Washington-on-the-Brazos State Historic Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Washington-on-the-Brazos State Historic Site clear sky chart from 2005-11-06 to 2024-01-13.

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	11% 6% 3 5% 5% 3 5% 5% 6% 8% 43%	74937
	Night	11% 5% 2 4 4 3 4 4 5% 8% 49%	74828
Transparency	Day	38% 7% 12% 19% 17% 7%	74662
	Night	34% 6% 12% 20% 18% 10%	74623
Seeing	Day	13% 6% 19% 26% 26% 10%	13754
	Night	14% 6% 24% 35% 18% 2	23751

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% 6% 2 5% 4 2 4 5% 4 5% 46%	1707
	11th to 20th	Day	19% 7% 2 4 5% 2 3 3 5% 5% 45%	1643
	21st to end	Day	17% 7% 3 6% 4 2 4 3 4 5% 45%	1782
February	1st to 10th	Day	22% 8% 4 5% 4 2 3 3 4 6% 39%	1755
	11th to 20th	Day	20% 8% 3 5% 6% 2 3 3 3 5% 41%	1749
	21st to end	Day	18% 8% 2 5% 6% 2 3 4 5% 6% 41%	1583
March	1st to 10th	Day	19% 8% 3 7% 5% 3 4 4 4 6% 36%	1980
	11th to 20th	Day	17% 7% 3 6% 5% 3 4 5% 4 6% 40%	1980
	21st to end	Day	15% 7% 3 5% 5% 3 4 4 5% 6% 43%	2178
April	1st to 10th	Day	14% 8% 4 7% 7% 3 5% 4 4 6% 38%	1980
	11th to 20th	Day	15% 7% 3 6% 6% 3 4 4 5% 6% 41%	2051
	21st to end	Day	11% 6% 3 6% 5% 3 4 4 5% 7% 45%	2160
May	1st to 10th	Day	10% 6% 3 6% 6% 3 6% 6% 6% 8% 39%	2302
	11th to 20th	Day	10% 6% 3 6% 6% 4 5% 5% 6% 8% 42%	2302
	21st to end	Day	10% 5% 3 6% 7% 5% 7% 8% 6% 10% 32%	2531
June	1st to 10th	Day	5% 4 3 5% 6% 4 5% 7% 8% 9% 44%	2340
	11th to 20th	Day	5% 3 2 3 5% 3 7% 8% 8% 9% 46%	2340
	21st to end	Day	5% 6% 3 6% 7% 6% 9% 8% 7% 9% 34%	2340
July	1st to 10th	Day	9% 4 4 7% 7% 4 7% 6% 8% 9% 36%	2340
	11th to 20th	Day	4 3 2 5% 6% 5% 7% 8% 7% 11% 41%	2340
	21st to end	Day	4 3 2 4 6% 5% 7% 7% 9% 12% 41%	2546
August	1st to 10th	Day	3 2 1 4 5% 4 5% 7% 8% 11% 51%	2328
	11th to 20th	Day	4 4 2 5% 5% 4 6% 6% 7% 11% 46%	2275
	21st to end	Day	5% 3 2 5% 7% 4 7% 6% 8% 10% 42%	2251
September	1st to 10th	Day	5% 4 3 6% 6% 4 7% 6% 7% 10% 43%	1980
	11th to 20th	Day	10% 7% 3 7% 8% 3 6% 5% 6% 8% 37%	1980
	21st to end	Day	6% 4 2 6% 6% 4 5% 6% 5% 7% 48%	1980
October	1st to 10th	Day	5% 3 2 5% 5% 3 4 5% 4 7% 57%	1913
	11th to 20th	Day	13% 4 2 4 5% 3 3 4 5% 5% 53%	1800
	21st to end	Day	13% 6% 2 4 4 2 2 4 4 4 56%	1980
November	1st to 10th	Day	10% 6% 2 5% 4 3 5% 5% 5% 6% 48%	1858
	11th to 20th	Day	11% 5% 2 5% 4 3 4 4 5% 6% 51%	1900
	21st to end	Day	16% 7% 3 5% 5% 4 4 4 4 5% 43%	1757
December	1st to 10th	Day	15% 8% 3 5% 5% 3 4 4 4 6% 43%	1710
	11th to 20th	Day	17% 7% 4 5% 5% 3 5% 4 4 6% 42%	2161
	21st to end	Day	16% 7% 3 5% 5% 3 3 3 4 5% 47%	3135
January	1st to 10th	Night	17% 6% 2 5% 3 2 3 3 4 6% 47%	2458
	11th to 20th	Night	17% 7% 2 5% 3 2 3 3 3 5% 48%	2379
	21st to end	Night	17% 7% 3 4 3 3 3 3 4 5% 48%	2574
February	1st to 10th	Night	17% 8% 4 6% 4 2 4 3 4 6% 42%	2218
	11th to 20th	Night	20% 8% 2 5% 4 2 3 2 3 5% 46%	2098
	21st to end	Night	21% 7% 3 5% 4 2 4 3 5% 6% 41%	1670
March	1st to 10th	Night	16% 7% 3 6% 5% 2 4 4 5% 8% 41%	1980
	11th to 20th	Night	15% 6% 4 5% 5% 3 5% 4 6% 5% 42%	1980
	21st to end	Night	12% 6% 3 5% 4 3 3 3 4 7% 49%	2178
April	1st to 10th	Night	11% 5% 3 5% 5% 3 4 5% 6% 9% 44%	1980
	11th to 20th	Night	13% 6% 3 5% 5% 3 4 3 5% 8% 46%	1857
	21st to end	Night	12% 7% 2 5% 5% 4 3 5% 5% 6% 48%	1800
May	1st to 10th	Night	11% 6% 3 6% 5% 3 5% 4 5% 9% 43%	1653
	11th to 20th	Night	9% 6% 2 5% 5% 3 3 5% 6% 8% 48%	1593
	21st to end	Night	7% 4 3 4 4 3 6% 6% 6% 11% 47%	1745
June	1st to 10th	Night	5% 3 1 4 4 3 4 4 6% 12% 54%	1620
	11th to 20th	Night	2 2 1 3 3 2 4 5% 7% 12% 58%	1620
	21st to end	Night	4 3 2 3 5% 4 5% 6% 8% 12% 47%	1620
July	1st to 10th	Night	6% 3 2 4 5% 4 5% 5% 9% 10% 49%	1620
	11th to 20th	Night	2 2 1 4 3 2 3 5% 6% 12% 59%	1620
	21st to end	Night	2 2 2 3 3 3 5% 6% 8% 12% 54%	1772
August	1st to 10th	Night	2 1 1 2 2 2 3 3 6% 13% 64%	1611
	11th to 20th	Night	4 4 1 3 4 3 4 5% 7% 12% 53%	1624
	21st to end	Night	4 2 2 5% 5% 3 6% 7% 7% 13% 47%	2098
September	1st to 10th	Night	5% 4 2 4 5% 2 5% 5% 8% 10% 50%	1980
	11th to 20th	Night	9% 6% 3 5% 5% 4 4 4 6% 8% 45%	1980
	21st to end	Night	8% 3 1 4 4 3 4 4 5% 10% 54%	1980
October	1st to 10th	Night	4 3 2 4 4 2 3 3 6% 7% 62%	2027
	11th to 20th	Night	10% 4 2 3 3 2 3 3 4 6% 59%	2160
	21st to end	Night	11% 6% 2 4 3 2 3 3 3 5% 59%	2376
November	1st to 10th	Night	10% 5% 2 4 4 2 4 4 4 6% 55%	2220
	11th to 20th	Night	10% 6% 2 5% 5% 3 4 3 5% 6% 51%	2280
	21st to end	Night	16% 6% 3 6% 4 3 4 4 4 6% 46%	2419
December	1st to 10th	Night	17% 7% 3 5% 4 2 3 3 5% 6% 45%	2470
	11th to 20th	Night	16% 9% 3 5% 5% 3 4 4 5% 6% 42%	3036
	21st to end	Night	15% 7% 2 5% 4 2 3 3 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% 4 8% 15% 22% 10%	1708
	11th to 20th	Day	42% 3 8% 16% 22% 10%	1643
	21st to end	Day	42% 4 5% 14% 21% 14%	1782
February	1st to 10th	Day	49% 3 6% 13% 18% 12%	1755
	11th to 20th	Day	47% 5% 7% 15% 17% 9%	1759
	21st to end	Day	45% 5% 9% 14% 16% 12%	1583
March	1st to 10th	Day	49% 6% 8% 12% 18% 7%	1980
	11th to 20th	Day	46% 5% 7% 13% 20% 10%	1980
	21st to end	Day	42% 4 8% 17% 21% 8%	2178
April	1st to 10th	Day	47% 5% 7% 18% 15% 9%	1980
	11th to 20th	Day	43% 5% 10% 14% 18% 10%	2051
	21st to end	Day	38% 6% 9% 19% 22% 7%	2160
May	1st to 10th	Day	41% 7% 10% 18% 16% 8%	2302
	11th to 20th	Day	38% 8% 11% 22% 18% 3	2306
	21st to end	Day	42% 10% 13% 22% 12% 1	2531
June	1st to 10th	Day	31% 9% 15% 28% 15% 2	2340
	11th to 20th	Day	29% 10% 16% 26% 16% 3	2340
	21st to end	Day	41% 11% 14% 23% 9% 1	2340
July	1st to 10th	Day	40% 9% 19% 24% 8% 1	2276
	11th to 20th	Day	31% 11% 22% 22% 14% 1	2271
	21st to end	Day	28% 12% 23% 27% 8% 1	2405
August	1st to 10th	Day	26% 9% 21% 30% 14% 1	2314
	11th to 20th	Day	29% 10% 22% 27% 10% 3	2275
	21st to end	Day	34% 10% 20% 23% 11% 2	2251
September	1st to 10th	Day	34% 10% 15% 21% 15% 6%	1980
	11th to 20th	Day	42% 9% 12% 16% 16% 4	1980
	21st to end	Day	34% 7% 11% 20% 16% 12%	1980
October	1st to 10th	Day	27% 7% 10% 23% 21% 13%	1913
	11th to 20th	Day	34% 5% 8% 17% 21% 17%	1800
	21st to end	Day	32% 4 6% 16% 24% 18%	1980
November	1st to 10th	Day	35% 6% 9% 18% 22% 10%	1858
	11th to 20th	Day	33% 6% 10% 17% 21% 13%	1900
	21st to end	Day	43% 6% 7% 15% 18% 11%	1757
December	1st to 10th	Day	42% 5% 11% 16% 19% 7%	1710
	11th to 20th	Day	43% 6% 6% 13% 23% 9%	2159
	21st to end	Day	41% 4 5% 14% 25% 10%	3135
January	1st to 10th	Night	39% 4 5% 18% 20% 13%	2458
	11th to 20th	Night	39% 3 7% 17% 21% 13%	2379
	21st to end	Night	39% 4 5% 15% 21% 16%	2574
February	1st to 10th	Night	45% 4 6% 12% 18% 14%	2218
	11th to 20th	Night	43% 4 6% 17% 17% 13%	2098
	21st to end	Night	47% 4 7% 14% 14% 14%	1670
March	1st to 10th	Night	42% 6% 9% 16% 18% 9%	1980
	11th to 20th	Night	42% 7% 9% 13% 19% 10%	1980
	21st to end	Night	37% 4 11% 16% 21% 10%	2178
April	1st to 10th	Night	37% 7% 10% 19% 17% 10%	1980
	11th to 20th	Night	38% 4 11% 18% 16% 13%	1857
	21st to end	Night	37% 6% 9% 20% 20% 8%	1800
May	1st to 10th	Night	39% 6% 12% 18% 18% 8%	1653
	11th to 20th	Night	33% 7% 13% 25% 18% 4	1594
	21st to end	Night	31% 8% 17% 29% 13% 2	1746
June	1st to 10th	Night	23% 8% 17% 30% 20% 1	1620
	11th to 20th	Night	18% 8% 21% 31% 18% 4	1620
	21st to end	Night	28% 9% 21% 28% 14% 1	1620
July	1st to 10th	Night	28% 11% 24% 29% 9%	1575
	11th to 20th	Night	18% 8% 29% 31% 13%	1566
	21st to end	Night	18% 12% 31% 31% 9%	1673
August	1st to 10th	Night	12% 7% 26% 38% 15% 1	1601
	11th to 20th	Night	22% 10% 25% 33% 9% 1	1624
	21st to end	Night	27% 11% 24% 28% 9% 2	2098
September	1st to 10th	Night	27% 9% 21% 22% 14% 6%	1980
	11th to 20th	Night	36% 9% 15% 21% 13% 6%	1980
	21st to end	Night	26% 6% 17% 20% 19% 12%	1980
October	1st to 10th	Night	20% 5% 12% 27% 21% 14%	2027
	11th to 20th	Night	28% 4 8% 20% 23% 17%	2160
	21st to end	Night	30% 4 6% 15% 23% 21%	2376
November	1st to 10th	Night	31% 6% 11% 21% 21% 11%	2220
	11th to 20th	Night	34% 5% 10% 19% 19% 13%	2280
	21st to end	Night	41% 4 8% 16% 19% 12%	2419
December	1st to 10th	Night	41% 4 8% 20% 16% 10%	2470
	11th to 20th	Night	43% 5% 7% 11% 19% 15%	3037
	21st to end	Night	38% 4 6% 14% 28% 9%	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% 9% 36% 23% 7% 1	270
	11th to 20th	Day	26% 8% 29% 27% 8% 2	248
	21st to end	Day	22% 15% 35% 20% 7%	264
February	1st to 10th	Day	29% 17% 28% 20% 6% 1	240
	11th to 20th	Day	29% 20% 27% 16% 8%	240
	21st to end	Day	28% 19% 34% 15% 4	198
March	1st to 10th	Day	23% 14% 34% 20% 9%	240
	11th to 20th	Day	17% 18% 40% 20% 5% 1	240
	21st to end	Day	20% 12% 38% 22% 8%	264
April	1st to 10th	Day	17% 16% 38% 22% 7%	240
	11th to 20th	Day	24% 12% 27% 25% 12% 1	329
	21st to end	Day	16% 11% 34% 25% 11% 2	420
May	1st to 10th	Day	14% 9% 36% 30% 11% 1	565
	11th to 20th	Day	13% 6% 26% 36% 16% 4	625
	21st to end	Day	11% 5% 24% 37% 21% 4	679
June	1st to 10th	Day	6% 3 11% 35% 36% 9%	630
	11th to 20th	Day	6% 2 11% 41% 30% 10%	630
	21st to end	Day	6% 3 10% 28% 40% 13%	630
July	1st to 10th	Day	6% 3 7% 30% 37% 17%	621
	11th to 20th	Day	3 3 29% 46% 18%	618
	21st to end	Day	4 1 3 23% 49% 20%	684
August	1st to 10th	Day	3 1 3 20% 50% 22%	626
	11th to 20th	Day	6% 1 2 17% 51% 23%	589
	21st to end	Day	7% 2 4 14% 36% 37%	370
September	1st to 10th	Day	6% 2 22% 37% 32%	270
	11th to 20th	Day	8% 1 6% 22% 39% 24%	270
	21st to end	Day	10% 2 5% 22% 40% 21%	270
October	1st to 10th	Day	5% 1 13% 31% 39% 11%	270
	11th to 20th	Day	13% 4 19% 32% 22% 10%	270
	21st to end	Day	24% 10% 21% 21% 19% 5%	297
November	1st to 10th	Day	15% 4 17% 37% 22% 4	270
	11th to 20th	Day	17% 6% 24% 34% 16% 3	270
	21st to end	Day	21% 14% 30% 23% 10% 2	270
December	1st to 10th	Day	20% 8% 31% 23% 13% 6%	270
	11th to 20th	Day	25% 15% 29% 23% 7% 1	270
	21st to end	Day	22% 10% 36% 22% 9% 1	297
January	1st to 10th	Night	21% 11% 29% 31% 8%	950
	11th to 20th	Night	20% 12% 30% 30% 8%	915
	21st to end	Night	21% 11% 31% 31% 7%	990
February	1st to 10th	Night	23% 12% 33% 27% 5%	778
	11th to 20th	Night	26% 9% 33% 28% 4	704
	21st to end	Night	28% 10% 41% 19% 2	486
March	1st to 10th	Night	19% 14% 36% 26% 5%	540
	11th to 20th	Night	18% 9% 37% 29% 6%	540
	21st to end	Night	15% 8% 43% 28% 6%	594
April	1st to 10th	Night	13% 10% 42% 31% 4	540
	11th to 20th	Night	16% 9% 34% 32% 8%	533
	21st to end	Night	15% 9% 39% 29% 9%	540
May	1st to 10th	Night	13% 9% 39% 31% 6% 1	540
	11th to 20th	Night	12% 4 32% 38% 13% 1	531
	21st to end	Night	8% 4 28% 42% 18% 1	584
June	1st to 10th	Night	6% 1 13% 50% 26% 4	540
	11th to 20th	Night	3 1 19% 53% 21% 2	540
	21st to end	Night	5% 10% 47% 33% 5%	540
July	1st to 10th	Night	5% 2 7% 44% 36% 6%	525
	11th to 20th	Night	3 1 6% 46% 41% 4	522
	21st to end	Night	3 1 5% 47% 39% 5%	591
August	1st to 10th	Night	2 4 51% 40% 3	537
	11th to 20th	Night	6% 1 2 46% 38% 6%	534
	21st to end	Night	4 1 5% 34% 49% 7%	594
September	1st to 10th	Night	6% 1 6% 32% 47% 7%	540
	11th to 20th	Night	10% 2 6% 32% 44% 7%	540
	21st to end	Night	10% 2 11% 32% 40% 6%	540
October	1st to 10th	Night	4 1 10% 46% 33% 6%	587
	11th to 20th	Night	11% 3 21% 41% 22% 2	720
	21st to end	Night	15% 7% 25% 32% 20% 2	792
November	1st to 10th	Night	11% 4 23% 39% 20% 2	740
	11th to 20th	Night	13% 7% 27% 36% 16%	760
	21st to end	Night	18% 8% 28% 31% 13% 1	899
December	1st to 10th	Night	21% 8% 27% 31% 12% 1	950
	11th to 20th	Night	25% 11% 25% 31% 7% 1	950
	21st to end	Night	22% 9% 31% 31% 8%	1045

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