| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 17°F | 37°F | 26°F | 1.8" |
| February | 19°F | 40°F | 29°F | 1.9" |
| March | 32°F | 55°F | 42°F | 3.1" |
| April | 42°F | 66°F | 53°F | 4.7" |
| May | 52°F | 74°F | 62°F | 4.8" |
| June | 62°F | 84°F | 72°F | 6.9" |
| July | 65°F | 87°F | 76°F | 4.3" |
| August | 63°F | 85°F | 73°F | 4.3" |
| September | 55°F | 79°F | 66°F | 4.1" |
| October | 44°F | 66°F | 54°F | 4.0" |
| November | 33°F | 54°F | 42°F | 2.6" |
| December | 22°F | 39°F | 30°F | 1.9" |
| Index | Greencastle | Missouri | National |
| Air quality index | 41 | 97 | 42 |
| Pollution index | 139,865 | 1,630,255 | 2,726,193 |
| Measurement | Total |
| Days measured | 241 |
| Days with good air quality | 208 |
| Days with moderate air quality | 33 |
| 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 | Population | Air Quality Index | National Avg. |
| Worthington, MO | 112 | 29 | 42.0% |
| Pollock, MO | 53 | 39 | 22.0% |
| Unionville, MO | 1,827 | 39 | 22.0% |
| Milan, MO | 2,429 | 41 | 18.0% |
| Winigan, MO | 26 | 41 | 18.0% |
| Greencastle, MO | 176 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Glacier View, AK | 176 | 31 | 38.0% |
| Point Lay, AK | 176 | 38 | 24.0% |
| Alpine, AZ | 176 | 44 | 12.0% |
| Little River, CA | 176 | 33 | 34.0% |
| San Geronimo, CA | 176 | 40 | 20.0% |
| Greencastle, MO | 176 | 41 | 18.0% |
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