| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 14°F | 32°F | 23°F | 1.7" |
| February | 15°F | 35°F | 25°F | 1.6" |
| March | 27°F | 50°F | 38°F | 2.7" |
| April | 37°F | 64°F | 50°F | 4.3" |
| May | 47°F | 75°F | 61°F | 3.4" |
| June | 58°F | 84°F | 71°F | 5.1" |
| July | 62°F | 86°F | 74°F | 4.9" |
| August | 61°F | 84°F | 72°F | 5.3" |
| September | 53°F | 79°F | 65°F | 2.8" |
| October | 42°F | 66°F | 53°F | 2.9" |
| November | 30°F | 51°F | 40°F | 2.4" |
| December | 18°F | 36°F | 27°F | 2.5" |
| Index | Rockford | Illinois | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 1,721,090 | 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 | 0.001% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.002% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Caledonia, IL | 195 | 42 | 16.0% |
| Poplar Grove, IL | 4,497 | 42 | 16.0% |
| Byron, IL | 3,607 | 42 | 16.0% |
| Monroe Center, IL | 384 | 42 | 16.0% |
| Pecatonica, IL | 1,957 | 42 | 16.0% |
| Rockford, IL | 147,441 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Joliet, IL | 147,323 | 55 | 10.0% |
| Naperville, IL | 147,734 | 55 | 10.0% |
| Guelph, ON | 147,931 | n/a | |
| Killeen, TX | 148,573 | 42 | 16.0% |
| Bellevue, WA | 146,145 | 45 | 10.0% |
| Rockford, IL | 147,441 | 42 | 16.0% |
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