	Night	17% 9% 2 6% 4 4 3 3 5% 5% 43%	3036
	21st to end	Night	16% 7% 2 5% 3 3 3 2 4 5% 49%	4532

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	40% 5% 6% 16% 22% 11%	2848
	11th to 20th	Day	36% 4 5% 20% 27% 8%	2850
	21st to end	Day	49% 3 4 13% 19% 12%	3135
February	1st to 10th	Day	50% 3 5% 12% 19% 12%	1880
	11th to 20th	Day	46% 5% 7% 15% 17% 10%	1859
	21st to end	Day	44% 4 8% 15% 17% 12%	1665
March	1st to 10th	Day	47% 7% 9% 14% 18% 5%	3436
	11th to 20th	Day	53% 6% 9% 11% 15% 6%	3480
	21st to end	Day	42% 6% 8% 17% 22% 5%	3814
April	1st to 10th	Day	46% 6% 7% 17% 14% 10%	2090
	11th to 20th	Day	44% 7% 8% 14% 18% 10%	2147
	21st to end	Day	40% 6% 7% 20% 20% 7%	2280
May	1st to 10th	Day	48% 8% 12% 17% 10% 5%	4088
	11th to 20th	Day	40% 8% 10% 17% 22% 3	4086
	21st to end	Day	38% 10% 12% 29% 10% 1	4489
June	1st to 10th	Day	32% 8% 14% 27% 17% 2	2470
	11th to 20th	Day	30% 9% 14% 25% 18% 3	2470
	21st to end	Day	41% 9% 14% 25% 10% 1	2470
July	1st to 10th	Day	31% 8% 25% 30% 4	4056
	11th to 20th	Day	36% 11% 17% 24% 12% 1	4051
	21st to end	Day	44% 11% 19% 21% 5%	4325
August	1st to 10th	Day	16% 7% 13% 40% 24% 1	4090
	11th to 20th	Day	22% 8% 31% 31% 8% 1	4053
	21st to end	Day	33% 11% 18% 23% 13% 1	3892
September	1st to 10th	Day	35% 12% 13% 20% 15% 6%	2090
	11th to 20th	Day	41% 9% 11% 16% 18% 4	2090
	21st to end	Day	34% 7% 9% 20% 17% 13%	2090
October	1st to 10th	Day	26% 4 10% 22% 22% 15%	2037
	11th to 20th	Day	33% 4 7% 17% 22% 16%	1890
	21st to end	Day	32% 5% 5% 16% 24% 18%	1980
November	1st to 10th	Day	36% 6% 9% 19% 21% 10%	1858
	11th to 20th	Day	33% 6% 10% 17% 21% 13%	1900
	21st to end	Day	44% 4 7% 15% 18% 11%	1742
December	1st to 10th	Day	43% 5% 9% 17% 19% 7%	1710
	11th to 20th	Day	43% 5% 8% 11% 23% 10%	2159
	21st to end	Day	41% 4 6% 14% 25% 11%	3135
January	1st to 10th	Night	39% 5% 7% 17% 18% 15%	4108
	11th to 20th	Night	34% 4 4 18% 27% 12%	4120
	21st to end	Night	46% 4 4 14% 22% 10%	4532
February	1st to 10th	Night	46% 4 6% 12% 18% 15%	2316
	11th to 20th	Night	43% 4 6% 16% 18% 13%	2218
	21st to end	Night	46% 5% 7% 13% 15% 13%	1787
March	1st to 10th	Night	50% 6% 9% 14% 15% 6%	3419
	11th to 20th	Night	53% 7% 8% 15% 12% 6%	3480
	21st to end	Night	41% 6% 8% 14% 22% 8%	3817
April	1st to 10th	Night	38% 7% 9% 19% 16% 11%	2090
	11th to 20th	Night	40% 5% 10% 16% 15% 14%	1980
	21st to end	Night	38% 5% 10% 21% 19% 8%	1900
May	1st to 10th	Night	50% 7% 9% 19% 10% 5%	2864
	11th to 20th	Night	36% 8% 10% 21% 20% 6%	2824
	21st to end	Night	37% 10% 14% 30% 9% 1	3099
June	1st to 10th	Night	22% 8% 16% 32% 20% 2	1710
	11th to 20th	Night	21% 8% 18% 30% 18% 5%	1710
	21st to end	Night	28% 9% 20% 28% 14% 1	1710
July	1st to 10th	Night	26% 10% 28% 31% 5%	2805
	11th to 20th	Night	27% 9% 23% 28% 12%	2796
	21st to end	Night	38% 9% 22% 24% 6%	2999
August	1st to 10th	Night	9% 5% 17% 44% 24% 1	2826
	11th to 20th	Night	17% 7% 33% 34% 9% 1	2848
	21st to end	Night	25% 12% 21% 28% 12% 1	3757
September	1st to 10th	Night	28% 10% 19% 21% 14% 7%	2090
	11th to 20th	Night	36% 8% 14% 22% 14% 6%	2090
	21st to end	Night	26% 6% 16% 20% 18% 14%	2090
October	1st to 10th	Night	20% 5% 11% 26% 21% 17%	2123
	11th to 20th	Night	27% 4 7% 20% 23% 18%	2275
	21st to end	Night	30% 4 6% 17% 22% 22%	2376
November	1st to 10th	Night	32% 6% 10% 21% 21% 11%	2220
	11th to 20th	Night	34% 4 9% 19% 20% 13%	2280
	21st to end	Night	40% 4 8% 16% 19% 12%	2435
December	1st to 10th	Night	41% 5% 7% 21% 16% 10%	2470
	11th to 20th	Night	43% 5% 6% 12% 20% 15%	3037
	21st to end	Night	38% 5% 4 15% 28% 10%	4532

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	23% 7% 40% 23% 6% 1	270
	11th to 20th	Day	25% 10% 30% 26% 7% 1	270
	21st to end	Day	22% 16% 32% 21% 8%	297
February	1st to 10th	Day	32% 14% 27% 20% 6%	270
	11th to 20th	Day	29% 17% 29% 19% 7%	270
	21st to end	Day	27% 20% 32% 19% 3	225
March	1st to 10th	Day	21% 14% 35% 23% 7%	270
	11th to 20th	Day	20% 15% 39% 20% 6% 1	270
	21st to end	Day	19% 14% 35% 23% 7% 1	297
April	1st to 10th	Day	15% 18% 37% 23% 5% 1	270
	11th to 20th	Day	22% 13% 31% 22% 10% 1	345
	21st to end	Day	18% 9% 32% 27% 13% 1	460
May	1st to 10th	Day	13% 10% 38% 27% 11% 1	634
	11th to 20th	Day	15% 6% 26% 31% 19% 3	675
	21st to end	Day	12% 4 25% 34% 21% 3	734
June	1st to 10th	Day	6% 1 13% 36% 34% 10%	680
	11th to 20th	Day	7% 1 11% 40% 31% 9%	680
	21st to end	Day	8% 1 10% 29% 39% 14%	680
July	1st to 10th	Day	7% 2 7% 27% 40% 17%	671
	11th to 20th	Day	4 1 3 30% 45% 18%	668
	21st to end	Day	5% 1 6% 24% 47% 17%	739
August	1st to 10th	Day	4 2 22% 50% 22%	676
	11th to 20th	Day	4 1 3 20% 48% 23%	651
	21st to end	Day	7% 1 4 17% 30% 41%	369
September	1st to 10th	Day	6% 1 7% 23% 34% 29%	300
	11th to 20th	Day	8% 8% 18% 38% 28%	300
	21st to end	Day	7% 2 7% 22% 39% 23%	300
October	1st to 10th	Day	4 2 9% 32% 38% 14%	300
	11th to 20th	Day	10% 4 17% 35% 22% 12%	297
	21st to end	Day	22% 10% 20% 23% 20% 5%	297
November	1st to 10th	Day	15% 4 16% 35% 21% 8%	270
	11th to 20th	Day	16% 8% 22% 33% 17% 4	270
	21st to end	Day	23% 14% 32% 19% 10% 3	270
December	1st to 10th	Day	17% 7% 30% 25% 15% 6%	270
	11th to 20th	Day	23% 17% 27% 21% 10% 2	270
	21st to end	Day	21% 9% 35% 25% 6% 3	297
January	1st to 10th	Night	21% 10% 32% 29% 8%	950
	11th to 20th	Night	23% 10% 29% 30% 8%	950
	21st to end	Night	22% 10% 30% 31% 6%	1045
February	1st to 10th	Night	23% 13% 33% 26% 4	796
	11th to 20th	Night	26% 10% 33% 27% 5%	744
	21st to end	Night	25% 14% 40% 18% 3	531
March	1st to 10th	Night	23% 9% 38% 26% 4	570
	11th to 20th	Night	19% 10% 36% 30% 6%	570
	21st to end	Night	15% 11% 41% 27% 5%	627
April	1st to 10th	Night	13% 9% 43% 30% 4	570
	11th to 20th	Night	16% 12% 35% 31% 7%	563
	21st to end	Night	16% 10% 37% 27% 9% 1	570
May	1st to 10th	Night	15% 11% 39% 29% 5%	570
	11th to 20th	Night	11% 4 32% 38% 13% 1	561
	21st to end	Night	7% 4 30% 41% 17% 1	617
June	1st to 10th	Night	5% 2 14% 51% 26% 2	570
	11th to 20th	Night	4 2 20% 52% 21% 1	570
	21st to end	Night	5% 1 12% 44% 34% 5%	570
July	1st to 10th	Night	7% 1 7% 44% 36% 5%	555
	11th to 20th	Night	3 1 7% 45% 40% 4	552
	21st to end	Night	4 1 6% 45% 40% 4	624
August	1st to 10th	Night	1 4 49% 42% 4	567
	11th to 20th	Night	4 4 46% 40% 5%	564
	21st to end	Night	4 6% 33% 50% 7%	627
September	1st to 10th	Night	6% 1 8% 32% 46% 7%	570
	11th to 20th	Night	10% 1 7% 34% 44% 5%	570
	21st to end	Night	9% 1 12% 32% 41% 6%	570
October	1st to 10th	Night	5% 1 10% 44% 33% 6%	603
	11th to 20th	Night	11% 3 22% 41% 21% 3	759
	21st to end	Night	13% 7% 26% 31% 20% 3	792
November	1st to 10th	Night	11% 4 23% 39% 21% 2	740
	11th to 20th	Night	13% 8% 26% 37% 16% 1	760
	21st to end	Night	18% 8% 28% 31% 14% 1	915
December	1st to 10th	Night	20% 9% 27% 31% 13% 1	950
	11th to 20th	Night	25% 9% 28% 30% 7%	950
	21st to end	Night	20% 10% 30% 31% 9% 1	1045

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