UTSA Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UTSA Observatory clear sky chart from 2005-11-06 to 2025-01-22.

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%	105232
	Night	16% 5% 2 4 3 3 3 3 4 6% 50%	104265
Transparency	Day	36% 6% 10% 21% 18% 10%	109076
	Night	34% 5% 10% 20% 19% 12%	108548
Seeing	Day	16% 9% 24% 30% 19% 3	14992
	Night	18% 7% 25% 35% 15% 1	25054

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% 6% 2 4 3 2 3 3 4 6% 52%	2937
	11th to 20th	Day	18% 5% 2 4 2 2 2 2 4 5% 55%	2940
	21st to end	Day	19% 7% 2 5% 3 3 3 3 5% 8% 43%	3469
February	1st to 10th	Day	21% 7% 2 4 3 3 3 3 4 5% 45%	1900
	11th to 20th	Day	20% 7% 2 5% 3 4 3 3 5% 6% 44%	1849
	21st to end	Day	18% 7% 2 4 2 3 3 2 4 5% 49%	1570
March	1st to 10th	Day	18% 7% 3 6% 4 4 3 3 4 5% 43%	3360
	11th to 20th	Day	23% 9% 3 8% 5% 4 4 3 4 4 33%	3480
	21st to end	Day	19% 6% 2 5% 3 3 3 3 5% 7% 46%	3814
April	1st to 10th	Day	15% 7% 3 6% 4 4 4 4 4 6% 44%	2090
	11th to 20th	Day	22% 6% 2 5% 4 3 3 3 5% 4 42%	2102
	21st to end	Day	17% 8% 2 6% 5% 4 4 2 4 6% 43%	2375
May	1st to 10th	Day	25% 8% 3 7% 4 4 4 3 4 6% 32%	4082
	11th to 20th	Day	15% 6% 2 6% 5% 4 4 3 6% 7% 42%	4082
	21st to end	Day	12% 5% 3 6% 5% 5% 5% 4 7% 10% 38%	4477
June	1st to 10th	Day	7% 4 2 5% 5% 5% 5% 5% 6% 8% 47%	2470
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 8% 44%	2470
	21st to end	Day	10% 6% 2 7% 6% 6% 6% 5% 6% 8% 38%	2470
July	1st to 10th	Day	8% 6% 2 6% 5% 4 4 4 5% 8% 47%	4120
	11th to 20th	Day	7% 4 2 5% 5% 5% 6% 5% 7% 10% 45%	4120
	21st to end	Day	10% 5% 2 7% 6% 5% 5% 5% 7% 10% 37%	4465
August	1st to 10th	Day	2 2 1 2 3 3 3 3 5% 8% 67%	3950
	11th to 20th	Day	4 2 1 4 3 3 4 4 7% 11% 56%	3726
	21st to end	Day	5% 4 2 6% 4 5% 6% 5% 6% 11% 47%	4037
September	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 7% 9% 39%	2090
	11th to 20th	Day	11% 6% 2 6% 5% 6% 6% 5% 6% 8% 39%	2090
	21st to end	Day	8% 5% 2 5% 4 5% 5% 4 6% 8% 48%	2090
October	1st to 10th	Day	7% 5% 2 4 5% 4 4 3 6% 6% 54%	2090
	11th to 20th	Day	14% 4 2 5% 3 4 4 3 4 6% 50%	2005
	21st to end	Day	12% 5% 2 4 3 3 3 3 3 6% 57%	2090
November	1st to 10th	Day	16% 7% 2 7% 4 4 3 3 4 6% 44%	1958
	11th to 20th	Day	15% 6% 2 5% 4 3 4 3 5% 6% 49%	1845
	21st to end	Day	19% 7% 2 5% 4 3 3 3 4 5% 46%	1800
December	1st to 10th	Day	29% 8% 3 6% 3 3 2 2 3 5% 36%	2940
	11th to 20th	Day	20% 7% 2 5% 4 3 4 3 4 5% 45%	3391
	21st to end	Day	15% 5% 1 4 2 2 3 2 3 4 58%	4488
January	1st to 10th	Night	19% 5% 1 3 2 2 3 2 4 6% 53%	4238
	11th to 20th	Night	13% 4 1 2 2 2 2 1 3 4 65%	4250
	21st to end	Night	24% 6% 2 5% 3 2 3 2 4 6% 44%	4248
February	1st to 10th	Night	20% 6% 1 4 3 3 3 3 4 5% 48%	2280
	11th to 20th	Night	20% 7% 2 4 3 3 3 2 3 4 49%	2218
	21st to end	Night	20% 5% 2 5% 4 2 3 3 4 6% 47%	1884
March	1st to 10th	Night	20% 8% 2 5% 4 3 3 3 4 5% 41%	3494
	11th to 20th	Night	22% 10% 3 7% 5% 4 3 3 4 4 34%	3480
	21st to end	Night	20% 7% 2 5% 3 3 2 3 4 5% 46%	3817
April	1st to 10th	Night	15% 6% 2 5% 3 3 4 2 6% 7% 46%	2090
	11th to 20th	Night	22% 6% 2 5% 3 3 3 3 3 4 46%	2022
	21st to end	Night	22% 5% 2 5% 4 2 3 3 4 5% 45%	1801
May	1st to 10th	Night	27% 8% 2 5% 3 3 3 2 4 6% 37%	2823
	11th to 20th	Night	16% 5% 1 5% 3 3 3 3 4 7% 49%	2823
	21st to end	Night	16% 6% 2 6% 3 4 3 3 5% 9% 43%	3090
June	1st to 10th	Night	7% 4 2 3 2 3 4 3 5% 9% 59%	1710
	11th to 20th	Night	7% 4 2 4 3 4 4 3 5% 9% 56%	1710
	21st to end	Night	9% 6% 2 5% 4 4 4 4 5% 10% 47%	1710
July	1st to 10th	Night	7% 4 1 4 4 3 4 4 6% 10% 52%	2850
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 8% 9% 50%	2850
	21st to end	Night	14% 6% 2 5% 4 4 4 4 7% 9% 42%	3098
August	1st to 10th	Night	2 1 1 3 2 2 2 2 4 8% 73%	2985
	11th to 20th	Night	3 3 1 4 3 3 4 4 7% 11% 57%	3113
	21st to end	Night	6% 4 1 4 3 4 4 4 7% 10% 54%	3611
September	1st to 10th	Night	9% 6% 2 6% 3 5% 5% 4 5% 8% 47%	2090
	11th to 20th	Night	11% 6% 2 7% 4 4 5% 3 5% 7% 46%	2090
	21st to end	Night	9% 4 2 5% 4 4 4 4 6% 8% 50%	2090
October	1st to 10th	Night	7% 4 1 3 3 4 3 4 5% 6% 59%	2090
	11th to 20th	Night	14% 4 2 4 4 3 3 3 4 5% 55%	2156
	21st to end	Night	16% 4 2 3 2 2 2 2 3 3 61%	2508
November	1st to 10th	Night	17% 6% 2 5% 3 2 3 3 3 5% 51%	2340
	11th to 20th	Night	13% 5% 2 4 3 4 4 3 4 5% 55%	2550
	21st to end	Night	19% 4 2 4 4 3 3 3 4 6% 47%	2600
December	1st to 10th	Night	28% 8% 3 6% 3 3 3 2 3 5% 37%	4250
	11th to 20th	Night	24% 7% 2 5% 3 2 2 2 3 3 47%	4816
	21st to end	Night	19% 5% 1 3 2 2 2 2 3 5% 56%	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	34% 4 4 16% 26% 16%	2938
	11th to 20th	Day	34% 3 5% 15% 25% 19%	2940
	21st to end	Day	40% 4 5% 12% 28% 10%	3469
February	1st to 10th	Day	43% 2 5% 13% 20% 16%	1900
	11th to 20th	Day	42% 4 6% 16% 19% 13%	1859
	21st to end	Day	38% 4 7% 17% 18% 16%	1570
March	1st to 10th	Day	44% 4 7% 15% 20% 10%	3360
	11th to 20th	Day	54% 6% 5% 10% 16% 9%	3480
	21st to end	Day	37% 6% 9% 16% 19% 13%	3814
April	1st to 10th	Day	42% 5% 7% 16% 17% 13%	2090
	11th to 20th	Day	43% 5% 6% 16% 16% 13%	2102
	21st to end	Day	43% 5% 6% 18% 17% 11%	2375
May	1st to 10th	Day	50% 8% 10% 13% 13% 6%	4120
	11th to 20th	Day	39% 7% 10% 18% 19% 7%	4086
	21st to end	Day	36% 9% 13% 30% 11% 1	4489
June	1st to 10th	Day	31% 9% 13% 25% 19% 3	2470
	11th to 20th	Day	35% 6% 12% 24% 18% 4	2470
	21st to end	Day	41% 7% 12% 24% 13% 3	2470
July	1st to 10th	Day	33% 6% 15% 38% 7% 1	4056
	11th to 20th	Day	31% 8% 18% 28% 14% 1	4051
	21st to end	Day	37% 8% 18% 28% 9%	4325
August	1st to 10th	Day	15% 5% 10% 46% 24% 1	3936
	11th to 20th	Day	21% 6% 23% 36% 12% 2	3762
	21st to end	Day	30% 9% 18% 23% 19% 2	4036
September	1st to 10th	Day	41% 7% 10% 18% 16% 8%	2090
	11th to 20th	Day	39% 8% 11% 19% 17% 5%	2090
	21st to end	Day	32% 7% 10% 21% 17% 13%	2090
October	1st to 10th	Day	18% 4 7% 11% 25% 35%	3480
	11th to 20th	Day	27% 4 8% 18% 22% 21%	3310
	21st to end	Day	28% 4 5% 17% 32% 14%	3428
November	1st to 10th	Day	42% 4 7% 19% 16% 12%	1958
	11th to 20th	Day	37% 4 8% 15% 20% 16%	1845
	21st to end	Day	42% 5% 6% 15% 19% 13%	1800
December	1st to 10th	Day	53% 3 5% 16% 14% 9%	2940
	11th to 20th	Day	42% 5% 7% 13% 20% 13%	3389
	21st to end	Day	32% 3 4 17% 27% 17%	4488
January	1st to 10th	Night	33% 5% 5% 15% 22% 21%	4238
	11th to 20th	Night	26% 2 4 18% 28% 22%	4250
	21st to end	Night	43% 4 4 13% 25% 12%	4248
February	1st to 10th	Night	38% 5% 6% 13% 20% 19%	2280
	11th to 20th	Night	40% 3 6% 17% 19% 16%	2218
	21st to end	Night	40% 4 6% 17% 18% 16%	1884
March	1st to 10th	Night	44% 6% 7% 16% 18% 10%	3494
	11th to 20th	Night	51% 7% 7% 11% 12% 10%	3480
	21st to end	Night	38% 6% 9% 17% 20% 10%	3817
April	1st to 10th	Night	36% 5% 10% 19% 15% 14%	2090
	11th to 20th	Night	42% 4 8% 14% 16% 16%	2022
	21st to end	Night	43% 5% 8% 19% 15% 11%	1801
May	1st to 10th	Night	47% 7% 9% 17% 13% 8%	2850
	11th to 20th	Night	35% 6% 11% 22% 19% 7%	2824
	21st to end	Night	36% 8% 13% 29% 12% 2	3099
June	1st to 10th	Night	23% 6% 17% 31% 21% 3	1710
	11th to 20th	Night	24% 7% 14% 29% 21% 5%	1710
	21st to end	Night	31% 7% 17% 28% 14% 2	1710
July	1st to 10th	Night	26% 8% 24% 35% 6% 1	2805
	11th to 20th	Night	27% 10% 20% 28% 14% 1	2796
	21st to end	Night	36% 9% 22% 25% 7%	2999
August	1st to 10th	Night	12% 5% 17% 46% 20%	2975
	11th to 20th	Night	19% 8% 31% 34% 7% 1	3143
	21st to end	Night	25% 11% 23% 24% 16% 2	3611
September	1st to 10th	Night	35% 7% 16% 20% 15% 7%	2090
	11th to 20th	Night	37% 7% 15% 21% 14% 5%	2090
	21st to end	Night	30% 7% 14% 21% 16% 12%	2090
October	1st to 10th	Night	16% 3 7% 16% 20% 37%	3480
	11th to 20th	Night	22% 4 10% 16% 30% 19%	3577
	21st to end	Night	26% 3 7% 17% 31% 16%	4120
November	1st to 10th	Night	36% 4 7% 18% 20% 14%	2340
	11th to 20th	Night	31% 4 9% 18% 20% 17%	2550
	21st to end	Night	39% 4 7% 17% 19% 14%	2600
December	1st to 10th	Night	53% 3 6% 14% 13% 12%	4250
	11th to 20th	Night	43% 4 5% 11% 19% 19%	4817
	21st to end	Night	34% 3 4 13% 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	27% 19% 33% 16% 3 1	300
	11th to 20th	Day	28% 15% 31% 20% 6% 1	300
	21st to end	Day	22% 17% 36% 17% 7%	300
February	1st to 10th	Day	35% 16% 26% 17% 6%	270
	11th to 20th	Day	31% 20% 32% 13% 3 1	270
	21st to end	Day	30% 20% 38% 11% 2	225
March	1st to 10th	Day	27% 19% 39% 13% 1	270
	11th to 20th	Day	20% 21% 43% 13% 3	270
	21st to end	Day	24% 21% 38% 15% 2	297
April	1st to 10th	Day	25% 17% 41% 16% 1	270
	11th to 20th	Day	23% 18% 37% 19% 3	300
	21st to end	Day	21% 13% 41% 20% 5%	555
May	1st to 10th	Day	21% 15% 34% 25% 5%	653
	11th to 20th	Day	18% 11% 32% 30% 9% 1	675
	21st to end	Day	16% 6% 34% 31% 12% 1	734
June	1st to 10th	Day	9% 3 20% 44% 23% 2	680
	11th to 20th	Day	9% 3 20% 48% 19% 2	680
	21st to end	Day	10% 4 16% 37% 28% 5%	680
July	1st to 10th	Day	11% 4 11% 34% 34% 6%	671
	11th to 20th	Day	8% 1 8% 36% 42% 5%	668
	21st to end	Day	7% 2 6% 37% 42% 6%	739
August	1st to 10th	Day	3 1 7% 36% 44% 9%	587
	11th to 20th	Day	7% 1 6% 44% 35% 8%	477
	21st to end	Day	5% 2 6% 35% 39% 12%	461
September	1st to 10th	Day	10% 1 12% 32% 39% 6%	300
	11th to 20th	Day	6% 3 6% 36% 40% 9%	300
	21st to end	Day	8% 1 12% 37% 32% 9%	300
October	1st to 10th	Day	8% 2 19% 41% 26% 4	300
	11th to 20th	Day	13% 4 22% 42% 18% 2	300
	21st to end	Day	21% 13% 22% 30% 11% 4	330
November	1st to 10th	Day	21% 10% 28% 29% 10% 2	300
	11th to 20th	Day	21% 12% 29% 26% 11% 1	300
	21st to end	Day	26% 16% 34% 15% 7% 2	300
December	1st to 10th	Day	23% 12% 34% 23% 6% 2	300
	11th to 20th	Day	29% 14% 28% 22% 6% 1	300
	21st to end	Day	21% 12% 36% 24% 5% 2	330
January	1st to 10th	Night	21% 9% 24% 34% 10%	1000
	11th to 20th	Night	23% 11% 29% 30% 7%	1000
	21st to end	Night	26% 10% 26% 32% 7%	870
February	1st to 10th	Night	27% 12% 26% 29% 7%	760
	11th to 20th	Night	28% 12% 30% 25% 5%	744
	21st to end	Night	26% 15% 32% 25% 3	628
March	1st to 10th	Night	25% 12% 32% 26% 4	623
	11th to 20th	Night	24% 12% 30% 29% 4	570
	21st to end	Night	20% 14% 38% 23% 4	627
April	1st to 10th	Night	17% 13% 42% 24% 4	570
	11th to 20th	Night	25% 12% 36% 25% 2	563
	21st to end	Night	25% 9% 36% 24% 5% 1	570
May	1st to 10th	Night	18% 11% 43% 24% 4	570
	11th to 20th	Night	17% 7% 37% 31% 8%	561
	21st to end	Night	16% 10% 34% 29% 11%	617
June	1st to 10th	Night	7% 4 26% 45% 17% 1	570
	11th to 20th	Night	7% 3 34% 44% 11% 1	570
	21st to end	Night	8% 3 25% 40% 21% 2	570
July	1st to 10th	Night	10% 3 18% 41% 25% 3	555
	11th to 20th	Night	4 1 13% 53% 28% 2	552
	21st to end	Night	8% 2 11% 50% 27% 2	624
August	1st to 10th	Night	2 1 11% 50% 33% 3	567
	11th to 20th	Night	6% 1 12% 49% 28% 4	564
	21st to end	Night	7% 1 7% 41% 39% 4	627
September	1st to 10th	Night	11% 2 10% 44% 30% 4	570
	11th to 20th	Night	12% 1 10% 41% 33% 3	570
	21st to end	Night	10% 2 13% 39% 32% 4	570
October	1st to 10th	Night	8% 2 14% 48% 25% 4	570
	11th to 20th	Night	15% 3 20% 39% 21% 2	636
	21st to end	Night	17% 8% 22% 32% 19% 3	836
November	1st to 10th	Night	17% 6% 22% 33% 20% 1	780
	11th to 20th	Night	15% 7% 22% 39% 16% 2	950
	21st to end	Night	23% 10% 23% 31% 13% 1	1000
December	1st to 10th	Night	24% 7% 24% 33% 12% 1	1000
	11th to 20th	Night	27% 8% 24% 30% 10%	1000
	21st to end	Night	22% 10% 26% 34% 9%	1100

Page generated at 2025-01-19 02:06:48. Version ac07

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