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 2018-02-22.

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%
16265
23%
48803
17%
16221
55%
16221
29%
30634

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
28%
974
18%
2716
10%
970
36%
970
22%
1875
January16th to endNight
30%
936
21%
2668
11%
936
38%
936
22%
1837
February1st to 15thNight
26%
764
17%
2292
9%
761
33%
761
20%
1610
February16th to endNight
25%
659
20%
1977
9%
659
33%
659
20%
1368
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
66%
585
23%
1845
24%
585
68%
585
32%
1100
September16th to endNight
65%
717
31%
1980
26%
717
67%
717
38%
1172
October1st to 15thNight
59%
720
33%
1980
26%
720
64%
720
40%
1161
October16th to endNight
51%
768
30%
2409
24%
768
59%
768
34%
1400
November1st to 15thNight
49%
760
30%
2471
27%
760
58%
760
33%
1417
November16th to endNight
44%
780
27%
2535
23%
780
52%
780
29%
1499
December1st to 15thNight
35%
780
19%
2535
15%
780
42%
780
20%
1712
December16th to endNight
31%
991
19%
2863
11%
991
39%
991
20%
2028

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%3445%7%41%
49445
Night
13%6%24424447%45%
48988
TransparencyDay
41%6%10%22%16%3
49132
Night
38%5%10%23%18%3
48805
SeeingDay
15%418%23%23%14%
6151
Night
16%5%22%32%20%3
16268

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
19%8%35%43435%5%35%
1552
16th to endDay
14%8%35%423346%40%
1902
February1st to 15thDay
17%9%36%424445%36%
1900
16th to endDay
17%9%46%5%3335%6%34%
1643
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%4345%36%5%5%7%39%
2103
16th to endDay
11%5%24424347%48%
1980
October1st to 15thDay
12%414434345%50%
1978
16th to endDay
14%423424325%53%
1814
November1st to 15thDay
15%5%2431235%6%50%
1717
16th to endDay
15%434323345%48%
1755
December1st to 15thDay
21%7%25%423446%37%
1755
16th to endDay
20%10%25%413346%35%
1709
January1st to 15thNight
18%8%35%5%34335%38%
2716
16th to endNight
17%8%46%5%23336%38%
2668
February1st to 15thNight
21%9%36%42334435%
2283
16th to endNight
17%8%46%5%34446%36%
1977
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
13%32441347%8%46%
1845
16th to endNight
9%5%24423446%53%
1980
October1st to 15thNight
11%413323346%55%
1980
16th to endNight
15%5%23322235%53%
2409
November1st to 15thNight
14%423313346%51%
2471
16th to endNight
14%6%23323335%49%
2535
December1st to 15thNight
19%10%25%423436%37%
2535
16th to endNight
22%8%35%423435%36%
2863

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%22%13%2
1553
16th to endDay
43%5%8%26%13%1
1902
February1st to 15thDay
49%39%23%11%2
1902
16th to endDay
50%311%20%12%2
1643
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
43%7%10%20%14%3
2103
16th to endDay
35%5%9%21%23%4
1980
October1st to 15thDay
35%48%23%22%4
1978
16th to endDay
34%35%22%27%5%
1814
November1st to 15thDay
35%37%19%26%6%
1717
16th to endDay
37%37%26%20%3
1755
December1st to 15thDay
47%49%21%14%2
1755
16th to endDay
50%412%19%11%1
1709
January1st to 15thNight
49%27%20%16%2
2716
16th to endNight
48%48%21%15%2
2668
February1st to 15thNight
51%39%21%12%1
2292
16th to endNight
49%5%8%21%13%2
1977
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
33%6%11%24%17%5%
1845
16th to endNight
31%48%21%27%6%
1980
October1st to 15thNight
30%38%22%28%6%
1980
16th to endNight
35%25%23%26%5%
2409
November1st to 15thNight
33%36%22%25%8%
2471
16th to endNight
37%37%24%22%4
2535
December1st to 15thNight
48%47%20%15%3
2535
16th to endNight
51%49%19%13%1
2863

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
27%8%42%11%7%1
135
16th to endDay
17%12%42%21%5%
144
February1st to 15thDay
29%14%34%14%5%
135
16th to endDay
29%11%41%12%6%1
100
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
15%16%19%33%25%
315
16th to endDay
15%27%19%37%17%
314
October1st to 15thDay
15%217%26%24%13%
313
16th to endDay
13%7%27%22%24%4
144
November1st to 15thDay
23%8%17%25%20%4
135
16th to endDay
21%5%28%22%18%2
135
December1st to 15thDay
14%10%43%17%11%1
135
16th to endDay
23%13%45%15%2
144
January1st to 15thNight
23%10%32%27%5%
975
16th to endNight
22%7%32%32%4
936
February1st to 15thNight
25%9%33%27%4
764
16th to endNight
20%10%37%27%3
659
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
16%17%24%40%9%
585
16th to endNight
11%113%26%39%5%
717
October1st to 15thNight
14%216%32%28%6%
720
16th to endNight
19%222%32%20%2
768
November1st to 15thNight
16%5%23%32%20%2
760
16th to endNight
18%6%27%33%13%
780
December1st to 15thNight
25%7%25%30%9%
780
16th to endNight
26%7%29%30%5%
991
Page generated at 2018-02-23 10:21:34. Version ac04

Copyright 2018 A. Danko


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