Arlington
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arlington clear sky chart from 2005-11-05 to 2018-03-11.

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 TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(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

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
50%
16326
30%
49072
20%
16282
59%
16282
40%
26709

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 15thNight
41%
972
31%
2522
14%
970
51%
970
41%
1465
January16th to endNight
37%
1040
34%
2704
13%
1040
50%
1040
41%
1566
February1st to 15thNight
34%
809
28%
2334
13%
804
45%
804
34%
1360
February16th to endNight
25%
678
29%
2044
9%
678
39%
678
32%
1175
March1st to 15thNight
32%
573
28%
2101
16%
573
47%
573
27%
1180
March16th to endNight
35%
576
25%
2112
15%
576
49%
576
30%
1126
April1st to 15thNight
32%
539
25%
1977
13%
539
50%
539
33%
1007
April16th to endNight
35%
540
27%
1707
17%
540
51%
540
37%
935
May1st to 15thNight
39%
540
23%
1620
14%
540
52%
540
34%
941
May16th to endNight
51%
562
25%
1684
21%
561
61%
561
39%
939
June1st to 15thNight
69%
540
24%
1620
23%
540
76%
540
53%
789
June16th to endNight
71%
540
26%
1620
29%
540
74%
540
46%
846
July1st to 15thNight
77%
510
25%
1530
28%
510
80%
510
49%
783
July16th to endNight
87%
564
31%
1594
37%
531
89%
531
56%
751
August1st to 15thNight
89%
539
38%
1610
44%
536
90%
536
61%
728
August16th to endNight
80%
573
29%
1899
34%
573
82%
573
40%
991
September1st to 15thNight
71%
540
23%
1980
28%
540
72%
540
35%
1045
September16th to endNight
73%
537
34%
1980
35%
537
78%
537
40%
967
October1st to 15thNight
64%
609
35%
2049
28%
609
71%
609
44%
1011
October16th to endNight
52%
768
39%
2304
23%
768
63%
768
48%
1146
November1st to 15thNight
50%
787
30%
2307
18%
787
59%
787
42%
1246
November16th to endNight
42%
975
36%
2535
16%
975
54%
975
42%
1460
December1st to 15thNight
38%
975
32%
2535
14%
975
49%
975
38%
1545
December16th to endNight
35%
1040
28%
2704
11%
1040
45%
1040
34%
1707

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 ForecastData
Points
Cloud CoverDay
10%42442445%7%51%
49553
Night
9%313323347%57%
49271
TransparencyDay
31%410%23%21%7%
49253
Night
27%39%22%24%11%
49076
SeeingDay
10%5%20%30%25%7%
6098
Night
10%6%28%35%16%1
16330

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
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
13%5%33212235%56%
1746
16th to endDay
12%423212236%57%
1872
February1st to 15thDay
12%7%25%313235%52%
1865
16th to endDay
17%5%1331123452%
1716
March1st to 15thDay
16%5%14423445%47%
2095
16th to endDay
12%5%24325%336%50%
2110
April1st to 15thDay
9%424425%337%51%
1971
16th to endDay
11%424325%447%49%
2251
May1st to 15thDay
12%5%35%5%25%45%7%43%
2331
16th to endDay
10%5%35%435%5%5%8%41%
2430
June1st to 15thDay
341343445%9%53%
2340
16th to endDay
7%2145%2446%9%50%
2340
July1st to 15thDay
6%324435%5%6%8%49%
2340
16th to endDay
3213436%6%7%11%49%
2466
August1st to 15thDay
313325%5%7%8%57%
2340
16th to endDay
6%4145%35%7%8%10%42%
2300
September1st to 15thDay
10%42442445%8%49%
1980
16th to endDay
7%41342446%7%53%
1980
October1st to 15thDay
8%31332345%7%57%
1899
16th to endDay
8%3122122469%
1920
November1st to 15thDay
9%4245%13335%55%
1879
16th to endDay
11%41321223461%
1755
December1st to 15thDay
16%6%12312345%51%
1755
16th to endDay
16%7%23423245%47%
1872
January1st to 15thNight
11%313312235%59%
2522
16th to endNight
11%412312235%60%
2704
February1st to 15thNight
12%5%23312235%56%
2327
16th to endNight
13%423422245%55%
2044
March1st to 15thNight
14%423323345%50%
2101
16th to endNight
9%5%13423346%55%
2112
April1st to 15thNight
7%324323348%56%
1977
16th to endNight
10%41331335%6%56%
1707
May1st to 15thNight
9%425%41335%8%50%
1619
16th to endNight
6%32332346%10%52%
1683
June1st to 15thNight
221231335%8%64%
1620
16th to endNight
22134245%6%9%56%
1620
July1st to 15thNight
42133244412%56%
1620
16th to endNight
21222347%10%62%
1702
August1st to 15thNight
211221225%9%69%
1620
16th to endNight
6%313325%47%12%50%
1899
September1st to 15thNight
10%32342345%7%53%
1980
16th to endNight
6%31331345%7%57%
1980
October1st to 15thNight
8%313223246%62%
2049
16th to endNight
8%21222222468%
2304
November1st to 15thNight
7%413323345%60%
2307
16th to endNight
9%412323235%60%
2535
December1st to 15thNight
12%5%13323245%54%
2535
16th to endNight
14%5%2432235%5%49%
2704

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
32%26%22%26%8%
1747
16th to endDay
29%26%19%30%11%
1872
February1st to 15thDay
36%36%22%22%9%
1866
16th to endDay
37%26%20%21%12%
1716
March1st to 15thDay
39%37%18%22%9%
2095
16th to endDay
36%310%20%21%8%
2112
April1st to 15thDay
33%49%22%17%11%
1971
16th to endDay
35%39%20%23%8%
2251
May1st to 15thDay
39%5%9%17%19%8%
2331
16th to endDay
39%5%12%21%17%2
2434
June1st to 15thDay
26%6%12%27%22%4
2340
16th to endDay
29%5%12%27%20%5%
2340
July1st to 15thDay
28%6%15%30%15%2
2201
16th to endDay
23%8%18%31%17%1
2311
August1st to 15thDay
19%6%15%30%24%2
2326
16th to endDay
31%8%15%27%14%2
2300
September1st to 15thDay
32%5%10%24%19%7%
1980
16th to endDay
29%48%24%24%9%
1980
October1st to 15thDay
25%411%23%20%14%
1899
16th to endDay
21%26%18%30%20%
1920
November1st to 15thDay
31%39%23%22%10%
1879
16th to endDay
28%2421%30%12%
1755
December1st to 15thDay
36%37%20%25%7%
1755
16th to endDay
39%26%21%22%7%
1872
January1st to 15thNight
28%26%19%29%12%
2522
16th to endNight
29%25%18%28%16%
2704
February1st to 15thNight
32%36%18%24%14%
2337
16th to endNight
33%27%19%21%14%
2044
March1st to 15thNight
35%37%17%21%13%
2101
16th to endNight
29%5%11%20%26%6%
2112
April1st to 15thNight
28%312%21%19%14%
1977
16th to endNight
29%410%20%24%11%
1707
May1st to 15thNight
32%5%12%19%20%10%
1620
16th to endNight
27%6%16%24%20%4
1685
June1st to 15thNight
18%413%33%23%7%
1620
16th to endNight
22%5%12%27%25%6%
1620
July1st to 15thNight
21%413%37%18%4
1530
16th to endNight
14%6%19%35%21%3
1594
August1st to 15thNight
13%313%33%30%6%
1610
16th to endNight
24%6%14%30%19%4
1899
September1st to 15thNight
28%5%11%22%21%9%
1980
16th to endNight
24%49%23%26%12%
1980
October1st to 15thNight
24%210%25%19%17%
2049
16th to endNight
21%2317%30%23%
2304
November1st to 15thNight
26%37%24%23%13%
2307
16th to endNight
28%25%17%31%14%
2535
December1st to 15thNight
33%27%20%23%11%
2535
16th to endNight
36%37%21%21%10%
2704

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
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
17%11%34%28%8%
135
16th to endDay
11%12%43%19%10%3
144
February1st to 15thDay
8%11%45%29%5%
135
16th to endDay
24%18%35%19%3
120
March1st to 15thDay
19%17%37%19%5%
121
16th to endDay
6%10%45%22%13%1
96
April1st to 15thDay
11%15%27%32%13%
90
16th to endDay
16%13%36%24%8%1
361
May1st to 15thDay
14%12%32%29%8%2
468
16th to endDay
14%6%33%28%15%3
517
June1st to 15thDay
6%113%45%26%8%
495
16th to endDay
6%112%35%32%10%
495
July1st to 15thDay
6%18%39%34%10%
475
16th to endDay
4326%49%16%
515
August1st to 15thDay
3433%44%13%
493
16th to endDay
12%6%32%35%11%
340
September1st to 15thDay
2420%45%27%
135
16th to endDay
9%224%43%20%
135
October1st to 15thDay
114%32%39%11%
135
16th to endDay
11%419%35%21%6%
144
November1st to 15thDay
17%329%29%15%4
135
16th to endDay
17%11%33%22%13%2
135
December1st to 15thDay
27%227%25%14%2
135
16th to endDay
25%11%34%23%5%
144
January1st to 15thNight
14%10%29%36%9%
975
16th to endNight
13%10%35%32%7%
1040
February1st to 15thNight
15%11%35%31%5%
809
16th to endNight
16%12%44%25%2
678
March1st to 15thNight
16%9%37%31%5%
573
16th to endNight
10%10%41%30%6%
576
April1st to 15thNight
7%12%45%29%5%
539
16th to endNight
12%7%42%29%6%1
540
May1st to 15thNight
9%6%39%35%8%1
540
16th to endNight
6%433%36%17%1
563
June1st to 15thNight
3121%48%23%1
540
16th to endNight
2116%48%28%2
540
July1st to 15thNight
5%10%47%33%2
510
16th to endNight
3346%43%3
564
August1st to 15thNight
2344%45%3
539
16th to endNight
6%442%38%6%
573
September1st to 15thNight
12%17%37%37%4
540
16th to endNight
8%11%38%36%5%
537
October1st to 15thNight
9%121%37%26%3
609
16th to endNight
9%6%29%34%18%1
768
November1st to 15thNight
9%5%30%36%17%1
787
16th to endNight
12%8%35%31%12%
975
December1st to 15thNight
17%7%34%33%7%
975
16th to endNight
17%8%34%31%7%
1040
Page generated at 2018-03-12 12:28:11. Version ac04

Copyright 2018 A. Danko


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