Port Isabel Lighthouse State Historic Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Port Isabel Lighthouse State Historic Site 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
Transparency	Day	27% 27% 15% 18% 10% 3	129763
	Night	30% 26% 14% 16% 10% 4	127610
Seeing	Day	11% 3 9% 27% 36% 14%	15438
	Night	14% 3 10% 30% 34% 10%	25369

By Month

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	40% 24% 9% 15% 8% 4	4078
	11th to 20th	Day	47% 24% 9% 11% 8% 2	4103
	21st to end	Day	47% 23% 9% 9% 10% 2	4994
February	1st to 10th	Day	29% 40% 6% 12% 9% 4	2000
	11th to 20th	Day	29% 41% 11% 9% 9% 2	1959
	21st to end	Day	23% 39% 8% 16% 11% 3	1755
March	1st to 10th	Day	32% 21% 13% 13% 9% 11%	4936
	11th to 20th	Day	32% 18% 10% 9% 22% 9%	4980
	21st to end	Day	27% 18% 10% 24% 13% 8%	5464
April	1st to 10th	Day	23% 36% 12% 16% 8% 4	2200
	11th to 20th	Day	26% 35% 10% 15% 10% 3	2176
	21st to end	Day	25% 36% 12% 14% 9% 4	2330
May	1st to 10th	Day	31% 21% 23% 16% 8% 1	5640
	11th to 20th	Day	22% 21% 16% 21% 16% 3	5866
	21st to end	Day	25% 24% 15% 29% 8%	6447
June	1st to 10th	Day	13% 36% 15% 23% 12% 1	2600
	11th to 20th	Day	14% 35% 11% 25% 13% 1	2600
	21st to end	Day	21% 37% 20% 18% 4	2600
July	1st to 10th	Day	35% 22% 26% 16% 1	5836
	11th to 20th	Day	27% 22% 34% 15% 2	5831
	21st to end	Day	28% 22% 28% 20% 3	6240
August	1st to 10th	Day	7% 23% 26% 41% 4	4067
	11th to 20th	Day	19% 27% 38% 14% 1	3793
	21st to end	Day	21% 29% 24% 21% 5%	3948
September	1st to 10th	Day	26% 37% 14% 15% 8%	2200
	11th to 20th	Day	28% 38% 12% 16% 6%	2178
	21st to end	Day	22% 37% 11% 17% 9% 3	2090
October	1st to 10th	Day	18% 24% 6% 13% 23% 16%	3480
	11th to 20th	Day	20% 24% 12% 15% 26% 5%	3214
	21st to end	Day	21% 24% 7% 22% 21% 5%	3428
November	1st to 10th	Day	16% 42% 7% 18% 15% 2	1958
	11th to 20th	Day	21% 43% 5% 13% 15% 4	2000
	21st to end	Day	28% 43% 8% 11% 8% 2	1955
December	1st to 10th	Day	48% 30% 8% 8% 5% 1	2940
	11th to 20th	Day	28% 28% 10% 16% 11% 6%	3389
	21st to end	Day	28% 23% 8% 22% 13% 6%	4488
January	1st to 10th	Night	41% 24% 9% 13% 10% 4	5888
	11th to 20th	Night	50% 22% 7% 9% 10% 2	5900
	21st to end	Night	52% 21% 9% 7% 7% 2	6002
February	1st to 10th	Night	29% 39% 6% 11% 10% 6%	2400
	11th to 20th	Night	33% 39% 9% 9% 9% 2	2338
	21st to end	Night	25% 37% 5% 15% 14% 3	1871
March	1st to 10th	Night	33% 20% 11% 16% 10% 9%	4919
	11th to 20th	Night	36% 17% 9% 9% 15% 14%	4980
	21st to end	Night	30% 17% 9% 20% 18% 6%	5467
April	1st to 10th	Night	28% 34% 12% 15% 7% 3	2200
	11th to 20th	Night	31% 34% 10% 11% 10% 4	2173
	21st to end	Night	31% 35% 10% 11% 8% 5%	2065
May	1st to 10th	Night	41% 20% 19% 11% 7% 2	4310
	11th to 20th	Night	30% 19% 15% 17% 15% 4	4054
	21st to end	Night	36% 20% 16% 23% 5%	4452
June	1st to 10th	Night	17% 36% 14% 20% 11% 2	1800
	11th to 20th	Night	15% 36% 11% 25% 10% 2	1800
	21st to end	Night	17% 37% 20% 21% 4	1800
July	1st to 10th	Night	38% 22% 23% 16% 1	4035
	11th to 20th	Night	29% 22% 34% 14% 2	4026
	21st to end	Night	24% 21% 30% 22% 2	4320
August	1st to 10th	Night	5% 23% 24% 42% 6%	3062
	11th to 20th	Night	17% 25% 38% 18% 1	3324
	21st to end	Night	14% 26% 32% 21% 7%	3949
September	1st to 10th	Night	24% 37% 15% 15% 7% 1	2200
	11th to 20th	Night	26% 38% 13% 17% 7%	2178
	21st to end	Night	20% 36% 10% 19% 10% 4	2090
October	1st to 10th	Night	16% 22% 6% 13% 21% 22%	3480
	11th to 20th	Night	17% 24% 10% 16% 23% 11%	3670
	21st to end	Night	19% 24% 7% 23% 23% 4	4120
November	1st to 10th	Night	15% 43% 5% 17% 16% 3	2340
	11th to 20th	Night	18% 43% 6% 14% 12% 6%	2400
	21st to end	Night	23% 43% 9% 14% 7% 3	2440
December	1st to 10th	Night	48% 29% 6% 9% 6% 1	4250
	11th to 20th	Night	23% 28% 13% 19% 12% 5%	4817
	21st to end	Night	28% 22% 8% 19% 12% 11%	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% 4 19% 30% 21% 2	300
	11th to 20th	Day	21% 6% 21% 34% 17% 1	300
	21st to end	Day	20% 5% 23% 34% 17% 1	330
February	1st to 10th	Day	20% 2 17% 38% 22% 1	300
	11th to 20th	Day	25% 5% 20% 31% 19%	300
	21st to end	Day	22% 2 16% 48% 12%	249
March	1st to 10th	Day	20% 5% 17% 37% 20% 1	300
	11th to 20th	Day	22% 5% 17% 40% 16%	300
	21st to end	Day	13% 4 26% 48% 9%	330
April	1st to 10th	Day	14% 2 24% 46% 14%	300
	11th to 20th	Day	12% 5% 22% 44% 15% 1	294
	21st to end	Day	14% 4 20% 45% 17% 1	430
May	1st to 10th	Day	10% 5% 18% 45% 20% 1	586
	11th to 20th	Day	9% 2 12% 52% 23% 1	725
	21st to end	Day	6% 2 8% 41% 40% 2	789
June	1st to 10th	Day	3 2 2 25% 58% 10%	730
	11th to 20th	Day	5% 1 2 28% 53% 11%	730
	21st to end	Day	6% 2 2 21% 47% 22%	730
July	1st to 10th	Day	7% 2 2 20% 48% 21%	721
	11th to 20th	Day	3 2 1 7% 56% 31%	718
	21st to end	Day	5% 2 1 6% 56% 30%	794
August	1st to 10th	Day	1 6% 50% 43%	638
	11th to 20th	Day	1 1 5% 51% 42%	467
	21st to end	Day	2 1 1 6% 39% 51%	363
September	1st to 10th	Day	10% 2 1 9% 41% 36%	330
	11th to 20th	Day	7% 2 10% 40% 40%	324
	21st to end	Day	8% 1 2 10% 50% 28%	300
October	1st to 10th	Day	5% 1 1 15% 57% 20%	300
	11th to 20th	Day	11% 1 5% 14% 52% 17%	300
	21st to end	Day	17% 2 9% 28% 34% 10%	330
November	1st to 10th	Day	12% 3 7% 25% 46% 8%	300
	11th to 20th	Day	26% 5% 5% 27% 28% 9%	300
	21st to end	Day	26% 6% 13% 27% 22% 6%	300
December	1st to 10th	Day	20% 3 14% 30% 24% 8%	300
	11th to 20th	Day	22% 4 13% 35% 23% 3	300
	21st to end	Day	17% 5% 14% 40% 20% 3	330
January	1st to 10th	Night	25% 5% 15% 33% 21% 2	1000
	11th to 20th	Night	22% 4 17% 39% 16% 1	1000
	21st to end	Night	23% 5% 16% 38% 16% 1	885
February	1st to 10th	Night	24% 4 18% 36% 18% 1	800
	11th to 20th	Night	25% 5% 16% 35% 18% 1	784
	21st to end	Night	22% 4 16% 44% 14% 1	551
March	1st to 10th	Night	20% 4 18% 39% 19% 1	600
	11th to 20th	Night	18% 5% 17% 42% 17% 1	600
	21st to end	Night	14% 3 17% 49% 15% 2	660
April	1st to 10th	Night	19% 4 19% 42% 17%	600
	11th to 20th	Night	14% 5% 18% 44% 18% 1	593
	21st to end	Night	13% 4 19% 45% 17% 1	600
May	1st to 10th	Night	14% 3 14% 50% 19% 1	600
	11th to 20th	Night	12% 3 15% 46% 25% 1	591
	21st to end	Night	9% 4 6% 42% 35% 3	650
June	1st to 10th	Night	2 1 1 26% 61% 9%	600
	11th to 20th	Night	4 1 2 30% 54% 10%	600
	21st to end	Night	5% 1 2 22% 49% 21%	600
July	1st to 10th	Night	8% 2 2 16% 55% 18%	585
	11th to 20th	Night	3 2 1 9% 59% 27%	582
	21st to end	Night	4 1 1 6% 62% 27%	657
August	1st to 10th	Night	2 6% 57% 35%	597
	11th to 20th	Night	2 2 5% 55% 36%	594
	21st to end	Night	4 1 2 6% 45% 43%	660
September	1st to 10th	Night	8% 1 1 6% 52% 33%	600
	11th to 20th	Night	10% 1 1 10% 52% 27%	594
	21st to end	Night	9% 1 2 13% 55% 20%	570
October	1st to 10th	Night	3 2 3 20% 54% 19%	570
	11th to 20th	Night	9% 2 4 22% 51% 11%	690
	21st to end	Night	11% 3 8% 31% 41% 6%	836
November	1st to 10th	Night	11% 2 7% 29% 43% 8%	780
	11th to 20th	Night	20% 3 9% 28% 37% 4	800
	21st to end	Night	19% 4 11% 36% 27% 3	840
December	1st to 10th	Night	22% 5% 10% 31% 25% 7%	1000
	11th to 20th	Night	20% 4 13% 34% 25% 3	1000
	21st to end	Night	21% 4 17% 35% 20% 2	1100

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

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