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 2024-11-05.

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% 32% 15% 17% 9% 3	104580
	Night	27% 31% 13% 15% 10% 4	102679
Seeing	Day	11% 2 8% 27% 37% 15%	14277
	Night	14% 3 10% 30% 34% 10%	24256

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	31% 32% 7% 14% 11% 5%	2848
	11th to 20th	Day	38% 32% 6% 10% 11% 3	2859
	21st to end	Day	40% 29% 8% 8% 12% 3	3487
February	1st to 10th	Day	28% 41% 6% 12% 9% 4	1900
	11th to 20th	Day	27% 43% 10% 9% 9% 2	1859
	21st to end	Day	22% 41% 8% 16% 11% 2	1670
March	1st to 10th	Day	39% 28% 12% 11% 7% 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% 37% 11% 13% 9% 5%	2213
May	1st to 10th	Day	23% 28% 26% 16% 5% 1	3944
	11th to 20th	Day	28% 29% 17% 14% 10% 2	4086
	21st to end	Day	15% 29% 14% 31% 11%	4489
June	1st to 10th	Day	13% 37% 14% 21% 13% 1	2470
	11th to 20th	Day	13% 37% 11% 25% 13% 1	2470
	21st to end	Day	20% 39% 18% 18% 4	2470
July	1st to 10th	Day	26% 27% 24% 22% 1	4056
	11th to 20th	Day	33% 28% 25% 12% 3	4051
	21st to end	Day	31% 28% 26% 14% 1	4325
August	1st to 10th	Day	7% 24% 25% 41% 4	3942
	11th to 20th	Day	19% 28% 38% 14% 1	3676
	21st to end	Day	21% 30% 23% 21% 5%	3827
September	1st to 10th	Day	26% 39% 12% 14% 8%	2090
	11th to 20th	Day	29% 39% 11% 14% 6%	2090
	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	15% 43% 7% 18% 15% 2	1898
	11th to 20th	Day	21% 45% 5% 13% 13% 3	1900
	21st to end	Day	27% 46% 8% 10% 8% 2	1858
December	1st to 10th	Day	24% 45% 9% 12% 8% 2	1710
	11th to 20th	Day	26% 39% 10% 7% 12% 6%	2159
	21st to end	Day	34% 30% 6% 16% 12% 1	3135
January	1st to 10th	Night	33% 31% 7% 9% 13% 6%	4108
	11th to 20th	Night	41% 31% 5% 8% 13% 3	4120
	21st to end	Night	46% 27% 6% 8% 10% 3	4198
February	1st to 10th	Night	28% 41% 6% 11% 9% 5%	2280
	11th to 20th	Night	31% 41% 8% 9% 9% 2	2218
	21st to end	Night	24% 39% 5% 14% 14% 3	1781
March	1st to 10th	Night	38% 27% 12% 12% 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% 36% 9% 11% 8% 6%	1962
May	1st to 10th	Night	32% 26% 22% 12% 7% 2	3015
	11th to 20th	Night	38% 27% 16% 12% 6% 1	2824
	21st to end	Night	27% 26% 15% 25% 7%	3099
June	1st to 10th	Night	16% 37% 13% 19% 12% 2	1710
	11th to 20th	Night	13% 38% 11% 25% 10% 2	1710
	21st to end	Night	17% 38% 19% 21% 4	1710
July	1st to 10th	Night	28% 27% 23% 20% 2	2805
	11th to 20th	Night	31% 29% 26% 12% 2	2796
	21st to end	Night	27% 29% 29% 13% 1	2999
August	1st to 10th	Night	4 23% 23% 43% 6%	2968
	11th to 20th	Night	17% 26% 38% 17% 1	3222
	21st to end	Night	14% 26% 32% 21% 7%	3828
September	1st to 10th	Night	24% 39% 14% 14% 7% 1	2090
	11th to 20th	Night	27% 39% 12% 15% 7%	2090
	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% 44% 5% 17% 17% 3	2280
	11th to 20th	Night	18% 45% 6% 14% 11% 5%	2280
	21st to end	Night	23% 46% 9% 13% 6% 3	2318
December	1st to 10th	Night	24% 46% 7% 12% 9% 2	2470
	11th to 20th	Night	23% 39% 8% 11% 11% 7%	3037
	21st to end	Night	33% 29% 8% 10% 14% 5%	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	15% 4 20% 45% 16% 1	393
May	1st to 10th	Day	10% 5% 17% 45% 22% 1	542
	11th to 20th	Day	10% 2 11% 52% 24% 1	675
	21st to end	Day	5% 2 9% 42% 40% 3	734
June	1st to 10th	Day	3 1 2 26% 57% 11%	680
	11th to 20th	Day	5% 1 2 27% 54% 11%	680
	21st to end	Day	6% 1 2 22% 46% 23%	680
July	1st to 10th	Day	7% 2 2 17% 50% 23%	671
	11th to 20th	Day	4 2 1 5% 56% 33%	668
	21st to end	Day	5% 1 1 6% 57% 29%	739
August	1st to 10th	Day	1 6% 52% 41%	593
	11th to 20th	Day	1 1 5% 52% 41%	430
	21st to end	Day	2 2 1 6% 40% 49%	330
September	1st to 10th	Day	11% 2 1 9% 41% 35%	300
	11th to 20th	Day	8% 2 11% 39% 40%	300
	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	11% 2 7% 25% 46% 9%	282
	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 19% 46% 18% 1	570
May	1st to 10th	Night	13% 3 12% 51% 20% 1	570
	11th to 20th	Night	12% 2 14% 45% 26% 1	561
	21st to end	Night	8% 4 6% 43% 36% 3	617
June	1st to 10th	Night	2 1 1 26% 61% 9%	570
	11th to 20th	Night	3 1 2 29% 55% 10%	570
	21st to end	Night	5% 1 2 22% 49% 21%	570
July	1st to 10th	Night	9% 1 2 15% 55% 19%	555
	11th to 20th	Night	3 2 1 7% 59% 28%	552
	21st to end	Night	4 1 1 6% 61% 27%	624
August	1st to 10th	Night	2 6% 58% 33%	567
	11th to 20th	Night	2 2 5% 55% 35%	564
	21st to end	Night	4 1 2 6% 44% 42%	627
September	1st to 10th	Night	8% 1 1 6% 51% 33%	570
	11th to 20th	Night	10% 1 1 10% 51% 28%	570
	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% 1 7% 30% 42% 8%	760
	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 2024-11-03 01:11:19. Version ac07

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