Arclight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arclight Observatory clear sky chart from 2005-11-05 to 2017-09-10.

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
49%
15571
22%
46734
17%
15527
55%
15527
29%
29325

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
29%
899
19%
2506
11%
895
36%
895
22%
1730
January16th to endNight
29%
864
20%
2463
10%
864
38%
864
22%
1707
February1st to 15thNight
27%
704
17%
2112
9%
701
33%
701
20%
1468
February16th to endNight
26%
631
20%
1893
10%
631
34%
631
21%
1299
March1st to 15thNight
34%
720
19%
2160
11%
720
40%
720
23%
1446
March16th to endNight
37%
665
17%
2027
12%
665
43%
665
22%
1355
April1st to 15thNight
41%
539
25%
1797
17%
539
54%
539
32%
1026
April16th to endNight
47%
540
23%
1791
19%
540
55%
540
29%
1077
May1st to 15thNight
49%
540
19%
1619
13%
539
54%
539
26%
1070
May16th to endNight
68%
561
19%
1683
16%
561
70%
561
33%
1019
June1st to 15thNight
72%
540
21%
1476
17%
540
74%
540
40%
868
June16th to endNight
80%
540
25%
1440
20%
540
81%
540
45%
833
July1st to 15thNight
72%
510
18%
1360
15%
510
73%
510
35%
853
July16th to endNight
79%
564
24%
1508
22%
531
80%
531
41%
867
August1st to 15thNight
77%
539
22%
1761
25%
536
79%
536
38%
974
August16th to endNight
77%
573
23%
1910
28%
573
78%
573
35%
1067
September1st to 15thNight
67%
566
23%
1791
24%
566
69%
566
32%
1058
September16th to endNight
63%
657
28%
1815
24%
657
65%
657
36%
1096
October1st to 15thNight
58%
660
34%
1815
27%
660
63%
660
40%
1058
October16th to endNight
50%
704
29%
2208
24%
704
58%
704
33%
1296
November1st to 15thNight
51%
700
31%
2276
28%
700
59%
700
35%
1275
November16th to endNight
41%
720
25%
2340
21%
720
50%
720
28%
1411
December1st to 15thNight
34%
720
19%
2340
15%
720
40%
720
19%
1602
December16th to endNight
32%
915
19%
2643
11%
915
39%
915
20%
1870

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
13%6%25%5%35%45%7%41%
47890
Night
13%6%24424447%45%
46919
TransparencyDay
41%6%10%22%16%3
47577
Night
38%5%10%23%18%3
46736
SeeingDay
14%417%23%24%15%
5622
Night
16%5%22%32%21%3
15574

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
20%8%35%423345%36%
1432
16th to endDay
14%9%35%5%23346%40%
1755
February1st to 15thDay
16%9%36%434445%37%
1750
16th to endDay
16%10%46%5%3335%6%35%
1580
March1st to 15thDay
20%10%25%424345%36%
1800
16th to endDay
16%7%37%5%35%45%6%34%
2207
April1st to 15thDay
12%5%25%5%34447%44%
2151
16th to endDay
13%6%26%6%3445%6%40%
2167
May1st to 15thDay
16%6%25%5%345%5%6%37%
2331
16th to endDay
10%425%6%45%6%5%9%38%
2431
June1st to 15thDay
8%325%435%5%6%9%44%
2479
16th to endDay
43135%35%6%8%11%44%
2520
July1st to 15thDay
9%5%25%6%46%7%6%9%35%
2520
16th to endDay
7%3145%37%6%8%11%38%
2554
August1st to 15thDay
6%4246%37%6%7%10%39%
2185
16th to endDay
8%425%5%35%6%7%9%39%
2292
September1st to 15thDay
13%4246%36%5%5%7%39%
2045
16th to endDay
11%6%25%5%24346%47%
1815
October1st to 15thDay
12%414424345%52%
1813
16th to endDay
15%413423225%53%
1663
November1st to 15thDay
14%5%1321235%6%52%
1582
16th to endDay
16%43432334447%
1620
December1st to 15thDay
22%7%25%423446%36%
1620
16th to endDay
19%10%25%413346%36%
1578
January1st to 15thNight
18%8%35%5%34335%38%
2506
16th to endNight
17%8%46%423336%37%
2463
February1st to 15thNight
20%9%36%42334436%
2103
16th to endNight
17%8%46%5%23446%37%
1893
March1st to 15thNight
16%7%25%5%24347%39%
2160
16th to endNight
14%8%36%5%24446%39%
2027
April1st to 15thNight
12%42432345%8%48%
1797
16th to endNight
12%425%425%447%46%
1791
May1st to 15thNight
13%5%35%5%345%47%41%
1619
16th to endNight
6%4145%25%5%6%11%45%
1682
June1st to 15thNight
7%315%32345%9%51%
1476
16th to endNight
2113436%6%6%12%51%
1440
July1st to 15thNight
7%4243345%6%9%46%
1440
16th to endNight
6%21342447%10%52%
1612
August1st to 15thNight
5%41442446%10%51%
1772
16th to endNight
5%3145%346%7%10%48%
1910
September1st to 15thNight
12%32441347%8%46%
1791
16th to endNight
9%5%24423446%51%
1815
October1st to 15thNight
11%323322346%56%
1815
16th to endNight
15%6%23312235%52%
2208
November1st to 15thNight
14%413313346%53%
2276
16th to endNight
15%6%23324335%47%
2340
December1st to 15thNight
20%11%35%423436%35%
2340
16th to endNight
22%8%35%423435%37%
2643

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
50%36%23%13%2
1433
16th to endDay
44%49%26%12%1
1755
February1st to 15thDay
49%38%23%12%3
1752
16th to endDay
49%311%20%12%2
1580
March1st to 15thDay
50%38%19%13%3
1800
16th to endDay
48%5%11%23%9%
2208
April1st to 15thDay
38%5%8%22%21%3
2151
16th to endDay
43%47%25%16%2
2167
May1st to 15thDay
45%6%10%21%14%1
2331
16th to endDay
41%8%13%22%12%2
2435
June1st to 15thDay
34%7%12%25%17%2
2479
16th to endDay
30%7%14%23%18%5%
2520
July1st to 15thDay
41%8%13%20%11%4
2371
16th to endDay
35%8%18%23%12%1
2395
August1st to 15thDay
36%8%15%21%16%2
2172
16th to endDay
38%7%12%21%17%2
2292
September1st to 15thDay
42%7%11%20%14%3
2045
16th to endDay
37%5%7%19%24%4
1815
October1st to 15thDay
34%47%23%24%5%
1813
16th to endDay
35%35%23%26%5%
1663
November1st to 15thDay
33%37%19%28%7%
1582
16th to endDay
39%37%26%18%3
1620
December1st to 15thDay
49%48%20%14%2
1620
16th to endDay
49%412%20%12%1
1578
January1st to 15thNight
49%37%20%16%2
2506
16th to endNight
48%48%22%13%2
2463
February1st to 15thNight
50%39%21%13%1
2112
16th to endNight
48%48%22%13%2
1893
March1st to 15thNight
44%5%9%19%16%4
2160
16th to endNight
45%5%11%23%12%1
2027
April1st to 15thNight
33%410%25%21%4
1797
16th to endNight
36%5%8%25%20%2
1791
May1st to 15thNight
42%5%9%23%15%2
1620
16th to endNight
31%8%17%22%16%3
1684
June1st to 15thNight
28%6%16%26%19%2
1476
16th to endNight
23%8%15%28%18%5%
1440
July1st to 15thNight
32%8%13%26%15%4
1360
16th to endNight
25%8%17%30%16%1
1508
August1st to 15thNight
26%7%16%26%21%1
1761
16th to endNight
28%7%14%26%19%4
1910
September1st to 15thNight
32%7%12%25%17%5%
1791
16th to endNight
33%47%19%27%7%
1815
October1st to 15thNight
30%38%21%29%6%
1815
16th to endNight
36%25%23%26%6%
2208
November1st to 15thNight
31%35%23%26%9%
2276
16th to endNight
38%37%25%20%4
2340
December1st to 15thNight
49%47%19%15%3
2340
16th to endNight
50%48%19%14%2
2643

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
31%8%40%8%8%2
90
16th to endDay
18%10%46%19%4
96
February1st to 15thDay
26%15%37%14%41
90
16th to endDay
24%11%43%12%7%1
81
March1st to 15thDay
27%11%37%18%4
90
16th to endDay
19%8%28%28%12%1
270
April1st to 15thDay
14%11%28%28%14%3
270
16th to endDay
18%327%25%19%5%
270
May1st to 15thDay
18%9%23%24%15%7%
288
16th to endDay
16%117%24%23%16%
331
June1st to 15thDay
8%112%23%27%27%
454
16th to endDay
6%112%31%30%17%
495
July1st to 15thDay
12%29%29%26%18%
475
16th to endDay
8%2416%32%35%
414
August1st to 15thDay
10%16%18%35%27%
313
16th to endDay
12%1421%38%21%
332
September1st to 15thDay
12%15%20%34%25%
294
16th to endDay
18%27%20%37%14%
254
October1st to 15thDay
13%219%25%24%14%
253
16th to endDay
16%6%18%26%27%5%
96
November1st to 15thDay
18%6%18%22%26%6%
90
16th to endDay
27%6%31%17%14%2
90
December1st to 15thDay
18%10%42%14%12%2
90
16th to endDay
17%14%48%15%3
96
January1st to 15thNight
23%10%32%27%5%
900
16th to endNight
22%7%34%31%4
864
February1st to 15thNight
24%9%34%27%3
704
16th to endNight
21%10%37%27%3
631
March1st to 15thNight
20%8%32%33%4
720
16th to endNight
18%7%28%32%11%1
665
April1st to 15thNight
15%6%32%33%10%1
539
16th to endNight
15%229%37%15%
540
May1st to 15thNight
16%322%37%16%2
540
16th to endNight
6%312%39%31%5%
563
June1st to 15thNight
8%18%40%36%4
540
16th to endNight
38%38%42%6%
540
July1st to 15thNight
9%15%31%46%6%
510
16th to endNight
6%1427%48%11%
564
August1st to 15thNight
7%429%48%9%
539
16th to endNight
6%336%45%7%
573
September1st to 15thNight
14%18%25%41%9%
566
16th to endNight
12%214%27%38%4
657
October1st to 15thNight
14%216%31%28%6%
660
16th to endNight
19%222%33%19%2
704
November1st to 15thNight
15%5%23%31%20%2
700
16th to endNight
19%6%28%33%11%
720
December1st to 15thNight
27%6%25%29%10%
720
16th to endNight
25%7%29%31%5%
915
Page generated at 2017-09-11 18:35:33. Version ac04

Copyright 2017 A. Danko


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