Big Bend Community College
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Bend Community College clear sky chart from 2005-11-06 to 2026-05-02.

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	7% 7% 3 7% 4 4 4 3 5% 6% 50%	135680
	Night	8% 8% 3 7% 4 4 4 4 5% 7% 45%	135526
Transparency	Day	34% 4 10% 23% 20% 9%	135408
	Night	38% 5% 11% 22% 17% 7%	135376
Seeing	Day	9% 7% 20% 27% 24% 13%	16102
	Night	11% 5% 22% 37% 22% 3	25265

By Month

Percentage Cloud Cover NW (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	14% 18% 9% 16% 6% 5% 4 2 4 5% 19%	4723
	11th to 20th	Day	10% 9% 4 9% 6% 5% 5% 3 5% 6% 38%	4716
	21st to end	Day	14% 13% 5% 10% 5% 4 4 3 4 5% 33%	5186
February	1st to 10th	Day	14% 14% 4 10% 6% 6% 5% 4 5% 7% 26%	1801
	11th to 20th	Day	9% 12% 4 10% 5% 4 3 4 5% 7% 35%	1890
	21st to end	Day	8% 11% 3 9% 6% 5% 5% 4 5% 7% 37%	1730
March	1st to 10th	Day	6% 7% 3 7% 6% 5% 6% 5% 7% 8% 40%	4618
	11th to 20th	Day	5% 7% 3 9% 5% 4 4 4 5% 7% 47%	4857
	21st to end	Day	8% 8% 4 9% 6% 6% 6% 5% 7% 9% 32%	6052
April	1st to 10th	Day	8% 8% 2 8% 6% 5% 5% 4 6% 8% 40%	2520
	11th to 20th	Day	5% 7% 2 6% 4 5% 5% 4 7% 8% 46%	2492
	21st to end	Day	5% 6% 2 6% 5% 5% 5% 5% 6% 8% 48%	2744
May	1st to 10th	Day	7% 4 2 4 3 3 3 3 5% 8% 58%	6335
	11th to 20th	Day	6% 5% 2 6% 4 4 4 4 6% 7% 52%	6277
	21st to end	Day	5% 6% 2 5% 4 4 4 3 5% 8% 53%	6903
June	1st to 10th	Day	8% 5% 2 6% 4 4 4 4 7% 8% 47%	2800
	11th to 20th	Day	5% 4 2 5% 4 4 5% 4 6% 7% 53%	2800
	21st to end	Day	3 3 1 4 3 3 3 4 4 7% 64%	2800
July	1st to 10th	Day	2 1 1 2 2 2 2 2 3 4 79%	6320
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 3 82%	6320
	21st to end	Day	2 2 1 2 1 2 2 2 3 6% 77%	6874
August	1st to 10th	Day	2 2 1 3 2 2 3 2 4 6% 74%	4541
	11th to 20th	Day	2 2 1 2 2 3 3 2 5% 6% 72%	4279
	21st to end	Day	3 4 2 4 4 3 3 2 4 5% 64%	4312
September	1st to 10th	Day	3 2 1 3 2 2 2 3 5% 7% 70%	2400
	11th to 20th	Day	4 4 2 5% 4 4 3 4 5% 7% 58%	2362
	21st to end	Day	5% 4 2 5% 3 3 5% 3 6% 6% 57%	2200
October	1st to 10th	Day	5% 3 1 4 3 3 3 3 5% 8% 62%	3291
	11th to 20th	Day	5% 4 1 6% 5% 5% 6% 5% 6% 7% 48%	3209
	21st to end	Day	9% 6% 3 7% 4 4 5% 4 6% 7% 45%	3200
November	1st to 10th	Day	13% 9% 3 9% 6% 5% 6% 4 5% 6% 36%	1854
	11th to 20th	Day	13% 11% 5% 11% 7% 5% 5% 4 5% 6% 29%	1697
	21st to end	Day	12% 12% 6% 11% 7% 6% 4 4 6% 5% 26%	1680
December	1st to 10th	Day	10% 12% 6% 14% 7% 6% 5% 4 6% 6% 25%	2700
	11th to 20th	Day	17% 20% 9% 14% 6% 4 4 3 4 4 15%	3105
	21st to end	Day	15% 19% 6% 15% 8% 5% 4 3 5% 4 16%	4092
January	1st to 10th	Night	16% 19% 7% 11% 5% 5% 5% 3 5% 6% 18%	8172
	11th to 20th	Night	9% 8% 4 8% 5% 4 4 3 5% 8% 43%	8182
	21st to end	Night	12% 15% 6% 10% 4 4 4 3 4 6% 34%	8994
February	1st to 10th	Night	10% 14% 4 8% 6% 4 6% 4 6% 7% 31%	2817
	11th to 20th	Night	9% 10% 3 8% 6% 5% 5% 4 6% 7% 36%	2616
	21st to end	Night	8% 10% 3 8% 7% 5% 5% 5% 7% 8% 33%	2076
March	1st to 10th	Night	5% 8% 2 7% 5% 5% 5% 4 8% 9% 42%	5496
	11th to 20th	Night	7% 9% 3 7% 5% 4 5% 3 5% 6% 47%	5348
	21st to end	Night	7% 8% 3 8% 6% 6% 6% 5% 9% 9% 32%	5164
April	1st to 10th	Night	8% 10% 3 7% 5% 5% 5% 4 6% 6% 41%	2100
	11th to 20th	Night	6% 7% 2 6% 5% 5% 4 4 5% 7% 49%	2079
	21st to end	Night	5% 6% 3 7% 5% 5% 5% 4 6% 9% 45%	1873
May	1st to 10th	Night	5% 4 1 5% 4 3 4 4 5% 6% 59%	3648
	11th to 20th	Night	6% 5% 3 6% 4 4 4 4 6% 7% 52%	3616
	21st to end	Night	5% 4 2 5% 3 3 3 4 5% 8% 58%	3972
June	1st to 10th	Night	7% 5% 2 5% 5% 4 5% 4 5% 8% 51%	1600
	11th to 20th	Night	5% 4 1 4 4 4 4 4 6% 7% 57%	1600
	21st to end	Night	4 4 1 4 3 4 4 3 6% 7% 60%	1600
July	1st to 10th	Night	2 2 1 1 1 2 2 3 5% 82%	3640
	11th to 20th	Night	1 1 1 1 1 1 2 3 4 84%	3640
	21st to end	Night	2 2 2 2 2 2 2 3 4 79%	3969
August	1st to 10th	Night	2 2 1 2 1 2 2 2 3 7% 76%	2609
	11th to 20th	Night	2 3 1 3 2 3 3 3 4 5% 70%	2769
	21st to end	Night	2 4 1 4 3 3 3 3 5% 5% 68%	3608
September	1st to 10th	Night	2 2 1 3 2 2 3 3 4 7% 71%	2000
	11th to 20th	Night	4 3 2 5% 3 4 4 3 6% 7% 60%	2037
	21st to end	Night	5% 4 2 4 3 3 3 3 6% 8% 60%	2200
October	1st to 10th	Night	5% 3 1 3 3 3 3 3 5% 9% 62%	3905
	11th to 20th	Night	6% 4 2 5% 3 4 5% 4 7% 8% 51%	3946
	21st to end	Night	10% 6% 2 6% 5% 5% 4 4 6% 7% 46%	4600
November	1st to 10th	Night	13% 8% 2 7% 5% 5% 5% 4 6% 7% 36%	2665
	11th to 20th	Night	12% 10% 3 8% 6% 7% 5% 4 6% 7% 34%	2919
	21st to end	Night	13% 13% 4 9% 6% 5% 5% 4 5% 6% 30%	2940
December	1st to 10th	Night	7% 9% 6% 8% 5% 4 5% 6% 7% 8% 34%	4700
	11th to 20th	Night	18% 17% 8% 13% 6% 6% 5% 4 5% 5% 13%	5320
	21st to end	Night	17% 18% 4 11% 6% 5% 5% 3 5% 6% 21%	7106

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	70% 4 8% 11% 7%	4723
	11th to 20th	Day	46% 5% 18% 23% 7%	4740
	21st to end	Day	55% 5% 15% 18% 7%	5187
February	1st to 10th	Day	56% 7% 16% 15% 5% 1	1801
	11th to 20th	Day	50% 6% 15% 18% 10% 1	1899
	21st to end	Day	47% 6% 16% 22% 9% 1	1730
March	1st to 10th	Day	38% 5% 12% 24% 20% 1	4618
	11th to 20th	Day	38% 3 10% 32% 16% 1	4857
	21st to end	Day	45% 6% 16% 21% 10% 2	6052
April	1st to 10th	Day	41% 5% 13% 25% 14% 2	2520
	11th to 20th	Day	34% 5% 14% 24% 17% 5%	2492
	21st to end	Day	32% 4 12% 26% 21% 5%	2744
May	1st to 10th	Day	25% 3 5% 21% 38% 9%	6335
	11th to 20th	Day	30% 5% 8% 28% 24% 5%	6280
	21st to end	Day	29% 4 12% 31% 21% 3	6904
June	1st to 10th	Day	34% 4 8% 24% 23% 6%	2800
	11th to 20th	Day	28% 4 10% 28% 22% 7%	2800
	21st to end	Day	21% 3 8% 30% 26% 10%	2800
July	1st to 10th	Day	12% 1 3 19% 30% 36%	6251
	11th to 20th	Day	10% 1 3 14% 32% 39%	6245
	21st to end	Day	11% 2 4 23% 37% 23%	6681
August	1st to 10th	Day	14% 2 4 16% 35% 28%	4524
	11th to 20th	Day	14% 3 7% 35% 32% 10%	4321
	21st to end	Day	22% 3 8% 27% 32% 9%	4311
September	1st to 10th	Day	15% 3 9% 27% 32% 14%	2400
	11th to 20th	Day	25% 4 12% 25% 26% 8%	2362
	21st to end	Day	26% 3 14% 33% 18% 5%	2200
October	1st to 10th	Day	21% 3 11% 38% 25% 2	3291
	11th to 20th	Day	34% 5% 18% 26% 16% 1	3209
	21st to end	Day	38% 5% 14% 27% 12% 4	3200
November	1st to 10th	Day	50% 7% 14% 20% 9% 1	1854
	11th to 20th	Day	56% 5% 16% 15% 7% 1	1697
	21st to end	Day	60% 6% 15% 11% 7% 1	1680
December	1st to 10th	Day	60% 6% 14% 16% 5%	2700
	11th to 20th	Day	75% 4 7% 9% 4	3108
	21st to end	Day	71% 5% 11% 10% 3	4092
January	1st to 10th	Night	67% 6% 12% 12% 4	8172
	11th to 20th	Night	39% 5% 19% 28% 9%	8220
	21st to end	Night	53% 5% 15% 20% 6%	8994
February	1st to 10th	Night	52% 6% 17% 17% 7% 1	2817
	11th to 20th	Night	45% 7% 17% 20% 10% 1	2618
	21st to end	Night	45% 6% 16% 21% 10% 1	2076
March	1st to 10th	Night	36% 5% 10% 27% 21% 2	5496
	11th to 20th	Night	38% 3 8% 33% 17% 1	5349
	21st to end	Night	44% 7% 15% 26% 8%	5164
April	1st to 10th	Night	41% 5% 13% 25% 14% 1	2100
	11th to 20th	Night	35% 4 9% 28% 19% 6%	2079
	21st to end	Night	34% 5% 9% 25% 21% 5%	1873
May	1st to 10th	Night	25% 3 5% 18% 41% 8%	3645
	11th to 20th	Night	31% 5% 6% 28% 26% 3	3616
	21st to end	Night	25% 4 12% 35% 21% 2	3972
June	1st to 10th	Night	31% 3 8% 25% 26% 7%	1600
	11th to 20th	Night	26% 6% 8% 28% 26% 6%	1600
	21st to end	Night	23% 3 7% 27% 29% 11%	1600
July	1st to 10th	Night	9% 1 3 18% 29% 40%	3600
	11th to 20th	Night	7% 1 2 13% 31% 46%	3592
	21st to end	Night	10% 2 3 17% 36% 31%	3857
August	1st to 10th	Night	13% 1 3 10% 39% 33%	2601
	11th to 20th	Night	19% 2 5% 27% 34% 13%	2793
	21st to end	Night	18% 3 5% 26% 31% 17%	3608
September	1st to 10th	Night	14% 3 7% 26% 28% 22%	2000
	11th to 20th	Night	22% 4 10% 24% 27% 12%	2037
	21st to end	Night	23% 4 10% 32% 23% 9%	2200
October	1st to 10th	Night	20% 3 8% 39% 29% 1	3905
	11th to 20th	Night	30% 4 15% 27% 21% 3	3946
	21st to end	Night	38% 4 10% 30% 16% 2	4600
November	1st to 10th	Night	46% 7% 15% 21% 10% 2	2665
	11th to 20th	Night	49% 7% 16% 18% 8% 1	2919
	21st to end	Night	54% 7% 16% 16% 7% 1	2940
December	1st to 10th	Night	44% 9% 19% 23% 4	4700
	11th to 20th	Night	73% 6% 12% 7% 3	5316
	21st to end	Night	64% 6% 12% 12% 6%	7106

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	25% 8% 20% 25% 21% 1	220
	11th to 20th	Day	20% 5% 17% 32% 19% 6%	220
	21st to end	Day	24% 2 19% 27% 22% 5%	242
February	1st to 10th	Day	19% 9% 26% 28% 16% 4	341
	11th to 20th	Day	16% 10% 24% 26% 18% 6%	450
	21st to end	Day	12% 13% 25% 29% 18% 4	446
March	1st to 10th	Day	12% 10% 28% 29% 16% 6%	540
	11th to 20th	Day	12% 12% 23% 26% 20% 7%	540
	21st to end	Day	13% 12% 27% 29% 15% 4	594
April	1st to 10th	Day	11% 10% 26% 24% 20% 9%	540
	11th to 20th	Day	8% 13% 28% 27% 17% 8%	532
	21st to end	Day	6% 9% 22% 29% 22% 12%	540
May	1st to 10th	Day	6% 8% 20% 30% 25% 11%	508
	11th to 20th	Day	10% 8% 17% 29% 24% 13%	528
	21st to end	Day	6% 5% 17% 27% 26% 19%	573
June	1st to 10th	Day	7% 7% 19% 25% 25% 17%	530
	11th to 20th	Day	5% 7% 25% 33% 19% 10%	530
	21st to end	Day	4 2 15% 30% 28% 21%	530
July	1st to 10th	Day	2 2 12% 34% 32% 19%	526
	11th to 20th	Day	1 3 15% 26% 35% 19%	524
	21st to end	Day	1 2 10% 24% 39% 24%	575
August	1st to 10th	Day	3 4 10% 25% 33% 26%	527
	11th to 20th	Day	2 2 10% 25% 33% 28%	517
	21st to end	Day	4 4 15% 25% 30% 22%	583
September	1st to 10th	Day	3 3 12% 27% 34% 21%	530
	11th to 20th	Day	5% 4 18% 23% 30% 20%	530
	21st to end	Day	7% 7% 17% 28% 28% 12%	530
October	1st to 10th	Day	8% 5% 22% 27% 24% 14%	530
	11th to 20th	Day	9% 8% 20% 25% 26% 12%	503
	21st to end	Day	15% 7% 22% 32% 18% 6%	363
November	1st to 10th	Day	15% 10% 22% 33% 17% 2	330
	11th to 20th	Day	17% 10% 25% 25% 17% 7%	228
	21st to end	Day	16% 5% 20% 32% 20% 7%	220
December	1st to 10th	Day	20% 7% 20% 26% 18% 10%	220
	11th to 20th	Day	21% 6% 27% 28% 15% 2	220
	21st to end	Day	19% 7% 26% 29% 18% 2	242
January	1st to 10th	Night	24% 6% 24% 31% 14% 1	840
	11th to 20th	Night	17% 3 21% 39% 18% 2	840
	21st to end	Night	15% 3 21% 40% 19% 2	924
February	1st to 10th	Night	17% 5% 28% 34% 14% 2	840
	11th to 20th	Night	12% 5% 26% 39% 17% 2	824
	21st to end	Night	11% 8% 27% 37% 15% 2	692
March	1st to 10th	Night	12% 7% 23% 41% 16% 2	840
	11th to 20th	Night	10% 5% 27% 36% 19% 2	840
	21st to end	Night	11% 7% 30% 37% 14% 1	924
April	1st to 10th	Night	11% 6% 24% 36% 21% 2	840
	11th to 20th	Night	9% 7% 31% 32% 18% 3	831
	21st to end	Night	7% 6% 29% 37% 18% 2	613
May	1st to 10th	Night	6% 8% 29% 36% 20% 2	402
	11th to 20th	Night	9% 6% 20% 34% 28% 3	394
	21st to end	Night	6% 3 19% 41% 27% 4	433
June	1st to 10th	Night	8% 4 20% 39% 27% 2	400
	11th to 20th	Night	5% 8% 28% 41% 16% 2	400
	21st to end	Night	5% 2 25% 40% 25% 3	400
July	1st to 10th	Night	3 1 17% 45% 32% 2	390
	11th to 20th	Night	2 3 19% 43% 31% 1	388
	21st to end	Night	2 1 14% 39% 40% 4	438
August	1st to 10th	Night	3 2 14% 37% 40% 5%	398
	11th to 20th	Night	3 2 11% 37% 39% 7%	510
	21st to end	Night	3 3 15% 36% 37% 5%	880
September	1st to 10th	Night	2 3 16% 40% 33% 6%	800
	11th to 20th	Night	5% 2 14% 33% 39% 7%	800
	21st to end	Night	7% 4 20% 34% 32% 4	800
October	1st to 10th	Night	11% 3 17% 39% 28% 2	800
	11th to 20th	Night	10% 4 22% 34% 26% 4	800
	21st to end	Night	12% 5% 25% 39% 16% 3	880
November	1st to 10th	Night	16% 6% 21% 44% 13% 1	820
	11th to 20th	Night	17% 7% 25% 34% 15% 2	840
	21st to end	Night	18% 5% 19% 37% 19% 2	840
December	1st to 10th	Night	16% 4 20% 41% 16% 4	840
	11th to 20th	Night	20% 6% 21% 38% 13% 1	840
	21st to end	Night	21% 6% 24% 38% 10% 1	924

Page generated at 2026-04-28 23:21:42. Version ac07

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