Houston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Houston clear sky chart from 2005-11-06 to 2024-12-02.

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	13% 6% 2 7% 5% 5% 5% 4 6% 8% 38%	101262
	Night	14% 6% 2 5% 4 4 4 3 5% 7% 45%	98439
Transparency	Day	40% 8% 12% 18% 15% 7%	104950
	Night	36% 7% 13% 20% 16% 9%	102878
Seeing	Day	13% 7% 21% 27% 23% 10%	14905
	Night	15% 6% 22% 33% 22% 2	24775

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	20% 9% 2 5% 4 3 4 3 4 5% 41%	2847
	11th to 20th	Day	17% 6% 2 4 3 2 3 2 3 6% 52%	2850
	21st to end	Day	24% 8% 3 6% 4 3 3 3 5% 7% 34%	3135
February	1st to 10th	Day	22% 9% 4 8% 4 4 4 3 4 4 35%	1800
	11th to 20th	Day	22% 10% 3 6% 4 4 3 3 4 5% 37%	1873
	21st to end	Day	21% 9% 2 5% 4 4 4 3 4 6% 39%	1727
March	1st to 10th	Day	19% 9% 3 8% 5% 6% 5% 4 6% 7% 29%	3436
	11th to 20th	Day	20% 7% 3 9% 6% 6% 5% 4 6% 6% 29%	3480
	21st to end	Day	19% 6% 2 6% 4 4 4 3 5% 6% 41%	3814
April	1st to 10th	Day	16% 8% 4 7% 5% 5% 4 4 5% 6% 37%	2090
	11th to 20th	Day	18% 7% 3 6% 5% 5% 4 3 5% 7% 37%	2211
	21st to end	Day	10% 7% 3 8% 6% 5% 4 4 5% 7% 40%	2280
May	1st to 10th	Day	17% 8% 3 9% 5% 6% 5% 4 6% 6% 31%	3820
	11th to 20th	Day	15% 6% 3 7% 5% 4 4 3 5% 7% 40%	4082
	21st to end	Day	9% 6% 2 6% 7% 5% 6% 6% 8% 10% 35%	4477
June	1st to 10th	Day	6% 5% 2 7% 6% 6% 6% 5% 8% 9% 40%	2470
	11th to 20th	Day	5% 4 2 6% 6% 7% 8% 7% 8% 9% 38%	2470
	21st to end	Day	6% 6% 3 9% 8% 7% 9% 6% 9% 9% 27%	2470
July	1st to 10th	Day	9% 5% 2 8% 6% 7% 8% 6% 9% 11% 29%	4120
	11th to 20th	Day	6% 7% 3 10% 8% 8% 8% 6% 9% 10% 24%	4120
	21st to end	Day	14% 8% 4 8% 6% 6% 6% 5% 8% 10% 25%	4465
August	1st to 10th	Day	2 2 1 4 4 5% 5% 5% 7% 11% 55%	4104
	11th to 20th	Day	3 2 1 4 4 5% 7% 6% 9% 13% 44%	3863
	21st to end	Day	8% 5% 2 8% 6% 6% 7% 6% 9% 9% 35%	4099
September	1st to 10th	Day	7% 6% 2 7% 7% 7% 7% 6% 8% 9% 34%	2090
	11th to 20th	Day	14% 7% 2 8% 6% 6% 6% 5% 6% 8% 34%	2090
	21st to end	Day	8% 5% 2 6% 5% 4 5% 4 7% 8% 46%	2090
October	1st to 10th	Day	7% 4 2 5% 4 4 3 4 6% 7% 54%	1972
	11th to 20th	Day	10% 6% 2 5% 4 4 4 4 5% 6% 50%	1900
	21st to end	Day	13% 6% 2 5% 4 3 3 2 5% 5% 52%	2090
November	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 4 6% 8% 43%	1958
	11th to 20th	Day	12% 6% 3 7% 4 4 4 3 5% 6% 46%	2000
	21st to end	Day	16% 7% 3 7% 4 4 5% 3 5% 5% 41%	1954
December	1st to 10th	Day	17% 8% 3 6% 5% 4 4 3 4 7% 38%	1719
	11th to 20th	Day	15% 7% 3 6% 4 5% 5% 4 4 7% 41%	2161
	21st to end	Day	17% 7% 2 5% 4 3 4 3 5% 6% 44%	3135
January	1st to 10th	Night	20% 8% 2 5% 3 4 3 3 5% 6% 42%	4108
	11th to 20th	Night	19% 6% 2 5% 3 3 2 2 3 5% 51%	4120
	21st to end	Night	25% 9% 2 5% 3 2 3 3 5% 7% 36%	4532
February	1st to 10th	Night	20% 9% 4 7% 3 3 4 2 4 6% 39%	2394
	11th to 20th	Night	20% 8% 3 6% 4 2 3 2 4 5% 43%	2191
	21st to end	Night	21% 9% 3 6% 4 3 4 2 4 5% 39%	1727
March	1st to 10th	Night	27% 12% 3 5% 3 3 4 3 5% 5% 30%	3419
	11th to 20th	Night	25% 9% 3 8% 5% 4 3 3 5% 6% 30%	3480
	21st to end	Night	15% 6% 2 6% 4 4 4 3 4 5% 46%	3817
April	1st to 10th	Night	16% 6% 3 6% 4 4 4 4 5% 8% 39%	2090
	11th to 20th	Night	20% 6% 3 5% 4 3 3 3 5% 7% 43%	1915
	21st to end	Night	14% 6% 2 6% 4 4 4 4 4 6% 44%	1900
May	1st to 10th	Night	27% 10% 2 6% 4 4 4 3 4 6% 30%	3071
	11th to 20th	Night	15% 6% 2 6% 4 4 5% 4 5% 8% 41%	2823
	21st to end	Night	13% 8% 3 6% 4 5% 5% 4 8% 10% 34%	3090
June	1st to 10th	Night	6% 3 1 4 3 4 3 4 6% 11% 57%	1710
	11th to 20th	Night	5% 2 2 4 3 3 4 3 7% 11% 56%	1710
	21st to end	Night	5% 5% 2 6% 4 4 5% 5% 9% 10% 44%	1710
July	1st to 10th	Night	6% 4 2 5% 5% 5% 5% 4 7% 11% 48%	2850
	11th to 20th	Night	3 4 2 6% 5% 5% 5% 5% 8% 11% 45%	2850
	21st to end	Night	10% 6% 3 8% 5% 4 5% 4 7% 10% 38%	3098
August	1st to 10th	Night	1 1 1 2 2 2 3 2 5% 9% 70%	2836
	11th to 20th	Night	2 2 1 2 2 3 3 4 8% 13% 59%	2969
	21st to end	Night	5% 4 1 5% 4 4 6% 4 8% 12% 48%	3537
September	1st to 10th	Night	5% 5% 2 7% 5% 4 5% 4 8% 10% 46%	2090
	11th to 20th	Night	11% 7% 2 6% 4 4 4 4 6% 7% 45%	2090
	21st to end	Night	8% 5% 1 5% 3 4 3 3 6% 8% 54%	2090
October	1st to 10th	Night	5% 4 1 5% 4 3 3 3 5% 7% 61%	2187
	11th to 20th	Night	9% 4 1 3 3 3 3 3 4 6% 61%	2280
	21st to end	Night	10% 5% 3 4 3 2 3 2 3 4 61%	2508
November	1st to 10th	Night	11% 6% 2 5% 4 3 3 4 5% 6% 51%	2340
	11th to 20th	Night	13% 6% 2 4 3 3 4 3 5% 5% 52%	2400
	21st to end	Night	16% 7% 2 5% 3 3 4 4 5% 6% 45%	2443
December	1st to 10th	Night	18% 7% 2 6% 4 3 4 3 4 6% 42%	2496
	11th to 20th	Night	15% 9% 3 7% 4 4 4 3 5% 5% 42%	3036
	21st to end	Night	17% 7% 3 5% 4 4 4 3 4 5% 45%	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	46% 4 5% 16% 19% 9%	2848
	11th to 20th	Day	34% 5% 6% 23% 22% 11%	2850
	21st to end	Day	50% 3 5% 15% 17% 10%	3135
February	1st to 10th	Day	53% 4 6% 11% 15% 11%	1800
	11th to 20th	Day	51% 4 6% 12% 17% 9%	1883
	21st to end	Day	47% 5% 8% 15% 14% 10%	1727
March	1st to 10th	Day	52% 7% 11% 13% 13% 4	3436
	11th to 20th	Day	54% 7% 9% 11% 14% 6%	3480
	21st to end	Day	42% 5% 9% 18% 21% 5%	3814
April	1st to 10th	Day	46% 8% 7% 16% 14% 9%	2090
	11th to 20th	Day	46% 5% 8% 15% 16% 9%	2211
	21st to end	Day	40% 7% 10% 18% 19% 6%	2280
May	1st to 10th	Day	50% 7% 11% 16% 11% 4	3855
	11th to 20th	Day	42% 7% 8% 18% 22% 3	4086
	21st to end	Day	38% 10% 13% 29% 9%	4489
June	1st to 10th	Day	35% 9% 18% 22% 15% 1	2470
	11th to 20th	Day	35% 11% 15% 22% 14% 2	2470
	21st to end	Day	45% 13% 16% 18% 7% 1	2470
July	1st to 10th	Day	41% 12% 26% 17% 3	4056
	11th to 20th	Day	48% 13% 17% 16% 5%	4051
	21st to end	Day	50% 12% 18% 17% 2	4325
August	1st to 10th	Day	20% 9% 16% 39% 16%	4090
	11th to 20th	Day	25% 12% 30% 28% 5% 1	3902
	21st to end	Day	40% 14% 15% 18% 13% 1	4098
September	1st to 10th	Day	42% 12% 14% 15% 13% 4	2090
	11th to 20th	Day	44% 9% 14% 16% 14% 4	2090
	21st to end	Day	33% 9% 14% 17% 16% 11%	2090
October	1st to 10th	Day	21% 4 6% 14% 15% 39%	3271
	11th to 20th	Day	22% 4 7% 16% 35% 15%	3140
	21st to end	Day	27% 4 6% 15% 35% 12%	3428
November	1st to 10th	Day	37% 7% 10% 18% 20% 8%	1958
	11th to 20th	Day	38% 6% 9% 12% 21% 13%	2000
	21st to end	Day	44% 6% 8% 14% 17% 11%	1954
December	1st to 10th	Day	45% 5% 8% 18% 16% 7%	1719
	11th to 20th	Day	42% 7% 9% 12% 21% 9%	2159
	21st to end	Day	41% 5% 6% 16% 20% 12%	3135
January	1st to 10th	Night	43% 5% 8% 16% 16% 12%	4108
	11th to 20th	Night	37% 4 6% 16% 25% 11%	4120
	21st to end	Night	49% 4 5% 16% 17% 10%	4532
February	1st to 10th	Night	48% 5% 7% 12% 16% 13%	2394
	11th to 20th	Night	45% 4 8% 13% 18% 12%	2191
	21st to end	Night	49% 4 6% 14% 17% 11%	1727
March	1st to 10th	Night	54% 6% 9% 14% 12% 5%	3419
	11th to 20th	Night	55% 6% 9% 13% 12% 5%	3480
	21st to end	Night	39% 6% 11% 14% 23% 7%	3817
April	1st to 10th	Night	43% 7% 9% 17% 15% 9%	2090
	11th to 20th	Night	42% 5% 10% 15% 15% 13%	1915
	21st to end	Night	40% 6% 9% 20% 18% 7%	1900
May	1st to 10th	Night	53% 7% 11% 15% 9% 5%	3101
	11th to 20th	Night	40% 7% 11% 18% 18% 7%	2824
	21st to end	Night	40% 11% 19% 23% 7% 1	3099
June	1st to 10th	Night	21% 7% 22% 31% 18%	1710
	11th to 20th	Night	22% 8% 18% 31% 18% 3	1710
	21st to end	Night	30% 10% 23% 26% 11% 1	1710
July	1st to 10th	Night	29% 9% 34% 25% 3	2805
	11th to 20th	Night	31% 11% 27% 26% 6%	2796
	21st to end	Night	40% 11% 24% 22% 3	2999
August	1st to 10th	Night	10% 7% 20% 48% 15%	2826
	11th to 20th	Night	15% 8% 37% 35% 5% 1	2996
	21st to end	Night	27% 15% 21% 27% 9% 1	3537
September	1st to 10th	Night	31% 11% 22% 20% 13% 3	2090
	11th to 20th	Night	36% 8% 14% 22% 16% 4	2090
	21st to end	Night	28% 8% 14% 22% 18% 11%	2090
October	1st to 10th	Night	16% 5% 8% 18% 17% 37%	3655
	11th to 20th	Night	15% 5% 6% 23% 32% 19%	3779
	21st to end	Night	21% 3 5% 18% 36% 17%	4120
November	1st to 10th	Night	32% 6% 11% 20% 21% 10%	2340
	11th to 20th	Night	33% 5% 11% 18% 20% 13%	2400
	21st to end	Night	40% 5% 9% 19% 17% 10%	2443
December	1st to 10th	Night	44% 6% 8% 19% 14% 9%	2496
	11th to 20th	Night	44% 5% 8% 11% 18% 14%	3037
	21st to end	Night	41% 5% 8% 13% 25% 8%	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	25% 11% 36% 21% 6% 1	270
	11th to 20th	Day	23% 11% 33% 25% 7% 1	270
	21st to end	Day	20% 19% 32% 23% 7%	297
February	1st to 10th	Day	27% 17% 32% 18% 7%	270
	11th to 20th	Day	29% 16% 31% 18% 4 1	270
	21st to end	Day	27% 17% 43% 11% 2	225
March	1st to 10th	Day	27% 15% 37% 17% 4	270
	11th to 20th	Day	17% 19% 43% 14% 5% 2	270
	21st to end	Day	20% 19% 35% 19% 8%	297
April	1st to 10th	Day	18% 21% 34% 23% 4	270
	11th to 20th	Day	24% 16% 28% 19% 12% 1	409
	21st to end	Day	14% 11% 35% 24% 14% 1	460
May	1st to 10th	Day	14% 15% 33% 24% 13% 1	492
	11th to 20th	Day	12% 7% 29% 33% 17% 2	675
	21st to end	Day	9% 5% 29% 36% 18% 3	734
June	1st to 10th	Day	8% 2 14% 35% 30% 11%	680
	11th to 20th	Day	7% 2 13% 38% 30% 10%	680
	21st to end	Day	9% 2 11% 32% 30% 16%	680
July	1st to 10th	Day	7% 3 9% 25% 37% 19%	671
	11th to 20th	Day	5% 1 5% 27% 38% 24%	668
	21st to end	Day	5% 1 6% 27% 39% 22%	739
August	1st to 10th	Day	3 4 24% 45% 24%	676
	11th to 20th	Day	4 1 3 19% 43% 30%	565
	21st to end	Day	11% 2 6% 20% 32% 29%	497
September	1st to 10th	Day	8% 3 8% 27% 33% 22%	300
	11th to 20th	Day	12% 2 7% 27% 34% 18%	300
	21st to end	Day	9% 9% 31% 36% 14%	300
October	1st to 10th	Day	7% 2 15% 40% 28% 7%	300
	11th to 20th	Day	10% 6% 17% 39% 21% 8%	300
	21st to end	Day	19% 8% 25% 25% 20% 3	330
November	1st to 10th	Day	15% 6% 24% 34% 17% 4	300
	11th to 20th	Day	16% 8% 26% 34% 12% 3	300
	21st to end	Day	18% 17% 31% 22% 10% 3	300
December	1st to 10th	Day	18% 7% 33% 27% 11% 4	273
	11th to 20th	Day	21% 15% 32% 24% 7% 1	270
	21st to end	Day	20% 11% 38% 22% 7% 2	297
January	1st to 10th	Night	22% 9% 31% 29% 9%	950
	11th to 20th	Night	22% 10% 27% 33% 8%	950
	21st to end	Night	21% 11% 28% 34% 6%	1045
February	1st to 10th	Night	24% 11% 31% 28% 5%	874
	11th to 20th	Night	25% 9% 29% 30% 7%	717
	21st to end	Night	24% 14% 36% 24% 3	471
March	1st to 10th	Night	24% 9% 31% 30% 6%	570
	11th to 20th	Night	19% 10% 32% 32% 7%	570
	21st to end	Night	16% 9% 34% 33% 8%	627
April	1st to 10th	Night	17% 9% 34% 35% 5%	570
	11th to 20th	Night	23% 8% 29% 30% 9%	563
	21st to end	Night	17% 9% 32% 30% 11% 1	570
May	1st to 10th	Night	16% 9% 31% 36% 8%	570
	11th to 20th	Night	12% 5% 28% 40% 15% 1	561
	21st to end	Night	6% 5% 26% 40% 21% 2	617
June	1st to 10th	Night	5% 2 12% 45% 35% 2	570
	11th to 20th	Night	6% 1 11% 49% 31% 2	570
	21st to end	Night	6% 2 11% 34% 42% 5%	570
July	1st to 10th	Night	6% 1 6% 36% 43% 8%	555
	11th to 20th	Night	4 1 3 33% 51% 9%	552
	21st to end	Night	4 4 32% 53% 7%	624
August	1st to 10th	Night	1 1 2 32% 57% 7%	567
	11th to 20th	Night	5% 2 29% 54% 9%	564
	21st to end	Night	6% 1 4 25% 53% 11%	627
September	1st to 10th	Night	7% 1 8% 23% 52% 9%	570
	11th to 20th	Night	13% 1 7% 27% 45% 6%	570
	21st to end	Night	9% 1 9% 32% 44% 5%	570
October	1st to 10th	Night	6% 2 11% 38% 39% 5%	667
	11th to 20th	Night	10% 2 23% 39% 24% 3	760
	21st to end	Night	12% 6% 22% 35% 22% 2	836
November	1st to 10th	Night	11% 5% 20% 38% 24% 2	780
	11th to 20th	Night	14% 7% 23% 38% 18% 1	800
	21st to end	Night	19% 8% 28% 30% 14% 1	843
December	1st to 10th	Night	19% 8% 26% 31% 15% 1	960
	11th to 20th	Night	21% 8% 30% 32% 9%	950
	21st to end	Night	22% 9% 30% 31% 8% 1	1045

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