San Antonio
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Antonio clear sky chart from 2005-11-06 to 2024-03-18.

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	14% 5% 2 5% 5% 3 4 4 5% 7% 46%	79182
	Night	14% 5% 2 4 4 2 3 4 4 7% 51%	80668
Transparency	Day	37% 5% 10% 20% 19% 9%	78907
	Night	33% 5% 11% 20% 19% 12%	80463
Seeing	Day	13% 6% 20% 30% 24% 7%	13814
	Night	15% 5% 24% 37% 17% 1	23941

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% 7% 2 3 3 2 3 3 4 6% 51%	2847
	11th to 20th	Day	18% 5% 2 3 3 2 2 2 4 5% 55%	2850
	21st to end	Day	20% 7% 3 4 3 2 3 4 5% 8% 42%	3456
February	1st to 10th	Day	21% 7% 3 5% 3 2 3 3 3 5% 46%	1900
	11th to 20th	Day	20% 7% 2 4 4 2 3 3 4 7% 44%	1849
	21st to end	Day	19% 6% 2 4 3 2 3 3 4 6% 48%	1570
March	1st to 10th	Day	20% 7% 2 5% 4 3 4 4 3 6% 42%	2044
	11th to 20th	Day	17% 6% 2 6% 4 3 3 4 4 6% 44%	2057
	21st to end	Day	18% 6% 2 5% 3 3 3 3 4 6% 46%	2178
April	1st to 10th	Day	16% 6% 3 5% 5% 3 3 4 6% 5% 43%	1980
	11th to 20th	Day	21% 6% 2 5% 4 3 5% 3 4 5% 43%	1972
	21st to end	Day	17% 6% 4 5% 5% 2 3 3 4 5% 46%	2256
May	1st to 10th	Day	16% 6% 3 5% 5% 3 4 5% 5% 6% 40%	2340
	11th to 20th	Day	14% 7% 3 7% 6% 3 5% 4 5% 7% 39%	2302
	21st to end	Day	15% 5% 3 6% 6% 4 5% 6% 6% 9% 34%	2531
June	1st to 10th	Day	6% 3 2 4 5% 3 6% 7% 7% 10% 46%	2340
	11th to 20th	Day	7% 4 3 5% 5% 4 5% 5% 6% 10% 45%	2340
	21st to end	Day	9% 5% 3 7% 7% 5% 6% 6% 7% 9% 36%	2340
July	1st to 10th	Day	12% 5% 4 6% 6% 4 6% 6% 6% 8% 37%	2340
	11th to 20th	Day	5% 3 1 5% 5% 3 6% 6% 7% 11% 48%	2340
	21st to end	Day	6% 3 2 3 5% 3 5% 5% 8% 12% 47%	2546
August	1st to 10th	Day	2 2 2 3 4 3 5% 5% 6% 10% 59%	2252
	11th to 20th	Day	6% 3 2 5% 6% 3 5% 6% 6% 11% 48%	2123
	21st to end	Day	6% 4 2 5% 6% 4 6% 6% 7% 11% 43%	2291
September	1st to 10th	Day	9% 6% 3 5% 6% 4 6% 7% 7% 7% 41%	1980
	11th to 20th	Day	11% 6% 3 5% 7% 4 6% 6% 6% 7% 39%	1980
	21st to end	Day	9% 4 2 4 5% 4 4 5% 6% 9% 48%	1980
October	1st to 10th	Day	8% 4 1 4 5% 3 5% 6% 6% 7% 53%	1980
	11th to 20th	Day	15% 4 2 4 4 3 4 3 5% 5% 50%	1892
	21st to end	Day	12% 5% 2 4 3 1 3 4 3 5% 58%	1980
November	1st to 10th	Day	17% 8% 3 5% 4 3 3 4 5% 6% 44%	1858
	11th to 20th	Day	15% 5% 2 5% 4 3 3 5% 4 6% 48%	1772
	21st to end	Day	21% 6% 2 5% 4 2 4 3 3 4 45%	1710
December	1st to 10th	Day	18% 6% 3 4 5% 2 3 4 5% 7% 44%	1710
	11th to 20th	Day	18% 7% 3 5% 4 2 5% 3 4 5% 44%	2161
	21st to end	Day	16% 6% 2 4 3 2 3 3 3 4 53%	3135
January	1st to 10th	Night	19% 6% 2 3 3 2 2 3 4 6% 51%	4108
	11th to 20th	Night	14% 4 2 2 2 1 2 2 2 4 65%	4120
	21st to end	Night	24% 6% 2 5% 4 1 3 3 4 6% 43%	4240
February	1st to 10th	Night	19% 5% 2 4 3 2 3 4 5% 6% 47%	2280
	11th to 20th	Night	21% 6% 1 3 3 2 2 2 4 5% 49%	2218
	21st to end	Night	20% 5% 2 4 4 2 3 3 4 6% 46%	1884
March	1st to 10th	Night	21% 6% 3 3 4 2 3 4 4 6% 44%	2133
	11th to 20th	Night	19% 6% 2 6% 5% 2 3 3 3 6% 44%	2068
	21st to end	Night	17% 6% 2 3 3 2 3 4 4 7% 49%	2178
April	1st to 10th	Night	14% 5% 2 5% 4 3 4 4 5% 8% 47%	1980
	11th to 20th	Night	20% 6% 2 4 4 2 3 3 4 6% 46%	1932
	21st to end	Night	18% 6% 3 4 4 2 3 5% 4 5% 45%	1702
May	1st to 10th	Night	15% 6% 3 5% 4 2 4 5% 4 7% 46%	1620
	11th to 20th	Night	14% 5% 3 5% 4 3 4 4 5% 10% 45%	1593
	21st to end	Night	14% 5% 2 5% 5% 3 3 4 6% 9% 45%	1745
June	1st to 10th	Night	6% 4 2 3 3 2 4 4 6% 8% 60%	1620
	11th to 20th	Night	6% 3 1 3 4 3 3 4 6% 10% 57%	1620
	21st to end	Night	8% 4 2 4 5% 3 4 5% 6% 10% 49%	1620
July	1st to 10th	Night	9% 5% 3 4 4 3 4 5% 6% 11% 46%	1620
	11th to 20th	Night	3 2 1 3 3 2 4 5% 5% 10% 61%	1620
	21st to end	Night	5% 3 1 3 3 2 4 5% 6% 11% 57%	1772
August	1st to 10th	Night	2 1 2 3 2 3 4 4 9% 70%	1682
	11th to 20th	Night	5% 3 1 4 4 3 4 5% 7% 11% 52%	1778
	21st to end	Night	7% 3 2 4 4 4 5% 5% 7% 10% 50%	2061
September	1st to 10th	Night	8% 5% 2 5% 5% 3 5% 5% 6% 8% 47%	1980
	11th to 20th	Night	12% 5% 2 6% 4 3 4 4 6% 8% 45%	1980
	21st to end	Night	8% 4 2 5% 5% 3 4 5% 6% 8% 51%	1980
October	1st to 10th	Night	7% 4 2 3 4 2 4 5% 5% 8% 57%	1980
	11th to 20th	Night	14% 5% 1 3 3 2 4 3 4 6% 54%	2047
	21st to end	Night	15% 4 2 3 3 2 2 2 2 4 61%	2376
November	1st to 10th	Night	14% 6% 3 5% 3 2 2 3 3 5% 54%	2220
	11th to 20th	Night	11% 6% 2 4 3 2 4 4 4 6% 54%	2403
	21st to end	Night	20% 5% 3 4 3 2 3 3 4 6% 47%	2470
December	1st to 10th	Night	18% 5% 2 4 3 2 3 3 4 6% 50%	2470
	11th to 20th	Night	20% 6% 2 5% 4 2 3 3 3 5% 46%	3036
	21st to end	Night	19% 5% 1 4 3 1 2 3 3 6% 52%	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	36% 2 5% 15% 26% 16%	2848
	11th to 20th	Day	33% 2 5% 17% 23% 19%	2850
	21st to end	Day	42% 3 5% 13% 28% 9%	3456
February	1st to 10th	Day	44% 3 5% 13% 19% 16%	1900
	11th to 20th	Day	42% 3 6% 17% 19% 13%	1859
	21st to end	Day	38% 4 8% 16% 18% 15%	1570
March	1st to 10th	Day	45% 4 9% 15% 16% 11%	2044
	11th to 20th	Day	42% 5% 7% 14% 16% 15%	2057
	21st to end	Day	40% 4 8% 17% 17% 14%	2178
April	1st to 10th	Day	41% 5% 9% 17% 17% 11%	1980
	11th to 20th	Day	42% 4 6% 17% 17% 13%	1972
	21st to end	Day	40% 4 6% 20% 18% 11%	2256
May	1st to 10th	Day	42% 5% 9% 19% 17% 8%	2340
	11th to 20th	Day	42% 6% 8% 20% 18% 5%	2306
	21st to end	Day	42% 7% 12% 21% 16% 1	2531
June	1st to 10th	Day	28% 9% 14% 28% 19% 3	2340
	11th to 20th	Day	31% 6% 14% 27% 17% 5%	2340
	21st to end	Day	40% 8% 14% 23% 12% 3	2340
July	1st to 10th	Day	42% 7% 14% 23% 11% 2	2276
	11th to 20th	Day	28% 7% 19% 26% 18% 2	2271
	21st to end	Day	25% 9% 21% 33% 12% 1	2405
August	1st to 10th	Day	20% 6% 17% 39% 17% 1	2238
	11th to 20th	Day	27% 9% 17% 30% 13% 3	2123
	21st to end	Day	34% 9% 18% 25% 13% 3	2291
September	1st to 10th	Day	40% 8% 11% 19% 15% 7%	1980
	11th to 20th	Day	40% 9% 10% 19% 17% 4	1980
	21st to end	Day	32% 7% 11% 21% 17% 11%	1980
October	1st to 10th	Day	29% 7% 10% 20% 25% 9%	1980
	11th to 20th	Day	36% 5% 9% 17% 18% 16%	1892
	21st to end	Day	29% 3 6% 16% 24% 21%	1980
November	1st to 10th	Day	42% 4 8% 18% 17% 11%	1858
	11th to 20th	Day	36% 5% 9% 16% 20% 14%	1772
	21st to end	Day	43% 3 7% 15% 19% 13%	1710
December	1st to 10th	Day	42% 5% 6% 19% 19% 10%	1710
	11th to 20th	Day	42% 5% 7% 10% 21% 14%	2159
	21st to end	Day	37% 2 5% 19% 26% 12%	3135
January	1st to 10th	Night	35% 3 6% 15% 19% 22%	4108
	11th to 20th	Night	27% 1 5% 17% 27% 22%	4120
	21st to end	Night	45% 3 4 12% 25% 11%	4240
February	1st to 10th	Night	39% 4 8% 13% 19% 18%	2280
	11th to 20th	Night	39% 3 6% 17% 19% 15%	2218
	21st to end	Night	39% 4 6% 17% 18% 15%	1884
March	1st to 10th	Night	40% 5% 11% 16% 16% 11%	2133
	11th to 20th	Night	43% 4 8% 13% 15% 17%	2068
	21st to end	Night	34% 4 13% 16% 18% 15%	2178
April	1st to 10th	Night	35% 6% 11% 20% 16% 13%	1980
	11th to 20th	Night	40% 4 10% 14% 16% 16%	1932
	21st to end	Night	40% 5% 8% 19% 16% 12%	1702
May	1st to 10th	Night	37% 5% 12% 19% 17% 10%	1620
	11th to 20th	Night	35% 7% 10% 24% 18% 5%	1594
	21st to end	Night	36% 7% 17% 25% 14% 1	1746
June	1st to 10th	Night	21% 6% 18% 30% 23% 2	1620
	11th to 20th	Night	22% 5% 15% 33% 21% 5%	1620
	21st to end	Night	30% 6% 19% 29% 14% 2	1620
July	1st to 10th	Night	31% 9% 20% 29% 10% 1	1575
	11th to 20th	Night	18% 6% 24% 32% 18% 1	1566
	21st to end	Night	17% 9% 29% 34% 10% 1	1673
August	1st to 10th	Night	13% 7% 22% 41% 16%	1672
	11th to 20th	Night	23% 10% 22% 32% 11% 2	1778
	21st to end	Night	27% 10% 23% 27% 11% 3	2061
September	1st to 10th	Night	33% 7% 18% 21% 15% 6%	1980
	11th to 20th	Night	35% 8% 14% 23% 14% 4	1980
	21st to end	Night	28% 7% 15% 23% 17% 10%	1980
October	1st to 10th	Night	23% 5% 11% 29% 20% 11%	1980
	11th to 20th	Night	29% 4 10% 20% 19% 17%	2047
	21st to end	Night	28% 3 6% 16% 24% 22%	2376
November	1st to 10th	Night	34% 4 8% 20% 21% 14%	2220
	11th to 20th	Night	31% 5% 11% 18% 20% 15%	2403
	21st to end	Night	39% 4 8% 16% 19% 13%	2470
December	1st to 10th	Night	36% 4 9% 20% 19% 13%	2470
	11th to 20th	Night	42% 3 6% 13% 17% 19%	3037
	21st to end	Night	34% 4 6% 14% 28% 13%	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% 10% 36% 23% 7% 1	270
	11th to 20th	Day	23% 12% 31% 26% 7% 1	270
	21st to end	Day	18% 13% 33% 26% 10%	297
February	1st to 10th	Day	29% 14% 26% 21% 9%	270
	11th to 20th	Day	26% 16% 31% 18% 7% 1	270
	21st to end	Day	23% 14% 41% 19% 2	225
March	1st to 10th	Day	20% 14% 38% 24% 4	270
	11th to 20th	Day	16% 12% 45% 21% 5%	261
	21st to end	Day	18% 15% 36% 27% 5%	264
April	1st to 10th	Day	21% 10% 38% 25% 5%	240
	11th to 20th	Day	17% 14% 35% 24% 9%	250
	21st to end	Day	16% 13% 36% 29% 6% 1	516
May	1st to 10th	Day	17% 9% 34% 31% 9%	603
	11th to 20th	Day	15% 9% 27% 35% 13% 1	625
	21st to end	Day	15% 5% 25% 38% 16% 1	679
June	1st to 10th	Day	6% 2 13% 41% 33% 5%	630
	11th to 20th	Day	7% 1 11% 47% 30% 5%	630
	21st to end	Day	9% 2 11% 34% 35% 10%	630
July	1st to 10th	Day	10% 3 7% 30% 39% 12%	621
	11th to 20th	Day	4 1 7% 26% 48% 14%	618
	21st to end	Day	5% 1 4 29% 46% 15%	684
August	1st to 10th	Day	2 1 3 30% 46% 17%	550
	11th to 20th	Day	6% 1 3 32% 42% 16%	437
	21st to end	Day	5% 2 5% 24% 40% 24%	410
September	1st to 10th	Day	9% 5% 24% 41% 21%	270
	11th to 20th	Day	6% 1 4 24% 44% 21%	270
	21st to end	Day	11% 1 7% 23% 41% 16%	270
October	1st to 10th	Day	8% 2 13% 32% 36% 10%	270
	11th to 20th	Day	12% 4 13% 40% 25% 5%	270
	21st to end	Day	18% 11% 22% 26% 20% 4	297
November	1st to 10th	Day	18% 9% 19% 33% 16% 6%	270
	11th to 20th	Day	22% 7% 22% 29% 16% 4	270
	21st to end	Day	24% 13% 34% 19% 9% 1	270
December	1st to 10th	Day	18% 9% 28% 28% 14% 3	270
	11th to 20th	Day	23% 16% 26% 27% 7% 1	270
	21st to end	Day	18% 8% 34% 31% 8% 1	297
January	1st to 10th	Night	21% 5% 27% 36% 11% 1	950
	11th to 20th	Night	21% 8% 27% 35% 9%	950
	21st to end	Night	23% 7% 28% 34% 8%	870
February	1st to 10th	Night	23% 9% 30% 32% 7%	760
	11th to 20th	Night	27% 8% 30% 30% 5%	744
	21st to end	Night	24% 12% 34% 28% 3	628
March	1st to 10th	Night	22% 10% 35% 29% 3 1	613
	11th to 20th	Night	21% 9% 34% 32% 4	564
	21st to end	Night	16% 10% 40% 28% 5%	594
April	1st to 10th	Night	15% 8% 44% 29% 4	540
	11th to 20th	Night	22% 10% 36% 28% 4	533
	21st to end	Night	20% 8% 41% 24% 7%	540
May	1st to 10th	Night	15% 9% 42% 29% 5%	540
	11th to 20th	Night	15% 6% 35% 36% 8%	531
	21st to end	Night	13% 4 36% 36% 11%	584
June	1st to 10th	Night	5% 2 22% 49% 22% 1	540
	11th to 20th	Night	6% 2 28% 51% 13% 1	540
	21st to end	Night	6% 2 18% 46% 25% 3	540
July	1st to 10th	Night	8% 4 13% 46% 27% 2	525
	11th to 20th	Night	4 1 9% 51% 33% 2	522
	21st to end	Night	5% 1 8% 49% 36% 2	591
August	1st to 10th	Night	1 1 8% 50% 38% 2	537
	11th to 20th	Night	5% 1 8% 50% 32% 4	534
	21st to end	Night	6% 1 5% 42% 41% 5%	594
September	1st to 10th	Night	9% 2 6% 41% 38% 4	540
	11th to 20th	Night	12% 1 9% 37% 37% 3	540
	21st to end	Night	7% 1 11% 41% 37% 3	540
October	1st to 10th	Night	6% 2 13% 46% 31% 3	540
	11th to 20th	Night	14% 3 16% 42% 23% 2	607
	21st to end	Night	13% 7% 23% 35% 19% 2	792
November	1st to 10th	Night	15% 5% 18% 38% 23% 1	740
	11th to 20th	Night	13% 4 23% 41% 18% 2	883
	21st to end	Night	22% 6% 24% 32% 15% 1	950
December	1st to 10th	Night	19% 7% 21% 37% 15% 1	950
	11th to 20th	Night	24% 8% 23% 33% 11%	950
	21st to end	Night	20% 6% 26% 37% 11%	1045

Page generated at 2024-03-15 13:01:45. Version ac07

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