Galveston Island State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Galveston Island State Park clear sky chart from 2005-11-06 to 2025-09-19.

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% 4 4 5% 4 6% 8% 41%	125978
	Night	13% 7% 2 6% 4 4 4 4 6% 8% 43%	123299
Transparency	Day	38% 8% 16% 20% 13% 6%	129612
	Night	37% 6% 14% 20% 15% 7%	127728
Seeing	Day	12% 4 15% 26% 29% 14%	15948
	Night	14% 4 15% 30% 29% 8%	25711

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	21% 11% 4 7% 4 3 3 3 4 5% 36%	4077
	11th to 20th	Day	17% 8% 3 6% 4 3 4 2 4 6% 43%	4080
	21st to end	Day	26% 9% 3 6% 4 3 4 3 5% 6% 31%	4488
February	1st to 10th	Day	20% 10% 3 6% 4 3 4 4 4 5% 36%	1876
	11th to 20th	Day	22% 11% 3 5% 4 3 3 3 3 5% 37%	2024
	21st to end	Day	22% 9% 3 7% 3 4 3 2 4 5% 37%	1815
March	1st to 10th	Day	18% 9% 3 8% 5% 4 4 4 5% 7% 34%	4936
	11th to 20th	Day	15% 7% 3 6% 5% 4 4 3 5% 5% 43%	4980
	21st to end	Day	17% 8% 3 7% 4 4 4 3 4 6% 41%	5462
April	1st to 10th	Day	17% 8% 3 6% 4 3 4 3 5% 7% 40%	2200
	11th to 20th	Day	14% 7% 3 6% 5% 4 4 3 6% 7% 41%	2350
	21st to end	Day	10% 7% 2 6% 5% 4 4 4 5% 7% 45%	2400
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 6% 4 7% 8% 28%	5405
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	5760
	21st to end	Day	9% 6% 3 7% 5% 6% 7% 6% 9% 12% 29%	6434
June	1st to 10th	Day	4 4 2 4 4 5% 5% 5% 9% 11% 48%	2600
	11th to 20th	Day	5% 4 1 4 4 5% 5% 6% 10% 13% 44%	2600
	21st to end	Day	7% 4 2 5% 5% 5% 5% 6% 9% 12% 40%	2600
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 12% 39%	5900
	11th to 20th	Day	5% 5% 3 7% 6% 6% 7% 6% 9% 12% 33%	5900
	21st to end	Day	8% 6% 2 7% 5% 5% 6% 5% 8% 10% 39%	6422
August	1st to 10th	Day	2 1 1 2 2 3 3 3 6% 10% 65%	4234
	11th to 20th	Day	3 2 1 3 3 4 5% 5% 10% 15% 52%	3913
	21st to end	Day	6% 4 2 6% 5% 6% 6% 5% 8% 11% 39%	4298
September	1st to 10th	Day	9% 5% 2 7% 5% 5% 6% 5% 7% 10% 38%	2200
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	2178
	21st to end	Day	8% 6% 2 6% 4 4 4 3 6% 7% 50%	2090
October	1st to 10th	Day	6% 4 1 5% 3 3 3 3 6% 8% 56%	1952
	11th to 20th	Day	10% 5% 2 4 3 4 4 3 5% 7% 55%	1900
	21st to end	Day	11% 4 2 5% 3 3 3 3 4 5% 57%	2090
November	1st to 10th	Day	10% 5% 2 6% 4 3 4 4 6% 7% 48%	1958
	11th to 20th	Day	13% 5% 2 5% 4 4 4 4 5% 6% 48%	2000
	21st to end	Day	15% 7% 3 6% 4 4 4 3 5% 7% 43%	2000
December	1st to 10th	Day	24% 8% 3 7% 3 4 3 3 3 6% 35%	2977
	11th to 20th	Day	12% 7% 3 6% 4 4 4 4 5% 7% 44%	3391
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 6% 45%	4488
January	1st to 10th	Night	20% 8% 2 7% 4 4 4 3 5% 5% 37%	5888
	11th to 20th	Night	17% 10% 3 6% 3 3 3 3 4 5% 43%	5900
	21st to end	Night	26% 10% 4 7% 4 3 3 3 5% 6% 29%	6490
February	1st to 10th	Night	19% 10% 3 6% 3 4 4 3 5% 6% 38%	2540
	11th to 20th	Night	20% 10% 3 5% 3 3 4 2 4 6% 42%	2262
	21st to end	Night	20% 9% 3 6% 3 4 4 4 4 5% 39%	1815
March	1st to 10th	Night	20% 10% 3 6% 3 3 4 3 5% 5% 38%	4919
	11th to 20th	Night	16% 8% 2 6% 4 4 3 3 4 5% 45%	4980
	21st to end	Night	15% 10% 2 5% 4 4 4 3 4 6% 43%	5467
April	1st to 10th	Night	14% 7% 3 6% 5% 4 4 4 5% 8% 42%	2200
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 5% 8% 46%	1996
	21st to end	Night	11% 6% 3 6% 4 4 5% 4 5% 8% 44%	2000
May	1st to 10th	Night	22% 9% 3 8% 5% 4 4 4 7% 7% 29%	4490
	11th to 20th	Night	13% 5% 2 5% 4 4 4 3 6% 8% 46%	4146
	21st to end	Night	10% 6% 3 7% 5% 5% 5% 5% 8% 12% 33%	4443
June	1st to 10th	Night	4 4 1 4 3 4 4 3 7% 10% 57%	1800
	11th to 20th	Night	5% 4 2 4 4 3 4 5% 8% 12% 50%	1800
	21st to end	Night	5% 4 2 5% 5% 6% 5% 5% 8% 14% 41%	1800
July	1st to 10th	Night	4 5% 2 6% 5% 4 5% 5% 7% 14% 43%	4078
	11th to 20th	Night	2 3 2 6% 5% 5% 6% 5% 9% 13% 44%	4077
	21st to end	Night	5% 4 2 6% 5% 5% 6% 5% 7% 12% 43%	4449
August	1st to 10th	Night	1 1 1 2 2 2 2 3 5% 8% 72%	2926
	11th to 20th	Night	2 2 1 3 3 3 4 4 7% 14% 57%	3135
	21st to end	Night	4 3 1 4 4 4 5% 5% 8% 14% 47%	3586
September	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 11% 41%	2194
	11th to 20th	Night	12% 6% 3 6% 5% 4 5% 4 6% 8% 43%	2178
	21st to end	Night	9% 4 2 5% 4 3 4 3 6% 8% 52%	2090
October	1st to 10th	Night	5% 3 2 5% 4 3 4 3 5% 8% 57%	2209
	11th to 20th	Night	10% 3 1 4 3 3 3 3 5% 6% 59%	2280
	21st to end	Night	10% 5% 2 4 3 3 3 3 5% 5% 59%	2508
November	1st to 10th	Night	10% 5% 2 5% 4 3 4 4 5% 6% 52%	2340
	11th to 20th	Night	14% 5% 2 5% 4 4 4 3 5% 6% 51%	2400
	21st to end	Night	14% 6% 2 6% 4 3 5% 4 4 6% 44%	2400
December	1st to 10th	Night	25% 9% 4 8% 5% 3 3 3 4 6% 30%	4207
	11th to 20th	Night	13% 7% 2 7% 4 4 5% 4 5% 7% 42%	4816
	21st to end	Night	14% 7% 3 7% 4 3 4 3 4 6% 46%	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	51% 4 6% 18% 15% 6%	4078
	11th to 20th	Day	41% 7% 8% 17% 19% 8%	4080
	21st to end	Day	53% 4 6% 13% 15% 10%	4488
February	1st to 10th	Day	50% 5% 7% 14% 14% 10%	1876
	11th to 20th	Day	50% 5% 9% 14% 15% 7%	2034
	21st to end	Day	48% 6% 10% 14% 13% 8%	1815
March	1st to 10th	Day	47% 8% 10% 15% 9% 10%	4936
	11th to 20th	Day	42% 7% 10% 11% 16% 13%	4980
	21st to end	Day	44% 6% 11% 18% 18% 3	5464
April	1st to 10th	Day	42% 8% 11% 17% 14% 8%	2200
	11th to 20th	Day	40% 8% 13% 17% 14% 8%	2350
	21st to end	Day	36% 8% 13% 21% 17% 5%	2400
May	1st to 10th	Day	49% 9% 13% 17% 9% 2	5440
	11th to 20th	Day	36% 7% 15% 24% 15% 3	5764
	21st to end	Day	39% 11% 18% 24% 7% 1	6447
June	1st to 10th	Day	24% 11% 21% 30% 13%	2600
	11th to 20th	Day	27% 11% 20% 29% 13% 1	2600
	21st to end	Day	32% 13% 23% 26% 6%	2600
July	1st to 10th	Day	36% 6% 37% 20% 2	5836
	11th to 20th	Day	40% 9% 32% 16% 2	5831
	21st to end	Day	39% 9% 23% 24% 5%	6240
August	1st to 10th	Day	15% 7% 21% 44% 13%	4220
	11th to 20th	Day	19% 9% 42% 27% 2	3950
	21st to end	Day	36% 12% 19% 23% 9% 1	4297
September	1st to 10th	Day	36% 14% 17% 17% 12% 3	2200
	11th to 20th	Day	40% 11% 13% 20% 14% 2	2178
	21st to end	Day	32% 8% 15% 20% 15% 9%	2090
October	1st to 10th	Day	21% 4 8% 17% 18% 32%	3238
	11th to 20th	Day	22% 4 7% 24% 33% 10%	3140
	21st to end	Day	24% 3 6% 20% 36% 10%	3428
November	1st to 10th	Day	33% 8% 12% 21% 21% 6%	1958
	11th to 20th	Day	35% 7% 12% 16% 19% 12%	2000
	21st to end	Day	41% 6% 10% 18% 16% 9%	2000
December	1st to 10th	Day	51% 5% 9% 20% 12% 3	2977
	11th to 20th	Day	40% 5% 10% 18% 17% 9%	3389
	21st to end	Day	39% 5% 8% 18% 19% 11%	4488
January	1st to 10th	Night	47% 6% 7% 16% 15% 9%	5888
	11th to 20th	Night	43% 4 6% 16% 22% 9%	5900
	21st to end	Night	56% 3 6% 11% 13% 10%	6490
February	1st to 10th	Night	47% 5% 9% 10% 18% 12%	2540
	11th to 20th	Night	44% 5% 9% 15% 17% 10%	2262
	21st to end	Night	47% 5% 10% 13% 17% 9%	1815
March	1st to 10th	Night	47% 6% 9% 16% 13% 10%	4919
	11th to 20th	Night	42% 7% 8% 13% 15% 15%	4980
	21st to end	Night	41% 6% 10% 16% 21% 7%	5467
April	1st to 10th	Night	40% 7% 11% 19% 15% 9%	2200
	11th to 20th	Night	35% 7% 13% 19% 14% 12%	1996
	21st to end	Night	36% 7% 13% 22% 15% 6%	2000
May	1st to 10th	Night	51% 8% 12% 17% 9% 3	4520
	11th to 20th	Night	34% 6% 13% 28% 14% 5%	4147
	21st to end	Night	39% 9% 21% 24% 7% 1	4452
June	1st to 10th	Night	22% 6% 21% 34% 17%	1800
	11th to 20th	Night	24% 7% 20% 31% 16% 2	1800
	21st to end	Night	31% 11% 25% 25% 9%	1800
July	1st to 10th	Night	31% 5% 40% 23% 2	4035
	11th to 20th	Night	31% 7% 37% 22% 3	4026
	21st to end	Night	35% 9% 24% 28% 5%	4320
August	1st to 10th	Night	12% 6% 18% 51% 14%	2916
	11th to 20th	Night	18% 6% 39% 33% 4	3164
	21st to end	Night	27% 14% 24% 25% 9% 1	3586
September	1st to 10th	Night	34% 11% 20% 19% 11% 3	2194
	11th to 20th	Night	39% 9% 14% 20% 15% 2	2178
	21st to end	Night	30% 8% 15% 22% 18% 7%	2090
October	1st to 10th	Night	21% 4 8% 17% 17% 32%	3690
	11th to 20th	Night	18% 3 7% 25% 33% 14%	3779
	21st to end	Night	20% 3 7% 17% 39% 15%	4120
November	1st to 10th	Night	32% 6% 13% 22% 19% 8%	2340
	11th to 20th	Night	34% 5% 10% 21% 19% 11%	2400
	21st to end	Night	39% 7% 10% 20% 17% 7%	2400
December	1st to 10th	Night	56% 4 8% 17% 11% 4	4207
	11th to 20th	Night	40% 5% 9% 18% 16% 12%	4817
	21st to end	Night	40% 5% 7% 15% 25% 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	26% 8% 32% 25% 9% 1	300
	11th to 20th	Day	20% 8% 29% 33% 9% 1	300
	21st to end	Day	20% 10% 34% 28% 8%	330
February	1st to 10th	Day	25% 8% 30% 26% 11% 1	300
	11th to 20th	Day	26% 7% 34% 25% 7% 1	300
	21st to end	Day	24% 6% 41% 24% 4	249
March	1st to 10th	Day	18% 4 38% 32% 7% 1	300
	11th to 20th	Day	18% 7% 42% 24% 7% 1	300
	21st to end	Day	16% 7% 36% 32% 8% 1	330
April	1st to 10th	Day	15% 12% 37% 26% 10% 1	300
	11th to 20th	Day	18% 8% 31% 26% 15% 2	468
	21st to end	Day	13% 5% 27% 36% 17% 2	500
May	1st to 10th	Day	15% 6% 26% 35% 17% 1	503
	11th to 20th	Day	11% 2 20% 42% 21% 4	675
	21st to end	Day	7% 3 16% 43% 27% 4	789
June	1st to 10th	Day	5% 1 6% 37% 38% 13%	730
	11th to 20th	Day	8% 1 6% 31% 41% 12%	730
	21st to end	Day	8% 2 5% 23% 40% 22%	730
July	1st to 10th	Day	8% 2 1 19% 44% 25%	721
	11th to 20th	Day	6% 1 2 15% 45% 31%	718
	21st to end	Day	6% 2 2 14% 48% 28%	794
August	1st to 10th	Day	2 1 1 12% 49% 35%	726
	11th to 20th	Day	4 1 1 6% 46% 42%	559
	21st to end	Day	12% 2 3 11% 35% 37%	582
September	1st to 10th	Day	13% 2 4 15% 37% 30%	330
	11th to 20th	Day	10% 2 16% 44% 28%	324
	21st to end	Day	10% 2 2 17% 46% 23%	300
October	1st to 10th	Day	7% 1 3 35% 39% 14%	300
	11th to 20th	Day	10% 2 8% 33% 32% 15%	300
	21st to end	Day	14% 5% 20% 28% 25% 8%	330
November	1st to 10th	Day	15% 2 14% 36% 26% 7%	300
	11th to 20th	Day	17% 3 19% 36% 21% 5%	300
	21st to end	Day	18% 6% 28% 30% 15% 3	300
December	1st to 10th	Day	21% 6% 22% 29% 16% 7%	300
	11th to 20th	Day	21% 8% 25% 32% 13% 2	300
	21st to end	Day	19% 4 29% 29% 16% 3	330
January	1st to 10th	Night	22% 7% 24% 32% 14% 1	1000
	11th to 20th	Night	20% 7% 26% 36% 11%	1000
	21st to end	Night	23% 7% 26% 34% 10%	1100
February	1st to 10th	Night	26% 6% 24% 33% 11% 1	940
	11th to 20th	Night	24% 7% 24% 33% 11%	708
	21st to end	Night	25% 9% 24% 33% 8%	495
March	1st to 10th	Night	21% 6% 26% 34% 13% 1	600
	11th to 20th	Night	17% 8% 23% 37% 15% 1	600
	21st to end	Night	15% 4 24% 38% 17% 2	660
April	1st to 10th	Night	17% 7% 24% 37% 15%	600
	11th to 20th	Night	17% 6% 23% 36% 18% 1	593
	21st to end	Night	13% 4 23% 41% 19% 1	600
May	1st to 10th	Night	15% 4 22% 42% 18%	600
	11th to 20th	Night	11% 2 17% 45% 24% 3	591
	21st to end	Night	7% 2 11% 40% 36% 5%	650
June	1st to 10th	Night	5% 2 3 32% 50% 8%	600
	11th to 20th	Night	5% 2 3 30% 50% 11%	600
	21st to end	Night	6% 1 3 19% 50% 21%	600
July	1st to 10th	Night	7% 2 2 12% 51% 26%	585
	11th to 20th	Night	3 1 1 8% 52% 36%	582
	21st to end	Night	3 2 2 10% 56% 27%	657
August	1st to 10th	Night	2 1 7% 60% 31%	597
	11th to 20th	Night	5% 1 1 5% 55% 34%	594
	21st to end	Night	8% 1 1 11% 49% 30%	660
September	1st to 10th	Night	8% 1 4 14% 49% 25%	600
	11th to 20th	Night	15% 1 3 17% 44% 20%	594
	21st to end	Night	10% 1 5% 21% 48% 16%	570
October	1st to 10th	Night	6% 1 5% 27% 50% 11%	689
	11th to 20th	Night	9% 3 10% 33% 37% 8%	760
	21st to end	Night	11% 5% 15% 33% 30% 7%	836
November	1st to 10th	Night	12% 3 12% 34% 33% 6%	780
	11th to 20th	Night	16% 3 17% 36% 28% 1	800
	21st to end	Night	15% 5% 23% 37% 18% 3	800
December	1st to 10th	Night	21% 6% 17% 33% 19% 3	970
	11th to 20th	Night	19% 6% 21% 37% 16% 1	1000
	21st to end	Night	22% 6% 23% 34% 14% 1	1100

Page generated at 2025-09-16 00:49:14. Version ac07

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