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 2025-07-06.

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% 6% 2 5% 4 4 4 3 5% 7% 46%	120233
	Night	16% 5% 2 5% 3 3 3 3 5% 7% 49%	118832
Transparency	Day	36% 5% 9% 21% 18% 11%	124080
	Night	34% 4 11% 20% 18% 13%	123115
Seeing	Day	13% 7% 21% 30% 23% 6%	15621
	Night	16% 6% 25% 37% 16% 1	25569

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% 8% 2 5% 3 3 3 2 3 5% 49%	4077
	11th to 20th	Day	17% 5% 2 5% 3 3 2 2 5% 5% 52%	4080
	21st to end	Day	27% 7% 2 4 2 2 2 2 4 6% 41%	4963
February	1st to 10th	Day	21% 7% 2 5% 3 2 3 3 3 5% 46%	2000
	11th to 20th	Day	20% 7% 2 5% 3 2 3 3 5% 6% 43%	1949
	21st to end	Day	19% 7% 2 4 3 2 4 2 4 6% 48%	1650
March	1st to 10th	Day	15% 6% 2 5% 3 3 3 2 4 6% 52%	4843
	11th to 20th	Day	16% 6% 2 6% 3 4 3 2 3 4 51%	4980
	21st to end	Day	20% 8% 2 5% 3 3 3 2 4 6% 45%	5462
April	1st to 10th	Day	15% 7% 3 6% 4 4 4 3 5% 5% 44%	2200
	11th to 20th	Day	19% 6% 2 5% 4 4 4 3 4 5% 42%	2194
	21st to end	Day	17% 7% 2 7% 5% 3 4 3 4 5% 43%	2508
May	1st to 10th	Day	22% 7% 3 7% 4 4 4 3 6% 6% 33%	5862
	11th to 20th	Day	10% 5% 2 6% 4 5% 4 3 5% 8% 48%	5862
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 9% 37%	6434
June	1st to 10th	Day	6% 3 2 5% 4 4 6% 5% 8% 10% 47%	2600
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 10% 45%	2600
	21st to end	Day	8% 6% 2 7% 6% 6% 5% 5% 8% 9% 37%	2600
July	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 4 6% 9% 45%	4186
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 5% 8% 11% 43%	4120
	21st to end	Day	9% 5% 2 7% 5% 5% 6% 4 8% 11% 36%	4465
August	1st to 10th	Day	1 2 1 2 2 3 3 3 5% 8% 68%	3958
	11th to 20th	Day	3 2 1 4 3 3 5% 5% 8% 11% 55%	3726
	21st to end	Day	5% 4 2 5% 4 5% 5% 5% 7% 12% 47%	4025
September	1st to 10th	Day	9% 6% 3 6% 5% 5% 6% 5% 7% 7% 41%	2090
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 5% 7% 8% 39%	2090
	21st to end	Day	9% 4 2 5% 5% 5% 4 4 6% 9% 48%	2090
October	1st to 10th	Day	8% 4 1 4 4 4 4 5% 6% 7% 54%	2090
	11th to 20th	Day	14% 5% 2 5% 4 4 4 3 5% 5% 51%	1997
	21st to end	Day	11% 5% 2 5% 3 2 3 3 3 5% 58%	2090
November	1st to 10th	Day	17% 8% 2 5% 4 4 3 3 5% 6% 43%	1958
	11th to 20th	Day	14% 5% 2 5% 4 3 3 4 4 6% 50%	1865
	21st to end	Day	20% 6% 2 5% 3 3 4 3 3 5% 46%	1800
December	1st to 10th	Day	31% 8% 3 6% 3 2 2 2 3 5% 35%	2940
	11th to 20th	Day	19% 7% 2 5% 4 3 4 3 4 5% 44%	3391
	21st to end	Day	14% 5% 2 4 3 3 3 2 3 4 57%	4488
January	1st to 10th	Night	20% 6% 1 4 2 2 2 2 4 6% 50%	5888
	11th to 20th	Night	12% 4 1 2 2 2 3 3 3 5% 62%	5900
	21st to end	Night	32% 5% 2 4 2 2 2 2 3 5% 40%	6058
February	1st to 10th	Night	19% 6% 1 4 3 3 3 3 5% 6% 48%	2400
	11th to 20th	Night	21% 7% 1 4 3 3 2 2 4 5% 48%	2338
	21st to end	Night	20% 6% 2 5% 3 3 3 3 4 5% 46%	1980
March	1st to 10th	Night	19% 6% 2 5% 4 3 3 3 4 5% 46%	5012
	11th to 20th	Night	16% 6% 2 6% 4 3 3 2 3 5% 51%	4980
	21st to end	Night	22% 8% 2 4 3 3 3 3 4 5% 43%	5467
April	1st to 10th	Night	14% 6% 2 6% 3 4 3 3 5% 7% 47%	2200
	11th to 20th	Night	20% 7% 2 5% 4 3 3 3 4 6% 46%	2149
	21st to end	Night	19% 7% 3 5% 4 3 3 3 4 5% 43%	1890
May	1st to 10th	Night	25% 9% 2 6% 3 3 3 3 4 5% 36%	4053
	11th to 20th	Night	11% 4 2 5% 3 4 4 3 6% 9% 49%	4053
	21st to end	Night	13% 6% 2 7% 5% 6% 4 4 6% 9% 38%	4443
June	1st to 10th	Night	6% 4 2 4 3 4 3 3 6% 8% 58%	1800
	11th to 20th	Night	6% 4 1 4 3 4 3 4 6% 10% 55%	1800
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 10% 50%	1800
July	1st to 10th	Night	6% 5% 2 5% 4 3 4 5% 6% 11% 50%	2895
	11th to 20th	Night	4 4 2 5% 4 5% 6% 5% 7% 10% 49%	2850
	21st to end	Night	10% 6% 2 7% 4 4 5% 4 7% 10% 42%	3098
August	1st to 10th	Night	1 1 2 2 2 3 3 4 8% 73%	2977
	11th to 20th	Night	3 2 1 4 3 4 4 4 7% 12% 57%	3113
	21st to end	Night	6% 3 2 4 3 4 4 3 6% 10% 54%	3633
September	1st to 10th	Night	9% 5% 2 5% 4 5% 5% 4 6% 8% 47%	2090
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 6% 8% 46%	2090
	21st to end	Night	8% 4 2 6% 4 4 4 4 6% 7% 51%	2090
October	1st to 10th	Night	6% 4 1 4 3 3 4 4 5% 8% 59%	2090
	11th to 20th	Night	13% 5% 1 4 2 2 4 2 4 6% 55%	2161
	21st to end	Night	14% 4 2 3 2 2 2 2 2 4 61%	2508
November	1st to 10th	Night	15% 6% 3 5% 3 3 2 3 4 5% 52%	2340
	11th to 20th	Night	11% 6% 2 4 2 3 3 3 4 6% 55%	2530
	21st to end	Night	19% 6% 2 5% 3 3 3 2 4 6% 48%	2600
December	1st to 10th	Night	30% 8% 3 5% 3 3 2 2 4 4 36%	4250
	11th to 20th	Night	22% 6% 2 5% 3 2 2 2 3 5% 47%	4816
	21st to end	Night	18% 5% 1 4 2 2 2 2 3 5% 55%	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	40% 2 5% 19% 23% 12%	4078
	11th to 20th	Day	35% 2 4 18% 21% 20%	4080
	21st to end	Day	47% 2 3 12% 24% 12%	4963
February	1st to 10th	Day	44% 3 5% 13% 19% 17%	2000
	11th to 20th	Day	42% 3 6% 17% 19% 12%	1959
	21st to end	Day	39% 4 8% 16% 19% 15%	1650
March	1st to 10th	Day	35% 3 6% 19% 15% 21%	4843
	11th to 20th	Day	38% 4 4 9% 16% 29%	4980
	21st to end	Day	42% 3 7% 20% 18% 11%	5464
April	1st to 10th	Day	40% 5% 9% 16% 17% 14%	2200
	11th to 20th	Day	43% 4 6% 17% 17% 13%	2194
	21st to end	Day	42% 5% 7% 19% 17% 11%	2508
May	1st to 10th	Day	49% 5% 9% 19% 13% 5%	5900
	11th to 20th	Day	33% 6% 11% 23% 19% 9%	5866
	21st to end	Day	38% 6% 10% 30% 14% 1	6447
June	1st to 10th	Day	27% 9% 15% 29% 18% 3	2600
	11th to 20th	Day	31% 6% 13% 26% 19% 5%	2600
	21st to end	Day	38% 8% 15% 25% 12% 3	2600
July	1st to 10th	Day	34% 5% 16% 37% 7% 1	4122
	11th to 20th	Day	34% 6% 18% 27% 14% 1	4051
	21st to end	Day	38% 8% 18% 28% 9%	4325
August	1st to 10th	Day	15% 4 11% 44% 25% 1	3944
	11th to 20th	Day	21% 6% 23% 36% 11% 2	3762
	21st to end	Day	30% 8% 19% 23% 18% 2	4024
September	1st to 10th	Day	40% 8% 10% 18% 16% 7%	2090
	11th to 20th	Day	39% 8% 10% 20% 18% 5%	2090
	21st to end	Day	32% 7% 11% 21% 17% 13%	2090
October	1st to 10th	Day	18% 4 6% 11% 24% 36%	3480
	11th to 20th	Day	26% 4 8% 18% 23% 21%	3302
	21st to end	Day	27% 4 6% 18% 29% 17%	3428
November	1st to 10th	Day	42% 4 8% 18% 17% 12%	1958
	11th to 20th	Day	35% 5% 8% 16% 20% 16%	1865
	21st to end	Day	42% 3 7% 16% 19% 13%	1800
December	1st to 10th	Day	55% 3 4 15% 14% 9%	2940
	11th to 20th	Day	42% 4 8% 15% 18% 13%	3389
	21st to end	Day	33% 2 4 19% 27% 15%	4488
January	1st to 10th	Night	37% 2 6% 19% 20% 16%	5888
	11th to 20th	Night	28% 1 5% 19% 27% 21%	5900
	21st to end	Night	50% 2 3 11% 20% 14%	6058
February	1st to 10th	Night	38% 4 8% 13% 19% 19%	2400
	11th to 20th	Night	40% 3 6% 17% 19% 15%	2338
	21st to end	Night	40% 4 6% 17% 18% 16%	1980
March	1st to 10th	Night	40% 3 8% 18% 15% 17%	5012
	11th to 20th	Night	37% 4 7% 12% 14% 26%	4980
	21st to end	Night	42% 4 10% 17% 17% 10%	5467
April	1st to 10th	Night	35% 6% 11% 19% 16% 14%	2200
	11th to 20th	Night	40% 4 9% 14% 16% 16%	2149
	21st to end	Night	43% 5% 9% 18% 15% 11%	1890
May	1st to 10th	Night	49% 5% 8% 18% 14% 6%	4080
	11th to 20th	Night	31% 6% 11% 22% 20% 10%	4054
	21st to end	Night	40% 7% 14% 28% 11% 1	4452
June	1st to 10th	Night	22% 6% 18% 31% 22% 2	1800
	11th to 20th	Night	23% 4 14% 32% 22% 5%	1800
	21st to end	Night	29% 6% 19% 29% 14% 2	1800
July	1st to 10th	Night	28% 7% 23% 35% 6% 1	2850
	11th to 20th	Night	28% 7% 22% 27% 14% 1	2796
	21st to end	Night	35% 7% 23% 26% 7%	2999
August	1st to 10th	Night	11% 5% 16% 47% 21%	2967
	11th to 20th	Night	19% 7% 32% 34% 7% 1	3143
	21st to end	Night	24% 10% 24% 26% 15% 2	3633
September	1st to 10th	Night	33% 7% 18% 20% 15% 7%	2090
	11th to 20th	Night	35% 8% 14% 23% 15% 5%	2090
	21st to end	Night	27% 7% 15% 23% 17% 12%	2090
October	1st to 10th	Night	16% 4 6% 17% 20% 37%	3480
	11th to 20th	Night	21% 3 10% 18% 28% 21%	3582
	21st to end	Night	24% 2 7% 20% 30% 16%	4120
November	1st to 10th	Night	35% 4 8% 20% 20% 14%	2340
	11th to 20th	Night	30% 5% 10% 18% 20% 17%	2530
	21st to end	Night	38% 4 9% 17% 19% 14%	2600
December	1st to 10th	Night	54% 3 6% 14% 12% 11%	4250
	11th to 20th	Night	42% 3 8% 12% 19% 17%	4817
	21st to end	Night	33% 3 5% 14% 27% 18%	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	23% 11% 36% 22% 6% 1	300
	11th to 20th	Day	23% 14% 31% 25% 6% 1	300
	21st to end	Day	20% 15% 32% 25% 9%	330
February	1st to 10th	Day	28% 12% 26% 23% 10%	300
	11th to 20th	Day	26% 16% 32% 18% 7% 1	300
	21st to end	Day	23% 14% 41% 20% 3	249
March	1st to 10th	Day	21% 16% 37% 23% 3	300
	11th to 20th	Day	15% 16% 43% 21% 5%	300
	21st to end	Day	19% 15% 36% 25% 4	330
April	1st to 10th	Day	20% 14% 36% 25% 5%	300
	11th to 20th	Day	15% 17% 35% 24% 8%	312
	21st to end	Day	17% 14% 37% 27% 5% 1	608
May	1st to 10th	Day	18% 12% 34% 29% 8%	703
	11th to 20th	Day	14% 11% 29% 34% 12% 1	725
	21st to end	Day	14% 6% 28% 37% 14% 1	789
June	1st to 10th	Day	6% 3 15% 42% 30% 5%	730
	11th to 20th	Day	8% 2 14% 45% 27% 4	730
	21st to end	Day	9% 2 12% 34% 34% 10%	730
July	1st to 10th	Day	11% 3 8% 30% 38% 11%	697
	11th to 20th	Day	5% 1 7% 26% 46% 13%	668
	21st to end	Day	6% 1 6% 29% 44% 14%	739
August	1st to 10th	Day	2 1 3 30% 46% 17%	595
	11th to 20th	Day	5% 1 4 34% 41% 15%	477
	21st to end	Day	5% 2 6% 25% 40% 22%	449
September	1st to 10th	Day	9% 1 9% 24% 37% 19%	300
	11th to 20th	Day	5% 1 6% 26% 42% 20%	300
	21st to end	Day	10% 2 8% 25% 40% 16%	300
October	1st to 10th	Day	7% 2 13% 32% 37% 9%	300
	11th to 20th	Day	11% 4 14% 40% 26% 5%	300
	21st to end	Day	17% 11% 21% 27% 20% 5%	330
November	1st to 10th	Day	18% 8% 21% 32% 15% 5%	300
	11th to 20th	Day	20% 7% 24% 29% 16% 4	300
	21st to end	Day	22% 13% 34% 20% 9% 2	300
December	1st to 10th	Day	21% 10% 28% 25% 13% 3	300
	11th to 20th	Day	23% 15% 25% 28% 8% 1	300
	21st to end	Day	17% 9% 34% 32% 8% 1	330
January	1st to 10th	Night	21% 7% 28% 34% 10% 1	1000
	11th to 20th	Night	21% 8% 29% 34% 8%	1000
	21st to end	Night	24% 6% 28% 34% 7%	915
February	1st to 10th	Night	23% 9% 30% 32% 6%	800
	11th to 20th	Night	27% 10% 30% 29% 4	784
	21st to end	Night	24% 12% 33% 28% 3	660
March	1st to 10th	Night	22% 11% 35% 29% 3 1	645
	11th to 20th	Night	20% 11% 33% 32% 5%	600
	21st to end	Night	18% 11% 40% 27% 4	660
April	1st to 10th	Night	16% 10% 43% 28% 4	600
	11th to 20th	Night	22% 11% 36% 27% 4	593
	21st to end	Night	21% 10% 39% 24% 6%	600
May	1st to 10th	Night	18% 10% 41% 27% 4	600
	11th to 20th	Night	15% 8% 35% 35% 7%	591
	21st to end	Night	13% 7% 38% 33% 10%	650
June	1st to 10th	Night	5% 3 25% 46% 20% 1	600
	11th to 20th	Night	6% 3 31% 48% 12% 1	600
	21st to end	Night	6% 3 20% 45% 25% 2	600
July	1st to 10th	Night	9% 4 14% 46% 26% 2	570
	11th to 20th	Night	4 1 10% 50% 33% 2	552
	21st to end	Night	5% 2 10% 47% 34% 1	624
August	1st to 10th	Night	1 1 9% 50% 38% 2	567
	11th to 20th	Night	5% 1 9% 51% 31% 4	564
	21st to end	Night	6% 1 6% 42% 41% 4	627
September	1st to 10th	Night	10% 2 8% 41% 36% 4	570
	11th to 20th	Night	12% 1 9% 39% 36% 4	570
	21st to end	Night	7% 1 12% 41% 37% 3	570
October	1st to 10th	Night	6% 2 13% 47% 31% 2	570
	11th to 20th	Night	14% 3 17% 42% 22% 2	641
	21st to end	Night	13% 7% 23% 35% 20% 2	836
November	1st to 10th	Night	16% 6% 19% 37% 21% 1	780
	11th to 20th	Night	13% 4 24% 41% 17% 2	930
	21st to end	Night	21% 7% 25% 32% 15% 1	1000
December	1st to 10th	Night	21% 7% 21% 36% 14% 1	1000
	11th to 20th	Night	24% 9% 23% 33% 11%	1000
	21st to end	Night	20% 6% 26% 37% 11%	1100

Page generated at 2025-07-03 00:30:11. Version ac07

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