Marfa
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marfa clear sky chart from 2005-11-05 to 2023-04-24.

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	4 2 1 3 3 2 4 4 5% 7% 59%	69355
	Night	3 2 1 3 3 2 4 4 5% 8% 60%	69326
Transparency	Day	22% 5% 9% 17% 25% 20%	69086
	Night	21% 5% 10% 18% 24% 20%	69095
Seeing	Day	5% 6% 20% 29% 26% 12%	12350
	Night	4 4 23% 42% 22% 2	22557

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 15th	Day	5% 3 1 2 3 1 3 3 3 6% 64%	2676
	16th to end	Day	3 3 1 2 2 1 3 3 4 5% 67%	2830
February	1st to 15th	Day	3 1 1 3 3 1 3 4 5% 6% 65%	2605
	16th to end	Day	3 2 1 2 3 2 3 4 4 6% 65%	2360
March	1st to 15th	Day	2 3 2 3 3 3 3 4 5% 7% 58%	2727
	16th to end	Day	2 3 1 2 2 1 3 4 4 7% 65%	3299
April	1st to 15th	Day	2 2 1 2 3 1 3 4 4 7% 66%	3195
	16th to end	Day	2 2 1 2 2 1 3 3 3 5% 71%	3105
May	1st to 15th	Day	2 2 1 2 3 1 3 3 4 5% 67%	3011
	16th to end	Day	2 1 2 2 1 3 4 6% 7% 68%	3226
June	1st to 15th	Day	1 1 1 2 2 1 4 4 5% 8% 66%	3315
	16th to end	Day	3 2 1 3 3 3 5% 5% 8% 9% 53%	3264
July	1st to 15th	Day	5% 3 1 4 6% 3 6% 7% 8% 9% 42%	3044
	16th to end	Day	4 2 2 4 5% 4 7% 7% 8% 10% 42%	3199
August	1st to 15th	Day	4 2 1 4 5% 4 7% 6% 7% 9% 44%	2944
	16th to end	Day	6% 4 2 6% 7% 4 7% 6% 8% 9% 37%	3233
September	1st to 15th	Day	9% 4 2 4 5% 3 6% 6% 6% 7% 42%	3031
	16th to end	Day	5% 4 2 3 4 3 4 5% 5% 7% 52%	2792
October	1st to 15th	Day	4 2 1 2 3 2 3 4 5% 6% 64%	2745
	16th to end	Day	5% 3 1 1 2 1 1 2 2 4 72%	2703
November	1st to 15th	Day	3 3 1 2 2 1 2 3 4 6% 70%	2532
	16th to end	Day	4 3 2 3 1 2 3 4 5% 68%	2410
December	1st to 15th	Day	4 3 1 3 3 1 3 3 5% 6% 63%	2429
	16th to end	Day	2 2 1 3 3 2 3 3 5% 7% 63%	2680
January	1st to 15th	Night	3 4 1 3 3 2 3 4 5% 5% 62%	3206
	16th to end	Night	4 2 1 3 3 2 3 3 5% 6% 63%	3424
February	1st to 15th	Night	3 2 1 2 3 2 3 3 4 7% 65%	3141
	16th to end	Night	3 3 1 3 3 2 4 4 6% 7% 60%	2820
March	1st to 15th	Night	3 3 1 3 3 2 3 4 5% 8% 59%	3187
	16th to end	Night	3 3 1 2 3 1 3 4 3 6% 65%	2982
April	1st to 15th	Night	3 2 1 2 3 2 3 3 4 7% 64%	2670
	16th to end	Night	2 2 1 2 2 1 4 3 3 5% 69%	2583
May	1st to 15th	Night	3 2 1 2 4 2 3 4 5% 6% 62%	2521
	16th to end	Night	2 2 1 2 3 1 4 4 5% 7% 63%	2468
June	1st to 15th	Night	2 1 1 2 3 2 5% 6% 7% 10% 54%	2295
	16th to end	Night	2 2 1 3 3 2 5% 7% 7% 9% 53%	2306
July	1st to 15th	Night	2 2 1 3 3 3 5% 8% 7% 11% 49%	2548
	16th to end	Night	3 2 1 2 4 3 4 4 8% 11% 52%	2647
August	1st to 15th	Night	2 2 1 2 3 2 4 6% 9% 14% 49%	2492
	16th to end	Night	3 2 2 4 5% 4 6% 7% 8% 11% 43%	2696
September	1st to 15th	Night	6% 3 1 3 5% 3 4 5% 7% 9% 47%	2552
	16th to end	Night	4 2 1 3 5% 3 5% 6% 7% 10% 50%	2787
October	1st to 15th	Night	3 2 1 3 3 2 4 5% 5% 8% 59%	2822
	16th to end	Night	4 2 1 2 1 1 2 3 3 4 70%	3258
November	1st to 15th	Night	3 2 2 2 1 3 2 4 6% 70%	3276
	16th to end	Night	3 2 1 2 2 2 3 2 5% 6% 67%	3485
December	1st to 15th	Night	5% 2 1 3 3 2 3 4 4 7% 62%	3501
	16th to end	Night	3 3 1 3 2 1 3 3 3 6% 66%	3659

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 15th	Day	21% 3 4 16% 32% 21%	2678
	16th to end	Day	18% 3 5% 14% 36% 20%	2829
February	1st to 15th	Day	18% 3 6% 14% 24% 33%	2608
	16th to end	Day	18% 3 4 11% 31% 29%	2360
March	1st to 15th	Day	23% 3 5% 14% 25% 27%	2727
	16th to end	Day	18% 2 6% 10% 28% 33%	3312
April	1st to 15th	Day	17% 3 6% 11% 27% 32%	3194
	16th to end	Day	15% 3 5% 10% 29% 36%	3106
May	1st to 15th	Day	18% 3 5% 13% 26% 32%	3016
	16th to end	Day	14% 4 7% 17% 25% 30%	3227
June	1st to 15th	Day	14% 4 9% 21% 29% 20%	3311
	16th to end	Day	22% 7% 12% 20% 22% 15%	3266
July	1st to 15th	Day	32% 10% 15% 23% 15% 3	2910
	16th to end	Day	28% 10% 19% 27% 12% 2	3043
August	1st to 15th	Day	31% 9% 14% 27% 15% 1	2932
	16th to end	Day	37% 10% 16% 18% 13% 3	3237
September	1st to 15th	Day	36% 8% 14% 18% 15% 5%	3035
	16th to end	Day	28% 7% 11% 19% 22% 10%	2791
October	1st to 15th	Day	19% 4 10% 25% 21% 19%	2747
	16th to end	Day	17% 1 4 15% 30% 30%	2703
November	1st to 15th	Day	15% 3 7% 22% 30% 21%	2532
	16th to end	Day	18% 2 6% 18% 29% 24%	2413
December	1st to 15th	Day	21% 3 5% 19% 32% 17%	2429
	16th to end	Day	19% 3 5% 19% 34% 18%	2680
January	1st to 15th	Night	22% 3 5% 14% 28% 24%	3208
	16th to end	Night	20% 3 5% 15% 35% 19%	3427
February	1st to 15th	Night	19% 2 6% 13% 23% 33%	3149
	16th to end	Night	22% 3 7% 8% 26% 31%	2821
March	1st to 15th	Night	22% 2 5% 15% 25% 28%	3187
	16th to end	Night	19% 3 6% 12% 26% 31%	2995
April	1st to 15th	Night	19% 3 7% 12% 25% 31%	2668
	16th to end	Night	17% 2 8% 12% 23% 35%	2581
May	1st to 15th	Night	19% 4 7% 16% 21% 28%	2525
	16th to end	Night	19% 6% 9% 20% 20% 24%	2470
June	1st to 15th	Night	20% 6% 11% 24% 23% 12%	2287
	16th to end	Night	21% 8% 13% 21% 20% 14%	2306
July	1st to 15th	Night	23% 8% 20% 28% 15% 3	2439
	16th to end	Night	22% 9% 20% 31% 15% 1	2517
August	1st to 15th	Night	20% 8% 20% 32% 16% 1	2483
	16th to end	Night	30% 11% 18% 22% 13% 4	2698
September	1st to 15th	Night	29% 9% 18% 21% 14% 6%	2552
	16th to end	Night	25% 8% 16% 20% 19% 10%	2786
October	1st to 15th	Night	21% 6% 13% 22% 18% 18%	2823
	16th to end	Night	17% 2 4 15% 27% 31%	3256
November	1st to 15th	Night	15% 2 7% 20% 32% 21%	3276
	16th to end	Night	17% 3 5% 16% 29% 27%	3486
December	1st to 15th	Night	21% 3 6% 19% 29% 19%	3501
	16th to end	Night	19% 3 5% 18% 33% 19%	3654

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 15th	Day	5% 9% 29% 35% 17% 2	360
	16th to end	Day	3 14% 32% 31% 14% 3	381
February	1st to 15th	Day	7% 8% 32% 27% 21% 2	360
	16th to end	Day	6% 14% 37% 30% 10%	318
March	1st to 15th	Day	7% 12% 33% 32% 10% 3	396
	16th to end	Day	5% 15% 38% 24% 12% 2	669
April	1st to 15th	Day	4 11% 39% 30% 11% 2	622
	16th to end	Day	4 14% 38% 31% 9% 2	588
May	1st to 15th	Day	3 8% 34% 33% 17% 2	584
	16th to end	Day	3 2 25% 41% 24% 1	735
June	1st to 15th	Day	2 10% 40% 37% 8%	866
	16th to end	Day	3 7% 30% 45% 13%	831
July	1st to 15th	Day	4 3 21% 38% 30%	601
	16th to end	Day	2 3 21% 42% 29%	640
August	1st to 15th	Day	6% 3 16% 38% 35%	581
	16th to end	Day	6% 1 3 19% 36% 32%	649
September	1st to 15th	Day	7% 1 5% 25% 37% 22%	601
	16th to end	Day	6% 1 8% 22% 34% 26%	360
October	1st to 15th	Day	6% 2 10% 26% 34% 19%	360
	16th to end	Day	6% 5% 17% 26% 28% 15%	384
November	1st to 15th	Day	6% 6% 21% 30% 23% 11%	360
	16th to end	Day	7% 11% 21% 31% 21% 6%	360
December	1st to 15th	Day	6% 9% 23% 37% 19% 2	360
	16th to end	Day	4 7% 34% 38% 13% 1	384
January	1st to 15th	Night	6% 4 27% 44% 16% 1	1070
	16th to end	Night	5% 5% 31% 40% 14% 1	1142
February	1st to 15th	Night	5% 6% 29% 41% 15% 1	1047
	16th to end	Night	5% 8% 39% 39% 6%	941
March	1st to 15th	Night	5% 7% 35% 38% 10% 1	1075
	16th to end	Night	4 8% 38% 36% 11% 1	977
April	1st to 15th	Night	3 7% 39% 41% 7%	802
	16th to end	Night	3 7% 38% 42% 8%	775
May	1st to 15th	Night	4 5% 36% 43% 9%	758
	16th to end	Night	3 3 27% 49% 15%	779
June	1st to 15th	Night	3 1 16% 58% 21%	760
	16th to end	Night	3 10% 55% 28%	752
July	1st to 15th	Night	3 5% 44% 44% 2	732
	16th to end	Night	3 4 45% 43% 2	796
August	1st to 15th	Night	2 2 34% 54% 5%	740
	16th to end	Night	4 5% 31% 52% 5%	810
September	1st to 15th	Night	6% 5% 39% 43% 3	762
	16th to end	Night	4 8% 44% 39% 2	756
October	1st to 15th	Night	5% 1 16% 48% 26% 2	793
	16th to end	Night	6% 3 17% 39% 26% 6%	1084
November	1st to 15th	Night	4 2 21% 43% 24% 3	1161
	16th to end	Night	4 4 24% 40% 22% 3	1341
December	1st to 15th	Night	6% 4 25% 44% 17% 2	1345
	16th to end	Night	4 6% 28% 44% 13% 2	1359

Page generated at 2023-04-24 19:39:32. Version ac05

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