Space Center Houston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Space Center Houston clear sky chart from 2005-11-06 to 2025-10-19.

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% 5% 5% 5% 4 6% 8% 39%	126340
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	123616
Transparency	Day	38% 8% 14% 19% 14% 6%	129988
	Night	37% 7% 13% 19% 16% 8%	128031
Seeing	Day	12% 5% 15% 26% 28% 14%	16083
	Night	14% 5% 17% 30% 28% 6%	25863

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	22% 11% 3 6% 3 3 3 3 3 5% 38%	4077
	11th to 20th	Day	14% 7% 2 5% 3 3 3 2 4 6% 49%	4080
	21st to end	Day	28% 8% 3 6% 3 3 3 3 4 6% 32%	4488
February	1st to 10th	Day	19% 10% 4 7% 4 4 4 4 5% 5% 34%	1877
	11th to 20th	Day	21% 11% 3 6% 5% 3 4 3 4 5% 35%	2003
	21st to end	Day	20% 9% 3 6% 4 3 4 3 5% 5% 37%	1815
March	1st to 10th	Day	16% 8% 3 8% 4 5% 4 3 6% 8% 36%	4936
	11th to 20th	Day	12% 7% 2 7% 5% 4 4 3 4 6% 45%	4980
	21st to end	Day	19% 7% 3 6% 5% 4 4 3 5% 6% 39%	5462
April	1st to 10th	Day	14% 9% 3 8% 5% 5% 4 3 5% 8% 38%	2200
	11th to 20th	Day	15% 8% 3 6% 5% 4 4 4 5% 8% 38%	2345
	21st to end	Day	10% 7% 3 7% 5% 4 6% 5% 6% 7% 41%	2400
May	1st to 10th	Day	16% 8% 3 9% 6% 6% 5% 5% 6% 7% 28%	5405
	11th to 20th	Day	11% 5% 2 7% 5% 4 5% 4 5% 7% 45%	5849
	21st to end	Day	9% 6% 2 7% 5% 6% 6% 6% 8% 11% 35%	6434
June	1st to 10th	Day	6% 5% 2 6% 5% 6% 6% 5% 8% 11% 41%	2600
	11th to 20th	Day	5% 5% 2 5% 6% 7% 8% 6% 9% 11% 37%	2600
	21st to end	Day	7% 5% 2 7% 7% 7% 8% 8% 9% 11% 30%	2600
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 7% 6% 10% 14% 30%	5900
	11th to 20th	Day	6% 6% 3 9% 6% 7% 6% 6% 8% 12% 31%	5900
	21st to end	Day	10% 6% 3 7% 5% 6% 5% 4 7% 10% 36%	6422
August	1st to 10th	Day	2 2 1 3 3 4 5% 5% 8% 11% 56%	4234
	11th to 20th	Day	2 2 1 4 4 5% 6% 6% 9% 13% 47%	3947
	21st to end	Day	8% 5% 2 6% 6% 6% 7% 5% 9% 11% 36%	4289
September	1st to 10th	Day	7% 6% 3 8% 7% 6% 7% 6% 7% 10% 35%	2200
	11th to 20th	Day	13% 6% 2 7% 5% 6% 6% 5% 7% 8% 36%	2200
	21st to end	Day	8% 5% 3 6% 5% 5% 5% 4 7% 8% 46%	2200
October	1st to 10th	Day	6% 4 2 5% 4 3 5% 4 6% 7% 55%	2060
	11th to 20th	Day	10% 5% 2 4 3 3 5% 3 5% 7% 52%	1980
	21st to end	Day	12% 5% 2 5% 3 3 3 3 4 6% 54%	2090
November	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 5% 5% 7% 45%	1958
	11th to 20th	Day	13% 5% 2 6% 5% 4 4 4 6% 6% 46%	2000
	21st to end	Day	15% 7% 2 7% 5% 4 4 4 5% 6% 43%	1990
December	1st to 10th	Day	24% 9% 2 6% 4 3 4 3 5% 7% 34%	2940
	11th to 20th	Day	12% 7% 3 6% 4 4 4 3 6% 7% 43%	3391
	21st to end	Day	15% 7% 2 6% 3 3 4 3 5% 5% 46%	4488
January	1st to 10th	Night	20% 8% 2 6% 4 3 4 3 5% 6% 40%	5888
	11th to 20th	Night	16% 8% 3 6% 3 3 3 3 4 5% 46%	5900
	21st to end	Night	28% 9% 3 6% 3 3 3 3 4 6% 32%	6490
February	1st to 10th	Night	18% 9% 4 7% 4 4 3 3 5% 5% 38%	2539
	11th to 20th	Night	20% 10% 2 6% 3 3 2 2 4 5% 41%	2282
	21st to end	Night	21% 9% 3 6% 4 3 4 3 4 5% 39%	1815
March	1st to 10th	Night	22% 10% 3 6% 4 3 3 3 5% 6% 36%	4919
	11th to 20th	Night	17% 8% 2 6% 4 3 3 3 4 5% 44%	4980
	21st to end	Night	18% 9% 2 6% 4 3 4 3 3 6% 42%	5467
April	1st to 10th	Night	15% 8% 2 7% 5% 4 4 4 5% 8% 39%	2200
	11th to 20th	Night	18% 6% 2 4 3 3 3 3 6% 7% 44%	2001
	21st to end	Night	11% 6% 2 7% 5% 5% 5% 4 5% 7% 43%	2000
May	1st to 10th	Night	25% 9% 3 7% 4 4 4 4 6% 7% 27%	4490
	11th to 20th	Night	15% 6% 2 7% 3 4 4 4 6% 7% 41%	4061
	21st to end	Night	16% 7% 2 7% 5% 5% 6% 5% 8% 11% 29%	4443
June	1st to 10th	Night	5% 3 1 4 3 4 4 4 7% 11% 56%	1800
	11th to 20th	Night	5% 3 2 4 3 3 5% 5% 7% 11% 54%	1800
	21st to end	Night	5% 4 2 6% 5% 5% 5% 5% 7% 12% 44%	1800
July	1st to 10th	Night	5% 5% 2 5% 4 4 5% 4 8% 14% 43%	4078
	11th to 20th	Night	3 3 2 6% 5% 5% 5% 4 8% 10% 48%	4077
	21st to end	Night	6% 5% 2 6% 4 4 6% 5% 7% 9% 45%	4449
August	1st to 10th	Night	1 1 1 2 2 2 3 3 4 10% 72%	2926
	11th to 20th	Night	2 2 1 2 2 3 4 4 8% 14% 59%	3103
	21st to end	Night	5% 3 2 5% 4 4 5% 4 8% 13% 47%	3604
September	1st to 10th	Night	5% 5% 2 7% 4 5% 5% 5% 8% 11% 44%	2200
	11th to 20th	Night	10% 6% 2 6% 4 3 4 4 6% 8% 45%	2200
	21st to end	Night	8% 4 2 5% 4 4 4 3 6% 8% 54%	2200
October	1st to 10th	Night	4 4 1 5% 4 2 4 3 5% 6% 62%	2318
	11th to 20th	Night	9% 4 1 3 3 3 3 3 5% 6% 61%	2377
	21st to end	Night	10% 5% 2 4 3 3 3 2 3 5% 60%	2508
November	1st to 10th	Night	10% 6% 2 5% 3 3 4 3 4 6% 54%	2340
	11th to 20th	Night	11% 5% 2 5% 4 4 4 4 5% 5% 52%	2400
	21st to end	Night	14% 7% 2 6% 4 4 4 2 4 5% 46%	2405
December	1st to 10th	Night	24% 10% 4 7% 5% 4 3 2 4 6% 32%	4250
	11th to 20th	Night	13% 8% 2 8% 4 4 3 3 4 5% 44%	4816
	21st to end	Night	15% 7% 2 6% 4 3 4 3 3 5% 47%	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	50% 4 4 18% 17% 6%	4078
	11th to 20th	Day	37% 5% 7% 21% 20% 11%	4080
	21st to end	Day	54% 3 4 12% 14% 12%	4488
February	1st to 10th	Day	50% 6% 7% 12% 15% 10%	1877
	11th to 20th	Day	50% 6% 7% 13% 16% 8%	2013
	21st to end	Day	49% 6% 9% 13% 14% 9%	1815
March	1st to 10th	Day	44% 9% 10% 15% 11% 11%	4936
	11th to 20th	Day	38% 8% 8% 11% 20% 15%	4980
	21st to end	Day	44% 7% 9% 18% 19% 4	5464
April	1st to 10th	Day	45% 7% 10% 15% 14% 9%	2200
	11th to 20th	Day	43% 6% 11% 16% 15% 8%	2345
	21st to end	Day	38% 9% 11% 19% 18% 5%	2400
May	1st to 10th	Day	50% 10% 13% 15% 9% 3	5440
	11th to 20th	Day	36% 7% 13% 22% 19% 3	5853
	21st to end	Day	37% 11% 17% 26% 8% 1	6447
June	1st to 10th	Day	31% 9% 19% 25% 15% 1	2600
	11th to 20th	Day	34% 9% 18% 25% 13% 1	2600
	21st to end	Day	39% 13% 20% 21% 7%	2600
July	1st to 10th	Day	37% 10% 33% 18% 2	5836
	11th to 20th	Day	41% 11% 29% 17% 3	5831
	21st to end	Day	41% 10% 18% 24% 6%	6240
August	1st to 10th	Day	17% 9% 17% 42% 14%	4220
	11th to 20th	Day	22% 11% 34% 29% 3	3985
	21st to end	Day	38% 13% 18% 20% 11% 1	4288
September	1st to 10th	Day	39% 13% 15% 16% 13% 3	2200
	11th to 20th	Day	41% 10% 14% 20% 13% 2	2200
	21st to end	Day	33% 8% 13% 20% 15% 10%	2200
October	1st to 10th	Day	21% 4 7% 15% 18% 35%	3359
	11th to 20th	Day	22% 4 6% 20% 37% 11%	3220
	21st to end	Day	25% 4 7% 18% 34% 12%	3428
November	1st to 10th	Day	35% 7% 11% 20% 19% 7%	1958
	11th to 20th	Day	36% 7% 11% 14% 20% 13%	2000
	21st to end	Day	43% 5% 9% 16% 17% 9%	1990
December	1st to 10th	Day	50% 5% 7% 17% 17% 4	2940
	11th to 20th	Day	38% 7% 11% 17% 18% 10%	3389
	21st to end	Day	39% 6% 7% 17% 20% 12%	4488
January	1st to 10th	Night	43% 6% 8% 18% 16% 9%	5888
	11th to 20th	Night	40% 5% 5% 16% 23% 11%	5900
	21st to end	Night	54% 4 6% 12% 14% 10%	6490
February	1st to 10th	Night	47% 5% 8% 10% 17% 12%	2539
	11th to 20th	Night	46% 5% 8% 12% 18% 11%	2282
	21st to end	Night	48% 6% 7% 12% 17% 10%	1815
March	1st to 10th	Night	48% 6% 8% 14% 14% 10%	4919
	11th to 20th	Night	42% 6% 8% 12% 16% 15%	4980
	21st to end	Night	43% 7% 9% 14% 20% 7%	5467
April	1st to 10th	Night	42% 7% 10% 16% 15% 9%	2200
	11th to 20th	Night	38% 7% 11% 17% 15% 13%	2001
	21st to end	Night	38% 6% 12% 21% 16% 7%	2000
May	1st to 10th	Night	53% 9% 10% 16% 9% 3	4520
	11th to 20th	Night	39% 7% 12% 23% 15% 4	4062
	21st to end	Night	42% 12% 19% 21% 6% 1	4452
June	1st to 10th	Night	21% 8% 22% 32% 17%	1800
	11th to 20th	Night	23% 7% 20% 31% 17% 2	1800
	21st to end	Night	31% 10% 24% 25% 10%	1800
July	1st to 10th	Night	30% 8% 37% 22% 2	4035
	11th to 20th	Night	29% 9% 33% 26% 3	4026
	21st to end	Night	34% 10% 22% 28% 5%	4320
August	1st to 10th	Night	10% 7% 20% 49% 14%	2916
	11th to 20th	Night	15% 9% 37% 35% 4	3131
	21st to end	Night	28% 14% 22% 26% 9% 1	3604
September	1st to 10th	Night	31% 12% 22% 19% 12% 4	2200
	11th to 20th	Night	35% 11% 13% 22% 16% 3	2200
	21st to end	Night	28% 7% 15% 22% 19% 9%	2200
October	1st to 10th	Night	19% 4 7% 18% 19% 33%	3786
	11th to 20th	Night	17% 4 7% 24% 32% 17%	3876
	21st to end	Night	20% 3 7% 16% 38% 17%	4120
November	1st to 10th	Night	31% 6% 12% 22% 20% 9%	2340
	11th to 20th	Night	33% 6% 10% 20% 20% 12%	2400
	21st to end	Night	40% 4 10% 20% 18% 9%	2405
December	1st to 10th	Night	54% 4 7% 16% 14% 5%	4250
	11th to 20th	Night	41% 6% 10% 13% 18% 13%	4817
	21st to end	Night	39% 6% 7% 14% 26% 8%	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	24% 9% 28% 29% 10% 1	300
	11th to 20th	Day	18% 11% 31% 31% 8% 1	300
	21st to end	Day	23% 11% 29% 29% 7% 1	330
February	1st to 10th	Day	22% 10% 29% 27% 12%	300
	11th to 20th	Day	28% 11% 25% 29% 6% 1	300
	21st to end	Day	26% 10% 35% 26% 4	249
March	1st to 10th	Day	22% 11% 29% 29% 10%	300
	11th to 20th	Day	16% 11% 39% 27% 5% 2	300
	21st to end	Day	16% 12% 29% 32% 9% 1	330
April	1st to 10th	Day	19% 11% 30% 27% 13%	300
	11th to 20th	Day	19% 11% 26% 27% 14% 2	463
	21st to end	Day	15% 7% 25% 33% 18% 2	500
May	1st to 10th	Day	14% 7% 26% 35% 17% 1	503
	11th to 20th	Day	12% 4 19% 38% 23% 4	712
	21st to end	Day	9% 3 17% 42% 25% 4	789
June	1st to 10th	Day	8% 1 8% 34% 37% 12%	730
	11th to 20th	Day	5% 2 8% 35% 35% 15%	730
	21st to end	Day	9% 7% 23% 42% 19%	730
July	1st to 10th	Day	7% 2 5% 18% 42% 26%	721
	11th to 20th	Day	6% 3 15% 43% 32%	718
	21st to end	Day	5% 1 4 16% 45% 30%	794
August	1st to 10th	Day	3 2 14% 45% 36%	726
	11th to 20th	Day	3 2 10% 43% 42%	581
	21st to end	Day	11% 2 6% 12% 34% 36%	573
September	1st to 10th	Day	12% 2 5% 17% 38% 25%	330
	11th to 20th	Day	10% 2 3 19% 44% 23%	330
	21st to end	Day	9% 1 6% 21% 41% 22%	330
October	1st to 10th	Day	7% 5% 35% 41% 12%	330
	11th to 20th	Day	11% 2 10% 29% 37% 12%	324
	21st to end	Day	16% 7% 17% 30% 25% 5%	330
November	1st to 10th	Day	15% 5% 15% 32% 28% 5%	300
	11th to 20th	Day	16% 5% 19% 34% 20% 6%	300
	21st to end	Day	15% 10% 30% 27% 14% 4	300
December	1st to 10th	Day	22% 8% 19% 32% 13% 6%	300
	11th to 20th	Day	18% 10% 29% 30% 11% 2	300
	21st to end	Day	18% 7% 25% 32% 15% 3	330
January	1st to 10th	Night	22% 7% 27% 33% 10%	1000
	11th to 20th	Night	22% 7% 27% 34% 10%	1000
	21st to end	Night	21% 8% 28% 35% 8%	1100
February	1st to 10th	Night	24% 7% 28% 33% 9%	939
	11th to 20th	Night	25% 9% 26% 30% 9%	728
	21st to end	Night	23% 11% 30% 29% 6%	495
March	1st to 10th	Night	23% 8% 26% 32% 12%	600
	11th to 20th	Night	18% 9% 25% 37% 11% 1	600
	21st to end	Night	17% 6% 25% 37% 14% 1	660
April	1st to 10th	Night	18% 8% 24% 40% 11%	600
	11th to 20th	Night	18% 7% 24% 35% 16%	593
	21st to end	Night	14% 5% 26% 38% 17% 1	600
May	1st to 10th	Night	14% 4 22% 47% 13%	600
	11th to 20th	Night	12% 3 17% 42% 23% 2	591
	21st to end	Night	6% 4 14% 39% 34% 4	650
June	1st to 10th	Night	7% 1 5% 32% 49% 6%	600
	11th to 20th	Night	6% 1 4 33% 50% 6%	600
	21st to end	Night	6% 1 4 22% 54% 14%	600
July	1st to 10th	Night	7% 2 4 15% 55% 18%	585
	11th to 20th	Night	3 1 12% 60% 23%	582
	21st to end	Night	4 1 2 13% 59% 20%	657
August	1st to 10th	Night	1 1 1 9% 65% 23%	597
	11th to 20th	Night	4 1 10% 63% 22%	594
	21st to end	Night	6% 1 3 13% 52% 24%	660
September	1st to 10th	Night	6% 2 4 17% 51% 21%	600
	11th to 20th	Night	14% 1 4 19% 48% 15%	600
	21st to end	Night	11% 1 7% 22% 49% 10%	600
October	1st to 10th	Night	6% 2 6% 30% 46% 9%	718
	11th to 20th	Night	9% 2 14% 35% 36% 5%	793
	21st to end	Night	12% 5% 14% 36% 29% 4	836
November	1st to 10th	Night	12% 4 13% 36% 31% 5%	780
	11th to 20th	Night	13% 5% 20% 36% 24% 2	800
	21st to end	Night	17% 5% 26% 35% 16% 2	805
December	1st to 10th	Night	20% 7% 23% 30% 18% 2	1000
	11th to 20th	Night	20% 8% 23% 35% 13%	1000
	21st to end	Night	21% 7% 24% 35% 12% 1	1100

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