Leaking Light Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Leaking Light Observatory clear sky chart from 2005-11-06 to 2024-03-06.

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
38%
24254
11%
80585
8%
24151
42%
24151
10%
63974

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 10thNight
18%
755
8%
4079
4%
737
24%
737
3%
3355
January11th to 20thNight
17%
759
6%
4074
3%
740
20%
740
3%
3398
January21st to endNight
18%
834
5%
4506
4%
814
24%
814
4%
3865
February1st to 10thNight
24%
760
9%
2470
5%
760
29%
760
8%
2021
February11th to 20thNight
16%
740
7%
2317
4%
740
21%
740
7%
1936
February21st to endNight
17%
628
8%
1884
4%
628
21%
628
8%
1571
March1st to 10thNight
25%
743
14%
2064
7%
743
32%
743
12%
1621
March11th to 20thNight
24%
720
11%
1980
7%
720
27%
720
10%
1634
March21st to endNight
23%
792
11%
2178
5%
792
29%
792
10%
1766
April1st to 10thNight
25%
720
11%
1957
4%
720
31%
720
13%
1535
April11th to 20thNight
31%
633
16%
1697
7%
633
37%
633
15%
1304
April21st to endNight
32%
540
14%
1620
7%
540
37%
540
12%
1270
May1st to 10thNight
35%
540
9%
1620
4%
540
39%
540
10%
1322
May11th to 20thNight
39%
531
11%
1593
6%
531
42%
531
12%
1287
May21st to endNight
54%
582
12%
1745
8%
582
57%
582
16%
1316
June1st to 10thNight
56%
540
12%
1620
10%
540
58%
540
17%
1200
June11th to 20thNight
55%
540
11%
1620
9%
540
57%
540
16%
1230
June21st to endNight
58%
540
12%
1620
11%
540
60%
540
17%
1200
July1st to 10thNight
60%
525
10%
1575
9%
525
61%
525
15%
1202
July11th to 20thNight
62%
522
12%
1566
8%
522
65%
522
17%
1164
July21st to endNight
61%
591
10%
1672
10%
558
63%
558
14%
1295
August1st to 10thNight
66%
537
12%
1600
11%
534
66%
534
16%
1207
August11th to 20thNight
63%
534
15%
1598
12%
534
65%
534
17%
1197
August21st to endNight
66%
650
14%
1838
11%
650
67%
650
18%
1361
September1st to 10thNight
63%
720
16%
1908
11%
720
64%
720
20%
1423
September11th to 20thNight
63%
720
20%
1980
15%
720
66%
720
20%
1452
September21st to endNight
49%
720
13%
1980
9%
720
53%
720
14%
1553
October1st to 10thNight
52%
720
12%
2096
10%
720
55%
720
16%
1565
October11th to 20thNight
39%
720
13%
2160
9%
720
45%
720
13%
1673
October21st to endNight
37%
792
14%
2376
13%
792
40%
792
12%
1868
November1st to 10thNight
43%
740
16%
2338
14%
740
47%
740
16%
1704
November11th to 20thNight
31%
760
14%
2470
9%
760
39%
760
10%
1958
November21st to endNight
30%
760
11%
2522
7%
760
35%
760
9%
2032
December1st to 10thNight
22%
760
7%
2660
4%
760
25%
760
6%
2245
December11th to 20thNight
24%
759
8%
3117
6%
759
28%
759
6%
2530
December21st to endNight
22%
827
5%
4485
2%
817
26%
817
3%
3714

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
17%10%47%6%345%5%7%33%
78527
Night
17%10%47%5%3445%6%35%
80980
TransparencyDay
34%36%7%13%8%1
78249
Night
35%35%7%13%8%1
80777
SeeingDay
22%13%26%22%13%5%
13895
Night
21%10%24%28%14%2
24277

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 10thDay
20%18%8%9%6%23346%21%
2836
11th to 20thDay
16%18%9%13%9%3333419%
2840
21st to endDay
24%21%7%8%7%3333416%
3124
February1st to 10thDay
23%13%48%7%34345%27%
1710
11th to 20thDay
19%14%5%10%5%35%45%6%26%
1748
21st to endDay
22%12%48%5%33445%30%
1570
March1st to 10thDay
18%10%5%6%433446%36%
2012
11th to 20thDay
19%12%47%5%34446%32%
1980
21st to endDay
21%10%47%5%345%5%5%31%
2178
April1st to 10thDay
15%9%37%6%35%5%5%7%34%
2002
11th to 20thDay
16%9%36%5%3435%7%38%
2227
21st to endDay
20%7%36%6%45%46%7%33%
2340
May1st to 10thDay
19%8%37%6%36%5%6%6%31%
2340
11th to 20thDay
19%7%47%7%35%5%5%7%31%
2301
21st to endDay
10%5%36%6%45%6%7%10%38%
2531
June1st to 10thDay
14%6%25%446%5%6%9%39%
2340
11th to 20thDay
11%6%36%6%35%6%7%9%37%
2340
21st to endDay
13%6%25%5%45%7%6%10%37%
2340
July1st to 10thDay
11%6%36%6%46%6%6%10%36%
2340
11th to 20thDay
11%5%46%6%46%6%6%9%39%
2340
21st to endDay
13%6%35%6%46%6%7%8%36%
2547
August1st to 10thDay
10%6%36%7%46%6%8%11%34%
2327
11th to 20thDay
11%7%26%6%46%7%8%11%33%
2303
21st to endDay
10%5%35%6%45%5%7%9%41%
2498
September1st to 10thDay
15%5%245%3446%6%46%
2049
11th to 20thDay
12%5%25%5%345%6%7%46%
1980
21st to endDay
18%8%35%42445%6%41%
1980
October1st to 10thDay
14%6%245%3446%9%42%
1864
11th to 20thDay
17%8%35%6%245%5%6%40%
1800
21st to endDay
23%7%36%5%34345%38%
1980
November1st to 10thDay
12%6%35%434446%49%
1736
11th to 20thDay
17%12%46%43345%6%36%
1710
21st to endDay
17%11%5%7%434447%34%
1651
December1st to 10thDay
24%15%6%9%5%33345%23%
1520
11th to 20thDay
20%15%6%9%5%23446%25%
2019
21st to endDay
23%18%5%8%6%35%5%5%5%16%
3124
January1st to 10thNight
15%19%6%11%7%4444420%
4109
11th to 20thNight
19%16%7%12%7%4443421%
4120
21st to endNight
27%15%6%9%7%2433320%
4532
February1st to 10thNight
21%13%5%8%6%35%345%28%
2470
11th to 20thNight
22%12%47%6%35%445%27%
2315
21st to endNight
23%14%48%434445%28%
1884
March1st to 10thNight
17%10%36%5%35%345%37%
2064
11th to 20thNight
20%10%47%6%3345%6%33%
1980
21st to endNight
22%9%36%5%2435%6%33%
2178
April1st to 10thNight
19%8%36%43445%6%37%
1957
11th to 20thNight
15%7%35%5%3446%8%39%
1697
21st to endNight
15%7%36%5%335%5%7%39%
1620
May1st to 10thNight
17%7%36%6%46%5%5%7%34%
1620
11th to 20thNight
16%8%46%6%2446%9%34%
1592
21st to endNight
10%6%25%5%35%5%6%9%44%
1744
June1st to 10thNight
10%5%25%5%35%5%5%9%46%
1620
11th to 20thNight
9%5%26%6%35%5%6%10%42%
1620
21st to endNight
13%4244345%8%10%44%
1620
July1st to 10thNight
10%6%36%6%446%6%10%41%
1620
11th to 20thNight
9%435%5%345%6%9%46%
1620
21st to endNight
11%426%6%46%7%8%8%39%
1771
August1st to 10thNight
6%6%25%8%35%6%8%8%42%
1611
11th to 20thNight
8%6%26%5%35%5%7%8%44%
1598
21st to endNight
10%4244345%5%10%49%
1838
September1st to 10thNight
12%415%43445%7%50%
1908
11th to 20thNight
10%5%144245%5%7%52%
1980
21st to endNight
14%6%36%4245%5%6%44%
1980
October1st to 10thNight
13%5%25%435%5%5%7%46%
2096
11th to 20thNight
16%7%15%5%3345%7%44%
2160
21st to endNight
23%7%25%5%23436%39%
2376
November1st to 10thNight
13%7%25%43345%449%
2338
11th to 20thNight
17%9%36%5%3445%7%36%
2470
21st to endNight
19%10%46%5%35%446%35%
2522
December1st to 10thNight
22%15%5%8%6%34445%25%
2660
11th to 20thNight
18%16%7%7%5%34435%28%
3158
21st to endNight
21%18%5%8%6%35%445%20%
4532

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 10thDay
52%26%10%9%3
2837
11th to 20thDay
55%25%6%9%4
2840
21st to endDay
59%23%5%10%2
3124
February1st to 10thDay
39%34%8%13%6%1
1710
11th to 20thDay
40%35%8%10%7%
1757
21st to endDay
42%34%7%12%6%1
1570
March1st to 10thDay
31%36%6%19%8%1
2012
11th to 20thDay
36%38%7%11%7%2
1980
21st to endDay
37%37%6%12%7%1
2178
April1st to 10thDay
30%38%7%12%9%3
2002
11th to 20thDay
31%36%8%10%13%3
2227
21st to endDay
31%38%7%14%8%2
2340
May1st to 10thDay
36%37%6%12%7%1
2340
11th to 20thDay
37%38%6%10%8%1
2305
21st to endDay
25%40%10%15%9%1
2531
June1st to 10thDay
26%39%9%14%10%2
2340
11th to 20thDay
26%39%10%14%9%1
2340
21st to endDay
27%39%10%14%9%1
2340
July1st to 10thDay
26%41%9%12%11%1
2275
11th to 20thDay
27%39%8%15%10%1
2271
21st to endDay
27%41%8%16%7%2
2405
August1st to 10thDay
26%39%11%15%8%
2314
11th to 20thDay
27%38%8%17%9%1
2303
21st to endDay
26%39%8%15%8%4
2498
September1st to 10thDay
25%36%5%15%15%3
2049
11th to 20thDay
25%38%6%18%12%2
1980
21st to endDay
31%37%6%13%12%1
1980
October1st to 10thDay
26%37%9%15%12%1
1864
11th to 20thDay
31%37%7%17%8%
1800
21st to endDay
32%37%5%14%11%1
1980
November1st to 10thDay
20%40%6%19%13%2
1736
11th to 20thDay
33%39%6%11%9%2
1710
21st to endDay
32%39%6%13%9%1
1651
December1st to 10thDay
40%40%7%11%2
1520
11th to 20thDay
44%32%6%13%5%
2016
21st to endDay
53%29%10%7%2
3124
January1st to 10thNight
51%27%8%10%3
4109
11th to 20thNight
53%26%9%9%3
4120
21st to endNight
58%23%412%2
4532
February1st to 10thNight
39%35%7%12%7%
2470
11th to 20thNight
40%35%6%11%7%
2317
21st to endNight
40%34%8%11%5%2
1884
March1st to 10thNight
32%35%418%10%1
2064
11th to 20thNight
36%37%6%13%7%1
1980
21st to endNight
34%36%6%13%8%3
2178
April1st to 10thNight
29%37%6%14%12%2
1957
11th to 20thNight
27%36%9%12%11%4
1697
21st to endNight
30%38%6%15%9%2
1620
May1st to 10thNight
33%39%7%11%8%2
1620
11th to 20thNight
32%39%8%10%9%2
1593
21st to endNight
24%38%10%17%9%1
1745
June1st to 10thNight
23%38%7%18%12%2
1620
11th to 20thNight
24%39%10%14%10%2
1620
21st to endNight
22%39%10%16%12%2
1620
July1st to 10thNight
25%39%9%15%10%2
1575
11th to 20thNight
22%40%9%16%12%2
1566
21st to endNight
24%42%10%14%10%1
1672
August1st to 10thNight
23%40%11%15%11%1
1600
11th to 20thNight
24%38%9%15%13%1
1598
21st to endNight
22%39%9%17%9%5%
1838
September1st to 10thNight
22%37%7%14%14%6%
1908
11th to 20thNight
20%38%7%17%15%2
1980
21st to endNight
28%38%7%11%14%1
1980
October1st to 10thNight
24%38%8%16%13%1
2096
11th to 20thNight
29%37%6%16%10%1
2160
21st to endNight
32%36%5%15%11%2
2376
November1st to 10thNight
21%40%5%16%15%3
2338
11th to 20thNight
30%40%6%12%9%2
2470
21st to endNight
33%40%6%13%8%1
2522
December1st to 10thNight
39%40%7%12%2
2660
11th to 20thNight
41%33%8%13%4
3160
21st to endNight
52%27%7%11%3
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
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
33%21%32%10%31
270
11th to 20thDay
38%23%29%9%2
270
21st to endDay
34%26%28%8%4
297
February1st to 10thDay
37%21%29%10%3
270
11th to 20thDay
42%25%27%42
270
21st to endDay
37%22%35%5%1
225
March1st to 10thDay
29%20%28%19%4
417
11th to 20thDay
27%23%30%13%5%1
420
21st to endDay
33%15%30%17%4
462
April1st to 10thDay
25%19%30%16%8%1
420
11th to 20thDay
28%16%27%18%8%3
412
21st to endDay
29%17%30%15%7%2
420
May1st to 10thDay
30%16%25%18%9%3
423
11th to 20thDay
23%11%30%21%12%3
447
21st to endDay
13%9%25%29%18%6%
485
June1st to 10thDay
16%5%26%29%18%5%
450
11th to 20thDay
15%10%22%30%15%7%
450
21st to endDay
14%9%29%26%18%5%
450
July1st to 10thDay
14%420%25%21%16%
445
11th to 20thDay
9%6%22%34%20%9%
444
21st to endDay
18%419%28%20%10%
488
August1st to 10thDay
12%5%17%33%23%11%
446
11th to 20thDay
14%5%18%32%20%12%
439
21st to endDay
13%421%26%25%11%
495
September1st to 10thDay
16%5%18%26%24%10%
450
11th to 20thDay
12%317%28%29%11%
450
21st to endDay
25%9%22%22%18%4
450
October1st to 10thDay
15%8%26%28%18%5%
450
11th to 20thDay
21%12%29%23%12%2
450
21st to endDay
27%15%28%20%9%1
495
November1st to 10thDay
18%14%34%21%9%3
334
11th to 20thDay
27%18%26%20%7%1
270
21st to endDay
23%20%36%18%21
241
December1st to 10thDay
29%18%36%12%31
180
11th to 20thDay
30%21%30%16%3
213
21st to endDay
36%19%27%14%5%
297
January1st to 10thNight
27%20%29%20%4
760
11th to 20thNight
34%15%28%21%3
760
21st to endNight
28%17%31%21%3
836
February1st to 10thNight
27%14%29%28%1
760
11th to 20thNight
32%19%29%18%1
743
21st to endNight
32%17%31%19%2
628
March1st to 10thNight
26%14%30%25%5%
743
11th to 20thNight
27%16%29%20%8%1
720
21st to endNight
28%15%28%22%6%
792
April1st to 10thNight
25%16%31%21%7%
720
11th to 20thNight
19%14%32%23%11%1
633
21st to endNight
19%9%34%31%7%1
540
May1st to 10thNight
21%9%29%28%12%1
540
11th to 20thNight
18%7%28%31%15%1
531
21st to endNight
13%420%34%24%5%
584
June1st to 10thNight
11%324%38%23%1
540
11th to 20thNight
11%420%37%25%4
540
21st to endNight
14%418%38%25%1
540
July1st to 10thNight
12%214%36%30%6%
525
11th to 20thNight
9%415%33%34%5%
522
21st to endNight
13%315%35%31%4
591
August1st to 10thNight
8%310%42%35%2
537
11th to 20thNight
11%313%36%32%6%
534
21st to endNight
13%312%34%33%6%
650
September1st to 10thNight
13%315%33%30%6%
720
11th to 20thNight
12%315%32%34%4
720
21st to endNight
17%6%19%33%21%4
720
October1st to 10thNight
17%5%21%32%23%3
720
11th to 20thNight
20%8%28%28%15%1
720
21st to endNight
26%11%22%29%11%1
792
November1st to 10thNight
17%9%26%33%15%1
740
11th to 20thNight
24%14%26%26%9%1
760
21st to endNight
25%12%26%28%7%1
760
December1st to 10thNight
34%13%25%23%5%
760
11th to 20thNight
25%15%29%25%6%
760
21st to endNight
29%14%29%26%2
836
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