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 2025-03-12.

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%	109134
	Night	14% 11% 4 8% 5% 5% 5% 4 5% 6% 33%	109862
Transparency	Day	49% 6% 13% 20% 11% 1	112777
	Night	49% 5% 12% 20% 12% 1	114359
Seeing	Day	17% 11% 29% 25% 14% 5%	15432
	Night	18% 10% 28% 29% 13% 2	24874

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	12% 15% 6% 16% 6% 6% 4 4 5% 6% 20%	3651
	11th to 20th	Day	13% 19% 8% 17% 8% 7% 5% 4 5% 4 10%	4060
	21st to end	Day	21% 16% 7% 12% 6% 4 5% 3 4 5% 17%	4466
February	1st to 10th	Day	15% 16% 5% 14% 7% 6% 4 4 4 4 21%	1800
	11th to 20th	Day	15% 13% 7% 11% 7% 5% 5% 5% 4 6% 23%	1788
	21st to end	Day	16% 14% 4 11% 7% 5% 5% 3 5% 5% 23%	1650
March	1st to 10th	Day	19% 12% 5% 10% 5% 5% 4 3 4 5% 27%	3435
	11th to 20th	Day	12% 12% 5% 10% 7% 4 5% 3 5% 7% 31%	3491
	21st to end	Day	17% 12% 4 9% 5% 4 4 4 4 6% 31%	3815
April	1st to 10th	Day	14% 13% 4 8% 5% 5% 5% 3 5% 8% 31%	2143
	11th to 20th	Day	18% 10% 3 7% 4 4 4 4 6% 7% 32%	2442
	21st to end	Day	18% 10% 3 8% 5% 4 4 4 5% 6% 33%	2470
May	1st to 10th	Day	17% 7% 3 7% 5% 5% 4 3 6% 7% 34%	4082
	11th to 20th	Day	16% 9% 3 7% 5% 5% 4 4 5% 9% 33%	4081
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 8% 46%	4715
June	1st to 10th	Day	10% 6% 3 7% 6% 5% 5% 4 6% 9% 39%	2660
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	2850
	21st to end	Day	10% 6% 2 6% 5% 4 4 5% 8% 9% 40%	2850
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	4637
	11th to 20th	Day	8% 5% 2 5% 4 5% 4 5% 8% 10% 42%	4230
	21st to end	Day	7% 5% 2 7% 6% 6% 5% 5% 7% 9% 41%	4466
August	1st to 10th	Day	13% 5% 2 6% 4 4 5% 5% 8% 9% 38%	4104
	11th to 20th	Day	8% 6% 2 7% 5% 6% 6% 5% 9% 10% 36%	4044
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	4503
September	1st to 10th	Day	10% 6% 2 6% 5% 6% 6% 4 6% 8% 42%	2190
	11th to 20th	Day	9% 6% 2 5% 4 4 4 4 5% 10% 46%	2090
	21st to end	Day	17% 7% 2 6% 5% 4 5% 4 6% 7% 36%	2090
October	1st to 10th	Day	13% 7% 3 7% 4 4 4 5% 7% 7% 39%	1962
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 4 6% 6% 32%	1900
	21st to end	Day	20% 10% 3 8% 6% 4 5% 3 5% 7% 27%	2037
November	1st to 10th	Day	12% 9% 3 8% 7% 5% 5% 5% 5% 7% 35%	1762
	11th to 20th	Day	15% 10% 4 9% 5% 5% 5% 4 6% 7% 29%	1698
	21st to end	Day	19% 13% 5% 10% 6% 6% 4 4 5% 6% 22%	1600
December	1st to 10th	Day	18% 13% 4 9% 5% 4 5% 3 7% 8% 23%	2600
	11th to 20th	Day	22% 17% 6% 10% 5% 4 4 4 5% 6% 18%	2990
	21st to end	Day	25% 20% 7% 12% 6% 5% 4 3 4 4 11%	3782
January	1st to 10th	Night	14% 12% 4 13% 6% 5% 5% 3 5% 6% 26%	6297
	11th to 20th	Night	11% 17% 8% 14% 8% 6% 6% 4 7% 5% 14%	5900
	21st to end	Night	19% 16% 7% 10% 4 4 4 3 4 5% 24%	6490
February	1st to 10th	Night	17% 15% 4 10% 7% 5% 5% 4 5% 5% 23%	2600
	11th to 20th	Night	16% 13% 5% 8% 6% 5% 5% 4 5% 6% 27%	2496
	21st to end	Night	18% 11% 4 9% 6% 5% 5% 4 6% 6% 26%	1980
March	1st to 10th	Night	18% 13% 4 7% 6% 4 4 3 4 6% 32%	3637
	11th to 20th	Night	10% 8% 3 7% 6% 5% 5% 3 6% 7% 39%	3498
	21st to end	Night	15% 10% 3 7% 5% 4 4 3 5% 4 38%	3817
April	1st to 10th	Night	16% 12% 4 7% 4 5% 3 4 5% 6% 35%	2044
	11th to 20th	Night	17% 7% 3 7% 5% 4 3 3 6% 8% 36%	1696
	21st to end	Night	15% 8% 3 8% 5% 4 5% 4 5% 7% 36%	1710
May	1st to 10th	Night	18% 9% 3 5% 4 4 5% 3 6% 8% 36%	2813
	11th to 20th	Night	11% 9% 4 8% 6% 7% 6% 4 7% 7% 33%	2812
	21st to end	Night	10% 5% 2 5% 4 4 4 4 5% 8% 50%	2836
June	1st to 10th	Night	11% 5% 3 6% 4 4 4 3 6% 10% 44%	1514
	11th to 20th	Night	10% 5% 2 5% 4 4 6% 5% 7% 10% 43%	1330
	21st to end	Night	9% 6% 1 5% 4 3 5% 4 5% 9% 48%	1330
July	1st to 10th	Night	7% 4 1 4 5% 4 4 3 6% 11% 50%	2328
	11th to 20th	Night	8% 5% 2 5% 3 4 3 4 7% 9% 51%	2716
	21st to end	Night	5% 4 2 5% 5% 4 5% 5% 8% 9% 48%	3087
August	1st to 10th	Night	10% 6% 3 5% 4 4 4 4 6% 8% 45%	2825
	11th to 20th	Night	7% 6% 3 6% 4 4 4 3 6% 7% 50%	2785
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 49%	3124
September	1st to 10th	Night	8% 7% 2 7% 4 4 5% 4 5% 7% 48%	1982
	11th to 20th	Night	9% 5% 2 5% 4 3 3 3 5% 6% 55%	2090
	21st to end	Night	14% 7% 2 8% 5% 4 5% 2 5% 6% 41%	2090
October	1st to 10th	Night	12% 6% 3 6% 4 4 4 3 5% 7% 46%	2218
	11th to 20th	Night	18% 9% 3 8% 4 4 4 3 5% 6% 38%	2280
	21st to end	Night	20% 9% 3 7% 5% 5% 4 4 6% 6% 31%	2559
November	1st to 10th	Night	13% 7% 3 6% 5% 4 4 4 5% 7% 41%	2536
	11th to 20th	Night	16% 11% 3 8% 5% 5% 5% 5% 5% 6% 32%	2700
	21st to end	Night	20% 12% 4 8% 5% 5% 4 3 5% 7% 28%	2800
December	1st to 10th	Night	14% 13% 5% 11% 7% 5% 6% 5% 6% 7% 22%	4580
	11th to 20th	Night	17% 18% 5% 10% 5% 5% 5% 3 5% 4 22%	5205
	21st to end	Night	24% 18% 6% 13% 5% 5% 4 3 4 5% 13%	7157

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	64% 7% 10% 11% 8%	3652
	11th to 20th	Day	75% 6% 8% 6% 5%	4060
	21st to end	Day	70% 4 11% 9% 6%	4466
February	1st to 10th	Day	65% 5% 11% 13% 5% 1	1800
	11th to 20th	Day	61% 5% 10% 15% 7% 1	1797
	21st to end	Day	62% 5% 12% 14% 6% 2	1650
March	1st to 10th	Day	59% 4 10% 22% 5%	3435
	11th to 20th	Day	52% 5% 17% 17% 7% 1	3491
	21st to end	Day	54% 5% 12% 17% 11% 1	3815
April	1st to 10th	Day	51% 6% 12% 19% 10% 1	2143
	11th to 20th	Day	49% 6% 11% 20% 13% 1	2442
	21st to end	Day	52% 4 13% 17% 11% 2	2470
May	1st to 10th	Day	47% 6% 11% 22% 11% 2	4120
	11th to 20th	Day	48% 5% 17% 21% 8% 1	4085
	21st to end	Day	36% 6% 13% 20% 19% 7%	4729
June	1st to 10th	Day	40% 6% 13% 26% 12% 2	2660
	11th to 20th	Day	37% 8% 15% 22% 17% 2	2850
	21st to end	Day	36% 8% 14% 25% 16% 1	2850
July	1st to 10th	Day	38% 7% 12% 25% 15% 3	4566
	11th to 20th	Day	32% 7% 15% 29% 15% 1	4157
	21st to end	Day	36% 8% 16% 19% 20% 2	4324
August	1st to 10th	Day	39% 7% 16% 23% 15% 1	4091
	11th to 20th	Day	38% 8% 14% 31% 8% 1	4083
	21st to end	Day	37% 6% 13% 31% 13% 1	4503
September	1st to 10th	Day	39% 8% 15% 24% 13% 1	2190
	11th to 20th	Day	32% 6% 14% 32% 15%	2090
	21st to end	Day	45% 7% 13% 24% 11% 1	2090
October	1st to 10th	Day	34% 5% 15% 26% 19% 1	3261
	11th to 20th	Day	43% 4 12% 23% 17%	3140
	21st to end	Day	46% 5% 17% 24% 8%	3339
November	1st to 10th	Day	47% 6% 13% 23% 9% 2	1762
	11th to 20th	Day	51% 6% 17% 15% 9% 1	1698
	21st to end	Day	62% 5% 14% 15% 3	1600
December	1st to 10th	Day	58% 6% 19% 14% 4	2600
	11th to 20th	Day	67% 5% 13% 9% 6%	2986
	21st to end	Day	78% 5% 8% 8% 2	3782
January	1st to 10th	Night	57% 6% 11% 18% 8% 1	6297
	11th to 20th	Night	67% 8% 12% 9% 4	5900
	21st to end	Night	63% 4 9% 14% 9%	6490
February	1st to 10th	Night	62% 6% 11% 15% 6% 1	2600
	11th to 20th	Night	57% 5% 11% 18% 9% 1	2498
	21st to end	Night	57% 4 12% 19% 7% 1	1980
March	1st to 10th	Night	54% 4 14% 21% 7%	3637
	11th to 20th	Night	43% 5% 16% 21% 14% 1	3498
	21st to end	Night	48% 4 11% 21% 15% 1	3817
April	1st to 10th	Night	49% 5% 11% 20% 13% 2	2044
	11th to 20th	Night	45% 5% 11% 21% 16% 2	1696
	21st to end	Night	46% 5% 11% 19% 16% 3	1710
May	1st to 10th	Night	46% 5% 10% 18% 15% 5%	2840
	11th to 20th	Night	46% 7% 11% 23% 11% 2	2813
	21st to end	Night	33% 6% 11% 23% 23% 5%	2843
June	1st to 10th	Night	35% 5% 12% 27% 17% 3	1514
	11th to 20th	Night	34% 6% 14% 23% 20% 2	1330
	21st to end	Night	33% 6% 13% 27% 19% 2	1330
July	1st to 10th	Night	29% 4 12% 30% 18% 6%	2290
	11th to 20th	Night	29% 4 15% 31% 20% 1	2666
	21st to end	Night	29% 8% 17% 21% 21% 3	2988
August	1st to 10th	Night	36% 5% 15% 25% 17% 2	2814
	11th to 20th	Night	33% 6% 14% 31% 14% 1	2812
	21st to end	Night	32% 5% 11% 33% 17% 1	3124
September	1st to 10th	Night	35% 7% 13% 27% 16% 3	1982
	11th to 20th	Night	29% 5% 13% 31% 20% 1	2090
	21st to end	Night	44% 4 12% 26% 13% 1	2090
October	1st to 10th	Night	27% 5% 9% 30% 28% 2	3699
	11th to 20th	Night	47% 3 11% 21% 18%	3779
	21st to end	Night	42% 5% 15% 25% 12%	4207
November	1st to 10th	Night	41% 6% 12% 26% 14% 1	2536
	11th to 20th	Night	51% 5% 14% 19% 10% 1	2700
	21st to end	Night	57% 5% 16% 17% 4 1	2800
December	1st to 10th	Night	60% 6% 17% 14% 3	4580
	11th to 20th	Night	63% 5% 12% 13% 6%	5208
	21st to end	Night	74% 4 11% 8% 2	7157

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% 36% 16% 6% 1	296
	11th to 20th	Day	23% 17% 43% 12% 4 2	300
	21st to end	Day	27% 17% 30% 22% 3 1	330
February	1st to 10th	Day	20% 21% 40% 15% 4	300
	11th to 20th	Day	24% 21% 38% 14% 3	300
	21st to end	Day	28% 19% 40% 13% 1	249
March	1st to 10th	Day	22% 17% 37% 18% 5% 1	424
	11th to 20th	Day	21% 14% 34% 24% 7%	464
	21st to end	Day	26% 14% 31% 23% 5%	506
April	1st to 10th	Day	18% 14% 36% 24% 7% 1	460
	11th to 20th	Day	25% 15% 27% 20% 8% 4	452
	21st to end	Day	23% 12% 31% 24% 8% 2	460
May	1st to 10th	Day	21% 14% 27% 27% 9% 3	463
	11th to 20th	Day	17% 9% 29% 25% 16% 4	487
	21st to end	Day	13% 5% 25% 30% 22% 5%	529
June	1st to 10th	Day	12% 8% 27% 34% 16% 4	490
	11th to 20th	Day	12% 7% 28% 36% 15% 3	680
	21st to end	Day	12% 6% 29% 35% 14% 4	680
July	1st to 10th	Day	10% 3 23% 30% 24% 10%	671
	11th to 20th	Day	12% 6% 24% 29% 19% 11%	551
	21st to end	Day	9% 3 21% 31% 25% 11%	532
August	1st to 10th	Day	10% 4 18% 29% 23% 15%	486
	11th to 20th	Day	9% 6% 19% 31% 20% 14%	479
	21st to end	Day	11% 4 21% 28% 26% 11%	539
September	1st to 10th	Day	11% 4 22% 29% 23% 11%	490
	11th to 20th	Day	8% 2 19% 26% 32% 12%	490
	21st to end	Day	19% 7% 24% 27% 15% 8%	490
October	1st to 10th	Day	18% 9% 26% 25% 18% 6%	490
	11th to 20th	Day	17% 15% 32% 22% 10% 3	490
	21st to end	Day	24% 14% 35% 20% 7% 1	486
November	1st to 10th	Day	16% 18% 36% 20% 7% 3	300
	11th to 20th	Day	22% 21% 31% 17% 7% 2	248
	21st to end	Day	22% 14% 42% 19% 5%	200
December	1st to 10th	Day	25% 18% 32% 18% 8% 1	200
	11th to 20th	Day	31% 17% 33% 18% 3	200
	21st to end	Day	29% 19% 31% 17% 4	220
January	1st to 10th	Night	23% 16% 36% 17% 6% 1	970
	11th to 20th	Night	28% 14% 33% 22% 4	800
	21st to end	Night	21% 13% 34% 26% 6%	880
February	1st to 10th	Night	24% 14% 35% 24% 3	800
	11th to 20th	Night	22% 16% 32% 24% 6%	783
	21st to end	Night	22% 17% 38% 21% 3	660
March	1st to 10th	Night	21% 13% 33% 28% 6%	800
	11th to 20th	Night	17% 13% 37% 25% 7% 1	767
	21st to end	Night	21% 13% 30% 27% 8% 1	836
April	1st to 10th	Night	20% 15% 32% 28% 5% 1	760
	11th to 20th	Night	20% 11% 32% 26% 10% 1	572
	21st to end	Night	20% 8% 33% 30% 9%	570
May	1st to 10th	Night	18% 6% 29% 33% 13% 1	570
	11th to 20th	Night	14% 6% 25% 39% 15% 1	561
	21st to end	Night	11% 3 22% 41% 21% 1	477
June	1st to 10th	Night	13% 5% 28% 39% 16%	380
	11th to 20th	Night	12% 4 22% 46% 16%	380
	21st to end	Night	11% 5% 24% 42% 17% 1	380
July	1st to 10th	Night	7% 1 15% 45% 30% 2	433
	11th to 20th	Night	10% 2 18% 38% 28% 3	552
	21st to end	Night	7% 3 16% 44% 28% 2	624
August	1st to 10th	Night	9% 2 14% 41% 30% 5%	567
	11th to 20th	Night	6% 2 15% 38% 34% 5%	564
	21st to end	Night	11% 2 17% 38% 30% 3	627
September	1st to 10th	Night	10% 4 21% 34% 26% 5%	755
	11th to 20th	Night	10% 4 15% 36% 29% 5%	760
	21st to end	Night	17% 6% 19% 34% 17% 6%	760
October	1st to 10th	Night	16% 6% 25% 33% 18% 3	760
	11th to 20th	Night	21% 10% 28% 26% 12% 2	760
	21st to end	Night	24% 12% 31% 28% 6%	836
November	1st to 10th	Night	17% 12% 29% 28% 13% 1	780
	11th to 20th	Night	20% 15% 36% 22% 6% 1	800
	21st to end	Night	27% 13% 31% 22% 7%	800
December	1st to 10th	Night	24% 16% 34% 19% 6% 1	800
	11th to 20th	Night	23% 15% 37% 21% 4	800
	21st to end	Night	29% 12% 33% 23% 3	950

Page generated at 2025-03-09 08:08:41. Version ac07

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