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Chicago
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chicago clear sky chart from 2005-11-05 to 2017-03-26.

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
38%
15439
16%
45294
10%
15399
43%
15399
23%
30733

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
20%
899
12%
2506
4%
895
27%
895
17%
1841
January16th to endNight
20%
960
11%
2673
4%
960
26%
960
15%
2025
February1st to 15thNight
23%
777
13%
2188
4%
777
29%
777
19%
1564
February16th to endNight
27%
631
17%
1893
8%
631
36%
631
17%
1346
March1st to 15thNight
28%
720
12%
2160
7%
720
31%
720
18%
1534
March16th to endNight
32%
629
15%
1969
10%
629
38%
629
19%
1381
April1st to 15thNight
32%
494
15%
1647
12%
494
39%
494
20%
1100
April16th to endNight
36%
495
18%
1576
12%
495
43%
495
21%
1056
May1st to 15thNight
40%
495
16%
1485
9%
495
45%
495
20%
1057
May16th to endNight
57%
513
16%
1370
11%
513
58%
513
32%
884
June1st to 15thNight
55%
495
16%
1320
11%
495
57%
495
32%
844
June16th to endNight
56%
495
15%
1320
10%
495
58%
495
31%
863
July1st to 15thNight
66%
465
17%
1240
13%
465
68%
465
39%
750
July16th to endNight
70%
522
18%
1305
14%
489
72%
489
39%
792
August1st to 15thNight
77%
494
22%
1438
22%
491
78%
491
41%
813
August16th to endNight
66%
525
19%
1750
19%
525
70%
525
33%
992
September1st to 15thNight
61%
511
26%
1666
26%
511
65%
511
33%
946
September16th to endNight
57%
657
25%
1815
19%
657
60%
657
34%
1120
October1st to 15thNight
49%
660
24%
1879
18%
660
54%
660
30%
1170
October16th to endNight
28%
704
16%
2252
9%
704
36%
704
20%
1496
November1st to 15thNight
32%
700
22%
2276
15%
700
42%
700
23%
1425
November16th to endNight
26%
738
15%
2358
9%
738
34%
738
18%
1612
December1st to 15thNight
19%
900
10%
2520
3%
900
25%
900
15%
1913
December16th to endNight
15%
960
10%
2688
3%
960
22%
960
11%
2209

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%35%5%5%7%36%
45690
Night
15%8%35%5%2445%6%38%
45462
Cloud CoverDay
13%7%36%5%35%5%5%7%37%
45699
Night
15%8%35%5%2445%6%38%
45467
TransparencyDay
45%7%13%20%11%1
45376
Night
45%5%13%21%12%1
45294
SeeingDay
15%5%26%31%15%4
5195
Night
18%8%28%29%13%1
15445

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
15%12%48%6%25%5%7%7%23%
1432
16th to endDay
11%16%7%11%6%34346%23%
1550
February1st to 15thDay
13%14%5%7%7%35%45%427%
1672
16th to endDay
13%9%47%6%45%5%5%6%29%
1580
March1st to 15thDay
15%9%48%5%245%5%6%30%
1790
16th to endDay
15%9%36%6%345%5%6%32%
2151
April1st to 15thDay
19%8%46%5%3445%5%33%
1971
16th to endDay
18%6%25%435%36%6%36%
2053
May1st to 15thDay
17%9%37%5%35%46%7%29%
2136
16th to endDay
11%5%35%5%345%5%9%39%
2392
June1st to 15thDay
11%5%25%5%35%5%6%7%41%
2310
16th to endDay
8%5%25%5%35%5%8%10%38%
2310
July1st to 15thDay
6%425%5%245%6%8%48%
2310
16th to endDay
5%3246%46%6%7%10%43%
2459
August1st to 15thDay
5%5%245%345%6%9%46%
2180
16th to endDay
8%425%5%35%5%6%9%43%
2100
September1st to 15thDay
13%314425%36%8%46%
1953
16th to endDay
8%5%25%5%36%5%6%9%40%
1815
October1st to 15thDay
14%6%25%6%34436%42%
1749
16th to endDay
16%7%46%5%44447%34%
1620
November1st to 15thDay
13%7%45%5%2345%6%41%
1582
16th to endDay
17%10%36%5%34436%35%
1599
December1st to 15thDay
17%13%47%6%46%6%45%23%
1440
16th to endDay
21%14%6%8%5%245%35%21%
1536
January1st to 15thNight
17%10%47%5%2445%5%31%
2506
16th to endNight
13%13%7%8%5%345%5%6%27%
2673
February1st to 15thNight
16%11%45%5%34345%35%
2186
16th to endNight
16%10%47%6%2434433%
1893
March1st to 15thNight
16%12%46%424335%34%
2147
16th to endNight
16%8%36%6%2445%7%33%
1969
April1st to 15thNight
17%7%25%6%25%447%36%
1647
16th to endNight
20%8%24423345%39%
1576
May1st to 15thNight
18%8%25%5%35%45%7%32%
1485
16th to endNight
10%335%4346%49%44%
1368
June1st to 15thNight
11%5%25%5%1445%9%44%
1320
16th to endNight
9%5%25%335%7%8%9%39%
1320
July1st to 15thNight
6%323425%6%5%8%50%
1320
16th to endNight
7%3144246%6%10%48%
1401
August1st to 15thNight
44134345%6%8%53%
1447
16th to endNight
9%323424447%52%
1750
September1st to 15thNight
11%424324447%51%
1666
16th to endNight
9%5%25%32345%7%48%
1815
October1st to 15thNight
13%5%34424347%46%
1879
16th to endNight
19%7%35%5%23435%38%
2252
November1st to 15thNight
13%8%24424445%44%
2276
16th to endNight
17%7%36%6%15%346%35%
2358
December1st to 15thNight
20%10%48%43446%5%26%
2520
16th to endNight
24%14%5%7%5%35%44420%
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
15%13%47%6%25%5%8%6%23%
1432
16th to endDay
11%15%7%10%7%24446%24%
1550
February1st to 15thDay
13%13%6%7%6%36%445%27%
1672
16th to endDay
14%9%5%7%6%35%5%5%6%30%
1580
March1st to 15thDay
15%10%49%5%25%5%5%6%29%
1800
16th to endDay
16%9%36%6%35%35%6%34%
2150
April1st to 15thDay
19%8%45%5%25%445%34%
1971
16th to endDay
18%6%25%43445%6%36%
2053
May1st to 15thDay
18%8%37%5%35%46%7%30%
2136
16th to endDay
12%5%26%5%35%5%5%8%40%
2392
June1st to 15thDay
11%5%25%5%345%6%7%42%
2310
16th to endDay
9%4246%346%7%10%40%
2310
July1st to 15thDay
6%425%5%245%6%8%49%
2310
16th to endDay
6%225%5%45%5%7%9%44%
2459
August1st to 15thDay
6%424445%5%6%8%46%
2180
16th to endDay
9%3245%245%6%9%45%
2100
September1st to 15thDay
13%31442445%7%47%
1953
16th to endDay
9%425%5%36%5%6%9%41%
1815
October1st to 15thDay
14%6%26%5%34445%42%
1749
16th to endDay
17%7%36%6%44437%34%
1620
November1st to 15thDay
13%8%35%423347%41%
1582
16th to endDay
16%9%37%5%35%336%35%
1599
December1st to 15thDay
17%12%47%6%36%6%5%6%23%
1440
16th to endDay
22%14%5%7%5%245%45%21%
1536
January1st to 15thNight
17%11%47%5%3445%5%31%
2506
16th to endNight
14%13%6%7%6%245%46%26%
2673
February1st to 15thNight
16%11%5%5%5%23345%35%
2179
16th to endNight
16%11%47%5%3434433%
1893
March1st to 15thNight
16%11%46%424335%35%
2160
16th to endNight
16%8%35%5%345%5%7%33%
1969
April1st to 15thNight
18%7%25%5%24447%36%
1647
16th to endNight
21%8%2442335%440%
1576
May1st to 15thNight
18%8%25%5%35%5%5%7%32%
1484
16th to endNight
10%3344245%5%9%44%
1368
June1st to 15thNight
11%5%25%4235%5%8%45%
1320
16th to endNight
10%5%15%325%6%8%9%40%
1320
July1st to 15thNight
7%213425%6%6%8%51%
1320
16th to endNight
6%4143345%7%10%48%
1401
August1st to 15thNight
431443446%9%53%
1447
16th to endNight
9%323423447%52%
1750
September1st to 15thNight
11%424424446%51%
1666
16th to endNight
10%425%423347%49%
1815
October1st to 15thNight
13%5%24423446%46%
1879
16th to endNight
20%7%35%5%23346%38%
2252
November1st to 15thNight
13%8%24424445%44%
2276
16th to endNight
17%7%46%5%24446%35%
2358
December1st to 15thNight
21%10%37%5%2445%6%26%
2520
16th to endNight
23%14%47%6%35%44421%
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
57%7%15%12%7%
1433
16th to endDay
61%5%13%13%5%
1550
February1st to 15thDay
59%411%15%7%1
1674
16th to endDay
53%7%11%20%5%1
1580
March1st to 15thDay
54%6%13%17%6%1
1800
16th to endDay
51%7%13%21%4
2151
April1st to 15thDay
52%5%9%19%12%
1971
16th to endDay
47%7%13%16%11%3
2053
May1st to 15thDay
53%6%12%20%5%
2136
16th to endDay
41%8%12%20%14%2
2396
June1st to 15thDay
39%8%12%21%16%1
2310
16th to endDay
37%9%14%20%15%2
2310
July1st to 15thDay
34%6%13%26%15%2
2161
16th to endDay
33%9%18%22%12%2
2292
August1st to 15thDay
33%8%14%26%15%1
2165
16th to endDay
35%8%14%24%15%3
2100
September1st to 15thDay
36%5%12%24%17%2
1953
16th to endDay
37%7%11%26%15%1
1815
October1st to 15thDay
42%5%12%23%14%1
1749
16th to endDay
50%5%13%21%8%1
1620
November1st to 15thDay
41%5%14%21%14%2
1582
16th to endDay
50%5%16%19%7%
1599
December1st to 15thDay
58%6%15%15%3
1440
16th to endDay
63%7%11%11%5%
1536
January1st to 15thNight
52%5%14%18%8%
2506
16th to endNight
56%5%15%15%6%
2673
February1st to 15thNight
51%414%19%9%1
2188
16th to endNight
53%311%21%8%1
1893
March1st to 15thNight
52%413%17%10%1
2160
16th to endNight
47%8%14%20%8%1
1969
April1st to 15thNight
47%411%20%15%1
1647
16th to endNight
47%410%19%14%3
1576
May1st to 15thNight
49%6%10%22%10%
1485
16th to endNight
35%7%15%20%17%3
1370
June1st to 15thNight
36%6%13%25%16%2
1320
16th to endNight
36%8%14%20%18%1
1320
July1st to 15thNight
29%6%14%26%19%3
1240
16th to endNight
27%8%17%24%18%3
1305
August1st to 15thNight
26%7%16%28%20%2
1438
16th to endNight
30%7%12%27%17%5%
1750
September1st to 15thNight
32%412%25%22%3
1666
16th to endNight
34%5%10%28%18%2
1815
October1st to 15thNight
36%5%10%26%20%1
1879
16th to endNight
47%314%22%10%1
2252
November1st to 15thNight
41%410%23%15%3
2276
16th to endNight
48%5%15%22%8%
2358
December1st to 15thNight
55%7%13%16%6%
2520
16th to endNight
66%5%10%12%4
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
25%6%41%22%4
90
16th to endDay
16%5%41%30%5%1
96
February1st to 15thDay
15%15%55%13%
90
16th to endDay
14%11%38%25%8%1
81
March1st to 15thDay
28%13%32%24%1
90
16th to endDay
27%5%31%22%11%1
235
April1st to 15thDay
24%12%30%20%10%
210
16th to endDay
20%6%27%26%13%4
210
May1st to 15thDay
22%7%30%24%11%3
228
16th to endDay
14%329%32%16%3
436
June1st to 15thDay
14%230%35%14%3
420
16th to endDay
11%230%35%13%5%
420
July1st to 15thDay
10%225%34%20%7%
400
16th to endDay
7%122%41%20%7%
445
August1st to 15thDay
7%12%41%25%11%
316
16th to endDay
8%414%36%25%10%
268
September1st to 15thDay
13%212%37%25%8%
255
16th to endDay
11%5%16%32%28%6%
254
October1st to 15thDay
16%6%23%29%19%4
189
16th to endDay
25%7%36%30%1
96
November1st to 15thDay
314%22%41%15%3
90
16th to endDay
30%12%40%12%5%
90
December1st to 15thDay
16%16%35%26%4
90
16th to endDay
28%19%45%33
96
January1st to 15thNight
22%13%37%20%4
900
16th to endNight
19%12%37%26%3
960
February1st to 15thNight
20%8%39%25%5%
780
16th to endNight
21%10%33%29%4
631
March1st to 15thNight
23%9%32%26%8%
720
16th to endNight
18%7%33%28%10%1
629
April1st to 15thNight
20%10%32%27%8%
494
16th to endNight
23%6%28%28%11%1
495
May1st to 15thNight
21%328%31%14%
495
16th to endNight
11%219%39%25%1
515
June1st to 15thNight
11%320%42%20%1
495
16th to endNight
11%121%38%25%1
495
July1st to 15thNight
9%215%39%29%3
465
16th to endNight
6%114%45%27%4
522
August1st to 15thNight
6%10%45%29%7%
494
16th to endNight
10%213%35%31%6%
525
September1st to 15thNight
11%415%32%27%7%
511
16th to endNight
12%5%17%31%28%4
657
October1st to 15thNight
16%8%20%31%20%2
660
16th to endNight
23%13%31%21%9%
704
November1st to 15thNight
18%11%34%23%11%1
700
16th to endNight
21%16%31%23%7%
738
December1st to 15thNight
26%13%36%19%4
900
16th to endNight
30%13%32%20%2
960
Page generated at 2017-03-27 10:15:16. Version ac04

Copyright 2017 A. Danko


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