This page made possible by the sponsorship of: Naperville Astronomical Association.

NAA Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the NAA Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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
36%
24600
17%
74705
10%
24547
43%
24547
22%
50542

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
20%
950
14%
2648
3%
946
30%
946
17%
1940
January11th to 20thNight
19%
926
12%
2591
4%
926
25%
926
14%
2008
January21st to endNight
20%
990
11%
2714
4%
990
27%
990
15%
2106
February1st to 10thNight
19%
860
10%
2300
3%
860
24%
860
16%
1747
February11th to 20thNight
24%
700
18%
2099
9%
700
32%
700
18%
1461
February21st to endNight
21%
592
18%
1776
6%
592
32%
592
17%
1265
March1st to 10thNight
23%
720
13%
2160
7%
720
29%
720
15%
1546
March11th to 20thNight
30%
699
19%
2118
10%
699
39%
699
23%
1368
March21st to endNight
25%
594
14%
1980
10%
594
33%
594
15%
1430
April1st to 10thNight
28%
540
16%
1800
10%
540
39%
540
21%
1165
April11th to 20thNight
30%
533
19%
1775
13%
533
40%
533
20%
1182
April21st to endNight
32%
540
15%
1710
7%
540
41%
540
20%
1189
May1st to 10thNight
34%
540
15%
1620
9%
540
41%
540
22%
1115
May11th to 20thNight
39%
531
17%
1477
9%
531
45%
531
25%
985
May21st to endNight
48%
583
16%
1551
9%
582
53%
582
29%
1040
June1st to 10thNight
52%
540
18%
1440
11%
540
57%
540
35%
902
June11th to 20thNight
52%
540
20%
1440
12%
540
57%
540
36%
881
June21st to endNight
48%
540
17%
1440
9%
540
53%
540
30%
956
July1st to 10thNight
64%
525
16%
1400
12%
525
66%
525
39%
856
July11th to 20thNight
62%
522
22%
1392
15%
522
66%
522
41%
817
July21st to endNight
66%
591
23%
1487
16%
558
70%
558
43%
872
August1st to 10thNight
70%
537
22%
1471
18%
534
72%
534
39%
881
August11th to 20thNight
70%
534
22%
1771
23%
534
73%
534
37%
967
August21st to endNight
67%
594
24%
1980
24%
594
72%
594
37%
1066
September1st to 10thNight
59%
540
24%
1800
23%
540
65%
540
32%
1022
September11th to 20thNight
60%
634
33%
1894
29%
634
66%
634
38%
1071
September21st to endNight
51%
720
23%
1980
15%
720
56%
720
32%
1248
October1st to 10thNight
49%
720
25%
2025
18%
720
53%
720
33%
1237
October11th to 20thNight
32%
720
22%
2164
11%
720
44%
720
25%
1372
October21st to endNight
27%
792
17%
2574
10%
792
37%
792
19%
1730
November1st to 10thNight
31%
740
25%
2406
16%
740
44%
740
26%
1448
November11th to 20thNight
26%
760
18%
2470
10%
760
37%
760
18%
1654
November21st to endNight
23%
817
18%
2527
9%
817
34%
817
19%
1729
December1st to 10thNight
23%
950
13%
2660
5%
950
29%
950
18%
1945
December11th to 20thNight
24%
950
14%
3216
5%
948
30%
948
14%
2334
December21st to endNight
15%
1036
6%
4849
3%
1026
22%
1026
6%
4007

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
14%9%47%6%35%5%5%7%36%
74831
Night
14%9%46%5%3445%7%39%
74986
TransparencyDay
47%6%12%21%12%2
74532
Night
46%6%11%21%14%2
74806
SeeingDay
18%11%29%25%13%5%
14633
Night
18%9%30%30%12%1
24613

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
16%12%5%8%7%35%45%6%28%
1517
11th to 20thDay
14%16%7%13%6%3445%5%23%
1480
21st to endDay
16%16%7%11%7%33446%22%
1638
February1st to 10thDay
17%14%6%8%7%46%446%24%
1659
11th to 20thDay
16%11%48%6%445%45%33%
1748
21st to endDay
17%14%6%9%5%3345%6%29%
1480
March1st to 10thDay
17%14%48%6%3345%6%30%
1800
11th to 20thDay
14%9%47%7%35%46%7%35%
1856
21st to endDay
20%11%5%8%7%45%5%5%6%24%
2376
April1st to 10thDay
17%11%5%8%6%35%46%6%31%
2160
11th to 20thDay
17%9%36%5%345%5%7%35%
2134
21st to endDay
20%8%37%6%345%5%7%32%
2250
May1st to 10thDay
18%8%36%5%36%6%6%8%30%
2340
11th to 20thDay
18%8%37%5%345%5%7%34%
2415
21st to endDay
14%6%26%6%35%5%7%8%37%
2725
June1st to 10thDay
10%5%25%445%5%6%8%45%
2520
11th to 20thDay
9%6%25%5%35%5%6%8%45%
2520
21st to endDay
12%5%36%6%47%7%7%9%36%
2520
July1st to 10thDay
7%425%5%35%5%7%9%46%
2520
11th to 20thDay
10%425%5%45%6%7%10%43%
2520
21st to endDay
6%5%26%6%46%6%8%10%42%
2743
August1st to 10thDay
7%5%35%5%446%7%10%42%
2458
11th to 20thDay
8%5%35%5%35%6%8%10%42%
2126
21st to endDay
8%424435%6%7%11%46%
2376
September1st to 10thDay
12%5%25%5%345%5%7%46%
2160
11th to 20thDay
11%424425%45%7%51%
2048
21st to endDay
9%6%35%5%35%5%6%7%45%
1980
October1st to 10thDay
13%6%36%5%34446%46%
1931
11th to 20thDay
18%8%47%5%44447%34%
1795
21st to endDay
21%9%36%5%33447%35%
1782
November1st to 10thDay
12%8%36%4245%5%6%45%
1672
11th to 20thDay
17%10%5%7%5%3435%8%33%
1710
21st to endDay
19%10%48%5%24445%35%
1651
December1st to 10thDay
15%12%5%8%6%35%45%6%31%
1520
11th to 20thDay
13%13%6%10%5%45%45%6%29%
1918
21st to endDay
21%22%9%12%7%3332414%
2783
January1st to 10thNight
15%12%5%8%7%3444434%
2648
11th to 20thNight
16%15%6%9%6%25%445%28%
2591
21st to endNight
16%12%8%10%6%3445%6%26%
2714
February1st to 10thNight
16%14%5%8%7%34445%30%
2300
11th to 20thNight
15%10%5%6%6%345%46%36%
2098
21st to endNight
18%11%48%5%2445%5%35%
1776
March1st to 10thNight
16%11%47%6%45%5%45%34%
2160
11th to 20thNight
14%7%36%5%25%45%7%42%
2118
21st to endNight
21%11%37%6%35%446%30%
1980
April1st to 10thNight
16%9%36%43445%8%38%
1800
11th to 20thNight
19%8%35%5%3345%6%38%
1775
21st to endNight
19%7%36%5%35%46%7%35%
1710
May1st to 10thNight
16%8%27%5%35%6%5%8%36%
1620
11th to 20thNight
15%6%35%6%35%5%5%7%41%
1476
21st to endNight
12%6%25%5%45%5%6%10%41%
1551
June1st to 10thNight
10%435%435%46%7%50%
1440
11th to 20thNight
8%5%24445%5%7%9%47%
1440
21st to endNight
11%5%25%6%36%5%6%8%43%
1440
July1st to 10thNight
7%426%5%25%48%9%50%
1440
11th to 20thNight
6%325%5%245%6%8%53%
1440
21st to endNight
5%3235%345%8%10%51%
1575
August1st to 10thNight
5%325%5%45%5%7%10%50%
1480
11th to 20thNight
7%4243345%7%9%53%
1771
21st to endNight
8%31443445%8%56%
1980
September1st to 10thNight
10%6%2442345%8%51%
1800
11th to 20thNight
9%424434447%55%
1894
21st to endNight
10%5%26%5%345%6%7%47%
1980
October1st to 10thNight
11%5%26%43345%6%50%
2025
11th to 20thNight
18%6%34433347%45%
2164
21st to endNight
21%10%36%423446%37%
2574
November1st to 10thNight
12%7%36%434446%49%
2406
11th to 20thNight
16%10%37%5%3435%7%37%
2470
21st to endNight
17%9%47%5%3435%6%37%
2527
December1st to 10thNight
17%10%5%8%6%34447%32%
2660
11th to 20thNight
16%13%5%9%6%34445%32%
3279
21st to endNight
24%18%6%9%6%35%445%16%
4884

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
57%5%13%16%10%1
1518
11th to 20thDay
63%5%13%13%5%1
1480
21st to endDay
64%5%10%13%8%
1638
February1st to 10thDay
61%6%12%12%8%2
1659
11th to 20thDay
54%410%21%9%2
1758
21st to endDay
57%5%9%19%7%2
1480
March1st to 10thDay
57%5%9%21%7%1
1800
11th to 20thDay
48%6%13%21%11%1
1856
21st to endDay
60%6%10%17%5%1
2376
April1st to 10thDay
53%6%11%18%11%
2160
11th to 20thDay
48%7%10%19%12%4
2134
21st to endDay
52%6%13%16%12%2
2250
May1st to 10thDay
50%8%13%21%8%
2340
11th to 20thDay
49%6%12%21%11%1
2419
21st to endDay
44%8%12%19%16%2
2725
June1st to 10thDay
35%8%11%26%17%2
2520
11th to 20thDay
36%7%10%23%20%3
2520
21st to endDay
41%9%14%20%13%2
2520
July1st to 10thDay
33%8%14%26%15%3
2451
11th to 20thDay
34%8%15%24%16%2
2445
21st to endDay
35%8%16%23%14%3
2591
August1st to 10thDay
34%9%16%28%11%2
2443
11th to 20thDay
34%9%15%25%15%1
2126
21st to endDay
30%8%14%25%19%4
2376
September1st to 10thDay
36%7%13%23%17%4
2160
11th to 20thDay
32%6%11%29%19%3
2048
21st to endDay
36%8%9%25%20%2
1980
October1st to 10thDay
39%5%12%24%17%2
1931
11th to 20thDay
48%5%11%20%13%1
1795
21st to endDay
50%5%11%22%10%2
1782
November1st to 10thDay
39%6%13%24%16%2
1672
11th to 20thDay
51%5%12%21%11%1
1710
21st to endDay
53%410%23%10%
1651
December1st to 10thDay
55%5%13%18%8%1
1520
11th to 20thDay
56%5%12%18%7%1
1915
21st to endDay
78%37%9%3
2783
January1st to 10thNight
53%5%10%19%12%1
2648
11th to 20thNight
59%6%13%14%8%
2591
21st to endNight
59%5%11%15%9%1
2714
February1st to 10thNight
58%411%17%9%2
2300
11th to 20thNight
50%5%10%20%13%1
2099
21st to endNight
53%411%18%11%3
1776
March1st to 10thNight
54%5%10%20%9%1
2160
11th to 20thNight
42%6%13%20%16%3
2118
21st to endNight
55%5%11%18%10%1
1980
April1st to 10thNight
45%5%13%24%12%2
1800
11th to 20thNight
46%5%8%21%15%5%
1775
21st to endNight
47%7%10%20%12%3
1710
May1st to 10thNight
45%7%12%24%11%
1620
11th to 20thNight
42%7%13%22%14%1
1477
21st to endNight
39%8%14%19%17%3
1552
June1st to 10thNight
33%7%11%25%20%4
1440
11th to 20thNight
32%6%13%25%22%2
1440
21st to endNight
38%8%15%21%16%3
1440
July1st to 10thNight
29%8%15%27%17%5%
1400
11th to 20thNight
27%7%14%27%21%4
1392
21st to endNight
25%8%16%26%19%5%
1487
August1st to 10thNight
28%8%17%30%16%2
1471
11th to 20thNight
26%8%16%27%20%2
1771
21st to endNight
28%6%12%29%20%5%
1980
September1st to 10thNight
31%7%11%25%23%3
1800
11th to 20thNight
29%5%8%30%25%2
1894
21st to endNight
35%7%11%29%16%3
1980
October1st to 10thNight
34%6%10%27%21%3
2025
11th to 20thNight
41%5%9%24%20%2
2164
21st to endNight
49%411%23%13%1
2574
November1st to 10thNight
38%5%9%25%19%3
2406
11th to 20thNight
48%412%21%13%2
2470
21st to endNight
50%412%23%12%
2527
December1st to 10thNight
53%5%11%21%9%1
2660
11th to 20thNight
55%412%18%9%1
3281
21st to endNight
71%5%8%11%4
4884

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
26%14%33%22%41
270
11th to 20thDay
27%15%36%16%41
255
21st to endDay
22%22%31%20%5%1
264
February1st to 10thDay
27%20%39%11%3
240
11th to 20thDay
27%19%34%18%2
240
21st to endDay
26%22%37%12%21
198
March1st to 10thDay
28%21%35%15%1
240
11th to 20thDay
30%15%35%15%5%1
260
21st to endDay
30%15%30%18%5%2
462
April1st to 10thDay
25%14%33%21%7%1
420
11th to 20thDay
24%18%25%18%12%3
412
21st to endDay
24%13%30%24%7%1
420
May1st to 10thDay
24%12%30%22%9%2
423
11th to 20thDay
20%11%33%23%10%4
561
21st to endDay
16%10%29%23%15%6%
679
June1st to 10thDay
10%8%30%31%15%6%
630
11th to 20thDay
13%8%30%29%17%4
630
21st to endDay
14%7%31%30%12%4
630
July1st to 10thDay
8%422%30%22%14%
621
11th to 20thDay
13%5%26%29%18%10%
618
21st to endDay
6%5%30%34%19%6%
684
August1st to 10thDay
8%221%35%23%11%
577
11th to 20thDay
11%5%18%29%25%13%
439
21st to endDay
8%5%19%32%25%11%
495
September1st to 10thDay
14%6%21%33%20%6%
450
11th to 20thDay
11%419%32%26%9%
450
21st to endDay
12%12%23%26%19%7%
450
October1st to 10thDay
17%9%24%29%15%5%
401
11th to 20thDay
21%23%34%16%6%
270
21st to endDay
33%14%33%16%5%
297
November1st to 10thDay
17%17%34%21%7%4
270
11th to 20thDay
26%19%33%14%6%1
270
21st to endDay
25%14%36%20%6%
270
December1st to 10thDay
20%18%40%14%8%
270
11th to 20thDay
22%17%39%16%5%1
270
21st to endDay
28%19%37%14%2
297
January1st to 10thNight
20%14%38%23%5%
950
11th to 20thNight
24%14%34%22%5%1
926
21st to endNight
21%12%35%28%41
990
February1st to 10thNight
24%13%35%23%4
860
11th to 20thNight
22%13%34%25%5%
703
21st to endNight
23%15%36%22%4
592
March1st to 10thNight
21%15%34%24%6%
720
11th to 20thNight
17%11%37%27%7%1
699
21st to endNight
25%10%33%24%7%1
594
April1st to 10thNight
20%13%34%26%7%1
540
11th to 20thNight
23%12%30%28%7%
533
21st to endNight
23%7%34%28%8%
540
May1st to 10thNight
19%6%36%30%8%1
540
11th to 20thNight
19%6%30%33%12%1
531
21st to endNight
14%5%27%35%18%1
584
June1st to 10thNight
11%427%43%15%1
540
11th to 20thNight
10%5%28%44%12%1
540
21st to endNight
11%428%38%17%1
540
July1st to 10thNight
7%315%44%28%3
525
11th to 20thNight
7%318%42%27%2
522
21st to endNight
6%218%47%26%2
591
August1st to 10thNight
6%312%46%29%4
537
11th to 20thNight
7%114%42%31%5%
534
21st to endNight
9%216%40%31%3
594
September1st to 10thNight
13%318%38%26%3
540
11th to 20thNight
10%420%35%27%4
634
21st to endNight
13%7%22%33%22%3
720
October1st to 10thNight
13%6%26%34%18%2
720
11th to 20thNight
23%10%31%26%10%1
720
21st to endNight
26%12%29%24%8%
792
November1st to 10thNight
17%10%38%24%10%1
740
11th to 20thNight
20%15%33%25%7%
760
21st to endNight
21%12%37%24%6%
817
December1st to 10thNight
24%11%34%25%5%
950
11th to 20thNight
23%11%33%27%5%
950
21st to endNight
30%14%32%20%3
1045
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