| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 27°F | 41°F | 34°F | 3.2" |
| February | 28°F | 45°F | 36°F | 1.5" |
| March | 36°F | 53°F | 44°F | 4.2" |
| April | 46°F | 65°F | 55°F | 4.4" |
| May | 56°F | 73°F | 64°F | 1.8" |
| June | 65°F | 82°F | 73°F | 4.5" |
| July | 70°F | 86°F | 77°F | 6.8" |
| August | 67°F | 83°F | 74°F | 1.2" |
| September | 62°F | 78°F | 69°F | 4.8" |
| October | 51°F | 68°F | 58°F | 1.2" |
| November | 39°F | 56°F | 47°F | 1.1" |
| December | 32°F | 47°F | 39°F | 0.4" |
| Index | Harrington | Delaware | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 1,358,370 | 7,274,307 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 293 |
| Days with moderate air quality | 72 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.002% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Greenwood, DE | 1,127 | 36 | 28.0% |
| Ellendale, DE | 751 | 36 | 28.0% |
| Bridgeville, DE | 3,508 | 36 | 28.0% |
| Farmington, DE | 184 | 36 | 28.0% |
| Milton, DE | 2,993 | 36 | 28.0% |
| Harrington, DE | 3,658 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Watergate, FL | 3,658 | 41 | 18.0% |
| Kelseyville, CA | 3,657 | 31 | 38.0% |
| Fairbury, NE | 3,657 | 31 | 38.0% |
| Richmond, Subd. A, NS | 3,657 | n/a | |
| Spencerport, NY | 3,656 | 37 | 26.0% |
| Harrington, DE | 3,658 | 42 | 16.0% |
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