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 2025-02-21.

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%	109207
	Night	11% 5% 2 4 3 3 3 3 4 6% 57%	109606
Transparency	Day	31% 4 9% 25% 23% 8%	112963
	Night	28% 4 8% 23% 25% 11%	113977
Seeing	Day	11% 8% 23% 30% 22% 7%	15462
	Night	12% 7% 29% 36% 15% 1	25501

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	14% 6% 3 6% 3 3 3 2 4 4 53%	4077
	11th to 20th	Day	8% 5% 1 3 2 2 2 2 4 6% 64%	4080
	21st to end	Day	22% 9% 3 5% 2 2 2 2 3 4 48%	4488
February	1st to 10th	Day	15% 7% 2 6% 4 3 3 3 4 5% 47%	1947
	11th to 20th	Day	15% 9% 3 6% 3 3 3 2 4 5% 48%	1949
	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% 47%	2090
	21st to end	Day	7% 4 1 4 2 4 4 3 6% 7% 58%	2090
October	1st to 10th	Day	4 2 1 4 2 3 4 3 5% 7% 63%	2057
	11th to 20th	Day	11% 4 1 4 2 2 3 2 4 5% 61%	1900
	21st to end	Day	13% 4 1 4 3 2 2 2 3 4 62%	2090
November	1st to 10th	Day	13% 6% 2 5% 3 2 3 2 4 5% 56%	1958
	11th to 20th	Day	7% 3 2 5% 4 3 3 3 4 6% 61%	1855
	21st to end	Day	14% 6% 2 4 3 3 3 2 4 5% 54%	1800
December	1st to 10th	Day	17% 7% 1 4 3 3 3 3 4 6% 50%	2940
	11th to 20th	Day	17% 6% 2 5% 4 3 4 3 4 4 47%	3391
	21st to end	Day	18% 7% 2 5% 3 3 2 2 3 4 50%	4488
January	1st to 10th	Night	16% 4 2 3 3 3 3 2 3 4 58%	5888
	11th to 20th	Night	8% 3 1 3 2 2 2 1 3 4 72%	5900
	21st to end	Night	23% 8% 2 5% 3 2 2 2 3 4 47%	6490
February	1st to 10th	Night	14% 7% 2 5% 3 3 3 2 4 5% 53%	2470
	11th to 20th	Night	16% 7% 2 5% 4 3 2 2 4 6% 50%	2338
	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 3 3 5% 7% 53%	2090
	21st to end	Night	6% 3 1 3 2 2 3 3 6% 7% 63%	2090
October	1st to 10th	Night	6% 2 1 3 3 3 3 3 5% 6% 66%	2103
	11th to 20th	Night	10% 4 1 3 2 2 2 2 4 5% 66%	2280
	21st to end	Night	15% 4 1 3 2 2 2 2 3 4 62%	2508
November	1st to 10th	Night	11% 5% 1 4 3 2 3 2 4 4 61%	2340
	11th to 20th	Night	6% 4 1 4 3 3 3 3 6% 6% 62%	2543
	21st to end	Night	14% 6% 2 4 3 3 3 3 3 5% 55%	2600
December	1st to 10th	Night	15% 7% 2 4 3 2 2 2 3 4 54%	4250
	11th to 20th	Night	16% 6% 2 6% 3 3 3 2 4 5% 51%	4816
	21st to end	Night	14% 7% 2 5% 3 3 2 2 3 4 55%	6490

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	35% 3 6% 26% 26% 5%	4078
	11th to 20th	Day	23% 3 6% 23% 36% 9%	4080
	21st to end	Day	43% 2 4 16% 22% 13%	4488
February	1st to 10th	Day	40% 4 6% 18% 21% 11%	1947
	11th to 20th	Day	41% 3 7% 20% 20% 9%	1959
	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% 22% 7%	2090
	21st to end	Day	24% 4 8% 23% 27% 14%	2090
October	1st to 10th	Day	13% 3 6% 15% 36% 28%	3421
	11th to 20th	Day	17% 2 5% 21% 34% 21%	3140
	21st to end	Day	22% 3 5% 27% 31% 11%	3428
November	1st to 10th	Day	32% 3 7% 21% 25% 11%	1958
	11th to 20th	Day	25% 3 8% 20% 30% 13%	1855
	21st to end	Day	34% 3 5% 22% 25% 11%	1800
December	1st to 10th	Day	37% 3 5% 26% 24% 5%	2940
	11th to 20th	Day	41% 3 5% 11% 28% 11%	3389
	21st to end	Day	40% 3 4 24% 21% 9%	4488
January	1st to 10th	Night	31% 3 6% 24% 28% 8%	5888
	11th to 20th	Night	20% 2 5% 20% 37% 17%	5900
	21st to end	Night	43% 2 4 13% 22% 15%	6490
February	1st to 10th	Night	36% 4 6% 18% 23% 14%	2470
	11th to 20th	Night	38% 3 7% 17% 25% 10%	2338
	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	30% 5% 12% 19% 22% 11%	2090
	21st to end	Night	20% 4 8% 23% 27% 17%	2090
October	1st to 10th	Night	13% 2 5% 16% 28% 36%	3506
	11th to 20th	Night	15% 2 5% 21% 32% 26%	3779
	21st to end	Night	24% 3 3 22% 33% 16%	4120
November	1st to 10th	Night	29% 3 6% 20% 26% 16%	2340
	11th to 20th	Night	23% 4 8% 22% 28% 16%	2543
	21st to end	Night	34% 3 6% 18% 27% 12%	2600
December	1st to 10th	Night	36% 3 5% 20% 27% 10%	4250
	11th to 20th	Night	38% 2 5% 14% 26% 14%	4817
	21st to end	Night	34% 3 6% 19% 26% 12%	6490

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	21% 15% 35% 24% 6%	300
	11th to 20th	Day	19% 15% 39% 20% 5% 2	300
	21st to end	Day	20% 16% 37% 19% 8% 1	330
February	1st to 10th	Day	19% 16% 39% 21% 5%	300
	11th to 20th	Day	21% 21% 39% 15% 4	300
	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% 1 6% 27% 42% 18%	300
	21st to end	Day	11% 2 8% 29% 34% 15%	300
October	1st to 10th	Day	1 3 19% 32% 34% 12%	300
	11th to 20th	Day	9% 6% 29% 35% 16% 5%	300
	21st to end	Day	21% 12% 22% 26% 15% 3	330
November	1st to 10th	Day	20% 8% 31% 28% 11% 4	300
	11th to 20th	Day	10% 9% 33% 31% 14% 3	300
	21st to end	Day	16% 16% 35% 22% 9% 2	300
December	1st to 10th	Day	17% 11% 33% 27% 11% 2	300
	11th to 20th	Day	19% 12% 37% 24% 8%	300
	21st to end	Day	22% 12% 38% 21% 7%	330
January	1st to 10th	Night	14% 9% 36% 33% 8%	1000
	11th to 20th	Night	17% 14% 32% 31% 5%	1000
	21st to end	Night	16% 11% 35% 31% 7%	1100
February	1st to 10th	Night	18% 12% 38% 28% 4 1	870
	11th to 20th	Night	20% 13% 39% 27% 2	784
	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% 42% 34% 3	570
	21st to end	Night	7% 1 15% 41% 32% 4	570
October	1st to 10th	Night	7% 1 20% 42% 28% 3	583
	11th to 20th	Night	11% 5% 27% 38% 17% 2	760
	21st to end	Night	17% 8% 29% 32% 14%	836
November	1st to 10th	Night	12% 4 30% 38% 14% 1	780
	11th to 20th	Night	8% 10% 33% 36% 13%	943
	21st to end	Night	16% 10% 36% 27% 10%	1000
December	1st to 10th	Night	16% 9% 36% 33% 6%	1000
	11th to 20th	Night	18% 9% 34% 32% 7%	1000
	21st to end	Night	17% 10% 36% 30% 7%	1100

Page generated at 2025-02-18 02:26:31. Version ac07

Copyright © 2025 Attilla Danko.