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 2024-04-25.

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	25% 36% 12% 15% 9% 3	83553
	Night	27% 34% 11% 14% 10% 3	85135
Seeing	Day	11% 2 9% 27% 37% 14%	13489
	Night	14% 3 10% 30% 33% 10%	23651

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	32% 29% 7% 14% 12% 6%	2848
	11th to 20th	Day	39% 29% 7% 11% 11% 3	2859
	21st to end	Day	42% 26% 8% 9% 12% 3	3487
February	1st to 10th	Day	30% 37% 6% 13% 10% 5%	1900
	11th to 20th	Day	29% 38% 11% 10% 9% 3	1859
	21st to end	Day	24% 37% 9% 17% 11% 2	1670
March	1st to 10th	Day	39% 26% 13% 12% 8% 3	3436
	11th to 20th	Day	43% 26% 13% 10% 5% 3	3480
	21st to end	Day	19% 24% 11% 22% 15% 10%	3814
April	1st to 10th	Day	23% 38% 13% 15% 8% 2	2090
	11th to 20th	Day	27% 36% 10% 14% 10% 2	2066
	21st to end	Day	25% 38% 10% 13% 9% 5%	2142
May	1st to 10th	Day	18% 40% 15% 17% 7% 3	2234
	11th to 20th	Day	22% 40% 11% 18% 9%	2306
	21st to end	Day	15% 40% 12% 18% 14% 1	2531
June	1st to 10th	Day	14% 39% 14% 20% 13% 1	2340
	11th to 20th	Day	13% 39% 11% 25% 11% 1	2340
	21st to end	Day	20% 40% 18% 17% 5%	2340
July	1st to 10th	Day	26% 41% 17% 14% 2	2276
	11th to 20th	Day	17% 41% 17% 20% 5%	2271
	21st to end	Day	15% 42% 23% 18% 2	2405
August	1st to 10th	Day	10% 39% 22% 25% 4	2236
	11th to 20th	Day	18% 40% 23% 17% 3	2079
	21st to end	Day	22% 42% 18% 14% 3 1	2178
September	1st to 10th	Day	23% 41% 12% 14% 9%	1980
	11th to 20th	Day	29% 41% 11% 12% 6%	1980
	21st to end	Day	22% 39% 11% 16% 9% 3	1980
October	1st to 10th	Day	21% 39% 6% 15% 17% 3	1980
	11th to 20th	Day	21% 38% 10% 16% 12% 3	1846
	21st to end	Day	19% 38% 6% 14% 16% 6%	1980
November	1st to 10th	Day	17% 41% 7% 17% 16% 2	1858
	11th to 20th	Day	25% 39% 7% 13% 13% 3	1900
	21st to end	Day	28% 41% 8% 10% 8% 5%	1858
December	1st to 10th	Day	27% 40% 10% 13% 8% 2	1710
	11th to 20th	Day	30% 35% 10% 7% 12% 6%	2159
	21st to end	Day	35% 27% 6% 17% 13% 2	3135
January	1st to 10th	Night	34% 28% 8% 10% 14% 7%	4108
	11th to 20th	Night	41% 28% 5% 9% 14% 3	4120
	21st to end	Night	48% 24% 6% 8% 10% 4	4198
February	1st to 10th	Night	29% 35% 7% 12% 11% 6%	2280
	11th to 20th	Night	33% 36% 9% 10% 9% 3	2218
	21st to end	Night	27% 34% 7% 16% 14% 3	1781
March	1st to 10th	Night	39% 25% 13% 13% 8% 2	3419
	11th to 20th	Night	47% 24% 13% 7% 6% 3	3480
	21st to end	Night	22% 23% 9% 20% 19% 8%	3817
April	1st to 10th	Night	28% 36% 12% 15% 7% 2	2090
	11th to 20th	Night	31% 35% 10% 11% 10% 3	2063
	21st to end	Night	30% 37% 9% 10% 8% 6%	1912
May	1st to 10th	Night	26% 39% 13% 11% 7% 3	1720
	11th to 20th	Night	26% 39% 12% 15% 7%	1594
	21st to end	Night	21% 40% 13% 14% 12%	1746
June	1st to 10th	Night	15% 39% 13% 19% 12% 2	1620
	11th to 20th	Night	13% 39% 12% 26% 8% 2	1620
	21st to end	Night	17% 40% 19% 20% 5%	1620
July	1st to 10th	Night	26% 43% 15% 14% 2	1575
	11th to 20th	Night	17% 41% 19% 19% 4	1566
	21st to end	Night	14% 41% 24% 18% 2	1673
August	1st to 10th	Night	6% 39% 23% 27% 5%	1673
	11th to 20th	Night	14% 40% 23% 20% 2	1818
	21st to end	Night	20% 40% 19% 15% 5% 1	2178
September	1st to 10th	Night	22% 41% 13% 15% 7% 1	1980
	11th to 20th	Night	27% 41% 13% 13% 7%	1980
	21st to end	Night	21% 38% 10% 18% 10% 3	1980
October	1st to 10th	Night	20% 37% 8% 15% 16% 5%	1980
	11th to 20th	Night	19% 38% 8% 20% 10% 5%	2093
	21st to end	Night	18% 38% 5% 16% 16% 6%	2376
November	1st to 10th	Night	16% 42% 6% 16% 17% 3	2220
	11th to 20th	Night	21% 40% 8% 15% 11% 5%	2280
	21st to end	Night	24% 40% 10% 14% 7% 5%	2318
December	1st to 10th	Night	28% 41% 8% 13% 9% 2	2470
	11th to 20th	Night	27% 35% 9% 11% 11% 7%	3037
	21st to end	Night	34% 26% 8% 11% 15% 6%	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	20% 4 20% 33% 21% 3	270
	11th to 20th	Day	20% 5% 19% 36% 19% 1	270
	21st to end	Day	20% 5% 24% 33% 17% 1	297
February	1st to 10th	Day	20% 2 18% 38% 20% 1	270
	11th to 20th	Day	23% 5% 19% 33% 20%	270
	21st to end	Day	21% 2 17% 49% 11%	225
March	1st to 10th	Day	21% 6% 15% 35% 22% 1	270
	11th to 20th	Day	23% 5% 16% 40% 16%	270
	21st to end	Day	12% 3 26% 48% 10%	297
April	1st to 10th	Day	15% 2 21% 48% 14%	270
	11th to 20th	Day	13% 6% 21% 44% 16% 1	264
	21st to end	Day	16% 4 18% 44% 18% 1	370
May	1st to 10th	Day	10% 4 17% 45% 23% 1	497
	11th to 20th	Day	10% 2 11% 52% 25% 1	625
	21st to end	Day	5% 2 8% 42% 41% 3	679
June	1st to 10th	Day	3 1 2 25% 58% 11%	630
	11th to 20th	Day	5% 1 1 27% 55% 10%	630
	21st to end	Day	7% 1 2 22% 45% 23%	630
July	1st to 10th	Day	7% 2 2 18% 49% 23%	621
	11th to 20th	Day	4 2 1 5% 56% 33%	618
	21st to end	Day	4 1 1 6% 57% 31%	684
August	1st to 10th	Day	1 6% 54% 38%	548
	11th to 20th	Day	1 1 4 53% 41%	393
	21st to end	Day	2 1 1 6% 40% 48%	297
September	1st to 10th	Day	10% 1 1 7% 44% 37%	270
	11th to 20th	Day	8% 1 11% 39% 40%	270
	21st to end	Day	9% 1 2 10% 51% 27%	270
October	1st to 10th	Day	5% 1 1 15% 56% 21%	270
	11th to 20th	Day	13% 1 4 14% 51% 17%	270
	21st to end	Day	19% 2 10% 28% 34% 7%	297
November	1st to 10th	Day	12% 3 6% 25% 46% 9%	270
	11th to 20th	Day	29% 4 4 27% 27% 9%	270
	21st to end	Day	26% 5% 13% 28% 21% 6%	270
December	1st to 10th	Day	20% 2 13% 30% 25% 9%	270
	11th to 20th	Day	24% 4 13% 38% 19% 2	270
	21st to end	Day	19% 6% 14% 39% 19% 3	297
January	1st to 10th	Night	24% 5% 15% 33% 22% 2	950
	11th to 20th	Night	22% 4 16% 40% 17% 1	950
	21st to end	Night	23% 4 17% 39% 17% 1	841
February	1st to 10th	Night	24% 4 18% 36% 17% 1	760
	11th to 20th	Night	24% 5% 16% 35% 18% 1	744
	21st to end	Night	21% 5% 17% 44% 14% 1	525
March	1st to 10th	Night	20% 4 17% 39% 19% 1	570
	11th to 20th	Night	19% 5% 16% 42% 17% 1	570
	21st to end	Night	14% 3 17% 49% 15% 2	627
April	1st to 10th	Night	19% 4 18% 42% 17%	570
	11th to 20th	Night	14% 5% 18% 45% 17% 1	563
	21st to end	Night	12% 4 18% 46% 19% 1	555
May	1st to 10th	Night	12% 3 12% 52% 21% 1	540
	11th to 20th	Night	12% 2 13% 45% 27% 1	531
	21st to end	Night	8% 4 5% 42% 38% 3	584
June	1st to 10th	Night	2 1 1 26% 61% 9%	540
	11th to 20th	Night	3 1 1 29% 55% 11%	540
	21st to end	Night	5% 1 2 22% 49% 21%	540
July	1st to 10th	Night	9% 1 1 14% 55% 19%	525
	11th to 20th	Night	3 2 1 7% 60% 28%	522
	21st to end	Night	3 1 1 6% 60% 29%	591
August	1st to 10th	Night	2 7% 58% 33%	537
	11th to 20th	Night	2 1 5% 55% 36%	534
	21st to end	Night	5% 1 2 6% 43% 42%	594
September	1st to 10th	Night	8% 1 6% 54% 32%	540
	11th to 20th	Night	10% 1 1 9% 50% 28%	540
	21st to end	Night	9% 1 2 13% 56% 19%	540
October	1st to 10th	Night	3 1 3 20% 54% 19%	540
	11th to 20th	Night	10% 2 4 22% 50% 11%	653
	21st to end	Night	12% 3 8% 32% 39% 5%	792
November	1st to 10th	Night	12% 1 7% 29% 42% 8%	740
	11th to 20th	Night	21% 3 9% 28% 36% 4	760
	21st to end	Night	19% 4 11% 37% 27% 3	798
December	1st to 10th	Night	21% 5% 10% 31% 26% 7%	950
	11th to 20th	Night	21% 4 14% 35% 24% 3	950
	21st to end	Night	22% 5% 17% 35% 19% 2	1045

Page generated at 2024-04-22 08:25:41. Version ac07

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