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 2024-01-22.

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% 5% 2 5% 5% 3 4 4 5% 7% 45%	74942
	Night	15% 5% 2 4 4 2 3 4 4 7% 51%	75020
Transparency	Day	38% 5% 10% 20% 18% 9%	74667
	Night	34% 5% 11% 20% 18% 12%	74815
Seeing	Day	16% 8% 23% 30% 19% 3	13662
	Night	17% 7% 25% 35% 15% 1	23752

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	17% 5% 2 4 4 2 3 4 3 5% 51%	1707
	11th to 20th	Day	19% 6% 3 5% 4 2 3 3 3 5% 48%	1710
	21st to end	Day	16% 5% 2 4 4 3 3 4 4 6% 50%	1975
February	1st to 10th	Day	21% 7% 2 4 3 2 3 3 4 5% 46%	1800
	11th to 20th	Day	20% 7% 2 4 4 3 4 3 5% 6% 43%	1749
	21st to end	Day	18% 7% 2 4 3 2 3 3 3 6% 49%	1480
March	1st to 10th	Day	22% 7% 2 5% 4 2 3 3 4 5% 42%	1927
	11th to 20th	Day	17% 6% 2 5% 5% 2 3 4 5% 6% 45%	1980
	21st to end	Day	19% 5% 2 4 4 2 3 3 5% 6% 46%	2178
April	1st to 10th	Day	15% 6% 3 5% 5% 3 4 5% 4 6% 44%	1980
	11th to 20th	Day	22% 5% 3 4 4 2 3 4 5% 5% 43%	1990
	21st to end	Day	17% 7% 3 5% 5% 3 3 3 4 6% 44%	2250
May	1st to 10th	Day	17% 5% 3 6% 5% 4 4 4 4 7% 40%	2340
	11th to 20th	Day	16% 6% 2 6% 6% 3 5% 4 6% 7% 38%	2302
	21st to end	Day	17% 5% 3 6% 6% 4 6% 5% 5% 9% 33%	2531
June	1st to 10th	Day	7% 4 2 5% 5% 4 6% 6% 6% 8% 47%	2340
	11th to 20th	Day	8% 5% 2 6% 6% 4 6% 6% 7% 8% 44%	2340
	21st to end	Day	10% 5% 3 6% 7% 4 6% 6% 6% 8% 37%	2340
July	1st to 10th	Day	12% 7% 3 6% 6% 4 5% 5% 5% 9% 37%	2340
	11th to 20th	Day	5% 3 2 4 5% 4 6% 6% 7% 10% 48%	2340
	21st to end	Day	6% 3 2 4 5% 3 5% 6% 7% 11% 48%	2546
August	1st to 10th	Day	2 2 2 3 4 3 4 5% 5% 9% 60%	2244
	11th to 20th	Day	7% 3 1 5% 6% 3 5% 5% 6% 10% 49%	2123
	21st to end	Day	6% 4 2 6% 6% 4 6% 7% 7% 10% 42%	2303
September	1st to 10th	Day	11% 7% 4 6% 5% 4 5% 5% 6% 9% 39%	1980
	11th to 20th	Day	12% 6% 2 6% 5% 4 6% 6% 6% 8% 39%	1980
	21st to end	Day	8% 5% 2 5% 5% 4 5% 5% 5% 8% 47%	1980
October	1st to 10th	Day	7% 5% 2 4 6% 3 4 5% 6% 7% 52%	1980
	11th to 20th	Day	15% 4 2 4 4 3 4 4 4 6% 49%	1900
	21st to end	Day	12% 4 2 4 3 2 3 4 3 6% 57%	1980
November	1st to 10th	Day	16% 7% 3 6% 5% 3 3 3 4 7% 44%	1858
	11th to 20th	Day	16% 6% 2 4 5% 2 4 4 5% 6% 48%	1753
	21st to end	Day	20% 7% 3 5% 4 2 3 3 4 5% 45%	1710
December	1st to 10th	Day	18% 5% 3 4 3 2 4 3 5% 7% 46%	1710
	11th to 20th	Day	19% 6% 2 4 5% 2 4 4 4 5% 45%	2161
	21st to end	Day	17% 6% 2 3 3 2 3 3 4 4 54%	3135
January	1st to 10th	Night	18% 5% 1 3 3 2 4 3 4 6% 53%	2458
	11th to 20th	Night	17% 6% 3 3 3 2 3 3 4 5% 51%	2470
	21st to end	Night	20% 5% 2 4 3 2 3 3 4 5% 49%	2429
February	1st to 10th	Night	20% 5% 2 3 3 2 3 3 4 6% 49%	2160
	11th to 20th	Night	20% 7% 3 4 3 2 3 3 3 5% 49%	2098
	21st to end	Night	21% 4 2 4 4 1 3 3 4 6% 46%	1776
March	1st to 10th	Night	20% 6% 3 5% 4 2 3 3 4 5% 45%	2031
	11th to 20th	Night	19% 6% 2 5% 4 2 4 3 4 5% 46%	1980
	21st to end	Night	19% 6% 2 4 3 2 3 4 4 6% 48%	2178
April	1st to 10th	Night	15% 5% 2 5% 4 2 4 3 5% 8% 46%	1980
	11th to 20th	Night	22% 6% 2 4 3 2 4 3 3 5% 47%	1915
	21st to end	Night	21% 4 3 4 5% 1 3 4 4 4 47%	1707
May	1st to 10th	Night	16% 6% 3 5% 5% 2 3 4 5% 7% 45%	1620
	11th to 20th	Night	16% 5% 2 4 4 3 5% 4 5% 8% 45%	1593
	21st to end	Night	15% 6% 2 5% 4 2 4 4 5% 11% 42%	1745
June	1st to 10th	Night	7% 4 1 3 3 2 4 4 4 8% 60%	1620
	11th to 20th	Night	6% 4 2 4 3 2 4 4 5% 9% 57%	1620
	21st to end	Night	8% 5% 2 5% 4 2 5% 5% 5% 11% 48%	1620
July	1st to 10th	Night	10% 5% 2 4 4 3 5% 5% 6% 10% 46%	1620
	11th to 20th	Night	4 2 1 3 4 3 4 5% 5% 9% 60%	1620
	21st to end	Night	6% 2 1 3 3 2 4 5% 7% 10% 57%	1772
August	1st to 10th	Night	3 1 1 3 3 1 3 3 4 9% 68%	1690
	11th to 20th	Night	5% 4 2 4 4 2 4 6% 7% 11% 52%	1778
	21st to end	Night	7% 4 1 5% 4 3 6% 5% 7% 10% 48%	2052
September	1st to 10th	Night	9% 6% 2 5% 4 3 5% 5% 5% 8% 48%	1980
	11th to 20th	Night	12% 5% 2 6% 5% 3 5% 4 5% 7% 45%	1980
	21st to end	Night	9% 4 3 5% 4 3 4 5% 6% 8% 50%	1980
October	1st to 10th	Night	8% 4 1 3 4 3 4 5% 5% 7% 58%	1980
	11th to 20th	Night	14% 4 2 4 4 2 2 4 4 5% 54%	2042
	21st to end	Night	16% 4 2 2 2 2 2 2 3 3 61%	2376
November	1st to 10th	Night	15% 6% 3 4 4 2 3 3 3 5% 53%	2220
	11th to 20th	Night	13% 5% 2 3 3 3 4 3 4 6% 54%	2422
	21st to end	Night	21% 4 3 4 4 2 3 4 4 6% 47%	2470
December	1st to 10th	Night	17% 5% 2 4 3 2 3 3 4 6% 50%	2470
	11th to 20th	Night	19% 7% 2 6% 4 1 3 3 3 5% 47%	3036
	21st to end	Night	20% 5% 2 3 3 2 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	37% 3 4 17% 22% 17%	1708
	11th to 20th	Day	41% 3 5% 14% 19% 17%	1710
	21st to end	Day	37% 3 7% 14% 23% 16%	1975
February	1st to 10th	Day	43% 2 6% 13% 20% 16%	1800
	11th to 20th	Day	43% 4 6% 16% 19% 13%	1759
	21st to end	Day	39% 4 7% 16% 18% 16%	1480
March	1st to 10th	Day	46% 3 8% 13% 17% 12%	1927
	11th to 20th	Day	41% 4 7% 15% 17% 16%	1980
	21st to end	Day	39% 5% 8% 17% 18% 13%	2178
April	1st to 10th	Day	43% 5% 7% 17% 18% 11%	1980
	11th to 20th	Day	43% 5% 6% 17% 16% 13%	1990
	21st to end	Day	42% 4 6% 19% 18% 11%	2250
May	1st to 10th	Day	44% 6% 8% 16% 18% 8%	2340
	11th to 20th	Day	44% 6% 9% 18% 18% 5%	2306
	21st to end	Day	47% 6% 11% 20% 15% 1	2531
June	1st to 10th	Day	31% 8% 13% 25% 19% 3	2340
	11th to 20th	Day	35% 6% 13% 25% 17% 4	2340
	21st to end	Day	42% 7% 12% 23% 13% 3	2340
July	1st to 10th	Day	42% 7% 14% 24% 10% 2	2276
	11th to 20th	Day	27% 8% 19% 26% 18% 2	2271
	21st to end	Day	25% 7% 21% 33% 13% 1	2405
August	1st to 10th	Day	20% 5% 16% 39% 19% 1	2230
	11th to 20th	Day	29% 7% 17% 30% 12% 4	2123
	21st to end	Day	35% 8% 16% 25% 13% 3	2303
September	1st to 10th	Day	41% 7% 10% 19% 16% 7%	1980
	11th to 20th	Day	41% 8% 11% 19% 16% 5%	1980
	21st to end	Day	33% 7% 10% 21% 18% 11%	1980
October	1st to 10th	Day	29% 7% 12% 18% 26% 9%	1980
	11th to 20th	Day	37% 4 9% 16% 18% 16%	1900
	21st to end	Day	30% 4 5% 15% 25% 21%	1980
November	1st to 10th	Day	42% 4 7% 20% 16% 11%	1858
	11th to 20th	Day	38% 4 8% 15% 20% 14%	1753
	21st to end	Day	43% 5% 6% 15% 19% 13%	1710
December	1st to 10th	Day	40% 4 8% 20% 19% 9%	1710
	11th to 20th	Day	42% 6% 6% 9% 22% 15%	2159
	21st to end	Day	35% 4 5% 16% 26% 14%	3135
January	1st to 10th	Night	34% 3 6% 16% 22% 19%	2458
	11th to 20th	Night	38% 2 5% 16% 21% 17%	2470
	21st to end	Night	40% 4 4 14% 20% 19%	2429
February	1st to 10th	Night	37% 5% 6% 13% 20% 19%	2160
	11th to 20th	Night	40% 3 6% 17% 19% 16%	2098
	21st to end	Night	40% 4 6% 16% 17% 16%	1776
March	1st to 10th	Night	41% 4 9% 16% 17% 11%	2031
	11th to 20th	Night	42% 5% 8% 13% 16% 17%	1980
	21st to end	Night	37% 5% 10% 16% 18% 14%	2178
April	1st to 10th	Night	37% 5% 10% 20% 15% 13%	1980
	11th to 20th	Night	41% 4 8% 14% 16% 16%	1915
	21st to end	Night	42% 5% 8% 19% 15% 11%	1707
May	1st to 10th	Night	38% 5% 11% 19% 16% 10%	1620
	11th to 20th	Night	37% 7% 10% 24% 17% 5%	1594
	21st to end	Night	38% 7% 16% 25% 14% 1	1746
June	1st to 10th	Night	23% 6% 16% 30% 21% 3	1620
	11th to 20th	Night	23% 7% 14% 30% 20% 5%	1620
	21st to end	Night	31% 7% 18% 27% 14% 3	1620
July	1st to 10th	Night	31% 9% 21% 28% 9% 1	1575
	11th to 20th	Night	19% 9% 23% 30% 19% 1	1566
	21st to end	Night	19% 10% 27% 33% 10% 1	1673
August	1st to 10th	Night	14% 7% 23% 40% 16% 1	1680
	11th to 20th	Night	23% 11% 23% 30% 11% 2	1778
	21st to end	Night	29% 8% 24% 24% 11% 3	2052
September	1st to 10th	Night	34% 7% 17% 20% 15% 6%	1980
	11th to 20th	Night	37% 8% 16% 21% 14% 5%	1980
	21st to end	Night	30% 7% 14% 22% 17% 11%	1980
October	1st to 10th	Night	26% 5% 12% 27% 20% 11%	1980
	11th to 20th	Night	31% 5% 10% 18% 19% 17%	2042
	21st to end	Night	30% 3 6% 15% 24% 22%	2376
November	1st to 10th	Night	35% 4 7% 19% 21% 14%	2220
	11th to 20th	Night	33% 5% 10% 18% 20% 16%	2422
	21st to end	Night	40% 4 7% 16% 19% 13%	2470
December	1st to 10th	Night	37% 4 8% 19% 20% 13%	2470
	11th to 20th	Night	42% 4 5% 13% 17% 20%	3037
	21st to end	Night	35% 4 5% 13% 28% 14%	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	27% 18% 34% 17% 4 1	270
	11th to 20th	Day	29% 14% 31% 20% 6% 1	270
	21st to end	Day	21% 18% 38% 18% 6%	267
February	1st to 10th	Day	35% 15% 28% 16% 7%	240
	11th to 20th	Day	31% 20% 33% 13% 3 1	240
	21st to end	Day	31% 21% 37% 10% 2	198
March	1st to 10th	Day	29% 18% 39% 12% 2	240
	11th to 20th	Day	20% 21% 43% 13% 3	240
	21st to end	Day	23% 20% 39% 16% 2	264
April	1st to 10th	Day	25% 15% 43% 16% 2	240
	11th to 20th	Day	23% 19% 35% 20% 3	268
	21st to end	Day	21% 14% 40% 21% 5%	510
May	1st to 10th	Day	19% 15% 34% 27% 5%	603
	11th to 20th	Day	19% 10% 31% 30% 9% 1	625
	21st to end	Day	17% 5% 33% 32% 12% 1	679
June	1st to 10th	Day	8% 2 19% 44% 24% 2	630
	11th to 20th	Day	8% 3 20% 48% 19% 2	630
	21st to end	Day	10% 3 16% 37% 28% 5%	630
July	1st to 10th	Day	11% 4 11% 34% 34% 6%	621
	11th to 20th	Day	6% 1 8% 36% 43% 5%	618
	21st to end	Day	6% 1 5% 37% 44% 7%	684
August	1st to 10th	Day	3 1 7% 37% 44% 8%	542
	11th to 20th	Day	7% 1 5% 42% 35% 8%	437
	21st to end	Day	5% 2 7% 35% 39% 12%	422
September	1st to 10th	Day	10% 1 9% 31% 43% 7%	270
	11th to 20th	Day	7% 3 6% 34% 41% 9%	270
	21st to end	Day	9% 1 12% 36% 33% 9%	270
October	1st to 10th	Day	9% 3 18% 42% 26% 4	270
	11th to 20th	Day	14% 4 23% 40% 17% 2	270
	21st to end	Day	22% 13% 23% 29% 9% 3	297
November	1st to 10th	Day	21% 10% 26% 30% 11% 3	270
	11th to 20th	Day	23% 13% 27% 27% 10% 1	270
	21st to end	Day	29% 17% 33% 14% 6% 1	270
December	1st to 10th	Day	20% 12% 34% 25% 7% 2	270
	11th to 20th	Day	30% 15% 29% 20% 6%	270
	21st to end	Day	22% 12% 36% 23% 4 2	297
January	1st to 10th	Night	21% 9% 24% 35% 10%	950
	11th to 20th	Night	23% 11% 28% 31% 7%	950
	21st to end	Night	26% 9% 26% 32% 7%	829
February	1st to 10th	Night	26% 11% 26% 30% 8%	720
	11th to 20th	Night	27% 12% 30% 25% 5%	704
	21st to end	Night	28% 14% 32% 24% 3	592
March	1st to 10th	Night	25% 12% 32% 27% 4	591
	11th to 20th	Night	23% 12% 31% 30% 4	540
	21st to end	Night	19% 14% 38% 24% 4	594
April	1st to 10th	Night	16% 13% 43% 24% 4	540
	11th to 20th	Night	24% 11% 36% 25% 3	533
	21st to end	Night	24% 9% 37% 24% 6% 1	540
May	1st to 10th	Night	16% 10% 44% 26% 4	540
	11th to 20th	Night	18% 6% 37% 31% 8%	531
	21st to end	Night	15% 9% 34% 30% 11%	584
June	1st to 10th	Night	7% 3 26% 46% 18% 1	540
	11th to 20th	Night	6% 3 34% 44% 11% 1	540
	21st to end	Night	8% 3 26% 39% 21% 2	540
July	1st to 10th	Night	10% 2 19% 41% 25% 3	525
	11th to 20th	Night	3 2 13% 53% 28% 2	522
	21st to end	Night	6% 1 10% 51% 28% 2	591
August	1st to 10th	Night	2 1 11% 52% 31% 3	537
	11th to 20th	Night	7% 1 13% 47% 29% 4	534
	21st to end	Night	7% 1 7% 41% 39% 4	594
September	1st to 10th	Night	10% 2 9% 43% 31% 4	540
	11th to 20th	Night	13% 1 10% 40% 33% 3	540
	21st to end	Night	10% 2 13% 38% 33% 4	540
October	1st to 10th	Night	8% 2 14% 47% 25% 4	540
	11th to 20th	Night	16% 4 20% 38% 21% 2	602
	21st to end	Night	17% 8% 22% 31% 18% 3	792
November	1st to 10th	Night	16% 6% 22% 34% 21% 1	740
	11th to 20th	Night	15% 7% 21% 38% 16% 2	902
	21st to end	Night	23% 9% 23% 31% 13% 1	950
December	1st to 10th	Night	22% 6% 24% 34% 12% 1	950
	11th to 20th	Night	27% 8% 25% 30% 10%	950
	21st to end	Night	22% 10% 26% 33% 9%	1045

Page generated at 2024-01-19 08:28:35. Version ac07

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