Big Bend Remote Observatories
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Bend Remote Observatories clear sky chart from 2005-11-06 to 2026-07-01.

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	3 2 1 3 3 3 3 3 5% 8% 66%	145710
	Night	3 3 1 3 3 3 4 3 5% 8% 64%	144142
Transparency	Day	17% 4 8% 20% 26% 25%	140824
	Night	18% 4 8% 20% 25% 25%	140035
Seeing	Day	4 4 13% 31% 31% 15%	16585
	Night	5% 3 18% 48% 25% 1	26752

By Month

Percentage Cloud Cover SW (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	2 2 1 3 2 2 2 2 3 6% 74%	5885
	11th to 20th	Day	3 3 1 3 2 2 3 2 4 5% 71%	5910
	21st to end	Day	4 5% 2 5% 3 3 3 3 4 6% 62%	6468
February	1st to 10th	Day	4 4 1 5% 3 3 3 3 5% 5% 65%	2100
	11th to 20th	Day	4 4 2 4 3 3 3 3 5% 6% 63%	2049
	21st to end	Day	3 4 1 2 2 2 3 3 5% 7% 68%	1730
March	1st to 10th	Day	3 4 2 4 2 3 3 3 5% 7% 64%	4600
	11th to 20th	Day	4 3 1 3 2 2 3 3 4 6% 69%	4858
	21st to end	Day	2 4 1 4 3 2 3 3 5% 7% 66%	6101
April	1st to 10th	Day	3 3 1 3 3 3 4 3 5% 7% 67%	2520
	11th to 20th	Day	3 3 1 3 2 2 4 3 5% 6% 69%	2491
	21st to end	Day	3 4 1 3 2 2 2 2 5% 6% 70%	2520
May	1st to 10th	Day	2 2 1 3 2 2 2 3 4 6% 74%	7057
	11th to 20th	Day	2 2 1 3 3 3 4 4 6% 7% 65%	7032
	21st to end	Day	2 2 1 3 2 3 4 4 6% 8% 65%	8161
June	1st to 10th	Day	1 1 1 2 2 3 3 4 6% 9% 69%	2730
	11th to 20th	Day	2 2 1 4 3 3 3 3 6% 8% 65%	2730
	21st to end	Day	2 2 1 4 3 3 4 5% 8% 11% 57%	2669
July	1st to 10th	Day	3 2 1 4 3 4 4 4 6% 10% 59%	5440
	11th to 20th	Day	2 2 1 3 3 4 5% 6% 9% 13% 54%	5440
	21st to end	Day	1 2 1 3 3 4 4 4 7% 11% 58%	5922
August	1st to 10th	Day	1 1 1 1 2 3 3 3 6% 12% 69%	5348
	11th to 20th	Day	2 2 1 4 4 4 5% 4 8% 12% 55%	5366
	21st to end	Day	1 1 1 3 3 4 4 4 8% 13% 58%	5984
September	1st to 10th	Day	4 5% 2 6% 5% 5% 5% 4 7% 10% 47%	2400
	11th to 20th	Day	7% 5% 1 4 4 5% 5% 4 6% 7% 52%	2250
	21st to end	Day	4 3 1 4 3 3 3 4 5% 7% 62%	2200
October	1st to 10th	Day	1 2 2 2 2 2 2 4 5% 79%	3590
	11th to 20th	Day	3 2 1 2 2 2 2 3 5% 5% 73%	3478
	21st to end	Day	3 2 1 1 1 1 1 1 2 4 83%	3548
November	1st to 10th	Day	4 2 1 3 1 2 2 2 3 6% 75%	2001
	11th to 20th	Day	4 3 1 3 2 2 3 3 5% 6% 69%	1890
	21st to end	Day	5% 4 2 3 2 3 3 3 5% 6% 65%	1890
December	1st to 10th	Day	3 3 2 8% 4 3 4 3 5% 7% 57%	3030
	11th to 20th	Day	2 3 1 3 2 2 4 3 4 6% 68%	3481
	21st to end	Day	2 2 1 2 2 2 2 2 5% 6% 74%	4841
January	1st to 10th	Night	2 2 1 2 2 2 2 2 3 6% 75%	7038
	11th to 20th	Night	4 3 1 2 2 2 2 3 4 5% 72%	7080
	21st to end	Night	6% 5% 2 4 3 3 3 4 5% 7% 58%	7746
February	1st to 10th	Night	4 4 1 4 3 3 3 3 5% 6% 65%	2520
	11th to 20th	Night	5% 3 2 3 3 3 4 3 5% 8% 61%	2458
	21st to end	Night	4 4 1 3 3 3 3 3 5% 6% 65%	2076
March	1st to 10th	Night	4 4 1 4 3 4 4 3 4 6% 63%	5494
	11th to 20th	Night	4 3 1 3 3 3 3 2 4 6% 69%	5329
	21st to end	Night	3 3 1 4 3 3 3 3 5% 6% 67%	5094
April	1st to 10th	Night	4 5% 1 3 3 3 4 4 5% 6% 63%	2100
	11th to 20th	Night	3 2 1 3 3 3 4 4 5% 6% 66%	2077
	21st to end	Night	3 3 1 2 3 2 4 3 4 5% 69%	2100
May	1st to 10th	Night	3 2 1 3 2 2 2 3 4 7% 72%	5890
	11th to 20th	Night	5% 3 2 4 3 3 4 4 6% 8% 59%	5860
	21st to end	Night	2 3 1 4 3 4 4 4 6% 7% 61%	6041
June	1st to 10th	Night	2 2 1 3 3 3 5% 5% 8% 12% 57%	1890
	11th to 20th	Night	2 1 1 5% 4 5% 5% 5% 8% 9% 54%	1890
	21st to end	Night	2 3 2 4 4 4 4 4 9% 11% 54%	1941
July	1st to 10th	Night	2 2 1 4 4 4 5% 5% 8% 11% 53%	4540
	11th to 20th	Night	1 1 1 3 3 4 5% 4 7% 12% 59%	4540
	21st to end	Night	1 2 1 4 4 4 6% 6% 8% 12% 53%	4951
August	1st to 10th	Night	1 1 2 2 2 3 4 7% 12% 67%	4464
	11th to 20th	Night	1 2 1 3 3 4 5% 5% 8% 13% 56%	4483
	21st to end	Night	1 1 1 3 3 4 5% 4 7% 13% 57%	4993
September	1st to 10th	Night	4 4 1 6% 4 5% 6% 4 7% 10% 49%	2000
	11th to 20th	Night	5% 4 2 6% 5% 5% 6% 5% 8% 11% 45%	2150
	21st to end	Night	4 4 1 4 4 4 5% 4 8% 10% 53%	2200
October	1st to 10th	Night	2 1 1 2 2 2 2 3 4 6% 76%	3590
	11th to 20th	Night	3 2 1 3 2 2 3 3 5% 7% 71%	3658
	21st to end	Night	2 2 1 2 1 1 1 1 3 4 82%	4252
November	1st to 10th	Night	4 2 1 2 2 2 2 2 3 4 77%	2492
	11th to 20th	Night	4 2 1 3 2 3 4 3 4 6% 70%	2730
	21st to end	Night	5% 3 1 3 2 3 3 3 4 6% 66%	2730
December	1st to 10th	Night	3 4 3 7% 4 4 4 5% 5% 8% 53%	4380
	11th to 20th	Night	2 4 1 3 2 2 3 2 4 6% 71%	4946
	21st to end	Night	2 3 1 3 2 2 3 2 5% 6% 71%	6419

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	14% 2 4 25% 31% 24%	5888
	11th to 20th	Day	17% 3 4 16% 36% 25%	5910
	21st to end	Day	24% 3 4 21% 29% 18%	6468
February	1st to 10th	Day	22% 3 5% 14% 26% 31%	2100
	11th to 20th	Day	23% 2 7% 15% 27% 27%	2059
	21st to end	Day	17% 2 4 12% 31% 34%	1730
March	1st to 10th	Day	20% 3 3 10% 21% 43%	4600
	11th to 20th	Day	17% 3 5% 10% 18% 47%	4858
	21st to end	Day	18% 3 3 8% 26% 42%	6101
April	1st to 10th	Day	17% 3 6% 12% 24% 37%	2520
	11th to 20th	Day	16% 3 5% 15% 23% 38%	2494
	21st to end	Day	17% 2 5% 13% 24% 38%	2520
May	1st to 10th	Day	13% 3 5% 15% 22% 44%	7057
	11th to 20th	Day	17% 3 6% 13% 26% 35%	7035
	21st to end	Day	15% 4 7% 22% 23% 29%	8162
June	1st to 10th	Day	11% 4 10% 26% 28% 21%	2730
	11th to 20th	Day	16% 4 8% 18% 30% 24%	2730
	21st to end	Day	19% 7% 13% 23% 25% 14%	2669
July	1st to 10th	Day	20% 7% 17% 31% 21% 4	5382
	11th to 20th	Day	18% 8% 17% 37% 15% 5%	5377
	21st to end	Day	17% 7% 18% 30% 21% 8%	5752
August	1st to 10th	Day	13% 4 10% 32% 34% 7%	3892
	11th to 20th	Day	18% 6% 15% 38% 20% 3	3882
	21st to end	Day	18% 7% 18% 29% 22% 6%	4311
September	1st to 10th	Day	30% 7% 17% 20% 15% 11%	2400
	11th to 20th	Day	29% 7% 12% 20% 21% 10%	2250
	21st to end	Day	20% 6% 10% 22% 25% 16%	2200
October	1st to 10th	Day	10% 3 7% 16% 28% 37%	3590
	11th to 20th	Day	14% 3 10% 20% 19% 34%	3478
	21st to end	Day	10% 2 3 12% 42% 32%	3548
November	1st to 10th	Day	14% 4 7% 19% 29% 27%	2001
	11th to 20th	Day	17% 3 8% 20% 29% 23%	1890
	21st to end	Day	21% 3 7% 24% 24% 21%	1890
December	1st to 10th	Day	23% 6% 10% 20% 26% 15%	3030
	11th to 20th	Day	18% 2 4 26% 30% 19%	3479
	21st to end	Day	13% 2 4 15% 38% 28%	4841
January	1st to 10th	Night	13% 2 3 21% 37% 25%	7038
	11th to 20th	Night	16% 3 3 12% 31% 35%	7080
	21st to end	Night	25% 3 5% 20% 26% 21%	7746
February	1st to 10th	Night	21% 2 5% 15% 25% 32%	2520
	11th to 20th	Night	23% 3 7% 14% 24% 29%	2458
	21st to end	Night	19% 3 4 11% 28% 35%	2076
March	1st to 10th	Night	23% 3 3 9% 22% 41%	5494
	11th to 20th	Night	18% 3 7% 10% 17% 45%	5329
	21st to end	Night	18% 4 4 10% 22% 43%	5094
April	1st to 10th	Night	22% 4 6% 14% 19% 36%	2100
	11th to 20th	Night	18% 4 7% 16% 18% 36%	2077
	21st to end	Night	16% 3 7% 18% 23% 33%	2100
May	1st to 10th	Night	13% 3 8% 18% 21% 36%	5888
	11th to 20th	Night	23% 4 8% 15% 20% 31%	5861
	21st to end	Night	21% 5% 9% 23% 18% 23%	6043
June	1st to 10th	Night	17% 6% 12% 28% 23% 13%	1890
	11th to 20th	Night	23% 6% 10% 17% 29% 16%	1890
	21st to end	Night	21% 7% 15% 22% 22% 14%	1941
July	1st to 10th	Night	21% 7% 20% 31% 18% 2	4490
	11th to 20th	Night	16% 6% 17% 39% 17% 4	4480
	21st to end	Night	19% 7% 18% 28% 21% 6%	4808
August	1st to 10th	Night	12% 5% 10% 35% 34% 4	3244
	11th to 20th	Night	17% 6% 16% 43% 17% 2	3243
	21st to end	Night	19% 8% 18% 29% 22% 4	3597
September	1st to 10th	Night	29% 7% 17% 23% 15% 10%	2000
	11th to 20th	Night	30% 8% 16% 19% 19% 9%	2150
	21st to end	Night	23% 7% 13% 22% 19% 15%	2200
October	1st to 10th	Night	11% 4 8% 15% 24% 38%	3590
	11th to 20th	Night	14% 4 10% 21% 17% 34%	3658
	21st to end	Night	10% 1 3 15% 38% 33%	4252
November	1st to 10th	Night	14% 3 6% 17% 30% 30%	2492
	11th to 20th	Night	16% 4 7% 19% 27% 28%	2730
	21st to end	Night	20% 3 10% 16% 24% 27%	2730
December	1st to 10th	Night	28% 5% 8% 19% 20% 19%	4380
	11th to 20th	Night	15% 2 5% 22% 33% 23%	4947
	21st to end	Night	16% 2 4 12% 39% 27%	6419

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	7% 3 19% 40% 24% 6%	330
	11th to 20th	Day	5% 6% 14% 42% 29% 4	330
	21st to end	Day	5% 9% 18% 41% 24% 4	363
February	1st to 10th	Day	7% 7% 15% 27% 35% 9%	330
	11th to 20th	Day	9% 8% 19% 40% 20% 4	330
	21st to end	Day	6% 5% 18% 44% 23% 4	273
March	1st to 10th	Day	6% 8% 19% 41% 22% 5%	330
	11th to 20th	Day	9% 9% 24% 30% 22% 6%	417
	21st to end	Day	7% 11% 21% 36% 19% 5%	594
April	1st to 10th	Day	5% 12% 27% 37% 17% 3	540
	11th to 20th	Day	4 13% 30% 33% 17% 3	532
	21st to end	Day	6% 8% 27% 38% 18% 2	540
May	1st to 10th	Day	4 8% 25% 41% 17% 5%	543
	11th to 20th	Day	3 7% 27% 37% 23% 4	568
	21st to end	Day	3 3 21% 38% 32% 3	757
June	1st to 10th	Day	2 2 12% 36% 38% 11%	780
	11th to 20th	Day	3 1 8% 34% 42% 11%	780
	21st to end	Day	2 1 6% 29% 46% 17%	719
July	1st to 10th	Day	3 1 5% 26% 31% 34%	527
	11th to 20th	Day	2 3 24% 38% 32%	524
	21st to end	Day	2 1 3 26% 41% 27%	575
August	1st to 10th	Day	2 1 3 17% 42% 35%	527
	11th to 20th	Day	4 1 4 23% 34% 34%	517
	21st to end	Day	1 1 5% 23% 35% 34%	583
September	1st to 10th	Day	6% 1 7% 26% 37% 24%	530
	11th to 20th	Day	5% 1 2 15% 43% 34%	380
	21st to end	Day	7% 6% 18% 36% 34%	330
October	1st to 10th	Day	4 1 5% 20% 39% 30%	330
	11th to 20th	Day	7% 1 7% 25% 32% 28%	330
	21st to end	Day	7% 4 11% 24% 34% 21%	363
November	1st to 10th	Day	5% 2 10% 28% 35% 20%	330
	11th to 20th	Day	6% 4 12% 22% 36% 19%	330
	21st to end	Day	7% 6% 15% 33% 32% 7%	330
December	1st to 10th	Day	7% 4 10% 33% 37% 8%	330
	11th to 20th	Day	4 6% 14% 34% 32% 11%	330
	21st to end	Day	5% 4 13% 37% 33% 9%	363
January	1st to 10th	Night	6% 2 21% 54% 16% 1	840
	11th to 20th	Night	8% 3 18% 55% 16% 1	840
	21st to end	Night	8% 4 24% 50% 14%	924
February	1st to 10th	Night	5% 5% 21% 50% 19%	840
	11th to 20th	Night	7% 4 26% 53% 10% 1	824
	21st to end	Night	6% 6% 28% 52% 8%	692
March	1st to 10th	Night	7% 5% 30% 48% 9% 1	840
	11th to 20th	Night	7% 5% 29% 45% 14%	836
	21st to end	Night	4 5% 33% 47% 10%	693
April	1st to 10th	Night	6% 6% 30% 48% 10%	630
	11th to 20th	Night	4 8% 33% 48% 7%	623
	21st to end	Night	3 7% 37% 46% 7%	630
May	1st to 10th	Night	5% 4 33% 50% 9%	630
	11th to 20th	Night	6% 5% 28% 52% 10%	621
	21st to end	Night	2 3 26% 54% 14%	683
June	1st to 10th	Night	3 2 14% 59% 22%	630
	11th to 20th	Night	2 1 16% 58% 22%	630
	21st to end	Night	4 1 10% 48% 37%	630
July	1st to 10th	Night	4 2 8% 38% 46% 3	585
	11th to 20th	Night	2 3 43% 50% 2	582
	21st to end	Night	3 4 42% 48% 2	657
August	1st to 10th	Night	1 1 3 34% 58% 3	597
	11th to 20th	Night	3 1 3 28% 59% 5%	594
	21st to end	Night	2 1 4 32% 56% 5%	660
September	1st to 10th	Night	6% 1 4 36% 52% 2	600
	11th to 20th	Night	6% 1 6% 38% 48% 1	600
	21st to end	Night	5% 8% 43% 42% 3	600
October	1st to 10th	Night	3 1 9% 45% 40% 1	600
	11th to 20th	Night	6% 1 11% 54% 27% 2	651
	21st to end	Night	5% 3 12% 45% 32% 4	880
November	1st to 10th	Night	6% 1 11% 45% 33% 3	852
	11th to 20th	Night	5% 2 12% 52% 27% 2	1050
	21st to end	Night	6% 3 21% 49% 19% 2	1050
December	1st to 10th	Night	7% 3 13% 53% 23% 1	1050
	11th to 20th	Night	5% 3 18% 54% 20% 1	1050
	21st to end	Night	4 3 17% 57% 18% 1	1058

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