| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 16°F | 31°F | 23°F | 2.8" |
| February | 17°F | 32°F | 24°F | 2.8" |
| March | 30°F | 47°F | 37°F | 4.6" |
| April | 39°F | 61°F | 49°F | 5.8" |
| May | 50°F | 72°F | 60°F | 5.3" |
| June | 59°F | 80°F | 69°F | 5.6" |
| July | 63°F | 82°F | 72°F | 5.8" |
| August | 61°F | 81°F | 70°F | 5.9" |
| September | 54°F | 75°F | 63°F | 3.7" |
| October | 43°F | 63°F | 52°F | 5.8" |
| November | 32°F | 49°F | 39°F | 4.0" |
| December | 21°F | 34°F | 27°F | 3.1" |
| Index | DeKalb | Illinois | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 1,874,490 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 341 |
| Days with good air quality | 240 |
| Days with moderate air quality | 100 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.002% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.010% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Lee, IL | 380 | 0 | 100.0% |
| Shabbona, IL | 937 | 0 | 100.0% |
| Big Rock, IL | 1,271 | 0 | 100.0% |
| Waterman, IL | 1,738 | 0 | 100.0% |
| Kaneville, IL | 518 | 0 | 100.0% |
| DeKalb, IL | 42,961 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Goose Creek, SC | 42,944 | 38 | 24.0% |
| Fond du Lac, WI | 42,944 | 36 | 28.0% |
| Winter Haven, FL | 42,933 | 36 | 28.0% |
| Freeport, NY | 42,993 | 51 | 2.0% |
| Vaudreuil-Dorion, QC | 42,920 | n/a | |
| DeKalb, IL | 42,961 | 42 | 16.0% |
Here are the best places to live in Illinois
Top 10 cities in Illinois with the best education system
These are the 10 most affordable cities in Illinois
10 Best cities in Illinois for employment