| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 31°F | 56°F | 43°F | 4.0" |
| February | 33°F | 59°F | 45°F | 3.1" |
| March | 40°F | 68°F | 53°F | 4.3" |
| April | 48°F | 76°F | 62°F | 4.1" |
| May | 57°F | 82°F | 69°F | 3.9" |
| June | 66°F | 90°F | 77°F | 3.8" |
| July | 69°F | 92°F | 79°F | 5.1" |
| August | 69°F | 90°F | 78°F | 4.8" |
| September | 63°F | 85°F | 72°F | 3.8" |
| October | 50°F | 76°F | 62°F | 4.0" |
| November | 39°F | 66°F | 52°F | 3.3" |
| December | 35°F | 58°F | 46°F | 5.6" |
| Index | Duncan | South Carolina | National |
| Air quality index | 44 | 37 | 42 |
| Pollution index | 7,804,180 | 3,620,438 | 2,726,193 |
| Measurement | Total |
| Days measured | 357 |
| Days with good air quality | 249 |
| Days with moderate air quality | 106 |
| Days w/ poor A.Q. for sensitive groups | 2 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.070% |
| Carbon Tetrachloride | 0.020% |
| Lead | 0.050% |
| Mercury | 0.003% |
| City | Population | Air Quality Index | National Avg. |
| Fairforest, SC | 1,484 | 44 | 12.0% |
| Inman, SC | 3,665 | 44 | 12.0% |
| Roebuck, SC | 2,768 | 44 | 12.0% |
| Startex, SC | 556 | 44 | 12.0% |
| Wellford, SC | 2,662 | 44 | 12.0% |
| Duncan, SC | 3,556 | 44 | 12.0% |
| City | Population | Air Quality Index | National Avg. |
| Sainte-Anne-des-Lacs, QC | 3,556 | n/a | |
| Kamloops 1, BC | 3,556 | n/a | |
| Blind River, ON | 3,555 | n/a | |
| Loxahatchee Groves, FL | 3,557 | 36 | 28.0% |
| Unadilla, GA | 3,557 | 38 | 24.0% |
| Duncan, SC | 3,556 | 44 | 12.0% |
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