Lake Ahern Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Ahern Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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
60%
24737
18%
74518
15%
24684
64%
24684
27%
48302

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
45%
758
21%
2459
15%
757
55%
757
27%
1477
January11th to 20thNight
50%
737
24%
2393
18%
737
60%
737
29%
1421
January21st to endNight
40%
792
22%
2574
15%
792
50%
792
25%
1598
February1st to 10thNight
49%
720
22%
2164
16%
720
56%
720
30%
1325
February11th to 20thNight
51%
700
23%
2073
14%
699
61%
699
37%
1181
February21st to endNight
53%
592
28%
1628
19%
592
60%
592
39%
959
March1st to 10thNight
53%
720
30%
1980
18%
720
62%
720
42%
1100
March11th to 20thNight
59%
720
27%
1980
20%
720
65%
720
41%
1127
March21st to endNight
56%
792
27%
2178
18%
792
64%
792
39%
1285
April1st to 10thNight
58%
720
24%
1980
19%
720
62%
720
42%
1130
April11th to 20thNight
57%
711
20%
1863
14%
711
61%
711
33%
1200
April21st to endNight
61%
720
25%
1800
16%
720
64%
720
41%
1097
May1st to 10thNight
69%
717
25%
1797
16%
717
71%
717
45%
1068
May11th to 20thNight
67%
531
15%
1592
11%
531
68%
531
29%
1014
May21st to endNight
71%
582
18%
1744
15%
582
71%
582
28%
1145
June1st to 10thNight
64%
540
11%
1620
9%
540
65%
540
20%
1167
June11th to 20thNight
72%
540
13%
1620
11%
540
72%
540
23%
1136
June21st to endNight
73%
540
10%
1620
9%
540
73%
540
23%
1140
July1st to 10thNight
73%
525
5%
1575
4%
525
73%
525
18%
1161
July11th to 20thNight
73%
522
7%
1566
7%
522
73%
522
20%
1131
July21st to endNight
75%
591
9%
1672
9%
558
76%
558
21%
1165
August1st to 10thNight
78%
537
8%
1600
7%
534
78%
534
24%
1116
August11th to 20thNight
75%
636
6%
1700
6%
636
75%
636
24%
1225
August21st to endNight
65%
792
8%
2116
7%
792
65%
792
18%
1619
September1st to 10thNight
66%
720
7%
1980
6%
720
66%
720
17%
1496
September11th to 20thNight
63%
720
5%
1980
5%
720
63%
720
16%
1520
September21st to endNight
62%
720
13%
1980
13%
720
63%
720
19%
1484
October1st to 10thNight
59%
720
17%
2052
17%
720
61%
720
23%
1423
October11th to 20thNight
67%
720
24%
2160
24%
720
71%
720
29%
1350
October21st to endNight
60%
792
23%
2376
21%
792
65%
792
31%
1454
November1st to 10thNight
56%
740
21%
2221
18%
740
61%
740
28%
1415
November11th to 20thNight
55%
760
23%
2280
20%
760
60%
760
27%
1466
November21st to endNight
52%
760
23%
2280
19%
760
58%
760
25%
1490
December1st to 10thNight
60%
760
25%
2470
21%
760
68%
760
30%
1432
December11th to 20thNight
53%
759
19%
2969
18%
758
58%
758
21%
1862
December21st to endNight
52%
831
14%
4476
20%
817
58%
817
15%
2923

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
8%5%36%7%47%7%8%10%36%
74916
Night
8%5%36%6%46%6%7%9%42%
74856
TransparencyDay
37%12%17%17%13%4
74639
Night
36%11%17%19%13%4
74652
SeeingDay
8%417%28%26%17%
14603
Night
9%417%30%30%11%
24750

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
11%6%25%5%345%6%8%44%
1706
11th to 20thDay
6%425%5%25%6%6%10%47%
1656
21st to endDay
11%5%45%5%35%45%8%43%
1782
February1st to 10thDay
11%6%26%6%35%6%5%8%43%
1791
11th to 20thDay
5%426%6%35%47%8%51%
1770
21st to endDay
9%5%25%445%5%6%9%46%
1628
March1st to 10thDay
4425%6%35%46%9%52%
1980
11th to 20thDay
7%3345%35%6%7%8%48%
1980
21st to endDay
6%4245%45%6%6%10%49%
2178
April1st to 10thDay
5%325%5%36%6%7%10%48%
1980
11th to 20thDay
8%5%25%7%46%6%8%10%38%
2044
21st to endDay
5%335%5%46%6%7%9%48%
2160
May1st to 10thDay
5%324435%5%7%9%53%
2175
11th to 20thDay
9%5%36%7%47%6%6%9%38%
2301
21st to endDay
8%5%37%8%5%8%8%7%10%32%
2531
June1st to 10thDay
13%7%38%7%6%8%7%8%11%22%
2340
11th to 20thDay
6%436%8%6%8%10%10%11%27%
2340
21st to endDay
8%6%38%8%6%8%9%9%11%25%
2340
July1st to 10thDay
8%6%37%8%5%9%8%9%11%24%
2340
11th to 20thDay
8%6%38%8%5%8%8%9%11%25%
2340
21st to endDay
7%5%48%9%6%8%10%9%11%24%
2547
August1st to 10thDay
6%426%7%5%9%10%12%12%28%
2327
11th to 20thDay
5%5%37%9%6%9%10%9%13%25%
2182
21st to endDay
10%6%48%9%5%7%8%8%12%23%
2233
September1st to 10thDay
7%6%48%9%5%8%9%8%11%25%
1980
11th to 20thDay
8%7%48%8%6%9%7%10%13%22%
1980
21st to endDay
9%7%37%8%6%8%9%8%9%25%
1980
October1st to 10thDay
11%6%37%8%5%7%6%8%11%28%
1905
11th to 20thDay
7%5%36%6%5%6%6%7%9%40%
1800
21st to endDay
9%425%5%45%5%6%11%43%
1980
November1st to 10thDay
8%436%6%5%6%7%7%11%39%
1857
11th to 20thDay
9%6%45%6%35%6%6%10%39%
1900
21st to endDay
9%5%36%7%36%6%7%9%38%
1896
December1st to 10thDay
8%446%6%45%5%6%9%44%
1710
11th to 20thDay
11%7%35%6%45%47%8%40%
2153
21st to endDay
11%6%47%7%47%8%6%8%32%
3124
January1st to 10thNight
10%5%35%5%345%6%8%46%
2459
11th to 20thNight
8%6%36%5%35%5%6%9%46%
2393
21st to endNight
11%6%36%5%36%5%6%6%44%
2574
February1st to 10thNight
10%6%46%5%345%6%7%46%
2164
11th to 20thNight
9%41443446%8%53%
2073
21st to endNight
7%4244345%5%8%53%
1628
March1st to 10thNight
5%2245%345%5%7%58%
1980
11th to 20thNight
7%42442445%9%54%
1980
21st to endNight
5%324435%6%6%9%52%
2178
April1st to 10thNight
5%3234345%6%8%55%
1980
11th to 20thNight
5%5%26%6%35%6%6%10%46%
1863
21st to endNight
5%3245%45%5%5%9%53%
1800
May1st to 10thNight
5%32342445%9%58%
1797
11th to 20thNight
7%5%25%6%35%7%7%10%43%
1592
21st to endNight
7%6%35%5%5%5%7%8%12%37%
1744
June1st to 10thNight
10%7%37%7%47%7%8%12%27%
1620
11th to 20thNight
6%5%37%7%46%7%9%14%32%
1620
21st to endNight
5%6%27%7%5%7%6%8%12%35%
1620
July1st to 10thNight
8%6%46%6%5%7%8%9%11%31%
1620
11th to 20thNight
7%5%46%7%5%7%8%8%11%33%
1620
21st to endNight
5%5%37%7%47%7%9%13%35%
1771
August1st to 10thNight
5%336%6%47%7%10%13%36%
1611
11th to 20thNight
45%26%7%47%8%10%12%34%
1700
21st to endNight
8%7%37%7%5%7%8%8%12%28%
2116
September1st to 10thNight
6%5%47%8%5%7%8%9%12%28%
1980
11th to 20thNight
8%7%39%8%5%7%7%8%11%27%
1980
21st to endNight
10%7%38%8%47%7%8%9%29%
1980
October1st to 10thNight
10%7%37%5%45%6%7%10%36%
2052
11th to 20thNight
6%5%36%5%45%5%6%8%45%
2160
21st to endNight
8%6%26%436%5%7%9%45%
2376
November1st to 10thNight
7%6%36%6%45%5%6%7%44%
2221
11th to 20thNight
8%5%36%6%45%5%7%9%41%
2280
21st to endNight
10%5%37%5%45%6%6%9%39%
2280
December1st to 10thNight
5%3246%46%6%7%9%47%
2470
11th to 20thNight
13%7%35%5%35%45%8%41%
3042
21st to endNight
11%8%48%7%46%6%6%8%34%
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
36%7%12%20%18%7%
1707
11th to 20thDay
31%6%9%26%22%6%
1656
21st to endDay
39%6%9%17%20%9%
1782
February1st to 10thDay
37%6%13%17%20%8%
1791
11th to 20thDay
27%6%14%22%23%8%
1780
21st to endDay
32%6%13%16%26%6%
1628
March1st to 10thDay
26%8%13%26%21%6%
1980
11th to 20thDay
29%7%16%26%16%6%
1980
21st to endDay
27%9%13%23%22%7%
2178
April1st to 10thDay
27%9%18%25%16%5%
1980
11th to 20thDay
36%11%13%18%17%4
2044
21st to endDay
28%8%13%23%23%6%
2160
May1st to 10thDay
24%7%15%23%24%7%
2175
11th to 20thDay
38%10%16%20%14%1
2305
21st to endDay
41%12%14%18%13%1
2531
June1st to 10thDay
49%12%21%14%4
2340
11th to 20thDay
39%17%25%17%2
2340
21st to endDay
43%16%25%13%3
2340
July1st to 10thDay
44%18%26%10%2
2275
11th to 20thDay
44%18%26%11%1
2271
21st to endDay
42%19%24%14%1
2405
August1st to 10thDay
36%20%33%11%1
2314
11th to 20thDay
38%21%29%10%1
2182
21st to endDay
45%19%23%10%3
2233
September1st to 10thDay
44%19%23%11%3
1980
11th to 20thDay
44%19%23%11%3
1980
21st to endDay
46%15%16%11%10%1
1980
October1st to 10thDay
43%12%15%14%11%4
1905
11th to 20thDay
36%7%12%21%20%3
1800
21st to endDay
32%7%14%19%21%8%
1980
November1st to 10thDay
34%9%14%19%18%7%
1857
11th to 20thDay
38%9%12%20%16%6%
1900
21st to endDay
38%8%13%19%18%4
1896
December1st to 10thDay
34%8%16%24%14%4
1710
11th to 20thDay
38%9%12%17%17%8%
2150
21st to endDay
44%11%13%15%12%6%
3124
January1st to 10thNight
35%7%13%20%17%8%
2459
11th to 20thNight
34%7%11%25%18%6%
2393
21st to endNight
38%7%11%21%14%9%
2574
February1st to 10thNight
35%7%14%17%17%10%
2164
11th to 20thNight
28%6%14%21%21%9%
2074
21st to endNight
27%9%14%21%24%6%
1628
March1st to 10thNight
24%7%13%28%22%6%
1980
11th to 20thNight
26%6%17%28%16%6%
1980
21st to endNight
26%9%13%26%19%7%
2178
April1st to 10thNight
24%8%20%26%16%7%
1980
11th to 20thNight
31%11%14%22%18%4
1863
21st to endNight
27%8%14%24%21%6%
1800
May1st to 10thNight
23%9%13%25%23%8%
1797
11th to 20thNight
34%11%20%22%12%1
1593
21st to endNight
35%12%18%19%15%1
1745
June1st to 10thNight
46%11%22%16%4
1620
11th to 20thNight
39%15%22%21%3
1620
21st to endNight
40%14%24%17%5%
1620
July1st to 10thNight
42%18%27%12%1
1575
11th to 20thNight
41%16%28%14%1
1566
21st to endNight
35%20%30%14%1
1672
August1st to 10thNight
33%20%33%13%
1600
11th to 20thNight
36%19%32%12%2
1700
21st to endNight
46%16%23%12%2
2116
September1st to 10thNight
42%21%21%14%3
1980
11th to 20thNight
47%17%20%14%2
1980
21st to endNight
46%14%16%14%8%2
1980
October1st to 10thNight
41%12%14%18%11%4
2052
11th to 20thNight
36%8%12%23%18%3
2160
21st to endNight
33%9%13%19%19%8%
2376
November1st to 10thNight
37%9%14%16%18%7%
2221
11th to 20thNight
37%9%13%20%14%7%
2280
21st to endNight
39%9%11%20%17%4
2280
December1st to 10thNight
30%9%16%28%11%5%
2470
11th to 20thNight
40%8%13%16%15%9%
3044
21st to endNight
44%10%12%17%12%5%
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
14%6%41%24%13%2
270
11th to 20thDay
5%10%37%30%15%4
252
21st to endDay
13%12%42%24%8%
264
February1st to 10thDay
12%9%32%30%14%4
240
11th to 20thDay
5%6%30%39%15%5%
262
21st to endDay
8%5%28%35%19%4
346
March1st to 10thDay
6%8%33%38%13%2
420
11th to 20thDay
9%7%27%34%20%3
420
21st to endDay
6%427%34%23%4
462
April1st to 10thDay
6%5%32%35%17%5%
420
11th to 20thDay
7%8%26%32%22%5%
412
21st to endDay
6%5%21%36%24%9%
420
May1st to 10thDay
6%222%39%22%9%
423
11th to 20thDay
11%312%30%30%14%
447
21st to endDay
9%38%27%31%22%
485
June1st to 10thDay
14%19%26%28%22%
450
11th to 20thDay
6%15%24%32%33%
450
21st to endDay
6%2417%32%40%
450
July1st to 10thDay
6%239%35%47%
445
11th to 20thDay
5%229%41%41%
444
21st to endDay
7%1417%32%38%
488
August1st to 10thDay
42313%40%37%
446
11th to 20thDay
5%1314%33%44%
439
21st to endDay
8%15%21%31%33%
495
September1st to 10thDay
8%35%15%34%36%
450
11th to 20thDay
8%18%25%30%29%
450
21st to endDay
9%17%22%34%27%
450
October1st to 10thDay
14%316%33%26%9%
450
11th to 20thDay
6%315%38%31%7%
450
21st to endDay
8%318%38%25%8%
495
November1st to 10thDay
9%8%18%36%22%7%
455
11th to 20thDay
12%425%34%20%4
460
21st to endDay
9%423%39%21%3
456
December1st to 10thDay
5%6%29%38%18%4
270
11th to 20thDay
9%9%27%34%18%3
270
21st to endDay
8%8%24%39%19%3
297
January1st to 10thNight
14%8%27%33%17%2
760
11th to 20thNight
10%7%27%35%19%2
737
21st to endNight
12%8%31%36%13%
792
February1st to 10thNight
12%7%24%37%20%1
720
11th to 20thNight
9%6%30%36%17%2
703
21st to endNight
10%327%42%18%
592
March1st to 10thNight
5%7%30%41%16%1
720
11th to 20thNight
7%5%24%40%22%3
720
21st to endNight
6%427%40%22%2
792
April1st to 10thNight
6%426%40%22%2
720
11th to 20thNight
7%5%24%40%23%2
711
21st to endNight
6%322%43%23%3
720
May1st to 10thNight
6%215%44%30%2
717
11th to 20thNight
9%211%32%39%8%
531
21st to endNight
8%17%26%45%13%
584
June1st to 10thNight
12%16%25%44%12%
540
11th to 20thNight
8%1418%44%26%
540
21st to endNight
6%414%46%29%
540
July1st to 10thNight
9%1311%46%31%
525
11th to 20thNight
7%128%43%39%
522
21st to endNight
5%139%47%36%
591
August1st to 10thNight
4311%48%34%
537
11th to 20thNight
439%46%38%
636
21st to endNight
11%15%16%39%27%
792
September1st to 10thNight
8%15%15%45%26%
720
11th to 20thNight
11%16%20%43%20%
720
21st to endNight
13%35%21%40%18%
720
October1st to 10thNight
14%112%30%34%8%
720
11th to 20thNight
7%411%30%39%9%
720
21st to endNight
11%317%32%32%6%
792
November1st to 10thNight
9%420%33%27%6%
740
11th to 20thNight
7%6%23%35%25%4
760
21st to endNight
11%422%39%21%2
760
December1st to 10thNight
6%421%36%29%4
760
11th to 20thNight
11%7%21%33%23%4
760
21st to endNight
10%6%23%34%24%2
836
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