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 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	3 3 1 3 3 3 4 3 5% 7% 65%	113195
	Night	3 3 1 3 3 3 4 3 5% 7% 65%	114604
Transparency	Day	19% 4 7% 19% 27% 24%	112981
	Night	18% 4 8% 19% 27% 24%	114407
Seeing	Day	5% 6% 20% 30% 27% 12%	14904
	Night	5% 4 24% 43% 22% 2	25221

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 3 1 2 2 2 3 2 4 7% 72%	4537
	11th to 20th	Day	2 2 2 1 2 3 2 4 5% 77%	4540
	21st to end	Day	2 3 1 3 3 3 4 3 4 6% 69%	4994
February	1st to 10th	Day	3 2 1 3 3 3 4 4 6% 6% 65%	2000
	11th to 20th	Day	3 3 1 4 3 3 3 3 5% 7% 65%	1949
	21st to end	Day	3 3 1 2 3 3 3 3 4 7% 68%	1650
March	1st to 10th	Day	2 4 1 4 4 4 4 3 5% 8% 60%	3230
	11th to 20th	Day	7% 5% 2 4 3 3 3 3 5% 6% 60%	3404
	21st to end	Day	1 2 1 4 3 3 4 4 6% 8% 62%	4124
April	1st to 10th	Day	3 3 1 3 3 2 3 4 6% 7% 65%	2280
	11th to 20th	Day	2 3 1 3 3 3 4 3 4 5% 68%	2251
	21st to end	Day	3 2 1 2 3 2 2 2 4 6% 73%	2280
May	1st to 10th	Day	1 2 1 2 2 2 2 2 3 4 78%	3800
	11th to 20th	Day	2 2 1 3 3 2 2 2 5% 6% 72%	3765
	21st to end	Day	2 1 2 2 2 2 3 5% 6% 76%	4358
June	1st to 10th	Day	2 1 1 2 3 3 4 5% 6% 8% 65%	2470
	11th to 20th	Day	3 2 1 3 3 3 4 4 6% 8% 63%	2470
	21st to end	Day	3 3 1 4 4 5% 5% 5% 8% 8% 53%	2430
July	1st to 10th	Day	3 3 1 5% 5% 5% 6% 6% 9% 10% 48%	3800
	11th to 20th	Day	3 2 1 3 4 4 6% 5% 8% 9% 55%	3800
	21st to end	Day	4 3 2 5% 4 6% 6% 5% 8% 10% 47%	4118
August	1st to 10th	Day	2 2 1 4 4 5% 5% 5% 8% 10% 55%	3784
	11th to 20th	Day	4 3 1 5% 4 5% 6% 5% 8% 9% 51%	3726
	21st to end	Day	4 3 1 6% 5% 6% 5% 5% 7% 8% 50%	4180
September	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 4 7% 8% 45%	2280
	11th to 20th	Day	8% 4 2 6% 4 5% 6% 4 6% 8% 46%	2171
	21st to end	Day	5% 5% 2 4 4 3 5% 3 5% 6% 57%	2090
October	1st to 10th	Day	2 1 1 2 2 2 2 3 3 4 78%	3480
	11th to 20th	Day	4 3 1 2 2 2 2 2 3 4 74%	3230
	21st to end	Day	3 2 1 1 1 1 1 3 4 82%	3438
November	1st to 10th	Day	4 2 1 2 2 2 2 2 4 6% 74%	1918
	11th to 20th	Day	2 4 1 3 2 2 3 3 4 6% 70%	1800
	21st to end	Day	6% 4 1 4 3 3 3 3 5% 6% 64%	1800
December	1st to 10th	Day	5% 8% 2 4 2 2 3 3 5% 8% 59%	2940
	11th to 20th	Day	1 2 1 3 3 4 4 2 4 6% 70%	3391
	21st to end	Day	2 2 1 2 1 2 2 2 4 5% 77%	4717
January	1st to 10th	Night	2 2 1 3 2 2 2 3 4 6% 71%	5428
	11th to 20th	Night	2 2 1 2 2 2 3 2 5% 5% 74%	5440
	21st to end	Night	3 3 1 3 3 3 3 3 5% 7% 65%	5984
February	1st to 10th	Night	3 3 1 2 3 3 3 2 5% 7% 67%	2400
	11th to 20th	Night	4 4 1 4 3 3 5% 4 6% 7% 59%	2338
	21st to end	Night	4 4 1 3 2 3 4 2 5% 7% 65%	1980
March	1st to 10th	Night	2 3 1 5% 3 4 3 3 7% 8% 61%	3854
	11th to 20th	Night	5% 5% 2 4 3 3 4 4 4 6% 60%	3584
	21st to end	Night	3 3 1 4 3 3 4 4 5% 6% 64%	3518
April	1st to 10th	Night	3 4 1 3 3 3 3 3 5% 7% 63%	1900
	11th to 20th	Night	4 3 1 4 3 3 4 3 5% 6% 65%	1877
	21st to end	Night	2 2 1 3 2 3 3 3 3 5% 72%	1900
May	1st to 10th	Night	2 2 1 2 2 2 3 2 3 5% 76%	3170
	11th to 20th	Night	3 3 1 3 3 3 3 4 5% 6% 67%	3140
	21st to end	Night	2 2 1 3 3 3 4 3 5% 6% 68%	3206
June	1st to 10th	Night	3 2 1 3 3 4 5% 5% 8% 10% 57%	1710
	11th to 20th	Night	3 3 1 4 4 3 6% 5% 8% 10% 52%	1710
	21st to end	Night	3 2 1 4 3 4 5% 6% 8% 9% 55%	1740
July	1st to 10th	Night	2 2 1 4 4 4 6% 6% 8% 12% 50%	3170
	11th to 20th	Night	2 1 1 2 2 3 4 6% 12% 66%	3170
	21st to end	Night	2 2 1 4 4 5% 5% 5% 8% 12% 51%	3445
August	1st to 10th	Night	2 1 1 3 3 3 5% 5% 9% 12% 57%	3155
	11th to 20th	Night	2 2 1 4 3 4 5% 4 8% 13% 55%	3113
	21st to end	Night	2 2 1 4 3 4 4 5% 7% 12% 56%	3487
September	1st to 10th	Night	5% 4 2 4 5% 4 5% 5% 7% 8% 50%	1900
	11th to 20th	Night	6% 4 1 4 5% 4 5% 5% 7% 10% 49%	2007
	21st to end	Night	3 2 1 3 5% 4 6% 5% 7% 10% 53%	2090
October	1st to 10th	Night	2 1 1 2 2 2 2 3 4 5% 75%	3480
	11th to 20th	Night	4 2 1 3 2 2 3 2 4 6% 71%	3677
	21st to end	Night	3 2 1 2 1 1 2 2 3 4 80%	4120
November	1st to 10th	Night	5% 2 1 2 2 1 2 2 4 6% 73%	2357
	11th to 20th	Night	2 3 1 3 2 3 4 2 5% 7% 68%	2600
	21st to end	Night	4 3 1 3 3 3 3 2 5% 6% 65%	2600
December	1st to 10th	Night	4 5% 2 7% 4 4 4 4 4 6% 56%	4250
	11th to 20th	Night	2 3 1 3 3 3 2 2 4 7% 70%	4816
	21st to end	Night	2 2 1 3 2 2 2 2 4 5% 77%	6288

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 3 18% 29% 35%	4538
	11th to 20th	Day	11% 2 3 16% 41% 25%	4540
	21st to end	Day	18% 2 4 17% 35% 24%	4994
February	1st to 10th	Day	19% 3 5% 14% 25% 35%	2000
	11th to 20th	Day	20% 2 5% 15% 29% 28%	1959
	21st to end	Day	17% 4 3 12% 31% 34%	1650
March	1st to 10th	Day	21% 4 3 11% 29% 32%	3230
	11th to 20th	Day	24% 3 5% 15% 26% 26%	3404
	21st to end	Day	18% 4 3 9% 32% 33%	4124
April	1st to 10th	Day	19% 3 5% 12% 28% 35%	2280
	11th to 20th	Day	18% 4 4 12% 28% 35%	2254
	21st to end	Day	15% 2 4 12% 33% 35%	2280
May	1st to 10th	Day	11% 2 3 11% 23% 50%	3800
	11th to 20th	Day	14% 4 6% 12% 24% 40%	3768
	21st to end	Day	9% 3 5% 15% 21% 46%	4358
June	1st to 10th	Day	15% 5% 10% 23% 28% 19%	2470
	11th to 20th	Day	18% 4 7% 18% 30% 23%	2470
	21st to end	Day	25% 7% 10% 23% 22% 13%	2430
July	1st to 10th	Day	26% 9% 17% 31% 15% 2	3741
	11th to 20th	Day	21% 7% 15% 33% 20% 4	3737
	21st to end	Day	27% 7% 15% 27% 19% 5%	3988
August	1st to 10th	Day	21% 7% 13% 29% 27% 4	3772
	11th to 20th	Day	26% 7% 15% 29% 19% 4	3762
	21st to end	Day	29% 8% 16% 26% 18% 3	4179
September	1st to 10th	Day	35% 6% 13% 20% 16% 10%	2280
	11th to 20th	Day	34% 6% 13% 19% 20% 8%	2171
	21st to end	Day	27% 5% 8% 22% 24% 14%	2090
October	1st to 10th	Day	12% 3 6% 15% 24% 40%	3480
	11th to 20th	Day	16% 3 6% 22% 21% 33%	3230
	21st to end	Day	9% 1 2 13% 47% 28%	3438
November	1st to 10th	Day	14% 2 6% 19% 31% 27%	1918
	11th to 20th	Day	17% 2 6% 21% 30% 23%	1800
	21st to end	Day	22% 2 6% 18% 30% 22%	1800
December	1st to 10th	Day	24% 3 5% 20% 29% 18%	2940
	11th to 20th	Day	17% 2 4 19% 35% 22%	3389
	21st to end	Day	11% 2 2 13% 38% 34%	4717
January	1st to 10th	Night	15% 3 3 13% 37% 30%	5428
	11th to 20th	Night	13% 2 3 16% 32% 33%	5440
	21st to end	Night	19% 3 4 19% 32% 23%	5984
February	1st to 10th	Night	18% 2 5% 15% 25% 36%	2400
	11th to 20th	Night	23% 4 7% 12% 26% 29%	2338
	21st to end	Night	20% 3 4 9% 28% 37%	1980
March	1st to 10th	Night	20% 4 2 12% 26% 35%	3854
	11th to 20th	Night	24% 6% 8% 16% 23% 24%	3584
	21st to end	Night	20% 4 4 10% 32% 31%	3518
April	1st to 10th	Night	21% 4 4 13% 27% 31%	1900
	11th to 20th	Night	20% 3 7% 13% 24% 33%	1877
	21st to end	Night	15% 2 6% 14% 26% 36%	1900
May	1st to 10th	Night	12% 3 3 11% 22% 50%	3170
	11th to 20th	Night	18% 5% 8% 14% 18% 37%	3141
	21st to end	Night	16% 4 7% 16% 19% 39%	3207
June	1st to 10th	Night	19% 5% 12% 28% 23% 14%	1710
	11th to 20th	Night	23% 6% 9% 19% 25% 19%	1710
	21st to end	Night	23% 6% 13% 25% 21% 13%	1740
July	1st to 10th	Night	24% 6% 21% 35% 12% 2	3120
	11th to 20th	Night	12% 4 17% 44% 20% 3	3110
	21st to end	Night	22% 7% 19% 33% 17% 2	3336
August	1st to 10th	Night	15% 6% 15% 34% 25% 4	3144
	11th to 20th	Night	19% 6% 22% 33% 19% 2	3143
	21st to end	Night	20% 7% 20% 32% 17% 3	3487
September	1st to 10th	Night	29% 7% 16% 22% 16% 10%	1900
	11th to 20th	Night	29% 7% 18% 21% 16% 9%	2007
	21st to end	Night	24% 5% 15% 22% 21% 13%	2090
October	1st to 10th	Night	12% 3 10% 12% 22% 41%	3480
	11th to 20th	Night	16% 3 10% 20% 18% 33%	3677
	21st to end	Night	11% 2 3 14% 42% 28%	4120
November	1st to 10th	Night	15% 2 6% 17% 34% 26%	2357
	11th to 20th	Night	17% 3 5% 21% 29% 26%	2600
	21st to end	Night	20% 3 6% 17% 30% 23%	2600
December	1st to 10th	Night	26% 5% 4 19% 28% 18%	4250
	11th to 20th	Night	17% 2 3 16% 35% 26%	4817
	21st to end	Night	13% 1 3 14% 39% 30%	6288

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% 34% 32% 16% 2	300
	11th to 20th	Day	4 11% 30% 36% 18% 2	300
	21st to end	Day	3 13% 34% 31% 15% 3	330
February	1st to 10th	Day	7% 8% 30% 26% 23% 6%	300
	11th to 20th	Day	6% 13% 34% 29% 17% 2	300
	21st to end	Day	6% 16% 36% 32% 10%	249
March	1st to 10th	Day	7% 12% 33% 34% 13% 2	300
	11th to 20th	Day	9% 14% 36% 24% 13% 4	406
	21st to end	Day	5% 19% 39% 26% 9% 2	506
April	1st to 10th	Day	5% 12% 39% 30% 12% 2	460
	11th to 20th	Day	4 15% 44% 27% 8% 3	452
	21st to end	Day	5% 13% 34% 35% 12% 1	460
May	1st to 10th	Day	3 9% 36% 31% 17% 3	463
	11th to 20th	Day	3 6% 32% 36% 20% 3	488
	21st to end	Day	3 2 25% 43% 26% 2	655
June	1st to 10th	Day	2 1 14% 42% 33% 8%	680
	11th to 20th	Day	3 1 8% 38% 41% 9%	680
	21st to end	Day	3 1 7% 28% 44% 17%	640
July	1st to 10th	Day	4 5% 23% 38% 29%	486
	11th to 20th	Day	2 1 3 20% 40% 34%	484
	21st to end	Day	3 3 24% 44% 25%	531
August	1st to 10th	Day	5% 1 3 16% 38% 37%	487
	11th to 20th	Day	6% 2 5% 19% 37% 32%	477
	21st to end	Day	6% 1 3 21% 36% 33%	539
September	1st to 10th	Day	7% 2 4 28% 38% 22%	490
	11th to 20th	Day	7% 2 8% 22% 34% 27%	381
	21st to end	Day	6% 1 9% 24% 35% 23%	300
October	1st to 10th	Day	6% 3 9% 24% 39% 20%	300
	11th to 20th	Day	7% 3 17% 29% 29% 15%	300
	21st to end	Day	5% 6% 19% 30% 25% 15%	330
November	1st to 10th	Day	5% 5% 22% 32% 26% 9%	300
	11th to 20th	Day	6% 8% 20% 29% 26% 10%	300
	21st to end	Day	8% 14% 26% 29% 20% 4	300
December	1st to 10th	Day	8% 8% 19% 39% 22% 4	300
	11th to 20th	Day	2 11% 30% 34% 19% 4	300
	21st to end	Day	5% 8% 32% 38% 14% 3	330
January	1st to 10th	Night	6% 6% 29% 44% 15% 2	800
	11th to 20th	Night	7% 5% 28% 43% 16% 1	800
	21st to end	Night	4 7% 34% 40% 13% 1	880
February	1st to 10th	Night	5% 7% 29% 40% 18% 1	800
	11th to 20th	Night	6% 6% 36% 41% 10%	784
	21st to end	Night	5% 10% 38% 41% 6%	660
March	1st to 10th	Night	5% 9% 36% 39% 10% 2	800
	11th to 20th	Night	7% 8% 35% 37% 13% 2	766
	21st to end	Night	3 8% 43% 36% 9% 1	655
April	1st to 10th	Night	4 7% 39% 41% 9%	570
	11th to 20th	Night	5% 8% 40% 41% 6%	563
	21st to end	Night	2 7% 39% 43% 8% 1	570
May	1st to 10th	Night	4 6% 35% 43% 12%	570
	11th to 20th	Night	4 6% 31% 47% 12%	561
	21st to end	Night	3 3 31% 49% 14%	617
June	1st to 10th	Night	3 2 17% 60% 18%	570
	11th to 20th	Night	4 1 15% 57% 23% 1	570
	21st to end	Night	3 2 11% 54% 29%	570
July	1st to 10th	Night	4 7% 44% 42% 3	555
	11th to 20th	Night	3 5% 47% 43% 1	552
	21st to end	Night	3 1 4 47% 43% 3	624
August	1st to 10th	Night	3 1 3 35% 53% 5%	567
	11th to 20th	Night	2 1 5% 33% 51% 8%	564
	21st to end	Night	5% 6% 35% 51% 4	627
September	1st to 10th	Night	6% 1 5% 40% 44% 4	570
	11th to 20th	Night	6% 1 6% 44% 41% 2	570
	21st to end	Night	4 1 11% 45% 38% 2	570
October	1st to 10th	Night	3 1 15% 50% 29% 2	570
	11th to 20th	Night	9% 2 19% 44% 24% 3	698
	21st to end	Night	5% 4 17% 41% 26% 7%	836
November	1st to 10th	Night	6% 2 18% 43% 25% 5%	797
	11th to 20th	Night	3 4 23% 44% 24% 2	1000
	21st to end	Night	5% 6% 28% 40% 19% 3	1000
December	1st to 10th	Night	7% 5% 23% 46% 18% 2	1000
	11th to 20th	Night	4 5% 30% 45% 15% 2	1000
	21st to end	Night	4 7% 28% 45% 13% 2	1015

Page generated at 2025-03-09 07:08:48. Version ac07

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