Sam Houston State University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sam Houston State University Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	12% 6% 2 6% 4 4 4 4 6% 8% 43%	126940
	Night	12% 6% 2 5% 4 4 4 3 5% 7% 48%	124006
Transparency	Day	37% 6% 11% 20% 18% 7%	130589
	Night	35% 5% 10% 20% 20% 10%	128420
Seeing	Day	12% 7% 21% 26% 25% 9%	16371
	Night	14% 7% 24% 34% 19% 2	26203

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% 9% 3 6% 3 3 3 3 4 6% 44%	4077
	11th to 20th	Day	14% 6% 2 4 3 3 4 3 4 5% 51%	4080
	21st to end	Day	27% 10% 2 6% 3 2 2 2 3 6% 37%	4488
February	1st to 10th	Day	19% 9% 3 6% 4 3 3 3 5% 5% 39%	1865
	11th to 20th	Day	21% 8% 3 6% 5% 3 3 3 4 4 40%	1949
	21st to end	Day	18% 8% 2 6% 4 3 3 3 5% 5% 41%	1814
March	1st to 10th	Day	16% 10% 3 8% 4 5% 3 3 5% 5% 37%	4936
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 5% 7% 47%	4980
	21st to end	Day	15% 8% 3 7% 5% 4 4 3 5% 6% 40%	5462
April	1st to 10th	Day	14% 9% 2 7% 5% 5% 4 4 4 7% 40%	2200
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 4 5% 6% 40%	2341
	21st to end	Day	9% 8% 2 6% 5% 5% 5% 5% 7% 6% 44%	2400
May	1st to 10th	Day	17% 7% 3 8% 5% 5% 5% 5% 6% 7% 33%	5629
	11th to 20th	Day	11% 5% 2 7% 4 5% 5% 4 6% 8% 42%	5862
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 10% 35%	6434
June	1st to 10th	Day	5% 4 2 6% 6% 6% 6% 6% 8% 11% 40%	2600
	11th to 20th	Day	5% 4 1 5% 5% 5% 5% 5% 8% 11% 45%	2600
	21st to end	Day	5% 5% 2 7% 7% 6% 7% 6% 8% 11% 36%	2600
July	1st to 10th	Day	8% 5% 2 6% 5% 6% 7% 5% 8% 12% 35%	5900
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 14% 39%	5900
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	6422
August	1st to 10th	Day	3 1 3 2 3 4 3 6% 9% 66%	4234
	11th to 20th	Day	3 2 1 4 3 4 5% 4 8% 12% 52%	4046
	21st to end	Day	6% 4 2 6% 5% 6% 6% 5% 8% 11% 42%	4265
September	1st to 10th	Day	6% 6% 2 6% 6% 5% 6% 5% 7% 9% 43%	2200
	11th to 20th	Day	11% 6% 2 6% 6% 5% 4 4 7% 8% 43%	2200
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 7% 53%	2200
October	1st to 10th	Day	6% 4 1 4 3 3 4 4 5% 6% 61%	2074
	11th to 20th	Day	11% 4 2 5% 3 3 3 3 4 5% 57%	2000
	21st to end	Day	14% 4 2 4 3 2 3 3 4 5% 56%	2200
November	1st to 10th	Day	10% 6% 2 5% 4 4 5% 4 5% 6% 49%	2058
	11th to 20th	Day	11% 5% 2 5% 4 3 4 3 5% 7% 52%	2100
	21st to end	Day	16% 7% 2 6% 5% 3 4 3 3 6% 45%	1987
December	1st to 10th	Day	21% 6% 2 4 4 3 3 3 4 6% 44%	2958
	11th to 20th	Day	15% 8% 3 6% 4 4 4 4 5% 6% 42%	3391
	21st to end	Day	15% 8% 3 7% 4 3 4 3 4 5% 45%	4488
January	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 4 5% 50%	5888
	11th to 20th	Night	13% 7% 2 4 2 2 3 3 3 5% 56%	5900
	21st to end	Night	26% 10% 3 5% 3 3 3 2 4 5% 37%	6490
February	1st to 10th	Night	16% 8% 3 6% 4 3 3 3 4 5% 43%	2550
	11th to 20th	Night	19% 9% 2 5% 3 3 3 2 4 5% 44%	2338
	21st to end	Night	18% 8% 3 5% 4 4 4 2 3 6% 43%	1815
March	1st to 10th	Night	19% 8% 3 6% 3 3 4 3 4 7% 40%	4919
	11th to 20th	Night	13% 7% 2 6% 4 3 3 3 4 7% 48%	4980
	21st to end	Night	15% 9% 3 7% 5% 4 3 2 4 5% 42%	5467
April	1st to 10th	Night	11% 7% 2 5% 5% 4 4 4 6% 7% 46%	2200
	11th to 20th	Night	11% 5% 2 7% 4 4 4 3 5% 7% 46%	2005
	21st to end	Night	12% 6% 2 6% 3 3 4 3 6% 7% 47%	2000
May	1st to 10th	Night	15% 9% 3 7% 5% 5% 4 4 6% 8% 33%	4271
	11th to 20th	Night	10% 6% 3 7% 5% 4 4 4 6% 8% 43%	4053
	21st to end	Night	9% 6% 2 6% 5% 5% 6% 5% 8% 11% 37%	4443
June	1st to 10th	Night	4 4 1 5% 4 3 4 4 7% 10% 54%	1800
	11th to 20th	Night	2 3 1 4 2 4 4 4 6% 12% 59%	1800
	21st to end	Night	4 4 1 5% 4 4 5% 4 9% 12% 47%	1800
July	1st to 10th	Night	6% 5% 2 6% 5% 4 5% 5% 8% 15% 38%	4078
	11th to 20th	Night	3 4 2 4 3 4 4 4 7% 11% 53%	4077
	21st to end	Night	8% 6% 2 6% 4 4 5% 4 6% 10% 46%	4449
August	1st to 10th	Night	1 1 1 2 1 1 2 2 4 10% 74%	2926
	11th to 20th	Night	3 2 1 3 2 3 4 3 7% 13% 60%	3006
	21st to end	Night	4 4 2 5% 4 5% 5% 4 8% 11% 47%	3614
September	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 5% 6% 9% 49%	2200
	11th to 20th	Night	9% 6% 3 6% 4 4 4 4 6% 7% 45%	2200
	21st to end	Night	8% 4 1 4 3 3 4 4 5% 8% 57%	2200
October	1st to 10th	Night	4 3 1 4 3 3 3 3 5% 7% 64%	2305
	11th to 20th	Night	10% 4 1 3 3 3 3 2 4 5% 63%	2400
	21st to end	Night	11% 4 2 4 2 3 3 2 3 4 61%	2640
November	1st to 10th	Night	11% 5% 1 4 3 3 3 3 5% 6% 55%	2460
	11th to 20th	Night	9% 6% 2 5% 4 5% 4 3 4 7% 53%	2520
	21st to end	Night	14% 6% 2 6% 4 3 3 3 5% 6% 47%	2629
December	1st to 10th	Night	20% 7% 3 7% 4 3 3 3 4 5% 41%	4277
	11th to 20th	Night	13% 9% 3 7% 4 3 3 3 4 5% 45%	4816
	21st to end	Night	13% 7% 2 5% 3 3 3 2 4 5% 53%	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	42% 4 4 24% 21% 5%	4078
	11th to 20th	Day	36% 3 3 19% 28% 11%	4080
	21st to end	Day	52% 2 3 13% 16% 14%	4488
February	1st to 10th	Day	48% 4 5% 14% 18% 10%	1865
	11th to 20th	Day	50% 4 5% 15% 17% 9%	1959
	21st to end	Day	45% 5% 7% 15% 17% 10%	1814
March	1st to 10th	Day	48% 5% 6% 11% 20% 10%	4936
	11th to 20th	Day	37% 5% 7% 12% 20% 19%	4980
	21st to end	Day	44% 6% 9% 20% 16% 5%	5464
April	1st to 10th	Day	47% 5% 6% 17% 16% 9%	2200
	11th to 20th	Day	44% 5% 9% 15% 18% 9%	2341
	21st to end	Day	39% 6% 8% 21% 22% 5%	2400
May	1st to 10th	Day	47% 8% 11% 21% 10% 3	5664
	11th to 20th	Day	38% 7% 15% 17% 22% 1	5866
	21st to end	Day	40% 8% 12% 24% 14% 1	6447
June	1st to 10th	Day	35% 9% 15% 24% 16% 1	2600
	11th to 20th	Day	31% 8% 15% 27% 16% 3	2600
	21st to end	Day	39% 10% 16% 22% 13% 1	2600
July	1st to 10th	Day	36% 11% 27% 23% 3	5836
	11th to 20th	Day	31% 10% 23% 26% 9%	5831
	21st to end	Day	39% 8% 16% 27% 10% 1	6240
August	1st to 10th	Day	15% 6% 15% 37% 26% 1	4220
	11th to 20th	Day	21% 8% 28% 34% 8% 1	4085
	21st to end	Day	33% 13% 17% 22% 13% 2	4264
September	1st to 10th	Day	36% 8% 13% 20% 17% 7%	2200
	11th to 20th	Day	40% 6% 13% 20% 16% 6%	2200
	21st to end	Day	29% 7% 10% 22% 19% 13%	2200
October	1st to 10th	Day	16% 3 6% 14% 24% 37%	3373
	11th to 20th	Day	20% 3 4 18% 35% 20%	3240
	21st to end	Day	24% 4 5% 17% 37% 14%	3538
November	1st to 10th	Day	35% 5% 8% 19% 24% 10%	2058
	11th to 20th	Day	34% 4 8% 19% 23% 13%	2100
	21st to end	Day	42% 3 6% 19% 19% 10%	1987
December	1st to 10th	Day	44% 3 6% 17% 24% 6%	2958
	11th to 20th	Day	43% 5% 7% 13% 22% 10%	3389
	21st to end	Day	42% 4 5% 14% 28% 8%	4488
January	1st to 10th	Night	37% 4 5% 23% 23% 8%	5888
	11th to 20th	Night	35% 2 3 19% 31% 11%	5900
	21st to end	Night	52% 3 3 12% 17% 13%	6490
February	1st to 10th	Night	45% 4 5% 14% 18% 15%	2550
	11th to 20th	Night	45% 4 6% 16% 19% 11%	2338
	21st to end	Night	45% 4 6% 14% 16% 15%	1815
March	1st to 10th	Night	45% 5% 5% 12% 21% 11%	4919
	11th to 20th	Night	37% 5% 6% 16% 16% 20%	4980
	21st to end	Night	46% 5% 7% 14% 20% 8%	5467
April	1st to 10th	Night	40% 5% 9% 17% 17% 12%	2200
	11th to 20th	Night	37% 6% 9% 19% 18% 12%	2005
	21st to end	Night	36% 6% 10% 18% 23% 6%	2000
May	1st to 10th	Night	47% 8% 10% 22% 11% 3	4301
	11th to 20th	Night	39% 6% 12% 20% 21% 3	4054
	21st to end	Night	37% 9% 15% 26% 11% 1	4452
June	1st to 10th	Night	25% 8% 18% 31% 18% 1	1800
	11th to 20th	Night	20% 7% 18% 32% 19% 4	1800
	21st to end	Night	27% 9% 18% 30% 15% 2	1800
July	1st to 10th	Night	32% 11% 31% 22% 4	4035
	11th to 20th	Night	24% 8% 25% 34% 8%	4026
	21st to end	Night	34% 8% 19% 28% 11%	4320
August	1st to 10th	Night	9% 4 17% 39% 30% 1	2916
	11th to 20th	Night	16% 7% 28% 40% 9% 1	3033
	21st to end	Night	29% 11% 20% 25% 13% 2	3614
September	1st to 10th	Night	30% 9% 18% 20% 15% 8%	2200
	11th to 20th	Night	36% 8% 14% 19% 16% 6%	2200
	21st to end	Night	25% 7% 12% 22% 22% 12%	2200
October	1st to 10th	Night	14% 4 6% 17% 20% 40%	3773
	11th to 20th	Night	16% 2 4 24% 30% 24%	3899
	21st to end	Night	22% 3 5% 16% 39% 16%	4252
November	1st to 10th	Night	31% 5% 10% 20% 24% 12%	2460
	11th to 20th	Night	33% 4 8% 19% 23% 13%	2520
	21st to end	Night	39% 4 8% 16% 21% 11%	2629
December	1st to 10th	Night	47% 3 5% 17% 23% 6%	4277
	11th to 20th	Night	41% 5% 8% 10% 23% 13%	4817
	21st to end	Night	35% 5% 5% 15% 30% 10%	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% 13% 39% 19% 5% 1	300
	11th to 20th	Day	23% 11% 37% 21% 6% 1	300
	21st to end	Day	26% 15% 35% 19% 5%	330
February	1st to 10th	Day	23% 15% 37% 19% 5%	300
	11th to 20th	Day	28% 19% 30% 20% 3	300
	21st to end	Day	24% 23% 36% 12% 5%	249
March	1st to 10th	Day	23% 19% 32% 19% 6%	300
	11th to 20th	Day	16% 18% 45% 16% 5%	300
	21st to end	Day	18% 17% 38% 20% 6%	330
April	1st to 10th	Day	20% 18% 35% 23% 4	300
	11th to 20th	Day	20% 15% 29% 25% 10% 1	459
	21st to end	Day	13% 13% 32% 32% 9% 1	500
May	1st to 10th	Day	14% 11% 35% 30% 10% 1	597
	11th to 20th	Day	14% 6% 28% 33% 17% 2	725
	21st to end	Day	10% 7% 26% 34% 19% 4	789
June	1st to 10th	Day	6% 3 13% 34% 34% 10%	730
	11th to 20th	Day	6% 2 13% 37% 32% 9%	730
	21st to end	Day	7% 2 12% 27% 37% 16%	730
July	1st to 10th	Day	7% 3 9% 25% 37% 19%	721
	11th to 20th	Day	4 2 4 29% 45% 17%	718
	21st to end	Day	6% 2 5% 22% 48% 18%	794
August	1st to 10th	Day	4 3 20% 50% 21%	726
	11th to 20th	Day	5% 5% 18% 46% 26%	642
	21st to end	Day	9% 1 6% 19% 38% 27%	562
September	1st to 10th	Day	3 2 9% 30% 36% 19%	330
	11th to 20th	Day	8% 2 8% 22% 42% 19%	330
	21st to end	Day	9% 2 8% 29% 36% 16%	330
October	1st to 10th	Day	5% 2 14% 37% 34% 8%	330
	11th to 20th	Day	9% 6% 20% 36% 21% 8%	330
	21st to end	Day	20% 10% 25% 24% 18% 5%	363
November	1st to 10th	Day	13% 8% 26% 30% 20% 4	330
	11th to 20th	Day	14% 8% 27% 36% 13% 2	330
	21st to end	Day	18% 18% 28% 22% 12% 2	330
December	1st to 10th	Day	21% 8% 35% 23% 10% 4	306
	11th to 20th	Day	25% 14% 32% 20% 8% 1	300
	21st to end	Day	23% 11% 35% 22% 8% 1	330
January	1st to 10th	Night	21% 11% 31% 31% 6%	1000
	11th to 20th	Night	21% 13% 32% 27% 8%	1000
	21st to end	Night	22% 10% 33% 31% 4	1100
February	1st to 10th	Night	21% 12% 35% 27% 5%	950
	11th to 20th	Night	26% 11% 34% 25% 4	784
	21st to end	Night	23% 11% 41% 23% 2	495
March	1st to 10th	Night	21% 12% 35% 27% 5%	600
	11th to 20th	Night	17% 12% 36% 29% 6%	600
	21st to end	Night	15% 11% 37% 32% 5%	660
April	1st to 10th	Night	15% 12% 38% 30% 6%	600
	11th to 20th	Night	15% 11% 35% 30% 9%	593
	21st to end	Night	15% 8% 37% 28% 11%	600
May	1st to 10th	Night	11% 11% 39% 31% 8% 1	600
	11th to 20th	Night	11% 7% 30% 36% 14% 1	591
	21st to end	Night	7% 5% 28% 39% 19% 2	650
June	1st to 10th	Night	5% 3 12% 44% 33% 4	600
	11th to 20th	Night	3 2 16% 51% 26% 2	600
	21st to end	Night	5% 1 10% 43% 35% 6%	600
July	1st to 10th	Night	5% 1 8% 41% 39% 6%	585
	11th to 20th	Night	2 1 7% 48% 37% 4	582
	21st to end	Night	4 1 7% 40% 42% 5%	657
August	1st to 10th	Night	3 3 47% 41% 6%	597
	11th to 20th	Night	5% 4 39% 45% 6%	594
	21st to end	Night	5% 1 6% 34% 46% 7%	660
September	1st to 10th	Night	7% 2 7% 35% 42% 7%	600
	11th to 20th	Night	12% 2 6% 31% 44% 6%	600
	21st to end	Night	9% 1 10% 33% 40% 6%	600
October	1st to 10th	Night	6% 2 11% 42% 34% 5%	705
	11th to 20th	Night	11% 4 20% 41% 21% 3	800
	21st to end	Night	13% 7% 26% 33% 19% 2	880
November	1st to 10th	Night	14% 3 22% 37% 22% 1	820
	11th to 20th	Night	12% 9% 25% 40% 14%	840
	21st to end	Night	18% 10% 31% 28% 12% 1	949
December	1st to 10th	Night	20% 8% 29% 32% 10%	1011
	11th to 20th	Night	23% 10% 31% 30% 5%	1000
	21st to end	Night	19% 10% 31% 31% 8%	1100

Page generated at 2025-11-30 00:00:55. Version ac07

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