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 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
38%
16664
16%
48886
11%
16624
44%
16624
23%
33040

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
21%
974
12%
2716
4%
970
28%
970
18%
1978
January16th to endNight
20%
1040
12%
2896
4%
1040
27%
1040
16%
2175
February1st to 15thNight
23%
843
14%
2374
4%
843
30%
843
19%
1705
February16th to endNight
26%
659
18%
1977
8%
659
35%
659
17%
1417
March1st to 15thNight
28%
720
12%
2160
7%
720
31%
720
18%
1534
March16th to endNight
32%
644
14%
2019
10%
644
38%
644
19%
1423
April1st to 15thNight
33%
539
14%
1797
11%
539
39%
539
20%
1209
April16th to endNight
36%
540
18%
1719
12%
540
44%
540
21%
1149
May1st to 15thNight
40%
540
15%
1620
9%
540
45%
540
21%
1143
May16th to endNight
55%
561
16%
1498
11%
561
57%
561
31%
972
June1st to 15thNight
57%
540
16%
1440
11%
540
59%
540
35%
897
June16th to endNight
56%
540
16%
1440
11%
540
59%
540
31%
940
July1st to 15thNight
68%
510
16%
1360
12%
510
70%
510
39%
823
July16th to endNight
71%
564
21%
1417
17%
531
74%
531
40%
848
August1st to 15thNight
76%
539
21%
1570
21%
536
77%
536
41%
888
August16th to endNight
66%
573
21%
1910
21%
573
71%
573
33%
1087
September1st to 15thNight
61%
558
25%
1818
25%
558
65%
558
34%
1021
September16th to endNight
58%
717
26%
1980
21%
717
62%
717
36%
1202
October1st to 15thNight
48%
720
23%
2050
17%
720
53%
720
30%
1289
October16th to endNight
28%
768
17%
2457
10%
768
37%
768
21%
1621
November1st to 15thNight
31%
760
21%
2471
14%
760
41%
760
22%
1575
November16th to endNight
25%
800
17%
2555
9%
800
35%
800
19%
1721
December1st to 15thNight
19%
975
11%
2730
3%
975
26%
975
16%
2044
December16th to endNight
15%
1040
10%
2912
2%
1040
22%
1040
11%
2379

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
12%7%36%5%35%5%5%7%36%
49376
Night
14%8%35%5%2445%6%38%
49054
Cloud CoverDay
13%7%36%5%345%5%7%37%
49385
Night
15%8%35%5%2445%6%38%
49059
TransparencyDay
45%7%13%20%11%1
49062
Night
45%5%13%21%12%1
48886
SeeingDay
15%5%27%31%15%4
6463
Night
18%8%28%29%13%1
16670

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%47%6%25%5%6%7%25%
1552
16th to endDay
11%15%6%11%6%34346%24%
1679
February1st to 15thDay
14%14%5%7%6%35%44427%
1816
16th to endDay
14%10%47%6%45%5%5%6%28%
1643
March1st to 15thDay
15%9%48%5%245%5%6%30%
1790
16th to endDay
16%10%36%6%345%5%6%31%
2215
April1st to 15thDay
19%8%36%5%3445%5%33%
2151
16th to endDay
18%6%26%435%46%6%36%
2240
May1st to 15thDay
17%9%37%5%3445%7%31%
2331
16th to endDay
12%5%35%5%345%5%8%39%
2616
June1st to 15thDay
10%5%25%5%345%6%7%44%
2520
16th to endDay
8%5%25%6%35%5%7%10%38%
2520
July1st to 15thDay
5%5%245%245%6%8%47%
2520
16th to endDay
5%3146%45%6%7%10%44%
2655
August1st to 15thDay
5%4246%35%6%6%9%45%
2378
16th to endDay
8%425%435%5%6%10%43%
2292
September1st to 15thDay
12%314425%35%8%47%
2130
16th to endDay
8%425%5%36%5%6%9%42%
1980
October1st to 15thDay
16%7%25%5%34435%41%
1908
16th to endDay
16%7%36%5%44447%35%
1767
November1st to 15thDay
15%7%45%5%2345%6%39%
1717
16th to endDay
16%9%36%5%34336%37%
1732
December1st to 15thDay
16%12%47%6%46%6%5%6%25%
1560
16th to endDay
21%14%6%8%5%245%35%22%
1664
January1st to 15thNight
17%10%47%5%24445%32%
2716
16th to endNight
13%13%7%8%6%245%45%28%
2896
February1st to 15thNight
16%12%46%423345%34%
2372
16th to endNight
17%10%37%6%2434432%
1977
March1st to 15thNight
16%12%46%424335%34%
2147
16th to endNight
16%8%36%6%2445%7%32%
2019
April1st to 15thNight
17%7%25%6%25%447%35%
1797
16th to endNight
19%7%24423345%40%
1719
May1st to 15thNight
18%7%35%5%35%45%7%34%
1620
16th to endNight
11%335%4345%5%9%43%
1496
June1st to 15thNight
10%5%25%414448%47%
1440
16th to endNight
8%5%24336%7%8%9%40%
1440
July1st to 15thNight
6%323425%6%5%8%50%
1440
16th to endNight
6%3144246%6%9%49%
1513
August1st to 15thNight
43144345%6%8%53%
1579
16th to endNight
9%323424447%52%
1910
September1st to 15thNight
10%423424447%51%
1818
16th to endNight
9%5%25%32345%7%49%
1980
October1st to 15thNight
14%5%34424347%45%
2050
16th to endNight
19%7%25%5%23435%39%
2457
November1st to 15thNight
14%9%24424445%43%
2471
16th to endNight
16%7%37%6%15%346%36%
2555
December1st to 15thNight
19%10%47%4345%6%5%28%
2730
16th to endNight
22%14%5%7%5%35%44421%
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
14%13%47%6%25%5%7%6%24%
1552
16th to endDay
11%15%6%10%7%24446%25%
1679
February1st to 15thDay
14%14%6%6%6%35%445%27%
1816
16th to endDay
14%9%5%7%6%35%5%5%6%29%
1643
March1st to 15thDay
15%10%49%5%25%5%5%6%29%
1800
16th to endDay
17%9%36%6%3435%6%33%
2214
April1st to 15thDay
19%8%35%5%24345%34%
2151
16th to endDay
18%6%25%43445%6%36%
2240
May1st to 15thDay
17%8%36%5%35%45%7%32%
2331
16th to endDay
12%5%26%5%35%5%5%8%39%
2616
June1st to 15thDay
10%425%5%345%6%7%44%
2520
16th to endDay
9%4246%346%7%9%39%
2520
July1st to 15thDay
6%425%5%245%6%8%48%
2520
16th to endDay
6%215%5%35%5%7%9%45%
2655
August1st to 15thDay
5%424445%5%7%9%46%
2378
16th to endDay
8%3245%245%6%9%46%
2292
September1st to 15thDay
12%3145%2445%7%48%
2130
16th to endDay
8%4245%35%5%6%9%43%
1980
October1st to 15thDay
16%6%26%5%34445%41%
1908
16th to endDay
17%7%36%5%34437%35%
1767
November1st to 15thDay
14%8%35%424347%40%
1717
16th to endDay
16%9%37%425%336%37%
1732
December1st to 15thDay
16%12%46%6%35%6%5%6%24%
1560
16th to endDay
21%14%5%7%5%245%45%21%
1664
January1st to 15thNight
17%11%47%5%3445%5%31%
2716
16th to endNight
13%13%6%7%6%24446%28%
2896
February1st to 15thNight
16%12%5%6%5%23345%34%
2365
16th to endNight
17%11%47%5%3433432%
1977
March1st to 15thNight
16%11%46%424335%35%
2160
16th to endNight
17%8%35%5%335%5%7%33%
2019
April1st to 15thNight
18%7%26%5%35%447%35%
1797
16th to endNight
20%8%2442345%440%
1719
May1st to 15thNight
18%8%25%5%3445%6%34%
1619
16th to endNight
11%3344245%5%9%43%
1496
June1st to 15thNight
10%5%244235%5%8%48%
1440
16th to endNight
9%5%15%326%6%8%9%40%
1440
July1st to 15thNight
6%213426%6%6%8%50%
1440
16th to endNight
6%3143245%7%9%49%
1513
August1st to 15thNight
4314435%46%9%53%
1579
16th to endNight
10%323424447%52%
1910
September1st to 15thNight
10%424424446%52%
1818
16th to endNight
9%425%42335%7%50%
1980
October1st to 15thNight
14%5%24423447%45%
2050
16th to endNight
19%6%35%423336%39%
2457
November1st to 15thNight
14%8%34324445%43%
2471
16th to endNight
16%7%36%5%24446%36%
2555
December1st to 15thNight
19%10%37%5%2445%6%27%
2730
16th to endNight
22%14%47%6%35%44421%
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
56%6%14%13%8%
1553
16th to endDay
60%5%14%13%5%
1679
February1st to 15thDay
59%411%15%7%1
1818
16th to endDay
54%7%11%19%5%1
1643
March1st to 15thDay
54%6%13%17%6%1
1800
16th to endDay
52%7%13%20%4
2215
April1st to 15thDay
52%5%9%19%12%
2151
16th to endDay
47%7%13%16%11%3
2240
May1st to 15thDay
52%6%12%21%6%
2331
16th to endDay
42%8%11%21%14%1
2620
June1st to 15thDay
37%8%11%22%19%1
2520
16th to endDay
38%9%14%19%15%2
2520
July1st to 15thDay
35%7%13%26%15%2
2371
16th to endDay
32%9%17%22%13%2
2488
August1st to 15thDay
33%8%15%25%15%1
2363
16th to endDay
34%8%14%24%15%2
2292
September1st to 15thDay
35%5%12%24%18%3
2130
16th to endDay
35%7%11%26%17%1
1980
October1st to 15thDay
43%5%11%23%13%2
1908
16th to endDay
49%412%21%9%1
1767
November1st to 15thDay
44%5%13%21%13%2
1717
16th to endDay
48%5%16%21%7%
1732
December1st to 15thDay
56%6%15%16%4
1560
16th to endDay
64%6%12%11%4
1664
January1st to 15thNight
52%414%17%10%
2716
16th to endNight
55%5%15%16%6%
2896
February1st to 15thNight
52%413%18%10%
2374
16th to endNight
54%311%20%8%1
1977
March1st to 15thNight
52%413%17%10%1
2160
16th to endNight
47%8%14%20%7%1
2019
April1st to 15thNight
47%411%19%14%1
1797
16th to endNight
47%410%19%14%3
1719
May1st to 15thNight
49%6%10%22%11%
1620
16th to endNight
36%7%15%20%17%3
1498
June1st to 15thNight
33%6%13%25%19%2
1440
16th to endNight
35%9%13%21%18%1
1440
July1st to 15thNight
29%7%14%26%19%3
1360
16th to endNight
27%8%16%25%18%4
1417
August1st to 15thNight
26%7%16%29%18%2
1570
16th to endNight
30%7%12%27%17%4
1910
September1st to 15thNight
31%412%25%23%3
1818
16th to endNight
32%5%10%28%19%2
1980
October1st to 15thNight
37%5%10%25%19%1
2050
16th to endNight
46%414%22%11%1
2457
November1st to 15thNight
43%410%23%15%3
2471
16th to endNight
46%5%15%23%8%
2555
December1st to 15thNight
54%7%13%17%7%
2730
16th to endNight
65%5%10%12%4
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
25%5%35%29%2
135
16th to endDay
15%441%31%4
144
February1st to 15thDay
22%14%48%14%
135
16th to endDay
19%12%40%21%7%1
100
March1st to 15thDay
28%13%32%24%1
90
16th to endDay
30%5%29%23%10%1
256
April1st to 15thDay
24%11%30%24%9%
270
16th to endDay
20%7%30%25%11%4
270
May1st to 15thDay
21%9%30%25%11%2
288
16th to endDay
15%431%30%14%3
516
June1st to 15thDay
12%227%38%16%3
495
16th to endDay
11%433%34%11%4
495
July1st to 15thDay
10%223%36%19%7%
475
16th to endDay
6%122%40%21%6%
515
August1st to 15thDay
7%15%40%24%11%
382
16th to endDay
8%414%37%25%9%
332
September1st to 15thDay
12%214%35%25%9%
315
16th to endDay
8%5%16%32%30%6%
314
October1st to 15thDay
20%6%22%31%15%3
243
16th to endDay
24%8%33%29%4
144
November1st to 15thDay
15%12%23%31%13%2
135
16th to endDay
24%11%43%15%5%
135
December1st to 15thDay
13%20%37%23%4
135
16th to endDay
24%18%49%42
144
January1st to 15thNight
22%13%36%21%4
975
16th to endNight
18%12%38%26%3
1040
February1st to 15thNight
21%8%38%25%5%
846
16th to endNight
22%10%33%28%4
659
March1st to 15thNight
23%9%32%26%8%
720
16th to endNight
18%7%32%28%10%1
644
April1st to 15thNight
20%9%31%28%8%
539
16th to endNight
22%5%29%28%11%1
540
May1st to 15thNight
21%328%31%14%
540
16th to endNight
12%220%38%23%1
563
June1st to 15thNight
10%319%44%20%1
540
16th to endNight
10%123%38%23%1
540
July1st to 15thNight
8%214%41%29%3
510
16th to endNight
6%114%45%28%4
564
August1st to 15thNight
6%10%46%29%7%
539
16th to endNight
10%213%35%31%6%
573
September1st to 15thNight
11%415%33%27%7%
558
16th to endNight
11%417%31%29%5%
717
October1st to 15thNight
18%7%20%32%19%2
720
16th to endNight
23%13%31%21%9%
768
November1st to 15thNight
19%11%34%22%11%1
760
16th to endNight
20%16%32%23%7%
800
December1st to 15thNight
25%13%36%19%4
975
16th to endNight
29%13%33%20%2
1040
Page generated at 2018-02-23 10:21:40. Version ac04

Copyright 2018 A. Danko


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