Arlington
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arlington clear sky chart from 2005-11-06 to 2026-06-16.

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	11% 5% 2 5% 3 3 4 3 5% 7% 52%	137720
	Night	11% 5% 2 4 3 3 3 3 5% 6% 56%	134649
Transparency	Day	31% 4 9% 24% 23% 8%	141437
	Night	29% 4 9% 22% 25% 11%	138999
Seeing	Day	12% 8% 23% 29% 21% 6%	17270
	Night	12% 7% 29% 36% 15% 1	27171

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	11% 5% 2 5% 3 2 3 3 4 5% 56%	5289
	11th to 20th	Day	7% 5% 2 3 2 2 2 3 4 6% 63%	5310
	21st to end	Day	22% 8% 3 6% 4 2 2 2 3 4 44%	5812
February	1st to 10th	Day	14% 7% 2 6% 4 3 3 3 4 5% 48%	2045
	11th to 20th	Day	15% 9% 3 6% 3 3 3 2 4 5% 48%	2049
	21st to end	Day	19% 7% 2 3 3 2 2 2 3 3 55%	1740
March	1st to 10th	Day	15% 6% 1 5% 3 3 3 3 5% 5% 51%	5046
	11th to 20th	Day	12% 5% 2 4 3 3 3 3 4 5% 56%	5090
	21st to end	Day	11% 6% 2 5% 3 4 4 3 5% 7% 50%	5583
April	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 5% 6% 48%	2310
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 6% 47%	2373
	21st to end	Day	13% 5% 2 5% 4 4 4 4 5% 7% 46%	2695
May	1st to 10th	Day	17% 7% 3 7% 4 4 4 3 5% 7% 39%	7617
	11th to 20th	Day	13% 6% 3 7% 4 4 4 3 5% 6% 45%	7627
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 5% 6% 8% 36%	8392
June	1st to 10th	Day	5% 4 2 5% 4 5% 5% 4 7% 10% 51%	2730
	11th to 20th	Day	5% 4 1 5% 4 5% 4 4 6% 9% 52%	2666
	21st to end	Day	5% 3 1 5% 4 4 4 3 6% 9% 56%	2600
July	1st to 10th	Day	8% 6% 3 8% 6% 5% 5% 5% 6% 8% 42%	5900
	11th to 20th	Day	3 2 1 3 3 3 3 3 6% 10% 63%	5900
	21st to end	Day	4 3 1 4 4 4 4 4 7% 10% 55%	6422
August	1st to 10th	Day	1 1 1 2 2 2 3 3 5% 8% 71%	4234
	11th to 20th	Day	4 2 1 2 2 2 4 4 6% 10% 62%	4171
	21st to end	Day	3 3 1 4 3 4 5% 5% 7% 10% 55%	4161
September	1st to 10th	Day	8% 6% 2 4 4 4 4 3 4 7% 55%	2200
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 6% 7% 48%	2200
	21st to end	Day	7% 4 1 4 2 4 4 3 6% 8% 58%	2200
October	1st to 10th	Day	4 2 1 4 2 3 4 3 5% 6% 65%	2165
	11th to 20th	Day	11% 4 1 4 2 2 3 2 4 5% 62%	2000
	21st to end	Day	13% 4 2 4 3 2 2 2 3 4 62%	2200
November	1st to 10th	Day	13% 5% 2 5% 3 2 2 2 3 5% 57%	2058
	11th to 20th	Day	7% 3 2 5% 4 3 3 3 4 6% 62%	1947
	21st to end	Day	14% 6% 2 4 3 3 3 3 4 5% 54%	1890
December	1st to 10th	Day	17% 7% 1 4 3 3 3 3 4 6% 50%	3030
	11th to 20th	Day	17% 6% 2 5% 4 3 4 3 4 4 48%	3481
	21st to end	Day	18% 7% 2 5% 3 3 2 2 3 4 51%	4587
January	1st to 10th	Night	13% 4 2 3 3 3 3 2 4 5% 59%	7634
	11th to 20th	Night	6% 3 1 3 2 2 2 2 3 5% 72%	7680
	21st to end	Night	23% 7% 2 5% 4 3 3 2 4 5% 42%	8402
February	1st to 10th	Night	14% 7% 2 5% 3 3 3 2 4 5% 53%	2593
	11th to 20th	Night	15% 7% 2 5% 4 3 2 2 4 6% 50%	2458
	21st to end	Night	16% 6% 2 4 3 2 3 2 3 4 55%	2064
March	1st to 10th	Night	16% 7% 3 5% 4 3 3 3 4 5% 47%	5029
	11th to 20th	Night	11% 4 2 4 3 3 3 3 5% 6% 56%	5090
	21st to end	Night	12% 5% 2 5% 3 4 3 3 4 5% 54%	5588
April	1st to 10th	Night	10% 5% 2 4 3 4 3 3 5% 8% 53%	2310
	11th to 20th	Night	13% 5% 2 5% 4 3 3 3 5% 6% 51%	2196
	21st to end	Night	11% 5% 1 5% 3 3 3 3 5% 6% 54%	1920
May	1st to 10th	Night	16% 7% 2 6% 4 4 4 4 6% 7% 40%	5266
	11th to 20th	Night	11% 6% 3 6% 4 4 4 3 5% 6% 48%	5265
	21st to end	Night	12% 6% 2 6% 4 4 4 4 6% 9% 42%	5793
June	1st to 10th	Night	5% 2 2 5% 4 4 4 3 6% 10% 56%	1890
	11th to 20th	Night	3 2 1 3 3 3 3 3 7% 8% 62%	1845
	21st to end	Night	2 3 1 4 3 3 4 4 6% 11% 59%	1800
July	1st to 10th	Night	6% 5% 2 6% 5% 4 4 4 6% 11% 47%	4078
	11th to 20th	Night	2 2 1 3 2 3 3 3 5% 10% 67%	4077
	21st to end	Night	2 2 1 3 3 4 4 3 6% 9% 62%	4449
August	1st to 10th	Night	1 1 1 1 1 2 2 4 9% 77%	2926
	11th to 20th	Night	3 2 1 3 1 2 2 2 6% 11% 67%	2888
	21st to end	Night	3 2 1 3 2 3 4 3 6% 9% 64%	3735
September	1st to 10th	Night	7% 5% 2 4 4 3 4 4 4 7% 57%	2200
	11th to 20th	Night	10% 5% 1 4 3 3 4 3 5% 8% 54%	2200
	21st to end	Night	6% 3 1 3 2 2 3 3 6% 7% 62%	2200
October	1st to 10th	Night	6% 2 1 3 2 3 3 3 4 6% 68%	2214
	11th to 20th	Night	10% 4 1 3 2 2 2 2 3 5% 67%	2400
	21st to end	Night	15% 4 1 3 2 2 2 1 3 4 62%	2640
November	1st to 10th	Night	11% 5% 1 4 3 2 3 2 4 4 62%	2460
	11th to 20th	Night	6% 4 1 4 3 3 3 3 5% 6% 62%	2670
	21st to end	Night	14% 6% 2 4 3 3 3 3 3 5% 54%	2730
December	1st to 10th	Night	15% 7% 2 4 3 2 2 2 3 4 54%	4380
	11th to 20th	Night	16% 6% 2 6% 3 3 3 2 3 5% 51%	4946
	21st to end	Night	14% 7% 2 5% 2 3 2 2 3 4 56%	6633

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	31% 3 7% 26% 28% 5%	5290
	11th to 20th	Day	23% 3 5% 26% 34% 9%	5310
	21st to end	Day	45% 3 4 18% 20% 10%	5812
February	1st to 10th	Day	40% 4 5% 19% 22% 11%	2045
	11th to 20th	Day	41% 3 7% 21% 20% 9%	2059
	21st to end	Day	36% 2 4 17% 27% 14%	1740
March	1st to 10th	Day	35% 4 5% 16% 23% 18%	5046
	11th to 20th	Day	31% 4 7% 18% 17% 23%	5090
	21st to end	Day	34% 4 10% 24% 21% 7%	5585
April	1st to 10th	Day	36% 5% 9% 20% 19% 10%	2310
	11th to 20th	Day	38% 5% 9% 19% 20% 9%	2373
	21st to end	Day	37% 5% 9% 20% 23% 6%	2695
May	1st to 10th	Day	44% 6% 12% 19% 17% 3	7655
	11th to 20th	Day	40% 5% 7% 19% 25% 4	7631
	21st to end	Day	44% 6% 14% 21% 11% 4	8405
June	1st to 10th	Day	28% 7% 15% 28% 18% 4	2730
	11th to 20th	Day	27% 6% 12% 30% 20% 5%	2666
	21st to end	Day	25% 6% 13% 28% 25% 4	2600
July	1st to 10th	Day	37% 7% 14% 30% 10% 1	5836
	11th to 20th	Day	18% 5% 15% 38% 22% 3	5831
	21st to end	Day	21% 7% 15% 33% 22% 2	6240
August	1st to 10th	Day	12% 4 12% 31% 36% 6%	4220
	11th to 20th	Day	17% 6% 15% 39% 21% 2	4210
	21st to end	Day	22% 8% 17% 35% 16% 2	4160
September	1st to 10th	Day	30% 5% 9% 23% 23% 11%	2200
	11th to 20th	Day	34% 6% 10% 21% 22% 7%	2200
	21st to end	Day	24% 5% 8% 24% 26% 14%	2200
October	1st to 10th	Day	13% 3 6% 15% 36% 27%	3529
	11th to 20th	Day	17% 2 5% 22% 34% 21%	3240
	21st to end	Day	23% 3 5% 27% 31% 11%	3538
November	1st to 10th	Day	31% 3 7% 21% 27% 11%	2058
	11th to 20th	Day	25% 3 8% 21% 30% 13%	1947
	21st to end	Day	35% 3 5% 23% 25% 10%	1890
December	1st to 10th	Day	37% 3 5% 26% 24% 5%	3030
	11th to 20th	Day	41% 3 5% 12% 28% 11%	3479
	21st to end	Day	39% 3 4 24% 21% 9%	4587
January	1st to 10th	Night	29% 3 7% 22% 32% 8%	7635
	11th to 20th	Night	18% 2 4 20% 38% 18%	7680
	21st to end	Night	46% 3 5% 15% 20% 12%	8402
February	1st to 10th	Night	35% 4 6% 18% 23% 14%	2593
	11th to 20th	Night	37% 3 7% 18% 25% 10%	2458
	21st to end	Night	35% 3 6% 15% 24% 17%	2064
March	1st to 10th	Night	40% 3 4 14% 21% 18%	5029
	11th to 20th	Night	30% 4 8% 17% 18% 23%	5090
	21st to end	Night	33% 4 7% 19% 29% 7%	5588
April	1st to 10th	Night	30% 4 10% 21% 20% 14%	2310
	11th to 20th	Night	34% 4 9% 19% 20% 14%	2196
	21st to end	Night	31% 5% 10% 22% 27% 6%	1920
May	1st to 10th	Night	42% 6% 11% 22% 15% 5%	5293
	11th to 20th	Night	37% 5% 12% 18% 24% 6%	5266
	21st to end	Night	38% 6% 15% 25% 9% 7%	5805
June	1st to 10th	Night	25% 5% 15% 30% 20% 5%	1890
	11th to 20th	Night	19% 6% 13% 33% 22% 7%	1845
	21st to end	Night	19% 6% 14% 28% 27% 6%	1800
July	1st to 10th	Night	31% 6% 15% 34% 12% 2	4035
	11th to 20th	Night	14% 5% 15% 37% 25% 4	4026
	21st to end	Night	19% 6% 16% 33% 23% 3	4320
August	1st to 10th	Night	8% 3 9% 33% 41% 6%	2916
	11th to 20th	Night	13% 3 14% 39% 25% 5%	2915
	21st to end	Night	16% 6% 20% 34% 20% 3	3735
September	1st to 10th	Night	27% 6% 10% 22% 25% 11%	2200
	11th to 20th	Night	29% 5% 12% 21% 22% 11%	2200
	21st to end	Night	21% 4 8% 23% 27% 17%	2200
October	1st to 10th	Night	12% 2 5% 16% 30% 35%	3617
	11th to 20th	Night	15% 2 5% 21% 32% 26%	3899
	21st to end	Night	23% 3 4 22% 33% 16%	4252
November	1st to 10th	Night	28% 3 6% 21% 28% 15%	2460
	11th to 20th	Night	23% 4 8% 22% 28% 16%	2670
	21st to end	Night	34% 3 6% 18% 28% 12%	2730
December	1st to 10th	Night	36% 3 5% 20% 27% 10%	4380
	11th to 20th	Night	38% 2 5% 14% 26% 14%	4947
	21st to end	Night	34% 3 6% 19% 26% 12%	6633

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	20% 14% 38% 22% 6%	330
	11th to 20th	Day	18% 15% 38% 21% 5% 2	330
	21st to end	Day	20% 17% 36% 19% 8% 1	363
February	1st to 10th	Day	18% 16% 39% 22% 5%	330
	11th to 20th	Day	21% 20% 41% 14% 4	330
	21st to end	Day	26% 19% 35% 17% 3	273
March	1st to 10th	Day	21% 18% 35% 22% 3	330
	11th to 20th	Day	20% 21% 35% 18% 4 1	330
	21st to end	Day	12% 17% 44% 21% 6%	363
April	1st to 10th	Day	18% 17% 35% 23% 7% 1	330
	11th to 20th	Day	21% 15% 36% 21% 5%	411
	21st to end	Day	17% 16% 37% 22% 6% 2	715
May	1st to 10th	Day	13% 14% 32% 33% 8% 1	753
	11th to 20th	Day	16% 9% 30% 31% 12% 3	775
	21st to end	Day	11% 7% 31% 30% 17% 4	844
June	1st to 10th	Day	6% 2 18% 40% 26% 8%	780
	11th to 20th	Day	6% 2 17% 44% 24% 6%	756
	21st to end	Day	4 2 12% 39% 35% 9%	730
July	1st to 10th	Day	7% 2 10% 36% 33% 13%	721
	11th to 20th	Day	3 1 8% 37% 39% 13%	718
	21st to end	Day	3 1 6% 31% 44% 14%	794
August	1st to 10th	Day	1 7% 34% 47% 11%	726
	11th to 20th	Day	8% 1 6% 32% 44% 10%	715
	21st to end	Day	4 1 5% 31% 36% 23%	497
September	1st to 10th	Day	7% 2 12% 22% 38% 20%	330
	11th to 20th	Day	6% 1 6% 27% 42% 18%	330
	21st to end	Day	10% 3 9% 30% 33% 15%	330
October	1st to 10th	Day	1 2 18% 33% 35% 12%	330
	11th to 20th	Day	8% 7% 28% 34% 18% 5%	330
	21st to end	Day	21% 12% 22% 27% 15% 3	363
November	1st to 10th	Day	19% 7% 31% 28% 12% 3	330
	11th to 20th	Day	10% 8% 33% 32% 13% 3	330
	21st to end	Day	17% 15% 36% 22% 9% 2	330
December	1st to 10th	Day	17% 12% 33% 26% 10% 2	330
	11th to 20th	Day	19% 12% 37% 23% 8% 1	330
	21st to end	Day	21% 11% 37% 23% 7%	363
January	1st to 10th	Night	14% 9% 37% 32% 8%	1050
	11th to 20th	Night	16% 14% 33% 32% 5%	1050
	21st to end	Night	17% 12% 35% 30% 6%	1155
February	1st to 10th	Night	17% 12% 38% 29% 4 1	913
	11th to 20th	Night	19% 14% 38% 26% 3	824
	21st to end	Night	20% 14% 42% 22% 2	680
March	1st to 10th	Night	18% 13% 37% 29% 4	630
	11th to 20th	Night	16% 10% 42% 27% 5%	630
	21st to end	Night	13% 12% 41% 29% 5%	693
April	1st to 10th	Night	11% 12% 44% 29% 4	630
	11th to 20th	Night	16% 12% 40% 28% 4	623
	21st to end	Night	14% 10% 43% 26% 6% 1	630
May	1st to 10th	Night	10% 10% 39% 34% 6%	630
	11th to 20th	Night	11% 7% 34% 34% 13% 1	621
	21st to end	Night	8% 4 34% 37% 15% 1	683
June	1st to 10th	Night	5% 3 22% 46% 23% 2	630
	11th to 20th	Night	4 2 27% 54% 13% 1	615
	21st to end	Night	3 1 17% 50% 27% 2	600
July	1st to 10th	Night	5% 1 12% 46% 31% 4	585
	11th to 20th	Night	3 1 12% 50% 33% 2	582
	21st to end	Night	2 1 8% 50% 37% 3	657
August	1st to 10th	Night	2 1 5% 52% 39% 2	597
	11th to 20th	Night	6% 7% 49% 32% 6%	594
	21st to end	Night	4 1 5% 46% 39% 6%	660
September	1st to 10th	Night	9% 2 10% 41% 35% 4	600
	11th to 20th	Night	12% 2 6% 43% 35% 3	600
	21st to end	Night	7% 1 16% 41% 32% 4	600
October	1st to 10th	Night	7% 1 19% 43% 28% 3	614
	11th to 20th	Night	11% 5% 27% 38% 18% 2	800
	21st to end	Night	17% 8% 29% 32% 14%	880
November	1st to 10th	Night	12% 5% 31% 38% 14% 1	820
	11th to 20th	Night	7% 10% 34% 36% 13%	990
	21st to end	Night	16% 10% 36% 27% 10%	1050
December	1st to 10th	Night	16% 9% 38% 32% 6%	1050
	11th to 20th	Night	18% 9% 34% 32% 7%	1050
	21st to end	Night	17% 10% 36% 30% 6%	1155

Page generated at 2026-06-13 01:31:45. Version ac07

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