Humble ISD Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Humble ISD Astronomical Observatory 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	12% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	101284
	Night	14% 7% 2 5% 4 4 4 3 5% 7% 45%	98421
Transparency	Day	40% 8% 12% 18% 16% 7%	104972
	Night	36% 6% 13% 20% 17% 9%	102860
Seeing	Day	12% 8% 20% 27% 23% 10%	14932
	Night	15% 6% 22% 33% 22% 2	24796

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	19% 10% 2 5% 4 3 3 2 5% 5% 42%	2847
	11th to 20th	Day	16% 7% 2 5% 3 2 2 2 4 5% 52%	2850
	21st to end	Day	23% 9% 3 6% 4 3 3 3 4 7% 35%	3135
February	1st to 10th	Day	23% 9% 3 6% 4 4 3 3 5% 5% 36%	1786
	11th to 20th	Day	21% 9% 3 7% 4 4 3 3 4 5% 38%	1877
	21st to end	Day	18% 10% 3 6% 4 3 4 3 5% 6% 39%	1727
March	1st to 10th	Day	18% 10% 3 8% 5% 5% 5% 4 6% 7% 29%	3436
	11th to 20th	Day	19% 8% 3 8% 6% 5% 5% 5% 6% 7% 28%	3480
	21st to end	Day	17% 7% 2 6% 5% 4 5% 3 5% 6% 41%	3814
April	1st to 10th	Day	15% 8% 2 9% 5% 5% 5% 4 4 6% 37%	2090
	11th to 20th	Day	17% 8% 2 7% 4 5% 4 3 6% 7% 38%	2221
	21st to end	Day	9% 7% 2 8% 5% 5% 5% 4 6% 7% 42%	2280
May	1st to 10th	Day	17% 8% 3 9% 6% 5% 5% 3 7% 7% 30%	3819
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 5% 8% 41%	4082
	21st to end	Day	9% 7% 2 6% 6% 5% 7% 6% 7% 10% 35%	4477
June	1st to 10th	Day	6% 5% 2 6% 6% 6% 6% 5% 7% 10% 40%	2470
	11th to 20th	Day	5% 4 2 6% 6% 7% 7% 7% 8% 11% 39%	2470
	21st to end	Day	6% 6% 3 7% 9% 8% 8% 7% 7% 9% 30%	2470
July	1st to 10th	Day	9% 5% 2 7% 6% 7% 7% 6% 9% 11% 31%	4120
	11th to 20th	Day	7% 7% 3 9% 7% 7% 8% 6% 10% 11% 27%	4120
	21st to end	Day	15% 8% 3 8% 6% 6% 6% 5% 8% 10% 26%	4465
August	1st to 10th	Day	2 1 1 3 3 5% 5% 4 7% 10% 58%	4104
	11th to 20th	Day	3 2 1 4 4 6% 7% 7% 10% 12% 45%	3860
	21st to end	Day	8% 5% 2 7% 6% 6% 7% 5% 8% 9% 36%	4133
September	1st to 10th	Day	7% 5% 3 9% 6% 6% 6% 5% 8% 9% 37%	2090
	11th to 20th	Day	13% 6% 3 7% 6% 6% 6% 5% 6% 6% 37%	2090
	21st to end	Day	7% 6% 2 6% 5% 4 6% 4 6% 8% 47%	2090
October	1st to 10th	Day	7% 4 1 4 4 4 4 3 5% 8% 56%	1966
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 4 4 7% 50%	1900
	21st to end	Day	13% 5% 1 6% 3 3 3 3 4 6% 53%	2090
November	1st to 10th	Day	11% 6% 3 7% 4 4 4 4 5% 7% 45%	1958
	11th to 20th	Day	11% 6% 2 7% 4 4 4 4 5% 6% 47%	2000
	21st to end	Day	16% 7% 2 8% 4 3 4 3 4 6% 42%	1952
December	1st to 10th	Day	15% 9% 3 6% 4 4 5% 3 4 7% 39%	1719
	11th to 20th	Day	14% 8% 3 5% 4 4 5% 4 5% 6% 41%	2161
	21st to end	Day	17% 7% 2 6% 4 3 4 3 5% 6% 44%	3135
January	1st to 10th	Night	18% 9% 2 6% 3 3 3 2 5% 6% 42%	4108
	11th to 20th	Night	18% 7% 2 5% 3 2 3 2 3 5% 51%	4120
	21st to end	Night	24% 10% 2 5% 3 3 3 3 4 7% 36%	4532
February	1st to 10th	Night	19% 9% 3 7% 4 4 2 3 4 6% 39%	2410
	11th to 20th	Night	20% 9% 3 5% 4 3 3 2 3 5% 43%	2187
	21st to end	Night	21% 9% 3 6% 4 3 3 3 5% 4 39%	1727
March	1st to 10th	Night	25% 12% 3 6% 3 4 3 3 6% 6% 30%	3419
	11th to 20th	Night	23% 10% 3 9% 4 4 4 3 5% 6% 30%	3480
	21st to end	Night	14% 7% 2 7% 4 4 4 3 4 6% 44%	3817
April	1st to 10th	Night	15% 6% 3 7% 5% 4 4 4 6% 7% 39%	2090
	11th to 20th	Night	19% 6% 2 5% 4 3 4 3 5% 7% 43%	1905
	21st to end	Night	15% 6% 2 7% 4 4 4 3 4 7% 45%	1900
May	1st to 10th	Night	25% 10% 3 7% 4 3 4 3 5% 8% 28%	3073
	11th to 20th	Night	13% 8% 2 6% 5% 4 4 3 5% 7% 42%	2823
	21st to end	Night	12% 8% 3 7% 4 4 6% 5% 8% 10% 34%	3090
June	1st to 10th	Night	5% 3 1 4 2 3 4 3 7% 10% 57%	1710
	11th to 20th	Night	4 3 1 3 3 4 4 4 7% 11% 56%	1710
	21st to end	Night	5% 5% 3 5% 4 5% 5% 5% 9% 11% 44%	1710
July	1st to 10th	Night	6% 4 1 4 4 5% 5% 5% 8% 12% 47%	2850
	11th to 20th	Night	4 5% 2 7% 5% 5% 6% 4 7% 11% 45%	2850
	21st to end	Night	10% 6% 3 7% 6% 4 4 5% 7% 9% 38%	3098
August	1st to 10th	Night	1 1 1 2 2 2 3 3 6% 10% 68%	2836
	11th to 20th	Night	2 2 1 2 2 2 4 4 7% 14% 61%	2971
	21st to end	Night	5% 3 2 4 4 5% 5% 5% 8% 11% 47%	3504
September	1st to 10th	Night	5% 5% 3 6% 4 5% 5% 5% 7% 10% 46%	2090
	11th to 20th	Night	11% 6% 3 6% 4 4 4 3 6% 8% 45%	2090
	21st to end	Night	7% 5% 2 4 3 3 4 3 5% 7% 55%	2090
October	1st to 10th	Night	5% 3 2 4 3 3 4 4 5% 6% 61%	2194
	11th to 20th	Night	9% 4 1 4 3 2 3 3 5% 6% 59%	2280
	21st to end	Night	11% 5% 2 4 3 3 3 2 3 4 60%	2508
November	1st to 10th	Night	11% 6% 2 5% 4 4 3 3 4 6% 52%	2340
	11th to 20th	Night	12% 6% 2 4 3 3 3 4 4 6% 52%	2400
	21st to end	Night	16% 6% 2 5% 4 3 3 3 5% 6% 46%	2445
December	1st to 10th	Night	17% 8% 2 5% 4 4 4 3 5% 5% 42%	2496
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 4 5% 43%	3036
	21st to end	Night	16% 7% 2 6% 4 3 4 3 5% 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	45% 5% 4 16% 20% 10%	2848
	11th to 20th	Day	34% 5% 5% 20% 25% 10%	2850
	21st to end	Day	50% 3 5% 15% 17% 11%	3135
February	1st to 10th	Day	52% 5% 5% 12% 16% 11%	1786
	11th to 20th	Day	49% 4 7% 13% 17% 10%	1887
	21st to end	Day	46% 5% 9% 15% 14% 11%	1727
March	1st to 10th	Day	52% 7% 11% 12% 15% 4	3436
	11th to 20th	Day	53% 7% 10% 11% 14% 6%	3480
	21st to end	Day	43% 5% 8% 18% 21% 5%	3814
April	1st to 10th	Day	47% 7% 7% 16% 15% 9%	2090
	11th to 20th	Day	46% 5% 8% 14% 17% 9%	2221
	21st to end	Day	39% 7% 10% 18% 20% 6%	2280
May	1st to 10th	Day	51% 7% 11% 16% 11% 4	3854
	11th to 20th	Day	41% 7% 9% 18% 22% 4	4086
	21st to end	Day	38% 10% 13% 29% 10% 1	4489
June	1st to 10th	Day	34% 11% 16% 22% 16% 1	2470
	11th to 20th	Day	34% 11% 16% 23% 14% 2	2470
	21st to end	Day	46% 12% 15% 19% 8% 1	2470
July	1st to 10th	Day	40% 12% 26% 19% 3	4056
	11th to 20th	Day	45% 13% 18% 18% 6%	4051
	21st to end	Day	50% 12% 17% 18% 3 1	4325
August	1st to 10th	Day	18% 9% 16% 38% 18%	4090
	11th to 20th	Day	25% 12% 29% 27% 6% 1	3899
	21st to end	Day	40% 14% 15% 18% 13% 1	4132
September	1st to 10th	Day	39% 12% 15% 15% 14% 4	2090
	11th to 20th	Day	44% 8% 14% 16% 15% 4	2090
	21st to end	Day	34% 7% 13% 18% 17% 12%	2090
October	1st to 10th	Day	19% 4 7% 14% 16% 40%	3265
	11th to 20th	Day	22% 5% 6% 15% 34% 18%	3140
	21st to end	Day	27% 4 5% 15% 35% 13%	3428
November	1st to 10th	Day	38% 6% 10% 18% 20% 9%	1958
	11th to 20th	Day	37% 6% 9% 12% 22% 14%	2000
	21st to end	Day	43% 5% 8% 15% 18% 11%	1952
December	1st to 10th	Day	47% 4 8% 17% 17% 7%	1719
	11th to 20th	Day	42% 6% 9% 13% 21% 9%	2159
	21st to end	Day	41% 5% 6% 16% 21% 11%	3135
January	1st to 10th	Night	42% 4 8% 18% 17% 12%	4108
	11th to 20th	Night	38% 3 6% 17% 26% 11%	4120
	21st to end	Night	49% 4 4 16% 18% 9%	4532
February	1st to 10th	Night	48% 4 7% 12% 16% 14%	2410
	11th to 20th	Night	45% 4 8% 13% 18% 12%	2187
	21st to end	Night	48% 5% 5% 14% 16% 12%	1727
March	1st to 10th	Night	54% 5% 9% 14% 13% 5%	3419
	11th to 20th	Night	55% 6% 9% 14% 12% 5%	3480
	21st to end	Night	41% 5% 11% 14% 22% 7%	3817
April	1st to 10th	Night	42% 7% 10% 17% 15% 10%	2090
	11th to 20th	Night	41% 5% 10% 16% 15% 13%	1905
	21st to end	Night	39% 6% 9% 20% 19% 7%	1900
May	1st to 10th	Night	53% 6% 11% 15% 9% 5%	3103
	11th to 20th	Night	41% 7% 10% 17% 20% 6%	2824
	21st to end	Night	40% 10% 18% 24% 7% 1	3099
June	1st to 10th	Night	21% 8% 20% 33% 18%	1710
	11th to 20th	Night	22% 8% 18% 31% 19% 3	1710
	21st to end	Night	29% 11% 22% 26% 11% 1	1710
July	1st to 10th	Night	29% 10% 34% 24% 4	2805
	11th to 20th	Night	31% 11% 25% 25% 8%	2796
	21st to end	Night	40% 12% 22% 23% 3	2999
August	1st to 10th	Night	11% 6% 21% 45% 17%	2826
	11th to 20th	Night	14% 8% 36% 36% 6%	2998
	21st to end	Night	28% 15% 20% 27% 9% 1	3504
September	1st to 10th	Night	29% 12% 22% 20% 12% 4	2090
	11th to 20th	Night	36% 8% 14% 21% 16% 5%	2090
	21st to end	Night	28% 7% 14% 22% 18% 11%	2090
October	1st to 10th	Night	17% 5% 8% 17% 16% 37%	3662
	11th to 20th	Night	16% 4 5% 22% 33% 21%	3779
	21st to end	Night	22% 3 5% 17% 38% 15%	4120
November	1st to 10th	Night	33% 6% 11% 20% 21% 9%	2340
	11th to 20th	Night	32% 5% 10% 18% 21% 13%	2400
	21st to end	Night	39% 5% 8% 19% 18% 10%	2445
December	1st to 10th	Night	43% 6% 8% 19% 14% 10%	2496
	11th to 20th	Night	45% 5% 7% 10% 19% 14%	3037
	21st to end	Night	41% 4 8% 12% 27% 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	27% 13% 33% 21% 6% 1	270
	11th to 20th	Day	24% 11% 32% 26% 4 2	270
	21st to end	Day	21% 19% 32% 22% 6%	297
February	1st to 10th	Day	31% 17% 29% 19% 5%	270
	11th to 20th	Day	27% 17% 34% 16% 5%	270
	21st to end	Day	23% 22% 41% 11% 2	225
March	1st to 10th	Day	26% 16% 36% 17% 5%	270
	11th to 20th	Day	19% 21% 41% 13% 5% 1	270
	21st to end	Day	21% 18% 36% 17% 7%	297
April	1st to 10th	Day	17% 22% 33% 23% 4	270
	11th to 20th	Day	25% 15% 28% 19% 11% 1	419
	21st to end	Day	12% 13% 36% 26% 12% 1	460
May	1st to 10th	Day	15% 15% 30% 26% 13%	491
	11th to 20th	Day	13% 8% 28% 32% 18% 2	675
	21st to end	Day	9% 7% 27% 35% 18% 3	734
June	1st to 10th	Day	7% 3 13% 36% 31% 10%	680
	11th to 20th	Day	6% 3 13% 39% 29% 10%	680
	21st to end	Day	7% 4 10% 31% 31% 16%	680
July	1st to 10th	Day	7% 3 10% 24% 37% 20%	671
	11th to 20th	Day	5% 1 5% 24% 42% 23%	668
	21st to end	Day	4 2 5% 25% 43% 21%	739
August	1st to 10th	Day	2 4 22% 47% 24%	676
	11th to 20th	Day	4 1 3 18% 46% 28%	562
	21st to end	Day	10% 2 6% 20% 33% 29%	518
September	1st to 10th	Day	8% 2 7% 28% 35% 21%	300
	11th to 20th	Day	9% 4 8% 29% 32% 18%	300
	21st to end	Day	9% 1 9% 30% 39% 11%	300
October	1st to 10th	Day	6% 4 12% 45% 28% 6%	300
	11th to 20th	Day	9% 5% 19% 38% 21% 8%	300
	21st to end	Day	20% 9% 23% 24% 18% 6%	330
November	1st to 10th	Day	14% 7% 25% 35% 16% 3	300
	11th to 20th	Day	14% 11% 25% 35% 11% 3	300
	21st to end	Day	20% 16% 29% 21% 11% 3	300
December	1st to 10th	Day	17% 7% 38% 24% 10% 4	273
	11th to 20th	Day	21% 16% 32% 25% 6% 1	270
	21st to end	Day	18% 11% 40% 23% 6% 2	297
January	1st to 10th	Night	22% 9% 32% 28% 9%	950
	11th to 20th	Night	25% 10% 26% 32% 7%	950
	21st to end	Night	23% 9% 29% 33% 6%	1045
February	1st to 10th	Night	26% 10% 31% 28% 5%	890
	11th to 20th	Night	27% 8% 30% 28% 6%	713
	21st to end	Night	24% 13% 36% 23% 3	471
March	1st to 10th	Night	24% 8% 32% 31% 4	570
	11th to 20th	Night	19% 10% 33% 32% 7%	570
	21st to end	Night	17% 10% 35% 29% 9%	627
April	1st to 10th	Night	17% 8% 36% 33% 5%	570
	11th to 20th	Night	22% 9% 31% 29% 8%	563
	21st to end	Night	18% 8% 33% 31% 10%	570
May	1st to 10th	Night	16% 11% 29% 36% 8%	570
	11th to 20th	Night	12% 5% 29% 38% 15% 2	561
	21st to end	Night	7% 5% 25% 38% 22% 2	617
June	1st to 10th	Night	5% 3 12% 43% 34% 3	570
	11th to 20th	Night	5% 1 12% 49% 31% 2	570
	21st to end	Night	6% 2 12% 35% 40% 5%	570
July	1st to 10th	Night	5% 1 6% 36% 43% 8%	555
	11th to 20th	Night	4 1 5% 33% 50% 7%	552
	21st to end	Night	3 1 3 36% 49% 7%	624
August	1st to 10th	Night	2 1 3 33% 55% 7%	567
	11th to 20th	Night	5% 1 2 33% 53% 7%	564
	21st to end	Night	6% 1 3 27% 55% 8%	627
September	1st to 10th	Night	6% 2 8% 25% 51% 9%	570
	11th to 20th	Night	13% 1 6% 29% 45% 5%	570
	21st to end	Night	8% 1 9% 34% 44% 4	570
October	1st to 10th	Night	6% 2 11% 38% 39% 5%	674
	11th to 20th	Night	10% 3 20% 39% 26% 2	760
	21st to end	Night	13% 7% 21% 36% 21% 2	836
November	1st to 10th	Night	12% 5% 21% 38% 23% 2	780
	11th to 20th	Night	14% 7% 26% 36% 17% 1	800
	21st to end	Night	18% 7% 29% 31% 14% 1	845
December	1st to 10th	Night	19% 8% 27% 31% 13% 1	960
	11th to 20th	Night	23% 9% 26% 33% 8% 1	950
	21st to end	Night	21% 9% 32% 30% 7%	1045

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