Champaign/Urbana Astronomical Society Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Champaign/Urbana Astronomical Society Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
40%
15765
19%
46473
12%
15721
46%
15721
25%
30666

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
22%
900
16%
2506
6%
895
32%
895
19%
1796
January16th to endNight
21%
960
14%
2607
5%
960
28%
960
18%
1929
February1st to 15thNight
24%
768
13%
2177
5%
766
31%
766
18%
1560
February16th to endNight
25%
631
18%
1893
8%
631
34%
631
21%
1292
March1st to 15thNight
26%
720
14%
2160
8%
720
33%
720
18%
1536
March16th to endNight
30%
615
15%
2008
9%
615
38%
615
20%
1364
April1st to 15thNight
31%
539
19%
1797
11%
539
43%
539
21%
1170
April16th to endNight
35%
540
19%
1770
13%
540
44%
540
22%
1166
May1st to 15thNight
37%
540
15%
1620
8%
540
43%
540
22%
1118
May16th to endNight
52%
562
18%
1521
12%
561
56%
561
31%
999
June1st to 15thNight
59%
540
19%
1440
14%
540
63%
540
39%
887
June16th to endNight
58%
540
19%
1440
14%
540
62%
540
36%
900
July1st to 15thNight
64%
510
18%
1360
15%
510
66%
510
39%
820
July16th to endNight
76%
564
27%
1417
21%
531
79%
531
48%
778
August1st to 15thNight
75%
527
22%
1623
24%
524
75%
524
38%
920
August16th to endNight
72%
525
21%
1750
22%
525
76%
525
33%
992
September1st to 15thNight
65%
498
29%
1653
30%
498
68%
498
32%
931
September16th to endNight
60%
657
28%
1815
21%
657
64%
657
36%
1095
October1st to 15thNight
54%
660
29%
1892
22%
660
60%
660
36%
1106
October16th to endNight
32%
704
21%
2215
12%
704
42%
704
24%
1400
November1st to 15thNight
36%
700
26%
2276
17%
700
48%
700
28%
1335
November16th to endNight
30%
720
19%
2340
13%
720
39%
720
20%
1548
December1st to 15thNight
24%
885
12%
2505
6%
885
30%
885
17%
1880
December16th to endNight
18%
960
12%
2688
4%
960
26%
960
12%
2144

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%7%36%5%3445%7%38%
47504
Night
14%7%35%5%24446%41%
46642
Cloud CoverDay
14%7%36%5%3445%7%38%
47513
Night
14%7%35%424446%41%
46647
TransparencyDay
44%5%12%21%12%2
47190
Night
43%411%22%15%2
46474
SeeingDay
15%6%24%25%20%8%
5751
Night
18%7%27%30%14%1
15767

By Month

Percentage Cloud Cover NE (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
17%12%5%8%7%45%336%24%
1432
16th to endDay
14%12%7%9%6%35%345%27%
1616
February1st to 15thDay
14%11%48%6%35%445%30%
1684
16th to endDay
13%13%5%9%6%24345%29%
1580
March1st to 15thDay
18%10%47%5%24446%31%
1790
16th to endDay
16%10%47%5%335%47%30%
2224
April1st to 15thDay
16%8%45%43345%6%36%
2151
16th to endDay
18%7%26%42445%5%35%
2190
May1st to 15thDay
19%7%35%6%3445%6%31%
2331
16th to endDay
12%426%5%45%5%5%8%38%
2591
June1st to 15thDay
9%425%435%5%6%8%45%
2520
16th to endDay
8%435%5%35%5%6%10%41%
2520
July1st to 15thDay
10%424435%5%7%9%41%
2520
16th to endDay
5%325%5%45%6%7%9%44%
2655
August1st to 15thDay
7%4245%35%5%7%9%45%
2238
16th to endDay
8%4245%346%6%9%43%
2100
September1st to 15thDay
13%5%245%345%5%6%42%
1964
16th to endDay
11%5%24424349%47%
1815
October1st to 15thDay
15%5%25%42445%6%44%
1737
16th to endDay
16%7%3442245%6%42%
1656
November1st to 15thDay
14%6%24313346%47%
1582
16th to endDay
18%7%46%423345%40%
1620
December1st to 15thDay
21%10%5%8%424445%26%
1452
16th to endDay
21%13%6%6%6%35%345%24%
1536
January1st to 15thNight
15%10%48%5%23445%34%
2506
16th to endNight
15%12%5%8%5%3334432%
2607
February1st to 15thNight
16%9%5%7%5%34346%32%
2175
16th to endNight
15%9%46%5%24345%37%
1893
March1st to 15thNight
18%10%35%424445%35%
2147
16th to endNight
16%8%26%5%245%5%8%35%
2008
April1st to 15thNight
15%6%35%5%34446%40%
1797
16th to endNight
18%7%25%434345%40%
1770
May1st to 15thNight
19%6%35%434446%37%
1620
16th to endNight
10%425%5%25%5%6%7%43%
1520
June1st to 15thNight
9%4144245%6%7%49%
1440
16th to endNight
7%4235%25%5%5%9%46%
1440
July1st to 15thNight
7%4143345%5%10%49%
1440
16th to endNight
33134145%5%9%57%
1513
August1st to 15thNight
4314425%6%6%9%50%
1632
16th to endNight
6%315%435%5%5%7%52%
1750
September1st to 15thNight
11%323423348%51%
1653
16th to endNight
8%42442445%7%50%
1815
October1st to 15thNight
12%42242335%6%53%
1892
16th to endNight
17%6%25%424345%43%
2215
November1st to 15thNight
11%6%23424345%49%
2276
16th to endNight
17%8%35%5%23346%38%
2340
December1st to 15thNight
21%10%47%5%24335%30%
2505
16th to endNight
23%13%5%7%6%3433425%
2688

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
17%12%5%8%8%45%336%24%
1432
16th to endDay
14%12%7%9%6%25%345%27%
1616
February1st to 15thDay
14%10%5%8%6%35%446%30%
1684
16th to endDay
13%12%49%7%34345%29%
1580
March1st to 15thDay
18%10%47%5%24445%31%
1800
16th to endDay
16%10%47%5%335%47%31%
2223
April1st to 15thDay
17%8%35%43336%6%36%
2151
16th to endDay
19%7%26%5%25%45%5%35%
2190
May1st to 15thDay
20%7%35%5%3445%6%32%
2331
16th to endDay
12%5%27%5%35%45%8%38%
2591
June1st to 15thDay
9%425%435%5%6%8%45%
2520
16th to endDay
9%425%435%5%6%10%41%
2520
July1st to 15thDay
10%5%15%435%5%7%9%41%
2520
16th to endDay
5%3245%45%6%7%9%44%
2655
August1st to 15thDay
7%4144345%7%9%45%
2238
16th to endDay
8%4245%346%6%9%43%
2100
September1st to 15thDay
13%5%246%345%5%6%43%
1964
16th to endDay
11%5%2443435%8%47%
1815
October1st to 15thDay
15%5%25%32345%6%44%
1737
16th to endDay
16%7%25%42245%6%42%
1656
November1st to 15thDay
15%5%34413346%47%
1582
16th to endDay
18%7%45%423345%40%
1620
December1st to 15thDay
21%10%48%424446%26%
1452
16th to endDay
21%13%6%7%6%24345%24%
1536
January1st to 15thNight
15%10%47%5%23445%34%
2506
16th to endNight
15%12%5%8%5%3434432%
2607
February1st to 15thNight
16%9%5%7%5%34346%32%
2168
16th to endNight
15%9%46%5%24445%37%
1893
March1st to 15thNight
19%10%35%5%24445%35%
2160
16th to endNight
17%7%26%5%245%5%7%35%
2008
April1st to 15thNight
15%6%35%5%34446%40%
1797
16th to endNight
18%7%25%434445%40%
1770
May1st to 15thNight
18%6%35%43435%7%37%
1619
16th to endNight
10%425%5%245%6%7%43%
1520
June1st to 15thNight
9%323425%47%7%50%
1440
16th to endNight
7%424435%5%6%9%46%
1440
July1st to 15thNight
7%4144245%5%10%49%
1440
16th to endNight
331341445%9%57%
1513
August1st to 15thNight
4314425%6%6%9%50%
1632
16th to endNight
6%315%435%45%7%52%
1750
September1st to 15thNight
11%324423348%51%
1653
16th to endNight
8%425%42445%7%50%
1815
October1st to 15thNight
12%42232335%6%53%
1892
16th to endNight
17%6%25%414345%43%
2215
November1st to 15thNight
12%6%23424345%50%
2276
16th to endNight
17%8%35%5%23336%39%
2340
December1st to 15thNight
21%11%47%5%24335%30%
2505
16th to endNight
23%12%5%8%5%3433424%
2688

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
61%39%15%8%
1433
16th to endDay
58%413%17%6%
1616
February1st to 15thDay
56%412%17%7%1
1686
16th to endDay
56%411%19%6%
1580
March1st to 15thDay
54%5%12%20%6%
1800
16th to endDay
52%5%13%21%5%1
2224
April1st to 15thDay
47%411%23%12%1
2151
16th to endDay
49%5%12%19%9%3
2190
May1st to 15thDay
51%6%11%20%9%
2331
16th to endDay
42%8%12%17%15%2
2595
June1st to 15thDay
34%8%12%25%16%2
2520
16th to endDay
37%6%13%21%16%3
2520
July1st to 15thDay
38%6%12%24%15%3
2371
16th to endDay
32%9%17%25%14%1
2488
August1st to 15thDay
32%7%14%26%16%2
2223
16th to endDay
35%7%12%23%14%7%
2100
September1st to 15thDay
41%5%8%21%18%5%
1964
16th to endDay
34%49%25%23%2
1815
October1st to 15thDay
38%5%10%26%17%2
1737
16th to endDay
41%412%24%15%1
1656
November1st to 15thDay
37%412%21%18%4
1582
16th to endDay
47%310%23%12%1
1620
December1st to 15thDay
59%311%17%7%
1452
16th to endDay
61%5%10%15%6%
1536
January1st to 15thNight
51%49%20%12%
2506
16th to endNight
54%313%19%8%
2607
February1st to 15thNight
52%313%18%9%1
2177
16th to endNight
48%312%24%9%
1893
March1st to 15thNight
51%5%10%20%10%1
2160
16th to endNight
47%6%14%21%9%1
2008
April1st to 15thNight
44%48%23%16%2
1797
16th to endNight
46%48%21%14%5%
1770
May1st to 15thNight
46%5%12%22%11%
1620
16th to endNight
36%8%15%18%17%3
1522
June1st to 15thNight
31%6%11%27%19%3
1440
16th to endNight
34%5%12%21%20%4
1440
July1st to 15thNight
31%5%13%26%18%4
1360
16th to endNight
22%5%15%34%19%2
1417
August1st to 15thNight
27%7%12%27%21%2
1623
16th to endNight
30%7%11%25%18%6%
1750
September1st to 15thNight
32%5%7%23%23%8%
1653
16th to endNight
32%5%8%24%24%4
1815
October1st to 15thNight
32%39%28%21%4
1892
16th to endNight
42%39%24%17%2
2215
November1st to 15thNight
36%39%23%20%6%
2276
16th to endNight
46%310%22%15%
2340
December1st to 15thNight
57%310%17%9%
2505
16th to endNight
63%47%15%7%
2688

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%15%27%18%6%
90
16th to endDay
17%15%38%22%5%
96
February1st to 15thDay
16%17%47%12%5%
90
16th to endDay
17%12%38%22%8%1
81
March1st to 15thDay
31%18%26%20%3
90
16th to endDay
26%9%29%23%8%1
238
April1st to 15thDay
21%12%32%21%11%
270
16th to endDay
25%5%26%24%14%4
270
May1st to 15thDay
21%10%26%27%10%3
288
16th to endDay
15%5%32%24%15%7%
491
June1st to 15thDay
11%422%29%26%6%
495
16th to endDay
10%5%32%28%14%8%
495
July1st to 15thDay
13%120%28%24%12%
475
16th to endDay
5%13%32%32%15%
515
August1st to 15thDay
9%9%25%36%18%
351
16th to endDay
7%116%24%32%17%
268
September1st to 15thDay
15%110%27%34%10%
255
16th to endDay
11%6%12%28%26%14%
254
October1st to 15thDay
11%5%24%25%24%7%
177
16th to endDay
19%7%38%26%8%
96
November1st to 15thDay
8%8%22%21%27%11%
90
16th to endDay
34%12%31%14%6%1
90
December1st to 15thDay
18%13%33%24%7%2
90
16th to endDay
29%13%42%12%2
96
January1st to 15thNight
22%12%37%24%3
900
16th to endNight
21%13%34%26%3
960
February1st to 15thNight
21%11%35%26%4
769
16th to endNight
20%10%37%27%3
631
March1st to 15thNight
25%10%33%25%5%
720
16th to endNight
19%10%36%24%8%
615
April1st to 15thNight
19%11%33%26%9%
539
16th to endNight
23%5%32%27%10%
540
May1st to 15thNight
22%329%33%10%
540
16th to endNight
11%426%34%21%1
563
June1st to 15thNight
11%220%47%19%
540
16th to endNight
9%122%43%22%
540
July1st to 15thNight
10%115%42%28%1
510
16th to endNight
311%41%37%4
564
August1st to 15thNight
6%8%42%37%4
527
16th to endNight
8%11%33%41%4
525
September1st to 15thNight
13%213%36%30%4
498
16th to endNight
9%5%17%31%31%4
657
October1st to 15thNight
14%6%19%35%20%3
660
16th to endNight
21%10%32%26%8%
704
November1st to 15thNight
15%9%33%29%11%
700
16th to endNight
23%12%29%26%7%
720
December1st to 15thNight
27%10%31%24%5%
885
16th to endNight
29%10%33%22%3
960
Page generated at 2017-08-13 13:16:14. Version ac04

Copyright 2017 A. Danko


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