Big Bend Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Bend Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	14% 10% 4 9% 5% 5% 5% 4 6% 7% 32%	138206
	Night	14% 11% 4 9% 5% 5% 5% 4 5% 6% 33%	134459
Transparency	Day	49% 6% 13% 20% 11% 1	141805
	Night	49% 5% 12% 20% 12% 1	138931
Seeing	Day	17% 11% 29% 25% 13% 5%	17291
	Night	18% 10% 28% 29% 12% 1	26521

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	14% 15% 7% 16% 7% 5% 5% 4 4 6% 19%	4738
	11th to 20th	Day	12% 19% 9% 17% 8% 7% 6% 4 5% 4 10%	5280
	21st to end	Day	17% 15% 6% 12% 7% 5% 6% 4 4 6% 18%	5780
February	1st to 10th	Day	15% 15% 5% 14% 7% 6% 4 4 4 4 22%	1890
	11th to 20th	Day	15% 13% 6% 11% 7% 6% 5% 5% 4 6% 23%	1880
	21st to end	Day	15% 14% 5% 12% 7% 5% 5% 3 5% 5% 23%	1730
March	1st to 10th	Day	17% 11% 6% 11% 7% 4 4 3 5% 5% 27%	4879
	11th to 20th	Day	12% 11% 4 10% 6% 4 5% 3 5% 7% 33%	5090
	21st to end	Day	17% 13% 5% 10% 5% 5% 4 4 4 5% 28%	5586
April	1st to 10th	Day	15% 12% 4 9% 5% 5% 5% 3 5% 7% 30%	2369
	11th to 20th	Day	17% 10% 4 7% 5% 4 4 4 6% 7% 32%	2702
	21st to end	Day	18% 10% 3 8% 4 4 4 4 5% 6% 34%	2730
May	1st to 10th	Day	17% 10% 3 8% 5% 5% 4 4 6% 7% 32%	7616
	11th to 20th	Day	13% 8% 3 7% 5% 4 5% 4 5% 9% 37%	7626
	21st to end	Day	14% 6% 2 7% 4 4 4 4 6% 7% 41%	8826
June	1st to 10th	Day	10% 6% 3 7% 6% 5% 5% 4 6% 9% 41%	2940
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	3150
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 9% 41%	3150
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 4 8% 10% 40%	6787
	11th to 20th	Day	7% 5% 3 6% 5% 5% 5% 5% 7% 9% 44%	6135
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	6424
August	1st to 10th	Day	13% 5% 2 6% 4 4 5% 5% 8% 9% 38%	4234
	11th to 20th	Day	8% 6% 2 7% 4 6% 6% 5% 9% 10% 37%	4174
	21st to end	Day	8% 6% 2 7% 5% 5% 6% 4 6% 9% 42%	4645
September	1st to 10th	Day	9% 6% 2 6% 5% 6% 6% 4 6% 8% 42%	2305
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 5% 10% 48%	2200
	21st to end	Day	17% 7% 2 6% 5% 4 5% 4 6% 7% 36%	2200
October	1st to 10th	Day	13% 6% 3 7% 3 4 4 5% 7% 7% 41%	2065
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 4 6% 6% 34%	2000
	21st to end	Day	20% 10% 3 8% 6% 4 5% 4 5% 7% 28%	2144
November	1st to 10th	Day	13% 9% 3 8% 6% 4 5% 5% 5% 7% 35%	1852
	11th to 20th	Day	15% 10% 4 9% 5% 5% 5% 4 6% 7% 30%	1783
	21st to end	Day	18% 13% 5% 10% 6% 6% 4 4 5% 6% 21%	1680
December	1st to 10th	Day	18% 13% 4 10% 5% 4 5% 3 7% 8% 23%	2680
	11th to 20th	Day	21% 17% 6% 10% 5% 4 5% 4 5% 6% 18%	3070
	21st to end	Day	25% 20% 7% 12% 6% 5% 4 3 4 4 11%	3866
January	1st to 10th	Night	14% 14% 5% 13% 6% 5% 5% 3 5% 6% 24%	8153
	11th to 20th	Night	9% 17% 8% 14% 8% 6% 6% 4 6% 5% 16%	7680
	21st to end	Night	15% 15% 6% 11% 5% 5% 5% 3 5% 4 25%	8401
February	1st to 10th	Night	16% 15% 4 10% 7% 5% 5% 4 5% 5% 25%	2730
	11th to 20th	Night	16% 13% 5% 8% 6% 5% 5% 4 5% 6% 27%	2624
	21st to end	Night	18% 12% 4 9% 7% 5% 5% 4 5% 6% 26%	2076
March	1st to 10th	Night	16% 12% 4 8% 6% 5% 5% 4 5% 6% 32%	5193
	11th to 20th	Night	9% 7% 3 8% 6% 5% 5% 4 6% 8% 39%	5090
	21st to end	Night	15% 11% 4 8% 5% 5% 4 3 5% 4 36%	5588
April	1st to 10th	Night	16% 13% 4 7% 5% 5% 4 4 5% 5% 33%	2259
	11th to 20th	Night	16% 7% 3 8% 5% 5% 4 3 6% 8% 35%	1876
	21st to end	Night	15% 8% 3 7% 5% 5% 5% 4 5% 8% 37%	1890
May	1st to 10th	Night	18% 9% 3 7% 5% 5% 5% 3 6% 7% 33%	5237
	11th to 20th	Night	13% 9% 4 8% 5% 5% 4 3 5% 7% 36%	5234
	21st to end	Night	18% 7% 2 5% 3 4 4 4 5% 7% 41%	5310
June	1st to 10th	Night	11% 5% 3 6% 4 4 4 3 6% 9% 45%	1673
	11th to 20th	Night	10% 6% 2 5% 4 4 5% 4 7% 10% 42%	1470
	21st to end	Night	9% 6% 1 5% 4 3 5% 4 5% 9% 48%	1470
July	1st to 10th	Night	6% 5% 2 6% 6% 4 5% 4 6% 11% 45%	3406
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 7% 9% 48%	3922
	21st to end	Night	7% 4 2 6% 4 4 4 4 6% 8% 51%	4427
August	1st to 10th	Night	10% 6% 3 5% 4 4 4 4 6% 8% 45%	2915
	11th to 20th	Night	7% 6% 3 6% 4 4 4 4 6% 7% 50%	2875
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 7% 8% 49%	3223
September	1st to 10th	Night	8% 6% 2 7% 5% 3 5% 4 5% 7% 49%	2087
	11th to 20th	Night	8% 5% 2 5% 4 3 3 3 5% 6% 56%	2200
	21st to end	Night	14% 8% 2 8% 4 4 5% 2 5% 6% 42%	2200
October	1st to 10th	Night	11% 6% 3 6% 4 4 4 3 5% 7% 47%	2335
	11th to 20th	Night	17% 9% 3 8% 4 4 4 3 5% 6% 39%	2400
	21st to end	Night	20% 9% 3 7% 5% 5% 4 4 6% 6% 32%	2694
November	1st to 10th	Night	13% 7% 3 6% 5% 4 4 4 5% 7% 41%	2666
	11th to 20th	Night	16% 10% 3 8% 5% 5% 5% 5% 5% 6% 32%	2835
	21st to end	Night	20% 12% 4 8% 5% 5% 4 3 5% 6% 27%	2940
December	1st to 10th	Night	14% 13% 5% 11% 7% 5% 6% 5% 6% 7% 22%	4720
	11th to 20th	Night	17% 18% 5% 10% 5% 5% 5% 3 5% 4 23%	5345
	21st to end	Night	24% 18% 6% 13% 5% 5% 4 3 4 5% 14%	7315

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	66% 6% 10% 11% 6%	4739
	11th to 20th	Day	76% 7% 7% 6% 5%	5280
	21st to end	Day	68% 4 11% 10% 7%	5780
February	1st to 10th	Day	64% 5% 10% 14% 5% 1	1890
	11th to 20th	Day	61% 5% 10% 15% 7% 1	1889
	21st to end	Day	62% 5% 12% 14% 5% 1	1730
March	1st to 10th	Day	59% 4 13% 18% 6%	4879
	11th to 20th	Day	50% 5% 18% 19% 7% 1	5090
	21st to end	Day	58% 4 12% 17% 9% 1	5586
April	1st to 10th	Day	53% 6% 12% 19% 10% 1	2369
	11th to 20th	Day	50% 6% 11% 20% 13% 1	2702
	21st to end	Day	51% 4 13% 19% 12% 2	2730
May	1st to 10th	Day	50% 5% 12% 20% 10% 3	7651
	11th to 20th	Day	45% 5% 14% 23% 12% 1	7630
	21st to end	Day	41% 4 10% 20% 20% 5%	8841
June	1st to 10th	Day	39% 6% 13% 26% 14% 2	2940
	11th to 20th	Day	38% 8% 15% 21% 16% 2	3150
	21st to end	Day	36% 7% 15% 25% 16% 1	3150
July	1st to 10th	Day	37% 7% 12% 27% 16% 2	6716
	11th to 20th	Day	34% 6% 14% 27% 17% 1	6062
	21st to end	Day	34% 6% 15% 23% 21% 2	6240
August	1st to 10th	Day	39% 7% 16% 23% 15% 1	4221
	11th to 20th	Day	38% 8% 14% 31% 8% 1	4213
	21st to end	Day	36% 6% 13% 31% 12% 1	4645
September	1st to 10th	Day	38% 8% 14% 25% 14% 1	2305
	11th to 20th	Day	32% 5% 14% 34% 15%	2200
	21st to end	Day	45% 6% 13% 24% 11% 1	2200
October	1st to 10th	Day	33% 5% 14% 26% 20% 1	3364
	11th to 20th	Day	42% 4 12% 24% 17%	3240
	21st to end	Day	46% 5% 16% 24% 9%	3446
November	1st to 10th	Day	47% 6% 13% 24% 9% 2	1852
	11th to 20th	Day	51% 6% 17% 16% 9% 1	1783
	21st to end	Day	62% 5% 14% 15% 3	1680
December	1st to 10th	Day	58% 6% 19% 14% 4	2680
	11th to 20th	Day	67% 5% 12% 9% 6%	3066
	21st to end	Day	77% 4 9% 7% 2	3866
January	1st to 10th	Night	60% 5% 12% 16% 7%	8154
	11th to 20th	Night	66% 7% 10% 10% 5% 1	7680
	21st to end	Night	61% 4 9% 17% 9%	8401
February	1st to 10th	Night	60% 6% 11% 16% 7% 1	2730
	11th to 20th	Night	57% 5% 11% 18% 9% 1	2626
	21st to end	Night	58% 4 12% 18% 7% 1	2076
March	1st to 10th	Night	52% 5% 14% 19% 9% 1	5193
	11th to 20th	Night	42% 4 16% 24% 12% 1	5090
	21st to end	Night	51% 4 10% 19% 16% 1	5588
April	1st to 10th	Night	52% 5% 10% 18% 13% 2	2259
	11th to 20th	Night	45% 6% 11% 21% 16% 1	1876
	21st to end	Night	45% 5% 11% 20% 15% 3	1890
May	1st to 10th	Night	50% 4 10% 15% 15% 6%	5264
	11th to 20th	Night	47% 5% 10% 23% 12% 3	5235
	21st to end	Night	43% 4 9% 21% 19% 3	5317
June	1st to 10th	Night	35% 5% 12% 26% 19% 3	1673
	11th to 20th	Night	34% 6% 14% 24% 19% 2	1470
	21st to end	Night	33% 6% 13% 27% 19% 1	1470
July	1st to 10th	Night	33% 5% 15% 27% 16% 4	3370
	11th to 20th	Night	32% 5% 14% 28% 20% 2	3875
	21st to end	Night	30% 6% 14% 21% 26% 3	4297
August	1st to 10th	Night	36% 5% 15% 25% 18% 2	2904
	11th to 20th	Night	33% 6% 14% 31% 14% 1	2902
	21st to end	Night	32% 5% 12% 34% 17% 1	3223
September	1st to 10th	Night	34% 6% 13% 28% 16% 2	2087
	11th to 20th	Night	28% 4 13% 33% 20% 1	2200
	21st to end	Night	44% 4 12% 26% 13% 1	2200
October	1st to 10th	Night	26% 5% 9% 30% 28% 1	3816
	11th to 20th	Night	46% 3 10% 21% 19%	3899
	21st to end	Night	42% 5% 15% 25% 13% 1	4342
November	1st to 10th	Night	41% 6% 12% 25% 14% 1	2666
	11th to 20th	Night	50% 6% 14% 19% 10% 1	2835
	21st to end	Night	57% 5% 16% 17% 4 1	2940
December	1st to 10th	Night	60% 6% 17% 14% 3	4720
	11th to 20th	Night	62% 5% 12% 14% 7%	5348
	21st to end	Night	74% 4 11% 9% 2	7315

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	21% 20% 37% 15% 6% 1	326
	11th to 20th	Day	24% 18% 42% 11% 4 2	330
	21st to end	Day	26% 19% 29% 21% 3 1	363
February	1st to 10th	Day	19% 20% 39% 17% 4	330
	11th to 20th	Day	24% 20% 38% 14% 3	330
	21st to end	Day	26% 21% 40% 12% 1	273
March	1st to 10th	Day	23% 18% 37% 18% 5% 1	460
	11th to 20th	Day	23% 16% 33% 22% 6%	540
	21st to end	Day	27% 15% 32% 21% 4	594
April	1st to 10th	Day	21% 15% 34% 22% 6% 1	540
	11th to 20th	Day	25% 15% 29% 19% 8% 4	532
	21st to end	Day	23% 11% 31% 25% 8% 2	540
May	1st to 10th	Day	22% 15% 29% 24% 8% 2	543
	11th to 20th	Day	17% 10% 30% 24% 14% 4	567
	21st to end	Day	15% 7% 27% 27% 19% 5%	617
June	1st to 10th	Day	12% 9% 28% 33% 15% 4	570
	11th to 20th	Day	12% 9% 30% 33% 14% 2	780
	21st to end	Day	11% 6% 29% 37% 14% 4	780
July	1st to 10th	Day	11% 3 25% 30% 23% 9%	771
	11th to 20th	Day	11% 6% 25% 29% 19% 10%	619
	21st to end	Day	9% 3 23% 31% 24% 11%	576
August	1st to 10th	Day	10% 4 18% 31% 23% 15%	526
	11th to 20th	Day	10% 6% 19% 32% 19% 14%	519
	21st to end	Day	11% 3 21% 29% 25% 11%	583
September	1st to 10th	Day	11% 5% 23% 27% 22% 12%	530
	11th to 20th	Day	8% 2 18% 26% 32% 13%	530
	21st to end	Day	18% 7% 23% 27% 16% 8%	530
October	1st to 10th	Day	17% 8% 25% 25% 18% 5%	530
	11th to 20th	Day	17% 15% 34% 22% 9% 3	530
	21st to end	Day	24% 14% 36% 20% 6% 1	527
November	1st to 10th	Day	18% 18% 36% 19% 6% 2	330
	11th to 20th	Day	23% 21% 31% 17% 6% 1	273
	21st to end	Day	22% 15% 40% 18% 5%	220
December	1st to 10th	Day	25% 19% 31% 18% 7% 1	220
	11th to 20th	Day	30% 19% 31% 16% 3	220
	21st to end	Day	30% 18% 31% 17% 4	242
January	1st to 10th	Night	23% 17% 36% 17% 5% 1	1018
	11th to 20th	Night	27% 15% 33% 21% 3	840
	21st to end	Night	20% 14% 35% 25% 5%	924
February	1st to 10th	Night	23% 13% 35% 24% 3	840
	11th to 20th	Night	22% 16% 32% 23% 6%	823
	21st to end	Night	22% 17% 37% 21% 2	692
March	1st to 10th	Night	21% 13% 32% 28% 6%	840
	11th to 20th	Night	19% 13% 37% 24% 7% 1	840
	21st to end	Night	21% 14% 31% 26% 7% 1	924
April	1st to 10th	Night	21% 15% 32% 27% 4	840
	11th to 20th	Night	19% 12% 32% 26% 10%	632
	21st to end	Night	19% 9% 32% 31% 8%	630
May	1st to 10th	Night	19% 7% 29% 33% 12% 1	630
	11th to 20th	Night	14% 6% 27% 38% 14% 1	621
	21st to end	Night	13% 3 24% 39% 19% 1	528
June	1st to 10th	Night	12% 5% 28% 39% 16%	420
	11th to 20th	Night	13% 4 24% 45% 15%	420
	21st to end	Night	11% 5% 24% 43% 17% 1	420
July	1st to 10th	Night	7% 1 14% 46% 29% 2	481
	11th to 20th	Night	10% 3 17% 39% 28% 3	597
	21st to end	Night	8% 3 16% 44% 28% 2	657
August	1st to 10th	Night	8% 2 13% 41% 30% 5%	597
	11th to 20th	Night	6% 3 14% 38% 34% 5%	594
	21st to end	Night	10% 2 17% 38% 30% 3	660
September	1st to 10th	Night	10% 4 21% 33% 27% 5%	795
	11th to 20th	Night	10% 4 15% 36% 31% 5%	800
	21st to end	Night	17% 6% 19% 35% 18% 6%	800
October	1st to 10th	Night	15% 6% 24% 34% 19% 3	800
	11th to 20th	Night	21% 10% 29% 27% 12% 2	800
	21st to end	Night	23% 12% 31% 28% 6%	880
November	1st to 10th	Night	17% 13% 29% 28% 12% 1	820
	11th to 20th	Night	20% 15% 36% 22% 6% 1	840
	21st to end	Night	27% 13% 31% 22% 7%	840
December	1st to 10th	Night	24% 16% 34% 20% 6% 1	840
	11th to 20th	Night	23% 16% 37% 21% 3	840
	21st to end	Night	29% 12% 33% 22% 3	998

Page generated at 2026-07-13 03:14:51. Version ac07

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