This page made possible by the sponsorship of: Stuart Levy.

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 2021-07-27.

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%
21091
19%
62202
12%
21047
46%
21047
25%
41084

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%
1200
16%
3346
6%
1195
31%
1195
18%
2436
January16th to endNight
20%
1280
13%
3476
4%
1280
27%
1280
17%
2615
February1st to 15thNight
21%
1029
13%
2918
4%
1027
27%
1027
16%
2166
February16th to endNight
22%
843
17%
2529
7%
843
32%
843
18%
1771
March1st to 15thNight
26%
960
16%
2880
8%
960
34%
960
19%
2008
March16th to endNight
30%
820
15%
2678
9%
820
38%
820
19%
1828
April1st to 15thNight
31%
719
20%
2397
12%
719
43%
719
21%
1571
April16th to endNight
33%
714
18%
2338
12%
714
43%
714
21%
1547
May1st to 15thNight
39%
720
17%
2160
10%
720
46%
720
24%
1451
May16th to endNight
49%
754
16%
2042
10%
753
54%
753
28%
1372
June1st to 15thNight
58%
720
21%
1920
15%
720
63%
720
38%
1189
June16th to endNight
58%
720
19%
1920
14%
720
61%
720
34%
1219
July1st to 15thNight
67%
690
20%
1840
16%
690
69%
690
40%
1103
July16th to endNight
76%
744
27%
1897
22%
711
78%
711
47%
1061
August1st to 15thNight
75%
665
23%
2044
25%
662
76%
662
39%
1139
August16th to endNight
69%
717
23%
2390
24%
717
74%
717
34%
1338
September1st to 15thNight
65%
679
28%
2254
29%
679
68%
679
34%
1252
September16th to endNight
60%
897
31%
2475
23%
897
64%
897
38%
1454
October1st to 15thNight
51%
900
27%
2578
19%
900
58%
900
35%
1534
October16th to endNight
31%
960
21%
3020
12%
960
42%
960
24%
1922
November1st to 15thNight
34%
940
25%
3056
16%
940
46%
940
27%
1835
November16th to endNight
30%
960
21%
3120
12%
960
41%
960
20%
2059
December1st to 15thNight
27%
1180
14%
3340
7%
1180
33%
1180
19%
2423
December16th to endNight
20%
1280
14%
3584
5%
1280
28%
1280
14%
2791

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%
63435
Night
14%7%35%5%24446%41%
62371
Cloud CoverDay
13%7%36%5%3445%7%38%
63444
Night
14%7%35%5%24446%41%
62376
TransparencyDay
44%6%12%21%13%2
63121
Night
43%411%22%15%2
62203
SeeingDay
16%7%25%25%18%6%
11164
Night
18%7%27%30%13%1
21093

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
16%13%6%8%8%45%336%23%
1912
16th to endDay
14%14%7%9%6%34345%26%
2155
February1st to 15thDay
16%12%5%9%6%34445%26%
2262
16th to endDay
16%12%48%6%34345%28%
2110
March1st to 15thDay
16%11%47%6%24446%32%
2390
16th to endDay
18%11%47%5%335%47%29%
2964
April1st to 15thDay
15%8%46%5%3445%6%36%
2871
16th to endDay
19%8%36%42445%5%35%
2887
May1st to 15thDay
18%6%35%6%35%45%7%32%
3111
16th to endDay
13%5%26%5%35%5%6%8%36%
3477
June1st to 15thDay
8%415%435%5%6%8%45%
3360
16th to endDay
8%435%5%36%5%6%10%41%
3360
July1st to 15thDay
8%414435%5%8%10%42%
3360
16th to endDay
7%324435%5%7%9%45%
3488
August1st to 15thDay
6%314435%5%6%9%48%
2813
16th to endDay
7%4245%346%6%9%44%
2868
September1st to 15thDay
12%5%245%24447%45%
2679
16th to endDay
9%42442445%9%48%
2475
October1st to 15thDay
14%6%25%42345%6%43%
2369
16th to endDay
17%8%35%422446%41%
2258
November1st to 15thDay
14%6%25%313346%45%
2122
16th to endDay
18%8%46%423345%38%
2160
December1st to 15thDay
17%10%48%5%35%446%28%
1936
16th to endDay
19%13%5%6%6%35%346%25%
2048
January1st to 15thNight
15%11%48%5%23445%33%
3346
16th to endNight
15%11%5%9%6%3434430%
3476
February1st to 15thNight
18%11%5%7%6%24345%29%
2916
16th to endNight
17%9%47%5%24445%34%
2529
March1st to 15thNight
16%10%35%5%24445%36%
2867
16th to endNight
18%8%25%5%2445%7%35%
2678
April1st to 15thNight
15%6%35%5%34446%40%
2397
16th to endNight
18%7%25%434345%39%
2338
May1st to 15thNight
17%5%35%43445%7%38%
2160
16th to endNight
11%5%25%6%345%6%7%40%
2041
June1st to 15thNight
8%4145%2446%7%50%
1920
16th to endNight
7%4245%25%6%6%9%45%
1920
July1st to 15thNight
7%3143345%5%10%50%
1920
16th to endNight
331341445%8%56%
1993
August1st to 15thNight
4313425%5%6%9%52%
2053
16th to endNight
6%31443445%8%53%
2390
September1st to 15thNight
10%324423348%51%
2254
16th to endNight
8%42432345%7%53%
2475
October1st to 15thNight
12%5%1242335%6%51%
2578
16th to endNight
17%7%25%423345%43%
3020
November1st to 15thNight
12%7%24423346%47%
3056
16th to endNight
17%8%45%423346%39%
3120
December1st to 15thNight
18%9%47%5%23445%32%
3340
16th to endNight
21%12%5%8%6%3433427%
3584

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
16%13%5%9%8%45%336%23%
1912
16th to endDay
14%14%6%9%6%34345%26%
2155
February1st to 15thDay
16%12%5%9%6%34445%26%
2262
16th to endDay
16%12%48%6%24345%28%
2110
March1st to 15thDay
17%11%47%5%24445%32%
2400
16th to endDay
18%11%46%434446%30%
2963
April1st to 15thDay
15%8%46%5%3446%6%36%
2871
16th to endDay
19%7%36%5%2445%5%35%
2887
May1st to 15thDay
18%6%35%5%35%5%5%7%32%
3111
16th to endDay
13%5%26%5%36%5%6%7%36%
3477
June1st to 15thDay
8%425%435%5%6%8%45%
3360
16th to endDay
9%425%5%35%5%6%10%41%
3360
July1st to 15thDay
8%414435%5%8%10%42%
3360
16th to endDay
7%3245%345%7%9%45%
3488
August1st to 15thDay
6%3144345%7%8%48%
2813
16th to endDay
8%4245%345%6%9%45%
2868
September1st to 15thDay
12%4245%34446%45%
2679
16th to endDay
9%42433445%8%48%
2475
October1st to 15thDay
14%5%25%43345%6%43%
2369
16th to endDay
17%8%35%422446%41%
2258
November1st to 15thDay
14%6%24413346%45%
2122
16th to endDay
18%8%46%423345%39%
2160
December1st to 15thDay
17%11%48%5%35%446%28%
1936
16th to endDay
19%12%5%7%6%25%346%26%
2048
January1st to 15thNight
16%11%5%8%5%23445%33%
3346
16th to endNight
15%11%5%9%6%3434430%
3476
February1st to 15thNight
18%11%5%7%6%34336%29%
2909
16th to endNight
17%9%47%5%24445%34%
2529
March1st to 15thNight
17%10%35%5%24445%36%
2880
16th to endNight
18%8%25%5%24447%35%
2678
April1st to 15thNight
15%6%35%6%34446%39%
2397
16th to endNight
18%7%25%434445%39%
2338
May1st to 15thNight
17%6%25%43435%7%38%
2159
16th to endNight
11%5%25%6%345%6%7%40%
2041
June1st to 15thNight
8%4245%2446%6%50%
1920
16th to endNight
7%424425%5%6%9%45%
1920
July1st to 15thNight
7%3144245%5%10%50%
1920
16th to endNight
431431445%8%57%
1993
August1st to 15thNight
43144245%6%9%52%
2053
16th to endNight
6%31443445%7%53%
2390
September1st to 15thNight
10%324423349%51%
2254
16th to endNight
8%41432345%7%53%
2475
October1st to 15thNight
12%41232335%6%51%
2578
16th to endNight
17%7%25%423345%43%
3020
November1st to 15thNight
12%7%24424346%47%
3056
16th to endNight
17%8%35%423336%39%
3120
December1st to 15thNight
18%9%47%5%24445%32%
3340
16th to endNight
21%12%5%8%5%3433427%
3584

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
62%48%14%9%
1913
16th to endDay
60%411%16%6%
2155
February1st to 15thDay
60%410%15%6%1
2264
16th to endDay
57%410%18%8%1
2110
March1st to 15thDay
53%5%10%20%8%1
2400
16th to endDay
53%412%20%7%1
2964
April1st to 15thDay
47%5%11%23%11%
2871
16th to endDay
49%411%19%10%3
2887
May1st to 15thDay
50%7%11%20%9%
3111
16th to endDay
43%9%13%17%13%2
3481
June1st to 15thDay
33%8%12%25%17%3
3360
16th to endDay
37%8%14%22%14%2
3360
July1st to 15thDay
34%7%13%27%13%3
3211
16th to endDay
32%8%15%25%16%1
3321
August1st to 15thDay
30%7%13%29%16%2
2798
16th to endDay
33%7%13%25%15%5%
2868
September1st to 15thDay
39%5%9%20%19%5%
2679
16th to endDay
33%5%10%25%23%2
2475
October1st to 15thDay
39%5%10%23%18%2
2369
16th to endDay
43%310%22%17%1
2258
November1st to 15thDay
38%5%12%21%17%4
2122
16th to endDay
49%310%23%11%1
2160
December1st to 15thDay
57%410%18%8%
1936
16th to endDay
59%5%10%17%6%
2048
January1st to 15thNight
53%39%20%12%
3346
16th to endNight
56%311%17%9%
3476
February1st to 15thNight
57%311%16%9%1
2918
16th to endNight
51%411%21%10%2
2529
March1st to 15thNight
50%49%21%12%1
2880
16th to endNight
48%5%13%20%10%1
2678
April1st to 15thNight
44%5%8%24%14%2
2397
16th to endNight
46%49%20%14%4
2338
May1st to 15thNight
44%6%12%22%12%1
2160
16th to endNight
39%8%15%19%14%2
2043
June1st to 15thNight
31%6%11%26%19%4
1920
16th to endNight
34%7%13%21%19%3
1920
July1st to 15thNight
29%6%14%27%17%4
1840
16th to endNight
23%5%15%31%20%2
1897
August1st to 15thNight
26%7%12%28%22%2
2044
16th to endNight
29%7%12%25%19%5%
2390
September1st to 15thNight
31%5%10%23%23%6%
2254
16th to endNight
30%5%9%26%23%5%
2475
October1st to 15thNight
33%49%25%22%4
2578
16th to endNight
43%38%22%18%2
3020
November1st to 15thNight
38%39%22%20%5%
3056
16th to endNight
46%310%23%15%
3120
December1st to 15thNight
53%411%19%11%
3340
16th to endNight
61%48%17%8%
3584

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
28%12%29%20%8%
270
16th to endDay
22%13%36%22%4
288
February1st to 15thDay
29%16%35%14%4
270
16th to endDay
28%12%35%19%4
240
March1st to 15thDay
26%17%31%20%4
270
16th to endDay
31%9%30%19%9%1
476
April1st to 15thDay
20%14%31%22%10%
510
16th to endDay
26%10%27%20%12%2
499
May1st to 15thDay
20%10%28%25%11%2
528
16th to endDay
15%5%28%26%15%6%
801
June1st to 15thDay
9%423%32%24%6%
795
16th to endDay
10%5%28%28%18%8%
795
July1st to 15thDay
9%116%25%29%18%
775
16th to endDay
9%12%31%32%13%
813
August1st to 15thDay
8%9%29%36%14%
546
16th to endDay
7%219%28%29%13%
524
September1st to 15thDay
13%313%28%32%8%
495
16th to endDay
9%5%18%28%26%12%
494
October1st to 15thDay
15%7%28%26%18%3
389
16th to endDay
25%12%32%21%6%1
288
November1st to 15thDay
17%11%31%20%13%5%
270
16th to endDay
27%10%35%19%6%
270
December1st to 15thDay
16%12%40%20%6%3
270
16th to endDay
21%13%39%18%6%
288
January1st to 15thNight
23%11%36%24%4
1200
16th to endNight
21%13%35%26%3
1280
February1st to 15thNight
26%11%35%23%3
1030
16th to endNight
23%11%37%25%2
843
March1st to 15thNight
24%11%34%26%4
960
16th to endNight
21%9%33%26%7%
820
April1st to 15thNight
20%10%33%28%7%
719
16th to endNight
23%7%31%27%9%
714
May1st to 15thNight
21%429%33%10%
720
16th to endNight
12%5%26%34%19%2
755
June1st to 15thNight
9%221%45%20%
720
16th to endNight
9%222%40%24%
720
July1st to 15thNight
9%113%37%32%5%
690
16th to endNight
411%42%36%3
744
August1st to 15thNight
7%8%44%35%3
665
16th to endNight
8%14%34%36%4
717
September1st to 15thNight
12%214%39%27%4
679
16th to endNight
9%418%33%29%4
897
October1st to 15thNight
14%6%23%36%17%2
900
16th to endNight
23%9%32%25%8%
960
November1st to 15thNight
17%9%34%28%10%
940
16th to endNight
23%12%29%26%6%
960
December1st to 15thNight
24%11%31%26%5%
1180
16th to endNight
27%10%33%24%4
1280
Page generated at 2021-07-28 17:31:18. Version ac04

Copyright 2021 A. Danko


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