| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 28°F | 46°F | 36°F | 5.4" |
| February | 30°F | 48°F | 38°F | 3.9" |
| March | 40°F | 60°F | 49°F | 7.8" |
| April | 49°F | 72°F | 60°F | 8.6" |
| May | 58°F | 79°F | 67°F | 6.3" |
| June | 67°F | 88°F | 77°F | 6.6" |
| July | 69°F | 88°F | 78°F | 6.4" |
| August | 67°F | 88°F | 77°F | 6.0" |
| September | 59°F | 82°F | 70°F | 5.5" |
| October | 48°F | 72°F | 59°F | 5.5" |
| November | 38°F | 59°F | 47°F | 5.8" |
| December | 32°F | 48°F | 39°F | 6.3" |
| Index | Simpson | Illinois | National |
| Air quality index | 44 | 41 | 50 |
| Pollution index | 125,359 | 3,347,295 | 5,066,862 |
| Measurement | Total |
| Days measured | 364 |
| Days with good air quality | 261 |
| Days with moderate air quality | 103 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.003% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City, State | Population | Air Quality Index | National Avg. |
| Golconda, IL | 819 | 44 | 12% |
| Goreville, IL | 1,178 | 44 | 12% |
| Stonefort, IL | 420 | 44 | 12% |
| Belknap, IL | 54 | 44 | 12% |
| Karnak, IL | 601 | 44 | 12% |
| Simpson, IL | 80 | 44 | 12% |
| City, State | Population | Air Quality Index | National Avg. |
| Penermon, MO | 80 | 26 | 48% |
| Stockholm, WI | 80 | 37 | 26% |
| Benedict, ND | 80 | 36 | 28% |
| Playas, NM | 80 | 12 | 76% |
| Maiden Rock, WI | 80 | 37 | 26% |
| Simpson, IL | 80 | 44 | 12% |
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