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 2017-04-01.

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
39%
15375
19%
45347
12%
15331
46%
15331
24%
30010

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%
497
20%
1657
12%
497
44%
497
22%
1068
April16th to endNight
35%
495
18%
1623
12%
495
44%
495
21%
1072
May1st to 15thNight
36%
495
16%
1485
9%
495
43%
495
22%
1033
May16th to endNight
53%
514
18%
1391
12%
513
57%
513
31%
914
June1st to 15thNight
56%
495
20%
1320
14%
495
61%
495
36%
837
June16th to endNight
59%
495
18%
1320
14%
495
61%
495
34%
836
July1st to 15thNight
64%
465
19%
1240
16%
465
66%
465
39%
752
July16th to endNight
76%
522
25%
1305
19%
489
78%
489
47%
725
August1st to 15thNight
75%
494
23%
1521
25%
491
76%
491
37%
865
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
14%7%36%5%3445%7%37%
45761
Night
14%7%35%5%24446%40%
45516
Cloud CoverDay
14%7%36%5%3445%7%37%
45770
Night
14%7%35%424446%40%
45521
TransparencyDay
45%5%12%21%12%2
45447
Night
43%411%22%14%2
45348
SeeingDay
15%5%23%25%20%8%
5152
Night
18%7%27%30%14%1
15377

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
17%8%45%43335%6%37%
1976
16th to endDay
18%7%26%424445%36%
2007
May1st to 15thDay
20%7%35%6%345%6%6%30%
2136
16th to endDay
12%427%5%35%5%5%8%38%
2369
June1st to 15thDay
10%425%5%35%5%6%8%42%
2310
16th to endDay
9%435%5%35%5%6%9%41%
2310
July1st to 15thDay
10%5%24435%5%7%9%41%
2310
16th to endDay
5%325%5%45%6%8%10%43%
2459
August1st to 15thDay
7%414435%5%7%9%45%
2096
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
14%6%45%5%24436%41%
1657
16th to endNight
18%7%25%424346%39%
1623
May1st to 15thNight
19%6%45%434447%36%
1485
16th to endNight
11%4245%25%5%6%7%44%
1390
June1st to 15thNight
10%414425%5%6%7%46%
1320
16th to endNight
7%4345%25%5%5%9%45%
1320
July1st to 15thNight
8%4144335%5%9%49%
1320
16th to endNight
33134145%5%9%56%
1401
August1st to 15thNight
5%313426%6%7%9%49%
1530
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%43335%6%37%
1976
16th to endDay
19%7%26%5%25%445%36%
2007
May1st to 15thDay
20%7%35%6%345%5%6%30%
2136
16th to endDay
12%427%5%35%45%8%38%
2369
June1st to 15thDay
10%425%435%5%6%8%42%
2310
16th to endDay
9%435%5%35%5%6%9%41%
2310
July1st to 15thDay
10%5%14435%5%7%9%41%
2310
16th to endDay
5%325%5%45%6%8%9%43%
2459
August1st to 15thDay
7%4144345%7%9%45%
2096
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
14%6%45%5%24446%41%
1657
16th to endNight
18%7%25%424445%39%
1623
May1st to 15thNight
19%6%35%5%3435%6%36%
1484
16th to endNight
11%425%5%245%6%7%44%
1390
June1st to 15thNight
10%424425%5%7%7%46%
1320
16th to endNight
7%5%245%35%5%6%9%45%
1320
July1st to 15thNight
8%4144245%5%9%49%
1320
16th to endNight
331331445%9%56%
1401
August1st to 15thNight
5%314425%6%7%9%49%
1530
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%410%24%12%1
1976
16th to endDay
48%412%20%9%3
2007
May1st to 15thDay
53%6%11%19%8%
2136
16th to endDay
42%8%13%17%15%3
2373
June1st to 15thDay
35%8%12%26%14%1
2310
16th to endDay
38%6%13%21%15%3
2310
July1st to 15thDay
38%6%12%24%15%2
2161
16th to endDay
32%9%17%26%12%
2292
August1st to 15thDay
32%7%14%26%17%2
2081
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%24%16%2
1657
16th to endNight
46%48%22%13%5%
1623
May1st to 15thNight
48%5%12%21%11%
1485
16th to endNight
36%8%15%17%17%4
1392
June1st to 15thNight
33%6%12%27%17%2
1320
16th to endNight
35%5%12%21%20%4
1320
July1st to 15thNight
32%412%27%18%3
1240
16th to endNight
23%5%16%34%18%1
1305
August1st to 15thNight
27%8%12%27%22%2
1521
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%29%22%12%
212
16th to endDay
22%424%24%17%6%
210
May1st to 15thDay
22%9%27%25%10%4
228
16th to endDay
14%5%29%24%18%8%
413
June1st to 15thDay
13%423%28%25%5%
420
16th to endDay
11%329%30%15%9%
420
July1st to 15thDay
14%118%27%25%12%
400
16th to endDay
6%112%33%32%14%
445
August1st to 15thDay
9%8%26%35%19%
303
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
18%12%33%26%8%
497
16th to endNight
23%5%31%28%10%
495
May1st to 15thNight
22%329%32%10%
495
16th to endNight
12%425%33%22%1
515
June1st to 15thNight
12%220%46%18%
495
16th to endNight
9%120%43%23%1
495
July1st to 15thNight
10%114%42%28%2
465
16th to endNight
411%41%37%5%
522
August1st to 15thNight
6%8%43%36%4
494
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 2017-04-02 06:39:03. Version ac04

Copyright 2017 A. Danko


Champaign/Urbana Astronomical Society Observatory Clear Sky Chart | Charts Home