| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 24°F | 42°F | 33°F | 3.6" |
| February | 26°F | 46°F | 35°F | 3.0" |
| March | 33°F | 54°F | 43°F | 3.9" |
| April | 43°F | 67°F | 55°F | 3.8" |
| May | 54°F | 76°F | 64°F | 4.6" |
| June | 62°F | 84°F | 73°F | 6.6" |
| July | 69°F | 89°F | 79°F | 5.2" |
| August | 65°F | 85°F | 75°F | 4.6" |
| September | 60°F | 81°F | 70°F | 3.7" |
| October | 48°F | 69°F | 58°F | 4.7" |
| November | 36°F | 58°F | 47°F | 2.6" |
| December | 31°F | 49°F | 40°F | 4.0" |
| Index | Middletown | Delaware | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 2,654,190 | 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 | 0.002% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.004% |
| Lead | 0.000% |
| Mercury | 0.005% |
| City | Population | Air Quality Index | National Avg. |
| Salem, NJ | 4,761 | 40 | 20.0% |
| Pennsville, NJ | 11,968 | 40 | 20.0% |
| Smyrna, DE | 11,694 | 42 | 16.0% |
| St. Georges, DE | 1,045 | 42 | 16.0% |
| Elkton, MD | 15,653 | 42 | 16.0% |
| Middletown, DE | 22,350 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Yellowknife, NT | 22,316 | n/a | |
| Garden City, NY | 22,384 | 51 | 2.0% |
| Patterson, CA | 22,309 | 49 | 2.0% |
| Westmount, QC | 22,392 | n/a | |
| Waynesboro, VA | 22,300 | 40 | 20.0% |
| Middletown, DE | 22,350 | 42 | 16.0% |
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