| 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 | Union | Illinois | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 543,516 | 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 | 0.000% |
| Benzene | 0.004% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Greenwood, IL | 352 | 37 | 26.0% |
| Ringwood, IL | 735 | 39 | 22.0% |
| McCullom Lake, IL | 1,193 | 39 | 22.0% |
| Wonder Lake, IL | 3,955 | 39 | 22.0% |
| McHenry, IL | 26,889 | 39 | 22.0% |
| Union, IL | 680 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Reynolds, IL | 680 | 46 | 8.0% |
| Ridgecrest, LA | 680 | 30 | 40.0% |
| Norwood, MO | 680 | 39 | 22.0% |
| Gibson, NC | 680 | 30 | 40.0% |
| Neapolis, OH | 680 | 40 | 20.0% |
| Union, IL | 680 | 42 | 16.0% |
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