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 2024-09-14.

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	10% 5% 2 5% 3 3 4 3 5% 7% 52%	100596
	Night	10% 5% 2 4 3 3 3 3 5% 6% 57%	97410
Transparency	Day	31% 5% 10% 25% 22% 7%	100410
	Night	29% 4 9% 24% 24% 10%	97267
Seeing	Day	11% 7% 23% 30% 22% 7%	14982
	Night	12% 7% 29% 36% 15% 1	24807

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	12% 5% 2 5% 3 2 3 2 4 5% 56%	2847
	11th to 20th	Day	11% 6% 2 3 2 2 2 2 4 6% 61%	2850
	21st to end	Day	21% 10% 3 5% 2 2 2 2 3 4 48%	3135
February	1st to 10th	Day	14% 8% 3 6% 4 3 3 3 4 5% 47%	1849
	11th to 20th	Day	15% 9% 2 5% 3 3 3 2 4 5% 49%	1849
	21st to end	Day	20% 8% 2 3 3 2 2 2 3 3 53%	1579
March	1st to 10th	Day	15% 6% 1 5% 3 3 3 3 5% 6% 50%	3436
	11th to 20th	Day	16% 6% 2 5% 5% 4 4 4 5% 6% 42%	3480
	21st to end	Day	12% 7% 2 5% 4 4 5% 4 6% 7% 44%	3814
April	1st to 10th	Day	11% 6% 2 6% 4 3 5% 4 4 6% 49%	2090
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 5% 6% 47%	2145
	21st to end	Day	11% 5% 2 5% 4 4 4 4 5% 7% 48%	2438
May	1st to 10th	Day	18% 7% 3 6% 4 3 4 3 5% 7% 39%	4082
	11th to 20th	Day	15% 6% 2 5% 3 3 4 3 5% 6% 49%	4082
	21st to end	Day	14% 7% 3 7% 4 5% 4 5% 6% 8% 38%	4477
June	1st to 10th	Day	4 4 1 4 4 5% 5% 4 7% 10% 53%	2470
	11th to 20th	Day	4 4 1 5% 4 5% 4 4 6% 9% 54%	2470
	21st to end	Day	5% 3 1 5% 4 4 4 3 6% 9% 55%	2470
July	1st to 10th	Day	5% 5% 2 6% 5% 4 4 4 5% 8% 53%	4120
	11th to 20th	Day	3 2 1 3 3 3 4 3 6% 9% 63%	4120
	21st to end	Day	5% 5% 2 6% 5% 5% 5% 5% 8% 11% 44%	4465
August	1st to 10th	Day	1 1 1 2 2 2 3 3 5% 8% 71%	4104
	11th to 20th	Day	4 2 1 2 2 2 4 4 6% 10% 62%	4041
	21st to end	Day	3 3 1 4 3 4 5% 5% 7% 10% 56%	4034
September	1st to 10th	Day	8% 6% 2 4 4 4 4 3 4 7% 55%	2090
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 6% 7% 46%	2013
	21st to end	Day	7% 4 1 4 2 4 4 3 6% 7% 58%	1980
October	1st to 10th	Day	4 2 1 4 3 3 4 3 6% 7% 61%	1949
	11th to 20th	Day	12% 4 1 4 3 2 3 2 4 5% 60%	1800
	21st to end	Day	14% 4 2 4 3 2 2 2 3 4 62%	1980
November	1st to 10th	Day	12% 5% 2 5% 3 2 3 2 4 5% 57%	1858
	11th to 20th	Day	7% 3 2 5% 4 3 3 3 4 6% 60%	1763
	21st to end	Day	15% 7% 2 4 3 3 3 2 4 5% 54%	1710
December	1st to 10th	Day	15% 6% 1 4 3 3 3 3 4 6% 51%	1710
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 4 5% 47%	2161
	21st to end	Day	18% 6% 2 5% 3 3 2 2 4 5% 51%	3135
January	1st to 10th	Night	14% 4 2 3 3 2 3 2 3 5% 60%	4108
	11th to 20th	Night	12% 4 1 3 2 2 2 1 3 4 65%	4120
	21st to end	Night	19% 7% 2 5% 3 2 3 2 4 5% 49%	4532
February	1st to 10th	Night	14% 7% 2 5% 3 3 3 2 4 5% 52%	2347
	11th to 20th	Night	14% 7% 2 5% 4 3 2 2 4 6% 51%	2218
	21st to end	Night	17% 6% 2 4 3 2 3 2 4 5% 53%	1874
March	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 4 5% 45%	3419
	11th to 20th	Night	15% 5% 2 5% 3 3 4 3 5% 6% 47%	3480
	21st to end	Night	12% 6% 2 5% 4 4 3 3 5% 6% 51%	3817
April	1st to 10th	Night	9% 5% 2 4 3 4 3 3 5% 8% 55%	2090
	11th to 20th	Night	13% 5% 2 5% 4 3 3 3 5% 6% 52%	1984
	21st to end	Night	10% 5% 1 4 3 3 3 3 5% 6% 56%	1738
May	1st to 10th	Night	20% 7% 2 5% 4 3 3 3 5% 6% 40%	2823
	11th to 20th	Night	12% 5% 3 6% 3 3 3 3 5% 7% 50%	2823
	21st to end	Night	12% 5% 2 4 4 3 4 4 6% 8% 49%	3090
June	1st to 10th	Night	4 2 1 4 4 4 4 3 5% 10% 58%	1710
	11th to 20th	Night	3 2 1 3 3 3 3 3 7% 8% 63%	1710
	21st to end	Night	2 3 1 4 3 3 4 4 6% 11% 58%	1710
July	1st to 10th	Night	4 3 1 4 3 3 4 3 5% 9% 62%	2850
	11th to 20th	Night	2 2 1 3 2 2 3 3 5% 9% 69%	2850
	21st to end	Night	3 3 2 4 4 4 4 3 7% 8% 56%	3098
August	1st to 10th	Night	1 1 1 1 1 2 2 4 9% 77%	2836
	11th to 20th	Night	3 2 1 3 1 2 2 2 6% 11% 67%	2798
	21st to end	Night	3 2 1 3 2 3 4 3 6% 9% 64%	3620
September	1st to 10th	Night	7% 5% 2 4 4 3 4 4 4 7% 56%	2090
	11th to 20th	Night	11% 5% 1 5% 3 3 4 3 5% 7% 52%	2024
	21st to end	Night	6% 3 1 3 2 3 3 3 6% 7% 63%	1980
October	1st to 10th	Night	6% 2 1 3 3 3 3 3 5% 6% 64%	1992
	11th to 20th	Night	11% 4 1 3 3 2 2 2 4 5% 64%	2160
	21st to end	Night	16% 4 1 3 2 2 2 2 3 4 61%	2376
November	1st to 10th	Night	10% 5% 1 4 3 3 3 2 4 4 62%	2220
	11th to 20th	Night	6% 4 1 4 3 3 3 3 5% 6% 61%	2415
	21st to end	Night	14% 6% 2 4 3 4 3 3 3 5% 54%	2470
December	1st to 10th	Night	12% 6% 2 4 3 3 2 2 4 5% 56%	2470
	11th to 20th	Night	12% 6% 2 5% 3 4 3 3 4 6% 52%	3036
	21st to end	Night	16% 6% 2 4 2 3 2 2 3 5% 55%	4532

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 6% 20% 34% 7%	2848
	11th to 20th	Day	27% 3 7% 23% 31% 9%	2850
	21st to end	Day	43% 2 5% 19% 23% 8%	3135
February	1st to 10th	Day	40% 4 6% 19% 21% 10%	1849
	11th to 20th	Day	40% 3 7% 20% 21% 10%	1859
	21st to end	Day	38% 2 4 15% 25% 15%	1579
March	1st to 10th	Day	35% 4 6% 16% 29% 10%	3436
	11th to 20th	Day	43% 5% 10% 22% 16% 4	3480
	21st to end	Day	38% 5% 12% 22% 18% 6%	3814
April	1st to 10th	Day	35% 5% 9% 20% 19% 11%	2090
	11th to 20th	Day	37% 5% 9% 19% 21% 9%	2145
	21st to end	Day	35% 5% 9% 21% 24% 6%	2438
May	1st to 10th	Day	44% 7% 8% 20% 17% 5%	4120
	11th to 20th	Day	37% 4 9% 24% 24% 2	4086
	21st to end	Day	43% 6% 12% 23% 11% 5%	4489
June	1st to 10th	Day	26% 7% 15% 29% 18% 4	2470
	11th to 20th	Day	27% 5% 12% 31% 20% 6%	2470
	21st to end	Day	25% 6% 12% 28% 25% 4	2470
July	1st to 10th	Day	27% 6% 14% 39% 13% 1	4056
	11th to 20th	Day	17% 5% 12% 40% 23% 3	4051
	21st to end	Day	29% 8% 14% 33% 16% 1	4325
August	1st to 10th	Day	12% 4 12% 30% 37% 6%	4090
	11th to 20th	Day	17% 6% 15% 39% 22% 2	4080
	21st to end	Day	21% 8% 17% 36% 16% 2	4033
September	1st to 10th	Day	30% 5% 9% 23% 23% 11%	2090
	11th to 20th	Day	36% 6% 10% 19% 21% 7%	2013
	21st to end	Day	25% 4 8% 23% 26% 14%	1980
October	1st to 10th	Day	22% 5% 11% 24% 22% 16%	1949
	11th to 20th	Day	27% 3 7% 17% 27% 19%	1800
	21st to end	Day	30% 2 6% 19% 26% 17%	1980
November	1st to 10th	Day	31% 3 8% 22% 26% 10%	1858
	11th to 20th	Day	26% 4 8% 21% 30% 12%	1763
	21st to end	Day	35% 3 5% 22% 25% 11%	1710
December	1st to 10th	Day	35% 3 8% 23% 24% 7%	1710
	11th to 20th	Day	40% 3 6% 15% 26% 10%	2159
	21st to end	Day	38% 3 5% 23% 22% 10%	3135
January	1st to 10th	Night	29% 4 5% 18% 34% 11%	4108
	11th to 20th	Night	26% 2 6% 20% 30% 15%	4120
	21st to end	Night	40% 2 5% 16% 26% 11%	4532
February	1st to 10th	Night	36% 4 6% 18% 22% 14%	2347
	11th to 20th	Night	37% 3 6% 17% 26% 11%	2218
	21st to end	Night	36% 3 6% 16% 22% 17%	1874
March	1st to 10th	Night	41% 4 5% 15% 22% 12%	3419
	11th to 20th	Night	37% 5% 11% 21% 19% 7%	3480
	21st to end	Night	35% 5% 8% 20% 26% 5%	3817
April	1st to 10th	Night	29% 4 11% 22% 20% 15%	2090
	11th to 20th	Night	34% 4 9% 19% 19% 15%	1984
	21st to end	Night	29% 4 10% 22% 29% 6%	1738
May	1st to 10th	Night	44% 6% 9% 21% 13% 8%	2850
	11th to 20th	Night	36% 4 12% 21% 25% 2	2824
	21st to end	Night	32% 7% 13% 28% 13% 6%	3099
June	1st to 10th	Night	23% 5% 16% 31% 20% 5%	1710
	11th to 20th	Night	18% 5% 12% 34% 23% 8%	1710
	21st to end	Night	19% 6% 13% 28% 27% 7%	1710
July	1st to 10th	Night	20% 5% 14% 44% 15% 2	2805
	11th to 20th	Night	13% 4 11% 42% 26% 4	2796
	21st to end	Night	23% 6% 15% 37% 18% 1	2999
August	1st to 10th	Night	7% 3 9% 33% 41% 7%	2826
	11th to 20th	Night	13% 3 14% 39% 25% 5%	2825
	21st to end	Night	16% 6% 20% 35% 20% 3	3620
September	1st to 10th	Night	27% 6% 10% 22% 25% 11%	2090
	11th to 20th	Night	31% 5% 12% 18% 21% 12%	2024
	21st to end	Night	20% 4 8% 23% 27% 17%	1980
October	1st to 10th	Night	21% 4 8% 27% 20% 19%	1992
	11th to 20th	Night	25% 3 8% 17% 26% 23%	2160
	21st to end	Night	29% 3 4 17% 27% 20%	2376
November	1st to 10th	Night	28% 3 6% 21% 27% 15%	2220
	11th to 20th	Night	24% 4 8% 22% 28% 15%	2415
	21st to end	Night	35% 3 6% 17% 28% 12%	2470
December	1st to 10th	Night	31% 3 7% 22% 24% 12%	2470
	11th to 20th	Night	35% 3 5% 16% 28% 13%	3037
	21st to end	Night	34% 3 5% 18% 26% 14%	4532

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% 34% 26% 6%	270
	11th to 20th	Day	20% 14% 39% 20% 5% 2	270
	21st to end	Day	20% 15% 37% 19% 9% 1	297
February	1st to 10th	Day	18% 17% 39% 21% 5%	270
	11th to 20th	Day	21% 19% 40% 15% 4	270
	21st to end	Day	29% 18% 36% 15% 2	225
March	1st to 10th	Day	20% 18% 35% 24% 3	270
	11th to 20th	Day	20% 19% 36% 20% 4 1	270
	21st to end	Day	13% 16% 45% 19% 6%	297
April	1st to 10th	Day	16% 17% 35% 24% 7% 1	270
	11th to 20th	Day	19% 14% 37% 24% 6%	343
	21st to end	Day	14% 17% 36% 24% 7% 2	618
May	1st to 10th	Day	12% 13% 32% 34% 8% 1	653
	11th to 20th	Day	16% 7% 29% 33% 12% 3	675
	21st to end	Day	12% 7% 31% 29% 18% 4	734
June	1st to 10th	Day	5% 2 15% 41% 28% 9%	680
	11th to 20th	Day	5% 2 17% 44% 25% 7%	680
	21st to end	Day	4 2 11% 39% 34% 10%	680
July	1st to 10th	Day	6% 1 9% 37% 34% 13%	671
	11th to 20th	Day	3 8% 36% 40% 13%	668
	21st to end	Day	3 1 5% 32% 45% 14%	739
August	1st to 10th	Day	1 7% 33% 47% 11%	676
	11th to 20th	Day	8% 1 6% 32% 44% 10%	665
	21st to end	Day	4 1 5% 30% 36% 24%	458
September	1st to 10th	Day	7% 1 10% 21% 40% 21%	300
	11th to 20th	Day	6% 2 5% 27% 41% 19%	279
	21st to end	Day	11% 3 6% 29% 35% 16%	270
October	1st to 10th	Day	1 3 20% 32% 33% 11%	270
	11th to 20th	Day	10% 7% 29% 33% 17% 5%	270
	21st to end	Day	23% 12% 22% 25% 15% 3	297
November	1st to 10th	Day	18% 7% 32% 28% 11% 4	270
	11th to 20th	Day	10% 9% 33% 32% 14% 3	270
	21st to end	Day	18% 16% 34% 22% 9% 1	270
December	1st to 10th	Day	16% 11% 33% 27% 11% 2	270
	11th to 20th	Day	19% 11% 38% 24% 8%	270
	21st to end	Day	22% 11% 38% 21% 8%	297
January	1st to 10th	Night	13% 9% 37% 33% 8%	950
	11th to 20th	Night	18% 13% 32% 31% 6%	950
	21st to end	Night	15% 11% 35% 32% 7%	1045
February	1st to 10th	Night	18% 12% 37% 28% 4 1	827
	11th to 20th	Night	18% 13% 40% 27% 2	744
	21st to end	Night	20% 15% 43% 20% 2	618
March	1st to 10th	Night	17% 13% 36% 29% 4	570
	11th to 20th	Night	16% 9% 41% 29% 5%	570
	21st to end	Night	13% 12% 41% 29% 4	627
April	1st to 10th	Night	10% 12% 45% 29% 5%	570
	11th to 20th	Night	15% 11% 41% 29% 4	563
	21st to end	Night	13% 9% 44% 26% 7% 1	570
May	1st to 10th	Night	11% 9% 40% 33% 7%	570
	11th to 20th	Night	11% 5% 34% 35% 14% 1	561
	21st to end	Night	8% 4 35% 37% 15% 1	617
June	1st to 10th	Night	4 2 20% 48% 24% 2	570
	11th to 20th	Night	4 1 26% 54% 14% 1	570
	21st to end	Night	3 1 16% 49% 28% 2	570
July	1st to 10th	Night	4 1 12% 46% 32% 4	555
	11th to 20th	Night	3 1 11% 50% 33% 2	552
	21st to end	Night	2 1 7% 51% 36% 3	624
August	1st to 10th	Night	2 1 5% 52% 39% 2	567
	11th to 20th	Night	6% 7% 48% 32% 5%	564
	21st to end	Night	4 1 5% 46% 39% 6%	627
September	1st to 10th	Night	9% 1 9% 40% 36% 4	570
	11th to 20th	Night	13% 2 6% 41% 35% 3	552
	21st to end	Night	7% 1 15% 41% 32% 4	540
October	1st to 10th	Night	7% 1 21% 41% 27% 3	552
	11th to 20th	Night	12% 5% 26% 38% 17% 2	720
	21st to end	Night	17% 8% 29% 32% 14%	792
November	1st to 10th	Night	12% 4 31% 38% 14% 1	740
	11th to 20th	Night	7% 10% 32% 37% 14%	895
	21st to end	Night	17% 10% 36% 27% 10%	950
December	1st to 10th	Night	16% 9% 36% 33% 6%	950
	11th to 20th	Night	18% 9% 34% 32% 7%	950
	21st to end	Night	17% 10% 36% 30% 7%	1045

Page generated at 2024-09-11 09:00:06. Version ac07

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