Marfa
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marfa 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	3 3 1 3 3 3 4 3 6% 7% 64%	145847
	Night	3 3 1 3 3 3 4 3 5% 8% 64%	144305
Transparency	Day	19% 4 7% 18% 26% 25%	140961
	Night	19% 4 8% 19% 25% 25%	140198
Seeing	Day	5% 6% 20% 29% 27% 13%	16757
	Night	4 4 24% 43% 22% 2	26909

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 2 2 2 3 2 4 7% 74%	5885
	11th to 20th	Day	3 2 1 2 1 2 2 2 4 5% 76%	5910
	21st to end	Day	4 4 2 4 3 3 4 3 5% 6% 64%	6468
February	1st to 10th	Day	3 2 1 4 3 3 4 3 6% 6% 64%	2100
	11th to 20th	Day	3 3 1 4 3 3 3 3 5% 7% 66%	2049
	21st to end	Day	3 3 1 2 3 3 3 3 4 6% 69%	1730
March	1st to 10th	Day	2 4 1 4 3 3 3 3 5% 8% 66%	4600
	11th to 20th	Day	5% 3 1 3 2 2 3 3 5% 6% 68%	4969
	21st to end	Day	1 3 1 4 3 3 4 4 5% 7% 64%	6045
April	1st to 10th	Day	3 2 1 3 3 2 3 4 6% 7% 65%	2520
	11th to 20th	Day	3 3 1 3 3 3 4 3 4 6% 66%	2491
	21st to end	Day	3 2 1 2 3 2 3 2 4 6% 72%	2520
May	1st to 10th	Day	2 3 1 3 2 3 3 3 4 6% 71%	7057
	11th to 20th	Day	3 3 1 3 3 3 3 2 5% 6% 68%	7032
	21st to end	Day	2 2 1 3 3 3 4 4 7% 8% 64%	8161
June	1st to 10th	Day	2 1 1 2 3 3 4 5% 6% 8% 64%	2730
	11th to 20th	Day	3 2 1 3 3 3 4 4 6% 8% 62%	2730
	21st to end	Day	3 3 2 4 4 5% 5% 5% 9% 8% 51%	2686
July	1st to 10th	Day	4 3 2 5% 4 5% 5% 5% 7% 9% 51%	5559
	11th to 20th	Day	2 2 1 3 4 5% 6% 5% 8% 10% 53%	5501
	21st to end	Day	3 2 2 5% 4 6% 7% 5% 8% 11% 47%	5922
August	1st to 10th	Day	2 1 1 3 3 4 5% 5% 7% 10% 59%	5348
	11th to 20th	Day	3 3 1 6% 5% 6% 8% 6% 9% 10% 43%	5366
	21st to end	Day	3 2 1 6% 5% 5% 5% 4 6% 8% 55%	5984
September	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 4 7% 8% 44%	2400
	11th to 20th	Day	8% 4 2 6% 4 6% 6% 5% 6% 8% 45%	2285
	21st to end	Day	5% 5% 2 4 4 3 5% 3 5% 6% 57%	2200
October	1st to 10th	Day	2 1 1 2 2 2 2 3 3 4 77%	3590
	11th to 20th	Day	4 3 1 2 2 2 2 2 3 4 74%	3333
	21st to end	Day	3 2 1 1 1 1 1 3 4 82%	3548
November	1st to 10th	Day	4 2 1 2 1 2 2 2 4 5% 75%	2016
	11th to 20th	Day	2 4 1 3 2 2 3 3 4 6% 70%	1890
	21st to end	Day	6% 3 1 4 3 3 3 3 4 6% 65%	1890
December	1st to 10th	Day	5% 7% 2 4 2 2 3 3 5% 7% 59%	3030
	11th to 20th	Day	1 2 1 3 3 4 4 2 4 6% 70%	3481
	21st to end	Day	2 2 1 2 1 2 2 2 4 5% 76%	4821
January	1st to 10th	Night	2 3 1 3 2 2 2 3 4 7% 71%	7038
	11th to 20th	Night	3 2 1 2 2 2 3 2 4 5% 74%	7080
	21st to end	Night	4 4 2 4 3 3 4 3 6% 8% 60%	7746
February	1st to 10th	Night	3 3 1 3 3 3 4 2 5% 7% 66%	2520
	11th to 20th	Night	3 4 1 4 3 3 5% 4 6% 6% 59%	2458
	21st to end	Night	3 3 1 3 2 3 4 2 5% 7% 66%	2076
March	1st to 10th	Night	1 2 1 4 3 3 3 3 6% 8% 67%	5494
	11th to 20th	Night	3 4 1 3 2 2 3 3 3 5% 71%	5228
	21st to end	Night	3 2 1 4 3 3 3 3 5% 6% 67%	5150
April	1st to 10th	Night	4 4 1 3 3 3 3 3 5% 7% 63%	2100
	11th to 20th	Night	4 3 1 4 3 3 4 3 5% 6% 65%	2077
	21st to end	Night	2 2 1 3 2 3 3 3 4 6% 72%	2100
May	1st to 10th	Night	2 2 1 3 3 3 3 2 4 6% 70%	5890
	11th to 20th	Night	3 3 1 4 3 3 4 4 6% 7% 63%	5860
	21st to end	Night	3 3 1 4 4 4 5% 4 6% 7% 60%	6023
June	1st to 10th	Night	2 2 1 3 3 4 5% 5% 8% 10% 57%	1890
	11th to 20th	Night	3 2 1 4 4 3 6% 5% 8% 10% 52%	1890
	21st to end	Night	4 4 2 4 3 4 5% 6% 7% 8% 52%	1924
July	1st to 10th	Night	4 3 2 4 4 4 5% 5% 7% 12% 51%	4640
	11th to 20th	Night	2 1 1 2 2 3 4 4 7% 13% 61%	4590
	21st to end	Night	1 2 1 4 5% 5% 6% 5% 9% 13% 47%	4951
August	1st to 10th	Night	1 1 1 2 2 3 4 4 7% 11% 64%	4464
	11th to 20th	Night	1 2 1 4 3 4 5% 5% 10% 16% 50%	4483
	21st to end	Night	2 2 1 3 3 3 4 4 7% 11% 60%	4993
September	1st to 10th	Night	5% 5% 2 5% 5% 4 5% 5% 7% 8% 49%	2000
	11th to 20th	Night	5% 4 1 4 5% 4 5% 5% 8% 10% 48%	2113
	21st to end	Night	3 2 1 4 5% 4 6% 5% 8% 10% 53%	2200
October	1st to 10th	Night	2 1 1 2 2 2 2 3 4 5% 76%	3590
	11th to 20th	Night	4 2 1 3 2 2 3 2 4 6% 71%	3794
	21st to end	Night	2 2 1 2 1 1 2 2 3 4 80%	4252
November	1st to 10th	Night	5% 2 2 2 1 2 2 3 6% 75%	2478
	11th to 20th	Night	2 3 1 3 2 3 4 2 5% 7% 68%	2730
	21st to end	Night	4 3 1 3 3 3 3 2 6% 6% 66%	2730
December	1st to 10th	Night	4 5% 2 6% 4 4 4 4 4 6% 56%	4380
	11th to 20th	Night	2 3 1 3 3 3 2 2 4 7% 70%	4946
	21st to end	Night	2 2 1 3 2 2 2 2 4 5% 77%	6427

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	13% 2 4 22% 30% 28%	5888
	11th to 20th	Day	13% 2 3 16% 38% 28%	5910
	21st to end	Day	22% 2 4 16% 33% 23%	6468
February	1st to 10th	Day	20% 3 5% 14% 25% 34%	2100
	11th to 20th	Day	20% 2 5% 15% 29% 29%	2059
	21st to end	Day	16% 4 3 11% 33% 34%	1730
March	1st to 10th	Day	18% 3 2 8% 24% 46%	4600
	11th to 20th	Day	18% 2 4 11% 24% 41%	4969
	21st to end	Day	18% 4 3 8% 30% 38%	6045
April	1st to 10th	Day	19% 3 5% 13% 27% 34%	2520
	11th to 20th	Day	20% 4 4 13% 26% 34%	2494
	21st to end	Day	16% 2 4 12% 32% 35%	2520
May	1st to 10th	Day	16% 3 5% 14% 23% 39%	7057
	11th to 20th	Day	17% 3 4 10% 25% 40%	7035
	21st to end	Day	16% 5% 7% 20% 20% 31%	8162
June	1st to 10th	Day	15% 5% 10% 23% 29% 17%	2730
	11th to 20th	Day	18% 5% 7% 18% 30% 22%	2730
	21st to end	Day	26% 7% 11% 23% 20% 12%	2686
July	1st to 10th	Day	26% 8% 15% 28% 20% 3	5501
	11th to 20th	Day	22% 7% 19% 35% 15% 3	5438
	21st to end	Day	27% 8% 20% 24% 18% 4	5752
August	1st to 10th	Day	21% 7% 12% 29% 28% 4	3892
	11th to 20th	Day	27% 7% 16% 29% 18% 4	3882
	21st to end	Day	29% 8% 15% 26% 18% 3	4311
September	1st to 10th	Day	35% 6% 13% 20% 15% 10%	2400
	11th to 20th	Day	35% 6% 13% 19% 20% 8%	2285
	21st to end	Day	28% 5% 8% 22% 24% 13%	2200
October	1st to 10th	Day	12% 3 6% 15% 25% 39%	3590
	11th to 20th	Day	16% 3 6% 22% 21% 33%	3333
	21st to end	Day	9% 1 2 12% 46% 29%	3548
November	1st to 10th	Day	14% 2 6% 19% 30% 29%	2016
	11th to 20th	Day	17% 2 6% 21% 30% 24%	1890
	21st to end	Day	22% 2 6% 18% 31% 21%	1890
December	1st to 10th	Day	24% 3 5% 20% 29% 19%	3030
	11th to 20th	Day	17% 2 4 19% 35% 23%	3479
	21st to end	Day	11% 2 2 14% 37% 33%	4821
January	1st to 10th	Night	15% 2 4 19% 34% 26%	7038
	11th to 20th	Night	15% 2 3 14% 31% 35%	7080
	21st to end	Night	24% 3 4 18% 29% 23%	7746
February	1st to 10th	Night	19% 2 5% 15% 26% 34%	2520
	11th to 20th	Night	24% 4 6% 12% 26% 28%	2458
	21st to end	Night	19% 3 4 9% 30% 37%	2076
March	1st to 10th	Night	16% 3 2 9% 25% 45%	5494
	11th to 20th	Night	17% 4 5% 11% 23% 40%	5228
	21st to end	Night	19% 3 4 9% 26% 39%	5150
April	1st to 10th	Night	21% 4 4 13% 27% 31%	2100
	11th to 20th	Night	21% 3 7% 14% 23% 32%	2077
	21st to end	Night	15% 2 6% 15% 26% 35%	2100
May	1st to 10th	Night	15% 4 6% 16% 22% 37%	5888
	11th to 20th	Night	19% 4 6% 15% 23% 33%	5861
	21st to end	Night	23% 5% 9% 19% 16% 29%	6025
June	1st to 10th	Night	19% 5% 13% 28% 23% 13%	1890
	11th to 20th	Night	23% 6% 9% 19% 25% 18%	1890
	21st to end	Night	26% 6% 13% 23% 20% 12%	1924
July	1st to 10th	Night	25% 6% 20% 33% 13% 3	4590
	11th to 20th	Night	14% 5% 22% 40% 16% 2	4530
	21st to end	Night	23% 9% 22% 27% 17% 2	4808
August	1st to 10th	Night	15% 6% 15% 34% 26% 4	3244
	11th to 20th	Night	19% 6% 23% 33% 18% 2	3243
	21st to end	Night	20% 7% 20% 32% 17% 3	3597
September	1st to 10th	Night	29% 8% 16% 22% 16% 10%	2000
	11th to 20th	Night	29% 8% 18% 21% 15% 9%	2113
	21st to end	Night	24% 5% 15% 22% 21% 12%	2200
October	1st to 10th	Night	11% 3 10% 13% 23% 40%	3590
	11th to 20th	Night	16% 3 11% 20% 18% 32%	3794
	21st to end	Night	11% 2 3 14% 41% 29%	4252
November	1st to 10th	Night	15% 2 6% 16% 34% 28%	2478
	11th to 20th	Night	17% 3 5% 20% 29% 26%	2730
	21st to end	Night	20% 3 6% 17% 31% 23%	2730
December	1st to 10th	Night	26% 5% 4 19% 27% 19%	4380
	11th to 20th	Night	17% 2 3 16% 35% 27%	4947
	21st to end	Night	13% 1 3 14% 39% 30%	6427

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	5% 11% 35% 31% 16% 2	330
	11th to 20th	Day	4 11% 28% 35% 20% 2	330
	21st to end	Day	4 12% 34% 32% 15% 3	363
February	1st to 10th	Day	7% 8% 29% 26% 23% 6%	330
	11th to 20th	Day	6% 13% 34% 28% 17% 2	330
	21st to end	Day	5% 14% 34% 32% 13% 1	273
March	1st to 10th	Day	7% 12% 32% 32% 15% 2	330
	11th to 20th	Day	8% 15% 35% 23% 14% 4	477
	21st to end	Day	5% 16% 37% 26% 13% 4	594
April	1st to 10th	Day	6% 13% 38% 28% 13% 2	540
	11th to 20th	Day	5% 15% 41% 27% 10% 2	532
	21st to end	Day	5% 13% 36% 34% 11% 1	540
May	1st to 10th	Day	3 10% 37% 30% 17% 3	543
	11th to 20th	Day	4 8% 33% 33% 19% 4	568
	21st to end	Day	3 2 25% 43% 25% 2	757
June	1st to 10th	Day	2 1 15% 42% 33% 8%	780
	11th to 20th	Day	3 1 8% 37% 40% 11%	780
	21st to end	Day	3 1 7% 29% 43% 18%	736
July	1st to 10th	Day	4 5% 25% 37% 27%	566
	11th to 20th	Day	3 1 3 20% 40% 33%	545
	21st to end	Day	3 3 23% 44% 27%	575
August	1st to 10th	Day	5% 1 3 16% 38% 37%	527
	11th to 20th	Day	6% 2 5% 19% 37% 32%	517
	21st to end	Day	5% 1 4 22% 35% 33%	583
September	1st to 10th	Day	7% 2 5% 28% 37% 22%	530
	11th to 20th	Day	7% 2 8% 22% 35% 27%	415
	21st to end	Day	6% 1 9% 25% 34% 25%	330
October	1st to 10th	Day	5% 2 8% 24% 40% 20%	330
	11th to 20th	Day	6% 3 16% 28% 29% 17%	330
	21st to end	Day	5% 6% 20% 30% 26% 14%	363
November	1st to 10th	Day	5% 5% 21% 31% 27% 11%	330
	11th to 20th	Day	6% 8% 20% 28% 27% 11%	330
	21st to end	Day	7% 13% 27% 28% 20% 5%	330
December	1st to 10th	Day	7% 8% 21% 39% 21% 3	330
	11th to 20th	Day	2 10% 29% 35% 19% 5%	330
	21st to end	Day	4 7% 30% 37% 16% 6%	363
January	1st to 10th	Night	6% 6% 28% 45% 14% 2	840
	11th to 20th	Night	7% 5% 28% 44% 16% 1	840
	21st to end	Night	4 6% 33% 42% 13% 1	924
February	1st to 10th	Night	5% 7% 28% 42% 17% 1	840
	11th to 20th	Night	6% 6% 37% 41% 10%	824
	21st to end	Night	4 10% 37% 43% 6%	692
March	1st to 10th	Night	5% 8% 36% 40% 10% 2	840
	11th to 20th	Night	6% 9% 34% 37% 13% 1	840
	21st to end	Night	3 7% 41% 37% 12% 1	723
April	1st to 10th	Night	4 7% 38% 41% 9%	630
	11th to 20th	Night	5% 8% 40% 41% 6%	623
	21st to end	Night	2 7% 39% 43% 8%	630
May	1st to 10th	Night	4 7% 36% 42% 11%	630
	11th to 20th	Night	4 6% 33% 46% 11%	621
	21st to end	Night	3 3 31% 49% 14%	683
June	1st to 10th	Night	2 2 18% 59% 19%	630
	11th to 20th	Night	3 1 16% 56% 23%	630
	21st to end	Night	5% 3 12% 52% 29%	630
July	1st to 10th	Night	4 7% 45% 40% 3	615
	11th to 20th	Night	3 6% 48% 42% 1	597
	21st to end	Night	2 1 4 46% 44% 2	657
August	1st to 10th	Night	3 1 4 36% 53% 5%	597
	11th to 20th	Night	2 1 5% 32% 53% 8%	594
	21st to end	Night	5% 6% 35% 50% 4	660
September	1st to 10th	Night	6% 2 5% 40% 44% 4	600
	11th to 20th	Night	6% 1 7% 44% 41% 2	600
	21st to end	Night	4 1 10% 45% 38% 2	600
October	1st to 10th	Night	3 1 15% 51% 29% 2	600
	11th to 20th	Night	8% 2 18% 44% 25% 3	735
	21st to end	Night	5% 4 17% 42% 26% 6%	880
November	1st to 10th	Night	6% 2 18% 43% 26% 5%	838
	11th to 20th	Night	4 4 22% 44% 24% 2	1050
	21st to end	Night	5% 6% 28% 41% 18% 2	1050
December	1st to 10th	Night	7% 4 24% 46% 17% 2	1050
	11th to 20th	Night	4 5% 29% 45% 15% 2	1050
	21st to end	Night	4 7% 28% 45% 15% 2	1066

Page generated at 2026-07-13 02:14:51. Version ac07

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