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 2018-02-22.

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%
16577
19%
48880
13%
16533
46%
16533
25%
32205

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
23%
975
16%
2716
6%
970
32%
970
19%
1947
January16th to endNight
21%
1040
14%
2824
5%
1040
29%
1040
18%
2081
February1st to 15thNight
24%
833
13%
2362
5%
831
30%
831
18%
1704
February16th to endNight
24%
659
18%
1977
8%
659
34%
659
20%
1366
March1st to 15thNight
26%
720
14%
2160
8%
720
33%
720
18%
1536
March16th to endNight
30%
615
15%
2008
9%
615
39%
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
44%
540
22%
1118
May16th to endNight
51%
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
61%
540
36%
900
July1st to 15thNight
64%
510
18%
1360
15%
510
65%
510
39%
820
July16th to endNight
76%
564
27%
1417
21%
531
79%
531
48%
778
August1st to 15thNight
75%
539
22%
1661
24%
536
76%
536
38%
936
August16th to endNight
71%
573
23%
1910
24%
573
76%
573
34%
1070
September1st to 15thNight
64%
544
28%
1804
28%
544
68%
544
33%
1013
September16th to endNight
61%
717
30%
1980
23%
717
65%
717
37%
1173
October1st to 15thNight
53%
720
28%
2064
21%
720
59%
720
35%
1224
October16th to endNight
32%
768
22%
2417
14%
768
42%
768
25%
1519
November1st to 15thNight
36%
760
25%
2471
18%
760
47%
760
27%
1473
November16th to endNight
31%
780
21%
2535
14%
780
41%
780
22%
1645
December1st to 15thNight
25%
959
12%
2714
6%
959
31%
959
18%
1999
December16th to endNight
18%
1040
12%
2912
4%
1040
26%
1040
13%
2317

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%
49377
Night
14%7%35%5%24446%41%
49049
Cloud CoverDay
14%7%36%5%3445%7%38%
49386
Night
14%7%35%424446%41%
49054
TransparencyDay
44%5%12%21%13%2
49063
Night
43%411%22%15%2
48881
SeeingDay
15%6%24%25%19%7%
6392
Night
18%7%27%30%14%1
16579

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%13%5%8%7%45%336%24%
1552
16th to endDay
14%12%7%9%6%34345%28%
1751
February1st to 15thDay
14%12%5%8%6%35%445%30%
1829
16th to endDay
15%13%49%6%24345%28%
1643
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%46%
2286
16th to endDay
8%4245%346%6%9%44%
2292
September1st to 15thDay
12%5%245%34446%44%
2142
16th to endDay
10%424424349%48%
1980
October1st to 15thDay
15%6%25%42345%6%43%
1895
16th to endDay
16%7%2432245%6%42%
1806
November1st to 15thDay
15%6%24313347%46%
1717
16th to endDay
17%7%45%423345%41%
1755
December1st to 15thDay
19%9%48%5%24446%28%
1573
16th to endDay
20%13%6%6%6%25%346%24%
1664
January1st to 15thNight
16%10%48%423445%34%
2716
16th to endNight
14%11%5%8%5%3334433%
2824
February1st to 15thNight
17%10%5%7%5%24346%32%
2360
16th to endNight
16%9%46%5%24345%36%
1977
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%5%6%9%50%
1670
16th to endNight
6%315%435%45%7%52%
1910
September1st to 15thNight
11%324423348%51%
1804
16th to endNight
8%42442345%7%52%
1980
October1st to 15thNight
12%42242335%6%51%
2064
16th to endNight
17%6%25%424345%44%
2417
November1st to 15thNight
12%6%24424346%48%
2471
16th to endNight
17%7%35%423346%40%
2535
December1st to 15thNight
19%10%47%5%24345%32%
2714
16th to endNight
22%13%5%8%6%3333425%
2912

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%13%5%8%8%45%336%24%
1552
16th to endDay
14%12%6%9%6%34345%28%
1751
February1st to 15thDay
14%11%5%8%6%35%435%29%
1829
16th to endDay
15%12%49%6%24345%28%
1643
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%46%
2286
16th to endDay
8%3245%346%6%9%44%
2292
September1st to 15thDay
13%5%245%34446%44%
2142
16th to endDay
10%42432435%8%49%
1980
October1st to 15thDay
16%5%25%42345%6%43%
1895
16th to endDay
16%7%2442245%6%42%
1806
November1st to 15thDay
15%6%34413346%46%
1717
16th to endDay
17%7%45%423345%42%
1755
December1st to 15thDay
19%10%48%5%24446%28%
1573
16th to endDay
21%13%6%6%6%24345%24%
1664
January1st to 15thNight
16%10%47%5%23445%34%
2716
16th to endNight
14%11%5%8%6%3434433%
2824
February1st to 15thNight
17%10%5%7%5%24346%32%
2353
16th to endNight
16%9%46%5%24445%36%
1977
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%
1670
16th to endNight
6%314435%45%7%53%
1910
September1st to 15thNight
11%324323348%51%
1804
16th to endNight
8%41442345%7%52%
1980
October1st to 15thNight
13%41232335%6%51%
2064
16th to endNight
17%6%25%424345%44%
2417
November1st to 15thNight
12%6%24424346%48%
2471
16th to endNight
17%7%35%423336%40%
2535
December1st to 15thNight
20%10%47%5%24345%32%
2714
16th to endNight
22%12%5%8%6%3433425%
2912

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%49%15%9%
1553
16th to endDay
57%413%17%6%
1751
February1st to 15thDay
57%411%17%7%1
1831
16th to endDay
57%411%19%6%
1643
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%17%2
2271
16th to endDay
34%7%13%24%14%6%
2292
September1st to 15thDay
40%5%8%21%19%5%
2142
16th to endDay
33%49%25%24%2
1980
October1st to 15thDay
40%5%10%25%16%2
1895
16th to endDay
41%412%24%16%1
1806
November1st to 15thDay
38%5%12%21%17%4
1717
16th to endDay
45%310%25%13%1
1755
December1st to 15thDay
57%411%18%8%
1573
16th to endDay
61%5%9%15%6%
1664
January1st to 15thNight
51%39%20%13%
2716
16th to endNight
54%412%19%9%
2824
February1st to 15thNight
53%312%18%10%1
2362
16th to endNight
49%312%23%9%
1977
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
26%7%12%28%21%2
1661
16th to endNight
29%7%11%25%19%6%
1910
September1st to 15thNight
31%5%8%23%22%8%
1804
16th to endNight
30%5%9%25%24%4
1980
October1st to 15thNight
33%39%27%20%4
2064
16th to endNight
42%39%24%17%2
2417
November1st to 15thNight
37%39%23%19%5%
2471
16th to endNight
44%310%24%15%
2535
December1st to 15thNight
54%411%17%10%1
2714
16th to endNight
63%47%15%7%
2912

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
27%14%28%19%9%
135
16th to endDay
17%15%38%24%3
144
February1st to 15thDay
23%13%45%13%4
135
16th to endDay
28%12%32%20%7%1
100
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%8%26%37%17%
367
16th to endDay
6%116%26%33%15%
332
September1st to 15thDay
14%311%25%35%9%
315
16th to endDay
9%5%14%27%26%16%
314
October1st to 15thDay
17%5%24%23%23%5%
230
16th to endDay
22%9%34%22%8%2
144
November1st to 15thDay
17%8%23%21%19%9%
135
16th to endDay
25%10%39%15%8%
135
December1st to 15thDay
14%17%38%20%8%2
135
16th to endDay
25%14%44%13%1
144
January1st to 15thNight
23%11%36%24%3
975
16th to endNight
20%12%36%26%3
1040
February1st to 15thNight
23%10%35%26%4
834
16th to endNight
22%10%36%27%3
659
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%43%36%4
539
16th to endNight
8%12%34%40%4
573
September1st to 15thNight
12%213%36%29%4
544
16th to endNight
9%417%31%32%4
717
October1st to 15thNight
15%6%19%36%19%2
720
16th to endNight
21%9%33%25%9%
768
November1st to 15thNight
16%8%33%28%11%
760
16th to endNight
22%12%30%26%7%
780
December1st to 15thNight
26%10%32%24%5%
959
16th to endNight
29%10%33%23%3
1040
Page generated at 2018-02-23 10:21:39. Version ac04

Copyright 2018 A. Danko


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